Effect of Zr on microstructure and mechanical properties of 304 stainless steel joints brazed by Ag–Cu–Sn–In filler metal

The effect of Zr on the microstructure and mechanical properties of 304 stainless steel joints brazed with Ag–Cu fillers was studied. The incorporation of Zr had little effect on the solid–liquid phase line of the fillers, and the melting temperature range of the fillers was narrowed, which enhanced their fluidity and wettability. The presence of Zr in the form of heterogeneous particles augmented the nucleation rate during solidification, transforming the intermittently distributed gray-black coarse dendrites into cellular crystals. This structural transformation led to fragmentation and refinement of the microstructure. The dissolution of Zr into Ag and Cu promoted the transformation of low-angle grain boundaries to high-angle grain boundaries (HAGBs), hindering crack propagation. Zr element in the brazing seam led to grain refinement and increased density of grain boundaries. The grain refinement could disperse the stress, and HAGBs could resist the dislocation movement, improving the joint strength. The results display that when Zr content was 0.75 wt.%, the maximum strength was 221.1 MPa. The fracture occurred primarily at the brazing seam, exhibiting a ductile fracture.