7005 artificial aging procedure questions

[Buck Kempner]

Good afternoon everyone, 

I don't know how many of you have experience with 7005 but here are my questions:

How do you recommend post weld heat treating a mix of 7005 tubing? Dedacciai specifies artificial aging at 248F for 12-16 hours while Fairing's artificial aging procedure is 6 hours at 200F followed by 4 hours at 320F. 

Does it matter which one I choose? It seems like both processes would work since they are both a proven process for the same alloy. But maybe there are slight percentage composition differences between the two companies and their specific 7005. I didn't do line by line comparison on each company's percent compositions of their respective 7005. 

Finally, Dedacciai says the artificial aging process needs to take place within 7 days from the manufacturing date. Does anyone know if Fairing has a recommended window within which the heat treatment needs to take place?

Any help is appreciated. Thank you,

  

Buck Kempner

Unified Bikes, LLC

45 Forge Road

Cambridge, Vermont 05444

Shop:   802-899-9188

Mobile: 802-318-0566

@unifiedbikes

[Dan Chambers]

Hi Buck,

Good to see some more builders taking on aluminium :-)

Here's what I've hoovered up over the years. Sorry for all the unit mixing, but as bike people, we're used to that, right?:

• Easton 7005 6h @ 200F(93C) + 4 h @ 320F(160C). This is what I use regardless of the brand of 7005 tubing, and with 7005/7020 mixes. 10 hours in the oven, so handy overnight. Carried out ASAP, usually same evening, just after alignment.
• Easton 7000 series Scandium 5h @ 275F(135C)+ 2h @ 300F(150C). This is what I use if there are any Sc tubes in the build, so old stock Easton or Deda 7***Sc.
• Columbus 135C(275F) for 5 hours + 150C(300F) for 2 hours.  Also in a different catalogue: 6h @ 90C(195F) + 4h @ 150C(300F). 4/5 days max after welding.
• Fairing: 6 hours @ 200F(93C) + 4 hours @ 320F(160C) within 7 days of welding (sooner is better, see graph below).
• Nova: 8 hours @200F(93C) + 4 hours @ 320F(160C)
• Dedacciai: 12-14 hrs @ 120C(250F) +/- 5C(8F). Seems to ignore the second solidus phase, and also takes longer, ie more costly running the oven.

You can see that it's essentially a matter of 'heat volume'; higher temps for shorter periods, or lower temps for longer periods, but it's all pretty much within the same ballpark.

The requirement to get the artificial ageing done within 4-7 days, earlier the better, is due to the natural age hardening process which will form precipitates. Any alignment should be done within hours, or preferably still warm. It takes around 3 months at room temp (22C) for the precipitates to fully form naturally, but the process is exponential, so the first few days are most significant (See below). Also, natural ageing is unreliable, produces poor yield strength and elongation recovery:

The second, more elevated 320F(160C) temp of the Easton/Fairing/Columbus/Nova method pushes the material through a new phase change. This creates a new solidus phase that is more stable than the one from the first cooler stage of aging (or natural ageing over time). This increases fatigue life and reduces the risk of stress corrosion cracking. Also, inter-granular stresses are reduced and made consistent, which helps yield strength recovery.

Several other friends with expertise on this in our Facebook Aluminum Frame Builder Group.

All the best,
Dan Chambers

[Buck Kempner]

Dan, 

Thanks! Yes, I'm taking on aluminum. It has been challenging for me.  I definitely have a lot of questions as I work my way through the first few frames I'm building. 

Thank you for sharing all this as well as explaining when to straighten the frames, that it is best to get the frames in the oven as soon as possible, why the second elevated temperature is more stable and that natural ageing is not stable. How did you learn about the phase change at the second elevated temperature? Do you have access to an MMPDS or something similar?

Buck Kempner

Unified Bikes, LLC

45 Forge Road

Cambridge, Vermont 05444

Shop:   802-899-9188

Mobile: 802-318-0566

@unifiedbikes

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[Dan Chambers]

Hi Buck,

I'll see if I can find a phase diagram for 7005/7020.
Quite a lot of information is from various handbooks of the Aluminium Federation (UK), Aluminium Deutschland, the Alcoa Aluminum Handbook and ASM book Aluminum and Aluminum Alloys.

The phase description etc was selected from Lon Kennedy of Nova, quoting Easton tech dept:

"Guys, the treatments do make a LARGE difference. 6061 loses about HALF of it's strength where the heat affected zone creates a fully annealed condition. The annealed condition 6061-O has a tensile (depending on how long held at temp around 800 degrees) of around 18-20KSi (fully T-6 is 42Ksi) To get 6061 back to T-6, you have to heat to 985F hold for 2-4 hours and quench in 22% glycol at 20 degrees per second- this cooling can be accomplished with glycol spray or in very special situations with forced air/gas cooling with inert gas. Bottom line is the rate of cooling must be 20 degrees/sec to get the hardness. Immediately after quench for bike frames typically there will typically be distortion that can be straightened within the first 4 hours (before onset of passive precipitation hardening). T-4 is worth about 32Ksi. To get to T-6 you have to age at about 350F for 8 hours. Back in day both GT and Trek had alignment tables within 20 feet of the heat treatment ovens. BTW - I'm from Nova and we sold around 12-14000 sets of 7005 in the heyday of Aluminum bike building. We also sold 6061, just not nearly as much. 7005 does not require the heat and quench cycle to regain it's strength, only the re- ageing process (8 hours at 200 followed by 4 hours @ 320. with 7005 the second 320 stage is vital because it not only initiates precipitation hardening- it pushes the material through a phase change that is essential to increase the fatigue life and to stop stress corrosion cracking (SCC), if this is done to spec, 7005 final tensile is 52Ksi- almost 24% stronger and fatigue life is 52% higher. (all my specs are from the Easton guys in BIKE when John Harrington was the VP of Bike.

For 7005 the first part of the treatment creates the onset of precipitates.This is where some controversy lies- Natural aging will get the formation of precipitates- takes about 3-6 months at room temp. BUT there is no guarantee of the phase change that is vital to fatigue and SCC resistance. The second elevated 320F temp pushes the material through a low temperature phase change. Easton's process was always a two stage with the second stage temp elevated. This has two effects- the phase change creates a new solidus phase that is more stable than the one from the first stage of aging. the net result is intergranular stresses are reduced and made consistent. For 7005 this has the effect of increasing fatigue life an reducing stress corrosion cracking.

I'm not the expert- but Easton had a guy with a PhD in materials with specialties in Aluminum alloys, and a couple of M.S. Mech E's that specialized in characterization of the alloys they used in all possible conditions. This included data not commercially available on area reduction (cold work) effects on fatigue, corrosion resistance, strength (tensile, yield, total elongation at failure, etc.)"

All the best,

Dan Chambers

[Dan Chambers]

Hi Buck,

All a bit above my pay grade technically. I really need to read up on metallurgy and phase diagrams.
Those steel brazing guys don't have to worry about this stuff :-)

https://www.researchgate.net/publication/221910585_Aluminium_7020_Alloy_and_Its_Welding_Fatigue_Behaviour

7020 is a Euro alloy, very similar in properties, and totally compatible with 7005 (which is more of a Taiwan/China alloy).
It's what Nicolai, Kavenz, Principia etc use in their frames.

All the best,

Dan Chambers

[Buck Kempner]

Dan, 

Thank you so much for all the info. Sorry for the delay getting back to you. I have been working on a 7005 fat bike. See attached pictures. This is the second one I have built to try and get my aluminum welding up to snuff for the two frames I will be welding today. The hardest thing to weld are those little water bottle bosses. I ended up welding on an extra 4 (8 total) to the frame in the picture just to figure out a technique for not melting over the top of the boss. 

I'm with you on the material science info. I have a lot to learn in that department as well. Speaking of which, I have a couple more questions if you don't mind answering them:

Do you verify mechanicals after heat treatment with a hardness tester or any other method?  As far as stress corrosion cracking, do you still coat your frames even after the 4 hours @ 320F? If so, what have you used and do you worry about coating the inside of the frame with anything?

Let me know if my questions are too much. I really appreciate you answering them. 

Thank you,

Buck Kempner

Unified Bikes, LLC

45 Forge Road

Cambridge, Vermont 05444

Shop:   802-899-9188

Mobile: 802-318-0566

@unifiedbikes

To view this discussion on the web visit https://groups.google.com/d/msgid/framebuilders/c41ce140-9e47-4178-bdb5-bfa58266310fn%40googlegroups.com.

[Dan Chambers]

Hi Buck,

Nice welds considering it's only your first few. I'd use a little more heat and cleaning action to get a flatter edge to the bead. Is that your own yoke?

Personally, I don't even try to weld the bottle bosses. I'm amazed at that neat job you did. I use aluminium rivnuts for all my ones. Many of the Taiwanese and Chinese factories will solder/braze their widgets with 4047 alloy and a flux, but I've never had any success using that with 7005 tubes. There are some low temp aluminium solders, but they all seem to be optimised for HVAC work, so on soft alloys.

If you really want to weld the bosses, then a bronze heatsink that plugs into the boss and holds it tight against the tube might be a way around to keep the heat more even.

The stress corrosion cracking is not a huge problem with 7005 frames, and they aren't really used in the corrosive atmosphere that's required. Here's a metallurgy paper on 7005 SCC.
I don't have any sort of coating on the inside of my aluminium frames, but I suppose some sort of passivation/chromate might help. It's certainly recommended for paint adhesion, but I prefer a good physical key from a glass bead blast, since those chromate chemicals are pretty nasty.

All the best,

Dan Chambers

[Buck Kempner]

Dan, 

Thanks for the tips on more heat and cleaning. Sharing the other options for braze ons and bosses, the metallurgy paper, the heat sink plug idea and what you do for frame finishing. I'm definitely going to avoid chromate chemicals. If it's anything like what made it into the groundwater in Hinkley, CA I would like to keep it out of my system. 

I turned up the heat today and added 5% more cleaning. I'm still struggling to get in a groove with consistency. For the same weld on one side of a tube the distance between the freeze pattern "dimes" is greater than the other side. The pattern will even be different between two consecutive welds. I'm assuming this is something that comes with practice? 

See the attached picture called "down tube to head tube weld.png". The gap in the freeze pattern at the bottom of the down tube/top tube weld is larger than the pattern on the side. Same weld settings. I think I'm just using a lot more pedal right under the head tube because I'm working the puddle down into that crotch so I add rod, work the puddle down to the root of the joint, and repeat. Then when I get to the side of the head tube, it's more of a matter of keeping the wall of the down tube from melting back so I just add more filler to keep it cool enough for the tube not to melt back.

The other things I have questions on are cleaning and these little pits I sometimes get in tacks and in weld stops. 

For weld prep I have been using acetone to clean off any oil, then mechanically clean with a stainless steel wire wheel and stainless wire brush, then wipe down again with acetone before welding. I've found that the stainless brushes really bring out the grain in the aluminum. Or at least in the way I'm using them. Is this normal? Do I need to ease off on the brush? Switch to scotch brite? See the attached picture "chainstay to yoke tack.png" it really shows the grain. 

In this same picture you can see the black soot I seem to frequently get on weld starts. Is this normal? If not, how do I avoid it? It seems to go away with the AC cleaning as the base metal heats up. 

Also in this picture you can see the little pits I get in tack welds and at the end or stop of some welds. These pits show up as I'm slowly letting off the pedal fight before the arc stops. 

The pictures "seat stay tack.png" and "yoke to bottom bracket.png" show more pitting and the "black crap" that shows up at arc starts. 

Finally thank you for letting me in the aluminum frame builders facebook group. Should I also post these questions there or move the conversation to that group?

Buck Kempner

Unified Bikes, LLC

45 Forge Road

Cambridge, Vermont 05444

Shop:   802-899-9188

Mobile: 802-318-0566

@unifiedbikes