Experimental Study of Secondary Dendrite Arm Spacing Vs Cooling Rate in Squeeze Casting of A356 alloys

摘要

Squeeze casting is a pressure assisted solidification process that has recently emerged as a near net shape manufacturing process for automotive parts in the USA. The applied pressure in squeeze casting ensures intimate contact between the dies and metal throughout solidification. In one variant of this process, a metered quantity of molten metal is slowly pushed into the die cavity and subsequently pressurized by a hydraulically actuated plunger until solidification is completed. Research in this field has identified die temperature, melt temperature, plunger speed, specific pressure level and its duration of application as some of the important parameters that control the final quality of cast parts. Although it is reported that squeeze casting results in a fine, equi-axed grain structure, no attempt has been made so far to correlate secondary dendrite arm spacing (SDAS) and cooling rate. The dendrite arm spacing is an important characteristic of a dendritic microstructure and has been used over the years as a measure of fine grain structure and hence casting quality. In the present study, a hockey puck-like shaped casting of aluminum alloy A356 was squeeze cast using a 350 t Ube vertical squeeze caster. Experiments were conducted at two melt temperatures and at two pressure levels, while the die temperature was maintained close to 400℉. Optical micrographs were taken at various locations of the casting and SDAS was measured using the line intercept method. Pressure inside the die cavity was continuously monitored using a Kistler pressure probe and recorded using the associated data acquisition system. The CAD model of the hockey puck-like casting was imported to a commercial software package and the solidification was simulated. Temperatures close to two mm from the casting-mold interface were measured using thermocouples mounted in the moving die half The interfacial heat transfer coefficient was estimated by closely matching the simulated and experimental cooling curves. Using this coefficient, the temperature distribution inside the casting was predicted. The models were validated by comparing predicted temperatures with measured temperatures in the mold.