2010 Volume 50 Issue 8 Pages 1165-1174
A new slurry-making method, called the nucleation-accelerated semi-solid slurry (NASS) method, was developed to fabricate high-quality semi-solid slurry with fine and uniform globular microstructures. The method's application to rheo-diecasting of ADC10 alloy was investigated. To enhance the heterogeneous nucleation of primary α particles in the initial stage of solidification, various types of funnels, including the pouring system in the slurry-making vessels, were designed and estimated by thermal analysis. It was found that the heterogeneous nucleation of primary α particles was closely related to the temperature–time history and the flow pattern. In the present study, the NASS method was set up based on the optimized funnel. It consisted of a specially designed conical-shaped funnel and the electromagnetic stirring (EMS) unit for artificial agitation. The conical-shaped funnel generated swirly flow pattern in the melt. This was further exaggerated by the EMS in the slurry-making vessel during the pouring stage, resulting in a copious heterogeneous nucleation. The qualities of the slurry were investigated using optical microscopy and rheo-diecasting. To study the effect of process parameters, the semi-solid slurry of ADC10 alloy was rheo-diecast using an 85-ton high-pressure die casting (HPDC) machine. Microstructural observation and hardness test were then carried out on the rheo-diecast specimens. The rheo-diecast products of the NASS method showed fine and uniform microstructures, with primary α-globules diameter averaging 39 μm and form-factor indicating a degree of globularity of 0.9. The optimized heat treatment condition for the rheo-diecast product of ADC10 alloy was achieved at the solution temperature of 490°C for 30–60 min, water quenching, and age hardening at 180°C for 7–8 h.