| In this paper,a new type of composite flame retardant is designed and prepared to protect the cast magnesium alloy from oxidative combustion during filling and solidification.The application of the composite flame retardant in different resin sand systems is discussed.In order to produce ZM6 magnesium alloy hydraulic valve for a certain aviation engine with high efficiency,the ProCAST simulation software was used to optimize the process parameters in the low-pressure casting process.And inkjet printing technology was used to prepare flame-retardant resin sand molds,finally the hydraulic valve castings with high quality were prepared successfully.The composite flame retardant was designed on the basis of three flame retardant mechanisms and prepared by a blending method.The influence of the amount of flame retardant on the mechanical properties and gas generation of inkjet printing flame retardant sand mold was studied.The research results indicates that the 24h tensile strength of sand mold is 1.146MPa,the flexural strength is 2.199 MPa,the amount of gas evolution is 8.88ml/g,and the gas permeability is 158.8% when the amount of flame retardant added in sand mold is 0.8% of the sand weight.At the same time,0.8% composite flame retardant is added into PEP-SET and Triethylamine resin sand and the performance of sand mold can meet the requirements of the casting.The sand-cast ZM6 alloy sample prepared by flame-retardant sand mold with inkjet printing has better surface quality than that of the no-added sand mold and the same amount of boric acid sand mold.The XRD results of the sample surface indicate that the oxide phase were Nd2O3 and MgO,the content of oxide phase of the casting prepared by adding 0.8% composite flame retardant sand mold was low;the EDS results of the sample surface show that the sand casting with 0.8% composite flame retardant has the lowest oxygen content.To realize the manufacture of a ZM6 magnesium alloy hydraulic valve,low-pressure sand casting process was used and a box of eight pieces of the central sprue pouring system including conformal top feeders and exhaust systems was designed.The quality requirements of castings such as dimensional tolerances and weight tolerances were clarified.The structure of the pouring system was optimized by using ProCAST simulation software.The filling and solidification processes of the optimized pouring system were simulated at the pouring temperatures of 740?,750?,and 760?,and the filling stability of the pouring system at different pouring temperatures was compared.Solidification process and shrinkage prediction were also done.It is found that the filling of the entire pouring system is the most stable at 750?,and the pouring temperature must be controlled strictly at 750? during the pouring.According to the optimized low-pressure casting process parameters,the casting process card was formulated,and the flame-retardant sand mold and core were prepared by inkjet printing technology.The casting was subjected to T6 heat treatment.There was no burning during casting.The results of the dimensional inspection of the casting showed that the overall structure of the casting was complete and the dimensional accuracy reached CT7.EDS and XRD inspection results showed that there were no metallurgical defects on the surface and inside of the casting.The microstructure observation and mechanical property inspection show that the casting has a good microstructure and high mechanical properties,which meets the standards of aviation parts.The above analysis and test results show that the casting meets the requirements for using.The composite flame retardant developed in this paper,the optimized flame retardant addition ratio,and the optimized low-pressure casting process can provide an experimental basis for the manufacture of thin-walled magnesium alloy castings with multiple pipes. |
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