As mentioned previously, based on the forging temperature, forging processes can be classified into hot forging, warm forging, and cold forging.
Based on the forming mechanism, forging can be categorized into open-die forging, closed-die forging, ring rolling, and special forging processes.
1. Open-Die Forging. This refers to a processing method in which simple, general-purpose tools-or the upper and lower anvils of forging equipment-are used to directly apply external force to a workpiece (billet), causing it to deform and thereby acquire the desired geometric shape and internal quality. Forgings produced using the open-die method are known as *open-die forgings*. Open-die forging is primarily utilized for producing forgings in small batches; forging hammers, hydraulic presses, and similar equipment are employed to shape the billet and produce qualified forgings. The basic operations involved in open-die forging include upsetting, drawing out, punching, cutting, bending, twisting, offsetting, and forge welding. Open-die forging is invariably performed using the hot forging method.
2. Closed-Die Forging. Closed-die forging is further subdivided into open-die closed-die forging (with flash) and closed-die forging (flashless). In this process, a metal billet is subjected to compressive deformation within the cavity of a forging die of a specific shape to produce a finished forging. Closed-die forging is generally employed for the production of parts that are relatively light in weight but required in large production volumes.
Closed-die forging can be classified into hot closed-die forging, warm closed-die forging, and cold closed-die forging. Warm and cold closed-die forging represent the future direction of development for this technology and serve as indicators of the advanced level of forging expertise.
Based on the material utilized, closed-die forging can also be categorized into ferrous metal forging, non-ferrous metal forging, and powder metallurgy forming. As the names imply, the respective materials involved are ferrous metals (such as carbon steel), non-ferrous metals (such as copper and aluminum), and powder metallurgy materials.
Extrusion is generally classified as a form of closed-die forging and can be further divided into heavy metal extrusion and light metal extrusion.
Closed-die forging (flashless) and closed-die upsetting represent two advanced processes within the realm of closed-die forging. Because they produce no flash (excess material), they offer high material utilization efficiency. Complex forgings can often be finished in a single operation or a limited number of steps. Furthermore, the absence of flash reduces the effective load-bearing area of the forging, thereby lowering the total load required for the process. However, it is crucial to ensure that the billet is not subjected to complete spatial constraint; to this end, strict control must be exercised over the billet's volume and the relative positioning of the forging dies, while also incorporating precise measurement of the forged part to minimize wear on the dies.
3. Ring Rolling. Ring rolling refers to the production of annular parts of various diameters using specialized equipment known as a ring rolling machine; it is also employed to manufacture wheel-shaped components such as automotive hubs and train wheels.
4. Special Forging. Special forging encompasses various methods-including roll forging, cross-wedge rolling, radial forging, and liquid die forging-all of which are particularly well-suited for producing parts with specific, complex geometries.
For instance, roll forging serves as an effective pre-forming process that significantly reduces the forging pressure required in subsequent forming stages; cross-wedge rolling is used to produce components such as steel balls and transmission shafts; while radial forging is utilized to manufacture large-scale forgings, such as cannon barrels and stepped shafts.
Forging Dies
Based on the kinematics of the forging dies, forging processes can be further categorized into orbital forging, rotary forging, roll forging, cross-wedge rolling, ring rolling, and skew rolling. Orbital forging, rotary forging, and ring rolling techniques can also be utilized for precision forging operations. To enhance material utilization efficiency, roll forging and cross-wedge rolling are frequently employed as preliminary processing steps for elongated workpieces. Similar to open-die forging, rotary forging is a localized forming process; its primary advantage is the ability to achieve the desired shape using significantly lower forging forces relative to the overall dimensions of the workpiece. In these localized forging methods-including open-die forging-the material flows outward from the vicinity of the die face toward the free surface during processing, making it inherently difficult to maintain high dimensional accuracy. Consequently, by employing computer control to precisely govern the die's trajectory and the sequence of the rotary forging process, it becomes possible to produce complex, high-precision components-such as a diverse range of large-scale turbine blades-while utilizing comparatively lower forging forces.
Forging equipment varies in terms of die kinematics and degrees of freedom. Based on the specific constraints governing material deformation-categorized into the four points listed below-forging equipment can be classified into the following four distinct types:
1. Force-Constrained Type: Hydraulic presses in which the ram (slide) is driven directly by hydraulic cylinders.
2. Quasi-Stroke-Constrained Type: Hydraulic presses in which the ram is driven via a hydraulic-powered crank-and-connecting-rod mechanism.
3. Stroke-Constrained Type: Mechanical presses in which the ram is driven by a mechanism comprising cranks, connecting rods, and wedges.
4. Energy-Constrained Type: Screw presses and friction presses that utilize a screw mechanism to generate forging force.
To achieve high dimensional accuracy, particular attention must be paid to preventing overloading at the bottom dead center (the lowest point of the ram's stroke), as well as to precisely controlling the ram's velocity and the die's position. This is because all these factors impact the tolerances, shape accuracy, and die life of the forged parts. Furthermore, to maintain precision, attention must be paid to measures such as adjusting the clearance in the slide guides, ensuring structural rigidity, calibrating the bottom dead center, and utilizing auxiliary drive mechanisms.
The Slide
Slides are also distinguished by their mode of vertical and horizontal movement-a feature typically employed in the forging of slender parts, in processes requiring specific lubrication and cooling, and in the high-speed production of components. Moreover, the use of compensation devices allows for the introduction of motion in additional directions. Given these varying configurations, the required forging force, process sequence, material utilization efficiency, production output, dimensional tolerances, and lubrication/cooling methods all differ accordingly; these factors, in turn, are key determinants of the overall level of automation.
