> >> > Stay away from the MIG. It will not deliver

 >> >  > > > aircraft quality welds.  TIG or Oxy/actyl are the two choices.
 >> >  > > > Jerry

 >> >  > I'm not a welding expert but would be interested in understanding
 why MIG
 >> >  > won't provide quality workmanship on aircraft.  --  That's
practically
 >> all
 >> > I
 >> >  > see them use at the race track.  Everyone keeps saying the
same thing,
 >> but
 >> >  > no one ever explains why.  If you can de-mystify this,
those of us who
 >> >  > aren't certified welders would appreciate it :-)
 >> >  >
 >> >  > Thanks,
 >> >  > Robin

 >> The MIG (metallic Inert Gas) welder is just fine for non heat treatable
 >> alloys and some aluminum. The rate of deposit and rate of cooling is very
 >> fast. On low carbon steels this is a good weld, since there
will be little
 >> strength lost from rapid cooling. You can weld mild (low
carbon) steel, and
 >> drop it into cold water without causing a problem. This is not
the case with
 >> SAE 4130 and others. On thicker pieces the material should be
warmed before
 >> welding and covered, or dropped into a lime box to cool very
slowly. On thin
 >> wall tubing the problem is much worse. A breath of air can chill the weld
 >> area quickly enough to install a near random selection of treated zones
 >> across the weld area. If TIG (Tungsten Inert Gas) is being
used, the joint
 >> should be reheated and cooled very slowly. This can be done by
covering the
 >> joint with a drape of nonflammable material (not touching the work). The
 >> reheat would include the weld and the tubing out from welded
area for an inch
 >> or inch and a half. Gas welding is so slow that a large area is
heated just
 >> doing the weld, so all that is left is the drape and slow cooling. This
 >> returns the material to close to the as shipped, annealed condition.
 >> Lynn E. Hanover

"John M. Larsen" wrote:

Hello Gang
Years ago I used to worry a lot about aircraft companies using MIG welders on their
4310 frames and components because of all the valid reasons mentioned on this tread.
Since then, I have studied a couple of dozen wrecked Kitfox, Avid and other 4130 tube
framed aircraft and found when these planes crash, the frame tubes always break or
bend near the middle of the tube. None ever failed at the weld.  Faced with these
facts, I pushed my opx-acetylene outfit in the corner and went out and bought a MIG
welder for my frame work.
John Larsen

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