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Reducing residual stresses and deformations in selective laser melting through multilevel multiscale optimization of cellular scanning strategy
Abstract
Residual stresses and deformations continue to remain one of the primary challenges towards expanding the scope of selective laser melting as an industrial scale manufacturing process. While process monitoring and feedback-based process control of the process has shown significant potential, there is still dearth of techniques to tackle the issue. Numerical modelling of selective laser melting process has thus been an active area of research in the last few years. However, large computational resource requirements have slowed the usage of these models for optimizing the process.
In this paper, a calibrated, fast, multiscale thermal model coupled with a 3D finite element mechanical model is used to simulate residual stress formation and deformations during selective laser melting. The resulting reduction in thermal model computation time allows evolutionary algorithm-based optimization of the process. A multilevel optimization strategy is adopted using a customized genetic algorithm developed for optimizing cellular scanning strategy for selective laser melting, with an objective of reducing residual stresses and deformations. The resulting thermo-mechanically optimized cellular scanning strategies are compared with standard scanning strategies and have been used to manufacture standard samples.
... The relative scarcity of research in this field is an outcome of the large computation time/resource requirements for solving the multi-scale multiphysics problem in selective laser melting. However, a few works [11] [12] exist that approach the optimization of scanning strategy as a heuristic problem, thereby achieving practical solutions within acceptable timeframes. ...
... In the current study, a real value genetic algorithm is used to optimize the scanning path with respect to the objective functions. Unlike previous studies [1] [9] [12], where the optimization problem was decomposed into two sub-problems, in the current study optimization of the scanning sequence and the scanning strategy for each cell is performed at the same time. While the previous studies chose to split the optimization problem into sequential optimization of sub problems so as to reduce the computation requirements, the quality of the optimized solutions can no doubt be improved by treating both the sequence and cell strategy together. ...
... One of the promising ways to produce complex parts of titanium aluminides is additive manufacturing (AM) since it reduces machining to a minimum degree and allows obtained parts with a complex shape (5,6). However, high residual stresses typical for most common AM processes promote cracks formation in TiAlalloys (7,8). Usually, high-temperature preheating is required to obtain crack-free intermetallic parts (9). ...
... Raster lines, contours, Medial Axis Transformation, fractals, etc.: in each case, the strategy is fixed independently from the part to build and the path adapted through few parameters optimization. If some works consider splitting into cells the area to melt, then matching them with scanning strategies [1,30,40], the optimization only focuses on these cells and the matching without considering any further scanning path modifications. Recently, three works proposed a different approach in which the path is fully optimized without any a priori restrictions. ...
In this paper, scanning paths optimization for the powder bed fusion additive manufacturing process is investigated. The path design is a key factor of the manufacturing time and for the control of residual stresses arising during the building, since it directly impacts the temperature distribution. In the literature, the scanning paths proposed are mainly based on existing patterns, the relevance of which is not related to the part to build. In this work, we propose an optimization algorithm to determine the scanning path without a priori restrictions. Taking into account the time dependence of the source, the manufacturing time is minimized under two constraints: melting the required structure and avoiding any over-heating causing thermally induced residual stresses. The results illustrate how crucial the part’s shape and topology is in the path quality and point out promising leads to define path and part design constraints.
... In this respect, these pure conduction models can be further exploited by coupling to other physics such as metallurgical or mechanical models. Even in some cases, one can use these models to perform an optimization to obtain the optimal processing condition to reduce final deformation, similar to the work by Mohanty and Hattel [72], based on a reduced-order pure conductive model. In the thermo-metallurgical models, the two independent solidification parameters, namely the solidification cooling rate and temperature gradient, are first calculated at the end and at the onset of solidification, respectively, see Fig. 8(a). ...
Numerical simulations have recently shown their potential as a robust, cheap and reliable tool for predicting the quality of components produced by metal additive manufacturing (MAM) processes. Despite the advantages of the MAM processes over conventional manufacturing methods, there is still a lack of thorough understanding on how different defects can form and originate during MAM processing. In that respect, advanced numerical techniques recently developed have the ability to predict the occurrence of such defects. These techniques have paved the way to efficiently obtain the optimal processing window for targeted mechanical properties to satisfy the end-use design requirements. The aim of this review paper is hence to present and classify numerical simulations of MAM, not solely based on their length-scale as often seen, but also based on the involved physics, as well as the modelling strategies at both the meso-scale and part-scale. The paper is arranged in the following way: First, literature describing purely conduction-based heat transfer simulations at meso-scale are presented. This is followed by a review of fluid-based simulations of increasing complexity based on the treatment of free surface of the melt pool at meso-scale. Finally, contributions based on different part-scale modeling approaches with a focus on thermo-mechanical behavior are reviewed.
... Various research groups have, therefore, focussed on different methods for reducing residual stresses in order to improve dimensional accuracy (Kruth et al. 2012, Mohanty and Hattel 2016, Robinson et al. 2018). In addition, several other process-dependent factors contributing to dimensional deviation and accuracy exist, such as part orientation and location, support structures and post-processing steps. ...
... The aim of these models is to determine the evolution of the temperature field and temperature gradients resulting in the buildup of thermal residual stresses [8]. It has been shown that residual stresses can be reduced by changing only the scan strategy [9], and therefore a fast tool for predicting the thermal responses of various scan patterns is key for determining optimal scan patterns [10,11]. For this purpose, models often neglect convection, radiation, and phase change, instead focusing on the heat conduction mechanism and adopting a linear form a the heat equation in an effort to speed up calculations further [12][13][14], the limitations of which have been explored by Promoppatum et al. [15]. ...
A unique and efficient semi-analytic method is presented for quickly predicting the three-dimensional thermal field produced by conduction from a heat source moving along an arbitrary path. A Green's function approach is used to decouple the solution at each time step into the analytical source contribution and a conduction contribution. The latter is solved numerically using efficient Gaussian convolution algorithms. This decoupling allows for boundary conditions on side boundaries to be satisfied numerically and lowers computational expenses by allowing calculations to be localized around the heat source. The thermal field resulting from arbitrary scan paths is constructed using analytical solutions for elementary linear segments. With focus on efficiency, a novel approach is used to store the calculated analytical line solutions for reuse on multiple other path segments. The results of various scan patterns are presented and successfully verified against finite element simulations. The computational times of predictions are shown to be faster than the corresponding finite element simulation by an order of magnitude with less than 1% average error. Given its ability to quickly predict the thermal history and changes in melt pool geometry due to arbitrary scan paths, this method provides a potentially powerful tool for exploration and optimization of laser powder bed fusion processes.
... The above governing equations and constitutive models are sufficient to develop a continuum model of the thermal conditions during SLM. 228,230,[234][235][236][237] The internal state variable approach 238 is well suited to the development of models for non-isothermal microstructural evolution. In general, a microstructure may be defined by different state variables such as grain size, volume fraction of grains, fraction of solid in solidification. ...
Numerical modelling is increasingly supporting the analysis and optimization of manufacturing processes in the production industry. Even if being mostly applied to multistep processes, single process steps may be so complex by nature that the needed models to describe them must include multiphysics. On the other hand, processes which inherently may seem multiphysical by nature might sometimes be modelled by considerably simpler models if the problem at hand can be somehow adequately simplified. In the present article, examples of this will be presented. The cases are chosen with the aim of showing the diversity in the field of modelling of manufacturing processes as regards process, materials, generic disciplines as well as length scales: (1) modelling of tape casting for thin ceramic layers, (2) modelling the flow of polymers in extrusion, (3) modelling the deformation process of flexible stamps for nanoimprint lithography, (4) modelling manufacturing of composite parts and (5) modelling the selective laser melting process. For all five examples, the emphasis is on modelling results as well as describing the models in brief mathematical details. Alongside with relevant references to the original work, proper comparison with experiments is given in some examples for model validation.
... This implies that any modelling effort in pursue of optimum SLM process parameters should explicitly take into account the 2 scanning pattern and be able to capture the effect of a given scanning pattern on the temperature evolution. Mohanty and Hattel [25,26,27,28] developed a fast low fidelity model and studied the temperature evolution of a single layer with various scanning strategies. Upon comparing their predictions with a FE analysis having a mesh size sufficiently small to capture characteristic gradients of the temperature field, the low-fidelity model was able to reproduce average and maximum temperatures of the domain. ...
Selective laser melting (SLM) wherein a metal part is built in a layer-by-layer manner in a powder bed is a promising and versatile way for manufacturing components with complex geometry. However, components built by SLM suffer from substantial deformation of the part and residual stresses. Residual stresses arise due to temperature gradients inherent to the process and the accompanying deformation. It is well known that the SLM process parameters and the laser scanning strategy have a substantial effect on the temperature transients of the part and henceforth on the degree of deformations and residual stresses. In order to provide a tool to investigate this relation, a semi-analytical thermal model of the SLM process is presented which determines the temperature evolution in a 3D part by way of representing the moving laser spot with a finite number of point heat sources. The solution of the thermal problem is constructed from the superposition of analytical solutions for point sources which are known in semi-infinite space and complimentary numerical/analytical fields to impose the boundary conditions. The unique property of the formulation is that numerical discretisation of the problem domain is decoupled from the steep gradients in the temperature field associated with localised laser heat input. This enables accurate and numerically tractable simulation of the process. The predictions of this semi-analytical model are validated by experiments and the exact solution known for a simple thermal problem. Simulations for building a complete layer using two different scanning patterns and subsequently building of multiple layers with constant and rotating scanning patterns in successive layers are performed. The computational efficiency of the semi-analytical tool is assessed which demonstrates its potential to gain physical insight in the full SLM process with acceptable computational costs.
Additive manufacturing (AM) is a fast-growing sector with the ability to evoke a revolution in manufacturing due to its almost unlimited design freedom and its capability to produce personalised parts locally and with efficient material use. AM companies, however, still face technological challenges such as limited precision due to shrinkage, built-in stresses, and limited process stability and robustness. Moreover, often post-processing is needed due to the high roughness and remaining porosity. Qualified, trained personnel are also in short supply.
In recent years, there have been dramatic improvements in AM design methods, process control, post-processing, material properties and material range. However, if AM is going to gain a significant market share it must be developed into a true precision manufacturing method. The production of precision parts relies on three principles:
1. Production is robust (i.e. that all sensitive parameters can be controlled).
2. Production is predictable (for example, the shrinkage that occurs is acceptable because it can be predicted and compensated in the design).
3. Parts are measurable (as without metrology, accuracy, repeatability and quality assurance cannot be known).
AM of metals is inherently a high-energy process, with many of sensitive and inter-related process parameters, making it susceptible to thermal distortions, defects and process drift. The complete modelling of these processes is beyond current computational power and novel methods are needed to practicably predict performance and inform design. In addition, metal AM produces highly textured surfaces and complex surface features that stretch the limits of contemporary metrology. With so many factors to consider, there is a significant shortage of background material on how to inject precision into AM processes. Shortage in such material is an important barrier for a wider uptake of advanced manufacturing technologies and a comprehensive book is thus needed.
This book aims to inform the reader how to improve the precision of metal AM processes by tackling the three principles of robustness, predictability and metrology, and by developing computer-aided engineering methods that empower rather than limit AM design.
Thermal analysis of laser processes can be used to predict thermal stresses and microstructures during processing and in a completed part. Thermal analysis is also the basis for feedback control of laser processing parameters in manufacturing. A comprehensive literature review of thermal analysis methods utilized in Laser Sintering (LS) has been undertaken. In many studies, experimental methods were commonly used to detect and validate thermal behavior during processing. Coupling of thermal experiments and FEM analyses were utilized in many of the latter studies. Analytical solutions were often derived from the Rosenthal solution and other established theories. In recent years, some temperature measuring systems have been implemented to validate the simulation results. The main characteristics of LS temperature distribution and effects of process parameters to temperature are also summarized and shown by a case study.
A transient three-dimensional beam-matter-interaction model was developed to simulate the process dynamics of laser
beam melting (LBM) of metals in the powder bed. The simulations were realized by the software OpenFOAM and
modified solvers. Based on the continuity equation, the equation of heat conduction and the Navier-Stokes equation the
laser material interaction is described. Furthermore, the volume of fluid method is used to characterize the free surfaces of
the multi-phase system. These process simulations were performed for steel powders. The parameters were chosen
according to those applied in industrial machines and the simulation results show good correlation to experimental data.
Selective laser melting (SLM) is an additive manufacturing technique in which metal products are manufactured in a layer-by-layer manner. One of the main advantages of SLM is the large geometrical design freedom. Because of the layered build, parts with inner cavities can be produced. However, complex structures, such as downfacing areas, influence the process behavior significantly. The downfacing areas can be either horizontal or inclined structures. The first part of this work describes the process parameter optimization for noncomplex, upfacing structures to obtain relative densities above 99%. In the second part of this research, parameters are optimized for downfacing areas, both horizontal and inclined. The experimental results are compared to simulations of a thermal model, which calculates the melt pool dimensions based on the material properties (such as thermal conductivity) and process parameters (such as laser power and scan speed). The simulations show a great similarity between the thermal model and the actual process.
Simulations of additive manufacturing processes are known to be computationally expensive. The resulting large runtimes prohibit their application in secondary analysis requiring several complete simulations such as optimization studies, and sensitivity analysis. In this paper, a low-fidelity pseudo-analytical model has been introduced to enable such secondary analysis. The model has been able to mimic a finite element model and was able to capture the thermal trends associated with the process. The model has been validated and subsequently applied in a small optimization case study. The pseudo-analytical modelling technique is established as a fast tool for primary modelling investigations.
Selective laser melting, as a rapid manufacturing technology, is
uniquely poised to enforce a paradigm shift in the manufacturing
industry by eliminating the gap between job- and batch-production
techniques. Products from this process, however, tend to show an
increased amount of defects such as distortions, residual stresses and
cracks; primarily attributed to the high temperatures and temperature
gradients occurring during the process. A unit cell approach towards the
building of a standard sample, based on literature, has been
investigated in the present work. A pseudo-analytical model has been
developed and validated using thermal distributions obtained using
different existing scanning strategies. Several existing standard and
non-standard scanning methods have been evaluated and compared using the
empirical model as well as a 3D-thermal finite element model. Finally, a
new grid-based scan strategy has been developed for processing the
standard sample, one unit cell at a time, using genetic algorithms, with
an objective of reducing thermal asymmetries.
A thermo-elastic model for calculation of the residual stresses is proposed and experimentally validated. Residual stresses depend on the shape of the remelted domain but are independent of its size. The maximum tensile residual stresses arise in the remelted region in the scanning direction. The calculated longitudinal tensile stresses are about twice greater than the transversal ones. The model explains formation of two systems of longitudinal and transversal cracks observed in experiments. The residual stresses are proportional to the difference between the ambient and melting temperatures, which justifies the known method to reduce the stresses by preheating.
In the selective laser melting process, one has to strike a balance between power and scan speed. When a small scan speed is used, thermal gradients are important and local solidification can lead to cracks. On the other hand, when high speed is used, the power has to be huge and phenomena due to heat transfer, like delamination or balling, arise. In this paper, we study different possible scanning strategies and we indicate those that lead to homogeneous heating of the part until its melting point. The results are compared to numerical simulations.
Porous structures are used in orthopaedics to promote biological fixation between metal implant and host bone. In order to achieve rapid and high volumes of bone ingrowth the structures must be manufactured from a biocompatible material and possess high interconnected porosities, pore sizes between 100 and 700 microm and mechanical strengths that withstand the anticipated biomechanical loads. The challenge is to develop a manufacturing process that can cost effectively produce structures that meet these requirements. The research presented in this paper describes the development of a 'beam overlap' technique for manufacturing porous structures in commercially pure titanium using the Selective Laser Melting (SLM) rapid manufacturing technique. A candidate bone ingrowth structure (71% porosity, 440 microm mean pore diameter and 70 MPa compression strength) was produced and used to manufacture a final shape orthopaedic component. These results suggest that SLM beam overlap is a promising technique for manufacturing final shape functional bone ingrowth materials.
Over the last decade, several studies have attempted to develop thermal models for analyzing the selective laser melting process with a vision to predict thermal stresses, microstructures and resulting mechanical properties of manufactured products. While a holistic model addressing all involved phenomena is yet to emerge, the existing partial models have already become computationally heavy. This is observed to go hand-in-hand with a trend across literature for the usage of finite element (FE) formulations for developing implicit 3D models. However, the 3D implicit FE models, though able to accurately simulate the process, are constrained by either the size or scale of the model domain. A second challenging aspect involves the inclusion of non-linear material behavior into the 3D implicit FE models.
An alternating direction implicit (ADI) method based on a finite volume (FV) formulation is proposed for modeling single-layer and few-layers selective laser melting processes. The ADI technique is implemented and applied for two cases involving constant material properties and non-linear material behavior. The ADI FV method consume less time while having comparable accuracy with respect to 3D implicit FE models. Drawing on the comparative results, appropriate models are recommended for different scenarios and modeling domains.
Over the last decade, several studies have attempted to develop thermal models for analyzing the selective laser melting process with a vision to predict thermal stresses, microstructures and resulting mechanical properties of manufactured products. While a holistic model addressing all involved phenomena is yet to emerge, the existing partial models have already become computationally heavy. This is observed to go hand-in-hand with a trend across literature for the usage of finite element (FE) formulations for developing implicit 3D models. However, the 3D implicit FE models, though able to accurately simulate the process, are constrained by either the size or scale of the model domain. A second challenging aspect involves the inclusion of non-linear material behavior into the 3D implicit FE models. An alternating direction implicit (ADI) method based on a finite volume (FV) formulation is proposed for modeling single-layer and few-layers selective laser melting processes. The ADI technique is implemented and applied for two cases involving constant material properties and non-linear material behavior. The ADI FV method consume less time while having comparable accuracy with respect to 3D implicit FE models. Drawing on the comparative results, appropriate models are recommended for different scenarios and modeling domains.
A three-dimensional thermomechanical finite element model is used to investigate effects of laser scanning patterns and chamber preheating on out-of-plane distortion and thermal stresses present in the parts built using a layer-by-layer solid freeform fabrication technique assisted with a moving laser source. It is shown that for a laser-processed component with large thermal gradients, a laser scanning pattern having its long scanning direction parallel to one axis leads to a saddle-shaped distortion. It is also found that the distortion is mainly caused by transient thermal stresses rather than residual thermal stresses. The out-of-plane distortion can be minimized with a proper selection of the laser scanning pattern combined with chamber preheating. The minimization of distortion so achieved is attributed to the reduction of transient thermal stresses and cancellation of effects of the alternated scanning directions.
Selective Laser Melting (SLM) is actually the most attractive technique in an Additive Manufacturing (AM) technology because of the possibility to build layer by layer up nearly full density metallic components without needing for post-processing. One of the main problems in SLM processes is represented by the thermal distortion of the model during forming; the part tends to be deformed and cracked due to the thermal stress. Therefore, it is important to know the effect of the process parameters on the molten zone and consequently on the density of the consolidated material. Great advantage can be obtained from the prediction of temperature evolution and distribution. The aim of this study is to evaluate the influence of the process parameters on the temperature evolution in a 3D model. The developed code evaluates the distribution and evolution of the temperatures in the SLM process and simulates the powder-liquid-solid change by means of a check of the nodes temperature.
Notice of RetractionAfter careful and considered review of the content of this paper by a duly constituted expert committee, this paper has been found to be in violation of IEEE's Publication Principles.We hereby retract the content of this paper. Reasonable effort should be made to remove all past references to this paper.The presenting author of this paper has the option to appeal this decision by contacting TPII@ieee.org.In selective laser melting (SLM) processing, the residual stress will be occurred inside the melted layer inevitable, and the deformation will be made simultaneously. The laser scanning strategy determined the deformation magnitude and the shape residual stress profiles. In order to optimize the manufacture and reduce the deformation of manufactured metal layers, helix scan strategy and progressive scan strategy for selective laser melting technology have been developed. In the result, the helix scan strategy helps to decrease the deformation of melted layer more effectively.
Selective laser melting (SLM), a powder metallurgical (PM) additive manufacturing (AM) technology, is able to produce fully functional parts directly from standard metal powders without using any intermediate binders or any additional post-processing steps. During the process, a laser beam selectively scans a powder bed according to the CAD data of the part to be produced and completely melts the powder particles together. Stacking and bonding two-dimensional powder layers in this way, allows production of fully dense parts with any geometrical complexity. The scanning of the powder bed by the laser beam can be achieved in several different ways, one of which is island or sectoral scanning. In this way, the area to be scanned is divided in small square areas (‘sectors’) which are scanned in a random order. This study is carried out to explore the influence of sectoral scanning on density, surface quality, mechanical properties and residual stresses formed during SLM. The experiments are carried out on a machine with an Nd:YAG laser source using AISI 316L stainless steel powder. As a result of this experimental study, it is concluded that sectoral scanning has some advantages such as lower residual stresses and better surface quality. However, the selection of parameters related to sectoral scanning is a critical task since it may cause aligned porosity at the edges between sectors or scanned tracks, which is very undesired in terms of mechanical properties.
Selective Laser Melting (SLM) of metallic powders, especially of high-strength nickel based alloys, allows for the manufacturing of components of high shape complexity and load capacity. However, due to high temperature gradients, induced during laser processing, the structural properties and geometrical accuracy of components can be affected. This paper aims to analyse different modelling approaches of the thermo-mechanical effects in SLM manufacturing of aero-engine components, in order to determine in advance possible shape distortions. Hereby, a methodical model reduction is proposed and evaluated to allow the finite element analysis of larger components with reasonable computational time. Major process characteristics such as heat input, molten region geometry (i.e. macrographs), material deposition (i.e. layer thickness), temperature dependent material and powder properties, phase transformation, process sequences and convection effects are taken into consideration. The proposed model reduction aims to decrease time consuming modelling effort and high computation duration and yet provide reliable structural results.
It has not been a simple matter to obtain a sound extension of the classical J
2 flow theory of plasticity that incorporates a dependence on plastic strain gradients and that is capable of capturing size-dependent behaviour of metals at the micron scale. Two classes of basic extensions of classical J
2 theory have been proposed: one with increments in higher order stresses related to increments of strain gradients and the other characterized by the higher order stresses themselves expressed in terms of increments of strain gradients. The theories proposed by Muhlhaus and Aifantis in 1991 and Fleck and Hutchinson in 2001 are in the first class, and, as formulated, these do not always satisfy thermodynamic requirements on plastic dissipation. On the other hand, theories of the second class proposed by Gudmundson in 2004 and Gurtin and Anand in 2009 have the physical deficiency that the higher order stress quantities can change discontinuously for bodies subject to arbitrarily small load changes. The present paper lays out this background to the quest for a sound phenomenological extension of the rateindependent J
2 flow theory of plasticity to include a dependence on gradients of plastic strain. A modification of the Fleck-Hutchinson formulation that ensures its thermodynamic integrity is presented and contrasted with a comparable formulation of the second class where in the higher order stresses are expressed in terms of the plastic strain rate. Both versions are constructed to reduce to the classical J
2 flow theory of plasticity when the gradients can be neglected and to coincide with the simpler and more readily formulated J
2 deformation theory of gradient plasticity for deformation histories characterized by proportional straining.
In selective laser melting processing, the residual stress will occur inside the melted layer inevitably, and the deformation will be made simultaneously. The laser scanning strategy determined the deformation magnitude and the shape residual stress profiles. In order to optimise the manufacture and reduce the deformation of manufactured metal layers, helix scan strategy and progressive scan strategy for selective laser melting technology have been developed. As a result, the helix scan strategy helps to decrease the deformation of melted layer more effectively.
Square areas, 15 mm × 15 mm in size, have been melted in the surface of powder beds made from H13 tool steel, using a raster-scanning CO2 laser focused to a beam diameter of 0.6 mm. Laser powers from approximately 50 to 150 W and scan speeds from 0.5 to 300 mm/s have been used, at two scan spacings, 0.15mm and 0.45 mm. The appearances of the layers in the different conditions, dense or porous, have been observed by low-magnification scanning electron microscopy. The masses of the layers have been measured and simulations have been carried out to predict the masses. The variation of mass with scan speed, at a constant laser power, has been found to be much less than might be expected from a constant absorptivity of laser energy into the bed. The simulations suggest that absorptivities range from 0.25 to approximately 1.0 and that, during any one scan, heating of the bed by previous scans must be considered in order even partially to explain the observations. The work is relevant to attempts to build metal parts without supports, by selective laser melting.
The process of direct manufacturing by selective laser melting basically consists of laser beam scanning over a thin powder layer deposited on a dense substrate. Complete remelting of the powder in the scanned zone and its good adhesion to the substrate ensure obtaining functional parts with improved mechanical properties. Experiments with single-line scanning indicate, that an interval of scanning velocities exists where the remelted tracks are uniform. The tracks become broken if the scanning velocity is outside this interval. This is extremely undesirable and referred to as the “balling” effect. A numerical model of coupled radiation and heat transfer is proposed to analyse the observed instability. The “balling” effect at high scanning velocities (above ∼20cm/s for the present conditions) can be explained by the Plateau–Rayleigh capillary instability of the melt pool. Two factors stabilize the process with decreasing the scanning velocity: reducing the length-to-width ratio of the melt pool and increasing the width of its contact with the substrate.
In an effort to optimize the manufacture of near fully dense metal parts, we have developed a sub-sector scan mode for selective
laser melting (SLM) technology. The working process of such a scan mode is realized by the way that the contour polygons of
a three-dimensional (3D) metal part obtained by slicing layer by layer are enclosed in a small quadrangle box, specifically
an encasing box, and sub-boxes (sub-sectors) in square are then divided by the enclosed quadrangle box with short scan lengths
on each side. Then these sub-sectors are hatched with bidirectional scanning lines which are normal with right-angles in adjacent
sectors. The algorithm is presented in this paper. In the SLM processes, this scan pattern helps to increase the temperature
in small sub-sectors and melt metal powder more effectively. Parts with complex structures from stainless steel 316L were
manufactured with this new scan mode.
For establishing Selective Laser Melting (SLM) in production technology, an extensive knowledge about the transient physical
effects during the manufacturing process is mandatory. In this regard, a high process stability for various alloys, e.g. tool
steel 1.2709 (X3NiCoMoTi 18-9-5), is realisable, if approaches for the virtual qualification of adequate process parameters
by means of a numerical simulation based on the finite element analysis (FEA) are developed. Furthermore, specific methods
to evaluate and quantify the resulting residual stresses and deformations due to the temperature gradient mechanism (TGM)
are required. Hence, the presented work contains particular approaches using the FEA for the simulation of transient physical
effects within the additive layer manufacturing (ALM) process. The investigations focus on coupled thermo-mechanical models
incorporating specific boundary conditions and temperature dependant material properties to identify the heat impact on residual
stresses and deformations. In order to evaluate the structural effects and simultaneously validate the simulation, analysis
on residual stresses based on the neutron diffractometry as well as considerations concerning part deformations are presented.
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.
In the SLM process, one has to balance between power and scan speed. When small scan speed is used, thermal gradients are important and local solidification can lead to cracks. On the other hand, when high speed is used, the power has to be huge and phenomena due to heat transfer, like delamination or balling, arise. In this paper, we study different possible scanning strategies and we point out those leading to an homogeneous heating of the part until its melting point. The results are compared to numerical simulations.
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Reproducibility for properties of selective laser melting products
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