TIG, stress relieving

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12notes

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I've already done most of the welding for the Hummelbird I'm building, so this is academic at this point, but the plans say to TIG or torch weld the 4130 parts, but then to stress relieve them (heat to cherry red and allow to cool) all the parts after welding.

If I'm stress relieving everything anyway, is there any reason I couldn't have used MIG instead of TIG?
 

BJC

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Several manufacturers of type certificated airplanes TIG weld 4160 truss fuselages but do not “stress relieve” them.

At least one uses MIG, also without “stress relieving”.


BJC
 

TFF

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Generally it is quality of penetration. TIG is a cleaner weld. If you weld with your feet, maybe not. The normal problem with MIG is if the need to change amps on the fly. The machines are not set up that way like a TIG. Champion uses a MIG when welding new Champs, Decathlons and Citabrias. They did have problems in the beginning of using MIG but figured it out. The welders there keep a hand on the amp adjust knob all the time and constantly twist it as thickness and angles change. You would think they would have come up with a foot pedal.

Some of the need to stress relive is quality of the welder person. If they are constantly overheating stuff, it’s a good idea. If you are good like pay to weld nuclear heat exchangers good, not so much. The plans are telling you to , to covers the bases.
 

Jay Kempf

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Most of the high volume welding robots I have seen use MIG without post heat treatment including aluminum, steel, stainless sheet and tube, thin wall and thick.

It is all about what the designer intended. If there isn't factor of safety to have the heat affected zone stay affected then you need to either do the math and test or follow plans. If you have any really critical welds in major load paths then I would follow the plans.
 

BBerson

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I use MIG for everything now. Just set the heat high for penatration on start, and then blip the trigger constantly to avoid burn through.
It should be red hot after the weld and then let it cool slowly same as a gas torch. I use a Millermatic 135.
Wouldn't use a cheap machine without some auto feed control.
 

PiperCruisin

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Why do stress relieving?... remember that the act of welding is adding molten metal to a joint and letting it cool. The result is high, residual tensile stress. This reduces the fatigue strength, a lot. Stress relieving, or post weld heat treatment (PWHT), can improve live about 2X, on average...but is not a guarantee. PWHT usually consists of heating to a particular temperature, holding for a time, and then gradual cool down.

I think what they are trying to do is normalizing the HAZ for better impact resistance.
 

BJC

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I think that 12notes’ question has been answered, so I thought I would add a question.

Has anyone seen a 1020-ish or 4130 tubular steel fuselage truss brazed with brass rather than welded?


BJC
 

TFF

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The British use to braze formula car frames before monocoque frames came out. I want to say they used a nickel rod but not sure. Low distortion because of lower heat. I think some types of joints were stronger than welding.
 
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Hot Wings

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Has anyone seen a 1020-ish or 4130 tubular steel fuselage truss brazed with brass rather than welded?
I don't know of any aircraft examples but there are plenty from the auto, motorcycle and bicycle world.


I found the use of fluxed gas in this example rather interesting. I intend to try it some day.
 

BJC

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Nice job on the Ariel Atom. Note that the tubing used is much larger than what is typical in small airplanes, and likely much thicker wall sizes. Ergo the large torch.

If you ever get a chance to drive an Atom, take it; they are lots of fun.


BJC
 

12notes

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Actually, I don't think my actual question has been answered. I'm not asking if I should heat treat, and I'm not asking about ease of competence with the two processes.

Assuming that both welds are done competently, and both welds are stress relieved, is there any difference in strength or ductility between a MIG weld and a TIG weld?

I MIG welded as a job for 5 years, and have been hobby MIG welding for about 15. TIG is new to me, the quality of the weld will be less with TIG given my lack of experience compared to MIG. While I can get a decent weld with TIG, it's not going to be nearly as pretty.

I've already TIG welded everything but the motor mount, and stress relieved the parts that matter.
 
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Hot Wings

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Assuming that both welds are done competently, and both welds are stress relieved, is there any difference in strength or ductility between a MIG weld and a TIG weld?
All else being equal there should be no difference. Metal doesn't care how it is heated*, just the heat profile.
Learning how to MIG thin metal, to me, was a longer learning curve than learning TIG for similar thickness metal. My MIG has basic (on off) pulse but I usually use the trigger as I find it more versatile. If I had a modern MIG that I could set the ratio between the cycles it might be different.

I also have a bit of trouble swapping between MIG with straight Co2 and MIG with Argon blend. I use the cheap stuff for 'common' welds where splatter isn't a concern.

*assuming an inert shield gas.
 

cheapracer

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Has anyone seen a 1020-ish or 4130 tubular steel fuselage truss brazed with brass rather than welded?
Not aircraft, no, but hundreds of highly stressed race car and motorcycle frames, Rickman motocross frames for example. They are still made the same way today (historic racing).

Rickman.jpg

Rickman 2.jpg
 

BJC

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I welded at least a half dozen broken frame tubes on my brother’s Bultaco. It had right side shifting which is much worse than driving on the left side of the road.


BJC
 

C Michael Hoover

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My understanding for the reason to heat the general area when TIG welded is that there is an abrupt heat gradation between the welded part and adjacent metal, and that "stress relieving" the welded area normalized the grain structure throughout the area. Is this incorrect?
Thanks for your input.
 

C Michael Hoover

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I welded at least a half dozen broken frame tubes on my brother’s Bultaco. It had right side shifting which is much worse than driving on the left side of the road.


BJC
I never broke a frame tube on my Bultaco Pursang. As for the right side shifting, How would possibly shift with a left side shifter when the steel shoe is on the left foot and your sliding into a corner on a flat track? The only Yamaha's that were competitive in those days were the converted road racers and they had a mechanism to move the shifter to the right side. (Flat track racing only, YMMV)
 

cheapracer

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I welded at least a half dozen broken frame tubes on my brother’s Bultaco. It had right side shifting which is much worse than driving on the left side of the road.
Bullys were steel welded frames, not ChM brazed AFIK.

I worked 3 years in a Brit bike shop, not only left and right gearchanges (Triumph changed to left with the 750), but Norton's gear pattern was 1 up, 3 down. Oh, and then there was the WLAs that came through occasionally, left foot clutch and hand gearchange :)
 

karmarepair

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I've already done most of the welding for the Hummelbird I'm building, so this is academic at this point, but the plans say to TIG or torch weld the 4130 parts, but then to stress relieve them (heat to cherry red and allow to cool) all the parts after welding.

If I'm stress relieving everything anyway, is there any reason I couldn't have used MIG instead of TIG?
I feel like TFF did a good job of answering your question. I HAVE heard (from a race car fabricator) though that TIG/MIG stress relieves on 4130 a lot cooler than cherry red. 250F was mentioned. And many do NOT bother, and seem to get along OK.

And about brazing, the relatively course microstructure of 4130 does not like braze metal; fatigue performance suffers.
 

BJC

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I never broke a frame tube on my Bultaco Pursang. As for the right side shifting, How would possibly shift with a left side shifter when the steel shoe is on the left foot and your sliding into a corner on a flat track?
My brother was racing motocross.


BJC
 

Hot Wings

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ong OK.

And about brazing, the relatively course microstructure of 4130 does not like braze metal; fatigue performance suffers.
There are a lot of bicycle frame builders that would like a citation to study. I'm suspecting "facts not in evidence" without documentation?
 
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