Metallurgical behavior and variation of vibro-acoustic signal during preheating assisted friction stir welding between AA6061-T6 and AA7075-T651 alloys

The present work investigated the effects of pin profiles (cylindrical and square), pin eccentricity (0.5 mm and 1 mm) in cylindrical tool and preheating (secondary heating) on metallurgical behavior, variation of vibro-acoustic signal pattern and joint strength during friction stir welding (FSW) between AA6061-T6 and AA7075-T651 alloys. The eccentric tool pins were observed to provide good flowability and intermixing between dissimilar metals, increased the size of stir zone, and the grains in stir zone were sufficiently finer with eccentric tool pin than concentric pin. The magnitude of vibro-acoustic signal increased when shoulder plunging started and drop in signal was noted when the tool shoulder reached its desired depth. The signal magnitude was noted to be higher in welding stage compared to tool plunging stage as the tool took in fresh material during tool movement along the weld path. Preheating the workpiece prior to pin plunging and during welding notably influenced the flow behavior and mixing pattern, and the grains in stir zone were slightly coarser than those in specimen without preheating. Significant reduction in the magnitude of the signal was also observed after preheating. Tensile and flexural strength of joints were also improved slightly when additional heating was employed.