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Centrifugal Casting

Centrifugal casting consists of producing castings by causing molten metal to solidify in  rotating  moulds.  The  speed  of  the  rotation  and  metal  pouring  rate  vary  with  the alloy and size and shape being cast. The following operations include in centrifugal casting  –  rotation  of  mold  at  a  known  speed,  pouring  the  molten  metal,  proper solidification rate, and extraction of the casting from the mold. The idea of employing centrifugal  force  to  make  castings  had  been  known  for  a  long  time,  it  was  A.  G. Eckhardt’s original patent of 1809 which revealed understanding the basic principles involved.  Centrifugal  casting  has  greater  reliability  than  static  castings.  They  are relatively free from gas and shrinkage porosity. Many times, surface treatments such as  case  carburizing,  flame  hardening  and  nitriding  have  to  be  used  when  a  wear resistant surface must be combined with a hard tough exterior surface.

One such application is bimetallic pipe consisting of two separate concentric layers of different alloys/metals bonded together.   Such pipes can be economically used  in  many  applications  and  can  be  produced  by  centrifugal  casting  process. Typically,  in  centrifugal  casting,  the  following  structure  or  zones  may  occur,  Chill Zone – This layer is of fine equiaxed structure which forms almost instantaneously at the mould wall, Columnar Zone – This is next to chill zone. It consists of directionally oriented crystals approx. perpendicular to the mould surface, Equiaxed zone – this region may occur next to columnar zone characterized by large number of uniformly grown crystals. Centrifugal casting is suitable for the production of hollow parts, such as pipes. The process is suited for producing structures with large diameters – pipes for oil, chemical industry installations and water supply, etc.

 

 

 

 

 

 

 

Thornton1  suggested 50 – 100 G speed range for die cast (metal mould) and 25 – 50 G  for  sand  cast  pots  and  shaped  castings.  Too  high  speed  results  in  excessive stresses and hot tears in outside surfaces.

Defects in Centrifugal Casting

Conventional   static   casting   defects   like   internal   shrinkage,   gas   porosity   and nonmetallic inclusions are less likely to occur in centrifugal casting.

Hot  Tears  –  Hot  tears  are  developed  in  centrifugal  castings  for  which  the  highest rotation  speeds  are  used.  Longitudinal  tears  occur  when  contraction  of  casting combined with the expansion of the mould, generates hoop stresses exceeding the cohesive strength of the metal at temperatures in the solidus region.

Segregation  –  Centrifugal  castings  are  under  various  forms  of  segregation  thus pushing less dense constituents at centre.

Banding  –  Sometimes  castings  produce  zones  of  segregated  low  melting  point constituents  such  as  eutectic  phases  and  sulphide  and  oxide  inclusions.  Various theories explain this, one states vibration is the main cause of banding.

Characteristics of Centrifugal Casting

1)  The casting is relatively free from defects.
2)  Non metallic impurities which segregate toward the bore can be machined off.
3)  Less  loss  of  metal  in  tundish  compared  to  that  in  gating  and  risering  in conventional sand casting.
4)  Better mechanical properties.
5)  Production rate is high.
6)  Can be employed to manufacture bimetallic pipes.
7)  Centrifugal  casting  process  can  be  used  for  fabricating  functionally  gradient metal matrix composite material.

Bimetallic pipes can be produced by centrifugal casting by using a cheaper material in place of a highly alloyed material. This will reduce cost of the bimetallic casting. Initially outer metal is poured in the rotating mould (mould is coated with a refractory coating)  followed  by  pouring  of  second  material  with  some  time  gap.  When  the freezing  is  complete  the  tube  has  an  annular  weld  or  diffusion  zone.  The  second metal should be poured in the rotating mould after the first metal has lost fluidity. If second metal is poured earlier then the composition and thickness of second metal will be changed. Also if second metal is poured late than the first metal then there won’t be good bonding.

Centrifugal Casting
Centrifugal Casting Set-Up
Centrifugal Casting Set up (Backside View)
Pouring Operation in Rotating Mould

 

 

 

 

 

 

 

 

The  centrifugal  casting  can  be  used  for  metal  matrix  composite  (MMC)  melts.  For example if stir cast aluminium/graphite melt is poured in the rotating mould, graphite particles  will  segregate  in the inner periphery of  the  centrifugal  casting  as  graphite density  is  lower  than  aluminium.  Thus  such  casting  can  be  employed  for  bearing applications5.

Applications

Pipes for water, gas and sewage; bearing bushes; cylinder liners; piston rings, paper making rollers; clutch plates; pulleys.

Some Example Castings

Aluminum Casting
Aluminum Casting

 

Aluminum Casting
Bimetallic Pipe (Outer layer – stainless steel, inner layer – mild steel)
Mild steel casing
Mild steel casing
Lead casing

References
1) Thornton, British Foundryman, 51, 559, 1958.
2) Cumberland J, ‘Centrifugal casting techniques’, British Foundryman, pg. 26-46, Jan 1963.
3) Jones M C, ‘Investigation of Centrifugal casting techniques’, Foundry trade journal, June, 1970.
4) Samuels M L, Schuh A.E, ‘Some recent developments in centrifugal casting’, Foundry, pg. 84, 1951.
5) P. K. Rohatgi, ‘Nonferrous Casting Alloys’, ASM Handbook, Volume 15: Casting.

Author – Amit M Joshi
(B.Engg. Mechanical, A.M.I.Prod.E, A.I.E)
Dept. of Metallurgical Engg. & Material Science,
Indian Institute of Technology – Bombay, India.

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