Development of compact copper-stainless steel heat exchanger using diffusing bonding techniques

The necessity of high effectiveness in a small volume has led to the development of perforated plate compact heat exchangers(CHE) for cryogenic applications. Although the basic principles have remained the same, the techniques of bonding have changed considerably during the last four decades. With the introduction of all metal construction, these exchangers are finding increasing use in cryogenic refrigerators. Besides soldering, brazing and adhesive bonding as joining techniques diffusion bonding of compact heat exchangers has been primarily applied. Diffusion bonding of copper to stainless steel(304) was carried out with interlayer metals such as Cr and Ni in order to investigate the influence of the interlayer metals on bonding conditions of the joints. The microstructure of the interfaces of diffusion-bonded bimetal was investigated by scanning electron microscopy(SEM) and X-ray diffraction (XRD). Solid state diffusion bonding technique is being applied in the construction of compact heat exchanger. In order to employ diffusion bonding processes for a construction of CHE, the process parameters which contribute to consistent formation of joints of the required strength have been critically examined. Process variables include temperature, pressure, time, surface roughness and, interlayer composition, density and thickness. Diffusion welding offers the unique possibility to achieve a metallurgical joining in the solid state without low melting filler, thus avoiding large brittle intermetallic phases, which form in the melt pool while cooling. The metals are joined by causing the coalescence of the metallic surfaces, by the application of pressure at elevated temperature for a finite interval. This technique produces joints with a minimum macroscopic deformation and without deterioration in the mechanical properties of the base metals, because the bonding is carried out below the solidus temperature(about 0.7Tin for the lowest melting-point metal) under a suitable pressure. Diffusion bonding of copper & SS were investigated to develop procedures for joining precision machined copper & SS components for the CHE. Diffusion bonds were tested over a range of temperatures from 800 0 C to 900 0 C, under different loading conditions(3 MPa-12 MPa ). The compact heat exchanger so fabricated has good strength, leak tightness and free flow of the heat-exchanging fluids