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Problem with reinforcing Al6061 with Alumina

Aluminium 6061 is an alloy made by age hardening, in which magnesium silicate is precipitated. These precipitates make the Aluminium harder, and this is the reason behind aging it. However, for some applications the alloy Al 6061 is not is not hard enough, and the problem is how to make it harder. The idea of reinforcement with Alumina was put under experiment and when the Alumina was introduced in the Al 6061 matrix, the latter became unexpectedly soft. The problem now is how to improve the reinforcement so that metal matrix composite becomes stronger. Another question is Why does the AL 6061 get softer when reinforced with Alumina? Thanks

Aboamar Alamgriby
- London, England, UK
2003

Some language problems first. Exactly what do you mean by alumina? You have somewhat intermixed hardness and strength. How did you measure each? Heat is the only thing that I am aware of that can change the strength of the material. A little more detail please, as I am curious. If you get enough information, Lee Gearheart will probably give you a very technical answer.

James Watts
- Navarre, Florida
2003

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2003

Not much info on your composite preparation, but from experience with aluminum alloy-alumina composites, I can make an educated guess as to why the metal matrix is softer than expected. Either

a) The alloy composition is off due to Mg loss by vaporization during melt processing (especially likely if there was prolonged stirring during alumina addition). Since Al-6061 depends upon magnesium silicide, Mg₂Si, formation for hardness/strength, a decrease below about 1.1 wt% Mg will limit hardness.

I suggest analyzing the metal matrix by electron microscopy, using only those results clearly 100% metal phase. Verify that the Mg₂Si phase is finely dispersed, and that little or no Si phase is present (Si phase demonstrates insufficient Mg). Or,

b)The heat treatment procedure was inadequate. Maximum strength of a heat-treatable All alloy requires proper solutionizing, quenching and artificial aging to the T6 temper. While these are standard procedures, an inadequate quenching speed limits the maximum strength developable during aging. As the alumina phase decreases the cooling rate within your composite, I suggest quenching in ice water. And of course, overaging would also soften the metal.

For those who may have wondered, alumina, Al₂O₃, is thermodynamically in equilibrium with Al-6061, so no side reaction such as MgAl₂O₄ (spinel) formation would occur. Also, addition of alumina (particles or fibers) to Al-6061, i.e., composite formation, increases the bulk (composite) strength, not that of the alloy phase

Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.