METAL FORMING AND POWDER MEALLURGY:

Forging:

Forging is a manufacturing process involving the shaping of metal using localized compressive forces. Forging is often classified according to the temperature at which it is performed: “cold”, “warm”, or “hot” forging. Forged parts can range in weight from less than a kilogram to 580 metric tons. Forged parts usually require further processing to achieve a finished part. Forging as an art form started with the desire to produce decorative objects from precious metals. Today, forging is a major world-wide industry that has significantly contributed to the development of man.

Principles of forging:

Forging is a massive forming process; the temperature of the workpiece is increased to such an extent that the deformation forces required are considerably less than would be needed to cold work it. The two most important forging processes are open-die forging (in which forming of the workpiece takes place locally and mostly using simple dies) and closed-die forging (where the workpiece is fully enclosed in a die whose form determines the shape of the forging).

For large-scale production, closed-die forging is usually used because it is regarded as being a very reliable process. Thanks to the superior mechanical properties obtained, the process can compete with the most advanced casting processes. Compared with casting, however, the range of possible shapes that can be produced is more limited. In particular, it is difficult to produce sharp corners, undercuts and cavities by use of forging.

For example, is used as the active medium. By using a sealed piston to increase the internal pressure, whilst at the same time closing both open ends of the hollow profile, the process can be used to produce hollow aluminium bodies having a uniform wall thickness. The forging process usually consists of the following steps:

  • sawing the extruded or continuously cast feedstock,
  • heating the blank,
  • upsetting or bending,
  • forging (rough and final forging),
  • deburring and, if necessary, punching,
  • heat treatment ,
  • pickling or blasting and
  • final inspection.

Unlike sheet forming, in forging there is always a change in the cross section of the feedstock. Generally speaking, changes to the cross-section are achieved either by material displacement (forging, extrusion,

rolling, and cold impact extrusion) or by material accumulation (upsetting).
At the same, the processes mentioned enable a change in the direction of flow of the material to occur.

Cavities are an added design possibility of massive forming.

Cutting processes are also always a part of the forging process. They are used to cut the feedstock, produce openings in forgings or produce blanks.

Applications of forging:

AUTOMOTIVE & TRUCK
The characteristics of forged parts strength, reliability and economy are what makes them ideal for vital automotive and truck applications. Forged components are commonly found at points of shock and stress such as wheel spindles, kingpins, axle beams and shafts, torsion bars, ball studs, idler arms, pitman arms and steering arms. Another common application is in the powertrain, where connecting rods, transmission shafts and gears, differential gears, drive shafts, clutch hubs and universal joints are often forged. Although typically forged from carbon or alloy steel, other materials such as aluminum and microalloyed steels are seeing great advances in forged auto and truck applications.

AGRICULTURAL MACHINERY & EQUIPMENT
Strength, toughness and economy are also important in farm implements. In addition to engine and transmission components, key forgings subjected to impact and fatigue range from gears, shafts, levers and spindles to tie-rod ends, spike harrow teeth and cultivator shafts.

VALVES, FITTINGS, OIL FIELD APPLICATIONS
Because of their superior mechanical properties and freedom from porosity, forgings are often associated with the high pressure applications of the valve and fitting industry. Corrosion and heat-resistant materials are used for flanges, valve bodies and stems, tees, elbow reducers, saddles and other fittings. Oil field applications include rock cutter bits, drilling hardware, and high-pressure valves and fittings.

HAND TOOLS & HARDWARE
Forged has traditionally been the mark of quality in hand tools and hardware. Pliers, hammers, sledges, wrenches and garden tools, as well as wire-rope clips, sockets, hooks, turnbuckles and eye bolts are common examples. Surgical and dental instruments are also often forged. Special hardware for electrical transmission and distribution lines such as pedestal caps, suspension clamps, sockets and brackets are commonly forged for strength, dependability and resistance to corrosion.

OFF-HIGHWAY EQUIPMENT/RAILROAD
Strength, toughness, machinability and economy account for the many uses of forgings in off-highway and heavy construction equipment, mining equipment, and material handling applications. In addition to engine and transmission parts, forgings are used for a wide variety of gears, sprockets, levers, shafts, spindles, ball joints, wheel hubs, rollers, yokes, axle beams, bearing holders and links.

GENERAL INDUSTRIAL EQUIPMENT
Forgings of great size are often found in industrial equipment and machinery used by the steel, textile, paper, power generation and transmission, chemical and refinery industries to name just a few. Typical forged configurations include bars, blanks, blocks, connecting rods, cylinders, discs, elbows, rings, T’s, shafts and sleeves.

ORDNANCE/SHIPBUILDING
Forged components are found in virtually every implement of defense, from rifle triggers to nuclear submarine drive shafts. Heavy tanks, missiles, armored personnel carriers, shells and other heavy artillery are common defense-related applications of forged components.

AEROSPACE
High strength-to-weight ratio and structural reliability can favorably influence performance, range, and payload capabilities of aircraft. Made of various ferrous, non-ferrous and special alloy materials, forgings are widely used in commercial jets, helicopters, piston-engine planes, military aircraft and spacecraft. Some examples of where a forging’s versatility of size, shape and properties make it an ideal component include bulkheads, wing roots and spars, hinges, engine mounts, brackets, beams, shafts, landing gear cylinders and struts, wheels, brake carriers and discs and arresting hooks. In jet turbine engines, iron-base, nickel-base and cobalt-base superalloys are forged into components such as discs, blades, buckets, couplings, manifolds, rings, chambers and shafts.

Rolling:

In metalworking, rolling is a metal forming process in which metal stock is passed through a pair of rolls. Rolling is classified according to the temperature of the metal rolled. If the temperature of the metal is above its recrystallization temperature, then the process is termed as hot rolling. If the temperature of the metal is below its recrystallization temperature, the process is termed as cold rolling. In terms of usage, hot rolling processes more tonnage than any other manufacturing process, and cold rolling processes the most tonnage out of all cold working processes.

There are many types of rolling processes, including ring rolling, roll bending, roll forming, profile rolling, and controlled rolling.

Extrusion:

Extrusion is a process used to create objects of a fixed, cross-sectional profile. A material is pushed or drawn through a die of the desired cross-section. The two main advantages of this process over other manufacturing processes are its ability to create very complex cross-sections and work materials that are brittle, because the material only encounters compressive and shear stresses. It also forms finished parts with an excellent surface finish.

Extrusion may be continuous (theoretically producing indefinitely long material) or semi-continuous (producing many pieces). The extrusion process can be done with the material hot or cold.

Commonly extruded materials include metals, polymers, ceramics, concrete, play dough, and foodstuffs.

Working principles of extrusion:

More plastic resin has processed through the extrusion process than through any other plastics manufacturing technique. This fact, however, is misleading. Very big amounts of plastics are processed by extrusion into pipes, films, sheets, coatings and so on. Extrusion is also used for producing pellets of thermoplastic materials. Therefore, the majority of plastic material produced in the world today has had at least one extrusion through an extruder. Then those materials will be used in other plastic processing methods, like injection moulding, blow moulding and other melt processing methods, including the extrusion.

Extrusion is a typical method to produce continuous products (pipes, sheets, films, coatings) of different kinds of materials. The first extruders were developed for food and building materials industry. Nowadays very many materials can be extruded into products. In the area of technical materials the polymeric materials are the most common ones.

During extrusion, a polymer melt is pumped through a shaping die into the final profile form. The first extruders were ram-type extruders and the first extruders were built in 1797 to extrude seamless lead pipes. The first ram-type extruders for rubber industry were designed 1845. In 1846 a patent for cable coating was filed for trans-gutta-percha and cis-hevea rubber and the first insulated wire was laid across the Hudson River for the Morse Telegraph Company 1849. The first screw extruder was patented 1879 for wire coating. The extrusion of thermoplastic polymers started in 1935.

Ram and screw extruders are both used to pump high viscous polymer melts through a die to form a continuous profile. They are based on different principles which are illustrated in figure 1.

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Figure 1 – Schematic of pumping principles

The extrusion process
The extruder is a melt pump. In the first stage the polymeric material is melted and then the melted material is forced via the die into the final form.

Today nearly all extruders are so called screw extruders, earlier there were different kinds of development stages. The most important step is the melting and homogenization of the polymeric materials. It can also be carried out with calenders. Multiscrew extruders have been developed for special purposes, especially for different kinds of mixing applications. One of the newest developments is the cone extruder, which will have many applications in pipe extrusion and other applications.

The extrusion process is best understood, when it is divided into key steps:

  1. Pretreatment of extruded material. This includes drying of materials, feeding of additives, preheating.
  2. Material is fed into the extruder through the throat, an opening that links the material hopper into the extruder barrel
  3. Forced feeders, if the feeding of materials is difficult or it is important that the material feed is very constant
  4. Inside the cylinder the raw material is conveyed from the feeding zone to the die. In this case it is very important that the friction between the cylinder is higher than the friction between the screw.
  1. The screw can be divided into three parts and a conventional screw is presented in fig. 11. The zones are: feeding zone, compression zone and homogenisation zone.
  2. The die. The melted material is pumped through the die into the final form.
  3. Calibration of the extrudate in the final dimensions and form
  4. Postprocessing of extrudates

Figure 2 is a schematic of a single screw extruder. In figure 3 there are some schematics of different twin screw extruders. Figure 4 is a schematic of a normal plasticating single screw extruder.

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Figure 2 – Schematic of a single screw extruder

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Figure 3 – Different twin screw extruders

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Figure 4 – A plasticating single screw extruder

The melting of viscoelastic polymeric material is carried out by external heating (outside the barrel by electricity or heated fluids) and internal heating (internal friction, molecular relaxations). The internal heating is the most important heating method in today’s extruders and its part of the heating energy is 80 – 100 %.

The melting process in the screw channel is presented in figure 5. There has been many attempts to intensify the melting process. Figure 6 presents the melt flow in the extruder cylinder.

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Figure 5 – Melting process in a screw channel

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Figure 6 – Melt flow in the cylinder

A modern extruder consists of different parts and the most important parts are:

The screw
The screw has the following tasks:

  • feeding of raw material from the feeder to the die
  • compression of the material
  • melting of the material
  • homogenization of the material
  • pressurising the material

A typical screw for rubber extrusion is presented in figure 7. Lots of studies have been made to develop more efficient screws for different applications. Nowadays the development of screws is based on the rheological facts of polymeric materials.

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Figure 7 – A typical screw for rubber processing

In figure 8 there are various screws for extrusion.

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Figure 8 – Screw structures for extrusion applications

The extrusion cylinder
The extrusion cylinders are normally significantly longer than in injection moulding. The cylinders are covered with heating (cooling) systems and the heating is made electrically or with heated liquids. The cooling is arranged normally with liquid or air.

The cylinders are made normally by using a bimetallic material which has high abrasion resistance. In many cases it can also be coated to increase the abrasion resistance.

The screen pack and breaker plate
The function of the screen pack is to filter out all possible contaminations from the polymer melt.

The breaker plate provides a controlled amount of back pressure for the extrusion process.

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Figure 9 – The screen pack and breaker plate

Gear pumps
Gear pumps are used in the extrusion process to provide a constant flow rate and pressure for the extrusion die. Figure 10 presents a conventional gear pump structure.

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Figure 10 – The screen pack and breaker plate

Tooling for extrusion
In the case of special products the tools are easiest to make. In the case of nonsymmetric products some die characteristics have to be taken into consideration.

Die swell: The extrudate is greater than the orifice from which it came. The die swell will also change the form of the extrudate (Figure 11).

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Figure 11 – Effect of die geometry

Flow surfaces: The surfaces which are in direct contact to the melt in the die have to be very smooth.

Heat: Dies are usually externally heated and this will eliminate the solidification of material in the die.

Wire drawing

Wire drawing is a metalworking process used to reduce the cross-section of a wire by pulling the wire through a single, or series of, drawing die(s). There are many applications for wire drawing, including electrical wiring, cables, tension-loaded structural components, springs, paper clips, spokes for wheels, and stringed musical instruments. Although similar in process, drawing is different from extrusion, because in drawing the wire is pulled, rather than pushed, through the die. Drawing is usually performed at room temperature, thus classified as a cold working process, but it may be performed at elevated temperatures for large wires to reduce forces. More recently drawing has been used with molten glass to produce high quality optical fibers.

Spinning:

Metal spinning, also known as spin forming or spinning or metal turning most commonly, is a metalworking process by which a disc or tube of metal is rotated at high speed and formed into an axially symmetric part. Spinning can be performed by hand or by a CNC lathe.

Metal spinning ranges from an artisan’s specialty to the most advantageous way to form round metal parts for commercial applications. Artisans use the process to produce architectural detail, specialty lighting, decorative household goods and urns. Commercial applications include rocket nose cones, cookware, gas cylinders, brass instrument bells, and public waste receptacles. Virtually any ductile metal may be formed, from aluminum or stainless steel, to high-strength, high-temperature alloys. The diameter and depth of formed parts are limited only by the size of the equipment available.

Powder metallurgy

Powder metallurgy is the process of blending fine powdered materials, pressing them into a desired shape or form (compacting), and then heating the compressed material in a controlled atmosphere to bond the material (sintering).

The powder metallurgy process generally consists of four basic steps:

  • Powder manufacture,
  • Powder blending,
  • And sintering.

Compacting is generally performed at room temperature, and the elevated-temperature process of sintering is usually conducted at atmospheric pressure. Optional secondary processing often follows to obtain special properties or enhanced precision.

The process of manufacturing of shaped components or semi-finished products such as bar and sheet from metal powder is called as Powder metallurgy.
The technique of powder metallurgy combines unique technical features with cost effectiveness and generally used to produce sintered hard metals known as ‘carbides’ or ‘tungsten carbides’.
This technique deals with the production of metal and non-metal powders and manufacture of components.
Powder metallurgy is generally used for iron based components.
The powders used as raw material can be elemental, pre-alloyed, or partially alloyed.
Elemental powders like iron and copper are more compressible and produce pressed compacts with good strength.
Pre-alloyed powders are harder but less compressible therefore require higher pressing loads to produce high density compacts.
Powder metallurgy technique has many advantage as well as limitation.
Some of the advantages of Powder Metallurgy are as follows;
1.    Powder metallurgy produces near net shape components. The technique required few or no secondary operations.
2.    Parts of powder metallurgy can be produce from high melting point refractory metals with less cost and difficulties.
3.    The tolerance of components produced by this technique have quite high tolerance, therefore no further machining is not required.
4.    This technique involves high Production Rate along with low Unit Cost.
5.    It can produce complicated forms with a uniform microstructure.
6.    Powder metallurgy has full capacity for producing a variety of alloying systems and particulate composites.
7.    This technique has flexibilities for producing PM parts with specific physical and mechanical properties like hardness, strength, density and porosity.
8.    By using powder metallurgy, parts can be produced with infiltration and impregnation of other materials to obtain special characteristics which are needed for specific application.
9.    Powder metallurgy can be used to produce bi-metallic products, porous bearing and sintered carbide.

10.  Powder metallurgy makes use of 100% raw material as no material is wasted as scrap during process.
Disadvantages of Powder Metallurgy:
1.    The production of powder for metallurgy is very high.
2.    The products of metallurgy can have limited shapes and features.
3.    This technique causes potential workforce health problems from atmospheric contamination of the workplace.
4.    The tooling and equipments require for powder metallurgy are very expensive, therefore becomes main issue with low production volume.
5.     It’s difficult to produce large and complex shaped parts with powder metallurgy.
6.    The parts produce by powder metallurgy have low ductility and strength.
7.    Finally divided powder like aluminium, magnesium, titanium and zirconium are fire hazard and explosive in nature.
8.    This technique is not useful for low melting powder such as zinc, cadmium and tin as they show thermal difficulties during sintering operations.

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