Stress Relief Annealing of 3003 Aluminum Discs for Nut Can Bottoms: Controlling Bulging under High-Temperature Filling Conditions

Abstract

This white paper explores the thermal-mechanical behavior and process control strategies of 3003 aluminum discs for nut can bottoms under high-temperature filling conditions. A recurring issue in the nut canning industry is thermal bulging or distortion at the bottom after filling with hot contents (typically 80–95 °C). This deformation compromises product integrity, consumer perception, and packaging safety.

To address this, the study investigates the role of stress relief annealing in minimizing residual stress accumulation within the 3003 alloy microstructure. By optimizing temperature, time, and cooling rates, manufacturers can effectively reduce bulging without sacrificing hardness or formability. The paper combines theoretical models, metallurgical principles, and empirical data derived from production-scale trials.

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1. Introduction

As the global nut and snack packaging market continues to expand, lightweight metal packaging has become a preferred choice for high-end and sustainable applications. Aluminum’s advantages—light weight, corrosion resistance, and excellent formability—make it a key material for nut can bottoms.

However, filling processes typically involve hot sealing and sterilization steps that expose can bodies and bottoms to high temperatures for extended durations. The mismatch between thermal expansion and pre-existing residual stresses can lead to thermal bulging, which disrupts the airtight seal and can deform the product geometry.

The objective of this white paper is to identify process-level solutions for this issue through stress relief annealing optimization. The chosen alloy, 3003 aluminum, is an Al-Mn series material with moderate strength, excellent corrosion resistance, and good plasticity—properties ideal for forming and stamping applications.

The focus of this study is to:

• Quantify how stress relief annealing affects internal stress distribution;
• Define optimal annealing parameters to minimize bulging;
• Ensure industrial feasibility with high productivity and consistent quality;
• Provide technical guidelines under ASTM B209, GB/T 3880, and YBB 00152002 standards.

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2. Material Overview: 3003 Aluminum Alloy

2.1 Alloy Composition

3003 aluminum alloy primarily consists of aluminum with manganese as the main alloying element, typically in the range of 1.0–1.5 %. Trace amounts of copper (0.05–0.2 %) are also included to enhance strength slightly.

This composition ensures a fine, stable grain structure that maintains strength after cold working and allows stress relief annealing without recrystallization cracking.

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2.2 Mechanical and Physical Properties

The softened (O) temper condition is ideal for deep drawing and forming into can bottoms. However, residual stress and localized strain from the stamping process must be carefully treated to avoid deformation under heat.

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3. Mechanism of Bulging Under High-Temperature Filling

During hot filling, nut cans are exposed to temperatures between 80–100 °C. The bottom panel, composed of thin 3003 aluminum discs, experiences nonuniform thermal expansion.

The following factors contribute to bulging:

1. Residual Stress from Cold Work:
   Stamping and deep drawing introduce compressive and tensile stresses across the disc thickness.
2. Thermal Gradient during Filling:
   The inner surface rapidly heats up while the outer remains cooler, creating differential expansion.
3. Insufficient Stress Relief:
   If annealing is incomplete, retained stress acts synergistically with thermal stress, pushing the bottom outward.
4. Inadequate Thickness or Grain Structure:
   Nonuniform grains or weak boundaries encourage local yielding under pressure.

Finite element models and metallographic analysis reveal that the bulging stress tends to peak at 70–90% of yield strength for unannealed samples, while annealed ones remain below 40%—indicating the effectiveness of stress relief.

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4. Fundamentals of Stress Relief Annealing

Stress relief annealing aims to reduce internal stresses without significant grain growth or loss of strength. For 3003 aluminum discs for nut can bottoms, the critical temperature range lies between 250–380 °C.

4.1 Principle of Recovery and Recrystallization

• Recovery Stage (250–310 °C):
  Dislocation rearrangement and annihilation occur, reducing residual stress without altering grain morphology.
• Partial Recrystallization (310–360 °C):
  New grains nucleate along high-energy boundaries, enhancing ductility and stress uniformity.
• Over-Annealing (>380 °C):
  Excessive grain growth weakens structural integrity and reduces mechanical resistance to bulging.

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4.2 Key Control Parameters

Adherence to these parameters aligns with GB/T 3190, ASTM B918, and YBB 00202003 process guidelines.

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5. Experimental Methodology

Industrial-scale trials were conducted at Shenzhen Huawi Aluminum Co., Ltd. using 0.65 mm-thick, 95 mm-diameter 3003 aluminum discs.

5.1 Experimental Setup

• Furnace: Continuous belt-type annealing furnace with PID control
• Atmosphere: Nitrogen + 0.5% hydrogen
• Measurement: Strain gauge and optical flatness detection
• Filling Simulation: 95 °C deionized water for 2 minutes, sealed immediately

5.2 Sample Matrix

5.3 Results Discussion

Samples annealed between 320–340 °C exhibited the lowest bulging (<1 mm). Temperatures above 370 °C caused microstructural coarsening, resulting in reduced mechanical stability.

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6. Microstructural Analysis

Optical microscopy and SEM analysis revealed that pre-annealed samples had elongated grains (aspect ratio >2.5), while optimally annealed samples displayed equiaxed grains (~15–20 µm).

Residual stress mapping via X-ray diffraction confirmed:

• As-rolled: 38–42 MPa residual stress
• After annealing at 330 °C/60 min: 12 MPa
• Reduction ratio: 70%

This correlates directly with the observed improvement in flatness and bulging resistance.

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7. Process Optimization Model

An empirical equation was developed to predict optimal annealing temperature (T_opt) as a function of cold reduction ratio (R) and target stress (σ_target):

[T_{opt} = 290 + 0.9R + 0.8\sigma_{target}]

Where R is in percent and σ_target in MPa.

Example:

For R = 35% and σ_target = 12 MPa,
[T_{opt} = 290 + 0.9(35) + 0.8(12) = 343.6 °C]

This correlates with the experimental optimum range of 320–340 °C.

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8. Industrial Application and Validation

8.1 Integration into Manufacturing

Industrial lines at Huawi Aluminum implemented the optimized annealing protocol:

• Continuous annealing furnace set at 335 °C
• Line speed: 1.2 m/min
• Cooling with controlled air flow (8 °C/min)

Post-process inspection showed a 92% reduction in bulging defect rate and a 12% improvement in dimensional stability.

8.2 Performance under Real Filling Conditions

• Hot fill: 92 °C peanut oil mixture
• Holding time: 3 min
• Cooling: natural air convection

No bulging observed in >98% of samples after 24 h storage.

8.3 Economic Impact

Energy efficiency increased by 8% due to reduced over-annealing, while product rejection rate decreased from 3.4% to 0.3%.

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9. Discussion

The findings confirm that the stress relief annealing mechanism significantly mitigates thermal deformation by stabilizing dislocation structures and refining grain boundaries.

Key Observations:

1. Bulging is mainly governed by residual stress magnitude, not alloy chemistry.
2. Thermal gradients during filling amplify pre-existing stress fields.
3. Controlled annealing (320–340 °C) delivers the optimal compromise between stress relief and mechanical retention.
4. Industrial scaling of this process enhances reliability and brand consistency.

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10. Conclusions

1. Residual Stress Reduction:
   Optimized stress relief annealing at 330 ± 10 °C for 45–60 min reduces internal stress by ~70%.
2. Bulging Control:
   The bulge height can be limited to <1.0 mm, ensuring visual and structural integrity.
3. Microstructure Stability:
   Fine equiaxed grains enhance yield strength uniformity while maintaining good formability.
4. Industrial Feasibility:
   Continuous annealing furnaces and controlled cooling systems ensure reproducibility at scale.
5. Economic Benefits:
   Implementation saves approximately USD 0.018 per can through reduced defects and improved throughput.

This optimized process enables 3003 aluminum discs for nut can bottoms to meet international quality benchmarks and extend their applicability in high-temperature filling environments.