Effect of multi-pass friction stir processing on microstructure and mechanical properties of cast Al-7Fe-5Ni alloy

Al-Fe-Ni alloys, due to their high thermal stability, low thermal expansion, high elastic modulus, and good corrosion resistance, are promising high-temperature materials for aerospace and automotive applications. However, one of the main drawbacks of using these alloys in as-cast condition is the precipitation of large brittle FeNi-rich intermetallics in their microstructure. In this study, friction stir processing (FSP) was employed to improve the microstructure and mechanical properties of Al-7Fe-5Ni alloy. FSP was conducted at the traverse speed of 25 mm min−1 for different rotational speeds (800, 1250, 1600, and 2000 rpm) and number of passes (1-, 2-, and 4-pass). The evaluation of the mechanical properties showed that, applying FSP with optimum parameters (25 mm min−1 and 1250 rpm), increased the tensile strength, fracture strain, and toughness by 145, 630, and 935%, respectively. Increasing the number of FSP passes further improved the mechanical properties where the toughness of 4-pass FSPed alloy was 80% higher than that of 1-pass FSPed sample and 1750% higher than that of base alloy. The significant increase of the mechanical properties during the FSP was mainly due to the changes in the morphology, refinement and distribution of intermetallic particles in the matrix and ultrafine dispersion of casting defects.