Analysis of the entire process of aluminum extrusion technology: from mold design to surface treatment

In the fields of modern industry and construction, aluminum profiles are ubiquitous, from aluminum alloy structural components to automotive parts. Their lightweight, high-strength, and corrosion-resistant properties make them an indispensable material. The magical process of transforming an ordinary aluminum rod into a profile with a specific cross-sectional shape is the aluminum extrusion process. This article will delve into the core principles, key equipment, process parameter control, common defects, and solutions throughout the entire process, from mold design to surface treatment.

1、 Process starting point: Mold design and manufacturing

The starting point of extrusion technology is the mold, which determines the final shape and surface quality of the profile. The design of molds must accurately calculate the flow laws of metals.

Mold structure design: For hollow profiles, a split combination mold is usually used. Its working principle is to place the heated aluminum ingot into an extrusion cylinder, and under pressure, the aluminum ingot is divided into two or more metal streams, flowing through a diversion bridge, and then re converging in the welding chamber, welded into a whole under high temperature and pressure, and finally flowing out through the gap formed between the mold core and the mold hole, forming a profile with a cavity. In order to prevent twisting of profiles during extrusion (especially profiles with poor cross-sectional symmetry, such as crossbeams with wide heads), special structures are used in mold design. For example, a protruding half core head structure is set on the discharge side of the upper mold, which faces the mold hole of the lower mold and is used to adjust and stabilize the metal flow rate, avoiding the phenomenon of fast flow rate on one side and slow flow rate on the other side due to differences in wall thickness, thereby ensuring the flatness of the profile.

Mold processing quality control: The processing accuracy of the mold directly affects the quality of the profile. CNC machining centers are widely used in modern machining to ensure accuracy. Common processing defects include: poor welding performance due to uneven chamfers of the shunt bridge, uneven feeding holes causing concave surfaces or bright lines on the profile surface, and uneven hardness of the working belt. To solve these problems, a series of measures are usually taken, such as using CNC programming to machine diversion holes and welding chambers to ensure the accuracy of three-dimensional shapes, and using slow wire cutting to machine complex mold holes to ensure the accuracy and smoothness of the working belt. In addition, heat treatment of molds is also crucial. The use of vacuum heat treatment and surface strengthening techniques (such as nitriding) can significantly improve the life of molds and the surface quality of profiles.

2、 Core Principles and Metal Flow

The core of extrusion technology is to induce plastic flow of metal under triaxial compressive stress state. According to the relationship between the direction of extrusion and the direction of metal flow, it is mainly divided into forward extrusion and reverse extrusion. Positive compression: This is currently the most commonly used method. The extrusion shaft pushes the gasket to push the ingot forward along the inner wall of the extrusion cylinder, causing it to flow through the mold hole and be extruded. The advantage of this method is flexible operation, but the disadvantage is that there is a huge frictional force between the inner wall of the extrusion cylinder and the ingot, which increases energy consumption, and the uniformity of metal flow is difficult to control.

Reverse extrusion: One end of the extrusion tube is closed, and the extrusion shaft drives the mold to move relative to the ingot. In this way, there is no relative motion between the ingot and the inner wall of the extrusion cylinder, and the friction is greatly reduced. This makes the metal flow more uniform, reduces the extrusion temperature, and allows for more precise dimensions and more uniform performance.

During the entire extrusion process, metal flow can be divided into three stages:

Filling and squeezing stage: The ingot is filled with the squeezing cylinder and mold hole under the action of the pier's coarse force until it is completely filled.

Horizontal extrusion stage: The metal flow tends to stabilize, forming a stable extrusion state, and the profile is uniformly extruded.

Turbulent extrusion stage: In the later stage of extrusion, the length of the ingot decreases, and the dead zone metal begins to participate in the flow, causing the metal flow to be turbulent and prone to defects such as bubbles and delamination.

3、 Key process parameter control

In order to obtain high-quality profiles, the following key parameters must be precisely controlled:

extrusion temperature

Temperature is one of the most important parameters in extrusion technology. Low temperature, high resistance to metal deformation, difficult extrusion, and easy damage to molds; If the temperature is too high, surface cracks, coarse grains, and even 'overburning' may occur. Usually, heating of aluminum alloy ingots

The temperature varies depending on the alloy grade, ranging from around 450 ℃ to 500 ℃. At the same time, the mold also needs to be preheated to a similar temperature to prevent rapid cooling when the hot aluminum comes into contact with the cold mold, which affects the metal flow.

extrusion speed

Squeezing speed refers to the speed at which the squeezing shaft advances. It directly affects the thermal equilibrium of the metal in the deformation zone. If the speed is too fast, it may cause a sharp increase in temperature due to deformation heat effects, which may lead to surface roughness, cracks, or increased mold wear; If the speed is too slow, the production efficiency will be low, and it may cause roughness on the surface of the profile similar to 'orange peel'. For hollow profiles that require good welding, sufficient time and pressure are needed in the welding chamber to ensure complete metal welding, so speed control is particularly critical.

Squeezing coefficient and specific pressure

Squeezing coefficient (λ): is the ratio of the cross-sectional area of the ingot to the total cross-sectional area of the profile. It represents the degree of deformation of the metal. The larger the compression coefficient, the more severe the deformation.

Unit extrusion pressure (specific pressure): It is the pressure applied by the extrusion shaft on the unit area of the ingot. The pressure must be strong enough to overcome the deformation resistance and friction of the metal, allowing it to pass through the mold. Insufficient pressure can lead to 'plug mold' or dimensional nonconformity.

4、 Common defects and solutions

During the extrusion process, various defects may occur due to the influence of mold design, process parameter settings, or equipment status.

Twisting and bending

Cause: This is one of the most common defects, mainly caused by uneven flow velocity when metal flows out of the mold hole. For example, when there is a significant difference in the thickness of the cross-section of the profile, the resistance at the thin wall is high and the flow velocity is slow, while the flow velocity at the thick wall is fast, resulting in the profile being 'bent' or 'twisted' at the moment of mold release Fried Dough Twists'.

Solution:

Repairing the mold working belt: By lengthening the working belt in the fast flow area, increasing the frictional resistance and slowing down its flow rate; Alternatively, thinning the working zone at slow flow rates can reduce resistance, increase flow velocity, and make the flow velocity of the entire cross-section more consistent.

Optimize the process: reduce the extrusion speed or use low-temperature extrusion to reduce the difference in metal flowability.

Mold structure optimization: As mentioned earlier, the 'half core head' design balances the flow rate from the source.

Welding line

Cause: Mainly occurs on hollow profiles or profiles extruded using a splitter die. After being divided by a shunt bridge, the aluminum ingots are reunited in the welding chamber. If the pressure in the welding chamber is insufficient, the temperature is too low, or there are oxides or oil stains on the metal surface, it will cause the metal to be unable to complete the welding process

Welding together creates a visible dark pattern or gap on the surface of the profile, known as the welding line. In severe cases, it can affect the mechanical properties of the profile and even crack.

Solution:

Increase welding pressure: Ensure that the welding chamber has sufficient cross-sectional area to establish sufficient static water pressure on the metal inside the welding chamber.

Keep clean: Ensure that the surface of the ingot is clean, free of oil stains and inclusions.
Raise temperature: Properly increase the temperature of the ingot and mold to promote the diffusion and recrystallization of metal atoms, achieving good welding.

Mold design: Optimize the shape of the diversion bridge (such as a droplet shape), reduce flow resistance, and ensure sufficient welding chamber volume.

Surface stripes (friction patterns, tissue patterns)

Friction pattern: When the profile flows out of the mold hole, it forms dry friction with the working belt, and the metal partially adheres to the working belt of the mold, resulting in periodic patterns on the surface. The solution includes optimizing the working zone angle (such as the exit angle of -1 ° to -3 °) and performing mold nitriding treatment to improve Hardness and reduced friction coefficient.

Organizational pattern: Due to uneven microstructure, compositional segregation, or insufficient homogenization treatment of the ingot, the color depth of the extruded profile varies during subsequent oxidation coloring. This requires starting from the casting process, optimizing the technology, and performing surface peeling treatment on the ingot.

Summary of Common Aluminum Extrusion Defects and Solutions

5、 Subsequent processing: from straightening to surface treatment

The extruded profile has not completed all the processes and still needs to undergo a series of subsequent treatments:

Online quenching: For aluminum alloys that require heat treatment strengthening (such as 6-series alloys), the profile needs to be cooled immediately after extrusion (air-cooled or water-cooled) to fix the solid solution and prepare for subsequent aging.

Stretching straightening: The extruded profile usually has slight bending or twisting. By using a stretching machine for a certain amount of permanent stretching (usually 0.5%~3%), internal stress can be eliminated and the profile can be straightened.

Sawing and sizing: Cut the profile into fixed length dimensions according to customer requirements.

Aging treatment: Through manual aging (such as heating to 175 ℃~200 ℃ for several hours), the strengthening phase in the alloy is precipitated, thereby achieving the required mechanical properties (T5 or T6 state) of the profile.

Surface treatment: In order to improve corrosion resistance and aesthetics, profiles are usually subjected to surface treatment. Common processes include anodizing (forming a protective oxide film), electrophoretic painting, or powder coating. Some innovative processes even attempt to heat immediately after extrusion molding

Spraying, achieving online surface treatment to improve efficiency.

Aluminum extrusion is a comprehensive technology that combines materials science, fluid mechanics, and precision machining. Starting from a small mold, to the final beautiful and durable profile, every step is full of technical challenges and wisdom. Currently, this field is developing towards energy conservation, emission reduction, high precision, and intelligence. For example, innovative research on reducing lateral welding waste through the use of irregular gaskets and irregular ingots is expected to elevate material utilization to a new level. Meanwhile, with the help of finite element simulation technology, engineers can use computers to
Predicting metal flow, optimizing mold design and process parameters have greatly shortened the development cycle and reduced trial and error costs. With the continuous advancement of technology, we will see more aluminum profiles with complex cross-sections, excellent performance, and efficient production applied in various industries.