Analysis of centrifugal casting using tin.
Metal casting is one of the basic manufacturing processes. Cylindrical and symmetrical hollow castings are produced by true Centrifugal casting. This casting technique has application across a wide range of industrial and artistic applications. When molten metal is poured onto the surface of a rotating centrifugal casting mold, all of it is not picked up (accelerated) immediately. Rotational velocity is imparted to it by virtue of the friction between the liquid metal and the mold. After the inner mold surface is covered, all succeeding melt will be accelerated by internal friction between the liquid surface already spinning and the melt to be picked up. This creates an appreciable tendency to slippage. An increase in mold speed above that which allows slipping and cascading (raining), but below the critical vibration range, tends to reduce the grain size. Surface quality of casting improves as spinning speed increases. With the optimum spinning speed the metal will be picked up rapidly and held firmly to the mold wall without slipping or raining. It is also important that the melt not slip on itself during its solidification period, as this could result in a defective casting. The properties of the centrifugally cast material is mainly governed on the speed of rotation. The solidification time of the casting also depends upon the speed of rotation of the mold. It decreases with an increase in speed of rotation.
The magnitude of the centrifugal force is regarded as of major importance in this process, as it causes segregation caused by the difference between the densities of the particle and matrix. The centrifugal action assists in the purification of the metal by forcing the light non- metallics to the in-side wall. The main benefits are the rapid directional solidification from the outer diameter to the inner diameter leading to a consistent grain structure with the excellent strength and toughness, lighter inclusions and gas porosity in the molten metal strongly migrate to the interior bore. Grain refining has been regarded as an effective means to upgrade material properties [1-3] as it gives rise to better mechanical properties of metals such as strength , toughness, ductility, fatigue strength and stress corrosion resistance. Mold speed plays a major role in controlling grain size and inner surface quality improves as spinning speed increases. The casting parameters that influence solidification structures can include the mold rotation and the mold dimension. Some cold modeling experiments were conducted to study the role of mold rotational speed (4). According to Nathan Janco  centrifugal casting process is more effective when compared to other casting process. Chang et al  did the simulation of microstructure for different speeds for vertical centrifugal casting.
The metal is introduced at one end of the mold and caused to flow by centrifugal action throughout the mold length. If the spinning speed is adequate and if the fluidity of the molten metal is satisfactory, the melt will distribute itself uniformly along the length of the mold before solidification.
If the temperature of the molten metal is too high or the fluidity of the molten metal is too high, the metal will not readily accelerate to the speed of the mold quickly. In a horizontal spinning mold, it will rotate around from the bottom of the mold to the top of the mold, when it does not have sufficient velocity. The force of gravity will tend to make part of the melt fall from the top of the mold down into the body of the melt at the bottom of the mold. Further in centrifugal casting, longitudinal cracks are encountered primarily during the early stages of solidification in where the high rotational speeds are employed.
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The experiment set up is shown in fig.1. Pure Tin was used for experiment because of low melting temperature. Experiments were conducted for different mould speeds and various thicknesses of castings. The experiments were conducted to find optimum spinning speed required to pick up the melt rapidly and hold firmly to the mold wall without slipping and raining. Pure Tin was melted in a furnace heated to a temperature of 500[degrees]C. After skimming the dross, it was poured in to the rotating mold quickly. After a few minutes the motor was turned off and casting pulled out. The thickness of the cast was controlled by taking suitable amount of melt. This procedure was repeated for different speeds and different thickness. Fig.2 shows cast in the mould.
With the optimum spinning speed the melt will be picked up rapidly and held firmly to the mold wall without slipping or raining. With the liquid metal in place, a pressure gradient is established radially across the wall thickness of the casting as a result of the centrifugal force. The pressure is lowest at the inner surface and increases to a maximum at the outside surface of the casting.
Generally pure metals are very soft and they have low hardness. The hardness of the material is expected to be a function of thickness of cast and also the speed of rotation of the mold, through grain size control.
Results and Discussion
Pure Tin was cast for 2, 4, 6 and 8 mm thickness cylinders. Fig.5. shows different centrifugally cast Tin cylinders. Vickers hardness of casts were determined. Hardness test was done for both outer and inner surface of the caste material.
Fig. 7. Shows microstructure of 4mm thickness cylinder cast at 200, 400, 600, 800, 1000 and 1200 rpm. Microstructure consists of equiaxed fine grains of tin. At 200 rpm the average grain size is 30-40 microns. At 400 rpm the average grain size is 40-50 microns. At 600 rpm the average grain size is 40-50 microns. 800 rpm the average grain size is 35-45 microns. Fig. e shows microstructure of pure tin cast at 1000 rpm. Microstructure consists of 15-25 microns fine grains. Fig. f shows microstructure of pure tin cast at 1200 rpm. The grain size 15-25 microns.
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The hardness of the material increases with increase in rotational speed (Fig.8.). Fig. 9. shows the variation of hardness with different thickness of the material. The hardness decreases with increase in thickness of the cylinder cast at the same rpm. The hardness of the outer surface was more compared with inner that of surface of cylinder (Fig.10.) due to chilling effect. Fig. 4.Shows the quality of the inner surface of the 4mm thickness cylinder cast at different speeds. As the speed increases the quality of inner surface improves. At 800 rpm the quality of the cylinder is good, but at 1200 rpm the metal distribution is not uniform. Cylinders cast above or below the optimum speed show poor inner surface.
Fig. 6. Shows the quality of the inner surface of the 6mm thickness cylinder cast at different speeds. At 1000 rpm the quality of the cylinder is good. At 1200 rpm the metal distribution is not uniform.
[FIGURE 8 OMITTED]
[FIGURE 9 OMITTED]
Microstructures consist of equiaxed fine grains of tin. The spinning speed influences the grain structure .The variation in grain size is more at lower speeds than at higher speeds of rotation. As the speed increases the hardness of the material also increases. As excepted the hardness decreases with increase in thickness of the casting. Hardness at the outer surface is slightly more compared with that of the inner surface of the cast. The quality of the inner surface improves with increase in rotational speed up to a point. The speed of rotation does not affect the outer surface quality of the cast. With the optimum spinning speed the metal will be picked up rapidly and held firmly to the mold wall without slipping are raining resulting in good castings.
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K.S. Keerthi Prasad *, P.G. Mukunda *, S.C. Sharma ** and M.S. Murali **
* Mechanical Engineering Department, NMIT, Bangalore E-mail : email@example.com,
** Mechanical Engineering Department, RVCE, Bangalore
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|Author:||Prasad, K.S. Keerthi; Mukunda, P.G.; Sharma, S.C.; Murali, M.S.|
|Publication:||International Journal of Applied Engineering Research|
|Date:||Feb 1, 2009|
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