In recent years, driven by the rapid development of electric powertrain systems and intelligent vehicle technologies, the penetration rate of new energy vehicles (NEVs) in China has increased significantly—from 14.2% in 2021 to 31.6% in 2023, and exceeding 50% for the first time in July 2024. Reducing vehicle body weight and improving the efficiency of battery energy utilization, thereby extending driving range, has become a key research direction in NEV development.

Due to its lower density and lighter weight compared with steel, aluminum sheet has become a major focus of research and application in NEVs, particularly in premium vehicle segments. However, aluminum sheets present a number of manufacturing and process-related challenges, including poor formability, pronounced aging effects, burr and chip generation during trimming, springback, and cracking during hemming operations. This paper summarizes and discusses aluminum sheet stamping production and process technologies, with the aim of providing both theoretical and practical guidance for addressing these issues in aluminum stamping applications.

Classification and Characteristics of Automotive Stamping Aluminum Sheets

Classification of Aluminum Sheets for Automotive Stamping

Aluminum sheets used for automotive stamping are typically classified into the 5XXX and 6XXX series, each with distinct characteristics:

5XXX Series Aluminum Alloys
The 5XXX series consists of Al–Mg alloys, which offer excellent formability, good corrosion resistance, and favorable weldability. However, annealed Al–Mg alloys may exhibit Lüders bands and delayed yielding during plastic deformation. As a result, they are primarily used for complex-shaped inner body panels.

Currently, commonly used grades include 5754-O and 5182-O. Some NEV manufacturers also use 5052-O to reduce costs. However, compared with 5754-O and 5182-O, 5052-O exhibits slightly inferior formability and carries a higher risk of cracking in complex stamping applications.

6XXX Series Aluminum Alloys
The 6XXX series consists of Al–Mg–Si alloys, characterized by high strength, good ductility, excellent corrosion resistance, and favorable bake-hardening properties. Common grades include 6014, 6016, 6111, and 6451. After forming and subsequent paint baking, artificial aging occurs, resulting in enhanced strength.

Grades 6014 and 6016 are commonly used for exterior body panels, while 6111 and 6451 are primarily applied in structural components. Typical aluminum grades used for different body components are shown in Table 1.

Table 1 Commonly used aluminum plate grades for automotive stamping parts

Surface Treatment Processes for Aluminum Sheets

(1) Surface Texture Types

Surface treatment processes for automotive aluminum sheets include MF (Milled Finish), EDT (Electron Discharge Texture), and EBT (Electron Beam Texture). The surface morphologies observed at 100× magnification are shown in Figure 1.

Figure 1: Surface morphology of an aluminum plate magnified to 100μm

• MF (Milled Finish): Surface roughness Ra 0.2–0.6 μm.

• EDT (Electron Discharge Texture): Produces a random textured surface that improves lubrication during stamping and eliminates directional rolling marks. Surface roughness Ra 0.7–1.3 μm.

Advantages of EDT Treatment:

1. Isotropic surface properties with no directional influence on formability.

2. Formation of lubrication pockets that provide:

   • Low and stable friction coefficients

   • Improved formability compared with MF

   • Higher stamping line efficiency

   • Reduced tool adhesion

   • More stable forming pressure

3. Improved surface quality after painting and better part fit.

• EBT (Electron Beam Texture): Produces a regular and well-defined texture pattern, offering superior forming and paint performance compared with EDT. EDT surfaces are random, while EBT surfaces exhibit discrete and ordered texture units, benefiting both forming and coating processes.

In practice, to ensure consistent vehicle paint quality, aluminum sheets for exterior panels are typically treated using the EDT process.

(2) Surface Pretreatment

To improve surface hardness, corrosion resistance, and wear resistance, automotive aluminum sheets commonly undergo titanium-zirconium (TiZr) coating pretreatment. This process uses arc ion plating or physical vapor deposition (PVD) to deposit titanium and zirconium onto the aluminum surface, forming a dense and uniform coating.

The main functions include:

1. Formation of a TiZr coating to prevent surface oxidation

2. Reduction of surface electrical resistance, improving weldability

3. Enhanced adhesion between the aluminum sheet and paint, resulting in a durable coating layer

(3) Surface Oiling Processes

To protect the aluminum surface, reduce friction, enhance lubrication, improve formability, lower the risk of micro-cracking, and extend tool life, aluminum sheets are typically pre-oiled before delivery.

Common oil types include solid oil (E1), semi-solid oil (AL200), and liquid oil (6130), as shown in Table 2 and Figure 2.

Figure 2: Commonly used oiling types on the surface of automotive aluminum plates

Table 2: Characteristics of Different Oils Commonly Used on the surface of Aluminum Plates

At present, these three oiling methods are the most widely used. European and North American manufacturers tend to prefer E1 solid oil, which requires dry cleaning systems during stamping. AL200 is compatible with both wet and dry cleaning systems and provides good lubrication for stamping operations. Oil 6130 is widely used in steel mills for pre-forming due to its low cost and is commonly applied by domestic OEMs. However, its low viscosity is less favorable for aluminum stamping formability. The characteristics of different oil types are summarized in Table 2.

Aluminum Sheet Stamping Production

Aluminum sheets are relatively soft, non-magnetic, and have limited ductility, with a maximum elongation of approximately 25% (compared to 40–50% for steel). As a result, aluminum stamping processes differ significantly from steel stamping.

Sheet Packaging

Due to their softness and chemical reactivity, aluminum sheets require stringent packaging, transportation, and storage conditions:

1. Use wooden or steel pallets, with corrugated board or cardboard placed at the bottom to absorb vibration

2. Secure sheets during transport using plastic or steel strapping to prevent relative movement, scratches, or oil stains

3. Protect edges with corner guards

4. Apply six-sided plastic film wrapping to minimize air exposure

5. Store under controlled temperature and humidity conditions where possible

Sheet Separation (Destacking)

Since aluminum is non-magnetic, conventional steel destacking methods cannot be used. Aluminum sheets are typically separated using high-pressure air knives. As standard factory compressed air pressure is approximately 0.6 MPa, booster systems are required to increase air pressure to 1.0–1.1 MPa at the destacking station.

For high-speed stamping lines, mechanical fingers are often used in conjunction with air knives to ensure reliable separation. Common air knife nozzle configurations are shown in Figure 3.

Key factors affecting destacking performance include:

1. Air pressure ≥ 1.0 MPa

2. Nozzle diameter: 1.5–2.5 mm

3. Nozzle spacing: 10–20 mm

4. Distance between air knife and sheet: 50–100 mm

5. Initial robot lifting speed: 5–15%

6. Consistent air knife height relative to the sheet during production

Sheet Cleaning and Oiling

Due to the soft material and limited formability of aluminum sheets, exterior panels are typically cleaned before stamping, while complex inner panels such as tailgate inner panels and door inner panels are usually oiled.

Aluminum Sheet Cleaning:Cleaning methods include dry cleaning and wet cleaning, depending on the oil type applied and available equipment. Solid oil coatings require dry cleaning, as wet cleaning systems may become clogged by solid oil residues, reducing drying efficiency. This is why many German OEMs, which predominantly use E1 solid oil, rely on dry cleaning systems. Domestic OEMs often use liquid oil to reduce costs, resulting in fewer constraints on cleaning equipment.

Figure 3: Common forms of air knife air nozzles

Aluminum Sheet Oiling:To ensure stable deep-drawing performance, localized oiling is often applied to deep-drawn components such as tailgate inner panels and door inner panels. Components with shallow draw depths, such as floor panels and hood inner panels, are typically stamped without additional oiling.

After oiling, excess oil may accumulate at lower edges and concave radii due to gravity. If not removed, this may cause paint defects such as pinholes during baking. Therefore, parts are usually wiped manually on both sides before storage.

Repair of Aluminum Parts

During grinding and repair operations, aluminum dust concentrations exceeding 30 g/m³ pose an explosion risk. As a result, aluminum repair areas must be enclosed and equipped with efficient dust extraction systems. Enclosures isolate the grinding area from external dust and ignition sources while ensuring effective ventilation.

To maintain dust concentrations below the minimum explosive limit, air exchange rates of at least 50 times per hour are typically required, with duct air velocities not less than 20 m/s.

Aluminum Stamping Process Technologies

Forming Characteristics

Due to material properties, aluminum sheets are prone to cracking, springback, and surface damage during forming. These risks must be addressed during die design, as summarized in Table 3.

1. Hidden Cracking: Product design requires stricter control of corner radii and draw depths compared with steel. Aluminum thinning rates are typically limited to ≤14% (vs. 20% for steel).

2. Springback Control: Die surface compensation combined with binder flange processes is commonly used. Figure 4 illustrates springback compensation for a hood outer panel based on CAE analysis. Through surface compensation and binder control, dimensional tolerances of ±0.5 mm can be achieved at mating interfaces.

Figure 4: The OP10 profile compensation and OP30 material clamping flanging structure of the outer panel of the hood of a certain model

Trimming Chip Control

Aluminum’s brittleness leads to premature fracture during trimming. Uneven fracture surfaces cause secondary shearing effects, as shown in Figure 5. Additionally, aluminum particles tend to adhere to tooling surfaces, leading to imprint defects and rework.

To reduce chip generation, DLC coatings are applied to cutting inserts, punches, and die sleeves after die tryout. Chrome plating is applied to draw dies, binder rings, and flanging blocks.

Figure 5: Fracture states of steel and aluminum at different cutting depths

Flanging and Hemming Radius Control

Flanging and hemming performance is commonly evaluated using bending tests, with the hemming factor defined as:f=Rmin⁡/tf = R_{\min} / tf=Rmin​/t

where Rmin⁡R_{\min}Rmin​ is the minimum bending radius and ttt is sheet thickness. Smaller fff values indicate better flanging and hemming performance. The first occurrence of unacceptable cracking is defined as the critical fff value. Key influencing factors include strain level and storage time. Test results are shown in Table 4.

Table 4: Edge wrapping coefficients for different durations and strains

Due to limited elongation, aluminum parts are prone to orange peel and cracking at flanging and hemming radii (Figure 6). Flanging quality is classified into four levels (Figure 7), with acceptable quality defined as Level 2 or higher. 

Figure 6: Flanging crack

Figure 7: Flanging morphology of different grades

Figure 8: Edge wrapping style

Key measures to reduce flanging cracks:

• Punch radius ≥ 1.6t

• Flanging clearance ≈ 1.1t

• Chrome plating of flanging blocks, Ra ≤ 0.6 μm, hardness ≥ 60 HRC

• Smooth material flow surfaces without sharp edges or tool marks

Key measures to reduce hemming cracks:

• Prefer “teardrop” hemming over flat hemming

• Use the largest possible hemming radius without affecting appearance or fit (typically f ≥ 0.65)

• Control thinning during flanging and hemming

Aging Behavior of Aluminum Sheets

6XXX series aluminum alloys exhibit natural aging behavior. At room temperature, yield strength increases by 20–25 MPa within the first three months, with slower changes after six months (Figure 9). Storage temperature and duration significantly affect material performance, potentially leading to orange peel or cracking during flanging and hemming.

To ensure stable production, stamping and welding of 6XXX alloys are typically completed within six months after heat treatment, with process windows expanded through die and hemming design optimization.

Figure 9: The performance trend of 6xxx aluminum plate over time

Figure 10: The appearance of the 6XXX series Luoping Line and the 5XXX series Ludus line

Although 5XXX alloys do not exhibit aging effects, prolonged storage may cause unstable drawing behavior, increasing the risk of hidden cracks. For aged 5XXX sheets, increased lubrication or pre-oiling is commonly used to mitigate defects.

Control Standards for Roping and Lüders Bands

Aluminum alloys have larger grain sizes than steel due to differences in grain growth during processing. In 6XXX alloys, roping is a surface defect caused by uneven grain size or inclusion distribution, resulting in surface waviness along the rolling direction after stamping and inconsistent paint appearance. This defect cannot be fully eliminated during stamping and can only be mitigated by reducing material flow.

Lüders bands occur in annealed 5XXX Al–Mg alloys due to localized yielding during deformation, forming band-like surface wrinkles. Since this defect cannot be avoided during stamping, 5XXX alloys are generally limited to inner panel applications.

Conclusion

Due to challenges such as poor formability, aging effects, trimming chip generation, springback, and hemming cracks, aluminum stamping requires tailored process adjustments in stamping operations, die design, and hemming processes. Through large-scale application and process optimization of hood panels, tailgate panels, and floor components for a specific vehicle model, the following results were achieved:

1. Key issues such as sheet destacking, cleaning, chip control, and cracking were addressed at the process source, achieving FTC ≥93% and ASPM ≥7.5 for exterior panels, and FTC ≥99% and ASPM ≥8.5 for inner panels.

2. Mass scrap caused by long storage times of 5XXX and 6XXX aluminum sheets was eliminated, enabling stable stamping and hemming within an 18-month window.

3. Aluminum stamping and hemming remain highly complex processes requiring integrated optimization and control of die design, tooling maintenance, stamping processes, and hemming operations.