Solidification Microstructure and Critical Conditions of Shrinkage Porosity Generation in Die Casting Process of JIS-ADC12 (A383) Alloy

Abstract

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 Al–Si binary eutectic phase are estimated to be 856 and 838 K 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: λ ₂ = 6.45θ _f^0.31. The primary alpha dendrites are also observed in the die cast sample. The average cooling rate estimated from λ₂ is approximately 60 K/s at the depth of 0.5 mm 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 200 µm is observed when the value of G/√R is less than or equal to 1.4 K^0.5s^0.5/mm.