artificial ageing 2

[ikikereiki]

Hi, I am investigating the influence of the accelerated or multi-step ageing of aluminium alloys (from T1 to T2 where T1>T2) to the time and strenght, does anyone have any article about this subject or any reccomendation
Best Regards
Alpay Karampas
alpay_karampas@hotmail.com

 

[metengr]

Aging mechanisms in aluminum or other metal alloys do indeed follow a distinct Arrhenius relationship between time and temperature, similar to a "C" curve behavior. In this case nucleation of precipitates and growth of precipitates are competing mechanisms. The strength level is established based on the size and distribution of precipitates. At lower temperatures nucleation rate is favored over growth so more time is necessary to achieve the same level of strength, whereas at higher temperatures growth rate is favored over nucleation rate so less time is required to achieve the same level of strength.

To answer your second question, there is information on the internet under key words like “age hardening of aluminum alloys” that might provide general information. If you have access to any college books on Materials Science/Engineering, this could be of help in explaining the precipitation hardening mechanism and dependency on time and temperature. I could direct you to metallurgical engineering text books, but I doubt that you would have these references handy. I also doubt that you will find any specific data that would answer your question.

Your last option is to run your own basic laboratory experiments regarding time and temperature for aging of a heat treatable aluminum alloy like 6061. You can vary the time and temperature and plot the hardness of the aluminum alloy. This would allow you to establish your own correlations.

 

[ikikereiki]

I am going to start the experiments next week on AA6063. Any suggestion about the T1 and T2 temperatures and time? Increasing the temperature from T1 to T2 can I get the same strenght with the T1's but in shorter time?
Best Regards
Alpay Karampas

 

[NickE]

Yes you can use shorter times at higher temps. The trouble is that things can start to happen VERY fast. Id think that aging for <2hrs on any part >1kg in mass might be teh extreme end of the spectrum. The difficulty here is that if the whole part doesnt come to temp and hold then you wont get the proper neucleation and growth.

Also AFAICR (As Far As I Can Remember) it is more important to have more homogeneous neucleation than it is to do it quickly. If the temp is too high you can get a few large precipitates instead of the preferred numerous small ones.

Also at some state of growth the precipitates can be come discontinuous with the matrix casing a consequent reduction in strength.

Nick
I love materials science!

 

[metengr]

If you have access to ASTM Standards, you can look up the specific ageing heat treatment for AA 6063. I don't have this Standard Specification handy at the moment. From what I have read, you are typically artificially aging 6XXX series aluminum alloys between 175 to 195 deg C at between 2 to 8 hours. The higher the ageing temperature, less time is required.

 

[ikikereiki]

yes but this would be two step ageing so is it ok if I choose the first temperature 170deg for 2 hours and then increase the temperature to 200deg for 5 hours, or do you have any suggestion about time and temperature. My aim is trying this accelerated ageing to get the same strenght but in shorter time.
Best Regards

 

[metengr]

For your proposed 2 step aging technique, you do not want to increase the aging temperature from 175 to 200 deg C on the same sample because it would result in an over aged or reduced strength condition when you are done. If anything, you would want to reduce the aging temperature from an initial temperature of 200 deg C for say 1-2 hours and drop back to 175 deg C for 2-3 hours for the same sample and see what happens. However, I don’t see how this would ultimately shorten your total aging time.

Why are you interested in a two step aging process? The shortest aging time would be accomplished by a single aging step at an optimized temperature. For example, select 180 deg C and hold for 5 hours.

 

[ikikereiki]

My project is two step artificial ageing. I have read an article which is accelerated ageing in AA 6xxx and shows that if a sample is aged at 165deg for 3 hours(underaged) and then the tempereture increased to 215deg the aged alloy reaches its peak about three hours earlier than conventionally aged and the strenght is higher than conventional ageing.Unfortunatelly I couldnt find any other article or research on the literature

 

[unclesyd]

According to ALCOA you can get 6063 to the T6 condition with a heat treatment of 6 hrs @ 182°C.

How was your material solution treated?

 

[ikikereiki]

a table at the article is as follows
solution temp. Ageing Temp1(deg) Ageing Temp2(deg)
500 165(2hours) 215(up to 3)
500 165(3hours) 215(up to 3)
500 165(4hours) 215(up to 3)
520 165(2hours) 215(up to 3)
520 165(3hours) 215(up to 3)
520 165(4hours) 215(up to 3)
540 165(2hours) 215(up to 4)
540 165(3hours) 215(up to 4)
540 165(4hours) 215(up to 4)

I wanted to send you the graph but I dont have it as a soft copy.The conventionally aged alloy reaches its peak some three hours later than this method.

 

[CoryPad]

ikikereiki,

You should perform a literature search on this subject. Try Scirus at:

http://www.scirus.com

metengr,

The reason for two-step ageing is to reduce costs. Theoretically, the best heat treatment will use a low temperature for the first step to form small, widely-dispersed particles. Then, a higher temperature is used to artificially age the structure to peak strength in less time (with less fuel and cost) than it would take at the low temperature. Many aluminium alloys use heat treatments that are on the order of 24 hours, so shortening them to just a few hours can be a huge cost savings.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[metengr]

Interesting subject and information.

 

[metengr]

ikikereiki,

Here is an article I found that might be of interest to you;

http://www.industrialheating.com/CD...story/BNPCoverStoryItem/0,2830,137729,00.html