Abstract

Interfacial Heat Transfer in Die Casting of Light Alloys

Anwar Hamasaiid - 5 July 2007

In die casting, heat transfer is controlled by the properties of the casting-die interface, due to the high thermal diffusivity and conductivity of the die. Higher heat transfer rates result in higher productivity in the die casting process along with finer microstructure with superior part qualities. In die casting processes, the use of numerical simulation to predict the mould filling, solidification, and the distribution of temperature in the die has become an important development in foundry technology and cast product developments. The effectiveness of the simulation is dependent on the accuracy of the heat transfer data at the metal-mould boundary used in the simulation software. For these reasons, accurate knowledge about the heat transfer between the casting and the die during the die casting process has become an important issue for most researchers in this field.

This investigation studies the interfacial heat transfer during the casting and solidification of light alloys in Gravity (GDC) and High Pressure (HPDC) Die Casting processes. Suitable experimental methods and sensors have been developed that enable the measurement of temperatures, in-cavity pressure and a range of process parameters. Heat Transfer Gauges have been manufactured, accurately calibrated and integrated into the dies for both investigated processes. These gauges measure the casting surface temperature with a pyrometric chain and the temperature through the die wall at different depths with very thin K-type thermocouples. Extensive industrial trials were performed using Al-7Si-0.3Mg, A-9Si-3Cu and AZ91 D alloys. From the temperature measurements, the Heat Transfer Coefficient (HTC) and heat flux density (q) at the casting-die interface have been evaluated using an inverse method. The obtained results have been correlated with the process parameters and to the microstructure of the castings for some selected cases.

In GDC, the results show that the applied coating on the die surface governs the heat transfer during solidification of the casting through its low effective thermal conductivity. The effective thermal conductivity of the coating is determined by the high level of porosity in its structure. For this reason, its chemical composition has a very limited influence on the peak value of heat transfer. However, the coating composition does influence the variation of the heat transfer coefficient with time during solidification. The variation of HTC at the casting-die interface has been divided into three main stages according to the conditions of contact during casting and solidification: liquid-solid, semisolid-solid and solid-solid contact. The HTC at each of these stages is dominated by different process parameters. A secondary peak in the evolution of the HTC has been observed at the end of the semisolid-solid stage. This secondary peak was attributed to the exudation of solute rich eutectic liquid onto the surface region of the casting enabling fresh liquid contact with the mould coating.

In HPDC the value of the HTC is much larger than that determined in GDC (10⁵ Wm^-2K^-1 compared to 10³ Wm^-2K^-1 in GDC). The variation of HTC with time is also very rapid since the heat flux density is much greater than that in GDC. The rapid variation of the HTC is attributed to the degradation of contact between the casting and the die occurring during solidification. The second stage velocity and the impact pressure caused by the sudden deceleration of the piston as the cavity was filled have a significant influence on the interfacial heat transfer during HPDC. This impact pressure is different from the intensification pressure which is applied during the third stage of the casting process. The values of the peak heat transfer coefficient showed no dependence on intensification pressure in our trials.

Analytical models have been developed to predict the thermal contact resistance (TCR) at the liquid-solid interface in general. Contact topography and interface characteristics are included in the model through die surface roughness and the mean trapped air layer between the casting and the die. The mean trapped air layer is determined from the mechanisms of contact at the liquid-solid interface. Two different contact mechanisms have been considered; liquid-porous solid and liquid-non porous solid contact. In the liquid-non porous case, the air is trapped and compressed inside the microcavities. However, when the solid surface is porous, the air is not trapped and can escape through the pores. In this situation, the contact conditions are determined by the pressure, surface tension of the liquid and the surface roughness characteristics of the die or coating. The proposed models determine the radius and the density of the microcontact points for a given condition of contact. The density and the radius of contact spots have been integrated into a classical thermal flux tube theory in order to calculate HTC at the casting-die interface.

The models have been applied to the casting-die interface in GDC and HPDC. The calculated TCR is found to agree well with the experimentally determined results. The models provide valuable information about the role of the casting-die interface in heat transfer during GDC and HPDC processes. The results show that the effect of the measured parameters such as pressure and roughness on the HTC at the casting-die depends on the mechanisms of contact.

For HPDC, the evolution of the HTC with time has been associated with the presence of a rapidly solidified region on the casting surface which forms early during solidification due to the high thermal gradient. However, in GDC the relatively low thermal gradients are associated with microstructures more consistent with the formation of a mushy zone across the casting.