I. Basic Knowledge of Aluminum Alloys

1.1 The Inevitability of Lightweighting

For every 1% reduction in vehicle weight, fuel consumption can be reduced by 0.6%–1.0%.
For every 100 kg of weight reduction, CO₂ emissions can be reduced by approximately 5 g/km.
Each 1 kg of aluminum used in a passenger car can reduce about 20 kg of exhaust emissions over its service life.
Vehicle exhaust emissions are positively correlated with fuel consumption.
Modern vehicles are becoming increasingly sophisticated with more additional equipment, making lightweighting an inevitable trend.

1.2 Main Targets of Automotive Lightweighting

The engine, chassis, and body (including interior and exterior parts) account for a large proportion of a passenger car’s total mass and therefore offer significant weight-reduction potential.
The body alone accounts for about 30% of the total vehicle mass, making aluminum substitution in the body structure particularly critical.

1.3 Characteristics of Aluminum Alloys

The density of aluminum is low (2.7 g/cm³), approximately one-third that of steel (7.8 g/cm³). Replacing steel with aluminum alloys can reduce weight by about 50%.
Aluminum naturally forms a dense and stable oxide film on its surface, giving it excellent corrosion resistance.
Due to its low melting point and good fluidity, aluminum has excellent castability and is suitable for manufacturing complex parts.
When one or more alloying elements are added, aluminum alloys are formed, which offer higher strength and hardness than pure aluminum. Some aluminum alloys can be heat treated, achieving tensile strengths above 600 MPa.
Thermal and electrical conductivity are approximately three times that of steel.

1.4 Classification of Aluminum Alloys

II. Introduction to Automotive Aluminum Materials

2.1 Performance Parameters

Automotive aluminum alloys mainly use heat-treatable 6000-series and non-heat-treatable 5000-series alloys. Among them, 5038 and 5183 are the most commonly used grades for car bodies. Tests show that the R-value has no obvious influence on the stamping formability of aluminum sheets.

2.2 Performance Requirements

Automotive aluminum sheets must meet mechanical and corrosion-resistance standards and also satisfy the following requirements:

1. Good formability, low yield ratio, and high forming limit.

2. Smooth surface quality.

3. Good weldability to meet the requirements of automotive component welding processes.

4. Good bake-hardening performance.

5. High dent resistance.

6. Excellent surface treatment and paintability.

7. Adequate resistance to natural aging.

The real stress-strain curve graph of the aluminum alloy sample

Aluminum Usage in Automotive Outer Panels in Europe and North America

Based on relevant data, the performance requirements for finished sheets used in automotive body panels are summarized as follows:

(a) In T4(p) condition:
Yield strength: 90–140 MPa
Tensile strength: 220–285 MPa
Total elongation: ≥20%
n-value: ≥0.27 (strain-hardening exponent)
r-value: ≥0.65 (0°), ≥0.40 (45°), ≥0.55 (90°)

(b) After 2% pre-strain + baking:
Yield strength: 160–260 MPa

2.3 Alloy Grades and Application Areas

Aluminum alloys for automotive body panels mainly include the 2000, 5000, and 6000 series.
The 2000 and 5000 series are mainly used for inner panels, while the 6000 series is mainly used for outer panels.
Typically, 5000-series and 6000-series aluminum sheets replace conventional steel sheets.

International grades and chemical compositions of automotive body aluminum alloys:

2.4 Comparison of 6xxx and 5xxx Series Aluminum Alloys for Automotive Bodies

Typical grades include 6016, 6111, 6022, 6181, 5052, 5182, and 5754, with thickness generally between 0.7–2.0 mm.

(1) 6xxx Series Al-Mg-Si(-Cu) Alloys
These alloys offer high strength and good formability and are widely used for automotive outer panels. They require good plasticity before painting and high precipitation hardening response during baking (bake-hardening).
They are heat-treatable alloys with good formability, corrosion resistance, strength, and high-temperature performance.
After stamping and paint baking, strength increases significantly. The yield and tensile strengths of T4 sheets are comparable to steel, while the n-value exceeds that of steel.

(2) 5xxx Series Al-Mg Alloys
These non-heat-treatable alloys feature medium strength, good corrosion resistance, good formability, and excellent weldability.
However, they are prone to Lüders elongation after room-temperature storage, surface wrinkling after stamping, reduced ductility with increasing Fe content, and softening after baking.
Their disadvantages include delayed yielding, Lüders lines, and orange-peel effect when grain size exceeds 100 μm.

Mechanical and stamping performance comparison between aluminum alloy and cold-rolled steel:

Aluminum alloy is widely used in automobiles

III. Equal-Strength Calculation Example: Aluminum vs. Steel

Note: t = sheet thickness.

To replace steel with aluminum while maintaining part performance, equal-strength calculations are required.
Based on bending stiffness and bending strength, thickness calculations are as follows:

(1) Based on bending stiffness:
tₐₗ / tₛ = (Eₛ / Eₐₗ)¹ᐟ³
Eₛ = 210 GPa, Eₐₗ = 61 GPa → tₐₗ / tₛ = 1.5

(2) Based on bending strength:
tₐₗ / tₛ = (σₛ / σₐₗ)¹ᐟ²
σₛ = 184.4 MPa, σₐₗ = 116.9 MPa → tₐₗ / tₛ = 1.26

Thus, aluminum sheet thickness increases from 0.7 mm to 1.0 mm.

IV. Production Process of Automotive Aluminum Sheets

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4.2 A typical process flow diagram for the production of ABS plates and strips in an aluminum processing plant

4.3 Schematic diagram of the process flow for manufacturing AVT body with ABS in an automobile factory

4.4 ABS Fully Continuous Production Line

Microstructural evolution during processing of 6xxx series aluminum sheets:

V. Surface Treatment of Aluminum Sheets

Purpose of surface treatment:

1. Improve lubricant adhesion and stamping formability.

2. Enhance paint adhesion.

3. Facilitate subsequent processing.

Process: MF / EDT / EBT + cleaning (alkaline + acid) + conversion treatment + lubrication.
Chemical conversion coatings include chromate (toxic), zirconium/titanium systems (Alodine, Garbond, Envirox), and anodizing.

Lubrication methods:
• Industrial oil (mainly North America)
• Dry lubrication film / wax (mainly Europe)

Advantages of dry lubrication: improved deep-drawing, stable transport, easy handling, and compatibility with welding and painting.
Disadvantages: lubricant selection per die, residue accumulation, and more difficult welding.

Aluminum automotive sheets and surface treatment practices in Japan, the US, and Europe: