Friction stir welding of dissimilar sheet materials

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There has been a growing need for improved fuel efficiency in the transportation industries, and this has motivated weight reductions through increased utilization of light-weight materials such as aluminum and magnesium alloys, and composites. However, during component fabrication the main challenge is welding of these materials, particularly when dissimilar combinations are encountered. Even similar alloy joining is not trivial, due to cracking, solidification porosity, and severe distortion during arc welding. Joints involving fiber reinforced polymer composites typically require fasteners or adhesives, which are more costly and time-consuming than welding. In recent years, the process of friction stir welding has emerged as a solid state welding technique which offers many advantages and new capabilities compared to fusion welding. The main feature of solid state welding is that many of the traditional problems like cracking and porosity are avoided by keeping materials below the melting point. The friction stir welding technique uses a cylindrical shaped tool with a pin shaped protrusion at the end, which is traversed along the materials to be joined, either in a lap weld or butt joint configuration. This proposal examines a variation of friction stir welding in which spot welding can be conducted using a pin that retracts into the cylindrical tool. Only a few studies have been reported on this so-called ‘refill friction stir spot welding’ approach, however it has been demonstrated that superior joint strength can be achieved compared to traditional friction stir spot welding. The process may be applied to dissimilar material combinations, and the main focus of this work is joining aluminum to fiber reinforced composite sheets. The objective will be to determine factors which control bonding during refill friction stir spot welding of these materials, and to identify processing parameters or possible interlays that provide higher joint strengths. A specialized refill friction stir welding system will be used to study the mechanisms of joint formation in thin sheet spot welds. The details of bonding in terms of reactions, intermixing, and mechanical interlocking will be compared to the welding parameters. Advanced electron microscopy techniques will be applied to evaluate the role of temperature and stress on bond formation. The properties of the joints will be investigated using overlap shear testing, as well as instrumented indentation testing using both macro-scale and nano-indentation techniques. This will allow overall joint strengths to be correlated with microscopic features and phases which form during welding. The proposed work will generate knowledge that is required in order to promote widespread implementation of this technology in industry. Since time, temperature, pressure and deformation control the joint strength, the role of each parameter needs to be understood. A thorough study of the interfacial bonding will help to identify which parameters dominate and contribute most significantly to joint strengths. The proposed work will allow processing maps to be developed for this technique which may also have applicability to other solid state joining processes, including conventional friction stir welding. These maps can be later utilized by the Canadian industries in order to rapidly develop solutions for joining difficult combinations by connecting processing parameters to the desired microstructures which provide highest strength. This will help transportations industries utilize advanced solid state joining technologies in order to increase their capabilities and reduce fabrication costs.