Cold spray deposition for fabrication of interlayers in stainless steel – titanium dissimilar metal laser welding

Dissimilar metal laser welding of stainless steel to titanium alloys remains one of the most challenging combinations because when directly joined, brittle intermetallic phases form in the fusion zone, which compromises the integrity of the joint. We have addressed this issue by applying interlayers using the cold spray (CS) deposition technique to form a barrier between stainless and titanium base materials. Cold spray is a solid-state deposition process, therefore unlike other interlayer techniques, there is no melting at the interface, enabling a wide variety of alloys for interlayer use. Several CS interlayer dissimilar joint configurations were explored in this work: 1) One of the two alloys in the dissimilar combination, e.g., CS stainless steel bonded to wrought titanium and vice versa. 2) A third dissimilar alloy utilizing alloys known to exhibit high CS density and good interfacial adhesion, e.g., CS Nb/Ta bonded to wrought titanium and subsequently laser welded to wrought stainless steel. Different processing conditions and post-CS heat treatments were utilized to affect several variables: 1) adhesion of CS interlayer to substrate, 2) density of CS interlayer, and 3) post-CS laser weldability. Microstructure characterization was performed using light optical and scanning electron microscopy to understand the effect of the processing conditions and heat treatments on mechanical properties and recrystallization of the CS interlayer. The interfacial adhesion behavior was evaluated utilizing a collar-pin pull-off testing methodology. The results are rationalized in terms of residual stress behavior in the overall joint. The mechanical properties of the overall dissimilar joints were evaluated with hardness testing and tensile testing. These results show that cold spray is a promising process for facilitating dissimilar joints with careful selection of processing parameters for both cold spray and subsequent laser welding.

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