Utilization of a Curly Toolpath in Friction Stir Welding

Abstract

Friction stir welding (FSW) is an advanced solid-state metal joining technique. This operation fuses adjacent materials through the use of a non-consumable, rotating tool, which is plunged into and travels along the seam of the materials. Since this joining method avoids the bulk melting of the base materials, it is considered a relatively energy efficient process. Additionally, the strength of the base material is often improved due to significant grain refinement resulting from the stirring action of the tool at relatively low temperatures. Another inherent benefit is that the joint thickness, which is dependent on the length of the pin, can be much greater than most other joining processes and can also be well controlled. This joining method conventionally relies on the friction at the tool-base material interface to stir materials. Other research has implemented complex tooling to mechanically enhance this stirring action. However, these tools are often expensive, requiring a high level of capability within industry. In order to improve the weld strength of FSW, a novel toolpath is utilized which significantly improves the mechanical mixing of the constituent materials without the need for complex tooling, such as tools with threaded pins. The path currently investigated forms a curl as it travels both perpendicular and parallel to the joint. This motion is used to extend the stirring action of the tool to regions outside the immediate joint area. It was found that this tool path is effective in improving weld strength under specific process parameters. Constraining the tool's axis normal to the workpiece surface resulted in a void that was formed in the majority of tests; however, this void was eliminated with modification of the process parameters. An uneven distribution of heat was recognized within this testing in which one side of the joint was hotter than the other. This observation may be used in future studies to perform multi-material joining where it is often necessary to increase the temperature of one material more than the other.