Statement of problem

1. Optimisation of diffusion bonding parameters of Ti-6Al-4V alloy through the structure and surface conditionsEvgenia Yakushinaa, Aleksey Reshetov, Andrzej Rosochowski Advanced Forming Research Centre, University of Strathclyde, 85 Inchinnan Drive, Inchinnan, Renfrewshire, PA4 9L J, United Kingdomaevgenia.yakushina@strath.ac.uk, Aim: estimate the effect of structure and surface conditions, as well as temperature and time, on the quality of diffusion bonded joints.

3. Statement of problem Processes of diffusion bonding (DB) and superplastic forming (SPF), often used in the aerospace industry for joining and shaping Ti alloys into complex parts, are very expensive because of high temperature (900°C - 960°C) and long duration (several hours) of both processes. However, these problems can be mitigated by using ultrafine grained (UFG) versions of Ti alloys. Improvement in the bonding process can be achieved by providing a higher diffusion rate in the contact zone. This rate depends on the contact surface area and properties of grain boundaries. Since grain boundaries play a major role in diffusion, it can be assumed that reducing the grain size and increasing the surface area of grain boundaries should improve diffusion. The main parameters describing the process of DB are temperature, pressure, duration of the process and surface roughness. Surface roughness of surfaces prepared for bonding is a very important but often disregarded factor. The effect of surface roughness is still not well characterized. A simple recommendation, based on the early DB experiments with Ti-6Al-4V carried out at Rolls Royce, is to avoid a surface roughness (Ra) above 1 μm [1].

4. Statement of problem

5. Initial material and experimantal procedure d=15±5μm d=240±60nm a) b) Microstructure of the CG Ti–6Al–4V alloy: a - optical microscopy of initial sample; b - TEM view after SPD-processing

6. Initial material andexperimantal procedure Surface roughness before DB Roughness measurements were performed using Alicona 3D Infinite Focus surface measurement optical microscope.

7. Surface roughness profiles • - grinding on P240 grit, Ra=200 nm, • - polishing with Meta Di Diamond suspension, Ra=50 nm

8. 3D surface profiles of the samples

9. Samples for bonding • - ground using P240 grit • - polished with Meta Di diamond suspension.

10. Initial material and experimantal procedure DB testing procedure Installation for DB: a - vacuum press; b - test chamber for DB; c - coupled samples put between dies Vacuum press GCA 20T Forgex

11. Ra = 200nm dv = 0.5 - 4 μm CG structure UFG structure Ra = 50 nm dv≤ 0.4 μm dv= 0.1 –0.9μm Samples bonded at 825 ºC, 2h, 3.2 MPa

12. Ra = 200nm CG structure UFG structure Ra = 50 nm Samples bonded at 825 ºC, 4h, 3.2 MPa

13. Influence of surface roughness on joint quality

15. Optimisation of DB regimes Time, h Temperature, ºC T = 825° C P = 3.2 MPa T = 2 hours Previous best result Present result

16. Shear test rig design

17. Simulation of shear test Complete bonding

18. Shear test data interpretation

19. Shear test experiment with different metals

20. Conclusions: • Under equal conditions, UFG material demonstrates better bondability compared with CG material. This can be explained by low-temperature superplasticity of UFG Ti-6V-4Al, as well as it's higher diffusion rate. • The reduction of DB temperature down to 825° C at low pressure of 3.2 MPa, was achieved in UFG condition, which is 50ºC lower than the temperature of DB achieved in the up to date publications. • It was established that surface roughness has a significant effect on the DB process. Thus, providing of high surface finish (Ra=50 nm) makes possible to obtain homogeneous joint without any optically detectable defects and uniform structure in the whole volume of sample already after 2 hours of DB at 825ºC .