铝含量及前处理对镁合金及其PEO膜显微结构和耐蚀性能的影响

发布时间：2018-07-28 17:34

【摘要】：镁合金密度较小,只有1.7 g/cm3。因此,它作为最轻的金属之一,在航空航天以及汽车、电子等对材料轻量化要求较高的领域具有日趋广泛的应用。然而,耐腐蚀性差作为瓶颈一直制约其发展以及应用。目前,为了提高镁合金耐腐蚀性能,设计新型镁合金以及表面处理是近年来采用的主要手段。添加了适量铝的镁合金在压铸性、疲劳性能、压缩性和强度等方面性能获得提高。然而铝元素作为镁合金中主要的合金元素,其对镁合金及其等离子体电解氧化(PEO)膜的显微结构及耐蚀性能影响的相关研究很少。本论文系统地研究了铝含量对镁合金及其PEO膜显微结构及耐蚀性能的影响,主要内容如下:(一)研究了共晶点附近的三种镁合金AZ91,AZ111和AZ141(亚共晶和过共晶结构)在3.5wt.%NaCl水溶液中的腐蚀行为。显微结构分析表明,铝含量影响了β相和共晶相(α+β)的含量和分布。不同铝含量的镁合金显微结构对耐蚀性能影响表现为:1、共晶相的增加促进了α基相的腐蚀。2、β相的含量和分布是影响镁合金耐蚀性能的主要因素,其含量影响腐蚀程度,其分布影响腐蚀进程;随着铝含量的增加,β相的体积分数增大,电偶腐蚀加速效应起主导作用。在腐蚀过程中,β相的网状分布,表现出一定的腐蚀阻挡作用;但是由于β相分布的不连续性,腐蚀总会沿着β相分布不连续处向α相继续腐蚀。(二)以不同铝含量的镁合金为基体,进行PEO膜层制备,并系统地研究了基体的显微结构、膜层的显微结构、膜层的耐蚀性能三者之间的关系。结果表明,在PEO过程中,基体的显微结构对其膜层显微结构影响显著,主要原因为基体中的铝含量影响了β相及共晶相(α+β)的含量和分布,不同相的含量以及分布对PEO放电过程影响显著,从而影响了膜层的显微结构包括致密内层和疏松外层的成分、含量以及膜层的孔隙率。腐蚀发生在膜层显微结构缺陷的位置。(三)前面的研究结果表明,在两相合金中,不同相的成分差异会导致合金表面电化学性质不同,在PEO过程中放电不稳定,导致介质阻挡层的显微结构缺陷。而介质阻挡层的形成是PEO过程中击穿放电进行的必要条件,其质量的好坏对放电的质量有直接影响,影响PEO过程中等离子体放电的稳定性及后续成膜能力。因此,考虑采用前处理的方法使镁合金的表面活性均匀。采用HF超声活化AZ91镁合金,在镁合金表面形成一层低溶解性的薄膜,使镁合金表面活性均匀一致。结果表明表面状态均匀的镁合金形成的致密层质量好,在PEO过程中放电稳定,后续成膜质量好:致密,均匀,没有显微结构缺陷。因此耐蚀性能显著提高。
[Abstract]:The density of magnesium alloy is small, only 1.7 g / cm ~ 3. Therefore, as one of the lightest metals, it has been widely used in the fields of aerospace, automobile, electronics and so on. However, poor corrosion resistance as a bottleneck has been restricting its development and application. At present, in order to improve the corrosion resistance of magnesium alloys, the design of new magnesium alloys and surface treatment is the main means used in recent years. The properties of magnesium alloy with proper amount of aluminum in die casting, fatigue, compressibility and strength were improved. However, as the main alloy elements in magnesium alloys, the effect of aluminum on the microstructure and corrosion resistance of magnesium alloys and their plasma electrolytic oxidation (PEO) films is seldom studied. The effect of aluminum content on microstructure and corrosion resistance of magnesium alloy and its PEO film was studied systematically in this paper. The main contents are as follows: (1) the corrosion behavior of three kinds of magnesium alloys AZ91, AZ111 and AZ141 (hypoeutectic and hypereutectic structure) near eutectic point in 3.5wt.%NaCl aqueous solution has been studied. Microstructure analysis shows that aluminum content affects the content and distribution of 尾 phase and eutectic phase (伪尾). The effect of microstructure of magnesium alloy with different aluminum content on corrosion resistance is as follows: 1. The increase of eutectic phase promotes the corrosion of 伪 -base phase. The content and distribution of 尾 phase are the main factors affecting the corrosion resistance of magnesium alloy, and the content of 尾 phase affects the corrosion degree of magnesium alloy. With the increase of aluminum content, the volume fraction of 尾 phase increases, and the accelerated effect of galvanic corrosion plays a leading role. In the process of corrosion, the net distribution of 尾 phase shows a certain corrosion blocking effect, but because of the discontinuity of 尾 phase distribution, the corrosion will continue to corrode the 伪 phase along the discontinuous distribution of 尾 phase. (2) the PEO film was prepared on magnesium alloy with different aluminum content, and the relationship between the microstructure of the substrate and the corrosion resistance of the film was systematically studied. The results show that in the PEO process, the microstructure of the substrate has a significant effect on the microstructure of the film. The main reason is that the content of aluminum in the matrix affects the content and distribution of 尾 phase and eutectic phase (伪尾). The content and distribution of different phases have a significant effect on the discharge process of PEO, thus affecting the microstructure of the film, including the composition of the dense inner layer and the loose outer layer, the content of the film and the porosity of the film. Corrosion occurs at the location of the microstructural defects in the film. (3) the results of the previous study show that in the two-phase alloy, the different phase composition will lead to different electrochemical properties of the alloy surface, discharge instability in the PEO process, leading to the microstructure defects of the dielectric barrier layer. The formation of dielectric barrier layer is a necessary condition for breakdown discharge in PEO process. The quality of dielectric barrier layer has a direct impact on the quality of discharge. It affects the stability of plasma discharge and the ability of subsequent film formation in the process of PEO. Therefore, the pretreatment method is considered to make the surface activity of magnesium alloy uniform. A low solubility film was formed on the surface of AZ91 magnesium alloy by HF ultrasonic activation, which made the surface activity of magnesium alloy uniform. The results show that the dense layer formed by magnesium alloy with uniform surface state is of good quality, the discharge is stable in PEO process, and the quality of subsequent film formation is good: dense, uniform and without microstructure defects. Therefore, the corrosion resistance is improved significantly.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2016
【分类号】：TG174.4

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