Microstructure Simulation And Mechanical Property Control Of Inconel 718 Alloy During Laser Powder Bed Fusion And Heat Treatment

Laser powder bed fusion(L-PBF)is a representative metal additive manufacturing technology.Compared with traditional castings and forgings,components fabricated via L-PBF have a finer grain structure and substructure,which improves their mechanical properties.However,the high cooling rate and temperature gradient during the L-PBF lead to columnar grain epitaxial growth in the componets.For engine hot-end components like discs and shafts,the coarse columnar grains have an adverse effect on the serviceability of the component.Therefore,the control of solidified microstructure during laser powder bed fusion is essential for the high-quality manufacturing of componets.The differences in microstructure results in that the stardard heat treatment for castings and forgings may not be suitable for Inconel 718 fabricated via L-PBF.It is necessary to develop a new heat treatment.Inconel 71 8 superalloy,which is widely used in aerospace field,is taken as the research object.An integrated calculation method combining temperature field model,multiphase field medel,Langer-Schwartz-Kampmann-Wagner(LSKW)model,and precipitation strengthening model was proposed in this paper to investigate the microstructure evolution with a large number of laser scanning and heat treatment parameters.The process windows of laser sanning and heat treatment of Inconel 718 alloy were quickly and efficiently to control the microstruction and mechanical property.The main results are as follows:To refine the solidification structure,suppress the epitaxial grain growth,and and prevent lack-of fusion(LoF)defects,a coupled computational model combining temperature field simulation of molten pool and grain structure simulation of phase field was proposed.In the phase field method,a LoF model was proposed to describe the LoF defects caused by insufficient remelting.Based on this model,the critical conditions for the occurrence of LoF defects were determined.The evolution of temperature field and grain structure with different laser scanning parameters were simulated to realize simultaneously the control of defect mitigation and grain-structure.From the simulation,when the interlayer rotation angle was 0～90°,the input laser energy density for the as-deposited structure with fine grains and without LoF defects varied from 55.0～62.5 J/mm3.The optimized process parameters(laser power of 280 W,scanning speed of 1160 mm/s,and rotation angle of 67°)for grain refinement and LoF-free were computationally screened through model calculation and experimental validation.The non-equilibrium solute capture parameters in the solidification model established via phase-filed method for the simulation of the microstructure of asdeposited Inconel 718 sample were determined.Based on the established solidification medel,the microstructure evolutions during homogenization and solution were simulated referring to the standard heat treatment(AMS 5383)proposed for castings.Phase-field simulation results show that the solutes distributions were uniform,and most of the Laves phase dissolves while retaining the morphology of deposited grain structure at the homogenization time of 0.5 h(H’),which have achieved the homogenization purpose.When the solution time was 20 min(S’),the volume fraction of δ phase was about 2%,which is mainly distributed at the grain boundaries.Under this condition,the high-temperature tensile fracture was a mixed fracture with gliding fracture as the main mode,and some cracks propagated through grains.The broken δ phase particles at the grain boundary improve the strength of grain boundary at high temperature.The optimized homogenization(H’)-solution(S’)parameters and TC-PRISMA(LSKW model)were used to reveal the size and volume fraction evolution of strengthening phases with different aging parameter and determin the optimal aging parameters.It was found that the critical length Lc was 21.2 nm for the coherent strengthening mechanism and Orowan strengthening mechanism of γ" phase.According to the nonlinear variation trend of the coherent strengthening and Orowan strengthening with increasing of size of γ" phase,an aging heat treatment(A’:720℃×12 h+FC×2 h+620℃×8 h+AC)with high strength and meets the plasticity requirements(12%)of AMS5663M was determined.Compared with the standard HSA heat treatment conditions,the ultimate tensile strength and yield strength of samples processed with the above H’S’A’ heat treatment were increased by 11%and 19%,respectively.