<p>When brazing YG18 carbide to 40Cr steel with a Cu–based active filler metal, the Co element diffused into the weld to form IMCs such as Co₂SnTi and CoTi, and the shear strength of the joint was decreased. To optimize the distribution of Co–rich compounds, in this study a Cu–Sn–Ti/Cu/Cu–Sn–Ti multi-layer filler metal was designed to join YG18 with 40Cr steel. The effect of Cu foil thicknesses on the microstructure and mechanical properties of YG18/40Cr joints was investigated. The microstructure of the joint was YG18/TiC/CoTi + Co₂SnTi/Cu [s, s]/CuSn₃Ti₅ + Cu [s, s]/TiC/40Cr. When the thickness of the Cu foil was 50&#xa0;µm, a small amount of Co–rich compounds were present in the YG18/Cu foil area as well as in the Cu foil/40Cr area, and the shear strength of the joint was lower. When the thickness of the Cu foil was 100&#xa0;µm, the Co–rich compounds were present only on the YG18 matrix side, and the residual stresses in the joint were released through the plastic deformation of the Cu foil. At this parameter, the shear strength of the joint was maximized at 268&#xa0;MPa. When the thickness of the Cu foil reached 150&#xa0;µm or 200&#xa0;µm, the thicker Cu foil generated a larger strain during brazing cooling, leading to the formation of joining defects at the interfaces and a decrease in the shear strength of the joint.</p>