REDUCTION OF SURFACE ROUGHNESS, SHELL CRACKING AND GAS POROSITY DEFECTS IN INVESTMENT CASTING PROCESS

Abstract

Investment casting is a promising technique used to manufacture thin and complex shape castings with good dimensional accuracy and excellent surface quality. It has minimum wastage of raw material which makes this process advantageous over other manufacturing processes like machining, forging, etc. Despite several advantages, the castings produced by investment casting process encounter with some problems such as shell cracking, shrinkage and gas porosity defects specifically in the case of Al-Si castings. The Al-Si casting has good mechanical properties which makes it a favorable material for various industrial application. But, the presence of aforesaid defects greatly affect the mechanical properties of Al-Si castings. Till date, several attempts have been made to overcome the aforesaid limitations. The current research work is aimed on reduction of surface roughness, shell cracking and gas porosity defects in Al-Si castings, thereby improving the mechanical properties and surface quality. The problem of shell cracking can be overcome by modifying the ingredients of investment slurry. Gas porosity defect occurs due to two major reasons i.e. poor permeability of shell and improper selection of casting parameters such as preheating temperature and thickness of ceramic shell. The permeability of the shell can be improved by modifying the ceramic slurry ingredients, followed by proper selection of casting parameters. In view of the above, the present experimental work was performed in three phases to fulfil the research objectives. The first phase of experimentation was focused on the selection of suitable pattern that resulted in minimum surface roughness and also in improving the dimensional accuracy of casting. Two pattern blends namely wax blend and coconut oil based hybrid blend were selected and their performance in terms of surface roughness, shrinkage and hardness was evaluated. The experiments were performed using response surface methodology with variation in three input parameters viz. injection temperature, injection flow rate and die temperature. This investigation revealed that the coconut oil based hybrid blend as a pattern material was better than the wax blend. Thus, coconut oil based hybrid blend was selected as the pattern material for the preparation of ceramic shell. The second phase of the experiments was focused on reduction of shell cracking defect by improving the shell properties. In this phase, a comparative study on shell properties was carried out in which four types of shells i.e. conventional, camphor modified, nano alumina modified and CNT modified shells were prepared. The modified shells were prepared by varying the proportions of the modifiers. The properties measured were shell thickness, green and fired v strength, corner strength, load bearing capacity and self-load deformation. The experimental results showed that the CNT modified shell exhibited in maximum shell thickness, strength, load bearing capacity and minimum self-load deformation as compared to the conventional and other modified shells. The third phase of experiments was focused on the reduction of gas porosity defects by improving the permeability of the shell. In this phase, hot permeability of the modified shells prepared in the previous phase were measured and compared. The results revealed that the camphor modified shell had maximum permeability, though the strength of the camphor modified shell was lowest among all the modified shells. Therefore, in order to set a trade-off between mechanical properties and permeability of shells, two types of hybrid shells (i.e. CNT based hybrid shell and nano alumina based hybrid shell) were prepared and their properties were compared with those of the other modified shells. The results revealed that the strength of the hybrid shells was improved without a significant reduction in permeability. The nano alumina based hybrid shell showed higher permeability than that of the CNT based hybrid shell. Further, in order to reduce the gas porosity defects during solidification process, A356 Al-Si castings were prepared by stir casting method using all the modified shells. The effect of casting parameters was investigated on surface roughness and porosity of the casting. The results revealed that both the surface roughness and porosity of castings were significantly affected by the casting parameters. The lowest surface roughness and porosity were found in the casting obtained by using camphor modified shell.