选区激光熔化Ti-6Al-4V钛合金的拉伸断裂行为研究

Fracture Behavior under Tensile Loading of Ti-6Al-4V Alloy Manufactured by Selective Laser Melting Processing

采用选区激光熔化(selective laser melting,SLM)工艺制备的Ti-6Al-4V合金构件具有相比传统工艺更加优异的力学性能,但其独特的显微组织对力学性能的影响机制尚需进一步研究。本文采用SLM工艺制备了4组棒状拉伸样品,各组样品轴向分别与基板平面呈0°,30°,45°和90°,拉伸结果表明,4组样品均获得优异的强塑性匹配,不同成形方向上性能呈现明显差异。成形过程中的激光能量密度较低,导致沉积态样品的显微组织无明显柱状晶晶界,平行于激光束入射方向截面呈现波状层间界,垂直于激光束入射方向截面呈现网格状道间界,两种边界的形成均与激光束对粉末的熔凝效应有关,边界内部为细小片状的α相,片层长轴近似与基板呈45°,且存在较弱的织构特征。小尺寸α相起到了细晶强化作用,使得4组拉伸样品综合性能优异;拉伸性能随样品轴向与基板角度不同而发生明显改变,是由于层间界和道间界与拉应力形成不同角度,造成初始裂纹沿边界α相扩展的难易程度不同所致,同时晶内α片层取向和较弱的织构也对性能的变化有一定的影响。

Ti-6Al-4V titanium alloy fabricated by selective laser melting(SLM)technology had good mechanical properties and unique microstructure,which was obviously different from that of α+β two-phase titanium alloy prepared by traditional technology. At present,the relationship between β-columnar crystal orientation,α-lamellar orientation and fracture mechanism of Ti-6Al-4V alloy formed by SLM was insufficiently studied,and the influence of interlaminar and interlaminar boundaries on properties was also lacking research. The data related to tensile samples of Ti-6Al-4V alloy with different angles were analyzed,and the influence of microstructure on tensile properties and fracture behavior of samples was discussed. Four groups of bars were prepared in EOS-M280 SLM system,and the long axis was 0°,30°,45° and 90° to the substrate respectively. The same process parameters(mainly including laser power,scanning speed,powder thickness and scanning distance)were used for all samples. The surface morphology of Ti-6Al-4V powder was analyzed by Zeiss ultra-scanning electron microscope(SEM),and the particle size distribution was measured by Mastersizer 2000 laser particle size analyzer. The interlaminar boundaries and track boundaries in the microstructure of the samples were observed by metallographic microscope(OM)in different directions,and the structural characteristics and causes of the boundaries were analyzed. The phase constitution of the alloy was determined by X-ray diffraction(XRD),and the relative strength change of the diffraction peaks was analyzed to determine the texture characteristics of the microstructure. The proportion of three strong peaks((100),(002),(101))of α phase changed obviously in XRD patterns. The increase of(100)and(002)peak intensities indicated an increase in the possibility of texture in SLM samples,especially in the direction perpendicular to the substrate surface(i.e. along the z axis). Compared with the traditional forged Ti-6Al-4V alloy,the strength of the samples in this paper was at least 60 MPa higher,and the plasticity index was greatly improved. The change of strength and plasticity was closely related to the angle between the long axis of the sample and the substrate. With the increase of stress direction and substrate angle,the strength was the highest at 0° and then decreased gradually. It was the minimum value at 45° and increased slightly at 90° and the yield/ultimate strength ratio of 90° sample was the highest. The plasticity index kept increasing from 0° to 90° and the angle of 30° and 45° were similar,and the plasticity of 90° sample increased obviously. A columnar structure with a diameter of ~80 μm appeared on the cross section of the 0° sample. The size of the structure was consistent with the width of the molten pool formed by laser beam scanning. Therefore,when the fracture occurred,the crack propagated along the track boundary. There was a groove structure intersecting with the track boundary in the fracture morphology. When the tensile process continued,dislocations gradually accumulated through the α-lamellae to the track boundary. When the accumulation reached a certain extent,microcracks occurred and separated between α cluster and α phase at the phase boundary.The cracks propagated rapidly along the inter pass boundary,forming intergranular fracture and finally fracture. The similar groove structure also occurred on the fracture surface of the 90° sample,so the inter channel boundary was the weak part of the whole material. The distribution of the track boundary and the interlayer boundary presented a specific angle. The track boundary was basically parallel to the z axis,and the interlayer boundary was approximately parallel to the substrate plane. According to Schmidt’s law,the maximum shear stress was located on the shear plane which was 45° to the tensile direction. When 30° and 45° specimens were stretched,the interlayer and track boundaries were approximately parallel to the maximum shear stress plane,especially for 45° specimens,the interlayer and track boundaries were almost parallel to the maximum shear stress plane. Therefore,the slip was more likely to occur along the interlayer and track boundaries,resulting in the decrease of yield strength and the decrease of tensile strength after the formation of microcracks. The tensile properties of Ti-6Al-4V alloy prepared by SLM process were related to the direction of stress. When the tensile stress was 90° to the substrate,the alloy exhibited the highest yield ratio and the best plasticity;when the tensile stress was 45°to the substrate,the strength was the lowest and the plasticity was acceptable;when the tensile stress was 0° to the substrate,the strength was the highest and the plasticity was the worst. The strength and plasticity of the alloy were better than those of the traditional forging process of the same alloy. α phase at the inter pass and interlayer boundaries was the main factor to reduce the properties of the alloy. The different anti strain coordination ability of α cluster and α phase at grain boundary led to premature crack initiation and intergranular fracture. At the same time,the angle between the stress direction and the inter pass and interlayer boundaries also affected the properties of the alloy.