Aluminum Alloys for a formed part and tube weldment

rcoope · Sep 3, 2018

I'm involved in the design of an electric vehicle that's at the bicycle end of the power spectrum. It looks like the best way to do the frame is a combination of laser cut and formed parts welded to stock tubes. This has been a common approach in the bike industry, see here for example:

. The entire upper part of this frame is two formed sections which are welded edge to edge to each other and to the head tube and down tube etc. Obviously a lot of motorcycles are made this way as well, with some mix of cast, extruded, formed and machined parts all welded together.

The question I have is what would be an appropriate alloy? I'm pretty sure I want to get 6061-O for the laser cut and formed parts, and then regular 6061-T6 for the tubes, weld and send the whole thing for heat treating. Obviously T6 sheet will crack when bent. 6061 was the classic allow for high performance welded parts in the bike industry, though 7005 is probably pretty common still. The manufacturers make up their own names for the alloys so you can't easily tell what is being used today. The one curious thing is 6061-O is not as commonly available as T6, eg. MetalSupermarkets doesn't have it on their website. But it is available. Also, my strategy is dependent on O being readily weldable to T6 and then properly heat treated together but I assume that should be fine. I haven't actually asked any potential experts like my favorite laser/forming shop what they think since this just came up on the weekend, but I'd welcome thoughts from PMers.

plastikdreams · Sep 3, 2018

Personally I'd go with billet aluminum...everyone knows it's the best.

Welding 0 to t6 should be ok, not sure if the heat treat will involve an annealing cycle before t6 treatment.

Fabworks · Sep 3, 2018

Since you are planning to heat treat post welding, look at 4943 filler as it gains strength from heat treating as well.

Product Details - Hobart Brothers Performance Welding Products

PlasmaOnTheBrain · Sep 6, 2018

T0 6061 only stays T0 till it comes up to room temp... Hence the lack of availability. It starts age hardening pretty quickly. I've heard tales from a co-worker that he had to drive to the heat treater with a cooler of dry ice, pick up his parts, run back to the shop and squish them in the break press before they got up to temp, otherwise they would crack.

Rob F. · Sep 6, 2018

I used to weld full suspension mountain bike frames very similar to what you are looking at doing back in 1990-'92 and it was like you are thinking 6061-T0. We had no problems having to keep stock cold so it would form. Blanks would sit for months before being formed.
Almost positive the tubes and lugs were all 6061-T6 and it all went to heat treat, came back as T6

Rob F. · Sep 6, 2018

I was remembering having a few scrap pcs of the T0 sheet that were left over from prototyping. I am curious to see what condition they are in now after aging for 25 or so years. I just need to remember where that stuff might be....

PlasmaOnTheBrain · Sep 10, 2018

I kinda hope you find them Rob, im curious my self lol
I know you can bend 6061 T6 with out it cracking, but it *Has* to be a large radius. Tight radius bends end up in 2 pieces...
I've made a few hooks out of 6061 T6 round bar and bent just find with no cracking issues. It was 1/2" round bar bent to about an 8" radius...

Rob F. · Sep 10, 2018

Depending on the job it can also be heated with a torch to help it bend without cracking. Kind of hurts the T6 aspect a bit though.
Bends and forges great at temperature. Be careful it will just crumble if to hot :mad5:

Lanso · Sep 11, 2018

Rob F. said:
I was remembering having a few scrap pcs of the T0 sheet that were left over from prototyping. I am curious to see what condition they are in now after aging for 25 or so years. I just need to remember where that stuff might be....

I can say for certain that 25 year old 6061-0 aluminum will still be soft and easily formed. To be sure I just went out to the shop and found some - bent a corner of .125 with my thumb!

rcoope · Sep 11, 2018

I have subsequently talked to various people who have done small projects by spot annealing 6061 with heat, bending it and then welding and just leaving the finished product to age. the folk wisdom is it gets up to T3 or so but that can still be formed OK on its own. So doing super pro deep drawn aircraft parts might require special handling of the annealed material but your overbuilt homebrew motorcycle frame is apparently OK. I have an email into my local laser shop about how easy it is for them to get the stuff and I have to find a heat treatment shop. I'll update the thread as this develops. It had better work since my formed part and tube design is getting pretty clever if I do say so myself!

Rob F. · Sep 11, 2018

How will the parts be formed? What thickness?

rogertoolmaker · Nov 2, 2018

Consider Alum. Alloy 5052H32. Forms and welds great.

Roger

adama · Nov 3, 2018

Don't quote me on this, but im pretty sure when it comes to bending tube, most benders buy the tube in a T4, T6 is to hard - will crack at tighter radius bends. T4 will bend and form just fine and only needs a bit more cooking to get to T6.

My understanding that whilst old T0 may be still bendable its not T0 after sitting around at room temp for long, hence why the suppliers can't sell it as such. Probably all depends on how extreme the forming is and how picky the customer is about cracking as to weather a T0 or a T3 grade matters.

rcoope · Nov 4, 2018

Hi Adama, I had come to more or less the same conclusion, you can only anneal to T0 and then form within a single controlled process so you'd have to do it all on-site under a proper SOP if you were doing anything serious like aircraft. But the T3-T4 temper does seem it's likely OK for reasonable bend radii. So the plan, (and in belated answer to Rob F's question) is to get get .125" 6061-T6 sheet, the only 6061 that's readily available around here at all, laser cut at our regular shop, send it for annealing to a specialist half hour away, then get it back to the laser shop for forming. Annealing will be $200 per batch so the prototype will be $$$ but once you do parts for five or ten frames in a batch it won't be bad. And the end result is you can have a proper welded and finally heat treated frame which is like a factor of two stronger than 5052 or un-heat treated 6061, so it can be properly light which will make the vehicle better overall. (roger see above but the formed parts have to welded to a bunch of machined parts and 5052 to 6061 is a bit of a mystery.) The prototype should hopefully come together by Christmas so I will post updates. I also have some sheet I"m going to experiment with in our shop to get some idea of bend radius and aging time. I expect we'll shoot for 90 degree bends with at least a .25" inner radius so reasonably generous.

Rob F. · Nov 4, 2018

I wouldnt worry to much about timeframe to form after anealing. The parts we made came as T-0 sheet, was sent to a stamping shop that stamped out the flat blanks in no real rush and they were then shipped to us for forming, and we did not rush at all to form them in a hurry, days and weeks to get the whole batch formed was no problem. it was .050 and .063 thick and fairly sharp bends at the edges, pretty sure the inside bend radius was less than material thicness and never a hint of any cracking. On one shape using elastomer to press blank over the form it would take 3 or 4 pressings, press it take out the form and hammer down some wrinkles and repete 3 times till part hugged the form. No work hardening or cracks ever there either, not enough to be noticable anyway. Very forgiving material. Get a cold wire feed for the tig torch and go to town.
.125 seems awful thick if you are making frames like the picture in post #1. We made the original version of that style frame in the early '90's and it was .050

rcoope · Nov 5, 2018

Rob F. said:
.125 seems awful thick if you are making frames like the picture in post #1. We made the original version of that style frame in the early '90's and it was .050

Thanks Rob, no it's a big long recumbent frame where the main formed parts make a 6' long box to house batteries and the novel drivetrain. .125 still might be on the thick side but we need to build one and do a bunch of testing.

adama · Nov 5, 2018

Cough, blow torch, cough laser thermometer, cough bucket of water, cough definitely no longer T6, lots of change from $200 to.

1/8" is massive over kill, look at ladders, look at there cross section and strength. Key thing in alu bike design - manufacture is weld location and placement, fatigue is your biggest issue. Make the tube too rigid and your going to massifly shorten the fatigue life of the wheel lugs and such, you want it all to flex nicely as one. This is the key bit about frame design, its why the old brazed chrome molly tubed frames do so well - were near indestructible.

Rob F. · Nov 5, 2018

adama said:
Cough, blow torch, cough laser thermometer, cough bucket of water, cough definitely no longer T6, lots of change from $200 to.

I agree completely with heating with torch to soften the temper. Dont overheat it and all is good enough to do do the first couple prototypes. I dont think that quenching in water is a requirement, just the act of heating will soften it weather air cooled or water cooled. Cooling in water just lets you handle it sooner, without waiting for it to air cool.

edit to add links:
Annealing (softening) Aluminium Plate | process.arts
Softening / Annealing Aluminum w/ Gene Winfield - SEMA 214 - Eastwood - YouTube
I have used the burn off the soot method many times and is very easy, permanent marker never completely goes away but will get quite a bit lighter, but will no longer come off with solvent.

Strostkovy · Nov 5, 2018

At my old job we had no issue bending 6061 T4 at 90 degrees with a punch radius the same as the material thickness. We also did T6 occasionally but that took double the radius.

rcoope · Nov 5, 2018

I am coming round to the thought that .125 is thick but it's hard to predict as this thing is really part way between a motorcycle monocoque and a bike frame. And we know in motorcycle frames they need to be more or less rigid but there is some subtle flex that's important for handling and took a long time for the manufacturers to get right.

Re torching vs oven annealing, it's partly a matter of logistics as the laser cutting and forming have to be outsourced and the laser guys would look at me uncomprehending me. Also we need a scalable process so if and when 100 get made then there is a straight forward workflow and at that point the batch costs are pretty low. Another option would be to see if thinner T6 with a large enough bend radius would be OK, but that also makes me nervous since you'd probably get progessive cracking at the bends that would be hard to initially see and then might vary with batches so annealing would be better. I'm almost done the design and then we'll go talk to the welder who is a highly experienced custom bike builder so more in due course.

Thanks for all the responses by the way.

Aluminum Alloys for a formed part and tube weldment

Stainless

Diamond

Hot Rolled

Cast Iron

Diamond

Diamond

Cast Iron

Diamond

Cast Iron

Stainless

Diamond

Cast Iron

Diamond

Stainless

Diamond

Stainless

Diamond

Diamond

Titanium

Stainless

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