Each advancement in industrial technology delivers significant economic and social benefits. Reducing energy consumption, pollution, and conserving resources remain critical challenges today.
Lightweight equipment reduces energy use and emissions, making aluminum alloys essential for achieving these goals. Developing heat and surface treatments for aluminum alloys is crucial.
Benefits
Aluminum alloys, known for weight reduction and energy savings, benefit manufacturing, operations, and equipment recycling in various industries. Aluminum alloy usage continues to rise annually, driving material innovation for diverse industrial applications.
Aluminum alloys serve in engine parts needing high thermal conductivity and moderate strength. Rail applications include high-speed train carriages and subway bodies, ensuring lightweight designs and safety.
In the energy field, aluminum alloys make photovoltaic brackets and wind turbine parts due to their light weight and durability.
Types and Characteristics
Cast Aluminum Alloys
The alloys, with 8–25% alloy elements, feature excellent fluidity for complex, low-performance components.
Forged Aluminum Alloys
This alloys, with lower alloy element content (5% or less), excel in heat and cold processing for vehicle chassis and safety parts. However, their costs exceed those of cast aluminum alloys.
Common Alloy Series
Aluminum-Copper Alloys (2XXX): These alloys offer high strength but low corrosion resistance, commonly used in aerospace.
Aluminum-Manganese Alloys (3XXX): With excellent corrosion resistance, these alloys suit food packaging and chemical equipment.
Aluminum-Silicon Alloys (4XXX): Durable and corrosion-resistant, these alloys are ideal for engine components.
Aluminum-Magnesium Alloys (5XXX): Lightweight and corrosion-resistant, these alloys are used in ships and automobiles.
Aluminum-Magnesium-Silicon Alloys (6XXX): Balancing strength, corrosion resistance, and workability, these alloys fit construction and rail applications.
Aluminum-Zinc Alloys (7XXX): The strongest alloys, with lower corrosion resistance, are used in aerospace and high-performance sports equipment.
Heat Treatment
Aluminum alloys like cast aluminum A356 and forged aluminum 6082 are widely used in automotive and rail industries. Their adaptability and superior properties promote lightweight development across various manufacturing sectors.
Cast Aluminum Alloy A356
Properties and Heat Treatment
Manufacturers produce A356 billets through processes like smelting, degassing, slag removal, and refinement. Its composition is shown in Table 3.
Solution Treatment Process
The alloy forms a supersaturated metastable matrix during solution treatment due to dissolved minor elements. Aging treatment precipitates strengthening phases like Mg₂Si.
Aging Stages and Hardness
Initial Stage: High supersaturation accelerates precipitation, sharply increasing hardness.
Equilibrium Stage: Precipitation slows as the matrix reaches balance with precipitates, stabilizing hardness.
Final Stage: Mg₂Si coarsens during extended heat retention, reducing hardness.
Figure 1–4 illustrate A356 castings’ heat treatment process and mechanical properties (average of three samples). The treatment process involves:
540°C × 2h water quench + 170°C × 8h aging.
Forged Aluminum Alloy 6082
Production Process
Producers cast, homogenize, anneal, extrude, and trim 6082 aluminum to create extruded rods. These rods undergo forging to manufacture components. Table 4 presents its composition.
Optimizing Solution Treatment
Solution Temperature
High temperatures enhance dissolution of strengthening phases but risk coarse grains or overheating.
Low temperatures reduce solubility, lowering strength and hardness.
Optimal solution temperature: 530°C, as per the 6082 alloy phase diagram.
Soaking Time
Cast aluminum alloys, with higher alloy content, need longer soaking due to slower dissolution rates.
Deformation level, sample size, and second-phase grain size influence soaking time for wrought aluminum.
Typical soaking time: 1–2 hours for medium-sized samples.
Quenching Delay Time
Minimizing delay improves mechanical properties. However, 0-minute delay is impractical.
Ideal quenching delay: 0.5 minutes.
Optimizing Aging Process
The aging process allows strengthening phases to precipitate and distribute finely, enhancing strength. Both cast and forged aluminum alloys use the same aging process:
170°C × 8h aging.
Surface Treatment Processes
The natural oxide film on aluminum alloys is about 0.005–0.015 mm thick. While it provides some corrosion resistance, this thin layer cannot adequately protect aluminum, especially under acidic or alkaline conditions.
To enhance adaptability, reduce corrosion, and extend service life, aluminum alloys undergo surface treatments to increase oxide film thickness by 100–200 times. The primary surface treatment methods include:
Chemical Oxidation
Process and Characteristics
Chemical oxidation uses oxidizers like dichromate or permanganate to form an oxide film.
The chemical oxide film is thinner than anodized films but offers advantages:
Economic and fast processing.
Simple production line setup.
No need for electrical equipment.
Ideal for low-cost, high-volume production of components.
Anodizing
Process Overview
Anodizing uses aluminum or its alloys as the anode and materials like lead, carbon, or stainless steel as the cathode. The process involves electrolysis in oxalic acid, sulfuric acid, or chromic acid solutions.
Electric anodizing achieves faster film formation compared to natural oxidation.
The film forms when the growth rate exceeds the dissolution rate, as determined by potential-pH equilibrium.
Film Properties
Anodized films vary in structure, properties, and color based on the electrolyte and conditions. Polished aluminum parts achieve smooth, bright, and transparent oxide films, which can be dyed for decorative purposes.
Micro-Arc Oxidation
Innovative Technology
Micro-arc oxidation grows ceramic films in situ on non-ferrous metal surfaces using advanced techniques.
It overcomes the voltage and current limitations of traditional anodizing, raising the anode potential to hundreds of volts.
Alternating current produces phenomena like corona, glow, and micro-arc discharges on the surface.
Enhanced Properties
High-temperature, high-pressure micro-plasma transforms Al₂O₃ into α-Al₂O₃ and γ-Al₂O₃.
The resulting oxide layer has superior hardness, density, and adhesion, significantly improving:
Wear resistance.
Corrosion resistance.
Heat shock resistance.
Insulation performance.
Comparison of Surface Treatments
Different treatments yield varying oxide film thicknesses:
Chemical oxidation: Thinnest (1–2 mm).
Anodizing: Moderate (30–40 mm).
Black micro-arc oxidation: Slightly thinner (20–30 mm).
White micro-arc oxidation: Thickest (50–60 mm).
Application Recommendations
Use micro-arc oxidation for high hardness and corrosion resistance.
Opt for chemical oxidation when decorative needs outweigh performance.
Anodizing offers the best cost-effectiveness for general purposes.
Conclusion
Although the automotive industry’s use of aluminum alloys is in its early stages, lightweight development demands will significantly increase their usage over the next decade.
Advancing smelting, extrusion, thermoforming, and post-treatment technologies remains crucial. For manufacturers, these challenges present immense opportunities to innovate and excel. The path forward is both demanding and rewarding.
What is aluminum alloy post-treatment?
Post-treatment refers to processes applied to aluminum alloys after machining or forming to enhance their surface properties, strength, or corrosion resistance.
Why is post-treatment necessary for aluminum alloys?
Post-treatment improves the durability, appearance, corrosion resistance, and mechanical properties of aluminum alloy components.
What are the most common post-treatments for aluminum alloys?
The most common post-treatments include anodizing, powder coating, painting, polishing, sandblasting, chemical conversion coating, and heat treatments.
What is anodizing, and why is it popular?
Anodizing is an electrochemical process that forms a protective oxide layer on the aluminum surface, enhancing corrosion resistance, hardness, and appearance.
What is powder coating, and how does it benefit aluminum alloys?
Powder coating involves applying a dry powder to the aluminum surface, followed by curing. It provides a durable, decorative, and corrosion-resistant finish.
How does chemical conversion coating work?
Chemical conversion coatings, such as chromate or phosphate coatings, provide a corrosion-resistant layer and improve paint adhesion without significant thickness.
What is sandblasting, and how is it used in aluminum post-treatment?
Sandblasting involves propelling abrasive material at the aluminum surface to clean it, create texture, or prepare it for further treatments like coating or painting.
What heat treatments are used for aluminum alloys?
Heat treatments like solution heat treatment, aging, and stress relieving enhance the mechanical properties, such as strength and toughness, of aluminum alloys.
How do you choose the right post-treatment for an aluminum alloy component?
The choice depends on factors like the application, desired properties (e.g., corrosion resistance, appearance), environmental conditions, and cost considerations.