How Is Metallographic Inspection Performed on Aluminum Alloys?

Macroscopic Inspection

Macroscopic inspection uses simple methods to detect internal defects over a large area of an aluminum product and is therefore an effective routine inspection method.

Preparing a Macroscopic Specimen

A low-magnification aluminum-alloy specimen is taken from a location selected according to the inspection purpose or applicable standard. The inspection surface is milled or otherwise prepared to a roughness Ra no greater than 3.2 μm. Oil is removed with gasoline, ethanol, acetone, or another solvent so that the macrostructure can be displayed clearly.

Alkaline etching is commonly used. Castings and cast-ingot specimens are etched at room temperature in 80-120 g/L NaOH solution, or 10%-15% by mass. Wrought-product specimens use 150-250 g/L NaOH at room temperature. Etching time varies by alloy from 3 to 30 minutes and continues until structural defects are clear. The ratio of etchant volume to immersed specimen volume should be at least 10:1. After etching, rinse with water, remove the black corrosion film with 20%-30% HNO₃ solution by mass, rinse again, and dry. A specimen may also be swab-etched, but immersion etching is the reference method.

Macroscopic Evaluation

Metallographic standards for low-magnification inspection of cast aluminum alloys divide pinhole severity into five grades and provide reference images. The specimen is compared visually with these images to determine its grade. Many other macroscopic defects occur in cast aluminum, but because standards do not yet define every type, inspection may refer to standards for defects in wrought-aluminum and aluminum-alloy ingots.

Macroscopic Defect Types

The relevant low-magnification standard covers specimen preparation for ingots, extrusions, forgings, sheet, and other wrought products; specimen etching; structural examination; defect classification; and test reports. It divides defects into 22 types.

Microstructural Inspection

Preparing Metallographic Specimens

Preparation should avoid creating a deep deformation layer. Cut the sample manually or mechanically, then flatten the inspection surface with a milling cutter, file, or coarse abrasive paper, removing 1-3 mm. Do not use a grinding wheel for cutting or flattening. Grind progressively from coarse to fine metallographic papers with moderate pressure and prevent coarse abrasive from contaminating the next stage. If a pregrinder is used, kerosene is preferred for cooling and lubrication.

Small samples or specimens whose surface must be observed, such as clad layers, can be mounted. Because mounting heat may artificially age some aluminum alloys, cold mounting is preferred. Aluminum may be polished mechanically, chemically, or electrolytically; mechanical polishing is most common. Rough polishing uses a concentrated suspension of approximately 5 μm alumina or chromium oxide in water, or diamond paste, on fine canvas or felt at 500-600 r/min. Fine polishing uses a dilute suspension with particles below 1 μm, or diamond paste, on silk at 150-200 r/min.

Improper preparation forms a gray-white oxide film that appears gray after etching and shows many small black spots in the aluminum matrix at high magnification. A correctly polished surface is mirror-bright and free from scratches and contamination. If fine polishing cannot remove scratches from commercially pure or high-purity aluminum, electrolytic polishing can be used.

Selecting an Etchant

Etchant selection depends on alloy composition, material condition, and inspection purpose. Common solutions include HF (1 mL) + H₂O (200 mL); HF (50 mL) + H₂O (50 mL); HF (2 mL) + HCl (3 mL) + HNO₃ (5 mL) + H₂O (190 mL); HNO₃ (25 mL) + H₂O (75 mL); H₂SO₄ (10-20 mL) + H₂O (80-90 mL); and H₃PO₄ (10 mL) + H₂O (90 mL). They can reveal general aluminum structure, grain structure, and various phases. Reagent-grade chemicals and distilled water should be used. Polished specimens are anodized before observing ingot grains, annealed samples, and deformed microstructures under polarized light.

Standards for Cast Aluminum Alloys

Cast-alloy metallography may be performed in the polished or etched condition. Chinese standard JB/T 7946.1-2017 evaluates modification of cast Al-Si alloys, JB/T 7946.2-2017 evaluates incipient-melting structures after heat treatment, and JB/T 7946.4-2017 evaluates grain size in cast Al-Cu alloys. Metallography also commonly includes examination of microporosity.

Evaluating Modification of Cast Al-Si Alloys

Modification evaluation is a principal aspect of cast-aluminum metallography. JB/T 7946.1-2017 provides separate methods for sodium and phosphorus modification; sodium modification is more common. Strontium modification is also widely used industrially, but China currently has no corresponding metallographic standard, so sodium-modification evaluation can be used as a practical reference.

After polishing, a sodium-modified specimen is etched for 5-10 seconds in 0.5% HF solution by mass. The entire inspection surface is examined at 200× and rated according to the reference grade representing most fields. Eutectic-silicon morphology defines six grades: unmodified, with needle-like silicon; undermodified, with short rods and needles; normally modified, with dots and worm-like silicon; fading modification, with coarsened silicon; slightly overmodified; and severely overmodified.

A phosphorus-modified cast Al-Si eutectic specimen is etched in 0.5% HF or a mixed-acid solution containing 0.5 mL HF + 1.5 mL HCl + 2.5 mL HNO₃ + 95.5 mL H₂O. Evaluation is performed at 100× against the standard grade representing most fields. The four grades are unmodified, well modified, normally modified, and undermodified.

Evaluating Incipient Melting in Cast Al-Si Alloys

This evaluation applies to heat-treatable alloys. Segregation and low-melting eutectics formed during solidification may remelt at the subsequent solution-treatment temperature. Features include incipient-melting triangles, grain-boundary melting, remelted globules, and remelted eutectics.

An incipient-melting triangle forms when the last low-melting eutectic to solidify at a grain intersection remelts during heat treatment and surface tension creates a sharp-edged triangle. If a low-melting eutectic within a dendrite melts and the liquid spheroidizes, a remelted eutectic globule forms. If the holding temperature is too high, low-melting constituents melt and resolidify as binary, ternary, or other remelted eutectics.

Five grades are defined: normal, overheated, slight incipient melting, incipient melting, and severe incipient melting. A sample is taken from a witness bar processed with the load. The entire polished surface is examined at 400×, and the most severe field determines the grade. A normal solution-treated structure has mainly particulate eutectic silicon with rounded edges and no agglomeration. An incipient-melting structure shows agglomerated, coarsened eutectic silicon with mostly straight edges, typical remelted globules, and multicomponent remelted eutectics.

Grain Size in Cast Al-Cu Alloys

Chinese standard JB/T 7946.4-2017 evaluates grain size in cast Al-Cu alloys. A specimen is generally cut from a tensile sample or taken as specified in technical documents. After grinding and polishing, it is etched in 0.5% HF or 0.5 mL HF + 1.5 mL HCl + 2.5 mL HNO₃ + 95.5 mL H₂O. The entire surface is examined at 100× and rated according to the standard grade representing most fields. Grain size is divided into eight grades.

Wrought Aluminum Inspection

Chinese standard GB/T 3246.1-2012 applies mainly to microstructural inspection of wrought aluminum and aluminum-alloy materials and products.

Additional Integrated Inspection Procedures

1. Ingot and heat-treated product microstructures. In the polished condition, inspect phase morphology, porosity, inclusions, and other defects. On etched specimens, examine dendritic structure, identify phases, and evaluate incipient melting in quenched material. Quenched and annealed wrought products are normally examined at 200×-500× for grain condition and incipient melting. GB/T 3246.1-2012 provides comparative micrographs for normal and incipient-melted ingots, cast-rolled sheet, and quenched or incipient-melted wrought products.
2. High-temperature oxidation. High furnace humidity during elevated-temperature treatment can produce surface blisters or intergranular pores just below the surface after heat treatment. This is called high-temperature oxidation.
3. Cladding thickness. Aluminum or aluminum alloy is often clad onto alloy sheet to improve corrosion resistance or meet processing requirements. Examine a transverse section and use a specimen clamp or mounting to protect the cladding during polishing. Average 5-10 thickness measurements and divide by total sheet thickness to obtain cladding percentage.
4. Copper diffusion. During prolonged high-temperature treatment of Al-Cu-Mg Alclad sheet, copper diffuses along grain boundaries into the cladding. Greater diffusion depth causes a greater loss of corrosion resistance. Electrolytically polish the specimen and examine the maximum copper-diffusion depth in the cladding on both faces.
5. Grain size. Grain-size inspection normally measures grains in the matrix, the α-aluminum solid solution. Minor phases, inclusions, and other constituents are ignored. Methods are generally the same as for other metals; rating may use standard comparison charts, the planimetric method, or the intercept method.