Understanding Aluminum Alloy Aging at High Stress and Elevated Temperatures

[LauriRauhala]

Hello,

I am designing a centrifugal heat pump, which compresses air. I'm considering making the impeller from aluminum, probably from 7075, since the stresses rise to about 385 MPa when rotating at high speeds. The input temperature of the air is 85 degrees Celcius and output 125 degrees Celcius. I am wondering how the impeller will age in the process over the years, and since the stress is quite high, is there a risk that the ageing will decrease yield strength lower than my max stress, 385 MPa?

Any tips or insights are very welcome, thanks in advance!Lauri

 

[Lnewqban]

Welcome, @LauriRauhala !

What will the process of construction of your impeller will be?
General dimensions?
Type of air seals?

 

[jrmichler]

If you want to use aluminum alloy, especially 7075, at high stress and elevated temperatures, an excellent resource is MIL HDBK 5 (search exactly that). It's free, legal, and readily available online. It has been superseded by the MMPDS which is expensive, but the information in MIL HDBK 5 is still good.

There is a whole chapter on aluminum alloys, and they have a large section on 7075. I suggest that you read the introductory chapters, the aluminum chapter, and then closely study the section on the 7075 alloy. These two graphs are only a small part of the information on 7075 alloy. This graph shows the effect of temperature on the yield strength of 7075-T6:

The second graph shows fatigue strength information on 7075-T6:

MIL HDBK 5 has discussions on how to properly use this information, but it does assume a knowledge of metal fatigue and mechanical properties.

 

[LauriRauhala]

Welcome, @LauriRauhala !

What will the process of construction of your impeller will be?
General dimensions?
Type of air seals?

Thanks for the welcome. The impeller diameter is about 400 mm, and we use labyrinth seals, either axial or radial. The gas is a refrigerant, properties close to ideal gas.

 

[LauriRauhala]

If you want to use aluminum alloy, especially 7075, at high stress and elevated temperatures, an excellent resource is MIL HDBK 5 (search exactly that). It's free, legal, and readily available online. It has been superseded by the MMPDS which is expensive, but the information in MIL HDBK 5 is still good.

There is a whole chapter on aluminum alloys, and they have a large section on 7075. I suggest that you read the introductory chapters, the aluminum chapter, and then closely study the section on the 7075 alloy. These two graphs are only a small part of the information on 7075 alloy. This graph shows the effect of temperature on the yield strength of 7075-T6:
View attachment 322381

The second graph shows fatigue strength information on 7075-T6:
View attachment 322382

MIL HDBK 5 has discussions on how to properly use this information, but it does assume a knowledge of metal fatigue and mechanical properties.

Thanks for these tips! I quickly went to their webpage, its not the easiest to navigate :) will have to reserve some hours to dig deeper into that.