Cooling curve thermal analysis of Al–Mg2Si–Cu–xSr composite

Abstract

In situ composites are today being considered for industrial use, owing to the fewer production steps involved, lower production cost, and better wetting of reinforcements. This study emphasises the characteristic features of an Al–Mg2Si–Cu in situ composite, with the addition of different amounts of Sr (0.01–0.1 mass%) as a modifier reagent, by employing computer-aided cooling curve thermal analysis. The identification of microstructures and phases was carried out using a scanning electron microscope equipped with an energy dispersive spectrometer. The results show that the nucleation temperature of the primary Mg2Si, eutectic Mg2Si, and Al5FeSi phases initially increased with the addition of 0.01 mass% Sr, and subsequently decreased with further addition of the element. Two new Sr-containing phases were detected after the precipitation of primary Mg2Si phase and prior to the formation of eutectic Mg2Si phase. A relationship between the cooling rate (CR) and solidification rate (SR) was established. Based on cell coherency point, it was found that the eutectic Al–Mg2Si cell required a longer time to grow with the increment of Sr. The solid fraction of Al5FeSi and Al5Cu2Mg8Si6 + Al2Cu phases remained constant at 8 ± 1% and 3 ± 1%, respectively. The increase in the terminal freezing range and the cracking susceptibility coefficient, by 182% and 16%, respectively, shows that Sr increases the probability of hot tearing.