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INFLUENCE OF TEEMING TEMPERATURE OF TIN IN CENTRIFUGAL CASTING

  
     
 

Shailesh Rao A

  
 

ABSTRACT
During centrifugal casting, when a mould is rotated at low and very high speeds defects are found in the final castings. Obtaining the critical speed for sound castings should not be a matter of guess or based on experience. The defects in the casting are mainly due to the behavior of the molten metal during teeming and solidification process. The studies of molten flow in partially filled rotating cylinders have indicated many features, which we do not find a mention in the literatures of centrifugal casting. Motion of molten metal at various speeds and its effect during casting are addressed in this paper. Tin is taken as an experiment fluid and its behavior during various rotational speeds are discussed.

Keywords: Fluid flow, Centrifugal casting, Teeming Temperatures
 
     
  One of the most crucial and active areas of research in fluids engineering today is that of material processing. The important underlying features involved in fluid flow and its effect in material processing needs to be studied thoroughly. Another aspect that was missing in the literature is the quantitative information on the dependence of quality of the product on the fluid flow. Centrifugal casting is one of the material processing techniques, in which the flow pattern of the molten metal during casting strongly affects the quality of the final product. Literature about fluid flow in centrifugal casting is very sparse. Depending upon the conditions of the molten metal, there must be an optimum speed, at which the molten metal will be picked up to form a true cylinder.

Jaluria [1] discusses the importance of fluid flow in material processing. He points out several aspects of fluid flow which changes the properties in various processing techniques. Most of the active researches are carried out on continuous casting. Janco
[2] indicates several important parameters in involved during the centrifugal casting process. From the literature, it is seen that most of the work has been carried out in the fluid behaviour of the molten metal during continuous casting [3-6]. They have investigated the parameters affecting the casting through cold modeling.

In this paper, the behaviour of molten Tin is studied to understand the role of optimum speed during this process. Influence of various casting variables such as mould rotating speeds and teeming temperature are studied in detail.
We start by describing a typical progression of phenomena encountered when the angular velocity of the cylinder is gradually increased. For 4mm thick casting, prepared at low rotational speeds Fig[2,a-b], a thin film of molten metal is pulled out of the pool and covers the entire cylinder inner surface. As the metal enters the bottom pool on the receding side a straight front is created. An accompanying recirculation region is formed in the pool and simultaneously the metal flows along the axis. The thin layer circulated on the surface of the mould gets solidified and forms Couette flow. Simultaneously, the melt gets a lift from the side wall and forms Ekmann flow. From the literature, these flows are formed at low viscous fluids [11,14,15]. With the gradual decrease in the viscosity, the metal pulled out from the pool also thickens. It eventually becomes unstable to a sloshing mode of motion on the rising side of the cylinder. The unstable liquid begins to slosh to and fro along the axis. During this stage the metal gets solidified and an irregular shaped casting is formed. At rotation speed of 800rpm, the centrifugal force dominates and the fluid coats the cylinder surface uniformly and rotates rigidly with the mould and thus forms a uniform hollow cylinder casting Fig[2,c]. With the further increase of speed to 1200rpm, the molten metal moves along the inner surface of the mould due to large driving force. Only small portions of the metal succeed in moving along the axis and immediately get solidified during its motion. Finally a thick casting was formed on one side and a thin casting on the other side. Moreover, the thickness of the resultant cylinder exhibits a non uniform, step-like formation as seen from Fig[2,d]. ]. The change in the pouring temperature also influences the appearance of the casting. The metal should be driven at higher rotational speed for the two reasons. The first is the increase in viscosity of the metal during teeming and the second is the less value in the super heat temperature. For teeming temperature of 3500C, a larger drive is required for a metal to spread along the axis and move along the side wall. At 600rpm, the metal spread out easily and tries to move along the side wall. The melt finally solidifies and Ekmann flow is observed during the final casting. With further increase of speed to 800rpm a uniform hollow due to uniform spread of the melt along the axis.
 
     
 

Fig[2]: Casting for 4mm thick Tin at (a) 200RPM  (b) 400RPM   (c) 600RPM  (d) 800RPM 

 

Fig 5: Casting of Tin of 4mm thick at (a) 600rpm  (b) 800rpm  and 6mm thick at (c) 600rpm and (d) 800rpm
       
 
 
Fig[6]: Microstructure of 4mm thick Tin at the inner surface for (a) 200rpm (b)400rpm (c) 600rpm (d) 800rpm
 
     
 

Fig[8]: Microstructure of 4mm thick of the inner surface of Tin melted at 3500C at (a) 600RPM  (b) 800rpm
 
     
 

3.2 Microstructure of the casting:
The solidification structure of the castings is of great importance because of its role on mechanical properties. A fine equiaxed grain structure is required in order to obtain homogenous and isotropic mechanical properties.

The microstructure at the inner surface of the casting is studied. Upon pouring the molten metal into the rotating mould having rotational speed of 200rpm, a lot of crystals nucleate on the cold wall mould as a result of super cooling. These chill crystals form the outer skin of the casting. The metal undercools at the middle and inner surface of the castings. During the solidification process, the structures form in dendrite shape and it can grow freely. These dendrites are growing in radial direction. The well oriented dendrite structure grows from the chill zone and continues towards the inner surface. The formation of dendrite begins to breakdown with increase in spinning speed to 400rpm. Here centrifugal force dominates and lifts the molten metal outwards leading to increase in cooling rates and hence size of the dendrite structure is less predominant. At the inner surface deflections of columnar grains are frequently observed in the centrifugal casting due to the Taylor flow which is exhibited by the molten metal.

With the increase in mould speed to 600 rpm, the metal moves in streamline along the axis and simultaneously gets lifted forming a uniform cylinder. The metal remains stable at their places during solidification and hence the solidification rate is more. Fine grains are observed at the inner surface of the cast. With an increase in speed to 800rpm, the casting formed will have thick section at one side and thin section at the other side. Taking the microstructure across thick section, fine structures are seen at the outer surface due to chilling effect of the molten metal. The solidification process at the middle and inner region takes place through conduction. Hence a dendrite structures are formed at the middle and inner region of the casting.

  
     
  REFERENCES  
  1. Yogesh Jaluria, “Fluid flow phenomena in Materials processing- The 2000 freeman scholar Lecturer”, Journal of Fluid      Engineering, ASME, Vol123, June 2001, 173-210.  
  2.G Bergeles, J Anagnostopoulos, “Three dimensional modeling of the flow and the interface surface in a continious casting    mold    model” Metallurgical and Materials Transaction B, Vol 30B, December1999, 1095-1105.  
  3. Nathan Janco, “Centrifugal Casting”, Americal Foundry men’s Society , 1988  
  4. B G Thomas R M McDavid, “ Flow and thermal Behaviour of the top surface Flux/Powder layers in Continious Casting molds”,     Metallurgical and Materials Transactions B, Volume 27B, August 1996, 672-685.  
  5. H Fredriksson, Raihle C M, “ In the formation of pipes and Centreline Segregates in continiously cast billets” ”, Metallurgical     and Materials Transactions B, Volume 25B, Febrauary 1994, 123-133.  
  6. Samarasekara I V, Brimacombe J K, ”, Canadian Metallurgical Quaterly, September 1978 , Vol 6, 1988, 279-282.  
 

 

  
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