Study Of The Residual Stress And Deformation Of GH4169 Fabricated By Selective Laser Melting

Selective Laser Melting(SLM)is a promising process which can fabricate metallic components with highly complex geometries and high performances through a layer-bylayer manner.Because of the high scanning speed and the small spot size of laser beam,the temperature field is extremely ununiform.The high temperature gradient results in the residual stress and deformation,which affect the forming quality,dimensional accuracy and the shape accuracy of the fabricated components.In this thesis,the residual stress and deformation of SLMed GH4169 were studied by using hole drilling method to measure the residual stress and by using digital caliper to measure the deformation along the deposited direction,respectively.The main research contents and the obtained results are as follows:(1)The influence of the laser energy density on the residual stress was investigated.It was found that the residual stress decreased as the laser energy density decreased while ensuring the density of the sample,but this effect weakened as the laser energy density decreased.(2)The influences of substrate characteristics such as introducing pre-compression stress to the substrate and the substrate material on the residual stress were investigated.It was found that the residual stress decreased as the pre-compression stress being introduced to the substrate.In this study,when the pre-compression stress was-143.6MPa,the residual stress decreased 130.4MPa,which decreased 18.7% of the residual stress as compared with that of without pre-compression stress.The residual stress of sample built on GH4169 was the smallest.The reasons are that the high thermal conductivity of 45 steel resultes in the high temperature gradient and the high difference of thermal expansion coefficient between the substrate of 304 stainless steel and the deposited material of GH4169 results in the unmatechable expansion and shrink of the substrate and deposited material.(3)The effects of hatch angle,pre-scanning and remelting on the residual stress were also investigated.The residual stress of the sample fabricated with the hatch angle of 113° was the smallest.The pre-scanning and remelting could reduce the residual stress.The residual stress increasd first and then decreased with the decreasing of the laser energy density of pre-scanning and remelting.When the remelting was applied to the sample,the transient heat treatment and the compression stress introduced by the remelting zone and high temperature zone decreased the residual tensile stress of the newly deposited layer.When the sample was pre-scanned with the low laser energy density,the residual stress released because of the abundant irregular pores.When the sample was pre-scanned with the high laser energy density,the second scanning acted as remelting,resulting in the residual stress decreasing.(4)The deformation and its distribution along the deposited direction of as-built,heattreated and cut sheet samples with different sizes were investigated sysmatically.The SLMed sheet sample showed a concave shape along the deposited direction,e.g.,“C” shape.There were three regions according the magnitude and variation of the deformation: at the bottom of the sample,the deformation was the lowest and the variation was the biggest;at the middle of the sample,the deforation was the highest and the variation was the lowest;at the top of the sample,the deformation and the variation were intermediate among the three of them.The location where the maximum deformation existed was at the top middle of the sample,which moved up with the increasing of the sample hight.The deformation of the as-built sheet sample increased with the increasing of the length,increased first then tended to be stable with the increasing of the height and decreased with the increasing of the thickness.The heat treatment and removal of the substrate only affected the magnitude of the deformation,the distribution and variation of deformation along the deposited direction were not affected.(5)The deformation and its distribution along the deposited direction of as-built inclined plate samples with different sizes were invrestigated.It was found that the inclined plate sample was also in a concave shape along the deposited direction.The tendencies of the deformation of the inclined plate sample with length and thickness were similar to that of the sheet sample,but the deformation increased with the increasing of the height.When the sample size was small,the deformation of the inclined plate sample was larger than that of the sheet sample with the same size.When the sample size was large,the low thermal conductivity of the powder under the inclined plate sample induced the high temperature,the large plastic deformation zone and the small compressive strain within the deformation zone.Furthermore,the number of the deposited layers of inclined plate was samller than that of the sheet sample with the same size,resulting in the deformation of the inclined plate being lower than that of the sheet sample.