Metallography of aluminum insight - Struers.com

Aluminum alloys

Adding very small amounts of alloying elements to aluminum can increase tensile strength, yield strength and hardness compared to pure aluminum. The most important alloying elements are Si, Mg, Cu, Zn and Mn. These mostly eutectic compounds must be finely dispersed through a hot working process before the alloy can be cold worked.

Ageing of aluminum alloys
Many aluminum alloys are age hardened to improve the mechanical properties. This can be done either naturally or artificially.

• Natural age hardening (example AlCuMg). After solution annealing, the workpiece is quenched and consequently the precipitation of the Al2Cu in the solid solution is sup- pressed. The workpiece is then left to age in ambient temperature. During this process the aluminum lattice precipitates the copper from the supersaturated solution. The resultant strain produced in the aluminum lattice leads to an increase in strength and hardness. The process takes 5-8 days.
• In artificial age hardening, ageing takes place at an elevated temperature, which reduces process time. With an AlMgSi alloy, for example, ageing occurs in 4-48 hours at 120-175 °C after solution annealing and quenching. The precipitation of the Mg2Si phase produces internal strain in the aluminum lattice, which results in an increase in strength and hardness.

Wrought aluminum alloys
The main alloying elements for wrought aluminum alloys are copper, magnesium, zinc and manganese. Silicon and iron affect the mechanical properties and corrosion resistance and can either be impurities or alloying elements, depending on the requested purity and application.

Common uses of wrought aluminum alloys:

• Plates in mechanical engineering and mold construction for rolled products, such as sheets and strips, as well as plated products like radiators and heat exchangers
• Plated sheets for specific semi-finished products for aircraft construction or for decorative applications such as trim and reflectors
• Mechanical engineering, conveying and electro technical applications, as well as high-strength sports and leisure products, such as snowboard bindings and mountain bike gears
• Fiber-reinforced aluminum in the aircraft and aerospace industries

Fig. 1: Aluminum alloy , cast, showing eutectic precipitation on grain boundaries, unetched, 200x


Fig. 2: As Fig. 1, homogenized, unetched, 200x


Fig. 3: As Fig. 2, hot rolled, unetched, 200x

Cast aluminum alloys
Aluminum casts are mainly alloyed to improve the metal’s mechanical properties and are differentiated according to their main alloying elements – silicon, magnesium and copper. Alloy contents that exceed the saturation of the solid solution are precipitated as pure metal, such as silicon, or as eutectics and inter-metallic phases.

Silicon increases the castability of aluminum. In eutectic alloys, such as AlSi12, small amounts of sodium are added before casting to refine the eutectic. In this refining process, instead of precipitating as coarse needles or plates (Fig. 4), the silicon forms a very fine eutectic with the α-solid solution (Fig. 5). The effect of hardening in these alloys is very low and therefore magnesium is added so that they can be age hardened.

Cast alloys with specific properties are used in various product groups, including the fabrication of pistons, slide bearings, parts for mechanical engineering, cylinder heads and brake shoes.

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 Some of the more important cast alloys and their properties  AlSi10Mg  Age hardened. Vibration and corrosion resistant  AlSi5Cu1  Age hardened. Good castability for welding and thin sections   AlMg3  Resistant to seawater AlSi25+ CuNi  Age hardened. Special alloy for pistons; wear resistant due to its high Si content   AlMgSiPb  Suitable for machining   AlSi9Cu3  Castable universal alloy and the most important alloy for pressure die casting 

Fig. 4: Aluminum-silicon cast, unrefined, 500x

Fig. 5: Aluminum-silicon cast, refined, 500x

The metallography of aluminum and its alloys

Metallography of aluminum is used in quality control for grain size determination and to determine microstructure defects on polished and etched specimens. In addition, specimens are often checked for impurities, such as oxides or zirconium aluminides.

Cast alloy aluminum is evaluated for shape, distribution of phases and possible porosity. In wrought material, defects from the rolling and extrusion process are investigated and plating thicknesses measured.

Fig. 6: Oxide in the surface of an aluminum pressure die casting, 50x

Challenges in the metallography of aluminum and its alloys

The metallographic challenges associated with aluminum and aluminum alloys change with the metal’s purity.

• As purity increases, aluminum becomes softer and more susceptible to mechanical deformation and scratches. In high purity aluminum, grinding can cause deep deformation, while grinding and polishing abrasives, such as silicon carbide and diamond particles, can be pressed into the surface.
• As alloying content increases, aluminum becomes harder. Cast alloys are relatively easy to prepare. However, the aluminum matrix must be well polished to avoid errors in structure interpretation.

 Overview of metallographic challenges and solutions  Challenge: Solution:  Pure aluminum is very soft and prone to mechanical deformation and scratching Plane grinding with the finest possible SiC Foil or Papers Silicon carbide and diamond particles can be pressed into the specimen surface Diamond polishing and/or final polishing need to be long enough to remove all embedded particles Severely worked and deformed wrought alloys are difficult to contrast, making structure interpretation difficult - Final polishing with colloidal silica suspension
- Anodization with Barker’s reagent

Read further for a detailed method description of how to prepare aluminum and its alloys for metallographic analysis quickly and accurately.

Fig.7: Embedded diamond particles in pure aluminum after polishing with 3 μm, 200x

Preparation of aluminum and its alloys: Mechanical grinding & diamond polishing

When working with aluminum and its alloys, we recommend mechanical grinding, followed by diamond polishing. For many pure aluminum and wrought alloy specimens, electrolytical polishing is also recommended.

Mechanical grinding

Plane grinding should be carried out with the finest possible grit to avoid excessive mechanical deformation.

• The hardness, size and number of specimens should be considered. However, even with large specimens of pure aluminum, plane grinding with 500# SiC Foil or Paper is usually sufficient.
• Large cast parts of aluminum alloys can be ground with 220# SiC or 320# SiC Foil. It is important that the grinding force is low to avoid deep deformation and to reduce friction between the grinding SiC Foil or Paper and specimen’s surface.

Diamond polishing

Diamond polishing should be carried out until all deep scratches from grinding have been removed. If water soluble constituents must be identified, we recommend polishing with water-free diamond suspension and lubricant.

Final polish for pure aluminum and aluminum alloys: The polish/check sequence

• Begin polishing. After 1 minute of polishing with OP-U suspension, check the specimen under the microscope.
• If necessary, continue polishing for another minute and check the specimen again.
• Continue this polish/check sequence until the required quality has been achieved.
• If diamond particles have been pressed into the surface during polishing, they can lead to erroneous interpretations of the structure. Therefore, the polish/check sequence may need to be relatively long. Continue the sequence until you can no longer see bright and dull areas on the surface of the specimen with the naked eye.
• Approximately 30 seconds before the end of polishing, pour water onto the polishing cloth to rinse the specimen and cloth.
• Finally, wash the specimen again with clean water and then dry it.

Note: Polishing for too long with silicon dioxide suspension OP-S NonDry can cause a pronounced relief, see Fig.11.

*Alternatively, MD-Dac

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* To avoid coarse scratches, the SiC Foil or Paper can be rubbed with wax before grinding.
** Alternatively, MD-Dac

* To avoid coarse scratches, the SiC Foil or Paper can be rubbed with wax before grinding.'

Fig. 9: Aluminum-silicon cast, after polishing with 3 μm diamond small scratches are still visible, 200x

Fig.10: Structure as in Fig. 9, but fine polished with OP-U suspension. The matrix is well polished, and the eutectic has more contrast, 200x

Fig.11: Aluminum-silicon cast polished for too long with OP-S suspension, silicon precipitates stand in relief, 100x

Electrolytic polishing

Electrolytic polishing of aluminum leaves a scratch-free surface and is often used in quality control as it delivers fast and reproducible results. However, it is not recommended for many cast alloy specimens, due to the many different phases in cast alloys.

Pure aluminum and wrought alloys
Electrolytic polishing is especially suitable for pure aluminum and wrought alloys.

• For routine grain size determination at 100x, pre-grinding with # SiC Foil is sufficient.
• Rolled or drawn surfaces do not need any grinding or polishing.
• For pure aluminum and precise examinations of grain shapes, the specimen should be fine ground to # and sometimes even to # before electrolytic polishing.

If you anodize the specimen with Barker’s reagent after the polishing, it will result in a color contrast that is particularly suited for grain size evaluation. To obtain the color effect, view the specimen under polarized light with a λ1⁄4 sensitive tint plate.

Fig.12: Plated sheet, anodized, grain areas are clearly visible and suitable for automatic image analysis, polarized light with λ1⁄4-plate, 100x

Discover the parameters for electrolytic polishing of aluminum in our application note here.

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Fig.13: Aluminum pressed part, macro etching, primary and heterogeneous precipitates are revealed

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