Study On Thermal Behavior And Microstructure Of H13 Tool Steel Fabricated By Laser Additive Manufacturing

As an important component of laser additive manufacturing(LAM)technology,selective laser melting(SLM)technology based on powder laying can realize rapid near net forming of high-performance complex metal parts,which has been applied in many fields and gradually become a new method of tool and die manufacturing.In this paper,H13 steel,a typical hot work tool steel,was taken as the research object,the complex thermal behavior of molten pool during SLM process of H13 tool steel was systematically studied based on numerical analysis method,and the influence and regulation mechanism of laser processing parameters on metallurgical defects,densification,microstructure and metallurgical bonding were systematically discussed based on experimental research method.The purpose of this paper is to provide theoretical basis and technical guidance for preparing H13 tool steel by SLM,and to promote the promotion and engineering application of this technology in tool,die and mould manufacturing industry.The main research contents and conclusions are as follows:(1)Based on ABAQUS software,a set of user subroutines were compiled,and a 3D finite element analysis(FEA)model for SLM process of H13 tool steel was developed to systematicallys study the complex thermal behavior of molten pool.The simulation results show that the powder material experiences a complex and rapid heating and cooling iterative behavior with heating and cooling rates as high as 106-107?/s during SLM process.The temperature distribution,structure evolution and thermal evolution behavior of the molten pool are significantly affected by laser power and scanning speed,but less affected by other laser processing parameters such as powder layer thickness and hatch spacing.(2)The metallurgical defect behavior,densification behavior and microstructure characteristics of H13 tool steel fabricated by SLM were studied.The formation mechanism of typical metallurgical defects in the forming process was discussed.And the regulation mechanisms of laser processing parameters on metallurgical defects,densification and microstructure were revealed.The results show that the SLM process is prone to induce some metallurgical defects such as pore,crack,lack of fusion or poor fusion.Among them,the cracks mostly originate from the side edges of the formed parts.With the growth of the main cracks,some secondary cracks would be induced to prevent the main cracks from continuing to grow.And the porosity defects could induce microcracks.The internal metallurgical defects and densification behavior are significantly affected by laser power and scanning speed.And the density initially increases and then decreases with increasing the applied laser power or scanning speed.Under the optimized laser processing parameters P=200 W and v=1000 mm/s,the H13 die steel samples fabricated by SLM with nearly full-dense cross-section have almost no defects,and the density is as high as 99.13%.The microstructure induced by SLM is composed of equiaxed crystals and columnar crystals with complex morphology and non-uniform distribution in the molten pool.There are obvious differences in the morphology,size and growth of the microstructure at different positions in the molten pool,and the columnar crystals have obvious epitaxial growth characteristics.(3)The influence of laser processing parameters on laser remelting behavior and premelting behavior during SLM process was quantitatively studied.The formation and regulation mechanism of metallurgical bonding were revealed.And the influence mechanism of laser volumetric energy density(FED)on metallurgical bonding property was discussed.It is found that the remelting behavior and premelting behavior are highly sensitive to laser power and scanning speed.The peak temperature of the remolten pool,remelting dimension,remelting index,premelting dimension,premelting index and lap ratio of the molten track are positively correlated with the applied laser power and negatively correlated with the applied scanning speed.When P?160 W or v?1500 mm/s,an effective metallurgical bonding could be obtained.The powder layer thickness mainly affects the laser remelting to the adjacent scanning layer,the hatch spacing mainly affects the laser remelting and premelting to the adjacent scanning track,and the substrate preheating temperature has little effect to laser remelting and premelting.Under the optimized energy density VED=111.1 J/mm3,there are no obvious interface defect between the adjacent scanning tracks or layers,and a good metallurgical bonding is obtained.