Vacuum brazing is a fabrication technique which is complimentary to welding. It is not always possible to or desirable to weld assemblies together but the requirement for strength or hermetic sealing may still exist. Vacuum Brazing is a high temperature process which, because of the absence of an oxidizing atmosphere, uses fluxless brazing alloys.
Components are held in their correct relationship by designing the joints to be self locating, using wire, tack welds or custom designed ceramic jigs. The joint design and clearances are important so consultation in the design stage is recommended.
VACUUM BRAZING is a term for various metal joining or brazing processes that take place in a chamber or retort below atmospheric pressure, otherwise known as a vacuum furnace. VACUUM BRAZING is brazing in a furnace using a vacuum atmosphere.
VACUUM BRAZING is broadly defined as a group of joining processes that occur above 840°F (400°C) and below the melting point of the base metal. Additionally, brazing produces coalescence by the melting and subsequent solidification of a filler metal or brazing alloy in the very narrow space between surfaces to be joined. The brazing alloy or brazing filler metal must have a lower melting point than the material being joined, because in brazing the base metal does not melt. In conventional brazing, molten filler metal is distributed between closely fitted surfaces of the base metal joint by capillary action – even against gravity.
Most of the common metals can be brazed, but not necessarily with the same brazing filler metal equipment, method, or procedure.
A vacuum furnace is a furnace using low atmospheric pressures instead of a protective gas atmosphere like most heat treating furnaces. Furnaces are categorized as hot wall or cold wall, depending on the location of the heating and insulating components. Cold wall furnaces are used in VACUUM BRAZING.
Assemblies are bright and clean (shiny) after vacuum brazing because the extremely low amount oxygen in a vacuum atmosphere prevents oxidation of parts. VACUUM BRAZING is particularly useful where base metals are processed that adversely react with other atmospheres, or where entrapped fluxes or gases are intolerable. VACUUM BRAZING is widely used to braze base metals of stainless steel, super alloys and carbon low alloy steels.
VACUUM BRAZING offers the combination of high cleanliness and uniform heating and cooling or rapid cooling. VACUUM BRAZING is ideal for oxidation sensitive materials such as those used in the aerospace industry.
VACUUM BRAZING: How to Get High-quality Brazed Joints
When doing VACUUM BRAZING, everyone wants to get perfect products. Knowing how to get high-quality brazed joints is the key. Today, let’s talk about vacuum brazing in detail.
What is VACUUM BRAZING
VACUUM BRAZING is a metal-joining process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the adjoining metal.
The American Welding Society defines brazing as:
“A group of welding processes that produces coalescence of materials by heating them to the brazing temperature in the presence of a filler metal having a liquidus above 840 deg F (450 deg C) and below the solidus of the base metal. The filler metal is distributed between the closely fitted faying surfaces of the joint by capillary action.”
What is the difference between VACUUM BRAZING and welding.
VACUUM BRAZING differs from welding in that it does not involve melting the workpieces and from soldering in using higher temperatures for a similar process, while also requiring much more closely fitted parts than when soldering.
The filler metal flows into the gap between close-fitting parts by capillary action. The filler metal is brought slightly above its melting (liquidus) temperature while protected by a suitable atmosphere, usually a flux. It then flows over the base metal (in a process known as wetting) and is then cooled to join the workpieces together. A major advantage of brazing is the ability to join the same or different metals with considerable strength.
About filler materials
A variety of alloys are used as filler metals for VACUUM BRAZING depending on the intended use or application method. In general, braze alloys are made up of 3 or more metals to form an alloy with the desired properties. The filler metal for a particular application is chosen based on its ability to: wet the base metals, withstand the service conditions required, and melt at a lower temperature than the base metals or at a very specific temperature.
Braze alloy is generally available as rod, ribbon, powder, paste, cream, wire, and preforms (such as stamped washers). Depending on the application, the filler material can be pre-placed at the desired location or applied during the heating cycle. For manual brazing, wire and rod forms are generally used as they are the easiest to apply while heating. In the case of furnace brazing, the alloy is usually placed beforehand since the process is usually highly automated. Some of the more common types of filler metals used are：
- Aluminium-silicon
- Copper
- Copper-silver
- Copper-zinc (brass)
- Copper-tin (bronze)
- Gold-silver
- Nickel alloy
- Silver
- Amorphous brazing foil using nickel, iron, copper, silicon, boron, phosphorus, etc.
VACUUM BRAZING
VACUUM BRAZING is a material joining technique that offers significant advantages: extremely clean, superior, flux-free braze joints of high integrity and strength.
Temperature uniformity is maintained on the workpiece when heating in a vacuum, greatly reducing residual stresses due to slow heating and cooling cycles. This, in turn, can significantly improve the thermal and mechanical properties of the material, thus providing unique heat treatment capabilities. One such capability is heat-treating or age-hardening the workpiece while performing a metal-joining process, all in a single furnace thermal cycle.
VACUUM BRAZING is often conducted in a furnace; this means that several joints can be made at once because the whole workpiece reaches the brazing temperature. The heat is transferred using radiation, as many other methods cannot be used in a vacuum.
Vacuum pumping in VACUUM BRAZING
The vacuum pumping capacity must be adequately sized in order to minimize the pump downtime of a new load to a deep vacuum level, so as to initiate the heating cycle and to have adequate throughput to keep up with the significant outgassing that takes place during the heating cycle due to magnesium vaporization. A deep vacuum level is an important process parameter because it ensures a relatively pure environment for brazing.
How to get high-quality VACUUM BRAZED joints
High-quality VACUUM BRAZED joints require that parts be closely fitted, and the base metals exceptionally clean and free of oxides. In most cases, joint clearances of 0.03 to 0.08 mm (0.0012 to 0.0031 in) are recommended for the best capillary action and joint strength.
However, in some VACUUM BRAZING operations, it is not uncommon to have joint clearances around 0.6 mm (0.024 in). Cleanliness of the brazing surfaces is also important, as any contamination can cause poor wetting (flow). The two main methods for cleaning parts, prior to brazing, are chemical cleaning and abrasive or mechanical cleaning. In the case of mechanical cleaning, it is important to maintain the proper surface roughness as wetting on a rough surface occurs much more readily than on a smooth surface of the same geometry.
Another consideration is the effect of temperature and time on the quality of brazed joints. As the temperature of the braze alloy is increased, the alloying and wetting action of the filler metal increases as well. In general, the brazing temperature selected must be above the melting point of the filler metal. However, several factors influence the joint designer’s temperature selection. The best temperature is usually selected to:
- Be the lowest possible braze temperature.
- Minimize any heat effects on the assembly.
- Minimize filler metal/base metal interaction.
- Maximize the life of any fixtures or jigs used.
In some cases, a worker may select a higher temperature to accommodate other factors in the design (e.g., to allow use of different filler metal, or to control metallurgical effects, or to sufficiently remove surface contamination).
The effect of time on the brazed joint primarily affects the extent to which these effects are present. In general, however, most production processes are selected to minimize brazing time and associated costs. This is not always the case, however, since in some non-production settings, time and cost are secondary to other joint attributes (e.g., strength, appearance).
