Dissimilar electron beam welding of the medium-entropy alloy Ni43.4Co25.3Cr25.3Al3Ti3 and 316 stainless steel for cryogenic application

Large-scale superconducting magnets in ITERs have an increasing need to develop hetero-structured components, which need to join those cryogenic strong, ductile high/medium-entropy alloys (H/MEAs) to traditional austenitic stainless steels (SSs). So far, investigation on the microstructure-strength relationship of HEAs dissimilar welded joints is still lacking, and high-strength ductile joints are being pursued in the welding field. However, the intermixed composition in the fusion zone (FZ) brings great uncertainty in phase stability. In this work, we investigated the dissimilar weldability of MEA Ni_43.4Co_25.3Cr_25.3Al₃Ti₃ and commercial 316 SS both in 2 mm thickness using electron beam welding (EBW). A full penetration and oxidation-free joint was produced with systematic columnar grains with an average size of 200 μm, which consists of f.c.c. matrix and a few titanium carbides as verified by both thermodynamic calculations and experimental observations. However, some cavities are present due to solidification shrinkage. The weld thermal cycling recrystallizes the MEA to form heat heat-affected zone (HAZ) with grain sizes of 6 μm and commonly observed (110)<112> texture. The 316 SS and its neighboring HAZ have comparable grain sizes of 11∼13 μm and (111)<101> texture. Strong, ductile dissimilar weld joins were developed, e.g. YS of 380 MPa, UTS of 691 MPa, a uniform strain of 17.1 %, and fracture strain of 24.5 % at 298 K, and YS of 480 MPa, UTS of 929 MPa, uniform strain of 17.8 %, and fracture strain of 21.7 % at 77 K. A severe localized strain concentration occurred in the FZ associated with two neighboring HAZs, which makes failure occur in the FZ by a ductile intergranular mode. The plastic deformation is mainly governed by a planar slip of dislocations along with a few stacking faults and deformation twinning events at both temperatures. Pronounced deformation-induced planar defects not only strengthen the strain hardening rate to ductilize but also strengthen the joint.