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Wrought Al-Zn-Mg alloys possess high mechanical properties,
hence it is worth to study and develop their casting materials.
However, owing to their poor fluidity and feeding
characteristic, Al-Zn-Mg alloys are liable to producing hot tear
and porosity in casting process. The purpose of this
research is to optimize an optimum composition for hot tear
resistance, and to study the factors which are conducive to the
hot tear resistance of the Al-Zn-Mg casting alloys. In this
study, we adopt a ring casting to evaluate the hot tear, and
evaluate the ability of hot tear resistance by the hot cracking
susceptibility (HCS). HCS is defined as the ratio of the total
cracks area to the radial cross-section area of the ring
casting. The experiment result show that Al-7wt%Zn-1.5wt%Mg
alloy exhibits the best ability of hot tear resistance. The HCS
of Al-Zn-Mg alloys varies with Zn and Mg contents. Factor of
solidification range, eutectic liquid fraction, elongation to
failure in solid-liquid condition, solidification shrinkage,
feeding characteristics and viscosity of alloys are correlated
with the hot tear resistance of alloys. Further, microstructure
observation show that grain boundary have a lot of porosities
and crack propagation path is along the grain boundary. Three
mechanisms can explain the hot tear：solidification shrinkage
stress, liquid feeding and grain boundary sliding. The alloy
induce shrinkage stress in solidification process. The alloy can
reduce stress concentration by grain boundary sliding, and
release the shrinkage stress.
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