RELATION BETWEEN AS-CAST MECHANICAL PROPERTIES, MICROSTRUCTURE AND SOLIDIFICATION CONDITIONS FOR ZN-AL ALLOYS

摘要

The columnar and equiaxed mode of solidification occur when equiaxed grains can nucleate and grow ahead of the columnar front provoking the columnar to equiaxed transition (CET). The prediction of the columnar to equiaxed transition is of great interest for the evaluation and design of the mechanical properties of castings products. Experiments are conducted to analyze the CET during the upward unsteady state directional solidification of Al-Zn and Zn-Al alloys, under different conditions of superheat and heat transfer efficiencies at the metal/mould interface. A combined theoretical and experimental approach is developed to quantitatively determine the solidification thermal parameters: growth rates, thermal gradients and cooling rates. The effects of solidification parameters like cooling rate, growth rate and temperature gradient on the CET position are investigated. Although the metallurgical and micromechanical aspects of the factors controlling microstructure, microhardness, strength and ductility of alloys are complex, it is well known that solidification processing variables are of a high order of importance. The present paper proposes general empirical expressions which permit the correlation between secondary dendrite spacings, growth rate, local solidification time and ultimate tensile strength for Zn-Al alloys.