Fundamental Study On Multi-beam Selective Laser Melting Of TA15 Alloy

Selective Laser Melting(SLM)technology is one of the most popular research areas in the field of additive manufacturing.Multi-beam Selective Laser Melting(MB-SLM)technology improves the forming size and efficiency of SLM technology,which provides a new route to solve the problem of building large and complex components in aeronautic and aerospace industries.Aiming at the weakness of basic research in TA15(Ti-6.5Al-2Zr-1Mo-1V)alloy formed by MB-SLM technology,the differences in forming qualities,microstructures and mechanical properties of TA15 titanium alloy under single and multi-laser beam conditions were studied systematically using the four-laser beam SLM system developed by our group.In addition,the effect of parameters on processing accuracy of SLMed components was also studied.The main conclusions are as follows:(1)A function model between the linear laser energy and the characteristic sizes of single tracks under conduction mode was established,which can guide the design of SLM processing parameters when the layer thickness is below 0.06 mm.The asynchronous scanning strategy can effectively improve the relative density of overlapped zones formed by the MB-SLM.Within the processing window,lower linear energy is helpful to reduce the pores caused by the trough effect near stitching lines.(2)The microstructure of SLMed TA15 alloys is mainly composed of acicular martensites ?? transformed from columnar prior ? grains.The martensites in the same sample have various sizes and obvious hierarchical characteristics.The average size of martensites increases with the increase of the number of laser beams applied in the SLM process.The martensites have various grain orientations and weak texture in the same sample.(3)The tensile anisotropy exists in the samples fabricated by single laser,where higher tensile strength forms in the horizontal direction and better elongation forms in the vertical direction.The mechanical properties of samples processed in four quadrants independently are consistent,and the tensile strength,yield strength and elongation are in the range of 1244 ~1272 MPa,997 ~ 1038 MPa and 7.7 ~8.7%,respectively.By selecting appropriate overlapped width and scanning direction,the tensile properties of samples fabricated by double laser beams can be as good as those of the samples fabricated by single laser.As for the samples fabricated by four laser beams,the order of the microhardness of XOZ section is as follows: single-beam forming area > double-beam overlap area > four-beam overlap area.After annealing at 900 ?,the SLMed sample can get a good match of strength and ductility.The tensile strength,yield strength and elongation are 1118 MPa,903 MPa and 11.6% respectively,which are beyond the standards for forging(4)The order of the processing parameters influencing in the dimensional accuracy is as follows: laser powder > scanning speed > layer thickness > hatch space.The relationship between the forming error and the laser energy density was established,based on which the precision of components can be controlled by scaling the contour of the model.For the sample under the same processing parameters,the qualities of different surfaces are different.The order of roughnesse values(Ra)is as follows: inclined sidewall > vertical sidewall > upper surface.With the increase of layer thickness and the decrease of slope angle,the roughness of sidewall increases gradually.