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Influence of interpass cooling conditions on microstructure and tensile properties of Ti-6Al-4V parts manufactured by WAAM
Abstract
Wire and arc additive manufacturing (WAAM) technology is growing in interest in the last years. The technology enables the manufacturing of real geometries by overlapping weld beads and is well suited for metallic parts with high buy-to-fly ratio. Manufacturing costs and time are critical issues which determine the business case. Therefore, it is necessary to develop strategies that minimise the production time while meeting quality requirements. In this regard, cooling conditions are a key factor to reduce time and determine mechanical properties and resulting microstructure. This study aims at investigating the effect of interpass cooling conditions on resulting mechanical properties and microstructure of Ti-6Al-4V alloy. The influence of dwell time between successive deposition of layers is investigated both for air and forced interpass cooling. Forced cooling is done by using water-cooled anvil under base plate. The goal is to find a minimum dwell time to maximise arc-on time and deposition rate while avoiding wall collapse, widening, oxidation and the need to apply post building heat treatment. Obtained mechanical properties are compared with standards for products manufactured by conventional manufacturing. Additionally, microstructure, surface finishing and part accuracy of WAAM samples are characterised.
... Obtained dimensions, microstructures, and mechanical properties highly depend on the cooling conditions during the manufacturing of a part by WAAM [27,28]. The successive deposition of layers of the process itself affects the previously deposited layer, creating complex microstructures, but also being a source of possible defects such as pores and segregations [29]. ...
... As observed in Figure 9, the effect of the use of the cooling plate in the interpass temperature can be clearly seen; however, it is more noticeable for shorter interpass dwell times, when the substrate is at higher temperatures. Other kinds of forced cooling [32] could be of help in decreasing the interpass temperature in a greater amount; however, the obtained microstructure can be affected by obtaining supersaturated solid solution hard phases [27] and affecting the mechanical properties. ...
... Real parts usually require manufacturing times of each individual layer equivalent or higher than the interpass dwell times. Moreover, a proper interpass dwell time helps increase the effective area as seen in Table 5 [27] and the height and thickness per layer grown also increases as shown in Table 10. This optimisation of interpass dwell time gave rise to a reduction in manufacturing time of 36%. ...
The CMT-Twin-based wire and arc additive manufacturing (WAAM) process for 5356 aluminium alloy has been investigated focusing on the optimisation of welding parameters to maximise the deposition rate while avoiding segregation-related problems during solidification. For that, different conditions have been studied regarding interpass dwell time and the use of forced cooling. The larger heat input produced by the double-wire CMT-Twin process, compared to the single-wire CMT, creates vast segregations for less intensive cooling conditions and short dwell times that can induce cracks and reduce ductility. Thermography has been applied to set a maximum local temperature between consecutive layers avoiding those segregations and pores, and to optimise the total manufacturing time by varying the interpass dwell time along the height of the wall. Only a constant interpass long dwell time of 240 s and the new optimised strategy were effective in avoiding merged segregations, reducing the latest total manufacturing time by 36%. Obtained tensile properties are comparable to other works using WAAM for this alloy, showing lower properties in the vertical orientation. The use of CMT-Twin-based welding technology together with variable interpass dwell time controlled by thermography is an interesting alternative to build up parts with wall thicknesses around of 10 mm in a reduced time.
... However, only a small number of studies exist so far, which deal with the influence of the temperature-time regime (e.g., interpass temperature or cooling rate) during additive manufacturing by CMT on the resulting component properties [13,34,35]. Li varied the interpass temperature up to 300 °C. ...
... The result is a significant reduction in elongation at break. Accordingly, high cooling rates and low interpass temperatures are beneficial for the resulting mechanical properties [13,35]. ...
... In order to avoid heat accumulation and to conduct the injected thermal energy out of the component as quickly as possible, a plate through which liquid flows was placed under the substrate in a cooling approach. A positive influence on manufacturing time and mechanical properties was observed [35]. ...
In a research project, the additive manufacturing process of components made of Ti-6Al-4 V using gas metal arc welding (GMAW), which is classified into the directed energy deposition–arc (DED-Arc) processes, was investigated. The project focused on the systematic development of economical additive build-up strategies and the analysis of the temperature–time regime during the build-up process, as well as the investigation of the resulting properties. A welding range diagram was created with recommendations for process settings for additive manufacturing with the controlled short circuit, as well as a presentation of possible defect patterns outside the range shown. For the fabrication of thick-walled structures, various build-up strategies were investigated by modifying the welding path and evaluated with regard to their suitability. Based on the results, additive structures were fabricated by varying the temperature–time regime in order to gain insights into selected geometrical, metallurgical, and mechanical properties. Different energy inputs per unit length, structure dimensions, and interpass temperatures (IPT) were used for this purpose. The research project provides comprehensive findings on the additive processing of the material Ti-6Al-4 V using metal inert gas welding, in particular with regard to the temperature–time regime and the resulting properties.
... The increased cooling employed on TW2 with an interpass temperature of 100 • C resulted in a phase balance of 36% ferrite and 64% austenite. This is consistent with previous work stating that increased cooling rates promote austenite, and fast cooling rates through T 12/8 increase the ferrite volume fraction content [2][3][4][26][27][28][29][30]. Additionally, these phase balance results for the two test samples quantify well the petroleum industry standards of 30-70% of ferrite-austenite balance based on ER2594. ...
... The increased cooling employed on TW2 with an interpass temperature of 100° C resulted in a phase balance of 36% ferrite and 64% austenite. This is consistent with previous work stating that increased cooling rates promote austenite, and fast cooling rates through T12/8 increase the ferrite volume fraction content [2][3][4][26][27][28][29][30]. Additionally, these phase balance results for the two test samples quantify well the petroleum industry standards of 30-70% of ferrite-austenite balance based on ER2594. ...
... Overall, this paper has demonstrated that relatively small changes to process parameters can have a large effect on the manufacturing defects and fatigue behaviour of the fabricated structures. This conclusion agrees well with the outcomes of the previous studies [27][28][29]. Therefore, a better control and consistency of the operation, which are the main features of the WAAM process, could facilitate the use of this manufacturing process for the fabrication of structural components working in challenging environments such as corrosion and fatigue [30,31]. ...
This study aimed to improve the overall fatigue properties of WAAM-produced SDSS by changing the interpass temperatures. Micro-computed tomography was used to quantitatively characterise the internal defects, such as porosity, in large-volume WAAM-fabricated SDSS materials. An increase in the interpass temperature led to a reduction in the ferrite phase balance by up to 20%. The fatigue anisotropy was still evident, but the fatigue limit in the weakest (transverse) direction was increased to 250 MPa or by approximately 40%. Meanwhile, the increased interpass temperature had no significant effect on fatigue resistance in the longitudinal direction. This study suggests that the interpass temperature can be critical for both achieving isotropic mechanical properties and increasing fatigue life of structural components fabricated with the WAAM method.
... A subsequent milling step ensures that the netshape and the surface quality comply with the part requirements. Accordingly, an important usage case in industrial applications, and one of the major challenges of near-netshape manufacturing [7], is ensuring geometric accuracy to achieve low BTF-ratios [7,8]. ...
... Vazquez et al. [8] implemented an interlayer cooling by using a water-cooled baseplate during the CMT-based WAAM of Ti-6Al-4V. The authors observed geometrical inaccuracies for short interlayer dwell times without the active cooling. ...
... Reproducible part geometries can be achieved by monitoring the interlayer temperatures and controlling them. The thermal process management (interlayer temperatures and cooling rates) affects not only the geometry, but also mechanical properties and metallurgical aspects (covered in [8,9,17,18,21]). These aspects are subject to ongoing research. ...
Wire and Arc Additive Manufacturing (WAAM) is a promising technology for the fabrication of large metal parts. During the process, the wire electrode is melted continuously or in a pulsating mode and deposited layer-by-layer onto a substrate. Due to the recurring energy input into the part during WAAM, adequate thermal management is crucial. The temperature distribution, especially the interlayer temperature in the part, is determined by the parameter settings as well as by the dwell times and can be monitored. This paper presents the cause-effect relationships between the interlayer temperature and the dwell times to enable a suitable temperature management. Thermal imaging was implemented during the manufacturing process, allowing the analysis of different interlayer dwell times and their effect on the interlayer temperatures. In addition, the influence of the temperature management on the geometric quality characteristics of the part was investigated. It was observed that a constant interlayer dwell time led to geometric irregularities in the part height and width. Monitoring the interlayer temperature is crucial in WAAM in order to maintain a constant temperature level along multiple layers for meeting the requirements for the geometry of the part and enabling near-net-shape manufacturing.
... Thus, GMAW is utilized in Ti-WAAM less frequently than GTAW or PAW due to complications regarding the metal transfer of the Ti-wire during manufacturing. Several studies did not report any issues concerning metal transfer during Ti-WAAM using CMT [35,36]. ...
... Grain boundary α forms at the prior β-grain boundary and there is a preferred orientation between the grain boundary α and the primary β-grain [41,42], which leads to the formation of lamellar α-colonies with an average α-lath thickness of about 1 µm (Figure 13b,d,f). Vazquez et al. [36] reported that this type of microstructure with the α-colonies along primary β-grains exhibits sufficient mechanical properties due to the assisted propagation of cracks along the thick α-colonies at the grain boundary. However, no significant difference between α-colony structures of the three different walls could be observed. ...
... entation between the grain boundary α and the primary β-grain [41,42], which leads to the formation of lamellar α-colonies with an average α-lath thickness of about 1 μm (Figure 13b,d,f). Vazquez et al. [36] reported that this type of microstructure with the α-colonies along primary β-grains exhibits sufficient mechanical properties due to the assisted propagation of cracks along the thick α-colonies at the grain boundary. However, no significant difference between α-colony structures of the three different walls could be observed. ...
In wire arc additive manufacturing of Ti-alloy parts (Ti-WAAM) gas metal arc welding (GMAW) can be applied for complex parts printing. However, due to the specific properties of Ti, GMAW of Ti-alloys is complicated. In this work, three different types of metal transfer modes during Ti-WAAM were investigated: Cold Metal Transfer, controlled short circuiting metal transfer, and self-regulated metal transfer at a direct current with a negative electrode. Metal transfer modes were studied using captured waveform and high-speed video analysis. Using these modes, three walls were manufactured; the geometry preservation stability was estimated and compared using effective wall width calculation, the microstructure was analyzed using optical microscopy. Transfer process data showed that arc wandering depends not only on cathode spot instabilities, but also on anode processing properties. Microstructure analysis showed that each produced wall consists of phases and structures inherent for Ti-WAAM. α-basketweave in the center of and α-colony on the grain boundary of epitaxially grown β-grains were found with heat affected zone bands along the height of the walls, so that the microstructure did not depend on metal transfer dramatically. However, the geometry preservation stability was higher in the wall, produced with controlled short circuiting metal transfer.
... They suggested that the interpass temperature should be precisely controlled during the deposition for better product quality. Vázquez et al. [9] investigated the influence of heat accumulation on the mechanical properties and microstructure of Ti6Al4V alloys manufactured by wire-arc DED and stated that reducing interpass temperature can avoid the coarsening of α GB and increase the elongation at break. Jimenez et al. [10] and Silva et al. [6] reported that severe heat accumulation leads to significant residual stress and distortion. ...
... Xiong et al. [11] showed that a lower interpass temperature is beneficial to increasing the surface quality of thin-walled parts fabricated by wire-arc DED. Many measures have been proposed to reduce heat accumulation, such as implementing active cooling systems [4,6,9], combining with conventional manufacturing processes like forge [12], and adjusting process parameters and deposition strategies [11,13]. ...
... Therefore, the in-process NDE can be integrated into the build process to leverage this inter-pass dwelling time to complete the inspection of the last pass without delaying the built process. Accordingly, a dwell time of 9 min was set to allow inter-pass cooling during the deposition of the experimental wall as suggested by [35]. This time was set to avoid the formation of coarse α GB phase grain microstructure, and thus, achieve optimal mechanical properties of this hypothetical component. ...
... inter-pass dwelling time to complete the inspection of the last pass without delaying the built process. Accordingly, a dwell time of 9 min was set to allow inter-pass cooling during the deposition of the experimental wall as suggested by [35]. This time was set to avoid the formation of coarse α GB phase grain microstructure, and thus, achieve optimal mechanical properties of this hypothetical component. ...
The demand for cost-efficient manufacturing of complex metal components has driven research for metal Additive Manufacturing (AM) such as Wire + Arc Additive Manufacturing (WAAM). WAAM enables automated, time- and material-efficient manufacturing of metal parts. To strengthen these benefits, the demand for robotically deployed in-process Non-Destructive Evaluation (NDE) has risen, aiming to replace current manually deployed inspection techniques after completion of the part. This work presents a synchronized multi-robot WAAM and NDE cell aiming to achieve (1) defect detection in-process, (2) enable possible in-process repair and (3) prevent costly scrappage or rework of completed defective builds. The deployment of the NDE during a deposition process is achieved through real-time position control of robots based on sensor input. A novel high-temperature capable, dry-coupled phased array ultrasound transducer (PAUT) roller-probe device is used for the NDE inspection. The dry-coupled sensor is tailored for coupling with an as-built high-temperature WAAM surface at an applied force and speed. The demonstration of the novel ultrasound in-process defect detection approach, presented in this paper, was performed on a titanium WAAM straight sample containing an intentionally embedded tungsten tube reflectors with an internal diameter of 1.0 mm. The ultrasound data were acquired after a pre-specified layer, in-process, employing the Full Matrix Capture (FMC) technique for subsequent post-processing using the adaptive Total Focusing Method (TFM) imaging algorithm assisted by a surface reconstruction algorithm based on the Synthetic Aperture Focusing Technique (SAFT). The presented results show a sufficient signal-to-noise ratio. Therefore, a potential for early defect detection is achieved, directly strengthening the benefits of the AM process by enabling a possible in-process repair.
... [4] To this aim, different methodologies have been studied: water bath, high-pressure air, aerosol, CO2 gas jet, combined system with air jet and water cooled platform. [4][5][6][7][8][9][10][11] However, when thinking to the industrial applicability of cooling strategies, the one performed with a highpressure air jet is the most feasible, especially in case of large-scale structures. Currently, the issue of forced interlayer cooling has been mainly addressed process-wise with the support of numerical simulations [5,6] or with the aim to reduce parts distortions, [7,8] while limited efforts have been devoted to assess the effect of different cooling conditions on final mechanical properties of printed parts. ...
... Currently, the issue of forced interlayer cooling has been mainly addressed process-wise with the support of numerical simulations [5,6] or with the aim to reduce parts distortions, [7,8] while limited efforts have been devoted to assess the effect of different cooling conditions on final mechanical properties of printed parts. Few studies have been focused on the microstructure and mechanical properties of interpasscooled WAAM Ti and Al alloys [9][10][11] , while lack of knowledge on the role of active or forced cooling on ferrous alloys exists, in particular in case of stainless steels, that have been widely reported as a suitable material for WAAM parts, by reason of their good printability and mechanical ...
Wire-and-Arc Additive Manufacturing (WAAM) is an innovative technology that involves deposition of subsequent layers of molten materials. Due to the high deposition rates, this technology is suitable for the production of large-scale complex structures. Further enhancement in the productivity can be achieved by an inter-layer cooling strategy that reduces idle time between depositions. However, the effect of the inter-layer cooling on microstructure and mechanical properties has to be addressed. In this view, the present work compares microstructural features and mechanical properties of WAAM-produced plates of austenitic AISI 304L, focusing on the effect of both active inter-layer air cooling and possible anisotropy induced by the additive process. Microstructural and mechanical characterization was carried out on samples extracted along the longitudinal, transverse and diagonal directions to the deposition layers of WAAM plates, processed with and without inter-layer active cooling. Results showed no remarkable influence of cooling conditions in the microstructure and mechanical properties of WAAM plates, that are indeed affected by the anisotropy induced by the additive process. The observed anisotropy in the elastic modulus, independent from different cooling conditions, was related to the crystallographic texture consequent to the highly oriented microstructure typically induced by the process.
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... In order to improve the geometry precision of parts, Xiong [12] used visual inspection of previous and current layers to achieve the excellent control of deposition height in WAAM via controlling the wire feed speed. The inter-pass cooling conditions have a significant influence on the morphology, microstructure, and mechanical properties of parts, as reported by [13]. Uwe Reisgen [14] proposed different process cooling strategies to increase the manufacturing efficiency of WAAM and pointed out that aerosol cooling can be a promising addition. ...
... (10), (11). (12), (13), (14), and (15). Similar to the droplet model, there is an induced magnetic field in the molten pool. ...
In this study, we develop a 3D transient mathematic model to simulate the heat transfer, fluid flow, and geometry morphology in GMAW-based wire arc additive manufacturing (WAAM). The processes of droplet formation, growth, and detachment from the end of wire electrode, who travels dynamically along the scanning direction, are coupled with molten pool for the first time by considering their own mechanical conditions and solving the transport equations in the whole solution domain. By the developed model, the simulations of single-pass multi-layer of WAAM of Al-5%Mg are performed. The calculated results indicate that when the droplet falls into the molten pool, the maximum velocity inside the droplet reaches 0.9m/s, resulting in that liquid metal in the middle flows toward the bottom of the molten pool and a depressed region is formed. On the surface of molten pool, the liquid metal dominated by Marangoni force flows from center to periphery, and on the bottom of molten pool, a clockwise circulation is formed. In addition, the interlayer idle time contributes to the formation of deposit with higher height and narrow width. Finally, to validate the model, the deposit profiles are also compared between simulated and experimental results.
... In order to improve the geometry precision of parts, Xiong [12] used visual inspection of previous and current layers to achieve the excellent control of deposition height in WAAM via controlling the wire feed speed. The inter-pass cooling conditions have a significant influence on the morphology, microstructure, and mechanical properties of parts, as reported by [13]. Uwe Reisgen [14] proposed different process cooling strategies to increase the manufacturing efficiency of WAAM and pointed out that aerosol cooling can be a promising addition. ...
... (10), (11). (12), (13), (14), and (15). Similar to the droplet model, there is an induced magnetic field in the molten pool. ...
In this study, we develop a 3D transient mathematic model to simulate the heat transfer, fluid flow, and geometry morphology in GMAW-based wire arc additive manufacturing (WAAM). The processes of droplet formation, growth, and detachment from the end of wire electrode, who travels dynamically along the scanning direction, are coupled with molten pool for the first time by considering their own mechanical conditions and solving the transport equations in the whole solution domain. By the developed model, the simulations of single-pass multi-layer of WAAM of Al-5%Mg are performed. The calculated results indicate that when the droplet falls into the molten pool, the maximum velocity inside the droplet reaches 0.9m/s, resulting in that liquid metal in the middle flows toward the bottom of the molten pool and a depressed region is formed. On the surface of molten pool, the liquid metal dominated by Marangoni force flows from center to periphery, and on the bottom of molten pool, a clockwise circulation is formed. In addition, the interlayer idle time contributes to the formation of deposit with higher height and narrow width. Finally, to validate the model, the deposit profiles are also compared between simulated and experimental results.
... Babu et al. [1] re-simulated the DED process conducted by Denlinger et al. [20] with the inclusion of a metallurgical model and obtained a similar trend of residual stresses according to the change of interpass time. It is critical to maintain a suitable interpass temperature because it influences the formation of the microstructure, porosity, geometric accuracy, and residual stresses within deposited parts [26][27][28][29]. ...
... By solving Equation (8), the proper interpass time that maintains a target interpass temperature T t is evaluated in Equation (9). The target interpass temperature for Ti-6Al-4V is assumed to be 400 • C in this study, as recommended [26][27][28][29]. Figure 10 shows proper interpass times estimated for different beads in order to maintain the target interpass temperature. Selected proper interpass times after the deposition of each layer are rounded to the nearest second with minimum value of 1 s, as indicated by solid black line in Figure 10, for practical implementation during a DED process. ...
Directed energy deposition (DED) provides a promising additive manufacturing method to fabricate and repair large metallic parts. However, it may suffer from excessive heat accumulation due to a high build rate, particularly during a wire feeding-type DED process. The implementation of interpass time in between two depositions of beads plays an important process role to passively control the interpass temperature. In this study, a method to estimate the proper interpass time using regression analysis from heat transfer finite element analysis is proposed for maintaining the interpass temperature during a wire feeding-type DED deposition of a planar layer. The overlapping beads of a planar layer are estimated using a polygonal-shaped bead profile in the finite element model. From the estimated proper interpass time, a selected proper interpass time scheme (PITS) is suggested for practical implementation. The selected PITS is applied in a thermo-mechanical finite element model to evaluate the temperature distribution and its effects on the depth of the melt pool, the depth of the heat-affected zone (HAZ), displacement, and residual stresses. By comparing the predicted results with those using a constant interpass time scheme (CITS), the selected PITS shows better control in reducing the depths of the melt pool and HAZ without severely inducing large displacement and residual stresses.
... Therefore, CMT has been selected as a promising arc-based AM method [27,47,48]. The successful application of CMT for the WAAM of various materials such as steels [41,46,49], titanium- [50,51], nickel-based [52][53][54], magnesium- [55][56][57], and aluminium alloys [58,59] has already been demonstrated. In general, the average deposition rate is lower in CMT, although the process is highly suitable for producing thin-walled components with challenging near-net shapes and geometric features [24]. ...
... The heat was removed from deposited parts via water, or high velocity gas. In studies, more groups of gas nozzles were used in order to a high cooling efficiency [12][13][14]. Alternatively, nozzles coaxial to the torch were developed, enabling simultaneous deposition and cooling [15]. ...
Components fabricated via wire-arc directed energy deposition (DED) currently suffer from a lack of dimensional accuracy. This can be controlled, via various advanced methods, including the introduction of external magnetic fields. This study developed and designed a miniaturised, parameter-adjustable magnetic field generation device. Further, the influence of materials and structures of this device on the magnetic field strength was analysed via a numerical simulation. In contrast, a steel device shell could provide high magnetic field intensity. An expanded bottom of the device benefits from a strength increase. Ultimately, an optical scanning system was used to evaluate dimensional accuracy of the component surfaces. The results show that a 5 mT magnetic field could effectively reduce dimensional errors and improve dimensional uniformity significantly.
... The findings in this work refer to individual layers and align with the findings on the effective total wall widths. Walls showing a V-shape in the first few layers were also observed in studies dedicated to active cooling applications by Kozamernik et al. [26] using steel and Vázquez et al. [27] using titanium material. Da Silva et al. [11] and Yang et al. [10] controlled the temperature of the part during the process and observed a more uniform geometry of the part. ...
Wire and Arc Additive Manufacturing (WAAM) of Ti-6Al-4V is becoming increasingly important in the aerospace industry for the production of large parts. Due to the high welding requirements of the material, high quality demands are placed on the process. To meet these high demands, quality assurance measures are applied to maintain mechanical and geometrical part properties. First, the interlayer temperatures that are applied influence the final geometry. The part must meet geometric accuracies in order to be machined after the WAAM process. Second, Ti-6Al-4V materials have a high affinity to absorb oxygen from the environment at elevated temperatures. This oxygen uptake results in a discoloration of the surface and an embrittlement of the material. Therefore, a defined and monitored oxygen content in the build chamber is crucial. This work presents an approach to determine limitations for the interlayer temperature of the part and the oxygen content in the build chamber. The influence of a temperature deviating from the set interlayer temperature on the layer width was analyzed. By varying the interlayer temperature, the layer width varied by up to 3 mm. It was shown that different restrictions for the oxygen content in the build chamber apply depending on the part size.
... For converting the Rockwell hardness to tensile strength, equations proposed by Petrenko [34,35] can also be used. ...
Wire arc additive manufacturing (WAAM) is an additive manufacturing process based on the arc welding process in which wire is melted by an electric arc and deposited layer by layer. Due to the cost and rate benefits over powder-based additive manufacturing technologies and other alternative heat sources such as laser and electron beams, the process is currently receiving much attention in the industrial production sector. The gas metal arc welded (GMAW) based WAAM process provides a higher deposition rate than other methods, making it suitable for additive manufacturing. The fabrication of mild steel (G3Si1), austenitic stainless steel (SS304), and a bimetallic sample of both materials were completed successfully using the GMAW based WAAM process. The microstructure characterization of the developed sample was conducted using optical and scanning electron microscopes. The interface reveals two discrete zones of mild steel and SS304 deposits without any weld defects. The hardness profile indicates a drastic increase in hardness near the interface, which is attributed to chromium migration from the SS304. The toughness of the sample was tested based on the Charpy Impact (ASTM D6110) test. The test reveals isotropy in both directions. The tensile strength of samples deposited by the WAAM technique measured slightly higher than the standard values of weld filament. The deep hole drilling (DHD) method was used to measure the residual stresses, and it was determined that the stresses are compressive in the mild steel portion and tensile in austenitic stainless steel portion, and that they vary throughout the thickness due to variation in the cooling rate at the inner and outer surfaces.
... This problem is partially solved by the application of Fronius Cold Metal Transfer technology, nevertheless that technology is patented, expensive and excludes other current sources [31][32][33][34]. Another approach is to introduce an additional cooling subsystem [35], yet that can result in unwanted changes in the microstructure [36,37], although Reisgen et al. reported that they observed no negative effects [38]. ...
Wear of the working surfaces of machinery parts is a phenomenon that cannot be fully countered, only postponed. Among surface lifecycle elongation techniques, hardfacing is one which is most often used in heavy load applications. Hardfaced coating can be applied using different welding approaches or thermal spraying technologies, which differ when it comes to weld bead dimensional precision, layer thickness, process efficiency and material. In this study the authors examine the geometrical behavior and hardness properties of two distinctive chromium-based Gas Metal Arc Welding (GMAW) cored wires. The stringer beads are applied numerically with five levels of linear energy, being a resultant of typical values of welding speed and wire feed, ranging between 250 mm/s to 1250 mm/s (welding speed) and 2 m/min to 10 m/min (wire feed). The samples were cut, etched and measured using a digital microscope and Vickers indenter, additionally the chemical composition was also examined. Hardness was measured at five points in each cutout, giving 40 measurements per sample. The values were analyzed using an ANOVA test as a statistical background in order to emphasize the divergent behavior of the cored wires. It appeared that, despite having less chromium in its chemical composition, wire DO*351 exhibits higher hardness values; however, DO*332 tends to have a more stable geometry across all of the heat input levels.
... The directional grain growth that occurs in titanium alloys in additive manufacturing can be reduced by an intermediate forming process [39,47,98,107,299,305,312,313], the addition of further elements like boron, silicon or other elements [303,311,317,321,322] or applying post-weld heat treatment [304,314]. Furthermore, like for other materials, the cooling rate or the inter-pass temperature has a significant influence on the mechanical properties of WAAM-made titanium alloy parts [309,310,318]. Furthermore, the machinability of Grade 5 titanium has been investigated, and it has been shown that the manufacturing strategy has a significant influence on the cutting forces [323]. ...
Wire arc additive manufacturing is currently rising as the main focus of research groups around the world. This is directly visible in the huge number of new papers published in recent years concerning a lot of different topics. This review is intended to give a proper summary of the international state of research in the area of wire arc additive manufacturing. The addressed topics in this review include but are not limited to materials (e.g., steels, aluminum, copper and titanium), the processes and methods of WAAM, process surveillance and the path planning and modeling of WAAM. The consolidation of the findings of various authors into a unified picture is a core aspect of this review. Furthermore, it intends to identify areas in which work is missing and how different topics can be synergetically combined. A critical evaluation of the presented research with a focus on commonly known mechanisms in welding research and without a focus on additive manufacturing will complete the review.
... Other work has shown [25] the trend associated with the growth of α W and its impact on increasing elongation. This was obtained after thermal treatment at 920 • C and a meandering strategy, and was attributed to the growth of α W and α GB making the whole structure more equilibrated, and hence the elimination of preferential paths. ...
Cold metal transfer (CMT)-based wire and arc additive manufacturing (WAAM) of Ti-6Al-4V alloy has been investigated to manufacture walls with two different building strategies. This study focuses on the influence of the application of thermal treatments on the resulting microstructure and mechanical properties. Deep microstructural analysis revealed different grades of growth of lamellae α phase after several thermal treatments at different temperatures, which lead to different tensile mechanical properties and better strength and ductility balance compared to the as-built condition. Results are compared with equivalent forged and casting standards and the state of the art for WAAM of Ti-6Al-4V alloy. At temperatures of 920 °C, anisotropy was maintained and elongation increased by 70% while yield strength and UTS was slightly decreased by 8%.
... To prevent the oxidation of the AM parts produced using the WAAM process, it is necessary to provide a high argon gas flow rate. This, in turn, leads to significant forced convection cooling, and careful attention needs to be paid to thermal management and interpass conditions [13][14][15]. Oxidation and dissolved oxygen in the titanium matrix causes an undesirably rapid reduction in the fracture elongation and increases its strength due to solid-solution hardening [16,17]. Electron beam processes serve as a useful alternative; because the process is carried out under vacuum, they are suitable for processing titanium and contamination by oxygen is avoided [18]. ...
The complex thermal cycles and temperature distributions observed in additive manufacturing (AM) are of particular interest as these define the microstructure and the associated properties of the part being built. Due to the intrinsic, layer-by-layer material stacking performed, contact methods to measure temperature are not suitable, and contactless methods need to be considered. Contactless infrared irradiation techniques were applied by carrying out thermal imaging and point measurement methods using pyrometers to determine the spatial and temporal temperature distribution in wire-based electron beam AM. Due to the vacuum, additional challenges such as element evaporation must be overcome and additional shielding measures were taken to avoid interference with the contactless techniques. The emissivities were calibrated by thermocouple readings and geometric boundary conditions. Thermal cycles and temperature profiles were recorded during deposition; the temperature gradients are described and the associated temperature transients are derived. In the temperature range of the α+β field, the cooling rates fall within the range of 180 to 350 °C/s, and the microstructural characterisation indicates an associated expected transformation of β→α'+α with corresponding cooling rates. Fine acicular α and α’ formed and local misorientation was observed within α as a result of the temperature gradient and the formation of the α’.
Wire arc additive manufacturing (WAAM) is presently growing as the major hub of research bodies throughout the universe. This is straightly noticeable in an enormous paper published recently pertaining to a myriad of distinct matters. Additive manufacturing is the quickest technique for the development of a product, and this is indicated by scientific industrial sections. It reinstated conventional ways in a few industrial circumstances by bringing down material utilisation. WAAM is much closer to welding in the process as it makes use of stratified deposition to design huge portions with less intricacy. Numerous experiments and estimates have evolved to ameliorate material properties concerning the remaining deformities like crackling and spattering. WAAM has acquired popularity as it has many benefits and very high efficiency. It increases the efficiency of the material and has a rate of deposition which is again very high and the lead time is shorter, the performance of the components is better, and the inventory cost is very low. This review is proposed to provide an appropriate summary of the field of WAAM. The inscribed matter in this review is embraced but not restricted to the materials. Various processes like monitoring, path planning, and modelling fall into the operations and techniques of WAAM. The alliance of detecting numerous authors into a consolidated form is the essence of this review. It is proposed to discover various fields in which the work is mislaid and in what ways the distinct topics can be manually integrated. A crucial estimation of introduced research along with welding research and a remarkable focus on additive manufacturing will accomplish this review.
This work investigates the anisotropy with the fatigue crack propagation behaviour of wire arc additive manufactured 316L stainless steel. Tension and fatigue crack growth tests were conducted on specimens prepared along the traverse, build and diagonal directions from single bead wall deposits. Anisotropy was observed with the fatigue crack propagation behaviour and tensile properties. The fatigue crack growth rates were comparable along the build and traverse directions. However, the slowest crack growth rate was observed along the diagonal direction. The high ductility observed along the diagonal direction limits the crack growth rate.
This work aims to present and explore thermal management techniques for the wire arc additive manufacturing (WAAM) of IN718 components. Excessive heat can be mitigated via air or water cooling. In this study, the material was deposited under four different heat-input conditions with air or water cooling. In air cooling, the layer is deposited in a normal atmospheric air environment, whereas with water cooling, the material is deposited inside a water tank by varying the water level. To validate the air and water cooling thermal management techniques, IN718 single-pass and multilayer linear walls were deposited using the bidirectional gas metal arc welding based WAAM setup under four different heat input conditions. During the deposition of single layers, the temperature profiles were recorded, and the geometric and microstructural features were explored. For multilayer wall structures, the mechanical properties (hardness, tensile strength, and elongation) were determined and assessed using the corresponding microstructural features explored through scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD) analyses. The microstructure observed through SEM analysis in the building direction was found to be nonhomogenous compared with that in the deposition direction. Moreover, water cooling was found to govern bead characteristics, such as wall width and height. The grain size and anisotropy of the mechanical properties also decreased in the water-cooled case. Hence, water cooling is an economical and efficient method to mitigate excessive heat accumulation in WAAM-deposited IN718.
WAAM (Wire Arc Additive Manufacturing) has been widely used due to its advantages of high processing freedom, low cost and high efficiency. However, one of the most relevant unsolved problems in WAAM is the heat accumulation caused by high heat input. In this study, the compressed argon‐based interlayer AC (Active Cooling) process is employed to reduce heat accumulation, and the influence mechanism on microstructure and mechanical properties of Ti‐6Al‐4V samples were revealed. The results show that the introduction of interlayer AC leads to the interlayer temperature decreases from 468 °C to 53 °C, and the widths of prior‐β columnar grains and α GB are refined. The increase of cooling rate (380 °C/s → 604 °C/s) results in the transformation of large‐sized colonies into finer basketweave structure, accompanied by the production of martensite α’ and a slight increase of β phase content. The finer basketweave structure increase the strength of the samples, while the narrower α GB and the high‐angle grain boundaries increase the resistance of crack propagation. The high dislocation density caused by the faster cooling rate increases the plastic deformation of the AC samples to a certain extent. As a result, the interlayer AC improves the strength and plasticity of the samples simultaneously, the samples change from brittle and ductile mixed fracture to ductile fracture mode.
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The wire arc additive manufacturing (WAAM) process is a 3D metal-printing technique that builds components by depositing beads of molten metal wire pool in a layer-by-layer style. Even though manufactured parts commonly suffer from defects, the search to minimize defects in the product is a continuing process, for instance, using modeling techniques. In areas where thermal energy is involved, thermomechanical modeling is one of the methods used to determine the input thermal load and its effect on the products. In the WAAM fabrication process, the thermal load is the most significant cause of residual stress due to the extension and shrinkage of the molten pool. This review article explores the thermomechanical effect and stress existing in WAAM-fabricated parts due to the thermal cycles and other parameters in the process. It focuses on thermomechanical modeling and analysis of residual stress, which has interdependence with the thermal cycle, mechanical response, and residual stress in the process during printing. This review also explores some methods for measuring and minimizing the residual stress during and after the printing process. Residual stress and distortion associated with many input and process parameters that are in complement to thermal cycles in the process are discussed. This review study concludes that the thermal dependency of material characterization and process integration for WAAM to produce structurally sound and defect-free parts remain central issues for future research.
Purpose
This paper aims to anticipate the possible development direction of WAAM. For large-scale and complex components, the material loss and cycle time of wire arc additive manufacturing (WAAM) are lower than those of conventional manufacturing. However, the high-precision WAAM currently requires longer cycle times for correcting dimensional errors. Therefore, new technologies need to be developed to achieve high-precision and high-efficiency WAAM.
Design/methodology/approach
This paper analyses the innovations in high-precision WAAM in the past five years from a mechanistic point of view.
Findings
Controlling heat to improve precision is an effective method. Methods of heat control include reducing the amount of heat entering the deposited interlayer or transferring the accumulated heat out of the interlayer in time. Based on this, an effective and highly precise WAAM is achievable in combination with multi-scale sensors and a complete expert system.
Originality/value
Therefore, a development direction for intelligent WAAM is proposed. Using the optimised process parameters based on machine learning, adjusting the parameters according to the sensors’ in-process feedback, achieving heat control and high precision manufacturing.
La fabrication additive de pièces métalliques a fait l'objet d'un vif intérêt ces dernières années comme une solution technologique importante pour la réalisation de pièces complexes. Parmi les différents procédés de la fabrication additive métallique, la fabrication additive arc-fil (FAAF) utilisant le soudage CMT (Cold metal transfer) est retenue pour notre étude grâce à son taux de dépôt important, faible coût des équipements et peu de perte de matière par projections lors de la fabrication. Dans la littérature, il est constaté que l'un des problèmes les plus importants qui empêchent l'application industrielle du procédé FAAF est la mauvaise précision géométrique des pièces fabriquées à cause de l'instabilité du procédé et du manque de contrôle-commande fiable pour traiter les irrégularités pendant le dépôt. L'objectif de ce travail est d'améliorer la stabilité et la performance géométrique du procédé. Dans ce travail, un système expérimental est mis en œuvre pour robotiser le procédé et contrôler la géométrie des pièces déposées. Le procédé est modélisé par les réseaux de neurones artificiels et un système contrôle-commande est développé permettant de commander la géométrie du dépôt et de réduire les erreurs de fabrication. De plus, une stratégie d'amélioration est appliquée afin de réduire les instabilités géométriques aux deux extrémités du cordon ; une méthode de contrôle in situ est également développée pour détecter les défauts internes des pièces déposées.
Ti-6Al-4V is a titanium alloy with excellent properties for lightweight applications and its production through Additive Manufacturing processes is attractive for different industrial sectors. In this work, the influence of mechanical properties on the notch fracture resistance of Ti-6Al-4V produced by Selective Laser Melting is numerically investigated. Literature data is used to inform material behaviour. The as-built brittle behaviour is compared to the enhanced ductile response after a heat treatment (HT) and a hot isostatic pressing (HIP) post-processes. A Phase Field framework is adopted to capture damage nucleation and propagation from two different notch geometries and a discussion on the influence of fracture energy and the characteristic length is carried out. In addition, the influence of oxygen uptake is analysed by reproducing non-inert atmospheres during HT and HIP, showing that oxygen shifts fracture to brittle failures due to the formation of an alpha case layer, especially for the V-notch geometry. Results show that a pure elastic behaviour can be assumed for the as-built SLM condition, whereas elastic-plastic phenomena must be modelled for specimens subjected to heat treatment or hot isostatic pressing. The present brittle Phase Field framework coupled with an elastic-plastic constitutive analysis is demonstrated to be a robust prediction tool for notch fracture after different post-processing routes.
To explore whether a copper-steel composite structure can replace cast nickel-aluminum bronze (NAB), the microstructure and mechanical properties of an arc deposited NAB/steel composite structure were systematically studied. The results demonstrate that in the composite structure, the precipitation of the rosette-like κI phase was inhibited, and the sizes of the α-Cu and κ phases were finer than those of the as-cast NAB. The texture density of the NAB layer was lower than that of as-cast NAB. The metallurgical layer dominated by Fe3Al formed at the copper-steel interface improved the strength of the composite structure. Compared with as-cast NAB, the yield strength and hardness of the composite structure increased by 51% and 30%. The Young's moduli of the α-Cu and β phases were higher than those of the as-cast NAB. The tensile cracks of the alloys were distributed around the κ phase and in the β phases. In the composite structure, the copper-layer cracked first and then extended to the copper-steel interface along with the β phase. Finally, the interface failed, and the copper-steel peeled off. In the tensile specimen, the κ phase was surrounded by dislocations and a large number of stacking faults and twins were generated.
In this study, the method to improve the bead quality and increase the wire-feed deposition efficiency propose in the CMT-GMA (cold metal transfer-gas metal arc) process of Ti-6Al-4V alloy deposition. The proposed method uses He shielding gas to improve the Ti deposition quality by increasing the wettability through the effective arc energy transfer realized by suppressing the adverse effect of cathode spot on the molten pool. The cathode spot is dispersed by the high ionization energy of He shielding gas, which improves the Ti deposition quality owing to the stability, narrow tip, and broad tail of the arc, which enables a bead shape that improves the sound deposition quality. Increasing the detachment current increased the deposition rate, attributed to the electromagnetic force-induced separation in the molten bridge and increase in droplet repetition. Under a detachment current of 290 A and when using the He shielding gas, the deposition rate was 2.69 kg/h, which was 20.5% higher than that under a detachment current of 50 A and when using Ar shielding gas. Through multilayer deposition, the deposition efficiency in single-bead multilayer deposition (1×5) was reduced to 30.1% under Ar shielding gas and to 12.5% under He shielding gas. In the case of multi-bead multilayer deposition (5×5), the Ar shielding gas induced cavities in the deposition structure. This result confirmed that the application of He shielding gas can help fabricate a high-density multilayer structure without interlayer defects.
Non-Destructive Evaluation (NDE) of metal Additively Manufactured (AM) components is crucial for the identification of any potential defects. Ultrasonic testing is recognised for its volumetric imaging capability in metallic components and high defect sensitivity. However, conventional ultrasonic techniques suffer from challenges when deployed on components with curved and non-planar geometries, such as those often encountered in AM builds.
The body of work introduces the concept of inspection of Wire+Arc Additive Manufacture (WAAM) components from their non-planar as-built surface, eliminating the requirement for post-manufacturing machining. In-situ or post-manufacturing inspection is enabled via an autonomously deployed conformable phased array roller-probe deploying Synthetic Aperture Focusing Technique (SAFT)-surface finding and multi-layer adaptive Total Focusing Method (TFM) algorithms, for fully focussed imaging of the as-built WAAM component.
The concept of the imaging approach is demonstrated by inspection, through the as-built surface, of two titanium WAAM components, one containing reference bottom-drilled holes, and the other with intentionally introduced Lack of Fusion (LoF) defects.
The TFM images of the WAAM components feature sufficient Signal-to-Noise Ratio to enable defect detection along with strong agreement against reference X-Ray CT data, confirming the competency of the approach for volumetric or layer-specific inspection of as-built WAAM components.
Finite element method (FEM) simulations are a powerful tool for understanding the thermal–metallurgical–mechanical effects of wire arc additive manufacturing (WAAM). Nonetheless, owing to the multiphysical nonlinear nature of welding coupled with the longer deposition time of WAAM, FEM simulations can be laborious and time-consuming, which makes it difficult to simulate the numerous procedural parameters of WAAM. Therefore, the present work aimed to employ an FEM mode to analyze the influence of idle time (30–240 s) on the interpass temperature (IT) of 20-layer single-bead walls produced via WAAM and use the FEM results to develop a predictive model for the IT based on an artificial neural network (ANN). The FEM simulations were performed using a heat source and a 20-layer single-bead wall model that was experimentally calibrated and validated. The first layers exhibited similar energy accumulation; however, as the wall height increased, the IT rapidly increased under to low idle times (≤ 120 s). The ANN was trained using the FEM simulations results, validated with FEM results (not included in the training database), and used to establish a process map (including the idle time, number of layers, and IT). This can help the manufacturers to obtain a suitable balance between productivity (lower idle times) and part behavior (e.g., microstructure and mechanical properties).
Cold metal transfer (CMT)–based wire-arc additive manufacturing (WAAM) is increasingly popular for the production of large and complex metallic components due to its high deposition rate, low heat input, and excellent material efficiency. The accurate prediction of the bead geometry is of great importance to enhance the stability of the process and its dimensional accuracy. Besides the wire feed speed (WFS) and travel speed (TS), the interlayer temperature is another key factor in determining the bead geometry because of the heat accumulation in the multilayer deposition. In this paper, considering the varying interlayer temperature, WFS, and TS as inputs, an artificial neural network (ANN) is developed to predict the bead width, height, and contact angle; then, by connecting the ANN model with a bead geometric model, a combined model is established to improve the ANN model. Based on experimental test data, with random combinations of input parameters, the combined model is demonstrated to be able to accurately predict the bead geometry (mean error < 5.1%). The general effect of interlayer temperature on the bead geometry was also investigated by experiment.
Over the past years, the demand for the wire arc additive manufacturing (WAAM) is potentially increased, and it has become a promising alternative to subtractive manufacturing. Research reported that the wire arc additively manufactured (WAAMed) material’s mechanical properties are comparable to wrought or cast material. In comparison with other fusion sources, WAAM offers a significant cost saving and a higher deposition rate. However, there are significant challenges associated with WAAM such as undesirable microstructures and mechanical properties, high residual stresses, and distortion. Thus, more research is still needed to handle the above challenges by optimizing the process parameters and post-deposition heat treatment. In line with the above, this paper attempts to fill the gap by presenting a comprehensive review of WAAM literature including stagewise development of WAAM, metals and alloys used, effects of process parameters, methodologies used by various researchers to improve the quality of WAAM component. Besides, this work proposes the areas that could be used as avenues for future research.
ABSTRACT
This research describes the process of manufacturing and machining of wire and arc additive manufactured (WAAM) thin wall structures on integrated and non-integrated WAAM systems. The overall aim of this thesis is to obtain a better understanding of deposition and machining of WAAM wall parts through an integrated system. This research includes the study of the comparison of deposition of WAAM wall structures on different WAAM platforms, namely an Integrated SAM Edgetek grinding machine, an ABB robot and a Friction Stir Welding (FSW) machine. The result shows that WAAM is a robustly transferable technique that can be implemented across a variety of different platforms typically available in industry.
For WAAM deposition, a rise in output repeatedly involves high welding travel speed that usually leads to an undesired humping effect. As part of the objectives of this thesis was to study the travel speed limit for humping. The findings from this research show that the travel speed limit falls within a certain region at which humping starts to occur.
One of the objectives of this thesis was to study the effect of lubricants during sequential and non-sequential machining/deposition of the WAAM parts. Conventional fluid lubricants and solid lubricants were used. In addition, the effect of cleaning of deposited wall samples with acetone was also studied. A systematic study shows that a significant amount of solid lubricant contamination can be found in the deposited material. Furthermore, the results indicate that even cleaning of the wire and arc additive manufactured surfaces with acetone prior to the weld deposition can affect the microstructure of the deposited material.
Keywords:
Wire and arc additive manufacturing, additive manufacturing, machining, solid lubricants, microstructure
Wire arc additive manufacturing (WAAM), utilizing welding arc to melt metal wire into shaped parts, has become a promising manufacturing technology recently. Tandem GMAW–based WAAM (TG-WAAM), in which two wires are fed into the molten pool simultaneously, has the potential to double the efficiency of traditional WAAM. However, the high wire-feed speed is accompanied with high heat input that is likely to cause molten pool overflowing, especially at upper layers because of decreased heat dissipation and increased heat accumulation. An in-process active cooling technology based on thermoelectric cooling is introduced into TG-WAAM in this research. Its effect on forming quality and efficiency of TG-WAAM is investigated experimentally. The results show that the additional cooling well compensates for the excessive heat input into the molten pool, which enables not only increased maximum wire-feed speed (9–15%) but also reduced inter-layer dwell time (42–54%), while maintaining the desired forming quality. The overall efficiency is improved by more than 0.97 times in the case study. This research provides a feasible scheme to solve the conflict between forming quality and efficiency during WAAM.
One of the challenges in additive manufacturing (AM) of metallic materials is to obtain workpieces free of defects with excellent physical, mechanical, and metallurgical properties. In wire and arc additive manufacturing (WAAM) the influences of process conditions on thermal history, microstructure and resultant mechanical and surface properties of parts must be analyzed. In this work, 3D metallic parts of mild steel wire (American Welding Society-AWS ER70S-6) are built with a WAAM process by depositing layers of material on a substrate of a S235 JR steel sheet of 3 mm thickness under different process conditions, using as welding process the gas metal arc welding (GMAW) with cold metal transfer (CMT) technology, combined with a positioning system such as a computer numerical controlled (CNC) milling machine. Considering the hardness profiles, the estimated ultimate tensile strengths (UTS) derived from the hardness measurements and the microstructure findings, it can be concluded that the most favorable process conditions are the ones provided by CMT, with homogeneous hardness profiles, good mechanical strengths in accordance to conditions defined by standard, and without formation of a decohesionated external layer; CMT Continuous is the optimal option as the mechanical properties are better than single CMT.
In-process deformation methods such as rolling can be used to refine the large columnar grains that form when wire + arc additively manufacturing (WAAM) titanium alloys. Due to the laterally restrained geometry, application to thick walls and intersecting features required the development of a new ‘inverted profile’ roller. A larger radii roller increased the extent of the recrystallised area, providing a more uniform grain size, and higher loads increased the amount of refinement. Electron backscatter diffraction showed that the majority of the strain is generated toward the edges of the rolled groove, up to 3 mm below the rolled surface. These results will help facilitate future optimisation of the rolling process and industrialisation of WAAM for large-scale titanium components.
Wire + Arc Additive Manufacture (WAAM) is an additive manufacturing technology that can produce near net-shape parts layer by layer in an automated manner using welding technology controlled by a robot or CNC machine. WAAM has been shown to produce parts with good structural integrity in a range of materials including titanium, steel and aluminium and has the potential to produce high value structural parts at lower cost with much less waste material and shorter lead times that conventional manufacturing processes. This paper provides an initial set of design rules for WAAM and presents a methodology for build orientation selection for WAAM parts. The paper begins with a comparison between the design requirements and capabilities of WAAM and other additive manufacturing technologies, design guidelines for WAAM are then presented based on experimental work. A methodology to select the most appropriate build orientation for WAAM parts is then presented using a multi attribute decision matrix approach to compare different design alternatives. Two aerospace case study parts are provided to illustrate the methodology.
Wire-feed additive manufacturing (AM) is a promising alternative to traditional subtractive manufacturing for fabricating large expensive metal components with complex geometry. The current research focus on wire-feed AM is trying to produce complex-shaped functional metal components with good geometry accuracy, surface finish and material property to meet the demanding requirements from aerospace, automotive and rapid tooling industry. Wire-feed AM processes generally involve high residual stresses and distortions due to the excessive heat input and high deposition rate. The influences of process conditions, such as energy input, wire-feed rate, welding speed, deposition pattern and deposition sequences, etc., on thermal history and resultant residual stresses of AM-processed components needs to be further understood. In addition, poor accuracy and surface finish of the process limit the applications of wire-feed AM technology. In this paper, after an introduction of various wire-feed AM technologies and its characteristics, an in depth review of various process aspects of wire-feed AM, including quality and accuracy of wire-feed AM processed components, will be presented. The overall objective is to identify the current challenges for wire-feed AM as well as point out the future research direction.
This paper presents an algorithm to automatically generate optimal tool-paths for the wire and arc additive manufacturing (WAAM) process for a large class of geometries. The algorithm firstly decomposes 2D geometries into a set of convex polygons based on a divide-and-conquer strategy. Then, for each convex polygon, an optimal scan direction is identified and a continuous tool-path is generated using a combination of zigzag and contour pattern strategies. Finally, all individual sub-paths are connected to form a closed curve. This tool-path generation strategy fulfils the design requirements of WAAM, including simple implementation, a minimized number of starting-stopping points, and high surface accuracy. Compared with the existing hybrid method, the proposed path planning strategy shows better surface accuracy through experiments on a general 3D component.
In this paper, the results of two different wire based additive-layer-manufacturing systems are compared: in one system Ti-6Al4V is deposited by a Nd:YAG laser beam, in the other by an arc beam (tungsten inert gas process). Mechanical properties of the deposits and of plate material are presented and evaluated with respect to aerospace material specifications. The mechanical tests including static tension and high cycle fatigue were performed in as-built, stress-relieved and annealed conditions.Generally, the mechanical properties of the components are competitive to cast and even wrought material properties and can attain properties suitable for space or aerospace applications.
Additive Manufacturing (AM) is an innovative manufacturing process which offers near-net shape fabrication of complex components, directly from CAD models, without dies or substantial machining, resulting in a reduction in lead-time, waste, and cost. For example, the buy-to-fly ratio for a titanium component machined from forged billet is typically 10-20:1 compared to 5-7:1 when manufactured by AM. However, the production rates for most AM processes are relatively slow and AM is consequently largely of interest to the aerospace, automotive and biomedical industries. In addition, the solidification conditions in AM with the Ti alloy commonly lead to undesirable coarse columnar primary β grain structures in components. The present research is focused on developing a fundamental understanding of the influence of the processing conditions on microstructure and texture evolution and their resulting effect on the mechanical properties during additive manufacturing with a Ti6Al4V alloy, using three different techniques, namely; 1) Selective laser melting (SLM) process, 2) Electron beam selective melting (EBSM) process and, 3) Wire arc additive manufacturing (WAAM) process.
The most important finding in this work was that all the AM processes produced columnar β-grain structures which grow by epitaxial re-growth up through each melted layer. By thermal modelling using TS4D (Thermal Simulation in 4 Dimensions), it has been shown that the melt pool size increased and the cooling rate decreased from SLM to EBSM and to the WAAM process. The prior β grain size also increased with melt pool size from a finer size in the SLM to a moderate size in EBSM and to huge grains in WAAM that can be seen by eye. However, despite the large difference in power density between the processes, they all had similar G/R (thermal gradient/growth rate) ratios, which were predicted to lie in the columnar growth region in the solidification diagram. The EBSM process showed a pronounced local heterogeneity in the microstructure in local transition areas, when there was a change in geometry; for e.g. change in wall thickness, thin to thick capping section, cross-over’s, V-transitions, etc. By reconstruction of the high temperature β microstructure, it has been shown that all the AM platforms showed primary columnar β grains with a <001>β || Nz fibre texture with decreased texture strength from the WAAM to the EBSM and SLM processes. Due to a lack of variant selection, the room temperature α-phase showed a weaker transformation α-texture compared to the primary β-texture with decreased texture strength in line with the reduction in β-texture strength.
The large β grains observed in the WAAM process were not significantly affected by changes in the GTAW (Gas Tungsten Arc Welding) process parameters, such as travel speed, peak to base current ratio, pulse frequency, etc. However, an increased wire feed rate significantly improved the grain size. Another important finding from this work was that by combining deformation and AM the grain size was reduced to a greater extent than could be achieved by varying the arc or, heat source parameters. It has been shown that the large columnar β-grain structure usually seen in the WAAM process, with a size of 20 mm in length and 2 mm in width, was refined down to ~ 150 μm by the application of a modest deformation, between each layer deposited.
The EBSM process showed consistent average static tensile properties in all build directions and met the minimum specification required by ISO 5832-3 (for the wrought and annealed Ti6Al4V). The WAAM samples produced using more effective shielding and the standard pulsed GTAW system also showed average static properties that met the minimum specification required by AMS 4985C for investment casting and hipped Ti6Al4V alloy. Overall, the fatigue life of the samples that were produced by AM was very good and showed a better fatigue performance than the MMPDS design data for castings. However, there was a large scatter in the fatigue life due to the effect of pores.
Wire and arc additive manufacturing (WAAM) enables the building of near net-shape components layer by layer by using arc welding technologies and wire filler metal as feedstock. The study aims at comparing the applicability of two innovative robotic arc welding technologies (cold metal transfer (CMT) and TopTIG) for additive manufacturing (AM) of stainless steel parts. Initially, a process development has been completed with the goal of optimizing material deposition rate during arc time. Both continuous and pulsed current programs were implemented. Then, different thick-walled samples composed of more than one overlapped weld bead per layer were manufactured in 316L stainless steel grade by CMT and TopTIG. Mechanical properties have been determined in as-build samples in different building orientations. WAAM applying CMT and TopTIG welding technologies is analyzed in terms of part quality (defined as the absence of defects such as pores, cracks, and/or lack of fusion defects); surface finishing; part accuracy; productivity; microstructural characteristics; and mechanical properties. Achieved mechanical properties and deposition rates are compared with the state of the art. Findings and conclusions of this work are applicable to the industrial manufacturing of stainless steel parts and requirements to apply these technologies to other expensive materials are finally discussed.
Wire + Arc Additive Manufacturing (WAAM) is a promising manufacturing process for producing large aerospace components. Based on welding technology, the process is highly affordable, has a very high deposition rate and is not limited by chamber size. Ti-6Al-4V is a promising candidate material for this technology given that it is extensively used in aerospace applications and some large, high buy-fly ratio components can be more efficiently produced by WAAM than via the conventional machining from billet approach. There is currently limited knowledge about whether additional post processes including heat treatments and hot isostatic pressing are necessary to unlock the optimal mechanical properties of Ti-6Al-4V components produced by WAAM. This work explores a range of different post process treatments and the effects on the microstructure and tensile properties of Ti-6Al-4V components produced by WAAM. The relatively slow cooling rate (10-20Ks⁻¹) during the β-α transformation produced Widmanstätten-α and offered an optimal balance between strength and ductility. Hot Isostatic Pressing (HIPing) removed gas porosity but was not effective in improving strength or ductility. Residual tensile stresses in as-built components severely impair ductility and should be removed through stress relief treatments.
Wire arc additive manufacturing (WAAM) offers a promising alternative to traditional subtractive manufacturing of metallic components, particularly in the case of large Ti6Al4 V structures for the aerospace sector that feature high buy-to-fly ratios. This study investigates the influence of heat accumulation on bead formation, arc stability, and metal transfer behaviour during the manufacture of Ti6Al4 V with the gas tungsten wire arc additive manufacturing (GT-WAAM) using localized gas shielding. An infrared pyrometer is used to measure the in-situ interpass temperature which is a key factor in determining the heat accumulation. Arc stability and metal transfer behaviour are monitored by means of a high speed camera. The results show that due to the various thermal dissipation paths along the building height, there exists a significant difference in temperature variation between substrate and in-situ layer. Owing to the influences of heat accumulation, the interlayer surface oxidation and bead geometries vary along the building direction, especially for the first few layers of the deposited wall, which lead to variation in arc shape and metal transfer behaviour. The research outcome provides a better understanding of the effects of heat accumulation on deposition stability during WAAM process, which benefits future process optimization and control.
A shielded environment is required during the wire+arc additive manufacture (WAAM) of titanium alloys to prevent oxidation. Applying local shielding can increase the flexibility of the WAAM process, however conventional devices do not provide adequate protection due to entrainment of the surrounding air. In this study, a new local shielding device based on laminar flow was developed and compared with a conventional device. The laminar local shielding device showed up to three orders of magnitude improvement with contamination levels below 2000 ppm being achieved with a stand-off distance of 30 mm. The performance was also assessed along a mock-up WAAM wall which showed that it could be protected up to 30 mm from the top. Finally, computational fluid dynamics models provided insight into the device performance and enabled the performance of an argon knife to be evaluated.
The present work investigates the anisotropic mechanical properties of a Ti–6Al–4V three-dimensional cruciform component fabricated using a directed energy deposition additive manufacturing (AM) process. The mechanical properties of the component in longitudinal and transverse orientations with respect to the build layers were measured under uniaxial tension. While the average ultimate tensile strength of ∼1060 MPa in both directions agrees well with prior studies on AM Ti–6Al–4V, the achieved elongations of 11% and 14% along the longitudinal and transverse directions, respectively, are higher. The enhanced ductility is partially attributed to the lack of pores present in these components. The anisotropy in ductility is attributed to the columnar prior-β grain morphology and the presence of grain boundary α, which serves as a path along which damage can preferentially accumulate, leading to fracture. In addition, the effect of oxygen on the strength and ductility of the component was studied. The findings indicate that a combined effect of an increase of 0.0124 wt.% oxygen and a decrease in α-lath width due to differential cooling at different heights within the component resulted in an increase of ultimate and yield strengths without a significant loss of ductility. Furthermore, this study demonstrates that quasi-static uniaxial tensile mechanical properties similar to those of wrought Ti–6Al–4V can be produced in an AM component without the need for post-processing heat treatments.
Wire and arc additive manufacturing (WAAM) is a novel manufacturing technique in which large metal components can be fabricated layer by layer. In this study, the macrostructure, microstructure, and mechanical properties of a Ti-6Al-4V alloy after WAAM deposition have been investigated. The macrostructure of the arc-deposited Ti-6Al-4V was characterized by epitaxial growth of large columnar prior-β grains up through the deposited layers, while the microstructure consisted of fine Widmanstätten α in the upper deposited layers and a banded coarsened Widmanstätten lamella α in the lower layers. This structure developed due to the repeated rapid heating and cooling thermal cycling that occurs during the WAAM process. The average yield and ultimate tensile strengths of the as-deposited material were found to be slightly lower than those for a forged Ti-6Al-4V bar (MIL-T 9047); however, the ductility was similar and, importantly, the mean fatigue life was significantly higher. A small number of WAAM specimens exhibited early fatigue failure, which can be attributed to the rare occurrence of gas pores formed during deposition.
Titanium alloys are attractive to the industrial world, as they offer the benefits of low density, great corrosion resistance, and relatively good strength, making them viable candidates for a multitude of applications. However, above 500 °C, oxidation and oxygen diffusion in titanium alloys need to be taken into account as they change their microstructure and then their mechanical properties. Oxidations were carried out between 600 and 750 °C on a specific titanium alloy: an α–β annealed Ti–6Al–4V. Oxidation kinetics and oxygen diffusion in the matrix were studied. SIMS analyses were realized on rotating specimens of this two-phase polycrystalline alloy in order to reduce roughness. Composition profiles along the sample thickness were compared to microhardness measurements. SIMS mappings were realized on the smooth slopes of the crater.
The microstructure and the mechanical properties of Ti–6Al–4V components, fabricated by two different wire based additive layer manufacturing techniques, namely laser-beam deposition and shaped metal deposition, are presented. Both techniques resulted in dense components with lamellar α/β microstructure. Large ultimate tensile strength values between 900 and 1000 MPa were observed. The strain at failure strongly depends on the orientation, where highest values up to 19% were obtained in direction of the building direction. Heat treatment increased the highest strain at failure up to 22%. The fatigue limit was observed to be higher than 770 MPa.
Effect of rolling on fatigue crack growth rate of wire and arc additive manufacture (WAAM) processed titanium school
- X Qiu
Robotic additive manufacture using the wire arc welding processes
- Z Pan
- J Norrish
Influence of post deposition heat treatments on microstructure and tensile properties of Ti-6Al-4V parts manufactured by CMT-WAAM
- L Vazquez
- N Rodriguez
- I Huarte
- P Alvarez
Strategies and processes for high quality wire arc additive manufacturing
- C R Cunningham
- J M Flynn
- A Shokrani
- V Dhokia
- S T Newman