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Vacuum Brazing of Silicon Nitride Ceramics and 45 Steel

In this paper, the Al-Si alloy foil is used as the intermediate reaction layer of vacuum brazing, and the vacuum brazing of silicon nitride ceramics and 45 steel is experimentally studied under the conditions of certain brazing temperature and holding time.
Selection of brazing filler metal: In this experiment, Al-Si alloy foil is used as brazing filler metal, which can not only reduce the research cost, but also does not have high requirements on the vacuum furnace.
Experimental equipment:Vacuum brazing is a relatively new brazing method, it does not require flux, the brazing joint is bright and dense, has good mechanical and corrosion resistance, and the obtained welded parts are beautiful. Vacuum brazing has strong adaptability to materials and structures, and is often used in the brazing of aluminum alloys, titanium alloys, stainless steel insoluble metals and ceramic materials, so it is widely used in aerospace and other fields.
Experimental procedure
Preparation of metallographic specimens inlay: In this test, we use casting inlay. The inlay material used is resin plus hardener. When pouring the inlay, the sample is placed in the prepared mold, and then the quantitative resin and curing agent are thoroughly mixed before pouring.

Sample surface treatment
First put a small amount of ceramic powder on the glass plate, drop water on the ceramic powder to make it into a slurry, and then grind the inlaid sample in the slurry. In the process of grinding, pay attention to pressing and pressing the sample with your fingers (to ensure that the sample is in close contact with the glass plate), and turning the grinding circle in the slurry, when the welding sample is completely exposed and invisible to the naked eye Obvious dents can be finely ground. Fine grinding is carried out on metallographic sandpaper. The 04# and 05# metallographic sandpapers were used in this experiment.
Analysis of Silicon Nitride Ceramic/45 Steel Vacuum Brazed Specimen
According to the combination of the brazing filler metal and the base metal, the spreading behavior of the brazing filler metal on the base metal can be divided into two types: wetting spreading and reactive spreading.
In the vacuum brazing process of this experiment, the principle of reaction spreading is used. At a suitable temperature, the active element Al in the Al-Si alloy foil preferentially diffuses on the surface of silicon nitride and steel and continuously participates in the eutectic reaction, thereby Realize the welding of silicon nitride and 45 steel.
Microstructure analysis of brazed seam
Al-Si solder and Si-N. There is a continuous distribution of black reaction structure on the interface, and there is a continuous distribution of product layer on the 45 steel side interface. Data analysis shows that the reactant phase at the Si3N4/Al-Si solder interface is AIN.
In conclusion
1. In the preparation process before vacuum brazing, it is necessary to ensure that the experimental material, especially the bonding surface, is clean and free from contamination: a suitable force should be applied to the sample, which is conducive to the close bonding and the inter-atomic diffusion, which promotes the interface The reaction proceeds.
2. From the analysis of 4 groups of metallographic diagrams, it can be seen that the temperature of vacuum brazing and the length of holding time will have a great influence on the microstructure and performance of the brazed joint. The brazing temperature and holding time mainly affect the strength by affecting the degree of interfacial reaction and grain size. The higher the vacuum brazing temperature and the longer the holding time, the interface reaction degree will increase, the grain size will become larger, the interface reaction layer will be thickened, and the joint strength will decrease. Therefore, it is very important to choose a suitable brazing temperature and a suitable holding time.
3. Al-Si fiber material with low hardness and high plasticity can braze Si3N4, ceramic/45 steel under pressure. The shear strength of the joint is not high, mainly because the strength of the Al-Si fiber itself is low, and during the brazing process, coarse coarse-crystalline silicon will be formed, and the brazing seam structure is not dense and loose.
4. The microstructure of Si3N4 ceramic/45 steel fiber welded joint is the layer distribution structure of Si3N4/AIN/Al-Si eutectic/45 steel.

Vacuum brazing equipment selection：Process technology must be matched with excellent equipment to play a role, and the brazing furnace selected in vacuum brazing is very important. The RHVB vacuum high temperature brazing furnace produced by SIMUWU is an excellent product for handling such processes. With its excellent temperature control accuracy and temperature uniformity, the workpiece can be brazed evenly and evenly, so as to achieve high-level brazing standards, reduce scrap rates, increase production efficiency and reduce costs.