Microstructure And Notch High-cycle Fatigue Properties Of 3D Printed TC4 Alloy

3D printing technology with the advantages of preparing complex components,short production cycles and high designability has been widely used in the field of aerospace.In the process of 3D printing,natural printing defects would inevitably appear,such as pores,lack of fusion,oxides,cracks,and so on,which can be very harmful to the fatigue performance of the material.At the same time,in practical application,the size and structure of aviation components are complex and changeable,and the fatigue cracks often initiate from the stress concentration site of components.Therefore,the research on the fatigue performance of 3D printing materials under stress concentration conditions is of great significance to the safety and reliability design of their components.In this paper,the microstructures and defects of TC4 alloys(MV and SV samples)prepared by two selective laser melting(SLM)processes were characterized and analyzed.The MV process parameters were laser power of 200 W,scanning speed of 1400 mm/s and layer thickness of 60μm;SV process parameters were laser power of 300 W,scanning speed of 1200 mm/s and layer thickness of 60μm.At the same time,the hardness,tensile properties and notch fatigue behavior of K_t=1,2 and 3 were systematically studied,and the key factors affecting notch fatigue properties were emphatically discussed.The microstructures and defects of the materials prepared by the two processes have been investigated.It is found that the microstructures of MV and SV samples perpendicular to the deposition direction are the"checkerboard"structure.And it is detected that there are shortβcolumnar crystals in MV samples parallel to the direction of deposition,and coarseβcolumnar crystals in SV samples parallel to the direction of deposition.The natural printing defects of MV and SV samples are uniformly distributed in the samples.Different batches of materials with the same process would differ in defect size,porosity and sphericity.The hardness,tensile properties and notch fatigue behaviors of the materials prepared by the two processes have been studied.It is found that the microhardness,tensile strength and elongation after fracture of MV material are higher than those of SV material.The notch fatigue properties of materials prepared by different processes are closely related to the defect states of materials.The fatigue cracks of all fractured samples initiate from the lack of fusion defects or oxides on the surface of the samples,and the fatigue lives of the fractured samples are less than10~6 cycles.The conditional fatigue strength of MV and SV samples decreases with the increase of the stress concentration factor.By analyzing the factors affecting notch fatigue performance,it is found that,for the same macro stress concentration factor,defect location and defect area,the sharpness of the defect edge can seriously affect the local stress concentration in the defect area.The fatigue performance of notched samples is determined by the combination effect of the macro stress concentration factor,defect area and defect sharpness.Based on Murakami’s classical formula,a defect sharpness correction coefficient can be coupled to describe the effect of defects on fatigue life.