What is the effect of ageing on the hardness of aluminium alloys?

[the_dialogue]

During the heat treatment of Aluminum alloys, two processes: Solution and Precipitation heat treatment were used.

Should the hardness increase or decrease from the original alloy after solution heat treatment? My reference text says that the material should be "soft and ductile" after solution heat treatment, but I am not sure if this is accurate.

What should be the effect on the hardness after Precipitation heat treatment? I was under the impression that this final hardness should be significantly higher than the original alloy and higher than the hardness measured after solution heat treatment.

Thank you.

 

[Gokul43201]

During the heat treatment of Aluminum alloys, two processes: Solution and Precipitation heat treatment were used.

Should the hardness increase or decrease from the original alloy after solution heat treatment? My reference text says that the material should be "soft and ductile" after solution heat treatment, but I am not sure if this is accurate.

It is accurate. Immediately after a solution heat treatment, you will have a soft alloy...for a few days at most. Remember, what you are doing is freezing a high temperature solution at a metastable low temperature state. It is the subsequent ageing that causes precipitation to occur, which increases the strength and hardness.

What should be the effect on the hardness after Precipitation heat treatment? I was under the impression that this final hardness should be significantly higher than the original alloy and higher than the hardness measured after solution heat treatment.

That sounds right.

 

[the_dialogue]

Thank you for replying Gokul. If you don't mind, I would like to ask a few more questions.

I seem to be confused with the whole idea of heat treatment. We start off with an aluminum matrix with precipitates in it. We heat the alloy and cool it, so that the precipitates (which blend with the aluminum to form one phase at a high temperature) "freeze" into the aluminum base. Therefore, i suppose I understand why the hardness is lower -- because the precipitates are what hinders dislocation movement in the first place, hence making the material harder. Upon the secondary treatment, which heats it to a lower temperature (150C), some of the precipitates are allowed to diffuse into the aluminum base, and once again the material is quenched. Now at this state, I would guess that there are a few more "frozen" percipitates? But why would this state be significantly stronger than the original alloy?

Why don't we simply leave an aluminum alloy lying around for years, until a lot of the precipitates have diffused and there is max. hindering of dislocation movement?

Thank you! I have absolutely no one to ask this question but you.

 

[Astronuc]

This article might help with some background -

Heat Treatable Aluminum Alloys
http://www.key-to-metals.com/Article39.htm

Thermomechanical Treatments of Aluminum Alloys
http://www.key-to-metals.com/Article105.htm

Compare temperatures with the melting point of Al - 933.47 [or 660.32 °C (1220.58 °F)].

Articles on Al and Al alloys - http://www.key-to-nonferrous.com/default.aspx?ID=Articles#p1

 

[rjholdsworth]

Having problems acheiving the correct hardness on LM25 TE, tried casting with the mag at the higher end, but made no diffrence. Any ideas would be greatly appreciated.

 

[andybarter043]

Ageing aluminium causes crystals to grow inside the aluminium crystals which interfere with the lattice and actually reinforce and harden the crystal structure of the alloy. In this new structure the aluminium alloys ability to shuffle its atoms and change shape is prevented making it harder and stronger.

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