Study On Materials And Precision Of Investment Casting Ceramic Shell/Core Via Double-head Extrusion Direct Forming

Modern investment casting is the main forming method of high-end precise castings such as aerospace,automobiles and so on.But the general process is considered as complex,with long cycles and high costs.In the investment casting process,not only the mold is required to prepare the wax mold,but also the ceramic coating needs to be dip-coated on the surface of the wax mold for multiple times.And the ceramic shell is obtained after drying,dewaxing and roasting.After the molten metal is filled,the ceramic shell can be removed to obtain the desired casting.In recent years,the rapid development of additive manufacturing technology provides a possible and novel method for the rapid and easy preparation of ceramic shell for investment casting.This work focuses on developing a double-head layered extrusion forming device,optimizing material compositions of the ceramic cores and the support structure,and building the theoretical model of the surface roughness regarding the ceramic shells.The research results have laid the foundation for materials and processes for the direct and integral preparation of complex ceramic shell/core for investment casting,which is expected to improve the traditional investment casting process,and possesses important theoretical and practical significance.The main contents of this paper are as follows:(1)A double-head layered extrusion forming device and collaborative control system was developed and constructed,consisting of a mechanical motion section,a slurry-driven extrusion module and a software control system.In order to minimize the influence of the reciprocating movement of printed substrate on the precision of green bodies,two extrusion heads move in X-Y plane and the printed substrate moves along Z direction.Additionally,two extrusion heads are tuned precisely in X,Y,and Z directions to ensure the positioning accuracy.The pneumatic plunger drive mechanism is used for slurry extrusion because of simple operation,low cost,and easy maintenance and cleaning.The flow model of the slurry in the needle is constructed.When the extrusion head is conical,the extrusion force required is smaller during the process of slurry stable extrusion.The software control system uses Marlin firmware,and the parameters in configuration are modified accordingly to flexibly control the collaborative printing of the two extrusion heads.The main parameters(baud rate,control board type,number of extruder nozzles,the printing stroke,movement speed,and the number of pulses required for the movement unit distance,etc)are modified to realize double-head collaborative extrusion forming.(2)The material system of CaO-based ceramic core via layered extrusion forming was studied.Effects of silica sol and silica powder on the performance of Cao-based ceramic core were tested,and the mechanism of improving the performance of calcium oxide-based ceramic core was discussed.It is proposed to use calcium carbonate as the matrix material,silica sol and polyethylene glycol as the composite binder,and octanol as the defoaming agent to prepare ceramic slurry.The CaO-based ceramic core was prepared by step sintering after layer extrusion forming.The higher the proportion of silica sol in the silica sol/polyethylene glycol composite binder,the greater the bending strength of the ceramic core body by layered extrusion forming after drying.When the proportion of silica sol is 30 wt.%,the prepared slurry is suitable for printing and the ceramic core green body possesses good performance with green bending strength of 4.2 MPa and drying shrinkage of 0.3%.When the molar ratio of CaO to SiO₂ is 2.45 and the sintering temperature is 1200?with 120min holding time,the comprehensive properties of CaO-based ceramic core is optimal and can quickly collapse in water.According to the microscopic morphology and thermodynamic analysis,the addition of silicon oxide promots the formation of Ca₂SiO₄,reduces the core hygroscopic property,linear shrinkage rate and bending strength,and increases the porosity of the ceramic core sample.(3)Two kinds of supporting materials and their properties as forming suspended structures were studied,including starch-based supporting material and salt-based supporting material.Suspended ceramic components were prepared by a double-head extrusion molding device.A starch-based slurry with 73.3 wt.%solid content was prepared by using starch as the main material and anhydrate ethanol solution of polyvinyl pyrrolidone as the binder.The size deviation and bending strength of the support samples are 1.1%and more than 11 MPa,respectively.The starch-based support material can be removed during the sintering process of the Al₂O₃-based ceramic sample.A Mg SO₄-based slurry with 60 wt.%solid content was prepared,and the bending strength of the salt-based sample is 9.94 MPa after drying.The salt-based sample can be dissolved and removed in water or weak acid.Suspended Al₂O₃samples were successfully fabricated using both starch-based and Mg SO₄-based supporting materials,and corresponding structures could be well maintained after removing supports.(4)Effects of needle diameter,forming layer height and inclination angle on the surface quality of ceramic shell samples were analyzed,and a theoretical model of the surface precision of the ceramic shell was constructed.A double-layer ceramic shell with different inner and outer layer accuracy was prepared.An Al₂O₃-based slurry with 73.3 wt.%solid content was prepared by using alumina as the main material,deionized water solution of polyvinyl pyrrolidone as the binder,octanol as the defoaming agent and glacial acetic acid as dispersant.When the ceramic shell samples were sintered at 1500?for 120 min,the bending strength is sufficient 40.5 MPa and the linear shrinkage rate is low 2.29%.The surface roughness of ceramic samples is reduced with smaller nozzle diameter,decreased layer height and the larger inclination angle during layered extrusion forming.The relationship between the maximum height difference on the surface of the ceramic shell sample and the layer height and inclination angle is established.A complex ceramic shell was prepared and an aluminum alloy casting experiment was conducted,which showed that the performance of the ceramic shell met the requirements of metal casting.Double-head extrusion is used to prepare a double-layer ceramic shell.The heights of the inner layer and outer layer are set as 0.2 mm and 0.4 mm,which ensures the accuracy of the inner shell and improves the preparation efficiency of the ceramic shell.