A new Brazing process for Semi-Cryo Engine established by ISRO
Manufacture of thrust chamber and pre-burner of Semi-Cryo engine requires joining of two shells by vacuum brazing. The inner shell is made of a copper alloy, whereas the outer shell is made of stainless steel. The inner shell has ribs on its outer surface which need to be joined to the inner shell. This joining is achieved by brazing which results in formation of active cooling channels in the finished hardware.
The usual process of brazing uses a metal foil placed between the two shells. When the assembly is heated, the foil melts and a braze joint is formed. This is a time consuming and labour intensive process. Moreover, the hardware has to be rotated during brazing to avoid accumulation of braze metal in the channels. In order to overcome these limitations, a new brazing process has been developed which involves the use of coated base metals. At appropriate brazing temperature, the sandwich layer melts and forms in-situ braze alloy between the joints. In the present development, an attempt is made to achieve braze joint through ‘static’ technique rather than hitherto followed method at ISRO of ‘rotary’ brazing.
For the thrust chamber, vacuum brazing is to be carried out between martensitic stainless steel and copper alloy. To form the braze joint, a layer of copper and silver coating is provided on the base materials to be joined. A schematic of the coating arrangement is given in Figure 1. Nickel coating is applied to act as a barrier between braze metal and steel. Figure 2 shows a cross section of electroplated steel and copper.
Initial experiments for process optimization were carried in coupon level in a vacuum furnace. Several such experiments were conducted to optimize the silver layer thickness, brazing temperature, brazing time and load. Using the optimized process, flat plates with milled channels were brazed to simulate thrust chamber configuration and pressure tested. Figure 3 shows the joint made with two flat plates. Pressure testing was done up to 600 bar and no de-bonding was observed after the pressure test.
Subsequently, pre-burner prototype hardware were fabricated and coated for the performance evaluation. A differential pressure is essential to ensure proper hugging of the cylindrical hardware assembly. For this purpose a Vacuum compression setup was designed and fabricated in-house in VSSC (Figure 4 and 5).
Using this setup, subscale hardware was realized and was evaluated through X-Ray radiography and was pressure tested by Semi Cryo Project team. It was confirmed that the hardware were free of blocks in the channels through X-Ray radiography. Pressure testing was done up to 500 bar and no de-bonding was observed. Figure 6 show the hardware and the cut cross section with typical rib fracture observed beyond 500 bar which implies the soundness of the brazed joint.
As it is a simpler process, brazing can be done at industries without any special equipment, and the advantages of this process are:
- This is a simple method of applying coatings by electroplating in the channels thus avoiding use of costly braze foils
- Time consuming and laborious brazing foil assembly on the contoured ribs is simplified.