Forging is the shaping of a piece of metal by pushing with open or closed dies. It is usually done hot in order to reduce the required force and increase the metal’s plasticity.
Open-die forging is usually done by hammering a part between two flat faces. It is used to make parts that are too big to be formed in a closed die or in cases where only a few parts are to be made and the cost of a die is therefore unjustified. The earliest forging machines lifted a large hammer that was then dropped on the work, but now air- or steam-driven hammers are used, since these allow greater control over the force and the rate of forming. The part is shaped by moving or turning it between blows. A forged ring can be formed by placing a mandrel through the ring and deforming the metal between the hammer and the mandrel. Rings also can be forged by rolling with one roll inside the ring and the other outside.
Closed-die forging is the shaping of hot metal within the walls of two dies that come together to enclose the workpiece on all sides. The process starts with a rod or bar cut to the length needed to fill the die. Since large, complex shapes and large strains are involved, several dies may be used to go from the initial bar to the final shape. With closed dies, parts can be made to close tolerances so that little finish machining is required.
Two closed-die forging operations given special names are upsetting and coining. Coining takes its name from the final stage of forming metal coins, where the desired imprint is formed on a smooth metal disk that is pressed in a closed die. Coining involves small strains and is done cold to enhance surface definition and smoothness. Upsetting involves a flow of the metal back upon itself. An example of this process is the pushing of a short length of a rod through a hole, clamping the rod, and then hitting the exposed length with a die to form the head of a nail or bolt.
Metallurgy
An important benefit of hot working is that it provides control over and improvement of mechanical properties. Hot-rolling or hot-forging eliminate much of the porosity, directionality, and segregation that may be present in cast shapes. The resulting “wrought” product therefore has better ductility and toughness than the unworked casting. During the forging of a bar, the grains of the metal become greatly elongated in the direction of flow. As a result, the toughness of the metal is substantially improved in this direction and somewhat weakened in directions transverse to the flow. Part of the design of a good forging is to assure that the flow lines in the finished part are oriented so as to lie in the direction of maximum stress when the part is placed in service.
The ability of a metal to resist thinning and fracture during cold-working operations plays an important role in alloy selection and process design. In operations that involve stretching, the best alloys are those which grow stronger with strain (strain harden)—for example, the copper-zinc alloy, brass, used for cartridges and the aluminum-magnesium alloys in beverage cans, which exhibit greater strain hardening than do pure copper or aluminum, respectively.
Another useful property that can be controlled by processing and composition is the plastic anisotropy ratio. When a segment of sheet is strained (i.e., elongated) in one direction, the thickness and width of the segment must shrink, since the volume remains constant. In an isotropic sheet the thickness and width show equal strain, but, if the grains of the sheet are oriented properly, the thickness will shrink only about half as much as the width. Since it is thinning that leads to early fracture, this plastic anisotropy imparts better deep-drawing properties to sheet material with the optimum grain orientation.
Fracture of the workpiece during forming can result from flaws in the metal; these often consist of nonmetallic inclusions such as oxides or sulfides that are trapped in the metal during refining. Such inclusions can be avoided by proper manufacturing procedures. Laps are another type of flaw in which part of a metal piece is inadvertently folded over on itself but the two sides of the fold are not completely welded together. If a force tending to open this fold is applied during the forming operation, the metal will fail at the lap.
The ability of different metals to undergo strain varies appreciably. The shape change that can be made in one forming operation is often limited by the tensile ductility of the metal. Metals with the face-centred cubic crystal structure, such as copper and aluminum, are inherently more ductile in such operations than metals with the body-centred cubic structure. To avoid early fracture in the latter type of metals, processes are used that apply primarily compressive stresses rather than tensile stresses.
Powder metallurgy
Powder metallurgy (P/M) consists of making solid parts out of metal powders. The powder is mixed with a lubricant, pressed into a die to form the desired shape, and then sintered, or heated to a temperature below the melting point of the alloy where solid-state bonding of the particles takes place. In the absence of any external force, sintering typically leaves the sample containing about 5 percent pores by volume, but, when pressure is applied during sintering (a process called hot pressing), virtually zero porosity remains. In some parts made by mixing two different elements, one component melts at the sintering temperature, and this liquid phase aids sintering of the solid particles.
Applications
The following roughly chronological account indicates the types of products that can be made by P/M.
The earliest commercial use of P/M was in the production of such high-melting-point metals as platinum, tungsten, and tantalum. Pure powders of these metals could be made by the low-temperature reduction of powders, usually oxides, and, since these metals melt at extremely high temperatures, it was easier to form solid parts by pressing and sintering the powders than by melting and casting. For example, P/M played an important role in the development of tungsten filaments for electric light bulbs.
Another early P/M product was porous-metal bearings and filters. In such parts sintering is conducted at a relatively low temperature so that the pores between the particles remain open and connected. Disks sintered in this way can serve as filters for liquids, or the sintered part can be impregnated with oil to make a self-lubricating bearing. In the latter case, the oil is held in the pores by surface tension. When the bearing heats up in use, some oil flows out and lubricates the surface, and, when the part cools, surface tension pulls the oil back into the fine channels.
Cemented carbides form another class of sintered product. Pure tungsten carbide (WC) is an extremely hard compound, but it is too brittle to be used in tools. However, useful tools can be made by mixing WC powder with cobalt powder and sintering at a temperature above the melting point of cobalt. Liquid cobalt then reacts with the surface of the WC, and when the part is cooled the cobalt freezes, holding the WC tightly together to form a composite structure with enough toughness to be used for tools and dies.
The greatest volume of P/M parts is now produced from iron powder, a process that was first developed during World War II. Small, complex parts, such as gears, require much work if machined from steel bars, and a significant volume of material is wasted as chips from the machining. However, if the part is made by P/M processes, little or no machining is necessary, there is less wasted material, and the cost is much lower. Many small parts for automobiles and appliances are produced in this manner. The second greatest volume of P/M parts is made from aluminum powder. These parts are light, corrosion-resistant, and (if alloy is used) can be heat treated to appreciably increase the strength. Small parts for automobiles and appliances are the most common applications.
A recent process uses P/M methods to improve the homogeneity and toughness of high-alloy tool steels. Cast ingots of these alloys contain a coarse network of brittle phases that are very difficult to break up by hot working, but if, instead of being cast into ingots, the liquid is atomized (solidified as small droplets), the rapidly solidified particles will be homogeneous. This powder can then be hot pressed into consolidated bars with better mechanical properties than those produced by ingot casting. Consolidation is often achieved by hot isostatic pressing, wrapping the pressed powder in an envelope of steel or glass, and heating it in a hot inert gas at high pressure. The consolidated metal is then worked into finished parts.
Processes
The most common method of producing metal powders is atomization of a liquid. Here a stream of molten metal is broken up into small droplets with a jet of water, air, or inert gas such as nitrogen or argon. Atomization in water yields irregularly shaped particles that can be pressed to a higher initial, or “green,” strength and density than can the spherical particles formed by atomizing with an inert gas.
In other atomization processes, centrifugal force is used. The metal can be poured onto a spinning disk that breaks up the stream, or a spinning rod can be melted by an electric arc so that it throws off particles as it spins.
The liquid metal being atomized may be an alloy or a pure metal that will subsequently be blended with other elements to form an alloy. After atomization, the powder must be separated into size ranges by passing it through a series of sieves. Powders of different sizes (and of different metals) are then blended for pressing into parts.
Powders are often produced by chemically reducing a powdered oxide—for example, iron oxide reduced with carbon or hydrogen. The resulting metal aggregate is then milled and sieved to obtain the desired powder. Powder also can be made by electrodepositing the metal at a high current density, followed by milling to break up the deposit.
The above processes often produce powders that are roughly 50 to 200 micrometres in diameter. Powders less than one-tenth this size can be found in the finest fraction of the powder produced by atomization. Such fine powders can be mixed with a wax, injection molded to form several parts at once, and then sintered. The resulting parts require very little machining to yield a finished product.
Heat treating
The properties of metals can be substantially changed by various heat-treatment processes. Depending on the alloy and its condition, heat treating can harden or soften a metal.
