Study On The Precision Control And Characteristics Of The Layered Extrusion Forming Casting Ceramic Mold Or Core

Compared with binder sand mold or core,ceramic mold or core holds the advantages of high strength and refractoriness,good air permeability,precise dimension control and bright casting surfce.Besides,ceramic mold or core will not produce sand inclusion or block defect during metal casting and is suitable to form all kinds of castings with complicated and thin flow internal structure,which is commonly adopted in complex castings such as hollow aeroengine blade.The use of rapid prototyping ceramic mold or core to achieve rapid casting is recognized as an important field of research and application.Layered extrusion forming technology as a novel rapid prototyping method based on the slurry extrusion has broadened the application of rapid prototyping technology in the field of casting.Compared with selective laser sintering and 3D printing method,layered extrusion forming fabricates ceramic in ambient temperature and without adoption of laser radiation,which entitles it outstanding advantages of low cost and green material system.Based on the scheme of layered deposition extrusion,this paper combines the high-strength ceramic core material with the overall layered deposition molding to produce complex ceramic casting mold or core,laying a theoretical and technical foundation for the precision and rapid casting of various complex castings.With the support of the National Natural Science Foundation of China project"Research on Layered Deposition Extrusion Molding and Precision Control of Complex Ceramic Casting Mold（Cores）（No.51775204）",this paper systematically studied several key issues such as precision control and performance control of ceramic cores for layered extrusion forming,which laid a theoretical and pratical foundation for rapid and low-cost forming of ceramic cores.The main contents of this paper are as follows:（1）A single-extruder layered extrusion forming device with the maximum molding size of 200mm*200mm*150mm has been developed,which is designed on the base of Cartesian coordinate system.The device consists of three major modules:mechanical system,extrusion system and control system.The motion feature of the mechanical system is that the Z axis is separated from the X axis and Y axis,which ensures the precision control requirements in the forming process and satisfies the load-bearing load in the mechanical design.The extrusion system adopts pneumatic piston extrusion mode.Through theoretical analysis and calculation,it is concluded that a larger diameter and a smaller length of the extrusion nozzle lead to a smaller gas pressure required for extrusion forming.Through ANSYS simulation of extrusion process,it is found that the flow rate is proportional to the applied pressure.The pratical air pressure can provide 0.01¹.0mpa forming pressure,which can meet the requirements for ceramic slurry extrusion.The control system connects the mechanical system and the extrusion system through relay,and adopts the upper and lower machine control mode to transmit the extrusion signal and movement signal.After repeated tests,the position accuracy of the equipment in the x-y-z three-axis can reach 0.05mm,the repeated positioning accuracy of the x-y axis can reach0.03mm,and the repeated positioning accuracy of the z-axis can reach 0.015mm.（2）The effects of slurry compositions and process parameters on the dimensional accuracy and surface quality of ceramic specimens formed by layered extrusion were systematically studied.The results show that the ceramic slurry suitable for layered extrusion must exhibit both extrudable and formable,that is,the ceramic slurry should have shear thinning rheological properties and sufficient support strength（yield stress>200Pa）.A prerequisite for improving the morphological integrity and dimensional accuracy of the part is to optimize the forming parameters so that the extruded ceramic filament possesses a plate structure and a suitable deposition rate.Single factor test and orthogonal test were used to study the influence law and mechanism of layer height value,extrusion head inner diameter and printing speed on the dimensional accuracy of ceramic parts.When the solid content of alumina is 50 vol.,extruder inner diameter is 0.40 mm,layer height is 70%of extruder inner diameter,and printing speed is 10 mm/s,the dimensional accuracy and surface roughness of layered extrusion forming samples can be significantly improved.（3）The effects of different polymer aqueous binder on layered extrusion forming of alumina ceramic specimens and the sintering performance were systematically studied.By using different kinds of polymer binders,it is proved that the binder concentration has a significant effect on the strength and sintering performance of laminated extruded ceramics.The strength of alumina ceramic blank is closely related to the three-dimensional network density formed by internal macromolecule.When 25wt.%PVP is used as the binder,the bending strength of alumina ceramic blank can reach 25.44mpa.After sintering at high temperature,certain bending strength and apparent porosity can be obtained,and the relationship between bending strength and apparent porosity is inversely proportional.The pratical aluminum alloy casting experiment verifies that the alumina ceramic shell after preheating 750℃can withstand the thermal shock of liquid metal and the shape and size of the castings are well maintained when the aluminium alloy is naturally poured.（4）In order to improve the bending strength of alumina ceramic specimen and reduce the sintering temperature of alumina,nano-oxide dispersion liquid-polymer nano-reinforced binder system was used to improve the green strength and facilicate sintering process.The results show that when nano-additives are introduced into the binder,nano-particles easily adsorb on the surface of alumina particles or even fill in the gap between alumina particles,increasing the contact between the particles inside the ceramic body and increasing the density of the ceramic body,which improves the bending strength of alumina ceramic body and reducing the drying shrinkage.Nano additives have an important influence on the sintering performance.The research results show that adding nano SiO₂,TiO₂ and ZrO₂ is beneficial to reduce the sintering temperature of alumina,so that alumina ceramics can obtain high strength at a lower sintering temperature.Adding nano-MgO reduces bending strength of alumina ceramic specimen because of brittle opposition（magnesium aluminum spinel）formed after sintering.When kaolin is added to the system,the high strength of alumina-kaolin-nano-MgO system can be obtained after sintering at 1300°C.