Research On Surface Alloying Of Lost Foam Casting Magnesium Alloy

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.