Understanding Aluminum Alloy Aging at High Stress and Elevated Temperatures

• Feb 16, 2023

• #1

LauriRauhala

3

4

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

 

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• Feb 16, 2023

• #2

Lnewqban

Homework Helper

Gold Member

3,349

1,882

Welcome, @LauriRauhala !

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

 

• Feb 16, 2023

• #3

jrmichler

Mentor

2,213

2,947

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 It has been superseded by the MMPDS which is expensive, but the information in MIL HDBK 5 is still good.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.

 

• Feb 17, 2023

• #4

LauriRauhala

3

4

Lnewqban said:

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.

 

• Feb 17, 2023

• #5

LauriRauhala

3

4

jrmichler said:

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.

 

1. What is aluminum aging in operation?

Aluminum aging in operation refers to the process of structural changes that occur in aluminum alloys when they are exposed to high temperatures or stress during their use. This process can cause changes in the mechanical properties of the aluminum, resulting in reduced strength and stiffness over time.

2. How does aluminum aging affect the performance of materials?

Aluminum aging can significantly impact the performance of materials by reducing their strength, stiffness, and fatigue resistance. This can lead to structural failures and decreased reliability in applications where aluminum is used, such as in aircraft, automobiles, and buildings.

3. What factors contribute to aluminum aging in operation?

Several factors can contribute to aluminum aging in operation, including exposure to high temperatures, cyclic loading, and exposure to corrosive environments. These factors can accelerate the aging process and lead to more severe degradation of the material.

4. Can aluminum aging be prevented or slowed down?

While it is not possible to completely prevent aluminum aging, it can be slowed down by carefully controlling the operating conditions and environment in which the material is used. This includes avoiding exposure to high temperatures and corrosive environments and implementing proper maintenance and inspection procedures.

5. How can the effects of aluminum aging be mitigated?

The effects of aluminum aging can be mitigated by implementing design strategies that take into account the potential for aging and selecting appropriate materials and alloys for specific applications. Additionally, regular maintenance and inspection can help identify and address any signs of aging before they lead to significant failures.