Strength and failure analysis of similar and dissimilar friction stir spot welds: Influence of different tools and pin geometries.

Tool and pin geometries play significant roles in material flow, stirring and weld quality in friction stir spot welding (FSSW) process. The present work investigates the influence of tool pin length (0 mm i.e. pinless, 0.2 mm and 0.4 mm) and pin diameter (3.3 mm and 4.95 mm) on lap shear strength during the FSSW of similar and dissimilar aluminum and copper alloy. Pinless tools are differently designed from H13 die steel single material and embedded aluminum and/or copper rods in H13. Results indicate that the maximum lap shear strength is achieved with pinless tools (single material). However, when tools with pins are used lap shear strength decreases with increase in tool plunge depth but increases with pin diameter. For pinless tool obscure hook formation is noted and crack is initiated from the joint interface. Asymmetric strain distributions as observed from the finite element analysis and hook formation observed during microscopic analysis may be responsible for quick failure when higher plunge depth of pin is used. Shear fracture in presence of micro-voids/dimples elongated along the loading direction are observed in similar FSSW whereas in case of Cu-Al dissimilar joints, formation of intermetallic compounds and quasi-cleavage fracture is observed. [ABSTRACT FROM AUTHOR]