The effects of laser scanning speed on the microstructure, microhardness, and corrosion behavior of Ni45 coatings were investigated by using optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness, and electrochemical measurements. The results showed that increasing laser scanning speed promotes the transformation from planar crystals to dendrites and refines the grains concurrently. The γ-(Ni, Fe), FeNi3, and M23(C,B)6 are identified as the primary phase composition in the Ni45 coatings regardless of the laser scanning speed. Thereinto, the formation and growth of M23(C,B)6 precipitates can be inhibited with increasing laser scanning speed due to the higher cooling rate, which affects the microhardness distribution and corrosion resistance of the coating. On the one hand, the microhardness of the whole coating presents a downtrend with increasing laser scanning speed due to the reduction of M23(C,B)6 phase. On the other hand, the corrosion resistance in 0.5 M NaCl solution is improved to some extent at higher laser scanning speed because the less precipitation of M23(C,B)6 reduces the depletion of Cr around the precipitates. In contrast, all the coatings exhibit undifferentiated but poor corrosion resistance in the highly corrosive 0.5 M NaCl + 0.5 M H2SO4 solution.

中文翻译：

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激光扫描速度对激光熔覆 Ni45 涂层的显微组织、显微硬度和腐蚀行为的影响

通过光学显微镜(OM)、扫描电子显微镜(SEM)、X射线衍射(XRD)、显微硬度和电化学测量研究了激光扫描速度对Ni45涂层的显微组织、显微硬度和腐蚀行为的影响。结果表明，提高激光扫描速度促进了平面晶体向枝晶的转变，同时细化了晶粒。无论激光扫描速度如何，γ-(Ni, Fe)、FeNi3 和 M23(C,B)6 被确定为 Ni45 涂层中的主要相组成。其中，提高激光扫描速度可以抑制M23(C,B)6析出物的形成和生长，这是由于较高的冷却速度，影响了涂层的显微硬度分布和耐腐蚀性能。一方面，由于M23(C,B)6相的减少，整个涂层的显微硬度随着激光扫描速度的增加呈下降趋势。另一方面，在 0.5 M NaCl 溶液中的耐蚀性在较高的激光扫描速度下有一定程度的提高，因为 M23(C,B)6 的较少沉淀减少了沉淀物周围 Cr 的消耗。相比之下，所有涂层在高腐蚀性 0.5 M NaCl + 0.5 M H2SO4 溶液中均表现出未分化但耐腐蚀性较差。