Effect of solution heat treatment and additives on the microstructure of Al-Si (A413.1) automotive alloys

Abstract

A study was carried out to determine the role of additives such as Mg, Cu, Be, Ag, Ni, and Zn on the microstructural characteristics of grain refined, Sr-modified eutectic A413.1 alloy (Al-11.7% Si) during solution heat treatment. For comparison purposes, some of the alloys were also studied in the non-modified condition. The alloys were cast in a steel permanent mold preheated at 425°C that provided a microstructure with an average dendrite arm spacing (DAS) of ∼22 μm. Castings were solution heat treated at 500 ± 2°C for times up 24 h, followed by quenching in warm water (at 60°C). Microstructural analysis of the as-cast and heat-treated castings was carried out using optical microscopy in conjunction with image analysis. Phase identifications were done using the electron probe microanalysis (EPMA) technique. In the as-cast condition, the addition of 0.42 wt% Mg to the unmodified alloy produced relatively large Si particles compared to the base A413.1 alloy. The Si particle size remained more or less the same with increase in solution treatment time and Mg level. Both Mg2Si and Al2Cu phases were observed to dissolve almost completely after 8 h solution time, while the Al5Cu2Mg8Si6 and α-Al15(Mn,Fe)3Si2 phases were found to persist even after 24 h. The β-Al5FeSi iron intermetallic platelets (possibly nucleated on SrO particles during solidification) underwent partial dissolution by the diffusion of Si atoms into the surrounding aluminum matrix after 24 h solution treatment. The presence of Ni and Cu (dissolved) in the α-Fe phase contributes to its stability during solution treatment. The modification effect of Sr on the β-Al5FeSi platelets is intensified in the presence of Zn.