Metallography: aluminum surface preparation

 

The study of microstructures is one of the chief means of study of various metals and alloys. This fully applies to aluminum and aluminum alloys.. Metallography makes it possible to determine the effect of various deformation and heat treatments on the properties of finished aluminum products., as well as to analyze the reasons of her marriage.

The main changes in the microstructure aluminum и aluminum alloys occur when:

• solidification from a liquid state
• gomogenïzacïï
• hot or cold working
• imprints
• aging.

A good interpretation of the structure makes it possible to reveal the full story of technological sample.

Aluminium metallography

Generally metallography of aluminum and its alloys is a rather difficult task. The thing is, that aluminum alloys are characterized by a wide variety of chemical composition, and also have a very wide range of hardness and other mechanical properties. Therefore, techniques, which are used for microscopic examination of samples of aluminum, can vary significantly for soft and hard aluminum alloy. Moreover, one and the same aluminum alloy may have multiple microstructural features and characteristics, such as:

• aluminum base
• secondary phase
• dispersoids
• grains
• subzerna, and
• grain and subgrain boundaries.

All these components of the microstructure are shown in various combinations depending on the type of alloy and its deformation and thermal history. At the same time, Some methods of sample preparation and research are common to all aluminum alloys. In some cases require special microstructure research methods.

Specimen for light microscopy

On practice, especially in the factory, commonly used light microscopy. electron microscopy, More sophisticated and expensive, often used in scientific research.

The principles of sample preparation for light microscopy of aluminum alloys are generally the same., like most other metals. Before cutting out the sample and etching is necessary to examine it carefully. If we study the surface of fracture, it is necessary to reliably protect it from damage and contamination.

The selected portion of the material was cut with an abrasive saw blade at a distance from the plane, which will be studied. It's necessary, as several tens of micrometers is removed by mechanical grinding,. To prevent heating of the sample and the sample structure changes coolant used during the cutting.

Sample surface preparation

First, an approximately flat surface is obtained (by filing, abrasive processing). For convenience, this operation, samples were placed in a special clamp of two plates or poured, for example, into epoxy resin (Fig. 1).

Fill carried out as follows. A round or square mandrel is mounted on a metal or ceramic plate (for example, steel). Inside the mandrel is placed in such a way the sample, prepares to face rested on the plate. Then, an epoxy resin with a hardener is poured into the mandrel.

Fig. 1 [3]

Sample grinding

After receiving the approximately flat surface of the sample was polished with sandpaper, which for this purpose is placed on a flat base (usually on glass) or fixed on a rotating circle.

Grinding is carried out successively sandpaper with different grain size - first coarse, and then fine-grained.

When changing the grade of paper - Skin - 90º changed to the direction of movement of emery paper sample relative to the direction the circle. This gives better removal of ridges and scratches from the previous grinding. abrasive particles from the surface of the sample after grinding is removed by blowing air or, it's better, washing with water.

In the grinding of most aluminum alloys - very soft - sandpaper pre-soaked in kerosene, rub paraffin or just moistened with water.

Sample polishing

Remaining after grinding small risks are removed by polishing. Typically mechanical polishing used, as well as chemical-mechanical and electrochemical.

Mechanical polishing to produce a rotating wheel with a stretched or glued polishing material - felt, velvet or broadcloth. The polishing material is continuously or intermittently coated with an abrasive substance with very small particles (aluminum oxide, iron oxide, chromium oxide).

Buff should be moist, and the pressure of the sample at him slightly. The rotation speed range in diameter 250 mm should be 400-600 rpm.

Polishing completed microsection believe, when its surface acquires a high gloss, and even under the microscope is not visible scratches or risks.

Sample drying

after polishing, regardless of the manner of its implementation, microsection washed with water, then wiped with alcohol and dried with filter paper.

Sample etching

After polishing microsection ready to be etched. Etching is substantially controlled process of electrolytic corrosion resulting from the interaction between the surface regions with different potentials. For a pure metal or single-phase alloys, the potential occurs between

• differently oriented grains
• grain boundaries and internal regions of grains
• impurity phases and aluminum matrix
• areas with different chemical composition.

These differences in the potentials and give a different dissolving various metal or alloy components and, in the end, identifying the microstructure. Therefore, the quality of the polishing effect on the development of the true microstructure. Incorrect preparation of the sample surface may distort the information on the structure.

Etchants for aluminum and its alloys

etchants, which are used for microscopic examination of aluminum alloys, quite a lot - at least, more than a dozen (table. 1 и 2). The greatest use of them in practice found Keller reagent, 1 %-ny NaOH solution and 0,5 %-hydrofluoric acid solution was, Barker's reagent.

reagent Keller

Keller Reagent:

• 2 ml hydrofluoric acid HF (48 %);
• 3 ml of hydrochloric acid HCl;
• 5 ml 190 ml water H₂O.

This etchant provides an opportunity to identify and highlight the grain boundaries in many wrought alloys.

A solution of caustic soda NaOH

1 %-caustic soda solution was used for detection of grain boundaries in the aluminum alloys of the 6xxx series, including, 6060/6063.

A solution of hydrofluoric acid

0,5 %-hydrofluoric acid solution (1 ml of hydrofluoric acid (48 %) on 200 ml of water) is used to identify the components of cast aluminum alloys, particularly those containing silicon.

It should be noted, what, for example, grain structure are not always easily detected by conventional etchants in all aluminum alloys. In thin sections of alloy with a low doping gives such etching step to diffuse the grain boundaries, which do not provide a good contrast microscope. In such cases, for the preparation of thin section surface used anodizing.

Table 1 – Etchants for use in microscopic examination of aluminum alloys [2]
See Table 2 for applicability to specific alloys

Table 2 – Applicability of etchants in Table 1 to microscopic examination of aluminum alloys [2]

Sample anodizing

Anodizing or anodic oxidation electrolytic process is, which results in the formation of an oxide film on the metal surface. the film growth direction oriented along the crystal lattice on the respective grain surface of the polished section. As a result, thin section surface anode film formed with varying thickness at different grains. This makes it possible to reveal the grain structure when illuminated with polarized light (Figs.. 1 and 2).

Anodizing apply a so-called Barker solution - 5 ml of HBF₄ (48 %) on 200 ml of water. The process is carried out at a current density 0,2 A / cm² during 40-80 seconds at room temperature.

Fig. 2 [3]

Fig. 3 – Plated sheet, anodized, grain areas are clearly visible, polarized light [3]

Sources:
1. TALAT 1202
2. Metallography, Microstructures, and Phase Diagrams // Aluminium and Aluminium Alloys – ASM Speciality Handbook / ed. J.R. Davis – 1996
3. struers.com/en/Knowledge/Materials/Aluminum