Basic Manufacturing Process - Ch.5.3

Q.9    List the advantage and disadvantages of extrusion.
Ans.    Application of extrusion: -
1. Production of lead sheathing: -

    One of the earliest applications of extrusion was in the production of the lead sheathing for cables used in electrical industries.
2. Production collapsible tubes: -
    Disposable tubes made of lead, tin and aluminium, are used as containers for a wide range of domestic materials, such as shaving cream, toothpaste, medicines greases and condensed milk, such as collapsible tubes are generally produced by impact extrusion in heavily constructed mechanical presses.
3. Production of various wrought products: -
    Extrusion process may be used to obtain final wrought products of various shapes and sizes in a single operation from the cast billet. The process becomes economical provided that the required output of material is large enough to justify the big equipment and installation cost.
4. Production of seamless pipe and tubing: -
    Pipe and tubing may be classified as weld or seamless depending upon the method of manufacturing. Welding pipe and tube are produced by lap butt and seam welding of roll bent sheets. White seamless tubes may be produced by extrusion, rolling and drawing operations. These operations can be used in three ways.
(a)    One of the methods of extruding a tube is to use a solid billet which is pierced and extruded in one step in the extrusion press.
(b)    Another method extruding a tube is to use a hollow billet.
(c)    A third method of extruding tubing, which is used with aluminium and magnesium alloys.
Extrusion Defects: -
Surface Cracking : When the surface of an extrusion splits. This is often caused by the extrusion temperature, friction, or speed being too high. It can also happen at lower temperatures if the extruded product temporarily sticks to the die.
Pipe : A flow pattern that draws the surface oxides and impurities to the center of the product. Such a pattern is often caused by high friction or cooling of the outer regions of the billet. 
Internal Cracking : The centre of an extruded product can develop cracks (also known as centreburst, centre cracking, arrowhead fracture of chevron cracking) due to a state of hydrostatic tensile stress at the centreline of the deformation zone in the die.

Q.10    Differentiate between hot and cold working of metals. Bring of the advantage and disadvantages of each of these techniques.                                                           (AKTU - 2009- 10)
Related Questions -
Q.    Compare hot working and cold working processes with suitable examples. Also discuss the advantages and disadvantages of each.                                                                (AKTU - 2011 - 12)
Ans.    Hot Working: -

        In materials science, hot working refers to processes where metals are plastically deformed above their recrystallization temperature. Being above the recrystallization temperature allows the material to recrystallize during deformation. This is important because recrystallization keeps the materials from strain hardening, which ultimately keeps the yield strength and hardness low and ductility high.  This contrasts with cold working.
Advantages And Disadvantages: -
    The advantages are:
  • Decrease in yield strength, therefore it is easier to work and uses less energy or force.
  • Increase in ductility.
  • Elevated temperatures increase diffusion which can remove or reduce chemical inhomogeneities.
  • Pores may reduce in size or close completely during deformation.    
In steel, the weak, ductile, face-centered-cubic, austenite microstructure is deformed instead of the strong body-centered-cubic, ferrite microstructure found at lower temperatures.
    Usually the initial workpiece that is hot worked was originally cast. The microstructure of cast items does not optimize the engineering properties, from a microstructure standpoint. Hot working improve the engineering properties of the workpiece because it replaces the microstructure with one that has fine spherical shaped grains. These grains increase the strength, ductility, and toughness of the material.
    The engineering properties can also be improved by reorienting the inclusions (impurities). In the cast state the inclusions are randomly oriented, which, when intersecting the surface, can be a propagation point for cracks. When the material is hot worked the inclusions tend to flow with the contour of the surface, creating stringers. As a whole the strings create a flow structure, where the properties are anisotropic (different based on direction). With the stringers oriented parallel to the surface it strengthens the workpiece, especially with respect to fracturing. The stringers act as “crack-arrestors” because the crack will want to propagate through the stringer and not along it.
The disadvantages are:
  • Undesirable reactions between the metal and the surrounding atmosphere (scaling or rapid oxidation of the workpiece).
  • Less precise tolerances due to thermal contraction and warping from uneven cooling.
  • Grain structure may vary throughout the metal for various reasons.
  • Requires a heating unit of some kind such as a gas or diesel furnace or an induction heater, which can be very expensive.
Cold Working: -
    A number of operations performed in hot working and cold working are common. However, cold working has an altogether different effect on the structure and physical properties of the worked metal. Cold working requires much higher pressures than hot working. It does not provide an appreciable reduction in size. In cold working the deformation of the metal takes place by a mechanism called slip. The extent to which a metal can be cold worked depends on its ductility. The higher the ductility of the metal, the more it can be cold worked. Cold working is carried out below the recrystallization temperature. Since recrystallization does not take place in cold working, the grains are permanently distorted. Residual stresses are setup during cold working and heat treatment is necessary to neutralize these stresses.
    Hot formed steel products are cold worked by processes like squeezing, bending, drawing and shearing to obtain thinner material, better surface finish, improved mechanical properties and greater dimensional accuracy.
Advantage of Cold Working: -
(1)     Cold working increases the strength and hardness of the material due to the strain hardening which would be beneficial in some situations. Further there is no possibility of decarburisation of the surface.
(2)     Since the working is done in cold state, hence no oxide formation on the surface and consequently, good surface finish is obtained.
(3)     Greater dimensional accuracy is achieved.
(4)     Easier to handle cold parts and also economical for small sizes.
(5)     Better mechanical properties are achieved.
Disadvantages/Limitations of Cold Working: -
(1)     Only small sized components can be easily worked as greater forces are required for large sections. Due to large deforming forces, heavy and expensive capital equipment is required.
(2)     The grain structure is not refined and residual stresses have harmful effects on certain properties of metals.
(3)     Many of the metals have less ductility e.g., carbon steel and certain alloy steels, cannot be cold worked at room temperature. It is therefore, limited to ductile metals and the range of shapes produced is not as wide as can be obtained by machining.
(4)     Tooling costs are high and as such it is used when large quantities of similar components are required.
Hot Working Vs Cold Working: -        

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