Electron Beam Percussive Forming (EBPF)

Recent research at TWI has resulted in the development of a novel Electron Beam (EB) materials processing technique, dubbed 'Electron Beam Percussive Forming' (EBPF).

In this process, an electron beam of a specific power density distribution is used to treat individual parts of a workpiece surface. The beam is allowed to interact with each point on the surface for an exact length of time. This is usually accomplished using a version of TWI's high speed EB deflection system to manipulate the beam. In this case, it is not necessary to turn the beam off between treatment positions, as the beam moves so quickly that its heat input in transit is negligible. During the treatment, there need not be any deliberate relative movement between the beam and the work, yet an exciting range of features may be produced, alone or in combination with other treatments.

In so many fields of engineering and science, the properties of an object are defined almost exclusively by its surface. Although at an early stage of development, EBPF looks set to add another powerful and speedy surface processing tool to those already available.

How does EBPF work?

The process works by utilising some of the same forces that are present in EB welding, EB texturing and ' Surfi-Sculpt ^® ' technologies, but in a different way.

The schematics below show what happens:

In stage a, the beam has spontaneously melted material, to a depth associated with the electron range into the material, and a diameter related to the size of the beam.

In stages b and c the molten zone extends.

In stage d, the beam is now boiling material in the centre of the interaction zone.

In stages e and f, the molten material is now displaced away from the centre of the point of action of the beam, thus forming a vapour-filled cavity. This process typically takes just a few microseconds, and is analogous to the initiation of an EB weld 'keyhole'.

Sequence f-m shows what can happen after the beam is turned off or moved elsewhere. Stage 'f' is not stable once the beam is no longer incident at this point. The forces of surface tension will attempt to flatten the melt pool somewhat. If the heat input is low, and the freezing rate high, then the material may freeze in stage g, as used in various texturing processes. However, if the heat input is a little higher, the melt pool may flatten appreciably before it freezes, e.g. stage h.

Should the heat input be high enough, and the viscosity be low enough, the concave 'g' melt pool may swing through the flattened 'h' stage and become convex as in 'j' and 'i'. If this process continues, the melt pool could oscillate several times (g-m, repeat) before freezing.

However, if the heat input is exactly right, and the freezing rate is high enough, the melt pool can be made to freeze in the 'j' stage, giving a protruding feature. In some cases there will be an accompanying shallow peripheral depression. In others, this may be more or less absent, even though there is a central protrusion. If the material in and around the melt pool is hot enough, the liquid metal will more than fill the hole from which it came. The apparent 'extra material' comes from a combination of elastic and plastic thermal stresses in the material; - this is the same effect that gives proud beads in EB welds.

Interestingly, the size and radius of the protrusion formed in EBPF may be just a small fraction of the beam diameter used to make it.

The process appears to work best in materials with a narrow freezing range, e.g. pure metals. These materials freeze quickly, and the liquid/solid interface is relatively smooth, leading to low viscous forces in the melt pool, and large amplitude oscillations. The speed of melt pool freezing is also enhanced if the melting point is high; radiant heat losses (which go as T ⁴) help to ensure speedy freezing.

By applying this type of treatment successively, in the same or different form, features of higher aspect ratio may be created and modified. Overlapping treatments may be used to create new types of surface.

A typical EBPF treated surface. Each feature is formed via a beam interaction lasting typically just a few micro-seconds or so. This means that several thousand such features may be formed every second in some cases. In this case a beam of ~100microns diameter has yielded consistent features protruding several tens of microns from the surface. Each feature has a tip with a radius of approximately 5microns- a tiny fraction of the beam diameter used to make them.

By utilising specially shaped beams, or modulating the treatment via beam focus or (x/y) deflection during interaction, further control and modification of the protrusion shape may be exercised.

By adding material before or during the process, a range of alloys and new materials may be created. Since the material in each feature grows epitaxially from the substrate, substrates with specific grain textures can be treated to give features with specific properties dependant on their physical shape and crystallographic orientation.

Features of a suitable size may be made in which there are specific optical interactions, or interactions with other types of wave or physical property. Control may be exercised over the properties of the treated surface, including magnetic, electromagnetic, thermal, mechanical, thermoelectric, ultrasonic, mechanical, emission, electrical properties etc.

Although some equivalent effects may also be produced using laser treatments, the fundamental interaction of the laser beam is different, since volumetric heating of thin surface layer does not occur in the same way. In addition, the speed and precision of laser beam manipulation technology is not sufficient at present to permit areas to be treated in the same way as is possible using an electron beam.

EBPF treatments, alone or in combination with other treatments are thought to have potential applications in a number of areas, including wettable surfaces (for printing etc), altered biocompatibility, the manufacture of cathodes, preparation of surfaces for bonding, surfaces with improved corrosion performance, preparation of surfaces prior to chemical conversion.

For further details, please contact Bruce Dance (e-mail to bruce.dance@twi.co.uk)