Influence of heat treatment on susceptibility to hot cracking of magnesium alloy EN-MCMgRE3Zn2Zr

Alloy EN-MCMgRE3Zn2Zr is gravity casted to sand casting moulds and is used mainly in aviation industry and car industry. In castings of magnesium alloy inconsistencies in form of porosities or misruns often appear. Such defects are repaired with the use of overlay welding and welding techniques. Main difficulty during welding magnesium alloys is their susceptibility to cracking in crystallization. These cracks are results of effects proceeded in Brittleness Temperature Range (BTR). It is temperature range between liquidus temperature and loss ductility temperature i.e. temperature on heating at which the ductility of material drops to zero. Technological factors, which decide above BTR are: welding condition and structure of cast formed in casting and heat treatment. Metallurgical factors are: chemical composition of alloys, chemical reaction during welding and morphology of joint. Constructional factors, such as shape of casting mould, his specific stiffness (quotient Young's modulus and density) decide about size of stresses and deformation accompanying formation of joint weld. In this paper an assessment is presented of the influence of technological, metallurgical and constructional factors and, in particular, the influence of heat treatment on the size and character of high-temperature brittleness range of the casted magnesium alloy EN-MCMgRE3Zn2Zr. The range of research included overlay welding and welding tests, metallographic research and the assessment of the mechanical properties in crystallisation temperature range. It was stated that EN-MCMgRE3Zn2Zr alloy is in a group of well weldable alloys with the use of tungsten inert gas welding, independently of the conducted heat treatment. A repair technology of the gravity castings made of EN-MCMgRE3Zn2Zr was worked out.