Microstructure of wrought aluminum alloys
As known, wrought aluminum alloys are divided into eight major series alloys by their main alloying elements. Therefore, each such Series will have its own characteristics of the microstructure. Another division of aluminum alloys refers to their ability to heat treatment (series 2000, 6000, 7000) or strain hardening - hardening (1000, 3000, 5000). It is also reflected in the microstructure of the alloys.
1000 series
The microstructure of this series are usually phase aluminum-iron and aluminum-iron-silicon. This is due to the fact, iron and silicon that have low solubility in aluminum and are impurity elements. Therefore, in this metal by etching 0,5 %-a saline solution of hydrofluoric acid easily detected phase such as FeAl3, Fe3SiAl or Fe2Si2Al9. If the interest is the grain structure of aluminum, then apply anodizing with Barker's reagent [1].
Figure 1 – Aluminium 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. 2.
0.5% HF.
Original magnification: 500x [2]
Figure 2 – Aluminium alloy 1100-O sheet, cold rolled and annealed.
Recrystallized, equiaxed grains and insoluble particles of FeAl3 (black).
Size and distribution of FeAl3 in the worked structure were
unaffected by annealing (see also Fig. 1).
0,5% HF.
Original magnification: 500x [2]
3000 series
These alloys have a major alloying element manganese. They therefore contain such phases, как (Mn, Fe) Al6 или (Mn, Fe)3unlucky12, which are identified by etching in 10 %-SG phosphoric acid solution H3PO4. To reveal the grain structure, obtained by cold working or annealing, apply anodizing [1].
Figure 3 – Aluminium aloy 3003-O sheet, annealed.
Longitudinal section shows recrystallized grains.
Grain elongation indicates rolling direction,
but not the crystallographic orientation within each grain.
Polarized light. Barker’s reagent.
Original magnification: 100x [2]
Figure 4 – Same alloy and condition as for Fig. 3, but shown at a higher magnification.
Dispersion of insoluble particles of (Fe,Mn)Al, (large) and
aluminum-mangenese-silicon (both large and small) was not changed by annealing.
0.5% HF.
Original magnification: 750x [2]
4000 series
Most of these alloys have high silicon content and are used as materials for soldering and welding, when they are melted. The casting phases are usually silicon Si and Fe particles2Si2Al9. When heat-treated silicon koalestsioniruet, whereas the iron-containing phases remain unchanged. These particles are etched 0,5 %-a saline solution of hydrofluoric acid.
Figure 5 – Aluminium alloy 4043 as-cast ingot with Fe2Si2Al9 (light) and
silicon (dark) in dendrite interstices.
0,5% hydrofluoric acid. Original magnification: 455x.
(Kaiser Aluminum & Chemical Corp.) [3]
Figure 6 – Aluminium alloy 4043 homogenized ingot.
Shows rounding and coalescence of the silicon constituent and .
the insoluble iron-rich phase remaining unchanged.
0,5% hydrofluoric acid. Original magnification: 445x .
(Kaiser Aluminum & Chemical Corp.) [3]
5000 series
Magnesium – the main element in these alloys. It has a significant solubility in aluminum;. When excess Content may be present in the form of eutectic particles of Mg2Al3. After cold rolling and annealing them can be found at the grain boundaries, and cold working, they can be allocated as a strain bands. In both cases, the structure is revealed by etching in 10 %-hydrochloric phosphoric acid H3PO4. Since this series of alloys chromium is a common additive, the compound of Cr2Mg3Al18 It may appear in the form of fine dispersoids.
Figure 7 – Aliminium alloy 5457-O plate 1O-mm thick, longitudinal section.
Annealed at 345 ºC. Polarized light. The grains are equiaxed.
See also Fig. 6, 7, and 8. Barker’s reagent.
Original magnification: 100x [2]
Figure 8 – Effect of cold rolling on alloy 5457-O plate,
originally 1O-mm thick, annealed at 345 ºC.
Polarized light. See Fig. 5 for annealed structure.
Barker’s reagent. Original magnification: 100x
10% reduction [2]
Figure 9 – Same as for Figure 6. 40% reduction [2].
Figure 10 – Same as for Figure 6. 80% reduction [2].
6000 series
This family of alloys is thermally hardened by precipitation particles Mg2Si. Etching of microstructures produced 0,5 %-a saline solution of hydrofluoric acid. Etching identifies insoluble iron type phase Fe3unlucky12 и Fe2Si2Al9, and coarse separation Mg2Si. An initial separation stage can only be seen through an electron microscope.
Figure 11 – Alloy 6061-F plate, 38 mm thick, as hot rolled (91% reduction).
Longitudinal section from center of plate thickness.
Particles are Fe3unlucky12 (gray, scriptlike) and Mg2Si (black).
See also Fig. 10 and 11.
0,5% HF. Original magnification: 250x [2]
Figure 12 – Some alloy and temper as Fig. 9,
but a longitudinal section from neat plate surface.
Porticles of Fe3unlucky12 and Mg2Si are more broken up and
uniformly distributed than in Fig. 9 (midthickness).
See also Fig. 10. 0,5% HF. Original magnification: 250x [2]
Figure 13 – Alloy 6061-F 6,4-mm sheet, hot rolled (reduced 98%);
midthickness longitudinal section.
Fe3unlucky12 and Mg2Si particles more broken and dispersed than in Fig. 10.
Most Mg2Si will dissolve during solution treating.
0,5% HF. Original magnification: 250x [2]
2000 series
The fully microstructure of these alloys can only be seen through an electron microscope. They have a very complex structure because of the large amounts of additives, which are used to increase strength, corrosion resistance or control grain size. Therefore the microstructure of these alloys multiphase, especially in the state of the cast. Etching of thin lead in 10 %-SG phosphoric acid solution. Typical phase - Al2CuMg и Al7Cu2Fe. When the content of copper 3,5-5 % they can be seen in the light microscope by etching reagent Keller.
Figure 14 – Alloy 2024-T3 sheet, solution heat treated at 495 ºC and
quenched in cold water. Longitudinal section.
Dark particles are CuMgAI2, Cu2MnAl20, and Cu2FeAl7.
Keller’s reagent. See also Fig. 13. Original magnification: 500x [2]
Figure 15 – Same alloy and solution heat treatment as Fig. 12,
but quenched in boiling water.
The lower quenching rate resulted in
precipitation of CuMgAI2 at grain boundaries.
Keller’s reagent. Original magnification: 500x [2]
Figure 16 – Same alloy and solution heat treatment as Fig. 12,
but cooled in an air blast.
The lower cooling rate resulted in
increased precipitotion of CuMgAI2 at grain boundaries.
Keller’s reagent. Original magnification: 500x
Figure 17 – Same alloy and solution heat treatment as Fig. 12,
but cooled in still air.
The slow cooling resulted in intragranular and
grain-boundary precipitation of CuMgAI2.
Keller’s reagent. Original magnification: 500x [2]
7000 series
This series of aluminum alloys containing zinc, magnesium and copper as main alloying elements, and chromium additives, zirconium, manganese, as well as iron and silicon. Therefore, the number of components and phases in the microstructure is quite large. For identification of used reagent Keller. Etched grain structure 10 %-a saline solution of phosphoric acid.
Figure 18 – Alloy 7075-O sheet, annealed.
The fine particles of MgZn2 (dark) were precipitated
at lower temperature during heating to or cooling from the annealing temperature.
The insoluble particles of FeAl3 (light gray, outlined)
were not affected by the annealing treatment.
25% HNO3. Original magnification: 500x.
Figure 19 – Alloy 7075-T7352 forging, solution heat treated,
cold reduced, and artificially aged.
Particles are insoluble (Fe,Mn)Al6 (dark gray).
Some unresolved Mg2Si may be present.
This is normal structure. Keller’s reagent.
Original magnification: 250x [2]
Sources:
1. TALAT 1202
2. Aluminum and Aluminum Alloys – ASM Speciality Handbook – 1993
3. Aluminum: Properties and Physical Metallurgy / ed. by John E. Hatch – ASM International – 1984
