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  1. #1
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    tube/welding

    anybody know if tack welding the frame with a mig, and the finish welding with gas will work?
    thanks,
    lou

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    MIG vs. 4130

    Generally, it is NOT a good idea to use MIG on the 4130 steel. Both hearsay and personal experience show that it leads to very brittle weld sites. Don't know why - TIG works well.
    Best way that I've found is to use the oxy-acetylene to make the tack welds. If you have a good welder person in the area who is familiar with 4130, let him/her do the final welding with a TIG setup. If not, practice a lot with the gases, or go to a vocational school class on welding.
    My fuselage (Bakeng Duce) is totally welded by me w/ oxy-acetylene, and I used tips ranging from #000 to #2 for the various tube junctions. The larger the junction - the more tubes running into it - the more heat you will need.
    Good luck with the project!

    Craig

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    yah, i new that mig produced a brittle weld with 4130, however, i was curious if rewelding (finishing after tacking) would reverse this. according to some of my reading, mig was actually used on some production aircraft! I'm not smart enough reinvent the wheel, so if someone has not already done it, is able to prove it works and can teach me how to duplicate it, i ain't doing it!!
    in any casethanks for the advice, and input.
    lou

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    Re: tube/welding

    Originally posted by lou mc carrell
    anybody know if tack welding the frame with a mig, and the finish welding with gas will work?
    thanks,
    lou
    Yes. only if you are good with the mig.

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    I took the opportunity last summer at Oshkosh 2002 to spend most of my time off the flight line and for a first time visiting the vendors and exhibitors booths almost exclusively. Both, Lincoln Electric and Miller (the most recognizable welding machine manufacturers) had side by side demo tents at the convention. They had almost all of the machines that they manufactured but a poor representation of MIG machines in both cases.

    The Lincoln people really went out of the way to demonstrate that they made the best products for welding as it relates to one off homebuilt aircraft. In the tent they had a lecturer who spoke about and demonstrated real time TIG welding on a very light gauge tube fuselage. The fuselage was a local EAA chapter project that had be previously shaped fit-up and tack welded, for the purpose of demo-ing at the show. The very knowledgeable and quite artful welder answered two of my most burning questions at the time. I’ll see if I can reiterate his general explanations here.

    I understood him to more or less say:
    “MIG is not the best choice for a tube frame construction because, the open circuit voltage has to be similar to the welding voltage,(especially true with less than top of the line machines). So, when the wire begins to feed at the moment of the initial contact, there is not enough heat (current flow) for effective parent metal penetration the first few millimeters of welded joint. The wire is cold and the parent metals are cold. Until the arc gets started for a few partial seconds…nothing welds, but cold wire feeds and melts onto the parent metals.

    The MIG machine is typically optimized (by the user) for the “midstream” welds to be the best. Once the arc is started, however, the heat travels through the metal ahead of the wire being fed just enough to bring the parent metal temperatures up to a point where the parent metals are “preheated” or more easily and correctly welded. Once the parent metal is in the correct heat range it is better able to be joined when the fed wire reaches it. Therefore a combination of the initial lack of heat and different temperatures down the weld, cause differences in the weld qualities from brittleness to radically varying tensile strengths and similar associated changes in the parent metal adjacent to the weld.”

    He went on to argue (and his demonstration convinced me) that better fit up and TIG tacking in a jig was the answer.

    My other question? Does a TIG welded 4130 frame need to be normalized? Its like the old saying, “If you get 10 pilots involved in any discussion, you’ll get 11 opinions”. After talking with the guys that sell the equipment, mine is that proper TIG welding keeps the HAZ (Heat affected Zone) small. The filler rod (brass coated mild steel as called for in my prints) usually alloys the metal some, so normalizing doesn’t recover total 4130 properties anyhow….any opinions?

    By the way this is not a plug for Lincoln….I own a Miller 250 dial-arc with a TIG solenoid and no foot pedal!
    Todd
    Working on a Christavia Mk1

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    Registered User Johnny luvs Biplanes's Avatar
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    Thumbs up

    Hi Todd, interesting post. There is a world-class source of welding information on The Welding Institute website: TWI It's well worth searcing through and covers every form of welding, John.
    Do ya thing in a 2 wing

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    Mig Welding

    Minor correction to Todd's reply.
    MIG & TIG are "home brew acronyms"
    MIG, or correctly: GMAW, (Gas Metal Arc Welding), has made numerous strides of late, including pulsing for non-ferrous metals, and at least one aircraft manufacturer uses it.
    TIG, or correctly: GTAW, (Gas Tungsten Arc Welding), Rods are not brass coated, it is copper.
    In fact there is a difference in steel welding rods used for GTAW.
    First is the fact that in heliarc welding the purity of the rod comes in several degrees, as well as less copper oxide preventing copper coat.
    Some of the more expensive rods have no coating at all, but are hermetically sealed in a container.

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    Archie;

    Let me ask this. What is the best filler rod to use when TIG (GTAW) welding 4130 cro-molly steel tube? I'd think 4130 filler rod with pure argon as shield gas, but yet my plans allow for the use of copper coated mild steel filler rod. Any idea why?

    Todd
    Todd
    Working on a Christavia Mk1

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    C-M Filler rod

    Numerous factors enter into the equation, but for simplification of the most misunderstood in GTAW is:
    If one were to use a 41xx to 44xx series filler, cooling stresses can/will develop, and must be relieved through the use of an autoclave or similar device. Heating with a torch applies some relief, but is not satisfactory.
    Welding with a mild steel filler such as 70s is acceptable, but requires heating with a torch as minimum for stress relief.
    Also acceptable, and in many cases preferred, is welding with stainless filler.
    In all cases, a gas lens should be incorporated, and any drafts in the welding area eliminated. When the weld is completed, hold the torch at the welded area until the shielding gas shuts off. This will eliminate oxides from forming around the HAZ.

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    Maybe you can explain why I have a few welds where the copper seems to have been boiled out of the alloy and floated to the top of the weld only to cool into a hard scale or cooled flux like blister? I know that it is not a tungsten inclusion, as it doesnt seem to happen where I have used no fill rod. Ocasionsaly however it's something that shows up after using the filler rod. Do some filler rods have a flux core or a flux layer between the copper and steel layers, like some mechanical fed weld wire?
    Todd
    Working on a Christavia Mk1

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    Tube welding

    You may have answered your own question.
    If it appears to have boiled, and has a "sandblasted" finish, it may have been overheated. A "blister" would indicate this.
    No filler rod with a flux core for GTAW welding.
    Tubular rod is incorporated in some rarer applications.
    Try turning the heat down, and fusing some scrap pieces without a filler rod on a lap weld.

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    Consider the information on the Lincoln Electric site at

    http://www.lincolnelectric.com/knowl...ly.asp?print=y

    The info has helped me make more consistent welds. But just like flying a nice insturment approach, Good welding gets better with some time under the hood!
    Todd
    Working on a Christavia Mk1

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    Thumbs up

    Hi, That's what I'm going to do when I start my Tailwind....Just have one of those Quick lighting propane torches to give it a good PRE heat before you tack with MIG.
    Gotta Fly...

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    I've been using 70S2 with a TIG welder, and I get those same little black spots where something boiled up (guessing it's the same thing Yankee is talking about). I assumed that it was some type of carbon deposit.

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    What welding rod to use wiith TIG? I think, it would be best to ask TIG welding equipment manufacturers for best rod to use with TIG. I know the proper rod works beter than the copper coated Like #7 or #11 . I am not sure of the rod number. Working at Oshkosh workshop, we did not expierence any boiling type residue. Pober Pixie and Acro builder.
    TC JohnLeitis

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