Systematic Phase-Field Study on Microstructure Formation During Brazing of Mar-M247 with a Si-Based AMS4782 Filler
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FFUSION brazing is a process which has gained more and more industrial importance, e.g. as build-up or repair brazing of gas and aircraft turbine components made of Ni-based high temperature materials.[1] In this type of process, joining or repair of identical or dissimilar materials is achieved by use of a filler which selectively melts on heating the component or assembly and which re-solidifies during isothermal holding and/or subsequent cooling. Crucial for the mechanical properties of the joint is the choice of a suitable filler material which is required to melt at a temperature which is low enough in order not to destroy the c/c¢-microstructure of the base material, but high enough to allow for considerable interdiffusion and thus for a stable bonding between the filler and the base material.
B. BO¨TTGER and M. APEL are with the Access, Aachen, Germany. Contact e-mail: [email protected] B. DANIELS, L. DANKL, and T. GO¨HLER are with the MTU Aero Engines AG, Materials Turbine (TEWT), Mu¨nchen, Germany. T. JOKISCH is with the Siemens AG, Power and Gas, Large Gas Turbines, Engineering, Berlin, Germany. Manuscript submitted October 9, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
This goal is typically achieved by choosing a filler material which is similar in composition to the base metal but modified by melting point depressing elements. Most prominent is the use of boron which not only strongly lowers the melting temperature of Ni-based alloys but also is a very fast interstitial diffuser, and therefore allows for isothermal solidification during the brazing process.[2–4] However, required brazing times are relatively long, and formation of brittle boride phases can be a serious problem. Another melting point depressor frequently used in filler materials is silicon, often in combinations with B.[5,6] Although its diffusivity in fcc-based Ni alloys is considerably lower compared to boron, this element has a much higher solubility in fcc as well as in the c¢ phase, so that formation of further intermetallic phases can be widely suppressed. When only Si is used like in case of the AMS4782 or BN-5 fillers, however, as a consequence of the low diffusivity of Si in the solid phases, solidification typically cannot be achieved during isothermal holding but mainly occurs upon cooling. This may lead to the formation of large blocky precipitates inside the braze gap[7] which can have a negative impact on the mechanical properties of the braze joint.[8] For identifying suitable filler materials and process parameters, a deeper understanding of the microstructural changes during the brazing process including melting is needed, which can be achieved by computer
simulation. For reaching this goal, among the different available simulation techniques, the phase-field approach appears to be most promising. Phase-field models have become very popular for the simulation of microstructure evolution during solidification.[9–11] The software MICRESS[12] is based on the phase-field concept for multiphase systems which has been ext
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