Aluminium metallography 

The microstructure of cast aluminum alloys

 

Classification of cast aluminum alloys subdivides them according to the main alloying elements into eight series:

  • 1hh.h series - Aluminum, no less 99,00 %
  • 2hh.h series - Copper
  • series 3xx.x - silicon (with copper and magnesium)
  • 4hh.h series - silicon
  • 5hh.h series - Magnesium
  • 7hh.h series - Zinc
  • series 8hh.h - lead
  • other elements - 9hh.h series.

Castings from cast aluminum alloys are produced by virtually all casting methods: in sand molds, chill, the low and high pressure, and so on. Most aluminum casting is heat treated. The quality control of cast aluminum products play an important role to control their microstructure.

2хх.х series alloys (copper)

Usually the aluminum alloys of this series have the worst casting properties, particularly low fluidity.

  • castings continuity control light microscope on polished samples without etching.
  • Main structural components - CuAl2, CuMgAl2 and Al7Cu2Fe.


Fig. 1 – Alloy A240-F, as investment cast.
The microstructure contoins large shrinkage voids (black), on interdendritic network of AI-Cu-Mg eutectic (mottled),
and some interdendritic porticles of CuMgAl2 (gray).
As-polished [2]


Fig. 2 – Allay 222-T61, sand cast, solution heat treated, and artificially aged.
The structure consists of an interdendritic network of rounded CuAI2 containing blades of Cu2FeAl7,
and same Fe3SiAl12 (dark gray script).
0.5% HF [2]


Fig. 3 – A.lloy 238-F, as permanent mold cast.
The structure consists of on interdendritic network of rounded CuAI2 (light gray)
containing blodes of CulFeAl7 (medium gray),
and some particles of silicon (dark groy).
0.5% HF [2]

3хх.х series alloys (silicon + copper + magnesium)

Casting aluminum alloys with silicon as a main alloying element are the most popular. They have the best casting performance when compared with other aluminum alloys. Moreover, These alloys have high corrosion resistance, weldability and low density.

  • Their microstructure consists essentially of silicon particles mesh, interdendritic are formed from aluminum-silicon eutectic. This structure is etched 0,5 %-a saline solution of hydrofluoric acid.
  • After the solution-annealed products appear spheronization and isolation of the aging process.


Fig. 4 – Alloy A332-F, as investment cast.
Interdendritic network of eutectic silicon (medium-gray script),
Mg2Si (black script),
Cu3NiAI6 (Iight-gray script),
and NiAI3 (dark-gray paticles).
0.5% HF [2]


Fig. 5 – Alloy 354-F, as investment cast.
Structure consists of a network of silicon porticles (dark gray, angular)
in a divorced interdendritic aluminium-silicon eutectic and
particles of Cu2Mg8Si6Al5 phase (light gray, scriptlike).
0.5% HF [2]


Fig. 6 – Alloy 355-F, as investment cast.
Structure consists of an interdendritic network of eutectic silicon (dark gray, sharp),
Cu2Mg8Si6Al5 (Iight-gray script),
Fe2Si2Al9 (medium-gray blades),
and Mg2Si (black, at left).
0,5% HF [2]

4хх.х series alloys (silicon)

Fig. 7 – Alloy 443-F, as sand cast.
Large dendrite cells resulted from slow cooling in the sand mold.
lnterdendritic structure: silicon (dark gray),
Fe3SiAl12 (medium gray script),
and Fe2Si2Al9 (light gray needles).
0,5% HF [2].

Fig. 8 – Alloy B443-F, as permanent mold cast.
The constituents are the some as those in Fig. 7 (a sand casting),
but dendrite cells are smaller because of faster cooling in the metal permanent mold.
See also Fig. 9.
0,5% HF [2].

Fig. 9 – Alloy C443-F, as die cast.
Same constituents as in Fig. 7 and Fig. 8,
but dendrite cells are smaller because of the very rapid cooling obtained in the water-cooled die-casting die.
0,5% HF [2].

5хх.х series alloys (magnesium)

Alloys of this series are known for high corrosion resistance, good machinability and attractive appearance after anodizing. Identify the structure of the casting 0,5 %-a saline solution of hydrofluoric acid.

Fig. 10 – Alloy 520-F, as sand cast.
Structure is insoluble porticles of FeAl3 (black) and
an interdendritic network of Mg2A13 phase (gray).
See Fig. 11 and 12 for the effect of solution heat treatment.
0,5% HF [2].

Fig. 11 – Alloy 520-T4, sand cast, solution heat treated at 4250C.
Constituents are the same as in Fig. 10, but the solution heat
treating has dissolved most of the Mg2Al3 phase (gray).
See also Fig. 12.
0,5% HF [2].

Fig. 12 – Alloy 520-T4, sand cast, solution heat treated.
Solidus was exceeded during solution heat treating, and melting of the eutectic has
formed a lacy network and rosettes of Mg2Al3 phase (gray).
See also Fig. 11.
0,5% HF [2].

7xx.x series alloys (zinc)

Typically 7hh.h series alloys have good mechanical machinability and high melting point. Therefore, they are used for parts, who are going to solder. These alloys during casting often susceptible to cracking.

Fig. 13 – Alloy D712-F, as sand cost.
Interdendritic network: porticles of CrAl7, Fe3SiAl12, and FeAl6.
Note the segregation (coring) of magnesium and zinc in the grains.
See also Fig. 14.
Keller’s reagent [2].

Fig. 14 – Alloy D712-F, as investment cast.
Same constituents as in Fig. 13.
Intergranular fusion voids (black) were cowed by eutectic melting
as a result of exceeding the solidus temperature during dip brazing.
Keiler's reagent [2].

Alloys of the 8xx.x series (tin)

Casting aluminum alloys 8hh.h series, containing silicon, commonly used for bearings. Etching patterns produce 0,5 %-a saline solution of hydrofluoric acid.

Fig. 15 – Alloy 850-F, as permanent mold cast.
Note hot tear, which occurred at or above the solidus,
and some Al-CuAl2 eutectic (gray) back filling of tear.
Particles of tin (rounded), NiAl3, and FeNiAl9 (both irregular).
0,5% HF [2].

Sources:

1. TALAT 1202

2. Aluminum and Aluminum Alloys / ed. J.R. Davis – ASM Speciality Handbook – 1996