Effect of tool concavity on flash formation during fabrication of tubes through friction stir welding

Tubular components are challenging to weld using friction stir welding (FSW) process due to its curvature. In the present work, a novel strategy involving concave shoulder tool with three different concavities (3°, 6° and 9°) coupled with double pass was devised to fabricate longitudinal FSWed tubes of AA5083-O. An increase in concavity resulted in formation of flash and thinning of weld zone (WZ) due to excessive heat generation caused by enhanced tool contact with tube curvature. The flash generation led to the formation of porosity in the WZ. The tool with 6° concavity resulted in spherical pores whereas long and needle shaped pores were witnessed in case of 9° concave tool. However, negligible pores were observed with 3° concave tool leading to excellent WZ strength. Therefore, it was inferred that concave shoulder tool with small concavity along with double pass was suitable for welding tubes using FSW.