【作者】 张雄；

【导师】 董选普；

【作者基本信息】 华中科技大学 ， 材料加工工程， 2011， 硕士

【摘要】 镁合金是一种轻质结构材料,被广泛应用于各领域,但由于耐蚀性差,其大规模应用受到限制。消失模铸造作为一种净无余量的绿色铸造技术,在成形镁合金铸件方面有独特优势。为改善镁合金铸件的防腐性能,本文采用消失模铸造表面合金化的方法对镁合金进行表面改性处理,并得到了一些有意义的结论。首先,本文对消失模铸造镁合金表面铝元素合金化进行了研究。单因素试验研究表明:一定工艺条件下,浇注温度及真空度的阈值分别处于750~780℃及0.02~0.04MPa之间,只有当两工艺参数取值大于其对应阈值时,镁合金表面才能形成合金化层。通过正交优化试验得到最优工艺条件:真空度为0.07~0.075MPa,粉体粒度为150μm,外层涂料为1层,粘结剂聚乙烯醇(PVA)含量为2.44%,浇注温度为780℃。该工艺下镁合金表面形成了厚度约为2mm、组织覆盖率接近100%的致密均匀合金化层。铝合金化层的形态呈菊花状或网状,主要由Mg17Al12、Mg和Al等物相组成。只有当组织覆盖率大于临界值约37%时,镁合金表面铝合金化层才能起到防腐作用,且组织覆盖率越高,镁合金耐蚀性越强。本实验最优工艺条件下镁合金表面合金化层的自腐蚀电位为-1.0628V,自腐蚀电流下降了两个多数量级。其次,本文对消失模铸造镁合金表面锌及铝锌复合合金化进行了探索。结果表明,在浇注温度为720℃或铸件模数为6.3mm的条件下,镁合金表面能形成以Mg17Al12、Al5Mg11Zn4、Zn、Mg为主要组成物相的具有一定防腐性能的锌合金化层。而当铝锌混合比例为3.5:1或铸件模数为10.4mm时,镁合金表面也能形成一层主要物相为Mg17Al12、Al、Zn、Mg的铝锌复合合金化层,腐蚀极化试验研究表明其自腐蚀电位相对AZ91D合金有所提高,自腐蚀电流下降一个多数量级,达到防腐蚀的效果。最后,以表面铝合金化为对象,建立了消失模铸造镁合金表面合金化模型,从理论上推导了表面合金化热力学及动力学条件,结果表明:热力学上,欲满足合金化过程的吸热量要求,镁合金熔体在铝熔点以上的降温幅度应至少为合金化层厚度的409.2倍;动力学上,镁合金的渗入深度与背压、粉体粒度、凝固时间、润湿角、金属液粘度及静压头等因素有关,且一定工艺条件下,真空度及粉体粒度均存在理论临界值。更多还原

【Abstract】 As a kind of lightweight structural material,magnesium alloys have been widely used in various fields. However, the corrosion resistance of magnesium alloys is very poor, which limits their large-scale application. Lost foam casting, a kind of near net-shaped green casting technology, is very suitable for magnesium alloys. In order to improve the corrosion resistance of magnesium alloy castings, this paper developed the method of lost foam casting surface alloying and made some meaningful conclusions.Firstly, surface aluminum-alloying of lost foam casting magnesium alloy was studied. Single factor experiments indicated that under certain processing conditions the threshold value of pouring temperature was between 750℃and 780℃, and that of vacuum degree was between 0.02MPa and 0.04MPa. Only when the pouring temperature and vacuum degree were greater than the threshold value, could the surface alloying layer be formed. The orthogonal and optimization experiments were applied to get the optimal processing conditions which were vacuum degree 0.07~0.075MPa, particle size 150μm, outer coating 1 layer, the content of PVA 2.44% and pouring temperature 780℃. In this processing, a dense and uniform alloying layer was formed on the surface of the magnesium alloy, and its thickness and microstructure coverage was about 2mm and 100% respectively. The surface aluminum-alloying layer whose main phases were Mg17Al12, Mg and Al showed in chrysanthemum or mesh structure. Only when the microstructure coverage was greater than the critical value of about 37%, could the surface alloying layer be anti-corrosive. And the higher the microstructure coverage was, the stronger the corrosion resistance of the magnesium alloy could be. Under the optimal processing conditions, the corrosion potential of the magnesium alloy was -1.0628V and the corrosion current decreased by more than two orders of magnitude.Secondly, surface zinc-alloying and aluminum-zinc-alloying of lost foam casting magnesium alloy were investigated. The results showed that when the pouring temperature was 720℃or the modulus of casting was 6.3mm, an anti-corrosive zinc-alloying layer which mainly consisted of Mg17Al12, Al5Mg11Zn4, Zn and Mg could be formed on the surface of the magnesium alloy. And when the weight ratio of Al-Zn was 3.5:1 or the modulus of the casting was 10.4mm, an aluminum-zinc-alloying layer whose main phases were Mg17Al12, Al, Zn and Mg could be also formed on the surface of the magnesium alloy. Corrosion polarization test showed that the corrosion potential of the aluminum-zinc-alloying layer was higher than that of AZ91D, and the corrosion current decreased by more than one order of magnitude.Finally, the surface alloying model of lost foam casting magnesium alloy was established based on the surface aluminum-alloying. And the surface alloying thermodynamic and kinetic conditions were deduced theoretically. The results showed that in order to meet the heat absorption requirement of the surface alloying process in thermodynamics, the dropping margin of the temperature of molten magnesium alloy which must be higher than the melting point of aluminum should be at least 409.2 times greater than the thickness of surface alloying layer. In kinetics the infiltrating depth of magnesium alloys was related to the factors of back pressure, particle size, coagulation time, wetting angle, viscosity and static head of liquid metal. And under certain processing conditions critical values of vacuum degree and particle size were existed in theory.更多还原