Effect of friction stir processing on the tensile and fatigue behavior of a cast A206 alloy

The microstructure of cast Al alloy A206 was modified using friction stir processing (FSP) to eliminate porosity, break down intermetallic particles and refine the grains. The inference from differential scanning calorimetry is that the nugget region of the as-FSP material contained primarily GP zones. Uniaxial tensile tests and bending fatigue tests were carried out at room temperature on A206 alloy in the cast and T4 conditions and after FSP with different heat treatments — as-FSP, FSP and aging, and FSP with solutionizing and aging. These produced varied microstructures with combinations of pores, grain sizes, precipitate types and particle sizes. The ultimate tensile strength and ductility improved after FSP, while the yield strength showed no improvement. A further inference from differential scanning calorimetry and microscopy was that the yield strength depended more on the matrix heat treatment condition; whereas the UTS was limited more by the size and distribution of intermetallic particles. FSP improved both the total fatigue life and the life to crack initiation. Elimination of pores and reduction of the intermetallic particle size contributed to the increased fatigue life; subsequent heat treatments changed the fatigue life only marginally.