Effects of Ni on the microstructure, hot tear and mechanical properties of Al–Zn–Mg–Cu alloys under as-cast condition

Abstract

The effects of Ni on the phases, microstructural evolution, hot tear susceptibility and mechanical properties of Al–6Zn-1.4Mg-1.2Cu alloys fabricated by gravity casting were investigated using X-ray diffraction (XRD), scanning electronic microscopy (SEM), optical microscopy, the N-Tec Hot tear and tensile test. The calculation of phase diagrams (CALPHAD) modelling was also conducted to understand the phase formation of the experimental alloys. The results showed that the secondary phases (including η-MgZn2, S–Al2CuMg and Al3Ni) are mainly distributed at grain boundary, while round S–Al2CuMg particles are also found in α-Al matrix. Ni only exists as Al3Ni with different morphologies and its number density ascends with the increase of Ni content in alloys, which also significantly improves the hot tear resistance. As a result, the increased Ni in the alloys remarkably improves the ultimate tensile and yield strength, while the elongation increases when Ni < 0.6 wt% followed by decreases when Ni > 0.6 wt%.