Effect Of Ca On Solidification Behavior Of Mg-Al-RE System Heat-resistant Magnesium Alloys

Since21Century, with the development of science and technology, energy andenvironmental issues have become increasingly prominent, and more lightweight has becomean inevitable trend of development of new materials. Therefore people have to turn theirattention to the magnesium alloy, which is the lightest in the structural material. But with thepoor heat resistance, the application of magnesium alloys is limited to the traditional areas suchas mechanical components, and few magnesium alloy was used in automobile engine andtransmission system.Therefore, all the researchers around the world have set their sights on improving the heatresistance of magnesium alloy. Up to now, the most common method is alloying. That is,adding the rare earth, alkaline earth and some other elements into the magnesium to improvethe heat resistant properties of magnesium alloy. However, the current research is focused onthe relationship between composition and properties, while study little about the effecting ofadding element in the solidification process. So many problems exist differences in the deeperdiscussion.In this paper, taking AE42magnesium alloy as a reference alloy. By using differentamount of cheap element Ca （0,0.5and1.2wt%） to instead part of the expensive rare earthelements in the alloy. AE42, AEC4105and AEC4112alloy was obtained. Then using thermalanalyzer （DSC）, XRD, OM, SEM and EDS and other detection methods to test the samples,study the phase transition temperature of alloys, especially researches the composition,morphology and distribution characteristics of the precipitates species, and then analyzes theinfluence of different content of Ca on the solidification behavior of the alloys.（1）The results show that: With the Ca content increasing, the beginning temperature ofthe solidification of the alloys decrease; the solid-liquid coexistence region became larger atfirst and then decreases; the formation temperature of Al-RE phase gradually decreases; whilethe precipitation temperature of Al₂Ca phase is increasing.（2）The solidification sequence of AE42, AEC4105and AEC4112alloys issubstantially the same. At first, the bright white cellular phases α-Mg solid solutionprecipitate from the liquid（Lâ†'α-Mg）; Then Al₂RE phase would directly forms from liquid（Lâ†'α-Mg+Al₂RE）; Later Al11RE3phase would form after Al₂RE phase precipitate（Lâ†'α-Mg+Al11RE3）; The difference is that, when the alloy does not contain Ca element，there exist three kind of phases（α-Mg,Al₂RE and Al11RE3）; When the alloy contain Caelement,in addition to the phase containing the above, it also has Al₂Ca phase. What more, Itshould be noted that a small amount of Ca atoms are solid dissolved in the Al-RE phase. And With the increase of Ca content, the number and size of bone shaped phase Al₂Ca willincrease.