Self-optimization in tool wear for friction-stir welding of Al 6061+20% Al2O3 MMC

Tool wear and the rate of wear for hardened, steel, right-hand screws rotating at 1000 rpm in the friction-stir welding of Al 6061+20 vol.%Al2O3 particles were observed to decrease for increasing weld or traverse speeds. When sufficiently long traverse distances were reached, tool wear became small or negligible, and an optimized tool shape emerged. This shape was slightly different at 6 and 9 mm s−1 weld speeds but in each case a self-optimized tool shape emerged. This self-optimizing wear phenomena and tool shape result by counter motions of solid-state flow regimes which depend upon both tool rotation speed and actual weld traverse speed. Although sound, porosity-free welds are obtained with both the unworn, threaded pin tool and the worn, unthreaded pin tool, microstructures vary and the worn pin tool produced a narrower heat affected zone with less drop in hardness than the threaded pin tool.