The first phase of the programme was to demonstrate a circumferential melt run on the inside diameter of a NPS 4 Schedule 10 duplex pipe.
After a short development programme, the resulting weld was of high quality and the interior weld profile much better than that which could have been achieved in a standard EB system.
(You can see a video of the 'EBOBend demonstration of interior Duplex tubular welds,' here.)
Repairing interior defects in niobium SCRF cavities with EBOBend
The second demonstration activity explored internal repair options for damaged Niobium super-conducting RF cavities using the internal processing opportunity that the EBOBend offers.
The internal surface finish of the cavities is a major factor in their performance. Vast effort is expended ensuring material quality is as high as possible and that welding and fabrication processes are at the highest quality levels for production. Chemical etching is also used during production to remove contamination and minor flaws [<100µm]. The presence of larger internal flaws such as scratches or pores directly impacts the performance factor of the cavities.
In this series of experiments simulated defects were introduced into a welded 1.3GHz niobium cavity using a vibro-etching tool. The damage was purposely made larger than normal to show the capabilities of the process.
A series of repair tests were performed and the results showed a complete removal of the defect, leaving a smooth weld root with a smaller profile than the original weld.
(You can see a video of the 'Internal repair processing of niobium SCRF cavities,' here.)
The only other conventional repair options are either a complex interior grinding process, a chemical etching treatment, and/or a low power, pulsed laser system capable of spot repair of shallow [60µm] pores. The successful demonstration of the repair of large flaws should enable future opportunities to recover damaged cavities.
Welding niobium cavities from the inside
A further phase of the programme explored the prospect of welding the final equator joint in the hemisphere/dumbbell assemblies. Traditionally, the final joint in the assembly of cavity structures is the equator weld on the hemispheres. This joint is welded using a tightly controlled process with higher vacuum and increased cleanliness than normal EB welding. This is done to avoid degrading the superconducting performance of the niobium via contamination during welding.
These welds must also be smooth on the interior and must not protrude heavily [<250µm] into the cavity, as these features impact their performance. Welding from the inside opens up several opportunities to change the traditional welding process. Thicker material can be welded without the risk of excessive weld roots, cosmetic weld passes can be made to further improve the weld smoothness and reduce any abnormal profiles.
Current results from a series of experiments show that excellent results can be obtained in ~3mm thick material. The full penetration weld made from the inside is extremely smooth after the cosmetic pass and, compared to the standard weld process, is of lower overall heat input. This will have additional benefits in reducing the size of the HAZ and minimising distortion.
Larger aerospace components welded with restricted access
One hot topic area that TWI has worked on recently is that of electron beam welding aerospace components, namely, fuel tanks for satellite systems and novel construction methods for landing gear applications. Both of these areas could benefit from the increased access and internal processing that the EBOBend kit offers. Normally, inaccessible weld roots are not machined or dressed due to difficulties in accessing these areas. Possible options for processing these areas with the EBOBend equipment could improve the root profile, increasing fatigue performance of the joints and/or offering novel construction routes to otherwise complex items.
The final two demonstration welds in the programme were a large demonstration vacuum vessel, which is similar in size to current satellite fuel tanks, and a thick section titanium alloy weld to gauge the penetration achievable in initial trials.
A large scale 400mm diameter stainless steel vacuum vessel was sourced for internal welding trials. A 100mm diameter vacuum port was used to gain access to the interior of the vessel to perform a weld simulation for novel construction/modification of the vessel.
(You can see a video of the 'Large vacuum vessel welded from the inside out,' here.)