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. © Editorial Office of Chinese Journal of Rare Metals. All right reserved.
