TIG, stress relieving

HomeBuiltAirplanes.com

Help Support HomeBuiltAirplanes.com:

dog

Well-Known Member
Joined
Dec 29, 2019
Messages
296
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?
4130 will get hard if quenched.welding in a cold (minus temps)shop could very well give a very hard(non ductile)
zone,a cold windy shop might guarantee this.Bad.
This can easily be tested with a file,or any other sharp and hard edge,a file will just slide over the hard zone,no grip,just what is needed for a cold chisle,stone drill ,etc.
Sand paper will work to test as well.
As to mig,tig,torch,or any welding or brazing, its irelivant to
"stress relieving","anealing","normalizing" ,whatever you want to call it.
Test your heat afected zones and decide if its waranted,or just follow the instructions.....keeping in mind that in a cold and windy shop it could just get worse,over heat and quick quench and now its scrap.
 

PMD

Well-Known Member
Joined
Apr 11, 2015
Messages
236
Location
Martensville SK
As to mig,tig,torch,or any welding or brazing, its irelivant to
"stress relieving","anealing","normalizing" ,whatever you want to call it.
Test your heat afected zones and decide if its waranted,or just follow the instructions.....keeping in mind that in a cold and windy shop it could just get worse,over heat and quick quench and now its scrap.
I did a lot of work with this stuff 40 years ago, and try to keep it in mind while building things today. I have included 2 links to save me summarizing a LOT of information, but to try to get it as short as possible, stress relieving does NOT happen in a few minutes in an uncontrolled environment. As an earlier post mentioned, one MIGHT improve the grain structure in the HAZ, but you are not going to relieve the (often very large) stresses in a welded structure.

Take a read of the code requirements - min of 2 hours at 450-600F. http://www.jflf.org/v/vspfiles/assets/pdf/keyconcepts4.pdf

To get a better understanding of the many, many changes of structure with temp: https://www.asminternational.org/documents/10192/1849770/ACF180B.pdf

Bear in mind, the weld bead starts as high as 3000F, then look at the phase diagram and see what it must go through. This why torch welding CAN be best for 4130 tubing (easiest to control deposit temp and a slower speed allows the feather of the flame to do more localized heating of substrate), TIG right behind, but MIG tends to have a hotter bead and faster travel, resulting in higher deltaT across HAZ and (IMHO) a sharper gradient of residual stress lines of force (thus shear loads within).

Oh: for the high strength brazing alloy used for Brit bike frames: Eutectic 16FC (fc just means flux coated). There are a fair number of comparable products that are Cu Zn Ni Ag alloys used for such purposes - and an earlier comment alluded to their strength - it can be comparable to steel weldments but with a lot less residual stress. In Canada you can even buy equivalent products at Canadian Tire and Princess Auto, one or two sticks at a time.

Oh..on edit: for the seemingly obligatory citation of credentials: I once worked for welding material manufacturer as tech guy and later was brit bike dealer/racer/fixer.
 
Last edited:

PiperCruisin

Well-Known Member
Joined
Jan 17, 2017
Messages
148
Location
Idaho
Here is an interesting document on weld fatigue from NACA TN No. 1262. See the Table and graph on page 52.
https://ntrs.nasa.gov/search.jsp?R=19930081881

Also, I remember most of the Avid Aircraft frames were MIG welded except for some of the smaller details and aluminum header tanks which were TIG'd. I don't remember if they did any stress relieving.
 
Last edited:

dog

Well-Known Member
Joined
Dec 29, 2019
Messages
296
I did a lot of work with this stuff 40 years ago, and try to keep it in mind while building things today. I have included 2 links to save me summarizing a LOT of information, but to try to get it as short as possible, stress relieving does NOT happen in a few minutes in an uncontrolled environment. As an earlier post mentioned, one MIGHT improve the grain structure in the HAZ, but you are not going to relieve the (often very large) stresses in a welded structure.

Take a read of the code requirements - min of 2 hours at 450-600F. http://www.jflf.org/v/vspfiles/assets/pdf/keyconcepts4.pdf

To get a better understanding of the many, many changes of structure with temp: https://www.asminternational.org/documents/10192/1849770/ACF180B.pdf

Bear in mind, the weld bead starts as high as 3000F, then look at the phase diagram and see what it must go through. This why torch welding CAN be best for 4130 tubing (easiest to control deposit temp and a slower speed allows the feather of the flame to do more localized heating of substrate), TIG right behind, but MIG tends to have a hotter bead and faster travel, resulting in higher deltaT across HAZ and (IMHO) a sharper gradient of residual stress lines of force (thus shear loads within).

Oh: for the high strength brazing alloy used for Brit bike frames: Eutectic 16FC (fc just means flux coated). There are a fair number of comparable products that are Cu Zn Ni Ag alloys used for such purposes - and an earlier comment alluded to their strength - it can be comparable to steel weldments but with a lot less residual stress. In Canada you can even buy equivalent products at Canadian Tire and Princess Auto, one or two sticks at a time.

Oh..on edit: for the seemingly obligatory citation of credentials: I once worked for welding material manufacturer as tech guy and later was brit bike dealer/racer/fixer.

Hadn't considered "stress relieving" a large complex structure as the furnace needed is so large for an aircraft.
My main point is that it is very easy to confirm
if there are very hard zones,too easy not to just make a routine check.
And infered is an agreement with other statements above that short of exceptional cercumstances,there should be no need to do post weld heat treat.
The original question mentions following the plans with a different process.....sorta open ended

My qualifications are aprenticeships with two Master Blacksmiths and now working in the trade doing anything and everything with iron and steel.
 

wsimpso1

Super Moderator
Staff member
Log Member
Joined
Oct 18, 2003
Messages
6,723
Location
Saline Michigan
If you are relieving residual stresses in welded steel, I suspect that 250 F won't be enough. Usual approach is dull red, then slowly back away with the torch. Here is why: 250-400 F is all you need to convert primary martensite (really brittle) to tempered martensite (strong and tough). But if you are welding in still air (and you are supposed to be welding in still air) you won't get these hard welds in the first place. So, the remaining thing to do is to soften the steel enough to let the stresses between pieces relax - that is going to take higher temps, around 1200-1300. Dull red in a well lit shop.

The biggest reason to revisit many welds is that the tube is bowed the wrong way, and you want to fix that so when you tauten the fabric later on the lines are straight, not scalloped.

As to concerns over brazing mid alloy tubes, check out the alloy content and heat treat condition on Reynolds bicycle tubes sometime. Looks a lot like 4130 and 4140 steel tubes we use in airplanes, and the industry has brazed tens of millions of bicycle frames using these tubes... I have looked, and I have never found a study that supports the tale that brazed joints are problematic in 4130. I have seen brazed joints that never had brazing flux removed and it is mostly boric acid. You must remove the flux - it looks almost like glass after brazing so it can fool you and cause corrosion later - abrasive blast is best. Confirm you are down to steel everywhere before putting on that primer where you are brazing.

Billski
 

dog

Well-Known Member
Joined
Dec 29, 2019
Messages
296
brazing is more resistant to cracking in some situations, takes less heat and skill
and done right is invisible,think bicycles.
If you need more strength then there is "silver solder" (not actual solder just called that),using borax works for many
types of welding,brazing,or as a barier when say
casting bronze or aluminum.Sand works in some
welding situations and the price is excellent.
When it comes to straitening things,there is heat,force or both, some have near magical skills
with a torch and can get metal to obey them.
The diferential expansion of metal can be used
to straighten metal or bend/curve it on purpose,with no mechanical force applied.
 

Hot Wings

Grumpy Cynic
HBA Supporter
Log Member
Joined
Nov 14, 2009
Messages
7,003
Location
Rocky Mountains
I have looked, and I have never found a study that supports the tale that brazed joints are problematic in 4130.
So have I, and it is the reason I am genuinely interested in a source of documentation.

I do know that if a brazed 4130 joint is overheated enough that the rod alloys with the steel the joint is junk. The only remedy is to cut out all of the ruined metal. On thin metal it is easy to do when the joint is under a boiling layer of borax. This may be where the idea that brazing causes a brittle joint comes from?

Putting the flux in the acetylene (Trimethyl Borate) looks like it removes this problem - along with the flux cleanup.
 

Lendo

Well-Known Member
Joined
Feb 6, 2013
Messages
486
Location
Brisbane
My research suggests welding Aluminium results in a hardened and weakened area adjacent to the weld due to the heat of the weld process. It will always crack and eventually fail there. The more it needs to flex the quicker it will fail. There is a product used for Brazing Aluminium, can't remember it, but looks good and strong.

Heating any Carbon Steel will harden it, the hardened area will be more brittle than the surrounding material, normalizing will neutralize that effect.

What is worse between MIG and TIG depends on the level of heat that goes into the process. How much heat goes into the process depends upon the operator.

I love welding with TIG because it gives such a neat and clean weld, but requires some skill.

I find MIG is so easy just turn up the heat to the point just before it blows holes in the metal and you have good penetration.
Hope that helps.
George
 

dog

Well-Known Member
Joined
Dec 29, 2019
Messages
296
My research suggests welding Aluminium results in a hardened and weakened area adjacent to the weld due to the heat of the weld process. It will always crack and eventually fail there. The more it needs to flex the quicker it will fail.

Hope that helps.
George
You meant cast magnisium?
Not aluminum of which millions of tons are welded
up just fine,like boats and ships welded from aluminum.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
12,849
Location
Memphis, TN
Everyone thinks it’s the welds are the problem. It’s really an alignment issue. If you jig stuff up beautiful and have a ridged jig, post heat is not a necessary problem. It’s about controlling warps. If it can’t warp when welding, it’s going to be ok. If your clusters or brackets are constantly fighting internally, good chance it can break. Pushing gaps closed and welding. Miss alignment tweaked straight. I fixed a frame twice. The original repair had a bind in it and it broke again. It did not break at welds it broke in the middle of the tube. The second time I post heated the joints. The airframe was 40 years old and had about 7000 hours on it at the time replaced with factory tubes. At 10,000 it got wrecked and the welds I put in did not break; it got bent bad enough that we replaced it instead of fixing, but I still have it and tube to fix it.

It’s not a definitive answer, it’s knowing your own strengths and weaknesses as a fabricator. That same brand was looking to improve the product so they sent a frame to a heat treat facility that could normalize a whole frame. Normal frame off the production jigs. The frame twisted really badly. If you want to normalize a whole fuselage frame, you need a jig that can go in the oven too. The normal production jig that this frame was built on probably weighs 10,000 lbs. made of 2x4 steel tubes. They were not going to build another just for heat treat when there was no problem to begin with.
 
  • Like
Reactions: dog

Lendo

Well-Known Member
Joined
Feb 6, 2013
Messages
486
Location
Brisbane
Dog, I have to admit your obviously right about boats etc. but for space frames welded aluminium is always problematic, like trailer frames etc. I have research this fairly extensively and can't see any way around the failure mode of welding, maybe the Al Brazing is the answer.
George
 

lr27

Well-Known Member
Joined
Nov 3, 2007
Messages
3,822
At some point, I did a little poking around about welded aluminum. I recall there were different alloys and tempers that were better and worse. Also different welding rod materials and welding techniques. Nothing gave me the warm fizzles about building an airframe with it, so I've forgotten the details.

OTOH, it's possible to make great stuff, like excellent bicycle frames, out of welded and heat treated 6061. I used to have such a homemade frame and knew others who did. If someone can afford to heat treat a whole fuselage, though, and the necessary jig to hold it in, they may have cubic money and can afford unobtainium. Or maybe there's a way to make a heat treating oven on the cheap?
 

dog

Well-Known Member
Joined
Dec 29, 2019
Messages
296
Dog, I have to admit your obviously right about boats etc. but for space frames welded aluminium is always problematic, like trailer frames etc. I have research this fairly extensively and can't see any way around the failure mode of welding, maybe the Al Brazing is the answer.
George
My info on brazing aluminum is second hand and historical,was considered a novelty....
As to welded "lumi" structures for aircraft use,it definitely isnt comon practice, and the reasons you sited are valid.....and would apply equaly to any metal that had been heat treated prior to welding
hence brazing carbide to steel for tooling or things like gears that are never weldements,etc.
I have seen lots of mig welded aluminum trailers hold up fine.Heavy trucks use all kinds of welded aluminum.
This is of course not the same deal as butt welded
.021 6061 t6, which is subjected to repeated bending and stretching.Not very likely.
There is though one member who is building a
aluminum flying boat that will have much of the primary structure welded, this has promise as
aluminum welding has come a long way, pulsed
mig in the right hands can produce welds that look like stacked dimes,perfect,flat,smooth,no pits,no craters.
Aluminum welding has a bad name firstly because it is very difficult to just pick up and do compared to
welding steel,second the equipment was very expensive,and thirdly when repairs were attempted
see number one.
Thats all changed and a push pull mig with a pulser
is something mere mortals can buy and use.
Any specific info on post weld strength is availible for any alloy or process.
Am I advocating welded spars or wing skins?nope
Welded fueselage skins in a presurised plane?nope
Rivits alow more movement and flex which is a nessesary design requirement for wings and pressure vessels that ordinary welding cant deliver.
The nessesary rigidity to prevent cracking would drive the weight of a fully welded aircraft past any acceptable level.
Welded hull in an amphibian?could be a good fit!
 

BBerson

Well-Known Member
HBA Supporter
Joined
Dec 16, 2007
Messages
13,111
Location
Port Townsend WA
The Alcoa handbook has about all the required info for welded aluminum and post weld strength.
 

Little Scrapper

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 3, 2014
Messages
5,674
Location
Wisconsin
Ok. Here’s the thing. You’re not going to get a answer you can dependably bank on. It’s impossible.

This whole question of heat treating 4130
Is only in certain home built airplane circles and the discussion goes round and round and eventually dies.

Understand this, there’s no possible way a amateur home builder that puts himself in charge of heat treating can know if whatever he does with his magic movement of a gas torch over a TIG weld actually works or hurts. Nobody has that answer.


There’s no data available that says you should or shouldn’t. It’s very common for people to overthink things so I understand the debate but really, does it matter? Again, nobody actually knows. My version of red is different than your version of red. What if I over heat? What if I under heat? See what I mean?


Here’s what I know for sure. Heat treating is a science for companies that understand heat treating. I can read a little, that’s about it. But I personally know people who race for a living and they build 4130 CM parts every day and NEVER heat treat anything. Some MIG and some TIG.

It’s a theory, nothing more. You’re on you own. I will be finishing a fuselage tomorrow and I TIG welded everything. I’m not gonna do the happy flame dance on the joints because I’m not gonna pretend I actually can heat treat 4130 CM with any degree of accuracy. It seems ridiculous to assume that but that’s me. I know people that do it and that’s great, whatever you feel comfortable with is the right answer. Understand that nobody is doing lab testing and even if they did the sample wouldn’t be the same as say a fuselage. Heat treating at a company is a exact science, heat treating at home is a art.

The good news is deaths in 4130 CM based airplanes don’t have wide spread problems with welds. We still have a major issue with pilot error though. Airplanes are overbuilt in my opinion, that’s why so many amateurs have such great success “structurally” speaking. It’s just not a problem.

So the right answer is to just build and be consistent I guess. That seems to work.
 

dog

Well-Known Member
Joined
Dec 29, 2019
Messages
296
Ok. Here’s the thing. You’re not going to get a answer you can dependably bank on. It’s impossible.

This whole question of heat treating 4130
Is only in certain home built airplane circles and the discussion goes round and round and eventually dies.
heat treating at a company is a exact science, heat treating at home is a art.

The good news is deaths in 4130 CM based airplanes don’t have wide spread problems with welds. We still have a major issue with pilot error though. Airplanes are overbuilt in my opinion, that’s why so many amateurs have such great success “structurally” speaking. It’s just not a problem.

So the right answer is to just build and be consistent I guess. That seems to work.
+ on consistency
tig in a jig? or just done on a pattern ,with clamps and blocks? the odd tube a little out of wack
and the persistant rumors of "weld stress induced cracking",plus the very real ability to move steel with just heat, and straighten out a tube,(while of course bending the rest of the assembly),only for those with huge experience
AND major mojo.
Granpa was a heat treater at the Aleganey national forge and I am a blacksmith who
can heat treat and temper...but not a great big
complicated tube welded structures.
Realy its about building a heavy jig, or learning
how to tack weld and finnish weld in a sequence that yields a (mostly) flat (acurate) structure, or
knowing that the furring is all thats going to show, and get that perfect.
what bothers me is that the focus tends to be on
trying to achive a ultra high level of accuracy on
every part withou any clue as to actual streangth,
no TTD testing of samples and then OCD on micro detail.
 

PMD

Well-Known Member
Joined
Apr 11, 2015
Messages
236
Location
Martensville SK
Has anyone ever built a 4130 fuselage and taken it to a boiler shop for stress relief? My neighbour has been building boilers for more than a century (OK, his grandpa started the biz long before he was borne) and he puts ever weldment into a huge stress relieving oven. One can often piggyback with a boiler for very reasonable cost - and be assured you have genuinely controlled conditions.
 

TFF

Well-Known Member
Joined
Apr 28, 2010
Messages
12,849
Location
Memphis, TN
To throw it in an oven, it has to be in a jig that can take the heat and stay straight. Ply table tops of homebuilt garage builders can’t do that. You can’t throw a free standing fuselage in an oven and get anything good out.

When you weld up something, you know what you have. Scraper is high end of quality, he does not need to post heat on any normal job he does. Me, I would take it joint by joint. I know when I forced something too much, fitting it. It’s about building style of the person. You have to have an opinion of your craftsmanship and where your good and bad qualities are.

Scrapper has built a couple of dozen fuselages. He is not a beginner.
A scientific perfect 4130 fuselage does not exist, not that perfection is bad. If everyone on the forum built the same fuselage all at once, they will all be different. They will all be with different positives and negatives. I would also bet 95% would be useable off the bat, but not one will be the same. Building is a fingerprint. You can change some, but for the most part if you do enough, your stuff could be recognizable. What the builder has to do is learn to recognize their work so they can grade their next step. What you can’t say is I quit.
 

Little Scrapper

Well-Known Member
HBA Supporter
Log Member
Joined
Jan 3, 2014
Messages
5,674
Location
Wisconsin
I’m kinda surprised nobody mentioned Steve Wittman. He brazed a lot of things with great success.

If anyone is reading this and hasn’t tig brazed you really should. It’s almost a magical experience and it’s really quite fun.

For the hell of it, well, to prove a point, I TIG brazed a pipe rack in my shop and I load the daylights out of it with hundreds of pounds. No cracks or breaks and my rack it literally vertical members with 90* arms extending out vertically with zero gussets.

TIG brazing, put that on your bucket list, you’ll thank me.
 
2
Top