Classification of Forging Forming Methods (I)
1. Cold and Warm Forging
The development of the automotive and motorcycle industries has greatly promoted the advancement of cold and warm forging technologies in China. For example, parts such as splines, gears, wheel hubs, connecting rods, and crankshafts can be formed using cold or warm forging. Cold-forged components have evolved from early products like piston pins, wheel nuts, and ball studs to more complex parts such as constant velocity (CV) joints, generator claw poles, spline shafts, starter gears, differential bevel gears, cross shafts, three-pin shafts, spiral bevel gears, and rear axles.
Single-piece gears formed through cold forging weigh over 1.0 kg and can reach a gear tooth accuracy of tolerance grade IT7. The largest cold-forged automotive part (a half-shaft sleeve) weighs over 10.0 kg. Shaft parts produced by cold extrusion can reach lengths exceeding 400 mm. In Japan and Germany, the total weight of cold-forged parts in a single vehicle can reach 40–50 kg, while in China it currently averages about 30.0 kg per vehicle.
Stepped shafts and spline shafts for automobiles, motorcycles, and general machinery are mostly manufactured using cold extrusion. Spiral spline shafts and worm-type parts are typically formed through cold rolling. End-face gears and small-sized straight bevel gears are often produced using cold orbital forging. Automotive gears manufactured using cold forging technology can achieve an accuracy of tolerance grade IT7. Complex internal cavities of CV joints are formed using cold or warm forging, with dimensional accuracy reaching 0.05–0.08 mm, allowing for direct assembly without further machining.
2. Closed Die Forging
Closed die forging is one of the advanced forging technologies. Unlike traditional forging methods, it involves unidirectional or bidirectional extrusion within a closed die cavity. This process produces flashless forgings with material utilization rates reaching 85–90%, productivity of 2,000–3,000 parts per shift, and manufacturing costs reduced by 15–20% compared to traditional processes. Dimensional accuracy is high, typically achieving: diameter < 0.04 mm, concentricity < 0.05 mm, and thickness < 0.15 mm.
This technology is applicable to the production of high-precision and complex forgings such as automotive differential planetary gears, axle gears, CV joint star-shaped sleeves, cross shafts, tripod CV joint housings, connecting rod caps, and clutch gears. These forgings require minimal machining allowance. For example, CV joint cross shafts only require a grinding allowance of 0.30–0.40 mm; the transmission accuracy of bevel gears can reach IT7, with tooth surfaces suitable for use without machining; and the internal ball groove diameter tolerance of the star-shaped sleeves is 0.05–0.08 mm.
3. Precision Forging of Aluminum Alloys
The development and application of mass-produced precision aluminum alloy forgings are closely tied to the rapid growth of the automotive industry. The growing demand for aluminum alloy forgings is mainly driven by the trend toward vehicle weight reduction.
China's overall aluminum alloy forging technology lags behind developed countries by 10–20 years. The current practice still relies on single-station upsetting and extrusion methods for producing relatively simple-shaped forgings. Since the 1960s, China has studied and widely applied extrusion processes for aluminum alloy pistons. However, few organizations are investing in the research and development of aluminum alloy forging processes for complex parts, especially for mass production and advanced practical forming technologies.
Aluminum alloy forgings for aircraft are generally produced by free forging in single or small batches. Due to low material utilization and high costs, this method is unsuitable for mass production. In recent years, as China's automotive industry-especially the passenger car sector-has developed, cold extrusion, warm stamping, and isothermal forging techniques have been increasingly applied to the large-scale production of complex aluminum alloy parts, such as brackets, fuse bodies, airbag housings, and communication device housings, meeting production demands.
4. Precision Hot Die Forging
Precision hot die forging is a widely used manufacturing method in China's automotive, motorcycle, general machinery, defense, and aerospace industries. It produces forgings that closely match the final shape of parts, saving materials and energy, reducing machining steps and equipment use, significantly improving productivity and product quality, and lowering production costs.
(1) Precision Hot Die Forging of Automotive Differential Gears:
As shown in Figure 1.9, differential gears (straight bevel gears) are among the most common applications of precision hot die forging technology. Currently, the straight bevel gears used in heavy-duty vehicles in China are primarily produced using this method, with tooth accuracy reaching IT8, completely eliminating the need for gear cutting.
(2) Precision Hot Die Forging of Automotive Front Axles:
The front axle is the largest forging on a heavy-duty vehicle, typically weighing between 70.0 and 130.0 kg. For heavy-duty vehicle front axles, the "form-rolling + integral hot die forging" technique is used. This process shapes the difficult-to-forge I-beam and spring seat using form-rolling, while hot die forging is applied only to the bent arms at both ends. This approach significantly reduces the required press capacity-using a 25 MN (2500-ton) screw press is sufficient to forge front axle blanks weighing around 120.0 kg. The forged product matches the quality of parts made on a 125 MN (12,500-ton) press, while die life is improved by 50%, and production costs are reduced by 20%.Multiple "form-rolling + integral hot die forging" front axle production lines have been established in China, becoming a key solution for technological upgrades in front axle forging enterprises. Figure 1.10 illustrates the production process for heavy-duty vehicle front axles by a certain company.
Figure 1.10(a) shows the form-rolling process on a 1000 mm automatic rolling machine.Figure 1.10(b) shows the bending and final forging process on a 25 MN (2500-ton) screw press.Figure 1.10(c) shows actual forgings from various stages of the "form-rolling + integral hot die forging" process.
