0.006mm Aluminum Foil Pinholes: Causes and Prevention of Non-Metallic Inclusions in Aluminum Matrix

HW-A. Introduction: Non-Metallic Inclusions – The “Innate Culprit” of 0.006mm Aluminum Foil Pinholes

Due to its ultra-thin nature (only 1/10 the diameter of a human hair), 0.006mm double-zero aluminum foil (thickness tolerance ±5%, grain size ≥ Grade 9) is extremely sensitive to non-metallic inclusions in the aluminum matrix. These aluminum-insoluble compound phases disrupt matrix continuity during 12–15 passes of cold rolling (cumulative deformation >99.2%), ultimately forming full-thickness 0.006mm aluminum foil pinholes. Industry data shows that 70% of pinhole defects in 0.006mm aluminum foil caused by raw material issues are directly attributed to non-metallic inclusions, with the equivalent diameter of these pinholes typically ranging from 20–30μm – precisely touching the critical control threshold (≤30μm) for 0.006mm aluminum foil pinholes in food packaging (GB/T 28118-2011) and electronic packaging (GB/T 36363-2018). Therefore, analyzing the types, formation mechanisms of non-metallic inclusions, and their correlation with 0.006mm aluminum foil pinholes is crucial for realizing source control of pinholes in this specific foil grade.

HW-B. Non-Metallic Inclusions in Aluminum Matrix: Three Core Causes of 0.006mm Aluminum Foil Pinholes

Non-metallic inclusions in aluminum used for 0.006mm foil (Grade 1070/1235, Al ≥99.35%) are categorized into three main types – oxides, carbides, and nitrides (total content ≤0.15%) – with minor sulfides and hydrogen-induced defects (accounting for <5%, negligible impact on 0.006mm aluminum foil pinholes). Each type differs significantly in its mechanism of causing 0.006mm aluminum foil pinholes and the resulting pinhole characteristics:

(A) Oxide Inclusions: Primary Source of 0.006mm Aluminum Foil Pinholes (>60% Contribution)

Oxide inclusions are dominated by α-Al₂O₃ (corundum phase), with small amounts of γ-Al₂O₃ and SiO₂. They are inevitable products of aluminum melt oxidation, and their volume fraction directly determines the density and size of 0.006mm aluminum foil pinholes.

1. Formation Mechanism and Correlation with 0.006mm Aluminum Foil Pinholes

• Entrainment of Surface Oxide Films: During melting (680–720°C), a double-layer oxide film forms on the aluminum melt surface (inner γ-Al₂O₃: 1–2μm; outer α-Al₂O₃: 3–5μm). If stirring is uncontrolled (speed >50r/min), the oxide film breaks into 10–50μm flaky aggregates. When cold-rolled to 0.006mm, these aggregates form “interface separation zones” due to low interfacial bonding energy with the aluminum matrix (0.8J/m²), ultimately evolving into clustered 0.006mm aluminum foil pinholes (diameter 25–30μm, 3–5 pinholes per cluster).

• Formation via Aluminum-Water Reaction: Micro-leakage in the chill roll cooling water circuit (>0.5mL/min) triggers a reaction between water and the aluminum melt, generating 5–10μm spherical α-Al₂O₃ particles. These inclusions form isolated 0.006mm aluminum foil pinholes (diameter 20–25μm, randomly distributed) in the final foil, as their high surface energy (1.2J/m²) maintains their discrete morphology during rolling.

• Inherent Inclusions from Raw Materials: 15–30μm blocky α-Al₂O₃ inclusions in low-purity aluminum ingots (Al <99.7%) – if not removed via refining – scratch the 0.006mm aluminum foil matrix during cold rolling, forming scratched pinholes (diameter 30–35μm, plow-like defects at edges).

2. Industry Cases: 0.006mm Aluminum Foil Pinholes Caused by Oxide Inclusions

• Food Packaging Scenario: In 2023, a North China factory producing 0.006mm aluminum foil for chilled meat packaging experienced excessive entrainment of 20–30μm α-Al₂O₃ aggregates due to uncontrolled melting stirring speed (65r/min, vs. standard 20–30r/min). Final inspection showed 0.006mm aluminum foil pinhole density reached 18 pinholes/m² (standard ≤5 pinholes/m²), with 70% being clustered pinholes. After optimizing stirring parameters (30r/min) and adopting a dual-layer ceramic filter plate (20μm + 15μm, filtration efficiency 96.5%), the pinhole density of 0.006mm aluminum foil decreased to 6 pinholes/m², meeting the oxygen barrier requirement for chilled meat (O₂ transmission rate ≤0.5cc/(m²·24h·atm)).

• Pharmaceutical Packaging Scenario: An East China pharmaceutical aluminum foil factory used Grade 1070 aluminum ingots with 0.28% Al₂O₃ content (standard ≤0.15%) to produce 0.006mm PTP aluminum foil. The final product exhibited 30–35μm pinholes, leading to 3 failed batches in sterility testing (YBB 00152002-2015). After switching to aluminum ingots with Al ≥99.8% and adding vacuum refining (≤10Pa, 30min), inclusion content dropped to 0.09%, and the pinhole rate of 0.006mm aluminum foil met standards.

(B) Carbide Inclusions: Inducers of Secondary 0.006mm Aluminum Foil Pinholes (20–25% Contribution)

Carbides are primarily Al₄C₃ (hexagonal crystal system), with small amounts of carbon particles from rolling oil. Although their direct contribution to 0.006mm aluminum foil pinholes is lower than oxides, they easily trigger “secondary pinholes” during annealing, disrupting batch stability.

1. Formation Mechanism and Correlation with 0.006mm Aluminum Foil Pinholes

• Reaction with Carbonaceous Refractories: Furnace linings (Al₂O₃ ≥85%) and graphite stirrers react with the aluminum melt at >700°C, generating 5–12μm short acicular Al₄C₃. When cold-rolled to 0.006mm, these inclusions separate from the matrix to form 1–3μm micro-voids due to poor plasticity (elongation at break <0.5%), eventually expanding into fine 0.006mm aluminum foil pinholes (diameter 15–20μm, accounting for 60% of carbide-induced pinholes).

• Carbonization Contamination from Rolling Oil: When rolling oil temperature exceeds 60°C (standard ≤50°C), thermal carbonization generates 3–8μm carbon particles, which are pressed into the surface layer (1–3μm) of 0.006mm aluminum foil, forming “carbon-imprinted pinholes” (diameter 15–20μm, carbon residues on hole walls).

• Decomposition of Annealing Atmosphere: Hydrocarbon impurities (>10ppm) in the annealing furnace nitrogen decompose into carbon, which reacts with aluminum to form 2–5μm Al₄C₃ precipitates. These precipitates react with hydrogen at 280–320°C to generate methane bubbles (diameter 5–10μm), which contract into secondary 0.006mm aluminum foil pinholes (diameter 20–25μm, carbon content 0.5–1.0% on hole walls) during cooling.

2. Typical Case: Secondary 0.006mm Aluminum Foil Pinholes Caused by Carbides

An electronics factory producing 0.006mm aluminum-plastic film substrates for lithium batteries experienced hydrocarbon contamination in the annealing furnace nitrogen (rising to 35ppm), resulting in 12 secondary pinholes/m² in the final product and a 2% electrolyte leakage rate (standard ≤0.1%). XRD testing detected Al₄C₃ characteristic peaks (2θ=33.4°, 38.1°) around the pinholes, while EDS analysis showed 0.8% carbon content on hole walls (normal <0.1%). After replacing the nitrogen purification column (hydrocarbon removal rate ≥99%) and adding vacuum degassing (≤5Pa, 20min), the secondary pinhole rate of 0.006mm aluminum foil decreased to <1%, and the leakage rate was controlled at 0.05%.

(C) Nitride Inclusions: Inducers of Linear 0.006mm Aluminum Foil Pinholes (10–15% Contribution)

Nitrides are mainly AlN (hexagonal crystal system), derived from impure nitrogen or grain refiner induction. Although their contribution is low, they easily cause “linear pinholes” in 0.006mm aluminum foil, impairing mechanical properties.

1. Formation Mechanism and Correlation with 0.006mm Aluminum Foil Pinholes

• Aluminum-Nitrogen Reaction: When oxygen content in casting/rolling melting nitrogen exceeds 50ppm (standard ≤30ppm), aluminum reacts with nitrogen to form 3–8μm acicular AlN (ΔG°(700°C)=-320kJ/mol, thermodynamically spontaneous). These AlN particles distribute along grain boundaries of 0.006mm aluminum foil, hindering dislocation movement during cold rolling and increasing the stress concentration factor to >2.0, triggering “grain boundary cracking” and forming linear 0.006mm aluminum foil pinholes (length 30–40μm, width 15–20μm).

• Refiner-Induced Precipitation: Ti and Zr in Al-Ti-B grain refiners (addition >0.02%) react with nitrogen to form TiN and ZrN, which act as heterogeneous nucleation sites for AlN. This promotes AlN precipitation at grain boundaries, forming a “grain boundary nitride layer” (1–2μm). This layer easily cracks during cold forming of 0.006mm aluminum foil (5mm depth), leading to post-forming pinholes (diameter 20–25μm, cracking rate >5%).

2. Prevention Key Points: Reducing 0.006mm Aluminum Foil Pinholes Caused by AlN

• Purify nitrogen via three-stage treatment (deoxidation, dehydration, decarbonization) to achieve purity ≥99.999% and oxygen content ≤30ppm, with online monitoring using a Siemens ULTRAMAT 23 analyzer (1 measurement/minute).

• Select low-nitrogen Al-Ti-B refiners (nitrogen content <0.002%) and control addition at 0.015–0.02%. Add 0.005–0.01% Ce to form more stable CeN, inhibiting AlN precipitation.

HW-C. Detection of 0.006mm Aluminum Foil Pinholes: Focus on Inclusion Tracing

For 0.006mm aluminum foil pinholes caused by non-metallic inclusions, a “offline precise characterization + online real-time monitoring” system must be established to clarify the correlation between pinholes and inclusions:

(A) Offline Detection: Identifying Inclusion Types Causing 0.006mm Aluminum Foil Pinholes

1. Metallographic Microscopy: Cut 20mm×20mm samples from 0.006mm aluminum foil, mount and polish them (Ra ≤0.02μm), then observe using a Leica DMi8 microscope (500–1000x magnification). Clustered pinholes correspond to Al₂O₃, fine isolated pinholes to Al₄C₃, and linear pinholes to AlN. Use Image-Pro software to count the diameter and density of 0.006mm aluminum foil pinholes (error ≤5%).

2. SEM-EDS Analysis: Observe the center of 0.006mm aluminum foil pinholes using a Zeiss Sigma 300 SEM (15kV, 0.8nm resolution), and analyze element ratios with an Oxford X-Max EDS detector. An Al/O ratio ≈2:3 indicates Al₂O₃, while Al/C ≈4:3 indicates Al₄C₃, enabling precise identification of inclusion phases (detection limit ≤0.1%).

3. Laser Particle Sizing: Dissolve 0.006mm aluminum foil in 5% hydrochloric acid (50°C, 30min), collect inclusions, and test their particle size distribution using a Malvern Mastersizer 3000 to determine the main size range of inclusions causing pinholes (e.g., 20–30μm Al₂O₃).

(B) Online Monitoring: Preventing Mass Production of 0.006mm Aluminum Foil Pinholes

1. Online Melt Filtration Detection: Install a PMS Lasair III laser particle counter (0.1–100μm, 100mL/min) in the tundish before casting. Trigger an alarm when inclusions >15μm exceed 5 particles/mL to prevent inclusions from entering the cast-rolled coil and reduce 0.006mm aluminum foil pinholes at the source.

2. Finished Product Pinhole Characterization: Observe the morphology of 0.006mm aluminum foil pinholes using a KEYENCE VHX-7000 ultra-depth microscope (20–2000x magnification). Use software to automatically match a database (clustered = Al₂O₃, linear = AlN), and test 3 samples (1m² each from the head, middle, and tail of each coil) to count the proportion of each pinhole type and guide process adjustments.

HW-D. Prevention of 0.006mm Aluminum Foil Pinholes: Full-Process Inclusion Blocking

 

Targeting the formation paths of non-metallic inclusions, full-process control from “raw materials – melting – cold rolling – annealing” is required to reduce their impact on 0.006mm aluminum foil pinholes:

(A) Raw Material Control: Reducing “Innate Risks” of 0.006mm Aluminum Foil Pinholes

• Select Grade 1070 (Al ≥99.7%) or 1235 (Al ≥99.35%) aluminum ingots with non-metallic inclusion content ≤0.15% (provide ICP-MS reports, detection limit 0.001%).

• Preheat aluminum ingots to 200–250°C before furnace charging (to remove adsorbed water), and manually sort to remove oxide scales (>0.1mm) and impurities to avoid inclusion introduction.

(B) Melting and Casting: Controlling the “Main Formation Zone” of 0.006mm Aluminum Foil Pinholes

• Oxidation Control: Purge the furnace with 99.999% nitrogen (oxygen partial pressure ≤0.001MPa), control melting temperature at 680–720°C (oxidation rate increases by 15% for every 10°C rise), and use dual-layer stirrers (20–30r/min) to avoid oxide film entrainment.

• High-Efficiency Filtration: Adopt dual-stage filtration (“20μm ceramic plate + 10μm foam ceramic”) before casting, control filtration speed ≤0.5m/min (to form a stable filter cake layer), and remove 5–50μm inclusions to reduce 0.006mm aluminum foil pinholes at the source.

• Casting Parameters: Control casting speed at 1.2–1.8m/min, cooling water temperature at 30–35°C (chill roll surface temperature difference ≤5°C), and cast-rolled coil thickness at 6–8mm (to avoid excessive cold rolling deformation and inclusion cracking).

(C) Cold Rolling and Annealing: Avoiding “Secondary Formation” of 0.006mm Aluminum Foil Pinholes

• Cold Rolling Control: Use hydrogenated mineral oil as rolling oil (viscosity 2.8–3.2mm²/s at 40°C), filter via three stages (particulate content ≤5mg/L), and control oil temperature ≤50°C. Grind work rolls (Cr5Mo1V, HRC 60–62) every 1500 tons of rolling to maintain Ra 0.2–0.4μm and prevent carbon particle indentation.

• Annealing Control: Purify nitrogen via “deoxidation – dehydration – decarbonization” (hydrocarbon content ≤5ppm), control heating rate at 5–8°C/min, hold at 280–320°C for 4–5h, and perform vacuum degassing (≤5Pa, 20min) before annealing to prevent Al₄C₃-hydrogen reactions and secondary pinhole formation.

HW-E. Conclusion: Non-Metallic Inclusions – Core Prevention Targets for 0.006mm Aluminum Foil Pinholes

Seventy percent of 0.006mm aluminum foil pinholes originate from non-metallic inclusions in the it matrix. Among these, α-Al₂O₃ is the primary inducer (60% contribution), causing clustered and isolated pinholes; Al₄C₃ triggers secondary pinholes (25% contribution), disrupting batch stability; and AlN forms linear pinholes (15% contribution), impairing mechanical properties.
Enterprises must adopt a full-process solution including “high-purity raw materials (Al ≥99.7%), melting filtration (removing 5–50μm inclusions), cold rolling temperature control (oil temperature ≤50°C), and annealing degassing (hydrogen ≤0.1mL/100g Al)” to control non-metallic inclusion content below 0.15%. This will reduce the pinhole density of 0.006mm aluminum foil to <5 pinholes/m², meeting quality requirements for food, electronics, and pharmaceutical industries, and enabling breakthroughs in high-end applications (e.g., lithium battery aluminum-plastic films, pharmaceutical PTP foils).