| Selective laser melting(SLM)is an advanced manufacturing technology. The structure is not limited,the process cycle is shortened,the forming efficiency is increased and the manufacturing cost is reduced by SLM process. This technique is particularly suitable for the manufacture of high-performance complex mold. 4Cr13 is a widely used corrosion-resistant mold steel. Its powder forming by SLM showed good performance. This study expanded research on mechanical properties of 4Cr13 steel formed by SLM. Discussed the tensile properties anisotropy and the effect of heat treatment on tensile properties. Analyzed the causes of the difference on the point of porosity and the pool border and the tensile properties tested directly and after heat treatment on the view of microstructure and fracture morphology. The main contents are as follows:(1) The tensile specimens of 4Cr13 were formed using the optimized process parameters and the tensile properties in the direction of X-Y, X-Z and Z-stack were tested. Along the X-Y plane tensile properties is the worst, the average tensile strength is only 469.1 MPa, extending rate is 3.0%. Stretching showed a brittle cleavage fracture. Along the X-Z plane tensile properties is better, the average tensile strength is only 1118.9 MPa, extending rate is 7.8%. Stretch is quasi-cleavage fracture. Along the Z-stack tensile properties is the best, the average tensile strength is only 1385.5 MPa, extending rate is 7.1%. Fine equiaxed dimples appear on the fracture surface, showing ductile fracture characteristics.(2) Many pores, special pool border, different grain orientation and organizational stress is common in the parts formed by SLM. According to these characteristics, this study analyzed the causes of tensile properties anisotropy. After uniforming the microstructure, removing these features, the tensile properties anisotropy has been reduced. Average tensile strength has become lower then before, showing 1067.9 MPa along the X-Z plane and 1054.6 MPa along the Z-stack. But plastic has greatly improved. Elongation respectively are 19.1% and 17.9%. Fracture are apparent fracture toughness.(3) After different heat treatment, tensile properties were tested along the X-Z plane direction. The results show that strength and ductility are significantly improved. After annealing treatment, uniform pearlite and ferrite generated. Strength was slightly reduced from 1118.9 MPa to 1067.9 MPa, by 4.7%. Plastic was greatly improved. Elongation increased from 7.8% to 19.1%, by 144.9%. 4Cr13 phase transition curves apparently rightward because of Cr. Coarse martensite was formed when was cooled in the air after normalizing and mixed in the pearlite. Uniformity of tensile properties was not improved, but strength was apparently improved to 1385.7 MPa, by 23.8%. Elongation was decreased to 5.9%,by 25.1%. Tempered martensite was formed after tempering treatment and relieve the brittle of quenched martensite. Then Strength and Plastic both are greatly enhanced. Strength was improved to 1837.1 MPa,by 64.2%. Elongation was improved to 13.8%,by 76.2%. After quenching and aging treatment, martensite is strong crisp hard. Strength and plasticity slightly worse than quenching and tempering process. Strength was improved to 1812.1 MPa,by 61.9%. Elongation was improved to 12.5%,by 59.1%.Through this study, the principle of tensile properties' anisotropy of 4Cr13 steel formed by SLM and the effect of heat treatment on tensile properties were obtained. Furtherly, the changes in the internal organization was characterized. This study made a foundation for further research of forming high performance complex mold parts using 4Cr13 by SLM for industrial applications. |
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