Developmet of Heat Resistant Mg-Zn-Al-Ca-RE Alloys for Die-casting

@Magnesium which is a light weight material is expected to be applied to automotive powertrain parts in order to enhance fuel efficiency through weight reduction. But, because of its inferior heat resistance which is required for such applications, the development of heat resistant magnesium alloys that can be used even at high temperatures is needed.

@In a previous research, cheap Mg-Zn-Al-Ca-RE(Mischmetal) alloys in which 1`3%RE was added to Mg-6%Zn-3%Al-0.6%Ca and Mg-8%Zn-4%Al-0.6%Ca alloys, were evaluated for the purpose of developing magnesium alloys having excellent cost performance index. Although the alloys have excellent heat resistance, hot tearing occurs during diecasting.

@Therefore, in this research in order to determine alloy compositions that do not contain low melting point eutectic compounds, which are thought to be responsible for hot tearing, Mg-0`1%Zn-5`9%Al-1%Ca-3%RE alloys were prepared and DSC analysis was used to investigate solidification characteristics. Consequently, alloys in which low melting point compounds do not crystallize were chosen and subjected to creep test. The result shows that the creep resistance of Mg-0.5%Zn-6“Al-1%Ca-3%RE(ZACE05613) alloy is the best among the examined alloys and it is comparable to that of ADC12 aluminum alloy, which is currently used for powertrain parts. In addition, diecasting was carried out using the ZACE05613 alloy and it was found out the alloy has excellent casting characteristics that is similar to the best diecasting magnesium alloy, AZ91D alloy.

@Furthermore, in order to investigate the influence of each alloying element on heat resistance and casting characteristics, Mg-0.5`1%Zn-4`6%Al-1`2%Ca-1`3%RE alloys were diecast and evaluated. The result shows that creep properties improve with decrease in the amount of Zn and Al contents. It is thought that this may be because the precipitation of Al-bearing compounds that cause reduction of creep resistance, becomes difficult as Al content decreases. Generally, increse in the amount of Al and Zn additions leads to increase in fluidity of the alloys because the crystallization of the low melting point compounds widens the solidification range. However large quentities of such compounds cause hot tearing.

Ca addition leads to the formation of Al-Ca or Mg-Ca compounds along the grain boundaries. Those compounds creep properties are remarkably improved, but cause hot tearing when the coverage ratio of grain boundary compounds grows big by excessive addition.

ZACE05613 alloy contains RE and Ca, which forms compounds that crystallize both within the grains and along the grain boundaries, and the compounds contribute to creep resistance. However the ratio of grain boundary compounds is not so large and that is why satisfactory creep properties as well as excellent casting characteristics are obtained at the same time with ZACE05613 alloy.

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