[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.

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" precipitates? 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?