Vacuum brazing– a high-quality joining process (Part 1)

More than 40 years ago the first industrial vacuum furnaces for the production of hot gas engines after the Stirling principle was purchased by Werkspoor in Amsterdam.

With the heater head made from stainless steel of the Stirling engine more than 300 braze joints with a temperature resistance of more than 800 °C must be produced. Vacuum brazing was the only joining technology with which this could be carried out successfully. Since this time vacuum brazing has grown to a high-quality joining technology with many-sided applications. With vacuum brazing there where new possibilities in material choice and constructions.

Vacuum brazing is a joining technology with which high-quality joints can be produced. The fundamental difference between welding and vacuum brazing is the fact that with the welding the base material becomes melted. With the vacuum brazing the components are connected to each other under use of an additional material with a lower melting point than the basic material. The vacuum brazing is executed in a vacuum furnace at temperatures of 800 °C till 1300 °C. No flux is used. One receives the necessary reduction effect for the decomposition of the metal oxide from the vacuum atmosphere. The use of high temperature and a reducing atmosphere is essential for the success of the brazing process. Different mechanisms are working for the reduction of the metal oxide:

Dissociation and/or vaporization of oxide
Diffusion of oxygen from the oxide layer into the basic material
Reduction of the oxide layer by the reaction with the carbon from the basic material
Cracking of the oxide layer through difference expansion with the base material
Remove of the oxide layer by a reducing residual gas atmosphere as for example hydrogen

Because with the brazing under vacuum the process parameters are exactly adjustable, therefor one receives reproducible metal joints. Primary features of braze under vacuum are:

The mechanical characteristics of the brazed joints correspond roughly those of the base material
Almost all material combinations are possible, also between metal and ceramics
A complete gap filling originates from the capillary effect
By the very uniform warming up in vacuum furnaces only low negligible deformations appear
Thin and thick connections are simply producible
The re -machining of brazed products is low or completely unnecessary
Vacuum-brazed connections have a low leakage rate
The brazing process can be combined very often with heat treatments like hardening, solution treatment and tempering
No post cleaning is necessary

Braze materials

For vacuum brazing filler materials are used on the basis of copper, nickel and precious metal. These filler materials contain no elements with high vapor pressure. Copper base materials are mostly reasonable and will be used for application temperatures up to 300 °C with less demand for the corrosion resistance. Nickel base filler materials have a very wide use.
Brazing alloys based on precious metals are expensive and are only used when high requirements on strength, corrosion resistance and vacuum leak rate.

Design aspects

For engineering mechanical characteristics like tensile strength, yield strength and fatigue strength of a brazed construction are important. These values are strongly influenced by the ductility of the braze joint. A joining is ductile if the connection refuses distortion only after one preceding. One receives optimum braze joints by a correct choice of the braze parameter such as

Design aspects
Braze gap
Surface conditions
Filler material
Braze temperature
Braze time
Complementary heat treatments

The strength of the joining becomes determined by the strength of the base material. With help of extensive investigations the following proportionality factors which serve with brazed joints as a directive have originated.

Tensile strength of the base material	100%
Fracture strength of the braze joining	90%
Shear strength of the braze joining	45%
Fatigue strength of the braze joining	30%

For brazed components it is important that the fracture strength of the brazed joints is higher than the yield strength of the base material. The suitability of a braze joint for its function are primarily determined by the chosen joint design. Several factors influence the selection of the type of joints to be brazed in a brazement. Brazed joints can be can be classified in three main groups which are based primarily on the appearance of one of the following mechanical stresses:

Tensile stress with butt joints
Shear stress with overlap joints
Compressive stress with unloaded joints

With good process control a strength which corresponds roughly to the base material is accessible by butt joints. If two different materials are brazed, in the general breakage will appear in the material with the slightest tensile strength. (Part 2 describes quality aspects and in the part 3 applications are described for different materials).

Author:Peter Steege