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Forging is a processing method that uses forging machinery to apply pressure to metal billets to cause them to undergo plastic deformation 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, optimize the microstructure, and 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. Important parts with high loads and severe working conditions in related machinery, except for simpler shapes that can be rolled plates, profiles or welded parts, mostly use forgings.

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 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, and 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 categories

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 the processing method of forgings that use simple general tools or directly apply 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 with small batches. Forging equipment such as forging hammers and hydraulic presses are used to form the blanks to obtain qualified forgings. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, twisting, displacement and forging. Free forging adopts hot forging.

2) Die forging

Die forging is divided into open die forging and closed die forging. The metal billet 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 direction of die forging, and also represent the level of forging technology.

According to the material, 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, which 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 material utilization rate is high. The finishing of complex forgings can be completed in one or several processes. Since there is no flash, the force 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. To this end, the volume of the blank should be strictly controlled, the relative position of the forging die should be controlled, and the forging should be measured to try to reduce the wear of the forging die.

3) Ring rolling

Ring rolling refers to the production of ring parts of different diameters by special equipment ring rolling machine. It is also used to produce wheel-shaped parts such as automobile wheels and train 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 of 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 swing rolling, swing rotary forging, roll forging, wedge cross rolling, ring rolling and oblique rolling. Swing rolling, swing 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. Rotary forging, like free forging, is also partially formed. Its advantage is that it can be formed even with a smaller forging force than 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 a wide variety of products and large sizes.

The die movement and degree of freedom of the forging equipment are inconsistent. According to the characteristics of the bottom dead point deformation limitation, the forging equipment can be divided into the following four forms:

Limited forging force form: hydraulic press with hydraulic direct drive of the slider.

Quasi-stroke limitation method: hydraulic press with hydraulic drive of crank-connecting rod mechanism.

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

Energy limitation 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 life of the forging die. In addition, in order to maintain accuracy, attention should be paid to adjusting the clearance of the slider 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 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. These factors also affect the level of automation.

3. Forging materials

Forging materials are mainly carbon steel and alloy steel of various compositions, followed by aluminum, magnesium, copper, titanium, etc. and their alloys. The original state of the material is bar, ingot, metal powder and liquid metal. 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 have a great impact on improving product quality and reducing costs.

Generally, small and medium-sized forgings use round or square bars as blanks. The grain structure and mechanical properties of the bars are uniform and good, the shape and size are accurate, the surface quality is good, and it 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, it is necessary to break the columnar crystals into fine grains and compact the looseness through large plastic deformation to obtain excellent metal structure and mechanical properties.

Powder metallurgy preforms that are pressed and sintered can be made into powder forgings by flashless die forging in a hot state. The forging powder is close to the density of general die forgings, has good mechanical properties, and has high precision, which can reduce subsequent cutting processing. The internal structure of powder forgings is uniform and without 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.

Apply static pressure to the liquid metal poured in the die cavity, so that it solidifies, crystallizes, flows, plastically deforms and forms under pressure, and then 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, 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, these alloys have a relatively narrow plastic zone, so the forging difficulty will be relatively large. The heating temperature, start forging temperature and final forging temperature of different materials have strict requirements.

4. Process flow

Different forging methods have different processes, among which the hot die forging process is the longest. The general order is: blanking; heating of forging; roll forging blank; die forging; trimming; punching; correction; intermediate inspection, inspection of forging size and surface defects; heat treatment of forgings to eliminate forging stress and improve metal cutting performance; cleaning, mainly to remove surface oxide scale; correction; inspection, general 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. Characteristics of forgings

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 organizations with finer grains and uniform size, so that the original segregation, looseness, pores, slag inclusions, etc. in the ingot are compacted and welded, and its organization becomes tighter, 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 make the metal streamlines 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 shape the required shape or appropriate compression force. This force is typically achieved by using a hammer or pressure. The forging process builds a delicate particle 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 forming 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, sand dropping, cleaning and post-processing are obtained to obtain objects with certain shapes, sizes and properties.