Steel-copper functionally graded material produced by twin-wire and arc additive manufacturing (T-WAAM)

SFRH/BD/144202/2019 UID/00667/2020In this work, a functionally graded material (FGM) part was fabricated by depositing a Cu-based alloy on top of a high strength low alloy (HSLA) steel by twin-wire and arc additive manufacturing (T-WAAM). Copper and steel parts are of interest in many industries since they can combine high thermal/electrical conductivity, wear resistance with excellent mechanical properties. However, mixing copper with steel is difficult due to mismatches in the coefficient of thermal expansion, in the melting temperature, and crystal structure. Moreover, the existence of a miscibility gap during solidification, when the melt is undercooled, causes serious phase separation and segregation during solidification which greatly affects the mechanical properties. Copper and steel control samples and the functionally graded material specimen were fabricated and investigated using optical microscopy, scanning electron microscopy, and high energy synchrotron X-ray diffraction. Retained δ-ferrite was found in a Cu matrix at the interface region due to regions with mixed composition. A smooth gradient of hardness and electric conductivity along the FGM sample height was obtained. An ultimate tensile strength of 690 MPa and an elongation at fracture of 16.6% were measured in the FGM part.publishersversionpublishe

Superelasticity preservation in dissimilar joint of NiTi shape memory alloy to biomedical PtIr

Laser microwelding was used to join, for the first time, superelastic NiTi to biomedical PtIr which can be used in multicomponent biomedical devices. By process optimization, it was possible to control the formation of the B2 NiTiPt, with no intermetallic compounds being formed. The NiTiPt phase inside the fusion zone had a strong metallurgical bonding with the NiTi base material due to the smooth transition of its grain orientation towards B2 NiTi. The major finding of the present work is the preservation of the NiTi superelastic response in the welded joint as evidenced by the load/unloading cycling up to 6 % strain, significantly higher than typically required for biomedical applications

Superelasticity preservation in dissimilar joint of NiTi shape memory alloy to biomedical PtIr

Laser microwelding was used to join, for the first time, superelastic NiTi to biomedical PtIr which can be used in multicomponent biomedical devices. By process optimization, it was possible to control the formation of the B2 NiTiPt phase, with no intermetallic compounds being formed. The NiTiPt phase inside the fusion zone had a strong metallurgical bonding with the NiTi base material due to the smooth transition of its grain orientation towards 〈111〉 B2 NiTi. The major finding of the present work is the preservation of the NiTi superelastic response in the welded joint as evidenced by the load/unloading cycling up to 6% strain, significantly higher than typically required for biomedical applications.This project has received funding from the EU-H2020 Research and Innovation program under Grant agreement No. 654360 having benefitted from the access provided by ICN2 (Spain) within the framework of the NFFA-Europe Transnational Access Activity. ICN2 is funded by the CERCA program/Generalitat de Catalunya and by the Severo Ochoa program of the Spanish Ministry of Economy, Industry and Competitiveness (MINECO, Grant No. SEV-2017-0706). JPO acknowledges Fundação para a Ciência e a Tecnologia (FCT - MCTES) for its financial support via the project UIDB/00667/2020 (UNIDEMI). The research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020 (proposal I-20160912)

Controlling intermetallic compounds formation during laser welding of NiTi to 316L stainless steel

Dissimilar laser welding of NiTi to stainless steel is of great importance in designing medical devices but the formation of hard and brittle intermetallic compound results in low strength joints. Normally, different interlayers are applied as physical and chemical barriers to control the microstructure and to improve the mechanical properties. However, this procedure is a cost and time consuming process and may cause the formation of other types of intermetallics depending on the interlayer used. In the present work, laser offsetting welding (LOW) was introduced without inserting any interlayer by shifting the laser beam 100 μm into the stainless steel from the NiTi/316L stainless steel interface. This led to a softer weld zone (~570 H V), due to the formation of less brittle intermetallics compounds (Fe2Ti, Cr2Ti and Ni3Ti) compared to that (~970 H V) when the laser beam was placed at the NiTi/316L stainless steel interface. For comparison purposes, an Ni interlayer was also used to control the chemical composition of the fusion zone. In terms of mechanical properties, both the laser offset welding and the use of an Ni interlayer, were seen to improve the tensile strength of the dissimilar joints (above 400 MPa) compared to the centerline welding condition (around 200 MPa). Hence, LOW was confirmed to be an effective method to laser weld the NiTi/Stainless Steels

Wire and arc additive manufacturing of 316L stainless steel/Inconel 625 functionally graded material: development and characterization

In this work, a 316L stainless steel to Inconel 625 functionally graded material (FGM) was built using different deposition strategies (named as direct and smooth-type interfaces) by Twin-Wire and Arc Additive Manufacturing (T-WAAM). This combination of materials is of interest in chemical plants, oil & gas, and nuclear applications, where high corrosion and wear resistance are essential requirements. Although these properties are superior in Inconel 625, replacing Inconel with stainless steel in strategic regions of structural components can reduce the overall costs and parts’ weight. Both direct and smooth transition interfaces were tested and characterized. Microscopic analysis revealed that each interface and the as-built samples had an austenitic matrix, and every sample was well bonded and free of defects. Different types of microstructures evolved at the interfaces due to distinct gradients in composition. Synchrotron X-ray diffraction measurements showed that the smooth-gradient produced secondary phases, such as δ-phase (Ni3Nb) and carbides, that were not present with the direct interface strategy. Overall, the properties were superior in the FGM with a direct interface, which experienced higher strengths and elongations upon failure. Moreover, neutron diffraction measurements revealed that lower residual stresses developed in the direct interface FGM than in the smooth gradient FGM

Wire and arc additive manufacturing of 316L stainless steel/Inconel 625 functionally graded material: development and characterization

In this work, a 316L stainless steel to Inconel 625 functionally graded material (FGM) wasbuilt using different deposition strategies (named as direct and smooth-type interfaces) byTwin-Wire and Arc Additive Manufacturing (T-WAAM). This combination of materials is ofinterest in chemical plants, oil & gas, and nuclear applications, where high corrosion andwear resistance are essential requirements. Although these properties are superior inInconel 625, replacing Inconel with stainless steel in strategic regions of structural componentscan reduce the overall costs and parts’ weight. Both direct and smooth transitioninterfaces were tested and characterized. Microscopic analysis revealed that each interfaceand the as-built samples had an austenitic matrix, and every sample was well bonded andfree of defects. Different types of microstructures evolved at the interfaces due to distinctgradients in composition. Synchrotron X-ray diffraction measurements showed that thesmooth-gradient produced secondary phases, such as $\delta$-phase (Ni$_3$Nb) and carbides, thatwere not present with the direct interface strategy. Overall, the properties were superior inthe FGM with a direct interface, which experienced higher strengths and elongations uponfailure. Moreover, neutron diffraction measurements revealed that lower residual stressesdeveloped in the direct interface FGM than in the smooth gradient FGM

Improving the ductility in laser welded joints of CoCrFeMnNi high entropy alloy to 316 stainless steel

© 2022 The AuthorsDissimilar joining involving high entropy alloys is currently being explored to evaluate the suitability of these novel advanced engineering materials in structural applications. Recently, joining of a CoCrFeMnNi high entropy alloy to 316 stainless steel was successfully attempted. However, the joint ductility was limited by the lack of deformation experienced by the cold-rolled CoCrFeMnNi base material during tensile loading. In this work, it is shown that by simply changing the base material condition, from cold-rolled to annealed, it is possible to significantly improve the joint fracture strain from ≈ 5 to ≈ 10 %, while preserving the strength at ≈ 450 MPa. Using electron microscopy, high energy synchrotron X-ray diffraction and mechanical testing aided by digital image correlation, the microstructure evolution across the welded joint is assessed and correlated to its mechanical performance. Moreover, thermodynamic calculations considering the compositional changes across the fusion zone were used to predict the microstructure evolution of this region.11Nsciescopu