Effects of bonding temperature on mechanical properties and interfacial morphology of YG8/40Cr joints fabricated using SPS diffusion bonding

The diffusion bonding of YG8 cemented carbide and 40Cr steel was performed using spark plasma sintering with Ni powder as the interlayer. The influence of bonding temperatures ranging from 700 to 1100 °C on the bonding interface morphology, mechanical properties, and failure mechanism of the joints was studied. By calculating the diffusion coefficients of Ni and Fe and observing the changes in the interface morphology, the pores at the Ni/40Cr interfaces were identified as Kirkendall voids formed because of the unbalanced diffusion of Ni and Fe. The diffusion activation energy of Ni in Fe was 87.37 kJ/mol, and that of Fe in Ni was 58.08 kJ/mol. The use of a pulsed current significantly reduced the diffusion activation energy, making diffusion easier. The bending strength of the joints increased sharply in the range from 700 to 900 °C, subsequently increased slowly, and tended to stabilise in the range from 900 to 1100 °C. The highest bending strength of 1080 MPa was achieved for the joint bonded at 1100 °C, which was attributed to the improved bonding of the YG8/Ni and Ni/40Cr interfaces and the resulting effects of the interlayer plastic constraint. The thickness of the CoNi diffusion zone at the YG8/Ni interface appeared to form a decisive factor in determining the failure locations of the joints. As the bonding temperature increased, the improved diffusion of Co and Ni enhanced the metallurgical bonding of the YG8/Ni interface, and the fracture position shifted from the polished surface of the YG8 base material (700 °C) to the Ni interlayer (1100 °C).