Aluminium metallography 

Aluminium metallography

Preparation of aluminum samples

Aluminum alloys cover a wide range of chemical compositions and therefore a wide range of hardness values. Therefore, the methods for preparing samples for microscopic examination - microclips - of different aluminum alloys can vary significantly.

Soft alloys and unalloyed aluminum are more difficult to polish mechanically. The main reasons for this are [1]:
a) deformation, which occurs during cutting and grinding extends to a great depth;
b) more likely the introduction of abrasive particles into the metal during polishing;
c) hard particles of the secondary phases are easily pulled out of the soft aluminum matrix during polishing.

Harder aluminum alloys have other problems. Preparing microsections is easier, but these alloys have significantly more different phases and their microstructure is much more complicated.

As known, aluminum and its alloys are divided into two categories - cast and wrought. Each of these groups is subdivided in turn into series according to the main alloying elements in their chemical composition. For details, see. Classification of aluminum alloys.

For all these aluminum alloys, the usual methods for preparing thin sections and microscopic studies are applied.. At the same time, for various series and groups of aluminum alloys there are some features, to be considered.

Return and recrystallization processes in aluminum alloys can occur at relatively low temperatures., near 150-300 oC. Such temperatures can easily occur during cutting operations., grinding and installing the sample in the mandrel. These operations rarely produce structural changes., which is visible under a light microscope, they can be seen in an electron microscope.

Metallography of aluminum for 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 (Fig. 1). In wrought material, defects from the rolling and extrusion process are investigated and plating thicknesses measured (Fig. 2, Fig. 3, Fig. 4).

Fig. 1 – Aluminum-silicon cast, color etched with molybdic acid [2]

Fig. 2 – Aluminum alloy 2024, cast, showing eutectic precipitation on grain boundaries, unetched [2]

 

Fig. 3 – As Fig. 2, homogenized, unetched [2]

Fig. 4 – As Fig. 2, hot rolled, unetched

The metallographic challenges with aluminum

The metallographic challenges associated with aluminum and aluminum alloys change with the metal’s purity [2]:

  • 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.

Features of metallography of technical aluminum

The structure of commercially pure aluminum can inherit its structure from a casting process or a cold or hot working process (Figs. 5 and 6). One of the problems is, that when observed under a light microscope, there is usually no contrast "picture" – there are too few structural elements in pure aluminum. This is a typical case., when can anodizing a surface of a section help.

The microstructure of unalloyed aluminum after cold working is best studied using electron microscopy. Then you can see the dislocations and the grain structure.


Fig. 5 – Alloy 1100-H18 sheet, cold rolled.
Note metal flow around insoluble particles of FeAI3 (black).
Particles are remnants of scriptlike constituents in the ingot that have been fragmented by working.
See also Fig. 6.
0,5% HF [3].


Fig. 6 – Alloy 1100-O sheet, cold rolled and annealed.
Recrystallized, equiaxed grains and insoluble particles of FeAl3 (black).
Size ond distribution of Fe-Al3 in the worked structure were unaffected by annealing
(see also Fig. 5).
0,5% HF [3]

Microstructure of wrought aluminum alloys

Wrought aluminum alloys originate from cast ingots. After casting and homogenization, they received appropriate mechanical and heat treatment., which changed the original cast structure. These changes are relatively small for large products after hot working., for example, forged parts, thick plates or massive extruded profiles. Changes become more noticeable with an increase in the drawing ratio and an increase in the degree of hot and cold deformation., as well as the number and type of heat treatments.

The most visible changes in the microstructure of wrought aluminum alloys include [1] (Figs. 7-10):

  • dissolution of soluble phases or their coalescence to reduce their surface energy;
  • precipitation of alloy components at elevated temperature, which were in a supersaturated solution;
  • mechanical fragmentation of brittle intermetallic phases and their stretching along the main directions of cold or hot working;
  • the processes return or recrystallization after cold working.


Fig. 7 – Alloy 2024-O sheet.
Structure consists of light gray particles of insoluble (Cu,Fe,Mn)Al6,
large block particles of undissolved CuMgAl2,
and fine particles of CuMgA12 that precipitated during annealing.
25% HNO3 [3]


Fig. 8 – Alloy 6061-F plate, 38 mm thick, as hot rolled (91% reduction).
Longitudinal section from center of plate thickness. Particles are Fe3SiAl12 (gray, scriptlike) and Mg2Si (black).
See also Fig. 9 and 10.
0,5% HF [3]

Fig. 9 – Some alloy and condition as Fig. 8, but a longitudinal section from neat plate surface.
Porticles of Fe3SiAl12 and Mg2Si are more broken up and uniformly distributed than in Fig. 8 (midthickness).
See also Fig. 10.
0,5% HF [3]


Fig. 10 – Alloy 6061-F 6,4-mm sheet, hot rolled (reduced 98%); midthickness longitudinal section.
Fe3SiAl12 and Mg2Si particles more broken and dispersed than in Fig. 9.
Most Mg2Si will dissolve during solution treating.
0,5% HF [3]

Sources:
1. TALAT 1202
2. struers.com/en/Knowledge/Materials/Aluminum
3. Aluminum and Aluminum Alloys – ASM Handbook / ed. J. R. Davis