Microstructural analysis and mechanical behavior of SS 304 and titanium joint from friction stir butt welding

Titanium and stainless steel components are assembled together in many industrial applications. This makes it desirable to join the titanium and stainless steel. The objective of this work is to analyze the microstructural evolution at the interface of titanium and stainless steel, upon friction stir butt welding. Fully consolidated friction stir butt welds are created at two tool rotation speeds for a given feed rate. The microstructure across the titanium and stainless steel interface is studied by performing EBSD and EDS scans. The welded joints and as-received titanium & stainless steel samples are subjected to tensile testing. Increase in temperature during the process led to the diffusion of iron, chromium and nickel from stainless steel into titanium and diffusion of titanium into stainless steel. This led to an interface with primarily beta-titanium with alpha-Ti precipitates and FeTi intermetallic compound on either side of the interface. For the welds performed at both the tool rotation speeds, the thickness of the FeTi and beta-titanium decreases from top to bottom in the joints. This is attributed to the higher temperatures near the top surface as compared to the weld bottom, during the process. Also, lower tool rpm leads to lower temperatures during friction stir welding. This results in reduced FeTi thickness in the joint, which translates into improved UTS of about 88% of the as-received CP-Ti for the weld performed at lower rpm. However, the fracture strain for both the welds are very low, owing to the presence of brittle FeTi intermetallics at the SS-Ti interface.