Intermixing in dissimilar friction stir spot welds

Dissimilar intermixing during friction stir spot welding of Al 5754 and Al 6111 sheets is investigated using a combination of experimentation and numerical modeling. Dissimilar intermixing is not observed in (a) spot welds made using a tool having a smooth pin with or without a dwell period and (b) spot welds made using a threaded tool without the application of a dwell period. It is proposed that dissimilar intermixing during the dwell period in spot welding results from the incorporation of upper (Al 5754) and lower (Al 6111) sheet materials at the top of the thread on the rotating pin. A ribbon of contiguous Al 5754 and Al 6111 lamella is moved downward via the pin thread as the tool rotates during the dwell period in spot welding. A helical vertical rotational flow is created during the dwell period in spot welding because the ribbon of contiguous Al 5754 and Al 6111 lamellae discharged from the bottom of the pin thread moves outward and upward before moving back toward the tool periphery and downward again.