| 316L stainless steel has good corrosion resistance and mechanical properties,and has application prospects in military,aerospace and medical and health industries.Traditional casting,rolling,machining and other methods have low material utilization rate,and it is difficult to manufacture special-shaped components with complex structures.Selective Laser Melting(SLM)is an advanced additive manufacturing technology that can form complex structural special-shaped parts,with short processing cycles and high material utilization.However,the change of forming process and post-treatment process will significantly affect the corrosion resistance of the material,resulting in poor material stability.Therefore,in this paper,316 L stainless steel forming parts with different process parameters were prepared by selective laser melting forming(SLM),and the samples with the optimal process parameters were heat treated.Through the potentiodynamic polarization curves,electrochemical impedance spectroscopy and electrochemical noise tests of different samples,the pitting behavior of different formed parts in corrosive medium was studied.(1)The pitting corrosion mechanism of 316 L stainless steel formed by selective laser melting and wrought 316 L stainless steel in corrosive medium was studied.Compared with wrought 316 L stainless steel,the microstructure of SLM formed 316 L stainless steel is transformed into an overlapping molten pool structure.And there is no Mn S-Al2O3 type inclusions,LOF holes were found on the SLM samples after increasing the scan speed.Using potentiodynamic polarization curves and electrochemical noise techniques,the pitting corrosion behavior of SLM316 L stainless steel and wrought 316 L stainless steel was compared.The pitting potential of the SLM 316 L sample is higher than that of the wrought sample.Three shapes of EN transients were determined to correspond to pitting pits induced by non-metallic inclusions,porosity,and out-of-fusion(LOF)pores.According to the wavelet and stochastic analysis results,The metastable pitting pit growth rate of the wrought samples is higher than that of the SLM samples,which reduces their pitting resistance.The LOF holes in the SLM samples increase the nucleation rate of metastable pitting pits and the growth probability of metastable pitting pits,resulting in a decrease in their pitting resistance.(2)The effect of heat treatment on the pitting behavior of 316 L stainless steel by selective laser melting was studied.Heat treatment can eliminate the molten pool structure of the sample.However,two kinds of inclusions,Mn-Si-S and Mn-Si,were found in the samples.Compared with the untreated samples,the pitting corrosion potential of the heat-treated samples decreased from 0.762 V to 0.509V(950°C)and 0.244V(1100°C).Statistical analysis of the electrochemical noise signal,Heat treatment results in a decrease in the number of transients and an increase in the amplitude of the transients.The pitting pit nucleation rate of heat-treated samples was lower than that of SLM 316 L samples,while the metastable pitting pit growth rate of SLM 316 L after heat treatment was higher than that of SLM 316 L samples.(3)The effects of remelting and high laser energy density on the pitting behavior of 316 L stainless steel formed by selective laser melting were investigated.After increasing the laser energy density,the number of pores in the sample increased and a deeper key-hole appeared,Mn-Si type inclusions found on remelted samples.Compared with the samples with optimal process parameters,the pitting potential of the samples with high laser energy density decreased from 0.98 V to 0.67 V,and the remelted samples decreased to 0.5V.The number of fast rising and slow falling transient peaks in the electrochemical noise spectrum of the high laser energy density sample increases,similar to the sample containing LOF holes;The number of slow-rising fast-falling transients increased in the remelted samples,similar to the heat-treated samples. |
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