Development of a Novel Boron free Filler Metal for Transient Liquid Phase Bonding of Ni base superalloys

Abstract

Joining of Ni base superalloys by transient liquid phase bonding (TLPB) has found limited application in aerospace due to the formation of brittle borides adjacent to the joint. Filler metals containing ~3 wt% B as a melting point depressant (MPD) are the industry standard due to fast isothermal solidification (IS) kinetics, low brazing temperature (1050 °C) and relatively low cost. In the current work, a boron-free filler metal using Cu and Mn as an alternative MPD is proposed as a novel filler metal (FM) for IN625 base metal (BM). A commercially available FM powder, NiCuMn37 was combined with pure Ni powder to create the novel Ni30 alloy, containing 30 wt% Ni. The Ni30 FM and prepared joints were analysed by in-situ thermal analysis using a differential scanning calorimeter (DSC). Using DSC analysis, the liquid fraction present in the joint was tracked and isothermal solidification (IS) kinetics was quantified against hold time at 1050 °C. Elemental analysis of Ni30/IN625 joints coupled with calculations of melting range using Thermo-Calc software confirmed the progress of IS at relevant hold times. Resulting braze joints had a solid solution microstructure with a diffusionally affected zone (DAZ) in the BM. IS of a ~110 µm braze joint completed after 80 minutes, and a ~380 µm braze joint completed IS after 1320 minutes. The kinetics of the Ni30/IN625 system was determined to be like that of boron containing FM BNi-2. Experimental diffusivity calculations determined that the IS kinetics are governed by the slower diffusing element Cu into IN625. An investigation of mechanical properties using single lap shear testing samples showed that Ni30/IN625 joints failed in the DAZ, and BNi-2/IN625 joints failed in the BM precipitated by brittle borides. Strength at failure showed that both joints had decreased strength compared to wrought IN625, but Ni30 joints had higher strength than BNi-2 joints. The comparable IS kinetics and increased strength of Ni30 are a promising combination indicating high potential for the novel Ni30 FM to replace boron containing FM alloys.