Effects of the aging temperature on the mechanical properties and microstructures of Al-5.1Zn-1.8Mg-0.4Ti wt.% alloy produced by squeeze casting

In this paper, the role of Ti in the precipitation strengthening of the Al-5.1Zn-1.8Mg-0.4Ti (wt.%) was studied. Al-Zn-Mg alloys (Al 7xxx series) have been widely used in industrial applications that require high-strength and low-density requisition, such as the aerospace industry. These alloys are commonly age-hardened, in which the diffusion of Zn and Mg atoms is incorporated from a supersaturated solid solution (SSSS), resulting in the formation of metastable precipitates. Ti is added to decrease the grain size, thus increasing the strength of the alloys. Fabrication of the alloy was carried out using squeeze casting, followed by homogenization at 400°C for 4 h. A subsequent solution treatment was employed at 440°C for 1 h, followed by water quenching. Aging was carried out at 90, 130, and 200°C for up to 200 h. Characterization was performed using Rockwell hardness testing, optical microscopy, and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS). The highest hardness was achieved in samples aged at 90°C. Higher aging temperatures decreased both the number and the time needed to achieve peak hardness. The addition of Ti was found to retard the strengthening effect by slowing down the kinetics of precipitation through decreasing the number of solute-vacancy complexes. The suggested major precipitation sequence is as follows: GP zones → η' → η (MgZn₂).