Multi-Principal Element Alloy Brazing Fillers with Reduced Microstructure Sensitivity to Joint Clearance

As investigation of multi-principal element alloys (MPEAs) continues to expand, there is congruent acceleration in development of MPEAs as brazing filler metals. Broad single-phase fields in MPEA composition spaces provide flexibility in MPEA filler chemistry design for melting point control, and the capability to interact with a variety of base materials while retaining a ductile microstructure dominated by a solid solution matrix. The authors’ previous work demonstrated three new MPEA brazing filler alloy systems, each tailored to a specific brazing temperature range, that produced single-phase brazed joint metal while using inexpensive elements.

Brazed assemblies in engineering applications often exhibit variable joint clearances, which may influence joint strength by impacting void fraction, stress state under load, and secondary phase distribution (e.g., borides and silicides in conventional Ni-B and Ni-Si brazing fillers). Systematic studies of joint clearance effects are needed to qualify a new brazing filler metal, but few previous studies on MPEA fillers have directly addressed this variable. This work assessed the impact of joint clearance on a candidate MPEA filler compared to BNi-5 (Ni-Cr-Si) on 316L stainless-steel. The MPEA filler exhibited an FCC matrix with minor secondary phases whose fraction remained relatively constant regardless of joint clearance, while BNi-5 filler exhibited strong dependence of silicide phase fraction upon joint clearance. Centerline voids were observed in the MPEA filler when the clearance exceeded 140-µm. Below this void formation threshold, the MPEA filler consistently produced mechanical property gains compared to BNi-5 in single-lap test joints with a fixed overlap-to-thickness ratio. Relative gains were 99%, 86%, and 620% in engineering sheer stress, base material tensile stress, and total elongation, respectively, at 100-µm fixed joint clearance. To target improved wide gap-filling ability, the roles of thermal cycle, product format, minor composition adjustment, and mixing with base material powders were additionally investigated for the MPEA.