Experimental and metallurgical analysis of the friction stir forming process

Abstract

The automotive industry requires numerous joints in light dissimilar materials. Since the automotive industry, as well as many others, is in need of improvements in this area, this project is focused on developing a new process that joins dissimilar materials with maximum strength conceivable by in part minimizing any brittle intermetallic region that may be formed. If successful, this research would produce a strong, lightweight, and efficient joint. Potentially, the Friction Stir Forming process can meet these criteria and overcome all of the current challenges that other processes are facing in a cheaper way without additional mass from rivets, bolts, or other fasteners. Fillers, chips, and substantial energy for heating can also be avoided. The concept of this process is to heat the top sheet by friction generated by a rotating tool, which simultaneously applies a downward force causing the top sheet to deform into a prefabricated hole in the bottom sheet. A mechanical interlocking joint can be formed with relatively simple machinery and techniques. The first part of this research was focused on exploring the concept feasibility and finding optimized processing parameters such as: spindle and plunge speed, torque, geometrical dimensions of the tools, etc. In the next part of the research, the knowledge gathered from the first part of the research was carried forward and the focus was applied to adjusting the FSF process for industrial conditions. An appropriate industrially relevant joint was evaluated once optimized processing parameters had been defined. These experiments were conducted at the General Motors Research and Development facility. The final part was material characterization using optical and electron spectroscopies. This provided a deeper insight into the macro and microstructural situation and the materials dynamics during the process including information about grain size, existing phases and their distribution, chemical uniformity, and deformation.