Forging is a processing method that uses forging machinery to apply pressure to metal blanks to cause them to undergo plastic deformation in order to obtain forgings with certain mechanical properties, shapes and sizes. It is one of the two major components of forging (forging and stamping). Forging can eliminate defects such as loose cast metal produced during the smelting process and optimize the microstructure. At the same time, because the complete metal flow lines are preserved, the mechanical properties of forgings are generally better than those of castings of the same material. For important parts in related machinery with high loads and severe working conditions, forgings are mostly used, except for simpler shapes that can be rolled plates, profiles or welded parts.
1. Deformation temperature
The initial recrystallization temperature of steel is about 727℃, but 800℃ is generally used as the dividing line. Forging above 800℃ is called hot forging; forging between 300 and 800℃ is called warm forging or semi-hot forging, and forging at room temperature is called cold forging. Forgings used in most industries are hot forgings, while warm forging and cold forging are mainly used for forging parts of automobiles, general machinery, etc. Warm forging and cold forging can effectively save materials.
2. Forging Category
As mentioned above, according to the forging temperature, it can be divided into hot forging, warm forging and cold forging. According to the forming mechanism, forging can be divided into free forging, die forging, ring rolling and special forging.
1）Free Forging
It refers to a processing method of forgings that uses simple universal tools or directly applies external force to the blank between the upper and lower anvils of the forging equipment to deform the blank to obtain the required geometric shape and internal quality. Forgings produced by free forging are called free forgings. Free forging is mainly used to produce forgings in small batches. Forging equipment such as forging hammers and hydraulic presses are used to form the blank to obtain qualified forgings. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, twisting, shifting and forging. Free forging is all hot forging.
2）Die Forging
Die forging is divided into open die forging and closed die forging. The metal blank is compressed and deformed in a forging die with a certain shape to obtain a forging. Die forging is generally used to produce parts with small weight and large batches. Die forging can be divided into hot die forging, warm forging and cold forging. Warm forging and cold forging are the future development directions of die forging, and also represent the level of forging technology.
According to the materials, die forging can also be divided into ferrous metal die forging, non-ferrous metal die forging and powder product forming. As the name suggests, the materials are ferrous metals such as carbon steel, non-ferrous metals such as copper and aluminum, and powder metallurgy materials.
Extrusion should be classified as die forging and can be divided into heavy metal extrusion and light metal extrusion.
Closed die forging and closed upset forging are two advanced processes of die forging. Since there is no flash, the utilization rate of materials is high. The finishing of complex forgings can be completed in one or several processes. Since there is no flash, the force-bearing area of the forging is reduced, and the required load is also reduced. However, it should be noted that the blank cannot be completely restricted. For this purpose, the volume of the blank, the relative position of the forging die, and the forging should be strictly controlled, and efforts should be made to reduce the wear of the forging die.
3）Rolling ring
Ring rolling refers to the production of ring parts of different diameters through special equipment ring rolling machines. It is also used to produce wheel-shaped parts such as automobile hubs and railway wheels.
4）Special forging
Special forging includes roll forging, wedge cross rolling, radial forging, liquid die forging and other forging methods, which are more suitable for producing parts with certain special shapes. For example, roll forging can be used as an effective pre-forming process to greatly reduce the subsequent forming pressure; wedge cross rolling can produce parts such as steel balls and transmission shafts; radial forging can produce large barrels, step shafts and other forgings.
5）Forging Die
According to the movement mode of the forging die, forging can be divided into pendulum rolling, pendulum rotary forging, roll forging, wedge cross rolling, ring rolling and oblique rolling. Pendulum rolling, pendulum rotary forging and ring rolling 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 front process of slender materials. Like free forging, rotary forging is also partially formed. Its advantage is that it can be formed even when the forging force is small compared with the size of the forging. In this forging method, including free forging, the material expands from the vicinity of the die surface to the free surface during processing. Therefore, it is difficult to ensure accuracy. Therefore, by controlling the movement direction of the forging die and the rotary forging process with a computer, products with complex shapes and high precision can be obtained with lower forging forces, such as forgings such as turbine blades with many varieties and large sizes.
The die movement and degree of freedom of forging equipment are inconsistent. According to the deformation restriction characteristics of the bottom dead point, forging equipment can be divided into the following four forms:
1）Limiting forging force form: hydraulic press with hydraulic pressure directly driving the slider.
2）Quasi-stroke limiting method: hydraulic press with hydraulic pressure driving crank-connecting rod mechanism.
3）Stroke limiting method: mechanical press with crank, connecting rod and wedge mechanism driving the slider.
4）Energy limiting method: spiral and friction press using spiral mechanism.
In order to obtain high precision, attention should be paid to preventing overload at the bottom dead center, controlling speed and die position, because these will affect the tolerance, shape accuracy and die life of the forging. In addition, in order to maintain accuracy, attention should also be paid to adjusting the clearance of the slider guide rail, ensuring rigidity, adjusting the bottom dead center and using auxiliary transmission devices.
There are also vertical and horizontal movement modes of the slider (used for forging of slender parts, lubrication and cooling, and high-speed production of parts forging), and the use of compensation devices can increase the movement in other directions. The above methods are different, and the required forging force, process, material utilization, output, dimensional tolerance and lubrication and cooling methods are different, which are also factors that affect the level of automation.
3. Forging materials
The materials used for forging are mainly carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, copper, titanium and their alloys. The original state of the materials includes bars, ingots, metal powders and liquid metals. The ratio of the cross-sectional area of the metal before deformation to the cross-sectional area after deformation is called the forging ratio. Correctly selecting the forging ratio, reasonable heating temperature and holding time, reasonable initial forging temperature and final forging temperature, reasonable deformation amount and deformation speed are very important for improving product quality and reducing costs.
Generally, small and medium-sized forgings are made of round or square bars. The bar has uniform and good grain structure and mechanical properties, accurate shape and size, good surface quality, and is easy to organize batch production. As long as the heating temperature and deformation conditions are reasonably controlled, forgings with excellent performance can be forged without large forging deformation.
Ingots are only used for large forgings. Ingots are cast structures with large columnar crystals and loose centers. Therefore, large plastic deformation is required to break the columnar crystals into fine grains and compact the looseness in order to obtain excellent metal structure and mechanical properties.
The powder metallurgy preforms that are pressed and sintered can be made into powder forgings by flashless die forging in hot state. The density of forging powder is close to that of general die forgings, with good mechanical properties and high precision, which can reduce subsequent cutting processing. The internal structure of powder forgings is uniform and has no segregation, and can be used to manufacture small gears and other workpieces. However, the price of powder is much higher than that of general bars, and its application in production is subject to certain restrictions.
By applying static pressure to the liquid metal poured into the die cavity, it solidifies, crystallizes, flows, plastically deforms and forms under pressure, and the die forgings with the required shape and performance can be obtained. Liquid metal die forging is a forming method between die casting and die forging, and is particularly suitable for complex thin-walled parts that are difficult to form by general die forging.
In addition to the usual materials, such as carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, copper, titanium and their alloys, iron-based high-temperature alloys, nickel-based high-temperature alloys, and cobalt-based high-temperature alloys are also deformed by forging or rolling. However, these alloys have a relatively narrow plastic zone, so forging is relatively difficult. There are strict requirements on the heating temperature, start forging temperature and final forging temperature of different materials.
4、Process
Different forging methods have different processes, among which hot die forging has the longest process flow. The general sequence is: blanking; heating of forging; roll forging preparation; die forging; trimming; punching; correction; intermediate inspection, to check the size 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 must undergo appearance and hardness inspection. Important forgings must also undergo chemical composition analysis, mechanical properties, residual stress and other inspections and non-destructive testing.
5、Forging characteristics
Compared with castings, metals can improve their organizational structure and mechanical properties after forging. After the casting organization is deformed by hot working by forging, due to the deformation and recrystallization of the metal, the original coarse dendrites and columnar grains are transformed into equiaxed recrystallized structures with finer grains and uniform size, and the original segregation, looseness, pores, slag inclusions, etc. in the steel ingot are compacted and welded, and the organization becomes more compact, which improves the plasticity and mechanical properties of the metal.
The mechanical properties of castings are lower than those of forgings of the same material. In addition, forging can ensure the continuity of metal fiber structure, keep the fiber structure of forgings consistent with the shape of forgings, and ensure that the metal streamlines are complete, which can ensure that the parts have good mechanical properties and long service life. Forgings produced by precision die forging, cold extrusion, warm extrusion and other processes are incomparable to castings.
Forgings are objects to which metal is subjected to pressure and plastic deformation to form a desired shape or appropriate compressive force. This force is typically achieved by using a hammer or a press. The forging process builds a refined grain structure and improves the physical properties of the metal. In the actual use of parts, a correct design can make the particles flow in the direction of the main pressure. Castings are metal shaped objects obtained by various casting methods, that is, the smelted liquid metal is poured into a pre-prepared mold by pouring, injection, suction or other casting methods, and after cooling, it is sanded, cleaned and post-processed to obtain an object with a certain shape, size and performance.
