In modern manufacturing, precision forging has become a highly favored metalworking process due to its exceptional precision and quality advantages. It is widely applied in various fields, including high-end components in aerospace and precision parts in automotive manufacturing. The precise control of deformation temperature and precision is crucial for ensuring the quality of precision-forged parts. This article will delve into these two core factors and reveal how they jointly shape high-quality precision-forged components.
The selection of deformation temperature in precision forging is of vital importance as it directly affects the surface quality, dimensional accuracy, and internal microstructure of the forged parts. Depending on different temperature ranges, precision forging is mainly divided into high-temperature precision forging, medium-temperature precision forging, and isothermal forging, which also plays a significant role in specific fields.
High-temperature precision forging is typically conducted above the recrystallization temperature of the material. This temperature range endows the metal with good plasticity, enabling it to fill the mold cavity more easily and thus produce complex-shaped forged parts. However, the high-temperature environment also poses numerous challenges, the most prominent of which are oxidation and decarburization. During high-temperature heating, the surface of the blank is prone to chemical reactions with oxygen in the air, forming an oxide layer. Meanwhile, the carbon element will also decrease due to high-temperature volatilization, known as decarburization. This not only degrades the material's properties but also affects the surface quality and dimensional accuracy of the forged parts. To effectively prevent these issues, heating furnaces with minimal or no oxidation, such as open-flame heating furnaces or those capable of controlling the atmosphere, can be employed. These heating devices utilize special atmosphere control techniques, such as introducing protective gases or using a vacuum environment, to significantly reduce the occurrence of oxidation and decarburization. In addition to protective measures during the heating process, the cooling of hot-forged parts also requires careful design. To avoid secondary oxidation during cooling, cooling can be performed in a protected device, such as an inert gas-protected cooling box. Moreover, orderly placing the forged parts in a gridded sand box is also an effective cooling method. When slow cooling is needed to reduce internal stress, the forged parts can be placed in a hot sand box or asbestos powder, which have good thermal insulation properties and can allow the parts to cool slowly in a relatively stable temperature environment, thereby achieving better microstructural properties and dimensional stability.
Medium-temperature precision forging is carried out below the recrystallization temperature of the material, generally within a temperature range where the material has good plasticity and moderate deformation resistance. Compared with high-temperature precision forging, medium-temperature precision forging can effectively reduce the degree of oxidation and decarburization while maintaining a certain strength and toughness of the material. This makes it an ideal processing method for forged parts with high requirements for surface quality and dimensional accuracy. Taking the commonly used 45 steel as an example, medium-temperature precision forging within the temperature range of 600–875°C can ensure that the material has low deformation resistance, facilitating forming, without causing severe oxidation. This allows the production of forged parts with high dimensional accuracy and good surface finish. In this temperature range, the working temperature of the mold is generally around 400°C. At this temperature, high-speed steel, a commonly used mold material, can well meet the requirements of medium-temperature precision forging. High-speed steel has high strength, high hardness, and good wear resistance, and can maintain stable performance at moderate temperatures, ensuring the service life of the mold and the quality of the forged parts.
Isothermal forging is a special precision forging process in which both the mold and the blank are heated to the optimal forging temperature of the blank and this temperature is maintained throughout the forging process. This process allows the blank to deform at a lower strain rate, thereby significantly improving the plasticity and flowability of the blank and reducing the deformation resistance. Isothermal forging is particularly suitable for high-performance alloy materials in the aerospace and aviation industries, such as aluminum alloys, magnesium alloys, and titanium alloys. These alloy materials usually have complex phase transformation processes and high deformation resistance, making it difficult for traditional forging processes to meet their high-precision and high-performance requirements. However, isothermal forging technology can fully exploit the performance advantages of these materials, ensuring the dimensional accuracy and surface quality of the forged parts while also enhancing their mechanical properties and service life. For example, in the manufacturing of blades for aero-engines, isothermal forging technology can precisely control the shape and size of the blades, enabling them to maintain stable performance in the harsh environment of high-speed rotation and high-temperature, high-pressure conditions, thus providing strong assurance for the safe operation of aero-engines.
Precision is a key indicator for measuring the quality of precision forging, involving multiple aspects such as dimensional accuracy, shape accuracy, positional accuracy, and surface roughness of the forged parts. Many factors affect the precision of precision-forged parts, including blank volume deviation, elastic deformation of the mold and forged parts, temperature fluctuations of the mold and blank (forged parts), mold accuracy, and equipment accuracy. Only by comprehensively controlling and optimizing these factors can high-precision precision-forged parts be manufactured.
Currently, the dimensional accuracy of precision-forged parts generally reaches IT15–IT12, with a surface roughness Ra of 3.2–1.6μm. For hot-forged parts, the precision is IT8–IT7, and the surface roughness Ra is 6.3–3.2μm; cold-forged parts have higher precision, ranging from IT6–IT5, with a surface roughness Ra of 0.8–0.4μm. Although these precision levels can meet the requirements of many industrial applications, in some high-end fields such as aerospace and medical devices, the precision requirements for precision-forged parts are continuously increasing. For example, in the aerospace field, the dimensional accuracy of some critical components is required to reach micrometers or even sub-micrometers, and the surface roughness is required to reach mirror levels. This poses higher challenges for the precision forging process, necessitating fine optimization and innovation in material selection, mold design, and forging process control.
To enhance the precision of precision forging, the following aspects can be considered.
The dimensional accuracy of the blank is a fundamental factor affecting the final precision of the forged part. During production, it is necessary to accurately calculate the size of the original blank and use advanced cutting equipment for precise cutting. Additionally, to ensure the surface quality of the blank, methods such as sandblasting and acid washing can be employed to remove oxide scale and impurities from the surface of the blank, preventing these defects from affecting the surface quality and dimensional accuracy of the forged part during the forging process.
During heating, oxidation and decarburization not only degrade the material's properties but also cause dimensional changes on the surface of the blank, thereby affecting the precision of the forged part. Therefore, employing minimal or no oxidation heating methods, such as vacuum heating or atmosphere-protected heating, can effectively reduce oxidation and decarburization on the surface of the blank, maintaining its surface quality and dimensional stability.
The precision and rigidity of forging equipment directly impact the stability and precision of the forging process. High-precision forging equipment can accurately control parameters such as forging force, speed, and stroke, ensuring the accuracy and repeatability of the forging process. Meanwhile, high-rigidity equipment can reduce elastic deformation during forging, improving the dimensional accuracy of the forged parts. For example, friction presses, crank presses, or high-speed forging hammers are all equipment with high precision and rigidity, suitable for precision forging production.
The closing accuracy of the mold is one of the key factors affecting the precision of the forged part. By using guiding devices such as guide pins and guide sleeves between the upper and lower dies of the forging mold, precise closing of the upper and lower dies can be ensured during the forging process, reducing mold misalignment and displacement, thereby improving the dimensional and shape accuracy of the forged part.
The mold is the core component of precision forging, and its design accuracy directly determines the precision of the forged part. The dimensional accuracy of the mold cavity should generally be about two grades higher than that of the forged part, with the roughness Ra of the important parts of the mold cavity being less than 1.6μm and that of the general parts being 3.2–1.6μm. Additionally, guiding and ejection devices should be designed on the mold to ensure accurate closing and smooth ejection of the forged part, reducing or completely eliminating the draft angle for forging. Moreover, mold design should also consider factors such as material selection, matching of hardness and strength, and heat treatment processes to ensure that the mold has good performance and a long service life under high-temperature, high-pressure forging conditions.
Precision-forged parts rely on molds for their completion, making the design accuracy of the mold a crucial factor affecting the precision of the forged parts. Modern precision forging molds are typically designed using CAD/CAM systems, which enable high-precision geometric shape design and comprehensive optimization of various factors such as mold structure, material, hardness, strength, and heat treatment. Based on the design requirements of the forged part, the CAD/CAM system can accurately calculate the size and shape of the mold to ensure the dimensional accuracy, positional accuracy, and surface quality of the forged part. During the design process, designers need to fully consider various potential adverse conditions that may arise during the forging process, such as material flow, mold wear, and forging force distribution, to design molds with high precision, efficiency, and long service life. For example, when designing the mold cavity, it is necessary to reasonably allocate the size and shape of the cavity according to the shape and size of the forged part to ensure that the material can fully fill the cavity during forging, avoiding defects such as underfilling and folding. Additionally, it is necessary to design reasonable diverting groove and vents on the mold to expel gases from the cavity during forging, preventing the formation of gas pores. Moreover, the selection of mold material is also of great significance. It is necessary to choose suitable mold materials based on the requirements of the forging process and the working conditions of the mold, and to carry out reasonable heat treatment processes to improve the hardness, strength, and wear resistance of the mold.
The forging process of precision-forged parts is influenced by various factors such as forging temperature, pressure, and speed. The control of these factors is crucial for ensuring forging precision. During the forging process, temperature changes can alter the plasticity of the material, while adjustments to pressure and speed can control the deformation and flow direction of the material. The mold design, on the other hand, can control the shape and position of the material. To ensure the controllability of the forging process, real-time monitoring and adjustment of these factors are necessary. For example, by using advanced temperature sensors and control systems, the forging temperature can be precisely controlled to ensure that it remains within the optimal range throughout the forging process. Meanwhile, through pressure sensors and speed controllers, the forging force and speed can be monitored and adjusted in real-time to ensure that the deformation process of the material meets the design requirements. Additionally, computer simulation technology can be employed to simulate and optimize the forging process, predict potential problems in advance, and take corresponding measures for adjustment and improvement.
The materials involved in precision-forged parts are usually high-strength, high-hardness metals such as titanium alloys, red copper, and aluminum alloys. These materials not only possess excellent physical and mechanical properties but also have deformation characteristics during the forging process that are well-suited for precision forging. In material selection, it is necessary to scientifically choose materials based on design requirements and the characteristics of the forging process to ensure the precision, toughness, and corrosion resistance of the forged parts. For example, titanium alloy has high strength, low density, good corrosion resistance, and high-temperature performance, and is widely used in the aerospace field. However, its forging performance is relatively poor, and it requires forging under suitable temperature and pressure conditions to obtain high-quality forged parts. Red copper has good electrical and thermal conductivity and is widely used in the electrical and electronic fields. Its forging performance is good, and it can be forged within a wide temperature range, but it is necessary to control the forging temperature and speed to avoid defects such as cracking. Aluminum alloy is a lightweight, high-strength material widely used in the automotive and aerospace fields. Although its forging performance is good, it is necessary to control the temperature and deformation speed during forging to prevent grain growth and performance degradation.
Precision-forged parts undergo several strict processing steps, such as preforming, rough forging, finish forging, annealing, and polishing. The details of each step can affect the precision of the forged parts. The preforming process is mainly to reduce the deformation of the material during forging, making the shape of the blank closer to that of the final forged part, thereby reducing the deformation amount and processing difficulty in subsequent steps. The rough forging process is mainly to process the burrs and cracks of the forged part, removing excess material and defects generated during forging to make the shape and size of the forged part closer to the design requirements. The finish forging process is the most important step, where the forged part can obtain precise shape and size with the help of the mold, while also improving the surface finish of the forged part. The annealing process is to eliminate internal stresses from the forging process and refine the crystalline structure, enhancing the toughness and corrosion resistance of the forged part. The polishing process is to make the surface of the forged part smooth and defect-free, achieving the required surface roughness as per the design. These steps work in coordination with each other to ensure the high precision of precision-forged parts. In actual production, strict quality control of each step is necessary to ensure that each step meets the design requirements, thereby ultimately manufacturing high-quality precision-forged parts.
Precision forging, as an advanced metalworking process, is playing an increasingly important role in modern manufacturing. The precise control of deformation temperature and precision is crucial for ensuring the quality of precision-forged parts. By reasonably selecting the deformation temperature, optimizing mold design, accurately controlling the forging process, and scientifically selecting materials and processing steps, high-precision and high-performance precision-forged parts can be manufactured. With the development of intelligent forging, new material applications, and green forging technologies, precision forging will continue to expand its application fields, providing stronger support for the development of modern manufacturing.
