& tricks from the Pros
Aluminum Welding Using Oxy-Fuel Welding on Aircraft Aluminum Sheet
We'll begin with a brief history of related welding processes, in order to present Oxy-fuel welding in context.
The heliarc (GTAW)
was discovered in November of 1942, and by 1946, together with
wire-feed (GMAW), helped pioneer the application of new exotic,
materials vital to supersonic flight. However, for nearly half
a century prior and half a century since, the common oxy-fuel torch
has been a significant mainstay in the aluminum joining process.
The chief advantages of OFW over GTAW are economy, speed, penetration, workability, and small flat weld beads requiring minimal dressing. Disadvantages are flux cleanup, large HAZ, and that fewer alloys lend themselves well to the process.
The fuel chart shown above indicates only part of the significant oxy-fuel heat difference between hydrogen and acetylene. Suffice it to say, acetylene is much hotter.
The choice of hydrogen as a fuel necessitates a completely separate tank, regulator, hose, and torch, because mixing acetylene residues with hydrogen gas invites explosive disaster. Further, hydrogen is not capable of producing soot, which can be used beneficially as a temperature indicator for the annealing process on aluminum sheet. The benefits of hydrogen may be fuel production cost (if an electrolysis plant is feasible), and a slightly cleaner weld zone appearance, due to the absence of carbon in the flame.
Another feature of this chart is to show general fuel consumption relative to oxygen consumption. The cost of fuels such as propane, MAPP, or natural gas might be based on this alone, but its also wise to consider that the excess oxygen in the process makes aluminum OFW more difficult -- something the flux may or may not handle too well.
Metal Filler Selection Chart
Note: Listed in order of increasing strength: 5356, 5183, 5556. 4047 has more Si than 4043, therefore less sensitivity to hot cracking, slightly higher weld shear strength and less ductility.
(A) 4043, because of its Si content, is less susceptible to hot cracking, but has less weld ductility and may crack when planished.
(B) For applications at sustained temperatures above 150 degrees F. because of intergranular corrosion.
(C) Low temperature service @ 150 degrees F. and below
(D) 5554 is suitable for elevated temperatures.
Virtually any torch may be considered viable, but this author tends to avoid giant "railway" torches, and, conversely, small jewelry torches. Most are fine, some are really good and comfortable, and a few might be considered for specialty work.
Hoses of choice are light and flexible, enabling both out-of-position work, and long periods requiring a steady hand.
Regulators, because of the low pressures
required, may not be accurate at the low
end, and so must be twiddled with the
torch lit to establish the best flame.
Do this with the torch valves open, setting
the largest, best flame for the tip. A
at the gauges will show the approximate
right pressure. Set the flame neutral, or
if the regs creep, slightly feathered (carburizing)
so as to avoid an oxidizing flame completely. (See Setting
Aluminum OFW flux must be of the highest
quality, and strictly specified as a welding
flux, not as a brazing or soldering flux.
Brazing fluxes, while providing a very
poor weld appearance, also re-alloy the
weld area with zinc from the zinc chloride
they contain. The presence of zinc in
this manner, makes the aluminum parent
metal weak and brittle.
Get the best fit-up possible to avoid large gaps, and select filler metal no thicker than the panel to be welded. Apply flux, and set flame a bit hot for tacking, then apply tacks 1 to 1 1/2 inches apart. Don't worry if distortion or stress causes some tacks to crack.
(Note: a professional 1 3/4-hour video is available from TM Technologies filmed through the special lens showing the OFW process clearly.) When tacks are complete, weld the panel completely from one end to the other.
Distortion can be controlled somewhat by clamping, joint design, hammering, or prying as you go. Chill blocks, while very good for GTAW are actually detrimental for OFW because of the additional heat required. When constructing tanks, for This gas weld on 3003 050 is three feet long and 1/4 inch wide. All distortion, softness and weld-based irregularities on the weld have been planished out. instance, and the 90 degree corners require welding, use the radius bend joint design, where a 45 degree radius bend on each panel allows for stiffness against distortion.
CLEANUP - GETTING STUFF TO STICK Flux cleanup begins by using hot (180 degrees F) water and the stainless steel brush immediately after welding, followed by liberal rinsing with fresh water. If only the filler was fluxed, the amount of cleanup will be minimal. Tanks and similar enclosed parts may be rinsed in this fashion, or as an additional precaution, soaked in the following for 5 to 10 minutes, cold:
1 gallon Technical Grade HNO3 (58-62%) @ 39.5 degrees Be added to: 1 gallon clean water, hot or cold.
Note: Any flux residue in voids or pinholes can be a painter's nightmare in 6 weeks. If any particular area is suspect, play a neutral flame over it, and any yellow-orange incandescence will betray residue.
As mentioned previously, an invisible oxide film appears nearly instantly on aluminum alloys. Proper scrubbing with an etching solution and waiting no longer than 20 minutes to prime, seal, or fill, will avoid such sundry unpleasantness as lifting, peeling, or blistering. A good acid etch also insures against any small traces of flux residue.
In summary, the simple economics of aluminum OFW have insured its longevity. Today, some professional tank builders often don't leak-test the parts produced using the OFW process, because their experience has proven its reliability. Conversely, every tank they produce by GTAW has an average leak count of 6 or 7, and thus must be tested. Workability of the HAZ can make or, literally, break the job. Planishing to remove distortion also saves cleanup and paint prep time, as well as, increasing the strength of the weld-softened area.
Follow the rules and take time to practice this century-old art, and if the parts don't turn out pretty darn nice, they can always be used to cook sand dabs!
Kent White has been a restoration metalman for 25 years. Kent has worked with many fine old craftsmen, and in 1976 became a Master Technician at Harrah's Auto Collection where he worked on boats, cars, and aircraft. He started TM Technologies in 1989 to help revive the traditions, tools, and methods of restoration metalwork.