| Magnesium alloys have been recognized as a light environmental-friendly material because of their low density, high specific strength, good casting performance. It is now widely used in aerospace, automobile industry, 3c products and medical equipment. However,magnesium alloy have the tendency to form hot tearing during the casting process, reduce the performance of magnesium alloy casting, results in the application limitation of magnesium alloy casting product, so research on the hot tearing behavior of magnesium alloys is of decisive significance.As a kind of magnesium alloy of Mg-Al-Mn series, AM50 magnesium alloy is widely used because it has a good toughness, plasticity and a strong impact resistant capability, but it is weak at resisting high temperature. Magnesium alloys with rare earth element have a good mechanical properties at room temperature and high temperature and excellent creep properties. As a kind of rare earth elements, Nd has the advantages of refining grain obviously,improving the microstructure of the alloy, and has been widely used in magnesium alloys. For the new AM50-Nd magnesium alloy, it is necessary to research the thermal cracking sensitivity of it. The present work first evaluates the hot tearing susceptibility(HTS) of AM50-Nd using thermodynamic calculations based on Clyne-Davies’ and Ramseyer’s model.The "T" shaped hot cracking mould is used for casting experiment, and the crack volume of AM50-Nd system is measured by paraffin melting method. Use computer to collect the signal data of temperature and shrinkage stress of AM50-Nd alloy, during the solidification process,then make a further processing and draw curves. The microstructure and morphology of hot tearing regions of the alloys are observed by X-ray diffraction and scanning electron microscopy. The effect of solid fraction, the range of temperature and the change of temperature on the thermal cracking sensitivity of AM50-Nd alloy is studied. And then compare AM50 magnesium alloy with traditional AZ91 magnesium alloy.Theoretical predictions and experimental results show that the order of the hot cra cking sensitivity coefficient(CSC) of AM50-Nd and AZ91 magnesium alloy is that: AZ91<AM50<AM50-0.5wt.%Nd<AM50-1wt.%Nd<AM50-2wt.%Nd<AM50-4wt.%Nd. Among them, the hot cracking sensitivity of AM50 alloy is higher than AZ91, after adding Nd, the hot cracking sensitivity of AM50 alloy enhance. The hot cracking sensitivity of AM50-4wt.%Nd is the largest.The main mechanism of hot tearing of AM50-Nd magnesium alloy is liquid film combined with solidification shrinkage repairing and dendritic bridging. With the increase of Nd content, the strength of the liquid membrane is reduced, liquidity becomes poor, it has no power to withstand the solidification shrinkage stress When the liquid membrane strength is not enough to resist solidification shrinkage stress, the hot tearing is forming because of the solidification shrinkage stress. Due to AZ91 and AM50 have much liquid fraction at the end of the solidification, the hot tearing can get better feeding effect. And there will appear dendritic bridging. And it can increase the binding force between the branches and the grain, and reduce the thermal cracking tendency of the alloy. |
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