ZM6铸造镁合金表面斑点缺陷形成机理分析

Formation Mechanism of Spot Defects on the Surface of ZM6 Magnesium Alloy

目的为探究ZM6铸造镁合金表面斑点缺陷产生的原因及其形成机理。方法通过扫描电子显微镜(SEM)对未经机加工和经机加工后的ZM6铸造镁合金表面不同形态的斑点缺陷进行形貌分析,并结合能量色散X射线谱(EDS)对斑点缺陷的化学成分进行检测,最后耦合合金铸态组织和力学性能进行综合分析。结果未经机加工的ZM6铸造镁合金表面斑点缺陷呈深黑色,而经机加工后的ZM6铸造镁合金表面斑点缺陷呈白色,且部分白色斑点中心有一小块黑色区域。不过未经机加工和经机加工后的ZM6铸造镁合金表面斑点缺陷都呈现龟裂状、其与基体的过渡界面处均未发现任何空隙存在,且其主要成分均为Mg的氧化物或氢氧化物、同时含有Cl、K等由溶剂引入的杂质元素,但未经机加工的ZM6铸造镁合金表面斑点缺陷中Cl元素的含量相对较高,而经机加工后的ZM6铸造镁合金表面斑点缺陷中K元素的含量相对较高。值得指出的是,经机加工后的ZM6铸造镁合金表面白色斑点中黑色区域存在微小气孔。结论ZM6铸造镁合金表面斑点缺陷的产生是由Mg的化学腐蚀及电偶腐蚀造成的,而不是由氧化夹杂造成的。因此,通过加强ZM6铸造镁合金精炼效果以获得化学成分均匀的熔体是获得高质量构件的有效途径。

Objective This paper aims to explore the causes and formation mechanism of spot defects on the surface of ZM6 magnesium alloy.Methods Spot defects with different morphologies on the surfaces of unmachined and machined ZM6 magnesium alloys were analyzed by scanning electron microscopy(SEM).Simultaneously,the chemical compositions of these spot defects were identified by energy dispersive X-ray spectroscopy(EDS).Finally,a comprehensive analysis was carried out combining the as-cast microstructure and mechanical properties of the alloy.Results The results showed that spot defects on the surface of the unmachined ZM6 magnesium alloy were deep black,while the spot defects on the surface of the machined ZM6 magnesium alloy were white,and there was a small black area in the center of some white spots.The spot defects on the surfaces of both unmachined and machined ZM6 magnesium alloys were cracked,and no voids were found at the transition interface with the substrate.The main components were oxides or hydroxides of Mg,while also containing impurity elements such as Cl and K introduced by the solvent.The content of the Cl element was relatively high in the surface spot defects of unmachined ZM6 Mg alloy,while the content of the K element was relatively low in the surface spot defects of machined ZM6 Mg alloy.It’s worth noting that there were tiny pores in the black area of the white spots on the surface of the machined ZM6 magnesium alloy.Conclusions It can be concluded that the spot defects on the surface of ZM6 magnesium alloy are caused by chemical and galvanic corrosion of Mg,rather than by oxidation inclusions.Therefore,strengthening the refining effect of ZM6 casting magnesium alloy to obtain a melt with uniform chemical composition is an effective way to obtain high-quality components.