Strategies to Improve Electron Beam Weld Quality for ISIS TS2 Targets Arghya Dey and Leslie Jones Target Design Group ISIS, STFC, RAL

ISIS TS2 Target Design ISIS TS2 target assembly: (a) target HIP assembly, (b) cooling sleeve, (c) cross flow guide and (d) window cap

Target Manufacturing Process W Core HIP: 1200 o C, 140 MPa, 3 hrs Joint 1 Joint 2 W Core Ta Clad EBW Water Cooling Arrangements Ta Flange

Problems Encountered the targets fail around 1 year of use. The aim is to achieve 5 years of service life * γ spectral analysis: Radioactive isotopes in the cooling water. 172 Lu, 175 Hf, 182 Ta [1], 187 W * Water leaks during operations [1] Goran Skoro, Stuart Ansell, Frances Burge

γ spectral analysis has been carried out on the water cooling system after a target had failed. (Comparison of intensity of gamma- lines): - activity of 175Hf is higher than activity of 182Ta ; - activity of 172Lu is below activity of 182Ta. Monte Carlo simulations: - activity of 175Hf is higher than activity of 182Ta (in Tungsten); - activity of 175Hf is two orders of magnitude lower than activity of 182Ta (in Tantalum); - activity of 172Lu is below activity of 182Ta (in Tungsten); - activity of 172Lu is two orders of magnitude lower than activity of 182Ta (in Tantalum). Conclusion: Comparison of experimental gamma spectrum with Monte Carlo simulations strongly indicates that radioactive material in the TS2 water cooling system is predominantly Tungsten. [1] Goran Skoro, Stuart Ansell, Frances Burge

Route Cause Analysis 29mm 2.3mm EBW penetration length << 1.3mm Too much reliance on HIP Bonding?

Weld penetration test Typical weld parameters

Microstructure analysis

Weld penetration tests & results Parameters varied: strip thickness, welding current (15–30mA), beam rotation (0 or some rotation) and focus of the electron beam (at surface or up to 40μm sub surface).

Observations Depth of weld penetration: high weld current or power sub surface focus of the electron beam low material volume around the weld line; e.g., thin strip size, grooves either side Low HAZ area: low weld power input focus of the beam to slightly below the surface thick weld specimen Fine grain size at HAZ: large material volume around the weld line to dissipate heat rotation of the electron beam focus of the beam to slightly below the surface

Key modifications In order to ensure a full circumferential weld on the previously manufactured target, a re-welding operation was carried out on the current Target. For all of the future targets, the material thickness at the weld joint is reduced down to 1.3mm from 2.3mm. Only 0.3mm material to be removed after HIP. Increasing the weld current to 25 – 30mA ( W power). Focusing the electron beam 40μm below the surface. A rotation of the electron beam is used during welding. The current TS2 target is in service since December 2014

Further work To establish the optimum material thicknesses for attaining full weld penetration in the joints of the tantalum cladding. To Improve the EB weld penetration depth and to reduce the HAZ by reducing the availability of materials through grooving around the joint. To understand the change of material property of tantalum at different manufacturing steps for the cladding (e.g. machining, EBW, HIP and combination of all). To evaluate the benefits of introducing some cold work at the HAZ area of the weld joint through shot peening.

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