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Wettability and stress concentration are the main challenges affecting the engineering application of copper/alumina dissimilar joints. In this paper, continuous drive friction welding of copper to alumina was conducted using 2.5 mm thick AA1100 aluminum as interlayer. The effects of friction pressure and friction time on the tensile strength of joints were evaluated, and the interface microstructure evolution and fracture morphologies were also analyzed. Obvious mutual diffusion occurred at the alumina/aluminum and aluminum/copper interfaces, enhancing wettability and proving metallurgical bonding. Microcracks formed in alumina base due to thermo-mechanical coupling effect in welding. As increasing friction pressure and friction time, the tensile strength increased till reaching a peak value of 35 MPa, and then decreased. The optimized parameters were determined as friction pressure of 12 MPa and friction time of 12 s. All samples failed at the aluminum/alumina interface, leaving a pit structure on alumina base. The application of pure aluminum interlayer is essential to obtain sound copper/alumina joints by diminishing stress concentration and wetting alumina, and the optimized residual aluminum layer is about 0.47 mm thick. The friction interface transferred from alumina/aluminum to inner plasticized aluminum layer, and then moved to aluminum/copper till the welding process finished. [ABSTRACT FROM AUTHOR]

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]