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.