With the continuous advancement of modern manufacturing toward higher precision and higher performance components, the cold forging process has gradually become an important technological route in the field of metal forming due to its high material utilization rate, excellent forming accuracy, and near-net-shape capability. However, performing large plastic deformation under room temperature conditions makes the cold forging process accompanied by extremely high contact stress and complex friction and wear behavior, which places stringent requirements on die life, forming stability, and product quality.
Therefore, a deeper understanding of the tribological mechanisms, lubrication functions, and surface condition effects in cold forging, as well as the rational selection of lubrication systems, is of great significance for improving process efficiency and reducing production costs. This article systematically analyzes the basic characteristics of the cold forging process, lubrication mechanisms, the influence of surface roughness, and types and applications of lubricants, aiming to provide useful references for engineering practice.
Cold forging is a metal forming method in which local compressive force is applied to metal at room temperature, causing plastic deformation into a required shape. Cold forging is a type of forging technology. According to processing temperature, forging can be classified into hot forging, warm forging, and cold forging. Hot forging is performed at high temperatures, warm forging lies between hot and cold forging, and cold forging is completed at room temperature.
Cold forging has developed rapidly in the field of steel products. It has several obvious advantages. First, the material utilization rate is high. Cold forging produces almost no waste, as most of the raw material is converted into the final product. Second, the forming accuracy is high. Cold-forged parts have precise dimensions and require minimal subsequent machining. Third, near-net-shape forming can be achieved, meaning the forged part is already very close to its final geometry and only requires minor finishing operations.
However, cold forging also presents challenges. Since forming occurs at room temperature, the flow stress of the metal is very high, making deformation difficult. This results in extremely high contact stress and severe wear on tools during operation. Local surface pressure can reach up to 3000 MPa. Under such conditions, common tool failure modes include overload failure, abrasive wear, adhesive wear, and fatigue fracture. The two most dominant mechanisms are cyclic load-induced fatigue and wear caused by high contact stress.
In cold forging processes, the tribological system plays a critical role in process stability. Tribology is the study of friction, wear, and lubrication. Proper lubrication can significantly reduce forming loads, minimize wear, and extend die life. It also prevents direct contact between the workpiece and the die, avoiding metal adhesion to the tooling surface.
The surface deformation during cold forging is extremely severe, and both interface temperature and contact conditions are harsh. Therefore, lubricants must have strong pressure resistance and temperature resistance. Poor lubrication can lead to product defects and even tool failure. However, with increasing environmental requirements and cost pressure, lubricant-free cold forging processes are also gaining attention.
The influence of lubrication is not limited to forging processes; similar principles exist in cutting operations. Under high-speed cutting conditions, dry cutting sometimes results in lower tool wear compared to cutting fluids. In cold forming processes, adjusting tool structure or lubrication conditions can effectively reduce forming forces and improve material flow behavior. Insufficient lubrication leads to increased forming forces, degraded surface quality, and higher surface roughness.
Surface roughness has a significant impact on tool life and product quality in cold forging. Surface roughness refers to the micro-scale peaks and valleys on a processed surface, including their height and spacing characteristics.
Higher surface roughness increases friction between the tool and the workpiece. Increased friction leads to several issues. First, higher energy consumption. Second, greater forming loads. Third, more severe tool wear. At the same time, under high-cycle fatigue conditions, higher surface roughness generally leads to lower fatigue life. High-cycle fatigue refers to material failure caused by repeated loading over long periods.
The initial surface morphology of the tool also significantly affects wear behavior. Rough surfaces can lead to premature tool failure and influence material flow behavior. Therefore, controlling surface roughness during die manufacturing is extremely important in cold forging processes.
Various types of lubricants are used in cold forging processes. Common categories include oil-based lubricants, water-based lubricants, and synthetic lubricants. In addition to reducing friction, some lubricants also provide anti-corrosion properties, further extending die life.
Graphite-based lubricants are the most widely used lubricating materials in forging, especially suitable for high-temperature hot forging processes. Graphite has excellent high-temperature lubrication performance and remains stable under extreme conditions. It is commonly available in three forms.
Dry graphite powder is mainly used in open-die forging operations and applied directly to the die surface by brushing or spraying. Its advantages include simplicity and excellent heat resistance, but it may cause dust pollution.
Water-based graphite suspensions are more environmentally friendly and are used in closed-die forging of steel and non-ferrous metals. At high temperatures, water evaporates rapidly, forming a uniform graphite lubricating film on the die surface, thereby reducing friction.
Graphite paste has higher viscosity and stronger adhesion, making it suitable for complex shapes or high-pressure areas. It provides more durable and localized lubrication and is commonly used in large forging components.
Glass-based lubricants are mainly used in ultra-high temperature forging processes, especially for high-temperature alloys and titanium alloys. Under high temperatures, they form a glassy film between the die and workpiece, effectively reducing friction and preventing sticking. These lubricants are typically in the form of glass powder, glass flakes, or coatings. They must be preheated before use to melt and spread evenly. While they offer excellent thermal resistance, they are costly and difficult to remove after forging.
Oil-based lubricants are widely used in cold and warm forging processes, including mineral oils, synthetic oils, and vegetable oils.
Mineral oils are low-cost and provide stable lubrication performance, effectively reducing friction and improving surface quality. However, they have poor environmental performance and low biodegradability.
Synthetic oils offer better thermal stability, oxidation resistance, and wear resistance compared to mineral oils, making them suitable for more demanding forming processes.
Vegetable oils are environmentally friendly and biodegradable but have weaker oxidation resistance and require proper storage and usage conditions.
Polymer-based lubricants are a rapidly developing class of new lubricating materials. They work by forming a thin, durable polymer film on the die and workpiece surfaces. These lubricants offer excellent wear resistance and adhesion, maintaining stable lubrication during forging and significantly reducing die wear. They can be used in both cold and hot forging depending on formulation and are increasingly applied in modern high-performance forging processes.
The selection of lubricants depends on several key factors.
First is forging temperature. High-temperature forging requires graphite-based or glass-based lubricants capable of withstanding extreme conditions. Cold and warm forging processes more commonly use oil-based or polymer lubricants.
Second is workpiece material. Different materials have different lubrication requirements. Steel requires good anti-oxidation and anti-friction performance. Non-ferrous metals focus more on anti-sticking properties. High-temperature alloys and titanium alloys require high-performance heat-resistant lubrication systems.
Third is the complexity of die geometry. For complex cavity dies, lubricants with good flowability and permeability are needed to ensure uniform coverage across the forming area.
Generally, oil-based lubricants are more commonly used under higher temperature conditions, while water-based lubricants are more suitable for lower temperature conditions. The lubricant must remain stable within a specific temperature range to ensure effective lubrication and reduced wear.
Good lubrication in cold forging provides multiple advantages.
Lubrication significantly reduces friction and wear between the die and workpiece, lowering tool replacement frequency and maintenance costs. It also prevents direct metal contact, reducing heat buildup and avoiding sticking.
High pressure and friction in cold forging can lead to metal fatigue and burr formation, affecting strength and durability. Lubrication reduces these effects, resulting in denser and smoother finished products.
With proper lubrication, die movement becomes smoother and forming processes more stable, reducing cycle time and improving precision. Equipment downtime is also reduced, lowering overall production costs.
Lubricants reduce high temperatures, sparks, and potential harmful interactions during tool contact, improving the working environment and protecting operators and equipment. They also reduce mechanical injury risks.
Good lubrication decreases frictional resistance and heat generation, thereby reducing energy consumption during equipment operation and lowering production costs.
Aluminum cold forging is an important processing method in modern manufacturing. It involves forging and extruding aluminum billets at room temperature. Its advantages include low surface roughness and improved strength due to work hardening.
Lubrication plays a crucial role in aluminum cold forging, directly affecting surface quality, forming accuracy, and production efficiency. Improving lubrication conditions is an economical and effective optimization method. The main function of lubricants is to reduce friction between the billet and die, minimize wear, and extend die life while improving product quality.
A mature lubrication method in aluminum cold forging is phosphating and saponification treatment. Phosphating forms a phosphate conversion layer on the metal surface. Saponification uses fatty acid soaps that react with zinc phosphate layers to form zinc stearate films, which provide excellent anti-friction performance.
Different materials use different lubrication systems. Carbon steel is typically phosphated and then lubricated with soap solution. Stainless steel uses oxalate treatment followed by a lubricant composed of 85% chlorinated paraffin and 15% molybdenum disulfide. Brass parts use soybean or rapeseed oil after passivation. Pure aluminum uses zinc stearate, while hard aluminum uses vegetable oils such as soybean, rapeseed, or castor oil after oxidation treatment.
Overall, cold forging is an advanced metal forming technology with high efficiency and high precision advantages, but it places extremely high demands on dies and lubrication systems under room temperature and high stress conditions. Lubrication, as a key element in the cold forging process, directly affects friction behavior and forming loads, and plays an essential role in extending die life, improving surface quality, and reducing energy consumption.
From traditional graphite and oil-based lubricants to modern polymer-based systems, different lubrication materials show unique advantages under different working conditions. Meanwhile, the coupling effect between surface roughness and lubrication conditions further influences forming stability and fatigue life. Therefore, in practical production, lubrication systems should be scientifically selected based on material properties, process temperature, and die structure to achieve efficient, stable, and sustainable cold forging processes.
