| Magnesium alloys, with low density, relatively high strength and rigidity, good electromagnetic shielding characteristics, high damping characteristics and ease of manufacturing. Now, it will play a more important role in the automobile industry and electrical industry. But magnesium and its alloys are known of their high affinity to oxygen, the structure of magnesia is loose and many small holes appear on the surface, so that the surface oxide film, which is made up of magnesia, cannot prevent the passage of oxygen and the oxidation of inner magnesium, so, oxidation magnesium makes the magnesium alloys oxidate and ignites easily during hot processing and magnesium alloys parts are different to make or produce. The chips of magnesium alloy are easy to ignition in processing,storage and transportation, it cause fire ,and it is difficult to process parts, that hinder magnesium alloy large scale extensive application. Reactive elements such as cerium, yttrium and other rare earth elements have been reported to be beneficial to high temperature oxidation resistance of metals. In this dissertation, The different cooling rate of AZ91D and AM50 magnesium alloys added with Mg-Ce and Mg-Y master alloy is prepared, effect of Ce (Y) on ignition point of AZ91D and AM50 magnesium alloys chips was studied, especially, the effect of the change of existing state of Ce (Y) in magnesium alloys, the sizes of the magnesium chip, the chemical components of magnesium alloy and cooling rate on ignition point of AZ91D and AM50 magnesium alloys chips, the results will provides foundation and new thoughts for Ce (Y) in magnesium alloy chips combustion.In the present study, the effect of Ce (Y) on ignition point of magnesium alloys chips is researched. The major research efforts of the present study are as follws:(1) Effect of the content of Ce (Y) on the ignition point of AZ91D and AM50 magnesium alloys is studied, the Ce (Y) can improve the ignition point of the magnesium alloy chips. The existing state of Ce (Y) in magnesium alloys has significant influence on ignition point. When the content of Ce (Y) in magnesium alloy reachs solid solution, the ignition point is highest; when the content of Ce (Y) in magnesium alloy is lower than solid solution, increasing the Ce (Y) content will increase the ignition point. When the content of Ce (Y) is higher than solid solution, leads to a decrease in the ignition point, but the ignition point is higher than AZ91D and AM50 alloys.(2) The solid solution of Ce (Y) in matrix improves the ignition point of the magnesium alloys chips. During heating, the Ce (Y) can react with oxygen piror to magnesium, and form Ce2O3 (Y2O3), the filling of Ce (Y) oxide into the holes of magnesia results in the formation of a composite and tightly coherent oxide film which can work as a protective layer at high temperature and reduce the oxygen inward diffusion rate, and retards the oxidation rate and improve the ignition point. When content of Ce (Y) is higher than solid solution, a lot of Al11Ce3 (Al2Y) phase in the microstructure of the magnesium alloy, increasing the Ce (Y) content, increasing Al11Ce3 (Al2Y) phase number, a limited depleted area of Ce (Y) begins to form around the Al11Ce3 (Al2Y) phase. During the heating, the tightness of oxide film in depleted area of Ce (Y) decrease, the more content of Ce (Y) is added, the more depleted areas of Ce (Y), the lowest the ignition point. The solid solution of Ce (Y) in matrix can improve the magnesium alloy ignition point and the Al11Ce3 (Al2Y) decrease the ignition point.(3) When the content of Ce (Y) in AZ91D and AM50 magnesium alloys is the same, the ignition point of AM50 magnesium alloy is higher than AZ91D alloy. The ignition point of magnesium alloys are relate to the content of Al in alloy, the less is the content of Al in alloy, the more the solid solution of Ce (Y), the less of depleted areas of Ce (Y), the less of theβ-Mg17Al12 phase, the higher of the ignition point of the alloy. The content of Al in AM50 alloys are higher than in AZ91D alloy, so the ignition point of AM50 alloy is higher than AZ91D alloy.(4) For the magnesium alloys, the variation in size of the chips in the range of 58-270μm have a limited influence on ignition point, the difference between maximum and minimum is less than 5℃. Both the chip specific surface area and the space between the chips play role in determining the ignition point, the change of the size has a limited influence on ignition point.(5) Ce (Y) has the same regularity on ignition point of AZ91D and AM50 magnesium alloys on different initiating heating temperature, when the content of Ce (Y) in alloy is lower than solid solution, the more Ce (Y) is added, the higher of the ignition; the content of is higher than the solid solution, the more Ce (Y) is added, the lower of the ignition point. The ignition point of alloy increase with increasing the initiating heating temperature.(6) The ignition time of the magnesium alloy chips in 500℃is relate to the existent form of Ce (Y) in magnesium alloy. When the content of Ce (Y) in alloy is lower than solid solution, the more rare earth is added, the longer of the ignition time; the content of Ce (Y) is higher than the solid solution, the more rare earth is added, the shorter of the ignition time.(7) The change of the cooling rate in magnesium alloy has remarkable influence on the ignition point, the faster the cooling rate, the higher of the ignition point. The solid solution of Ce (Y) in matrix is the main cause in improving the magnesium alloy ignition point. The cooling rate of rapid cooling is 1.5×105℃/s, the cooling rate is quickest, the solid solution of Ce (Y) and the ignition point is highest; the cooling rate of furnace cooling is 4.32×10-2℃/S, the cooling rate is lowest, the solid solution of Ce (Y) and the ignition point is lowest.(8) Calculation of thermodynamics shows that the Ce (Y) has much higher affinity to oxygen than Mg during heating. The isothermal oxidation kinetics indicates that the different cooling rate of different content of Ce (Y) on magnesium alloy has different oxidation rate. When the content of Ce (Y) is lower than the solid solution in magnesium alloy with same cooling rate, the more Ce (Y) is added, the lower of the mass gain, the slower of the oxidation rate; when the content of is higher than the solid solution, the more Ce (Y) is added, the higher of the mass gain and the quicker of the oxidation rate. When the magnesium alloy has the same content of Ce (Y) with different cooling rate, the quicker of the cooling rate, the more of the solid solution, the lower of the mass gain, the rapid cooling of magnesium alloy has much lower mass gain than furnace cooling. So the solid solution of Ce (Y) in matrix can decrease the magnesium alloy oxidation rate. Based on the above analysis, the oxidation model is established to demonstrate the oxidation process on the surface of the magnesium alloy.
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