GB/T 3246.1-2024Inspection method for structure of wrought aluminium and aluminium alloy products—Part 1: Inspection method for microstructure (English PDF)

3.1 Eutectic melting

The phenomenon that the metal temperature reaches or exceeds the melting point or solidus of the low-melting eutectic in the alloy, causing the eutectic or solid solution grain boundary to remelt.

3.2 Heat treat induced porosity

When aluminum alloy materials are treated at high temperatures, due to the high humidity in the furnace (caused by the moisture contained in the furnace itself, the oil on the surface of the product, or the cooling medium brought into the furnace during the solution treatment process, etc.), the surface of the wrought aluminum and aluminum alloy materials after heat treatment will blister or pores will appear near the surface along the grain boundary.

Note: Heat treat induced porosities are also called high temperature oxidation (HTO). The typical microstructure characteristics are as shown in Figure 2.

3.3 Cladding layer

A layer of aluminum or aluminum alloy clad on the surface of wrought aluminum alloy plate, to improve corrosion resistance or certain process performance.

Note: The cladding layer is also called aluminum cladding layer when it is pure aluminum. See Figures 3 and 4 for the cladding layer.

3.4 Copper diffusion

The phenomenon that, in aluminum-copper-magnesium alloy cladding aluminum plate, the copper atoms in the alloy diffuse into the cladding layer along the grain boundary, after high temperature and long-term heating treatment.

3.5 Sell zone

A zone formed, during casting, when the solidified shell produces local softening and remelting, filling the low-melting eutectic phase that has not yet solidified between dendrites and grain boundaries, flows along the dendrites and grain gaps to the surface layer of the ingot.

4.1 Method overview

The optical (metallographic) microscope is used to analyze, evaluate, characterize the phase and organizational composition of the constituent materials, the number, morphology, size, distribution orientation, spatial arrangement of grains (including possible sub-grains), inclusions, etc.

4.2 Reagents or materials

Warning - The inorganic acids used in the analysis are corrosive to the human body. The test personnel must wear personal protective equipment to reduce the risk of direct exposure.

4.2.1 Water, GB/T 6682, grade 3.

4.2.2 Anhydrous ethanol.

4.2.3 Nitric acid (1 + 4).

4.2.4 Nitric acid (1 + 1).

4.2.5 Electrolytic polishing solution. Perchloric acid (rho = 1.77 g/mL)-ethanol solution (1 + 9).

4.2.6 Magnesium oxide.

4.2.7 Etching agent. See Table 1 for reagent ratios and applicable tests.

4.2.8 Membrane forming solution. See Table 2 for reagent ratios and applicable tests.

4.2.9 Anhydrous ethanol cotton ball. Prepared by dipping anhydrous ethanol in absorbent cotton ball.

4.2.10 Sandpaper. The particle size is 68 µm ~ 100 µm.

4.2.17 Synthetic leather.

4.2.18 Diamond. The particle size is 3 µm.

4.2.25 Water-based lubricant.

4.2.26 Kerosene.

4.3 Instruments and equipment

4.4 Specimen

4.4.1 Sampling

Determine the specimen size and sampling quantity according to the provisions of the product standard and the test purpose, heat treatment method, use requirements of the product; cut the specimen from the appropriate position; select the inspection surface.

If the product standard does not specify, cut the specimen according to Table 3 and Table 4.

4.4.2 Specimen clip and mount

For specimens that need to check the surface structure of the product (such as measuring the cladding thickness and copper diffusion depth, etc.), specimen clip or mount shall be performed. When the specimen is small, it should mount the specimen. When clamping the specimen, annealed pure aluminum sheets shall be placed between the specimens and on the outside, to ensure that there is no gap between the specimens after clamping and the specimen surface is flat.

4.4.3 Specimen grinding and polishing

4.4.3.1 Manual specimen preparation

4.4.3.2 Semi-automatic or automatic specimen preparation

Automatic or semi-automatic grinding and polishing machines can be used instead of manual grinding and polishing. Examples of specimen preparation processes are shown in Table 6.

4.4.3.3 Electrolytic polishing

4.4.3.4 Vibration polishing

After mechanical polishing, the specimen shall be mounted and then vibrated according to the process parameters in Table 8. For specimens used for electron backscatter diffraction (EBSD), in addition to electrolytic polishing, vibration polishing can also be used.

4.4.4 Specimen etching

4.4.4.1 Refer to Table 1 to select a suitable specimen etchant.

4.4.5 Anodizing film

4.4.5.1 The specimen can be anodized and filmed only after polishing. The anodizing device is as shown in Figure 15.

4.4.5.2 Refer to Table 2 to select the appropriate specimen film-making solution.

4.5 Organization inspection

4.5.1 Eutectic melting inspection

Inspect the microstructure. If any of the three characteristics of remelting eutectic balls, local remelting widening of grain boundaries, remelting triangles formed at the junction of three grains appears, that is, the appearance of rose-shaped eutectic structure, the microstructure is judged to be eutectic melting. The normal structure and eutectic melting structure of the ingot (or billet)'s homogenization treatment are shown in Figures A.89 ~ A.96 in Appendix A.

4.5.2 Inspection of heat treat induced porosity (high temperature oxidation)

Inspect the microstructure. If the wrought aluminum and aluminum alloy products have bubbles on the surface or pores as shown in Figure 2 appear along the grain boundaries near the surface, it is judged that there are heat treat induced porosities.

4.5.3 Cladding layer inspection

4.5.3.1 On the microscopic cross section of the cladding layer of the specimen, measure the cladding layer thickness along the length direction using an eyepiece micrometer.

The measurement points shall be no less than 5 and the average value shall be calculated.

4.5.3.2 When required by the purchaser, the cladding ratio V can be calculated according to formula (1). The value is expressed as a percentage. The calculation result is expressed to two decimal places. The value is rounded off according to the provisions of GB/T 8170.

4.5.4 Copper diffusion inspection

When preparing the specimen for copper diffusion depth inspection, electrolytic polishing can be used to check whether the copper diffusion penetrates or the maximum depth of copper diffusion in the aluminum cladding on both sides can be measured with an eyepiece micrometer.

4.5.5 Inspection of remaining casting structure

Inspect the microstructure. If the wrought aluminum and aluminum alloy products show a microstructure in which the dendrites are not completely broken [see Figure 14b)], it is judged that there is remaining casting structure.

4.5.6 Determination of grain size

The grain size determination is carried out in accordance with Appendix B. The typical anodic film polarized grain structure is shown in Figure A.1 ~ Figure A.88.

4.5.7 Determination of the second phase and particle size

The determination of the second phase and particle size is carried out in accordance with Appendix C.

4.5.8 Observation and analysis of the morphology and distribution of the second phase

4.5.8.1 The type of the second phase (such as primary phase, eutectic phase, precipitation phase, etc.) can be determined, by observing the microstructural characteristics of the second phase, such as morphology, size, distribution; then the phase composition of some phases can be preliminarily determined. The method for determining the phase composition of the primary phase and eutectic phase is shown in T/CNIA 0161.

4.5.8.2 Observation and analysis of the aging phase is carried out in accordance with the provisions of YS/T 1623.

4.5.9 Homogenization effect evaluation

The homogenization effect evaluation method shall be carried out in accordance with the provisions of YS/T 1624.

4.5.10 Microscopic slag inspection

Use 200 times magnification to scan within the inspection area of the specimen (preferably 10 mm × 10 mm). Use 500 times magnification to judge the observed suspected slag inclusions. Use 200 times magnification to measure and count the maximum intercept size and number of slag inclusions for each slag inclusion.