Experimental Studies on Wire Arc Additive Manufacture of SS 308L and Inconel 718 Part

Abstract

Wire Arc Additive Manufacturing (WAAM) belongs to the category of Direct Energy Deposition (DED)-based Additive Manufacturing (AM) in which the feedstock material is in the form of metallic wire. The wire melts under the action of a welding arc; the molten material is deposited on a substrate. Successive bead deposition in a layer-upon-layer manner (along the build direction) develops the final 3D part. In order to build a cross-section/ layer, multiple beads need to be deposited one-after- another onto the horizontal plane in accordance with a given pattern. WAAM is characterized by its high material deposition rate (when compared to the powder- based AM techniques) and is suitable towards fabricating parts which are medium- to-large sized, possessing low-to-medium level of design intricacy. The present dissertation focuses on microstructure and mechanical property characterization of Cold Metal Transfer (CMT) + Metal Inert Gas (MIG) welding based WAAMed SS 308L and Inconel 718 parts. The complex heat interaction phenomena that take place during execution of WAAM process is also studied herein. In the first part of the dissertation work, the SS 308L-T1 wire is utilized to fabricate a 3D slab (149 22 125 mm3). Evolution of location-dependent microstructure and thereby anisotropic static mechanical properties (tensile properties and microhardness) is witnessed. WAAMed SS 308L as fabricated microstructure is composed of grain boundary 𝛿 ferrite of varied morphologies (vermicular/ skeletal and lathy) within the inter-dendritic regions of 𝛾 austenite. The average value of the Ultimate Tensile Strength (UTS) of the horizontally-cut specimens (~ 542 MPa) exceeds to that of the vertically-cut specimens (~ 520 MPa). Post-heat treatment (at 1200 °C, 4 h + furnace cooling) causes significant grain growth; therefore, reduction in the microhardness value from 178 HV0.1 to 143 HV0.1 is witnessed. It is also experienced that the proportion of 𝛿 ferrite phase gets reduced considerably upon the said post-heat treatment. In the second part of the dissertation, an attempt is made to fabricate a 3D slab (15  25 18 mm3) of Inconel 718 through CMT + MIG based WAAM. WAAMed Inconel 718 exhibits dendritic microstructure decorated with Laves phase, 𝛿 phase and MC- type carbides. In the as fabricated condition, the average value of the UTS along horizontal direction obtained is ~ 824 MPa. Post-heat treatment cycle attempted herein consists of homogenization (at 1080 °C, 1.5 h + air cooling) followed by solution treatment (at 980 °C, 1 h + air cooling) and finally, double stage ageing – the first stage ageing is at 720 °C, 8 h + furnace cooling and the second stage ageing is at 620 °C, 8 h + air cooling. It is experienced that detrimental Laves phase is completely dissolved during homogenization treatment although residues of 𝛿 phase is witnessed after the solution treatment. Ageing treatment causes precipitation of the strengthening phase 𝛾′′ (Ni3Nb). Thus, post-heat treatment imparts 50 % improvement in the UTS and 85 % increment in the microhardness value when compared to the as fabricated condition. However, post-heat treatment lowers ductility. In comparison with the as fabricated specimen, better wear resistance is attributed to the post-heat treated specimen as evidenced through the dry sliding wear test (at room temperature). The wear rate of the as fabricated specimen obtained is ~ 0.0035 mm3/Nm; on the other hand, the post-heat treated specimen corresponds to the wear rate of ~ 0.0025 mm3/Nm.