Stress Relief Time and Temp for Aluminum

[Caity]

I'm looking into my company's long-standing Stress Relieving Process, and I don't understand what I'm seeing. In the absence of a specific call out (i.e. 'Stress Relieve' vs. 'Stress Relieve iaw SPEC at 000F for Xhrs'), how do we know what temperatures to use?

We have a posted time and temperature chart for various aluminum alloys, but it doesn't reference any specifications. It lists the time/temp for 6061 as 350F for 4h. The 'normal' Aluminum Heat Treating specification, AMS 2770 references Stress Relieving as a 'Partial Anneal' and suggests temperatures in the range of 630-660F for 6061. I've found several other forum posts and customer specifications (including top AS&D companies) that agree with our lower temperature, some of which even acknowledge that the higher temperatures given in AMS 2770 will negatively affect the strength of the material.

I asked our guys where our chart numbers came from, the answer was 'the local heat treating expert told us to stay below the lower limit for aging in AMS 2770'. I have no reason to double the process - it produces results and lines up with our customers' instructions, but this answer doesn't really fly from a quality perspective.

What's going on? It seems to be generally accepted that you'd stress relieve something like 6061 at 350F, but there is no specification referenced by any of the sources I found permitting that. Where are these temperatures coming from? Is this really a 'Stress Relief', or is it something else? Can anyone shed some light on this?

 

[swall]

350F is a typical ageing temperature for 6061-Txxx. If you stress relieve on top of this, you will reduce the mechanical properties of the product. What are you trying to accomplish? What is the product form?

 

[blacksmith37]

Sounds like an age hardening cycle. Four hours is not necessary. For steels ASME generally calls for one hour per inch thickness.

 

[Guest102023]

I agree, it sounds like someone carelessly called age hardening ’stress relief’.

I would start over from first principles:
Determine what alloy you have and research an appropriate heat treatment for the properties you want. ASM handbook would have the info, but you can also google.

"If you don't have time to do the job right the first time, when are you going to find time to repair it?"