2017 Volume 58 Issue 11 Pages 1624-1627
Al-Si-Mn-Mg alloy, AA365 (Silafont-36), has been recently developed for automotive parts produced by the high-pressure die-casting process. During the die-casting process, differences in section thickness cause uneven cooling, which results in different mechanical properties and cause the build-up of residual stresses and defects in the part. In the present study, we have attempted to identify the microstructural changes of α-aluminum dendritic phase and eutectic region, and the mechanical property changes in AA365 alloy at different cooling rates during solidification. The alloy cooled at 9000 K/min (water quenching) acquired a secondary dendrite arm spacing (SDAS) of 3.4 µm and contained over 75% of dendritic α-aluminum phase, whereas the alloy having a cooling rate of 77 K/min (air cooling) showed 12 µm SDAS and 65.5% of α-aluminum phase. The ultimate tensile stress and the elongation of AA365 cooled at 9000 K/min went up to 262.3 MPa and 4.4%, respectively, when compared with the alloy cooled at 77 K/min, which had 192.3 MPa tensile strength and an elongation of 2.9%. The water quenching increases the hardness of dendritic α-aluminum phase by about 130% compared to that of the air-cooling, and it was confirmed that the fast cooling rate could increase the solubility of the elements that can be dissolved in the α-aluminum phase. The hardness of the alloy increased with an increase in the cooling rate during solidification due to uniform and fine size of the silicon bearing intermetallic phases in the eutectic region, caused by fast solidification.