Anybody here done this, soldering steel to aluminum?

All yooze soldering type folks, I want to solder a steel tube into an aluminum one. I'm thinking that if I tin the steel with the mostly tin lead free plumbing solder and
the aluminum with the mainly zinc aluminum solder I should then be able to join them together with the lead free plumbing solder. I know, because I have done it more than once, that I can get the lead/tin solder and lead free plumbing solders to wet aluminum but it is not nearly as easy as using the zinc based stuff. I know there are other ways to do this but I am looking for a fairly simple, low tech, strong and cheap, way to join the one tube into the other with a reliable method using widely available materials that won't be affected by solvents like gasoline, alcohol, acetone, benzene, and the other typical components of pump gas. This is not just for me. I also want to be able to show other folks how to do this, folks who have limited resources and education. Thanks, Eric
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On 4/29/2016 9:54 PM, snipped-for-privacy@whidbey.com wrote:

Try Sal-Met flux. It worked well for me using Lead/Tin solder.
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wrote:

That stuff seems hard to find. Eric
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It's easy enough to solder steel to aluminum, as others have explained. But there are some more fundamental problems:
1. The difference in temperature coefficients of linear expansion, which will cause such assemblies to try to tear themselves apart mechanically. If the contact area is small enough, the mushiness of solder will handle the mismatch, but the joint will eventually fatigue and break.
2. This is a built in galvanic couple, and if not dry and isolated, the assembly will rapidly corrode. The aluminum will corrode first, protecting the steel.
What are you trying to build? There are ways to avoid these problems.
Joe Gwinn
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wrote:

Gas tanks for bicycle motors. I can see how corrosion might be a problem if the joint was wet a good deal of the time but this joint will be dry most of the time. The joint is a steel filler neck soldered into an aluminum tube. With the diameters involved, and a 70 degree F rise the steel would expand .0008 and the aluminum .0015, so basically twice as much. Do you think the joint would fatigue quickly? I'm trying to picture this joint in my mind, the stresses involved. When soldering the aluminum will expand quite a bit so when the joint cools to room temp. the steel will be in compression. As the joint warms the steel will just be under slightly less compression. What would you do in this case in order to meet the requirements in my original post? Eric
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On Sat, 30 Apr 2016 09:15:55 -0700, snipped-for-privacy@whidbey.com wrote:

This is not from personal experience, but when making solder and brazed joints between dissimilar materials (particularly those with widely different TCoE, like aluminum and steel), industry practice is to design as much overlap in the joint at possible. Two pieces joined along a narrow seam line are considered to be the least desirable situation.
You want as much shear area as you can get. This is just a general design guideline, though. Different circumstances can be different in joining.
--
Ed Huntress

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On Sat, 30 Apr 2016 12:34:56 -0400, Ed Huntress

The idea is to have the steel filler neck inserted into the aluminum tube about 1/2". That oughta give me plenty of overlap. Eric
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On Sat, 30 Apr 2016 12:42:25 -0700, snipped-for-privacy@whidbey.com wrote:

Some possibles:
http://tinyurl.com/btnwayp Alumiweld
http://tinyurl.com/zba93td Alloy 5
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On Sat, 30 Apr 2016 09:15:55 -0700, snipped-for-privacy@whidbey.com wrote:

Get an aluminum filler neck or a steel tube???
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wrote:

Mounting flange for the filler neck and fuel filler hose connecting the two?
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wrote:

I suppose I could go that route, but it would look cleaner if the steel is soldered into the aluminum tube. There are lots of folks putting little two and four stroke motors on bicycles. The kits come with a gas tank. Some like the tanks and some others either fab their own or buy a custom one. Lots of the people mounting these kits are doing so on a tight budget and get really creative. I thought it would be cool if I hammer form an aluminum tank and post the process on youtube or a forum. My goal is to show how this can be done with minimal tools, skill, and cost. That's why the tank halves need to be soldered instead of welded. And the filler neck needs to be sawn off of the stock tank and soldered into the fabbed up neck on the new tank. Eric
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On Sat, 30 Apr 2016 12:49:59 -0700, etpm wrote:

Overall, I would expect that mild steel would be as easy to form as aluminum, and a lot easier to solder and/or weld when everything is pounded out to shape.
For chuckles, do a search on "hydroforming" on YouTube, and look at what people are doing in backyard shops with flat pieces of sheet metal and pressure washers.
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On Sat, 30 Apr 2016 09:15:55 -0700, snipped-for-privacy@whidbey.com wrote:

JB weld? Maybe it's flexible enough to handle the temp cos.
http://www.fillernecksupply.com/weld-in-filler-necks/ Weld-in aluminum necks for $3.42. Got helium?
What surprises me is that there are two different diameters. I thought gas filler necks were standardized many decades ago. <shrug>
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On Friday, April 29, 2016 at 7:51:50 PM UTC-7, snipped-for-privacy@whidbey.com wrote:

Fluxes for aluminum (fluorides) and for steel (acid) are very different. Probably you'd have to tin the parts separately, then use a milder flux while heating the whole assembly, and let solder wick in from one end or the other of the joint. There are many steps to this, it's probably easier to knurl the steel, apply some compatible sealant/adhesive (silicone?) and press the assembly together. That has the advantage, too, that you can anodize the aluminum beforehand.
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