Microstructures in interface region and strengths of friction stir lap Al-to-Steel and Al-to-Ti welds

Fracture strength of aluminium alloy to steel (Al-to-steel) welds made using friction stir lap welding (FSLW) relating to friction stir (FS) conditions and to the microstructures formed in the interface region has recently been further studied. As expected, intermetallic formation is a condition for joint strength to develop. Joining intermetallics form a single layer as the pin approaches and a mix stir zone (MSZ) of irregular Fe-Al intermetallic layers and alpha-Fe forms as the pin penetrates to the bottom plate during FS. The single layer could bear a relatively high load. The reason for this is not very clear but the restricted growth of the layer may be the reason of a low level of defects allowing the layer to bear a comparatively high load. Fracture strength reduces when a MZS forms instead of a single layer. This relates to the mode of fracture in the MSZ in which crack growth along the brittle intermetallic layers dominates. For welds with a MSZ but without other macro-defects, the strength value is not influenced significantly by FS parameters. The study has been extended to aluminium to Ti alloy (Al-to-Ti) welds. For welds made with penetrating tool pin to form a MSZ, a good joint strength is also obtained. Al-Ti interface intermetallic layer grows considerably slower, in comparison to that during Al-to-steel FSLW, when penetration depth is close to zero. It will be shown that with the very thin interface layer, deformation and fracturing deviate more deeply into the aluminium side and thus fracture strength increases considerably.