ENHANCING THE PROPERTIES OF WAX PATTERN AND PERMEABILITY/STRENGTH OF CERAMIC SHELL IN INVESTMENT CASTING PROCESS

Abstract

Investment casting is an effective way to produce very thin and most complex components with very high dimensional accuracy and surface finish. In this process, wax is commonly used as the pattern material. The wax pattern is repeatedly dipped into ceramic slurry and coarse stucco is sprinkled over it to develop a ceramic shell. After dewaxing and firing, molten metal is poured into the shell. After solidification, the ceramic shell is removed to obtain the final cast part. Very fine ingredients are used for the primary coating slurry to achieve smooth internal surface of the shell. These fine ingredients of the inner coat slurry reduce the shell’s permeability, which lead to porosity defects on the cast part. The present research work is aimed on improving the surface finish and dimensionally accuracy of wax pattern, enhancing the shell’s permeability, reducing shell cracking by enhancing shell strength, thereby improving the quality of Al-Si alloy casting. Dimensional accuracy and surface properties of the wax pattern could be improved by proper proportions and proper selection of wax blend. The permeability of the ceramic shell could be improved by modifying the ceramic slurry ingredients, followed by proper selection of process parameters, such as firing temperature, shell thickness, etc. The shell cracking could be minimised by proper selection of slurry ingredients and additives, followed by proper selection of process parameters. The present research work has been done in four stages to fulfil the research objectives. The first stage of the experimental work was focussed on the selection of best wax blend that resulted in excellent surface finish and dimensional accuracy of wax pattern. L27 orthogonal array was used to design the experiments. Fuzzy logic was used to optimize the process parameters, such as, surface roughness, shrinkage and hardness of the wax pattern. The results indicated that proper proportion of waxes in wax blend resulted in better properties with optimized process parameters. The second stage of the experimental work was focussed on enhancing the shell permeability by modifying the inner and outer coat ceramic slurries ingredients. In this phase, a comparative study on shell properties such as permeability, strength, surface roughness etc. was carried out in which four type of shells i.e. 1. conventional shell, 2. iodine and needle coke modified shell, 3. ABS powder iv and needle coke modified shell, 4. fine needle coke and coarse needle coke modified shell were prepared. The modified shells were prepared by varying the proportions of inner and outer coat additives. The results revealed that the iodine and needle coke modified ceramic shell exhibited higher shell permeability, as compared to other modified shells. Field-Emission Scanning Electron Microscope (FE-SEM) was used to examine the microstructure of conventional and modified shells and it was found that some pores were developed at the inner and outer layers of the modified shell, which lead to increment in permeability. Iodine and needle coke modified shell also possessed improved shell strength, inner surface finish as compared to other shells. The third phase of the experimental work was focussed on enhancing the shell strength by modifying the outer coat ceramic slurry ingredients. In this phase, conventional slurry of the outer coat was modified by adding glass-fibers. These glass-fibers provided the additional strength to the ceramic shell. The modified ceramic shell had slightly higher shell thickness as compared to the conventional ceramic shell. Glass-fibers were thermally stable at higher temperature even at 1200˚C. Fiber reinforced modified ceramic shell had a larger bonded area between the fiber modified ceramic slurry and stucco coating. Uniformly distributed glass-fibers act as an obstacle to fracture under bending force and required a higher load to fracture. These fibers formed a stronger bond to the matrix and required a higher load to pull off from the ceramic shell. These characteristics of glass-fibers to the modified ceramic shell provided higher strength to the shell. One more property of the glass-fiber modified ceramic shell is that it formed a rough internal surface when pulled off from the shell surface. This property increased the friction force between the fibers and matrix, and required higher force to fiber pull off from the shell. As the quantity of the glass-fibers increased beyond 0.50%, it formed bundling and clusters that reduces the strength of the shell. These modified ceramic shell possessed higher shell strength but poor shell permeability. Fourth phase of the experimental work was focussed on the reduction of gas porosity defects in Al Si castings. Iodine and needle coke modified shell, ABS powder and needle coke modified shell, fine needle coke and coarse needle coke modified shell, glass-fibers modified shells with optimum quantity of additive were used to cast Al-Si alloy. The effect of shell coats and firing temperature were investigated on porosity, surface roughness and hardness of the castings. The results indicated that the parameters like shell coats and shell firing temperature affect the porosity, surface roughness and hardness of the castings.