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:
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.
Features of metallography of technical aluminum
The structure of technically pure aluminum can inherit its structure from a foundry process or a cold or hot working process. 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.
Microstructure of 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 aluminum alloys include:
– 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 of the cast structure;
– 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.