Dissertation > Excellent graduate degree dissertation topics show

Influence of Microstructure on Pull-Compression Fatigue Properties at Room Temperature of NZ30K Cast Magnesium Alloys

Author: LiZhiFeng
Tutor: PengLiMing
School: Shanghai Jiaotong University
Course: Materials Processing Engineering
Keywords: Mg-Nd Magnesium Alloy Tensile-Compression Fatigue Grain size distribution Forming process Fatigue Fracture surface
CLC: TG292
Type: Master's thesis
Year: 2011
Downloads: 49
Quote: 0
Read: Download Dissertation

Abstract


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.

Related Dissertations

  1. Prominent coal seam breaking mechanism and adjustable pore Process,TD823.82
  2. Gas injection process of assimilation of blast furnace charge,TF538.63
  3. The Theory and Practice Research of Iron and Steel Material Forming Process Based on Saving Type Reducing Quantity,TF301
  4. Colourful Phosphating Technics and Film-Forming Mechanism Research of Steel,TG177
  5. Research on Continuous Multi-point Forming Method, Equipment and Experiment,TG306
  6. Study on Yanshanian Magmatic to Hydrothermal Mineralization Processes of Major Tin-Polymetallic Deposit-concentrated Area in South Hunan,P618.44
  7. Study on the One-Step Solution-Based Synthesis and Properties of Core-Shell Structure Nanocomposite,TB383.1
  8. Research on Key Technologies of Intelligent Design in Sheet Metal Forming Processes,TG386
  9. A Fundamental Study on Laser Forming of Sheet Metal Without Mould Based on Thermal Stress,TN249
  10. Analysis of Tourmaline Resources Potential and the Research on Its Comprehensive Development and Utilization,P619.2
  11. Research on Preparation and Forming Mechanism of Trivalent Chromate and Chromate-free Passive Films on Electroless Ni-p Coating,TG174.44
  12. Robust Design of Sheet Metal Forming Process and Topology Optimization Die Structure High Strength Steel,TG386
  13. Research on Forming Simulation for Bilayer Composite Bushes by Finite Element Method Based on MSC.Marc,TG386.3
  14. Study on the Changes of Suspended Sediment Grain-size Distribution in the Middle and Lower Yangtze Mainstem,P343.1
  15. The FEM Simulation on Forming Process of High Pressure Vessel of Titanium Alloy,TG30
  16. Geology and Geochemical Characteristics and Mineralization Potential Estimation of Bajiatan Intrusion in Luzong Volcanic Basin, Anhui Province,P588.1
  17. Research on the Autonomy of West European Cites in Middle Ages,K503
  18. Toughening Piezoelectric Ceramics,TQ174
  19. Study of Forming Parameters of Helix Welded Pipe,TG441.7
  20. The Research for Drawing Process of High Speed Railway’S Track Template,TG386.32
  21. Research on the Upset Forging Technology of Marine Crank of 305 Crankshaft,TG316

CLC: > Industrial Technology > Metallurgy and Metal Craft > Casting > Non-ferrous metal casting > Light metal casting
© 2012 www.DissertationTopic.Net  Mobile