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Thread: Brazing vs. TIG

  1. #11
    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.
    David

    Torotrak (Development) Ltd
    University of Newcastle upon Tyne Graduate
    Newcastle Racing 2003-2006

  2. #12
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    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.

  3. #13
    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.
    Mike Duwe
    UWP Alumni

    Former Drivetrain Leader and Team Captain

  4. #14
    Don't forget the most versatile type of welding of them all.. JB Welding.

  5. #15
    <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 about welding 4130, the need for heat treatment, etc. Hopefully, there's some useful info in there that you haven't seen before.....

  6. #16
    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.
    'engine and turbo guy'
    Cornell 02-03

  7. #17
    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?

  8. #18
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    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.

  9. #19
    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

  10. #20
    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.
    ...we got emergency on planet earth

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