Compatability of metals?

Gary_in_Indiana · Jun 28, 2003

I'm looking to build a little project and, not being at all familiar with this topic, thought I'd better check it out with the TBN brain trust.

I'm looking to make a frame out of aluminum for its light weight and some mounting brackets for it out of steel for its strength. I'm currently thinking of just bolting, or screwing or rivetting the two metals together. Is any method or type of connector superior to another? Is there anything of which I need to be aware when doing this? Thanks. /forums/images/graemlins/smile.gif

bgott · Jun 29, 2003

That's a tough question. I went to Google and punched in " galvanic action" and came up with a ton of informative sites. Here is one site with a chart at the end that says "maybe". It depends on what you're going to use it for. I've used steel brackets on aluminum on fittings for commercial fishing boats in the ocean and had the aluminum last longer than the steel. If whatever you're building never gets wet and you have no stray electrical currents floating around you probably won't have a problem.

http://www.clihouston.com/howmetals.htm

bgott · Jun 29, 2003

Here's a site with a fastener chart.

http://www.coastalfasteners.co.nz/galvanic_action.htm

bgott · Jun 29, 2003

I'm on a roll now! /forums/images/graemlins/laugh.gif Here's a site with some hints on fittings.

http://members.eaa.org/home/homebuilders/authors/bingelis/Making%20Fittings%20-%20Part%203.html

I chased back through the site and found the other two fitting articles along with a lot more neat stuff.

http://members.eaa.org/home/homebuilders/building/basics/articles.html

Anonymous Poster · Jun 29, 2003

Gary, hard to say without knowing what kind of strength you need for the brackets, but a couple of thoughts. Some of them might apply.

Aluminum can be as strong as steel if you have enough of it in the right shape. You might think of making the brackets from aluminum. If you don't need to be able to remove the brackets, they can then be welded to the frame and the strength is further improved.

Stainless steel fasteners are tough to hurt, and they don't hurt other things. Of course, they're more expensive, but they're probably the best choice if you are going to mix and match materials. We have a problem with that sort of thing in tropical South Florida, and it works here. You will have a lot less problem in Indiana.

If you just need to keep things together, rivets are OK. I assume you are talking about pop rivets; the sort of rivets you peen over are strong enough to hold skyscrapers, bridges, ships, etc. together, but the strength is needed in shear, not tension. Rivets don't do a real good job of pulling things together. Screws are better, but they can loosen. Bolts are best, especially if you need tension as well as shear, or if you need to pull things tighter. Bolts with lock nuts or loctite don't loosen as easily, if at all in the real world.

You can reduce or eliminate galvanic action between the aluminum and steel with an insulating gasket. Could be as elaborate as rubber or as simple as roofing felt; nowadays a bit of plastic is probably easier to obtain. Got an old waste basket splitting open? Cut strips out of it. Use SS fasteners and the galvanic action won't be transferred through the connection. Test the fasteners with a magnet - some SS is better than others. If a magnet won't pick them, it's as good as you're going to get from the home supply store.

hazmat · Jun 29, 2003

Bgott hit it on the head - galvanic corrosion. Easier to only have to match bolts to the aluminum than bolts to the steel & aluminum.

Pound for pound "regular" (6061-T6 extruded) Aluminum is stronger than "regular" (A36) steel. I say regular because there are lots of space age alloys that break the "rules of thumb". Steel is used because it is cheaper or if you need lots of strength - weight be dammed.

Let us know what this project is - we can help make sure you've got enough aluminum.

Bolting is the easiest joint to predict the strength of vs rivets, welding, brazing, gluing etc. - thus it is the most "conservative"

PS Any progress on the 5th wheel convertible car hauler?

Gary_in_Indiana · Jun 30, 2003

Hazmat,

I'm just looking to improve upon the wooden frame DIYGuy used for his canvas canopy. His frame was wooden (birch, maybe?) and I'm looking to make one of aluminum. The stock is flanged 3/4" square tubing. There is a manufacturer just up the road which manufactures plastic connectors for such things so I don't need to worry about welding aluminum, etc., to get my corners done.

For the canopy material itself, I was planning on having the shop which does my seat repairs make something with boxed corners and either an elastic edge or ties not unlike a fitted bottom sheet for a bed.

As I have a backhoe, I'll make this so it can slide back and offer some sun protection for using that, too. I already have the frame assembly designed and drawn and know it'll do all I ask of it (but I keep asking more, of course). /forums/images/graemlins/smile.gif

If I knew how to do something CAD-like and show it as an attachment, I would. For now, I guess it's something I'll have to make them photograph to post.

As to the gooseneck trailer, that's not even in the design stage yet. /forums/images/graemlins/blush.gif I'll need to get quite a few projects done before I can even begin to work on that. /forums/images/graemlins/frown.gif It's in the "I'd like to build one of these" stage right now. /forums/images/graemlins/crazy.gif

hazmat · Jul 1, 2003

Sounds like the aluminum just has to support itself & the canopy. Shouldn't be a problem as long as you keep in in the garage during tornados /forums/images/graemlins/tongue.gif