Study On Phosphating Process Of Mg-Zn-Y-Ca Alloy Applied For High-Speed Railway’s Swing Bolster

With the rapid development of the railway transportation industry,lightweight and high-speed have become an important symbol of modern high-speed trains.As a key component of the EMU,the structure of the swing bolster of the high-speed EMU is complex,and it is generally made of aluminum alloy.Lightweight trains can be realized if magnesium alloy,currently the lightest metal structure material,can be used instead of the existing material.However,due to the poor corrosion resistance of magnesium alloy,the application of magnesium alloy is largely limited.Therefore,according to the structural characteristics of the swing bolster,this paper proposes anti-corrosion measures to use surface phosphating treatment of the inner cavity of the swing bolster.Using Mg-Zn-Y-Ca alloy for swing bolster as matrix material,the phosphating film was prepared by two kinds of phosphating processes,and the mechanism of phosphating film formation in different systems was described.The effects of phosphating solution formula,phosphating temperature,phosphating time,rare earth additive concentration and silicate sealing process parameters on corrosion resistance and microstructure of zinc and calcium phosphating processes were investigated by orthogonal test and single factor variable method.And the optimal phosphating process parameters were determined.The results showed that the optimized zinc phosphating process parameters were as follows:p H=2.5,Zn O=9g/L,H₃PO₄=23ml/L,C₄H₆O₆=1g/L,Na F=1.5g/L,Na NO₃=7g/L,phosphating tempeture T=50℃,phosphating time t=20min;The optimized calcium phosphating process parameters were as follows:p H=3.5,Ca O=10g/L,H₃PO₄=60ml/L,Ca（NO₃）₂=14g/L,C₆H₄O₅NSNa=1g/L,Na₂Mo O₄=1g/L,phosphating tempeture T=50℃,phosphating time t=20min.Both the phosphated compounds of the two systems can increase the corrosion resistance of the Mg-Zn-Y-Ca alloy,and the calcium phosphating film is better than the zinc phosphating film.The addition of rare earth additives can increase the content of phase on the surface of phosphating film,improve the surface morphology and enhance the corrosion resistance of the film.The optimal addition amount of Ce（NO₃）₂in zinc and calcium phosphating solution is 1.6g/L and 0.8g/L,respectively.After rare earth treatment,a new petal-shaped structure was formed on the basis of the original structure of the zinc phosphating film.The phase of the film mainly included Zn₃（PO₄）₂·4H₂O and a small amount of elemental Zn.The calcium phosphorus film is made of fine needle-like tissue to the compact,structurally dense block tissue.The phase of the film mainly included Ca HPO₄·2H₂O and a small amount of Ca₃（PO₄）₂.The optimal silicate sealing process parameters are:Na₂SiO₃=70g/L,sealing temperature T=70℃,sealing time t=10min.After the phosphate film of the two systems is sealed,Na₂SiO₃and Mg SiO₃are formed on the surface of the film.These two new phases fill up the surface defects of the film and further improve the corrosion resistance of the film.The optimal corrosion resistance of Mg-Zn-Y-Ca alloy samples can be obtained by sealing after calcium phosphating,with the average drop time of 1002s,the corrosion rate of 0.0372mg/cm²·h,and the corrosion current density of 4.971×10^-6A/cm²,and the dense layer resistor R_fis 4854Ω·cm².Its corrosion resistance is compliant with the anti-corrosion criteria for the existing swing bolster.Therefore,this study laid the theoretical and experimental basis for the actual application of magnesium alloy lightweight swing bolster,with strong engineering application value.