mig welding 4130 fuse

[Bill Berson]

Instead of just making comments "off the top of your head" perhaps some research is advised.
Plenty of OEM's use MIG, especially Kitplane companies. Any dedicated homebuilder with the proper quality equipment and training could duplicate the conditions and processes used in a typical Kitplane factory. It isn't a question of the process, only the skill and knowledge of the welder. And that applies to all welding methods.
As an inspector, I have been scanning for accident reports dealing with weld failure for 30 years. The only fatal weld failure incident I can recall was a crash that killed two EAA pilots in a Lycoming powered Wright Flyer replica a few years ago. In that case the NTSB provided detail photos of the failed TIG welds (lack of full penetration). I have not read or heard of any fatal MIG weld failures in any official NTSB report yet. Please report if you have anything.
It's sadly funny to read these comments. MIG can be ideal for many airframe parts. Maybe not all, but what is?
MIG is FAA approved, see AC 43.13

[Mike Switzer]

My comments were not "off the top of my head", they are based on my experience as a Professional Engineer, several years of which were in a fabrication shop where I was the primary engineer responsible for the design of welds. I never said you can't use a MIG, I said make sure it is inspected by a qualified individual, because inexperienced welders MAY make a weld that looks good on the surface but does not penetrate properly. We primarily used MIG and the guys did that every day & were very good, but some welds still did not pass inspection.

[akroflyer]

Perhaps you and the others "knowledgeable" in mig welding should tour the American champion aircraft factory and report back to us uneducated home builders. F.Y.I. I can't think of an OEM that doesn't use mig.

[Aaron Novak]

Perhaps you and the others "knowledgeable" in mig welding should tour the American champion aircraft factory and report back to us uneducated home builders. F.Y.I. I can't think of an OEM that doesn't use mig.

I have toured it. If you will notice I said "critical weldments". American Champion, Maule, Bellenca (back in the day) use a combination of Mig and Tig on their fuselages. Mig in non critical and Tig on critical. Heck there are fuselages that are arc welded that are just fine, however that was under factory controlled conditions. There are probably some small kit manufacturers that use all mig, and if their design analysis allows for it, that's fine.

Bill, you are incorrect. The skill of the welder has little to do with the resultant condition of the base metal with the mig process (providing a constant process schedule). It is all engineering control. Cold starts and laps can be somewhat controlled. That being said, a simple tee cluster welded in 2 passes might be ok, while the same welded in 3 or 4 might not be due to the difference in energy input vs. time.

There are actually numerous failures of both Tig and Mig, luckily most are caught before they cause a fatality and most end up as service bulletins or AD's. Again, not saying Mig cant work, just saying that for some pretty serious reasons, It has better applications than a homebuilt fuselage. The only reason people have ever given me to using it, has been speed, and cost of equipment. Pretty weak arguments considering the implications.

[akroflyer]

There are NO non critical weldments on an airframe, even if its just a bracket holding a data plate if it fails it could jam the controls. The skill of the welder has everything to do with good welds in homebuilding, and I have never heard of anyone making 2,3 or 4 passes on a T cluster. Every time I get in my plane (mig welded) I know my life depends on the welds, its worked for 25 years.

[Aaron Novak]

Sorry, should have said segments, not passes. What I am saying, is that with some materials and processes, the "skill" of the welder has less to do with structural integrity of the weld, than the process and method itself. 4130 and Mig are one of those combinations that can be real touchy. Now there are many designs out there that are heavily over engineered, and so the clusters can withstand some discontinuity and loss of ductility (good for homebuilding). Some can not. There is no dispute of the technical nature of welding, only as to a designs ability to cope with it. I.E. what works on a Whitman design may not on a Stolp due to different calcs used during design.

[eiclan]

Gday Chaps, As a fabricator of long standing , this discussion is very interesting. I am very experienced withe mig and oxy, tig not so much. As I intend to begin a build in the near future I will go with what I do best and that is the mig. The mig seems to be disparaged as unworkable with 4130 but while I have never welded it before I will do as I have always done and practice and set up the machine and get on with it.
I think the problem is , is that the mig is hard to do well. Unlike the tig or oxy, the mig wire speed is what it is while the weld is happening and the same with the amps. To do a competent weld on thin material requires skill and loose trousers, you need the ball room!
The tig and oxy by comparison are so much easier, the heat is easy to control, the input of filler wire much more controlled.
This is an argument that could go on forever and all I know is that I feel better with that mig handpiece in my hand than anything else, and yes as I cut and cope 4130 tube I will be practicing my skills and machine setup on the off cuts because I know my mig welds will always be better than my tig welds and the oxy is just too slow. Cheers Ross

[Aaron Novak]

Ross,
Very understandable that you want to do what you are comfortable with, and there is value in that. From a technical perspective the issue with mig is low total heat input, and so a rapid cooling rate through the transitional phase near the weld toes in the HAZ. Preheat temp can be a big influence here as it can help mitigate cold starts, and reduce the post weld cooling rate. Some caution should be used when building a design that was proven on O/A welding (or other), with another process. The mechanical properties can be quite different between processes. Designs with less fatigue margin typically fare better with a correctly used O/A or Tig process.

[Bill Berson]

Aaron, you have repeatedly stated that MIG is incompatible with 4130 for some reason. Can you please provide a document link to this?
Apparently, FAA, EAA and all the factories have not seen this document.
It sounds like you may not have ever actually welded a fuselage with MIG. Some can, some can't, just like some can't land a taildragger. Landing a taildragger takes specialized knowledge and skill and it's fun. Same with MIG, it's fun.
I don't have time to participate in every nonsensical MIG discussion on all these web forums.
But this is EAA. People at EAA get the facts and build airplanes with the same skills, materials, processes and tools as the factories.

[Aaron Novak]

Bill,
Go back and re-read my posts please. Notice my point of concern is limited to homebuilts, not factory. This is simply due to mig being the most difficult process to use effectively for this application and material, for the average homebuilder.