Some Studies on Process Parameters in Centrifugal Casting

Abstract

In a centrifugal casting process, the fluid behavior of the melt plays an important role in determining the quality of the final products. There are many parameters which influence the centrifugal casting process namely pouring temperature of the melt, initial temperature of the mold, thermal conductivity of the mold material, rotational speed of the mold, size of the mold and time taken for pouring the melt into the mold, etc. Rotational speed of the mold is one such parameter amongst the important process variables which affect the rate of solidification of the molten metal. When the liquid is rotated in partially filled cylindrical mold at different rotational speeds, it exhibits various flow patterns, namely Ekmann flow, Couette flow and Taylor flow; these are disturbing flows inside the cylinder. A brief survey of the earlier literature indicates that many investigations have been carried out to study the behavior of the liquids and its effect on the casting process. The microstructures of the castings are influenced mainly by the behavior of molten metal flow during rotation of the mold. To get a uniform hollow cylinder, the molten metal must spread along the axis after being poured and must slide along the inner surface of the mold. But the factors involved in fluid instabilities that influence the quality of the casting and rate of cooling need to be investigated. The analysis of liquid flow during centrifugal casting is very difficult to comprehend due to the opaque nature of the melt and the mold and the viscosity of the liquid varies with time. The phase change, due to heat transfer also adds to the complexity of the problem. Since the mold and melts are opaque, motion of the melt cannot be visualized and analyzed with conventional measurement techniques. Therefore in order to study the various liquid patterns and the rate of cooling at different rotational speeds, it is necessary to make preliminary examinations about the nature of the liquid flow in a partially filled rotating cylinder at various rotational speeds. Cold modeling experiments have been carried out using liquids with different viscosities to study the fluid behavior. Influence of rotational speed of the mold, its optimization to form a liquid cylinder and regular cooling rates at different rotational speeds have been carried out. In this research work, an attempt is made to study the process of solidification and the effect of solidification structures on the mechanical properties of centrifugal castings. The three types of solidification processes have been carried out. One being the solidification of pure Tin and the others being the solidification of alloys such as eutectic Al-12wt%Si alloy and a hypereutectic Al-17wt%Si alloy they have a range of temperature to solidify. Tin metal is used to study the metallurgical behavior of pure metal, as it is having low melting temperature of 231oC, Al-12wt%Si alloy of melting temperature 577oC and Al-17wt%Si alloy of melting temperature 577oC to 620oC are used to analyze the metallurgical behavior of the alloys and also to study the particle segregation in the cylinder across the thickness of the casting. Eutectic as we know is a reversible isothermal reaction of a liquid metal which forms two different solid phases in a binary system upon cooling, i.e., L= α +β. This is an invariant reaction in which liquid phase transforms to two solid phases. The rate of solidification of any melt during centrifugal castings is of great importance because of its role in determining the microstructure and mechanical properties. The rate of solidification of pure metal in centrifugal casting is measured based on the grain size and for the Al-Si alloys it is measured based on the Secondary Dendrite Arm Spacing (SDAS). In this work properties like grain size, solidification rate, hardness, and specific wear rate are determined. It is found that the regions where rapid solidification occurs, there fine equi-axed grains are observed and coarse grains are observed at regions where slow solidification takes place. In casting experiments the cooling curves were drawn for the gravity castings which were made initially by monitoring the cooling rate. The microstructures were analyzed using image analyzer. Grain sizes have been measured and a graph is plotted for rate of solidification verses grain size. Using this data the rates of solidification of centrifugal castings are inferred, based on the grain size of the castings. The effect of mold wall thickness on rate of solidification is evaluated by making the castings using molds with varying wall thickness. Experiments have been conducted to study the effect of mold preheating on rate of solidification. As the mold temperature increases the temperature difference between the die and the molten metal decreases and hencerate of solidification decreases. Fast solidification rate leads to the fine grain formation leading to an increase in hardness and decrease in specific wear rate. For all the cases the variation in hardness along the radial direction is determined. Specific wear rate have been determined at the inner and outer surfaces of the cylindrical castings. For the Al-12wt%Si centrifugal casting by evaluating SDAS the solidification rates have been calculated. Variation in hardness and specific wear rate were also studied. Fine grains were observed at the outer surface of the cylindrical tin casting which is due to the chilling effect and hence the hardness was found to be higher compared to the hardness at the inner surface. It is also found that hardness is gradually decreasing towards the inner radius of the casting. But in case of Al-12wt%Si at the outer surface the hardness is higher due to chilling effect with the cold mold wall. At the inner surface hardness is higher due to the segregation of Si particles at the inner surface, because of its lower density with the matrix and also due to the centrifugal effect on the Si particles at higher speeds of rotation of the mold. Similar results have been obtained in case of Al-17wt %Si castings with slightly higher hardness at the inner surface of the casting.