In recent years, with the rapid advancement of electric drivetrain technologies and intelligent vehicle systems, the penetration rate of new energy vehicles (NEVs) in China has increased significantly—from 14.2% in 2021 to 31.6% in 2023, and surpassing 50% for the first time by July 2024.

Reducing vehicle body weight, improving energy utilization efficiency, and ultimately extending driving range have become key research priorities in the development of NEVs.

Compared to conventional steel, aluminum sheet offers lower density and lighter weight, making it an increasingly attractive material for automotive body structures. As a result, the adoption of aluminum body panels—particularly in high-end NEVs—has become a major focus of research and development in recent years.

However, aluminum sheets present a number of manufacturing and process challenges, including relatively poor formability, pronounced aging effects, a tendency to generate debris during trimming, springback in formed parts, and cracking during hemming operations.

This paper aims to provide a systematic overview and discussion of aluminum sheet stamping processes and production practices, with the goal of offering both theoretical insights and practical guidance for addressing these challenges in real-world manufacturing.

Classification and Characteristics of Automotive Stamping Aluminum Sheets

Classification of Aluminum Sheets for Automotive Stamping

Aluminum sheets used in automotive stamping are typically categorized into the 5xxx and 6xxx series, each with distinct characteristics and application scenarios.

The 5xxx series aluminum alloys, based on Al–Mg compositions, are known for their excellent formability, good corrosion resistance, and strong weldability. However, in the annealed (O) temper, Al–Mg alloys may exhibit Lüders lines and delayed yielding during forming, which can affect surface quality. As a result, these alloys are primarily used for inner panels and components with complex geometries.

Common grades include 5754-O and 5182-O, which are widely adopted in the industry. In some cases, certain new energy vehicle manufacturers have also used 5052-O to reduce material costs. However, compared with 5754-O and 5182-O, 5052-O offers relatively lower formability and carries a higher risk of cracking when used in stamping complex parts.

The 6xxx series aluminum alloys, based on Al–Mg–Si compositions, provide a balanced combination of high strength, good ductility, excellent corrosion resistance, and favorable bake-hardening properties. Typical grades include 6014, 6016, 6111, and 6451. After forming, these alloys can undergo artificial aging during the paint baking process, resulting in increased strength through bake hardening.

Among them, 6014 and 6016 are commonly used for outer body panels due to their surface quality and formability, while 6111 and 6451 are more suitable for structural components requiring higher strength.

The selection of aluminum grades varies depending on the specific function and requirements of different automotive body parts, as summarized in Table 1.

Table 1. Common Aluminum Grades for Automotive Stamping

Alloy Series	Outer panel	Inner panel / structural component	Other components
5XXX Series	/	5182/5754/5154/5454	5052
6XXX Series	6016/6014/6111	6016/6451/6111/6022	6061/6063/6181

Surface Treatment Processes of Aluminum Sheets

(1) Surface Texture of Aluminum Sheets

The surface treatment processes for automotive aluminum sheets are typically categorized into MF (Milled Finish), EDT (Electrical Discharge Texturing), and EBT (Electron Beam Texturing).

The surface morphology of these finishes, observed at a magnification of 100 μm, is illustrated in Figure 1.

Figure 1. Aluminum Sheet Surface Morphology (100 μm Magnification)

Surface Finishing and Treatment of Aluminum Sheets

(1) Surface Texture Types

MF (Milled Finish):
MF surfaces are relatively smooth, with a surface roughness (Ra) ranging from 0.2 to 0.6 μm.

EDT (Electrical Discharge Texturing):
EDT produces a random surface texture, which enhances lubrication during stamping while eliminating directional rolling marks from the finishing process. The surface roughness typically ranges from Ra 0.7 to 1.3 μm.

Advantages of EDT:

1. Isotropic surface behavior, ensuring no directional influence on formability.
2. Formation of micro-lubrication pockets that generate hydrodynamic pressure, resulting in:
   • A low and stable coefficient of friction
   • Improved formability compared to MF
   • Increased stamping efficiency
   • Reduced material adhesion on tool surfaces
   • More stable forming pressure conditions
3. Improved surface quality after painting and better dimensional matching of components.

EBT (Electron Beam Texturing):
EBT creates a regular and controlled texture pattern, which is beneficial for both forming and paint performance.

Compared to EDT’s random texture, EBT features well-defined and isolated beam-patterned textures, offering enhanced consistency in forming and coating processes.

In practice, to ensure optimal paint appearance across the vehicle body, EDT-treated sheets are commonly used for all outer body panels (closures).

(2) Surface Pretreatment

To improve surface hardness, corrosion resistance, and wear resistance, automotive aluminum sheets are typically treated with titanium-zirconium (Ti-Zr) coating systems.

These coatings are applied using techniques such as arc ion plating or physical vapor deposition (PVD), where titanium and zirconium are evaporated and deposited onto the aluminum surface to form a dense and uniform coating.

The key functions include:

1. Formation of a Ti-Zr protective layer to prevent surface oxidation
2. Reduction of surface electrical resistance, improving weldability
3. Enhancement of paint adhesion, resulting in a more durable coating system

(3) Oiling Process

To protect the aluminum surface, reduce friction, and improve lubrication during stamping, aluminum sheets are typically coated with oil prior to delivery.

This process helps:

• Improve stamping performance
• Reduce the risk of micro-cracks during forming
• Enhance product quality
• Extend tool life

Common oil types include:

• Solid oil (E1)
• Semi-solid oil (AL200)
• Liquid oil (6130)

Details are typically summarized in Table 2 and Figure 2.

Figure 2. Typical Oiling Types for Automotive Aluminum Sheets

Figure 2. Typical Oiling Types for Automotive Aluminum Sheets 

Oil products	Characteristics
E1	1. Solid fat, not easily volatile 2. Easy to be drawn and formed 3. Solidification point temperature: 60℃ 4. At 100℃, kinematic viscosity: 11 mm²/s 5. Due to being solid fat, only dry cleaning machines can be used for sheet material cleaning during stamping.
AL200	1. Special lubrication products for metal processing, with strong lubrication capacity. 2. Suitable for deep drawing, rolling, extrusion and rolling processes of aluminum and other non-ferrous metals. 3. Can maintain good lubricity at extreme temperatures and has a wide working temperature range. 4. At 40℃, the kinematic viscosity is 100mm²/s. 5. Freezing point temperature: -15℃. 6. Can be cleaned for sheet materials using wet cleaning machines or dry cleaning machines.
6130	1. At 40℃, the kinematic viscosity is 30mm²/s. 2. Excellent rust prevention and lubrication properties. It can be pre-applied on aluminum sheets, cold-rolled sheets, and galvanized sheets, and is widely used for oiling the raw materials of steel. 3. Low viscosity, easy to clean. 4. The sheet materials can be cleaned using wet cleaning machines and dry cleaning machines.

Oiling Selection and Stamping Production Considerations for Aluminum Sheets

(1) Selection of Pre-Oiling Types

At present, three main types of pre-oiling are commonly used for automotive aluminum sheets: E1 (solid oil), AL200 (semi-solid oil), and 6130 (liquid oil).

European and American manufacturers tend to prefer the E1 solid oiling process, although it requires the use of dry cleaning systems during stamping.

AL200 offers greater flexibility, as it is compatible with both wet and dry cleaning systems, while also providing effective lubrication that supports stable forming performance.

6130, due to its lower cost, is widely used in raw material pre-treatment by steel mills and is also commonly adopted by domestic automotive manufacturers. However, its relatively low viscosity makes it less suitable for demanding aluminum stamping applications.

A comparison of the characteristics of these oil types is provided in Table 2.

(2) Characteristics of Aluminum Sheets in Stamping Production

Compared to steel, aluminum sheets exhibit:

• Lower hardness
• Non-magnetic properties
• Reduced ductility, with a maximum elongation of approximately 25% (compared to 40–50% for steel)

As a result, the stamping process for aluminum sheets differs significantly from that of steel.

(3) Packaging, Transportation, and Storage Requirements

Due to the softness and chemical activity of aluminum, stricter handling requirements are necessary:

1. Pallet selection:
   Wooden or steel pallets are used, with corrugated board or cardboard at the bottom layer to reduce vibration and material loss.
2. Securing during transport:
   Steel or plastic strapping is required to prevent sheet displacement, avoiding scratches and oil staining.
3. Edge protection:
   Protective corner guards should be applied to prevent edge damage.
4. Full wrapping:
   Six-sided plastic film packaging is recommended to minimize air exposure.
5. Controlled storage (if possible):
   Temperature- and humidity-controlled storage conditions are preferred.

(4) Sheet Separation (Destacking)

Since aluminum is non-magnetic, conventional magnetic separation methods used for steel cannot be applied.

Aluminum sheet separation typically relies on high-pressure air systems:

• Standard plant compressed air is around 0.6 MPa, so booster systems are required to increase pressure to 1.0–1.1 MPa at the destacking station.
• For high-speed stamping lines, mechanical fingers (separators) are often used in combination to ensure reliable sheet separation.

Typical air knife configurations are shown in Figure 3.

Key factors affecting separation performance:

1. Air pressure ≥ 1.0 MPa
2. Nozzle diameter: typically 1.5–2.5 mm
3. Nozzle spacing: 10–20 mm
4. Distance from air knife to sheet: 50–100 mm
5. Initial lifting speed of robotic gripper: 5–15%
6. Stable air knife height relative to the sheet during operation

(5) Cleaning and Re-Oiling Before Stamping

Due to the relatively low strength and formability limitations of aluminum, pre-stamping surface preparation is critical.

• Outer panels are typically subjected to cleaning processes
• Complex parts such as tailgate inner panels and door inner panels often require additional lubrication (re-oiling)

Cleaning methods include:

• Dry cleaning
• Wet cleaning

The choice depends on the pre-oiling type and available equipment:

• If solid oil (E1) is used, dry cleaning is mandatory
• Wet cleaning systems cannot effectively handle solid oils, as they may clog the pores of squeeze rollers and reduce cleaning efficiency

This is why many European OEMs and joint ventures, which predominantly use E1 oiling, rely on dry cleaning systems.

In contrast, domestic manufacturers often use liquid oils for cost considerations, allowing greater flexibility in selecting cleaning equipment.

Figure 3. Common Air Knife Nozzle Designs Used for Sheet Separation

Oiling Application and Rework Considerations for Aluminum Stamping

(1) Local Oiling for Forming Stability

To ensure stable deep drawing performance during aluminum stamping, localized oiling is typically applied to components such as tailgate inner panels and door inner panels, where forming depth is significant.

For parts with relatively shallow draw depth—such as floor panels and hood inner panels—direct stamping without additional oiling is generally sufficient.

However, after oiling, the thickness of the surface oil film increases. During storage, gravity causes the oil to accumulate at lower edges and concave radii (R zones). If not properly removed, this can lead to paint defects such as shrinkage or pinholes during baking.

Therefore, after oiling and before storage, it is common practice to:

• Assign dedicated personnel
• Wipe both sides of the stamped parts
  to ensure a uniform and controlled oil film prior to painting.

(2) Rework and Grinding Safety for Aluminum Parts

During rework operations such as grinding, aluminum dust poses a significant explosion risk. When dust concentration exceeds approximately 30 g/m³, explosive conditions may occur.

To mitigate this risk, aluminum rework areas should incorporate:

• Enclosed grinding environments
  to isolate the operation from external dust and ignition sources
• Efficient dust extraction systems
  to maintain safe air quality and prevent dust accumulation

Key operational requirements include:

• Air exchange rate of at least 50 air changes per hour within the grinding area
• Air velocity within ducts not less than 20 m/s

These measures help ensure that dust concentration remains below the lower explosive limit (LEL), significantly improving workplace safety.

Stamping Process Considerations for Aluminum Sheets

(1) Forming Characteristics

Due to inherent material properties, aluminum sheets are prone to several forming defects during stamping, including:

• Cracking
• Springback
• Surface galling or denting

To address these challenges, stamping dies must be carefully designed from the outset to mitigate potential defects. Key design strategies and process controls are summarized in Table 3.

Table 3. Key Die Design Parameters for Aluminum Sheet Forming

Category	Steel plate	Aluminum sheet
Product Corner Radius /mm	Rₘᵢₙ 0.2	Rₘᵢₙ5
Drawing Depth	R > H	R > 1.5H
Draft Angle /°	min 8	min15
Punch-Die Corner Radius /mm	Rₘᵢₙ 6	Rₘᵢₙ10
Draw Bead Corner Radius /mm	Rₘᵢₙ 1	Rₘᵢₙ 2
Thinning Rate /%	max 20	max14

(1) Hidden Cracking (Subsurface Damage)

Compared to steel, aluminum requires stricter design control over parameters such as corner radii and forming depth during product development.

In simulation analysis, the allowable thinning rate for aluminum sheets is typically limited to within 14%, whereas steel components can tolerate up to 20%.

Failure to control thinning may result in subsurface damage or hidden cracking, which can compromise part integrity during subsequent forming or service.

(2) Springback Control in Stamping

Springback is a critical issue in aluminum stamping due to the material’s lower elastic modulus. To mitigate this effect, die design typically incorporates:

• Surface compensation (geometry compensation)
• Restraining flanging (binder-assisted flanging processes)

As shown in Figure 4, based on CAE-predicted springback behavior for a vehicle hood outer panel, targeted compensation was applied in key areas, including:

• Front bumper joining region
• Headlamp joining region
• Rear windshield joining region

In addition, based on prior experience with center panel sinking, extra compensation was introduced in the central region of the hood. The specific compensation values are illustrated in Figure 4(a).

Through a combination of die surface compensation and controlled flanging techniques, the dimensional accuracy of peripheral mating surfaces can generally be maintained within ±0.5 mm.

Figure 4. OP10 Surface Compensation and OP30 Binder Flanging for a Hood Outer Panel

Control of Burrs and Debris in Aluminum Trimming Operations

Due to its relatively brittle behavior during shearing, aluminum tends to fracture prematurely in trimming operations. As a result, when the upper die cutting edge penetrates the material, the uneven fracture surface can lead to a secondary shearing effect, as illustrated in Figure 5.

In addition, aluminum particles exhibit a strong adhesion tendency. During repeated trimming cycles, fine aluminum debris can accumulate and disperse across the die surface, leading to:

• Surface impressions on parts
• Increased rework requirements
• Reduced overall production quality

To minimize debris generation and improve process stability, the following measures are commonly adopted after die tryout:

• DLC (Diamond-Like Carbon) coating applied to cutting components such as:
  • Inserts
  • Punches
  • Die sleeves
• Chrome plating applied to forming components, including:
  • Draw dies (male and female)
  • Binder surfaces
  • Flanging blocks

These surface treatments help reduce friction, prevent material adhesion, and improve tool durability, thereby enhancing overall stamping quality.

Figure 5. Fracture States of Steel and Aluminum at Varying Penetration Depths

Flanging and Hemming Radius Control for Aluminum Sheets

The flanging and hemming performance of aluminum sheets is typically evaluated using bending tests.

A key parameter used is the hemming factor, defined as:

• f = Rₘᵢₙ / t

where Rₘᵢₙ is the minimum bending radius and t is the sheet thickness. This parameter is also referred to as the minimum relative bending radius.

A smaller f value indicates better flanging and hemming performance of the aluminum sheet.

During testing, the point at which the first unacceptable crack appears is defined as the “First Crack”, and the corresponding f value is used to characterize the material’s bending limit.

The primary factors influencing the f value include:

• Pre-strain (tensile strain)
• Storage time (aging effect)

Test results for different strain levels and storage durations are summarized in Table 4.

Table 4. Hemming Factor (f) as a Function of Aging Time and Strain

Time limit (months)	Pre-stretching amount	Bending limit Rₘᵢₙ / t	Bending limit Rₘᵢₙ / t
Time limit (months)	Pre-stretching amount	0° direction	90° direction
3	0	0.33	0.22
	5	0.44	0.33
	7	0.56	0.44
	10	0.60	0.50
	15	0.67	0.56
5	0	0.44	0.44
	5	0.50	0.44
	7	0.67	0.56
	10	0.67	0.56
	15	0.78	0.67

Surface Defects in Flanging and Hemming

Due to the relatively low elongation of aluminum sheets, defects such as orange peel surface texture and cracking are likely to occur at the flanging radius (R) and hemming radius (R) during stamping, as illustrated in Figure 6.

Based on the morphology of the flanged edge, the surface quality is typically classified into four levels, as shown in Figure 7.

In practice, acceptable quality standards generally require Level 2 or above.

The main factors affecting flanging cracks, along with corresponding countermeasures, are summarized as follows.

Figure 6. Typical Cracking in Flanged Aluminum Parts

Figure 7. Classification of Flanging Morphology at Different Quality Levels

Figure 8. Common Hemming Types for Aluminum Sheet Components

Key Factors and Countermeasures for Flanging and Hemming Cracks

(1) Factors Affecting Flanging Cracks and Countermeasures

To reduce the risk of flanging cracks in aluminum stamping, die design and surface quality must be strictly controlled:

• Flanging radius (R):
  The punch radius should satisfy R ≥ 1.6t
• Flanging clearance:
  Typically controlled at 1.1t
• Surface treatment of flanging blocks:
  • Chrome plating is recommended
  • Surface roughness: Ra ≤ 0.6 μm
  • Hardness: ≥ 60 HRC
• Material flow surfaces (R zones of punch and die):
  Should be as smooth as possible, free from defects such as:
  • Hard spots
  • Sharp edges
  • Tooling marks or mismatches

These measures help ensure smooth material flow and reduce localized stress concentrations that can lead to cracking.

(2) Factors Affecting Hemming Cracks and Countermeasures

Hemming cracks are influenced by both material properties and process design:

• Hemming type selection:
  A “teardrop hemming” profile is generally preferred over “flat hemming”, as it reduces strain concentration
• Hemming radius (R):
  Should be as large as possible without affecting appearance or assembly requirements
  • Typically, the hemming factor should satisfy f ≥ 0.65
• Thinning control:
  • Thinning at the flanging area
  • Thinning during hemming

Excessive thinning significantly increases the risk of cracking during hemming operations.

Aging Behavior of Aluminum Sheets

6xxx series aluminum alloys exhibit noticeable aging characteristics.

At room temperature:

• Yield strength typically increases by 20–25 MPa within the first 3 months
• After 6 months, the rate of change gradually stabilizes

This behavior is illustrated in Figure 9.

Storage conditions—particularly temperature and duration—have a significant impact on material performance. Improper storage can lead to defects such as:

• Orange peel surface
• Cracking during flanging and hemming

Production Recommendations for 6xxx Aluminum Alloys

To ensure stable production performance, the following practices are recommended:

• Complete stamping and welding within 6 months after heat treatment
• During early process development:
  • Optimize die design
  • Increase hemming robustness
  • Expand the manufacturing process window

These measures help mitigate the effects of aging and improve overall forming reliability.

Figure 9. Mechanical Property Changes of 6XXX Aluminum Sheets over Time

Figure 10. Lüders Lines in 5XXX Alloys and Piobert Lines in 6XXX Alloys

Storage Effects and Surface Defect Control in Aluminum Sheets

Although 5xxx series aluminum alloys do not exhibit significant aging effects, prolonged storage can still lead to unstable material flow during deep drawing, increasing the risk of subsurface damage and cracking during stamping.

For 5xxx sheets with extended storage time, it is common practice to:

• Increase lubrication levels, or
• Apply pre-oiling before production,

to mitigate defects such as hidden damage and cracking during forming.

Control of Piobert Lines and Lüders Lines

Compared to steel, aluminum alloys generally have larger grain sizes, as grain growth during processing is influenced by multiple metallurgical factors. In contrast, steel typically exhibits finer and more uniform grains.

Piobert lines are a surface defect commonly observed in 6xxx series aluminum alloys. They are caused by non-uniform grain size or uneven distribution of inclusions, leading to localized deformation during forming. These defects are often associated with the accumulation of cube-oriented grains, resulting in inconsistent surface appearance after painting.

During stamping, if grain size distribution is uneven, deformation along the rolling direction may become non-uniform, producing visible Piobert lines. This defect cannot be fully eliminated during stamping and can only be mitigated through process optimization, such as reducing material inflow.

Lüders lines, on the other hand, are typical of 5xxx series (Al–Mg) aluminum alloys in the annealed condition. They result from localized yielding during deformation, which produces band-like surface irregularities.

Similar to Piobert lines, Lüders lines cannot be completely avoided during stamping. For this reason, 5xxx alloys are primarily used for inner panels, where surface appearance requirements are less stringent.

Conclusion

Due to inherent material characteristics, aluminum sheets present multiple challenges in stamping applications, including:

• Limited formability
• Aging effects
• Debris generation during trimming
• Springback
• Cracking during hemming

As a result, stamping processes, die design, and hemming operations must be carefully adapted to accommodate the unique behavior of aluminum materials.

Based on mass production applications of components such as hood inner and outer panels, tailgate panels, and floor structures in a specific vehicle program, a series of process optimizations and technical improvements have achieved the following results:

1. Process stabilization at the source:
   Key issues such as sheet separation, cleaning, debris control, and cracking were effectively resolved.
   • Outer panels: FTC ≥ 93%, ASPM ≥ 7.5
   • Inner panels: FTC ≥ 99%, ASPM ≥ 8.5
2. Extended material usability:
   Scrap issues caused by long-term storage of 5xxx and 6xxx aluminum sheets were eliminated, enabling stable stamping and hemming performance for up to 18 months of storage.
3. Integrated process optimization:
   Aluminum stamping and hemming are highly complex processes requiring coordinated optimization across:
   • Die process design
   • Tooling structure and maintenance
   • Stamping parameters
   • Hemming processes

Only through system-level control and continuous optimization can stable production quality and efficiency be achieved.

For projects requiring custom aluminum materials, small-batch supply, or fast turnaround , MOOPEC provides flexible and efficient solutions tailored to your needs. Contact us today to explore how we can support your production.