JAR 66 CATEGORY B1 MODULE 6
6 METHODS USED IN SHAPING METALS
engineering
uk
6 METHODS USED IN SHAPING METALS
There are four basic methods of converting raw material into the required manufactured shape whilst also achieving the desired material structure. They are casting, deformation, machining, and various forms of fabrication (i.e. the joining together of smaller pieces or particles of material to form a larger object).
Welding, adhesive bonding, mechanical fasteners or even powder metallurgy come under this latter heading.
Casting exploits the fluidity of a liquid as it takes shape and solidifies in a mould.
Deformation exploits the remarkable property of materials (mostly metals) to flow plastically in the solid state without deterioration of their properties. Processes such as these, result in a minimum of material waste.
Machining processes provide excellent precision, but the process generates a large amount of waste material. Fabrication techniques enable complex shapes to be constructed from simpler particles or units.
6.1 CASTING
This involves the pouring of molten material into a shaped mould and allowing it to solidify to that shape. It is an ancient process, which enables complex shapes to be produced in a wide range of materials in a single-step operation. Cast components can range in size from the small teeth of a zip, to large casings of several metres in diameter. Ocean-going ships’ propellers, up to 10 metres in diameter, are produced this way. Modern casting techniques have resulted in:
high quality (i.e. minimum porosity and reasonably defect-free products)
high production rates
good surface finish
small dimensional tolerances
the ability to cast a very wide range of materials.
Moulds are made in a variety of materials including plaster and ceramics but, by far, the most widely used are those of sand and metal.
6.1.1 SAND-CASTING
The two basic types of sand-casting are:
Removable/re-usable pattern (usually wood or metal)
Disposable pattern (e.g. polystyrene patterns, which vaporise when the metal is poured).
JAR 66 CATEGORY B1 MODULE 6 MATERIALS AND
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Although sand-casting is simple in principle, there are many vital aspects of the technique, which are necessary to produce good castings. The sand, for
example, must have:
Adequate binding qualities (to achieve this, a small percentage of clay is added).
Suitable porosity characteristics (to permit the escape of gas/steam, formed in the mould). There are different requirements for different metals (e.g. steel and aluminium).
Correct grain size and sufficient strength (the sand is graded by means of a sieve and the strength is controlled by the amount of bonding agent present).
Suitable temperature resistance (i.e. the sand must withstand the molten metal temperature without fusing/melting).
Adequate hardness (the hardness may be checked by the resistance to indentation by a spring-loaded ball).
Acceptable moisture content levels (this is usually in the range of 2% to 8%
and is checked by weighing the sand before and after drying).
While the characteristics of the sand are important, the design of the mould must also meet certain standards, some of which are:
The top and bottom halves of the mould (‘cope’ and ‘drag respectively), must incorporate positive alignment features.
The pattern must be shaped such that withdrawal from the sand leaves a perfect impression. Tapered faces are, therefore, better than perpendicular faces.
Suitable feed channels must be provided for the molten metal to enter the mould. These channels are called the ‘sprue’ and the ‘runners’.
Strategically placed reservoirs (called `risers') must be incorporated to ensure proper filling of the mould as the metal shrinks and begins to solidify. Typical steel shrinkage is around 3%-4% and aluminium shrinkage, 6%-7%.
The incorporation of vents, where necessary, to permit the escape of gas and steam when the molten metal contacts the sand.
Local ‘chills’ are sometimes included in the mould, to encourage more rapid, local solidification of the metal.
JAR 66 CATEGORY B1 MODULE 6 MATERIALS AND
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6.1.2 ADVANTAGES/DISADVANTAGES OF SAND-CASTING
The advantages of sand-casting are that it is a simple process, which does not require elaborate equipment and is economical for small batches. It is also
suitable for most metals. The major shortcomings are that the process is not very rapid, it is not particularly accurate (due to lack of sand rigidity) and it is not suitable for thin-wall sections.
6.1.3 TYPICAL CASTING DEFECTS
Casting defects vary to some extent, depending on the casting process used, but the most common ones are:
Inclusions (e.g. sand or mould lining material sticking to the surface)
Porosity (usually caused by gas/vapour, which is unable to escape before solidification)
Cold Shuts (when local areas of metal are not molecularly joined, due to solidification occurring too rapidly).
Hot Tears (where the material is cracked by excessive tensile stresses, resulting from thermal contraction).
6.1.4 SHELL-MOULDING
Shell-moulding is a process in which a thin shell is produced, by bringing a mixture of sand and a thermosetting resin into contact with a heated pattern.
When a sufficiently thick shell has been produced, the shell is finally cured (backed up by sand or steel shot in a moulding box). The subsequent casting process is then the same as for normal sand-casting. The advantages of shell- moulding over conventional sand-casting are:
it can be semi-automated, which reduces cost
finer sand can be used, which results in a smoother surface finish.
6.1.5 CENTRIFUGAL-CASTING
This technique involves the molten metal being poured into a rotating mould. The process is used for the manufacture of hollow cylinders (e.g. cylinder liners), bronze or white metal bearings etc. The rotation can result in acceleration forces of up to 60g and this produces high-quality, dense castings, since all of the slag migrates to the bore (due to it being of lower density than the metal) and it can then be machined out.
JAR 66 CATEGORY B1
This process uses a permanent metal mould, which results in more accurate, and better finished, castings than those produced in sand. Die-casting, can be sub- divided into ‘gravity’ or ‘pressure’ processes, depending on how the metal is fed into the mould.
Gravity Die-Casting - sometimes known as ‘Permanent-Mould Casting’.
This casting process is virtually identical to sand-casting except that the mould (die) is metal. A wide range of metals can be cast and hollow castings are possible if a sand core is used. Fine grain structures are produced, due to the more rapid rate of cooling, compared to that achieved in sand-casting.
Pressure Die-Casting - as implied, molten metal is fed under high pressure (thousands of psi) and held during solidification. Most die-castings are in non-ferrous materials (aluminium, magnesium, zinc, copper and their alloys), because steels have too-high a melting temperature for the metal dies to accommodate. The dies are, usually, made from hard, tool-steels and are water cooled. This process can achieve excellent detail, super finish, low porosity, and thin sections. Expensive equipment is necessary, but very high production rates are possible. Automatic ejection occurs and, on small components, 100 units per minute is not uncommon. Hollow castings cannot be made by die-casting.
6.1.7 INVESTMENT-CASTING (LOST WAX)
This is a very old method of casting (which was used by the ancient Chinese), but it only became of great industrial importance in the 1950's, when gas turbine manufacturing began to increase. The process was ideally suited to the production of complex-shaped nozzle guide vanes and turbine blades which, often, contained tortuous inner passages, very thin sections and had to be cast in exotic materials. The basic process is as follows:
A master die is made first from an easily worked metal such as brass.
Hot wax is then injected into the die, under pressure, to produce a wax pattern.
The wax pattern is then removed from the die and coated with a layer of investment material (a ceramic slurry or paste), usually by dipping a number of times.
When the investment coating is set, it is then heated to allow the wax to run out, and molten metal is then poured into the investment mould.
When cool, the investment coating is then broken away from the cast, metallic component.
JAR 66 CATEGORY B1 MODULE 6 MATERIALS AND
HARDWARE
engineering
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For obvious, reasons this investment-casting process is often referred to as the
‘Lost Wax’ process. It is a technique, which is capable of producing precision castings with a dimensional accuracy of less than 0.1 mm. Surface finish is also excellent, but the major advantage, that the process offers, is the ability to produce accurate, complex shapes which would be impossible by machining.
6.2 FORGING
This is a squeezing/hammering technique, which is intended to achieve large deformation/shaping of the material. The process is usually carried out hot (i.e.
above the re-crystallisation temperature), so that these large deformations can be attained without being accompanied by any massive, residual stresses.
Sometimes a cold forging operation may be necessary but, in this instance, the material will be harder, stronger and pre-stressed (i.e. still containing unrelieved internal stresses).
Forging ranges from the simplest form of the hand operations, conducted by the blacksmith, to the massive, mechanical, powered rams, used for very large forgings. The forging hammer will often have a relatively low strike rate, but sometimes high-speed, pneumatic hammers are used for High-Energy-Rate Forming.
Forging not only shapes the metal, but also reduces grain size and produces a directional control of grain flow. Both of these are desirable features for many engineering applications, particularly for highly-stressed components, such as crankshafts and especially if they are subject to a mechanical fatigue
environment.
JAR 66 CATEGORY B1 MODULE 6 MATERIALS AND
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6.2.1 DROP-STAMPING
Drop-stamping, or drop-forging (refer to Fig. 16), involves the use of shaped dies and a heavy drop-hammer, which usually falls under gravity. The piece of
material, to be forged, is placed between the top and bottom dies and the drop-hammer is allowed to fall the necessary number of times for the contact faces of the dies to come together. ‘Flash gutters’ are provided, to accommodate the excess metal (flash), which squeezes out between the top and bottom dies.
Connecting rods are typical components made by the drop-forging process.
6.2.2 HOT-PRESSING
Hot-pressing is similar, in principle, to drop-forging, but is actuated by one, long, steady, squeezing operation, as compared to a number of blows. This process tends to affect the whole structure of the component, whereas some forging processes, using multi- (but light) blows will, mainly, affect the material closest to the surface.
6.2.3 UPSETTING
Upsetting is, sometimes, called ‘Heading’ and usually involves locally heating of the end or ends of the material, immediately prior to forging. Poppet valves are formed in this way, as well as forged bolts. Sometimes this process is done cold (in which case it is referred to as ‘Cold Heading’), and some rivet heads are formed in this way.
The Drop-Forging Process Fig. 16
JAR 66 CATEGORY B1 MODULE 6 MATERIALS AND
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engineering
uk
6.3 ROLLING
Rolling can be carried out hot or cold.
When done hot ,it is capable of achieving major re-forming/re-shaping, and slabs can be reduced to plate or sheet while bars of circular or rectangular cross
section can also be produced. Hot rolling can also produce structural shapes such as ‘H’ or ‘I’ section beams.
If the rolling is done cold, it is aimed at improved surface quality, better accuracy, and increased hardness/strength. Hot, dilute, sulphuric acid is used to remove the hot scale from steel prior to cold rolling. The rolling process would also be used to produce the clad (and unclad) sheets of aluminium alloys.
6.4 DRAWING
Drawing is a purely, tensile operation, usually carried out hot. Wire, rod and tubing, can be produced by this process, where the material is pulled through a shaped, hardened die. A ductile material is essential.
6.5 DEEP DRAWING/PRESSING
This process uses a ram, to deform a piece of sheet metal into a recessed die and is usually done hot.
6.6 PRESSING
Pressing involves the use of male and female formers for shaping sheet material.
The sheet is placed between the formers, which are then forced together by a powered ram. Pressing is usually done hot (except for the soft, ductile materials).
6.7 STRETCH-FORMING
This is a technique used for shaping sheet metal over a stretch-block or former.
The sheet metal is firmly gripped by clamps and the sheet is then stretched over the former (by moving the clamps or the former) and the material is stretched beyond its elastic limit so that permanent deformation occurs.
This process is convenient for small batches of material (and is particularly financially attractive since only one former is needed) but, local changes of form (concave/convex or vice versa) cannot be produced by this process.
6.8 RUBBER-PAD FORMING
In principle this process uses a flexible, rubber-pad, attached to a hydraulic ram, which forces a piece of sheet metal to conform to the shape of a forming block.
Like stretch-forming, the process only uses one former, so it eliminates critical
JAR 66 CATEGORY B1 MODULE 6 MATERIALS AND
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batches (e.g. aircraft production), low-cost, easy to machine, materials can be used for the forming block.
Rubber-Bag forming (Hydro-forming) uses the same principle, but incorporates a flexible diaphragm and hydraulic pressure in place of the rubber pad.
6.9 EXTRUDING
The extrusion process, forces hot metal through a shaped die, to produce circular, rectangular, tubular, angular, half-round sections etc.
In some respects, the process is similar to drawing, but extruding forces metal from a heated billet, through hardened dies by compression, whereas, in drawing, it is achieved by tension. Malleability is, therefore, an essential material property for the extrusion process.
Extruding is normally restricted to aluminium alloys and copper alloys, where extrusion temperatures of 400ºC-500ºC and 650º-1000ºC respectively are used.
Steel is extremely difficult to extrude, due to the excessive pressures required.
6.9.1 IMPACT-EXTRUSION
This process is, usually, a cold-forming operation, which is suitable to very soft and malleable materials (e.g. aluminium). The shaped component is formed, by forcing a punch onto a ‘blank’ of material within a shallow recess. The extruded shape results from the metal being forced to escape through the small gap, between the punch and the recess.
6.10 SINTERING
Sintering; involves metal, in powder form, which is heated to approximately 70%-80% of its melting temperature and then squeezed to shape in a die.
The process is often used to form components made from materials with a very high melting temperature (e.g. tungsten). It also allows non-metallic materials, such as graphite and carbon, to be incorporated into the mixture.
The operation is usually conducted in a controlled atmosphere (typically argon or nitrogen) to prevent oxidation. Under the high pressures used, a metallurgical bond occurs (diffusion bonding), between the particles of powder. The sintered end-product is, typically, around 10%-20% porous and can then be impregnated with graphite (or high melting-point grease), to provide excellent, self-lubricating properties for plain bearings, bushes etc.
Sintering can be used where the combined properties of materials are required, as when copper and graphite are used for electrical brushes (i.e. copper to carry the current and graphite to act as a low-friction contact)
JAR 66 CATEGORY B1 MODULE 6 MATERIALS AND
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Tungsten carbide cutting tools can also be produced in this way, by incorporating tungsten carbide particles within a cobalt matrix.
Hot, Isostatic-Pressing, uses a similar technique to sintering, but uses higher temperature and very much higher pressures to produce zero porosity. The technique is sometimes used to heal micro-porosity in super-critical castings.) 6.11 SPINNING
Spinning is an old process, in which a piece of sheet metal may be formed, to shape, around a rotating former, which is mounted on the spindle of a lathe. The necessary force to deform the sheet metal is generated by a long tool, which is levered about a suitably positioned fulcrum.
For thin gauge, soft metals, the tool can be manipulated by hand, while, for
thicker gauge materials, a hydraulic actuator is used on a purpose-built machine.
Cones, flares, bowls and bell-mouth shapes, are produced by spinning.
6.12 CHEMICAL MILLING
Chemical milling is, sometimes, referred to as chemical etching. It is a purely chemical process, not electro-chemical.
Although simple in principle, chemical milling offers a method of producing complex patterns and lightweight parts and is used for incorporating integral ribs and stiffeners in sheet metal. Tapered sections can also be easily formed - the unwanted material being eaten away by a suitable chemical.
The process is ideally suited to aluminium alloys. The chemical, in this instance, is a hot alkaline solution (usually caustic soda) and, while it is a relatively slow process, its unique advantages make it very attractive for airframe components.
The areas, which must not be eaten away by the fluid, are simply protected by a thin layer of plastic, which can be brushed or sprayed on.
Although the chemically etched surface is not very rough, a drop in fatigue strength does result and, in critical applications, restoration of fatigue strength is desirable. A light, peening operation, using glass beads or steel shot, achieves this.
6.13 ELECTRO-CHEMICAL MACHINING
Using electrolysis and, by making the workpiece the anode of the dc electrical circuit, an electrolyte is pumped rapidly (under pressure) through the gap between the shaped cathode (also referred to as the tool) and the workpiece.
The tool is moved slowly towards the workpiece, by a ram, so that metal is progressively removed from the workpiece, until the desired shape is achieved
JAR 66 CATEGORY B1 MODULE 6 MATERIALS AND
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engineering
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The process is ideal for metals, which are difficult to machine by conventional methods, and the finish achieved is good. High electric current is required, and other, essential, requirements for the process are that the tool needs to be a good conductor (copper or brass) and it must resist corrosion, because the electrolyte is often a salt solution.
6.14 ELECTRO-DISCHARGE MACHINING E.D.M.
This process is, sometimes, called spark machining (or spark erosion), because, rather than using electrolysis, the technique involves the removal of metal by the energy (and heat) of electrical sparks, which travel from the electrically negative tool electrode, through a dielectric fluid, and explosively strike the electrically positive workpiece.
The intense heat of the strike, causes local particles of metal to instantaneously vaporise, without a molten metal phase (a process known as ‘sublimation’), though, away from the actual centre of the explosion, molten fragments of metal are washed away, with the vapour, by the dielectric fluid.
A suitable fluid (usually kerosene) is fed, under pressure, between the electrode and the workpiece, to maintain a uniform electrical resistance. The spark rate is around 10,000 per second and the gap between the tool and the workpiece is critical and must be maintained, throughout the operation, at approximately 0.025 mm - 0.075 mm (0.001 in - 0.003 in).
The real advantage of EDM is that, not only is it suitable on materials which are difficult to machine conventionally, but it also excels in its ability to produce high-aspect ratio, very small holes of any cross-sectional, in very hard metals.
Typical holes achievable, by this method, are in the regions of 0.025 mm diameter x 750 mm deep (0.010 in x 3 in).
A novel variation of EDM is a technique sometimes referred to as ‘wire-cutting’, which uses a moving, fine piece of copper or nickel wire as the electrode. The wire, 0.05 mm - 0.25 mm in diameter (0.002 in - 0.010 in), is positioned by, and fed over, two pulleys and resembles a simple band-saw operation. The workpiece is mounted on a table, which can be moved in two axes and, when the table is computer controlled, the wire-cutting process can cut accurate, complex shapes in metals (e.g. dovetails, fir-trees etc.) which are difficult to machine with
A novel variation of EDM is a technique sometimes referred to as ‘wire-cutting’, which uses a moving, fine piece of copper or nickel wire as the electrode. The wire, 0.05 mm - 0.25 mm in diameter (0.002 in - 0.010 in), is positioned by, and fed over, two pulleys and resembles a simple band-saw operation. The workpiece is mounted on a table, which can be moved in two axes and, when the table is computer controlled, the wire-cutting process can cut accurate, complex shapes in metals (e.g. dovetails, fir-trees etc.) which are difficult to machine with