S. James Felsted and Dr. Tracy W. Nelson, Mechanical Engineering
Friction Stir Welding is a solid state joining process that makes possible the joining of materials that were previously difficult or impossible to weld. A closer examination of friction stir welding shows that welding occurs because of mechanical mixing, frictional heating, and forging. Welds are made on a conventional milling machine that powers a steel tool consisting of a shank, shoulder, and pin (1). This process is being investigated in order to better understand the interaction of the tool with the material and the nature of the material deformation.
A 5000 series Aluminum was used as a marker material, embedded within a 6061 base metal. The markers were fitted into narrow slots and staggered in order to avoid mixing between markers during the welding process. Welds were performed on a conventional milling machine by stopping the tool at different positions along the marker material. Weld specimens were sectioned and then polished and etched with a chemical reagent. Digital images were captured with a camera mounted on an optical microscope to reveal the path of material deformation. The samples were cross-sectioned multiple times in order to provide a greater understanding of the material movement as the tool moved though the weld.
The threaded shape of the tool pin is shown embedded within the material (Fig. 1). As the tool continues to travel along the weld, material is drawn from the outer edges of the tool and mixed in a circular pattern at the center of the tool path (Fig. 2). The screw thread of the pin enhances plastic deformation of the material and a circular “nugget” of consolidated material appears (Fig. 3). Further cross sections along the weld path reveal how the “nugget” is smoothed over by the shoulder section of the tool as it follows the path of the pin (Fig. 4).
The formation of the “nugget” was a point of particular interest in this study. A better understanding of how this nugget forms and its effects on the material properties could lead to enhancements in tool design. Some tool design considerations include the pin thread pitch and the diameter of the pin. Further study could include a quantitative analysis of the amount of material that is stirred by the pin relative to the amount of material remaining behind the tool.