Flow and Microstructural Evolution of Co-Based Filler Metals on Stainless Steel and Intermetallic Phases Mechanical Property Determination by Nanoindentation

Madeni, J.C.

Cobalt-based brazing filler metals have been used in a wide range of high temperature applications. During brazing, filler metal, substrate and environment factors affect the filler metal flow. These factors are density, viscosity, wettability, alloying, surface condition, geometry, heat transfer/distribution on braze coupon, inert gas, gas flow rate, or vacuum utilized during brazing. This study investigates the flow characteristics resulting from short brazing cycles of thin foil Cobalt-based filler metals on 304 stainless steel substrates and the development of the interfacial microstructure during thermal cycling as the braze joint ages. In addition, the mechanical properties of the intermetallic phases within the joint are determined using nanoindentation.

The experiments used 304 stainless steel substrates and two cobalt-based filler metals. Coupons were machined to provide a 100 mm braze gap with 5 mm length and 6 mm width brazing area. Images were acquired during filler metal flow; filler metal displacement was measured and correlated with time to determine the velocity. Joint coupons were placed under thermal cycling conditions at 200, 500 and 800^oC for 10 and 20 hours. Then, these coupons were sectioned for metallography. Optical microscopy 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 within the braze joint and their mechanical properties were determined.

Filler metal flow results show that the two filler metals have approximately constant average velocities of 1.2x10-2 mm/s and 8.6x10-3 mm/s, from beginning to end of braze channel. 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.