A novel local shielding approach for the laser welding based additive manufacturing of large structural space components from titanium

The Advanced Telescope for High-ENergy Astrophysics (ATHENA) will observe 'the hot and energetic universe', which was determined one of the most urgent scientific topics for a major future space mission by The European Space Agency (ESA). One of its three main components is the optical bench, a monolithic titanium structure that accommodates 678 mirror modules and has to keep them accurately aligned. The immense but slender structure in the range of a 2.5 to 3 m diameter at a height of 300 mm proves a challenge to manufacturing. A hybrid robot cell is developed using additive build up via laser welding, combined with high-performance machining and state of the art process and metrology monitoring and control. The present work focuses on the shielding of the laser induced melt-pool, a key concern when processing titanium. The sensitive metal with unusual low heat conductivity requires a large area of high purity atmosphere to prevent embrittlement. However, the large hybrid system prohibits the use of a sealed enclosure and therefore a local shielding system is developed for the challenging case of the ATHENA optical bench's hollow-chamber design. As the thin wall design poses a worst-case scenario in terms of heat dissipation and shielding flow, the effectiveness of the system can be applied to the flexibility of lot size one as well. The key features of the novel approach are the prevention of turbulence while keeping operation economical despite the large shielding area. The first is achieved by means of an integrated honeycomb screen, the latter by employing a layered flow with a higher velocity outer curtain and an air deflecting co-flow. This system was numerically optimized, tested and the effectiveness proven by means of visual inspection, microstructural analysis and measurement of material properties.