In the present work, in order to refine the microstructure of repair GTAW welded AZ91C cast alloy and to optimize its microstructure and hardness properties, friction stir processing was applied on the longitudinal direction along the welded direction. Mictrostructral evaluations showed that FSP destroyed the coarse dendritic microstructure and dissolved the secondary hard and brittle-? Mg 17 Al 12 phase existing at grain boundaries of weld zone. Decrease in porosities and mean value of grain size to 11 microns was also resulted. Optical microscopy results showed an increase in amount of ? phase in the weld zone in comparison with base metal and the significant role of FSP in eliminating the? -phase in weld zone. Hardness profiles of different processed zones revealed a uniform increase in hardness value particularly in the stir zone of FSPed region where the mean value of hardness was 65, 67 and 82 Vickers for base metal, weld zone and stir zone, respectively. Wear results showed a decrease in wear rate of friction stir processed area and reaching to steady state. Keywords: Casting, Optimize, Repair weld, Friction stir processing, GTAW
