Author Topic: Experimental Study on Cooling Rate of Centrifugal Casting Based on Grain Size  (Read 2579 times)

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Author : Madhusudhan, Narendranath S, G C Mohan Kumar
International Journal of Scientific & Engineering Research Volume 3, Issue 1, January-2012
ISSN 2229-5518
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Abstract- The rate of solidification affects the microstructure, quality and mechanical properties of the castings. The analysis of heat transfer in centrifugal casting is very complex due to rapid solidification, rotating mould, opaque mould and high temperature. As the grain size is directly depending on the rate of solidification of the casting, based on grain size the rate of solidification of the centrifugal casting can be determined. Grain size has been measured for the gravity castings at different cooling rates and using this result rate of solidification of the centrifugal castings have been determined which are produced at different rotational speeds.

Keywords- Centrifugal Casting, Microstructure, Rate of solidification, Rotational speed.
Centrifugal casting is a process of producing hollow castings by causing molten metal to solidify in rotating mould. The operations include in centrifugal casting are rotation of mold at a known speed, pouring the molten metal and extraction of the casting from the mold. Solidification is quite rapid and hence good metallurgical quality is achieved as solidification starts from mould inner surface corresponding to casting outer surface, so low melting point impurities are carried by the solidification front to the casting inner surface and gas porosity is also forced at the casting inner surface because of its low density and also fine grain structures are formed [1]. The inner impurity surface can be removed by machining. The casting parameters which influence the solidification structures include, the mold rotational speed, mold dimension, preheating temperature of the mold, pouring temperature and metal composition. But rotational speed of the mold is one of the important process variables which affect the rate of solidification of the molten metal [2]. Determination of temperature distribution during centrifugal casting process and further determination of solidification time of centrifugal castings by experimental techniques is very difficult as the mould rotates at a very high speed during solidification. In view of this accurate data on solidification time of centrifugal casting are not available [3]. But simulation by CFD program can be treated as an attractive and useful tool for modeling centrifugal casting process[4].As the rotational speed is increased the centrifugal force is increased by a square proportion, which may create a strong convection in the liquid pool and then producing a As As a  result, the growth of equiaxed grains is favored [5].The rate of solidification is very important phenomenon as it has a great influence on the So one of the method to study the solidification rate is based on grain size. In this experiment initially gravity castings are produced at different solidification rates and corresponding grain sizes have been determined. Using this results the solidification rates of the centrifugal castings are determined corresponding to different grain sizes of the centrifugal castings produced at different process conditions.

Initially gravity castings are made and grain size has been determined corresponding to different solidification rate. Using these results solidification rates of centrifugal castings have been determined.

2.1 Gravity Die Casting

Three types of dies are used with wall thickness of 10 mm, 20 mm and 30 mm which causes different cooling rates cooling rate 1, cooling rate 2, and cooling rate 3. Thermocouple junction is kept inside the mould and molten Tin at about 450o is poured to these moulds. Cooling curves are drawn for the above three cases and slopes of these cooling curves represents the solidification rates (oC/Sec.). The microstructures are obtained and graph of solidification versus grain size has been plotted.

2.2 Centrifugal Casting
formation of a fine grain structure by increasing the degree of constitutional supercooling [6]. Generally an area of the casting which is cooled quickly will have a fine structure and area which cools slowly will have coarse grain structure. Several studies have explained about the effect of above mentioned process variables of centrifugal casting, but the influences of casting parameters on solidification morphology and its theoretical and quantitative description are still far from clear understanding.  homogenization of temperature in the bulk liquid [5].   

The   Figure 1 shows the experimental setup of centrifugal casting, which consists of a mild steel cylindrical die fixed to a driving flange. This driving flange is connected to the shaft of a DC motor, where the speed can be varied from 0 to 2000 rpm with high accurate speed controller. The flow of metal into the mould is confined in the horizontally oriented, axially rotating cylindrical die. Centrifugal castings are obtained at three different speeds, 200 rpm, 400 rpm and 800 rpm.

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