How to Perform an Analysis of an Aluminum Zinc Alloy

To perform an analysis of an aluminum zinc alloy, you need to determine its elemental composition, verify its mechanical properties, and examine its microstructure. The most reliable methods are optical emission spectrometry (OES) for chemistry and tensile testing for strength, combined with metallography for grain structure. This article explains the step-by-step process, the equipment commonly used, and how to interpret the results for quality control or failure analysis.

Preparing the Sample

Proper sample preparation is the foundation of any accurate analysis. For an aluminum zinc alloy, the sample must be representative of the bulk material. If you are testing a casting, take a slice from the center of a runner or a separately cast test bar. For wrought products like sheet or extrusion, cut a transverse section that includes the full thickness.

Surface preparation depends on the analytical method. For optical emission spectrometry, the sample needs a flat, clean surface free of oxides and contaminants. Grind the face on a belt sander using 60- or 80-grit silicon carbide paper, then finish with a clean, dry cloth to remove dust. For metallographic examination, mount the sample in epoxy, then grind and polish through progressively finer abrasives down to 1-micron diamond paste. Etch the polished surface with a reagent such as Keller’s etch (2 mL HF, 3 mL HCl, 5 mL HNO₃, 190 mL water) to reveal grain boundaries and second-phase particles.

Determining Chemical Composition

The primary goal of an analysis of an aluminum zinc alloy is to measure the weight percentages of zinc, aluminum, and any alloying or impurity elements. Optical emission spectrometry (OES) is the standard method in production and quality control labs. The instrument generates a spark that vaporizes a small amount of the sample surface, and the emitted light is analyzed by a spectrometer to identify and quantify elements.

For aluminum zinc alloys, you typically expect zinc in the range of 2 to 12 weight percent, with aluminum making up the balance. Common alloying additions include copper (0.5–3%), magnesium (0.5–2%), and sometimes silicon or iron as impurities. The OES instrument reports results in weight percent, and you compare these against the specification for the specific alloy designation, such as those listed in standards from ASTM International.

A quick alternative for field work or when OES is not available is X-ray fluorescence (XRF) spectrometry. Handheld XRF analyzers can give semi-quantitative results for zinc and other elements above atomic number 11. However, XRF is less accurate for light elements like aluminum and magnesium, so it is best used as a screening tool.

Interpreting the Results

If the zinc content falls outside the specified range, the alloy may not meet strength or corrosion resistance requirements. For example, a zinc content below 2% might produce an alloy that is too soft for structural use, while above 12% can cause brittleness and poor castability. Copper and magnesium levels also affect mechanical properties, so check each element against the target specification.

Testing Mechanical Properties

Once you know the composition, you need to confirm that the alloy’s strength and ductility match the intended application. The most common test is the tensile test, performed according to ASTM International standard E8. Machine a round or flat specimen from the sample, pull it in a universal testing machine until it fractures, and record the yield strength, ultimate tensile strength, and elongation.

For a typical aluminum zinc alloy such as 7075 (Al-Zn-Mg-Cu), the yield strength in the T6 temper is around 500 MPa, with an ultimate tensile strength near 570 MPa and elongation of about 11%. Lower-zinc alloys like 7005 have lower strength but better weldability. If your test results deviate significantly from these values, it may indicate incorrect heat treatment or a composition problem.

Hardness testing is a faster, non-destructive alternative. Use the Rockwell B scale (HRB) or Brinell method. For the same 7075-T6 alloy, the Brinell hardness is typically 150 HB. A hardness test alone cannot replace a tensile test for design data, but it is useful for quick quality checks on production parts.

Examining the Microstructure

Metallography reveals the grain structure, the distribution of second-phase particles, and any casting defects. After etching, examine the sample under an optical microscope at magnifications from 50x to 500x. In an aluminum zinc alloy, you will see the aluminum matrix (light areas) and zinc-rich phases (darker regions). In alloys with copper and magnesium, you may also see fine precipitates of MgZn₂ or Al₂CuMg.

Grain size affects mechanical properties. A fine, equiaxed grain structure generally gives higher strength and better fatigue resistance. If you see large, elongated grains, the alloy may have been worked or heat-treated incorrectly. Porosity appears as round or irregular dark spots and indicates gas entrapment during casting. Excessive porosity will lower ductility and cause failure under load.

For a more detailed analysis, use scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). This lets you identify the exact composition of individual phases. For example, you can confirm that a bright particle is a zinc-rich phase or that a gray inclusion is an oxide film. Resources from ASM International provide reference micrographs and phase diagrams for common aluminum zinc alloys.

Key Takeaways

• The standard methods for analysis are optical emission spectrometry for composition, tensile testing for mechanical properties, and optical microscopy for microstructure.
• Sample preparation must be careful and consistent to avoid contamination or surface artifacts.
• Compare measured composition and properties against the specified alloy standard to determine if the material is acceptable.
• Unexpected elements or phases can indicate contamination, incorrect alloy selection, or processing errors.
• For critical applications, combine multiple analytical techniques to confirm the material’s suitability.