[Flydude]

I think this is an old subject and I couldn't find a forum discussion for it. However, after watching the video "Mig Like Tig", I thought I would weigh in on the subject.
First of all, I am a certified airplane nut. I build them and fly them (not RC stuff - the real thing). So, about 20 years ago I had the opportunity to visit an airplane factory in Nampa Idaho. The company was called Kitfox and they manufactured an airplane kit called the Kitfox. It is a small two-seat airplane that would be used for light bushplane type flying. Anyway, I was invited to the factory to learn how they weld their fuselages together. The Kitfox Fuselage is fabricated with 4130 thin-walled aircraft tubing. And even though the FAA does not recognize the MIG welding process as an acceptable "aircraft grade" welding process, Mig was used to make the steel tube fuselages. I won't go into all the why details right now. The point is, Kitfox used MIG as their weld process of choice. They demonstrated and taught me how they weld. The effect of their procedure makes mig welds look like tig welds. However, that wasn't their objective.

If anyone is interested in the procedure and information I have please let me know. I realize this isn't for everyone but it is great for aircraft, or any application where thin-walled 4130 tubing is required. I have used this method to fabricate aircraft structures, suspension arms, and fine or precise work of all kinds. The procedure isn't fast but it does make a full penetration weld on 4130 tubing. The heat effected zone is very small (about 1/16" wide) and the welds are more ductile than a normal weld; this is important for strength and longevity of an airframe; especially if you are going to trust your life to it.

[Otto Nobedder]

Welcome, Flydude,

You'll find there are a few EAA members and builders here. Jody, our founder, is one of them and does attend Sun-N-Fun.

There are a few active builds among our members, and airframe questions do come up from time to time.

There is a "Mig like TIG" thread, but airframes did not come up... I'd have to say this is the first I've heard of a MIG procedure for 4130 airframes, and I, for one, would love to know the details.

Steve S

[Flydude]

Full Penetration

P2080276.JPG (24.51 KiB) Viewed 1712 times

So here is how I was taught and how I have done all my aircraft welding, so far.

Machine used: Lincoln SP 130 Plus and SP 175 Plus
Wire: L-tec Easy-Grind (ER70s-3) 0.23" diameter
Gas: CO2-Argon mix (pretty common for mild steel)

Procedure: (not too scientific)
Optimize welder for normal welding (test on 4130 sheet/plate 1/8"), then set a little hotter.
Prepare the ends of the tubing ("fish-mouth" for very precise fit. clean with emery cloth 320 grit).
Fit the tube cluster together and clamp in place.

Strike an arc and hold until puddle is 1/8" - 3/16" diameter. Release trigger.
Before the puddle grows dark, strike another arc at the edge of the first puddle and drag the new puddle half way back into the first one.
Repeat this process, never letting the previous puddle go completely dark before creating the new one.
Continue until the bead is complete.

It takes a little practice to make the welds without burning through or over cooking the joint (that makes it brittle and weak - don't fly with those).

I won't go into detail about how to weld a tube cluster or the sequence/direction of welds. This information can be had from a number of other sources that describe these details better than I can.

I have attached some photos of some sample welds I made. Notice the small heat effected zone. Also notice the inside or back side of the weld (inside the tube). You can see that there is full penetration around the joint (beads or bumps inside the tube).

An interesting comparison test I did a few years ago was to weld identical joints using Oxy-Acetylene, Mig, and Tig.
I made a simple T-joint (90 degree). After welding three clusters (one for each process), I put a piece of round bar inside the tube that was welded perpendicular to the main tube. I placed the main tube in a vise and bent the welded tube back and forth with the round bar. This was not a controlled test nor did I measure the forces I was using to bend the tube. I just wanted to see which joint was the most ductile. The ideal joint will have the same properties as the parent material, which was 4130 normalized. What I didn't want was to introduce a brittle welded joint to an airframe. The Oxy welded joint was the most ductile and therefore the most reliable joint. Followed by the Mig welded joint. Last was the Tig welded joint.
The Oxy welded cluster had the benefit of some amount of post weld heat treatment (not to harden but to anneal). The Mig welded cluster had intermittent heat applied due to the "stich" or "leap-frog" procedure (one puddle at a time) and so the weld bead and surrounding metal had not completely turned to a Martensite grain structure (the hard brittle stuff). The Tig cluster had similar heat as the Oxy but without the post weld annealing action. That being said, there is no reason why a Tig welded tube cluster could not be post-weld annealed with a torch. Or the Mig for that matter. However, it has been proven that the Mig method does not require and post-weld annealing (talk to the Succesors of Kitfox, and the Avid Flyer folks).

Well, that's all for now. Kind of a long post. It is hard to describe this well in writing; I wish I could just invite you all over to my shop and demonstrate and share.

[Otto Nobedder]

Make sense to me.

The testing, even without controls, is hard to argue, as well.

Thanks for sharing!

I did finally find my small EAA library amongst my stored stuff....

Steve S

[flyingmaniac]

Hey Flydude can you give me Somme advice on filler rod for a PA- 18 fuselage from scratch, on this forum peoples say to used ER70s-3 but Mister Tig witche is a aerospace professional welder say ER80S-sb what is best for Tig and way are peoples having different opinion , and what's yours ?,

[Demented]

Hey Flydude can you give me Somme advice on filler rod for a PA- 18 fuselage from scratch, on this forum peoples say to used ER70s-3 but Mister Tig witche is a aerospace professional welder say ER80S-sb what is best for Tig and way are peoples having different opinion , and what's yours ?,

From what I've been reading, people are finding ER80S welds just as well as ER70S, and the additional rigidity in the weld is worth it. I think it was CJ here on the forums who said he's got a friend who welds NASCAR cages and uses it as his preferred rod.
At least in the motorsports world, properly welded joints tend to break not at the weld itself, but around the HAZ, which makes one believe the more ridged weld is of no problem. I think it really comes down to personal preference. I've seen some guys switch back and forth between aerospace and building race bikes and they all tend to stick with ER70S just to make it easy on them. None have had any issues as far as I am aware of.

[cj737]

Hey Flydude can you give me Somme advice on filler rod for a PA- 18 fuselage from scratch, on this forum peoples say to used ER70s-3 but Mister Tig witche is a aerospace professional welder say ER80S-sb what is best for Tig and way are peoples having different opinion , and what's yours ?,

From what I've been reading, people are finding ER80S welds just as well as ER70S, and the additional rigidity in the weld is worth it. I think it was CJ here on the forums who said he's got a friend who welds NASCAR cages and uses it as his preferred rod.
At least in the motorsports world, properly welded joints tend to break not at the weld itself, but around the HAZ, which makes one believe the more ridged weld is of no problem. I think it really comes down to personal preference. I've seen some guys switch back and forth between aerospace and building race bikes and they all tend to stick with ER70S just to make it easy on them. None have had any issues as far as I am aware of.

One builder I know uses ER80-SD2 on CroMo. I do also know a welder at another large Team but they use ER70, Everything The is MIG’ed. To each their own I guess

[WeldingJunkie]

Sounds great in theory but are you suggesting your welds look like tig? or just close ? many times when you use gun manipulation to get a mig like tig look the welds are cold or too much heat that makes brittle from gun manipulation. How is that avoided with your process? Its almost impossible to get a tig weld from a mig welder defect free.