Aluminum Alloy Foil Testing

Aluminum alloy foil inspection mainly conducts scientific testing and analysis on the chemical composition analysis, thickness and thickness deviation testing, mechanical property testing, surface wetting tension testing, pinhole degree testing and surface quality inspection, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Chemical composition analysis: Precisely determining the contents of the various elements (such as the Si, Fe, Cu, Mn, Mg and Zn) in the aluminum alloy foils is the basis of ensuring the material designation conformity and subsequent processing properties. Mainly according to the standards such as the GB/T 20975 series or ISO 2107, the ratios of the aluminum matrix and alloying elements are ensured to meet the requirements of the specific designations (such as 1100, 1235, 8011 and 8079), thereby affecting their electrical conductivity, corrosion resistance and mechanical properties.

Thickness and thickness deviation testing: The thickness is the most core dimension indicator of the aluminum alloy foils, directly affecting their barrier properties, mechanical strengths and costs. The testing usually uniformly selects the multiple points in the width directions, using the high-precision contact or non-contact thickness gauges for the measurement, and evaluates the average thicknesses and the longitudinal and transverse thickness deviations according to the standards such as GB/T 22638.1 or ASTM B479, ensuring that their uniformity meets the requirements of the subsequent deep processing (such as the lamination and coating).

Mechanical property testing: Evaluating the behavior of the aluminum alloy foils under the forces, mainly including the tensile strengths, proof strengths at the non-proportional extension (Rp0.2) and elongations after fracture. Through the universal material testing machines according to GB/T 228.1 or ASTM E8 for the tensile testing, whether the different states such as the soft (O state) and hard (H state) meet the requirements is judged. This is crucial for the stamping and forming of the battery foils and the flexibility of the food packaging foils.

Surface wetting tension testing: For the packaging aluminum foils that need the subsequent printing, coating or lamination, the surface cleanliness and adhesion are extremely critical. The dyne pens or dyne liquid method (such as GB/T 22638.7) is commonly used to quickly determine the surface wetting tension values, evaluating the effects of the surface treatments (such as the cleaning and passivation), ensuring that the inks or adhesives can spread and adhere well, preventing the problems such as the delamination and missed coating.

Pinhole degree testing: The pinholes are the tiny holes penetrating the foil materials, which will seriously affect their barrier properties, especially when used for the high-barrier packaging (such as the pharmaceuticals and foods) and battery current collectors. The testing is usually carried out on the light transmission testing tables in the dark boxes, and according to the standards such as GB/T 22638.3, the numbers and sizes of the pinholes within the specified areas are counted. It is the important indicator of evaluating the metallurgical qualities and rolling process levels of the foils.

Surface quality inspection: Through the visual inspection or optical equipment, the defects on the aluminum alloy foil surfaces are checked, including the scratches, oil stains, corrosion spots, bright lines, dark lines, wrinkling and foreign matter indentation. These defects not only affect the appearances, but also may become the weak points of the mechanical properties or cause the interruptions of the subsequent processing. The inspection standards usually refer to the technical agreements of the supply and demand parties or the relevant industry specifications.

Electrical conductivity/resistivity testing: For the aluminum foils used as the conductive materials (such as the power capacitor foils and battery current collectors), the electrical conductivity is the key performance indicator. The eddy current conductivity instruments or DC resistance measuring instruments are used, according to GB/T 12966 or ASTM B193 for the testing. The electrical conductivity is closely related to the alloy compositions, heat treatment states and grain structures, directly affecting the conductive efficiencies and heat generation of the components.

Testing Scope

Packaging aluminum alloy foils: Mainly including the flexible packaging foils for the foods, pharmaceuticals, cigarettes and daily chemical products, as well as the rigid packaging foils used to make the container lid materials and easy-tear lids. The testing emphases are the thickness uniformity, pinhole degrees, surface cleanliness (wetting tensions), hygienic indicators (heavy metal migration and microorganisms), as well as the lamination peel strengths, ensuring that they have the excellent barrier properties, safety and processing adaptability.

Electric power and electronic aluminum alloy foils: Covering the power capacitor foils, electrolytic capacitor anode foils and cable shielding foils. This type of foils has the extremely high requirements for the electrical properties, mechanical properties and geometric dimensions. The core testing items include the electrical conductivity/resistivity, tensile strengths, elongations, thicknesses and cross-section burrs (for the etched foils), to ensure their stability and long lives in the high-voltage and high-frequency environments.

Battery current collector aluminum alloy foils: Mainly used as the positive electrode (such as the lithium iron phosphate batteries) current collectors of the lithium-ion batteries. In addition to the routine thicknesses and mechanical properties, the special attention is paid to the surface roughnesses (affecting the coating adhesion), the balance of the tensile strengths and elongations (meeting the electrode sheet rolling requirements), cleanliness (low metal impurity contents to prevent the battery self-discharge), as well as the interface resistances after the coating.

Building and decorative aluminum alloy foils: Applied to the building insulation material facings, decorative panel composite layers and air conditioning ventilation ducts. The testing focuses on the weatherability (such as the UV aging resistance), corrosion resistance (salt spray testing), composite strengths with the substrates, fire resistance (such as the combustion grades), as well as the consistency of the surface gloss and colors.

Household and heat exchange aluminum alloy foils: Including the household foils (for the cooking), air conditioning heat dissipation fin foils and automotive heat exchanger foils. For the household foils, the heavy metal dissolution amounts and heat resistance need to be tested; for the heat exchanger foils, the hydrophilic or hydrophobic coating properties, corrosion resistance (copper-accelerated acetic acid salt spray testing), brazing properties and the fin forming dimension precision are emphatically tested.

Special industrial aluminum alloy foils: Such as the ultra-thin high-strength foils for the aerospace, carrier aluminum foils of the rolled copper foils for the flexible circuits and electromagnetic shielding foils. The testing scope is determined according to the specific applications, possibly involving the extreme mechanical properties (ultra-high strengths or superplasticity), special electromagnetic properties, extremely high dimension stability or vacuum properties.

Testing Methods

Spectroscopic analysis method (chemical compositions): The spark direct-reading spectrometers (OES) or inductively coupled plasma optical emission spectrometers (ICP-OES) are used for the fast and multi-element simultaneous analysis. The OES is suitable for the fast on-site screening of the block or thick foil samples; the ICP-OES needs to dissolve the samples, with the higher sensitivity, especially suitable for the trace element analysis. It is the key method of verifying the material designations and impurity control.

Weighing method (thickness): As the arbitration method of the thickness measurement, according to GB/T 22638.1. The specimens of the known areas (such as 100cm²) are selected and weighed with the precision balances. According to the density of the aluminum foils (usually taken as 2.71 g/cm³), their average thicknesses are calculated. This method has the high precision, but the process is cumbersome, mainly used for the instrument calibration and dispute resolution.

Tensile testing method (mechanical properties): According to GB/T 228.1, the electronic universal testing machines are used. The aluminum foil specimens are prepared into the specified shapes and sizes (usually the rectangular or dumbbell shapes), and the tensile forces are applied at the constant rates until the fracture. Through the force-displacement curves, the tensile strengths, yield strengths and elongations after fracture are automatically calculated. It is the basic method of evaluating the load-bearing and deformation capabilities of the material.

Dyne testing method (surface wetting tensions): A semi-quantitative fast testing method. The dyne pens or dyne liquids (unit: mN/m) of the series of known surface tensions are used to draw on the aluminum foil surfaces. If the liquid films continuously shrink within 2 seconds, the surface tensions are lower than the testing values; if the continuous liquid films can be maintained, they are higher than or equal to the values. Through the testing liquids of the different values, the surface tension ranges can be roughly determined.

Light transmission testing method (pinhole degrees): The standardized visual inspection method. The aluminum foil specimens are spread flat on the testing tables with the uniform light sources inside. In the dark room environments, the operators observe from above. The numbers of all the visible light-transmitting pinholes within the specified areas (such as 0.1 m²) are counted, and the sizes of the maximum pinholes are measured. This method has the simple equipment, but has the requirements for the environmental light and operator vision.

Eddy current testing method (electrical conductivity and thickness): A non-destructive and fast non-destructive testing method. The eddy current thickness gauges or conductivity instruments generate the high-frequency electromagnetic fields through the probe coils, inducing the eddy currents in the aluminum foils. The secondary magnetic fields generated by the eddy currents interact with the probes, and the changes are related to the electrical conductivities and thicknesses of the material. After the calibration, these two parameters can be measured simultaneously or separately, widely applied to the online testing of the production lines.

Metallographic microscope method (micro-structures): Through the sampling, mounting, grinding, polishing and chemical etching to prepare the metallographic specimens, the grain sizes, morphologies, second phase distributions and rolling fiber structures of the aluminum alloy foils are observed under the optical microscopes or scanning electron microscopes. This method is the important means of analysing the material heat treatment states, evaluating the rationality of the processing processes and studying the failure mechanisms (such as the fracture and corrosion).

Testing Standards

Domestic Standards:

1. GB/T 20975, Methods for chemical analysis of aluminium and aluminium alloys

2. GB/T 22638, Test methods for aluminium and aluminium alloy foils

3. GB/T 12966, Method of eddy current measurement of electrical conductivity of aluminium alloys

4. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

5. GB/T 10125-2021, Corrosion tests in artificial atmospheres — Salt spray tests

International Standards:

1. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

2. ISO 2107, Aluminium and aluminium alloys — Wrought products — Temper designations

3. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

4. ASTM B479, Standard specification for annealed aluminum and aluminum-alloy foil for flexible barrier, food contact, and other uses

5. ASTM E8, Standard test methods for tension testing of metallic materials

6. ASTM B193, Standard test method for resistivity of electrical conductor materials

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Direct-reading spectrometer: Used for the fast quantitative analysis of the chemical compositions of the aluminum alloy foils. The instrument excites the sample surface atoms through the spark discharge, measures the intensities of the characteristic spectral lines of the various elements, and compares with the calibration curves established by the standard samples. It can simultaneously obtain the contents of the main and trace alloy components except the gas elements within tens of seconds. It is the core equipment of the incoming raw material inspection and process control.

High-precision thickness gauge: Divided into the contact type (such as the dial gauge type and inductive type) and non-contact type (such as the radiometric, laser and eddy current types). The contact thickness gauges have the high precision, often used for the laboratory arbitration; the non-contact thickness gauges (especially the radiometric and laser types) can realize the high-speed and non-destructive online continuous measurement, monitoring the thickness fluctuations during the rolling processes in real time. They are the key equipment of ensuring the product thickness uniformity.

Electronic universal material testing machine: The core equipment of carrying out the mechanical property testing such as the tensile, peeling and tearing. Equipped with the high-precision load transducers and extensometers, the computer control systems control the crosshead movement speeds and collect the force and deformation data in real time, automatically generating the test reports. For the ultra-thin aluminum foils, the dedicated film fixtures and micro force transducers need to be used to ensure the testing accuracy.

Surface tension tester/dyne testing kit: Used to evaluate the wettability of the aluminum foil surfaces. The dyne testing kits have the low costs and simple operation, suitable for the fast on-site judgment. The more precise automatic surface tension testers calculate the surface free energies by measuring the contact angles of the liquid droplets on the solid surfaces, with the more objective and quantitative results, often used for the development and quality arbitration.

Pinhole degree testing table: Consisting of the LED light source plates with the uniform brightness, frosted glass or milky white acrylic diffusion plates, light-blocking dark boxes and observation windows. The uniformity and stability of the light sources are the keys of ensuring the consistency of the pinhole testing results. The advanced equipment may be equipped with the CCD cameras and image analysis software, realizing the automatic recognition, counting and size measurement of the pinholes.

Eddy current conductivity instrument/thickness gauge: Based on the electromagnetic induction principle, used for the non-contact measurement of the electrical conductivities and thicknesses of the aluminum foils. The instruments are light, and the probes can move and scan on the product surfaces, quickly finding the areas with the uneven properties. Need to be calibrated with the standard sheets of the known electrical conductivities and thicknesses. The measurement results are affected by the material temperatures, curvatures and the distances from the probes.

Metallographic preparation and analysis system: Including the cutting machines, mounting machines, grinding and polishing machines, metallographic microscopes and image analysis software. This system is used to prepare the micro-structure specimens of the aluminum foils, and through the microscope observation and software analysis, the grain sizes and second phase ratios are quantitatively measured, providing the direct micro-structure evidence for the process optimization and failure analysis.