One-off EB machine tailored to non-welding

TWI Bulletin, January/February 1992

Keith Nightingale graduated from Cambridge with a degree in natural sciences, specialising in chemistry. His involvement with electron beams started at the research laboratories of EMI, Hayes, where he worked on cathode materials for klystrons and other high power microwave devices.

After a short period of secondment to the Services Electronic Research Laboratory at Harlow, he joined the embryo Electron Beam Section at BWRA, as it was then, in 1966. From that time he has been involved in many EB equipment development programmes, particularly glow discharge EB welding, the first non-vacuum EB system at TWI in the late 1970s, and the current EUREKA sponsored NVEBW development.

There's a continuing interest in using the electron beam process for non-welding applications, heat treatment and surface melting. Keith Nightingale reviews beam deflection systems and describes a novel electron beam gun.

A focused electron beam designed for welding can have a power density above 10 ⁵ W/mm ² and will be able to penetrate from a few hundredths of a millimetre to a few hundred millimetres in a single pass depending on the beam power being used ( Fig.1a). However, for heat treatment and metal evaporation, deep penetration must be avoided, so the power density must be reduced by several orders of magnitude; this can be accomplished by defocusing the beam ( Fig.1b), or, to give better control of the heat pattern, by deflecting the beam magnetically ( Fig.1c).

a) Focused beam - deep penetration
b) Defocused beam - surface melting
c) Focused and deflected beam - controlled surface melting

In the last few years, power supplies to drive magnetic deflection coils have become progressively more sophisticated so that now there is a choice of commercially available analogue or digital equipment; analogue deflection is carried out simply by applying a sine, square or any regular shaped wave to the X and Y deflection coils at any frequency up to a limit imposed by the amplifier/coil combination, usually about 5kHz.

Digital deflection is normally accomplished by designing a pattern as a series of points with a given order and duration on a microcomputer from where it can be stored on tape or disc and then downloaded to the deflection unit. When the beam is manipulated at high frequency with the given pattern, the workpiece resolves this as a homogeneous heat source whose shape is defined by the pattern. ^[1]

All these deflection systems refer to any electron beam, in or out of vacuum; however, because the throw distance of an electron beam at atmospheric pressure (at 200kV or less accelerating voltage) is limited to a few tens of millimetres, only in-vacuum systems are commercially viable.

Practical applications

Practical applications of pattern deflected electron beams for surface heating fall into three categories in order of increasing energy density:

• Heating with no surface melting;
• Controlled surface melting;
• Surface evaporation.

Surface heating without melting is used for transformation hardening steels by heating the surface to above the transformation temperature and allowing it to cool as quickly as possible by self-quenching to promote formation of martensite and form a hard, wear-resistant surface layer. ^[2,3,4]

Surface melting can be used as an extension of the above surface modification technique with the added flexibility of being able to incorporate an alloying material in the form of wire, strip or powder. ^[5]

Surface evaporation is a widely used industrial technique for coating a thin metal layer on to a solid or film substrate (typical applications being aluminium on to solids or plastic film) for reflective and decorative coatings, and cobalt alloys on to plastic film for magnetic recording tape. In these applications, a polyester film on a roll up to several metres wide is passed round a cooling drum above the evaporation source and on to a take-up roll. The source is heated by a linearly deflected electron beam with a power between 50 and 250kW allowing film speeds up to 10 m/sec ( Fig.2).

Whereas aluminium has a low enough evaporation temperature for resistance heating to be used as an alternative to EB heating, higher melting point metals such as copper, cobalt and chromium can be evaporated in vacuum only by EB.

Compared with other heating processes, energy from the electron beam couples efficiently with the surface. Coupling efficiency is affected both by the atomic number (AN) of the surface material, e.g. the ratio of backscattered electrons to primary electrons is 0.18 for Al (AN 14) and 0.33 for Co (AN 27), ^[6] and also by the angle of incidence of the primary EB, being maximum at normal incidence.

The greater the beam deflection angle, i.e. the wider the roll on to which material is to be evaporated, the greater is the angle of incidence, resulting in a further reduction of coupling efficiency. These backscattered electrons are adverse in two ways. Not only do they represent a loss of heating efficiency but also, if they strike the surface being coated, they may cause unacceptable thermal damage to it.

Linear electron beam guns

• No dynamic deflection system is required;
• It reduces the problems of space charge which occur with the point source beam currents of several amps which are required for high powered systems at the relatively low accelerating voltages used (30-50kV);
• A constant and low angle of incidence of electron impingement is maintained along the target.

TWI was recently asked to design and construct just such a linear electron beam gun by an Industrial Member for a metal evaporation application where a deflected point source was not acceptable.

Prototype gun

A 50kV power supply was available at TWI but its maximum current rating was well below that required for the full power specification, so the prototype gun was made considerably shorter in order that the beam current per unit length was the same as in the final design. Results from the prototype gun showed that beam width and linearity on the target as revealed by X-ray pinhole camera photography were within specification and gave confidence to proceed with the full scale gun.

Full scale gun

Two particular problems required attention at the design stage of the full scale gun; first was the tensioning of the filament. Tungsten expands by about 1% over the ambient to electron emission temperature (2200K) range so that, if supported rigidly at each end, the filament would have sagged to such an extent that electrons emitted from its centre would be launched from an incorrect position relative to the gun electrodes and hence would not follow the correct trajectory to the target. Various tensioning devices were tried before a solution was found, operation being complicated by having to work at >250°C.

The second problem was that of heating of the cathode shield by radiant energy from the filament; this shield structure, insulated to 50kV from earth, was by default also well insulated thermally, and relied almost entirely on radiation for cooling. Again, a finite element computer program - this time a proprietary version called MARC - was used to calculate the heat transfer characteristics involving thermal conduction within the shield and radiation from its surface, and to estimate the maximum temperature reached under steady state conditions. Foreknowledge of this temperature allowed a correct choice of gun materials to be made.

HV power supplies

For roll coating applications, where the film speed is typically 10 m/sec, it is essential that gun discharges are minimised because every millisecond that the bath is not evaporating fully leads to a 100mm 'gap' on the film. TWI now has several years' experience in specifying and operating high power switch mode power supplies for EB welding where a similar 'minimum discharge' specification is required. ^[7] These supplies have the advantage that, because they operate at 5.5kHz, the stored energy is considerably reduced compared with 50Hz systems; hence fast control of the high voltage is possible leading to suppression of microdischarges in the electron gun before they turn into prolonged discharging and power supply shut down. An additional benefit is a reduction in the transformer size, the tank for the 50kV, 250kW power supply being only 1m long X 0.7m wide X 1m high.

Performance

After manufacture, the gun was tested at TWI to check operation of the electrode cooling features and a beam of 1A at 50kV was achieved. Upon successful completion of these tests, the gun was installed in the sponsor company's vacuum chamber, connected to the 250kW switch mode power supply and evaporation trials started. In the presence of metal vapour, some discharging occurred, but this was minimised by additional shielding between the evaporant bath and the high voltage components of the gun and by fast control of the high voltage during discharges; the beam power has now been worked up to close to the design value. The design and construction of an electron gun as described demonstrates the ability of the Electron Beam Department at TWI to provide Industrial Member companies with a service for producing one-off equipment, even though not directly related to welding applications.

References