Rate of cooling of aluminum parts after a glycol heat process

[dsaun777]

I work for an aerospace supplier across different departments. I was recently shown how the production workers straighten aluminum parts after a glycol heat solutionizing process. The aluminum parts undergo a heat treatment for a few hours and are quenched in glycol.

I can't get too in-depth in the process because it is for a government contractor. But after treatment, the parts are cooled to about room temperature without refrigeration and then straightened only within a short period of time. If the parts are not treated within that short period of time they are placed in sub-zero coolers to be straightened later. Wouldn't the cooling rate of the freezers make the grain size smaller and the part harder to straighten? Would it not be easier to let the parts cool at a slower rate?

I am puzzled by this because it seems to go against what I thought I learned in a manufacturing class. What is happening on the molecular level that would make the parts easier to straighten if they are rapidly cooled as opposed to slowly cooling? Thank you.

 

[Baluncore]

Steel requires rapid chilling to freeze it with a hard fine grain with carbide. Copper and aluminium alloys are different, they must be cooled rapidly to preserve the annealed state.

Heating the aluminium anneals the alloy, quick cooling preserves the soft state at room temperature for a few hours, or until it is work hardened, for example, by bending or cold drawing. If it was cooled too slowly, secondary crystals would have time to precipitate at grain boundaries in the alloy, making it strong and hard, faster than it was cooled. Only a few aluminium alloys behave in that way, hardening over time.

By quenching it quickly to room temperature, then placing it in a freezer, the precipitation of secondary crystals that harden and strengthen the alloy is delayed, so it can remain workable for more than just a few hours.

https://en.wikipedia.org/wiki/Precipitation_hardening

 

[dsaun777]

Great response, thank you. How does quickly cooling the alloy delay the precipitation of secondary crystals?

 

[Baluncore]

How does quickly cooling the alloy delay the precipitation of secondary crystals?

The rate of the migration and precipitation is reduced at lower temperatures, so the alloy must be taken quickly from the annealing temperature to cold. That explains why it is rapidly quenched in a cold liquid that has an extended liquid temperature range.

It is the secondary precipitation that locks the crystal boundaries and prevents cracks from propagating, which strengthens the material.

 

[dsaun777]

The heating of the alloy is what make the FCC into BCC, then the quench further prevents the FCC from forming again. Right?