| In recent years,with the increase of per capita car ownership,road transport carbon emissions account for 70% to 80% of transportation.Studies have shown that reducing vehicle weight can effectively reduce vehicle exhaust emissions.As the lightest metal structural material,magnesium and its alloys have become the main research object of automobile lightweight.In order to meet the requirements of automobile lightweight design and reflect the low density advantage of magnesium alloy,this paper takes AM60 alloy and new magnesium alloy AM60-0.2Ca-0.2Y(wt.%)(AMXW)as the research object to study the die casting process,mechanical properties and corrosion behavior of high pressure die casting(HPDC)magnesium alloy car cross beam(CCB),so as to provide theoretical basis and technical reference for the development of new HPDC magnesium alloy.The main work in this paper are as follows:The mold filling process of liquid metal in CCB pressing chamber was studied by using Pro CAST software,and the acceleration position of HPDC punch was controlled to680 mm to reduce the entrainment of molten metal.In order to verify the effect of different production processes on the casting results of CCB,the die casting process of CCB was optimized by orthogonal experiment.The results show that the injection speed is 5.3 m/s,the shortest filling time is 0.054 s,the pouring temperature of liquid metal is 660 ℃,the shortest solidification time is 48.33 s,the mold preheating temperature is 230 ℃,the minimum hot crack volume is 67.98 cc,the pouring temperature,mold preheating temperature and injection speed have little effect on the shrinkage cavity.Combining the simulation results with the trial production results,the process parameters suitable for the production of HPDC AMXW alloy CCB are as follows: pouring temperature 680 ± 5 ℃,mold preheating temperature 230 ± 5 ℃,injection speed 4.8 ± 0.1 m/s.The finished product rate of CCB castings produced by this process is 90%,and the HPDC CCB is only4.18 kg.Comparing the microstructure of CCB produced by different die casting processes,it was found that lower pouring temperature and higher mold preheating temperature lead to abnormal growth of primary grains in the central region of castings.When the injection rate is 5.3m/s,the coarse primary grains of molten metal diffuse to the surface of castings.The mechanical properties of CCB produced by different high pressure die casting processes were studied.The mechanical properties of die castings can be improved by increasing pouring temperature and decreasing mold preheating temperature.The mechanical properties of die castings are the best when pouring temperature is 700 ℃ and mold preheating temperature is 190 ℃.Based on the fact that the vehicle was easily affected by low-order vibration frequency when driving,the first-order modal frequency of AMXW alloy HPDC CCB is 212 ± 1 Hz,and the average stiffness of CCB under dynamic impact is575.37 N/mm,which accords with the practical application condition of CCB.The corrosion resistance of magnesium alloy was tested by immersion,hydrogen evolution,salt spray,weight loss and electrochemical tests.It was found that the addition of Ca and Y elements to gravity casting(GC)AM60 alloy could improve the corrosion resistance of magnesium alloy.HPDC process can improve the overall corrosion resistance of the alloy.Compared with GC,HPDC process can reduce the grain size by 5 to 10 times,make the second phase continuously distributed at the grain boundary,and improve the corrosion resistance of AMXW alloy by more than 6 times.However,HPDC AM60 alloy is prone to local corrosion,and the surface corrosion defects are mostly pitting.HPDC AMXW alloy is prone to large-area shallow corrosion. |
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