HightemperatureoxidationofAZ31+0.3wt.%Ca andAZ31+0.3wt.%CaOmagnesiu malloys

increases the oxidation resistance of Mg alloys, however the reason for this has not been adequately pursued [8–10]. The use of lowcost, stable CaO instead of Ca is strongly desirable for industrial applications. In case of Ca, special precaution is needed during packing, shipping, and handling due to its high reactivity with air. In contrast, CaO can be handled safely without any fire hazard, and is much cheaper than Ca. Both Ca and CaO are known to improve the mechanical properties of Mg alloys through grain refinement owing to the formation of secondary phases such as Mg2Ca in pure Mg [7,9,11] or Al2Ca in Mg–Zn–Al alloys [10] along grain boundaries during casting. In this study, AZ31 alloys were cast under a CO2 + 1%SF6 protective atmosphere, whereas AZ31 + 0.3 wt.%Ca and AZ31 + 0.3 wt.%CaO alloys were cast in atmospheric air. Casting in air would be a considerable advantage in light of global warming, since the SF6 that has been used together with CO2 for casting in magnesium production industries is the most potent greenhouse gas, with a global warming potential of 23,900 times that of CO2 and a life-time of 3200 years [12,13]. In this study, AZ31 alloys with and without 0.3 wt.%Ca or CaO were oxidized between 450 and 650 °C in air. The AZ31 alloys are widely used in industries to produce lightweight parts because of their high level of toughness and ductility. The microstructure, oxide scales formed, and the oxidation mechanism of the alloys were investigated. The effect of CaO on the casting and the subsequent oxidation of AZ31 alloys was compared with that of Ca. This study was performed in order to develop oxidation-resistant, environmentfriendly CaO-added Mg alloys.
2. Experimental procedures
The commercial AZ31 alloy ingot (Al = 3.0, Zn = 0.7, Mn = 0.3, Si = 0.02, Cu = 0.002, Ni = 0.001, Fe = 0.001 in wt.%, Mg = balance)
Previously, the addition of Ca was found to suppress the vigorous reaction of molten Mg with oxygen during casting, and increase the oxidation resistance of Mg alloys. The formation of a thin CaO-rich outer layer on the surface effectively retarded oxidation and ignition of Mg alloy [5–7]. The inner layer was either a mixture of MgO and CaO in the case of Mg + (0.5–3) wt.%Ca alloys [5], or a mixture of MgO, CaO, and Al2O3 in the case of AZ91 + (0.3– 5) wt.%Ca alloys [6,7]. Recently, Kim et al. has found that CaO also
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1. Introduction
Magnesium alloys are used as light-weight structural components, because magnesium has the lowest density (1.7 g/cm3) among metallic materials in practical use, high specific strength, good castability, good machinability, and a low elastic modulus (45 GPa). However, the widespread usage is limited due to poor oxidation resistance [1–4]. When magnesium alloys are exposed to hot air, the oxide scales that form are initially thin and protective, but they suffer from severe oxidation or even burning after certain time. This results in complete disintegration into oxide powders. The origins of poor oxidation resistance are as follows [1–4]. (1) The MgO scales cannot cover the whole sample surface because its Pilling– Bedworth ratio is 0.81, and as such does not act as a barrier to oxidative attack. (2) The vapor pressure of Mg keeps increasing as the temperature increases. At high temperatures, the MgO layer is entirely nonprotective, leading to ever increasing linear oxidation rates. (3) The oxidation of Mg to MgO is an exothermic reaction that gives off a large amount of heat. Hence, initially thin, adherent MgO scales soon become highly porous, and poorly adherent at high temperatures, eventually leading to ignition.
abstract
Magnesium alloys of AZ31 + 0.3 wt.%Ca and AZ31 + 0.3 wt.%CaO were cast and oxidized between 450 and 650 °C in atmospheric air. The initially added Ca and CaO enabled to cast the alloys in air without using environmentally hazardous SF6 gas, by forming a thin CaO-rich barrier layer at the surface during casting. A thin CaO-rich barrier layer was also formed at the surface during oxidation in air, thereby increasing the oxidation resistance of the AZ31 alloy considerably. The initially added Ca and CaO reacted with Al to become Al2Ca along the grain boundaries of the AZ31 alloy during casting.
article info
Article history: Received 30 October 2012 Accepted 22 January 2013 Available online 1 February 2013