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Defect-free (single-pass multi-layer) maraging steel components were prepared by wire arc additive manufacturing (WAAM). The study on microstructure evolution and strengthening behavior of the components at different conditions was deeply investigated. It was found that the as-printed components consisted of lath martensite, residual austenite, and a few in-situ strengthening phases. There was significant element segregation among intragranular and intergranular, and the ultimate tensile strength (UTS) of the components reached 1184 MPa with an elongation of 16.8 %. After direct aging treatment at 480 °C for 4 h, unevenly distributed nanoprecipitates formed within the lath martensite, resulting in an increase of UTS about 1484 MPa. The presence of over 10 % reverted austenite in the microstructure led to elongation reached 14.8 %. 840 °C for 4 h solution treatment was applied to eliminate element segregation and reduce the content of retained austenite to below 3 %. Subsequent aging treatment at 480 °C for 4 h resulted in the dispersed precipitation of strengthening phases such as Ni 3 Ti and Ni 3 Mo, which lead to UTS increased to 1686 MPa while the elongation decreased to 9.2 %. Finally, the Orowan-modified model was used to calculate the theoretical yield strength (YS) of the aged components. The calculated values were found to be in good agreement with the actual results. • The ultimate tensile strength and elongation of maraging steel fabricated by wire arc additive manufacturing (WAAM) after heat treatment are reached to1684 MPa and 9 % respectively, which is the highest strength and elongation in currently researches on WAAM-processed maraging steel of the same grade. • The solution treatment at high temperature (840 °C for 1 h) was used to eliminate the element segregation in the intragranular and intergranular regions of the maraging steel components prepared by WAAM, making the element distribution tend to be uniform. • By conducting different heat treatments on the components and changing the size and distribution of strengthening phases, the performance control of WAAM-processed maraging steel components has been achieved. [ABSTRACT FROM AUTHOR]

The GH4169 Ni-based superalloy is prepared by wire arc additive manufacturing and the sizes and morphologies of Laves phases under different heat treatment strategies are obtained to further investigate the influence of residual Laves phases on the tensile properties of the superalloy. After the solution treated at a higher 1150 °C and followed by aging, the Laves phase is completely dissolved, with limited δ phase with varying sizes distributed at the grain boundaries. While for lower 1020 °C solid solution heat treatment temperature, the Laves phase in the sample changes from chain-like to particle-like shapes by partially dissolving, and the needle-like δ phase exists around the particle-like Laves phase. Such residual "Laves + δ" impede high-temperature tensile crack propagation, significantly enhancing plasticity. This mechanism makes the lower temperature solid solution treated sample exhibit similar tensile stress but ∼2 times of elongation when compared to the sample treated by higher solid solution. • Novel Heat treatment strategy to benefit from dense Laves phase in WAAM GH4169. • Lower solutionized treatment forms bulk Laves phase surrounded by needle δ phase. • Laves/δ structure rises the plasticity without lower the strength at high temperature. • Micro-cracks initiate inside residual Laves phase and inhibit by surrounding δ phase. [ABSTRACT FROM AUTHOR]

Thin-wall samples of 17-4 PH stainless steel were prepared by pulsed TIG wire arc additive manufacturing technology (P-TIG WAAM), and the microstructure and mechanical properties of the samples in different regions (bottom, middle, top) were studied. The results show that the microstructure of the thin-wall samples are mainly consisted of martensite matrix embedded by worm-like (bottom) and strip-shaped (top) δ-ferrite, with a small amount of austenite. The δ-ferrite content gradually decreases along the building direction, resulting in a gradual increase in microhardness from the bottom to the top. Along the building direction, the precipitation density and size distribution of nano-scale CRPs (Cu-rich precipitates) in δ-ferrite and martensite, as well as at the δ-ferrite/martensite interface are significantly different: the CRPs in δ-ferrite is precipitated with lower density, larger size, and inhomogeneous distribution at the bottom region, while that in martensite is precipitated with higher density, smaller size, and homogeneous distribution. At the bottom region, a chain of large size fcc-CRPs (∼65 nm) is precipitated along the δ-ferrite/martensite interface to coordinate the strain between them and improve the strength and ductility simultaneously, thus excellent mechanical properties (tensile strength = 1356 ± 24 MPa, elongation = 15.16 ± 2.6 %) are obtained. The relationship between the evolution of CRPs and the intrinsic heat treatment (IHT) during P-TIG WAAM is also discussed. [ABSTRACT FROM AUTHOR]

In this study, a hybrid wire arc additive manufacturing (WAAM) and interlayer friction stir processing (IFSP) approach was utilized to enhance the quality of 2024 aluminum alloy components. The defects characteristic, microstructure evolution, and mechanical properties of the fabricated components and after T6 heat treatment were analyzed systematically. Our analysis reveals that IFSP can break and dissolve the eutectic phases, significantly reduce porosity defects and refine the grain size of the as-deposited materials, thus improving the overall mechanical properties. Notably, compared to the WAAM as-deposited component, the yield strength (YS) and ultimate tensile strength (UTS) in the top-layer stir zone (SZ) of IFSP component increased from 154 ± 3 to 267 ± 2 MPa, and from 258 ± 12 to 360 ± 3 MPa, respectively. However, in the middle-layer SZ, S-phase precipitation and coarsening induced by thermal cycling during additive manufacturing slightly reduced the tensile properties. Post-T6 heat treatment led to uniformly distributed properties in the IFSP component along both horizontal and vertical directions, with the YS, UTS, and elongation (EL) averaging 353 ± 9 MPa, 470 ± 4 MPa, and 10.5 ± 1.6%, closely matching those of wrought counterparts. The significant improvements in mechanical properties are primarily attributed to the elimination of porosity and precipitation of fine needle-like S-phase. This research not only demonstrates the potential of IFSP in enhancing the mechanical properties of WAAM-fabricated 2024 aluminum alloy but also offers a promising technique for producing high-quality components suitable for high-performance aerospace applications. [ABSTRACT FROM AUTHOR]

In this work, P-MIG is used as additive process, a new low-cost Co-free maraging steel wire is used as raw material to additive manufacturing block component, and the microstructure and mechanical properties of the component is investigated. The microstructure of the component is composed of martensite and austenite, and the closer to the bottom, the more serious the elemental segregation and the more austenite content. From the top to the bottom the high-angle grain boundaries increase and the grain size decrease, the difference of the heat dissipation condition and the thermal cycle are the reasons for this phenomenon. The texture of austenite is mainly Cube texture, and the texture of martensite is Rotated Cube texture and Goss texture. The tensile strength of XD, YD, ZD three directions is 1172 ± 15 MPa, 1021 ± 7 MPa, 1139 ± 16 MPa, respectively, impact toughness is 64.7 ± 1.6 J/cm2, 51.1 ± 5.6 J/cm2, 51.4 ± 3.1 J/cm2, respectively, TRIP effect occurs in the process of tensile, the main mode of fracture in tensile and impact is toughness fracture. The microhardness decreases gradually from bottom to top, which is due to the short-lived aging strengthening by thermal cycle, the change of grain size and GND density. The results of the work reveal the successful application of the new low-cost Co-free maraging steel wire in WAAM and provide an important reference for the adjustment of additive process, subsequent heat treatment and the adjustment of wire composition. [ABSTRACT FROM AUTHOR]

The high-performance additive manufacturing of relatively small aluminum alloy components has been widely proven successful. However, wire arc additive manufacturing (WAAM) has always been challenging in achieving uniform microstructure and high mechanical properties when manufacturing large-sized parts. Therefore, this study used cold metal transition wire arc additive manufacturing (CMT-WAAM) to prepare Al-5.49Cu-0.4Mn-0.29Cd (ACMC) alloy to obtain a uniform microstructure and improve its mechanical properties. The results show that the as-deposited (AD) samples have an obvious layered deposited structure. The grains in the microstructure are all equiaxed crystals and a large amount of copper-rich eutectic phase precipitates at the grain boundaries. After heat treatment, the vast majority of the eutectic phase dissolves, and θ'phase, with a size of 100–200 nm, uniformly precipitates in the α-Al matrix. The ultimate tensile strength (UTS) and yield strength (YS) of the heat-treated (HT) specimen can reach 483.7 MPa and 412.3 MPa, respectively. There is no significant difference in the mechanical properties between the transverse and longitudinal directions of thin-walled samples. Based on experimental results and theoretical analysis, the strengthening mechanism and fracture mechanism of CMT-WAAM aluminum alloy were revealed. This work provides a theoretical basis for the microstructure optimization and mechanical performance improvement of ACMC alloy, which is of great significance for the high-performance preparation of large-sized components in the aerospace field. • The microstructure consisting of equiaxed grains was obtained. • Al3(Zr, Ti) hindered the change of grain size after heat treatment. • The ultimate tensile strength after heat treatment reached 483.7 MPa. • The strengthening mechanism of Al–Cu–Mn–Cd alloy was revealed. • The fracture mechanism of Al–Cu–Mn–Cd alloy was analyzed. [ABSTRACT FROM AUTHOR]

A new deposition strategy combining interlayer cooling with isothermal transformation was studied using wire arc additive manufacturing based on cold metal transfer technology with high-strength low-alloy steel as the research object. The effects of the deposition method on the geometrical characteristics, microstructure and mechanical properties of the alloys were systematically evaluated. The results show that the use of interlayer cooling combined with isothermal transformation is effective in accelerating the cooling rate, preventing the spreading of the melt pool, and reducing the overflow, thus obtaining higher geometrical accuracy compared with the air-cooled method. The use of interlayer cooling can control the temperature gradient well. The microstructures of different deposition processes are almost acicular ferrite, M-A group elements and bainite structure. The heat treatment process transforms part of the retained austenite into bainite structure, resulting in a more uniform structure in the middle and upper parts of the sample. The samples prepared by the new deposition strategy showed no obvious overlapping region during the tensile process, which improved the mechanical properties of the overlapping region, effectively increased the ductility of the thin-walled parts in the vertical direction, and reduced the anisotropic mechanical behavior of the samples. • A new deposition strategy combining interlayer cooling with isothermal transformation was adopted. • The faster cooling rate results in a partial transformation of the retained austenite into bainite. • Thin-walled parts prepared by a new deposition strategy improve ductility in the vertical direction. • No significant overlapping regions were observed on the vertical samples after the isothermal transition. [ABSTRACT FROM AUTHOR]

Obtaining a reliable NiTi–Ti6Al4V(TC4) heterogeneous alloy component exhibiting excellent strength and elasticity via integrating distinct material benefits can provide design flexibility in multifunctional device fabrication. However, the fabrication of this materials couple is challenging due to the development of excessive Ti 2 Ni intermetallic compound. Inspired by the bioheterogeneous layered structure, Nb interlayer was applied for the wire arc additive manufacturing (WAAM) of NiTi and TC4 to restrict the mixing of alloying elements on both sides. However, the strength of NiTi–Nb-TC4 alloy component was limited by pure Nb characteristics when the NiTi and TC4 was completely separated by the unmolten Nb interlayer. In this work, the tensile strength perpendicular to the interface of NiTi–Nb-TC4 alloy component was enhanced to introduce appropriate amount Ni and Ti in Nb interlayer by adjusting the composite WAAM process at the expense of Ti 2 Ni. Microstructure evolution, composition change and mechanical properties of composite WAAM NiTi–Nb-TC4 alloy component were investigated. It was shown that the compression strength and fracture strain perpendicular to the interface of WAAM NiTi–Nb-TC4 sample was 1549.0 ± 17 MPa and 22.3 ± 8 %, and the compressive strength can reach 63.6 % and 94.5 % of WAAM NiTi alloy and TC4 alloy, respectively. The composite WAAM NiTi–Nb-TC4 sample maintained 65.3 % recovery rate after ten compression cycles, which confirmed that partial superelasticity was maintained in the WAAM NiTi–Nb-TC4 multilayer structure. This fact further proved that WAAM can be successfully applied to deposit shape memory/superelastic alloys. The use of the Nb interlayer enhanced the tensile strength perpendicular to the interface to 460.7 ± 16 MPa. The ultimate tensile strength perpendicular to the interface of composite WAAM NiTi–Nb-TC4 sample can reach 72.0 % and 60.0 % of WAAM NiTi alloy and TC4 alloy, respectively. The formation of NiTi, TiNb and β-Nb phases in Nb interlayer and the network distribution of Ti 2 Ni and TiNb phases explained the improved mechanical performance of the composite WAAM NiTi–Nb-TC4 alloy component. • Integrated additive manufacturing NiTi-TC4 using composite WAAM with Nb interlayer. • Uncover microstructural evolutions and mechanical properties of composite WAAM NiTi–Nb-TC4 heterogeneous alloy component. • The introduction of Ni and Ti in the Nb interlayer improved the strength and toughness of the interlayer. • Provide first-hand observations on NiTi-TC4 WAAM and new insights for dissimilar metal additive manufacturing. [ABSTRACT FROM AUTHOR]

The present study demonstrates the suitability of wire arc additive manufacturing (AM) for hot work tool steel processing. Different arc welding techniques and energy inputs were applied and systematically compared to determine the deposition characteristics, microstructure and mechanical properties. All AM deposits show a sound visual appearance and full density without macroscopic imperfections, i.e. cracking. By adhering to a pre-defined interpass strategy, the cold metal transfer process can be used to achieve higher weld beads with lower dilution and faster build-up rates than the metal active gas process. The microstructure of the AM parts is comparable for all process configurations and consists of an α/α′-matrix with a finely dispersed vermicular and polygonal δ-ferrite network; no notable amount of retained austenite could be measured, but it could be observed by transmission electron microscopy embedded within the laths. Intensive precipitation of multiple molybdenum-based precipitates is observed along the interface matrix to δ-ferrite. In contrast, iron-based precipitates are predominantly found inside and at the boundaries of the laths of the matrix. Similarities are also evident in the mechanical properties, resulting in an average hardness of 380–390 HV1 and absorbed impact energy of 10–12 J at room temperature. High yield strength values of 1000–1100 MPa and ultimate tensile strength of 1200–1400 MPa were obtained. No significant differences in the measured mechanical properties could be noted regarding the specimen orientation, indicating the isotropy of the properties. [ABSTRACT FROM AUTHOR]

In this study, a hybrid wire arc additive manufacturing (HWAAM) process that combines low-frequency vibration with cold metal transfer (CMT) arc is proposed to address the problems of porosity, coarse grains, and low mechanical performance encountered in WAAM aluminum alloys. The effects of different vibration frequencies on the microstructure and mechanical properties were investigated. The results indicated that as the vibration frequency increases, the grain size refinement reaches a maximum of 19.1% compared with the non-vibrated specimen. Vibration can enhance the fluidity and the cooling rate of the weld pool. The fatigue fracture of dendrite arms is the main mechanism of microstructure grain refinement caused by vibration. The dynamic bending stress induced by vibration on the dendrite arms and the root remelting phenomenon are the dominant factors for the dendrite fragmentation. Increasing the vibration frequency did not result in an obvious improvement in the tensile strength of the specimens, but did significantly increase the elongation. With the increase in vibration frequency, the maximum transverse and longitudinal elongation reached 22.2% and 23.5% respectively, a raise of 15.6% and 38.2% compared with non-vibrated specimen. Vibration reduced the size and distribution density of pores on the fracture surface, and increased the dimpled microstructure, improving the mechanical performance of the aluminum alloy. [ABSTRACT FROM AUTHOR]

Two Ni Cu alloys with various contents of Mn, Ti, Al and C were deposited in a shape of single-bead multi layered walls using wire arc additive manufacturing technology. To modify solute atom concentrations and particle number density values, the as-welded alloys were subjected to annealing at 1100 °C and age-hardening heat treatment in the 610-480 °C temperature range. Microstructure characterisation was carried out using optical, scanning, conventional transmission and atomic resolution transmission electron microscopy. Work hardening behaviour was studied using tensile testing. For similar deposition and heat treatment conditions, an alloy with higher C and Al, and lower Mn contents exhibited a higher number density of >20 nm TiC particles, higher number density of <20 nm γ′-Ni 3 (Al, Ti) particles, and, associated with these, superior hardness, tensile strength, strain hardening rate and toughness. The comparative effect of solid solution and precipitation strengthening on work hardening behaviour and fracture mode is discussed. [ABSTRACT FROM AUTHOR]

Abstract Wire arc additive manufacturing (WAAM) features advantages such as low cost and high disposition rate, and thus WAAM is a feasible additive manufacturing process. Although some characteristics of WAAM have been documented in the literature, the process stability, structural integrity, component morphology, microstructure, and mechanical properties during WAAM under different arc modes are not comprehensively demonstrated and understood. Here, we performed WAAM experiments with 316L stainless steel under different arc modes and a constant deposition rate, and then we discussed the mechanism and impact of the arc mode on the manufacturing process stability, structural integrity, microstructures, and mechanical properties. The results indicate that the SpeedPulse and SpeedArc additive manufacturing processes are relatively stable, significantly efficient, and structurally sound. Although the deposition rate and scanning speed of SpeedPulse WAAM and SpeedArc WAAM are the same, SpeedArc WAAM has a lower heat input and a higher cooling rate. Therefore, SpeedArc WAAM produces a finer solidification structure than SpeedPulse WAAM. The ultimate tensile strengths of the SpeedPulse and SpeedArc additive manufactured specimens along the horizontal direction are greater than 540 MPa and slightly greater than previously reported results. Due to the lower heat input and finer solidification structure, a component produced by SpeedArc WAAM has greater tensile strength and hardness than a component produced by SpeedPulse WAAM. [ABSTRACT FROM AUTHOR]

The critical application value of AISI H13 tool steel in the die and mold industry and the great application potential of additive manufacturing (AM) technology in die manufacturing and repair have attracted extensive attention from researchers. However, there is no consensus on the solidification mode and microstructure evolution mechanism of H13 during AM. The relationship between the unique cellular/dendritic substructures and the grain structures in AM-fabricated H13 components has yet to be fully understood. This work focused on the microstructure and mechanical properties of different regions of the H13 component manufactured by wire arc additive manufacturing technology. Thermodynamic calculations and experimental results confirmed the formation of δ-ferrite during the non-equilibrium solidification of AM. The cellular/dendritic substructure is caused by the solidification of δ-ferrite and austenite together with the accompanying segregation. The results of optical microscopy and scanning electron microscopy, as well as the reconstructed prior austenite grain structure by electron backscatter diffraction (EBSD) all confirmed that the formation and distribution of cellular/dendritic substructures were not directly related to the prior austenite grain. The EBSD pole figures show that although the martensite transformation weakened the preferred orientation of crystals, the martensite phase in the typical structure of the deposition area still had a high texture strength, which may be the main reason for the anisotropic mechanical properties of the H13 component. • The evolution mechanism of cellular/dendritic substructures and grain structures was investigated. • The formation of δ-ferrite during non-equilibrium solidification of H13 was confirmed. • The anisotropy of mechanical properties originated from the preferred orientation rather than the γ R films. [ABSTRACT FROM AUTHOR]

A parameter design of experiments was undertaken to study the impact of Si content in the wire feedstock, weld speed, and interpass temperature on the microstructure and mechanical properties of high deposition rate Wire-Arc Directed Energy Deposition (WA-DED) of 316L stainless steel. Small-scale, representative builds were constructed using a high deposition rate pulsed spray transfer mode. Across conditions, WA-DED 316L builds exceeded American Society of Mechanical Engineers (ASME) SA-240 minimum values for yield strength, elongation, and tensile strength at room temperature, tested both parallel and perpendicular to the build direction. It was found that 316LSi samples (higher Si content version of 316L) displayed significantly higher strengths and ductilities than samples produced with 316L, while the impact of weld speed and interpass temperature were less significant. A heat transfer model of the WA-DED process was created to allow for microstructure predictions. An infrared thermal camera system was used to calibrate this model by taking temperature measurements at fixed points during the deposition of several layers. Solidification models were developed to allow for predictions of microstructural features in the as-built condition. Predictions of dendrite spacings and growth morphologies show good agreement with experiments, demonstrating the potential for modeling the influences of the WA-DED parameters for process optimization. Based on microstructural analysis, it was concluded that the increase in strength and ductility in 316LSi compared to 316L is due to effects of composition on solid solution strengthening and stacking fault energy, suggesting opportunities for developing new stainless steel alloys for WA-DED with improved mechanical performance. [ABSTRACT FROM AUTHOR]

In this paper, titanium-aluminide functionally graded material with a designed composition range from pure Ti to Ti-50 at% Al is successfully fabricated using the double-wire arc additive manufacturing method (WAAM). Due to the influence of Al concentration, the morphology, microstructure, mechanical properties and oxidation behaviour vary greatly along the gradient direction of the manufactured bulk. With increasing Al content from the bottom to the top, the bulk exhibits a layered structure consisting of α–β duplex structure, α-α 2 lamellar structure, large α 2 grains, α 2 -γ duplex lamellar structure and γ interdendrities structure in sequence from the bottom to the top. Microhardness and tensile strength exhibit similar trends and are comparable to those of mono-composition components. The oxidation resistance degrades at an increasing rate with decreasing Al content due to oxide breakaway occurring in the TiAl alloy matrix that consists of single α 2 or α 2 + α. The experimental results indicate that the WAAM method is able to produce defect free TiAl functionally graded material with the desired composition gradient, suitable mechanical properties and acceptable oxidation behaviour. [ABSTRACT FROM AUTHOR]

The wire arc additive manufacturing (WAAM) 2Cr13 thin-wall part was deposited using robotic cold metal transfer (CMT) technology, and the location-related thermal history, densification, phase identification, microstructure, and mechanical properties of the part were explored. The results show that pre-heating effect from previously built layers can be effectively used to reduce residual stresses; cooling rate firstly decreased rapidly and then kept stable in the 15th–25th layers. The peaks of the α-Fe phase of the AM part drifted slightly toward a relatively smaller Bragg's angle as a result of solute atoms incorporation when compared with that of the base metal. Small amounts of pores were present with the absence of cracks in different layers, being indicative of a high densification level. As-deposited microstructure consisted of martensite and ferrite, along with (Fe, Cr) 23 C 6 phase precipitated at α-Fe grain boundaries. Martensite content increased gradually from the 5th layer to the 25th layers, indicating that metastable martensite partly decomposed into stable ferrite due to the carbon atoms diffusion. The hardness and UTS changed slightly in the 05th–15th layers and then increased quickly from the 20th layer to the 25th layers at the expense of ductility; the fracture process transformed from ductile (01st–10th layers) to mixed-mode (15th–20th layers), and finally to brittle fracture (25th layer). The findings above suggest that, despite the emergency of few pores and slightly inadequate ductility, this robotic CMT technology is a feasible method to obtain desired microstructures and enhanced mechanical properties for the WAAM 2Cr13 part in comparison with its as-solutioned counterpart. [ABSTRACT FROM AUTHOR]

A variable polarity cold metal transfer (VP-CMT) arc power source with different arc modes was employed in additive manufacturing Al-6Mg alloy parts. The microstructures were characterized using scanning electron microscopy with electron back-scattered diffraction. Even equiaxed grains in size of 20.6–28.5 µm with random orientation were obtained under VP-CMT mode, while a large number of columnar grains in bigger size exist in samples under other arc modes. Tensile strength of the VP-CMT sample with a maximum of 333 MPa is higher than that of the Al-6Mg wrought alloys due to fine-grain strengthening. However, the tensile strength of the VP-CMT sample in different tensile direction was anisotropic, with a percentage of 8–27%. The comprehensive analysis of defects and grain orientation showed that the micro pores in interlayer pore region lead to the anisotropy. [ABSTRACT FROM AUTHOR]

Selective laser melting (SLM) and wire arc additive manufacturing (WAAM) are two representative metal additive manufacturing technologies that fabricate the net shaping of small complex parts and near-net shaping of large components. To explore the feasibility of SLM-WAAM hybrid fabricating large and complex components, the preliminary hybrid SLM-WAAM process of Ti-6Al-4V is investigated. In this study, horizontal (H-) and vertical (V-) SLM samples are used as substrate for WAAM process to analyze microstructure and tensile properties, and to be compared with that of SLM samples and WAAM samples. The results show that Ti-6Al-4V sample formed by hybrid fabrication consists of three typical zones: SLM zone, WAAM zone and interface zone. The fine and short β columnar grains consisting of primarily martensite α´ exist in SLM zone, then these fine grains grow coarse in the about 3 mm-thick interface zone because of the repeated heating process, in contrast, the coarse β columnar grains awash with α lamellae lead to epitaxial growth in WAAM zone. There is a good metallurgical bonding in the interface zone of both H-SLM-WAAM and V-SLM-WAAM specimens and all the fractures are located in the WAAM zone during the tensile test. The mean yield strength and ultimate tensile strength of the H-SLM-WAAM specimens are 850 MPa and 905 MPa, respectively. In comparison, the V-SLM-WAAM samples exhibited slightly higher tensile strengths; the mean yield strength and ultimate tensile strength are 890 MPa and 995 MPa, respectively. The elongations of different directions are both more than 10%, satisfying the requirements of engineering applications. These tensile properties of SLM-WAAM samples are comparable with or even better than that of WAAM samples. [ABSTRACT FROM AUTHOR]

Large-size lattice sandwich structure (LSS) has broad application prospects in the industrial field because of its specific stiffness and high strength. Quasi-static compression behavior is one of the basic mechanical properties of LSS. In this paper, large size titanium pyramid lattice sandwich structures were fabricated by pulse hot-wire arc additive manufacturing (PHWAAM). The mechanical properties and failure modes of pyramid LSS were studied by quasi-static axial compression tests. Theoretical and experimental analyses showed that the compression strength of 4-cells LSS differs greatly from that of 5-cells LSS, and the specific strength and specific stiffness also differ significantly. 5-cells LSS has better compression behavior, strength is 608.23 MPa higher than the 4-cells LSS. The process of lattice damage during the experiment was observed, and the failure mode was analyzed by theoretical calculation together with metallography and fracture. The main failure mode is brittle fracture, but there is plastic deformation in some lattice cells. The compression properties of the two kinds of LSS are studied by finite element method (FEM) and compared with the theoretical and experimental results. The research findings presented in this study provide a concise and effective guidance for quasi-static compressive behaviors of large-size LSS. The proposed conclusion is of great significance to the explore of the mechanical properties of LSS manufactured by WAAM. [ABSTRACT FROM AUTHOR]

Wire arc additive manufacturing (WAAM) technology has potential advantages in the production of large-scale aluminum alloy components. However, as-deposited Al–Cu alloys produced by WAAM often suffer from porosity defects and coarse eutectic phases distributed along the grain boundaries, resulting in poor mechanical properties and hindering the applicability of WAAM to prepare aluminum alloy components. Herein, 2219 aluminum alloy was prepared using WAAM assisted by interlayer friction stir processing (FSP). Porosity defects in the as-deposited materials were eliminated, and coarse eutectic phases were broken and dissolved into the α-Al matrix via interlayer FSP, improving the mechanical properties of the components. The specimens processed by WAAM + interlayer FSP were subjected to different thermal cycling at different locations. The top-layer stir zone (SZ) was not affected by thermal cycling during the additive manufacturing process, and mechanical properties in the SZ were superior to those in the middle region. Compared with the as-deposited materials prepared by WAAM, the yield strength (YS) and ultimate tensile strength (UTS) in the middle region along the horizontal direction of the components manufactured by WAAM + interlayer FSP increased from 118 to 143 MPa and from 255 to 277 MPa, respectively. Further, the ultimate fatigue strength increased from 97 to 139 MPa. The YS and UTS along the vertical direction were 140 and 268 MPa, respectively, which are slightly lower than those along the horizontal direction. This is due to the small overlapping between the two adjacent SZs. The partially melted zone (PMZ) between the SZs was not completely eliminated, and the θ phase in the PMZ was mostly distributed along the grain boundaries. [ABSTRACT FROM AUTHOR]

An innovative wire arc additive manufacturing method using pulsed arc plasma (PAP-WAAM) has been proposed to manufacture high-performance metal parts by effective heat management in our previous study. In general, enhancing the strength of 316L stainless steel (SS) tends to reduce its useful ductility. Here, we reported that 316L SS additively manufactured via the PAP-WAAM method exhibited simultaneously higher strength and ductility than 316L SS fabricated by traditional WAAM based on gas tungsten arc welding (GT-WAAM). The PAP-WAAM 316L SS exhibited a high yield strength (YS) (0.2% offset) of 425 ± 13 MPa and ultimate tensile strength (UTS) of 609 ± 7 MPa, as well as a large elongation (EL) of 45 ± 7%. By comparison, the GT-WAAM 316L SS exhibited relatively low tensile properties, including YS (0.2% offset) of 396 ± 40 MPa, UTS of 585 ± 25 MPa, and EL of 32 ± 3%. High strength is attributed to fine microstructure, high-density dislocations, and low-angle grain boundaries generated during PAP-WAAM, while superior ductility correlates with a steadily high strain hardening rate that originates from the occurrence of abundant deformation twinning. Deformation twinning also contributes to the strength of additively manufactured 316L SS, because the twin boundary can act as a barrier to dislocation slip. This work demonstrates the potential of PAP-WAAM to fabricate high-performance metallic components with superior strength and ductility. • PAP-WAAM 316L SS exhibited simultaneously higher strength and ductility than GT-WAAM 316L SS. • The variation in molten pool behavior and interpass temperature during additive manufacturing was investigated. • The mechanism for the simultaneously enhanced strength and ductility of 316L SS fabricated by PAP-WAAM was investigated. [ABSTRACT FROM AUTHOR]

Wire arc additive manufacturing (WAAM) is a potential technology gradually applied in the aerospace, automobile, and military fields due to its advantages of high deposition efficiency, low manufacturing cost, and unrestricted manufacturing space. However, it is still challenging to fabricate large-size high-performance aluminum alloys using the WAAM method. Here we introduced TiC nanoparticles into the AA7075 wire to improve the performance of Al–Zn–Mg–Cu alloys fabricated by WAAM. TiC nanoparticles tended to be incorporated with the second phases on the grain boundary and strengthened these phases. Besides, TiC nanoparticles can as the heterogeneous nucleation sites for the Al matrix due to the small lattice mismatch between them, resulting in refined grain. Finally, the large-size TiC/AA7075 part was fabricated by optimized manufacturing processes. Compared to AA7075 samples, the TiC/AA7075 samples showed fine equiaxed grains and strengthened second phases, as well as superior (strength ∼ 435 ± 10 MPa, elongation ∼ 7.8 ± 0.8% for the deposited state) and isotropic mechanical properties. The outstanding properties are comparable to those of heat-treated Al–Zn–Mg–Cu alloys fabricated by WAAM in existing studies. This method can be widely applied in the WAAM of aluminum alloy to improve the microstructure, and it provides a novel strategy for manufacturing large-size high-performance aluminum alloy. [ABSTRACT FROM AUTHOR]

Ultrasound as a novel auxiliary means has been gradually used in additive manufacturing (AM) process. Ultrasonic cavitation and acoustic streaming can effectively control the columnar to equiaxed transition of grain structures, which can improve the anisotropy of mechanical properties. In addition, post-heat treatment (PHT) was considered to improve the microstructure uniformity and mechanical properties of alloys prepared by AM. In this study, we investigated the effect of PHT on the microstructure and mechanical properties of IN625 superalloy fabricated by ultrasonic-assisted wire arc additive manufacturing (UA-WAAM). The results showed that ultrasonic successfully inhibited the growth of columnar grains, and the microstructure was replaced by equiaxed grain structure. When the PHT temperature reached 1200 °C, the grain morphology changed, and a large number of annealing twins were generated in the alloy. Further, with the increase of PHT temperature, Laves phase disappeared and carbide particle size decreased. The results of mechanical properties showed that the microhardness and yield strength of the IN625 superalloy decreased with increasing PHT temperature, but the elongation and strain hardening ability increased significantly. The combination of UA and PHT provided an opportunity to achieve excellent strength-ductility balance in metals and alloys fabricated by AM. [ABSTRACT FROM AUTHOR]

Wire Arc Additive Manufacturing (WAAM) is an emerging group of methods for producing large parts with complex geometries and varying wall thicknesses. These parts usually exhibit anisotropic material behavior due to their intrinsic heterogeneous microstructure. To fully exploit the versatility of WAAM, a rigorous understanding of the relationship between processing conditions, microstructure, and mechanical response of WAAM parts is necessary. To this end, this paper investigates the structure-property relationship for thick-walled austenitic stainless steel WAAM parts experimentally and numerically using a mean-field crystal plasticity model. The major microstructural features are studied using optical microscopy and electron backscattered diffraction. A representative microstructure volume element is obtained with averaged features to study spatial variations in the microstructure across the WAAM part. Uniaxial tensile tests assisted with Digital Image Correlation along the transverse direction, diagonal (45o from the transverse direction), and building direction within the transverse direction-building direction plane are used to study the mechanical properties and associated deformation fields. The resulting heterogeneous microstructure with periodically alternating microstructural features reveals a clear anisotropic material behavior. Furthermore, distinct plastic deformation patterns for different loading directions arise from the spatially varying microstructure. The proposed crystal plasticity model adequately describes the crystallographic texture-induced orientation-dependent yield strength. • Additively manufactured part shows heterogeneous and periodic microstructure. • A representative microstructure description with averaged features is obtained. • Anisotropic plasticity is captured by experiments and by a mean-field crystal plasticity model. • The relation between the microstructure and anisotropic response is described. [ABSTRACT FROM AUTHOR]

The microstructure of wire arc additive manufactured (WAAMed) titanium alloy was characterized by brittle martensitic, which was significantly different from that of wrought alloy. It was generally believed that brittle martensitic was detrimental to the toughness of titanium alloys. In order to obtain WAAMed titanium alloy with excellent combination of strength and toughness, Ti5111 alloy, which is a newly developed deformation twinning induced plasticity(TWIP) alloy with an excellent combination of impact toughness and strength, was prepared by arc additive manufacturing(WAAM) technology and then annealed at 900 °C + air cooling(AC) and 950 °C+ air cooling. Microstructure, tensile properties and impact toughness of WAAMed Ti5111 alloy were investigated. Electron backscatter diffraction (EBSD) observation was adopted to identify the deformation twins activated in impact samples. Results showed that as-built WAAMed Ti5111 with excellent toughness was obtained, which was 42.8% higher than that of WAAMed Ti–6Al–4V alloy. After annealing, more deformation twins and secondary twins were activated during impact loading, which could effectively improve the toughness of WAAMed Ti5111. [ABSTRACT FROM AUTHOR]

In this study, the microstructure, mechanical properties and fracture mechanism of wire arc additive manufactured Inconel 718 alloy fabricated by cold metal transfer process under the action of the alternating magnetic field (AMF) are studied by changing the excitation current. The results show that the AMF deflects the droplet and changes the direction of the molten pool maximum temperature gradient, which inhibits the growth of large columnar crystals and refines the grains so that the index of {100}<001> texture decreases significantly. The microstructure of the 0 A and 40 A samples is composed of γ(Ni) + (TiN) [nucleus]/(NbC + TiC)[shell] + NbC + Laves. The length of γʹʹ-Ni 3 Nb of the 0 A sample (98 nm) is greater than that of the 40 A sample (84 nm). The AMF suppresses the large columnar crystals, and the average grain size is reduced to 161 μm. The volume fraction of the Laves phases is significantly reduced from 2.4% to 1.3%, and the Laves phases morphology changes from long stripe to granular under the action of the AMF. AMF increases the ultimate tensile strength and decreases the anisotropy. [Display omitted] [ABSTRACT FROM AUTHOR]

The balanced ferrite:austenite (α/γ) ratio is essential for duplex stainless steels (DSSs) to achieve superior strength and corrosion resistance. Nevertheless, the chemical compositions of commercial welding wires are not especially designed for the wire arc additive manufacturing (WAAM) process, leading to excessive austenite formation (∼74%) in their as-built DSSs. In this research, the chemical composition of a flux-cored wire arc additively manufactured super duplex stainless steel (FCWA-AM SDSS) was especially designed based on its thermal-history to achieve the balanced α/γ ratio. Mechanisms describing the microstructure evolution of α and γ during the FCWA-AM process were also proposed. Cr 2 N was the secondary phase formed in the as-built SDSS, which acted as the nucleating sites of intragranular austenite during the cooling of the deposition layer and precipitated at Widmanstätten austenite boundaries in the subsequent intrinsic heat treatment (IHT). The corrosion tests suggested that a thicker and more stable passive film was formed on the surface of the as-built SDSS than the hot-rolled 2205 DSS but their pitting corrosion resistance were comparable because the precipitation of Cr 2 N hindered the improvement of pitting corrosion resistance. The as-built SDSS showed higher strength than previously reported WAAM DSSs because of the enhanced solid solution strengthening effect and the balanced α/γ ratio. [ABSTRACT FROM AUTHOR]

A Ni–Ti–Cr–Mo–Nb wall component was fabricated for the first time by dual-wire arc additive manufacturing (in situ alloying) using TA1 and Inconel 625 welding wires as feedstock for the potential application in fields characterized by low temperature environments, such as polar research and space exploration. The microstructure and properties of the as-built component were studied. The results show the as-built component consists of NiTi (B2), Cr 2 Ti, and Ti 2 Ni phases. In the upper region, Ti 2 Ni phase is present as nanoparticles mainly embedded in Cr 2 Ti phase. Cr 2 Ti phase is present as long oriented strips or granular precipitates. Due to the presence of secondary Cr 2 Ti and Ti 2 Ni phases, the hardness of this component is 600–650 HV 0.2. The average compression strength is ∼1970 MPa and it exhibits outstanding superelasticity with a recoverable strain of 5.5%. The austenite finish temperature is −62 °C, which indicates that Ni–Ti–Cr–Mo–Nb alloy has the superelasticity at low temperature. • A new material wall-component, i.e. Ni–Ti–Cr–Mo–Nb alloy, was in-situ manufactured. • Ti 2 Ni phase as a nano-particle was found. • Hardness of this component was 600–650 HV 0.2. • As-built component exhibited the outstanding superelasticity. [ABSTRACT FROM AUTHOR]

In this study the microstructure evolution during wire arc additive manufacturing (WAAM) with high strength low-alloy (HSLA) steel was studied to employ this knowledge in the WAAM parts microstructure control, which was demonstrated by the functionally graded specimen manufacturing with only ER110S-G grade wire. The experiment design was based on the application of two WAAM modes: the cold metal transfer (CMT) and a conventional self-regulated gas metal arc welding (C-GMAW) mode. Material thermal history was estimated with the use of finite element modeling and the microstructure transformations were assessed using the calculated continuous cooling transformation (CCT) diagram. The microstructure was investigated with the use of optical and scanning electron microscopy. Mechanical properties were determined by tensile and microhardness tests, digital image correlation analysis was applied to detect the deformation zones during tensile tests of the functionally graded specimen. The results of the study showed a strong correlation between cooling rate, tempering time and microstructure and hence mechanical properties: depending on the thermal history the microhardness varied from 283 to 411 HV. Both C-GMAW and CMT modes resulted in the formation of two main zones: the no-diffusion upper zone and the diffusion lower zone. In the upper zone the microstructure is determined by cooling rate from 800 °C down to 500 °C. The upper zone consists of martensite, bainite, a mixture of martensite and bainite, martensite-austenite phase (MA) and retained austenite (RA). In the lower zone the microstructure is determined by the tempering time in the range 700–200 °C. The lower zone consists of a tempered martensite, troostite, sorbite, alpha ferrite, MA-phase and carbites. Both C-GMAW and CMT modes resulted in the relatively high ultimate tensile strength (UTS): 807 MPa for CMT and 759 MPa for C-GMAW. CMT and C-GMAW resulted in the different elongation in the horizontal and vertical directions, it ranged from 5 to 10%. Cooling rate and tempering time regulation allowed to manufacture the functionally graded specimen with the relatively high strength and with the predetermined fracture localization. • Precise microstructure evolution for WAAM with ER110S-G grade steel is characterized. • Microstructure evolution is controlled by the thermal history regulation. • Functionally graded sample printed using single material with distinct microstructure. [ABSTRACT FROM AUTHOR]

Regionalization of microstructure regulation and mechanical property adjustment during the thermal cycle of the deposition process plays a vital role in the grain morphology and orientation of aluminum alloy components manufactured using wire arc additive manufacturing (WAAM) technology. However, the microscopic mechanism of the mechanical property difference caused by uneven microstructure in different regions remains unclear for the WAAM 2319 alloy. The geometric properties of grains are described quantitatively in different regions for the WAAM 2319 alloy through EBSD data processing and analysis. The effect of grain orientation on grain boundary, including subgrain boundaries and the boundary of coincidence site lattice (CSL boundary), are explored with EBSD data. The grain boundary and dislocations are characterized by transmission electron microscopy (TEM). The distribution of geometrically necessary dislocation (GND) in the WAAM block at different regions is studied through EBSD analysis. The high Schmidt factor or Taylor factor (M) percentage was quantitatively calculated in different regions. Then the relationship between grain morphology and plastic deformation ability is established through these factors. The results indicate that the average grain size is small and located in the top and middle of the WAAM sample with a low critical Taylor factor (M c). The Taylor factor in most grains easily exceeds M c (M c < M), which causes most dislocations to pass through grain boundaries quickly, and plastic deformation easily occurs in these regions. There are various CSL boundaries except Σ3 boundaries existing in the middle of the sample, which can maintain the high mobility of the grain boundaries. Therefore, more slip transmission in the middle of the WAAM sample can provide the driving force for grain boundary migration. Most grains with a high subgrain boundary density (above 2.5) and the geometrically necessary dislocation (GND) density (5 × 1014 m−2) can hinder the movement of dislocations and improve the strength of materials at the sidewall of the sample. The Schmidt factor of columnar crystals is generally low (below 0.25), and the sliding system does not start rapidly in columnar crystals, especially at the bottom of the WAAM sample. • Regionalization of grain geometric properties after the WAAM process is fitted. • The influence mechanism of grain morphology on plastic deformation is revealed. • The relationship between grain misorientation and slip transmission is built. • The mechanisms of slip transmission around the grain boundary are explored. [ABSTRACT FROM AUTHOR]

NiTi alloy exhibits superior superelastic properties but its wide application is limited by its poor workability. In this study, Ni-rich NiTi wire was used as the feedstock and a wall-shaped NiTi component with height of 130 mm was deposited on Ti substrate by cold metal transfer (CMT) based wire arc additive manufacturing (WAAM) process. The microstructure and mechanical properties of the as-deposited NiTi alloy specimens in the horizontal and vertical directions, as well as their correlations were discussed in detail. In particular, the superelastic properties of the deposited materials were evaluated by cyclic loading-unloading tensile test. It was found that a very high superelastic recovery rate of more than 90% can be reached after 25 cycles for samples tested along both the horizontal and vertical directions. The mechanism that governs the high superelasticity in the alloy was explored in terms of microstructure, chemical composition and phase transformation analyses. [ABSTRACT FROM AUTHOR]

In the present study, the variation in molten pool thermal history with layer number during wire arc additive manufacturing of Inconel 625 wall was investigated. Further, its effect on the evolution of microstructure and anisotropy in mechanical and corrosion properties was reported. During the deposition process, the molten pool thermal history was monitored using a non-contact type IR pyrometer operating at 1.6 μm wavelength. A total number of 40 layers were deposited, building a wall of 60 mm height. With an increase in layer number, the molten pool lifetime and cooling rate were found to increase and decrease, respectively, resulting in coarse grains and increased elemental segregation or Laves phase formation. To investigate its effect on mechanical properties, the samples were collected in a skewed fashion along the height with orientation in the deposition direction. The tensile specimens collected close to the substrate exhibited better strength and ductility, while the samples from the top location of the wall exhibited a relatively brittle mode of fracture, which was investigated by carrying out the fracture surface analysis using SEM. Corrosion test was also conducted along the height of the wall, wherein the samples close to substrate exhibited better corrosion resistance due to refined microstructure and low elemental segregations. Further, EDS, XRD analysis and hardness test were carried out to investigate the elemental composition, variation in phases and hardness with layer number, respectively. [ABSTRACT FROM AUTHOR]

This study investigated the effects of using shielding gases with different nitrogen concentrations on the microstructures and mechanical properties of SHER-120G maraging steel fabricated by wire arc additive manufacturing (WAAM). With increases in the nitrogen concentration, the contents of nitrogen and austenite in the bulk specimens increased significantly, while δ-ferrite formation was inhibited. In addition, nitrogen was absorbed by the molten pool and decomposed into N atoms during WAAM. Some N atoms formed a solid solution with the matrix and the rest reacted with Cr to form Cr 2 N, which plays an important role in the process of martensite lath refinement. The optimal shielding gas composition was 94% Ar + 2% O 2 + 4% N 2 , which resulted in a tensile strength and micro-hardness of 1324 MPa and 441 HV, respectively. These values are 12.3% and 12.5% higher, respectively, than those of samples fabricated without nitrogen in the shielding gas. [ABSTRACT FROM AUTHOR]

The 300M steel straight wall parts (SWPs) are fabricated by wire arc additive manufacturing based on cold metal transfer technology. The effects of interlayer temperature on microstructure evolution and mechanical performance in the different regions of the SWPs are investigated. The results show that the surface waviness of the SWP first decreases and then increases with the increase of interlayer temperature, and the lowest surface waviness is controlled in 0.47 mm at the interlayer temperature of 200 °C. In the top region, the microstructure mainly consists of untempered martensite and slightly changes with the increase of interlayer temperature. In the middle and bottom regions, the microstructure is mainly composed of tempered martensite as the interlayer temperature is 100 or 200 °C. However, when the interlayer temperature is increased above 400 °C, the needle-like bainite and feather-like bainite are gradually formed due to the slow cooling rate and long dwell time in the bainite transformation zone. The microhardness and ultimate tensile strength in the top region are significantly higher than those in the middle and bottom regions, which are attributed to the enhanced solid solution strengthening effect of untempered martensite. The tensile fracture morphology is transformed from the quasi-cleavage fracture mode to the ductile mode with the increase of interlayer temperature or the decrease of distance from the substrate. [ABSTRACT FROM AUTHOR]

This study investigated the effects of using shielding gases with different nitrogen concentrations on the microstructures and mechanical properties of SHER-120G maraging steel fabricated by wire arc additive manufacturing (WAAM). With increases in the nitrogen concentration, the contents of nitrogen and austenite in the bulk specimens increased significantly, while δ-ferrite formation was inhibited. In addition, nitrogen was absorbed by the molten pool and decomposed into N atoms during WAAM. Some N atoms formed a solid solution with the matrix and the rest reacted with Cr to form Cr 2 N, which plays an important role in the process of martensite lath refinement. The optimal shielding gas composition was 94% Ar + 2% O 2 + 4% N 2 , which resulted in a tensile strength and micro-hardness of 1324 MPa and 441 HV, respectively. These values are 12.3% and 12.5% higher, respectively, than those of samples fabricated without nitrogen in the shielding gas. [ABSTRACT FROM AUTHOR]

In this study, a novel Cu-reinforced SHER120-G maraging steel was developed for the wire arc additive manufacturing (WAAM) process. The microstructural evolution, mechanical properties, and Ɛ-copper particle-strengthening mechanisms of the samples were investigated. The content of δ-ferrite and austenite increases with the increase of deposition height due to the unique thermal cycle characteristics of WAAM process. High-resolution transmission electron microscopy (HRTEM) images of the age hardened specimens clearly exhibit that there is a small amount of fine Ɛ-copper particles in the as-deposited sample due to the transient ageing effect. After direct ageing, a uniform and dense distribution of Ɛ-copper particles with the size about 6 nm formed in the martensite matrix. The yield strength and microhardness were significantly increased owing to the Orowan bypass mechanism and strain strengthening. The ultimate tensile strength, elongation and impact toughness reach 1221 MPa, 11.76% and 63 J, respectively. Additionally, improvements in the elongation and impact toughness characteristics were caused by the finer lath martensite, the disappearance of δ-ferrite, and the formation of higher content of reversed austenite during ageing. • A novel Cu-reinforced maraging steel for WAAM-process is developed. • The Ɛ-Cu particle may nucleate and grow up during 2.5-h age treatment. • The increase in the concentration of Ɛ-Cu significantly improves the yield ratio. • The main strengthening mechanism in WAAM SHER120-G is Orowan bypass mechanism. [ABSTRACT FROM AUTHOR]

A feasibility research was firstly carried out to deposit SS316L nuclear nozzles directly on the large-curvature PWR (pressurized water reactor) main pipeline by using the well-designed spatial scanning path derived from self-developed slicing algorithm for 3D model during wire arc additive manufacturing (WAAM) and CMT (cold metal transfer) process. This study provides comprehensive and systematic microstructure and mechanical datum of the as-deposited WAAM-CMT SS316L nozzles. Microstructure characterizations indicate that almost no precipitates or hot crack except micron pores remain inside, and the molten pools present typical hierarchical structures: 1.26% granular-δ within cellular γ sub-grains in the bottom region, lathy-δ within cellular γ grains in the central region and 6.49% skeletal-δ within column dendritic γ grains in the top region, respectively. This can be attributed to the synergistic effects of temperature gradient and super-cooling degree under the repeated rapid solidification and annealing treatment cycle inherent to WAAM-CMT. Both 25 °C (RT) and 350 °C (HT) tensile properties are well above the ASTM A370 criteria due to the dispersion-strengthening of dendritic ferrite within hierarchical-structure molten pools, coupled with the pinning effects of high-density dislocation tangles and cellular sub-grain boundaries as well as the annihilation of harmful intermetallic precipitates like σ phase and carbides that could become crack initiation. Further studies should pay attention to inhibiting the surface oxidation of each depositing layer and reducing the amounts of internal micron pores and δ-ferrite so as to suppress Cr-Mo-Ni atomic segregation and improve inter-granular corrosion resistance preferably without post-weld heat treatment. [ABSTRACT FROM AUTHOR]

In this study, microstructural features and mechanical properties of a wire arc additively manufactured 420 martensitic stainless steel were investigated in as-printed and heat-treated conditions. Initial microstructural investigations on the as-printed part revealed the formation of residual δ-ferrite during the solidification process, which is known as a deleterious phase to both mechanical and corrosion performance of stainless steels. To remove the residual δ-ferrite and obtain a fully martensitic microstructure, the as-printed samples were subjected to different austenitizing temperatures of 950, 1050, 1150, and 1300 °C. Austenitizing at 1150 °C was selected as the optimum cycle due to removal of undesirable phases, such as δ-ferrite and carbides, resulting in a fully martensitic microstructure. Following the austenitizing heat treatment, the samples were tempered at different temperatures including 200, 300, 400, 500, and 600 °C. Increasing the tempering temperature was found to vary the size, morphology, and distribution of chromium carbides precipitated during the tempering process. Although, tempering at lower temperatures (200 and 300 °C) decreased the hardness due to the formation of tempered martensite and stress relieving of the structure, the intermediate temperature of 400 °C increased the hardness value by virtue of the formation of carbides at optimum size and distribution. However, tempering at 500 and 600 °C decreased the hardness as compared to 400 °C due to intergranular segregation and coarsening of carbides. The results of uniaxial tensile testing were consistent with the hardness measurements and confirmed that the tempering temperature of 400 °C led to the optimal combination of strength and ductility ascribed to the formation of fine and homogenously distributed chromium carbides embedded in a moderately tempered martensitic matrix. [ABSTRACT FROM AUTHOR]

Due to the intrinsic high room temperature brittleness and cold-cracking susceptibility, the fabrication and forming of γ-TiAl intermetallic alloy component is extremely difficult. Therefore, in recent years, a wire-arc additive manufacturing (WAAM) technique has been developed to fabricate the γ-TiAl intermetallic alloy by depositing the Ti and Al wires into a single tungsten arc generated molten pool with specific wire feed ratios. However, the WAAM fabricated γ-TiAl intermetallic alloy has been found having inhomogeneous layer-by-layer microstructure and the excessive heat input of tungsten arc would induce significant residual stress in the bulk sample. In the present paper, the previous WAAM has been further upgraded and an innovative twin-wire plasma arc additive manufacturing (TW-PAAM) process has been developed. Afterwards, a γ-TiAl intermetallic alloy wall component with specific chemical composition of Ti-48Al has been fabricated and the metallography, phase composition and tensile properties are characterized subsequently. It has been found that a significantly more uniform microstructure is obtained in the TW-PAAM fabricated γ-TiAl intermetallic alloy than the previous WAAM technique. The content of α 2 phase, lamellar colony size and lamellar spacing exhibited the tendency of decreasing from the lower to upper part along building direction. And the tensile strength and ductility of the lower section are lower than the middle and top sections. In general, the present TW-PAAM technique has shown promising capability of fabricating γ-TiAl intermetallic alloy with lower cost, and the investigation results would become a valuable reference for understanding the evolution mechanism of microstructure and mechanical properties of the additively manufactured TiAl alloy. [ABSTRACT FROM AUTHOR]

The columnar to equiaxed transition (CET) of grain structures has been a great challenge during titanium alloy additive manufacturing (AM), especially for wire arc additive manufacturing (WAAM) with the highly localized heat input and large temperature gradient. In this work, three combined methods (ultrahigh frequency pulse arc + growth restricting solutes (GRS) additions (HP–S method), ultrasonic vibration + GRS additions (UV–S method) and low frequency pulse arc + GRS additions (LP-S method)) were investigated and compared for the capacity of refining β columnar grains and weakening α texture during the titanium alloy WAAM process. Compared with the other two methods, the LP-S method has a dramatic effect on achieving CET and weakening α texture. During the solidification process of LP-S method, three stages can be divided: (1) nucleation stage (2) competitive growth stage (3) re-melting stage, which promote CET and weaken α texture. It can be anticipated that further optimization in the selection of alloy elements and the process parameters of the low frequency pulse arc (base current, peak current and pulse frequency, etc.) can achieve grain morphology control and mechanical properties improvement. [ABSTRACT FROM AUTHOR]

The metallic component parts fabricated by wire arc additive manufacturing (WAAM) usually present columnar grain structures along the build direction showing anisotropy in both microstructure and mechanical response. In this work, a thin-walled sample is made by the WAAM method using Ti-6.5Al–1Mo–1V–2Zr-0.1B wires, where 0.1 wt% B element is added into the near-α titanium alloy. The microstructure of the as-deposited sample exhibits an equiaxed prior-β grain structure. The dynamic response of the WAAM sample is studied systematically along both scanning direction (OY) and building direction (OZ). The sample exhibits a much low anisotropy in mechanical response at a strain rate of 3000 s−1 along the OY and OZ directions. The differences are less than 1% and 6% along the two directions for the average flow stress and the uniform plastic strain, respectively. A preferred orientation of <0001> α along the loading direction is found after the dynamic compression. The formation mechanism of the crystal preferred orientation is discussed. [ABSTRACT FROM AUTHOR]

Functionally graded materials (FGM), whose structural properties are varied along their volume to perform an intended function, have a full application in the marine industry. Under the corrosive marine environment, a corrosion-resistant alloy of duplex stainless steel, functionally graded with carbon-manganese steel along with the inner layer, is proposed here for the risers. The interface strength of FGM, fabricated using Cold Metal Transfer – gas metal arc welding, based on Wire Arc Additive Manufacturing (WAAM) in the lab-scale, is compared with an X-52 Carbon Manganese steel to highlight the superiority of the proposed FGM. The role of metal transfer characteristics on the microstructure formation at the layers adjacent to the interface is analyzed and correlated with the voltage-current waveform those obtained during the deposition process. Based on the experimental studies carried out, it is seen that the yield strength of the FGM interface is very close to that of X52 steel, with a marginal increase of about 6% in the ultimate strength. Based on the energy dispersive X-ray analysis, it is observed that there is no enrichment of chromium across the interface towards the carbon-manganese steel side. It is vital for the corrosion-resistance of duplex stainless steel. Proposed FGM shows advantages both in strength and durability and hence seen as a promising candidate for marine riser applications. [ABSTRACT FROM AUTHOR]