| Magnesium alloy is the lightest metal structural materials,which has low density,high specific strength and specific stiffness,and is considered as "the green engineering material of the 21st century".Using high performance magnesium alloy parts is an effective way to realize the lightweight of automobile and aircraft.At present,large and complex thin-walled magnesium alloy castings have been applied in the parts of aircraft engines.Due to the advantages of lightweight and high strength,engine weight is reduced and trust-weight ratio is increased.However,there are still some problems in the preparation of the casting technology for thin-walled magnesium alloy castings.Problems are mainly:(1)The filling problem of complex thin-walled magnesium alloy castings is solved by improving the fluidity of magnesium alloy;(2)The mechanical properties of thin-walled magnesium alloy castings often fail to meet service requirements due to the problems of the microstructures and defects,and the low corrosion resistance limits their applications;(3)The preparation period of the complex structure sand core is long,and the size accuracy of the sand core is difficult to control.(4)The optimization processes of the casting process and heat treatment process of thin-walled castings are complicated,long period and costly.Since 201 2,the idear of "material genetic engineering"has been gradually applied to the research and development of magnesium alloys and their components.Among them,Integrated Computing Materials Engineering aims to integrate the tools of computational materials science and the simulation softwares of material forming process into a whole system,which can quickly predict and optimize the material composition,processing process and product performance,and gradually change from "Traditional Design"model to "Predictive Design" model.Therefore,it is an urgent need to develop complex thin-walled magnesium alloy castings with good quality and standard performance by efficient research and development mode.In this paper,magnesium alloy castings for aircraft engines are studied by theoretical analysis,numerical simulation and experiment test.Taking ZE41 magnesium alloy as a research object,the effects of alloying elements Ca and Sr on the microstructure,fluidity and mechanical properties are investigated.The effects of heat treatment processes on the microstructure,mechanical properties and corrosion resistance are investigated.The microstructure and mechanical properties of heat-treated magnesium alloys are predicted by building the "heat treatment process/composition-microstructure/mechanical property" neural network and regression analysis models for ZE41 magnesium alloys.The gating system of gearbox casting are optimized by simulating the filling process,solidification process and shrinkage porosity.By combining the scripting language with the command-line operation of the simulation software,an integrated computing platform for the "process-microstructure-properties" simulation is constructed.The microstructure and mechanical property are predicted quickly by using the integrated computing platform,optimizing the casting process parameters and heat treating technology.Finally,the complicated sand cores and moulds are fabricated by Profile Failure-based Rapid Prototyping(PFRP)technology.Based on the optimized composition,casting process and heat treatment process,the magnesium alloy gearbox casting are acquired,which possesses good internal and external quality and mechanical properties meeting the service requirements(ultimate tensile strength≥200MPa,yield strength≥135MPa,elongation≥2%).Specific contents are as follows:(1)The effects of alloying elements Ca and Sr on the microstructure,fluidity and mechanical property of ZE41 magnesium alloy are studied.The results indicate that when the content of Ca is 0.2 wt.%,the average grain size of the as-cast alloy is reduced from 48.4μm to 35.9μm,and the concentric triple helix filling length is increased from 101mm to 157mm.However,the improvement in mechanical properties is not significant.On this basis,the effect of minor Sr on the microstructure,fluidity and mechanical property of the as-cast ZE41-0.2Ca alloy is investigated.The results show that when the content of Sr is 0.2 wt.%,the average grain size of the as-cast alloy is reduced from 36.7μm to 31.3 μm,and the filling length is increased from 157mm to 225mm.The main reason for the improvement in the fluidity is:fine rounded grains increase the critical solid fraction of the dendrites when they form a strong skeleton,which can postpone the dendrite coherency point.Moreover,Ca has a strong affinity with O to form a dense oxide film on the surface of the melt,reducing oxide inclusions in the melt and increasing the fluidity of the alloy.In addition,the as-cast ZE41-0.2Ca-0.2Sr alloy possesses the optimal mechanical properties.(2)The ZE41-0.2Ca-ySr(y=0,0.1,0.2,0.4 wt.%)alloys are aged at 325℃,which exhibits good age hardening ability.Minor Ca and Sr are effective in accelerating the kinetics of the initial ageing-hardening process and delaying the dynamics of the over-aged process.The peak-aged alloys consist of a-Mg matrix,T-phase,Mg51Zn20 phase,minor interdendritic Mg2Ca phase,and intragranular Zn-Zr particle phase.Rod-like β’1 and disccoid β’2 precipitates are obtained in the matrixs of peak-aged alloys.Due to the effective pinning effect of the β’1 precipitate,the peak-aged ZE41-0.2Ca-0.2Sr alloy obtains the highest ultimate tensile strength and yield strength.According to electrochemical test,weight loss measurement,hydrogen evolution test and corrosion morphology analysis,corrosion resistance of the alloys is improved by adding minor Ca and Sr.The peak-aged ZE41-0.2Ca-0.2Sr alloys obtains the optimal corrosion resistance,which is mainly due to the corrosion blocking effect of the second phase is greater than the galvanic corrosion effect.(3)Based on the experimental data about the heat treatment microstructure and mechanical properties,input variables are Ca and Sr contents,aging temperature and aging time,and the outputs are average grain size,ultimate tensile strength,elongation and microhardness.The BP neural network model is established by the artificial neural network method.The optimal BP network model is obtained by optimizing the number of hidden layer nodes.The "heat treatment process/composition—microstructure/mechanical properties" regression analysis model are obtained by regression analysis.The regression model provides a mathematical model for multicriteria optimization program.The heat treatment process and alloying element content when the tensile strength is the highest and the grain size is the smallest are acquired by the multicriteria optimization program.(4)The filling process,solidification process and shrinkage porosity are simulated to optimize and design the vertical slitting gating system.By combining the scripting language with the command-line operation of the simulation software,an integrated computing platform for the "process-microstructure/defect-property" simulation is constructed,realizing the full-process simulation calculation from CAD model data input to "process-microstructure/defect-property" simulation.Among them,the "casting process—microstructure/defect" model is implemented using the pre-processing,solving and post-processing modules,while the"microstructure—property" model is implemented using MATLAB.In addition,the integrated computing platform software batch calls MATLAB,realizing the integrated optimization calculation of "heat treatment technology/composition-microstructure/property" for the gearbox casting.The heat treatment process optimization based on the mechanical property prediction is realized.The complicated sand cores and moulds are fabricated by Profile Failure-based Rapid Prototyping(PFRP)technology.Based on the optimized composition and casting process,the "near net shape,zero defect casting" magnesium alloy thin-walled gearbox casting is acquired,which possesses good internal and external quality and the mechanical properties meeting the service requirements. |
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