1. Overview of SiC ceramic welding
Among modern high-temperature structural materials, silicon carbide ceramics have a wide range of excellent properties such as high temperature strength, strong oxidation resistance, low density, wear resistance, corrosion resistance, etc., so they have a wide range in aerospace, automotive, chemical and nuclear energy fields. Application prospects. However, because silicon carbide is a compound mainly composed of covalent bonds, its inherent brittleness makes it difficult to prepare large and complex-shaped parts. Therefore, it is usually necessary to produce these parts through the connection technology between ceramics.
The ceramic material connection process mainly needs to solve two problems: one is the wettability of the connection interface; the other is the stress buffering problem of the joint. At present, many process methods have been developed for the connection of silicon carbide (SiC) ceramics, such as diffusion connection, mechanical connection, physical and chemical vapor deposition connection and brazing. In the above connection method, brazing has the advantages of simple process, low equipment investment and suitable for production requirements.
The brazing connection of SiC ceramics mainly uses Ag-Cu, Cu-Au or Cu-based medium and low temperature solder. However, the low temperature of brazed SiC ceramic joints (generally not exceeding 500℃) limits the performance of SiC ceramics' excellent high temperature performance. There are few studies on the high-temperature brazing connection of SiC ceramics. Although some people have done some work, the production welding of SiC ceramics has become the bottleneck of SiC ceramic applications.
In the research of SiC ceramic welding, one method is to study the wetting and interfacial reaction of Co-based solder and Ni-based solder to SiC ceramic, and successfully connect SiC ceramic with Co-based solder; another The method is: using the prepared Ti-Si eutectic solder to successfully connect the SiCf/SiC composite material, and make the shear strength of the joint after normal temperature reach (71±10) MPa; another method is: use 22Ti-78Si high temperature eutectic solder was prepared by non-consumable arc melting technology (Ts=1342℃, Tl=1376℃), and on the basis of the wetting and interfacial reaction of the material on SiC ceramic, the SiC ceramic was brazed connection.
Second, the brazing operation
Taking the 22i-78si (ωt%) high temperature eutectic solder prepared by non-consumable arc melting technology as an example to connect SiC ceramics, the basic steps of connecting SC ceramics are introduced:
1. Use a diamond saw to cut the SC to the required size, and use wire cutting to cut the 22Ti-78i solder prepared by the non-consumable arc melting technology into the appropriate size for welding SiC ceramics.
2. Reduce the brazing material to about 50~200pm by mechanical means.
3. Polish the SC ceramics and brazed parts to be connected with diamond abrasive paste of about 1μm.
4. Before assembling the reactive connection, SiC and brazing material are further polished with 0.5m diamond abrasive paste.
5. Put the SiC and solder in the acetone solution and clean them with ultrasonic waves. After cleaning, rinse with deionized water.
6. When assembling the reaction pair, apply a slight pressure of about 150 Pa to the connecting parts.
7. Under the condition that the vacuum degree is better than 10-2Pa, increase the welding temperature to a brazing temperature (1380℃~1420℃) at a heating rate of 10℃/min;
8. After welding, the weldment will be naturally cooled with the furnace after 5-20 minutes of heat preservation.
According to the above process, brazing can be achieved under the conditions of brazing temperature 1380℃~1420℃, holding time 5~20min, material thickness 50~200m; at brazing temperature 1400℃, holding time 10min When brazing with the thickness of 100m, the maximum shear strength of the obtained SiC/22Ti-78si/SiC brazed joint can reach 125MPa.
Three, summary
Practice has proved that in the process of brazing SiC ceramics, whether the brazing temperature and holding time are appropriate is the key to successful brazing of SiC ceramics. Only suitable brazing temperature and holding time can achieve SiC ceramic welding. Otherwise, if the brazing temperature is too high or the holding time is too long, it will lead to the loss of brazing filler metal and cause voids in the brazing joint. With the increase of the brazing temperature and the extension of the holding time, the morphology of the phase structure in the brazing seam changes. These changes in the morphology of the phase structure will have an important impact on the strength of the brazed joint.
The influence of the thickness of the brazing material on the strength of the joint is that when the thickness of the brazing material is thin, the residual thermal stress of the joint cannot be well buffered; when the thickness of the brazing material is too large, it will be expanded due to the brazing material and the SiC ceramic The difference in performance leads to an increase in the residual thermal stress of the joint, so only a suitable thickness of the solder can help buffer the residual thermal stress of the joint and obtain a higher joint strength.