| Maraging steel is a kind of fe-Ni martensitic die steel with low carbon content.It not only has excellent ductility and toughness,but also has good welding and machining properties.It is widely used in aerospace,atomic energy and high-end die and other important fields.Selective laser melting(SLM)as a typical metallic additive manufacture technology has many attractive advantages over traditional manufacturing method,and is regarded as the flag of a new round of industrial revolution.This work takes grade 300 maraging steel(MS)as the research object,and uses SLM technology to prepare high-performance maraging steel from three aspects: paramenter optimization,ceramics reinforced MS metallic matrix composites and heat treatment.The main contents include:(1)The optimum process parameters of SLM forming MS were studied.Firstly,the causes of SLM forming parts defects were analyzed,and the process parameters needed to be optimized were determined.Finally,the systematic optimization of process parameters was designed to obtain high density maraging steel.The effects of laser power and scanning speed on the morphology and size of molten track were studied by single track test.The optimal range of laser power and scanning speed was selected according to the two conditions of continuous and smooth molten track,and verified by single factor boundary test.Then,under the premise of keeping the laser power and scanning speed unchanged,the single factor test of hatch spacing was carried out.The results show that the spatter decreases first and then increases with the increase of the hatch spacing,and the reasonable range of the hatch spacing should be greater than 50μm and less than 90μm according to the number of spatter and the overlap rate.Finally,the effects of various factors and their interactions on the density of MS samples were analyzed by orthogonal-response surface test,and the optimal process parameters were obtained: laser power 150 W,scanning speed 793.7mm/s,scanning distance 65μm.Under this parameter combination,the actual density of MS samples was up to 99.80%.(2)The surface morphology,microstructure and mechanical properties of SLM formed SiC/ maraging steel composites(MMCs)were investigated.The results suggest that with the increase of SiC volume fraction,the swelling and spheroidization of the top surface are aggravated,the surface roughness and pores become larger,the side powder adhesion is serious,the solidification structure changes from cellular crystals to columnar crystals and then to dendrites,and the samples of hardness and tensile strength are higher than those without SiC powders.When the vol.% of SiC powders is0.5,the samples of hardness and tensile strength are the highest,reaching 371.1 HV and1.51 Gpa respectively,which are 7% and 54.8% higher than that of the sample without SiC powders.Therefore,although the addition of SiC powders increases the top surface roughness and porosity,it can refine the grain and increase dislocation density to improve mechanical properties,and then enhance the comprehensive performance of the sample.(3)The effects of different heat treatments on the microstructure,microhardness and tensile strength of MMCs samples formed by SLM were studied.The results show that the solution treatment,solution and aging treatment and aging treatment all change the original microstructure,the grain structure is replaced by fine lath martensite,and the precipitates at the boundary of the melt channel dissolve and dissipate.After solution treatment,the original cellular lattice crystal structure is transformed into white oxidized particles,but the number of white oxidized particles can be reduced or eliminated by aging treatment and increasing the volume fraction of SiC.The mechanical properties of the samples after the three heat treatments were compared and analyzed.It was found that the solid solution treatment would increase the grain size and reduce the mechanical properties of the samples.The hardness and tensile strength of the samples would be significantly increased by the precipitation of strengthened phase after the solid solution and then the aging treatment,while the direct aging treatment would be improved again on the basis of the solid solution and aging treatment.Among them,the hardness and tensile strength of M0.5 sample after direct aging treatment reach 615.26 HV and 2.316 Gpa,respectively,which are increased by64.75% and 53.4% respectively before specific heat treatment,and further increased by9.55% and 15.8% compared with solution and aging treatment. |
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