Microstructure and Mechanical Properties of Cooling Slope Rheocast Al-7Si-0.3Mg Alloy and Development of a Rheo Pressure Die Cast Knuckle Housing

Abstract

The present work studies the microstructure and mechanical properties of the Al-7Si-0.3Mg alloy, at cooling slope rheocast state, during isothermal holding of the cooling slope processed slurry, as well as, in the Rheo pressure die cast state. Significant effect of process variables, prevails during cooling slope slurry generation technique, is evidenced in the mechanical and fracture behavior of the rheocast alloy. Further improvement in the semi-solid slurry morphology in terms of enhanced sphericity of primary Al particles is evidenced during isothermal holding stage. Combined grain refiner and modifier addition prior to slurry generation is found to be helpful to achieve improved mechanical properties in the rheocast billets and also found to yield higher sphericity of primary Al grains during isothermal globularization stage. Out of the process conditions studied here, 60 deg slope angle and grain refiner + modifier addition is found to be ideal to achieve highest tensile, impact properties, and spheroidal microstructural morphology. Approximately, 7 to 14 minute of isothermal holding after cooling slope processing of the slurry is found to be ideal for component development, using in-house developed Rheo pressure die casting (RPDC) system. Here, automobile knuckle housing is cast as a prototype component, while melt pouring temperature into the shot sleeve is varied between 590 °C and 610 °C, and final injection velocity is maintained at 3.2 m/s. Finally, microstructure and mechanical properties of the Rheo pressure die cast component are studied at as-cast and T6 heat-treated state, by sectioning out samples from chosen locations of the solidified component.