Forging is a processing method that uses a forging machine to apply pressure to a metal blank to plastically deform it to obtain a forging with certain mechanical properties, certain shapes, and sizes. It is one of the components of forging (forging and stamping). Through forging, defects such as as-cast looseness generated in the metal smelting process can be eliminated, and the microstructure can be optimized. At the same time, due to the preservation of the complete metal streamline, the mechanical properties of forgings are generally better than those of castings of the same material. For important parts with high load and severe working conditions in related machinery, forgings are mostly used in addition to rolling plates, profiles, or welded parts with simple shapes.
1 Deformation temperature
The initial recrystallization temperature of steel is about 727 °C, but 800 °C is generally used as the dividing line, and hot forging is higher than 800 °C; between 300 and 800 °C, it is called warm forging or semi-hot forging, and forging is carried out at room temperature. is called cold forging. Forgings used in most industries are hot forging. Warm and cold forging is mainly used for forging parts such as automobiles and general machinery. Warm and cold forging can effectively save materials.
2 Forging categories
As mentioned above, the forging temperature 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 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 the free forging method are called free forgings. Free forging is mainly based on the production of forgings in small batches. Forging equipment such as forging hammers and hydraulic presses are used to form and process the blanks to obtain qualified forgings. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, torsion, offset, 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 cavity 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 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 implies, the materials are ferrous metals such as carbon steel, non-ferrous metals such as copper and aluminum, and powder metallurgy materials.
Extrusion should belong to die forging, which can be divided into the heavy metal extrusion and light metal extrusion.
Closed die forging and closed upsetting are two advanced processes of die forging. Since there is no flash, the utilization rate of materials is high. It is possible to complete the finishing of complex forgings in one or several operations. Since there is no flash, the stressed 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 reason, the volume of the blank should be strictly controlled, the relative position of the forging die should be controlled, the forging should be measured, and efforts should be made to reduce the wear of the forging die.
3) Rolling ring
Ring rolling refers to the production of ring-shaped parts of different diameters through special equipment ring-grinding machines and is also used to produce wheel-shaped parts such as automobile hubs and train wheels.
4) Special forging
Special forging includes roll forging, cross wedge rolling, radial forging, liquid die forging, and other forging methods, which are more suitable for the production of parts with special shapes. For example, roll forging can be used as an effective performing process to greatly reduce the subsequent forming pressure; cross wedge rolling can produce parts such as steel balls and drive shafts; radial forging can produce large forgings such as barrels and stepped shafts.
5) Forging die
According to the movement mode of the forging die, forging can be divided into pendulum rolling, pendulum rotary forging, roll forging, cross wedge rolling, ring rolling, and skew rolling. Pendulum rolling, pendulum swaging, 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 front-end processing of slender materials. Rotary forging, like free forging, is also partially formed, and its advantage is that compared with the size of the forging, the forming can be achieved even when the forging force is small. In this forging method, including free forging, the material expands from the vicinity of the die surface to the free surface during processing, so it is difficult to ensure accuracy. The forging force can be used to obtain products with complex shapes and high precision, such as forgings such as steam turbine blades of many varieties and large sizes.
Die motion and degrees of freedom of forging equipment are inconsistent, and can be divided into the following four forms:
The form of limiting forging force: a hydraulic press that directly drives the slider by hydraulic pressure.
Quasi-stroke limitation method: hydraulic press that drives the crank connecting rod mechanism by hydraulic pressure.
Stroke limitation method: a mechanical press in which the crank, connecting rod, and wedge mechanism drive the slider.
Energy limitation method: screw and friction press with a screw mechanism.
In order to obtain high precision, care should be taken to prevent overload at the bottom dead center, and to control the speed and mold position. Because these will have an impact on forging tolerances, shape accuracy, and forging die life. In addition, in order to maintain the accuracy, attention should also be paid to adjusting the clearance of the slider guide rail, ensuring the rigidity, adjusting the bottom dead center, and using the auxiliary transmission device.
There are also differences in the vertical and horizontal motion of the slider (for forging of slender parts, lubricating cooling, and forging of parts for high-speed production), and the compensation device can be used to increase the motion in other directions. The above methods are different, and the required forging force, process, material utilization rate, output, dimensional tolerance, and lubrication and cooling methods are different. These factors are also factors that 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 raw state of the material is a 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. Correct selection of forging ratio, reasonable heating temperature and holding time, reasonable initial forging temperature and final forging temperature, reasonable deformation amount, and deformation speed has a lot to do with 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 bar are uniform and good, the shape and size are accurate, and the surface quality is good, which is convenient for mass 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. The ingot is an as-cast structure with large columnar crystals and a loose center. Therefore, the columnar crystals must be broken into fine grains through large plastic deformation, and then loosely compacted to obtain excellent metal structure and mechanical properties.
The powder metallurgy preforms that have been pressed and sintered can be made into powder forgings by forging without flash in the 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 operations. Powder forgings have a uniform internal structure and 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 limited.
Applying static pressure to the liquid metal poured in the die cavity makes it solidify, crystallize, flow, plastically deform and form under the action of pressure, and then die forgings with the desired shape and properties can be obtained. Liquid metal dies forging is a forming method between die casting and die forging, and is especially suitable for complex thin-walled parts that are difficult to form in general die forging.
In addition to the usual materials and aluminum, magnesium, copper, titanium, etc. and their alloys, iron-based superalloys, nickel-based superalloys, and deformed alloys of cobalt-based superalloys are also completed by forging or rolling, but these alloys are due to their plasticity. The area is relatively narrow, so the forging difficulty will be relatively large, and the heating temperature, opening forging temperature and final forging temperature of different materials have strict requirements.
4 Process flow
Different forging methods have different processes. Among them, hot die forging has the longest process flow. The general sequence is: blanking of forging billet; heating of forging billet; roll forging preparation billet; die forging forming; trimming; punching; straightening; 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, general forgings are subject to appearance and hardness inspections, important forgings also It must undergo chemical composition analysis, mechanical properties, residual stress, and other inspections and non-destructive testing.
5 Features of forgings
Compared with castings, the metal structure and mechanical properties can be improved after forging. After the casting structure is deformed by the forging method, due to the deformation and recrystallization of the metal, the original coarse dendrites and columnar grains become an equiaxed recrystallized structure with finer grains and uniform size, which makes the original segregation and recrystallization in the ingot. Porosity, pores, slag inclusions, etc. 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, the forging process can ensure the continuity of the metal fiber structure, so that the fiber structure of the forging is consistent with the shape of the forging, and the metal streamline is complete, which can ensure the parts have good mechanical properties and long service life. Precision die forging and cold extrusion are used. Forgings produced by processes such as warm extrusion and warm extrusion are incomparable to castings.
Forging is an object to which a metal is subjected to pressure to shape the desired shape or suitable compressive force through plastic deformation. This force is typically achieved through the use of a hammer or pressure. The forging process builds a refined grain structure and improves the physical properties of the metal. In real-world use of the component, a proper design enables particle 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 injected into the pre-prepared casting mold by pouring, injection, suction, or other casting methods. Processing, etc., the resulting objects of a certain shape, size, and properties.
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