The effect of Ti and Zr elements and cooling rate on the microstructure and tensile properties of a new developed super high-strength aluminum alloy

In this investigation, the effect of Al–5Zr and Al–5Ti–1B grain refiners and cooling rate on an Al–Zn–Mg–Cu alloy with high amount of zinc was studied. Microstructural examinations were conducted by X-ray diffractometry, optical and scanning electron microscopy coupled with energy dispersive spectrometry. Constituents were identified as α(Al), η-MgZn2, T-Al2Mg3Zn3 and S-Al2CuMg phases. The optimum level of Zr as Al–5Zr and Ti as Al–5Ti–1B grain refiners was found to be 0.3 and 0.05 wt.%, respectively. Increasing cooling rate through reduced sections, decreases grain size and dendrite arm spacing, but Ti-refined specimens showed lowest sensitivity to cooling rate. After homogenization at 455 °C for 24 h, constituents were α(Al) and Zn-free S-phase. T6 heat treatment including quenching to room temperature and aging at 120 °C for 24 h were employed to reach to maximum strength. Increasing cooling rate results in higher strength and Ti-refined specimens showed highest tensile strength and elongation values. Data scatter of tensile tests attributed to casting defects and fractography studies revealed a predominantly intergranular brittle fracture.