Fabrication of metals- forming operations,casting

Fabrication of metal

Metal fabrication technique are normally consistof metal forming method, casting,powder metallurgy, wielding. This process is proceed by refining alloy and upon heat treatment.

Metal fabrication technique:

1. Forming operation

Forging

Extrusion

Drawing

2. Casting

Send

Die

Investment

Lost foam

Continuous

3. Miscellaneous

Powder Metallurgy

Wielding

The metal fabrication technique depends upon several factors such as properties of matter, size and shape of metal to be finished, its cost and its uses.

Forming operation

Metal forming operation is those in which the shape of metal piece is changed by plastic deformation. Metal forming consists of different process:

• Forging
• Rolling
• Extrusion
• Drawing

Depending upon the temperature, the deformation of metal is achieved by two ways:

• Hot working
• Cold working

When deformation is achieved at a temperature above the temperature of re-crystallization, then the process is called hot working and if the deformation is achieved below the temperature at which re-crystallization occurs is known as cold working. For hot working operation, the large deformations are possible due to the fact that metals become soft and ductile. Cold working produced on decrease in strain which decreases in ductility, the metal becomes hard at low temperature.

Forging

It is the mechanically working or deforming a single piece of hot metal by the application of successive blows or by continuous squeezing, forging are classified as closedie or open die. In close die a force is applied to two die halves having the finished shape such as the cavity as shown in figure.

Rolling

It is the must widely used deformation process in which a piece of metal is placed between teo rolls. There is reduction in thickness due to compressive stress rxerted by rolls. Cold rolling may be used in the production of sheet, strip and thin foil with high quality surface finish.

Extrusion

It is the technique in which a metal bar is force through a die orifice (hole) by compressive a compressive stress applied to a ram as shown in figure. The piece that emerges has the desired shape and reduced cross sectional area. Extrusion product looks as rod tubing or it may have completed cross section geometry.

Drawing

It is the pulling of metal piece through a die having tapered bored by tensile force that is applied on the exist exit end. There is reduction in cross section and length is increased as shown In figure. The total drawing operation may consist of number of drawing in series. Rods , wire and tubing products are fabricated by this method.

• Casting operation

Casting is the fabrication process in which a totally molten metal is produced into a mold cavity having a desired shape upon solidification. Casting technique are employed when

• The finished shaped s so large or so complicated that any other method would be impracticable.
• A particular alloy is so low in ductility that forming by either hot or cold working would be difficult.
• In comparison to other fabrication process casting is the most economic.

The commonly used casting method are:-

1. Sand casting
2. Die casting
3. Investment casting
4. Lost foam casting
5. Continuous casting

Sand casting

With send casting ordinary sand is used as mold material. A two pieced mold is formed by packing same around of pattern that has the shape of intended casting. Sand cast parts include automotive cylinder blocks, fire hydrants and large pipe fitting.

Die casting

In die casting liquid metal is force into a mold under pressure and at high velocity and the material is allowed o solidified with the pressure maintained. Two piece of permanent steel mold a die is employed in the desired shape. When there is complete solidification the die is open and the piece is ejected. The technique is used to alloy aluminum and magnesium alloy zinc and other metals.

Investment casting

It is sometimes called ‘lost wax’ casting. The pattern is made from a wax or plastic that has a low melting temperature. Around the pattern is poured a fluid slurry which set up form a solid or investment plaster of paris is usually used.

The mold is then heated, such that pattern melts and is burned out leaving behind a mold cavity having the desired shape. This technique is used in jewelry, dental, crown and inlays.

Lost foam casting

A variation of investment cast is lost foam ( or expendable pattern) casting, here foam is formed by compressing polystyrene beads into desired shape and bounded them together by heating. Sand is packed around the patter to form the mold. As the molten metal is poured into the mold it replaces the pattern which vaporizes the compacted sense remains in place and upon solidification assumes the shape of mold. This technique is used in automobile engine block, cylinder heads and electric motor frames .

Continuous casting

Many molten metals are solidified by casting in to large ingot molds. The ingot are normally subjected to a primary hot rolling operation. The product of which is a flat sheet or slab. These casting and rolling steps are combined casting. Using this technique refine and molten metal is formed as rectangular or cross section (circular) geometry.

References:

Callister, W.D and D.G Rethwisch. Material Science and Engineering. 2nd. New Delhi: Wiley India, 2014.

Lindsay, S.M. Introduction of Nanoscience . New York : Oxford University Press, 2010.

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Poole, C.P. and F.J. Owens. Introduction To Nanotechnology. New Delhi: Wiley India , 2006.

Raghavan, V. Material Science and Engineering. 4th . New Delhi: Pretence-Hall of India, 2003.

Tiley, R.J.D. Understanding solids: The science of Materials. Engalnd : John wiley & Sons , 2004.