I have aluminum 5052, thickness 1.5 mm.
I want to make it harder in certain critical points. I have heard on a concept using only Geometrical tools to do that, someone make it clear for me?

 

5000 series aluminum hardens via something called cold work hardening.

Essentially you have push and distort the material to make it get stronger.
Rolling it out thinner, embossing a waffle type pattern etc.

That being said, I don't think you will be too successful. Better off using a different grade of material that can be heat treated to the strength you require.