ELECTRON BEAM WELDING

DEFINATIONA metal joining process that uses a narrow stream of electrons to produce a highly concentrated heat source.Electron beam welding (EBW) is a fusion welding process in which a beam of high-velocity electrons is applied to two materials to be joined. The workpieces melt and flow together as the kinetic energy of the electrons is transformed into heat upon impact. EBW is often performed in conditions of vaccum in order to prevent dissipation of the electron beam.

INRODUCTION TO EBMHigh energy density beam welding refers to electron or laser processes where a beam of electrons or photons, respectively, can be focused to power densities high enough to melt and vaporize the metals being joined. The high power densities and associated metal vaporization can be used to produce welds with high depth-to-width aspect ratios, small heat-affected zones, and reduced distortion. In certain applications, high energy density beam welds can offer both high quality and cost-effectiveness when compared to other welding methods. This article provides a brief history of electron and laser beam welding, discusses the properties of electrons and photons used for welding, and contrasts electron welding by way of an introduction to the sections that follow, where each process is described separately. In addition, a section is included on the growing field of microjoining with electron and laser beams, which is becoming more important as miniaturization of components continues in the areas of microelectronics, implantable medical devices, sensors, optoelectronics, and micromechanical systems. Although many of the principles are the same between macro- and microjoining, when weld dimensions become less than a millimeter, spot size, beam control, and part manipulation become more challenging, joint preparation and cleanliness become more important, and not all of the material properties scale at the same rates. These factors bring new challenges to both the technological and metallurgical aspects of microjoints made by electron and laser beams.

Electron beam welding equipment

Electron beam welder

Any electron beam equipment comprises:

1 - : electron gun, generating the electron beam,

2 - : working chamber, mostly evacuated to "low" or "high" vacuum,

3 - : work-piece manipulator (positioning mechanism),

4 - : supply and control/monitoring electronics.

Electron gun

In the electron gun, the free electrons are gained by thermo-emission from a hot metal strap (or wire), which are then accelerated and formed into a narrow convergent beam by electric field produced by three electrodes: the electron emitting strap, the cathode, connected to the -pole of the high (accelerating) voltage power supply (30 - 200 kV) and the +H.V. electrode, the anode. There is a third electrode charged negatively with respect to the

cathode, called Wehnelt or control electrode. Its negative potential controls the portion of emitted electrons entering into the accelerating field, i.e. the electron beam current.

After passing the anode opening the electrons move with constant speed in a slightly divergent cone. For the technological applications the divergent beam has to be focused, which is realized by the magnetic field of a coil, the "magnetic focusing lens.

For the proper function of the electron gun, it is necessary that the beam is perfectly adjusted with respect to the optical axes of the accelerating electrical lens and the magnetic focusing lens. This can be done by applying magnetic field of some specific radial direction and strength, perpendicular to the optical axis before the focusing lens. This is usually realized by a simple correction system consisting of two pairs of coils. By adjusting the currents in these coils any required correcting field can be produced.

After passing the focusing lens the beam can be applied for welding directly or after being deflected by the deflection system. This consists of two pairs of coils, each pair for one of the X and Y directions. These can be used for "static" or "dynamic" deflection. The static deflection is useful for exact positioning of the beam by welding. The dynamic deflection is realized by supplying the deflection coils by currents which can be controlled by the computer. This opens new possibilities of electron beam applications, like e.g. surface hardening or annealing, exact beam positioning, etc.

The fast deflection system can also be applied (if provided with appropriate electronics) for imaging and engraving. In this case the equipment is operated similarly as a scanning electron microscope, with the resolution of about 0,1 mm (limited by the beam diameter). In a similar mode the fine computer controlled beam can "write" or "draw" a picture on the metal surface by melting a thin surface layer.

Working chamber

Since the publication of the first electron beam welding machines at the end of 1950s, the application of electron beam welding spread rapidly into industry and research in all highly developed countries. Up to nowadays uncountable number of various types of electron beam equipment have been designed and realized. In most of them the welding takes place in the working vacuum chamber in high or low vacuum environment.

The vacuum working chamber may have any desired volume from a few liters up to hundreds of cubic meters. They can be provided with electron guns supplying electron beam with any required power up to 100 kW, or even more if needed. In micro-electron beam devices the components in the tenths of a millimeter dimension range can be precisely welded. In welders disposing with high enough power electron beams, welds up to 300 mm deep can be realized.

There are also welding machines in which the electron beam is brought out of vacuum into the atmosphere. With such equipment very large objects can be welded without huge working chambers.

Work-piece manipulators

The electron beam welding can never be "hand-manipulated", even if not realized in vacuum, as there is always strong X-radiation. The relative motion of the beam and the work-piece is most often rotation or linear travel of the work-piece. In some cases the welding is realized by the beam being moved by the computer controlled deflection system. The work-piece manipulators are mostly designed individually to meet specific requirements of the welding equipment.

Power supply and control/monitoring electronics

Any electron beam equipment must be provided with appropriate supply of power for the beam generator. The accelerating voltage may be chosen between 30 and 200 kV. Usually it is about 60 or 150 kV, depending on various conditions. With rising voltage the technical problems and the price of the equipment are rising rapidly, hence, whenever it is possible the lower voltage about 60 kV is to be chosen. The maximum power of the H.V. supply depends on the maximum depth of weld required.

The high voltage equipment must also supply the low voltage above 5 V for the cathode heating, and negative voltage up to about 1000 V for the control electrode.

The electron gun also needs low voltage supplies for the correction system, the focusing lens, and the deflection system. The last one may be very complex when it should provide the computer controlled imaging, engraving and similar applications of the beam.

Complex electronics may also be needed for the control of the work-piece manipulator

HISTORYElectron beam welding was developed by German physicist Karl-Heinz Steigerwald of Stuttgart, West Germany before 1958. Steigerwald had been conducting experiments on their electron microscope to increase power of the scope and found out to their surprise, that the specimen which was being examined would disappear. Later, Steigerwald determined that by regulating the power settings, the specimen would melt and resolidify. Thus the electron beam welder was born.

In 1959 through agreements with the Carl Zeiss Co., United Technologies Corporation (then called United Aircraft) and its division, Hamilton Standard, sold "Zeiss" electron beam welders in the United States. Hamilton Standard then sold E.B. machines with a label "Hamilton - Zeiss" and finally produced machines labeled "Hamilton Standard".

In 1976 Hamilton Standard sold its interest in the electron beam welding business to Leybold - Heraeus Vacuum Systems Inc. In 1989 the entire operation was taken over by PTR - Precision Technologies Inc., Enfield, CT. To date, over 800 electron beam welders have been installed around the world through the combined efforts of Zeiss Co./ Hamilton Standard/ Leybold-Heraeus/ PTR Precision Technologies Inc.

The various electron beam systems available are: high vacuum, partial vacuum, and non-vacuum and are used accordingly depending upon the application.

Today, electron beam welding is used in many diversified industries throughout the world such as: nuclear, aerospace, aircraft, automotive, instrumentation, medical, electronics, commercial and job shops

PHYSICS OF ELECTRON BEAM WELDING

Electrons are elementary particles possessing a mass m = 9.1E10−31 kg and negative electrical charge e = 1.6E10−19 C. They exist either bound to an atomic nucleus, as conduction electrons in the atomic lattice of metals, or free electrons in vacuum.

Free electrons in vacuum can be accelerated and their orbits controlled by electric and magnetic fields. In this way we can form narrow beams of electrons carrying high kinetic energy, which upon collision with atoms in solids transform their kinetic energy into heat. Electron beam welding provides excellent welding conditions because it involves:

Strong electric fields, which can accelerate electrons to a very high speed. Thus, the electron beam can carry high power, equal to the product of beam current and accelerating voltage. By increasing the beam current and the accelerating voltage, the beam power can be increased to any practically desirable value.

Using magnetic lenses, by which the beam can be shaped into a narrow cone and focused to a very small diameter. This allows for a very high surface power density on the surface to be welded. Values of power density in crossover (focus) of the beam can be as high as 104 – 106 W/mm2.

Shallow penetration depths in the order of hundredths of a millimeter. This allows for a very high volumetric power density, which can reach values of the order 105 – 107 W/mm3. Consequently, the temperature in this volume increases extremely rapidly, 108 – 1010 K/s.

The effect of the electron beam is dependent upon many conditions. The most important factors in determining the effectiveness of EBW are the physical properties of the material to be welded, especially the ease with which the material is melted or turned to gas in low-pressure conditions. Electron beam welding can be so intense that loss of material due to evaporation or boiling during the process must be taken into account when welding. At lower values of surface power density (in the range of about 103 W/mm2) the loss of material by evaporation is negligible for most metals, which is favorable for welding. In the upper region of the power density the material affected by the beam may be evaporated totally in a very short time; while unsuitable for welding, this is occasionally used in machining to remove material.

Beam formationCathode - the source of free electrons

Tungsten cathodes: strap - wire

Conduction electrons (that are not bound to the nucleus of atoms) move in crystal lattice of metals with velocities distributed according to Gauss law, depending on temperature. They can not leave the metal unless their kinetic energy (in eV) is higher than the potential barrier at the metal surface. Number of electrons fulfilling this condition increases with increasing temperature of the metal exponentially, according to Richardson's rule.

As a source of electrons for electron beam welders, the material must fulfill more requirements:

to achieve high power density in the beam, the emission current density [A/mm2], hence the working temperature, should be as high as possible,

to keep evaporation in vacuum low, the material must have low enough vapour pressure at working temperature.

The emitter must be mechanically stable, chemically not sensitive to gases present in vacuum atmosphere (like oxygen and water vapour), easily available, etc. EBW also called electron beam welding

These and some other conditions limit the choice of material for the emitter to metals with high melting points, - practically only two of them, tantalum and tungsten. With tungsten cathodes, emission current densities about 100 mA/mm2 can be achieved, but only a small portion of emitted electrons takes part in beam formation, depending on the electric field produced by

anode and control electrode voltages. The type of cathode most frequently used in electron beam welders is made of tungsten strip, about 0.05 mm thick, shaped as shown in Fig. 1a. The appropriate width of the strip depends on the highest required value of emission current. For the lower range of beam power, up to about 2 kW, the width w=0.5 mm is appropriate.

Acceleration of electrons, current control

Beam generator

Electrons emitted from the cathode possess very low energy of only a few eV. To give them the required high speed, they are to be accelerated by strong electric field applied between the emitter and another, positively charged, electrode, - the anode. The accelerating field must also navigate the electrons to form a narrow converging “bundle” around the axis. This can be achieved by an electric field in the proximity of the emitting cathode surface, which has, in addition to an axial component also a radial one, forcing the electrons in the direction to the axis. Due to this effect, the electron beam converges to some minimum diameter in a plane close to the anode.

For practical applications the power of the electron beam must, of course, be controllable. This can be accomplished by another electric field produced by another, with respect to the cathode negatively charged

At least this part of electron gun must be evacuated to "high" vacuum, to prevent "burning" the cathode and emergence of electrical discharges.

Focusing

After leaving the anode the divergent electron beam does not have power density sufficient for welding metals and has to be focused. This can be accomplished by magnetic field produced by electric current in a cylindrical coil.

Magnetic lens

The focusing effect of a rotationally symmetrical magnetic field on the trajectory of electrons is the result of complicated influence of magnetic field on a moving electron. This effect is a force proportional to the induction B of the field and electron velocity v. The vector product of the radial component of induction Br and axial component of velocity va is a force perpendicular to those vectors, making the electron to move around the axis. Additional effect of this motion in the same magnetic field is another force F oriented radially to the axis, which is responsible for the focusing effect of the magnetic lens. The resulting trajectory of electrons in magnetic lens is a curve similar to a helix. In this context it should be mentioned that variations of focal length (exciting current) shall course a slight rotation of the beam cross-section.

Beam deflection system

Correction & deflection coils

As mentioned above, the beam spot should be very precisely positioned to the joint to be welded. This is commonly accomplished mechanically, by moving the workpiece with respect to the electron gun, but sometimes it is preferable to do this by deflecting the beam. Most often a system of 4 coils positioned symmetrically around the gun axis behind the focusing lens, producing magnetic field perpendicular to the gun axis, are used for this purpose.

There are more practical reasons why the most appropriate deflection system is such that is used in TV CRT or PC monitors. It applies to both the deflecting coils, as well as to the necessary electronics. Such a system enables not only “static” deflection of the beam for positioning purposes mentioned above, but also precise and fast dynamic control of the beam spot position by a computer. This enables e.g.:

welding joints of complicated geometry, image enlarged pictures of objects in the working chamber on TV or

PC monitor.

Both possibilities do find many useful applications in EB welding practice.

JOINING DISSIMILAR MATERIALS

Joining two metal components by welding, i.e. by melting part of both in the vicinity of the joint, in case of two materials with very different properties is often not applicable because of unsuitable properties of their alloy, due to creation of brittle inter-metallic compounds. This fact cannot be changed even by electron beam heating in vacuum, but nevertheless it makes possible to realize joints meeting high demands on mechanical compactness that are perfectly vacuum-tight. The principal rule of the method is not to melt both parts, but only that one with lower melting point, while the other remains in solid state. Advantage of the electron beam is in the possibility to localize the heating to a proper point and to control exactly the energy needed for the

process. High vacuum atmosphere, no doubt, substantially contributes positively to the success. General rule of the construction of joints that should be made in the way mentioned above is that the part with the lower melting point should be directly accessible for the beam.

EXPLANATION

Electron Beam Welding is a method which uses a focussed stream of high energy electrons generated by a filament and directed to the joint requiring to be welded. The heating is very localised and the bulk of the assembly therefore remains cold and stable. This results in a very narrow weld with a minimal heat affected zone. There is no need to use filler metal as the parent metal of the assembly is melted. As this is a line of sight method it is not possible to weld around corners or re-entrant angles. Weld depths of up to 30mm can be produced and computer controls ensure minimal operator dependence, thus providing good reproducibility throughout a batch of components, even though this is a piece part process. Since the heat input is very localised it is possible to weld together previously heat treated components, which is a very economical method for producing composite gear shafts, with for example a case hardened gear on a hardened and tempered shaft.

The key benefits

Minimal Distortion - Welds finish machined parts Welds difficult and dissimilar materials Exceptional Quality and Repeatability High Weld Speeds Deep narrow welds of close to Parent Metal strength

PROCEDURE

When fast moving electrons hit a metal surface they are decelerated  which transforms the kinetic energy of each individual electron in the beam into thermal energy in the component. This transformation is stable in the high 90% range for all metals regardless of whether the electrons hit the surface at a perpendicular or shallow angle. As a practical matter, this physical

behavior makes the process very robust and reliable.When electrons in a focused beam hit a metal surface, the high energy density instantly vaporizes the material, generating a so-called key hole (Figure 1). A characteristic of

 

Figure 1 : Key-Hole

 

this phenomenon is that it allows the unique capability for deep, narrow welds with very small heat affected zones (HAZ) and minimized thermal distortions of welded assemblies (Figure 2). Depth-to-width ratios of up to 40:1 have been achieved in production for many years.

ADVANTAGES

The advantages of electron beam welding includes the following:

Vacuum process, yields clean reproducible, high integrity joints Low heat input with minimal distortion and a narrow heat-affected zone Weld penetration of up to 30mm+ Metals of dissimilar melting points

and thermal conductivities can be welded Quantities from one-off development to large production batches can be

accommodated. 14 machines, range from 9” cube to 9’x5’x6’

Low heat input for the welded parts; Minimal distortion;

Narrow melt zone (MZ) and narrow heat affected zone (HAZ); Deep weld penetration from 0.05 mm to 200 mm (0.002” to 8”) in

single pass; High welding speed; Welding of all metals even with high thermal conductivity; Welding of metals with dissimilar melting points; Vacuum process yields in clean and reproducible environment; Natural welding process for oxygen greedy materials such as titanium,

zirconium and niobium; Machine process guaranteed for reliability and reproducibility of the

operating conditions; Cost-effective welding process for large production in automatic

mode; and

Parts can mostly be used in the as welded condition - no sub-machining required

Size Capability:

Our Sciaky CNC Electron Beam Welder has the capability of welding diameters to 20 inches in a twenty five (25) foot long chamber with the electron gun located in the center.

DISADVANTAGES

The material melted by the beam shrinks during cooling after solidification, which may have unwanted consequences, like cracking, deformations and changes of shape, depending on conditions.

The butt weld of two plates will result in bending of the weldment due to the fact that more material has been melted at the head than at the root of the weld. This effect is of course not as substantial as by arc welding.

Another potential danger is the emergence of cracks in the weld. If both parts are rigid, the shrinkage of the weld produces high stress in the weld which may lead to cracks if the material is brittle (even if only after remelting by welding). Consequences of the weld contractions should always be considered by the construction of the parts to be welded.

APPLICATIONS Aerospace

o Jet engine components o Parts of structures o Transmission parts o Sensors

Power generation Space

o Titanium tanks o Sensors

Vacuum systems Medical Automotive

o Transmission parts o Gears o Parts of turbocharger

Electrical/electronic industries o Parts in copper material

Nuclear o Fuel housing o Parts of structure o Valves o Instrumentations

Research centres o Copper parts o Superconductivity material components

All metals even with high thermal conductivity o Steel and stainless steel o Aluminium and alloys o Copper and alloys o Nickel alloys and refractory metals o Titanium and alloys o Zr, Mo, Ta, Hf, W, Nb, etc

SCOPE

Those considering electron-beam welding (EBW) have three basic techniques from which to choose: nonvacuum, partial vacuum, and high vacuum. These methods use a stream of high-energy electrons to locally heat metals to welding temperatures exceeding 10,000°C. And all need a high vacuum at the beam source. The difference between the three lies downstream of the beam source.

Nonvacuum EBW uses large-capacity vacuum pumps and blowers with differentially pumped sections to separate the vacuum from atmospheric pressure. Weld quality suffers because gas molecules scatter and diffuse the electron beam as it travels from the source to the work surface.

Partial-vacuum electron beam welding is typically done at a chamber pressure of about 75 to 300 microns. The gun portion where the beam is generated is held at about 1310 -4 Torr. The chief advantage of partial-vacuum EBW is a short pump downtime required prior to welding. Welds made with the method are not as narrow or clean as from high-vacuum EBW.

High-vacuum EBW avoids the scattering problem and produces higher-quality welds because welding takes place in a hard vacuum (about 10 -4

Torr). As such, part size is limited to that of the vacuum chamber. The pumps that evacuate the chambers are expensive, slow, and consume a lot of power. And all equipment associated with the welding process inside the weld chamber must be capable of operating in a vacuum, which further raises costs.