Brazing vs. TIG

[rjwoods77]

I just helped a buddy fix a tractor with proper brazing rod and a torch. He says he prefers the process over TIG on his go-kart frames because of less warpage from the heat of TIG. I have always read that a braze joint is typically stronger than a TIG joint if the proper rod is used as well as the process being more shock resistant. Along with that it just looks better. I was also watching "How It's Made" last year and watched how they manufactured a bike by dropping a braze slug into a 4 tube coped joint, hit it with some heat for about 10 seconds and the joint was welded. I always wondered if this could be applied to a fsae car or just straight braze rod welding. I often think most people reach for a TIG out of fashion rather than thinking manufacturing process through. I have often questioned why nobody brazes since the advantages look good and since so many bike frames and go-kart frames in the past were done and are still done this way. The weld he did on the tractor went way quicker than a tig would have with half the hassle. It was a tube to a flat plate of metal.

[rjwoods77]

I just helped a buddy fix a tractor with proper brazing rod and a torch. He says he prefers the process over TIG on his go-kart frames because of less warpage from the heat of TIG. I have always read that a braze joint is typically stronger than a TIG joint if the proper rod is used as well as the process being more shock resistant. Along with that it just looks better. I was also watching "How It's Made" last year and watched how they manufactured a bike by dropping a braze slug into a 4 tube coped joint, hit it with some heat for about 10 seconds and the joint was welded. I always wondered if this could be applied to a fsae car or just straight braze rod welding. I often think most people reach for a TIG out of fashion rather than thinking manufacturing process through. I have often questioned why nobody brazes since the advantages look good and since so many bike frames and go-kart frames in the past were done and are still done this way. The weld he did on the tractor went way quicker than a tig would have with half the hassle. It was a tube to a flat plate of metal.

[VFR750R]

http://en.wikipedia.org/wiki/Brazing

http://en.wikipedia.org/wiki/Gas_tungsten_arc_welding

Not that wikipedia is 'expert' information, it seems to point toward brazing as efficient and 'strong enough' welds. Tig is more difficult and can get similar strength welds with much less filler.

If you intend to heat treat a part after welding, tig is the only choice, and both are hot enough to normalize heat treated material. Tig welding may be hotter, but done correctly the part is subjected to the heat for much less time, so there is less heat soak into adjacent material.

Both have faults, many of which can be attributed to amateur welders, such as undercutting, too much or not enough heat ect.

[mtg]

I'm not a brazing expert, but don't you need very tight fits for brazing? I don't think the filler is really considered structural, to fill a gap.

I do have a lot of experience with TIG, and yes, tight fits are preferable, but not completely necessary. I've probably filled 3/16" gaps between 0.035" wall 4130 tubing- not that I recommend it, but it is possible.

Rob, I'm going to have to disagree with you on brazed joints looking better, but that's just an opinion. I think good TIG welded joints look badass.

This is a good topic, though.

[exFSAE]

No. People think it through. TIG is by far better way to go IMO.

The big selling point being with a braze joint to be good, as I'm aware your fitup has to be on the order of .005" to get the capillary action to work. Good luck with that, with (relatively) inexperienced fabricators working either late at night, workin with some cheap beer in em, or both! We had hard enough time keeping gaps < .063", though we did it all by hand.

On the other end of the spectrum, you can MIG all sorts of nastyness. Not exactly all that great for really thin material or aluminum though (for the most part).

TIG is a nice in between. If you got decent fitup ( < .065") the welds go real quick if you're good. If you got big gaps, you can fill em but it takes a while. Can do 4130, can do stainless, can do aluminum. Good TIG welds are SEXY, too. Stack o' dimes. Mmhm.

[rjwoods77]

Just a word on gaps and amature welding. I have read a couple welding handbooks/ reference books and they state that there has to be near zero gap on a fusion weld or else you aren't getting the fusion of the two pieces and the rod together just the rod. One of the books I have and the guy repeats that a proper aerospace certified weld has to start with near no gap (less than 1/32 i would assume) or else it isn't a certifiable weld. I have been party to gap filling as well as any FSAE guy but it seems it is an amature way of getting a compromised weld. I am also have been part of the "It should be fine" thought to since redos are hard to do. For those that actual heat treat a 4130 frame how do you deal with warpage and suspension mount accuracy after the fact.

http://www.amazon.com/Welders-Handbook-Complete-Oxyacet...elding/dp/1557882649 (http://www.amazon.com/Welders-Handbook-Complete-Oxyacetylene-Welding/dp/1557882649)

[Chris Clarke]

I think there is some confusion on brazing here.

Brazing and bronze welding are two different processes.

A brazing joint needs to be similar to one used by an adhesive, where there is a very tight fit loaded in shear to wick the material in. This process is generally used with lug fabrication in bikes.
Brazing (http://www.henryjames.com/lugbig.html)

Bronze Welding, or fillet welding is similar to mig, tig, etc. in its prep. Generally the weld is a little large than tig, due to the lower yield stress of the weld material.

Personally I like the look of a good bronze fillet weld more.
Bronze Welding (http://www.kirkframeworks.com/raw%20fillet%20seatcluster%20400.jpg)

I think the advantage of both types is that there is less heat pumped into the joint, so there is no need for heat-treating. As well as the fact that you get a more ductile joint.

Another big bonus is the fact that the equipment is dirt cheap in comparison to a tig.

[Steve O]

Am I the only one who thinks that it is funny that the first 2 posts of this topic were at least partially based on a TV show and wikipedia. Not to knock either of them cause they are great, just noting that I think its funny.

Other than that, I don't really disagree with anything posted here.

Steve

[James Morris]

Id always go for the T.I.G.weld over brazed for one reason. heat.
There is no way a brazed joint from an oxy-acet set will put less heat into the material than a T.I.G. set up to pulse the weld bead.
Could you weld up your 20SWG stainless exhaust with an oxy-acet kit without turning it into a mess? i have my doubts...

[Brian Evans]

The British tube frame race car industry grew up on and still uses nickle-bronze welding. Van Diemen still uses it today. It's a great method for welding race car chassis - it's stronger than the parent material, and you can repair it easily. If you have to remove a tube, a little heat and it's gone. If you have to remove a Tig's tube, saws, grinders, and if it's a complicated joint - good luck.

Proper bronze welding is far more difficult than tig welding. Excellent bronze welding is truly the mark of an expert weldor. Getting a perfect, gorgeous bead consistently is still beyond me and I've bee trying for years. I learned how to get that bead on thin aluminium with Tig in a tenth the time. As far as heat is concerned, you don't melt the parent metal with bronze welding, you do with Tig. So the heat has got to be less. If you're doing it right, you don't even get the metal bright red hot.

For the right application, bronze welding is an excellent way to go. BTW, with the right filler material, a Tig torch and no other flux is an excellent way to braze.

[D J Yates]

I recently attended a few evening classes at a local college to get a bit of practical welding experience. I was brazing for a couple of classes and can clearly recall the instructor saying that the college's Formula Ford chassis (which he had to repair regularly!) were a brazed space frame construction. A brazed joint is allegedly 'stronger' (than a weld)(in stiffly sprung applications) due to ductility of the joint. Note that this was an evening class for the general public, not engineers, so i was never going to get a scientific answer, but having thought about this it makes sense:

A perfect welded butt joint in pure tension would be stronger due to the higher yield strength of steel. However, in reality, and when joining tubes of all different angles and sizes, the stress concentrations and thermal stresses introduced by welding would reduce the ultimate strength of the joint. Also, with a correctly triangulated frame, i would expect lower stresses with brazed joints - the relative stiffness (elastic modulus) of the brazed joint and steel tubes will alter the loading such that, for a given amount of chassis bending or torsion, the cross-bracing members will see higher tension/compression forces and the joints will see lower moments. Basically, a brazed frame will be closer to a pin-jointed structure than its welded equivelent.

As mentioned by earlier posts, there are several benefits to brazing - reduced cost and heat-treatment. But for an inexpereinced FSAE team or individuals, brazing is particularly well suited because it's so quick and easy - there's no UV arc or potential spatter so you can work in comfort and see what your doing. Also, probably most important, particularly for a prototype, is that if you decide a tubes in the wrong place then you can just heat the joint up and remove the tube - easy and far less aggressive than cutting out tubes and grinding joints down.

[rjwoods77]

Very interesting points on repair and changing after the fact. In terms of heat we would have to check a phase diagram to see what is going on with 4130 to see the effects of 800F heat and whatnot. My main aim at posting this question was about reducing "creep" when welding up a spaceframe. Some do it better than others and I wanted to pose this question to see if there is any benefit to it. We always joke about gaps but all the pros say that welding on gaps is an amature and incorrect thing to do. I always joke that everyone knows how to weld but nobody really knows how to weld. I went looking for that video of the bicycle frame construction that was on "How its made" and I have to say it was a real eye opener just like the video of the vacumm formed plastic panels that ETS mini baja did on their mini baja car in terms of "wow that looks like a huge time saver". Ill keep looking for it. Also just to note again in the Welding Handbook the author goes out of his way to show that brazing is a viable and possibly stronger way to do things.

[drivetrainUW-Platt]

Cool topic...After I read the first sentance where it said...I was fixing a tractor...I thought oh goodness.

Something that hasnt really been discussed is the metal you are welding. I know nothing about brazing or braze welding. Does it work good with chromoly, mild steel, aluminum even?

It might be the best process out there but if it does't work with the metals we are using on these cars its not useful.

Then theres heat treating, can you braze 4130 and then heat treat it, or is it not necessary with a braze weld. Would hate to see someone braze a frame, throw it in an oven, then come back 12 hours later to a pile of tubes on the floor and a puddle of bronze because it all melted and fell apart.

[exFSAE]

Don't forget the most versatile type of welding of them all.. JB Welding.

[vreihen]

<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">Originally posted by drivetrainUW-Platt:
Then theres heat treating, can you braze 4130 and then heat treat it, or is it not necessary with a braze weld. </div></BLOCKQUOTE>

In the 1940's, 4130 was used in aircraft production with oxy-acetylene welding.

Here's a seemingly credible series of articles (http://www.netwelding.com/Welding%204130.htm) about welding 4130, the need for heat treatment, etc. Hopefully, there's some useful info in there that you haven't seen before.....

[VFR750R]

i don't recall us ever mass heat-treating frames but we used to heat-treat uprights after welding when we made steel uprights. Finish machining was done after heat-treating to compensate for warpage/shrinkage.

[James Morris]

This topic has now got me intrigued, so it think i will do some tests on saturday with test pieces of 2 lengths of 1 inch T45 18SWG welded/brazed together to create a T. I think i'll try three methods of joining;
TIG welding - A31 filler
Brazing using an oxy-acet set
Brazing using a TIG set
Bosh 2 M10 holes in the top of the T, clamp it down, set up a DTI and put some force on untill failure.
Any other sudgestons?

[rjwoods77]

Moz22,

Remember that braze weld strength is all dependant on what rod you use. IF you want to do a comparison of strength then make sure you research your rod correctly and get the strongest stuff that would be applicable for spaceframe brazing.

[Brian Evans]

The rod considered the standard for nickle-bronze welding of race car chassis is SIF-Bronze No. 2.

http://www.sifbronze.co.uk/ie/menu/products.htm

I use Allstate 11, which is a silver-bronze rod with similar strength characteristics, but more eutectic so harder to get a good fillet.

Brian

[Christopher Catto]

regarding pin jointed structures and spaceframes with large offsets between tube centreline and joint:

1) if you TIG your frame, joints will take bending loads and increase stiffness at the cost of brittleness if you overload it

2) if you braze, you may well put less bending on the joint but axial forces along the tube may well have other effects like pulling the joint free

I would not want to run a frame with offset joints in any of the two cases (brazing or TIG with very hard brittle material). If your frame is well designed, then I don't think there's a big disadvantage in TIG.

I've seen various welded aluminium structures fail, now that's bad! I've seen a beautiful weld pool...that's just come completely away from the parent material! The only thing I would trust is welded frames on Yamahas and all the modern bikes or the usual oil tanks etc.

[John_Burford]

Braze vs. TIG tubular spaceframes

The main differences are skill, prep, and weight.

A proper TIG joint requires a skilled operator (not everybody can TIG), lots of prep work to closely fit joints, and post heat treatment (using a gas torch to anneal the joint MUST done). Failure to anneal a TIG joint leaves a highly brittle zone near the root of the weld.

A braze joint requires little skill, minimal prep work, and no post heat treatment. Weight is the penalty. Brazed joints need generous fillets of brass to be strong which means more weight.

A brazed FSAE frame will weigh 3 to 5 lbs more than a TIG frame. Most of UTA's frames in the 90's were brazed including two Spirit of Excellence champions. The 92 or 93 team did a fatigue test on TIG and brazed samples. The brazed sample lasted longer. Although, looking at the samples years later, it was clear the TIG samples were not annealed.

So, brazed frames are faster to make, require less skill, and weigh more that a TIG frame for similar strength.

My opinion is, if you have the skill and the time, save the weight and TIG the frame.

John Burford
UTA Alumni