Diffusion bonding and brazing of high purity copper for linear collider accelerator structures

Diffusion bonding and brazing of high purity copper were investigated to develop procedures for joining precision machined copper components for the Next Linear Collider (NLC). Diffusion bonds were made over a range of temperatures from 400 \ifmmode^\circ\else\textdegree\fi{}C to 1000 \ifmmode^\circ\else\textdegree\fi{}C, under two different loading conditions [3.45 kPa (0.5 psi) and 3.45 MPa (500 psi)], and on two different diamond machined surface finishes. Brazes were made using pure silver, pure gold, and gold-nickel alloys, and different heating rates produced by both radiation and induction heating. Braze materials were applied by both physical vapor deposition (PVD) and conventional braze alloy shims. Results of the diffusion bonding experiments showed that bond strengths very near that of the copper base metal could be made at bonding temperatures of 700 \ifmmode^\circ\else\textdegree\fi{}C or higher at 3.45 MPa bonding pressure. At lower temperatures, only partial strength diffusion bonds could be made. At low bonding pressures (3.45 kPa), full strength bonds were made at temperatures of 800 \ifmmode^\circ\else\textdegree\fi{}C and higher, while no bonding (zero strength) was observed at temperatures of 700 \ifmmode^\circ\else\textdegree\fi{}C and lower. Observations of the fracture surfaces of the diffusion bonded samples showed the effects of surface finish on the bonding mechanism. These observations clearly indicate that bonding began by point asperity contact, and flatter surfaces resulted in a higher percentage of bonded area under similar bonding conditions. Results of the brazing experiments indicated that pure silver worked very well for brazing under both conventional and high heating rate scenarios. Similarly, pure silver brazed well for both the PVD layers and the braze alloy shims. The gold and gold-containing brazes had problems, mainly due to the high diffusivity of gold in copper. These problems led to the necessity of overdriving the temperature to ensure melting, the presence of porosity in the joint, and very wide braze joints. Based on the overall findings of this study, a two-step joining method is proposed for fabricating the NLC structures. The structure would be assembled with pure silver braze inserts using a self-aligning step joint design, then the assembly would be vacuum diffusion bonded at 700 \ifmmode^\circ\else\textdegree\fi{}C and 3.45 MPa pressure to seal the critical inner portion of the assembly. Finally, during the same furnace cycle, the temperature would be increased to 800 \ifmmode^\circ\else\textdegree\fi{}C in order to react the silver with the copper to form a liquid braze alloy that would join and seal the outer portion of the cells together.