Article

Comparisons of laser powder bed fusion additive manufacturing builds through experimental in situ distortion and temperature measurements

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  • Autodesk Inc.
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Abstract

In situ experimental measurements of the laser powder bed fusion build process are completed with the goal gaining insight into the evolution of distortion in the powder bed fusion build process. Utilizing a novel enclosed instrumented system, five experimental builds are performed. Experimental builds compare materials: Ti-6Al-4V and Inconel® 718, differing build geometries, and manufacturing machines: EOS M280 and Renishaw AM250. A combination of in situ measurements of distortion and temperature and post-build measurements of final part geometry are used to compare and contrast the different experiments. Experimental results show that builds completed using Inconel® 718 distort between 50%-80% more relative to Ti-6Al-4V depending on substrate size and build geometry. The experimental build completed on the Renishaw AM250 distorted 10.6% more in the Z direction when compared with the identical build completed on the EOS M280 machine. Comparisons of post-build XY cross-sectional area show a 0.3% contraction from the predefined build geometry for the Renishaw AM250 as compared with the 4.5% contraction for the part built using the EOS M280. Recommendations and future work are also discussed.

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... The first approach is to embed thermocouple(s) inside the part or in the substrate, for measuring temperature trends at a few discrete locations. The second approach uses thermal imaging to measure the part surface temperature [12,13]. This work applies the second strategy. ...
... Researchers have acquired the temperature trends at the underside of the part by brazing thermocouples on the surface of the build plate in a manner such that the head of the thermocouple is barely exposed [12,13]. However, it is observed that the temperature signals obtained by the thermocouple inside the substrate is considerably attenuated as the part grows in size. ...
... For example, Dunbar et al. embedded both a thermocouple and strain gage array within the build plate of an EOS M270 machine to validate their predicted distortion trends [12,13]. In their setup, the sensor array is coupled to a battery-powered data acquisition system incorporated underneath the build plate. ...
Article
The objective of this work is to provide experimental validation of the graph theory approach for predicting the thermal history in additively manufactured parts that was recently published in these transactions. In the present paper the graph theory approach is validated with in-situ infrared thermography data in the context of the laser powder bed fusion (LPBF) additive manufacturing process. We realize this objective through the following three tasks. First, two types of test parts (stainless steel) are made in two corresponding build cycles on a Renishaw AM250 LPBF machine. The intent of both builds is to influence the thermal history of the part by changing the cooling time between melting of successive layers, called interlayer cooling time. Second, layer-wise thermal images of the top surface of the part are acquired using an in-situ a priori calibrated infrared camera. Third, the thermal imaging data obtained during the two builds were used to validate the graph theory-predicted surface temperature trends. Furthermore, the surface temperature trends predicted using graph theory are compared with results from finite element analysis. As an example, for one the builds, the graph theory approach accurately predicted the surface temperature trends to within 6% mean absolute percentage error, and approximately 14 Kelvin root mean squared error of the experimental data. Moreover, using the graph theory approach the temperature trends were predicted in less than 26 minutes which is well within the actual build time of 171 minutes.
... This technology has been employed by end users that are in need for rapid part fabrication with many features that can be easily altered or customized. However, there remain some challenges due to high energy densities applied on the thin layer of powder repeatedly [1,2] and resultant thermal gradients [1,3], dynamic region of molten material [1,4], spatter and defect formation [1,2], stress-induced distortions in relatively large parts with high aspect ratios [2], solidified microstructure [4]. In order to obtain a fuller process understanding, microstructure investigation and modeling for solidification [4], thermal measurements for meltpool and spatter, and modelling for thermal predictions [5] have been reported. ...
... This technology has been employed by end users that are in need for rapid part fabrication with many features that can be easily altered or customized. However, there remain some challenges due to high energy densities applied on the thin layer of powder repeatedly [1,2] and resultant thermal gradients [1,3], dynamic region of molten material [1,4], spatter and defect formation [1,2], stress-induced distortions in relatively large parts with high aspect ratios [2], solidified microstructure [4]. In order to obtain a fuller process understanding, microstructure investigation and modeling for solidification [4], thermal measurements for meltpool and spatter, and modelling for thermal predictions [5] have been reported. ...
... This technology has been employed by end users that are in need for rapid part fabrication with many features that can be easily altered or customized. However, there remain some challenges due to high energy densities applied on the thin layer of powder repeatedly [1,2] and resultant thermal gradients [1,3], dynamic region of molten material [1,4], spatter and defect formation [1,2], stress-induced distortions in relatively large parts with high aspect ratios [2], solidified microstructure [4]. In order to obtain a fuller process understanding, microstructure investigation and modeling for solidification [4], thermal measurements for meltpool and spatter, and modelling for thermal predictions [5] have been reported. ...
Article
Full-text available
L-PBF is an additive manufacturing process which can produce nearly fully dense parts with complex geometry by using laser which follows layer-to-layer scanning on powder material. In-process statistical monitoring techniques are required to detect localize material spatter and control the meltpool. High speed video imaging provides process insights for identifying meltpool and spatter and can be integrated into process monitoring for L-PBF process. We demonstrate the use of high speed camera videos for in-situ monitoring of L-PBF of nickel alloy 625 to detect spatter and over melting regions to improve the process control capability. The quantities that can be measured via in-situ sensing can be referred to as process signatures and can represent the source of information to detect possible defects. The video images are processed for temporal-spatial analysis by using principal component analysis and T2 statistical descriptor for identifying the history of pixel intensity levels through the process monitoring. These results are correlated as over melting and spatter regions.
... When more heat is applied than needed to completely fuse the deposited material, the thermal gradient increases causing higher residual stresses after the component has cooled. 1 Six different scanning strategies were investigated toward the expansion of part distortion during the SLM operation, and it was discovered that the island scanning strategy can cause less distortion than the line scanning strategy. 2 A lot of investigations have been discovered toward distortion on LPBF process experimentally and numerically. ...
... This research on combined TMM-ISM with optimization method uses all principles as described below. In general, the governing equations for TMM-based simulation are based on transient 3D heat conduction equation as defined in Equation (1) in which T, q, C p , k, Q, and v are temperature, density, specific heat capacity, thermal conductivity, absorbed heat, and laser scanning speed. ...
Article
This research concerns on the application of combined thermomechanical—inherent strain method (TMM-ISM) in predicting the distortion of additively manufactured component. The simulation and experimental verification were conducted in the form of vertical cylinder using selective laser melting, which was subsequently cut in the middle section. The setup and procedure of simulation approaches followed the actual process parameters such as laser power, layer thickness, scan strategy, and temperature dependent material, including flow curve retrieved from specialized computational numerical software. The investigation began with virtual calibration test using TMM, followed by manufacturing process simulation using ISM. Based on the maximum deformation result of simulated calibration and accuracy consideration from previous equivalent study, the inherent strain values used in ISM analysis were obtained using self-developed optimization algorithm with direct pattern search Nelder–Mead method in finding the minimum error of distortion using MATLAB. The error minima were measured between transient TMM-based simulation and simplified formulation in calculating the inherent strain values with respect to longitudinal and transverse laser directions. Furthermore, the combined TMM-ISM distortion results were compared to fully TMM with equivalent mesh number and verified based on experimental investigation conducted by renowned researcher. It can be concluded that the result of slit distortion from TMM-ISM and TMM showed good agreement with the error percentage of 9.5% and 3.5%, respectively. However, the computational time for combined TMM-ISM was reduced tremendously with only 63 min if compared to TMM with 129 min in running full simulation on solid cylindrical component. Hence, combined TMM-ISM-based simulation can be considered as an alternative method to replace time-consuming and cost-intensive calibration preparation and analysis.
... techniques have ranged from embedded transducers measuring distortion at selected points [34] to melt pool tracking via photodiodes [35]. The unique capability of the three-dimensional digital image correlation (3D-DIC) system developed in this work is the ability to provide full-field distortion measurements, resulting in matching full-field RS measurements. ...
... These initial temperatures were consistent with in-process data for laser additive builds [34,49]. ...
Article
Article link: https://www.sciencedirect.com/science/article/pii/S2214860418300812 The severe thermal gradients associated with selective laser melting (SLM) additive manufacturing (AM) generate large residual stresses (RS) that geometrically distort and otherwise alter the performance of printed parts. Despite broad research interest in this field, it has remained challenging to measure warpage in general as well as RS distributions in situ, which has obfuscated the mechanisms of stress formation during the printing process. In pursuit of this goal, we have developed a non-destructive framework for RS measurement in SLM parts using three-dimensional digital image correlation (3D-DIC) to capture in situ surface distortion. A two-dimensional analytical model was developed to convert DIC surface curvature measurements to estimates of in-plane residual stresses. Experimental validation using stainless steel 316L “inverted-cone” parts demonstrated that residual stress varied across the surface of the printed part, and strongly interacted with the component geometry. The 3D-DIC based RS measurements were validated by X-ray diffraction (XRD), with an average error of 6% between measured and analytically derived stresses. Systematic variation in RS was attributed to the sector-based laser raster strategy, which was supported by complementary finite element calculations. Calculations showed that the heterogeneous RS distribution in the parts emerged from the sequential re-heating and cooling of the new surface and changed dynamically between layers. The unique DIC based RS methodology brings substantial benefits over alternatively proposed in situ AM RS measurements, and should facilitate enhanced process optimization and understanding leading towards AM part qualification.
... Khairallah et al. [1] reported that the molten pool is dynamically changing involving a complex melt flow which is difficult to control due to the stochastic nature of high-energy laser beam and powder material interaction. Dunbar et al. [2] identified that sections of fused part experience rapidly rising and cooling temperatures resulting in thermal deflections and part distortions. Similarly, Bartlett and Li [3] reported that high-energy thermal processing on the powder bed through heating, melting, and fusion results in a large and fluctuating thermal gradient. ...
... Tian et al. [5] reported large variations in mechanical properties and solidified microstructures are other concerns for parts and products that demand prolonged fatigue life in especially nickel-based superalloys. In addition, as reported by Dunbar et al. [2] and Bartlett and Li [3], thermal deflections and uneven cooling-related distortions are common in relatively large parts with high aspect rations that are built on a metal platform in the powder bed. These and some other studies also relate dynamically changing molten flow behaviour and meltpool size to formation of defects, pores and even delamination among the fused layers. ...
Article
Full-text available
Additive manufacturing is being adopted to produce metal parts directly from digital design with applications in automotive, aerospace, and biomedical products. Laser powder bed fusion (LPBF) process is a viable technology for this purpose that utilizes a high-power laser beam which follows layer-to-layer scanning of predefined paths on a metal powder bed. In situ monitoring of LPBF process is essential to detect the localized meltpool, its vicinity, and material spatter around it with a goal to control the health of the meltpool. Among many alternatives, high-fidelity video monitoring can provide cost-effective insights to the on-going process to detect changes in meltpool and its vicinity that can further be integrated into an adaptive control system. For this purpose, a high frame rate camera was employed for in situ viewing of meltpool regions during laser fusion of a super alloy, Inconel 625, powder material to be able to improve the process control capability. The size and shape of the meltpool and the heat affected region detected via in situ viewing represent the sources of information to detect possible anomalies and defects. These acquired video volumes were processed and analysed using statistical process control (SPC) charts. The results indicate that some occurrences of undermelting, overmelting, and material spatter can be detected that can then be correlated to localized defects, delamination, and layer separation.
... YS for 304 stainless steel. The experiments of Dunbar et al. [21] demonstrated that IN718 parts distort 50% more than Ti6Al4V parts for the same geometry and experiment condition. It results from here that, in manufacture of complex, thin-walled parts of IN718, preventing a distortion is the main criterion of process parameter selection, apart from porosity. ...
... For IN718 parts fabricated by LPBF, the standard ASTM F3055− 14a recommends stress-relief annealing temperature within 1065 • C ± 15 • C for 1.5 h, leaving however to the manufacturer the decision on matching annealing temperature and time with overall dimensions and shape of the part, dimensional accuracy requirements and type of the PBF-LB/M machine. According to Dunbar et al. [21] experiments, two identical IN718 parts made by different manufacturers can be distorted in the z direction with a 10.6% difference. Mercelis and Kruth [23] reported that stress level in the 316 L manufactured components on the build platform reaches the material yield point. ...
Article
Full-text available
During manufacture of Inconel 718 (IN718) parts with complex geometry and high dimensional accuracy by laser powder bed fusion (LPBF), effective stress relieving is required before cutting off the parts from the build platform, in order to prevent distortion. The residual stress can be removed completely only by full recrystallization of structure, induced by annealing at the temperature close to that used in hot isostatic pressing (HIP). This work was aimed at investigating the effect of stress-relieving at 1150 °C for 6 h on properties of LPBF-ed IN718, obtained after each of successive thermal processing steps: optional HIP, solution annealing and double aging, in relation to analogous research performed with use of the thermal processing parameters recommended by ASTM standards. In addition, the effect of heating and cooling rates during individual heat-treatment steps on dissolution and precipitation of secondary phases was investigated. It was shown that an advantage of stress relieving at 1150 °C in comparison to the standard annealing at 1065 °C is higher effectiveness of stress elimination, higher resistance to liquid-film formation during heating-up, lack of delta phase, significantly higher elongation (48%) and area reduction (30%) of IN718 in final (without HIP) aged condition. Although the high-temperature stress relieving and HIP treatments resulted in coarse-grained microstructures, the tensile strength values determined at room and elevated temperatures (425 °C and 650 °C) were higher than the nominal values of wrought IN718 with similar grain size and much higher than those of cast and HIP-ed IN718.
... Distinct build platform materials were reported in the studies collected. Five studies [24,40,61,81] used stainless steel, three studies used carbon steel [28,71,82] and IN718 [30,83,84], and only one used aluminum [73]. Preferably, the platform material should be the same as the built part (so, IN718) to ensure a homogeneous Table 2 Hall flow rate, density, and mechanical properties of LPBF parts using the virgin and recycled IN718 powders [26] heat flow and contractions/dilations and a greater chemical affinity at the first printed powder layers and to avoid the diffusion phenomenon. ...
Article
Full-text available
The substantial growth of laser powder bed fusion (LPBF) technology has partly been driven by its opportunity to provide high-performance complex design solutions with outstanding benefits for the aerospace industry. The key opportunities for metal additive manufacturing in aerospace applications include significant cost and lead-time reductions in addition to the possibility of highly efficient complex and lightweight designs. Inconel 718 (IN718) alloy is one of the most common materials usually employed in rocket engines, turbine blades, and turbocharges. The high geometrical complexity of the type of components demanded a detailed exploration of the LPBF of IN718 parts in the last years. As-built and post-processed IN718 LPBFed parts are covered both in terms of the processing parameters as for the metallurgical and physical properties and the mechanical properties (tensile, hardness, and fatigue properties). These complex inter-relations are presented in such a manner (graphs and tables) that can act as engineering tools for helping engineers and designers to obtain near-net-shape parts made of IN718 with the desired properties. This comprehensive overview of the influence of LPBF processing parameters on the final properties of IN718 alloy allows understanding that there is no straightforward relation between energy density and final properties of LPBFed IN718 parts. Thus, the combination of different parameters must be considered and studied individually based on the requirements of each final application. Based on these observations, challenges and future opportunities are also highlighted for the LPBF production of multi-functional IN718 aerospace parts.
... The cooling time per layer is the dominant factor that influences the final temperature as well as the stress distribution. Dunbar et al. [61] suggests that implementing a cooling step period between each layer can reduce the final residual stress and distortion, and the current work concurs. ...
Article
The Laser Beam Powder Bed Fusion (PBF-LB) category of Additive Manufacturing (AM) is currently receiving much attention for computational process modelling. Major challenges exist in how to reconcile resolution, energy and time in a real build, with the practical limitations of resolution (layer height and mesh resolution), energy (heat format and magnitude) and time (heating and cooling step times) in the computational space. A novel thermomechanical PBF-LB process model including an efficient powder-interface heat loss mechanism was developed. The effect of variations in layer height (layer scaling), energy and time on the temperature and stress evolution was investigated. The influence of heating step time and cooling step time was characterised and the recommended ratio of element size to layer scaling was presented, based on a macroscale 2D model. The layer scaling method was effective when scaling up to 4 times the layer thickness and appropriately also scaling the cooling step time. This research provides guidelines and a framework for layer scaling for finite element modelling of the PBF-LB process.
... Some simplifications to predictive modeling have been proposed to reduce the three-dimensional (3D) heat transfer problem into a subset of twodimensional (2D) solutions [5]. Also, it has been reported that some thermal models can lead to prediction of thermal stress-induced part distortions [6]. However, there is no numerical thermal or physical modeling method available to predict the topography of the surfaces generated with LPBF. ...
Article
Full-text available
The powder bed fusion-based additive manufacturing process uses a laser to melt and fuse powder metal material together and creates parts with intricate surface topography that are often influenced by laser path, layer-to-layer scanning strategies, and energy density. Surface topography investigations of as-built, nickel alloy (625) surfaces were performed by obtaining areal height maps using focus variation microscopy for samples produced at various energy density settings and two different scan strategies. Surface areal height maps and measured surface texture parameters revealed the highly irregular nature of surface topography created by laser powder bed fusion (LPBF). Effects of process parameters and energy density on the areal surface texture have been identified. Machine learning methods were applied to measured data to establish input and output relationships between process parameters and measured surface texture parameters with predictive capabilities. The advantages of utilizing such predictive models for process planning purposes are highlighted .
... Some simplifications to predictive modeling have been proposed to reduce the three-dimensional (3D) heat transfer problem into a subset of two-dimensional (2D) solutions [5]. Also, it has been reported that some thermal models can lead to prediction of thermal stress induced part distortions [6]. However, there is no numerical thermal or physical modeling method available to predict the topography of the surfaces generated with LPBF. ...
Article
Full-text available
The powder bed fusion based additive manufacturing process uses a laser to melt and fuse powder metal material together and creates parts with intricate surface topography that are often influenced by laser path, layer-to-layer scanning strategies, and energy density. Surface topography investigations of as-built, nickel alloy (625) surfaces were performed by obtaining areal height maps using focus variation microscopy for samples produced at various energy density settings and two different scan strategies. Surface areal height maps and measured surface texture parameters revealed highly irregular nature of surface topography created by laser powder bed fusion (LPBF). Effects of process parameters and energy density on the areal surface texture have been identified. Machine learning methods were applied to measured data to establish input and output relationships between process parameters and measured surface texture parameters with predictive capabilities. The advantages of utilizing such predictive models for process planning purposes are highlighted.
... In this example, a = 100/5 = 20 Hz, and frequency spikes at { a } ( ∈ ℤ J ) appear in the fast Fourier transform of the disturbance. It is noteworthy that besides the raster scan that was used as a unit problem in this study, other scanning patterns yield similar repetitive disturbance patterns (see, e.g., experimental results in [27]). As such, automatic control algorithms [26] can be applied to attenuate those undesired repetitive spectra of peak temperature, as will be illustrated next. ...
... Therefore, the disturbance periodicity-defined by the scan speed, part geometry, and path planning-has no guarantees to be an integer multiple of the sampling time of the molten pool sensors. It is also important to recognize that besides the proof-of-concept bidirectional trajectory, other scanning patterns yield repetitive disturbance components in a similar fashion (see, e.g., experimental results in [21]). These fractional-order disturbances challenge conventional RC and demand new theoretical designs for RC to maximize performance in PBF. ...
Conference Paper
Full-text available
This paper studies control approaches to advance the quality of repetitive energy deposition in powder bed fusion (PBF) additive manufacturing. A key pattern in the nascent manufacturing process, the repetitive scanning of the laser or electron beam can be fundamentally improved by repetitive control (RC) algorithms. An intrinsic limitation, however, appears in discrete-time RC when the exogenous signal frequency cannot divide the sampling frequency. In other words, the disturbance period in the internal model is not an integer. Such a challenge hampers high-performance applications of RC to PBF because periodicity of the exogenous signal has no guarantees to comply with the sampling rate of molten-pool sensors. This paper develops a new multirate RC and a closed-loop analysis method to address such fractional-order RC cases by generating high-gain control signals exactly at the fundamental and harmonic frequencies. The proposed analysis method exhibits the detailed disturbance- attenuation properties of the multirate RC in a new design space. Numerical verification on a galvo scanner in laser PBF reveals fundamental benefits of the proposed multirate RC.
... Deformations can take place in a wide variety of forms, such as cracks and warping. [26] If the internal stresses do not reach higher values than the yield strength, they can be observed as residual stresses on the part after manufacturing. Both residual stresses and deformations are important problems for additive manufacturing. ...
Article
The study is focused on the manufacturing of fully dense and porous Ti6Al4V parts by Powder Bed Fusion (PBF) method. In the scope of the study, the reasons of the impact of the powder recoating system and internal stress-induced failures in the manufacturing of tensile specimens were investigated and solution proposals were presented. Results that were determined by implementing the most suitable solution were investigated. In this study, failures that occurred during manufacturing were prevented by analyzing the powder recoating system. As a result of the study, the negative effects, which were caused by the impact of the recoater blade on the parts, were prevented. It was observed that the samples were bent due to the forces that were generated by the recoating system movement during manufacturing. The bending conditions were eliminated when the samples were manufactured in a barrel. Thus, no deformation occurred in the part. The negative effects of internal stresses in samples were removed by using the cooling rate concept and FEA analysis. Finally, assumptions of simulations were verified by manufacturing samples with the same machine parameters.
... The literature survey also reports a large number of in-situ measurements as well as numerical simulations to investigate distortions and residual stresses in AM. Denlinger et al. [16][17][24][25] measured vertical displacements using Laser Displacement Sensors (LDS) and calculated both the deformations and the residual stresses of Ti-6Al-4V cantilever substrates induced by electron beam and laser in AM processes. They also investigated the vertical displacement of the substrate using a Differential Variable Reluctance Transducer (DVRT) in Selective Laser Melting (SLM). ...
Article
Full-text available
Residual stresses and distortions are two technical obstacles for popularizing the Additive Manufacturing (AM) technology. The evolution of the stresses in AM components during the thermal cycles of the metal depositing process is not yet clear, and more accurate in-situ measurements are necessary to calibrate and validate the numerical tools developed for its simulation. In this work a fully coupled thermo-mechanical analysis to simulate the Laser Solid Forming (LSF) process is carried out. At the same time, an exhaustive experimental campaign is launched to measure the temperature evolution at different locations, as well as the distortions and both the stress and strain fields. The thermal and mechanical responses of single-wall coupons under different process parameters are recorded and compared with the numerical models. Good agreement between the numerical results and the experimental measurements is obtained. Sensitivity analysis demonstrates that the AM process is significantly affected by the laser power and the feeding rate, while poorly influenced by the scanning speed.
... Residual stresses have been found to be very low in regions subjected to plastic deformation due to stress and expansion. As a result of deformation, it is assumed that the stressed regions are exposed to stress relaxation [8,9]. ...
Article
Full-text available
Direct metal laser sintering (DMLS), one of the laser powder bed additive manufacturing technologies produces solid metal parts from 3-D CAD data, layer by layer, by melting/sintering and bonding metal powders with a focused laser beam. In this processes isn't complete melting of powder particles in micro melt pools as well as selective laser melting (SLM) and electron beam melting (EBM). Thus some different stress conditions and defects occur depending on the temperature changes during manufacturing. In this study, this problems is investigated aspect cooling rate. Cooling rate affects the solidification process in the melting (sintering) process such as casting, welding, laser assisted processes. Therefore, it also affect part quality and properties. In the scope of study, it is tried to explain how occurring the internal stresses and distortions differ depending on the cooling rates of geometrically different parts in additive manufacturing. The residual stresses and deformations are analyzed by FEA to see relation with geometry (volume, area) to cooling rate for Ti6Al4V materials. Cube shaped samples at 20, 40, 60, 80 and 100 mm edge dimensions have analysed by using FEA. Besides 10mm cube sample is manufactured as solid and verified both as experimental and numerical. Based on the FEA results, cooling rate values are changed from 1.67 to 16.67. In conclusion, the reasons of the problems occurring during laser powder bed fusion are investigated in terms of the cooling rate in relation with the samples geometry.
... The geometric integrity of the thin-wall can be affected by the restricted heat flux due to their smaller cross-section area. The restricted heat flux leads to sharp thermal gradients, which in turn may cause cracking and warping (distortion) defects [11,12]. Another common reason for the frequent failure of thinwalls is the interaction of thermal and mechanical factors; the geometric distortion of the thin-wall due to the thermal gradients causes the part to protrude out of the powder bed, a phenomenon called super-elevation, which leads to contact of the part with the re-coater [13,14]. ...
Conference Paper
The goal of this work is to quantify the link between the design features (geometry), in-situ process sensor signatures, and build quality of parts made using laser powder bed fusion (LPBF) additive manufacturing (AM) process. This knowledge is critical for establishing design rules for AM parts, and to detecting impending build failures using in-process sensor data. As a step towards this goal, the objectives of this work are two-fold: 1) Quantify the effect of the geometry and orientation on the build quality of thin-wall features. To explain further, the geometry-related factor is the ratio of the length of a thin-wall (l) to its thickness (t) defined as the aspect ratio (length-to-thickness ratio, l/t), and the angular orientation (θ) of the part, which is defined as the angle of the part in the X-Y plane relative to the re-coater blade of the LPBF machine. 2) Assess the thin-wall build quality by analyzing images of the part obtained at each layer from an in-situ optical camera using a convolutional neural network. To realize these objectives, we designed a test part with a set of thin-wall features (fins) with varying aspect ratio from Titanium alloy (Ti-6Al-4V) material — the aspect ratio l/t of the thin-walls ranges from 36 to 183 (11 mm long (constant), and 0.06 mm to 0.3 mm in thickness). These thin-wall test parts were built under three angular orientations of 0°, 60°, and 90°. Further, the parts were examined offline using X-ray computed tomography (XCT). Through the offline XCT data, the build quality of the thin-wall features in terms of their geometric integrity is quantified as a function of the aspect ratio and orientation angle, which suggests a set of design guidelines for building thin-wall structures with LPBF. To monitor the quality of the thin-wall, in-process images of the top surface of the powder bed were acquired at each layer during the build process. The optical images are correlated with the post build quantitative measurements of the thin-wall through a deep learning convolutional neural network (CNN). The statistical correlation (Pearson coefficient, ρ) between the offline XCT measured thin-wall quality, and CNN predicted measurement ranges from 80% to 98%. Consequently, the impending poor quality of a thin-wall is captured from in-situ process data.
... Residual stresses have been found to be very low in regions subjected to plastic deformation due to stress and expansion. As a result of deformation, it is assumed that the stressed regions are exposed to stress relaxation [8,9]. ...
Conference Paper
Full-text available
Direct metal laser sintering (DMLS), one of the laser powder bed additive manufacturing technologies produces solid metal parts from 3-D CAD data, layer by layer, by melting/sintering and bonding metal powders with a focused laser beam. In these processes isn't complete melting of powder particles in micro melt pools as well as selective laser melting (SLM) and electron beam melting (EBM). Thus some different stress conditions and defects occur depending on the temperature changes during manufacturing. In this study, this problem is investigated aspect cooling rate. Cooling rate affects the solidification process in the melting (sintering) process such as casting, welding, laser assisted processes. Therefore, it also affects part quality and properties. In the scope of study, it is tried to explain how occurring the internal stresses and distortions differ depending on the cooling rates of geometrically different parts in additive manufacturing. The residual stresses and deformations are analysed by FEA to see relation with geometry (volume, area) to cooling rate for Ti6Al4V materials. Cube shaped samples at 20, 40, 60, 80 and 100 mm edge dimensions have analysed by using FEA. Besides 10mm cube sample is manufactured as solid and verified both as experimental and numerical. Based on the FEA results, cooling rate values are changed from 1.67 to 16.67. In conclusion, the reasons of the problems occurring during laser powder bed fusion are investigated in terms of the cooling rate in relation with the samples geometry.
... The part sides and bottom were held at 80°C, simulating the retained heat in the surrounding powder bed and base plate. Though arbitrary, these temperature boundary conditions are appropriate approximate values for laser additive processes as determined experimentally [27,32]. The final layer was then added and heated to the material's melting point of 660°C for 0.1 s and allowed to cool for 1 s. ...
Article
https://www.sciencedirect.com/science/article/pii/S2214860418305293. Selective laser melting (SLM) has become one of the most commonly utilized processes in metal additive manufacturing (AM). Despite its widespread use and capabilities, SLM parts are still being produced with excessive volumetric defects and flaws. The complex dependence of defect formation on process parameters, geometry, and material properties has inhibited effective quality assurance in SLM production. Exacerbating these issues are the difficulties thus far in accurately detecting and identifying defects in-process so that parts may be qualified without destructive testing. Some of the most detrimental defects produced during SLM processing are lack of fusion (LoF) defects, which are frequently found to be in excess of 100 μm in size, thus these defects are of critical importance to detect and remove. In this work, we have developed and demonstrated the capabilities of a novel in situ monitoring system using full-field infrared (IR) thermography to monitor AlSi10Mg specimens during SLM production. Using layerwise relative surface temperature measurements, subsurface defects were identified via their retained thermal signature at the surface; transient thermal modeling was performed, which supported these observations. Parts were characterized using ex situ scanning electron microscopy (SEM) to validate data identified defects and, critically, to estimate detection success. The IR defect detection method was highly effective in identifying defects, with an 82% total success rate for LoF defects; detection success improved with increasing defect size. The method was also used statistically to analyze the presence of systematic process errors during SLM production, expanding the capabilities of IR monitoring methods. This unique analysis method and simple integration for in situ IR monitoring can immediately improve non-destructive qualification methods in SLM processing.
... However, neglecting the pre-placed powder layer in their simulations resulted in predicting temperatures 30% higher than measured ones and they suggested including the placement of powder layer for more accurate predictions. Later, Dunbar et al. [11] conducted insitu experimental measurements with a goal of gaining insight into the evolution of distortion in the powder bed fusion build process. Their experimental results showed that builds completed using nickel alloy IN718 distort between 50% and 80% more relative to titanium alloy Ti-6Al-4V depending on substrate size and build geometry. ...
... Earlier simulations of DED of Ti-6Al-4V from the group are presented in [23,33]. They also modelled PBF of Ti-6Al-4V and Alloy 718 [34,35], and built cylindrical shapes with an outer diameter of 15.88 mm and a wall thickness of 1.59 mm, and a height up to 12.27 mm. Each layer was 50 μm in thickness and remeshing was applied to reduce the amount of required elements. ...
Article
The development of computational welding mechanics (CWM) began more than four decades ago. The approach focuses on the region outside the molten pool and is used to simulate the thermo-metallurgical-mechanical behaviour of welded components. It was applied to additive manufacturing (AM) processes when they were known as weld repair and metal deposition. The interest in the CWM approach applied to AM has increased considerably, and there are new challenges in this context regarding welding. The current state and need for developments from the perspective of the authors are summarised in this study.
... While the investigations shown in Figure 9 show techniques for DED, the same techniques have been applied to PBF techniques [89]. Other temperature measurement techniques for both investigating the thermal phenomena and verifying/validating thermal models have been investigated such as infrared thermography and pyrometry [90][91][92][93], which have equally shown strong agreement between experiment and modeling. ...
Article
Next generation, additively-manufactured metallic parts will be designed with application-optimized geometry, composition, and functionality. Manufacturers and researchers have investigated various techniques for increasing the reliability of the metal-AM process to create these components, however, understanding and manipulating the complex phenomena that occurs within the printed component during processing remains a formidable challenge—limiting the use of these unique design capabilities. Among various approaches, thermomechanical modeling has emerged as a technique for increasing the reliability of metal-AM processes, however, most literature is specialized and challenging to interpret for users unfamiliar with numerical modeling techniques. This review article highlights fundamental modeling strategies, considerations, and results, as well as validation techniques using experimental data. A discussion of emerging research areas where simulation will enhance the metal-AM optimization process is presented, as well as a potential modeling workflow for process optimization. This review is envisioned to provide an essential framework on modeling techniques to supplement the experimental optimization process.
... Therefore, the disturbance periodicity -defined by the scan speed, the part geometry, and the laser path planninghas no guarantees to be an integer multiple of the sampling rate of the molten pool sensors. It is also important to recognize that besides the proof-of-concept bidirectional trajectory, other scanning patterns yield repetitive disturbance components in a similar fashion (see, e.g., experimental results in Dunbar, Denlinger, Gouge, Simpson, and Michaleris (2017). These fractional-order disturbances challenge conventional RC and demand new algorithmic designs for RC to maximize performance in SLS. ...
Article
Full-text available
This paper discusses fractional-order repetitive control (RC) to advance the quality of periodic energy deposition in laser-based additive manufacturing (AM). It addresses an intrinsic RC limitation when the exogenous signal frequency cannot divide the sampling frequency of the sensor, e.g., in imaging-based control of fast laser-material interaction in AM. Three RC designs are proposed to address such fractional-order repetitive processes. In particular, a new multirate RC provides superior performance gains by generating high-gain control exactly at the fundamental and harmonic frequencies of exogenous signals. Experimentation on a galvo laser scanner in AM validates effectiveness of the designs.
... A wealth of knowledge exists in the literature on the numerical and experimental approaches to analyze the thermal field and part distortion in metal additive manufacturing processes. Alexander et al. [1] performed a serial of experiments to measure the temperate field and distortion of laser powder bed fusion based on different machines, materials and process parameters. Thermocouples were used to capture the temperature history at the substrate, and one self-designed ruler was used to measure the displacement of build layers during the SLM process. ...
Conference Paper
Due to rapid cyclic heating and cooling in metal additive manufacturing processes, such as selective laser melting (SLM) and direct metal deposition (DMD), large thermal stresses will form and this may lead to the loss of dimensional accuracy or even cracks. The integration of numerical analysis and experimental validation provides a powerful tool that allows the prediction of defects, and optimization of the component design and the additive manufacturing process parameters. In this work, a numerical simulation on the thermal process of DMD of 0Cr18Ni9 stainless steel is conducted. The simulation is based on the finite volume method (FVM). An in-house code is developed, and it is able to calculate the temperature distribution dynamically. The model size is 30mm × 30mm × 10.5mm, containing 432,000 cells. A DMD experiment on the material with the same configuration and process parameters is also carried out, during which an infrared camera is adopted to obtain the surface temperature distribution continuously, and thermocouples are embedded in the baseplate to record the temperature histories. It is found that the numerical results agree with the experimental results well.
... Note that besides the bidirectional scan used here, other scan patterns yield similar repetitive disturbances (see, e.g., experimental results in [22]). Here, automatic control algorithms [1,20] can be brought in to handle those undesired repetitive spectra, as will be discussed in Section 5. ...
Preprint
Although laser-based additive manufacturing (AM) has enabled unprecedented fabrication of complex parts directly from digital models, broader adoption of the technology remains challenged by insufficient reliability and in-process variations. In pursuit of assuring quality in the selective laser sintering (SLS) AM, this paper builds a modeling and control framework of the key thermodynamic interactions between the laser source and the materials to be processed. First, we develop a three-dimensional finite element simulation to understand the important features of the melt-pool evolution for designing sensing and feedback algorithms. We explore how the temperature field is affected by hatch spacing and thermal properties that are temperature-dependent. Based on high-performance computer simulation and experimentation , we then validate the existence and effect of periodic disturbances induced by the repetitive in-and cross-layer thermomechanical interactions. From there, we identify the system model from the laser power to the melt pool width and build a repetitive control algorithm to greatly attenuate variations of the melt pool geometry. 1 Introduction Different from conventional subtractive machining, additive manufacturing (AM, also called 3D printing) builds up a part from its digital model by adding together materials layer by layer. This paper studies laser-based AM technologies , with a focus on the selective laser sintering (SLS) subcategory. This AM technology applies laser beams as the energy source to melt and join powder materials. A typical workpiece is built from many thousands of thin layers. Within each layer, the laser beam is controlled to follow tra-jectories predefined by the part geometry in a slicing process. After the sintering of one layer is finished, a new thin layer of powder is spread on top, and then another cycle begins. SLS accommodates a broad range of materials (e.g., metals , polymers, and ceramics) and can build customized parts with complex features and high accuracy requirements. Despite the advantages and continuously emerging applications, * Corresponding author broader adoption of the technology remains challenged by insufficient reliability and in-process variations. These variations are induced by, for example, environmental vibrations, powder recycling, imperfect laser-material interactions, and mechanical wears [1-3]. Predictive modeling and process control have thus been key for mitigating the variations and enhancing the energy deposition in SLS. Several existing strategies employ numerical and control-oriented modeling to understand SLS and other laser-based AM processes such as laser metal deposition. In numerical modeling, most researchers adopt finite element analysis (FEA) to investigate thermal fields of the powder bed and substrate, melt pool geometries, and mechanical properties of the printed parts in response of various scanning patterns, scan speeds, number of lasers, and over-hanging structures [4-6]. In control-oriented modeling, current researches often implement low-order system models obtained from system identification techniques, taking laser power or scan speed as the input and melt pool temperature or geometry as the output [2, 7-9]. Furthermore, [8, 10] connect a nonlinear memoryless submodel in series with the linear system model to account for nonlinearities. [9] builds a spatial-domain Hammerstein model to identify the coupled repetitive in-and cross-layer dynamics. The Rosenthal equations give the analytical solutions for a moving laser source in thick and thin plates and have been used to predict the temperature distribution of the powder bed [11-14]. Based on the reduced-order models, existing researches [2, 15, 16] apply PID control to regulate the process parameters and reduce the in-process errors. From there, [17] adds a feedforward path for tracking improvement. Other controllers have also been shown capable in improving the dimensional accuracy of the printed parts, including but not limited to the sliding mode controller [10], predictive controller [7], and iterative learning controller [18]. Note that except for [2], which was developed for SLS, all the other reviewed controllers were tailored for laser metal deposition. Stepping beyond current architectures, this study builds 1 Copyright c by ASME
... The restricted heat flux leads to sharp thermal gradients, which may in turn cause cracking and warping (distortion) defects. 11,12 Another common reason for the frequent failure of thin-walls is the interaction between thermal and mechanical factors. The geometric distortion of the thin-wall due to thermal gradients causes the part to protrude out of the powder bed, a phenomenon called super-elevation, which leads to part contact with the recoater. ...
Article
Full-text available
The goal of this work is to mitigate flaws in metal parts produced from the laser powder bed fusion (LPBF) additive manufacturing (AM) process. As a step toward this goal, the objective of this work is to predict the build quality of a part as it is being printed via deep learning of in situ layer-wise images acquired using an optical camera instrumented in the LPBF machine. To realize this objective, we designed a set of thin-wall features (fins) from titanium alloy (Ti-6Al-4V) material with a varying length-to-thickness ratio. These thin-wall test parts were printed under three different build orientations, and in situ images of their top surface were acquired during the process. The parts were examined offline using X-ray computed tomography (XCT), and their build quality was quantified in terms of statistical features, such as the thickness and consistency of its edges. Subsequently, a deep learning convolutional neural network (CNN) was trained to predict the XCT-derived statistical quality features using the layer-wise optical images of the thin-wall part as inputs. The statistical correlation between CNN-based predictions and XCT-observed quality measurements exceeds 85 %. This work has two outcomes consequential to the sustainability of AM: (1) it provides practitioners with a guideline for building thin-wall features with minimal defects, and (2) the high correlation between the offline XCT measurements and in situ sensor-based quality metrics substantiates the potential for applying deep learning approaches for the real-time prediction of build flaws in LPBF.
... The disturbance periodicity is closely related to the recurring laser scanning trajectories and the repetitive in-layer thermomechanical interactions. Besides the bidirectional scan, other scan patterns yield similar repetitive disturbances (see, e.g., experimental results in [32]). To deal with these undesired repetitive spectra, we develop the closed-loop simulation in Section 2 to bring automatic control algorithms [1,33] into FEM. ...
Article
Full-text available
A high-precision additive manufacturing process, powder bed fusion (PBF) has enabled unmatched agile manufacturing of a wide range of products from engine components to medical implants. While finite element modeling and closed-loop control have been identified key for predicting and engineering part qualities in PBF, existing results in each realm are developed in opposite computational architectures wildly different in time scale. This paper builds a first-instance closed-loop simulation framework by integrating high-fidelity finite element modeling with feedback controls originally developed for general mechatronics systems. By utilizing the output signals (e.g., melt pool width) retrieved from the finite element model (FEM) to update directly the control signals (e.g., laser power) sent to the model, the proposed closed-loop framework enables testing the limits of advanced controls in PBF and surveying the parameter space fully to generate more predictable part qualities. Along the course of formulating the framework, we verify the FEM by comparing its results with experimental and analytical solutions and then use the FEM to understand the melt-pool evolution induced by the in- and cross-layer thermomechanical interactions. From there, we build a repetitive control algorithm to attenuate variations of the melt pool width.
... Several studies have been recently conducted to utilize physical experiments for discovering the suitable build and process factors in LPBF of thin walls [22,23]. Dunbar et al. [24] designed an experiment on the effect of thin wall orientation angle regarding recoater blade direction as well as the build platform on porosity within the thin wall. Kranz et al. [25] performed an experiment on thin walls to obtain guidelines for good build quality. ...
Article
Despite the flexibility offered by additive manufacturing (AM), various process factors ranging from design to material can lead to inconsistent repeatability and quality issues from build-to-build. Currently, AM process monitoring mainly focuses on gaining an understanding of physical phenomena involved in a single quality indicator, ignoring the correlation between heterogeneous characteristics that can provide insight useful for accurate analysis. In this study, a multimodal Gaussian process methodology is designed to simultaneously quantify interactions between AM design parameters at play (e.g., orientation, thickness, height, and contour spacing) and heterogeneous quality characteristics (e.g., distortion, porosity, and roughness) when fabricating thin walls using AM. A real-world study is performed on Ti6Al4V thin wall structures fabricated with laser powder bed fusion, multiple design parameters are varied to evaluate the capability of the proposed method in quantifying such interactions. Experimental results using x-ray computed tomography show that the proposed multimodal Gaussian process provides an improvement of 21.7% compared to the single response model with respect to the root mean square error. The proposed methodology shows great potential to quantify heterogeneous characteristics of quality according to the process factors in a variety of AM builds, such as lattice and honeycomb structures.
... A variety of strategies have been widely investigated to reduce RS, such as process parameters [4,5], base plate preheating [6,7], and structure optimization [8]. For better control of the thermal history, microstructure, and mitigation of the RS [9], a novel duallaser beam PBF-LB was recently presented as a promising technique [10], where a laser beam (which works as a main melting laser) with enough power to melt the powders and a secondary laser (which acts as an auxiliary heat source) with reduced power to slow the cooling process during PBF-LB. ...
Article
Full-text available
Laser beam powder bed fusion (PBF-LB) additive manufacturing (AM) is an advanced manufacturing technology that manufactures metal components in a layer-by-layer manner. The thermal residual stress (RS) induced by the repeated heating–melting–cooling–solidification processes of AM is considered to limit the wider uptake of PBF-LB. A dual-laser beam PBF-LB strategy, with an additional auxiliary laser and reduced power, working in the same powder bed simultaneously, was recently proposed to lower RS within the manufactured components. To provide insights into the optimum PBF-LB AM configurations and process parameters for dual-laser PBF-LB, this study proposed three different coordinated heating strategies (i.e., parallel heating, post-heating, and preheating) of the auxiliary heat source. The temperature fields and RS of dual-laser beam PBF-LB, for Ti-6Al-4V with different process parameters, were computationally investigated and optimized by the thermo-mechanically coupled 3D models. Compared with the single beam PBF-LB, parallel heating, post-heating, and post-heating strategies were proved as effective approaches to reduce RS. Among these, the preheating scanning is predicted to be more effective in mitigating RS, i.e., up to a 10.41% RS reduction, compared with the single laser scanning. This work could be beneficial for mitigating RS and improve the mechanical properties of additively manufactured metal components.
... They found that a minimum thickness of 0.4 mm is required to have a good build quality. Dunbar et al. [30] designed an experiment on the effect of thin wall orientation angle regarding recoater blade direction as well as the build platform on porosity within the thin wall. They achieved that to attain thin walls with consistent thickness, the orientation angle concerning the build platform and recoater blade direction should be 90°and 45°, respectively. ...
Conference Paper
The capability of metal additive manufacturing (AM) to produce parts with complex geometries manifests its potential to revolutionize manufacturing. However, the presence of heterogeneous defects even under optimized part design and processing conditions imposes critical barriers towards scaling metal AM to production environments. The recent advancement in imaging technology leads to a data-rich environment in AM and provides a unique opportunity to enhance understanding of design-quality interactions. However, the state-of-the-art image-guided methodologies focus on the characterization of the individual defect and are less concerned about transferring knowledge across heterogeneous defects to improve modeling and prediction ability. This study introduces a novel data-driven methodology to simultaneously quantify interactions between design parameters (e.g., orientation, thickness, and height) and heterogeneous defects (e.g., geometry distortion, discontinuity, and porosity) in the metallic AM build. A real-world case study on complex thin wall structures is evaluated with the single-response characterization models. Experimental results show average improvement of 64.8%, 70.4%, 73.3% in RMSE in comparison with single response model with inducing percentage u = 0.25, 0.5, and 0.75, respectively. The proposed model enables providing practitioners with a guideline to understand multimodal defects interrelation and fabrication of thin walls with minimal defects.
... As reported in previous studies, the defects either on, or just beneath the free surface, were found to be the CIR for all HCF samples [44]. Since the critical defects in individual specimens are different in size, location and shape, the fatigue life under a lower stress level also varies between individual specimens [45,46]. Uncertainty of defect sizes in AM treated alloys is regarded as the critical factor in the scatter of fatigue S-N data [25,47]. ...
Article
The mechanical and fatigue resistance of additively manufactured (AM) metals is based on its microstructural features and defects, that can be mitigated by post-processing. Currently, understanding the effect of metallurgical defects on fatigue life is a significant step towards the wider application of AM alloys. Here, the tensile properties of laser powder bed fused (L-PBF) Ti-6Al-4V alloys, with varied annealing temperatures (650–950 ℃) were studied, and fatigue resistance under the optimized annealing temperature was investigated. The results show that after the optimized heat treatment, the high cycle fatigue (HCF) data of this alloy still has a large variation. Surface defects typically act as crack origin sites. The largest defect size of HCF samples was predicted by extreme value statistics, based on observation at the fatigue fracture origin. This was used to establish the relationship between defect size and fatigue limits. The El-Haddad model is proven to better fit the HCF data than the Classical linear elastic fracture mechanics and standard Murakami approach. Finally, a fatigue life prediction model based on a normalized fatigue S-N curve is proposed, to well describe the relationship between critical defect size and loading stress.
... The furnace temperature traces were verified with many vendors, but not all. Additionally, the build plate thickness can have an impact on the geometry, but limited studies are available that compare similar conditions to varying part geometry to fully understand [85,86]. Many build plates from the current study were observed to be a minimum of 18 mm up to 32 mm although exact dimensions not captured. ...
Article
Full-text available
This study evaluates a series of additively manufactured geometric feature build plates to baseline variations and process capability between multiple Laser Powder Bed Fusion (L-PBF) machine configurations. The geometric feature build plate has dimensions of 140 mm in X-orientation, 140 mm in Y-orientation, and 31.8 mm in Z-height. Geometric features incorporated include but are not limited to X- and Y-distances; varying angle walls; horizontal holes; and vertical features including round holes, concentric hollow cylinders, protruding cylinders, square channels, thin wall thicknesses, freeform surface, and slots. Select features are based on standardized ISO/ASTM test artifact geometry. Additional features were included that are of interest for various aerospace or industrial applications and to understand practical limitations of various machine configurations. All the features are designed and located to be visually and geometrically accessible for inspection without removal from the base plate and to have high probability for successful printing among L-PBF platforms. Features from each of the 16 build plates were measured using optical and mechanical measurement methods and data compiled for a detailed evaluation of each feature. The systematic error for build accuracy across all features was 23.8±5.5 μm with a 99.9% confidence interval.
... Moreover, heat transfer mechanism, geometry and positioning of samples have a strong in uence on the temperature gradients experienced by the material. [24,25]. Temperature values obtained by the thermal camera were consistent with the literature [26]. ...
Preprint
Full-text available
Among additive manufacturing technologies, Laser Powder Bed Fusion (L-PBF) is considered the most widespread layer-by-layer process. Although the L-PBF, which is also called as SLM method, has many advantages, several challenging problems must be overcome, including part positioning issues. In this study, the effect of part positioning on the microstructure of the part in the L-PBF method was investigated. Five Ti6Al4V samples were printed in different positions on the building platform and investigated with the aid of temperature, porosity, microstructure and hardness evaluations. In this study, martensitic needles were detected within the microstructure of Ti6Al4V samples. Furthermore, some twins were noticed on primary martensitic lines and the agglomeration of β precipitates was observed in vanadium rich areas. The positioning conditions of samples were revealed to have a strong effect on temperature gradients and on the average size of martensitic lines. Besides, different hardness values were attained depending on sample positioning conditions. As a major result, cooling rates were found related to positions of samples and the location of point on the samples. Higher cooling rates and repetitive cooling cycles resulted in microstructures becoming finer and harder.
... Biegler et al. [10] and Xie et al. [11] employed Digital Image Correlation (DIC) to monitor the in-situ distortion in metal AM processes. Dunbar et al. [12] used a differential variable reluctance transducer (DVRT) to measure the in-situ distortion in the laser powder bed fusion process. Heigel et al. [7], Dunbar et al. [13] used the coordinate measurement machine (CMM) to measure the post-process distortion in metal AM process. ...
Article
Undesired distortion often occurs in metal additive manufacturing due to the high temperature gradient resulting from repeated thermal cycles. A good understanding and fast predictions of in-situ distortion are essential to achieve high dimensional accuracy and prevent delamination or failure of build parts. Experimental investigations and numerical methods have been employed to study the in-situ distortion. However, the complex measurement systems and high computational cost limit their applications. An analytical modeling method with closed-form solutions is proposed in this paper to predict the in-situ distortion of laser cladding process without using iteration-based numerical calculations. The effects of build edges and geometry are considered, which include thermal convection and radiation at boundaries. Heat input and heat sink solutions modified from the point moving heat source model are added together to predict the temperature profile of the build and substrate. The die-substrate assembly model is used to calculate the deflection during the manufacturing process. Alloy 625 is selected to test the predictive accuracy and computational efficiency of the presented analytical model. The predicted results are close to the experimental data of in-situ distortion in literature. The computational time is less than 30 s. The good predictive accuracy and low computational cost make the presented method a promising approach to study the full-field temperature and distortion of a geometrically complex part.
... The disturbance periodicity is closely related to the recurring laser scanning trajectories and the repetitive in-layer thermomechanical interactions. Besides the bidirectional scan, other scan patterns yield similar repetitive disturbances (see, e.g., experimental results in [32]). To deal with these undesired repetitive spectra, we develop the closed-loop simulation in Section 2 to bring automatic control algorithms [1,33] into FEM. ...
Preprint
Full-text available
A high-precision additive manufacturing process, powder bed fusion (PBF) has enabled unmatched agile manufacturing of a wide range of products from engine components to medical implants. While finite element modeling and closed-loop control have been identified key for predicting and engineering part qualities in PBF, existing results in each realm are developed in opposite computational architectures wildly different in time scale. This paper builds a first-instance closed-loop simulation framework by integrating high-fidelity finite element modeling with feedback controls originally developed for general mechatronics systems. By utilizing the output signals (e.g., melt pool width) retrieved from the finite element model (FEM) to update directly the control signals (e.g., laser power) sent to the model, the proposed closed-loop framework enables testing the limits of advanced controls in PBF and surveying the parameter space fully to generate more predictable part qualities. Along the course of formulating the framework, we verify the FEM by comparing its results with experimental and analytical solutions and then use the FEM to understand the melt-pool evolution induced by the in-and cross-layer thermomechan-ical interactions. From there, we build a repetitive control algorithm to attenuate variations of the melt pool width.
... Moreover, the laser power was 280 W, and the laser scan speed was 1200 mm/s. Details on the processing parameters were reported in 16 . All the specimens were fabricated on the pure titanium substrate of 250 × 250mm 2 and 15 mm in thickness. ...
Article
In the additive manufacturing (AM) process, metal powder can be directly used to produce metal components. Unfortunately, a large thermal gradient is developed during the AM process, which leads to the generation of residual stress and complex shape-distortions. In this study, the influence of the geometrical size and structural features of a hollow Ti-alloy blade prepared by the AM process on the shape-distortion behavior was systematically investigated using the three-dimensional (3D) blue-light scanning technology. The results indicated that the concentrated residual stress was developed on the surface of the blade. The compressive residual stress induced a bulging distortion, while the tensile residual stress resulted in denting distortion on the blade surfaces. When the blade height and torsion angle increased, the shape-distortion was aggravated owing to the accumulation of microscopic strain and the elevated temperature gradient. However, the shape-distortion mitigated when the wall thickness significantly increased or the stiffened plates were set within the blade cavities, owing to a strengthening structural constraint which inhibited the distortion behavior. In addition, a control method for the shape-distortion during AM process was able to implement based on the proper optimization of the geometrical sizes and structural features of complex 3D-printed components.
... The geometric integrity of the thin wall can be affected by the restricted heat flux due to its smaller cross-sectional area. The restricted heat flux leads to sharp thermal gradients, which in turn may cause cracking and warping (distortion) defects [13,14]. Another common reason for the frequent failure of thin-walls is the interaction of thermal and mechanical factors; the geometric distortion of the thin-wall due to the thermal gradients causes the part to protrude out of the powder bed, a phenomenon called super-elevation, which leads to contact of the part with the re-coater [15,16]. ...
Article
Full-text available
The goal of this thesis is to quantify the link between the design features (geometry), in-process signatures, and build quality of parts made using the laser powder bed fusion (LPBF) additive manufacturing (AM) process. This knowledge is the foundational basis for proposing design rules in AM, as well as for detecting the impending build failures using in-process sensor data. As a step towards this goal, the objectives of this work are two-fold: 1) Quantify the effect of the geometry and orientation on the build quality of thin-wall features. To explain further, the geometry related factor is the ratio of the length of a thin wall (𝑙) to its thickness (𝑡) in the X-Y plane along which powder is deposited (raked or rolled), termed as the aspect ratio (length-to-thickness ratio, 𝑙/𝑡), and the angular orientation (θ) of the part which refers to the inclination of the part in the X-Y plane to the re-coater of the LPBF machine. 2) Monitor the thin-wall build quality by analyzing the images of the part obtained from an in-process optical camera using a convolutional neural network. To realize these objectives, we designed a test part with a set of thin-wall features (fins) with varying aspect ratios from Titanium alloy (Ti-6Al-4V) material – the aspect ratio 𝑙/𝑡 of the thin-walls ranges from 36 to 183 (11 mm long [constant], and 0.3 mm to 0.06 mm in thickness). These thin-wall test artifacts were built under three angular orientations, 0°, 60°, and 90°. Further, the parts were examined offline using X-ray computed tomography (XCT). Through the offline XCT data, the build quality of the thin-wall features in terms of its geometric integrity was quantified as a function of the aspect ratio and orientation angle, which helped codify a set of design guidelines for building thin-wall structures with LPBF. The resulting geometric design rules are summarized as follows. 1) The orientation angle (θ) of 90° should be avoided while building thin-wall structures. 2) The aspect ratio (𝑙/𝑡) of a thin wall should not exceed 73 (11 mm / 0.15 mm). 3) The height of a thin wall should not be more than nine times its thickness. To monitor the quality of the thin-wall, in-process images of the top surface of the bed were acquired during the build process. The online optical images were correlated with the offline quantitative measurements of the thin walls through a deep learning convolutional neural network (CNN). The statistical correlation (Pearson coefficient, 𝜌) between the offline XCT-measured thin-wall quality and the CNN predicted measurement ranged from 80% to 98%. Consequently, the impending poor quality of a thin wall was captured from in-process data. Advisor: Prahalada Rao
... Direct energy deposition (DED) also completely melts the powder. This method shares similarities with welding and is known to affect the material's properties [12]. Sandia National Research Lab integrated a laser beam into the DED in a method later called LENS (laser engineered net shaping) [13]. ...
Article
Full-text available
Additive manufacturing (AM) has recently been accorded considerable interest by manufacturers. Many manufacturing industries, amongst others in the aerospace sector, are already using AM parts or are investing in such manufacturing methods. Important material properties, such as microstructures, residual stress, and surface topography, can be affected by AM processes. In addition, a subtractive manufacturing (SM) process, such as machining, is required for finishing certain parts when accurate tolerances are required. This finish machining will subsequently affect the surface integrity and topography of the material. In this research work, we focused on the surface integrity of Ti-6Al-4V parts manufactured using three different types of AM and finished using an SM step. The aim of this study was to gain an understanding on how each process affects the resulting surface integrity of the material. It was found that each AM process affects the materials’ properties differently and that clear differences exist compared to a reference material manufactured using conventional methods. The newly generated surface was investigated after the SM step and each combination of AM/SM resulted in differences in surface integrity. It was found that different AM processes result in different microstructures which in turn affect surface integrity after the SM process.
... The disturbance periodicity is closely related to the recurring laser scanning trajectories and the repetitive in-layer thermomechanical interactions. Besides the bidirectional scan used in this study, other scan patterns yield similar repetitive disturbances (see, e.g., experimental results in [23]). To deal with these undesired repetitive spectra, we implement the closed-loop simulation by bringing automatic control algorithms [1,21] into finite element modeling, as will be discussed in Section 4. ...
Conference Paper
Full-text available
Powder bed fusion (PBF) additive manufacturing has enabled unmatched agile manufacturing of a wide range of products from engine components to medical implants. While high-fidelity finite element modeling and feedback control have been identified key for predicting and engineering part qualities in PBF, existing results in each realm are developed in opposite computational architectures wildly different in time scale. Integrating both realms, this paper builds a first-instance closed-loop simulation framework by utilizing the output signals retrieved from the finite element model (FEM) to directly update the control signals sent to the model. The proposed closed-loop simulation enables testing the limits of advanced controls in PBF and surveying the parameter space fully to generate more predictable part qualities. Along the course of formulating the framework, we verify the FEM by comparing its results with experimental and analytical solutions and then use the FEM to understand the melt-pool evolution induced by the in-layer thermomechanical interactions. From there, we build a repetitive control algorithm to greatly attenuate variations of the melt pool width.
... Note that besides the bidirectional scan used here, other scan patterns yield similar repetitive disturbances (see, e.g., experimental results in [22]). Here, automatic control algorithms [1,20] can be brought in to handle those undesired repetitive spectra, as will be discussed in Section 5. ...
Article
Full-text available
Although laser-based additive manufacturing (AM) has enabled unprecedented fabrication of complex parts directly from digital models, broader adoption of the technology remains challenged by insufficient reliability and in-process variations. In pursuit of assuring quality in the selective laser sintering (SLS) AM, this paper builds a modeling and control framework of the key thermodynamic interactions between the laser source and the materials to be processed. First, we develop a three-dimensional finite element simulation to understand the important features of the melt-pool evolution for designing sensing and feedback algorithms. We explore how the temperature field is affected by hatch spacing and thermal properties that are temperature-dependent. Based on high-performance computer simulation and experimentation, we then validate the existence and effect of periodic disturbances induced by the repetitive in- and cross-layer thermomechanical interactions. From there, we identify the system model from the laser power to the melt pool width and build a repetitive control algorithm to greatly attenuate variations of the melt pool geometry.
Article
Element-free Galerkin (EFG) method is applied for the first time to numerical modeling of additive manufacturing (AM) process, in which the superiorities of the EFG method in implementing adaptive computation and in constructing high order approximation are exploited. Second-order moving-least squares (MLS) approximation is constructed for both temperature and displacement. A non-linear coupled thermo-mechanical model considering moving heat sources, radiation boundaries, elastoplastic materials and temperature-dependent parameters is adopted. Adaptive coarsening based on the background integration mesh is developed to efficiently model the layered deposition process. In addition, an efficient integration scheme using background hexahedral elements is proposed to evaluate the domain integrals of the weak forms. Numerical results show that the developed method is able to effectively model the AM process with substantially reduced number of approximation nodes. The effects of laser power, scan speed and scan path on temperature and distortion are investigated. The improved accuracies due to the proposed integration scheme and the adopted second-order approximation are also demonstrated.
Thesis
Full-text available
Titanium alloy can meet the urgent needs of advanced aircraft for high reliability, high maneuverability and long life due to its light weight, high specific stiffness, high specific strength and good corrosion resistance. Therefore, the application level of titanium alloy often represents the advance of the aircraft. Laser Solid Forming (LSF), one of typical metal Additive Manufacturing (AM) technologies, can achieve the accurate forming of high-performance complex parts, and has been gradually applied to the fabrication of the large and complex titanium alloys in aerospace high-end equipment such as advanced aircraft. However, LSF process is a non-uniform, fast and multi-scale thermal-microstructure-stress coupled process. During LSF process, the materials undergo repeated heating, melting, solidification and cooling. The dynamic non-uniform temperature field in the component results in large residual stresses and distortion, which affects the geometric accuracy and mechanical properties of the fabricated compnents. The thermo-mechnical coupling model based on finite element method is expected to effectively predict the mechanical behavior of components during LSF, control the geometry and properties of LSF parts in real-time and reduce residual stress and distortion. Hence, this paper focuses on the prediction of thermo-mechanical behavior of LSF Ti-6Al-4V alloy and combines numerical simulation with benchmark evaluation experiments to reveal the influence of the process parameters on the evolution of the thermo-mechnical fields during LSF. The main conclusions obtained are as follows: 1) Different mechanical properties coming from different literatures for the same Ti-6Al-4V alloy have been tested. The results obtained showed large discrepancies. The most accurate response to correctly characterize the mechanical behavior of the metal deposition in LSF is obtained. The sensitivity analysis of the mechanical properties of Ti-6Al-4V alloy shows that the distortion and residual stresses are strongly influenced by the thermal expansion coefficient while are slightly affected by the Young’s modulus. The influence of the elastic limit is also very significant, because it changes the formation and evolution of the plastic strains. 2) The in-situ measurement platform of full-field thermal and mechanical responses in LSF process were further improved. Thermocouples and infrared thermal imaging were combined to achieve accurate measurement of the surface temperature of the fabricated part during LSF. The displacement sensor was used to monitor the distortion of the substrate in manufacturing process. Meantime, digital image correlation technology was used to monitor the strain field o the single-walls in real-time. 3) The LSF thermo-mechanical coupled model was experimentally calibrated and validated by in-situ measurements. The numerical results are in agreement with the experimental measurements. In the LSF process, the generation and development of distortion and stress are mainly resulted from the large temperature gradient and high cooling rate. Note that in the initial stage the extremely high temperature gradient occured, and the distortion increment in final cooling process accounted for 60% of the residual distortion. The maximum temperature gradient and the maximum tensile stress occured in the deposition of the first layer. With the increase of the number of the deposited layers, the temperature gradient and the maximum tensile stress first decreased rapidly and then remained stable. The heat accumulation, residual stress and distortion of the built were strongly affected by the laser power but slighjtly affected by the scanning speed. 4) The effects of preheating methods, cooling rates and deposition locations on the residual stress and distortion of the parts were investigated. The results show that the laser preheating substrate can reduce residual stress, but may lead to the larger distortion. The entile preheating of the substrate is an effective method to reduce distortion and residual stress. Increasing the pre-heating temperature, the mitigation is more marked. In addition, it is advantageous to reduce residual stress and distortion by optimizing the deposition position and designing specific substrate structures. 5) The effect of different scanning paths on the residual stress and distortion of the built was investigated. The results show that when the 90º alternating scanning method is adopted, the residual stress and distortion of the part can be effectively alleviated. Additionally, increasing the heat accumulation of the substrate and reducing the length of the scanning line are beneficial to reduce residual stress and distortion. 6) During the LSF process, the physical properties of Ti-6Al-4V thermo-mechanical model will obviously affected by many factors such as laser, component size and external environment, among others. Therefore, for different AM processes, specific material types and forming environments as well as other factors mentioned are used to determine the boundary conditions and physical properties used in the AM model.
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Additive manufacturing (AM) has gained extensive attention and tremendous research due to its advantages of fabricating complex-shaped parts without the need of casting mold. However, distortion is a known issue for many AM technologies, which decreases the precision of as-built parts. Like fusion welding, the local high-energy input generates residual stresses, which can adversely affect the fatigue performance of AM parts. To the best of the authors’ knowledge, a comprehensive review does not exist regarding the distortion and residual stresses dedicated for AM, despite some work has explored the interrelationship between the two. The present review is aimed to fill in the identified knowledge gap, by first describing the evolution of distortion and residual stresses for a range of AM processes, and second assessing their influencing factors. This allows us to elucidate their formation mechanisms from both the micro- and macro-scales. Moreover, approaches which have been successfully adopted to mitigate both the distortion and residual stresses are reviewed. It is anticipated that this review paper opens many opportunities to increase the success rate of AM parts by improving the dimension precision and fatigue life.
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This work presents high-speed thermographic measurements of the melt pool length during single track laser scans on nickel alloy 625 substrates. Scans are made using a commercial laser powder bed fusion (PBF) machine while measurements of the radiation from the surface are made using a high speed (1800 frames per second) infrared camera. The melt pool length measurement is based on the detection of the liquidus-solidus transition that is evident in the temperature profile. Seven different combinations of programmed laser power (49-195 W) and scan speed (200-800 mm/s) are investigated, and numerous replications using a variety of scan lengths (4-12 mm) are performed. Results show that the melt pool length reaches steady-state within 2 mm of the start of each scan. Melt pool length increases with laser power, but its relationship with scan speed is less obvious because there is no significant difference between cases performed at the highest laser power of 195 W. Although keyholing appears to affect the anticipated trends in melt pool length, further research is required.
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A laser/powder deposition process has been added to the Shape Deposition Manufacturing system at Stanford University. This process is more robust than previous SDM metal deposition processes, consistently producing fully dense, near-net shape deposits with excellent material properties Material is deposited by scanning the laser across a surface while injecting metallic powders into the melt-pool at the laser focus. A number of parts have been produced with the system, including an injection molding tool, multimaterial structures and simple mechanisms. Currently research is being performed to improve the finish quality of the parts. One of the main areas of research involves controlling thermal stresses which can lead to warpage and delamination. Selective deposition techniques and the use of low coefficient of thermal expansion materials such as INVAR™ show promise for reducing deformations caused by internal stresses.
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The pore structure, the hardness and the mechanical properties of three-dimensional titanium models formed by the selective laser melting method with a neodymium-doped yttrium aluminium garnet (Nd:YAG) pulsed laser are investigated. The optical and scanning electron micrographs show that pore structure depends on the peak power, the scan speed and the hatching pitch. The Vickers hardness of the laser formed specimens is around 240 HV (0.2 kgf), higher than that of the wrought material (125-160 HV). Depth profiling by X-ray photoelectron spectroscopy (XPS) indicates that oxygen pick-up occurs during laser forming of the titanium model processed in a closed chamber filled with argon. The fatigue strength of the titanium models formed by changing the hatching pitch and the laser power were measured. It is possible to improve the fatigue strength of the as-formed models by decreasing the hatching pitch or by hot isostatic pressing (HIP). The specimens after HIP have a fatigue strength comparable to the wrought material.
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A method for modeling the effect of stress relaxation at high temperatures during Laser Direct Energy Deposition processes is experimentally validated for Ti-6Al-4V samples subject to different inter-layer dwell times. The predicted mechanical responses are compared to those of Inconel® 625 samples, which experience no allotropic phase transformation, deposited under identical process conditions. The thermal response of workpieces in additive manufacturing is known to be strongly dependent on dwell time. In this work the dwell times used vary from 0 to 40 s. Based on past research on ferretic steels and the additive manufacturing of titanium alloys it is assumed that the effect of transformation strain in Ti-6Al-4V acts to oppose all other strain components, effectively eliminating all residual stress at temperatures above 690°C. The model predicts that Inconel® 625 exhibits increasing distortion with decreasing dwell times but that Ti-6Al-4V displays the opposite behavior, with distortion dramatically decreasing with lowering dwell time. These predictions are accurate when compared with experimental in situ and post-process measurements.
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Build part certification has been one of the primary roadblocks for effective usage and broader applications of metal additive manufacturing (AM) technologies including powder-bed electron beam additive manufacturing (EBAM). Process sensitivity to operating parameters, among others such as powder stock variations, is one major source of property scattering in EBAM parts. Thus, it is important to establish quantitative relations between the process parameters and process thermal characteristics that are closely correlated with the AM part properties. In this study, the experimental techniques, fabrications, and temperature measurements, developed in recent work (Cheng et al., 2014,"On Process Temperature in Powder-Bed Electron Beam Additive Manufacturing: Model Development and Experimental Validation," ASME J. Manuf. Sci. Eng., (in press)) were applied to investigate the process parameter effects on the thermal characteristics in EBAM with Ti-6Al-4 V powder, using the system-specific setting called “speed function(SF)” index that controls the beam speed and the beam current during a build. EBAM parts were fabricated using different levels of SF index (20–65) and examined in the part surface morphology and microstructures. In addition, process temperatures were measured by near infrared (NIR) thermography with further analysis of the temperature profiles and the melt pool size. The thermal model, also developed in recent work, was further employed for EBAM temperature predictions, and then compared with the experimental results. The major results are summarized as follows. SF index noticeably affects the thermal characteristics in EBAM, e.g., a melt pool length of 1.72mm and 1.26mm for SF20 and SF65, respectively, at 24.43mm build height. SF setting also strongly affects the EBAM part quality including the surface morphology, surface roughness and part microstructures. In general, a higher SF index tends to produce parts of rougher surfaces with more pore features and large b grain columnar widths. Increasing the beam speed will reduce the peak temperatures, also reduce the melt pool sizes. Simulations conducted to evaluate the beam speed effects are in reasonable agreement compared to the experimental measurements in temperatures and melt pools sizes. However, the results of a lower SF case, SF20, show larger differences between the simulations and the experiments, about 58% for the melt pool size. Moreover, the higher the beam current, the higher the peak process temperatures, also the larger the melt pool. On the other hand, increasing the beam diameter monotonically decreases the peak temperature and the melt pool length.
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The effect of the beam scanning speed on part microstructures in the powder-bed electron beam additive manufacturing (EBAM) process was investigated in this research. Four levels of the beam speed were tested in building EBAM Ti–6Al–4V samples. The samples were subsequently used to prepare metallographic specimens for observations by optical microscopy and scanning electron microscopy. During the experiment, a near-infrared thermal imager was also used to acquire build surface temperatures for melt tool size estimates. It was found that the X-plane (side surface) shows columnar prior β grains, with the width in the range of about 40–110 µm, and martensitic structures. The width of columnar grains decreases with the increase of the scanning speed. In addition, the Z-plane (scanning surface) shows equiaxed grains, in the range of 50–85 µm. The grain size from the lowest beam speed (214 mm/s) is much larger compared to other samples of higher beam speeds (e.g., 376–689 mm/s). In addition, increasing the beam scanning speed will also result in finer α-lath. However, the porosity defect on the build surface also becomes severe at the highest scanning speed (689 mm/s).
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In situ measurements of the accumulation of distortion during additive manufacturing (AM) of titanium and nickel base alloys are made as a function of changes in dwell time between the deposition of individual layers. The inclusion of dwell times between individual layers to allow for additional cooling during the deposition process is a common technique utilized in AM processes. Experimental observations made here in Inconel® 625 laser deposited builds show that the accumulation of distortion occurs with a consistent trend over the course of the builds and both distortion and residual stress levels decrease with increasing dwell times from 0 to 40 s. On the other hand, changes in dwell time for the Ti–6Al–4V laser deposited builds have a significant impact on the accumulation of distortion, with shorter and no dwell times minimizing the distortion accumulation and even reducing it over a range of build heights. These shorter dwell times also produce builds with significantly lower residual stress and distortion levels, particularly when no dwell time is applied. Based on these results, the materials to be deposited should be considered when developing appropriate path planning schedules.
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This paper reviews the state-of-the-art of an important, rapidly emerging, manufacturing technology that is alternatively called additive manufacturing (AM), direct digital manufacturing, free form fabrication, or 3D printing, etc. A broad contextual overview of metallic AM is provided. AM has the potential to revolutionize the global parts manufacturing and logistics landscape. It enables distributed manufacturing and the productions of parts-on-demand while offering the potential to reduce cost, energy consumption, and carbon footprint. This paper explores the material science, processes, and business consideration associated with achieving these performance gains. It is concluded that a paradigm shift is required in order to fully exploit AM potential.
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This paper presents a numerical analysis technique for predicting welding-induced distortion. The technique combines two-dimensional welding simulations with three-dimensional structural analyses in a decoupled approach. The numerical technique is particularized on evaluating welding-induced buckling. The numerical predictions can be utilized either as a design evaluation or manufacturing analysis tool. As a design tool, the effect of the welding procedures can be determined and incorporated into the evaluation and optimization of the design configurations. As a manufacturing analysis tool, for a fixed design, different welding processes and procedures can be evaluated to minimize welding distortion. Experimental results obtained from small- and large-scale mock-up panels verify the numerical modeling approach.
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Modelling and simulation of metal deposition (MD) poses several challenges to the modeller in addition to the usual challenges in modelling of welding. The aim of the work presented in this paper is to enable simulation of metal deposition for large three-dimensional components. Weld paths that are created in an off-line programming system (OLP) can be used directly to prescribe the movement of the heat source in the model. The addition of filler material is done by activation of elements. Special care must be taken to the positioning of the elements, due to large deformations. Nodes are moved to ensure that the added material has correct volume and shape. A physically based material model is also implemented. This material model is able to describe the material behaviour over a large strain, strain rate and temperature range. Temperature measurements and deformation measurements are done in order to validate the model. The computed thermal history is in very good agreement with measurements. The computed and measured deformations also show quite good agreement. It has been shown that the approach yields correct results, providing that flow stress and heat input models are calibrated with sufficient accuracy. The method reduces the modelling work considerably for metal deposition and multipass welding. It can be used for detailed models but also lumping of welds is possible and often necessary for industrial applications.
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This paper discusses high precision microscale laser bending and the thermomechanical phenomena involved. The use of a pulsed and a CW laser for microscale bending of ceramics, silicon, and stainless steel is demonstrated. For each laser, experiments art conducted to find out the relation between bending angles and laser operation parameters. Changes of the ceramics surface composition after laser irradiation are analyzed using an electron probe microanalyzer (EPMA). Results obtained by the pulsed and the CW laser are compared, and it is found that the CW laser produces more bending than the pulsed laser does. However, the pulsed laser causes much less surface composition change and thermomechanical damage to the targets. Numerical calculations based on the thermo-elasto-plastic theory are carried out and the results ore used to explain the phenomena observed experimentally.
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Residual thermal stresses and distortion are frequently present in the parts built using a layer-by-layer solid freeform fabrication techniques assisted with a moving laser source. This study uses finite element analysis to investigate the effect of laser scanning patterns on residual thermal stresses and distortion. It is shown that the out-of-plane distortion of a layer, processed by a moving laser beam can be minimized with a proper selection of the laser scanning pattern. A scanning pattern having changes in its scanning direction frequently by 90° at every turn can lead to the cancellation of concave upward and downward distortions. As a result of this cancellation, very small distortion is present in the laser processed plate. It is also found that distortion is mainly caused by transient thermal stresses rather than residual thermal stresses.
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Welding induces residual stresses in structures and may cause buckling distortion if the stresses exceed the critical buckling stress of the structure. Reducing the welding heat input or increasing the structural stiffness reduces or eliminates buckling distortion. However, where, because of the design constraints, structure geometry and weld size are fixed, the transient thermal tensioning process is effective in reducing buckling distortion. An experimental verification and demonstration of transient thermal tensioning for minimising welding induced buckling distortion is presented. Conventional welding was carried out to demonstrate buckling distortion and establish a baseline case. Buckling distortion was eliminated using transient thermal tensioning during welding under the same welding conditions. After buckling distortion was eliminated, angular distortion became evident, which was eliminated using mechanical restraints alongside transient thermal tensioning. Residual stress measurements were obtained using the blind hole drilling method and a comparison of residual stresses for the baseline panel and for the panel with transient thermal tensioning is presented.
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This overview will focus on the direct fabrication of metal components by using laser-forming techniques in a layer-by-layer fashion. The main driving force of rapid prototyping (RP) or layer manufacturing techniques changed from fabrication of prototypes to rapid tooling (RT) and rapid manufacturing (RM). Nowadays, the direct fabrication of functional or structural end-use products made by layer manufacturing methods, i.e. RM, is the main trend. The present paper reports on the various research efforts deployed in the past decade or so towards the manufacture of metal components by different laser processing methods (e.g. selective laser sintering, selective laser melting and 3-D laser cladding) and different commercial machines (e.g. Sinterstation, EOSINT, TrumaForm, MCP, LUMEX 25, Lasform). The materials and applications suitable to RM of metal parts by these techniques are also discussed.
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Temporal and spatial distributions of temperature and strain–stress have been modelled and investigated experimentally for the laser cladding process. The model corresponded to experimental conditions where the multilayer protective coatings were prepared by direct laser cladding of stellite SF6 powder on X10Cr13 chromium steel by means of a 1.2 kW CO2 laser. For calculations the effect of base preheating, temperature dependent material properties, and also influence of time-break between cladding of the consecutive layers were taken into account. The calculated temperature fields indicated good bonding of the substrate and coating, which was in agreement with the micro-analytical test results. A decrease of the number of microcracks in the coating with an increase of substrate preheating temperature was concluded from stress calculations and confirmed in the experiment. Moreover, an increase of the cracking susceptibility with an increase of the time delay between cladding of the consecutive layers was evidenced by modelling. The best technological results were obtained for the case of single-layer coatings prepared on a preheated substrate and for higher coating thickness required the processing of consecutive layers with a possibly short time delay is advisable due to effective usage of laser beam energy for preheating and lower temperature gradients.
Article
Selective laser melting (SLM) is a powder-based additive manufacturing capable to produce parts layer-by-layer from a 3D CAD model. Currently there is a growing interest in industry for applying this technology for generating objects with high geometrical complexity. To introduce SLM process into industry for manufacturing real components, high mechanical properties of final product must be achieved. Properties of manufactured parts depend strongly on each single laser-melted track and each single layer. In this study, effects of the processing parameters such as scanning speed and laser power on single tracks formation are explored. Experiments are carried out at laser power densities (0.3–1.3) × 106 W/cm2 by cw Yb-fiber laser. Optimal ratio between laser power and scanning speed (technological processing map) for 50 μm layer thickness is determined for stainless steels (SS) grade 316L (−25 μm) and 904L (−16 μm), tool steel H13 (−25 μm), copper alloy CuNi10 (−25 μm) and superalloy Inconel 625 (−16 μm) powders. A considerable negative correlation is found between the thermal conductivity of bulk material and the range of optimal scanning speed for the continuous single track sintering.
Article
A multiphase model for Ti–6Al–4V is proposed. This material is widely used in industrial applications and so needs accurate behaviour modeling. Tests have been performed in the temperature range from 25 °C to 1020 °C and at strain rates between 10−3 s−1 and 1 s−1. This allowed the identification of a multiphase mechanical model coupled with a metallurgical model. The behaviour of each phase is calibrated by solving an inverse problem including a phase transformation model and a mechanical model to simulate tests under thermomechanical loadings. A scale transition rule (β-rule) is proposed in order to represent the redistribution of local stresses linked to the heterogeneity of plastic strain. Finally this model is applied to two laser assisted processes: direct laser fabrication and laser welding.
Article
In situ X-ray diffraction experiments using synchrotron radiation were performed on Ti–6Al–4V samples to directly observe the α → β phase transformation during heating. These experiments were conducted at the Advanced Photon Source (APS) using a 30 keV synchrotron X-ray beam to monitor changes in the α and β phases as a function of heating time under different heating rates. The results were compared to computational thermodynamic predictions of the phase fractions versus temperature, providing information about the kinetics of the α → β transformation in Ti–6Al–4V. The measured transformation rates were shown to be consistent with a diffusion-controlled growth mechanism, whereby diffusion of V in the β-Ti phase controls the rate. Based on the X-ray diffraction data, real time measurements of the α and β phase lattice parameters were made. Dramatic differences were observed in the changes of the lattice parameters of the two phases during the transformation. These changes are believed to be due to the partitioning of V and its strong effect on the lattice parameter of the β phase. An unexpected contraction of the lattice parameter of the β phase was further observed during heating in the temperature range between 500 and 600 °C. The origin of this contraction is most likely related to the annealing of residual stresses created by the different thermal expansion behaviors of the two phases.
Article
Tolerance loss due to residual stress-induced warping is a major concern in solid freeform fabrication (SFF) processes, particularly those which involve direct deposition of molten metals. An understanding of how residual stresses develop and how they lead to tolerance loss is a key issue in advancing these processes. In this paper, results are presented from warping experiments on plate-shaped specimens created by two direct metal deposition methods, which are utilized by a particular SFF process termed shape deposition manufacturing (SDM). Results from these experiments give insight into the differences between the two deposition methods, the role of preheating and insulating conditions during manufacture and the influence of deposition path on magnitudes and distributions of warping displacements. Results are then compared to numerical predictions from both one and two-dimensional residual stress models, which are applicable to SDM and similar direct metal deposition processes. Results from the experiments and numerical models suggest that a combination of initial substrate preheating and part insulation can be applied to SDM and similar SFF processes to limit warping deflections, which is substantially simpler than active control of part temperatures during manufacture. Results also suggest that 3-D mechanical constraints are important in achieving precise control of warping behavior in SFF processes.
Finite element model of pulsed laser forming
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