| Selective Laser Melting(SLM)is one of the mainstream technologies in additive manufacturing today.It has a very high degree of design freedom and can form various complex structures that cannot be produced by traditional processing methods.The SLM parts have good precision and performance,do not require post-processing,and are suitable for the manufacture of single-piece and small-batch products.However,the characteristics of rapid prototyping by selective laser melting lead to a great thermal gradients and extremely high cooling rates during the forming process,and improper process parameters will lead to a large number of defects in SLM parts,which greatly affect the performance of the formed parts.Fracture is one of the common failure forms,especially in the key parts with defects,sudden fracture will cause very serious consequences,so it is necessary to study the fracture property of the parts fabricated by selective laser melting.In this paper,316 L stainless steel was used as raw material to prepare compact tensile specimens.Orthogonal experiments were designed to study the effects of process parameters(laser power,scanning speed,powder thickness and scanning strategy)on the fracture properties of SLM 316 L stainless steel samples.The defects,microstructure and fracture morphology of SLM samples were analyzed.The main conclusions are as follows:The relationship between load and crack opening displacement of fracture parameters was obtained by tensile test and digital image correlation technique.Within the currently selected range of parameters,most SLM samples exhibited greater fracture loads and smaller crack opening displacements at the same load compared to conventionally machined 316 L stainless steel sample,indicating enhanced fracture property.The results of range and variance analysis through orthogonal experiments show that the effect of process parameters on the fracture properties of 316 L stainless steel samples fabricated by SLM follows the order of laser power,layer thickness,scanning strategy and scanning speed.The fracture performance was optimal at the process parameters with a laser power of 200 W,a scanning speed of 500mm/s,a layer thickness of 0.03 mm and a parallel scanning strategy.The surface morphology,internal defects,microstructure and fracture morphology of SLM316 L stainless steel samples were analyzed by scanning electron microscope.The results show that the formation and lap quality of the melt track are mainly related to the energy input density,because it affects the surface tension and wettability of the fluid in the molten pool.The defects in the SLM samples mainly include air holes and incomplete fusion,and the content of defects is positively correlated with its fracture performance.The grains of SLM 316 L stainless steel samples are mainly columnar dendrites.There is epitaxial growth of columnar dendrites,and the grain growth direction in the molten pool is complex,mainly related to the crystal orientation and local heat flow direction.We also found sub-micron-level substructures that do not exist in the traditional mechanical parts under a high magnification scanning electron microscope.The substructures are mainly columnar and honeycomb shapes.They are actually the shapes of a structure at different angles.Adjusting the process parameters to make them cross can effectively reduce the anisotropy of the SLM parts.When there are a few defects in the formed simples,the grain size of the microstructure can be used as the evaluation basis of the fracture property.The smaller the grain size of the microstructure,the better the fracture property of the formed simple.The fracture surface of SLM 316 L stainless steel samples shows obvious necking phenomenon,and there are dimples and cleavage steps on the surface of the fracture,indicating that the fracture is a mixture of ductile fracture and brittle fracture or transgranular fracture.In addition,there are a lot of defects on the surface of the samples with poor fracture performance. |
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