Combined effect of calcium and zinc on the hot cracking of Al – Mg аlloys

Abstract

This paper examines the effect of calcium and zinc on the hot cracking of cast magnaliums and how their hot cracking behaviour is related to their structure. The authors rely on the Thermo-Calc methods (TTAl5 database) to substantiate the composition of the Al – Mg – Ca – (Zn) alloys in view. Pure burden components were smelted in a resistance furnace and then cast in graphite and steel moulds. Structural and compositional analyses relied on optical and scanning electron microscopy. Specimens for structural and characterization studies were made out of flat castings with a 10×20 mm section, which were then remelted and cast into pencil-shaped specimens for hot cracking tests. It was found that as the concentration of calcium in the alloys rises, their eutectic becomes coarser, i.e. certain dendrite branches tend to get shorter and thicker. Combined inoculation with calcium and zinc does not modify the structure. A hot cracking study showed that a shorter crystallization interval applied when selecting an alloy did not impact that parameter in the studied system. For example, alloy Al – 10 Mg – 4 Ca has a shorter crystallization interval compared with alloy [АМг10ч]; at the same time the former is much more prone to hot cracking. Inoculating the experimental alloy Al – 6 Mg – 2 Ca – 2 Zn with zinc ensures a better resistance to hot cracking compared with the commercial alloy [АМг10ч]. A study that looked at the effect of superheating on castings showed that the best effect is achieved when heating 50 oC above the liquidus temperature. As the superheat temperature increases, so does the grain size of the Al – 6 Mg – 2 Ca – 2 Zn alloy. The hardness and density of this alloy are similar to those of the base one.This research was funded by the Russian Science Foundation under Grant No. 21-79-00134.