Solidification microstructure and critical conditions of shrinkage porosity generation in die casting process of JIS-ADC12 (A383) alloy

Differential thermal analysis (DTA), analyses of unidirectional solidification process and die casting process, and computational numerical simulations were conducted for JIS-ADC12 (A383) alloy to investigate the generation conditions of shrinkage porosities which occasionally appear in JIS-ADC12 die castings and affect the quality of products. The crystallization temperature of the primary alpha phase and AlSi binary eutectic phase are estimated to be 856 and 838K from DTA, respectively. The multi-phase eutectic reaction of JIS-ADC12 alloy starts from 826 K, and finishes at 801 K. The primary alpha dendrites are clearly observed in the unidirectional solidification microstructures, and the relationship between the secondary dendrite arm spacing λ₂ and the local solidification time θ_f is expressed by the following equation: λ₂ = 6:45θ^0:31_f . The primary alpha dendrites are also observed in the die cast sample. The average cooling rate estimated from λ₂ is approximately 60K/s at the depth of 0.5mm from the surface of die castings. The computational numerical simulation reveals that the values of solidification parameter G= /√R increases around the surface of the JIS-ADC12 die castings in where the amount of shrinkage porosities decreases. Especially, shrinkage porosities of the size lager than 200μm is observed when the value of G= /√R is less than or equal to 1.4K ^0.5s^0.5/mm.