Study of Mechanical Properties of Both Magnesium Alloys AZ91D and AM50A Cast by Thixomag Process Influence of Microstructure

摘要

The forming of alloys from the liquid state can be realized thanks to various casting technologies where the temperature of the metal is high enough to fill a given die correctly. A variety of casting defects often occur in the final part, such as gas porosity, shrinkage porosity and hot cracking. In the case of the magnesium foundry, all handling of molten metal requires serious safety precautions to avoid explosion and burning. Protective gas mixtures (SF{sub}6, SO{sub}2, CO{sub}2) are used to minimize the ignition risks but have a bad impact on the environment. The semi-solid forming operates using a lower temperature in the solidus-liquidus range and seems to be interesting, in particular, for the quality requests of the die caster and for safety and a clean environment. The purpose of this paper is to present the mechanical results of tensile tests carried out on magnesium standard samples, which are cast by Thixomag process from the semi-solid state. Two magnesium alloys AZ91D and AM50A are characterized. The static and dynamic elastic modulus are determined by tensile tests equipped with an extensometer and by an ultrasonic method respectively. In the case of semi-solid samples, the mechanical properties increase and, in particular, the Young's modulus. The porosity ratio of semi-solid parts is lower than that of conventional parts, cast by conventional process. The microstructure and the X-ray analysis display aluminum segregation between the globular particles and the eutectic phase. The aluminum content is around 20 wt% in the eutectic phase. This means that the aluminum is supersaturated in the eutectic and precipitated discontinuously like thin lamella of Mg{sub}17Al{sub}12. This is an intermetallic cubic structure. The size and the shape of globules influence the mechanical properties. This study confirms the first results announced two years ago on the wiper parts. The presence of Mg{sub}17Al{sub}12 lamella reinforces the solid solution, having a hexagonal structure, and improves the Young's modulus.