Effect of Zr on microstructures and mechanical properties of an AlMgSiCuCr alloy prepared by low frequency electromagnetic casting

The Al1.6Mg1.2Si1.1Cu0.15Cr (all in wt. %) alloys with and without Zr addition prepared by low frequency electromagnetic casting process were investigated by using the optical microscope, scanning electron microscope and transmission electron microscope equipped with energy dispersive analytical X-ray. The effects of Al3Zr phases on the microstructures and mechanical properties during solidification, homogenization, hot extrusion and solid solution were studied. The results show that Al3Zr phases reduce the grain size by ~ 29% and promote the formation of an equiaxed grain structure during solidification. Numerous spherical Al3Zr dispersoids with 35-60 nm in diameters precipitate during homogenization, and these fine dispersoids change little during subsequent hot extrusion and solid solution. Adding 0.15 wt. % Zr results in no recrystallization after hot extrusion and partial recrystallization after solid solution, while the recrystallized grain size is 400-550 μm in extrusion direction in the Zr-free alloy. In addition, adding 0.15 wt. % Zr can obviously promote Q′ phase precipitation, while the β″ phases are predominant in the alloy without Zr. Adding 0.15 wt. % Zr, the ultimate tensile strength of the T6 treated alloy increases by 45 MPa, while the elongation remains about 16.7%.