Diffusion bonding of steels with a homogeneous microstructure throughout the joint

Cold-finished carbon steel bars were bonded by means of the transient liquid phase bonding (TLPB) process using amorphous metallic foils of the eutectic Fe-B composition as filler material. A homogeneous microstructure throughout the joint was obtained. Traces of borides in the middle of the joint were the only distinguishable microconstituent from the base metal due to the TLPB process. The B concentration profile across the joint was measured by neutron radiography and was found to be composed of a central sharp peak with a maximum concentration of 15.9 ppm B superimposed over a broad peak (base width of ≈ 5 mm) with a maximum concentration of 13.3 ppm B. Owing to this low range of B concentrations, boride precipitation was almost suppressed, and only a scarce number of borides were observed at the joint. The resulting boride structure was identified as Fe₂₃B₆ by synchrotron microfocused X-ray diffraction, and its stabilization at room temperature is discussed. The bonded samples were subjected to a bend test, with a bending angle of 180°, and no cracks were observed. In tension tests, the bonded samples attained an ultimate tensile strength (UTS) of 434 MPa, an elongation of 32.3% and a reduction area q of 51.2%—78.6%, 165.6% and 75.4%, respectively, of the base metal. The fracture of the bonded samples occurred at the joint. It was determined that the decrease in UTS compared with that of the base metal was due to the recovery, recrystallization and grain growth that occurred during the TLPB thermal cycle. In addition, from fracture surface observation, it was found that the decrease in q in bonded samples was caused by the presence of traces of borides at the joint, which were the result of the liquid phase that solidified during the cooling stage.

Graphical abstract