The metallographic challenges associated with aluminum and its alloys vary greatly, depending on the metal’s purity. Here, you can learn how to prepare any aluminum product for metallographic examination quickly and effectively.
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Fig. 1: Aluminum alloy 2024, 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.
| 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
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
| 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
*Alternatively, MD-Dac
* 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
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
Fig.13: Aluminum pressed part, macro etching, primary and heterogeneous precipitates are revealed
When working with chemicals the standard safety precautions must be observed.
| Macro etching | |
| For pure aluminum | 90 ml water |
| 15 ml hydrochloric acid | |
| 10 ml hydrofluoric acid | |
| Deep etching to reveal the primary dendritic structure | |
| 100 ml water | |
| 10-25 g sodium hydroxide | |
| Micro etching | |
| Flick’s reagent: Grain boundary etching for most types of aluminum and alloys | |
| 90-100 ml water | |
| 0.1-10 ml hydrofluoric acid | |
| Dix and Keller reagent: Grain area etching for Al-alloys with copper, also suitable for pure aluminum | |
| 190 ml water | |
| 5 ml nitric acid | |
| 10 ml hydrochloric acid | |
| 2 ml hydrofluoric acid | |
| Color etchants: | Molybdic acid solution according Klemm or Weck |
Fig.15: Experimental aluminum alloy with 6% Si and 10% copper, unetched
Fig.16: Same as Fig.15 but etched for 30 seconds with 1 g molybdic acid in 200 ml water + 6 g ammonium chloride. The silicon is dark blue and is distinguishable from the greyish CuAl2
All images by Xiuping Jiang, Application and Laboratory Manager, China
For specific information about the metallographic preparation of aluminum, contact our application specialists.