Diffusion bonding and brazing of high purity copper for linear collider accelerator structures - art. no. 053502

Diffusion bonding and brazing of high purity copper were investigated to de velop procedures for joining precision machined copper components for the N ext Linear Collider (NLC). Diffusion bonds were made over a range of temper atures from 400 degreesC to 1000 degreesC, under two different loading cond itions [3.45 kPa (0.5 psi) and 3.45 MPa (500 psi)], and on two different di amond machined surface finishes. Brazes were made using pure silver, pure g old, and gold-nickel alloys, and different heating rates produced by both r adiation and induction heating. Braze materials were applied by both physic al vapor deposition (PVD) and conventional braze alloy shims. Results of th e diffusion bonding experiments showed that bond strengths very near that o f the copper base metal could be made at bonding temperatures of 700 degree sC or higher at 3.45 MPa bonding pressure. At lower temperatures, only part ial strength diffusion bonds could be made. At low bonding pressures (3.45 kPa), full strength bonds were made at temperatures of 800 degreesC and hig her, while no bonding (zero strength) was observed at temperatures of 700 d egreesC and lower. Observations of the fracture surfaces of the diffusion b onded 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 are a under similar bonding conditions. Results of the brazing experiments indi cated that pure silver worked very well for brazing under both conventional and high heating rate scenarios. Similarly, pure silver brazed well for bo th the PVD layers and the braze alloy shims. The gold and gold-containing b razes had problems, mainly due to the high diffusivity of gold in copper. T hese 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 assemb led with pure silver braze inserts using a self-aligning step joint design, then the assembly would be vacuum diffusion bonded at 700 degreesC and 3.4 5 MPa pressure to seal the critical inner portion of the assembly. Finally, during the same furnace cycle, the temperature would be increased to 800 d egreesC 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.