Heat Treating of Aluminum Extrusions (6063, 6061, 6005 alloys) 3

[Joules1971]

I work for an aluminum extruder and have been here for almost 3 years - not a long tenure here, but I have learned a lot and am not having some concerns about some of our practices. Coming from a materials engg. background, I am fairly familiar with aluminum heat treating, but here we have an in-house published spec. that shows much shorter heating cycles than I am used to. I brought it up to our extrusion manager and got the typical "that's just how we've always done it here" response and not much else. My concern is that we may not be meeting required properties for some of our customers, but am not sure how to go about making the right people aware of the situation without stepping on too many toes. I would like to know how common it is to undercut the heating time during aging like this at other companies - anyone have any thoughts?

 

[COKA]

What about the temperature? Is it the same, or is it somewhat higher? I am aware of some superalloys specs that have a long age and a short age cycles. The long age is performed at a lower temperature for a longer period of time, and the short age is performed at a higher temperature for a shorter period of time. A temperature difference of about 50 F gives say about 8 houres shorter cycles. I don't know if the same can apply to aluminum, but it can be a good start.

Coka

 

[metengr]

Way back when in one of my MET labs, we had performed a lab experiment on artificial aging (precipitation hardening) of an aluminum alloy (can't remember which series). We had to show the affects of aging time and temperature on peak hardness. As it turned out, you can achieve similar peak hardness by either increasing the aging temperature and shortening the time or vice versa. Whew, that was a long time ago.

 

[unclesyd]

As stated by previous posts there is some latitude in the heat treatment of Al alloys to achieve the desired physical properties.

6063 as an example of an alloy with a lot of latitude.
The solution treatment can be done by controlling the extrusion temper and quenching from that temperature.
The precipitation treatment for tempers T5 and T52 are 400°F and 450°F for 1 Hr while the T6 treatment is 8 Hrs at 350°F. The alternate for T6 is 6 Hrs at 360°F.

6061(extruded) doesn’t have the latitude of 6063, but there is different heat treatment for plate, rolled or drawn, extruded, and forgings.

I don’t have data on 6005.

All these alloys have a general requirement of rapid heating to solution temperature and rapidly quenching to RT. The times at temperature are started when the metal temperature is within ± 10°F of the desired .

Make sure when you HT aluminum read the notes and general requirements.

 

[Joules1971]

My concern is that the temperatures are not higher than recommended, but the times are shorter. Basically what happens is that we put the parts in the age oven, close the doors and start 'er up. The timer starts right then, and for sure the oven isn't instantly at temperature. From what I can tell we're doing the cycles "at temp" about 2-4 hours short of recommended and then using a Webster instrument to determine resulting hardness. Just seems rather flimsy to me, maybe I'm just anal.

 

[Joules1971]

Also, there is no quench at all on any of these parts after HT.

Thanks to all for your responses so far!! (c:

 

[metengr]

The only other option is to run some test coupons with several different batches of Aluminium alloys and determine if you are indeed providing the required minimum mechanical property requirements per your customer's specifications. Of course, you would have to present it in such a way that it is more of a QA/QC check than trying to confront your boss that their process of doing things in the past is messed up.

 

[COKA]

When you say that you don't quench after HT, you mean after solution HT, right? Because in your question, you are talking about ageing... This is just to make it more clear to me. Because from what I remember about aluminum, parts are air cooled after ageing. However, if in your company they are not quenching after solution HT, then I would start to get worried. What are the parts used for? Are those critical components? Aerospace? Although, hardness is a good indication that the material is OK, but I would still have a hard time letting this issue go. Why don't you make a report on the importance of quenching and ageing, and what can happen if not respected and so on. You can start by giving it to the person in charge and then go up the ladder, especially if you are responsible for that department. It can be bad for your career down the line if something ends up happening and there is no record that you did anything about it.

Coka

 

[unclesyd]

Come back with which alloy and the size(largest cross section) of the part and the heat input of the aging oven.

As I posted the parts can be solution treated fron the extrusion process. This is quite common for the 6063 material as stated.

 

[Joules1971]

OK, going back to do a little more homework on this, and I'll let you know what I come up with. Thanks so much for all your responses, you've given me some good things to think about so far. And just so you don't worry too much, we have very little automotive work here, and nothing safety related (as yet). I've decided not to let this drop though, I am determined to prove my case and make any necessary changes no matter what. What's the worst that can happen - I get fired? LOL I could use a vacation!

 

[Joules1971]

One last update...

After doing more research into this whole debacle, I found out we do have a solution HT on some alloys, but it's a forced air "quench" at the extrusion press rather than H2O. (Labeled acceptable in the ASM documentation.) Nearly everything we make also gets stretched (stress relieved) which is not called out as Tx51 anywhere in our in-house documentation that I can find (not sure if it's a big deal). Most everything also gets artificially aged at very close to the ASM spec. for time and temp. Now, though, my battle will be the "at temp." part of the procedure. At least it narrows the fight down to a more specific set of data that I can gather and provide as evidence of my concerns. Hopefully my words will not fall on deaf ears around here!

Again, thanks to all who posted, I have learned a lot from all of you AND from doing all this homework!

 

[unclesyd]

The 6061 tempers for streched are or were
6061 T4 T451X
6061 T6 T651X 300°F for 8 Hrs.

 

[IFRs]

Have you reviewed the Aluminum Association Specifications for heat treatment?

 

[TVP]

FYI, two excellent articles were published in LIGHT METAL AGE on this topic. The authors, A.J. "Bill" Bryant and Roger A. P. Fielding, both have considerable experience in this field. The first one appeared in the April 2001 issue, with the title "Interpretation of The American National Standards Institute Temper Designations for Extruded Aluminum Alloys". The second one was published in the April 2003 entitled "Downstream Extrusion Process Control: Quenching, Cooling, Aging".

The following is an excerpt from the 2001 article:

"Cooling rate for this practice [TVP-meaning press quenching] is defined as the average temperature drop when subjected to a constant cooling system from initial extrudate temperature (exit temperature) to 400 F (204.4 C). Lower cooling rates may be used if supported by satisfactory documentary evidence that such lower rates yield mechanical properties complying with all specification requirements. This also applies to the die exit-quench entry time interval. Permitted quench media include, but are not limited to, water or water/glycol mixture in a standing wave, quench tank, spray, pressurized water jets, or as fog, air blast, or combinations of these."

You can contact the publishers of LMA to order individual issues by using the following link:

http://www.lightmetalage.com/