| Magnesium alloy is developed as the third kind of metal materials after steel and aluminium alloys, and it is the lightest structural alloy. It has low density, high strength weight ratio and MOE density ratio that compared with other materials, besides, the physical and mechanical properties of magnesium alloy are excellent. So it is applied in many fields as the structural materials. But the chemical activity is so active for magnesium that it is easily to oxidate in air, and the oxidation layer is non-protective, which restricts the range of application to some extent.At present, the studies on oxidation behavior of magnesium alloys in room and liquit environment are more mature, however, there are few reports on the oxidation behavior at high temperature. But in some processing process, such as heat treatment, welding, casting and thixomolding, magnesium alloys are usually exposed to high temperature and touch with oxidant inevitably, which lead to oxidate. In the study, the oxidation dynamic and process of AZ61and AM60magnesium-aluminium alloys at400～550℃was investigated by static increase weight method. The oxidation dynamic laws are discussed by isothermal oxidation dynamic curves. The changes of surface morphology are observed by Tsview. The surface morphology, phase composition and dimensions of oxidation were analyzed by SEM, XRD and LPSA, respectively. Finaly, the oxidation resistance of the same and different alloy system are compared horizontally and longitudinally, respectively. The main conclusions are drawn as follows:1. At400℃, the alloy oxidizes entiretily, and the oxidation layer is preservative; above450℃, oxidation dynamic follows the deviation square parabola line rules, the oxidation production increases constantly and superposites each other with the rise of oxidation temperature and the extension of oxidation time, so the oxidation weight increases continuously.2. The critical factor affected on oxidizing is the melt temperature of eutectic phase. The (3phase occurs melting when the temperature is to its melt point, and the selective oxidation happenes. At450℃, the speed of β phase’s melting is slower than the formation of oxidation film, so the oxides presents flakiness; at500℃, β phase melts seriously, and the alloy sufferes damage from punctation to grain boundary, which not only destories the original film, but aslo restrictes the formation of new film simultaneously, then the oxidation sustains, the oxides are granular. At550℃, the liquid ingredients increase, and oxidation aggravates and the alloy burns in the end.3. Oxidation temperature and time effect on the phase composition and surface morphology of oxides. At450℃, the oxides is flakiness MgO, while at500℃, it is granular MgO. With the rise of oxidation temperature, only the alloy oxidizes seriously and even burus, the oxides just includes MgAl2O4, Mg3N2and Mg6MnO8, then the morphology changed.4. For the same magnesium-aluminium alloy system, because the amount of β phase is more and distribution is along the grain boundaries in as-cast alloys, the alloy occurs corrosion in the grain boundaries during oxidation at high temperature, for the extruded, however, the β phase refining during extrusion process, and distribution scattered, so it is impossible to surffer from corrosion in grain boundaries as as-cast, instead it shows pointlike ruin, therefore the oxidation resistance of extruded alloy is better than that of as-cast alloy. For the different magnesium-aluminium alloy system, comparing with AM60alloy, the content of Zinc is higher in AZ61alloy, which increases the tendency to form microporosities, and easies to form pores in oxidation layer, so the oxidation resistance is less than that of AM60alloy.5. In general, the brand of magnesium alloy is different, but the specific feature of magnesium alloy is common, like the high vapor pressure, the higher the oxidation temperature, the higher the vapor pressure, and oxidation speed of alloys is superimposed. |
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