Forging is a metal processing method that uses forging machines to apply pressure to metal parts, producing plastic deformation that results in forged parts with specific mechanical properties, shapes and sizes. It is one of the two components of forging, along with stamping.
Forging eliminates defects such as casting porosity in the casting process, while optimizing the microstructure. Furthermore, since the complete aerodynamics of the metal are preserved, the mechanical properties of forgings are generally superior to those of castings made from the same material.
Except for rolled plates, profiles or weldments with simple shapes, forgings are mainly used for crucial components that experience high loads and severe working conditions in relevant machines.
1. Deformation temperature
The initial recrystallization temperature of steel is approximately 727°C. However, 800°C is normally considered the limit for hot forging. Forging above 800°C is called hot forging, while forging between 300°C and 800°C is called hot or semi-hot forging. Forging at room temperature is called cold forging.
Hot forging is the most commonly used method for manufacturing forgings in most industries. Hot and cold forging, on the other hand, is mainly used in the automotive, general machinery, and other parts production industries. These methods can efficiently save materials.
2. Types of Forging
As mentioned earlier, forging can be classified into hot forging, hot forging and cold forging based on temperature. Furthermore, it can be categorized into free forging, die forging, ring rolling and specialized forging based on the forming mechanism.
1) Free forging
Free forging is a processing method that involves the use of simple universal tools or the direct application of external force to deform a blank between the upper and lower anvil of forging equipment to obtain the required geometry and internal quality.
Forgings produced using this method are known as free forgings and are typically produced in small batches.
To create qualified forgings, various forging equipment such as forging hammers and hydraulic presses are used to shape and process the blank.
The fundamental processes of free forging include upsetting, drawing, punching, cutting, bending, twisting, shifting and forging. This method typically employs hot forging techniques.
2) Die forging
Die forging can be categorized into two main types: open die forging and closed die forging. During this process, a metal part is deformed and pressed into a specifically shaped forging mold chamber to create forged parts.
Typically, die forging is used to manufacture parts with small weights and in large batches. This process can be divided into three types: hot forging, hot forging and cold forging.
Both hot forging and cold forging are considered the future direction of die forging and represent advances in forging technology. Die forging can also be classified based on the materials used, including ferrous metal die forging, non-ferrous metal die forging, and powder product forming.
Ferrous metals such as carbon steel, non-ferrous metals such as copper and aluminum, and powder metallurgy materials are used in this process.
Related Reading: Ferrous vs Non-Ferrous Metals
Extrusion is a type of forging that can be categorized into heavy metal extrusion and light metal extrusion.
Closed die forging and closed upsetting are two advanced die forging processes. A significant advantage of these processes is the high material utilization rate since there is no burr.
With one or multiple processes, complex forgings can be completed.
Furthermore, the absence of burrs reduces the stress area of the forging, resulting in lower required loads.
However, it is important to note that white space cannot be completely limited. Therefore, it is necessary to strictly control the volume of the blank, manage the relative position of the forging die, and measure the forging to minimize the wear of the forging die.
3) Grinding ring
Ring grinding is the process of producing ring parts of varying diameters using specialized equipment known as a ring grinder. It is also used in the production of wheel parts, including automobile hubs and train wheels.
4) Special forging
Special forging techniques include roll forging, cross wedge rolling, radial forging, liquid die forging and other methods that are more suitable for producing certain complex shaped parts.
Roll forging, for example, can serve as an efficient preforming process that significantly reduces the amount of pressure required for subsequent forming operations.
Cross wedge rolling is used to produce steel balls, drive shafts and other similar components.
Radial forging, on the other hand, is used to manufacture large barrels, stepped shafts, and other types of forgings.
5) Forging die
According to the movement mode of the forging die, forging can be divided into oscillating rolling, rotating oscillating forging, roll forging, cross wedge rolling, ring rolling and cross rolling.
Rotary forging, rotary forging and ring rolling can also be processed by precision forging.
In order to improve the utilization of materials, roll forging and cross-rolling can be used as previous processes for thin materials.
Just like free forging, rotary forging is also formed locally.
Its advantage is that compared to the size of the forging, it can also be formed when the forging force is small.
In this forging method, including free forging, the material expands from close to the die surface to the free surface during processing.
Therefore, it is difficult to guarantee accuracy.
By controlling the movement direction of the forging die and the rotary forging process by computer, products with complex shapes and high precision can be obtained with low forging force, such as producing forged parts such as steam turbine blades with many varieties and large sizes.
The movement of forging equipment may not be consistent with the degree of freedom, which can be categorized into the following four types:
- Forging force limiting mode: An oil press with a sliding block driven directly by oil pressure.
- Near stroke limit mode: An oil press that drives a crank connecting rod mechanism by oil pressure.
- Stroke limit mode: A mechanical press with a slider driven by a crank, connecting rod and wedge mechanism.
- Power limiting mode: A screw and friction press using a screw mechanism.
To achieve high precision, attention should be paid to preventing overload at the bottom dead center by controlling the speed and position of the die, as these factors can affect the forging tolerance, shape accuracy and die life.
Furthermore, maintaining accuracy requires adjusting the clearance of the sliding block guide rail, ensuring rigidity, adjusting the bottom dead center, and using an auxiliary transmission device.
For forging thin parts, lubricating, cooling and forging parts for high-speed production, the slider can move vertically or horizontally. Compensation devices can also be used to increase movement in other directions.
The above methods differ in required forging force, process, material utilization, production, dimensional tolerance, and lubrication and cooling methods. These factors also affect the level of automation.
3. Forging materials
Forging materials mainly include carbon steel and multi-component alloy steel, as well as aluminum, magnesium, copper, titanium and their alloys. These materials are available in the form of bars, ingots, metal powder and liquid metal.
The forging ratio refers to the ratio of the cross-sectional area of the metal before deformation to the cross-sectional area after deformation. Correct selection of forging rate, reasonable heating temperature and holding time, reasonable initial and final forging temperature, and reasonable deformation and deformation speed are essential to improve product quality and reduce costs.
Round or square bars are generally used as blanks for small and medium-sized forgings. These bars have uniform and good grain structure and mechanical properties, accurate shape and size, and good surface quality, making them convenient for mass production. With reasonable heating temperature and deformation conditions, excellent performance forgings can be produced without large forging deformations.
In comparison, ingots are only used for large forgings. The ingots have a cast structure with large columnar crystals and a loose center. Therefore, it is necessary to break the columnar crystals into fine grains and compact them through large plastic deformation to obtain excellent metallic microstructures and mechanical properties.
Powder metallurgy preform can be transformed into powder forging by non-flash forging in hot state. Powder forging has similar properties to general die forging, including good mechanical properties and high precision, and can reduce subsequent cutting. The internal structure of powder forging is uniform without segregation, making it ideal for small gears and other workpieces. However, the price of the powder is much higher than that of bars in general, limiting its application in production.
Liquid metal pressure forging is a forming method between pressure casting and pressure forging. By applying static pressure to the liquid metal poured into the die hole to solidify, crystallize, flow, plastic deform and mold under the action of pressure, forgings with the required shape and properties can be obtained. This method is particularly suitable for complex thin-walled parts that are difficult to form by general forging.
Finally, wrought alloys of iron-based superalloys, nickel-based superalloys and cobalt-based superalloys can also be completed by forging or rolling. However, these alloys are relatively difficult to forge due to their narrow plastic zone. Therefore, there are strict requirements for the heating temperature, open forging temperature and final forging temperature of different materials.
4. Process flow
Various forging methods employ different processes, and among them, hot forging has the longest process flow.
The typical sequence is as follows: molding the forging blank → heating the forging blank → preparing the forged blank by rolling → forging forming → cutting → punching → correction → intermediate inspection to check the size and surface defects of the forging → heat treatment of forging to remove stresses and improve metal cutting performance → cleaning to eliminate surface oxide scale → correction → inspection.
Typically, forgings undergo inspections for appearance and hardness, while important forgings also undergo inspections for analysis of chemical composition, mechanical properties, residual stress and other non-destructive testing (NDT).
5. Characteristics of forgings
Compared to castings, forging can improve the microstructure and mechanical properties of metals.
When the metal is deformed and recrystallized by the hot forging method, the original coarse, columnar dendritic grain structures transform into equiaxed recrystallization structures with finer, more uniform grains. This process makes the segregation, porosity, slag inclusion and other original imperfections of the ingot more compact and welded, which improves the plasticity and mechanical properties of the metal.
The mechanical properties of castings are generally lower than those of forged parts from the same material.
Furthermore, forging ensures the continuity of the metal fiber structure and maintains the consistency of the fiber structure with the shape of the forged parts. The process completes the metal flow line and ensures that the parts have good mechanical properties and a long service life.
Forgings produced by precision forging, cold extrusion, hot extrusion and other methods are superior to castings.
Forging involves pressing the metal into the desired shape or applying an appropriate compressive force through plastic deformation, typically using a hammer or pressure. The forging process refines the particle structure and improves the physical properties of the metal. In practical applications, a correctly designed part can direct particle flow in the direction of primary pressure.
Casting is the process of obtaining an object shaped from metal using various casting methods. Molten liquid metal is injected into a prepared mold through casting, injection, suction, or other casting techniques. The object is then cooled, sand dropped, cleaned, and undergoes post-treatment to achieve a specific shape, size, and performance.
