Study on Fatigue Life and Fracture Behaviour of Similar and Dissimilar Resistance Spot-Welded Joints of Titanium Grade 2 Alloy and Austenitic Stainless Steel 304

Resistance spot welding (RSW) is now the primary joining process used in the automobile and aerospace sectors. Mechanical parts, when put into service, often undergo cyclic stress. As a result, avoiding fatigue failure should be the top priority when designing these parts. Given that spot welds are a type of localised joining that results in intrinsic circumferential notches, they increase the likelihood of stress concentrations and subsequent fatigue failures of the structure. Most of the fatigue failures in automotive parts originate around a spot weld. To that end, this study seeks to examine the mechanical properties and fatigue behaviour RSW joints made of titanium (Ti) grade 2 alloy and AISI 304 austenitic stainless steel (ASS) with equal and unequal thicknesses of 0.5 and 1 mm. Based on the mechanical properties and fatigue life results, the maximum tensile shear strength and fatigue life for the RSW titanium joint were 613 MPa and 7.37 × 105 cycles for the 0.5–0.5 mm case, 374.7 MPa and 1.39 × 106 cycles for the 1–1 mm case, and 333.5 MPa and 7.69 × 105 cycles for the 1–0.5 mm case, respectively. The maximum shear strength and fatigue life of ASS welded joints were 526.8 MPa and 4.56 × 106 cycles for the 1–1 mm case, 515.2 MPa and 3.35 × 106 cycles for the 0.5–0.5 mm case, and 369.5 MPa and 7.39 × 105 cycles for the 1–0.5 mm case, respectively. The assessment of the shear strength and fatigue life of the dissimilar joints revealed that the maximum shear strength and fatigue life recorded were 183.9 MPa and 6.47 × 105 cycles for the 1 mm Ti–0.5 mm ASS case, 115 MPa and 3.7 × 105 cycles for the 1 mm Ti–1 mm ASS case, 156 MPa and 4.11 × 105 cycles for the 0.5 mm Ti–0.5 mm ASS case, and 129 MPa and 4.11 × 105 cycles for the 0.5 mm Ti–1 mm ASS case. The fatigue life of titanium and stainless steel welded joints is significantly affected by the thickness, particularly at maximum applied stress (0.9% UTS), meaning that similar thicknesses achieve a greater fatigue life than unequal thicknesses. Conversely, the fatigue life of the dissimilar joint reached the greatest extent when an unequal thickness combination was used. The ductile failure of similar Ti and ASS welded joints was demonstrated by the scanning electron microscopy (SEM) examination of fatigue-fractured surfaces under the high-cycle fatigue (HCF) regime, in contrast to the brittle failure noticed in the low-cycle fatigue (LCF) regime. Brittle failure was confirmed by the SEM fatigue of dissimilar joint fractured surfaces due to interfacial failure. The Ti and ASS fractured surfaces presented river-like cleavage facets. On the Ti side, tiny elongated dimples suggest ductile failure before fracture. The topography results showed that the roughness topography parameters of similar and dissimilar fractured specimens made from Ti grade 2 and AISI 304 for the HCF regime were lower than those of the fractured specimens with LCF. The current study is expected to have practical benefits for the aerospace and automotive industries, particularly the manufacturing of body components with an improved strength-to-weight ratio.