Comparison of laser and electron beam welding for safety-critical space applications

Welded joints for space applications must be of high quality, to tight dimensional requirements, and of high performance, e.g. in terms of their mechanical and corrosion properties.

Examples are welded valves, which are used to control the flow of hazardous propellants through thrusters to alter the position of satellites. Nevertheless, in recent years, laser weld quality anomalies during manufacture have occurred, jeopardising projects, qualification activities and (in some cases) missions, resulting in substantial costs and delays.

The supply chain to the European Space Agency (ESA) now uses electron beam (EB) welding for critical welds in valves, consigning laser welding to secondary applications. ESA is keen to re-develop laser welding as a viable alternative to EB, and has been working with TWI to develop and compare both processes on representative materials and thicknesses, using a number of different laser beam welding approaches.

Trials have been carried out on thin sheet coupons of 347, 430 and/or 17-7 stainless steels, to develop suitable welding parameters. The weld qualities and microstructures that result have then been analysed and compared, using conventional X-ray film radiography, metallographic cross-sectioning and hardness surveying, as well as digital micro-focus X-ray radiography, laser weld profile scanning, scanning electron microscopy (SEM), electron backscattered diffraction (EBSD) pattern analysis and electron dispersive X-ray (EDX) analysis.

In work going forwards, comparative mechanical property testing of welded samples will be taking place, both at the coupon level, and in a simple breadboard demonstrator design.