Vacuum Brazing

Vacuum brazing is a particular type of furnace brazing, where vacuum is created in the furnace for carrying out brazing process. Unlike other heat treating furnaces which uses protective gas atmosphere, vacuum furnace make use of comparatively low atmospheric pressures. This metal joining technique provides much clean, superior, and flux-free brazed joints along with high strength and integrity. The process is expensive because it is performed inside a vacuum chamber vessel but the advantages are quite significant. Vacuum brazing takes place in a chamber or retort, known as a vacuum furnace, below the atmospheric pressure. Instead of a protective gas atmosphere in the furnace, the vacuum furnace have low atmospheric pressures. These furnaces are categorized as hot wall or cold wall depending on the location of the heating and insulating components.

Vacuum brazing is actually a three step process, which finally creates a bond that is leak tight, non-corrosive, and stronger than alternative bonding methods.

Types of Materials for Joining
Vacuum brazing can be used for joining the following list of materials:

 

Materials for Joining
Stainless Steel	Carbide	Graphite
Carbon steel	Ceramic	Titanium
Metalized ceramic	Tantalum	Glass to metal
Copper	Stellite	Molybdenum

 

Alloys for Joining
Copper	Nickel	Silver
Copper/Gold	Pure metal

 

Vacuum Brazing: Three in One Process
Vacuum brazing is a very efficient brazing process, which is a combination of three chemical process i.e. bonding, cleaning, and heat treating.

Bonding
Usually nickel braze alloy is used as a filler material in the vacuum brazing to fill the gap between the parts being joined. The melting temperature of filler alloy is lower than the parts to be joined. It melts and diffuses into the metal and create a non-corrosive bond. After the brazing is completed, the transgranular diffusion of elements forms a new alloy, which melts at a higher temperature than the original braze alloy.

Nickel alloy fillers are used in a number of applications, including all commonly used stainless steels, low carbon steel, kovar, and tool steels. It is also used for extremely corrosive applications, like harsh oil or chemical environments, but the need must justify the cost. Nickel alloys provide a non-corrosive bond at a very reasonable cost in most of the applications. Apart from nickel, other brazing filler options are silver, copper, gold, and several combinations of these three. These filler alloys are available in many forms, such as powder, paste, foil, etc. Due to the ally variety these fillers can be used to join even non-metal materials, such as ceramic to metal, diamond and other minerals to metals.

Cleaning
A defining characteristic and advantage of the vacuum brazing process is its brightening and cleaning ability. It immediately become evident when the product comes out of the brazing furnace. During the vacuum brazing, the assembly goes through a bake-out process that promptly removes oxides, oils, and other contaminants from the part assembly. The cleaning process is effective than chemical cleaning. This bake-out manages to reach the internal dimensions of tubing, short capillary tubes, and machined part crevices. This helps increase the productivity because after the vacuum braze process no flux or other contaminants need to be removed. This cosmetic advantage is particularly important for the medical and instrument device industries.

Heat Treating
The heat treatment of the total assembly is up to 2175F for nickel alloy joining of stainless steel. This makes the part assembly metallurgically consistent meaning it has consistent tensile strength throughout & is coupled with enhanced overall assembly strength & ductility. There are many simultaneous thermal treatments with some stainless steels and/or precipitation hardened alloys in Vacuum brazing. A vacuum furnace can effectively braze either single or large assemblies which even enables the effective brazing of a small assembly by a commercial vacuum braze operation. This kind of vacuum braze production is appropriate for the daily draw of parts for just in time delivery.

Why Vacuum Brazing?
Once the vacuum brazing technique is applied, the assemblies become quite bright, clean & shiny simply because of extremely low amount of oxygen in a vacuum atmosphere. This prevents the oxidation of parts allowing the technique to be quite useful when it comes to its wide application. Vacuum brazing's application to base metals of stainless steel, super alloys & carbon low alloy steels, further make these base metals processed which adversely react with other atmospheres, or where entrapped fluxes or gases are intolerable.

Vacuum brazing is apt since it offers a bombastic combination of high cleanliness and uniform heating and cooling or rapid cooling. This is ideal for oxidation sensitive materials. With surface oxides containing elements such as Cr, Mn, Ti, V, Al and Si, these appear upto more than 3% in alloy steels which can be fluxless brazed satisfactorily in vacuum.

 

Generally, by making use of a gas fired or electrically heated furnace, work load placed in the hot retort vacuum furnace is sealed, evacuated and heated from the outside. Such a furnaces has been specially designed to work at temperatures up to about 1100°C. The vacuum pump in this case may be oil sealed mechanical type which can take pressures ranging from 10 Torr to 0.1 Torr while turbo mechanical vacuum pumps takes maximum 10-2 Torr to 10-3 Torr pressure.

 

Advantages of Vacuum Brazing

• Vacuum brazing provides extremely clean, superior, flux-free braze joints while providing high integrity and strength.
• Temperature is maintained at a uniform rate on the work piece when heating. It greatly reduces the residual stresses because of slow heating and cooling cycles. It can have a significant impact on the thermal and mechanical properties of the material.
• This method can be expensive because it is performed inside a vacuum chamber vessel but the advantages are significant.
• The flow of filler metals are well provided without the use of fluxes

Tags:- Vacuum Brazing Technique, Vacuum Furnace Brazing Process

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