Delta Air Lines Technical Operations Center in Atlanta (Delta TechOps; 866-MRO-Delta or TechSales.Delta@delta.com) is the largest airline MRO in North America. In addition to providing maintenance and engineering support for Delta’s fleet of aircraft, Delta TechOps serves more than 100 aviation and airline customers from around the world, specializing in high-skill work like engines, components, base and line maintenance.
Delta TechOps considers much of its work sheet metal. Metal thicknesses average 0.030 inch, with 0.012 inch being considered extremely thin and 0.063 inch considered “thick” sheet metal. Applications include:
- All of the ducting. Ducting is made from Inconel 625, Titanium (commercially pure and Ti-6Al-4V grades), and 6061-T6 aluminum.
- “Hot section” parts (combustion area) made from nickel and cobalt alloys.
- The APU (auxiliary power unit) which uses several different grades of stainless steel.
For best results when welding sheet metal in aircraft applications, consider the following advice from Jody Collier, a senior certified welding inspector (SCWI) and instructor/developer, welding training/certification with Delta TechOps.
Rule of thumb: 1 for 1
To estimate required heat input, a good rule of thumb is to use 1 amp per 1 thousandth (0.001) inch of thickness for carbon steel. Stainless steel and other nickel-based metals require slightly less heat — perhaps just 2/3 of an amp per thousandth — because nickel conducts heat poorly. For aluminum and magnesium (excellent heat conductors), increase heat input slightly.
Use copper chill bars with argon (shielding gas) ports. Both the copper and the gas draw heat away from the weld area. Delta TechOps fabricates its own chill bars in a wide variety of shapes and sizes. It’s OK to use a copper shoe attached to a steel s tructure, but configure the bar so that the copper touches the weldment.
Caution: Do not put copper in direct contact with cobalt alloys, as any zinc in the copper may cause issues in high-temperature applications. In these instances, nickel-plate the chill bars.
Cover your backside
When it comes to welding sheet metal, Delta TechOps treats most applications (except aluminum) as it would titanium. Titanium always requires shielding the backside of the weld to prevent oxidation (“sugaring”). In thin applications, shielding gas on the backside of the weld reduces heat input. It also improves weld quality by eliminating metal surface imperfections (the irregularity of sugaring) that could be a potential source of cracking in high-fatigue applications.
Love the back-step
The standard practice when TIG welding involves pushing the weld puddle in one continuous direction for the entire length of the weld. With back-stepping, the welder breaks up the seam in sections to dissipate the heat. Welders start at position 1 and weld out to the right edge. They then move to position 2 and weld toward position 1, tying-in with the first weld. The length of each step varies by application; thicker material and better part restraint (fixturing) may permit a longer weld. At Delta TechOps, the average step is about 1-inch long and never longer than 2 inches. Do not confuse the back-step method — a planned series of steps along a single joint — with the randomness of skip welding.
Other sheet metal applications (e.g., stainless steel in food service) typically call for a size #7 TIG cup. For welding sheet metal in aerospace applications, use a #15 cup (15/16-inch diameter) or larger. A large cup provides better gas coverage of the weld puddle and heat affected zone, thus reducing total heat input and improving bead quality. It should also improve bead appearance and reduce the need for post-weld wire brushing to remove oxidation.
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