The causes of tiger skin and orange peel patterns on aluminum discs used in kitchenware during stamping and stretching

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The causes of tiger skin and orange peel patterns on aluminum discs used in kitchenware during stamping and stretching

1. Introduction: Industrial Impacts of Surface Texture Defects on Aluminum Discs for Kitchenware

Aluminum discs for kitchenware (mainly made of 3003, 5052, and 1100 alloys, with a thickness of 0.8-3.0mm) often develop two typical surface defects after stamping and drawing: “tiger stripes” (periodic, strip-shaped light and dark patterns with a spacing of 2-5mm) and “orange peel” (irregular, uneven rough surfaces with a roughness Ra >1.6μm). Their hazards are reflected in three aspects:
  1. Appearance Failure: Failure to meet the industry standard for kitchenware surfaces (“no visible patterns, Ra ≤0.8μm”) (e.g., GB/T 32073-2015 Aluminum and Aluminum Alloy Kitchenware), with a maximum defect rate of 25%;
  1. Performance Risks: Oil stains and microorganisms easily accumulate in the textured areas, and local stress concentration (stress in tiger stripe areas is 30% higher than in normal areas) reduces the corrosion resistance of kitchenware (salt spray test life shortened by 40%);
  1. Cost Losses: Defects increase the rework rate, raising the unit processing cost by 15-20% (e.g., a kitchenware factory suffers annual losses exceeding 2 million yuan due to orange peel).
The essence of these two defects lies in uneven plastic deformation of the material, but their formation mechanisms differ, requiring precise traceability from multiple dimensions. Notably, analyzing these defects is a key part of addressing Problems in processing aluminum discs for kitchenware.

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2. Definition and Feature Differentiation of Tiger Stripes and Orange Peel

(1) Tiger Stripes

  • Appearance Features: Periodic light and dark stripes distributed along the drawing direction, mostly appearing on deep-drawn parts (e.g., pot sidewalls). The stripe spacing decreases as the drawing depth increases (2mm spacing at 50mm depth, 1.2mm spacing at 80mm depth);
  • Testing Indicators: Visible to the naked eye (observed at a 45° angle under natural light), with a thickness deviation >8% in stripe areas (≤5% in normal areas). Metallographic observation shows that grain orientation in stripe areas is arranged in a directional manner;
  • Typical Scenario: Highly prevalent when processing deep-drawn pots (drawing ratio >2.0) using 3003 O-temper aluminum discs.

(2) Orange Peel

  • Appearance Features: Irregular uneven rough surfaces, similar to orange peel texture, mostly appearing on shallow-drawn parts (e.g., frying pan bottoms) or stamped parts, with an uneven height of 0.05-0.2mm;
  • Testing Indicators: Surface roughness Ra >1.6μm (qualified kitchenware requires Ra ≤0.8μm), with a distinct granular feel when touched. Microscopic observation reveals a large number of tiny protrusions (0.1-0.5mm in diameter) on the surface;
  • Typical Scenario: Highly prevalent when processing thin-walled frying pans (thickness <1.2mm) using 5052 H14-temper aluminum discs.

3. Core Causes of Tiger Stripes: Periodic Stress Fluctuations and Directional Deformation

(1) Directional Differences in Material Microstructure

  1. Uneven Grain Orientation: During the rolling of aluminum discs, uneven rolling speed (e.g., ±5% fluctuation in cold rolling mill speed) causes grains to align directionally along the rolling direction (texture degree >0.8). During stamping and drawing, directional grains easily undergo “batch slip” in the stress direction, forming periodic deformation bands. When the spacing of these deformation bands interferes with the light reflection wavelength (400-760nm), alternating light and dark tiger stripes appear.
Example: Due to cold rolling speed fluctuations (20m/min → 22m/min) of 3003 O-temper aluminum discs in a factory, the texture degree reached 0.85, and the incidence of tiger stripes in deep-drawn pots increased from 5% to 32%.
  1. Periodic Distribution of Second-Phase Particles: In 3003 aluminum alloy, AlFeSi second-phase particles (1-3μm in diameter) may form periodic aggregates (2-5mm spacing) during rolling. During drawing, stress concentration easily occurs in the matrix around the particles, forming deformation stripes consistent with the particle distribution period. When the volume fraction of particles >1.2%, the incidence of tiger stripes increases significantly (28% incidence at 1.5% volume fraction, only 8% at 1.0%).

(2) Periodic Fluctuations in Process Parameters

  1. Pulsating Changes in Drawing Speed: If the pressure of the hydraulic system in a hydraulic drawing machine is unstable (±0.5MPa fluctuation), the drawing speed will change in a pulsating manner (e.g., 1.0m/s → 1.2m/s → 1.0m/s). When the speed rises sharply, the local deformation rate of the material exceeds the dynamic recovery rate (approximately 0.8m/s for 3003 O-temper), forming “over-deformation bands”; when the speed drops sharply, deformation slows down to form “weak deformation bands”. The alternating appearance of these bands forms tiger stripes.
Laboratory Data: When the drawing speed fluctuates by ±10%, the incidence of tiger stripes increases from 6% to 21%.
  1. Periodic Imbalance of Blank Holder Force: If the blank holder force fluctuates periodically with the stamping stroke (e.g., ±15% force fluctuation caused by wear of the cam-type blank holder mechanism), the flow speed of the aluminum disc edge material will change periodically. Fast flow speed forms “thick bands”, while slow flow speed forms “thin bands”. The thickness difference leads to different light reflectivity, presenting stripe patterns. Example: When the blank holder force fluctuates from 5000N to 5750N, the thickness deviation of the pot sidewall increases from 5% to 12%, resulting in obvious tiger stripes.

(3) Periodic Defects of Molds

  1. Periodic Wear of Mold Surfaces: If the punch surface has periodic scratches (spacing consistent with the stamping stroke, approximately 3-5mm) due to insufficient lubrication, the scratches will be “copied” onto the aluminum disc surface during drawing, forming mechanical tiger stripes. When the mold surface roughness Ra increases from 0.4μm to 1.2μm, the incidence of such tiger stripes rises from 3% to 18%.
  1. Periodic Poor Ventilation in Mold Cavities: If the mold vent holes are distributed periodically (e.g., one φ1mm hole every 4mm) and some holes are blocked, the cavity pressure will change periodically (-0.01MPa → -0.03MPa) during drawing, leading to differences in local forming density of the material and forming light and dark stripes.

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4. Core Causes of Orange Peel: Local Plastic Inequality and Micro-Protrusions

(1) Material Aspect: Uneven Microstructure and Hard Particle Distribution

  1. Excessive Difference in Grain Size: Improper annealing process of aluminum discs (e.g., 3003 O-temper annealed at 320℃ < standard 340-360℃, holding time 0.5h < standard 1.5h) leads to uneven grain size (maximum grain 50μm, minimum grain 10μm). During drawing, large grains easily undergo “intragranular slip”, while small grains are difficult to deform due to high deformation resistance, forming an orange peel-like surface with “large grain protrusions and small grain depressions”.
Measured Data: When the grain size difference >40μm, the incidence of orange peel reaches 45%; when the difference <20μm, the incidence is only 12%.
  1. Presence of Hard Particles and Inclusions: Hard particles formed by elements such as Fe and Si in aluminum alloys (e.g., AlFeSi phase, hardness HV180, matrix hardness HV30) or foreign inclusions (e.g., aluminum oxide particles, 5-10μm in diameter) are difficult to deform with the matrix during drawing and will “push up” the surrounding matrix to form tiny protrusions. When the hard particle content >0.7% (3003 alloy standard ≤0.6%), the incidence of orange peel increases from 15% to 38%.
  1. Uneven Thickness of Oxide Film: Uneven thickness of the oxide film on the aluminum disc surface (5μm → 12μm) due to improper pretreatment (e.g., alkali washing time fluctuation 10s → 20s) leads to high deformation resistance in thick film areas and easy over-deformation in thin film areas, forming an uneven surface. In a factory, due to uneven temperature in the alkali washing tank (50℃ → 60℃), the oxide film thickness difference reached 8μm, and the orange peel rate exceeded 40%.

(2) Process Aspect: Local Stress Concentration and Lubrication Failure

  1. Excessive Local Drawing Ratio: During stamping and drawing, if the local curvature of the mold cavity is too large (e.g., R=2mm < standard 5mm at the pot handle connection), the local drawing ratio will reach 2.8 (far exceeding the 3003 O-temper limit of 2.2). Local “plastic over-deformation” of the material forms wrinkled protrusions, which overlap to present orange peel. Example: When the frying pan edge fillet R=3mm, the local drawing ratio is 2.5, and the orange peel rate is 28%; when R=6mm, the drawing ratio is 1.8, and the rate drops to 9%.
  1. Local Friction Difference Caused by Uneven Lubrication: Uneven lubricant spraying (e.g., local missed spraying) increases the friction coefficient from 0.05 (normal) to 0.18 (unlubricated areas). The material in unlubricated areas directly rubs against the mold, producing “drag deformation” and forming irregular protrusions. In a kitchenware factory, due to blockage of the automatic sprayer nozzle, the lubrication coverage rate decreased from 98% to 85%, and the orange peel rate increased from 12% to 35%.
  1. Local Insufficiency of Blank Holder Force: Parallelism deviation of the blank holder (>0.1mm/100mm) causes local insufficient blank holder force (e.g., 3000N < standard 5000N in a certain area). The material in this area flows too fast, forming “accumulated protrusions” that create height differences with surrounding normal areas, presenting an orange peel shape.

(3) Mold Aspect: Surface Roughness and Cavity Defects

  1. Excessive Mold Surface Roughness: Insufficient polishing of the die surface (Ra=1.6μm > standard 0.4μm) causes its rough surface to be “imprinted” onto the aluminum disc surface, forming orange peel consistent with the mold texture. Measured Results: When the mold Ra=0.8μm, the finished aluminum disc Ra=1.2μm (orange peel); when Ra=0.2μm, the finished product Ra=0.6μm (qualified).
  2. Tiny Cavities in Mold Cavities: Tiny cavities remaining in the mold cavity due to processing errors (e.g., CNC milling residual tiny depressions, depth 0.05mm) prevent the material from filling the depressions during drawing, forming “reverse protrusions”. The superposition of multiple depressions presents an orange peel shape. In a mold factory, due to milling cutter wear, the cavity depression rate reached 15%, leading to an orange peel rate of over 30% in kitchenware.

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5. Industry Case Verification: Cause Identification and Solutions

Case 1: Tiger Stripes on Deep-Drawn Pots with 3003 O-Temper Aluminum Discs

  • Problem Phenomenon: Light and dark stripes with 3mm spacing on pot sidewalls, 28% defect rate;
  • Cause Investigation:
    1. Cold rolling mill speed fluctuation ±8%, aluminum disc texture degree 0.88 (standard ≤0.7);
    1. Hydraulic drawing machine pressure fluctuation ±0.8MPa, pulsating drawing speed;
  • Verification Measures: Adjust cold rolling speed stability to ±2%, replace hydraulic system seals (pressure fluctuation reduced to ±0.2MPa);
  • Result: Tiger stripe rate reduced to 6%, meeting industry standards.

Case 2: Orange Peel on Shallow-Drawn Frying Pans with 5052 H14-Temper Aluminum Discs

  • Problem Phenomenon: Frying pan bottom Ra=2.2μm, rough to the touch, 35% defect rate;
  • Cause Investigation:
    1. Annealing temperature 310℃ (below standard 370-390℃), grain size difference 55μm;
    1. Mold surface Ra=1.2μm (exceeding standard);
  • Verification Measures: Adjust annealing process (380℃ × 2h), polish mold to Ra=0.2μm;
  • Result: Orange peel rate reduced to 10%, Ra stabilized at 0.7-0.8μm.

6. Cause-Oriented Prevention Directions (Extended Application)

  1. Material End: Control Fe content in 3003 alloy ≤0.6%, grain size difference after annealing ≤20μm, oxide film thickness 5-8μm (deviation ±1μm);
  1. Process End: Maintain drawing speed stability ±5%, blank holder force fluctuation ±5%, lubricant coverage rate ≥98%;
  1. Mold End: Ensure mold surface Ra ≤0.4μm, no cavities in the cavity, uniform vent hole spacing (≤3mm) and no blockage.
These prevention measures are crucial for solving Problems in processing aluminum discs for kitchenware and improving product quality.

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7. Conclusion

The core cause of tiger stripes in the stamping and drawing of aluminum discs for kitchenware is uneven deformation caused by periodic factors (grain orientation, process fluctuations, mold periodic defects); while orange peel is caused by micro-protrusions induced by non-periodic factors (uneven grains, hard particles, local process imbalance). Tracing the causes of these two defects requires integrating material microstructure, process parameters, and mold conditions. Through “quantitative control + targeted adjustment”, the defect rate can be effectively reduced, providing protection for the surface quality of aluminum kitchenware products. Addressing these texture defects is a key step in comprehensively resolving Problems in processing aluminum discs for kitchenware.

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