TWI Ltd has recently completed a general support technology programme (GSTP), funded by the European and UK Space Agency, for the development and qualification of a linear friction welding based manufacturing route for a satellite application.
Background
The manufacture of critical aerospace structures is traditionally accomplished by machining rolled plates or forgings, sourced to fulfil the envelope of each component. As such, any external features like reinforcement ribs, bosses or mounting brackets expand the envelope of the structure, significantly increasing the raw material costs. In addition to lacking design flexibility, this approach contributes to very high buy-to-fly ratios, especially when considering large-scale structures for spacecraft.
Aiming to save raw material costs, designers may choose to attach external features using traditional joining techniques, namely fusion welding, mechanical fastening or adhesive bonding. However, these processes either require increasing the thickness of the joint to compensate for a local reduction of parent material strength or add mass to the final assembly. In parallel, there are promising developments on the use of additive manufacturing (AM) for aerospace structures. However, there are well documented technical and commercial challenges when considering this technology to build large-scale components: (i) the relatively slow deposition rates; (ii) the extent and practicality of post processing and inspection; (iii) process size limitations; (iv) limitations on the range of eligible materials which can be additively manufactured; and (v) the mechanical properties achieved.
Friction welding is an attractive option for producing solid-state joints with high structural integrity. In particular, linear friction welding (LFW) has been researched by the aerospace sector for joining safety critical structures and fabricating close to net-shape preforms from which high integrity components can be extracted. Furthermore, new design possibilities arise from the ability to join dissimilar combinations or even materials previously considered to be unweldable.
When considered at the design stage, LFW has provided numerous examples of savings in raw materials and manufacturing costs. The use of LFW to produce high integrity near net-shape fabrications for spacecraft demonstrated savings in raw materials of 73% and 66% for a satellite fuel tank application and cryogenic tanks for launch vehicles, respectively [Gandra et al. 2023].
Objectives
This investigation was carried out in partnership with Airbus Defence and Space Stevenage (Airbus) and funded by European and UK space agency. The aim of this investigation was to develop and qualify an LFW based manufacturing route for space applications by demonstrating technology readiness level 6, i.e. fabrication of a fully functional prototype or representational model.
The objectives for this investigation were as follows:
- Perform a survey of candidate applications/parts aimed at reducing the current manufacturing cost and time.
- Develop a LFW procedure that produced welds which met the acceptance criteria for the selected application.
- Validate the LFW based manufacturing route by fabricating of a representative technology demonstrator.
- Qualify the LFW technology demonstrator using a testing protocol based on the relevant space standard (ECSS-Q-ST-70-39C1)
- Determine the potential improvements in manufacturing cost, efficiency and environmental friendliness of the LFW based manufacturing route via a life cycle and economic assessment