304 stainless steel sound-absorbing honeycomb vacuum brazing

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The sound-absorbing honeycomb structure is composed of a perforated panel, a honeycomb core and a non-porous panel, which are connected by vacuum brazing or bonding. The sound-absorbing principle of the sound-absorbing honeycomb is mainly to use the sound wave to enter the Helmholtz resonant cavity formed by the honeycomb core through the perforated panel, and generate reflection on the bottom panel and the honeycomb core, and the sound wave will rub against the panel and the honeycomb core to convert the sound energy into heat energy Reduce noise. In addition to sound-absorbing properties, the sound-absorbing honeycomb sandwich structure also has excellent properties such as light weight, high specific strength and specific stiffness, and has been widely used in the field of aviation manufacturing, such as aircraft cockpit sound-absorbing structures, engine nacelle inlet sound-absorbing liners, and engine shorts. Cabin exhaust nozzle sound-absorbing lining, etc. Compared with aluminum alloy glued sound-absorbing honeycomb structure, stainless steel and titanium alloy brazed sound-absorbing honeycomb structure can withstand higher working temperature and load level, and has been widely studied and widely used.

When the metal honeycomb is vacuum brazed, in order to obtain a good welding rate, it is required to strictly control the brazing gap between the panel and the core. When stainless steel materials are vacuum brazed with nickel-based solder, the suitable brazing gap range is usually 0.02-0.1 mm. When the gap between the panel and the honeycomb core is too large, unwelded defects are likely to occur. After the welding is completed, secondary or even multiple brazing repairs are required. Although two or more brazing repairs can improve the welding rate, they have a certain impact on the mechanical properties of the honeycomb structure. It is recommended that the maximum number of repairs for 304 stainless steel honeycomb brazing defects be one time.

1 experiment
The panel with holes in the experiment is 0.2 mm thick 304 stainless steel, the hole diameter is 1.0 mm, and the perforation rate is 2.5%, and the non-porous panel is 0.5 mm thick 304 stainless steel. The honeycomb core is a regular hexagonal honeycomb composed of 0.05 mm thick 304 stainless steel foil strips, the core cell size is 8.0 mm, and the height is 6.0 mm.
The 304 stainless steel sound-absorbing honeycomb was prepared by vacuum brazing. Before welding, grind and clean the panels with holes and the panels without holes, and assemble and fix them as shown in Figure 1.

The brazing material is a commercial BNi2 non-product foil strip brazing material, the chemical composition is shown in Table 1, and the thickness of the brazing material is 50 m. The melting point of BNi2 filler metal is 971~999 ℃. Through many vacuum brazing process explorations, the vacuum brazing process of 304 stainless steel sound-absorbing honeycomb BNi2 filler metal is determined as: brazing temperature 1050 ℃, heat preservation 15 min. Use SIMUWU’s RHVB high temperature vacuum brazing furnace for vacuum brazing process.

2 Results and Analysis
2.1 Microstructure and analysis of brazing interface
304 stainless steel panel base metal structure is shown in Figure 2. It can be seen from Figure 2 that before vacuum brazing, the base metal structure of the panel is composed of austenite grains, and some grains are distributed as twins.

The interfacial structure of honeycomb core and panel in 304 stainless steel sound-absorbing honeycomb sandwich structure is shown in Fig. Panel area (Ⅲ) composition.
The structural photo of the foil-solder area (I) under a high-power optical microscope is shown in Figure 3(b). It can be seen that this area consists of the solder solidification area (A area) and the solder-foil reaction area ( Area B) composition. When the temperature rises above the melting point of the BNi2 solder, the solder at the interface between the core and the panel melts, and the liquid solder wets and spreads on the surface of the foil due to capillary action, and solidifies after cooling, forming the solder solidification zone A. At the same time, in the high temperature stage, element diffusion reaction occurs between the brazing filler metal wetted and spread on the foil surface, and the chemical elements in the brazing filler metal diffuse into the foil.

The brazing seam area (Ⅱ) is located between the honeycomb core foil and the panel (as shown in Figure 3(a)), and the EDS analysis results of the composition of the brazing seam area are 67.26 Ni, 17.44 Fe, 7.64 Cr, 7.66 Si (atomic fraction, %). It can be seen that the brazing seam area is mainly formed by mutual dissolution of Ni, Cr, Si, etc. Ni-based solid solution structure, no brittle eutectic structure or intermetallic compounds are formed, and the entire brazing seam area is composed of solid solution, indicating that the vacuum brazing process is better.

The structure of the solder-panel region (Ⅲ) under a high-power optical microscope is shown in Figure 3(c). It can be seen that this region consists of an interface reaction region C and a base metal diffusion region D. The EDS analysis results of the components in the C area of the interface reaction zone are 7.38 Ni, 42.57 Fe, 15.57 Cr, 5.06 Si, 33.99 B (atomic fraction, %), and the B and Si elements in the solder and the panel material diffuse significantly. Area D of the diffusion zone of the base metal shows obvious characteristics of grain boundary infiltration. The EDS analysis of the composition at the grain boundary shows that it is 14.86 Ni, 29.15 Fe, 26.01 Cr, 1.16Si, 37.37 B (atomic fraction, %), which can be seen There are more B elements, and the diameter of B atoms is small, and it mainly diffuses into the base metal in the way of inter-product infiltration, and is distributed at the grain boundary of the base metal, and the diffusion is fast. At the same time, due to the high brazing heating temperature, the grain size of the 304 stainless steel panel is larger than that of the base metal.

3 Conclusion
(1) BNi2 amorphous foil strip brazing filler metal is suitable for the preparation of 304 stainless steel honeycomb sandwich structure, and the stainless steel honeycomb sandwich structure with good interface welding quality can be obtained when the brazing temperature is 1050 °C and the heat preservation time is 15 min.

(2) The brazing interface of the prepared 304 stainless steel honeycomb sandwich structure consists of foil-brazing area (I), brazing seam area (II) and brazing material-panel area (Ⅲ). In the foil-solder area, due to the capillary action of the liquid solder, the solder spreads along the surface of the honeycomb core box material and undergoes a significant elemental diffusion reaction with the honeycomb core foil. The brazing seam area is composed of Ni-based solid solution structure formed by mutual dissolution of Ni, Cr, Si, etc., and no brittle eutectic structure or intermetallic compound is formed. For the solder-panel region, the B and Si elements in the solder diffused significantly in the panel material, and due to the rapid diffusion effect of B element along the grain boundary, the panel material showed obvious grain boundary infiltration characteristics.

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