Effect of B on the microstructure and mechanical properties of Cu–Cu joints with in-situ Ag–Cu–Zn–Sn filler metal

Abstract

With the development of advanced manufacturing, traditional Ag-based filler metals cannot reconcile the contradiction between high performance and complex processing. It also unable to meet the requirements of new materials, new structures and new processes for brazing connection performance. In this paper, based on the Ag–Cu–Zn–Sn flux-cored filler metal, the effect of the addition of B on the microstructure and mechanical properties of Cu–Cu brazing joints was investigated using FESEM‒EDS, EPMA, EBSD, TEM, microindenter and universal tensile testing machine. The results indicated that the microstructure in the brazing seam was mainly composed of Ag (s.s), Cu(s.s) and (Ag + Cu) eutectic. With increasing B content, the grain size and texture strength of Ag(s) and Cu(s) gradually decreased. The refinement of Ag-rich dendrites was mainly due to the enrichment of B atoms around the dendrite, which limited the diffusion of solute atoms in the melt and the release of solidification latent heat in front of the liquid‒solid interface, further inhibiting the growth of Ag-rich dendrites. The refinement behavior of Cu-rich dendrites was mainly attributed to the increase in B content, which reduced the tip radius of Cu-rich dendrites. The mass transfer path of B → B2O3 and B → liquid CuSn alloy → mixed melt of CuSn alloy + solder skin alloy → liquid filler metal in the brazed joint → Cu rich phase and grain boundary in the brazed joint → Ag rich phase and grain boundary in the brazing seam was clarified. The addition of B can improve the microhardness of the brazing seam. The shear strength reached a maximum value of 237.2 MPa when B was added at 2%. The fracture models mainly included intergranular fracture, dimple fracture and cleavage fracture. The strength of brazed joints can be improved by solid solution strengthening, process hardening strengthening and fine grain strengthening. The tangle and jog of dislocations within the crystal, the presence of a large number of grain boundaries and B-rich areas can prevent crack tip extension and enhance the toughness of brazing joints.