Fabrication Characteristics and Processes of Fabrication - Materials Suited for Specific Fabrication Processes
The fabrication characteristics of metals are explained under the heads: formability, castability, machinability and weldability.
(a) Formability:
The ability of a metal to be formed is based on ductility of the metal. Ductility is based on its crystal structures. The metal that has the face centred cubic crystal structure is most ductile because the crystal has the greatest opportunity for slip in four distinct nonparallel planes and three directions of slip in each plane.
The other factors which control ductility of the material are grain size, alloying elements and softening heat treatments such as annealing and normalizing. The small grain sizes are recommended for shallow drawing of copper and relatively large grains for heavy drawing on the thicker gauges.
Hot and cold working, also have an effect on ductility. The high pressure applied in hot drawing distorts the grains which determine the ductility; cold working also results in distortion of crystals. Generally, cold worked crystals are more distorted and are usually less ductile than the hot worked crystals.
Alloying elements in a pure metal normally reduce its ductility, because if they replace the atoms of pure metal it reduces the number of slip planes as it occurs in steel, which is an alloy of carbon and iron and so steel is less ductile than iron. If the alloy finds its room in the spaces between the atoms of pure metal it offers increased resistance to slip, which happens in steel when iron carbide precipitates in slip planes when steel solidifies. By softening heat treatment such as annealing which consists of heating the metal to the re-crystallisation temperature at which at first the grains may be very small but they grow in size as long as the metal is exposed to the high temperature, when the desired size is obtained the metal is allowed to cool. During recrystallization ductility of metal is
restored because distorted crystals are reformed in re-crystallisation.
The processes using the property of formability of metal are under two major categories: (i) Hot working and (ii) Cold working.
Hot working processes:
Rolling, forging, extrusion and hot pressing are hot working processes. In hot working the metal is heated sufficiently to make it plastic and easily worked. The temperature of the heated metal or alloy should be above the re-crystallisation temperature. This temperature is different for different metals.
Hot rolling is used to create a bar of material of particular shape and dimensions. The principal rolled steel sections arc plates, angles, tees, channels and joists; round, hexagonal and square bars for forging and machining operations; sheets, rails, etc. All of them are available in many different sizes and in different materials. The materials most available in the hot rolled bar sizes are steel, aluminium and copper alloys. Tubes may be manufactured by hot rolling of strips or plates; the product may be butt welded or lap welded.
Forging is the hot working of metals by hammers, presses or forging machines. For small work forging is carried out with hand hammers but for large work hammers and forging machines are used. Forging alters the internal structure of metals which results in increased strength and ductility. Compared with castings, forgings have greater strength for the same weight. Forging should be
carried out within proper temperature range. If the temperature is too high the metal will be weak and brittle. If the temperature is too low, there will be internal stresses which may lead to distortion or cracking.
Many small parts are drop forged. In drop forging, solid lump with little or no previous treatment by hand is squeezed between dies to the shape required with one or more blows from a drop hammer. The component can be made to dimensions and with a good surface so that machining may be unnecessary. The limitations of this process are that the number of parts should be great (production volume has to be high) and complicated shapes cannot be produced as they can not be removed from dies.
Extrusion is a process where a heated blank is caused to flow through a restricted orifice under great pressure. Very complicated shapes may be produced by the extrusion process. The process is restricted to materials of low melting points such as brass, aluminium and certain alloys of tin, lead and other soft metals.
Hot pressing consists of forming metal to shape in a very rigid type of power press. A hot piece of metal is pressed and extruded in suitable dies into a smoothly finished piece to accurate dimensions. Automobile valves are formed by this process.
Cold working processes:
In cold working is the forming of a metal is done usually at room temperature. In some cases, higher temperatures are used but always, the temperature is lower than re-crystallisation temperature of the material. Cold working may vary from a simple bend to great deformation produced by deep pressing and tube drawing. The result of cold work is to increase hardness and tensile strength but to decrease ductility and shock resistance. Cold worked parts have a bright new finish, are more accurate and require less machining. Where cold work is considerable, the part may be annealed at some intermediate stage or stages of work. In cold working the surface of a material is very important as scale may be worked into the finished article with serious results. Some of cold working processes are drawing, heading, spinning, stamping, etc.
Drawing is a process by which the cross section of a metal is diminished by pulling it through an accurately formed hole in a drawing die. The operation is performed cold and only simpler forms can be produced without excessive resistance and tearing.
Heading is a cold working process in which the metal is gathered or upset . This operation is commonly used to make screw and rivet heads. The blank is usually a piece of wire of suitable length and cross section; one end is cold forged in dies to form the desired shape of the head. Annealing may be required after cold heading.
Spinning is the operation of working sheet material around a rotating form into a circular shape. Pressure is applied to the sheet by means of a blunt nosed tool which presses it against the former. This is an economical method of forming parts if the quantities are small.
Stamping is the term used to describe punch press operations such as blanking, coining, forming and shallow drawing.
Powder metallurgy :
It is the art of making small components by heat treatment of compressed metallic powders, sometimes with inclusion of non-metallic material.
The powdered metals in desired proportions are compressed in moulds under a very high pressure varying from 700 to 14,000 kg/sq cm depending on the metal. The compacted part is heated at a temperature which is less than melting point of the major ingredient. T'he disadvantages of this method are (i) low strength of the component (ii) higher cost of material and (iii) the limited range of materials which can be used.
Filaments of refractory metals such as tungsten, self lubricating bearings, tungsten carbide tips for cutting tools and iron alloys for permanent magnets are examples of articles made from powdered metal. By this process small components can be made out of some metals whose melting point is too high to allow use of die casting.
(b) Castability:
Castability of a metal is judged to a large extent on the following factors: solidification rate, shrinkage, segregation, gas porosity, and hot strength.
Solidification Rate:
The ease at which a metal will continue (o flow after it has been poured in the mold depends on its analysis and pouring temperature. Some metals such as grey iron are very fluid and can be poured into thin sections of complex castings.
Shrinkage :
Shrinkage refers to the reduction in volume of a metal when it goes from a molten to a solid state. For steel, the amount of contraction amounts to about 6.9 to 7.4% by volume, or 2 cm per metre; grey iron contracts half as much. This shrinkage factor has to be taken into account by the pattern maker and designer, not only to allow for the proper finished .size, but also to sec that undue strains will not be encountered during shrinkage due to the mould design. Various elements can be added to the alloy to control fluidity and shrinkage as discussed later in this chapter.
Segregation :
As the metal starts to solidify tiny crystal structures resembling pine trees and referred to as dendrites start to form at the mold edges. As they form, they tend to exclude alloying elements. Subsequent crystals that form are progressively richer in alloy content as the metal solidifies. Thus the surface of the casting is not of the same quality as that in the centre. This is overcome in part at least by subsequent heat treatment, or very slow cooling.
Some metals in the molten state have a high affinity for oxygen and nitrogen. These gases become trapped as the metal solidifies creating voids or pinholes.
Hot Strength:
Metals are very low in strength right after solidification. This is especially true of the non-ferrous metals. Precautions must b(‘ taken at the lime of casting to avoid stress concentration that causes flaws and hot tears to develop as the metal solidifies .
Casting is the oldest form of metal shaping and is still the basic engineering process since most metals are melted and cast from ores. Castings are made of iron, steel, various brasses and bronzes, aluminium and its alloys and the various white metal alloys.
Patterns may be made of wood or metal and with its help the sand mould is formed in which molten metal is poured. The mould is dried before the metal is poured. Metal in cooling solidifies to the form outlined in the mould.
In die casting process the mould is usually made of steel and molten metal is poured or forced under pressure into the mould. This method is used for mass production only.
Non-ferrous alloys arc sometimes cast centrifugally. Molten metal is poured into a rapidly rotating cylindrical mould and is held against the mould by centrifugal force so that core is not required. On cooling the casting is complete. Such castings are generally denser and more homogeneous than ordinary sand castings. This process is limited to simple shapes and to fairly large quantities.
The following precautions should be observed in design of castings :
(i) All sections should be designed as far as possible with a uniform thickness.
(ii) All walls should be sufficiently thick to allow the molten metal to flow freely into all corners.
(iii) Adjoining sections should be designed with generous fillets or radii.
(iv) Parts should be designed so that patterns may be drawn readily from the moulds.
(v) A complicated part should be designed in two or more castings. These castings are assembled by fasteners.
(vi) Where the section uniformity is not possible, light sections should be blended into heavy sections.
Thickness of casting determined by calculations is often too small to permit production of good castings. Minimum values of the thicknesses for various castings are prescribed.
Material Minimum thickness in mm
Grey cast iron 6
Malleable cast iron 6
Steel casting 6
Brass 3
Bronze 3
Aluminium 3
(c) Machinability :
Machinability is the ease with which metal can be removed in operations such as turning, drilling, reaming, etc. Ease of metal removal requires that the forces acting against the cutting tools should be relatively low and the chips will be broken up, a good finish should result and the tools should last a reasonable period of time before it has to be replaced or resharpened. Machinability is also expressed as a machinability rating for each material.
This rating is given for most ferrous metals using steels 13S25 in the cold drawn conditions as the basis of 100% machinability. This value involves turning at a cutting speed of 54.9 surface metre per minute for feeds upto 0.1778 mm per revolution and depths cut upto 6.35 mm using appropriate cutting fluid with high speed steel T70W18Cr4V1 tools. Machinability of other metals will be judged with respect to this basis.
This property plays a predominant role in deciding the selection of material for components manufactured using machining on automatic machine for mass production. By adding alloying materials like sulphur and lead in steel its machinability can be increased, however, some reduction in tensile strength in takes place.
(d) Weldability:
It may be said that all metals are weldable by one process or another. However, the real criterion in deciding on the weldability of a metal is weld quality and the ease with which it can be obtained.
In deciding on weldability of a metal, the characteristics commonly considered are the heating and cooling effects on the metal, oxidation, and gas vaporization and solubility.
Heat and Cooling:
The effect of heat in determining the weldability of a material is related to the change in microstructure that results. For example, steels are sometimes considered weldable or not weldable on the basis of the hardness of the weld. The deposited weld metal may pick up carbon or other alloys and impurities from the parent metal that make it hard and brittle so that cracks result upon cooling.
The opposite effect may also be considered. A metal may have a certain hardness temper that will be changed by the heat of the weld. Although both of these conditions can be corrected by added precautions and heat treatment, they add to the cost and hinder the simplicity of the weld.
Oxidation :
Oxidation of the base metal, particularly at elevated temperatures, is an important factor in rating weldability of a metal. Metals that oxidize rapidly, such as aluminium, interfere with the welding process. The oxide has a higher melting point than the base metal, thus preventing the metal from flowing. It also may become entrapped in the weld metal, resulting in porosity, reduced strength, and brittleness
Gas:
Large volumes of troublesome gases may be formed in the welding of some metals. These gases may become trapped in the weld because certain elements vaporize at temperatures below those needed for welding. Not only will this cause porosity, but some of the beneficial effects of these elements are lost.
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