Article

Influence of interpass cooling conditions on microstructure and tensile properties of Ti-6Al-4V parts manufactured by WAAM

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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.

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... 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. ...
Article
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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.
... 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. ...
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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.
... 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. ...
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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.
... 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. ...
Article
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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.
... 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. ...
Article
Full-text available
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.
... [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 ...
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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. This article is protected by copyright. All rights reserved.
... 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. ...
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... 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]. ...
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... 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]. ...
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Thesis
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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.
Thesis
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.
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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.
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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.
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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.
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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.
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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.
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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.
Article
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.
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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.
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Robotic additive manufacture using the wire arc welding processes
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  • N Rodriguez
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