热处理对激光选区熔化Ti6Al4V合金电化学腐蚀行为的影响

Effect of Heat Treatment on Electrochemical Corrosion Behavior of Selective Laser Melted Ti6Al4V Alloy

通过激光选区熔化技术制备沉积态的Ti6Al4V合金,并在800℃下进行热处理,优化组织结构,探索两种状态下Ti6Al4V合金在3.5%(质量分数) NaCl溶液中的电化学腐蚀行为。结果表明,沉积态的Ti6Al4V合金相组织主要由细针状α’马氏体相以及初生β相组成;而经热处理后,转变为板条状的α相+剩余β相,组织更为均匀、单一。电化学测试结果表明,两种状态的Ti6Al4V合金在3.5%NaCl溶液中均发生自发钝化。热处理对合金表面阴极过程没有明显影响,但通过降低阳极反应速率使开路电位及自腐蚀电位升高,自腐蚀电流密度下降至沉积态的1/5。两种状态下Ti6Al4V合金表面均可形成致密钝化膜,且热处理后表面形成的钝化膜更致密、更厚。热处理后合金表面极化电阻值约是沉积态的3.8倍,表明钝化膜保护性更为优异。这一结果表明,热处理可显著优化合金组织结构,提高激光熔化技术制备的Ti6Al4V合金耐蚀性。在较为苛刻的服役环境中,建议采用热处理态的合金作为结构件。

Ti6Al4V alloy was selectively laser melted(SLM) to produce the deposited alloy and which was subsequently heat treated at 800 ℃ to optimize its microstructure. The electrochemical corrosion behavior of the as deposited and post-heat treated Ti6Al4V alloys was investigated in 3.5%(mass fraction) NaCl solution. The results show that the as deposited alloy composes of dominantly acicular α’-martensite and some prior β-grains. After heat treatment, the microstructure transforms to a combination of lath-like α-phase and residual β-phase. Both alloys exhibit spontaneous passivation in 3.5%NaCl solution. Based on the polarization behavior, it is evident that the influence of heat treatment on the cathodic process is insignificant. However, the anodic reaction rate is greatly reduced, which results in the significant decrease of corrosion rate and ennoblement of open circuit potential and corrosion potential. In both cases, a protective film on the alloy surface is confirmed. However, the passive film formed on the heattreated alloy is much thicker and compact. The polarization resistance of Ti6Al4V alloy after heat treatment is 3.8 times higher than that of the as deposited ones. Our results suggest that the as deposited Ti6Al4V alloy produced by SLM should undergo suitable heat treatment in order to prolong its service life in corrosive circumstances.