Abstract
Advanced solidification processes like welding, soldering, and brazing are often characterized by their specific solidification conditions. But they also may include different types of melting processes which themselves are strongly influenced by the initial microstructures and compositions of the applied materials and therefore are decisive for the final quality and mechanical properties of the joint. Such melting processes are often not well- understood because - compared to other fields of solidification science - relatively little research has been done on melting by now. Also, regarding microstructure simulation, melting has been strongly neglected in the past, although this process is substantially different from solidification due to the reversed diffusivities of the involved phases. In this paper we present phase-field simulations showing melting, solidification and precipitation of intermetallic phases during diffusion brazing of directionally solidified and heat-treated high-alloyed Ni- based gas turbine blade material using different boron containing braze alloys. Contrary to the common belief, melting of the base material is not always planar and can be further accompanied by detached nucleation and growth of a second liquid phase inside the base material leading to polycrystalline morphologies of the joint after solidification. These findings are consistent with results from brazed laboratory samples, which were characterized by EDX and optical microscopy, and can be explained in terms of specific alloy thermodynamics and inter-diffusion kinetics. Consequences of the gained new understanding for brazing of high- alloyed materials are discussed.
Highlights
Diffusion brazing is a process which has gained industrial importance, e.g. as build-up or repair brazing of gas and aircraft turbine components made of Ni-/Co-based high temperature materials [1]
In this paper we present phase-field simulations showing melting, solidification and precipitation of intermetallic phases during diffusion brazing of directionally solidified and heat-treated high-alloyed Nibased gas turbine blade material using different boron containing braze alloys
Contrary to the common belief, melting of the base material is not always planar and can be further accompanied by detached nucleation and growth of a second liquid phase inside the base material leading to polycrystalline morphologies of the joint after solidification
Summary
Diffusion brazing is a process which has gained industrial importance, e.g. as build-up or repair brazing of gas and aircraft turbine components made of Ni-/Co-based high temperature materials [1]. To achieve an isothermal solidification process and to avoid formation of brittle eutectic phases, a filler material similar to the base metal is applied which has been modified by melting point depressing elements (typically boron) [2,3,4]. The task is to minimize the brazing time, and to avoid formation of new grain boundaries (epitaxial brazing) For reaching this goal, among the different available simulation techniques, the phase-field approach appears to be most promising. Characteristic microstructural features are the polycrystalline nature of the molten zone with two or more rows of newly formed grains as well as boride precipitates on both sides of the gap which reach further into the base material with increasing brazing time.
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