Dissimilar interface and joint strength of SS 304 and titanium friction stir spot welds: a numerical and experimental analysis.

The joining of Ti to stainless steel would allow cost-effective and light weight designs for many applications. For friction stir spot welding of Ti to steel with lap configuration, dwell time is an important process parameter. The higher dwell time promotes higher heat generation, which helps material movement and mixing for joint creation. However, higher dwell time also leads to higher peak temperature and incubation time, which encourage undesirable IMC formation. This research focuses on the impact of dwell duration on the elemental diffusion and IMC growth during FSSW of CP-Ti and SS 304. This research numerically and experimentally analyzes the diffusion of elements, intermetallic compound formation, and shear strength of stainless steel 304 and titanium dissimilar friction stir spot welds. The temperature profiles from numerical thermal model show that the peak temperatures at the dissimilar joint interface increases as dwell time increases. The diffusion model and characterization results reveal that Ni, Fe, and Cr diffuse across the dissimilar joint interface from steel to titanium side. Since Ni, Fe, and Cr are β-Ti stabilizers, the β-Ti stabilizes next to the dissimilar interface. The α-β Ti needles are observed between the stabilized β-Ti and α-Ti region towards base material. The numerical predictions of β-Ti and Fe-Ti IMC compare well against the experimental results. From shear strength (tensile) tests, the maximum shear strength (tensile) of ~ 8 kN is achieved from the SS 304-CP-Ti joint performed with 10 s dwell time. At the joint interface, the presence of β-Ti strengthens the joint and the presence of Fe-Ti IMCs encourages abrupt brittle failure and reduces strength. With dwell times less than 10 s, the Fe-Ti IMC thickness is less than 1 μm, which leads to higher shear strength (tensile) with higher β-Ti thickness as dwell time increases. With 20 s dwell time, the Fe-Ti IMC thickness of about 1 μm leads to reduced shear strength (tensile). The EDS analysis of the fracture surfaces reveal that dissimilar SS 304-CP-Ti joints failed from the dissimilar interface containing Fe-Ti IMC. [ABSTRACT FROM AUTHOR]