Effect of Ni Interlayer Thickness on the Welding Morphology and Mechanical Properties of SPS Diffusion-Welded YG8/40Cr Joints

Diffusion welding of YG8 and 40Cr was carried out by spark plasma sintering. The influence of Ni interlayer with different thicknesses varying from 0 to 500 μm on the welding morphology and mechanical properties of joints was studied. The fracture position moved from the cemented carbide substrate toward the connection interface with the increase in the thickness of the Ni intermediate layer. The joint with nickel-free interlayer and 100-μm thick fractured during electrical discharge machining due to the large residual stress. The bending strength of the joints enhanced with the increase in the Ni intermediate layer from 200 to 300 μm, and a maximum flexural strength of 982 MPa was obtained with the thickness of the Ni intermediate layer to be 300 μm, which was up to 97% of the strength of the cemented carbide substrate. However, the further increase in Ni intermediate layer from 300 to 500 μm resulted in a decrease in the bending strength, which was due to the coupled effects of residual stress and interlayer plastic constraint. Besides, fracture cracks were also observed at the interface of Ni/40Cr with the thickness of Ni intermediate layer to be 400 and 500 μm. The formation of Kirkendall pores in the interface of Ni/40Cr due to different diffusion coefficients of Fe (1.468 × 10−14 m2/s) and Ni (2.15 × 10−14 m2/s) was responsible for the crack initiation and propagation in the Ni/40Cr interface.