Metallurgical and mechanical behavior of brazed thin alloys sheets assemblies
Abstract
Heat exchangers included in air conditioning systems for aircraft are produced by brazing stamped thin alloys sheets made of nickel-based alloys, Alloy 600 and Ni 201, or stainless steel, AISI 444. Separation metal sheets and locking bars of Alloy 625 are used to complete the system. The brazing filler metal, mainly composed of nickel, manganese, silicon and copper, is referred as BNi-8. In order to control brazing process, a good knowledge of both the brazing filler metal metallurgical behavior and of the interaction with the base metal is essential. The study of the brazing filler metal melting behavior in itself reveals that the melting point is highly dependent on the chemical composition and especially on silicon content. Microstructures analysis showed the presence of several phases with significant differences in terms of mechanical properties at a small scale which could induce local embrittlement. Interactions between the brazing filler metal and the different alloys constitutive of the assembly induce chemical composition evolutions related to the local configuration of the assembly. Dissolution and interdiffusion processes as well as chemical exchanges with the furnace environment occur. Finally, due to this set of phenomena, significant brazing defects can affect the mechanical integrity of the component.
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Introduction
Air to air heat exchangers assembled on aircraft are manufactured by brazing process. This joining process, carried out at high temperature and under controlled atmosphere, enables welding between different base metals due to brazing filler metal. During the brazing process, the temperature is chosen to be higher than the brazing filler metal liquidus but lower than the base metal solidus. According to Tuah-Poku et al. [1] and to J. Ruiz-Vargas et al. [2] most recently, brazing process could be divided in four steps: 1) melting of the filler metal, 2) widening of the liquid joint through dissolution of a part of the base metal, 3) isothermal solidification through diffusion from the liquid joint into the base metal and 4) final homogenization. During the second stage, the brazing filler metal wets base material surfaces, spreads along the joint between the base metals through capillary forces and diffuse into them [3] [4]. Finally, as explained in the last two steps, the joint is therefore made firstly by diffusion of the brazing filler metal in the base metal and secondly by solidification of residual liquid during the final cooling [5].
Air to air heat exchangers are complex structural components allowing to cool hot air collected from the engine by using cool air ducted from the engine fan. In the core of the pre-cooler, owing to an assembly of fluted thin metal sheets, interlayer plates and locking bars, heat transfers are optimized. To reduce the weight of component and optimize heat exchanges, thin metal sheets made of nickel-based alloys (Ni 201, Alloy 600 and Alloy 625) and ferritic stainless steel (AISI 444) are used as base metals together with a nickel-based brazing filler metal called BNi-8.
Brazing defects occur during the brazing cycle and can lead to a loss of mechanical integrity of the component. They are linked with the thermal cycle but also with the metallurgical transformations and deformations which the system is subjected to. To better understand the complex and multi-processes phenomena that occur during the brazing process, the metallurgical and mechanical behavior of the brazing filler metal and base metals were studied independently before focusing attention on the interaction between them. The aim of this multi-scale approach was to identify the first order microstructural variables controlling the macroscopic mechanical properties of the assembly.
Conclusion
Before studying the interactions between base metals and brazing filler metal, it has been observed that small variations of chemical composition induce different solidification paths and thus different microstructures and mechanical behavior. For base metals, it has been seen that brazing conditions in terms of temperature had an effect on the microstructure and hence on the mechanical properties. The brazing cycle induces a decrease in mechanical strength. Finally, for the assemblies, the precipitates formed in the interaction zone between the brazing filler metal and the metal sheets seems to play a first order role in the final mechanical response of the assembly. Nevertheless, precipitation is also present in the core of the brazing filler metal. When precipitates are in a continuous eutectic form, the mechanical strength is greatly reduced. It is therefore essential to define selection criteria in terms of brazing filler metal chemical composition and thickness of the deposit to optimize the mechanical behavior of the considered assemblies.