In researching the welding methods used for fuselage construction, I soon realized this was controversial subject among builders.
There are basically two primary methods: Tungsten inert gas (TIG) and Oxygen Acetylene (Gas). TIG is the more modern, technologically based method, while Gas is the traditional method. Each method seems to have it's proponents, and they don't seem to agree on much of anything. Until recently, I hadn't decided which method I would use. "I’am leaning strongly toward Gas" "OK now I’m going to TIG weld?"
Most of my experience was with TIG. So that's what I was most comfortable with, and assumed I would use. The first thing I did though, was buy a Gas rig, as you need one for heating tubing. Also, since the physical techniques of melting in filler rod are basically the same for each method, I thought I could practice my technique with the gas rig until I purchased the TIG machine I wanted.
Each of these two methods has certain advantages and disadvantages. Each method also seems to be wrapped in rumor, bias and misinformation. To be better prepared to make an informed decision regarding this matter, I would strongly suggest becoming familiar with the terms Normalize, Annealed, and Stress Relive.
Normalized is the condition the 4130 tube and sheet comes in noted on the material as Condition N and has a tensile strength of 95,000PSI.
Condition A, or annealed 4130, has a tensile strength of around 50,000PSI and us used to make parts that are stamped etc. and later is changed to Condition N by being subjected to a normalizing process in a heat treat oven. Anneal is a term that refers to having the metal in its softened state.
Stress Relieve is a term that applies to relaxing a structure that has a lot of stored energy in it. If you welded a square frame, the cut it on one side and it sprung open with great force, it had stored stresses. These stresses could have been caused by misalignment, excess material shrinkage during welding etc.
Stress Relieving requires the metal to be hot enough to become soft like if you are bending it. Most Gas guys will swear that you MUST stress relieve TIG welds to prevent cracking. (They say that the argon post flow, quench hardens the weld?) TIG guys will advise against this practice. This is done with a gas torch in the field. The problem is that in the field, it is very difficult to know if you performed this function correctly. There is a product called a TEMPIL stick-Temperature indicator, it’s like a crayon that melts at a given temperature. These supposedly tell you when you have reached the correct temperature.
In the book “Performance Welding” the author, Richard Finch, talks about hundreds of airplane motor mounts that were TIG Welded and not stress relieved. None cracked after thousands of hours of use. So, based on this, I would have to believe that stress reliving a TIG weld is not necessary. All these welds were also probably very good quality. Poor welds are just that, and you have to ask yourself what you would feel comfortable with, in turbulence, at 2500’ AGL?
July 4, 2009: For Dan at @yahoo.com, I tried to e-mail you back, but your address wouldn't work. Please read what Lincoln Electric says about pre and post heating of 4130 steel .120 thick and less. There are allot of wife's tails out there on this subject, with not much back them up. I'll take the advice of the weld engineers at Lincoln and TIG depot. As for Finch's book, your right, it's not perfect. It does however give a pretty good overview of allot of stuff. Therefore, it will remain as recommended reading.
TIG vs. Gas
Gas welding (oxygen/acetylene) is the traditional method of welding steel tube airframes. Thousands of airplanes, to include the infamous Piper Cub, have been built using this method. Gas welding has several advantages.
TIG welding (Tungsten Inert Gas) is the modern technology that many builders use. If you go to your local welding supply store, this is probably the method they will say you must use. Don’t be surprised when they tell you it is impossible to use a gas torch to construct a tubing airframe.
There are several reasons for this. One is that they make a lot more money by selling a TIG machine. Most TIG machines are overpriced at stores like these. The other is the salesmen just don’t know what they are talking about. So, unfortunately, the welding store is not necessarily the place to get your education regarding this matter.
TIG machines can make it much easier to control the puddle than can Gas welding, so you can produce some very nice looking welds early on. They also have the capability to more easily weld aluminum and "stick" weld. They also have several drawbacks. First they are expensive. The Lincoln Square Wave 185 can be purchased through TIG Depot for around $1700. (This is one of the better prices I have been able to find, with good customer service) By the time you get an argon cylinder, mask and other items, you are probably looking at $2100. The Gas rig, on the other hand, can be purchased for a fraction of the cost. You also need to have one anyway, primarily to make bends, and flatten tubing in some areas. I sell a kit that has everything you need to get started. It costs $429 and includes my DVD on how to build your airframe. You would still need to lease your tanks from your local welding store. You will usually get a refund of around $250 when you return them if you choose to end your lease. I would get the biggest tanks you can afford, to keep from running out of gas frequently. If you don’t have anyone to mentor you, my DVD would be very valuable. There is a LOT of information contained in the 120 minutes, and it was recently picked-up the the EAA SportAir Workshops!
I practiced quite a while with the gas torch and a “bargain bag” of 4130 tubing I bought from Wicks Aircraft Supply. If I had it to do-over again, I would just purchase several feet of the exact tubing you will be working with. The bargain bag includes a lot of junk pieces that are really thick, so they aren't very realistic for practice.
I hadn't gas welded for many years, and it really showed when I started to practice. However, with practice, you can make significant improvements in a short period of time. One older guy told me there were three things required to become a good gas welder. The first one is practice. The second one is practice. You get the picture.
Anyway, in a short time, you can produce welds that will pass the airworthiness test. It is difficult to produce gas welds that will come close to the “stack-of-dimes” beads that a TIG welder will produce with the same amount of skill. But, occasionally, you can surprise yourself! And with practice, you can get there pretty fast.
So the real question is which method is the best? (update as of 3/23/05)
I guess it really comes down to what you really want to do? You really need to have the gas rig for bending and flattening tubing in some areas. And, while I tried to rationalize my way into gas welding, I just couldn't help but buy the TIG welder.
The TIG just has too many nice features (to include fairly easy aluminum welding) to keep me from buying it. And to be honest, I just really wanted one! I plan to update this page when I get my welder set-up, and have started to practice.
10/06/05 Since the last time I updated this page, I have final welded about 2/3's of the Main Fuselage. I started at the tail, and worked forward in a circular pattern. The typical set-up for my welder has turned out to be a #5 cup with 1/16 tungsten. Argon set about 10 cfm. The amount of tungsten stickout really varies depending upon the joint. To get in the crotch of some joints, you need about 7/16". This doesn't seem to be too much, as when you don't have gas coverage, you can really tell. The other technique I have been using is to start the filler closer to the crotch, then back hand the weld to the proper point. I have also been using ER80S-D2 1/16" rod. I had to get it from TIG depot, since no local welding stores carry it. They are really reasonable though! I just ordered another 3 lbs and with shipping it was only $18.
If I had one thing to do-over, it would be the way I tacked. A lot of my tacks were too big. I did this because during the building process, I had to cut a few back out to relieve distortion. So I started tacking to make that easy. I didn't have to cut that many out, maybe 3? So, I would tack as if I was starting a weld at this point. The other thing was that I used the tacking process to help me practice the TIG. It was good practice, but nothing like running the first bead on the real thing!
As for amperage, I have been using around 46 amps for the .035 wall tubing and around 58 amps for the .049. Then, when using the pedal, you can't go any higher that the setting on the digital meter. The more practice I have, the nicer it is to have the ability to bump-up the amperage. Especially when the joint starts to get thicker due to filler. I am probably using a little more filler than I need, but I would rather error on the heavy side of the weld. I also prepare about 12 pieces of tungsten at a time. That way, when one gets contaminated, I can switch out quickly.
With each joint, I start by wire brushing then wipe down with acetone. Once welded and the joint has cooled to room temperature, I wire brush and spray it down with Rustlick. If you don't do this, it will start to surface rust almost immediately. The Rustlick is great stuff. I learned about it from postings from Kevin Kimball, who builds the Pitts Model 12. They even weld through the Rustlick?
My welding proficiency has really improved since I started to final weld my fuselage. For that reason, I'm glad I started on the tail, as it doesn't have to absorb as much abuse as the front of the airplane. I feel comfortable with all the welds, but the forward ones look much better. They will also be visible from inside the cockpit, while the tail will be covered with fabric.
Anyway, very happy with the Lincoln Tig welder. Well worth the investment!