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Thread: Welding vs Rivet

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    Welding vs Rivet

    Hello all,

    First off there is alot of information on this site and I would like to thank all those who contribute to the posts.

    I am interested in building an all metal plane. I am curious about the outer skin I am a TIG welder by trade and I have access to everything related to TIG welding this is why I have chosen to go the all metal route. I was thinking of welding all of the outer skins instead of using rivets. Is this acceptable or is there some reason as to not weld the outer skin ? Any feedback on this matter is appreciated.

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    CAB
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    The type and "thinness" of aluminum used for aircraft skins can't be welded.

    If you have any questions, try e- mailing this guy; he knows everything sheetmetal.

    http://www.tinmantech.com/index.php

    CAB
    Bearhawk#862

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    Thanks
    Last edited by BlackWidow; May 14th, 2006 at 11:32 AM.

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    Super Moderator orion's Avatar
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    In my line of work, this question has come up a number of times over the years. If we look at candidate materials, it may not necessarily be out of the realm of possibilities. Due to rising costs, more construction today is based on 6061-T6 than the stronger 2024-T3. If we considered only the latter, then the likelyhood of a weld working over a period of time would be virtually zero. However 6061 is weldable and even if the welding process degrades the T6, fuselage skin structures are not highly loaded so if we really stretched our imaginations, and used materials with sufficient weldable gauges, it just may be possible. Historically though, I'm not sure anyone has actually tried this.

    The strength-critical areas such as the wings would however suffer from the weld due to the significant degradation of the material's strength, so there I would not even consider the possibility.

    But I think the real problems would have to do with the welding process itself. First off, by going to thicker gauges the airplane would have to be rather sizeable, especially in horsepower, since the eventual structural weight would most likely be quite significant. I'm assuming the thinnest weldable gauge might be around .040", which is twice the thickness that we commonly use. Would the weight penalty be prohibitive? That of course depends on the design. If the airplane is designed to be light and optimized around the thinner standard gauge, then yes, it probably would be prohibitive unless you re-engineered the kit or design for a heavier empty and gross weight, and of course, a heavier engine.

    But the main issue I might guess is the quality of the final product. Given the temperature based expansion of aluminum, I can't imagine how you'd weld all that material without the final product looking like a pretzel.

    The only realistic possibility I can think of is to design a truss type fusleage structure based on 6061-T6 extruded tube. I've done this exercise a couple of times and think it's feasible. It would require quite a bit of jigging in order to keep things straight and aligned throughout the process, but the overall integrity of the aluminum truss could be made so that it is quite airworthy.

    Will it be lighter than a steel truss? For equivalent strength and stiffness, not likely. Will it be cheaper? No - steel tubes are quite a bit cheaper than aluminum. Will it look cool? Quite possibly. With a nice blue anodize after welding, it would be quite eye catching.

    In short, within significant limits it could probably be done - it's just dependent on what you're after.
    Last edited by orion; May 14th, 2006 at 12:09 PM.

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    CAB
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    If you must weld............

    Take a look at the various "rag-n-toob" planes out there. TIG is an excellent way to put these together.

    CAB
    Bearhawk#862

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    Super Moderator orion's Avatar
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    Re-reading what I wrote, I should probably clarify that my initial comments have to do with welding a monocoque skin aluminum structure as you would have in something like an RV. It seems to pertain directly to what BlackWidow was originally asking.

    For several reasons I think that welding a monocoque structure is unlikely so that leaves a tubular truss, which could be feasible. And yes, TIG is a very good process for trusses, be they aluminum or steel.

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    Re: Welding vs Rivet

    HI,

    Bridges are not riveted any moreWelding is much more expensive than riveting/bolting in large construction which is the main reason. For a given connection design in a large structure, to get the same strength it just costs more to weld.The main type of connection used these days for large structure involves bolts squeezing the two or more plates together so that the load is carried by friction and not bearing. The reason they got away from rivets is that it is much harder to tell how much clamping force you get from a rivet.




    Thank you...

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    Re: Welding vs Rivet

    The biggest reason that aircraft structures are riveted or bolted is that welding generally drives you from a strong temper to a weak temper, taking fatigue strength down with it. Where the strength is not needed, you might be able to get away with welding...

    Billski

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    Registered User vortilon's Avatar
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    Re: Welding vs Rivet

    The alloys used for aircraft construction don't lend themselves to conventional weld like TIG or others. Now Cessna and Beech both have used extensive spot welding of 2024 T3 with good success. As far as I can tell it holds up as good as riveting in the areas I have seen it used on, mostly secondary structure.

    The Eclipse jet used some magical process called friction stir welding for the skins. I am clueless as to what that is.
    We the unwilling led by the unqualified have been doing the impossible now for so long with so little we now feel it's possible to do anything with nothing.

    http://www.azairframe.com/index.html

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    Registered User Starman's Avatar
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    Re: Welding vs Rivet

    Some problems with welding thin sheet structures of the type needed for aircraft, in addition to the others mentioned:

    For TIG or MIG the weld always shrinks when it cools because it always goes on hot. Therefore you will see all the weld lines as pucker lines that you can spot a mile away (it will never win any good looking or even descent looking awards) and the sheet between the welds will bulge, but not as predictably as a fabric wing under lift because the metal will bulge in or out, and it can bulge out on one end of a bay and in on the other. If the pucker lines don't align with the airflow it will cause extra drag. It can also induce forces which will twist or curve a thinner structure even if it is jigged and well clamped while it is being welded. Welding it evenly, back and forth, in small sections, will help control this twisting warping problem.

    In a thin structure like a wing you can only weld one surface because everything needs to be welded on the inside and when the second skin goes on you can't get to the weld joints. This can be a possible problem on fuselages too, and requires a design intended for the welding process from the start.

    Friction stir welding requires some large and expensive machinery. Basically what happens is that the two sheets must be precision butted together (it may only be able to do flat butt joints and no angled joints) against a backing bar and then a hard metal tool like tungsten is 'rubbed' along the joint in small circles at high speed so fast that it heats the metal up to melting. This process greatly reduces shrinkage and warpage but no homebuilder or small company can afford either the machinery and the jig/backing bar development is not picnic either. The machine needs to be CNC programmed to know where the weld is and the thhckness of metal, etc.

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