| The rapid development of industrial technology has promoted the manufacturing industry to gradually develop towards smart,green and personalized directions.As a typical metal additive manufacturing technology,laser selective melting(SLM)technology has the advantages of high density of formed parts,high forming size and surface accuracy,and excellent mechanical properties of formed parts.Therefore,it is very suitable for processing parts with complex shapes.The conformal cooling mold can improve the cooling unevenness of the traditional cooling system,and can effectively improve the cooling efficiency and the quality of the formed parts.However,the internal waterway shape is complex,and the traditional processing technology is difficult to achieve.SLM has a considerable advantage in the preparation of conformal cooling molds by additive manufacturing and layer-bylayer forming.It can greatly improve the utilization rate of materials,but the cost is a little high.Besides,there are few types of high-strength die steel powder which is suitable for SLM processing.Consequently,in this paper,18Ni300 maraging steel(abbreviated as MS1)is selected as the research object,and according to the different carbon equivalents,NAK80,S136,H13,17-4ph are selected as the base material for the bimetal printing forming research.The purpose of this article is to study the influence of printing process parameters on the interface structure and performance,and to carry out the heat treatment strengthening research of the formed bimetallic parts to promote the industrial production application of 3D printing on high-strength die steel bimetallic materials.The specific contents are as follows:(1)We carry out some laser remelting strategy research on the interface transition layer to obtain a good metallurgical bonding and defect-free interface.Based on this,the process parameters of laser printing on the interface transition layer are optimized.The results of the best printing process parameters are: the first powder layer is scanned twice by laser,and then each the laser is scanned once in each layer.And the printing process parameters of the laser power of 300 W,the scanning distance of 0.12 mm,the powder thickness of 50?m,the scanning rate of 720mm/s can get the highest interface connection strength when the laser energy density at this time is 69.4J/mm3.The overlap rate between the layers of the molten pool near the interface is 28%,which is close to 30% of the stable forming part of MS1.(2)Under the optimal forming process parameters,the length of the element diffusion region near the interface is affected by the difference of carbon equivalent between the base material and MS1.The lager the difference,the longer the element diffusion region.With the increase of scanning rate,the tensile properties of the connection interface show a trend of firstly increasing and then decreasing,and reach the maximum when the optimal forming process parameters are reached.The maximum strength of the interfaces of NAK80-MS1,H13-MS1,S136-MS1 and 17-4ph-MS1 are 1342 MPa,953MPa,1022 MPa and 1203 MPa respectively.The hardness test results show that the hardness value near the interface is the highest,but there is a tremendous unevenness.The morphology of the interface junction is affected the carbon equivalent of base materials.As the carbon equivalent increases,the interface changes from a fine needle-like structure to a cell-like structure.(3)After the solution + tempering + aging treatment,the matrix structure has changed and a large number of dispersion-strengthened second phases have precipitated.The stability and uniformity of the structure have increased,and the overall mechanical properties of the sample have also been greatly improved.The ultimate tensile strength of the NAK80-MS1 is increased to 1364 MPa,the H13-MS1 is increased to 1716 MPa,the S136-MS1 is increased to 1914 MPa,and the 17-4phMS1 is increased from to 1314 MPa,while the elongation of the four samples are all decreased.The hardness of NAK80-MS1 reached 42 HRC,the overall hardness of H13-MS1 and S136-MS1 samples reached more than 48 HRC,and the minimum hardness of 17-4ph-MS1 reached 44 HRC.The fracture analysis results show that the fracture behavior of the specimen after heat treatment has changed from toughness to brittleness.(4)Comparative analysis of the mechanical performance data of the overall heat treatment of the bimetallic materials formed by SLM and the printed sample after heat treatment strengthening are carried out.The results show that the tensile strength of the overall heat-treated samples is slightly higher than the samples after heat treatment strengthening and then printing,but the elongation is reduced.In general,the performance difference is not very huge.The comprehensive mechanical properties of the sample after the overall heat treatment can be basically equivalent to that of the sample after heat-treated and then printed.In addition,this treatment method can also eliminate the residual thermal stress inside the material and increase the service life of the mold. |
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