Aluminum extrusion is a core plastic forming process in the aluminum processing industry chain. It produces architectural and industrial profiles with predefined cross-sectional shapes and dimensions by applying axial pressure to aluminum alloy billets to force them through die cavities. This process features high material utilization, high forming efficiency and dense product microstructure, and its products are widely used in construction, rail transit, new energy vehicles, electrical and electronics, aerospace and other fields. The matching of extrusion process parameters and process control directly determine the microstructure, mechanical properties, dimensional accuracy and surface quality of extruded profiles, which is a concentrated reflection of the core technical capability of aluminum processing enterprises.
I. Core Extrusion Process Flow and Key Links
Aluminum extrusion is a systematic process with multi-process collaboration, which can be divided into three stages: billet pretreatment, extrusion forming, and post-extrusion treatment. The process precision of each link jointly determines the final product quality.
1. Billet Pretreatment Process
(1) Homogenization Treatment
During semi-continuous casting, aluminum alloy ingots are prone to microstructure defects such as intragranular segregation, coarse second phases and concentrated internal stress. Direct extrusion will lead to uneven deformation resistance, poor surface quality and large performance fluctuations. Homogenization treatment involves heating the billet to a temperature below the solidus line and holding it for a long time (for 6xxx series alloys, usually 540~580℃, holding for 4~12 hours). It eliminates composition segregation through atomic diffusion, and dissolves and precipitates coarse intermetallic compounds, significantly improving the plasticity and deformation uniformity of the billet, which is the prerequisite for ensuring extrusion quality.
(2) Billet and Die Preheating
The billet heating temperature shall be set according to the alloy system: 460~530℃ for 6xxx wrought aluminum alloys, and 400~480℃ for 7xxx high-strength aluminum alloys. The principle is to ensure good plasticity of the alloy while avoiding overburning. Meanwhile, the extrusion container and die shall be preheated synchronously. The die temperature is generally 10~30℃ lower than the billet temperature, so as to reduce the sudden increase of forming resistance and die thermal shock caused by temperature drop.
2. Extrusion Forming Process
According to metal flow and stress characteristics, mainstream extrusion processes are divided into three categories, adapting to different product positioning:
Direct Extrusion: The most widely used process. The billet moves forward with the extrusion stem, and the metal flow direction is consistent with the extrusion stem. It features simple operation and convenient die change, but has large friction resistance on the container wall, and there are certain differences in microstructure and properties between the head and tail of the profile.
Indirect Extrusion: The die moves synchronously with the extrusion stem. There is no relative sliding between the metal and the extrusion container, resulting in low friction resistance, uniform deformation and low extrusion force. It is suitable for high-strength alloys and high-precision industrial profiles, but the equipment structure is complex and the maintenance cost is high.
Isothermal Extrusion: By presetting the axial temperature gradient of the billet and regulating the extrusion speed in real time, the metal temperature in the deformation zone is kept constant. It completely solves the coupling contradiction between speed and temperature, and realizes uniform microstructure and properties along the full length of the profile. It is the mainstream process for high-end industrial profiles.
The core parameter of the extrusion process is the extrusion ratio (the ratio of the cross-sectional area of the extrusion container to the cross-sectional area of the profile), which reflects the degree of deformation. The extrusion ratio of architectural profiles is usually 20~50, and that of industrial structural profiles can reach 50~100. Insufficient extrusion ratio will lead to weak grain refinement effect and low strength, while excessive ratio is limited by equipment tonnage and die strength.
3. Post-Extrusion Treatment Process
(1) On-line Quenching
The temperature of the profile exiting the die is close to the solution treatment temperature, so it needs to be cooled and quenched immediately to make alloy elements dissolve in the aluminum matrix in a supersaturated state, creating conditions for subsequent age strengthening. Cooling methods are divided into three categories: air cooling, water mist cooling and water cooling, corresponding to different product tempers:
T5 Temper: Adopting air cooling or water mist cooling with moderate cooling rate, combined with subsequent artificial aging, it has good forming accuracy and is widely used in architectural decorative profiles.
T6 Temper: Adopting warm water or strong water mist quenching with fast cooling rate and sufficient solution treatment, it has higher strength after aging and is used for load-bearing industrial profiles.
The cooling rate shall match the alloy composition: insufficient cooling will lead to premature precipitation of the second phase and reduce the age strengthening effect; excessive cooling will easily generate large internal stress, causing deformation and cracking of the profile.
(2) Stretch Straightening and Finishing
The quenched profile has residual internal stress and bending/twisting defects. A tensile deformation of 0.5%~2% is applied by a stretcher to eliminate stress and correct straightness and dimensional accuracy. After that, fixed-length sawing, artificial aging, and surface treatment (anodizing, electrophoretic coating, powder coating, etc.) are carried out according to requirements.
(3) Artificial Aging
Artificial aging is the core process of heating the profile to a set temperature and holding it, so that the supersaturated solid solution precipitates nano-scale strengthening phases, significantly improving the strength. Typical processes include: T5 aging of 6063 alloy at 170~180℃ for 6~8 hours; T6 aging of 6061 alloy at 175℃ for 8~10 hours. The precise control of aging temperature and time is the core means to adjust the matching of strength and plasticity.
II. Mechanism of Process Parameters on Microstructure and Properties
The extrusion process is a hot deformation process with multi-field coupling of temperature, speed and deformation. Process parameters ultimately determine the microstructure and properties of the profile by changing the recrystallization behavior and the precipitation state of the second phase.
1. Temperature Parameter
Increasing the billet temperature will improve the alloy plasticity and reduce the deformation resistance, which is conducive to forming complex cross-sections. However, when the temperature exceeds the critical value, it will cause grain coarsening, excessive recrystallization, and even overburning defects, leading to a cliff-like decline in mechanical properties. At the same time, the plastic deformation work during extrusion will be converted into heat, making the die exit temperature of the profile significantly higher than the initial billet temperature. The faster the extrusion speed, the more obvious the temperature rise effect, which is the core difficulty of process control.
2. Extrusion Speed
The extrusion speed shall be dynamically matched with temperature: too fast speed will lead to concentrated deformation heat, surface defects such as tearing and orange peel on the profile, and large temperature difference between the core and surface, resulting in uneven microstructure and properties; too slow speed will lead to low production efficiency, and the profile stays in the high temperature zone for a long time, which is prone to recrystallized grain growth. The industry generally adopts the matching strategy of "low temperature + fast extrusion" or "high temperature + slow extrusion", and combines isothermal extrusion technology to achieve the balance between efficiency and quality.
3. Deformation Degree
According to plastic deformation theory, increasing the extrusion ratio will increase the dislocation density, promote dynamic recrystallization, refine the grain structure, and improve the comprehensive properties of strength and plasticity of the profile. When the extrusion ratio is less than 10, the grain refinement effect is not obvious, and coarse grain ring defects are prone to occur; excessive extrusion ratio will cause a sharp increase in extrusion force, which is limited by equipment tonnage and die strength.
4. Quenching and Aging Regime
The quenching cooling rate directly determines the degree of solution treatment: the faster the cooling rate, the higher the concentration of the supersaturated solid solution, and the more dispersed the strengthening phases precipitated after aging, resulting in higher strength. For 6xxx series alloys, the critical cooling rate is about 1~5℃/s. Below this value, Mg₂Si phase precipitates in advance and loses the strengthening effect.
The aging process determines the precipitation state of strengthening phases: under-aging leads to insufficient precipitation of strengthening phases and low strength; peak aging reaches the maximum strength but with decreased plasticity; over-aging causes coarsening of strengthening phases, reduced strength and improved plasticity. The corresponding regime shall be selected according to product performance requirements.
III. Precise Control Methods for Core Performance Indicators
The performance control of aluminum extruded products focuses on four core dimensions: mechanical properties, dimensional accuracy, surface quality, and corrosion resistance & electrical conductivity.
1. Mechanical Property Control
Mechanical properties (tensile strength, yield strength, elongation after fracture) are the core indicators of industrial profiles. The full-process control path includes:
Composition Source Control: Precisely control the content of strengthening elements in the alloy, such as controlling the total content of Mg and Si and the Mg/Si ratio in 6xxx series alloys to ensure sufficient formation of Mg₂Si strengthening phase; strictly control impurity elements such as Fe and Cu to avoid brittle phases reducing plasticity.
Homogenization Adequacy: Ensure uniform composition and microstructure of the billet and eliminate local performance fluctuations.
Extrusion Process Matching: Control reasonable extrusion temperature and extrusion ratio to ensure sufficient dynamic recrystallization and grain refinement.
Quenching-Aging Closed-Loop Control: Ensure the quenching cooling rate meets the standard, and precisely control the temperature uniformity and holding time of the aging furnace to achieve optimal precipitation of strengthening phases.
2. Dimensional Accuracy Control
The dimensional accuracy of aluminum profiles is affected by many factors such as die, equipment and process. The control points include:
Die dimension: Adopt high-precision machining and mirror polishing, design reasonable bearing land length and porthole structure to compensate for the difference in metal flow speed.
Process dimension: Stabilize extrusion temperature and speed to avoid cross-sectional deformation caused by uneven flow rate; control the stretching amount of stretch straightening to prevent dimensional deviation.
Equipment dimension: Ensure the alignment accuracy of the extruder and the straightness of the guide rail to reduce twisting and bending caused by equipment clearance.
The dimensional accuracy of high-end industrial profiles can reach IT6~IT7 grade, meeting the requirements of precision assembly.
3. Surface Quality Control
Common surface problems include scratches, pits, streaks, blisters, etc. The core prevention and control measures are:
Improve the metallurgical quality of billets to reduce casting defects such as pores and inclusions.
Maintain the extrusion container and die regularly, clean up surface debris and aluminum chips, and keep the die bearing land in a mirror state.
Optimize the matching of extrusion speed and temperature to avoid surface tearing caused by excessive metal flow rate.
Optimize the conveying structure of the discharge table and cooling bed to reduce mechanical scratches.
4. Corrosion Resistance and Electrical Conductivity Control
Corrosion resistance: Reduce the tendency of electrochemical corrosion by controlling the uniformity of grain structure and reducing coarse second phases; combined with surface treatments such as anodizing and electrophoretic coating, the salt spray resistance and acid-base resistance can be greatly improved.
Electrical conductivity: For aluminum profiles for electrical conduction, it is necessary to control the content of impurity elements and reduce lattice distortion; at the same time, avoid coarsening of the second phase caused by over-aging, so as to ensure the balance between conductivity and strength.
IV. Common Quality Defects and Prevention Solutions
| Defect Type | Core Cause | Prevention & Control Solution |
|---|---|---|
| Surface blisters / peeling | Pores and shrinkage cavities in billets; residual oil or aluminum chips in extrusion container | Improve billet casting quality; clean the extrusion container and die before extrusion; control billet heating temperature to avoid overburning |
| Coarse grain ring / grain coarsening | Excessively high extrusion temperature; too small extrusion ratio; insufficient deformation degree | Reduce billet and die temperature; increase extrusion ratio; optimize extrusion speed to inhibit recrystallization grain growth |
| Unqualified mechanical properties | Insufficient quenching cooling rate; deviation of aging regime; composition segregation | Increase quenching cooling intensity; calibrate temperature uniformity of aging furnace; ensure sufficient billet homogenization |
| Profile twisting / bending | Unreasonable design of die bearing land; uneven metal flow rate; improper stretch straightening | Optimize die portholes and bearing lands; adjust extrusion temperature and speed; precisely control stretching amount and direction |
| Surface scratches / drag marks | Wear of die bearing land; hard protrusions on discharge table and cooling bed | Polish or replace dies regularly; clean the contact surfaces of conveying equipment; optimize discharge guide structure |
V. Industry Technology Development Trends
With the upgrading of demand from high-end downstream manufacturing, aluminum extrusion technology continues to evolve towards high precision, high performance, low carbonization and intelligence.
Integrated Extrusion of Large Complex Cross-Sections Demand from fields such as new energy vehicle chassis and rail transit car bodies promotes the popularization of 10,000-ton large-tonnage extruders, realizing integrated forming of large thin-walled, porous hollow, and complex cross-section profiles, replacing traditional welding assembly, and improving structural strength and lightweight level.
Extrusion Technology for High-Strength and High-Toughness Alloys Breakthroughs in extrusion technology for 7xxx, 2xxx high-strength aluminum alloys and aluminum-lithium alloys support the lightweight demand of aerospace and new energy vehicles. The core difficulties lie in controlling deformation cracking and improving microstructure uniformity. Indirect extrusion, isothermal extrusion, gradient heating and other technologies have become standard configurations.
Intelligent Extrusion Production Lines Through digital twin, on-line infrared temperature measurement, visual dimension inspection and other technologies, real-time closed-loop control of extrusion parameters is realized; combined with AI algorithm to optimize die design and process parameter matching, manual trial and error is reduced, and the yield and production efficiency are significantly improved.
Green and Low-Carbon Extrusion Process Promote energy-saving technologies such as electromagnetic induction heating and waste heat recovery to reduce energy consumption in the extrusion process; develop short-flow extrusion processes, cancel the billet reheating link, and directly use casting waste heat for extrusion, which can reduce comprehensive energy consumption by more than 30%.
