1. Basic Principles
Hot aluminum extrusion: Preheat homogenized aluminum billet to plastic forming temperature. Under triaxial compressive stress from the extrusion press, metal flows through the die orifice to form profiles.
Core logic: Alloy Composition → Billet Microstructure → Thermo-mechanical Extrusion Parameters → Microstructure Evolution → Mechanical Properties / Dimensional Accuracy / Surface Quality
Main extrusion technologies:
Forward Hot Extrusion: Dominant process in mass production
Backward Extrusion: Less friction, more uniform microstructure; widely adopted for high-end profiles
Key Note: Heat-treatable 6xxx series alloys (6063 / 6061 / 6082) rely heavily on three factors: solution effect at extrusion exit, on-line quenching cooling rate, artificial aging regime. 1xxx & 3xxx non-heat-treatable alloys obtain mechanical properties mainly via extrusion deformation and work hardening.
2. Complete Process Flow (Full-chain Property Control)
Melting & Casting → Billet Homogenization → Billet Heating → Container & Die Preheating → Extrusion Forming → On-line Quenching (Air Cooling / Mist Cooling / Water Quenching) → Traction & Cooling Table Cooling → Stretch Straightening → Cut-to-length Sawing → Artificial Aging → Inspection & Warehousing
2.1 Pre-process: Billet Homogenization (Root Control of Properties)
Non-homogenized billets feature casting segregation, coarse Mg₂Si phases and intergranular impurities, resulting in uneven hardness, low elongation and anodizing color difference after extrusion.
6063 Homogenization: 560～575℃, holding 4～6 h
6061 / 6082 Homogenization: 565～580℃, holding 6～8 h
Control target: Eliminate composition segregation, dissolve coarse secondary phases into matrix to ensure consistent extrusion performance of full-length billets.
2.2 Four Critical Extrusion Temperatures (Coupled Control)
Reference window for 6063-T5
Billet Temperature: 470～510℃ Low temperature: Higher extrusion load, extrusion stall, surface cracking and poor weld seam quality. Excessively high temperature: Grain coarsening, overburning risk, reduced strength and surface pitting.
Extrusion Container Temperature: 410～440℃ Maintain lower than billet temperature to avoid surface chilling and uneven metal flow.
Die Temperature: 430～460℃ Large temperature difference induces severe thermal shock and die cracking. Low die temperature increases flow resistance and causes surface streaks.
Profile Exit Temperature (Determines solution treatment efficiency) ✅ T5 requirement: Exit temperature ≥ 500℃ for sufficient dissolution of Mg₂Si into aluminum matrix. Below 490℃: Insufficient solid solution; hardness cannot reach target after aging. Above 555℃: Rapid grain growth leading to degraded strength and toughness.
Advanced industrial solution: Isothermal Extrusion Control. The system automatically adjusts extrusion speed according to deformation heat to stabilize exit temperature within target range.
2.3 Extrusion Speed (Strongly coupled with temperature)
Extrusion speed refers to ram speed. Profile exit speed = Extrusion Ratio × Ram Speed
Thin-wall, hollow & complex cross-sections: Low speed
Solid thick-wall profiles: Higher allowable speed
Too fast: Excessive deformation heat, overheating, surface cracking and twisting. Too slow: Heavy heat loss, dropping exit temperature and failed solid solution.
Empirical range for 6063: Solid profile exit speed: 25～80 m/min Hollow porthole die profiles: 10～40 m/min
Operation standard: Low startup speed → gradual acceleration → stable extrusion → deceleration at billet tail. Constant full-speed extrusion is prohibited.
2.4 Extrusion Ratio λ
\(λ=\frac{Billet\;Cross-sectional\;Area}{Profile\;Cross-sectional\;Area}\)
Normal range: λ = 10～60
Low λ: Insufficient deformation, coarse microstructure and inferior mechanical performance
High λ: Sharply increased extrusion pressure, cracking risk and heavy die load
2.5 On-line Quenching (Performance boundary for T5 / T6)
Mg and Si atoms dissolved in matrix at high extrusion temperature must be rapidly cooled to form supersaturated solid solution, which enables precipitation strengthening during aging.
Minimum cooling rate requirement for 6063-T5: ≥38℃/min (cooling from 500℃ to 200℃)
Cooling modes:
Air cooling: Decorative profiles, avoids water stains with limited cooling capacity
Mist cooling: General industrial profiles
Water quenching (T6 process): High-strength thick-wall 6061 / 6082
Risk: Uneven cooling leads to high residual stress, difficult straightening and post-machining distortion.
2.6 Stretch Straightening
Start stretching after profile cools down ≤50℃
Standard stretching rate: 0.5%～2.0%
Functions: Correct bending & twisting, partially release residual stress, slightly boost strength via mild work hardening
Over-stretching (>2.5%): Grain damage, microcracks and dramatic drop in elongation.
2.7 Artificial Aging (Final property setting process)
Aging mechanism: Nanoscale Mg₂Si strengthening phases precipitate from supersaturated solid solution, improving tensile strength, yield strength and hardness.
Common industrial aging cycles:
6063-T5: 175～185℃, holding 6～8 h
6061-T6: 160～170℃, holding 8～12 h
6082-T6: 165～175℃, holding 7～10 h
Common issues:
Under-aging: Insufficient hardness
Over-aging: Coarsened precipitates, irreversible strength reduction
Control requirement: Furnace temperature deviation ≤±5℃. Avoid dense stacking to guarantee hot air circulation.
3. Correlation Between Mechanical Properties and Process Parameters
Standard inspection items: Ultimate Tensile Strength (Rm), 0.2% Offset Yield Strength (Rp0.2), Elongation (A), Vickers Hardness (HV), Residual Stress, Microstructure (Grain size, precipitates)
Low strength / insufficient hardness ① Non-homogenized billets; ② Insufficient exit temperature causing poor solid solution; ③ Inadequate quenching cooling rate; ④ Low aging temperature or insufficient holding time; ⑤ Mg & Si content close to lower specification limit.
Satisfactory strength but poor elongation & brittleness ① Over-high exit temperature → grain coarsening; ② Excessive stretching rate; ③ Ultra-fast cooling leading to high residual stress; ④ Severe uneven metal flow forming banded microstructure.
Wide hardness fluctuation within one batch Large temperature gradient along billet length, unstable extrusion speed, uneven cooling air flow, unreasonable loading layout inside aging furnace.
4. Hidden Die Influence on Material Properties
Unbalanced bearing length: Uneven metal outflow velocity → heterogeneous cross-section microstructure, local stress concentration and twisting.
Insufficient welding chamber pressure for porthole dies: Weak weld seams, significant anisotropy, easy fracture along weld lines under load.
Uneven die preheating: Asymmetric temperature distribution causing differential microstructure.
5. Defect & Property Correlation Table
| Defect | Root Process Cause | Consequence on Performance |
|---|---|---|
| Periodic surface cracking | Over-high extrusion speed, temperature window out of range | Surface microcracks, degraded fatigue resistance |
| Poor weld seam of hollow profiles | Low extrusion temperature, insufficient welding chamber pressure | Low transverse strength, risk of rupture under pressure |
| Hardness difference between head & tail of profiles | Axial temperature gradient of billet, varying exit temperature | Scattered mechanical performance in batch |
| Severe distortion after machining | Uneven quenching, high residual stress | Poor dimensional stability |
| Anodizing streaks / color difference | Uneven metal flow, inconsistent grain size | Unqualified appearance, variable corrosion resistance locally |
6. Advanced Industrial Control Technologies
SCADA Full-process Data Monitoring Collect billet temperature, die temperature, infrared exit temperature, ram speed, extrusion load, cooling fan power; build parameter-hardness correlation curve for early abnormality warning.
Closed-loop Isothermal Extrusion System Real-time infrared measurement of exit temperature; PLC dynamically adjusts ram speed to maintain constant exit temperature and narrow property deviation.
Backward Extrusion Technology Eliminate friction layer between billet and container; deliver uniform microstructure and lower residual stress, suitable for new energy & precision structural components.
Numerical Simulation (DEFORM / Altair) Predict flow field, temperature field and stress field; optimize die design and process parameters to reduce trial production cost.
7. Workshop Quality Control Logic
Main control chain: Stable Alloy Composition → Stable Billet Homogenization → Stable Extrusion Temperature & Speed Matching → Stable Quenching Rate → Stable Aging Cycle
Property failure is rarely caused by a single parameter; it mostly results from mismatched multi-process parameters. Tuning principle: Stabilize temperature window first, then fine-tune extrusion speed. Do not blindly raise production speed at the cost of exit temperature.
