Effect of cadmium-free filler metal on mechanical properties and microstructure of PDC cutters

Objective PDC bits are critical components of geological support equipment and play a vital role in advancing technology and improving efficiency in the coal industry. Currently, PDC bits are brazed using silver filler metals containing cadmium, a toxic and polluting element. This study introduces the use of environmentally friendly, cadmiumfree filler metals for brazing PDC bits. The study investigates the wetting mechanism between brazing filler metals and base materials, utilizing Ag-Cu-Zn series brazing filler metals containing high silver and Sn, Mn and Ni, and only Ni for flame brazing of PDC cutters and steel. The shear strength of different brazing solders and the performance of PDC cutters post-welding were analyzed. Microstructure, element content, and Raman spectra of PDC cutters with the highest melting temperature were examined pre- and post-brazing to assess potential thermal damage. Interface morphology, element diffusion, section features, and phase composition of brazing joints were analyzed to elucidate the formation mechanism of cadmium-free Ag-Cu-Zn series filler metal joints. Additionally, the wetting angle and chemical affinity parameters of the filler metal and interfaces were measured and calculated to comprehensively evaluate brazing performance. The results indicate that: (1) All four cadmium-free Ag-Cu-Zn brazing alloys satisfy the strength requirements for PDC cutter brazing, with Ag-Cu-Zn brazing alloys containing Mn and Ni achieving a strength of 322.989 MPa. (2) The brazing mechanism of Ag-Cu-Zn solder containing Sn involves micro-diffusion of elements at the interface. For brazing joints containing Mn and Ni or only Ni, the formation mechanism involves creating a reaction layer at the interface, which increases bonding strength. (3) The fracture morphology of Ag-Cu-Zn series brazing metal containing 3% Sn, Mn and Ni, and only Ni is ductile fracture, while the Ag-Cu-Zn series brazing alloy containing 5% Sn exhibits a brittleductile mixed fracture. (4) Mn reduces the tension between the Cu-rich phase and the base metal on both sides of the interface, whereas Ni only reduces the tension at the steel interface. Consequently, cadmium-free Ag-Cu-Zn brazing joints containing Mn and Ni have the highest strength. These findings provide theoretical and experimental support for the engineering application of environmentally friendly cadmium-free brazing filler metals in PDC bit brazing, promoting a transition towards more sustainable and advanced brazing technologies.