热处理对SLM-316L不锈钢组织结构及钝化行为的影响机制

对SLM-316L不锈钢在900℃下进行不同时间的热处理，结合热处理后SLM-316L不锈钢的组织结构和腐蚀行为研究，揭示了SLM-316L不锈钢在900℃热处理过程中组织结构的演变规律以及其对钝化行为的作用机制.研究结果表明，900℃热处理时，在组织结构方面，SLM-316L不锈钢晶粒的基本形状和尺寸没有明显变化，但是随着保温时间延长，SLM-316L不锈钢中的位错和亚晶界逐渐消失，同时伴有MnS颗粒物沿晶界析出；在耐蚀性能方面，热处理对SLM-316L不锈钢的耐蚀性能产生重要影响，在含有NaCl的缓冲溶液中，SLM-316L不锈钢的点蚀电位随着保温时间延长逐渐降低，同时电化学阻抗逐渐减小；此外，在钝化膜性质方面，不同热处理时间试样上形成的钝化膜有明显差异，随着保温时间延长，SLM-316L不锈钢钝化膜的厚度逐渐减小，载流子的密度以及扩散系数变大.最后，通过构建不锈钢钝化膜能带结构和空间电荷层的理论模型，讨论了热处理对SLM-316L不锈钢钝化行为的影响机制.

Effect of heat treatment on the microstructure and passive behavior of 316L stainless steel fabricated by selective laser melting

Selective laser melting (SLM), a rising additive manufacturing technology, has extensive application potential because of its advantage in fabricating components with individual and complex shapes. During the SLM progress, the laser molten pool cools very quickly, leading to non-equilibrium microstructure formation and high thermal residual stress in the SLM components. Therefore, a suitable heat treatment is required to reduce the residual thermal stress and obtain excellent mechanical properties after the SLM process. In this paper, the 316L stainless steel fabricated by SLM (SLM-316L SS) is first heat-treated at 900 ℃ for 0, 0.5, 1.0, 3.0, and 5.0 h. Then, the microstructures of SLM-316L SS treated at different times are investigated using SEM, TEM, and EDS, and their corrosion resistance is estimated through electrochemical measurements. Finally, the microstructure evolution of SLM-316L SS during heat treatment at 900 ℃ is discussed based on the experimental data, and the effect of the microstructure on the formation kinetics and properties of the passive film on SLM-316L SS is explained. The results of the microstructure analysis reveal that the dislocations and sub-grain boundaries in SLM-316L SS disappeared with increasing holding time, accompanied by the precipitation of MnS inclusions, carbides, and σ phases along the grain boundaries. The potentiodynamic polarization in the buffer solution with 0.1 mol·L-1 NaCl reveals that a sample with a longer holding time shows a more negative pitting potential. According to the EIS test results, the shape of the curve in Nyquist diagram is not completely circular. The calculated film thicknesses decrease with increasing holding time. The potentiostat polarization under 0.1, 0.2, 0.3, 0.4, and 0.5 V vs SCE was used to form passive films on SLM-316L SS after heat treatments. By fitting the Mott-Schottky curves, the negative slopes demonstrate that the passive films formed on the samples are an n-type semiconductor, and the calculated point defect densities in the sample increase with the holding time. In addition, a logarithmic relationship holds between the carrier densities and the formation potentials of the passive films, and, using this relationship, the calculated diffusion coefficient of point defects across the passive film of SLM-316L SS increases with the holding time. A theoretical model related to the energy band structure and space charge layer is obtained based on the Mott-Schottky results to explain the electrochemical reaction on the passive film/solution interface.