Relation between cooling rates and microstructures in gravity-die-cast AZ91D disks
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I. INTRODUCTION
THE trend toward weight reduction in transport equipment has led vehicle manufacturers to produce various components made of magnesium alloys, particularly the alloy known as AZ91D. The microstructure of that alloy is essentially an a-Mg matrix, with Al and Zn dissolved in it, and a eutectic structure of a-Mg 1 b(Mg17Al12) on the a-Mg grain boundaries. Aluminum contributes to the alloy’s corrosion resistance and castability, but the presence of the intermetallic phase, Mg17Al12, reduces its mechanical properties, particularly at elevated temperatures. The Al content of the alloy is, therefore, necessarily limited, and casting difficulties must be overcome with controlled casting conditions: the feeding system, the casting temperature, and the mold’s coating, dimensions, thermophysical properties, and preheating temperature. The heat flux was measured, and the cooling capacity of the metal mold or a metal chill with different alloys determined, by measuring the temperature in them and applying the inverse solution of the transient heat-conductance equation.[1–6] Based on this research, the following cooling mechanism is suggested. In the first stage, close contact exists between the melt and the metal mold, and the heat emitted by the casting is absorbed by the chill close to the casting/mold contact area. In that stage, the heat flux depends mainly on the heat capacity of the chill rather than on its heat conductance. In the second stage, following the establishment of a thermal gradient in the mold, the heat flux is governed by the thermal conductivity of the chill wall. In the last stage, an air gap forms between the casting and the mold, which then governs the cooling rate of the casting. The higher the ratio between the thermal masses of the mold and of an Al-Cu-Si casting (via their wall thicknesses), the shorter the duration of solidification;[2,3] but when the ratio exceeds about 2, any further reduction will be small. A. AVISHEI and M. BAMBERGER, Professor, are with the Department of Materials Engineering, Technion, Haifa 32000, Israel. Manuscript submitted October 8, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
When casting brass in a steel mold, it was found that a thin-walled mold ceases to be effective, whereas a mold with a thicker wall contributes to the cooling of the casting, other conditions being equal. In the former case, the mold is said to be “blocked,” it no longer serves as a heat sink.[1] The preheating temperature also exerts an influence on the heat flux; raising that temperature entails a reduction in the cooling rate and leads to coarser microstructures.[4,5] In all the research cited, the alloy involved had a much larger specific volumetric heat capacity than the AZ91D magnesium alloy. It could, therefore, be expected that the heat transfer and the solidification of magnesium alloys would differ from those found for Al alloys or brass. The goal of this article is to present the heat flux, solidification times, and the microstructure of AZ91D disks gravity-diecast i
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