GTAW-P Helps Aircraft Production Fly
An aerospace facility improved rework time and part quality with this process
Reprinted with permission: The AWS Welding Journal
By ANDREW PFALLER, a segment manager, Miller Electric Mfg., TIG Solutions, Appleton, Wis. PFALLER is also a Certified Welding Inspector and the vice chair of the AWS D17 Committee on Welding in the Aircraft and Aerospace Industry and vice chair of the D17K Subcommittee on Fusion Welding in the Aircraft and Aerospace Industries.
Many passengers use air travel regularly with little thought as to the amount of care put into manufacturing each and every component of the aircraft. Fabrication in the aerospace industry has evolved significantly over the years with the addition of new processes and technologies. One of these processes, often overlooked and underutilized, is pulsed gas tungsten arc welding (GTAW-P). The Safran Nacelles group in Burnley, England, started implementing GTAW-P in their operations with success.
The facility manufactures thin-gauge titanium components used in nacelles — the parts on an aircraft that protect the engine. The Burnley location is part of Safran, Paris, France, a large aerospace organization that produces parts for customers around the world.
“We work to a lot of internal standards with high-quality requirements,” said Mark Durkin, the welding specialist who oversees weld quality at the Burnley plant. “Tolerances are very stringent.”
As part of those rigorous standards, the facility is accredited by the National Aerospace and Defense Contractors Accreditation Program (NADCAP) and undergoes regular audits of its weld quality management systems.
“The welding quality side is a very big part of the business, especially when it’s aircraft components. We have a nondestructive testing site, and everything does go through there to get inspected,” said Chris Hill, manufacturing manager at the Burnley plant. “What sets us apart from other suppliers is our work with thin-gauge titanium. Not many facilities can weld down to 0.4 titanium, so, obviously, our skill set and our welders support that skill.”
While trying to fabricate the titanium parts, which range from 0.4 mm (0.016 in.) to 1.6 mm (0.063 in.) in thickness, the Burnley facility experienced issues with distortion that resulted in added time for rework and higher costs for scrapped parts. Leveraging its long-standing relationship with Miller Electric and the Dynasty® family of machines, the facility implemented the pulsing function on the Dynasty 400 GTAW machine to help reduce the heat input.
Traditional direct current (DC) GTAW is a constant current process in which the welding output is either preset or controlled directly by the welder with a foot or fingertip control. For many, this works acceptably well.
In pulsed welding, the amperage is precisely modulated between a higher peak current and a lower background current by the control circuitry of the welding equipment. The five most common parameters that would be associated with this welding process are the following:
- Peak amperage determines the highest amperage that will be achieved.
- Pulse frequency or pulses per second (PPS) determines how often the amperage completes the modulation per second. Lower settings can provide a broader arc better for outside corners, whereas higher settings, above 100 PPS, provide a stiffer column that will give the welder a more controllable arc.
- Peak % time is how long the weld remains at the higher amperage of the pulse cycle. A 40% value is often considered optimal with higher settings as it results in the widening of the weld bead and higher heat input.
- Background % amperage controls the amount of welding current, in relationship to the peak amperage, during the lower power cycle. Lowering this will give the pool a colder feel, and increasing it will help provide a more fluid pool. Most applications will start around a 25% setting.
- Pulse waveshape varies the profile of the modulation. Most inverter welding machines utilize only a square waveform, which is a fast transition between the background and peak. At low-pulse frequencies, welders often like a smoother feel, so they may choose a sine or triangular waveform. With both additional waveshape options (sine and triangular), users will notice a reduction in the distinctive buzzing sound associated with pulsed DC welding due to the QuietPulse™ technology in the latest Dynasty GTAW machines.
When utilizing the parameter appropriately, it can increase travel speed and reduce the heat input/heat-affected zone, which are highly correlated to the effects of distortion. Other secondary benefits can include pool agitation and a stiffer arc, which make it easier to weld on sluggish materials such as high-nickel alloys and titanium.
Welding Thin Titanium Parts
The Burnley facility has areas for sheet metal, pre- and postproduction, welding, nondestructive examination, heat treatment, and pressing. It makes more than 200 different products that go to different areas of the operation.
In the welding operation, 90% of the parts being fabricated are 0.4-mm titanium. One specific component for a customer was causing issues due to the very tight geometric tolerance, the amount of welding required on the part, and the area on the part where the welding was needed. The team was seeing problems with weld distortion causing a lot of rework and parts being scrapped.
Before switching to utilizing pulsed welding for that part, the rework in the sheet metal operation required 45 minutes to an hour to remove the distortion on each part. According to Durkin, GTAW-P “allows us to lower the heat input, which then leads to less weld distortion. We saw a massive time savings.”
Time savings wasn’t the only advantage. The facility aims to have zero welding rework in meeting customer quality and timeline requirements. The welding operation has 12 welding bays and employs 19 welders who qualify to ISO 24394, Welding for aerospace applications — Qualification test for welders and welding operators — Fusion welding of metallic components.
“Because it’s so thin, we have to be really careful to not blow holes in the material and make sure we’re not getting any color in the weld,” GTAW Welder Samuel Chadwick said. “We get a stamp and put our name on every job, so you better make sure it’s perfect every time. We don’t want anything coming back to us.”
“We could hit first-time yield on every part rather than scrapping some off due to weld deformation,” Hill said. “That’s really key. If we have any scrap fallout due to a geometric issue, we’ve then got to replace that part through our production plan.”
Continuing Improvement
Visual LCD color displays have also revolutionized the way welders approach parameter selection.
“We use complex jigs here at Safran,” said GTAW Welder Michael Whittaker. “With [our current welding machines], you can change the parameters very easily and very precisely, which is what you need in aerospace.”
Durkin sees more benefit in the latest GTAW welding machines at the Burnley facility because they provide visual feedback on how adjustments will affect the welding current and the welding arc. This helps welders understand the arc and predict the results before they even strike an arc.
The welders and managers at the Burnley plant have been very happy with the results they have achieved using GTAW-P. They are passing quality testing on the first try for each part, helping the operation hit quality goals and reducing scrap.
Fig 1 Thin titanium parts are joined in a fixture using GTAW-P to minimize distortion and achieve the required quality.
