Effects of element diffusion on microstructure evolution and residual stresses in a brazed joint: Experimental and numerical modeling

The microstructure evolution induced by element diffusion and residual stresses in a brazed joint were studied by energy dispersive spectrometer, electron backscatter diffraction, nanoindentation and numerical modeling. The results show that the B element is diffused from filler metal to base metal and reacts with the Cr element around grain boundaries. A diffusion affected zone (DAZ) with phase transformation and boride precipitates CrxBy was generated and the mechanism of microstructure evolution was discussed. In addition, many coincident site lattice boundaries in the DAZ and base metal were found, but less were observed in the filler metal, which indicates that the stress corrosion cracking is prone to occur in the brazing seam. Affected by element diffusion, the residual stress distribution in the DAZ was found to be extremely complex, and the local residual stresses in the DAZ are higher than that in the filler metal. However, the material properties of DAZ are always assumed to be the same as the base metal, and the maximum residual stress is found in the filler metal. Thus an improved numerical model considering phase transformation was proposed, which has a pretty good agreement with the experimental results.