REDUCTION OF SHRINKAGE AND POROSITY DEFECTS IN INVESTMENT CASTING

Abstract

Investment casting (IC) is regarded as precise fabrication process for components, which have intricate shape and requires excellent surface finish with high dimensional accuracy. It is used to manufacture parts from turbocharger wheels to golf club heads, from electronic boxes to hip replacement implants, general engineering to aerospace and defence outlets. Among the non-ferrous alloys, there is a wide range of applications of aluminium and its alloys. Largest applications of aluminium investment castings are in automobile and aerospace industries. Within last few years, there has been a rapid increase in the utilisation of aluminiumsilicon (Al-Si) alloys, particularly for many commercial automotive applications, due to their high strength to weight ratio, high wear resistance, low density and low coefficient of thermal expansion. Literature reveals that hypoeutectic Al-Si investment castings are widely used for making impeller wheels, track landing flaps for Airbus A-330, fan housing used in military expeditionary vehicles, etc. However, occasional minute defects like macro and micro pores including shrinkage porosity always persist in investment casting. These micro-structural defects cause adverse effects on mechanical properties of hypoeutectic Al-Si investment cast parts. In the present investigation, attempts have been made to reduce the shrinkage and gas porosity defects in investment cast parts for improving the mechanical property namely, tensile strength of the hypoeutectic Al- Si alloy investment casting. Shrinkage is a result of the inability of the casting to feed the required amount of metal during the contraction from liquid to solid. In case of investment casting, shrinkage defect arises in two different stages. In first case, it occurs in disposable wax patterns and secondly, it occurs in the castings during solidification. In order to eliminate the shrinkage defects in investment cast parts, steps have been taken in this study to at both the abovementioned stages. In this regard, the shrinkage (dimensional contraction) of the wax pattern is first compensated by adding filler, namely soluble starch powder to the wax blend. It has been found that the starch powder was surfacially compatible with the pattern waxes and the wax blends containing starch powder possessed excellent pattern characteristics, such as least v shrinkage and surface roughness, as desired by the investment casting industries around the globe. Besides pattern wax compositions, influence of injection process parameters on pattern shrinkage has also been studied using Taguchi method. Further, multi-response optimization of wax pattern has been done using many statistical techniques and the method predicting the best result in terms of pattern properties has been identified. A fuzzy prediction model has also been developed to forecast the wax pattern properties namely linear shrinkage, surface roughness and penetration. The efficacy of the developed fuzzy model was verified by confirmatory experiments and it was found that the absolute percentage error of the predicted results by the model was below 10 %, which justifies its suitability for predicting the wax pattern characteristics. Electromagnetic stirring (EMS) has been recognized as the most efficient way to reduce grain size and, consequently, the mechanical properties of the castings were improved. The influence of EMS and casting process parameters on controlling casting shrinkage on IC parts has not been previously studied. Hence, the impact of EMS on casting shrinkage reduction in investment castings has been investigated. The process parameters chosen were shell preheat temperature, stirring current, stirring time and pouring temperature. The optimal combination of EMS and casting process parameters for dimensional and volumetric shrinkage reduction and tensile strength improvement were determined by Taguchi method. Confirmatory experiments were later conducted with the obtained optimal settings. In a nut shell, it was found that the final product quality has been improved by the combinational treatments adopted in this research work. From the micro-structural analysis, it has been found that EMS casting led to fine and equiaxed grains, instead of dendritic structures and exhibited more uniform distribution in the eutectic matrix. The gas porosity defects in the investment casting occurs in two different stages namely, ceramic shell building and pouring/ solidification of the castings. One of the key requirements of an IC shell is sufficient shell permeability and thermal conductivity to maintain an adequate heat transfer through the shell wall and allowing the entrapped gases inside the castings to escape out from it, thereby reducing the porosity defects in the cast product. In this regard, the permeability of vi the ceramic shells has been increased by the addition of naturally available ingredients, such as coconut fiber and saw dust to the ceramic slurries. SEM micrograph of coconut fibers at higher magnification revealed rough fibrous surface indicating higher green strength, which is very much essential for dewaxing process. SEM micrograph of fired surface of coconut modified ceramic shell revealed void in the place of coconut fiber, thus indicating increase of porosity inside the ceramic shells. It was further confirmed by computing apparent porosity for coconut fiber modified ceramic shells, which was found to be 27.07 %. Similarly, the apparent porosity for the saw dust modified shells was found to be 29.19 %. Besides adding coconut fibers and saw dust to the ceramic slurries, an innovative approach of micro-drilling of ceramic shells has also been employed in this study to make the shell pervious. The apparent porosity for the USM drilledceramic shells was found to be 18.64 %, which is significantly lesser as compared to that of coconut and saw dust modified shells. Finally, it is concluded that the ceramic shell containing saw dust has shown highest porosity, which makes it the most suitable naturally available filler for investment casting ceramic shells. In order to reduce the gas porosity related defects associated with pouring and solidification of hypoeutectic Al-Si alloy in the conventional investment casting, the process parameters chosen were shell preheat temperature, firing temperature, firing time and pouring temperature. The overall apparent porosity was found to be 3.26 % and the most significant process parameter controlling apparent porosity was shell preheat temperature, followed by melt pouring temperature. Firing temperature was the least significant process parameter influencing apparent porosity in conventional IC. Grain structure in case of conventional IC, was mostly dendritic and the grain size was large i.e. 185 μm and a large number of irregular voids in interdendritic region were found in case of conventional IC. Further, trace element namely, sodium (Na) (belonging to Group IA) addition, along with EMS processing of the molten alloy, prior to pouring in the pre-heated porous ceramic shells, was performed to observe its effect on reducing the casting porosity. The maximum quantity of addition of the sodium to the molten alloy was limited to 0.05 %. From the micro-structural analysis, it was found that the sodium modified IC part had refined eutectic Si in form of fibers. It possessed regular shaped large vii void and the grain size was reduced to 165 μm. The grain size was further reduced to 85 μm, when EMS process was combined with Na modification in hypoeutectic Al-Si alloy investment casting. Consequently, less shrinkage porosities were seen. The optimal conditions for EMS processed Na modified IC parts were determined by Taguchi method and the apparent porosity was 2.14 %, which is less than that obtained by conventional IC process. It was found that the ceramic shell thickness was the most important and the stirring current was the least important process parameter influencing the apparent porosity in IC. From the micro-structural analysis, it was evident that there should be appropriate percentage of sodium content in the melt, which was its critical content. If it was added below the critical value, complete arrest of the eutectic Si in the alloy was not possible. Its addition above the critical limit led to over modification of eutectic Si.