The Effects of Surface Roughness and Metal Temperature on the Heat-Transfer Coefficient at the Metal Mold Interface

Abstract

This article focused on the effects of surface roughness and temperature on the heat-transfer coefficient at the metal mold interface. The experimental work was carried out in a unique and versatile apparatus, which was instrumented with two types of sensors, thermocouples, and linear variable differential transformers (LVDTs). The monitoring of the two types of sensors was carried out simultaneously during solidification. The concurrent use of two independent sensors provided mutually supportive data, thereby strengthening the validity of the interpretations that were made. With this type of instrumentation, it was possible to measure temperature profiles in mold and casting, as well as the air gap at the metal mold interface. Commercial purity aluminum was cast against steel and high carbon iron molds. Each type of mold had a unique surface roughness value. Inverse heat-transfer analysis was used to estimate the heat-transfer coefficient and the heat flux at the metal mold interface. A significant drop in the heat-transfer coefficient was registered, which coincided with the time period of the air gap formation, detected by the LVDT. An equation of the form h ¼ 1 b à Aþc þ d was found to provide excellent correlation between the heat-transfer coefficient and air gap size. In general, an increase in mold surface roughness results in a decrease in the heat-transfer coefficient at the metal mold interface. On the other hand, a rise in liquid metal temperature results in a higher heat-transfer coefficient.

Comparisons of the Effects of Air and Helium on Heat Transfer at the Metal-Mold Interface

“…In particular, the coupling of TC1 with LVDT1 and TC3 with LVDT2 was to detect the movement of the thermocouple tip as the material expanded and contracted in the course of an experiment. In fact, this coupling was first introduced in the literature, [1] which serves as the springboard for the present work. The core of each LVDT used was attached to the coupling unit (C in Figure 1).…”

“…[1] These modifications were introduced in an effort to increase the uniformity of the surface finish of the castings and, as such, reduce the variability of the experimental results. Figure 3 displays two castings obtained under the current version (A) vs the previous version (B) described in the literature, [1] which illustrates this difference. A few other comparisons between these two experimental setups are shown in Table I.…”

“…In an effort to make further improvements, it was of interest to develop the IHCP so that it might require fewer iterations than that in the earlier study. [1] In the present study, the IHCP used is a modified and upgraded version of the IHCP used in the literature. [1] The following describes the one-dimensional transient heat-conduction equation:…”

“…[1] In the present study, the IHCP used is a modified and upgraded version of the IHCP used in the literature. [1] The following describes the one-dimensional transient heat-conduction equation:…”

“…The controlling factors of HTC have been outlined in a recent publication. [1] Helium gas is nontoxic and nonflammable and is widely available at relatively low cost. [2] In addition, for a range of temperatures from 25°C to 500°C, helium's thermal conductivity is at least 5 times higher than that of air.…”