A new strategy for preparing high strength diffusion-bonded Ni-based superalloy joints at ultra-low temperature via surface nanocrystallization and spark plasma sintering

The challenge of low temperature and rapid diffusion bonding of a Ni-based superalloy was hereby addressed by using a Ni nano-coating and a spark plasma sintering (SPS). It successfully produced a Ni-based superalloy joint with 337 MPa shear strength at 500°C for 30 min, which is approximately 400°C lower than the traditional hot pressure diffusion bonding (HPDB) temperature. The microstructure and mechanical properties of the joints were systematically investigated. It is revealed that the pulsed current and ultra-fine grains (19 nm) in the Ni nano-coating could significantly facilitate voids closure. The voids closure mechanisms involved (i) pulsed current strengthened plastic deformation, (ii) pulsed current strengthened surface source diffusion, (iii) pulsed current strengthened bonding interface diffusion, (iv) grain growth dividing the initial large voids into nano-voids, and (v) massive grain boundaries (GBs), lattice defects, and local high-temperature strengthened GBs diffusion. Furthermore, the GBs migration across the interface was investigated, and the results revealed that the GBs migration and fine grains (350 nm) near the bonding interface together increased the joint strength.