Seam welding - brazing?

[DirtyLude]

I have a 89 CRX which I'm prepping for racing. I'm doing the chassis right now, it's nearly stripped and ready for sandblasting.

I'm planning on seam welding the chassis, but I was thinking about brazing it instead. Nearly all of the joints are lap joints and perfect for brazing. The brazed lap joint would be stronger than a fusion welded lip and with a low temp braze I'm sure I could get away without warping the metal anymore than fusion welding would.

http://www.higginstribe.com/gallery/crx/crx-strip13.jpg

Anyone have any comments for or against? I'm new at this and I don't want to make a mistake, jumping in with the oxy-acet torch. Since I haven't heard of anyone else doing it, I'm sure there's must be something I'm missing.

The car wont be ready for this season. It'll be driveable for lapping sometime in the summer and I'll decide what the final modifications will be based on the 2004 rulebook.

Where can I get dry ice from? I have a crapload of tar paper in the bottom of this car that needs to come out.

[Marsh]

Why do you say brazing is stronger than welding?

My concern with brazing would simply be that it's much easier to screw up. Welding is relatively easy and much more likely to be done right by a non-pro.

[DirtyLude]

I'm still in NY and nowhere near my welding books, but two books I have sing the praises of brazing, especially for lap joints. Factors like the distance between the metals and the length of the lap can effect the strength but a brazed lap joint will have a 40,000 to over 70,000 psi tensile strength. The base metal will fail before the joint fails.

Doing a quick look over the chassis, most of the joints are lap joints that have been spot welded together. They all seem perfect for brazing.

It's also a pretty simple process, pretty hard to screw up. Apply flux, heat the area to be brazed and apply the filler metal.

[rainman]

Don't you need cleaned and prepped surfaces before brazing? If not, this sounds like an interesting alternative to welding.

[DirtyLude]

That's true, it does need to be cleaned. I usually wire brush surfaces I'm going to braze. I'm not sure how much that it would effect the join. Maybe it can be cleaned with an acid etch before hand.

When I think about it, tensile strength I'm sure doesn't really mean that much. Has anyone seen a stitch welded seam break?

[ctheo]

You can get dry ice at Prax Air. Look them up on the net. There are lots of them around.

[Gary]

First, and most important point. I am not a welder. I can MIG weld, but anyone with a MIG welder can do that. What follows is book learning, because I've never gas welded or brazed. So take it for what it's worth. The following points are relevant, and are excerpted from Welder's Handbook Chapter 7 (HP books):

"Because brazing requires less heat , and therefore results in less warpage than fusion welding, brazing is extensively used in autobody repair..."

Fusion welding mild steel - 1482 degrees C
Brazing - 566-580 degrees C

Finally, on the strength issue, the book says: the (properly) "....brazed joint is stronger than the (surrounding) metal itself".

On the strength of what to book says, I'd braze the seams. One other point. " You must never breathe brazing vapours". In the book, this point is in italics. It got my attention.

Hope this is helpful.

Gary

[Marsh]

Originally posted by Gary

Finally, on the strength issue, the book says: the (properly) "....brazed joint is stronger than the (surrounding) metal itself".

On the strength of what to book says, I'd braze the seams. One other point. " You must never breathe brazing vapours". In the book, this point is in italics. It got my attention.

Gary

The warpage factoid is true. As for a braze being stronger than the metal, that is true for some metals but not all, besides it's true of welding, for most mild steels, as well. I'm no welding expert by any means, but I can tell you that we have debated brazing the chassis on the SAE car for a couple of years and decided not to because of strength and reliability issues. The concessus is that TIG is best, MIG is a easiest and brazing is a band-aid solution to our warping problem.

[DirtyLude]

Except on the SAE car, I can't imagine alot of the joints in the chassis being lap joints. Most are butt, fillet, and t-joints, aren't they? This is a tube frame car, right? You really can't successfully braze these types of joints.

You would have to be an excellent welder to ensure that your fusion welding was high enough quality with enough penetration to be as strong or stronger than the surrounding metal and not be a weak point. With brazing it takes relatively little skill. As it is, road car chassis' are usually stitch welded to avoid warpage, so the joint will not have a full continuous weld bead, anyway.

Unless you can come up with some of the reasons brazing was shelved in your SAE project, it doesn't really help me, because the application is different.