Microstructural Evolution and Mechanical Properties of Ti Alloy/Stainless Steel Lap Joints during Cold Metal Transfer Technique with CuSi₃ Filler Wire

Joining of dissimilar titanium (Ti) alloy to stainless steel is a very significant challenge during conventional fusion welding. In this study, cold metal transfer (CMT) technique is developed for lap joining of Ti alloy to stainless steel with CuSi₃ filler wire. The microstructure evolution, mechanical properties and fracture mechanism of Ti alloy/stainless steel joints are investigated. When low heat input is applied, many fine spherical particles are distributed in the fusion zone, which are indicated as iron-rich phase in the Cu matrix. With an increase in the heat input, the particles become irregular and coarser in the fusion zone, inferring as Fe-Si-Ti ternary phase and Fe₂Ti phase. Since the fractions of the melted Ti alloy and stainless steel increase, the reaction of Ti with Fe and Si becomes more intense and Fe-Si-Ti ternary phase and Fe₂Ti phase thus appear in the fusion zone. The shear strength of the lap joints decreases with an increase in the heat input. There are two fracture modes in the lap joints, suggesting different fracture mechanisms. The fracture path of the joints is along the Ti-Cu interface under low heat input. The brittle Ti-Cu intermetallics are observed on the fracture surface, indicating that the shear strength and fracture feature of the joints are associated with the compound layer. The fracture failure of the lap joints occurs in the fusion zone under large heat input. The Fe-Si-Ti ternary phase and Fe₂Ti phase have detrimental effects on the joint strength, resulting in a brittle fracture in the fusion zone.