Effect of Incubation Coupled with Artificial Aging in T6 Heat Treatment of A356.2 Aluminum Casting Alloy

A356.2 is one of the most popular Al casting alloys for structural applications in automotive and aerospace sectors. Typically, the cast components from these alloys undergo a T6 heat treatment to significantly improve the mechanical properties. Solution heat treatments, incubation at room temperature, followed by artificial aging at an elevated temperature are the typical steps in a T6 heat treatment process. The incubation process (IP) is typically carried out between 4 to 12 hours on commercial foundry products and typically longer on aerospace castings. There has been a few studies on the mechanisms of precipitation reactions occurring during the incubation and natural aging processes, however all these are for the Al-Si-Mg wrought alloys which are markedly different in composition from Al-Si-Mg casting alloys such as the A356.2. A hypothesis for the sequence of precipitation reaction in the primary Al phase during incubation and artificial aging processes, and the quantified effect of these aging treatments on the resultant tensile properties of the cast components with A356.2 Al alloy is presented. The results show that depending on incubation time, the formation of individual clusters of Si and Mg, breakdown of the individual-clusters of Mg and formation of the co-clusters of Si and Mg (Mg5Si6) would take place. The formation of Mg1.8Si and the final stable Mg2Si phases occurs during artificial aging. The sequence of the precipitation reactions were formulated from micro-hardness data on the primary Al phase matrix, tensile properties and information from the prior-art in this field of study. Further, the time of incubation has a significant effect on the resultant mechanical properties because the precipitation reaction during artificial aging is directly affected by the duration of incubation time.