Assessing the Influence of Cr and Fe in the Filler Metal on Dissolution and Isothermal Solidification Kinetics During TL
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TRANSIENT liquid phase bonding (TLPB) is a joining method which relies on the diffusion of atomic species between a liquid and solid, enabling joining via diffusion-induced isothermal solidification (IS).[1,2] TLPB provides a means of joining advanced components used in a wide assortment of demanding applications ranging from dissimilar metals, ceramics, and more often nickel-based superalloys. Usually Ni-superalloys are joined using low-melting Ni-based filler metals (FM) containing Boron, in addition to other elements which act as potent melting point depressants, enabling joining at relatively low temperatures.[3] Although Boron-containing FMs display an excellent combination of wetting, spreading, cost, and self-fluxing properties compared to more costly Au and Pd FMs, they form intermetallic borides along the joint which can greatly deteriorate mechanical performance.[4–12] The most detrimental boride formation at the centreline of the joint can be avoided by complete isothermal solidification in the joint gap. The detrimental affects of the fine distribution of borides forming in the diffusionally
E.D. MOREAU and S.F. CORBIN are with the Department of Mechanical Engineering, Dalhousie University, 1360 Barrington Street, P.O. Box 15,000, Halifax, NS B3H 4R2, Canada. Contact e-mail: [email protected] Manuscript submitted 29 June 2020; accepted 5 October 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
affected zone (DAZ) can be minimized through post-processing or manipulation of the FMs.[13–19] Two key stages of TLPB are base metal dissolution (BMD) and isothermal solidification (IS). BMD is essentially the chemical equilibration between the liquid phase (e.g., FM) and the base metal (BM) during heating from the solidus to the peak braze temperature. Significant BMD can cause liquid gap widening and an increase in the time to achieve isothermal solidification. The subsequent IS stage involves diffusion of the melting point depressants and solute atoms from the liquid phase into the BM, leading to diffusion-induced solidification. The rate of isothermal solidification is a determining factor in the time required to reach complete isothermal solidification and the avoidance of brittle eutectic regions at the joint centreline. Although it is well established that the nature of the BM has a marked influence on TLPB behavior, little systematic studies have been performed which examine the influence FM composition has on BMD and rate of IS in particular, despite the growing number of FMs continuously developed. Examining such effects in any detail is inherently difficult as the melting range (solidus and liquidus) of FM’s is governed by their composition, making TLPB comparisons at the same braze temperature difficult. Furthermore, differences in the FM liquidus and braze temperature could potentially aggravate BMD which can significantly alter both the composition and the quantity of the resulting liquid phase.
The goal of this work was to directly compare the dissolution and isothermal solidification rate during TLPB of IN718 bra
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