How does magnesium aid recrystallization in aluminum alloys?

[darkelf]

I understand that Magnesium aids recrystallization in Aluminium alloys. Can anyone please explain how this occurs or better still refer me to a paper that does?

 

[Astronuc]

Start with the binary alloy phase diagram of Al-Mg. Is Mg a substitution or insterstitial alloying element? Are there any intermetallic compounds of Al-Mg?

Also, the alloys contain impurities, and these impurities, e.g. Si or O can for compounds such as silicides or oxides which have high melting points, which means they will solidify first in a cooling melt, and these sites will nucleate precipitation of the solid phase.

 

[darkelf]

Thanks, the information was quite helpful. But at high temperatures in Al - Mg alloys does Mg diffuse? You mentioned impurities such as oxygen, are there always oxygen impurites? Are there any papers on this issue?

 

[Astronuc]

Sure there are always impurities. This is inherent in manufacturing, especially if a company uses recycled material. There are probably papers on impurities in particular alloys, but I wouldn't know specifically in which journal.

One can look at manufacturing specs, say from Alcoa or Reynolds Aluminum, or look at ASTM/SAE(AMS) specs for impurity allowances. Chemically pure materials (>99.999%) would be prohibitively expensive, so many alloys will have impurity levels in the ppm range, with total impurity levels reaching about 1000 ppm. Impurities like P, S, H, N in many alloys might be in the range of 10-50 ppm for each, depending on the alloy system, and some impurities may be set in the ppb range.

Impurity levels are not specified for all elements, but rather are based on what elements (materials) may come on contact with the particular alloy or element constituent during the manufacturing processes.

Vacuum arc melting (usually triple melt) is one way to remove impurities which are volatile.

 

[darkelf]

Thank you. That explains a lot, but I think maybe I was asking the wrong question. I am more interested in the formation of oxides within the alloy and how this would take place. Within the alloy. Thats one of the reasons I was asking about the diffusion of magnesium in aluminium alloys and how this occurs.
Again, many thanks

 

[Astronuc]

If oxides are used in an alloy, e.g. a dispersion-strengthened alloy, then there are at least two ways to add them. One would be to simply add an oxide of one of the metal constitutents, e.g. alumina (Al₂O₃) or magnesia (MgO), then melt the alloy. One would need to know the solubility of oxygen in the melt. Then the melt is solidified.

Another way is to use metal powders, e.g. in an Al-alloy, one would use metal Al particles, Mg particles, and perhaps alumina or magnesia, and whatever other alloying elements are to be added, e.g. Si. The Al powder could be oxidized so that the surface of the metal Al is alumina. The powders are blended for homogeneity and then sintered. Silicon forms silicides with high melting temperatures, and they would be sites around which solidification/freezing occurs when a melt cools.

Now the powders are prepared from melts, so the alloying may occur at the powder stage, using an atomization and rapid solidification process.

During sintering, the alumina may form into particles in or between the metal grains. One should research sintered aluminum powder/product (SAP).

Aluminum alloys are not my area of expertise, but I have familiarity with various manufacturing processes.

ASM Interational (formerly American Society for Metals) has a handbook on Aluminum and Aluminum Alloys, which would answer many questions and provide many references.
Aluminum and Aluminum Alloys

See also Sintered Aluminum Powder (SAP), which is very basic.