Investigation On Residual Stress Behavior And Control Methods In Selective Laser Melting Of 316L Stainless Steel

The recent developments in manufacturing industry require agile,delicate and flexible fabrication techniques that can meet the demands of present era.Through conventional techniques,the complex 3D geometrical features required to meet current demands are difficult to attain.Additive manufacturing processes on the other hand can be appropriate alternative which can overcome these issues.These additive manufacturing processes can produce components by customized approaches without increasing the production costs and hence change the existing manufacturing models.Selective laser melting(SLM)is one such powder bed fusion based additive manufacturing process that can produce full density 3D complex metallic components with high accuracy and low material losses.SLM is the most widely used additive manufacturing technique for various metallic materials and many components have been mass produced through SLM.Nevertheless,SLM process still require further improvements to enhance the process efficiency and produce superior components.In particular,the SLM process induced residual stresses should be investigated in detail as they can significantly influence the component accuracy,life and performance.Eliminating or minimizing these residual stresses in SLM fabricated components can effectively eradicate workpiece distortion,prevent crack initiation and propagation,and extend the component life span.At present several researchers have conducted investigations on the prediction and control of SLM generated residual stresses however,the research on SLM process induced residual stresses in 316L stainless steel,a commonly used metallic material for SLM is still in preliminary states and many aspects needs to be further revealed and investigated.This dissertation focuses on the residual stress generation,evolution mechanism and control techniques in SLM fabricated 316L stainless steel to improve the quality of SLM fabricated 316L stainless steel components.Therefore,a mesoscale finite element method(FEM)model has been established to reveal the residual stress generation mechanism in SLM.In the established FEM model,enhanced thermal conductivity approach,layer built up with element life and death technique,continuum material modeling,FORTRAN based phase change subroutines and other techniques have been used which can effectively enhanced the accuracy as well as the resolution of residual stress generation simulations.The residual stress findings of numerical model were also compared and verified with experimental values measured for samples fabricated at identical process and size parameters.The complicated thermal phenomena happening during the SLM fabrication process are the main controlling factor that govern the generation of residual stresses.Therefore,initially different thermal variables in SLM process of 316L stainless steel have been studied by the established FEM model.The effects of parameters i.e.,laser power,scanning speed,and hatch spacing on thermal variables of melt pool temperature,heating rate,cooling rate,melt pool life time,melt pool width and melt pool depth have been investigated and relationships were established.These relationships can provide basic foundation for the understanding of the influence of SLM parameters on residual stresses.Through uncoupled thermo-mechanical simulation of SLM fabrication process,residual stress behavior in SLM of 316L stainless steel and its variation with different processing parameters have been studied.The influence of laser power,scanning speed,hatch spacing,scanning pattern,scan vector length and scanning sequence on residual stress was investigated.A flexible modeling approach i.e.,adopting different model dimensions for laser-based parameters investigation and scanning based parameters investigation instead of unified model dimensions,have been used to ensure high resolution of the numerical model.Afterwards,correlations between the SLM parameters and residual stress have been identified.In addition to the uniform stress values,variation of equivalent residual stress,X and Y stress components along different axial directions and locations have been discusses as well.Since the magnitude of SLM induced residual stresses are considerably high,it is difficult to minimize or eliminate them entirely by just changing the SLM process parameters.Therefore,auxiliary stress reduction and homogenization techniques used to eliminate SLM induced residual stresses have been numerically investigated through the established FEM model.For a wide range of preheating temperature,influence of baseplate preheating and powder bed preheating on residual stresses have been studied.Moreover,the influence of in-situ rescanning on the residual stress of SLM fabricated 316L stainless steel components has also been numerically explored.The influence of rescanning on residual stress and mechanical behavior have been experimentally investigated as well.Through XRD phase analysis and microstructural variations,the residual stress reduction characteristics after rescanning were observed.The effects of rescanning on mechanical behavior of SLM fabricated 316L stainless steel components in terms of hardness and tensile properties were also observed.Post processing stress relief treatments have been studied as well and the influence of different post processing treatments on the mechanical properties have been discussed.The SLM fabricated 316L stainless steel components were deep cryogenic treated i.e.,immersed in liquid nitrogen at-196? for 24 and 48 hours to investigate the residual stress change characteristics.The results stated that the residual stress values were reduced by 23%and 28%by deep cryogenic treatment for 24 hours and 48 hours respectively.In order to explore the effect of post annealing cooling conditions,SLM fabricated 316L stainless steel samples were kept at 1050? for 1 hour followed by air cooling(high cooling rate)and furnace cooling(slow cooling rate)to access the influence of different cooling conditions.A reduction of 27%and 42%in residual stress was observed by air cooling and furnace cooling respectively.At the same time,the influence of aforementioned post fabrication treatments on the mechanical properties i.e.,hardness and tensile behavior of SLM fabricated 316L stainless steel samples were also researched.