The process of brazing is, in many branches of industry, the only method that ensures high-quality joints in fuel, hydraulic, and pneumatic systems of modern aerospace engines.
Broad application in contemporary industry of hitherto rarely used metals such as titanium, zirconium, magnesium, and tantalum, as well as corrosion- and heat-resistant alloys, calls for the development of new brazing alloys, methods, and equipment.
The aerospace industry has applied new materials; however, their application has required the mastering of high-tech metal joining methods. One of those methods is fluxless vacuum brazing. This technology is applied, among other purposes, to the brazing of pipe ducts used in aero engines. These are ducts or systems of ducts that form part of fuel, oil, and air systems; components of hydraulic installations; and anti-icing systems.
In practice, vacuum brazing is used to join two groups of materials:
• Austenitic steels (18-8 type): grades AMS 5557, AMS 5570 (AISI 321), used for the manufacture of seamless piping.
• Nickel-based alloys of the Inconel type.
The most commonly used filler materials are AgCu42Ni2 (AMS 4765) and Au-18Ni (AMS 6574).
For hard brazing of components, a silver-based filler usually is employed—one with a eutectic composition or with a small difference between the solidus and liquidus temperature. Such materials are relatively easy to apply. However, for special-purpose applications, such as strength at elevated temperatures, there is a need to use other fillers, even though they may be more difficult to braze.
Fillers with a large difference between the solidus and liquidus temperatures are rarely used, and they constitute a field as yet little known and investigated. Such a material is Ag42Cu2Ni with a chemical composition of Ag=55.0-57.0%; Cu=41.0-43.0%; Ni=1.5-2.5%. The range of solidus-liquidus temperatures of this alloy is 771-893°C.
Quality control of the joint obtained comprises checking whether, as the result of brazing, 100% of the meniscus line was obtained around the circumference of the joint and the degree of coverage by the filler material (which should be at least 80% of the surface). The joint is also tested for cracks, nicks, etc. Excessive flow-out of the brazing material is not recommended, and in such cases is removed mechanically.
Surface roughness, which affects the flow properties of the filler, and thereby wetting ability, is one of the main factors affecting quality. Research conducted in Poland shows that in the case of Ag42Cu2Ni filler alloy, the best flow characteristics were exhibited by specimens which were electroplated with nickel prior to the brazing operation (although surface roughness in this version was higher than for other methods of surface preparation).
Isotropic surface preparation methods, conducive to roughening, are more advantageous than anisotropic preparation methods. This stems from the easier displacement of the filler material along channels formed on the surface of the metal. Of these preparation methods, two stand out: rolling in the process of manufacture in the steel mill and surface preparation using loose abrasives. In both methods, the distance of displacement of the filler ally front was at its maximum.
Of significant importance is the width of the brazing gap. This is because of the presence of a dual phase structure of the filler alloy, comprising a high melting phase and a free-flowing one. The blockage of brazing gaps by the high melting phase grains, carried by the flowing liquid phase with a low melting point, was observed. The effect, shown by X-ray, is that there are zones in which there is a lack of the filler material, despite a multifold excess of the alloy relative to the volume that is to be filled.
Quite different phenomena occurred during the brazing of austenitic 18-8 grade steel with gold-based filler alloys, which is the preferred alloy when what is needed is not only high strength at elevated temperatures but also high corrosion resistance. In the case of Au-18Ni, although the effect of surface preparation on the flow characteristics of the alloy was determined, no effect was observed of the preparation method on the isotropic character of displacement of the filler alloy front. A very interesting phenomenon observed during brazing was the occurrence of alloying of the chrome-nickel stainless steel with the filler material. A clear effect was determined of temperature and duration of brazing on the displacement of the phase boundary between the filler material and the steel in the direction of the steel core. This phenomenon deserves further investigation because of the properties of the obtained brazed joints and, hence, flight safety.
This article was based on SAE technical paper 2009-01-3173 by T. Babul and A. Nakonieczny of the Institute of Precision Mechanics and J. Senkara of the Warsaw Technical University.