High-strength dissimilar laser welding of TC4/316 L stainless steel via in-situ interfacial high-entropy design

This study designed novel CoCrNi and CoNiV medium-entropy alloy interlayers for laser welding of titanium-steel dissimilar joints, fundamentally avoiding the corrosion-induced failure common in copper-containing joints under pressurized water reactor conditions. Both joints achieved their maximum strength under identical processing parameters, with the CoNiV interlayer exhibiting a broader parameter window for robust bonding. The two joints exhibited distinct bonding mechanisms: fusion-brazing for the CoCrNi joint and full fusion welding for the CoNiV joint. These mechanisms accounted for the differences in the average thickness and through-thickness uniformity of the reaction layer on the TC4 side, as well as the resulting fracture paths. The reaction layer was composed of two sublayers: a Ti-rich layer and a Ti-depleted layer. In the CoNiV joint, the substitution of V for Cr reduced the hardness and elastic modulus of the constituent phases, improved interfacial phase compatibility, and lowered thermal residual stresses. As a result, the CoNiV joint achieved a tensile strength of 331.98 MPa, representing an increase of 94.5% over the CoCrNi joint. All fractures occurred within the reaction layer on the TC4 side and displayed a brittle fracture mode.