Microstructures in interface region and mechanical behaviours of friction stir lap Al6060 to Ti-6Al-4V welds

There has been uncertainty about the best pin position for friction stir lap welding (FSLW) of Al-to-Ti so that weld samples can achieve the highest attainable load during tensile-shear testing. In this study, tests were conducted to evaluate how much tensile-shear load an Al6060 to Ti-6Al-4V (6060-Ti64) FSLW sample of a set width (Fm/ws) can support before fracture depends on the pin positioning related microstructures in the weld interface region. Microstructures differ depending on whether or not the tool pin penetrates the lapping interface. It has been found that Fm/ws values of the present macro-defect free weld samples vary quite significantly but in general are significantly higher than those reported in the literature. When the penetration depth is zero a thin Al6060-Ti64 interface layer forms and this layer does not grow beyond 250 nm. It will be shown that the thin interfacial layer can support a high tensile-shear load and thus the adjacent Al6060 material shears to fracture. When the pin penetrates during FSLW and thus the commonly observed mix stir zone (MSZ) forms, values of Fm/ws are lower than that of zero pin penetration welds but remain quite high. This can be explained by cracks needing to propagate through the tough and high strength α(Ti) regions in MSZ, requiring a high load to cause fracture.