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.
- The first step in vacuum brazing is very easy. The parts to be joined
are positioned together. Due to tight tolerances, several components fit
together tightly and are ready for brazing filler material to be applied
to the bonding area. If the components to be joined are complex, then
they are assembled with special tack welding, fixturing, staking, or a
combination of all these methods.
- The second step requires the application of brazing alloy to the
joining area. Most of the braze joint areas lend themselves to a
suspension of a gel binder and braze alloy powder. The slurry is usually
applied by an expert using needle point tips with foot controlled
pneumatic pumps supplying the alloy. Some other forms of alloys, such as
wire, preforms, or foil can be manually applied to the braze area.
- Vacuum furnace treatment is the final step in vacuum brazing. It is a
programmed computerized cycle based on the component size, material or
quantity of assemblies, and alloy composition. The vacuum thermal
process includes a number of serial processes heat up, preheat, holding
period, braze alloy solidification, and quenching.
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
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