TECHNICAL FEATURE

WeldingIt's Not Black Magic

BY BUDD DAVISSON

11 right, everyone out therewho thinks welding was in-vented by Merlin the Magicianand is an exotic combination

of alchemy and sorcery, hold up yourhands. No takers? Okay, let's put it inother terms which mean the samething...do you view welding as a skillyou might (underline "might") be ableto learn but would never, as in NEVER,use in building an airplane you yourselfwould fly? Would you always call in apro to do the welding?

The foregoing, unfortunately, is thewildly inaccurate mental image manyhave of welding. For some reason it ex-udes a mystical aura which says wemere mortals could never get goodenough that we would trust our ownwelds. Hog wash! Welding is a ski l llike any other and is one of the mostsatisfying, and useful, you'll ever learn.

Before composites steamrolleredover every other type of homebuilt air-craft construction, welded tubestructures were by far the most popularand no one, it seemed, feared welding.However, at that time when you spokeabout welding, you were only speakingabout one type of welding process, oxy-acctylcnc.. .old fashioned torch welding.Today, however, we have at least threedistinct types of welding which are verymuch a part of the sport aviation sceneand each has its devotees and its detrac-tors. Each has its applications, strongpoints and shortcomings.

Just as the proliferation of materialsand aircraft kits has confused the sportaviation builder by introducing somany new decision paths, the samething has happened with welding. Forthat reason, we thought we'd touch onwelding as a concept and then look ateach of the different types of weldingand how they fit that concept. All thetime we're discussing the subject,however, remember we are speaking

about welding as it exists in the sportavia t ion envi ronment where we'reworking with relatively thin pieces andthe material is usually 4130 chromoly.

Some of the comments we're goingto make may not apply when buildingbridges or battleships.

First, a definit ion of welding. It isthe process of joining two pieces ofmetal in which a narrow band alongthe joint is melted so the two pieces ofmetal flow together. Additional metalis added for strength in the form offiller rod.

The most important word in theforegoing definition and which differ-entiates it from brazing or soldering isthe world "melted." The parent metalis actually melted, rather than just hav-ing the two pieces joined by a different

material, such as lead or brass, whichadheres to the surface acting as a metal-lic form of glue.

As it happens, there arc some appli-cations in aircraft for both solderingand bra/ing but none of them are inprimary structure. Soldering is strictlyfor sticking avionics wires togetherand brazing must be limited to attach-ing non-critical tabs.

As it happens, there is a new formof brazing utilizing Eutectic Corpora-tion's No. 16, nickel-brass rod, whichmay at sometime in the future be us-able in light structural applications. Ithas a cast strength of 30,000 psi, whichis all a joint would be good for if alarge gap, .025", is bridged and ig-nored. However, they report if the gapis brought down to .002-.005", the

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Many equipment companies are now building less expen-sive TIG units which can also do "stick" or arc welding.Most are also plumbed for water-cooled torch handles.

There is a wide variety of MIG units available starting at very lowprices. The wire feeds through the hose. Some welders feel MIG re-quires more operator skill and precautions.

joint strength goes up to nearly 100,000psi, which is actually better than a nor-mal welded joint. 4130 chromoly, bythe way, has a strength of approximately94,000 psi in the form we usually use itand the most common mild steel alloy,1020, is in the 60,000 psi range. How-ever, the Eutectic rod should not beconsidered structural unless the struc-ture was designed specifically for it andthe joint allowables lowered to a safe,average number.

When speaking about aircraft weld-ing, it is important we take into accountthe material characteristics of chro-moly because the very factors whichgive it its high strength-to-wcight ratio,including its carbon content, are alsowhat make it more critical to weld thanlower carbon steels.

For one thing, 4130 is not very tol-erant of thermal shock. It doesn't liketo be heated up and cooled quickly, es-pecially in a narrow band, as around aweld. When this is done the crystallinestructure of the material will be al-tered. If, for instance, it is heated up towelding temperature and then allowedto cool quickly to room temperature,the weld and the parent material willdevelop a coarse grainy texture (mart-inzite) internally which makes it brittleand prone to cracking and breakage.

In speaking with various experts inthe field, including Bill Ridgway, the48 SEPTEMBER 1995

Director of Technology for EutecticCorporation-North America, they allagreed 4130 should be brought up totemperature slowly and left to coolslowly. That is just a characteristic ofthe material.

The structure surrounding the weldcompounds this heating and coolingproblem in several ways. One of theseis the heat-sinking, or thermal mass,characteristic of the joint. If one tubeis being joined to another, the materialwhich is distant from the weld and issti l l cool, wil l be trying to soak theheat away from the weld. So, when theheat source is removed, the heat israpidly sucked out into the rest of thestructure and the weld cools qu i tequickly. The more tubes or the largermass of metal which is attached to theweld, the larger the heat sink and themore rapidly it wi l l cool. The entirestructure is a sponge trying to soak upthe heat.

Another characteristic of a rapidlyheated or cooled weld is the internalstresses which are locked into it by ex-pansion and contraction. As the metalaround the joint is heated in prepara-tion for welding, it expands and thenarrower the band heated, the more vi-olent the difference is between theexpansion right in the weld area andthe expansion further away in thestructure. One part of the metal is

pushing hard on the other. Then, whenthe weld is completed and the moltenmetal of the bead and the parent metalcool, they contract. The real problemis they contract at a different rate thanthey expanded because much of thematerial is now locked in place by theweld and can't move. As if that isn'tbad enough, much more of the struc-ture is now part of the heat sink so theweld area has the potential of coolingmore rapidly than it was heated.

What we have in a weld is a situationnot unlike an earthquake which is aboutto happen. The Earth's tectonic platesare pushing against one another likecraxy and eventually, usually at a faultline where the stresses are focused,movement occurs causing an earth-quake and earthquakes are alwaysaccompanied by cracks. That's whathappens in a 4130 weld in which thecontraction and expansion are ignored.All those stresses are locked in and are amini-earthquake just waiting to happen.

So, how do you avoid or get rid ofthe stresses? According to the experts,the first step in keeping 4130 happy isto preheat it before welding. Put plentyof heat in the metal covering a sizabledistance (1" or more) from the point ofwelding. That accomplishes twothings. First, it puts heat into the restof the structure so it isn't so heat hun-gry that it sucks heat out as fast as you

can put it in. This makes the weld goeasier and slows cooling down. Sec-ondly, preheating expands a muchlarger area and sets the stage for amore gradual contraction over a largerarea as the weld cools. If only the nar-row band of the weld is heated, whenit cools, all the contraction is concen-trated in that one tiny area and theinterface between the hot and cool ar-eas really gets tweaked from thecontraction stresses. Every metal actsthe same in this situation, but withhigh carbon steels like 4130 thestresses right at the interface are moreviolent. By spreading out the expan-sion/contraction area, we've spreadout the area to transfer the stresses andthey don't concentrate as much.

Preheating helps avoid stresses byspreading everything out, but if theweld is left to cool too rapidly, thestresses can still be quite high. On topof that, rapid cooling promotes mart-inzite bui ld-up in the weld whichmakes it more brittle. To put it simply,4130 prefers to cool down gradually.

If a weld is kept hot, letting the redcolor remain over the entire weld for10-20 seconds and then the heat isgradually moved off over the next 20-30 seconds, the material is allowed tomove around and seek its own comfortlevel before it is completely locked inplace. Many welders wi l l finish theweld and then play the torch on thearea, raising all of it up to a uniformdull red, then let it cool slowly whileplaying the torch on it. Others wi l lcome back later with a rosebud heatingtip and bring the entire joint back up todull red and give everything a chanceto re-align before letting it slowly cooloff. This is called stress relieving andthe term says it all.

Now that you know how finicky4130 can be, let's talk about the vari-ous different types of welding open tous in sport aviation.

Oxy-AcetyleneThe old fashion oxy-acetylene torch

has been with us since around the turnof the century and it hasn't changedmuch, other than becoming more andmore precise and compact. This hasworked to the sport aviator's advan-tage in that it has given us a number ofnice little torch handles like the SmithAirline series. They work best for usbecause of their relatively small size

In TIG welding the arc is struck between a non-consumable tungsten electrode andthe rod fed in the usual fashion. Argon gas flows through the handle.

and light weight and the placement ofthe adjusting knobs up front where wecan reach them without changing handposition.

We also have a few new exotic pis-tol-grip torches which involve a tricknew tip design which works well insome areas. The ergonomics of thehandle, however, make them difficultto use in tight quarters.

To get into the game, the prospec-tive welder needs two tanks, acetyleneand oxygen, which may be owned orrented, a regulator (dual or s inglestage), torch handle, tips (size 0, 1,2,3), 15 feet of hose, glasses or gogglesand in-line check valves. The entire rigshould cost about $250 or less plus awelding cart.

Oxy-acetylene is the type of weldingusually seen in sport aviation for a lotof reasons, tradition and cost being thefirst two. Also, it has been around solong it is easy to find someone to teachyou and it's relatively easy to learn.

In terms of heating aspects of atorch, they are crude to the extremewhen compared to the pin-point heatsources of both TIG and MIG welding,the other two processes we'll discuss.A torch blows heat all over the place,which is precisely why it works so wellfor the amateur welder. It takes somuch heat to get a weld going and itcovers such a large area that it has theleast preheating problems of any of themethods. However, it is still advisedthat he entire area be brought up to adull red before the bead is started. Withan oxy-acetylene torch all that requiresis playing the flame around the area fora while before actually welding.

The typical join welded with oxy-acetylene has much more heatpumping through its various bits oftubing and steel than the other twotypes because the torch can't bring anarrow area up to welding temperaturewithout also blowing heat into sur-rounding areas. This means the heat

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The so-called "rosebud" tip for an oxy-acetylene torch is indespensible for supplyinglarge quantities of heat for bending, stress relieving, etc.

sink of the surrounding area has ab-sorbed much more heat so it won't beso heat-thirsty when the weld is fin-ished. Still, it will be thirsty.

When the weld is finished, all thewelder has to do to stress relieve thejoint is back the torch off a little andplay it around the bead just as he didwhen preheating. Heat will be put intothe joint at a decreasing rate until thetorch is totally removed. By that time all

the microscopic bits and pieces insidethe joint will have had time to find acomfortable place to sit before the wholejoint is cooled and locked up tight.

As a normal rule the rod used inthe weld is copper coated mild steel.The copper is there to prevent rusting.There's been a lot of discussion aboutusing 4130 rod or worrying about thecopper contaminating the weld. Indiscussing these factors with the met-

allurgists, they said the bead willprobably be diluted approximately50% with the melted 4130 anyway, sowe shouldn't worry.

Contamination and dilution mightbe a worry, if we were driving tiny lit-tle beads down into the openings in thejoints. But we're not. The width of thebead is always enormous compared tothe thickness of the material. We havea 5/16" bead running around .035" or.049" material, so the size of the beadby itself outweighs any serious con-cern. This factor alone is why so fewwelds, even the really ugly ones, break.

TIG (Tungsten Inert Gas)Welding

A lot of folks call TIG "Heli-Arc"which is actually a trade name, but itall means the same thing. In this sys-tem the torch held by the welder is anelectrode holder that in some versionslooks suspiciously like a wood burningset. A non-consumable tungsten elec-trode is centered in the middle of aceramic housing and connected to thepower unit by an electrical cable andseveral small hoses. Electricity ispumped through the electrode and anarc is struck between the electrode andthe grounded parent metal. Heat is gen-erated instantaneously and is usuallycontrolled by a foot pedal which is ac-tually controlling the juice going to it.

When the parent metal is melted bythe arc, which can be theinstant the arc is struckif the pedal is to themetal, filler rod is fed innot un l ike using a con-ventional torch.

While all the fire-works are going on, aninert gas (Argon) is be-ing pumped through theceramic housing anddown over the weld inprocess. This puts the ac-tual welding process in alittle dome of inert at-mosphere and keeps itfrom being contaminatedby outside elements. This

Regulators can be eithersingle or two-stage and ei-ther work well for aircraftwelding. The two-stagemaintains constant pres-sure as the tank level goesdown.

50 SEPTEMBER 1995

fly ID

The most commonly used oxy-acetylene torch is the Smith Airline AW1A (top) "air-line" torch because of its small size and light weight. It is seen here with theirLite-O-Matic and normal size AW10.

gives even a better view of what is actu-ally happening in the weld puddle thanan oxy-acetylene torch which is alreadyexcellent. The view of the puddle is crit-ical because the entire character andstrength of the weld is determined bywhat is happening in the tiny band at theleading edge of the puddle.

The better TIG torches have a cool-ing fluid pumped through the handle tokeep it cool. This isn't really necessaryfor aircraft welding because our weldsare so short, but it sure makes thingsmore comfortable. The cost of the en-tire rig varies greatly but bottoms out atabout $850 with $1200-51500 beingmore average.

The Eutectic folks say they oftensuggest TIG welding 4130 with 3 12stainless steel rod because it is more

ductile and tolerates the internal stressesbetter than mild steel or chromoly.

The devotees of TIG have somepretty good points in their favor. Forone thing, if you're working a joint thathas a lot of mass connected to it and istough to weld for a normal torch be-cause it takes so much heat you canhop on it with a TIG torch and almostinstantly be welding. It can really bringthe surface up to temperature in a hurryso it is a much faster means of welding.

TIG also generates a really nice,clean weld and it is possible to maketight, beautiful little beads.

The foregoing two strong points arealso pointed to by detractors as themethod's shortcomings. The very factthat a stone-cold joint can be welded in-stantly means a huge amount of heat is

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being driven into a tiny band so quicklythe weld is finished before the area iseven close to being finished expanding.

Then, because the heat zone is sonarrow and surrounded by nice coolsteel, the heat is sucked out like it is at-tached to a vacuum cleaner. In thissituation, the experts say the internalstresses are enormous because they areconcentrated in such a tiny area.

An additional danger mentioned bythe professionals is the possibility ofhydrogen embrittlement. If there are52 SEPTEMBER 1995

any oxides, oil or moisture on the metalwhen it is welded, the heat can be so in-stantaneous the hydrogen in thecompounds may become trapped in theweld. When that happens, the finishedweld may look and test just fine but ifthe tests are repeated a few days later,microscopic cracks may be found.

Hydrogen problems don't usually ex-ist with gas welding because so muchheat is driven into the area prior to weld-ing much of the hydrogen present iscooked off. TIG is also fairly slow,

Oxy-acetylene welding will always bethe chosen method for homebuildersbecause of lower cost and greater flexi-bility. It's not as fast as TIG but hasother advantages.

when compared to MIG, so the heatbuild-up will cook off a lot of the hydro-gen, but not necessarily all. It dependson how quickly the weld is started.

So, here we see the experts making(wo different arguments for preheatingwhen using TIG: it reduces both ther-mal shock and hydrogen embrittlement.They also make the same arguments asbefore for stress-relieving the welds af-terwards by reapplying heat.

It is theoretically possible to preheatand maybe even stress relieve to a cer-t a i n ex ten t w i t h the TIG uni t bykeeping the arc flame playing over thearea but wi th the power too low tomelt the surface. However, the areabeing effected is still fairly small.That 's why some TIG welders whohave concerns in this area come backin with a rosebud tip on an acetylenetorch and heat a larger area in a moreuniform fashion.

MIG (Machine Inert Gas)Currently MIG welding is a contro-

versial welding technique because it isfairly new to the aviation industry andhas the ability to violate some of thelong held tenets of 4130 welding un-less great care is taken. One of theprimary concerns in sport aviation ap-plications by the amateur welder isthat the basic units are relatively inex-pensive ($300-$400) and appear easyto use. There is a real temptation to useit without understanding some of thecaveats involved.

In many ways the system resemblesTIG in that an arc is struck between theparent metal and an electrode in thehandle while an inert gas is flowingover it. The big difference, however, isthat the electrode in this case is a con-sumable filler rod which is being fedthrough the handle automatically as itis being melted into the weld. There isno separate f i l ler rod so it is a one-handed operation.

Because it is a consumable elec-trode, the weld commences almost assoon as the arc is struck, so the weldmoves very quickly. This is why MIGwas developed in the first place: It wasdesigned as a production system for

assembly lines where time reallycounted. It is also better suited to join-ing material in which both pieces areapproximately the same thickness orhave the same thermal mass because itis difficult to direct the heat.

It is hard to see exactly what is hap-pening in the puddle so it is difficult tojudge the actual penetration of theweld. Also, everything is happeningquite quickly so it is easy to screw upbefore you realize it.

It is the instantaneous heating na-ture of the MIG weld that concernsthose who say 4130 requires preheat-ing and stress relieving. The heat goesin ins tant ly , the weld is f i n i shedquickly, and then the rest of the struc-ture is free to quickly suck as muchheat out as it can. Also, any hydrogenwhich may be present in whateverform doesn't have a chance to escape,so the metal has to be as clean as pos-sible, inside and out, to eliminate thepossibility of embrittlement.

According to Eutectic's Bill Ridg-way, MIG also has the ability toproduce welds with an elevated hard-ness which effects the duct i l i ty andbrittleness of the joint. He says a hard-ness over Re 30 indicates a possibilityof martinzite build-up caused by toorapid air cooling of the weld. TheAmerican Petroleum Institute has ap-parently set a max limit of Re 27-30for their pipeline welds for this reason.

For comparison, most sources say apiece of 4130N tubing will test Re 19-23 before welding.

On the other side, it is of interest tonote that according to Jerry Mehlhaffat American Champion, the ent i reCitabria line has been MIG weldedsince the 1960s. He also says theydon't stress relieve and their weldersarc re-certified every six months. Theysay the FAA tests their welds and arehappy with Re 38-41 in the weld. Thetubing will test about Re 3 next to theweld while the same tubing next to agas weld will test almost zero.

Many kit manufacturers arc usingMIG in their airframe fabrication be-cause it is fast which becomes a majorcost factor. Some of them report theystress relieve specific componentswhile others don't.

Obviously what we have here is a se-rious difference of opinion concerningMIG welding on 4130. However, if ageneral statement can be made bout it, itis that it appears to require an experi-

enced hand to produce a weld that is cor-rect and safe. Even then certainpreparation techniques must be followedand stress relieving seems advisable.

The biggest single danger of MIGwelding in the sport aviation environ-ment is that it gives the in i t ia limpression it is easy to do. That maybe the case when doing body work orbuilding wrought-iron stairs, but it isdefinitely not the case when weldingup a 4130 structure on which yourlife depends.

Summary

In reality, it is extremely difficult tofind an aircraft weld which has actuallybroken in flight. This is in part becausethe way structures are designed weldsusually spend most of their lives in com-pression, so they don't sec much load.Also, the way most joints are designedthere is easily 200-300% more weldlength than is necessary, even after ap-plying normal safety factors.

However, we have an unknownwhen it comes to lockcd-in stressesand possible hydrogen embrittlement,especially in MTG welds. The experts

tell us the types of failures we'll seefrom improper heating and cooling aswell as embrittlemcnts will almost al-ways be fatigue failures. That beingthe case, flight time is the only truejudge of what works and what doesn't.Many kit builders can point at the hun-dreds of MIG welded airplanes theyhave f lying which have had no fail-ures. That is true this year. What aboutnext year, or the year after that? Kitplanes don't fly very much and thosewhich use MIG welding fly even lessthan the average. So, if an airframe hasa point at which it will begin experi-encing failures, we may not reach thatpoint for years to come. That being thecase, it would appear we don't haveenough experience in the sport avia-tion environment yet to say whetherthe possible problems associated withTIG or MIG pose a threat or not.

Since what we're talking about herearc flying machines, which are gener-ally unforgiving of structural failure, itseems to us we ought to err on the sideof conservatism. This is another way ofsaying regardless of the type of weld-ing system employed, when in doubtpreheat and then stress relieve. 4

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