The Effect of Polymer Content of Quenchant on Microstructural and Mechanical Characteristics of AA-7075 Plates Before Age Hardening

Abstract

In the heat treatment of AA 7075, residual stress, in some cases, increases the mechanical properties of the parts, such as resistance to fatigue, and in most cases, distorts the parts. This research tried to investigate the effects of residual stress on microstructure after the solutionizing process. After homogenizing, 7075 Al alloy samples with 6 and 20 mm thicknesses were solutionized at 485 °C for 30 and 90 minutes, respectively. Then samples were quenched in 10, 30, and 50% Polyalkylene Glycol (PAG) solutions with water. Afterward, the hole drilling method was used to measure post-quench residual stress. The results showed that the hardness number dropped in 6-mm samples as the tensile residual stress increased. This stemmed from the decrease in the quench rate due to the rise in the amount of PAG. However, this trend in 20-mm samples was upward and then dropped as the PAG amount increased from 30% to 50%. The results also demonstrated that in the 6-mm sample, tensile residual stress has an inverse proportion to the hardness. As the quench rate decreased, and subsequently, the tensile residual stress raised, 6-mm samples' grain size decreased, while grains in 20-mm samples were hardly distinguishable. A decrease in 6-mm samples' grain size could be the effect of more nucleation during the recrystallization under the elastic force's influence, and consequently, raise in overall stored energy created by tensile residual stress. Therefore, in the quenched AA 7075, the tensile residual stress can contribute to the recrystallization phenomenon. This effect is created by the rise in the system's overall stored energy due to the lattice's elastic force.