Behavior of Cobalt and Nickel Based Filler Metals in Brazing Applications

Nickel and Cobalt-based brazing filler metals are being used in a wide range of high temperature and high precision applications.  During brazing, filler metal, substrate, and environment factors affect filler behavior. This study investigates the 1) wettability and filler metal flow, 2) microstructural development of braze joints upon thermal aging, and 3) intermetallic phases mechanical properties.

The experiments used 304 stainless steel substrates, four nickel-based filler metals and two cobalt-based filler metals. Coupons of 100 mm braze gap with 5 mm length and 6 mm width brazing area were prepared. Images were acquired during filler metal flow from which their velocity was determined. Joint coupons were placed under thermal cycling conditions at 200, 500 and 800^oC for 10 and 20 hours. Optical and scanning electron microscopy (SEM) were used to characterize the joint interface. Energy Dispersive X-ray Spectroscopy (EDS) and dot mapping techniques were used to identify the intermetallic phases that form at the joints interfaces. Nanoindentation was performed on the intermetallic phases to determine their mechanical properties.

Wettability studies resulted on the nickel-based alloys lowest contact angles (10^o to 4.5^o); compared to (15^o to 9^o) for the cobalt-based alloys. Filler metal flow analyses resulted in the nickel-based alloys higher velocities (1.4x10^-2 – 4.2x10^-2 mm/s) than the cobalt-based (0.8x10^-2 – 4.0x10^-2 mm/s). The microstructural characteristics at the base metal/filler metal interface, in particular the stability and morphology of intermetallic compounds, change significantly as the joint ages at high temperature cycles. In addition, large amounts of boron diffuse into the substrate, not only through the grain boundaries but within the grains themselves. Results from nanoindentation display the significant mechanical properties difference among the microstructural features. Young’s Modulus values varied from 203 to 282 GPa, and from 232 to 348 GPa for the intermetallics in the nickel and cobalt based alloys, respectively.