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NESIUM alloys are widely used in many industry fields due to their excellent intrinsic properties, such as low density, high specific strength, and high specific rigidity. However, the oxidation of the magnesium during the melting process limits the alloy’s application in industry. To combat the oxidation problem, certain protective methods are applied during the melting and casting process. Such methods include the addition of flux to the melt, protective gas, alloying with ignition-proof materials, and vacuum protection.[1,2]

WEIHONG LI, JIANHUA WU, and JIXUE ZHOU are with the Shandong Key Laboratory for High Strength Lightweight Metallic Materials, Advanced Materials Institute, Shandong Academy of Sciences, Jinan, 250014, Shandong, China. Contact e-mail: [email protected] BAICHANG MA and JINWEI WANG are with the Shandong Engineering Research Center for Lightweight Automobiles Magnesium Alloys, Advanced Materials Institute, Shandong Academy of Sciences, Jinan, 250014, Shandong, China. YUANSHENG YANG is with the Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, Liaoning, China. Manuscript submitted December 15, 2016.

METALLURGICAL AND MATERIALS TRANSACTIONS B

In the early stage of oxidation prevention, fluxes were widely used for protecting the magnesium alloys due to exhibition of effective prevention and ease of application. However, fluxes release harmful gases at high temperatures, which will not only pose a serious threat to human health, but also corrode the equipment.[2] Recently, gases, such as SO2,[3,4] SF6,[5] HFC-134a,[6] and HFC125,[7,8] have been used to protect molten magnesium alloys. Unfortunately, gases cause environmental problems because they act as greenhouse gases when released into the atmosphere.[9,10] Regarding the alloying method, the addition of the alloying elements deteriorates the mechanical properties of the magnesium alloys significantly.[11] Vacuum protection has no harmful side effects to the environment but the process raises the production cost of magnesium. The goal therefore is to find a cost-effective environmentally friendly solution to prevent the magnesium oxidation issue. CO2, which is not toxic or corrosive and cheaper than SF6 and SO2, has a lower degree of green house effect than SF6.[12] Research has shown that CO2 can protect molten magnesium in different forms. Fruehling[13] claimed that an atmosphere of pure CO2 gas could effectively protect the magnesium melt. Emami and Sohn[12] used CO2/air mixture to protect molten magnesium from oxidation. Yang and Lin[14] used CO2 snow

Table I.

Chemical Compositions of AZ91D Alloy (Weight Percent)

Al

Zn

Mn

Si

Fe

Cu

Ni

Mg

9.1

0.67

0.16

0.02

0.003

0.01

0.002

bal.

to develop a highly efficient method for protecting magnesium melts. Thus, past research has shown that CO2 is effective in protecting the molten magnesium. Graphite can react with oxygen to produce CO2 rather than CO below 1273 K (1000 °C) for the lower activation energies of CO2 desorption than those of CO desorption.[15,16] The produc