| Magnesium alloy has many advantages over other metal materials,and has a broad application prospect in the aerospace industry.It is very suitable for the application in the shell,damping system and other components of aerospace vehicles.In the process of precision casting,magnesium alloy complex castings are easy to produce defects such as insufficient pouring,air entrainment,cold shut,hot spot,shrinkage porosity,etc.,which make it difficult to form.The research object of this paper is magnesium alloy satellite support,which is made by gravity casting and investment casting.The satellite support is a combination of frame and mesh structure.The thickness of the thin wall is only 3mm,the structure is compact,casting is difficult,wax mold forming is difficult,and deformation is easy to occur.In this paper,photosensitive resin was selected as wax mold material,and the wax mold was prepared by 3D printing light curing molding(SLA).After the completion of the wax mold preparation,the shell was prepared.There was a high activity reaction between magnesium and oxide ceramics for investment mold materials and bonding materials,and it was easy to oxidize and burn.In this study,considering the particularity of magnesium alloy investment casting,a high-performance ceramic shell was prepared for investment casting of magnesium alloy satellite support.The surface layer adopts zirconia and boron nitride refractories with particle size of 325 mesh,the middle layer and back layer adopt mullite refractories with wide source,low price,convenient preparation and good comprehensive performance.It can prevent the interface chemical reaction between magnesium alloy and shell material and ensure the surface quality of casting.Combined with the structural characteristics of the satellite support and the design principles of all kinds of pouring systems,this study proposes three different pouring system schemes: slot type,ladder type and deluge type,and uses the finite element simulation software Pro CAST to carry out the numerical simulation of investment casting for the three schemes respectively,and to carry out various physical fields in the process of filling and solidification,such as flow field,temperature field and solidification defects The distribution is simulated and analyzed.The results show that the yield of step process is the highest,38.18%.The defect volume of the deluge type is the least,which is 0.12cm3.Based on the defect results of three kinds of gating systems,the ladder gating system and deluge gating system with less defects were optimized,and the actual production gating was carried out.The structure of the exhaust port was designed reasonably,considering the influence of the air entrapment on the quality of the casting.The optimal scheme of pouring system is rain pouring riser.Considering the influence of liquid metal temperature,mold shell temperature and pouring speed on the quality of castings,the orthogonal test was designed,and the optimal process plan was found as follows: pouring speed of liquid metal 75mm/s,preheating temperature of mold shell 250 oC and pouring temperature of liquid metal 740 oC.The results show that after solution treatment at 500 oC and 6h,the second phase dissolves basically,and the grains do not grow up obviously;after aging treatment at 220 oC and 12 h,Mg-Y binary compound precipitates out in the grains of the alloy,showing dispersion distribution.After heat treatment,the strength of the casting is increased by about 50% to over 300 MPa,and the elongation is not less than 10%.The properties of the casting meet the requirements of the product. |
PDF Full Text Request