Influence of Heat Treatment on Microstructure and Mechanical Properties of Laser Selective Melting Bimetal Structure Connection Interface
Objective Selective laser melting(SLM)technology offers a wide range of design freedom,high density,and strong metallurgical bonding;thus,it is highly suitable for processing workpieces with complex shapes.A conformal cooling mold formed via SLM can improve the cooling efficiency and decrease the injection cycle time.However,only a few types of mold steel materials are suitable for 3D printing because of long processing time and high costs.First,conventional processes can be employed to manufacture conventional parts;subsequently,complex parts can be built using SLM.With this approach,the manufacturing efficiency can be improved and costs can be reduced.In this study,a new type of 3D printing die steel material(AM40)is deposited on a commercial H13 substrate using SLM.The effects of heat treatment(HT)on the microstructure and mechanical properties of AM40/H13 bimetallic structural materials are studied,and the deformation and cracking behaviors of the bimetallic molds are revealed.Methods In this study,AM40 steel powder and annealed H13 steel sheets are used.SLM is used to deposit AM40 onto the H13 substrate.Subsequently,quenching and tempering are performed to study the effects of the heat treatment.The particle size distribution is characterized using a laser particle size analyzer,whereas the microstructure and fracture morphology are characterized using optical microscope(OM)and scanning electron microscope(SEM).The grain morphology,orientation,and local misorientation of the bonding interface are characterized using electron backscatter diffraction(EBSD).Additionally,a Vickers microhardness tester is employed to measure the microhardnesses of the as-built and heat-treated samples.Tensile tests are performed using a fatigue testing machine.Results and Discussions No crack defect is observed at the interface of the as-built AM40/H13 bimetallic structure and the unique Marangoni molten pool at the interface(Fig.7).Moreover,fine cellular and columnar martensite structures are observed in the AM40 region(Fig.8).The microstructure of H13 is coarsened austenite(Fig.5),and the distinct microstructural inhomogeneity is observed at the bonding interface.After quenching and tempering,the characteristics of the molten pool disappear,and uniform lath martensite microstructures form in the H13 region(Fig.8).The inhomogeneity of the grain size and misorientation at the interface are eliminated(Fig.10).Moreover,the diffusion width of element at the interface increases from 440 μm to 500 μm(Fig.9).Additionally,the hardness of the as-built AM40/H13 at the bonding interface is 642 HV,which is higher than those of AM40(529 HV)and H13(202 HV).The average hardness of HT-AM40/H13 at the bonding interface decreases to 480 HV(Fig.11),thus indicating that the hardness difference between AM40 and H13 is eliminated by the heat treatment.The tensile strength of HT-AM40/H13 increases significantly from 644 MPa to 1436 MPa(Fig.12).Furthermore,some dimples,along with a cleavage pattern,are observed in the fracture(Fig.14),thus indicating that the fracture mode is a combination of ductile and brittle.The increase in the tensile strength and ductility of the heat-treated AM40/H13 bimetallic alloy is analyzed based on the microstructure and fracture morphology of the bonding interface.Conclusions In this study,the as-built AM40/H13 bimetallic structure does not exhibit crack defects at the interface,and the microstructure is heterogeneous.Marangoni convection and cellular and columnar structures are observed in the weld pool at the interface.The alloying elements are evenly distributed at the interface,thus indicating good metallurgical bonding.After heat treatment,the grain size and dislocation density near the interface are similar,thus eliminating the inhomogeneity of the interface structure.The elements at the interface diffuse,and the diffusion width increases by 60 μm.The hardness at the as-built AM40/H13 bimetallic H13 side is the lowest(202 HV),followed by that at the AM40 side(529 HV);by contrast,the interface hardness is the highest(642 HV).Tensile deformation and cracking of the bimetal preferentially occur at the H13 side,with a strength of 644 MPa and fracture elongation of 29％,thus indicating ductile fracture.After heat treatment,the hardness of H13 increases to 483 HV,which is equivalent to that of AM40(479 HV)after heat treatment,and the inhomogeneity of the hardness is eliminated.In addition,the tensile strength of HT-AM40/H13 increases significantly from 644 MPa to 1436 MPa,which is between those of AM40 and H13.The fracture is preferentially located at the AM40 side,far from the interface.Further,some dimples and cleavage patterns are observed,thus indicating that the fracture mode is a combination of ductile and brittle.
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