预热对选区激光熔化316L不锈钢力学性能的影响

Effect of Preheating on Mechanical Properties of 316L Stainless Steel Fabricated by Selective Laser Melting

为了提升激光增材制造316L不锈钢的综合性能,比较了室温、预热100℃和200℃三种条件下选区激光熔化(SLM)316L不锈钢试样的组织、拉伸性能、冲击韧性和疲劳性能。结果表明:SLM制备的316L具有显著的各向异性。沿水平面方向和沿垂直方向取样时,试样室温冲击功分别约为轧制板材的47%和44%;室温制备SLM试样承受垂直方向载荷时的抗拉强度约为其承受水平方向载荷时的抗拉强度的81.6%。预热对致密度和冲击韧性的影响很小;预热使显微硬度和抗拉强度略微降低、延伸率略微提高,但总体上变化不显著。在所设置的试验条件下,预热对试样承受垂直方向载荷时的疲劳寿命的影响很小,但使试样承受水平方向载荷时的疲劳寿命得到明显提高。分析认为,预热后试样在疲劳试验中需要经历更多循环载荷加载次数才能达到临界裂纹尺寸。

Objective The fundamental research of the selective laser melting(SLM)technology is a research hotspot at home and abroad.Because of its good weldability,corrosion resistance,mechanical properties,and ductility,316L stainless steel has become one of the most widely concerned SLM materials.The characteristics of the SLM process and its negative effects on fatigue performance have been the bottleneck of popularization and application of the SLM technology.Preheating is a simple and easy method to control the microstructure and properties.It is used for SLM forming of die steel,tool steel,and aluminum alloy with a large cracking tendency,which can effectively reduce the residual stress,warping deformation,and local cracking tendency.However,the effect of preheating on the mechanical properties of the selective laser melted 316L stainless steel has not received much attention.In this paper,the microstructure,microhardness,tensile properties,impact toughness,and fatigue properties of the selected lasermelted 316L stainless steel at room temperature,preheated at 100℃and 200℃are systematically investigated in order to expand the possible ways for improving the comprehensive properties of 316L stainless steel manufactured by SLM.Methods Using the JG-SLM260 selective laser melting equipment and the 316L stainless steel powder prepared by the gas atomization method,the SLM tests of 316L stainless steel are carried out under the conditions of no preheating,preheating at 100℃and preheating at 200℃,respectively.First,the relative density of the samples is measured by the Archimedes drainage method.Then,the cross-sectional microstructure is observed under the Nikon MA200optical microscope.Vickers microhardness on the cross section of the sample is measured by the Qness semi-automatic microhardness tester with the load of 200g and the load retention time of 15s.The tensile test and the Charpy impact test are carried out at room temperature.In the tensile test,the tensile rate is 0.5mm/min.The tensile test and the Charpy impact test are repeated three times in each direction.The room temperature fatigue tests are conducted by using Instron-1341,and the loading stress varies periodically in 150--0 MPa according to a sinusoidal waveform,and the loading frequency is 20Hz.The fracture morphologies of the tensile specimen,the Charpy impact specimen,and the fatigue specimen are observed by the VEGA II XMU tungsten filament scanning electron microscope(SEM).In addition,the properties of the 316L stainless steel are compared between SLM and conventional rolling.Results and Discussions It is found that preheating has little impact on the grain size.Preheating can increase the density slightly and reduce the average Vickers microhardness by about 4%.The relative density of the sample with a preheating temperature of 100℃is the same as that of the sample prepared at room temperature,and the relative density of the sample with a preheating temperature of 200℃is higher by just 0.1%than that of the sample prepared at room temperature.The average hardnesses of the SLM samples at room temperature,preheated at 100℃and preheated at 200℃are 366.1,350.5,and 350.0HV,respectively.The tensile strength of the rolled 316L sheet is 739.12 MPa.In the horizontal tensile direction,the tensile strengths of the SLM samples obtained at room temperature,preheated at 100℃,and preheated at 200℃are 816.58,784.18,and 789.78 MPa,respectively.All of them are higher than the tensile strength of the rolled sheet.In the vertical tensile direction,the tensile strengths of SLM samples obtained at room temperature,preheated at 100℃,and preheated at 200℃are 658.9,625.28,and 636.18 MPa,respectively.All of them are lower than the tensile strength of the rolled sheet.The tensile strength increases slightly by preheating,but not significantly.In addition,the elongation of the SLM-formed 316L sample is about 50%of that of the rolled sheet,and its tensile strength under vertical loading is about 80%of that under horizontal loading.After preheating,the elongation of the 316Lsample formed by SLM in different directions increases by about 6%,and the tensile strength decreases by about 4%.Preheating has little effect on the impact absorbing energy of the SLM sample at room temperature.The impact absorbing energy of the SLM sample sampled horizontally at room temperature is about 47%of that of the hot-rolled 316Lstainless steel,and that of the SLM sample sampled vertically is about 44%of that of the hot-rolled 316Lstainless steel.When sampling in the horizontal direction,the numbers of cycles of the fatigue fracture of the SLM samples obtained at room temperature,preheated at100℃,and preheated at 200℃are 560890,1446426,and 1478941,respectively.When sampling in the vertical direction,the numbers of cycles of the fatigue fracture of the SLM samples obtained at room temperature,preheated at100℃,and preheated at 200℃are 236070,208394,and 218500,respectively.The fatigue life of the SLM samples prepared at room temperature under a vertical cyclic load is about 42%of that under a horizontal cyclic load.Preheating has little effect on the fatigue life of the SLM specimens subjected to a vertical cyclic load.The number of cycles of the sample subjected to the horizontal cyclic load increases obviously due to preheating.The main reason may be that preheating can inhibit the formation of large size defects.Conclusions Mechanical properties of the SLM 316L sample have significant anisotropy.The impact absorbing energy of the sample prepared at room temperature is about 47%and 44%of that of the rolled plate,respectively,when the sampling direction is in the horizontal direction and the vertical direction.The tensile strength of the SLM samples prepared at room temperature under a vertical load is about 81.6%of that under a horizontal load.Preheating has little effect on the relative density and impact toughness.Preheating reduces microhardness by about 4%.After preheating,the tensile strength of the 316L sample formed by SLM decreases by about 4%and the elongation increases by about 6%.Under the experimental conditions in this paper,preheating has little effect on the fatigue life of the sample under a vertical load,but significantly improves the fatigue life of the sample under a horizontal load.The main reason may be that preheating can inhibit the formation of large-size defects.