Thrust Chamber Head Titanium Brazing News

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The thrust chamber head of the liquid rocket engine requires light weight, compact structure, high strength, good heat resistance and reliable operation. Therefore, most of the small and medium thrust engines working in space use the structural design of the titanium alloy straight-through injector head. For this head structure scheme, if automatic pulse welding, laser welding and other methods are used in the process, the core body and the head are likely to cause spatter and thermal deformation during welding, which will lead to the blockage of the nozzle holes or the deterioration of the impact accuracy of the nozzle jet, which will eventually lead to Affects the spray and working performance of the engine head. In order to solve the above technical problems, the head of the thrust chamber of the liquid apogee engine of the Dongfanghong-3 satellite used an interference fit structure in the pre-research stage, the core parts were placed in liquid nitrogen, and the injector frame and other parts were placed in the heating furnace In the middle, the temperature difference between the two parts is about 500C for press fitting, and the interference amount reaches 0.03~0.05mm. Such a design structure and process method can only ensure the product strength requirements, but cannot meet the air tightness requirements, and can not meet the design requirements, so Be eliminated.

According to the brazing experience of the thrust chamber head of various types of stainless steel structures under development, after repeated demonstrations, when the liquid apogee engine was transferred to the model development stage, the product was changed to a brazed structure, and the process adopted a vacuum brazing scheme.

There are many factors affecting the quality of brazing parts, such as: the structural form of brazing parts, the selection of the size of the brazing seam gap, the performance of brazing filler metals, the amount of brazing filler metals used, the control of needle welding temperature, the degree of brazing vacuum, Brazing holding time, surface treatment of parts, etc. In this paper, the results of the brazing process are given under the conditions of the determined brazing structure of the thrust chamber head, titanium alloy materials and the amount of brazing material used.

Head brazing structure and brazing material
The thrust chamber brazing consists of the injection core, the injector frame, the support and the pressure measuring nozzle and other parts by vacuum brazing, and the parts of the head are made of titanium alloy 7715D. The brazing structure is shown in Figure 1. The brazing filler metal used is TZNC-1 titanium alloy brazing filler metal.

2.2 Brazing vacuum degree
Since titanium alloy materials are particularly easy to oxidize at high temperatures, which affects the quality of the weld, and even causes the product to be oxidized in severe cases, the vacuum degree of the brazing process is one of the key process parameters. The surface of the initial welding test piece is gray and seriously oxidized, indicating that the vacuum degree of the product does not meet the requirements during the high temperature brazing process. After improving the vacuum pumping system, the diameter of the pipeline is increased from φ60 to φ100, the power of the vacuum pump is increased, and the vacuum degree of brazing is improved, so that the vacuum degree reaches the level of 2×10-4Pa. After many process tests, the product quality has been greatly improved. After that, the appearance of the brazed head product and the brazed seam have no oxidation phenomenon, and all of them have passed the strength and air tightness test.

Solving the problem of micro-cracks in product brazing seam
At that time, vacuum brazing of dry titanium alloys lacked domestic materials and mature experience for reference, especially for two-component liquid rocket engines, which was the first time in China. In the case of metallographic structure, etc., after brazing the test piece for many times and conducting metallographic analysis on the brazing seam of the test piece, it is found that there are micro-crack defects in the welding seam.

Cracks are serious defects that are never allowed to exist in welding, and even microcracks in brazing seams pose a potential threat to the mechanical properties of the product. So it is necessary to find out the reason and eliminate the existence of cracks. After many process tests, careful observation and analysis, it was found that the reason was that during the brazing process, the container and the weldment product kept shaking, and the solidification process of the needle solder, which eventually led to micro-cracks in the brazing seam. After finding the reason, the tooling equipment was modified, the mechanical vibration source of the equipment was separated from the vacuum container of the brazed product, and the vibration source was isolated. After many tests, no cracks were found.

Brazing process of titanium alloy
After many process tests and multiple batches of product production, it is shown that the following brazing process can be used for brazing, and stable product quality can be obtained.

assembly
The interference between the two parts is 0.02~0.03mm. The amount of clearance in the gap part of the brazing seam is 0.01~0.02mm.

Brazing
Brazing temperature: 980±5℃
Vacuum degree: 1.0×10-2～5.0×10-2Pa Holding time: 10～30min
Immediately after the brazing heat preservation is completed, it is transferred to the strengthening treatment.
After many brazing tests and metallographic analysis, the following conclusions are drawn:
The brazing seam has poor toughness and extremely low strength without strengthening treatment. There is a layer of eutectic needle solder structure in the brazing seam, which belongs to the brittle phase. However, after the strengthening treatment at 920℃ for 4h, the eutectic needle solder structure in the brazing seam gap basically disappeared, and the joint surface was welded to form co-produced grains, so the strength and toughness of the brazing seam were improved.

Enhanced treatment
Furnace temperature: 920±5℃ Holding time: 4h
Vacuum degree: 1.0×10-2～5.0×10-2Pa
The 36 head products that have been brazed so far using the above process technology all meet the design requirements, and the engines have all been successful after the hot test run. Among them, two engines in the long-range reliability hot test run have accumulated more than 18,000 seconds, and no problems with the brazing head were found. The three engines have been delivered to the satellite for flight, and all have successfully completed their missions.

in conclusion
The titanium alloy brazing process not only produces reliable products for the satellite liquid apogee engine, but also provides a technological basis for subsequent types of titanium alloy needle welding. The 25N, 150N, 2500N dual-element thrust chamber head of the propulsion subsystem has also successfully adopted the above-mentioned brazing materials and process specifications, indicating that the above-mentioned brazing process is mature and stable.

Equipment selection: The RHVB series vacuum brazing furnace produced by SIMUWU is a high-quality product for the vacuum heat treatment of tooling and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum brazing process. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing. The product line includes vacuum air quenching furnace, vacuum oil quenching furnace, vacuum brazing furnace, etc., which are widely sold in developed and developing countries.