Aluminum and its alloys play a key role in many fields with their excellent properties, including low density, high strength, good conductivity, superior corrosion resistance and excellent forming performance. In addition, it maintains its performance at low temperatures, does not magnetize, does not spark on impact, has sound absorption and nuclear radiation resistance. These advantages make the aluminum industry have much development space and move towards glory.
Aluminum and Aluminum Alloys International Designation System
| Major Alloying Element | Series | Description |
|---|---|---|
| Pure Aluminum (Al ≥ 99.00%) | 1XXX | Non-heat-treatable; the last two digits indicate the minimum aluminum percentage after the decimal point. |
| Copper (Cu) | 2XXX | Heat-treatable alloys strengthened by precipitation hardening (aging). |
| Manganese (Mn) | 3XXX | About 20% stronger than 1XXX series alloys; non-heat-treatable. |
| Silicon (Si) | 4XXX | Commonly used as filler materials for aluminum welding; non-heat-treatable. |
| Magnesium (Mg) | 5XXX | Excellent corrosion resistance and weldability; non-heat-treatable. |
| Magnesium + Silicon (Mg + Si) | 6XXX | Heat-treatable; Mg₂Si is the primary strengthening phase. |
| Zinc (Zn) | 7XXX | High-strength, heat-treatable alloys. |
| Lithium (Li) | 8XXX | Special-purpose alloys, often used in aerospace. |
| Reserved Group | 9XXX | Reserved for future development. |
Explanation of Numbering System:
1XXX Series (Pure Aluminum):
The last two digits (XX) indicate the minimum aluminum content to the hundredths decimal place.
The second digit indicates impurity control:
- If it is 0, there is no special control of impurity limits.
- If it is 1–9, it means specific control over one or more impurities or alloying elements.
2XXX to 8XXX Series (Alloys):
The last two digits have no special meaning. It is used only to distinguish different alloys within the same series.
The second digit indicates the alloy version:
- 0 means the original alloy.
- 1–9 indicates modifications of the original alloy.
Classification by Heat-Treatability:
Non-Heat-Treatable Alloys:
- 1000 Series – Pure aluminum
- 3000 Series – Aluminum-Manganese
- 4000 Series – Aluminum-Silicon
- 5000 Series – Aluminum-Magnesium
Heat-Treatable Alloys:
- 2000 Series – Aluminum-Copper-Magnesium
- 6000 Series – Aluminum-Magnesium-Silicon
- 7000 Series – Aluminum-Zinc-Magnesium
Temper Designations (Suffixes):
- A dash (-) followed by letters and numbers (e.g., -Hn or -Tn) indicates the temper, i.e., the processing condition of the material.
- Hn: Cold-worked (strain-hardened) temper.It is used for non-heat-treatable alloys
- Tn: Thermally treated (heat-treated) temper. It is used for heat-treatable alloys
Aluminum Alloys Properties and Applications
1000 Series:
Examples: 1050, 1070
Characteristics: High-purity aluminum, excellent electrical and thermal conductivity, good corrosion resistance
Applications: Electrical conductors, heat exchangers, chemical piping systems
2000 Series
Examples: 2011, 2014, 2017, 2117, 2024
Characteristics: Excellent machinability, high strength, poor corrosion resistance
Applications: They are known as “Duralumin” series; It is ideal for machining parts such as structural components, screws, aircraft materials, forged stock, hydraulic parts for automobiles and machinery, and sports equipment
3000 Series
Examples: 3003, 3203
Characteristics: Better heat resistance than pure aluminum, higher strength, good corrosion resistance
Applications: Chemical equipment piping, heat exchangers, photoconductor drums for copiers
4000 Series:
Example: 4032
Characteristics: Good heat resistance and excellent wear resistance
Applications: VCR magnetic heads, piston components, forging materials
5000 Series:
Examples: 5052, 5056
Characteristics: Medium-strength alloys, excellent corrosion resistance, good weldability
Applications: Chemical plant piping, machinery parts, camera lens barrels
6000 Series
6061
Characteristics: Medium-strength structural alloy with excellent corrosion resistance, good weldability and workability
Applications: Land vehicles, ships, marine equipment, road construction materials, building structures, sports equipment
6063
Characteristics: Excellent corrosion resistance, good surface finish, outstanding extrudability. They are widely used in extruded products.
Applications: Building materials, construction components, decorative materials, home appliances, and various general-purpose applications
7000 Series
7003
Characteristics: Medium-strength structural alloy suitable for welding
Applications: Vehicle frames, automotive and motorcycle parts
7075
Characteristics: They are known as “Super Duralumin”, one of the highest-strength aluminum alloys; poor corrosion resistance and weldability
Applications: High-strength applications such as aircraft components, mechanical parts, sports equipment
8000 Series
Examples: 8090, 8091
Characteristics: Limited commercial use
Applications: Specialty applications, typically in aerospace or other niche fields
9000 Series
Characteristics: High-performance alloys with special elements (e.g., rare earth metals). It is designed for exceptional or specialized properties
Applications: Advanced applications requiring ultra-high performance or unique functional characteristics.
Aluminum Extrusions Introduction
Aluminum extrusions are one of the four major categories of primary aluminum processed products. The other three being cast aluminum, forged aluminum, and rolled aluminum products. They are made by heating aluminum or aluminum alloy billets to a plastic state (approximately 400–500°C), and then using hydraulic force to extrude the billet through a die to form the desired shape.
The resulting forms include profiles, tubes, bars, and wires, with profiles and tubes accounting for the majority of applications.The main product types :
Profiles
These can be solid, hollow, or semi-hollow. You can see common shapes include angle, T-shaped, channel, and other more complex cross-sections.
Tubes
Tube cross-sections may be round, square, rectangular, hexagonal, octagonal, sector-shaped, or elliptical.
A. Welded Tubes:
These are formed using dies that include a welding chamber. The solid billet is divided into multiple metal streams, which are then rejoined and welded in the chamber to form hollow shapes. Common dies for this process include bridge dies, porthole dies, and taper-type dies.
B. Seamless Tubes:
These are made using hollow billets, or by first piercing a solid billet before extrusion. Since the process is more complex, costly, and limited by the mandrel length. The seamless tubes are usually produced as larger hollow sections that are then drawn down to smaller sizes. Due to higher costs, seamless tubes are typically used only when required; otherwise, welded tubes are preferred.
Bars
Aluminum bars can be produced through hot rolling or extrusion. They are typically undergo cold working to reach final dimensions.
Rod: Circular or near-circular cross-section with diameter greater than 9.5 mm (3/8 inch).
Bar: Square, rectangular, or regular polygonal cross-section, with at least one dimension exceeding 9.5 mm.
Wires
Aluminum wire is drawn from bars or rods and has a diameter less than 9.5 mm. Cross-sections can be square, round, rectangular, hexagonal, or octagonal.
Advantages and Applications
With proper design and die manufacturing, aluminum extrusions can form complex shapes directly and achieve high strength. As a result, their applications are extremely wide-ranging.
Building and Construction:
Aluminum doors and windows, curtain walls, railings, signage boards, etc.
Transportation Components:
Bicycle rims and frames, automotive radiators, marine vessel parts, vehicle bodies, aircraft frames, and seat structures.
Consumer Durable Goods:
Sports and leisure equipment such as aluminum baseball bats, inline skate frames, badminton rackets, etc.
Machinery and Industrial Equipment Components:
Structural frames for conveying systems in electronics, food processing, and synthetic fiber industries; heat exchangers, pneumatic tool parts, air compressor cylinders, forging blanks, industrial piping, and elevator housings.
Electronics Components:
Heat sinks, disk drive read/write heads, optical scales, copier photoconductor drums, optical rails, magnetic tape drums, etc.
Defense and Weaponry Parts:
Missile casings, firearm components, and other military hardware.
Classification in the Industry
Extruded products are generally divided into two categories:
- General Extrusions: Includes products from categories 1 and 3 above (e.g., architectural and consumer products).
- Industrial Extrusions: Includes the remaining categories with higher technical demands.
General architectural extrusions must meet performance standards including:
- Aesthetic durability
- Wind pressure resistance
- Air tightness
- Water tightness
- Sound insulation
- Thermal insulation
- Condensation resistance
- Fire resistance
- Smooth and durable operability (e.g., windows and doors)
- Safety performance
Industrial extrusions are subject to even stricter technical requirements.
Aluminum Extrusion Basic Elements
Die Components
The die backing set includes components such as the die holder, bolster, sub-bolster, and pressure platen. These components have a critical impact on the quality of the extruded product. A high-strength and rigid die backing system helps reduce die deflection, ensuring reusability and extended die life.
Since the die is the first and most directly pressured component during the extrusion process, die designers must carefully determine the mandrel and die height based on the profile’s cross-section and the ram dimensions. Thererefore, Your goal is to prevent die deflection or cracking, as these issues may result in unpredictable metal flow and deviations from the intended profile shape.
To minimize deflection, You should need a well-fitted backing set. Currently, embedded bolster blocks are commonly used. Designers typically reinforce areas under high stress with additional support. It can reduce deflection and enhance tool stability.
Die Design Fundamental Principles
Non-uniform wall thickness is possible, but uniform thickness is generally preferred for consistency.
Solid profiles are less costly to produce than hollow profiles.
Corners and transitions should be designed with small-radius fillets instead of sharp angles to prevent stress concentrations.
Symmetrical profiles improve extrusion efficiency and stability.
Incorporating grooves or ribs can increase structural strength and reduce wear on the extrusion tooling.
For complex shapes, consider hollow or semi-hollow cross-section designs.
Near-net shapes help reduce or eliminate the need for secondary machining.
Screw attachments Designing can facilitate assembly and integration with other components.
Manufacturing Tolerances Basic Principles
Linear tolerance: ±0.008 of the total profile length (approx. 0.8%).
Angular tolerance: Ranges from 1° to 2°, depending on the profile configuration.
Twist tolerance: Approximately 0.5° per inch.
Wall thickness tolerance: ±10% of the wall thickness at the specified section.
Flatness tolerance (horizontal): ±0.004 of the profile width (approx. 0.4%).
Straightness tolerance (vertical): ±0.001 of the length at the measured section (approx. 0.1%).
Aluminum Extrusion Manufacturing
Extrusion manufacturing mistakes are often well-known by those in the industry. For example, when a factory is rushing to fulfill an urgent order for a key customer, operators may hurriedly start the extrusion press without properly verifying all temperatures beforehand. In such cases, the billet container liner may become significantly hotter than its outer shell, essentially turning the liner into a secondary heater, which can easily crack under stress.
For large billet containers, it is especially important to spend sufficient time preheating the system to a proper operational temperature. If not adequately preheated, internal stress will build up inside the container. Although this is not a frequent issue, high-level aluminum extrusion manufacturers ensure such problems never occur.
Top extrusion factory face the same markets as ordinary manufacturers. They may use similar equipment and operate under similar cost pressures. Yet, they can achieve superior efficiency, generate minimal scrap, experience shorter downtimes, and significantly higher productivity. So It can also allow them to secure better margins on every order.
The key difference?
Elite manufacturers obsess over temperature.
They constantly monitor, record, and control temperatures throughout the process. Moreover, they understand the thermal behavior at every stage of production and treat temperature management. It is a critical success factor.
Productivity vs. Extrusion Speed
Assuming no unexpected shutdowns, productivity mainly depends on extrusion speed, which is influenced by four factors:three fixed, and one variable.
Extrusion Press Tonnage
Larger presses allow extrusion at lower temperatures, which can improve material properties and reduce thermal defects.
Die Design
During extrusion, friction between the aluminum alloy and die can raise the material’s temperature by around 38°C. The most common issue at the die surface is wear, particularly in the bearing areas. At temperatures exceeding 450°C, even nitrided surfaces begin to corrode.
Therefore, die designers must ensure the profile can be extruded at high enough speeds without exceeding the material’s critical failure temperature, and without causing cracks, sticking, or surface defects. If you want to achieve this,you must require high-quality dies. Although It is more expensive, their cost is quickly offset by improved productivity.
Alloy Characteristics
The alloy type significantly affects extrusion but is less controllable. For instance, when the exit temperature of the profile reaches 538°C, surface degradation may occur, including oxide buildup or cracking within the die liner. If the press force is insufficient or the die is too hard, billet temperature may rise, you should reduce the extrusion speed to prevent overheating and surface flaws.
Temperature (Controllable Factor)
Of the four speed-limiting factors, temperature is the one that can be controlled. The optimal extrusion temperature depends heavily on the specific aluminum alloy being processed.
Below are the appropriate extrusion temperatures for 6000 series aluminum alloys:
| Aluminum Alloy Type | Extrusion Ingot Temperature | Extrusion Temperature |
|---|---|---|
| 6063 | 415－440℃ | 500℃ |
| 6061 | 425－455℃ | 525℃ |
| 6005 | 415－445℃ | 510℃ |
Importance of Billet Container Temperature
The billet container temperature is critically important in the extrusion process, and preheating must not be taken lightly. To minimize thermal stress and prevent liner shrinkage, both the container and its liner must be gradually and uniformly heated to the operating temperature. The temperature increase rate should not exceed 37°C per hour.
We recommended that during preheating maintain the container at 232°C for 8 hours, then further heat it to 427°C for an additional 4 hours before extrusion begins. By this, you can ensure uniform temperature distribution and relieve internal thermal stress within the container.
In addition, the ideal preheating method is to use a baking oven. It also helps prevent stem breakage due to thermal stress.
Note:
We advised that the extrusion stem undergo stress relief every six months by standing it vertically in an oven at 427–428°C for 12 hours, followed by slow cooling inside the furnace.
Extrusion Temperature Control for Optimal Results
To achieve optimal extrusion quality, the billet container temperature should generally be 10–38°C lower than the billet temperature. If, due to high extrusion speed, the container becomes hotter than the billet, active cooling of the container should be implemented. When the container temperature reaches 468°C, scrap generation increases significantly.
For surface finish extrusion, controlling the temperature of the dummy block or pressure pad is also essential. If the billet tail becomes too hot, impurities may be extruded along with the product. In this case, you can inject air through the stem to cool the center of the dummy block by about 52°C, reducing impurities and increasing productivity. The same airflow can also reduce the container temperature by about 24°C, further improving process efficiency.
Die Temperature Management
Top Aluminum Extrusion manufacturers also place great importance on die temperature. The typical preheating temperature range for dies is 450–480°C.
- Flat dies require at least 2 hours of heating.
- Hollow dies require at least 4 hours of heating.
Even if the die is preheated properly, if the die support block,which is typically twice the size of the die. If it is not preheated, it can act as a heat sink, reducing the effectiveness of die preheating.
If the die temperature falls below 427°C, it may crack and may require the use of special billets to achieve the desired extrusion. On the other hand, excessively high die temperatures can reduce die hardness and cause oxide formation, particularly on load-bearing surfaces.
For effective preheating, we recommended that use a furnace. If the furnace is too small and dies are stacked too close together, airflow is restricted, reducing heating efficiency. A box-type furnace that heats dies individually is now commonly used and provides excellent results.
Billet Temperature Considerations
The optimal billet temperature depends on factors such as:
- Extrusion press tonnage
- Alloy type
- Profile complexity
- Die temperature
- Extrusion speed
Billets at lower initial temperatures have more room to absorb heat during the process, allowing for higher extrusion speeds.
Profile Exit Temperature
Exit temperature is another important factor closely monitored by top Aluminum Extrusion manufacturers. This temperature is influenced by:
- Ram (stem) speed
- Die friction
During extrusion, temperature typically rises by about 38°C, largely depending on die design. You can optimize die design to achieve maximum productivity, while maintaining dimensional accuracy and surface finish.
Emerging Trends in Extrusion Technology
Variable Die Technology During Extrusion
Allows adjustment of die components or cavity positions in-process
Supports controlled variation in profile width and thickness.
Wide-Section Extrusion Technology
a. Expansion Plate Techniques
b. Use of angled billet containers combined with reshaped billets for larger cross-section profiles.
Finite Element Analysis (FEA) in Extrusion
Advanced computer simulations using algebraic models to analyze stress and strain during extrusion.
6000 Series Extrusion Alloys Basic Metallurgical Theory
6000 series aluminum alloys Extrusion is a thermally driven process, which involves complex interactions. To efficiently produce high-quality extrusions, you should unstand the metallurgy involved .
Role of Magnesium and Silicon
6000 series alloys are typical heat-treatable aluminum alloys. Their strength can be significantly improved through thermal processing rather than mechanical work. The key alloying elements in this series are magnesium (Mg) and silicon (Si), which combine to form magnesium silicide (Mg₂Si) precipitates. These precipitates exist in several morphologies within the alloy matrix, primarily as:
β″ (Beta double prime) Mg₂Si: The finest needle-like precipitates. When uniformly and densely distributed within the alloy, they significantly enhance mechanical properties.
β′ (Beta prime) Mg₂Si: These are larger, needle-shaped precipitates grown from β″. Their contribution to mechanical strength is minimal.
β (Beta) Mg₂Si: The coarsest form, typically cubic in shape. Due to their large size, these precipitates offer no benefit to the alloy’s mechanical performance.
When selecting alloy compositions, using a silicon-rich formulation is generally more beneficial than a magnesium-rich one. Why?
Higher magnesium content does not significantly enhance the final mechanical properties.
Excess magnesium increases flow stress, making extrusion more difficult.
In contrast, increased silicon content promotes a more effective aging response, thereby improving strength.
Effects of Individual Alloying Elements
Iron (Fe)
Iron is commonly present as an impurity in aluminum alloys. Combining with aluminum and silicon to form intermetallic compounds such as Al-Fe-Si. These compounds do not contribute to mechanical strength. If not properly managed, it can negatively impact extrusion performance. So you should precise control of iron content, especially in products requiring high surface finish quality. Because iron also affects anodizing results and electrical conductivity.
Manganese (Mn)
Manganese is used in many 6000 series alloys for several beneficial effects:
Reduces homogenization time by transforming β-AlFeSi into α-AlFeSi.
Helps inhibit coarse grain growth during post-extrusion heat treatment in high-strength alloys like 6061 and 6082.
Enhances crack resistance by preventing silicon particle nucleation at grain boundaries, which can cause embrittlement.
Chromium (Cr)
Chromium offers effects similar to manganese but is even more effective in reducing quench sensitivity.
Copper (Cu)
Copper improves electrical conductivity and machinability in extrusion alloys. In high-strength alloys such as 6061, copper helps counteract the negative effects of natural aging at room temperature after artificial aging. However, when copper content exceeds 0.2%, corrosion resistance significantly deteriorates.
Zinc (Zn)
Zinc does not negatively impact mechanical properties in 6000 series alloys. However, when its concentration exceeds 0.02%, it may cause “mottling” or surface patterning during anodizing, due to localized etching variations.
Effects of Major and Minor Elements
| Alloying Element | Extrudability | Quench Sensitivity | Strength/Hardness | Ductility/Toughness |
|---|---|---|---|---|
| Mg | ↓ | ↑ | ↑↑ (with Si) | ↓ (at high levels) |
| Si | → | ↓ | ↑↑ | → |
| Fe | ↓ | → | ↓ | ↓ |
| Mn | → | ↓ | ↑ (grain refinement) | ↑ |
| Cr | → | ↓↓ | ↑ | ↑ |
| Cu | → | ↑ | ↑ (at low %) | ↓ (at high %) |
| Zn | → | → | → | → |
Thermal Cycle of 6000 Series Alloys
To achieve maximum mechanical strength, 6000 series alloys must undergo the following thermal processes:
Solution heat treatment during extrusion: It is typically integrated with the extrusion process.
Quenching: Rapid cooling at an appropriate rate depending on the specific alloy to retain the dissolved elements in solid solution.
Artificial aging (precipitation hardening): Controlled heating to allow precipitation of fine strengthening phases.
Without proper temperature control during these steps, it is impossible to develop optimal mechanical properties. You must accurate thermal management to ensure magnesium silicide precipitates form with desirable characteristics.
Key Factors Affecting the Extrusion Process
Extrusion Press Capability (Available Pressure)
To achieve the highest productivity from an extrusion press, it is typically operated near its maximum working pressure. This maximum pressure often corresponds to the highest achievable extrusion speed.
When using softer alloys or when higher available pressure is applied for the sake of maximizing efficiency, the extrusion boundary curve shifts to the left, indicating increased productivity.
Conversely, when working with harder alloys, higher extrusion ratios, or more complex profile shapes, the boundary curve shifts right, thereby expanding region A, reducing the available pressing capacity (i.e., extrusion speed), and lowering productivity.
Using higher billet temperatures can help partially compensate for slower extrusion speeds, though the temperature increase is often constrained by other variables.
Mechanical Properties
Mechanical property requirements also impose constraints on the extrusion process. Under given limits of extrusion speed and billet temperature, a stronger correlation arises between mechanical performance and processing parameters. This interaction also affects surface finish, required extrusion force, and overall product quality.
A green boundary curve is introduced to represent the threshold for meeting mechanical property requirements. When using billets containing coarse Mg₂Si particles or extruding profiles with thick cross-sections, you need higher mechanical strength. Therefore, the curve shifts to the right.
This implies that both the extrusion speed and billet temperature must be increased to supply more thermal energy, allowing better dissolution of Mg₂Si within the billet and ultimately improving mechanical strength.
In contrast, if the billet contains a fine distribution of β″ and β′ Mg₂Si precipitates, and the extruded profile has a thinner cross-section or lower strength requirement, so the green curve moves to the left.
In such cases, both extrusion speed and billet temperature can be reduced while still meeting the mechanical performance target with ease.
The Extrusion “Process Window”
The area enclosed by the three boundary curves for mechanical properties resembles a “window frame.” Within this window lies the range of processing conditions capable of producing high-quality extrusions. The exact position within this window must be determined by considering factors such as surface finish (smoothness, anodizing quality, extrusion accuracy) and the mechanical property limits.
These boundaries also indicate the maximum productivity achievable under given conditions. Other variables,such as
alloy type
- extrusion ratio
- profile geometry
- mechanical strength
- These may enlarge or shrink the process window.
Forced Quenching (Cooling)
After extrusion, alloy profiles must be rapidly cooled to ensure magnesium and silicon remain in solid solution. It can enable the alloy to achieve maximum strength during subsequent aging. The appropriate cooling rate depends on the cross-sectional thickness of the profile and may be carried out using different methods, such as:
- Natural air cooling
- Fan-assisted air cooling
- Water mist spray
- Water bath quenching
Aging Treatment
When you need higher mechanical properties , 6000 series alloys must undergo aging treatment. The improvement in mechanical strength depends on both the alloy type and the specific aging process.
- Natural aging at room temperature
- Artificial aging at elevated temperatures
In terms of strength, the key factor is the alloy’s resistance to dislocation motion. When stress is applied to a material, dislocations begin to move and multiply. As stress increases, this dislocation activity becomes more severe until material failure occurs. The presence of Mg₂Si precipitates hinders this movement, thereby increasing strength.
The size and density of these precipitates can be precisely controlled through aging treatment conditions. A small amount of fine β″ precipitates can partially inhibit dislocations, while a higher density provides greater resistance and enhances strength.
However, if the precipitates grow too large,becoming β′ or β phase Mg₂Si. This depletes the available Mg and Si, reducing the total number of strengthening precipitates. With fewer precipitates, dislocations can move more freely, resulting in lower mechanical strength.
To achieve peak mechanical performance, artificial aging should promote the maximum formation of fine β″ precipitates. For typical 6000 series alloys, the recommended conditions are:
- 170°C for 8 hours, or
- 185°C for 6 hours
You can optimize these conditions to maximize precipitation hardening and improve alloy strength.
Heat Treatment of Aluminum Alloys
What do the common temper designations of heat-treatable aluminum alloys represent?
F – As Fabricated: Products have undergone cold working, hot working, or casting without any special thermal treatment.
O – Annealed: Wrought products annealed to the lowest strength level or castings annealed to improve ductility and dimensional stability.
H – Strain Hardened: Wrought products have been strengthened through cold working (strain hardening).
W – Solution Heat Treated: Indicates an unstable temper following solution heat treatment and natural aging for a limited period (e.g., W 1/2 hour).
T – Thermally Treated: Indicates a stable temper achieved through heat treatment other than F, O, or H conditions.
What are the classifications under the “T” temper designations?
T1: Cooled from an elevated-temperature shaping process (such as casting or extrusion) and naturally aged to a stable condition.
T2: Cooled from an elevated-temperature shaping process, cold worked, and then naturally aged.
T3: Solution heat treated, cold worked, and then naturally aged.
T4: Solution heat treated and naturally aged.
T5: Cooled from an elevated-temperature shaping process and then artificially aged.
T6: Solution heat treated and then artificially aged.
T7: Solution heat treated and then overaged to a stable condition (for improved dimensional or thermal stability).
T8: Solution heat treated, cold worked, and then artificially aged.
T9: Solution heat treated, artificially aged, and then cold worked.
T10: Cooled from an elevated-temperature shaping process, cold worked, and then artificially aged.
Stress-relieved variants:
TX51: Solution heat treated and stress relieved by stretching.
TX52: Solution heat treated and stress relieved by compressing.
TX53: Stress relieved by a combination of stretching and compressing after solution heat treatment.
What is the most common strengthening mechanism for heat-treatable aluminum alloys?
Precipitation Hardening (Age Hardening):
Aluminum alloys from the 2xxx, 6xxx, and 7xxx series rely on precipitation hardening. Through quenching and aging treatments, a phase transformation occurs within the alloy, resulting in the formation of fine precipitates. These precipitates obstruct dislocation motion, thereby strengthening the material. This process is known as precipitation hardening or age hardening.
Solution Strengthening:
Non-heat-treatable alloys do not undergo precipitation hardening (although they may still form precipitates). Instead, their strength is improved through mechanisms such as solid solution strengthening and grain refinement.
Standard Procedure for Precipitation Hardening of Aluminum Alloys
A practical precipitation hardening process involves the following three essential steps:
Solution Heat Treatment
The material is heated to a temperature where it enters a single-phase solid solution region (α-phase), allowing the alloying elements to fully dissolve into the aluminum matrix.
Quenching
The material is then rapidly cooled to “freeze” the dissolved elements in a supersaturated solid solution.
Aging Treatment
The quenched material is held at a specific temperature to allow gradual precipitation of strengthening phases.
If this is done at room temperature, it is known as natural aging.
If it is performed in a furnace at elevated temperature, it is called artificial aging.
What is Overaging in Aluminum Alloys?
During aging, the material’s hardness initially increases as precipitates form and their number. The distribution become more effective at impeding dislocation motion. This reaches a peak where the precipitate spacing is optimal and strength is maximized.
Overaging occurs when these precipitates begin to coarsen, increasing in size while decreasing in number. The spacing between precipitates widens, reducing their effectiveness at blocking dislocations. As a result, the material’s hardness and strength begin to decline.
This loss of strength is due to a reduced resistance to dislocation movement, which is a key mechanism in material hardening.