Inhomogeneous microstructure and mechanical properties of friction stir processed NiAl bronze

As-cast Cu–9Al–4.5Ni–4Fe NiAl bronze (NAB) alloy was subjected to friction stir processing (FSP) in a wide range of tool rotation rates of 800–2000 rpm and traverse speeds of 50–200 mm/min. After FSP, the initial coarse microstructure of the as-cast NAB was transformed to fine structure, and the porosity defects were eliminated. However, the stir zones were characterized by inhomogeneous structure and could be divided into four regions: fine Widmanstätten primary α phase in the surface layer, banded primary α and β′ phases in the subsurface layer, equiaxed α and β′ phases in the center, and streamlike α and β′ phases at the bottom. The heterogeneous microstructure could be alleviated by adjusting the FSP parameters, but could not be completely eliminated under investigated FSP parameters. The FSP NAB exhibited significantly improved hardness, tensile strength, and ductility compared to the base metal. When the NAB was subjected to two pass FSP, its microstructure was further homogenized, resulting in apparently increased ductility with similar hardness and tensile strength.