Sorry guys for the lenght of this answer and the delay. Had to discuss it with my son as I stole the info from him (his book). Feel free to buy it! Pat
> > TIG and oxyacetylene are the methods used in certified aircraft repair and > by homebuilders for a few reasons. Note that different factories do use > TIG, MIG, oxyacetylene, and stick for apparently similar jobs, but there > are > big differences. > The real difference bewteen MIG and TIG is the > > 1) size of the weld bead > 2) Control of heat input and rate of heat onset > > 1) MIG produces a much larger weld bead than TIG or oxyacetylene. The > extra > filler material isn't needed and may add up to 5 LBS more to a large > tubular > steel fuselage. In addition, MIG weld beads often do not blend in > smoothly > with the base metals. This creates a 'stress riser' on highly elastic > structures (fuselage or light engine mount for example) that will > eventually > crack in service. Some structures are designed to be very rigid and the > increased stress brought on by a sudden change in cross-section (big fat > MIG > bead) isn't a factor. Some structures are so overdesigned to comply with > certain loading requirements that certain stress risers don't matter. A > MIG > weld bead doesn't lend itself to visual inspection because it often gives > the appearance of a 'cold weld' (insufficient penetration). It would not > meet the visual inspection standards for normal aircraft welds because > there > is no way to tell if there is complete penetration. Factories have clever > and expensive methods to inspect the welds (Xray, destroying lots of > samples, etc.). MIG is faster and that is why factories use it when they > can. > > 2) Because MIG is often started and stopped at a constant amperage, the > beginning and end of the weld won't have the same structural properties as > the middle of the weld. Again a structure may be so overdesigned to meet > certain requirements that this is not going to be a factor. Also when a > weldment is composed of various thicknesses or dissipates heat in odd ways > (like when welding near an edge), the inability to vary the heat input > while > welding may create poor welds in certain areas. This can also be a > problem > when welding very small or delicate pieces in that the starting and > stopping > of the weld create blobs of filler material in areas they aren't desired, > or > burns away the base metal. > > MIG welding produces the same chemical change to the base metal as TIG > welding and the two welds should be the same basic strength, apart from > the > factors discussed previously. The difference bewteen the two welding > methods is the application of the heat and filler material. It is > important > to differentiate bewteen strength, stiffness, ductility, etc. when > discussing materials and their weldments. > > Even stick welding has been used in the construction of aircraft parts > where > it was found to be advantageous by the engineers. > > Note that in structural analysis of welded aircraft parts, the US > government > no longer allows estimates of weld strength based on historical or > published > data (see MMPDS-01 and MIL HNDBK 17 in comparison to ANC-5). An entity > wishing to gain approval to fabricate new aircraft parts that include > weldments must submit many samples to a destructive testing lab that will > produce real data about the strength of the joints, that data being used > to > create analyses of the structures. This is due to the fact that welding > technique, welding method, and the materials used vary widely from one > organization to another. > > An exception to this is that the FAA will approve TIG and oxyacetylene > welds > on 4130 structures as long as the welds meet the visual inspection > requirements for aircraft welds. This is probably because airplanes have > been built this way for a hundred years and the same materials have been > used since the 1930's. The welds produced by these methods have been > demonstrated to not only have the strength, but the correct stiffness, > ductility, etc. for nearly every type of structural loading condition. > > The reheating of 4130 steel (normalization)after welding has been > investigated in the not too distant past and it appears that no structural > advantages are to be had by doing this. In addition, it is impossible to > uniformly heat a weldment to the correct temperature with a torch and > Tempilstick. Some factories do it (in an big oven) because of their > specific manufacturing process (special welding rod, etc.) and one cannot > make a comparison without knowing all the facts. The FAA does not require > the normalization of welds made with TIG or oxyacetylene on 4130 > structures > when the correct filler rod is used (ER70S2). > > > Hope this gives some idea about the factors involved. Read the book, > Construction of Tubular Steel Fuselages, for more information. > > David Russo, A&P, aircraft welder, pilot, BS Aero, etc. and > coincidentally, > the author of Construction of Tubular Steel Fuselages >> >> Any tips? >> >> P.S. Even if I were never to finnish this plane,( Oh I >> will) I have the some of the coolest tools now!!! >> >> Ron Smith, >> kr2ssxl, boat stage >> Cypress Ca
