Friction Stir Welding (FSW) is a solid state joining process using a rotating cylindrical shoulder and threaded pin which is plunged into the work piece to traverse along the join line. Friction Stir Processing (FSP) is a comparatively new process derived from FSW, which can be used to refine and modify as-cast microstructures for superior properties. In the last decade, several researchers have attempted to evaluate the microstructure and properties inside the stir zone. However, it is important to explain the features occurring outside the stir zone, in particular during multipass FSP (MP-FSP). The aim of this proposed research is to explain the MP-FSP process parameters and their relations to mechanical properties of the processed material, and hence to explain the fracture behaviour of the MP-FSPed material.

Experimentally, cast plates of A356 (Al-7Si- 0.3Mg) alloy were made. Initial set of experiments with a high tool rotation speed (ω rpm) was conducted to produce maximum stir zone area. However, on finding that this parameter was not suitable (as will be explained), lower ω range FSP experiments were conducted. First, single pass FSP experiments were carried out to understand the flow, particularly outside the stir zone followed by MP-FSP under varying process parameters. Tensile testing was conducted and factors/FS parameters relating to fracture strain, and thus the quality of MP-FSP stir zone were studied and fracture features were examined.

It was found that the use of high ω values resulted in the less deformed cast material wedging up into the stir zone of the previous pass. This wedging resulted in low tensile fracture strain. Explanation of Si particle alignment due to wedging and subsequently causing low fracture strain will be given. It will be shown that reducing ω reduces the degree of wedging, as the downward stir flow reduces. However, low ω values are shown to have a detrimental effect, incapable of completely disrupting / healing of tunnel wall oxide films, thus resulting in low fracture strain. It will be shown that with the current FS condition, ω=1000 rpm is the optimum value.