Evaluation of Metal-Mold Interfacial Heat Transfer Coefficient in a Low-Pressure Permanent Mold (LPPM) Aluminum Casting Process

摘要

The low-pressure permanent mold (LPPM) casting process is a reliable and cost-effective choice for the manufacture of lightweight cast aluminum wheels and other high-strength, performance-critical parts. The most critical factor that controls the metal-mold interfacial heat transfer in Permanent Mold (PM) casting is the Heat Transfer Coefficient (HTC). Simulation of any solidification process demands accurate values of interfacial HTC as an input to the computer model of a casting. The importance of estimating an appropriate HTC for modeling the LPPM process need not be over emphasized. However, at this point, no such study has been reported in the foundry literature.A systematic study of the LPPM casting of aluminum alloy A356 has been conducted in this work to estimate the HTC for the process. A wheel-like shaped casting was made in a low-pressure PM casting machine, in which molten metal from a pressurized furnace is pushed into the mold via a stalk tube. The air pressure inside the furnace is increased to pressurized the metal to feed porosity as the casting solidifies. As many as 23 thermocouples were embedded in the die cavity to monitor the cooling behavior of the casting and to record the temperature history using the associated data acquisition system. The CAD model of the wheel-like casting and to a commercial software package and the 3D solidification process was simulated. Different IHTC distributions were applied to different segments of the casting-die interfaces. A semi-empirical equation was used to characterize the air gap formed between the die and casting upon contraction of the casting. The interfacial heat transfer coefficient (IHTC) was estimated by closely matching the simulated and experimental cooling curves. It was observed that the air gap does not influence the IHTC after a lapse of about 300 seconds while the IHTC approaches a constant value of about 400 W/m~2 K, whatever may be the air gap. The heat transfer at this stage is controlled by thermal conductance at the interface, which is a function of the thermal properties and thickness of die coatings, as well as the surface roughness of the interfaces.