These results indicate that the addition of this solid “rivet” like section of the weld significantly impacts the overall strength of the joint. So,to achieve maximum strength with the process, it is critical that the top and bottom sheet join to one another in the through hole.This “riveted” structure provides additional clinching or normal force that helps hold the weld together similar to how a bolt holds sheets of metal together. This is likely due to expansion of the aluminum from heat during the process and its subsequent cooling and contraction. Additional research is needed to more fully understand the bonding characteristics of this joint.