Study on the influence of solution temperature on the synergistic effect of precipitation strengthening and fine grain strengthening of AA6061

Abstract

The effect of solution temperature on the synergistic effect of precipitation strengthening and fine grain strengthening of AA6061 was studied. It was found that when the samples were heat treated at 510 ℃ or below, silicon-containing phases could not be fully dissolved, and the amount of Si involved in precipitation strengthening was too small, resulting in low strength. The best comprehensive properties (tensile strength 322 MPa, hardness 110 HB, elongation 9.5%) could be obtained at 530 ℃, because silicon-containing phases dissolved more fully at this temperature, and more Si atoms participated in aging strengthening. Although eutectic silicon could be more fully dissolved at 550 ℃, the grain grown rapidly at this temperature, and the negative effect of grain growth on properties exceeded the positive contribution of more fully dissolved silicon. The results of metallographic analysis were consistent with those calculated by first principles. The results showed that β-Mg2Si, θ-Al2Cu and Q- Al4Cu2Si7Mg8 phases had higher cohesive energy density and were easier to dissolve during heat treatment. The cohesive energy density of eutectic silicon was very low, so it needed very high temperature to decompose. Compared with Mg atom and Cu atom, even though Si atom had stronger binding ability with aluminum matrix, the premise of its binding with aluminum matrix was to separate from silicon-containing phase. Therefore, eutectic silicon was the most difficult to dissolve in the common as-cast structure of 6061 aluminum alloy. In this study, it was found that precipitation strengthening and fine grain strengthening were excellent at the solution temperature of 530 ℃.