How the Vacuum Brazing Process Works
The popularity of the vacuum furnace brazing process stems from the clean vacuum atmosphere or gaseous environment used during the brazing cycle, which mostly eliminates the use of fluxes and post-braze cleaning. Various types of furnace are used for brazing, mostly employing either a gaseous atmosphere or typical vacuum environment, created with a vacuum pump to achieve the desired vacuum level. The overall furnace construction is based on either batch type or continuous operation. Batch operation includes retort type furnaces used for hydrogen brazing and vacuum chambers for vacuum brazing.
Vacuum furnaces are widely used, and often companies will utilise them for the heat treatment process, using heat exchangers, as well as for brazing. Furnaces today are based on a cold wall construction, with internal heating elements, usually carbon or molybdenum. The cold wall refers to the water-cooled double skin construction used to keep external temperatures down to room temperature (or less). Braze furnaces can be either horizontal or vertical and can be either top or bottom loading. High temperatures of up to 1300°C and vacuum levels < 1x10-5 mbar can be achieved.
Most materials can be brazed in a furnace, although high vapour pressure elements should be avoided in vacuum brazing (zinc, cadmium, lead, etc). All brazing materials need to be cleaned prior to insertion in the furnace to remove surface scale, grease and other contaminants (high quality in, high quality out). The most widely used fillers for furnace brazing are based on silver, copper, nickel and gold, with the latter two being most applicable to stainless steels and heat and corrosion-resistant alloys.
Different Types of Brazing
Because brazing uses a variety of heat sources, it is often classified by the heating method used. To achieve brazing temperature, some methods heat locally (only the joint area), others heat the entire assembly (diffuse heating). Some of the more widely used methods include:
Localised Heating Techniques
Torch Brazing
In this method, the heat required to melt and flow filler metal is supplied by a fuel gas flame. The fuel gas can be acetylene, hydrogen, or propane and is combined with oxygen or air to form a flame. This process is readily automated and requires low capital investment. Torch brazing requires the use of a flux, so a post-braze clean is often required.
Induction Brazing
High frequency induction heating for brazing is clean and rapid, giving close control of temperature and location of heat. Heat is created by a rapidly alternating current which is induced into the workpiece by an adjacent coil.
Resistance Brazing
This is a process in which heat is generated from resistance to an electrical current (as for induction brazing) flowing in a circuit which includes the workpieces. The process is most applicable to relatively simple joints in metals which have high electrical conductivity.
Diffuse Heating Techniques
Furnace Brazing
Furnace brazing offers two prime advantages; protective atmosphere brazing (where high purity gases or vacuum negate the need for flux) and the ability to control accurately every stage of the heating and cooling cycles. Heating is either through elements or by gas firing.
Dip Brazing
This involves immersion of the entire assembly into a bath of molten brazing alloy or flux. In both cases the bath temperature is below the solidification point of the parent metal, but above the melting point of the filler metal.
Vaccum Brazing Advantages
When compared to the other heating methods, vacuum brazing offers significant advantages:
- clean, flux free, quality brazed joints, with oxide free joint surfaces
- able to form multiple joints at once
- joining of dissimilar materials e.g. ceramic to metal
- ability to control accurately every stage of the heating cycle and cooling cycle, resulting in reduction in residual stress in the brazed parts
- temperature uniformity leads to minimal distortion or movement, hence ideal for high precision assemblies
- step brazing using multiple braze alloys
- process repeatability
- heat-treatments, e.g. hardening or annealing, of the parts in a single furnace thermal cycle while performing a metal-joining process - saving time and cost
Further Information (TWI Members Only)
TWI Members can download our exclusive guide to best practice in brazing, here: Brazing - a guide to best practice.