The precision welding of metallic joints plays an important role in the production of components for space applications. In order to meet the high levels of performance required, metallic joints must be produced which have the correct dimensional tolerance, are free from defects and exhibit the correct mechanical and corrosion performance. In propulsion systems in particular, all valves and pipes have welds which must be leak tight and safely contain hazardous propellants.
In recent years, there have been anomalies related to weld quality issues with valve manufacturing, and weld failures have jeopardised space projects, qualification activities and (in some cases) missions, causing substantial costs and delays.
Electron beam (EB) welding is currently used for the critical welds in flow control valves, whilst laser welding is applied to the secondary (internal) welds. The overall objective of this project is to demonstrate the current state of the art for both EB and laser welding in terms of weld quality for the types of weld and materials currently utilised in the valves. A key aim for the future is laser welding becoming a viable alternative for selected critical welds within flow control valves.
After a thorough review of the process with ESA and Airbus Defence and Space GmbH in Lampoldshausen, Germany, the main areas of the project are ‘baseline material characterisation’, ‘optimisation of the welding process parameters’ for both electron beam and laser welding, ‘property evaluation of optimised welding processes’ and ‘manufacturing and testing of demonstrators’.
A breadboard demonstrator design has been agreed and work has commenced to develop the welding processes for it. The materials being assessed are 347 stainless steel, 430 stainless steel, Ti6Al-4V and 17-7 CH900 stainless steel.
Parameters are being developed to compare the results of laser and electron beam welding. Preliminary melt run trials in each material, followed by butt welds in both heterogeneous and dissimilar material combinations, have already taken place.
Results of these trials are now being evaluated through a combination of visual inspection, X-ray radiographic inspection (using both conventional and microfocus X-ray techniques), scanning acoustic microscopy, destructive sectioning, hardness surveying and electron microscopy, to confirm that stable, acceptable conditions have been established. The quality of the welds is being assessed to the aerospace standard DIN 29595:2007-04 ‘Welding in aerospace – Fusion welded metallic components – Requirements’.
Figure 2: EB and laser welding trials have been undertaken on the agreed materials of interest to achieve specific weld penetration depths and qualities. Images 1–5 (not all to same scale) show example results.
Once the processing windows have been confirmed, a detailed analysis will be carried out on the properties of a selected number of further weld coupons, including fracture toughness, fatigue crack growth rate and stress corrosion cracking.
The work programme outlined above will lead to selected welding processes being taken forward to the manufacturing and testing of the demonstrators, to commence in 2017.
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