Summary Of Cold Heading Forming Knowledge, Quickly Put It Away!

Cold heading (extrusion) belongs to metal pressure processing and is one of the non-cutting metal pressure processing processes.

In production, under normal temperature, external force is applied to the metal to form it in the predetermined mold. This method is usually called cold heading (extrusion).

In the forming process of fasteners, the cold heading (extrusion) technology is a main processing technology. The cold heading technology is most suitable for producing bolts, screws, nuts and rivets.

Today, Xiao Bian introduces the basic concept of cold heading, the development history of cold extrusion, the advantages and disadvantages of cold heading, and the comparison of cold heading, hot heading and warm heading.

Basic concept of cold heading

Cold heading (extrusion) is an important part of precision plastic volume forming technology. Cold extrusion refers to putting metal blank into the mold cavity under the cold state, forcing metal material to produce plastic flow under the action of strong pressure and certain speed, so as to obtain the required shape, size and certain mechanical properties of the extrusion parts.

Obviously, the cold extrusion process relies on the mold to control the metal flow, and relies on the massive transfer of metal volume to form parts.

In fact, the forming of any fastener can be realized not only by cold heading, but also by forward and reverse extrusion, compound extrusion, punching, rolling and other deformation methods in addition to upsetting deformation.

Therefore, the term "cold heading" in production is just a customary term. More specifically, it should be called "cold heading (extrusion)".

Development history of modern cold extrusion

The modern cold extrusion technology began at the end of the 18th century. The French began cold extrusion by extruding lead from small holes into bullets during the French Revolution.

In 1830, some people in France began to use mechanical presses to produce lead and tin tubes by reverse extrusion.

In 1906, in order to manufacture brass suit buttons in the United States, someone has obtained the patent right of the hollow cup blank of the forward extrusion.

The Hooker method, which was patented by Americans in 1909, is the forward punching extrusion method. The metal flow direction is the same as the punching extrusion direction. It was developed after the 1906 patent was purchased. The cup blank in the patent is manufactured by the deep drawing method.

In the First World War, the Hooker method was used to manufacture the brass cartridge case. In 1934 before the Second World War, the Germans used this method to trial-manufacture the steel cartridge case, but failed because of the serious thermal adhesion.

It was not until the middle of the Second World War that the extrusion method was successful in manufacturing the steel cartridge case due to the use of a new surface lubrication treatment method - to form a phosphate film on the surface of the workpiece.

Since then, cold extrusion technology has become practical and has become the most widely used method in cold forging technology.

In the 1960s, the growth of Japan's automobile industry created favorable conditions for the development of cold extrusion technology. From the perspective of cold extrusion equipment, since Japan's first 2000kN PK precision press (elbow press) was produced by Japan's Keida Corporation in 1933, more than 2000 PK series presses have been produced so far.

With the development of the automobile industry, the demand for high-precision press becomes more and more urgent. Huida Co., Ltd. has also developed various forging presses.

At the same time, Komatsu of Japan has developed LIC and LZC series cold forging forming presses with high precision and easy operation as the goal.

From the perspective of cold extrusion products, Japan successfully cold extruded the starting clutch gear, drive shaft spline and alternator pole core in the 1970s. In the 1980s, it also successfully cold-extruded large high-precision constant velocity ball outer race, inner race, cross shaft, automobile differential bevel gear and other high-precision parts. It has made great contributions to the high performance of Japanese cars and the reduction of production costs.

The cold extrusion technology in China has a similar starting time with that in Japan. In the 1970s, China used to promote the supercooled extrusion technology in the batch production of bicycles, automobile electrical appliances and other products, and successfully developed the extrusion forming of the starting gear and put it into batch production.

However, a series of technical problems such as process, equipment, materials, molds, lubrication, automation devices and the original size, original state and post-treatment of the blank have not been fundamentally solved, so it has not been greatly developed. In the 1980s, with the rapid development of household appliances and automobile and motorcycle industries, the introduction, digestion and absorption of cold extrusion process equipment and production technology, scientific researchers overcame many problems of cold extrusion technology through production practice, and at the same time, the cold forging equipment has also developed greatly.

At present, China has been able to produce watch cases, bicycle flywheels, center shafts, precision forged gears, constant velocity universal joints for automobiles, spark plugs and piston pins for internal combustion engines, automobile tappets, camera parts, automobile starter directional sleeves, starting gears, etc. with cold extrusion technology, and has reached the same level at home and abroad.

Advantages of cold heading (extrusion) process

Cold extrusion technology is an advanced production technology with high precision, high efficiency, high quality and low consumption, which is mainly used in the large-scale production of small and medium-sized forgings. Compared with other processing processes, cold extrusion has the following advantages:

a) Save raw materials. Cold extrusion is to use the plastic deformation of metal to make parts of required shape, which can greatly reduce cutting and improve material utilization. The material utilization rate of cold extrusion can generally reach more than 80%.

b) Improve labor productivity. Using cold extrusion process instead of cutting to manufacture parts can increase productivity several times, dozens of times, even hundreds of times.

c) Parts can obtain ideal surface roughness and dimensional accuracy. The precision of parts can reach IT7~IT8, and the surface roughness can reach R0.2~R0.6. Therefore, parts processed by cold extrusion are rarely re-cut, and only need to be finely ground at places with special requirements.

d) Improve the mechanical properties of parts. The cold work hardening of the metal after cold extrusion and the formation of a reasonable fiber streamline distribution inside the parts make the strength of the parts much higher than that of the raw materials. In addition, a reasonable cold extrusion process can form compressive stress on the surface of parts and improve fatigue strength. Therefore, the heat treatment process can be omitted for some parts that originally need heat treatment strengthening after cold extrusion process. Some parts originally need to be made of high strength steel, and can be replaced by low strength steel after cold extrusion process.

e) It can process parts with complex shape and difficult to cut. Such as irregular section, complex internal cavity, internal teeth and invisible internal groove.

f) Reduce part costs. Because the cold extrusion process has the advantages of saving raw materials, improving productivity, reducing the cutting amount of parts, and replacing high-quality materials with poor materials, the cost of parts is greatly reduced.

Difficulties in application of cold extrusion technology

1) High requirements for moulds. During cold extrusion, the blank is subjected to three-dimensional compressive stress in the die, which significantly increases the deformation resistance, which makes the stress of the die much greater than that of the general stamping die. When cold extrusion of steel, the stress of the die often reaches 2000MPa~2500MPa. In addition to high strength, the mold should also have sufficient impact toughness and wear resistance. In addition, the strong plastic deformation of the metal blank in the mold will raise the mold temperature to about 250 ℃~300 ℃. Therefore, the mold material needs certain tempering stability. Due to the above conditions, the life of cold extrusion die is far lower than that of stamping die.

2) Large tonnage press is required. Due to the large deformation resistance of the blank during cold extrusion, hundreds or even thousands of tons of press are required.

3) Due to the high cost of cold extrusion die, it is generally only applicable to parts produced in large quantities. Its suitable minimum batch size is 50000~100000 pieces.

4) The blank needs to be surface treated before extrusion. This not only increases the number of processes and takes up a large production area, but also makes it difficult to realize production automation.

5) It is not suitable for processing high-strength materials.

6) The plasticity and impact toughness of cold extrusion parts become poor, and the residual stress of parts is large, which will lead to the reduction of deformation and corrosion resistance of parts (stress corrosion).

Development trend of cold extrusion technology

1) With the increasingly serious energy crisis, people will pay more attention to the environmental quality, and the increasingly fierce market competition will promote the forging production to develop in the direction of high efficiency, high quality, refinement, energy saving and material saving. Therefore, the output of refined forgings produced by extrusion and other technological means will be greatly developed in the market competition.

2) With the development of automobile in the direction of light weight, high speed and smoothness, higher requirements are put forward for the dimensional accuracy, weight accuracy and mechanical properties of forgings. For example, in addition to the requirements for the error between the big and small ends, the weight error of each connecting rod forging for car engine is also required to be no more than 8g. The high requirements of new products will promote the development of refined production technology.

3) Specialized and large-scale organization of production is still the development direction and trend of cold extrusion production. In France, the total labor productivity of professional manufacturers that produce forgings by extrusion process from 1991 to 1994, that is, the output and output value of extrusion parts per person, are higher than those of general manufacturers that produce die forgings or free forgings. Take 1994 as an example, the per capita output of extrusion parts of professional manufacturers was 51024KG, creating an output value of 775688 francs. In the same period, the average output per person of the manufacturers producing die forgings was only 39344KG, with an output value of 592384 francs, which was only 77.1% and 76.37% of the professional manufacturers of extrusion parts. Compared with the free forging factory, it is lower.

4) Special extrusion machine will become a development trend. With the development of refined production of medium and small forgings and the promotion and application of cold extrusion and warm extrusion processes, multi-station cold extrusion presses, precision presses and special machines designed and manufactured for certain forgings will be greatly developed.

Common extrusion methods can be divided into the following categories

a) During forward extrusion, the metal flow direction is consistent with the movement direction of the punch. The forward extrusion can be divided into two types: solid forward extrusion and hollow forward extrusion. The forward extrusion method can produce solid and hollow parts of various shapes, such as screws, mandrels, tubes and cartridge cases.

b) Back-extrusion: During extrusion, the metal flow direction is opposite to the movement direction of the punch. Back-extrusion can be used to manufacture cup-shaped parts with various cross-section shapes, such as instrument housing, universal joint bearing sleeve, etc.

c) Compound extrusion: During extrusion, part of the metal flow direction of the blank is the same as the movement direction of the punch, while the other part of the metal flow direction is opposite to the movement direction of the punch. The compound extrusion method can produce double cup parts, also can produce cup and rod parts.

e) Reduced diameter extrusion is a kind of abnormal forward extrusion method with small deformation, and the blank section is only slightly reduced. It is mainly used for manufacturing stepped shaft parts with small difference in diameter and as the finishing process of deep-hole cup parts.

The common feature of the above extrusion methods is that the flow direction of the gold chips is parallel to the punch axis, so it can be collectively referred to as the axial extrusion method. In addition, there are radial extrusion and upsetting extrusion.

Comparison of cold extrusion, hot extrusion and warm extrusion

a) Although the cold extrusion method has many advantages, the large deformation resistance limits the size of parts, and also restricts the use of cold extrusion technology for materials with large deformation resistance.

b) Although the hot extrusion forming method can reduce the deformation resistance of the material, it can reduce the dimensional accuracy and surface quality of the product due to the problems of oxidation, decarburization and thermal expansion caused by heating. Therefore, it generally needs a lot of machining before it can be used as the final product.

c) Warm extrusion method is to heat the blank to an appropriate temperature below the metal recrystallization temperature for extrusion. Due to metal heating, the deformation resistance of the blank is reduced, forming is easy, the tonnage of the press can also be reduced, and the life of the die is extended. However, it is different from hot extrusion, because the possibility of oxidation and decarburization is small when heated in the low temperature range, and the mechanical properties of the product are not different from those of cold extrusion. In particular, materials that are difficult to machine at room temperature, such as stainless steel, high carbon steel, some steels with high chromium content, and superalloys that precipitate hardened phases, may become machinable or easy to machine during warm extrusion.

d) Warm extrusion is not only suitable for hard to process materials with high deformation resistance, but also suitable for low carbon steel suitable for cold extrusion, because warm extrusion has the advantage of facilitating continuous production. During cold extrusion, including cold extrusion of low carbon steel, pre-softening annealing is generally required before processing, and annealing is also required between cold extrusion processes. Passivation treatment shall be carried out before cold extrusion. This makes it difficult to organize continuous production. During warm extrusion, pre-softening annealing and annealing between various processes can be avoided, and surface treatment can also be avoided, which makes continuous production of microstructure possible. At least, many auxiliary processes can be reduced.

e) Warm extrusion can adopt large deformation, which can reduce the number of processes. Die costs can also be greatly reduced, and universal forging equipment can be used instead of high-priced forging equipment with extremely high rigidity. So although warm extrusion needs to heat the metal, the total processing cost is relatively cheap, especially when manufacturing non-axisymmetric shaped parts with complex processes, warm extrusion can play its role.

f) At present, the lubricant used in warm extrusion is not completely satisfactory. At the same time, there is also a lack of practical data on processing, and there are many technical problems to be solved.

Comparison of hot and cold upsetting processes for fasteners

Hot upsetting

In the process of hot upsetting, the billet is heated by induction or in the forging furnace or oven to the temperature above the crystallization point of the metal.

This extreme high temperature is necessary to avoid strain hardening of metal during deformation. Because the metal is in the shaping state, it can make quite complex shapes. The metal maintains ductility and toughness.

The average forging temperature required for hot heading of different metals is:

Steel up to 1150 ° C

Aluminum alloy 360 to 520 ° C

Copper alloy 700 to 800 ° C

In order to forge some metals, such as superalloy steel, a hot upsetting called isothermal forging is adopted.

Here, the mold is heated to the temperature close to the billet to avoid the surface cooling of the parts during the forging process. Forging is sometimes carried out in a controlled atmosphere to minimize the formation of oxide scale.

Generally speaking, complex parts are manufactured by hot upsetting because it allows the material to deform in its plastic state and the metal is easier to process.

Factors to consider hot heading include:

Production of complex parts

Medium and low precision dimensions

Low stress or low work hardening

Uniform grain structure

Increased ductility

The disadvantages of hot heading include:

Less precise tolerances

Material may warp during cooling

Changing metal grain structure

Possible reaction between surrounding atmosphere and metal

Cold heading (or cold forming)

Cold heading causes the metal to deform below its crystallization point. Cold heading reduces ductility and improves tensile strength and yield strength. Cold heading is usually carried out at room temperature.

The most common metal in cold heading applications is usually carbon steel or carbon alloy steel. Cold heading is usually a closed die process.

Cold heading is usually cheaper than hot heading, and the final product requires little finishing. Due to the improvement of metal strength by cold heading, lower grade materials can sometimes be used to produce parts that cannot be machined or hot headed.

Cold heading is also less susceptible to pollution, and the final part has better overall surface finish.

Disadvantages include:

The metal surface must be clean and free of oxide scale before forging

Poor ductility of metal

Residual stress may occur

Need heavier and larger equipment

Need higher strength mold

Warm upsetting

Warm upsetting is carried out below the recrystallization temperature but above the room temperature, overcoming the disadvantages of hot upsetting and cold upsetting and gaining its advantages.

The formation of a small amount of oxide scale can be controlled more accurately than hot heading. Compared with cold heading, the processing cost is lower and the pressure required for manufacturing is also lower.

Compared with cold working, work hardening is reduced and ductility is improved.