Experimental study on heat-affected zones of aluminum alloys in flow drill riveting

Flow drill riveting (FDR) is a novel one-step and single-sided joining process. In FDR, a rivet penetrates through workpieces at a high rotation speed and a certain feed rate, which generates heat through friction to soften the local material. However, the frictional heat usually brings a softening effect to heat-treatable aluminum alloys, which results in heat-affected zones in joints. In this work, temperature evolution of AA6061-T6 during FDR was measured and analyzed, which exhibited a nonlinear relationship with time and was characterized by two peaks. According to the temperature evolution, the FDR process was divided into two stages: penetration of rivet (stage 1) and stop of rivet rotation (stage 2). Electron backscatter diffraction and microhardness results show that the first rise of temperature in stage 1 was attributed to friction stir between rivet and workpieces, and the friction stir at contact surface between rivet head and flash on the top surface of the upper workpiece resulted in the second rise of temperature in stage 2, which caused a larger heat affected zone on the upper workpiece. Furthermore, the effects of process parameters (i.e., rotation speed and feed rate) on temperature evolution of AA6061-T6 were investigated. A higher rotation speed results in higher temperatures, a larger heat-affected zone and a greater softening effect on the upper workpiece, while the lower workpiece is less affected, resulting in lower joint strength. A larger feed rate contributes to a smaller heat affected zone on both upper and lower workpieces, resulting in higher joint strength.