Effect Of In-situ Heat Treatment On Microstructure And Mechanical Properties During SLM Forming Of IN738 Alloy

The additive manufacturing technology had unique technical advantages in producing high value-added parts with complex shapes.However,due to the influence of printing process parameters and part geometry,the products underwent different cyclic thermal effects,so they showed obvious differences in microstructure and mechanical properties.In the process of SLM forming,a unique cyclic thermal effect was formed layer by layer,which could produce intrinsic heat treatment on the formed parts,and had the potential to adjust the microstructure and improve the comprehensive mechanical properties.SLM was a multi-scale and multi-physical field coupling process,and there were many factors affecting cyclic thermal effects.Therefore,it was crucial to understand the changes in temperature and stress fields caused by cyclic thermal effects and their effects on microstructure and mechanical propertiesIn this paper,the influence of cyclic thermal effects on the in-situ tissue transformation and precipitation phase precipitation during SLM forming of IN738LC alloy was investigated through the design of SLM printing experiments with different energy densities and interlayer times as well as simultaneous thermal cycling numerical simulations,and the response relationship between temperature and stress field changes and microstructure and mechanical properties was revealed.The research work provided theoretical basis and technical supports for understanding the influencing factors and mechanisms of cyclic thermal effects,optimizing the printing process parameters of IN738LC alloy,establishing a matching heat treatment regime,and promoting the application under actual working conditions,and the following conclusions were drawn:The samples prepared with low volume energy density(EIVED)and long interlayer time(LILT)had the best print quality,the lowest crack density and porosity,the HAGBs grain boundary ratio is 7.8%,the average grain size was 10.97 μm,the grain size above 50 μm was 1.2%,the residual stress/strain was low,and the geometric dislocation density was 4.923×1013 m-2.The crack density and porosity of the samples prepared with high volume energy density(E3VED)and long interlaminar time(LILT)were increased.The proportion of HAGB grain boundaries in the samples was 4.8%,the average grain size was 14.63 μm,and the proportion of grains above 50 μm was 4.52%.The strain accumulation of the samples was increased significantly.The samples had a geometrically necessary dislocation density of 6.104×1013 m-2.Influenced by printing process parameters and component geometry,nano-primary MC in the alloy correspondingly transformed into spherical secondary MC phase in the grain and polygonal secondary MC phase on the grain boundary.The average equivalent size of MC carbide in grain and grain boundary of low volume energy density(E1VED)and long interlaminar time(LILT)samples was 63 nm and 1.05%,respectively.The average equivalent size of MC carbide in grain boundary was 140 nm and 15.07%.The yield strength of the samples was 917 MPa,and the tensile strength and section shrinkage were 1086 MPa and 5.6%,respectively.The average equivalent size of MC carbides in grains of high volume energy density(E3VED)and long interlaminar time(LILT)samples was 47 nm and 1.43%,and the average equivalent size of MC carbides in grain boundaries was 105 nm and 6.08%.The yield strength of the samples was 685 MPa,and the tensile strength and section shrinkage of the samples were 779 MPa and 2.9%,respectively,with a decrease of 28%and 48%.The simulation results of thermal cycle showed that improving energy density would increase the peak temperature of characteristic points,while prolonging interlayer time would decrease the peak temperature under the same energy density;When liquid-solid or solid-liquid transition occurred,stress would be generated,and stress cycle would be induced under the action of thermal cycle.Under the action of laser heat source,the powder melting would not produce stress,and when the heat source was far away,the internal stress would accumulate,as the heat source approached,the internal stress would be released gradually.