Determining the Mechanisms Governing Transient Liquid Phase Bonding of Nickel-based Superalloys using Boron Containing Filler Metals
During TLPB, an initial boron uptake (IBU) phenomena was quantified using DSC and microstructural analysis of a IN625 base metal (BM)/BNi-2 filler metal (FM) joint. A conceptual model was developed to quantify both the extent of isothermal solidification (IS) and IBU, in terms of the redistribution of boron from the liquid phase into the base metal, as precipitated borides. An explanation for the formation of precipitated borides as a diffusionally affected zone (DAZ), was developed using diffusion path theory applied to a simple ternary Ni/Ni-Si-B (BNi-3) braze joint. The combined use of Thermocalc modelling and microstructural analysis confirmed that the DAZ formed in accordance with equilibria requirements in a multi-component system. Ni200/BNi-3 and IN600 (Ni-Fe-Cr)/BNi-3 couples were examined to isolate the effects BM Cr-content has on the IBU, DAZ and IS rate. A substantial reduction in the rate of IS resulted as a result of the Cr in the BM. This was due to the formation of a low volume fraction of CrB versus, high volume-fraction, Ni3B in the DAZ of Ni200, as predicted by calculated phase equilibria. A broader examination on the TLPB behaviour of IN718/BNi-2, IN718/BNi-3 Ni200/BNi-2 and IN600/BNi-2 allowed a systematic, comprehensive assessment of BM and FM composition on TLPB behaviour. From this work, the IBU appears to be independent of BM/FM composition, leading to an IS zone (2WISZ) in the range of 23 to 43 µm. A base metal dissolution (BMD) process, which lead to braze gap widening, was governed by the compositional similarity between BM and FM. A greater BM to FM compositional difference equated to a greater degree of BMD. Gaps with no BMD (i.e. IN625/BNi-2) remained at 80 µm, while BMD lead to gaps as large as 167 µm (i.e. Ni200/BNi-2). Isothermal solidification was controlled by the rate at which boron is redistributed as metal borides within the BM - the DAZ. This was dependent on BM boride former content (Ni in the case of Ni200 and Cr, Nb, Mo for IN625, IN600, and IN718). The rate of IS was shown to increase linearly with the boride former concentration.