Sunday, February 12, 2012

Welding and Brazing: a Visual

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When I wrote about fillet brazing (a lugless form of brazing) earlier, some wanted to know what was the difference between that process and welding. The quick and simplified answer is like this: welding involves heating up and joining two pieces of metal to one another directly; brazing involves using another material (one with a lower melting point) to join them, sort of like a hot glue. Last week I photographed an unpainted frame made for Josh of Bike Safe Boston that shows the two methods alongside.

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Here is a shot of the underside of the frame, by the bottom bracket area. On the right is a welded kickstand plate, and next to it on the left are two little braze-ons that are (I think) cable guides. Notice the pools of golden liquid (melted brass) that surround the braze-ons, but not the welded plate. Instead, the welded plate and the chainstays it is attached to are a sort of rainbow colour, from the weld pool that is formed when joining the two pieces.

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Similarly, here you can see that the seat cluster joint is welded (weld pool rainbow), but the little rack attachment point on the chainstay is brazed on (brassy border).

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My description is simplistic and leaves a great deal out, but it's an easy visual for a layperson to identify brazing vs welding on a "raw" or unpainted frame. I love the colourful look the juxtaposition of both techniques creates on this one - built by Ted Wojcik and designed by DBC City Bikes. More pictures here.

31 comments:

  1. Yes the welding process does create an interesting image to view, I wonder why people are so quick to cover it over with paint it would be nice to see bikes just clear coated for preserving the metal and showing off the builders handy-work grind marks and all. =)

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    1. there are a couple of companies that do sell unpainted or clear coat only frames......most of the time paint hides less than stellar workmanship, take a peek at a chinese mass produced frame sometime, not pretty. another difference between welded and brazed frames is the amount of heat that is put to the tubing, excess heat changes the molecular structure of the base material & in some cases can cause embrittlement. In addition, brazing is more like to produce perosity in the weld joint...this is when tiny air pockets form in the weld, it happens primary due to a lack of shielding gas or in the case of brazing, flux....these tiny holes weaken the joint, increasing the chance of failure....ride free :0)

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    2. I think that the owner of this particular frame should consider getting it clear-powdercoated. The more I look at my pictures, the more I think the rainbow pools and coppery outlines would look great "in the nude."

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  2. That's mostly correct. The two methods work very similarly in most regards. When welding, a current is passed through a metal wire that will melt when it is in close enough proximity to the pieces being welded to 'ark', that is, when the pieces of metal are in close enough proximity that the electric current can overcome the static potential of the air buffer between them to complete the circuit by jumping to the metal being bonded. The wire electrode will melt and be carried over the arc with the electricity and bond to the welding pieces. When this cools, the characteristic bead pattern is left behind.

    When fillet brazing on the other hand, a similar wire is used to join the two pieces together, only it does not act as an electrode to carry the voltage across which heats up both the wire and the pieces being joined. I instead, a blowtorch is used to heat te receiving pieces until hot and then thermal wire is also put under the fire to melt into the weld.

    In both cases the metal being welded and the filler wire are heated until melting, and then the filler wire is allowed to seep into the gap and cool, acting as a glue. In fillet brazing, the heat source is a blowtorch, while in welding the heat source comes from the resistance of the metal to the electric current being applied to it.

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  3. I could be wrong but welding generally involves a filler material. It is heated to a greater degree than brazing and it penetrates both parts that are welded and the filler material melts into them.

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    1. Yes there is a filler material in the weld pool. I did not want to complicate things by mentioning that.

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  4. You might want to also note that brazing can often be accomplished without weakening the tube while welding cannot be done without affecting the tube properties. That difference is why the best frames used silver braze back in the day. That being said, in many locations, the local effects of a weld are not important - such as for a kickstand plate.

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    1. Is there anything in the way of statistics comparing actual failure rates for lugged, fillet brazed and welded frames? I have seen loads of broken lugged frames in my day, but not many broken welded frames.

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    2. Anna - As Richard Risemberg suggests below, in the past, higher end tubing was not necessarily designed to withstand welding. The cheaper bikes of the era used tubing thick enough it could probably withstand a nuclear blast.

      Changes in tube manufacturing and better TIG welders today have for the most part eliminated all but aesthetic differences in bicycle tube joining.

      My two bikes are lugged steel. Well done fillet brazing make the best looking bike in my opinion. Only a few have the skill necessary though.

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    3. During welding steel we have about 1300 Degrees of Celsius. Around weld creates structure called "thick grain" (in microscopic scale). Area around weld is in fact weaker than the weld itself. To balance this welded parts should be normalized in a oven. Steel is being normalized around 800 degrees of Celsius. During brazing temperature is about 900 Degrees of Celsius and during brazing there isn't created structure called "thick grain" (in microscopic scale). Brazed details thus don't need to be normalized. But technically speaking weld (+ normalizing) is mechanically stronger than brazing. Not every welding facility takes care of normalizing steel frames after welding.

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  5. On some of the shots, the kickstand plate one in particular, the brazed joint hasn't been cleaned of flux residue - the black area immediately surrounding the brass is an area where the flux got nice and hot, and the white area past that are ares where the flux stayed very cool. Cleaned up, the brazed joints would look much nicer.
    BTW, the guides are hydraulic cable guides for disc brakes, but an be used for other things as well: cables are held in place with either little plastic lips or zip ties. I've used a set on my commuter to attach my hub dynamo cable to my frame and fork.
    Mark

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    1. It does look like the frame should be cleaned before going to powdercoat.

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    2. I think people are missing an essential point. In a weld, the metal is raised to a high enough temperature to actually MELT it. More melted metal is added to the joint. The welding process does not "glue" the pieces together, but melts them together and in effect makes them one piece of metal.

      The heat to do this can come from an electric arc or a gas flame, usually acetylene.The high heat will modify the properties of some metals, but not stainless steel or aluminium. Alloy steels should be welded with a filler rod that matches the metal being welded. High carbon steels will lose most or all of its "springiness" by being melted.

      Brazing is more or less like glueing two pieces of wood together with liquid glue, except the "glue" is another metal that has a lower melting point than the metal being brazed. There is no melting of the metal being brazed.

      This is a bottomless subject. Google it if you are really curious. I put in seven years as a welder in industry at one point in my life. With the proper equipment and technique, one can weld a metal and change the charactoristics of it to a very minimal extent. My area was high pressure pipes.

      Leo

      Leo

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    3. Thank you Leo for your informative post.

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  6. interesting frame! is it a single speed?

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    1. I think it will be set up with a 3-speed drum brake hub.

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  7. The welding vs brazing debate is like the gin vs vodka martini debate: all a matter of taste!

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    1. Um... What debate? a "vodka martini" is not a martini!!

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    2. I think this comparison should be thoroughly tested.

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  8. Many of the newer steels (ie Reynolds 7xx and 8xx series) are specifically designed to be welded without becoming brittle. They can also be very strong at lower wall thicknesses, thus giving a light yet sturdy frame.

    They can, of course, be brazed as well, with or without lugs. Lugs do add a bit of strength--I have seen them referred to in technical literature as "stress distribution collars"--but welds are strong too, and the techniques of welding are widespread and well-developed.

    An acquaintance of mine who has a very high end machine shop says that his welder can weld two soda pop cans together without harming the metal, and has done so to show off.

    For very light tubing, such as bike frames, I prefer lugs, but I have one of each right now. However, the lugged bike is 45 years old, I have put over 30,000 miles on it myself on rough LA roads (who knows many miles it covered before me), and it is a very light ex race bike, so I'm pretty comfortable with lugged frames! (In fact, it has fenders, rack, & lamps, and is STILL light!)

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    1. Basically, it's the air-hardening steel alloys that are said to actually benefit from welds. In the Reynolds range, these are the 853 (heat-treated) and 631 (cold-worked.) The current 725 stuff is heat-treated chromoly; the classic 753 was heat-treated manganese-molybdenum steel. These can be safely welded, but they don't share the air-hardening benefits of the 631 and 853.
      -rob

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    2. Just so I understand this, are you saying there is an advantage to welding those particular tubing models as opposed to brazing them? or only that there is no disadvantage to welding them?

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    3. There used to be a disadvantage to welding bicycle tubes and now it's not an issue if tubes designed for welding are used.

      Lugs still function quite well as stress distribution collars as RR puts it. For frames that will be used and abused for many years lugs still have some theoretical advantages. Realizing those advantages requires a lot of labor and a certain amount of weight.

      Making a frame weld work requires tight mitering. It's an absolute precondition. A bad miter will be difficult to weld and will usually show. Lugged frames should oughta be mitered just as precisely but you never really know. The welded frame also can be mitered and built to any angle while the lugged frame has to conform to the angles built into the lug, or the builder has to do significant work and/or make significant compromises to adjust the lug.

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  9. Veloria,
    I think the pictures of the Wojcik frame illustrate the visual differences between welds and braze-ons. I also think your explanation was good; surely a simplified explanation, but that was the intention, wasn't it?

    I think it might also be helpful to show some pics of a frame where the main tubes have been joined by fillet brazing next to a frame where the main tubes were TIG-welded together. Of course, your readers can easily images.google that themselves, to compare the smooth and seamless fillet brazing to the stack-of-dimes (on good examples) of a TIG weld...
    -rob

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    1. It is surprisingly difficult to find pictures I've taken that are comparable side-by-side. Ideally I would love to photograph 2 unpainted frames representing each method next to each other, but that kind of opportunity has not presented itself yet.

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    2. It's true that these opportunities don't present themselves often, unless you find yourself in a frame shop that produces frames with both TIGged and fillet brazed junctures. I found these 2 close-up pics of unpainted BB shells, one TIGged into a frame, the
      other brazed. (I'm currently a student intern in a special education classroom, and it's helping me figure out methods to use photos to clearly illustrate concepts. Now, if only I could *take* a decent photo.)

      hth
      -rob


      http://www.kirkframeworks.com/images/bb_for_index.jpg

      http://t3.gstatic.com/images?q=tbn:ANd9GcTRvLN9dEULegn9TKX-7HzlTJ1ZqUOb3L0jHTS9f_Rnjn1ZW0BF1pTF_7Th

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  10. The fork crown in the flickr set is gorgeous! Is it a Pacenti or did the framebuilder make it?

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    1. I was wondering that myself. I assumed it was a Pacenti initially, but it does not seem to resemble any of their models.

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  11. I just really like the look of the welding pool I guess. Kinda like the oil-sheen on a puddle of water. A little strange, I know, maybe it's my way of celebrating not being color-blind. I've been told I'm tone-def so I try not to sing too loud.

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    1. I like it as well. A raw or clear-coated welded frame could look really nice.

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  12. You're correct that welding bonds one tube directly to another tube and brazing bonds two tubes together via either a lug or a fillet so that the two tubes are actually bonded to either the lug or the filet rather than directlt to each other. I think it bears clarifying (and I'm only about 97% certain of this) that a lug is an external sleeve that the tubes to be joined slide into whereas a filet slips inside the two tubes to be joined. Nicely carved lugs can add visual appeal to frame (like all of those frames at Rivendell) but a properly done filet joint is a seemless joint that's pretty much invisible.

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