Study On Preparation Of Ceramic Shells And Its Interface Reaction With Direction Solidification Alloy

Nickel based directional solidification superalloy with additions of hightemperature chemical active elements Hf, Cr and C, require the investment castingceramic shell with high chemical stability and mechanical property. The coating slurryand the preparing process have great influence on the performance of ceramic shell.Therefore, the whole process of investment casting and the interface reaction betweenthe casting and the ceramic shell have been studied in this paper.By adding Al2O3 particles of different sizes, silica sol of different concentrationand mineralizer of different proportion, the ceramic shells were made by currenttechnics. The particle size of Al2O3 and the rheological property of slurries werestudied by particle analysis and rheological property analysis. The mechanicalproperty was studied by three-point bending test. The phase compositions andmicrostructure of ceramic shells and the products of interface reaction betweenceramic shells and casting were investigated by the means of XRD, SEM, DES, DSCin this paper. To get optimal processing parameters, great attention have been focusedon the drying process and sintering process in this paper.The results showed that the strength of ceramic shell became harder by increasing theproportion of mineralizer, and its gravitational distortion at 1500℃was decreased.The property of slurry was affected significantly by particle sizes of the Al2O3, Al2O3powders with wide size distribution could make high powder-liquid ratio and improvethe brushability of the slurry. When the proportion of Na2O increasing, the bendingstrength and high temperature creep resistant properties were decreased. Whensintering the shell, more phase of mullite was formed by increasing the concentrationof silica sol. The silica sol of 24% concentration could get better mechanicalproperties and lower gravitational distortion. The technology specification wasoptimized by studying the process of drying and sintering. The ceramic shell made bythis method had finish inner face, appropriate strength, high chemical stabilities andso on. The result of the interface reaction between superalloys and ceramic shellsshowed that the reaction products were HfO2 and Cr2O3, and the thickness of theHfO2 layer was up to 5~10μm when the proportion of mineralizer increased.Furthermore, the infusion process was enhanced by the interface reaction. The conversion zone of the colour pink was formed by the solid solution process of Al2O3and Cr2O3. The main reasons of sand fusion were the interface reactions and theinfusion process between superalloy and ceramic shells. The problem of sand fusioncould be solved when the proportion of mineralizer was 3wt%.