The welding process that will be used to repair flow liner cracks found in the orbiter fleet is a Gas Tungsten Arc Welding technique that results in a very pure weld with minimal impurities in the final product. This type of technique is typically used for the welding of oxygen sensitive materials. In the case of the orbiter flow liner cracks, the weld is a full penetration weld that will need to go completely through the metal even though the welder will only have access to one side. This increases the difficulty of the process.

Since cleanliness is critical to the Shuttle Main Propulsion System, advantages of Gas Tungsten Arc Welding over conventional arc welding are the lack of weld spatter and minimal debris generation. This type of weld leaves no slag after the weld pass, and minimal debris on the backside of the repair where access is extremely limited. The liquid hydrogen lines require a 400 micron cleanliness level, meaning that no particles bigger than 400 microns (0.016 inches) can be observed in the line after the repair process. Safeguards and protective measures have been put in place, and a final inspection after completion of the work will verify lines are clean and ready for flight.

The limited access in the repair area creates a number of additional challenges for the welder. Due to the argon gas inerting process necessary for the weld repair, the welder will be instrumented with an oxygen sensor to ensure he is able to breath adequately. Another oxygen sensor will be placed near the weld to verify that no oxygen is in the immediate area of the welding as it is taking place.

The detailed weld process for the repair of the flow liners has been developed specifically for this repair at the Marshall Space Flight Center, Huntsville, Ala. The process consists of several autogenous (no filler material) heat passes to reduce and eliminate any residual stresses created by the actual filler weld pass.

The flow liners on Atlantis, Endeavour and Discovery are crafted from Inconel 718, a nickel and iron super alloy that has high strength and is very resistant to the extreme ranges of temperatures in Main Propulsion System fuel flow lines during ascent. The flow liners themselves are 0.05 inches thick. Flow liners on Columbia, the first orbiter built, are made from Corrosion Resistant Stainless Steel 321 (CRES), which has similar performance characteristics.

Because of limited technical data available for Gas Tungsten Arc Welding of welding on the specific material used in Atlantis, Endeavour and Discover flow liners (Inconel 718), an empirical test program was developed and performed at Marshall to both certify the welder and the repair process.

More than 100 "coupon" samples were created with slots stamped in them just as the actual part. These coupons were then run on a cyclic load-testing device, capable of inducing millions of fatigue load cycles in only a couple of days, until they developed cracks of a similar size and location as the actual parts on the orbiters. These cracked coupons then were subjected to a variety of weld repair techniques, and placed back into the testing machine to attempt to create the crack again. The weld repair technique, which performed as well as the parent material in the cyclic load testing, was selected as the process for this repair. Additional coupons were run through the entire process and tested at many different load values to establish a base line technical data, and to familiarize the welder with the repair process. The testing program is ongoing and will be repeated for material used for Columbia's flow liners.

To further ensure the viability of the flow liners, the holes in the flow liner have been carefully polished to relieve any of the residual stress resulting from the stamping of the holes into the liner and to reduce the probability of micro cracks growing into actual cracks. This polishing effort is similar to efforts taken in the polishing of internal parts in racing engines. Racing engines also are subjected to very high stresses, and it has been demonstrated that polishing eliminates surface irregularities, which in turn decreases their susceptibility to cracking and failures.

Throughout the welding operation and following its completion, a number of non-destructive inspections and evaluations using X-Rays, Ultrasonic Testing, Eddy Currents and Borescopes will ensure that the final repair is consistent with the parent material and with the coupons from the test program.