Characterization Of Defects And Fatigue Behavior Of Al-Mg-Sc-Zr Alloy By Selective Laser Melting

In recent years,additive manufacturing technology has been paid more and more attention by governments,manufacturers and researchers,because its unique forming method can create complex structural components by stacking up layers without the need for additional tools such as molds,which has obvious advantages in structural weight reduction and structural integration.The refined grain structure of Selective Laser Melting(SLM)alloy makes it have excellent tensile properties,but the fatigue properties of SLM alloy are poor due to the defects such as pores.It is of great significance to improve the fatigue performance of SLM alloy by characterizing and analyzing the three-dimensional characteristics of defects and studying the influence of defects on fatigue performance.In this thesis,Al-Mg-Sc-Zr alloy formed by SLM was taken as the research object,and the micro structure observation,three-dimensional characterization of pore defects,basic mechanical properties testing,fatigue test and fatigue fracture analysis were carried out.The characteristics of pore defects and their effects on fatigue properties of Al-Mg-Sc-Zr alloy formed by SLM were mainly studied.The micro structure of Al-Mg-Sc-Zr alloy formed by SLM was characterized by Scanning Electron Microscopy(SEM),Electron Back Scattering Ddiffraction(EBSD)and Transmission Electron Microscopy(TEM).The micro structure of Al-Mg-Sc-Zr alloy formed by SLM is composed of molten pool.The inner of the molten pool is coarse-grained region composed of columnar crystals,and the boundary of the molten pool is fine-grained region composed of sub-micron equiaxed crystals.The overall grain size of the alloy is small,and the refined grain structure makes the SLM-formed Al-Mg-Sc-Zr alloy possess excellent tensile properties and microhardness.Based on X-ray Computed Tomography(XCT),the three-dimensional characteristics of pore defects in SLM-formed Al-Mg-Sc-Zr alloy were characterized.It was found that the pores were divided into small pores and large unfused pores.The porosity of the alloy is between 0.004%and 0.102%.The size of the pore defects is generally small,but there are some unfused defects with large size.The maximum equivalent diameter of the pore defects is 189μm.The sphericity of pore defects is between 0.2 and 0.6,and the sphericity is generally small.It is found that the maximum estimated value of defect characteristic size based on fatigue fracture is greater than that based on XCT.The fatigue properties of Al-Mg-Sc-Zr alloy formed by SLM are poor due to the large unfused defects.The fatigue fracture morphology of Al-Mg-Sc-Zr alloy formed by SLM was characterized and analyzed.It was found that the fatigue cracks initiated and propagated from the sample surface or near the surface defect,and when the size of the fatigue crack source defect was larger,the fatigue cracks initiated from the defect could propagate faster.Based on Paris formula,the fatigue life prediction model based on pore size was obtained by fitting the fatigue life data and the defect characteristic size of the fatigue crack source in the fatigue fracture,and the calculated results were in good agreement with the test life.