Characterizations of precipitation behavior of Al-Mg-Si alloys under different heat treatments

Abstract

The solidification-precipitation behavior of Al-Mg-Si multicomponent alloys has long been an absorbing topic. Experiments were carried out to analyze the precipitation behaviors of Al-Mg-Si alloys under different heat treatments. All specimens were homogenized at 570 °C for 8 h, and then solution treated at 540 °C for 55 min. Subsequently, the specimens were age treated for different times at temperatures of 100 °C, 150 °C and 180 °C, respectively. The experimental results show that the occurrence of dispersed free zones (DFZ) is caused by the uneven distribution of dispersed phase. During the aging process, pre-β" phases form at the initial stage and an aging temperature of 100 °C is too low to complete the transformation of pre-β" to β". At 150 °C, the precipitation sequence is concluded as ssss-pre-β"-pre-β"+β"-β"-β'-β. Moreover, changes in sizes and densities of the pre-β", β"and β' phases during the aging process has an important influence on the evolution of microhardness and electrical resistivity. The microhardness peak value of 150 °C is similar to that of 180 °C, which is -141 HV. While, at 100 °C, the microhardness increases slowly, and the attainable value is 127 HV up to 19 days. When the aging temperature is 100 °C, the electrical resistivity has the highest average value. When the aging temperature exceeds 100 °C, with the occurrence and growth of P'and P', the resistivity has a distinct decrease with prolonged aging time.