| Magnesium alloy is widely used for the higher specific strength, specific stiffness and good mechanical property. A lot of studies of Mg alloy are conducted to develop new Mg alloy with higher strength. The latest study revealed that Mg-3.0Nd-0.2Zn-Zr (wt. %) alloy is a magnesium rare earth alloy with high strength, low rare earth content and good formability. The alloy has a better mechanical property at both room and high temperature and corrosion resistance property than AZ91D alloy. It is expected the alloy will be used in the field of aerospace, automobile etc. However, fatigue property of NZ30K alloy hasn't been investigated systemly as a key factor of material, which will be focused by this article.The dimension and distribution of grain size, the influence of high cycle fatigue property of NZ30K alloy by different microstructure of normal formability process were determined. Different grain size and distribution of gravity casting NZ30K magnesium alloy can be made by adjusting the Zr content. NZ30K Mg alloys with different microstructure were made by four type of formability casting process: gravity permanent cast (GPC), gravity sand cast (GSC), low pressure permanent cast (LPC) and low pressure sand cast (LSC). Microstructure, tensile property, high cycle fatigue property at room temperature and fatigue fracture surface were investigated by optical microscope (OM), tensile test machine, SincoTec MAG50KN and scanning electric microscope (SEM).NZ30K-T6 alloy with grain sizes 40μm, 60μm, 100μm and 180μm were made by adjusting the Zr content. The range of grain size distribution is getting smaller and going to be normal distribution with the decrease of average grain size. Yield strength increases with grain size reducing, and shows H-P relation. Tensile strength increases first and then decreases with the decrease of grain size, and gets its peak at 60μm. Elongation decreases first and then increases with the decrease of grain size, and the result shows the min. value at 100μm, and max. value at 60μm. The fatigue strength of NZ30K-T6 alloy up to 107 cycles increases at room temperature with the the decrease of grain size. The variety of fatigue strength is a little with the range of grain size from 180μm to 100μm, only 1MPa enhancement. The obvious increment is 12MPa when the avearge grain size decreases from 100μm to 60μm. While the fatigue strength decreases a little from 60μm to 40μm, the reduce volume is less than 1.5MPa.The difference of microstructure and mechanical properties for NZ30K magnesium alloys cast by GPC, GSC, LPC and LSC could be understood as the influence of different cooling rate of solidification condition. If the solidification cooling speed is fast (metal mold cast), grain size distribution is normal distribution, and the average grain size is smaller. When the solidification cooling rate decreases to sand cast, the grain size distribution range is broader and deviating from normal distribution obviously, and the grain size distribution is uneven in different area and the average grain size is larger. For tensile properties, tiny and uniform microstructure is helpful for improving mechanical properties of alloy, including yield strength, tensile strength and elongation. Compare with the yield strength, the increases of tensile strength and elongation are more remarkably. For fatigue properties at room temperature, there is no large increase because of tiny and uniform microstructure. The fatigue strength of GPC-NZ30K-T6 is 76.39±7.04MPa, and that of GSC-NZ30K-T6 is 75.27±0.75MPa. The average value of fatigue strength for GPC-NZ30K-T6 is just improved 1.12MPa comparing with GSC-NZ30K-T6. However, the fluctuation range of fatigue strength property becomes wider. Considering of fluctuating range of alloy fatigue properties, the uneven grain size distribution is favorable for stabilize the fatigue property of NZ30K alloy.SEM investigation results of NZ30K alloy fatigue fracture show that fatigue cracks initiation specimen surfaces. The fracture surface consists of three typical areas, which are fracture initial area, crack propagation area and collapse area. The surface morphology of fracture initial area is generally smooth. The stripe is clear and regular, and the distribution of fracture surface stripe is parallel, with short the stripe space. The surface morphology of crack propagation area shows more fluctuation comparing with fracture initial area, and the stripe distribution is disordered. The surface morphology of collapse area shows more fluctuation than the above two areas, and the stripe distribution is more disordered. The surface morphology of collapse fracture region is similar to those of tensile fracture surface, which are all characterized by cleavage planes. |
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