Quoted from 509

Just a little word of caution when working aluminum parts, I rough cut everything with a Milwaukee bandsaw and  then get it to its final shape with the side wheel on the belt sander. Don't let the part heat up above what is comfortable to hold in your hand or you will remove the temper or hardness and make a weaker part, just sayin!

Good point there.

Now it looks I followed a not very suitable method to cut my aluminum parts. I had used an angle grinder with a metal cutting blade to cut all my metal parts. Obviously the metal got nice and hot, and so I cooled the part in a tub of water before further processing the part ( sanding etc.)  It now seems, I got a more brittle / weaker part.

https://www.nutecbickley.com/blog/rdi-news/quenching-aluminum-parts/

I am certainly not the authority on metalurgy keith, so take what I say with a grain of salt. As long as you don't bring the part to around 800 degrees you will be fine. aluminum conducts and gives up its heat very easily.  I always keep my finger on the part, and when I can't keep my there any longer I dunk the part in water and it instantly cools to room temperature. if you can bend the aluminum part with your fingers, yeah you got it way too hot!

If you test the temp of the part with a finger it will never get above 150 degrees.  But, if you let the part get much hotter than that the T6 temper will be lost.  Certainly 800 degrees would be way too hot.