This study aims to understand the uncertainty about the optimum or best pin position (Dp) for friction stir lap welding (FSLW) of Al-Cu. Tensile shear testing is used to determine the Mechanical strength of FSL welds under static loading. Fracture strength (σLap) corresponding to the maximum load in a test over the sample width is used as the strength value. Interface microstructures differ depending on whether the tool pin penetrates the lapping interface. It has been found that σLap values of the defect free weld samples vary quite significantly and in general are significantly higher than those reported in the literature. When the pin penetration is close to zero no intermetallic layers were formed, hence the value of σLap was zero. When the pin penetration is 0.4mm, the commonly observed a thin Al–Cu interface layer forms and this layer does not grow beyond 3µm. It is shown that the thin interfacial layer can withstand a high tensile-shear load and thus the adjacent Al material shears to fracture. When the pin penetrates more than 0.4mm, the commonly observed mix stir zone (MSZ) forms and values of σLap are lower than that of 0.4mm pin penetration welds but remain quite high.

Friction Stir Welding (FSW) is a relatively new solid state joining technique which is used not only for joining the aluminum and its alloys but also has potential for joining dissimilar materials with very different physical and mechanical properties which are hard to weld using conventional fusion welding processes. Tensile shear testing is used to determine the Mechanical strength of friction stir lap (FSL) welds under static loading, fracture strength (σLap) corresponding to the maximum load in a test over the sample width is widely used as the strength value. During friction stir lap welding (FSLW) of dissimilar metals with large differences in melting temperatures, a metallurgical bond is established through the formation of interfacial intermetallic compounds. However, as these intermetallic compounds are generally believed to be brittle with little ductility, they are generally considered to have detrimental effect on fracture strength. The aim of the present research is to study how the interface structure is affected by FSW parameters and how the formation of interface structure affects fracture of Al-Steel and Al-Ti FSL welds.