Forging can be broadly classified into open-die forging, closed-die (impression die) forging, hot forging, cold forging, and seamless ring forging. Based on the process of applying force, forging techniques can further be classified as drop, press, upset, automatic, roll, precision, multi-directional, and isothermal forging.
This article discusses various metal forging techniques and their applications.
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Types of Forging Processes Explained
Forging is the process of applying force to shape the workpiece without removing any material or changing the state of the matter.
Unlike die punches, forging applies force to shape the workpiece, instead of shearing and removing the material.
As a result, there are various advantages of forging over other metalworking processes, and depending upon your requirement, you can select the best-suited type of forging for your application.
However, you must be wary of forging defects to prevent them from weakening the workpiece and reducing its reliability.
Forging processes are broadly classified based on the type of die used and the temperature of the workpiece.
Based on the Type of Die
Open-die Forging
Open-die forging is one of the simplest forms of forging, which uses a hammer and an anvil to deform the workpiece in the desired shape.
This type of forging does not use a die cavity to shape the workpiece, instead, the orientation and movement of the workpiece are used to get the required form.
The dies usually have a flat surface but may also have some specific surface shape for different applications.
Open-die forging can make various shapes, such as discs, shafts, flats, custom shapes, etc. It is also used to prepare workpieces for further operations.
This process is usually used in small shops and DIY applications to make a small number of pieces where replicability and dimensional tolerance are not a significant concern.
Closed-die Forging
Closed-die forging, also known as impression die forging, involves two dies shaped in the desired form.
The heated workpiece is placed on the lower die, which resembles a mold, and is mounted on the anvil.
While the other half of the mold/die is mounted on the hammer that forces the hot metal workpiece to deform and fill the die cavity.
Excess material is squeezed out from the die and is referred to as flash.
Unlike open-die forging, closed-die forging shapes the material according to the shape of the die, providing the ability to produce identical parts with no or minimal deviation.
However, the initial cost of closed-die forging is comparatively higher due to the need for specially designed dies.
It is suited for comparatively higher volume production and is commonly used in automobile and tool manufacturing.
Seamless Ring Forging
Seamless ring forging requires blanking a piece of metal under a punching machine and then feeding the ring structure to the forging rollers.
It is used to produce seamless ring structures that can withstand heavy loads without failure.
This type of forging requires special type of roller dies that apply compressive forces on the workpiece to alter its cross-section and produce the desired ring structure.
The rollers can consist of unique shapes and designs to produce ring structures with unique contours, as per the design requirement.
Generally, this type of forging can be used to produce rings having an internal diameter ranging from 5 inches to 350 inches.
However, forging large-sized rings requires large industrial forging machines capable of exerting extreme forces to forge metal, and are, therefore, suitable for industrial applications only.
Based on the Temperature of the Workpiece
Forging usually involves heating the workpiece to enhance its ductility, followed by hammering to mold it into the desired shape.
However, forging a cold workpiece produces different results, in terms of the mechanical properties and the surface finish of the part.
When comparing forging against machining, the forged parts generally have better strength than machined parts, irrespective of the type of forging performed.
Therefore, it is important to select the best-suited forging for your application.
Hot Forging
Hot forging is the process of heating a metal workpiece beyond its recrystallization temperature, which enhances its ductility, making it suitable for forging operation.
It is one of the most commonly used types of forging as it reduces the force required for molding the workpiece, thereby minimizing the effort and time required for forging.
The hot metal also provides a better flow of material during forging, making it ideal for die-forging operations where intricate patterns are to be forged.
Furthermore, heating the workpiece beyond its recrystallization temperature results in the annealing of the workpiece, thereby relieving the internal stresses and making it suitable for further operations.
As a result, this technique is best suited for forging hard materials, such as forging steel workpieces.
However, non-uniform cooling of the hot metal can result in the formation of scales on the surface of the workpiece, leading to a poor surface finish.
Cold Forging
Unlike hot forging, cold forging does not involve heating the workpiece, eliminating the need for a furnace.
This type of forging produces parts with a better surface finish and higher strength than hot-forged parts.
However, the hammering of metal workpieces can develop internal stresses, rendering the workpiece unsuitable for further operations without undergoing a heat-treatment process like annealing.
Apart from that, cold metal has comparatively low ductility and requires strong force to shape the metal.
Therefore, it is suitable for forging soft metals such as aluminum, brass, bronze, alloy steel, etc.
Generally, cold forging is carried out at temperatures varying from room temperature to a few hundred degrees.
Different Forging Techniques
| Forging Technique | Overview |
|---|---|
| Drop forging | Process of shaping metals using a mechanical hammer and dies |
| Press forging | Deforming metal using continuous pressure on the dies |
| Upset forging | Axial forces are applied to increase the cross-section of rods into a specified geometry. |
| Automatic hot forging | Automated forging of metal bars into symmetrical components |
| Roll forging | Rolling of hot metal bars into the desired profile |
| Precision forging | High-accuracy and surface finish forging, which requires little or no further operation |
| Multi-directional forging | Used to improve mechanical properties by deforming material in all axial directions |
| Isothermal forging | Heated dies are used to achieve uniform deformation, producing precise products |
Drop Forging
| Drop forging | Details |
|---|---|
| Tools required | Mechanical hammer, Dies |
| Material capability | Steel, Magnesium, Aluminum, Brass, and Copper |
| Application | Discs, shafts, flats, and customized shapes |
Drop forging is a process of shaping metal using dies and hammers.
It consists of a mechanical hammer that is dropped on the hot workpiece to shape it.
The workpiece is heated and placed in a stationary die, having the impression or die cavity of the desired shape.
Dropping the mechanical hammer on the hot workpiece forces it to take the shape of the die cavity, thereby resulting in the formation of the desired part.
Generally, for industrial applications, hammers are powered using compressed air or hydraulic systems to add to the gravitational force for faster production rates.
Drop forging results in parts with refined grain structure and a high strength-to-weight ratio, making it ideal for forging parts that require high strength, toughness, and durability
However, drop forging is limited by size, as large forgings will require a larger hammer and a heavy-duty mechanism to lift and drop that heavy hammer.
Press Forging
| Press forging | Details |
|---|---|
| Tools required | Forging press, Dies |
| Material capability | Steel, Aluminum, Titanium, Brass, Copper |
| Application | Wheels, Bushings, Gears, and other similar shapes |
The press forging process involves applying continuous pressure to the workpiece between two dies.
A forging press machine is used to apply pressure, and it plastically deforms the workpiece to attain the shape of the die.
Unlike impact force used in drop forging, press forging applies gradually increasing pressure to the workpiece until the desired shape is attained.
The elimination to achieve the final shape in a single stroke of the forge press instead of repeated blows produces a uniform deformation throughout the workpiece.
This process is applicable with open and closed dies, while providing the ability to hot or cold forge the workpiece.
Press forging allows a high degree of control over the pressure applied, enabling you to make various shapes and sizes with minimal scrap in the form of flash.
Although press forging involves a higher setup cost than drop forging, it is economical for higher production runs, easily automated, and provides comparatively better dimensional accuracy.
Upset Forging
| Upset forging | Details |
|---|---|
| Tools required | Upsetter, clamp, dies |
| Material capability | Steel, Aluminum, and other forgeable metals |
| Application | Fasteners, Torsion bars, Gear blanks, etc. |
Upset forging is a type of forging that is used to partially forge a long workpiece, such as a metal bar.
This means that the bar is locally heated and clamped firmly while axial pressure is applied at the end to be forged. This force increases the diameter of the workpiece by reducing its length.
Upsetting is done using specialized machines called crank presses and split dies which facilitate the workpiece movement from one die to the other, and successive deformations are used to achieve the desired shape.
The pre-formed part is usually a rod held with the tooling, and the end to be formed is locally heated. The piece is inserted in the die, and the heading tool applies pressure to upset the material into the die cavity.
Parts made from upset forging have higher strength and high fatigue resistance than machined components as the grain pattern follows the shape of the part.
The localized heating of the workpiece makes it energy efficient, whereas the elimination of the need for machining saves material wastage and time.
This process is limited by the largest length that can be upset without buckling. The maximum upset length cannot be more than thrice the diameter of the workpiece.
It is widely used to manufacture screws, bolts, valves, and other fasteners. Most materials can be forged using this technique except copper and some aluminum alloys.
Automatic Hot Forging
| Automatic forging | Details |
|---|---|
| Tools required | Induction heater, dies, automatic presses, rollers, and other specialized tools |
| Material capability | Iron, steel, and other forgeable metals |
| Application | Gears, bearings, and symmetrical parts |
Automatic hot forging is a process in which long bars are fed into the system, and finished forged products come out of the other end, without the need for human interaction.
The process involves heating the bars using induction heaters, followed by rolling and cutting the material into required blank sizes. It then undergoes several forming processes to get the required shape.
The process can also involve a cold-forming operation for finishing the part. The final product has high dimensional accuracy, a good surface finish, and is readily machinable.
There is no flash produced in this method resulting in material savings.
This type of forging is best suited for manufacturing small symmetrical parts with quick cycle time and minimal labor requirements.
However, the equipment used for automatic hot forging is comparatively more costly than other processes, making it ideal for large-scale production.
It is widely used in industries to manufacture gears, bearings, flanges, and other small symmetrical parts.
Roll Forging
| Roll forging | Details |
|---|---|
| Tools required | Roll forging machine, roll dies |
| Material capability | Iron, Steel, Aluminum |
| Application | Axle, shafts, leaf spring, hand tool, etc. |
Roll forging is a technique where heated round or flat bars are fed into rolls rotating in the opposite direction, to shape the part.
The rollers consist of grooves in the desired shape, that deforms the material and forges the piece with the required geometry.
These grooves are present on the circumference of the rolls, usually ranging between one to three-quarters of the rim, making them suitable for forging small size and repetitive patterns.
The desired shape is achieved by passing the workpiece through successive rollers or re-feeding into the same rollers.
There is no flash involved in this process, thereby minimizing wastage and enhancing productivity.
It is commonly used in the automotive industry to produce axles and leaf springs. It is also used to manufacture shafts, hand tools, etc.
Precision Forging
| Precision forging | Details |
|---|---|
| Tools required | Precision dies, forge press |
| Material capability | Titanium, Steel, and other exotic alloys |
| Application | High-grade machine components |
Precision forging is used to produce parts with extremely tight tolerance.
Generally, precision forged parts do not require further machining to get the desired dimensions, except for a finishing process, like polishing, to improve its surface finish.
Unlike traditional forging, precision forging is the process of refining the shape of an already forged part to meet the dimensional requirements.
Additional parameters, such as temperature control, lubrication, descaling, etc., are incorporated in precision forging to improve product quality.
However, specialized machinery and dies are used in this process leading to high initial costs and increased set-up time required to achieve close tolerances.
This makes it suitable for industrial applications where the overall production cost counters the high initial cost and set-up time of the machinery.
This process is commonly used to manufacture machine components in various industries such as automotive, agriculture, railway, mining, etc.
Multi-directional Forging
| Multi-directional forging | Details |
|---|---|
| Tools required | Forge press, flat dies |
| Material capability | Steel, copper, magnesium, aluminum, etc. |
| Application | High-strength bulk materials |
Multi-directional forging involves applying load to bulk material in all axial directions by either changing the orientation in each pass or redistributing the forces in all directions in a single pass.
The workpiece is subjected to high strain by plastically deforming it in all directions without changing the overall cross-section.
This results in reduced grain size with a homogenous structure, leading to increased ductility, while strain hardening enhances its strength.
Multi-directional forging does not require any special tools, making it suitable for different types of industries.
However, it results in strain accumulation at the core, which can be overcome by repeated passes.
Apart from that, the process is time-consuming and requires an automated setup or manual reorientation of the workpiece after each forging pass.
Isothermal Forging
| Isothermal forging | Details |
|---|---|
| Tools required | Heated dies, vacuum chamber, forge press, |
| Material capability | Aluminum and Titanium alloys |
| Application | Components for aerospace and jet engines |
Isothermal forging uses dies that are heated to the workpiece's temperature, and the temperature is maintained throughout the process.
Using a heated die eliminates the cooling effect from the die surface, and helps achieve a uniform deformation.
This temperature is maintained throughout the process, producing parts with a high surface finish and good dimensional accuracy.
Maintaining the temperature throughout the process results in gradual straining, leading to uniform mechanical properties throughout the forged workpiece.
The dies are made from heat-resistant alloys to withstand high temperatures and pressure leading to increased costs.
Apart from that, a vacuum or inert atmosphere is to be maintained to minimize oxidation, leading to high setup and operational costs with low production rates.
It is used for forging aluminum and strain-rate-sensitive materials like titanium alloys. Due to its high overall costs, this method is only suitable for forging critical components in aerospace and jet engines.
Hand Forging vs Power Forging
| Hand Forging | Power Forging |
|---|---|
| Manual hammer and anvil are used | Power hammer and anvil are used |
| Varied strike force | Consistent strike force |
| Low impact force and less deformation | High-impact force and high material deformation |
| Finished products can be made | Mainly used for rough shaping |
| More precision and control | Less control and variability |
Hand forging is the simplest form of forging that involves hammering the workpiece on an anvil, whereas power forging uses electric or hydraulic powered hammers to shape the workpiece.
Generally, hand forging involves open-die forging where the workpiece's orientation and the hammer's strike determine the shape of the forged part.
Unlike power forging, hand forging is not suitable for closed-die forging that involves dies with intricate patterns.
This is because the strike force is not sufficient to force the metal in every intricate detail of the die, making it suitable for forging parts with simple geometry.
On the other hand, power forging involves the use of open- as well as closed dies to forge the metal workpiece.
Power forging provides a considerably higher and consistent strike force which allows for closed die forging of intricate patterns.
Usually, open-die power forging is used for the rough shaping of the workpiece, followed by hand forging to finish the product.
Power forging has higher set-up costs due to powered hammers and specialized tooling, while hand forging uses simple tools and requires a lower investment.
The forging process can be selected based on the production volume and the required geometry of the forged part.
Hand forging is suited for limited production of unique parts where making dies is not economically feasible while power forging is recommended for production runs.
Final Thoughts
The forging techniques have considerably evolved over the years with technological advancements. A range of processes, complex dies, and heavy machines are used to produce various parts.
Hand forging or smith forging can be an easy and cost-effective way for DIY forging applications. As your skills improve, you can invest in a power hammer for a faster production rate and reduced effort.
Drop forging can also be an alternative for small production volumes. The parts produced will have better quality and strength when compared with hand-forged parts.
Although closed-die drop forging will require higher investment in tooling, superior products can be made with reduced production costs.
For small-scale manufacturing, drop and press forging can be used, while hobbyists are better off working with hand or power forging.
Other processes, such as roll forging and precision forging, are suited for industrial manufacturing due to the high costs and complexity associated with the process.
Frequently Asked Questions (FAQ)
What tools do you need to start metal forging?
To start metal forging, you will need a furnace to heat the workpiece, hammers of different sizes and shapes to deform the material, an anvil to place the part for striking, and tongs/clamps to hold the workpiece.
What are the advantages of forging over casting?
Forgings produce mechanically stronger components than casting due to superior grain structure and refinement. It is also free of casting defects such as porosity, cavities, shrinkage, etc. You can use cheaper materials in forging and provide higher strength than casting.
What are scale pits in forgings?
Scale pits are surface defects common in forging operations in an open environment. These are tiny cracks or deformations in forged parts caused by using stock and dies without cleaning the surfaces. The scale and oxide on the forging surfaces get embedded into the piece and cause these defects, which become more pronounced after the pickling process.
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