Study On Technology Of The Expendable Pattern Shell Casting Process With Vacuum And Low Pressure For Aluminum (Magnesium) Alloy

With the rapid development of aerospace and automotive industry, the application of larger complicated and thin-walled aluminum and magnesium alloys precision castings are greatly increased. Currently, die casting, permanent mold casting, sand casting, investment casting and lost foam casting (LFC) are usually employed for manufacturing the aluminum and magnesium alloys components. However, these casting processes are difficult to simultaneously meet the requirements of larger size, complicated and thin-wall, precision forming as well as high quality.This study combines foam pattern preparation of LFC, shell precision fabrication of investment casting and vacuum and low pressure casting technology, and the expendable pattern shell casting process with vacuum and low pressure (EPSC-VL) for aluminum (magnesium) alloys is developed to produce the larger complicated and thin-walled aluminum (magnesium) alloys precision castings and possesses some advantages, which are flexible design, low cost and shrinkage of foam pattern, high shell precision of investment casting as well as better formability of vacuum and low pressure casting. Some key technologies of this new casting process including materials and process of shell fabrication based on foam pattern, shell cracking mechanism in removing process, forming process optimization as well as microstructure and mechanical properties characteristics, etc. were investigated in this paper, and a set new technology for producing larger complicated and thin-walled aluminum (magnesium) alloys precision castings was developed.The foam patterns with high quality were obtained by using foaming molding technology in order to make a better preparation for the shell fabrication. The new method by means of coating the wax based organic smooth agents outside foam pattern to improve the surface quality of foam pattern was firstly proposed. Influences of surface quality of foam pattern, ceramic shell and wettability between metal and mold on the surface quality of castings were investigated. The results show that the surface quality of castings is greatly improved by using foam patterns with high density, coating with high quality, silica sol as well as finer powder to fabricate shell. In addition, the surface quality of castings can also be increased by means of pouring under vacuum level and pressure casting and increasing shell discharge port, etc.According to analyzing microstructure and properties of shell prepared by using different powders and silica sol and discussing the influence of powder solution ratio on properties of coating and shell, the key materials and process of shell fabrication were investigated, and the optimized process of compound shell fabrication was processed. Moreover, the modification study of silica sol shell was also carried out. The results indicate that with the powder solution ratio increasing, viscosity, density and smear weight of coatings are increasing gradually, meanwhile shell strength is also increased incessantly. The suitable powder solution ratio for alumino-silicate and silica sol coating is 2.2-2.4, for zircon and silica sol coating is 3.2-3.4. The three-layer compound thin-shell was adopted with zircon and silica sol as surface coating, alumino-silicate and silica sol as transitional coating as well as alumino-silicate and sodium silicate as back-up coating. This compound shell has better surface quality, higher strength, shorter preparation time, lower cost and exuviating easily. The modification study of silica sol shell shows that PVA should be added into stucco materials, and this modification method can improve stability of coating, increase room temperature strength of shell and shorten drying time of shell.The material characteristics of foam pattern including loss weight, vitrification transition, producing-gas quantity, producing-gas velocity and volume change as well as removing pattern process were synthetically investigated, and the mechanism of shell cracking in removing foam pattern process was discussed. The removing-roasting integrative process was proposed. The results indicate that producing-gas quantity, producing-gas velocity and volume change of foam pattern are gradually increased with increasing of density of foam pattern, and the foam pattern with higher density is prone to leading to shell cracking. The density of foam pattern should be controlled at 0.05-0.065 g/cm3. The shell mould is not easily cracked when the pattern removing process is carried out with the furnace being heated little by little. The shell mould is soon destroyed when it is set directly into the furnace at above 400℃temperature because of thin shell mould rapidly shrinking and foam pattern hindering. Moreover, when the shell mould is directly set into the furnace at lower temperatures 250 to 300℃, the shell shrinks slowly and the shell mould would experience an expansion force from the foam pattern for a long time. However, the shell mould has no cracking when it has been preheated at 70℃for a long time even if the furnace temperature is above 400℃and the shell is put into the furnace directly. The expansion force is related to removing pattern temperature, holding temperature time and foam pattern size. The removing- roasting integrative process is as follows. Firstly, foam pattern is removed at 250℃for 30 min with the furnace being heated little by little, and then temperature is increased to 500℃for holding 30 min to democompose foam pattern. Lastly, temperature is increased to 800℃holding 60 min to roast shell.Influence of process parameters on the filling ability and internal quality of A356 aluminum alloy in EPSC-VL was investigated, and the forming process was optimized. The relationships between filling velocity, pressure speed and gas flowrate were deduced, and they are both linear relationships each other. Therefore, we can regulate gas flowrate to control pressure speed and filling velocity. It was found that the effect order on the filling ability of A356 alloy in EPSC-VL from strong to weak is gas flowrate, casting temperature, gas pressure and vacuum level, respectively. The process parameters ranges of A356 alloy in EPSC-VL are as follows:casting temperature 720-750℃, gas flowrate 12-19 m3/h, gas pressure 0.03-0.04 MPa, vacuum level 0.02-0.03 MPa. Process parameters for larger complicated and thin-walled castings should choose the upper limit value of process parameters.The microstructures and mechanical properties of A356 alloy obtained by EPSC-VL were investigated. The results show that the phases of A356 alloy fabricated by EPSC-VL mainly consist of a-Al primary solid phases, eutectic silicon particles and Mg2Si phases. The grains size of EPSC-VL castings is obvious finer compared to other LFC casting processes, and the grains size of as-cast is only 45.0% of the lost foam casting under gravity casting (LFC-G),54.3% of the expendable pattern shell casting process under gravity casting (EPSC-G) and 56.9% of the lost foam casting with vacuum and low pressure (LFC-VL), respectively. The internal quality of EPSC-VL castings is obviously improved compared to other LFC casting processes, and its porosity is very lower (only 0.16%). The porosity of EPSC-VL is only 8.1% of LFC-G,24.2% of EPSC-G and 20.2% of LFC-VL, respectively. Furthermore, the mechanical properties of EPSC-VL castings has greatly improved compared to other LFC casting processes, and the tensile strength, elongation and hardness of EPSC-VL castings are respectively up to 278.27 MPa,8.1% and 93.1 HB, and there are respectively 20.2%,166.4% and 17.6% higher than that of LFC-G castings, and are respectively 6.8%,31.7% and 8.3% higher than that of EPSC-G castings, and then are respectively 10.4%,145.4% and 15.1% higher than that of LFC-VL castings. In addition, the EPSC-VL castings perform better surface quality than that of LFC castings.The gating system design of the complicated and thin-walled typical castings was investigated, and the filling and solidification processes were analyzed using the numerical simulation software to forecast casting defects, and the optimized gating system design was obtained. The complicated and thin-walled aluminum and magnesium alloys parts with high quality has been successfully produced by using EPSC-VL, and it proves that EPSC-VL has great advantages in the forming of complicated and thin-walled castings. The defect characteristics and formation mechanism in EPSC-VL were investigated, and the prevention measures based on casting defects were proposed.