Influence of additional heating in friction stir welding of dissimilar aluminum alloys with different tool pin profiles

Influence of additional heating, besides the frictional heat generated during the friction stir welding (FSW) process, was investigated to study the force variation, material flow/mixing, and metallurgical behavior. Dissimilar FSW with additional heating (prior to plunging as well as during welding) was performed between AA6061-T6 and AA7075-T651 with cylindrical (CYL), cylindrical tapered (CT), square (SQ), cylindrical full threaded (CFT), and threaded with three intermittent flat faces (TIF) tool pins. Results indicate that each tool pin affects the material flow in different ways and better flowability and intermixing are observed when additional heat source was employed. Significant reduction in forces was recorded with the application of additional heat source for all tool pin profiles. Additional heat source strongly influenced the flow behavior, mixing pattern, distribution, and metallurgical characteristics in stir zone (SZ). Formation of tunnels/voids in SZ was also significantly affected by different tool pin profiles and for some tool pins, decrease in size of tunnels was observed after employing additional heating. The material flow visualization was carried out through horizontal and vertical (longitudinal) sectioning of weld nugget region which indicated the formation of ring pattern for joint prepared with CFT tool pin whereas discontinuous flow pattern was observed for joint prepared with TIF tool pin. The flow was also visualized for some joints using tracer powders in slots on advancing, along faying surface and retreating side and tracer powders were found to be distributed in an arc-like manner when observed on plan view. Microhardness mapping in and around the weld nugget regions indicated a better flow/distribution/mixing of the two different base materials with the application of additional heating.