Working with the companies TWI Ltd. and Airbus UK, researchers from the European Space Agency have used the technique of Friction Stir Welding (FSW) to build the components of a prototype spacecraft propellant tank. This new method would have lower costs and lower lead times than conventional manufacturing techniques.
FSW is a high quality, more sustainable, manufacturing method and has been used for building aluminium-based aerospace components. This study, published in CEAS Space Journal, uses the technique to join components made of a titanium alloy.
Residual stress caused during manufacture can cause distortion and degrade the mechanical performance of the component, leading to potential failure. It's therefore important for the engineers to understand the residual stress that is present once the parts have been joined together.
Previous studies have looked at the residual stress inside flat FSW components, but this is the first to study curved components. Using X-ray and neutron diffraction and hole drilling, the collaborators were able to determine the magnitude of the residual stresses near the joint around the tank circumference and evaluate these in the context of the requirements defined by the space industry.
The three methods are fundamentally different, and so the researchers were also able to compare the results from each. Thanks to the highly penetrating nature of neutrons, neutron diffraction on Engin-X was used to study the bulk residual stresses, whereas X-ray diffraction and hole drilling were used to study the surface of the propellant tank.
They found that the residual stresses present were lower than for conventionally welded tanks, or for previously reported FSW examples. This confirms that using this new method for joining hemispherical and cylinder structures was successful for manufacturing a propellant tank that meets the stringent acceptance criteria of the space industry with regards to residual stresses in the weld.
“The collaboration between ESA, ISIS and the Engin-X beamline is a great example of the kind of research that can be initiated on the Harwell research campus," explains Martina Meisnar from ESA. “We had a new technology and were facing a challenging research problem with the structural integrity of these propellant tanks. The scientists and experts at Engin-X did an outstanding job and helped us design the experiment and evaluate the data. We are certainly hoping for future collaborations on similar topics."
The full paper can be found online at DOI: 10.1007/s12567-021-00398-w