Activity and action principle of lubricants
The activity of a lubricant is a key concept. Specifically, it refers to the ability of polar molecules in the lubricant to form a solid protective layer on the friction surface. This ability is no simple matter. It directly affects the lubricating performance of the lubricant and also determines the magnitude of the attractive force between the lubricant and the friction object.
When polar substances are present in the lubricant, a series of wonderful changes will occur. It will reduce the surface tension of the pure solvent, which is like loosening the bonds of the solvent and weakening the intermolecular constraints. At the same time, the polar substances will enhance the adsorption force between the metal (including tools and deformed objects here) and the lubricant molecules. This enhanced adsorption force is like building a solid bridge between the metal and the lubricant, making them closely connected.
Generally speaking, when animal and vegetable oils and mineral oils containing oily additives come into contact with the metal surface, the polar groups in the lubricating oil will have physical adsorption with the metal surface. This is as natural and tight as a magnet attracting iron filings, thus forming an oil film in the deformation zone. This oil film is like a protective layer, isolating the metal surface from external friction.
If the lubricant contains active elements such as sulfur, phosphorus, and chlorine, these polar substances will exhibit even greater power. They can undergo a chemical reaction with the metal surface, that is, chemical adsorption, and then form a chemically adsorbed film. This chemically adsorbed film is like a layer of super glue, firmly adhering to the surfaces of metals and tools, and playing a good lubricating role. For example, stearic acid reacts chemically with the extremely thin oxide film on the metal surface to form fatty acid salts, and its chemical reaction formula is 2RCOOH + MeO = (RCOO)₂Me + H₂O. Through the chemical adsorption effect, a metal fatty acid soap with very small frictional stress will be generated on the surface of the metal oxide film, greatly reducing the friction force.
The viscosity of lubricants and its influence
The viscosity of a lubricant is an important indicator for measuring its own stickiness and thickness. It is like a ruler for measuring the flow resistance of lubricating oil. During the metal plastic processing, the viscosity of the lubricant has a huge impact.
If the viscosity is too low, that is, the lubricating oil is overly thin, then it will flow as easily as water. In the deformation zone of metal plastic processing, such lubricating oil can be easily squeezed out and cannot form an effective protection on the metal surface. Naturally, it cannot play a good lubricating role.
On the contrary, if the viscosity is too high, the lubricating oil will become overly thick. Such lubricating oil will encounter greater shear resistance when flowing, just as difficult as walking on a muddy road. Moreover, the oil film it forms will be too thick, which will make the surface of the product rough and fail to obtain a smooth surface. Similarly, it cannot achieve a good lubrication purpose.
In addition, the increase in viscosity also makes it difficult for the lubricant to enter the deformation zone. For example, in the drawing process, thinner lubricants are usually used more (except for some metals), or the metal or tools are completely immersed in the tank of liquid lubricant to ensure that the lubricant can function smoothly. Therefore, in actual production, it is very crucial to select lubricating oil with appropriate viscosity according to the process conditions and product quality requirements.
Selection of lubricants
Requirements for lubricants in plastic forming
When selecting and formulating lubricants, a series of strict requirements must be met. First of all, the lubricant should have good pressure resistance. Under high pressure, the lubricating film should be able to tightly adhere to the contact surface like a loyal guard, maintain a good lubrication state, and ensure the smooth progress of the metal processing process.
Secondly, good high-temperature resistance is also essential. During thermal processing, the high-temperature environment is like a huge furnace. The lubricant cannot decompose or deteriorate in such an environment and must always maintain the stability of its performance.
Moreover, the lubricant also needs to have the function of cooling the mold. During the metal processing, the mold will generate a large amount of heat due to friction. If it is not cooled in time, the mold will be damaged. The lubricant is like a cooling fan, cooling the mold and extending its service life.
In addition, the lubricant should not have a corrosive effect on metals and molds. This is just like we cannot use corrosive liquids to clean precious items. The lubricant should be like a gentle caregiver, protecting metals and molds from damage.
At the same time, from the perspectives of environmental protection and health, lubricants should be non-toxic to the human body and not pollute the environment. This is a principle that must be followed in modern industrial production to ensure that the production process is both safe and environmentally friendly.
Finally, the lubricant should be easy to use and clean, with abundant sources and low prices. Only in this way can it be widely used in actual production and reduce production costs.
Commonly used lubricants
Liquid lubricant
Liquid lubricants are the most widely used and can usually be divided into two categories: pure oils (mineral oils or animal and vegetable oils) and water-soluble types.
Mineral oil includes machine oil, cylinder oil, spindle oil, gear oil, etc. Its molecular composition only contains two elements, carbon and hydrogen, and it is composed of non-polar hydrocarbons. When it comes into contact with metal, only the physical adsorption between non-polar molecules and the metal surface will occur, and no chemical reaction will take place. Therefore, its lubricating performance is poor. In pressure processing, it is rarely directly used as a lubricant. Usually, it is only used as the base oil for formulating lubricants. Various additives are added, or it is mixed with solid lubricants to form a liquid - solid mixed lubricant.
Animal and vegetable oils include beef tallow, lard, soybean oil, castor oil, cottonseed oil, palm oil, etc. The fatty acids they contain are mainly stearic acid (C₁₇H₃₅COOH), palmitic acid (also known as soft fatty acid C₁₅H₃₁COOH) and oleic acid (C₁₇H₃₃COOH). These fatty acids all contain polar groups (such as COOH, COONa) and belong to polar substances. One end of their molecules is a non - polar hydrocarbon group, and the other end is a polar group. They can be arranged directionally on the metal surface to form a lubricating oil film, which strengthens the adsorption force of the lubricant on the metal and makes it difficult to be squeezed out during plastic processing.
Emulsion is a two-phase system in which soluble mineral oil and water are uniformly mixed. Since oil and water are difficult to mix with each other, in order to make the oil suspend in water in the form of tiny droplets and form a stable emulsion, an emulsifier must be added. An emulsifier is a compound composed of lipophilic groups and hydrophilic groups. It is like a bridge that can connect oil and water and prevent them from separating easily. To improve the lubricity of the mineral oil in the emulsion, an oiliness additive also needs to be added. At present, in the rolling process of copper, aluminum and their alloys, oleic acid - triethanolamine emulsion is mostly used. Its components are roughly as follows: 80% - 85% of machine oil or transformer oil, 10% - 15% of oleic acid and about 5% of triethanolamine. First, make a cream (preparation), and then add 90% - 97% of water and stir it into an emulsion. Among them, water plays a cooling role, machine oil or transformer oil is the basic lubricating oil. Oleic acid acts as an oiliness agent to improve the lubricating performance of the mineral oil, and at the same time reacts with triethanolamine to form amine soap, which acts as an emulsifier. The emulsion is mainly used in the processes of strip cold rolling, high - speed wire drawing, deep drawing, etc.
Solid lubricant
Solid lubricants include graphite, molybdenum disulfide, soap, etc. Theoretically, any solid substance with a shear strength lower than the flow shear strength of the metal being processed can be used as a solid lubricant in plastic processing. For example, the thin copper sheets placed on the end faces of cold - forged steel billets, the thin pure aluminum sheets wrapped during the hot - rolling of aluminum alloys, the copper plating on the surface when drawing high - strength wires, and the paraffin, beeswax, fatty acid soap powder used in drawing all belong to solid lubricants. However, the most commonly used ones are still graphite and molybdenum disulfide.
Graphite belongs to the hexagonal crystal system and has a multi - layer scaly structure with a greasy feeling. The atomic spacing within the same layer is 1.2, and the binding force is strong, while the spacing between layers is 3.35, and the binding force is weak. When the crystal lattice is subjected to shear stress, inter - layer slip is very likely to occur. Therefore, when graphite is used as a lubricant, the friction between the contact surfaces of the metal and the tool is essentially the internal friction between graphite layers. This internal friction is much smaller than the friction when the metal is in direct contact with the tool, thus playing a lubricating role. Graphite also has good thermal conductivity and thermal stability. Its friction coefficient increases to some extent with the increase of normal pressure, but has little relation with the relative sliding speed. In addition, after graphite adsorbs gas, the friction coefficient will decrease, so its lubrication performance under vacuum conditions is not as good as that in the air. The friction coefficient of graphite generally ranges from 0.05 to 0.19.
Molybdenum disulfide also belongs to the hexagonal crystal system structure, and its lubrication principle is the same as that of graphite. However, its friction coefficient in a vacuum is smaller than that in the atmosphere, so it is more suitable as a lubricant in a vacuum. The friction coefficient of molybdenum disulfide is generally 0.12 - 0.15. In the atmosphere, graphite begins to oxidize when the temperature exceeds 500℃, while molybdenum disulfide oxidizes at 350℃. In order to prevent the oxidation of graphite and molybdenum disulfide, boron trioxide is often added to them to increase the service temperature. Graphite and molybdenum disulfide are currently commonly used high - temperature solid lubricants in plastic processing, and they can be made into water - based or oil - based agents when used.
Commonly used soaps include sodium stearate, zinc stearate, and ordinary soap. Zinc stearate is used for cold extrusion of aluminum and aluminum alloys. Sodium stearate is used as a lubricant for the processing of drawing non - ferrous metals, and is also used in the saponification treatment process after the phosphating treatment of steel billets.
Liquid - solid lubricant
Liquid - solid lubricants are suspensions in which solid lubricating powders are suspended in lubricating oil or working oil to form a solid - liquid two - phase dispersion system. For example, the graphite emulsion used in drawing tungsten and molybdenum wires, and the molybdenum disulfide (or graphite) oil - based (or water - based) agents used in hot extrusion all belong to this type of lubricants. It involves adding fine powder of molybdenum disulfide (or graphite) with relatively high purity and a particle size of less than 2 - 6μm into oil (or water). The weight of the powder accounts for about 25% - 30%. During use, it is diluted with lubricating oil (or water) according to actual needs, and the general concentration is controlled within 3%. To reduce the precipitation of solid lubricating powders, a small amount of surface - active substances can be added to reduce the tension at the liquid - solid interface, improve the lubricity between them, and act as a dispersant.
Melt lubricant
Melt lubricants are a type of lubricants that emerged relatively late. When hot - working (hot forging and extrusion) metals and alloys such as tungsten, molybdenum, tantalum, niobium, titanium, zirconium, etc., which have high strength at high temperatures, strong adhesion to the tool surface, and are easily contaminated by gases such as oxygen and nitrogen in the air, glass, asphalt, or paraffin are often used as lubricants. The essence is that when glass comes into contact with the high - temperature billet, it can melt into a liquid film between the contact surfaces of the tool and the billet, achieving the purpose of separating the two contact surfaces. Therefore, glass is both a solid lubricant and a melt lubricant.
Glass lubricants have the following characteristics:I. Poor thermal conductivity. When melted at high temperatures, the glass acts like a heat shield, surrounding the surface of the billet, preventing direct contact between the billet and the mold, reducing the temperature of the billet, and also avoiding overheating of the mold.II. A wide operating temperature range. They can be selected for a working temperature range from 450 - 2200℃. The viscosity of glass decreases as the temperature rises, and different compositions result in different viscosity - temperature characteristics. Therefore, the appropriate glass composition can be selected according to the processing temperature and the required viscosity.III. Good chemical stability and no chemical reaction with metals. They can be used alone in forms such as powder, mesh, filament, and glass cloth, or mixed with other lubricants.However, glass lubricants also have a drawback. A layer of glass adheres to the surface of the processed parts, which is not easy to remove.
IV. Additives in lubricants
In the lubrication field, in order to comprehensively improve the performance of lubricating oil and make it perform better in aspects such as lubrication, wear resistance, and corrosion prevention, a small amount of active substances are often added to the lubricating oil. These active substances are collectively referred to as additives.
Additives play a crucial role in lubricating oil, which is like injecting powerful "energy" into the lubricating oil. Generally speaking, an ideal additive needs to have multiple characteristics. Firstly, it should be easily soluble in engine oil. Only in this way can it be evenly distributed in the lubricating oil and fully exert its effectiveness. Secondly, thermal stability is also one of the key factors. Under various complex working conditions, the lubricating oil may face high - temperature environments. If the thermal stability of the additive is poor, it is prone to decomposition or deterioration, thus losing its due function. In addition, good physical and chemical properties are also essential, which can ensure that the additive can stably perform its functions when in contact with the lubricating oil and the metal surface.
Common types of additives are rich and diverse, and each has its unique function.
Extreme pressure agent
Extreme pressure agents are a type of special additives. They are usually organic compounds containing sulfur, phosphorus, and chlorine. For example, chlorinated paraffin, sulfurized olefins, etc., all fall into this category. Under extreme working conditions of high temperature and high pressure, extreme pressure agents will undergo decomposition reactions. The decomposed products will react chemically with the metal surface, and then form iron chloride and iron sulfide films. These films have remarkable characteristics. They have a relatively low melting point, are easy to melt, and present a layered structure. This special structure enables them to still play a good lubricating role under relatively high pressure. However, there are also certain disadvantages in using chlorinated paraffin. It has a corrosive effect on the metal surface, which needs special attention in practical applications.
Oiliness agent
Oily agents generally refer to substances such as natural esters, alcohols, and fatty acids, all of which contain carboxyl (COOH) active groups. These active groups are like small "hooks" that form a lubricating film on the metal surface through adsorption with the metal surface. This lubricating film can effectively reduce the friction and wear between metals, improving the operating efficiency and service life of equipment.
Anti-wear agent
Anti-wear agents play an important role in protecting metal surfaces and reducing wear. Commonly used anti-wear agents include sulfurized cottonseed oil, sulfurized whale oil, etc. These sulfides can generate free radicals at the S - S bond, and then the free radicals react chemically with the metal surface to form a lubricating oil film with anti-corrosion and anti-wear functions. This film is like a strong "armor" that can effectively protect the metal surface from wear and corrosion.
Rust inhibitor
The main function of rust inhibitors is to prevent metals from rusting. Barium petroleum sulfonate is one of the commonly used rust inhibitors. When it is added to lubricating oil, it will form an adsorption film on the metal surface. This film acts like a barrier, which can isolate moisture and oxygen, thus effectively preventing metals from rusting.
In addition, in graphite and molybdenum disulfide, diboron trichloride is often used as an additive, which can improve the oxidation resistance and service temperature of graphite and molybdenum disulfide, enabling them to maintain good performance even in harsher environments.
In plastic processing, different types of additives have different addition amounts and functions. The specific details are shown in the following table:
Type Function Compound Name Addition Amount
Oiliness agentForm an oil film to reduce frictionLong-chain fatty acids, oleic acid0.1 - 1%
Extreme pressure agentPrevent the adhesion of contact surfacesOrganic sulfides, chlorides5 - 10%
Anti-wear agentForms a protective film to prevent wearPhosphate ester5 - 10%
Rust inhibitorPrevent lubricating oil from rustingCarboxylic acid, alcohol0.1 - 1%
EmulsifierEmulsify oil and stabilize emulsionSulfuric acid and phosphate esters~3%
Pour point depressantPrevent the solidification of paraffin in oil at low temperaturesChlorinated paraffin0.1 - 1%
Viscosity agentIncrease the viscosity of lubricating oilPolymers such as polymethyl methacrylate2 - 10%
After adding appropriate additives to the lubricant, many benefits can be brought. It can reduce the friction coefficient, reduce the occurrence of metal sticking to the mold, improve the deformation degree of the metal, and at the same time improve the surface quality of the product. Therefore, at present, lubricating oils containing additives are widely used in industrial production.
V. Improvement of lubrication methods
In the plastic forming process, reducing friction and wear is the key to improving production efficiency and product quality. In addition to continuously improving the performance of lubricants and developing new lubricants, improving lubrication methods is also of great significance.
Fluid lubrication
Fluid lubrication has a wide range of applications in some specific processing technologies, such as wire drawing. During the wire drawing process, a sleeve is added at the entrance of the die, and there is a very small gap between the sleeve and the billet. When the billet passes through the sleeve at a high speed, it will carry the lubricant into the die hole. At the entrance of the die hole, due to the reduction of the gap, high pressure will be generated in the lubricating oil. When the pressure reaches a certain value, a fluid lubricating film will be formed and maintained between the billet and the die. This film can play a good lubricating role and reduce friction and wear.
In the backward extrusion process, by designing the die and the billet into specific shapes, the lubricant can persistently and stably isolate the punch from the blank during the backward extrusion process, thus producing a good lubrication effect. In addition, in processes such as hydrostatic extrusion and hydroforming, the high-pressure liquid not only serves as a medium for transmitting the deformation force but also plays a role in forced lubrication. This dual effect makes the extrusion force much lower than that of ordinary extrusion, greatly improving the production efficiency.
Surface treatment
Surface treatment is an important measure for improving lubrication methods. It can enhance the bonding force between the metal surface and the lubricant and improve the lubrication effect. The common surface treatment methods are as follows:
Surface phosphating treatment: When cold-extruding and cold-drawing steel products, even if additives are added to the lubricating oil, the oil film may still be damaged or squeezed out, thus losing its lubricating effect. To solve this problem, a phosphate or oxalate film, namely the phosphating film, needs to be chemically formed on the surface of the billet. This phosphating film is porous and can adsorb the lubricant. The thickness of the phosphating film is generally between 10 - 20μm. It is firmly bonded to the metal surface and has a certain plasticity, and can deform together with the steel during the processing. After the phosphating treatment, a lubrication treatment is also required. Commonly used treatment agents include sodium stearate, soap, etc. This process is called saponification.
Surface oxidation treatment: For some difficult-to-process superalloys, such as tungsten wire, molybdenum wire, tantalum wire, etc., anodizing or oxidation treatment is required before drawing. Through this treatment, an oxide film will be formed on the metal surface. This film can serve as a lubricating substrate and has an adsorption effect on the lubricant, thereby improving the lubrication effect.
Surface coating: Electroplating is a commonly used surface treatment method. The coating obtained through electroplating has a fine structure, high purity, and good bonding force with the substrate. Currently, copper plating is a relatively commonly used coating method. After the blank is copper - plated, the coating can serve as a lubricant. This is because the friction coefficient of the coating is much smaller than that of the part metal, which can effectively reduce friction.