What is forging?

Forging is a processing method that uses forging machinery to apply pressure to metal billets, causing them to undergo plastic deformation in order to obtain forgings with certain mechanical properties, shapes, and sizes. One of the two major components of forging (forging and stamping). Forging can eliminate defects such as casting porosity in the smelting process of metals, optimize the microstructure, and, due to the preservation of complete metal flow lines, the mechanical properties of forgings are generally better than those of castings of the same material. Important parts in related machinery with high loads and severe working conditions, except for simple shapes that can be rolled into plates, profiles, or welded parts, are mostly made of forgings.

Classification of Forging

1、 According to the movement of the billet, forging can be divided into free forging, upsetting, extrusion, die forging, closed die forging, and closed upsetting.

1. Free forging. There are mainly two types of forging, manual forging and mechanical forging, which use impact force or pressure to deform the metal between the upper and lower anvil blocks to obtain the required forgings.

2. Die forging. Die forging can be divided into open die forging and closed die forging. Metal billets are obtained by compression deformation in a forging die chamber with a certain shape to obtain forgings. It can also be divided into cold heading, roll forging, radial forging, and extrusion, etc.

2、 According to the deformation temperature, forging can be further divided into hot forging (forging temperature higher than the recrystallization temperature of the billet metal), warm forging (forging temperature lower than the recrystallization temperature of the metal), and cold forging (room temperature). The recrystallization temperature of steel is about 460 ℃, but 800 ℃ is commonly used as the dividing line, and hot forging is used for temperatures higher than 800 ℃; Between 300 and 800 ℃, it is called warm forging or semi hot forging.

According to the movement of the billet, forging can be divided into free forging, upsetting, extrusion, die forging, closed die forging, and closed upsetting. Closed die forging and closed upset forging have high material utilization due to the absence of burrs. It is possible to complete the precision machining of complex forgings with one or several processes. Due to the absence of burrs, the stress area of the forging is reduced, and the required load is also reduced. However, it should be noted that the billet cannot be completely restricted. Therefore, it is necessary to strictly control the volume of the billet, control the relative position of the forging die, and measure the forgings, striving to reduce the wear of the forging die.

According to the movement mode of the forging die, forging can be divided into swing forging, swing rotary forging, roll forging, cross wedge rolling, ring rolling, and oblique rolling. Rotary forging, rotary forging, and ring forging can also be processed by precision forging. In order to improve the utilization rate of materials, roll forging and cross rolling can be used as the preceding processes for processing slender materials. Rotary forging, like free forging, is also locally formed, and its advantage is that it can be formed even under smaller forging forces compared to the size of the forging. This forging method, including free forging, involves the expansion of materials from the vicinity of the mold surface to the free surface during processing, making it difficult to ensure accuracy. Therefore, by controlling the movement direction of the forging die and the rotary forging process with a computer, complex shaped and high-precision products can be obtained with lower forging force, such as producing forgings with multiple varieties and large sizes of steam turbine blades.

The mold movement and degrees of freedom of forging equipment are inconsistent. According to the characteristics of deformation limitations at the bottom dead center, forging equipment can be divided into the following four forms:

Limited forging force form: an hydraulic press that directly drives the slider with oil pressure.

Quasi stroke limit method: an hydraulic press that drives the crank connecting rod mechanism by hydraulic pressure.

Stroke limitation method: a mechanical press with a crank, connecting rod, and wedge mechanism driving the slider.

Energy limitation method: Utilize the spiral mechanism of the screw and friction press.

In order to achieve high accuracy, attention should be paid to preventing overload at the bottom dead center, controlling the speed and mold position. Because these will have an impact on the tolerance, shape accuracy, and die life of forgings. In addition, in order to maintain accuracy, attention should also be paid to adjusting the clearance between the slider guide rails, ensuring stiffness, adjusting the lower dead center, and utilizing auxiliary transmission devices.

In addition, according to the movement mode of the slider, there are also vertical and horizontal movements of the slider (used for forging, lubrication and cooling of slender parts, and forging of high-speed production parts), and the use of compensation devices can increase movement in other directions. The above methods differ in terms of required forging force, process, material utilization, output, dimensional tolerance, and lubrication and cooling methods, which are also factors that affect the level of automation.

Materials and processes for forging

The main materials used for forging are carbon steel and alloy steel with various compositions, followed by aluminum, magnesium, copper, titanium, and their alloys. The original state of materials includes bars, ingots, metal powders, and liquid metals. The ratio of the cross-sectional area of a metal before deformation to the cross-sectional area after deformation is called the forging ratio. The correct selection of forging ratio, reasonable heating temperature and insulation time, reasonable initial and final forging temperatures, reasonable deformation amount and deformation speed are closely related to improving product quality and reducing costs.

Generally, small and medium-sized forgings use round or square bars as billets. The grain structure and mechanical properties of the bar material are uniform and good, with accurate shape and size, good surface quality, and easy to organize mass production. As long as the heating temperature and deformation conditions are reasonably controlled, high-performance forgings can be forged without significant forging deformation.

Ingots are only used for large forgings. The ingot is an as cast structure with large columnar crystals and a loose center. Therefore, it is necessary to break the columnar crystals into fine grains through large plastic deformation, and compact them loosely to obtain excellent metal structure and mechanical properties.

Powder metallurgy preforms made by pressing and firing can be forged into powder forgings without burrs in the hot state. The density of forging powder is close to that of general die forgings, with good mechanical properties and high accuracy, which can reduce subsequent cutting processing. The internal structure of powder forgings is uniform without segregation, and can be used to manufacture small gears and other workpieces. However, the price of powder is much higher than that of ordinary bars, and its application in production is subject to certain limitations

Applying static pressure to the liquid metal poured into the mold cavity, which solidifies, crystallizes, flows, undergoes plastic deformation, and forms under pressure, can obtain the required shape and performance of the die forging. Liquid metal forging is a forming method that lies between die casting and forging, especially suitable for complex thin-walled parts that are difficult to form in general forging.

Different forging methods have different processes, among which the hot die forging process is the longest, and the general order is: forging blank cutting; Forging billet heating; Roll forging blank preparation; Die forging forming; Cutting edges; Punching; Correction; Intermediate inspection, checking the dimensions and surface defects of forgings; Heat treatment of forgings to eliminate forging stress and improve metal cutting performance; Cleaning, mainly to remove surface oxide scale; Correction; Inspection: Generally, forgings need to undergo appearance and hardness inspection, while important forgings also need to undergo chemical composition analysis, mechanical properties, residual stress testing, and non-destructive testing.

In addition to common materials such as carbon steel and alloy steel with various compositions, followed by aluminum, magnesium, copper, titanium, and their alloys, the deformation alloys of iron-based high-temperature alloys, nickel based high-temperature alloys, and cobalt based high-temperature alloys are also completed by forging or rolling. However, due to the relatively narrow plastic zone of these alloys, the difficulty of forging is relatively high, and the heating temperature of different materials is different, There are strict requirements for both the opening forging temperature and the final forging temperature.