Forging is a manufacturing process in which metal is shaped by compressive forces applied by hammers, presses, dies or rolls. The process has developed from traditional smith forging into a wide range of controlled industrial processes capable of producing components from relatively small parts to very large engineering forgings.
Many ductile engineering metals can be forged. Depending on the material, component and process, forging may be carried out hot, warm or cold. Starting material may include billet, bar, bloom or other suitable wrought or preformed stock.
An important advantage of forging is that deformation can produce a directional grain-flow pattern which follows the shape of the component. With appropriate material selection, forging practice and heat treatment, this can provide excellent mechanical properties and makes forging particularly important for highly loaded engineering components.
Forging processes can be classified in several ways, including the degree to which the workpiece is constrained by the tooling and the method used to apply the forging force. Important processes include:
In open-die forging the workpiece is compressed between relatively simple dies without being completely enclosed within a die cavity. Flat dies are commonly used, although shaped tools may be employed for particular operations.
The required component is produced by repeatedly deforming and manipulating the workpiece between forging operations. The operator or handling system may rotate, translate or reposition the workpiece so that deformation is progressively distributed through the required regions.
Open-die forging has relatively low tooling requirements compared with closed-die forging and is particularly suitable for low production quantities and large components. Shafts, rings, discs, cylinders and other relatively simple forms can be produced by this method.
Dimensional control and repeatability are generally lower than those obtainable from well-controlled closed-die processes, and subsequent machining is commonly required to produce finished functional surfaces.
In closed-die forging, shaped dies progressively constrain the workpiece and force the material to flow into the required component geometry. The process is also commonly associated with impression-die forging.
The component may require several forging stages. Starting stock can first be distributed into a suitable preform before intermediate and finishing dies produce the final forged shape. Excess material may flow from the die cavity as flash, depending on the particular forging process.
Closed-die forging provides greater control of component shape and repeatability than open-die forging and is suitable for relatively high production rates. It can also be used to control material flow through important regions of a component.
The principal disadvantage is the cost of the dies and associated production equipment. Closed-die forging therefore becomes increasingly attractive when the production quantity is sufficient to justify the tooling investment.
Plastic deformation during forging can cause the grain-flow or fibre-flow pattern of wrought material to follow the shape of the component. Where the forging sequence is properly designed, this flow can be directed around highly stressed features rather than being interrupted by machining from a larger block of material.
This is one reason forgings are widely used for components subjected to high mechanical loads, fatigue or impact. The benefit depends on the material and the complete manufacturing route and should not be treated as a substitute for appropriate material specification, heat treatment and component design.
Press forging applies compressive force through a controlled pressing action rather than the repeated high-velocity blows associated with hammer forging. Hydraulic or mechanical presses may be used depending on the required force, speed, component and production process.
The relatively sustained application of force can produce deformation through a substantial proportion of the workpiece. Press forging is therefore important for both large engineering forgings and controlled production of smaller components.
Press forging may be performed using either open or closed dies. Large open-die hydraulic presses are used for substantial shafts, rings and other heavy engineering components, while closed-die presses can provide high repeatability for production components.
Typical applications include highly loaded structural components, wheels, shafts and aircraft landing-gear components.
Upset forging increases the cross-sectional area of a workpiece by compressing it along its longitudinal axis. Bar or wire stock is commonly held in a die while material projecting beyond the die is compressed by a moving tool.
Simple shapes may be produced in a single operation, while more complex components can require several progressive forming stages to distribute the material without producing unacceptable defects.
Upsetting is extensively used for producing headed components such as bolts, screws, rivets and other fasteners. When performed without heating the material, the process is commonly referred to as cold heading or cold forming.
Swaging is a forming process used to reduce, size, point, taper or otherwise alter the cross-section of rods, tubes and similar components. Repeated radial blows or die movements progressively deform the workpiece.
The process can be used to form external profiles and, with suitable internal tooling or mandrels, can also control the internal dimensions of tubular components.
Roll forging uses shaped rotating rolls to progressively reduce and redistribute the cross-section of heated stock. Unlike conventional flat rolling, the rolls contain profiles which progressively form the workpiece as it passes between them.
The process can be used to produce finished components or, commonly, to manufacture preforms which distribute material appropriately before subsequent closed-die forging.
Roll forging can provide efficient material utilisation and controlled material flow for components whose cross-section varies along their length.
Forging may be classified as hot, warm or cold according to the temperature of the workpiece relative to the behaviour of the material.
Hot forging is carried out at an elevated temperature which allows large plastic deformation with relatively low forming forces. Recrystallisation during hot working can modify the grain structure as deformation proceeds.
Warm forging is performed between conventional hot- and cold-working temperature ranges. It can provide a compromise between the reduced forming forces associated with hot working and the dimensional and surface advantages associated with colder processes.
Cold forging is performed without heating the workpiece to conventional hot-forging temperatures. It can provide good dimensional accuracy, surface finish and high production rates, but requires greater forming forces and sufficient material ductility for the required deformation.
A component intended for forging should be designed with both the final engineering requirements and the practical flow of material during manufacture in mind.
Important considerations include:
Forging is capable of producing high-integrity engineering components, but poor material flow, unsuitable temperatures or incorrect process design can produce defects. Potential problems include laps, folds, underfilling, cracking, scale-related surface defects and undesirable grain flow.
Inspection requirements depend on the application and consequences of failure. Critical forgings may require combinations of dimensional inspection, surface examination and volumetric non-destructive testing in addition to material certification and mechanical testing.
Many engineering components can be produced either by forging followed by machining or by machining directly from bar, plate or billet. The most appropriate route depends on production quantity, component geometry, material cost, mechanical-property requirements and tooling investment.
Machining from solid can be attractive for prototypes and small quantities because dedicated forging tooling may be avoided. For larger production quantities, a forging can reduce material removal and machining time while providing a material-flow pattern appropriate to the component.
The decision should therefore be made from the complete manufacturing and engineering requirement rather than assuming that either route is inherently superior.
Rolling is closely related to forging because both processes shape metal by compressive plastic deformation. In conventional rolling, material passes between rotating rolls which reduce its thickness or alter its cross-section while drawing the material forward.
Hot rolling is extensively used to produce semi-finished and finished products including plate, strip, bar and structural sections. Rolled billets, blooms and other products may subsequently provide the starting material for forging operations.
Cold rolling can provide closer dimensional control and improved surface finish for suitable products, while also changing the mechanical properties through cold work.
Rolling is a substantial manufacturing subject in its own right. The important distinction on this page is that roll forging uses profiled rolls to progressively form individual components or forging preforms, whereas conventional rolling principally produces continuous or long-length products.
The choice between open-die, closed-die, press, upset and roll forging depends on the component and manufacturing requirements. Selection should consider the material, component size, geometry, required mechanical properties, production quantity, dimensional requirements, tooling cost and subsequent machining operations.
Open-die forging is particularly attractive for large components and relatively small production quantities, while closed-die forging becomes increasingly attractive where component geometry and production quantity justify dedicated tooling. Upset forging is particularly effective for headed components, while roll forging can efficiently redistribute material along the length of a workpiece.