Forging - WikiHQ

thumb|upright=1.4|Hot metal [[ingot being loaded into a hammer forge]]

thumb|A [[Semi-finished_casting_products#Billet|billet in an open-die forging press]]

Forging is a manufacturing process involving the shaping of metal using localized compressive forces. The blows are delivered with a hammer (often a power hammer) or a die. Forging is often classified according to the temperature at which it is performed: cold forging (a type of cold working), warm forging, or hot forging (a type of hot working). For the latter two, the metal is heated, usually in a forge. Forged parts can range in weight from less than a kilogram to hundreds of metric tons.

History

thumb|Forging a nail. Valašské muzeum v přírodě, Czech Republic

Forging is one of the oldest known metalworking processes.

Temperature

All of the following forging processes can be performed at various temperatures; however, they are generally classified by whether the metal temperature is above or below the recrystallization temperature. If the temperature is above the material's recrystallization temperature it is deemed hot forging; if the temperature is below the material's recrystallization temperature but above 30% of the recrystallization temperature (on an absolute scale) it is deemed warm forging; if below 30% of the recrystallization temperature (usually room temperature) then it is deemed cold forging. The main advantage of hot forging is that it can be done more quickly and precisely, and as the metal is deformed work hardening effects are negated by the recrystallization process. Cold forging typically results in work hardening of the piece.

Drop forging

thumb|left|Boat nail production in [[Hainan, China]]

Drop forging is a forging process where a hammer is raised and then "dropped" into the workpiece to deform it according to the shape of the die. There are two types of drop forging: open-die drop forging and impression-die (or closed-die) drop forging. As the names imply, the difference is in the shape of the die, with the former not fully enclosing the workpiece, while the latter does.

Open-die drop forging

thumb|upright|Open-die drop forging (with two dies) of an ingot to be further processed into a wheel

thumb|A large 80 ton cylinder of hot steel in an open-die forging press, ready for the upsetting phase of forging

Open-die forging is also known as smith forging. In open-die forging, a hammer strikes and deforms the workpiece, which is placed on a stationary anvil. Open-die forging gets its name from the fact that the dies (the surfaces that are in contact with the workpiece) do not enclose the workpiece, allowing it to flow except where contacted by the dies. The operator therefore needs to orient and position the workpiece to get the desired shape. The dies are usually flat in shape, but some have a specially shaped surface for specialized operations. For example, a die may have a round, concave, or convex surface or be a tool to form holes or be a cut-off tool. Open-die forgings can be worked into shapes which include discs, hubs, blocks, shafts (including step shafts or with flanges), sleeves, cylinders, flats, hexes, rounds, plate, and some custom shapes. Open-die forging is typically employed when the workpiece is very large or when only a small batch size is required. Open-die forging lends itself to short runs and is appropriate for art smithing and custom work. In some cases, open-die forging may be employed to rough-shape ingots to prepare them for subsequent operations. Open-die forging may also orient the grain to increase strength in the required direction.

Better response to thermal treatment
Improvement of internal quality
Greater reliability of mechanical properties, ductility and impact resistance

"" is the successive deformation of a bar along its length using an open-die drop forge. It is commonly used to work a piece of raw material to the proper thickness. Once the proper thickness is achieved the proper width is achieved via "edging". "" is the process of concentrating material using a concave shaped open-die. The process is called "edging" because it is usually carried out on the ends of the workpiece. "" is a similar process that thins out sections of the forging using a convex shaped die. These processes prepare the workpieces for further forging processes.

File:Forging-edging.svg|Edging

File:Forging-fullering.svg|Fullering

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====Impression-die forging====

Impression-die forging is also called "closed-die forging". In impression-die forging, the metal is placed in a die resembling a mold, which is attached to an anvil. Usually, the hammer die is shaped as well. The hammer is then dropped on the workpiece, causing the metal to flow and fill the die cavities. The hammer is generally in contact with the workpiece on the scale of milliseconds. Depending on the size and complexity of the part, the hammer may be dropped multiple times in quick succession. Excess metal is squeezed out of the die cavities, forming what is referred to as "flash". The flash cools more rapidly than the rest of the material; this cool metal is stronger than the metal in the die, so it helps prevent more flash from forming. This also forces the metal to completely fill the die cavity. After forging, the flash is removed.

Impression-die forging has been improved in recent years through increased automation which includes induction heating, mechanical feeding, positioning and manipulation, and the direct heat treatment of parts after forging. One variation of impression-die forging is called "flashless forging", or "true closed-die forging". In this type of forging, the die cavities are completely closed, which keeps the workpiece from forming flash. The major advantage to this process is that less metal is lost to flash. Flash can account for 20 to 45% of the starting material. The disadvantages of this process include additional cost due to a more complex die design and the need for better lubrication and workpiece placement.

{| class="wikitable"

|+ Dimensional tolerances for impression-die forgings

Upset forging

Upset forging increases the diameter of the workpiece by compressing its length.

The length of unsupported metal that can be upset in one blow without injurious buckling should be limited to three times the diameter of the bar.
Lengths of stock greater than three times the diameter may be upset successfully, provided that the diameter of the upset is not more than 1.5 times the diameter of the stock.
In an upset requiring stock length greater than three times the diameter of the stock, and where the diameter of the cavity is not more than 1.5 times the diameter of the stock, the length of unsupported metal beyond the face of the die must not exceed the diameter of the bar.

Automatic hot forging

The automatic hot forging process involves feeding mill-length steel bars (typically long) into one end of the machine at room temperature and hot forged products emerge from the other end. This all occurs rapidly; small parts can be made at a rate of 180 parts per minute (ppm) and larger can be made at a rate of 90 ppm. The parts can be solid or hollow, round or symmetrical, up to , and up to in diameter. The main advantages to this process are its high output rate and ability to accept low-cost materials. Little labor is required to operate the machinery.

There is no flash produced so material savings are between 20 and 30% over conventional forging. The final product is a consistent so air cooling will result in a part that is still easily machinable (the advantage being the lack of annealing required after forging). Tolerances are usually ±, surfaces are clean, and draft angles are 0.5 to 1°. Tool life is nearly double that of conventional forging because contact times are on the order of 0.06-second. The downside is that this process is only feasible on smaller symmetric parts and cost; the initial investment can be over $10 million, so large quantities are required to justify this process.

The process starts by heating the bar to in less than 60 seconds using high-power induction coils. It is then descaled with rollers, sheared into blanks, and transferred through several successive forming stages, during which it is upset, preformed, final forged, and pierced (if necessary). This process can also be coupled with high-speed cold-forming operations. Generally, the cold forming operation will do the finishing stage so that the advantages of cold-working can be obtained, while maintaining the high speed of automatic hot forging.

Examples of parts made by this process are: wheel hub unit bearings, transmission gears, tapered roller bearing races, stainless steel coupling flanges, and neck rings for liquid propane (LP) gas cylinders. Manual transmission gears are an example of automatic hot forging used in conjunction with cold working.

===Roll forging===

Roll forging is a process where round or flat bar stock is reduced in thickness and increased in length. Roll forging is performed using two cylindrical or semi-cylindrical rolls, each containing one or more shaped grooves. A heated bar is inserted into the rolls and when it hits a spot the rolls rotate and the bar is progressively shaped as it is rolled through the machine. The piece is then transferred to the next set of grooves or turned around and reinserted into the same grooves. This continues until the desired shape and size is achieved. The advantage of this process is there is no flash and it imparts a favorable grain structure into the workpiece.

Examples of products produced using this method include axles, tapered levers and leaf springs.

Net-shape and near-net-shape forging

Also known as precision forging, it was developed to minimize cost and waste post-forging such that the resultant product needs little or no final machining. Costs are reduced by using less material, producing less scrap, less energy used, and less or no additional machining. Precision forging also requires less draft, 1° to 0°. The downside of this process is cost, it is only implemented when significant cost reduction can be achieved.

Cold forging

Near net shape forging is most common when parts are forged without heating the slug, bar or billet. Aluminum is a common material that can be cold forged depending on final shape. Lubrication of the parts being formed is critical to increase the life of the mating dies.

Induction forging

Unlike the above processes, induction forging is based on the type of heating style used. Many of the above processes can be used in conjunction with this heating method.

Multidirectional forging

Multidirectional forging is forming of a work piece in a single step in several directions. The multidirectional forming takes place through constructive measures of the tool. The vertical movement of the press ram is redirected using wedges which distributes and redirects the force of the forging press in horizontal directions.

Isothermal forging

Isothermal forging is a process by which the materials and the die are heated to the same temperature (iso- meaning "equal"). Adiabatic heating is used to assist in the deformation of the material, meaning the strain rates are highly controlled. This technique is commonly used for forging aluminium, which has a lower forging temperature than steels. Forging temperatures for aluminum are around , while steels and super alloys can be .

Benefits:

Near net shapes which lead to lower machining requirements and therefore lower scrap rates
Reproducibility of the part
Due to the lower heat loss smaller machines can be used to make the forging

Disadvantages:

Higher die material costs to handle temperatures and pressures
Uniform heating systems are required
Protective atmospheres or vacuum to reduce oxidation of the dies and material
Low production rates

Materials and applications

thumb|Solid forged billets of steel (glowing incandescently) being loaded in a large industrial chamber furnace, for re-heating

Forging of steel

Depending on the forming temperature steel forging can be divided into:

Hot forging of steel
Forging temperatures above the recrystallization temperature between 950–1250 °C
Good formability
Low forming forces
Constant tensile strength of the workpieces
Warm forging of steel
Forging temperatures between 750–950 °C
Less or no scaling at the workpiece surface
Narrower tolerances achievable than in hot forging
Limited formability and higher forming forces than for hot forging
Lower forming forces than in cold forming
Cold forging of steel
Forging temperatures at room conditions, self-heating up to 150 °C due to the forming energy
Narrowest tolerances achievable
No scaling at workpiece surface
Increase of strength and decrease of ductility due to strain hardening
Low formability and high forming forces are necessary

For industrial processes steel alloys are primarily forged in hot condition. Brass, bronze, copper, precious metals and their alloys are manufactured by cold forging processes; each metal requires a different forging temperature.

Forging of aluminium

Aluminium forging is performed at a temperature range between 350–550 °C
Forging temperatures above 550 °C are too close to the solidus temperature of the alloys and lead in conjunction with varying effective strains to unfavorable workpiece surfaces and potentially to a partial melting as well as fold formation.
Forging temperatures below 350 °C reduce formability by increasing the yield stress, which can lead to unfilled dies, cracking at the workpiece surface and increased die forces

Due to the narrow temperature range and high thermal conductivity, aluminium forging can only be realized in a particular process window. To provide good forming conditions a homogeneous temperature distribution in the entire workpiece is necessary. Therefore, the control of the tool temperature has a major influence to the process. For example, by optimizing the preform geometries the local effective strains can be influenced to reduce local overheating for a more homogeneous temperature distribution.

Application of aluminium forged parts

High-strength aluminium alloys have the tensile strength of medium strong steel alloys while providing significant weight advantages. Therefore, aluminium forged parts are mainly used in aerospace, automotive industry and many other fields of engineering especially in those fields, where highest safety standards against failure by abuse, by shock or vibratory stresses are needed. Such parts are for example pistons, chassis parts, steering components and brake parts. Commonly used alloys are AlSi1MgMn (EN AW-6082) and AlZnMgCu1,5 (EN AW-7075). About 80% of all aluminium forged parts are made of AlSi1MgMn. The high-strength alloy AlZnMgCu1,5 is mainly used for aerospace applications.

Forging of magnesium

Magnesium forging occurs at a temperature range between 290–450 °C

Magnesium alloys are more difficult to forge due to their low plasticity, low sensitivity to strain rates and narrow forming temperature. This forging method has shown to improve tensile properties but lacks uniform grain size. Even though the application of magnesium alloys increases by 15–20% each year in the aerospace and automotive industry, forging magnesium alloys with specialized dies is expensive and an unfeasible method to produce parts for a mass market. Instead, most magnesium alloy parts for industry are produced by casting methods.

Equipment

thumb|Hydraulic drop-hammer

thumb|(a) Material flow of a conventionally forged disc; (b) Material flow of a counterblow (impactor) forged disc

The most common type of forging equipment is the hammer and anvil. Principles behind the hammer and anvil are still used today in drop-hammer equipment. The principle behind the machine is simple: raise the hammer and drop it or propel it into the workpiece, which rests on the anvil. The main variations between drop-hammers are in the way the hammer is powered; the most common being air and steam hammers. Drop-hammers usually operate in a vertical position. The main reason for this is excess energy (energy that is not used to deform the workpiece) that is not released as heat or sound needs to be transmitted to the foundation. Moreover, a large machine base is needed to absorb the impacts.

===Forging presses===

A forging press, often just called a press, is used for press forging. There are two main types: mechanical and hydraulic presses. Mechanical presses function by using cams, cranks and/or toggles to produce a preset (a predetermined force at a certain location in the stroke) and reproducible stroke. Due to the nature of this type of system, different forces are available at different stroke positions. Mechanical presses are faster than their hydraulic counterparts (up to 50 strokes per minute). Their capacities range from 3 to 160 MN (300 to 18,000 short tons-force). Hydraulic presses, such as the four-die device, use fluid pressure and a piston to generate force. The advantages of a hydraulic press over a mechanical press are its flexibility and greater capacity. The disadvantages include a slower, larger, and costlier machine to operate.

! Force (tonnes)

! Ingot size (tonnes)

! Company

! Location

| 16,500

| 600

| Shanghai Electric Group

| Heilongjiang, China

| 13,000

| Doosan

| South Korea

{| class="wikitable"

|+List of large forging presses, by force

! Force (tonnes)

! Force (tons)

! Ingot size (tonnes)

! Company

! Location

| 90,718

| (100,000)

| Giva Group

| Rho, Lombardy, Italy

| 80,000

| (88,200)

| >150

| China Erzhong

| Deyang, China

| 75,000

| (82,690)

|VSMPO-AVISMA

| Russia

| 65,000

| (71,660)

|Aubert & Duval

| Issoire, France

| 53,500

| (60,000)

| Weber Metals, Inc.

| California, United States

| (45,350)

| 50,000

| 20

| Alcoa 50,000 ton forging press Alcoa, Wyman Gordon

| US

| 40,000

| (44,100)

| Aubert & Duval

| Livingston, Scotland

| 30,000

| (33,070)