Electron Beam Welding:
Introduction:
Electron Beam Welding (EEW) is a unique way of delivering large amounts
of concentrated thermal energy to materials being welded. It became viable,
as a production process, in the late 1950's. At that time, it was used
mainly in the aerospace and nuclear industries. Since then, it has become
the welding technique with the widest range of applications. This has
resulted from the ability to use the very high energy density of the beam
to weld parts ranging in sizes from very delicate small components using
just a few watts of power, to welding steel at a thickness of 10 to 12
inches with 100 Kilowatts or more. However, even today most of the applications
are less than 1/2" in thickness, and cover a wide variety of metals
and even dissimilar metal joints
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How it works:
The most common Electron Beam systems used in manufacturing today are
of the high vacuum design. The other machine types are:
1-Partial vacuum equipment.
2-Non-vacuum equipment.
These two types are used in mass production where high output is important.
The diagram shown, shows the classic triode gun and column assembly. The
triode gun design consists of the cathode (Filament), Bias cup (Grid)
and Anode. Other sub-assembly components that contribute to the triode
are: High voltage insulator Feed-through, high voltage cable and deflection
coils. All these components are housed in a vacuum vessel called the upper
column. The column assembly is held under a high vacuum by an isolation
valve positioned below the anode assembly.
The vacuum environment provides several benefits:
•Removes the bulk gas molecules necessary for a stable triode.
•Provides protection for the incandescent filament against oxidization.
•Provides a controlled environment to protect the gun against welding
byproduct.
Beam Formation: Upper Column
The beam formatting begins with the emission of electrons from the incandescently
heated tungsten filament.
During this process the filament is saturated by a determined amount of
the electrical current. Electrons boil off the filament tip as it reaches
operating temperatures and gathers in the grid cup assembly. A negative
high voltage potential (acceleration voltage) is applied to the filament
cathode assembly, with the cathode assembly charged at 150 kV the only
force preventing the electron beam from propagating is a secondary negatively
charged voltage that resides on the grid cup or bias assembly. This voltage
respectively lower than the accelerating voltage acts as a valve that
controls the volume of electron energy that can flow from the cathode
emitter to its attracting target. The anode at a positive potential is
one of the attracting targets in the triode but its role is more of a
beam formation device rather than a collector of electrons. The secondary
target is the workpiece which is usually metallic and offers a conductive
path to earth to complete the circuit. The electron gun assembly design
is a result of some extensive engineering studies and experimentation.
Some of the early triode designs were mathematically modeled and their
designs still produced today.
Beam Delivery: Lower Column
Other important components of the beam delivery column are the focus and
deflection coils and isolation valve. The magnetic focus coil located
beneath the anode assembly provides the means for squeezing the beam into
a tightly focused stream of energy or can be used to widely dispersed
energy resource. The deflection coil is another very important component
that will contribute to the latter discussion of beam control parameters
but for now we will simply say that it is a steering device. The focus
coil is circular in design and is concentric with the column. An electrical
current is passed through the coil which produces the resultant magnetic
fluxes that act to converge the electron beam. The deflection coil is
configured with four separately wound coils positioned at right angles
to the column. The four coils are segmented as sets (x and y ) each axis
becomes a separate control allowing the energizing of each axis on command,
thus steering the beam. Many industrial applications require the precise
manipulation of the beam energy so as to provide a pattern for processing.
This is usually accomplished by superimposing an AC signal onto the four
coils simultaneously therefore creating a specific pattern. The isolation
valve serves to isolate the vacuum environment in the upper column from
the lower. After the electron beam has passed through the lower column,
it enters the chamber cavity. Another important part of the lower column
of the (EBW) machine is the viewing optics, the optics are arranged in
the lower column in such a manner that when viewing the beam energy through
a video camera or magnified optics it gives the view from a parallel plane,
giving the viewer the perception of looking down the column.
Beam Interaction in Chamber Cavity:
As the beam enters the chamber cavity it is aimed onto a target material
placed at a determined height representative of the actual workpiece.
This procedure is typical in most pre-weld set-up requirements. The welding
technician would then follow a process of beam alignment and beam parameter
calibration. Unlike laser, the preparation is quite different in the fact
that the technician must view the actual beam through the optical system
in order to verify the beam alignment and focus. With a laser beam, the
technician could not view the beam quality and therefore must rely on
instrumentation to profile the beam energy. Once the beam has been tuned
and calibrated the equipment is now ready for part processing.
The focused beam of electrons is impinged at a targeted location on the
weld joint at which point the kinetic energy of the electrons is converted
to thermal energy. The workpiece can either be stationary and the beam
energy deflected or the workpiece can be traversed along a desired axis
of motion. This motion can be computer controlled such as a CNC table,
or, simply a rotating mechanism can be employed.
As the beam energy is applied to the moving part several physical transformations
take place. The material instantly begins to melt at the surface, then
a rapid vaporization occurs followed by the resultant coalescence.
Two welding modes are used in the (EBW):
1-Conductance mode:
Mainly applicable to thin materials, heating of the weld joint to melting
temperature is quickly generated at or below the materials surface followed
by thermal conductance throughout the joint for complete or partial penetration.
The resulting weld is very narrow for two reasons:
a- It is produced by a focused beam spot with energy densities concentrated
into a .010 to.030 area.
b- The high energy density allows for quick travel speeds allowing the
weld to occur so fast that the adjacent base metal does not absorb the
excess heat therefore giving the E.B. process it's distinct minimal heat
affected zone.
2-Keyhole mode:
It is employed when deep penetration is a requirement. This is possible
since the concentrated energy and velocity of the electrons of the focused
beam are capable of subsurface penetration. The subsurface penetration
causes the rapid vaporization of the material thus causing a hole to be
drilled through the material. In the hole cavity the rapid vaporization
and sputtering causes a pressure to develop thereby suspending the liquidus
material against the cavity walls. As the hole is advanced along the weld
joint by motion of the workpiece the molten layer flows around the beam
energy to fill the hole and coalesce to produce a fusion weld. The hole
and trailing solidifying metal resemble the shape of an old fashion keyhole.
Both the conductance and keyhole welding modes share physical features
such as narrow welds and minimal heat affected zone .The basic difference
is that a keyhole weld is a full penetration weld and a conductance weld
usually carries a molten puddle and penetrates by virtue of conduction
of thermal energy.
Advantages:
1-Deeper and narrower: Ability to achieve a high depth-to-width ratio
eliminating multiple-pass welds.
2-Low heat input: Minimal shrinkage and distortion as well as ability
to weld in close proximity to heat sensitive components.
3-Superior strength: Vacuum melt quality can yield 95% strength of base
material.
4-Versatility: From .001" to 3" deep penetration welds, each
performed with exceptional control and repeatability.
5-High purity: Vacuum environment eliminates impurities such as oxides
and nitrides.
6-Superior process: Permits welding of refractory metals and combinations
of many dissimilar metals not easily weld with conventional welding processes.