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An Introduction to Surfi-Sculpt Technology - New Opportunities, New Challenges (April 2007)

 
Bruce G I Dance, MA, Dr Anita L Buxton, TWI Ltd, Cambridge, UK

Paper presented at 7th International Conference on Beam Technology, Halle, Germany, 17-19 April 2007.

1 Introduction

It has been said; 'the only thing that stays the same is that everything always changes'. And so it is, in the field of Electron Beam (EB) technology.

Although the basic principles of thermionic electron beam generation and control have been understood for many years, new methods of application, refinements and control are constantly emerging.

To this constellation of EB techniques, we now add the recently developed Surfi-Sculpt technology [ Figures 1-5].

Fig.1. An example in Titanium alloy of the possibilities using Surfi-Sculpt processing
Fig.1. An example in Titanium alloy of the possibilities using Surfi-Sculpt processing

This paper reports aspects of the development and application of this exciting new technology, as well as some of the technical developments that underpin it.

2 Surfi-Sculpt development and application

2.1 Background

In EB welding technology, we clearly understand that there is typically a 'keyhole' that is traversed through the workpiece ( Figure 2). It is this keyhole effect that gives both EBW and LBW (Laser Beam Welding) the ability to make deep, narrow welds, with all the advantages that this can bring.

Any experienced power beam welding engineer will be familiar with the peculiarities that arise at the start and the finish of such welds; at the start, there is usually a 'bump' of displaced material, and at the end, there may be a slight depression, or even a significant cavity. Normally we look at these features as imperfections or even 'problems' to be solved.

Fig.2. Typical power beam welding keyhole mechanism
Fig.2. Typical power beam welding keyhole mechanism

However, it is also clear that the keyhole action has in fact moved material in the reverse of the welding direction, albeit not very efficiently. Many years ago, we were discussing this 'problem' phenomenon yet again at TWI, but this time we asked ourselves some questions: What if we set out to enhance this effect? What if we repeated this type of action, in order to enhance this effect?

At the time, there was not sufficient control over the EB system to do what was required. However, it was clear that we could contrive a controlled material displacement on surfaces using the facilities then available. This first led to the development and later industrial application of the Electron Beam Texturing (EBT) process. [1 and 3]

2.2 The EBT process

The EBT process ( Figure 3) can operate at extremely high speed, and allows the generation of a range of surface textures. Typically these textures have ~1:1 aspect ratio, and may include overhangs to give 're-entrant' features suitable for bonding. All this may be implemented with just a single beam, to give (typically) 500 to 2000 features per second. In some cases even higher speeds are possible.

Fig.3. EBT examples, with unsynchronised (L) and synchronised (R) deflection strategies
Fig.3. EBT examples, with unsynchronised (L) and synchronised (R) deflection strategies

The EBT process is now being industrially applied, with several suitably retro-fitted EB machines as well as dedicated new builds equipped for the task. This transition from 'laboratory' to 'production' necessitated the development of improved equipment, both in beam generation and control.

With the first EBT equipment, we could not normally achieve a well-controlled process that incorporated repeated beam visits to a single location on the work. However, the development of more sophisticated controls suited to industrial EBT helped in the development of more sophisticated processing strategies, eventually leading to the development of the Surfi-Sculpt process.

2.3 The Surfi-Sculpt process - first steps

In the Surfi-Sculpt process, typically repeated visits of the beam to overlapping or adjoining locations on the work are used to create a wide range of features.

Once we could exercise an approximation of the correct control of the beam, development of these processing strategies was possible.

It soon became clear the best of the previously developed EBT parameters did not necessarily produce a cumulative effect when repeated at the same locations. It seems that the most consistent EBT parameters may therefore impose conditions that make the beam-metal interaction in some way self-limiting.

However, by employing beams of the correct power density, and by using the correct beam manipulation, we found it was possible to generate a well controlled, - and more importantly cumulative - 'build-up'/'excavation' process.

Fig.4. Surfi-Sculpt process operation; after two 'swipes' (L) and after four 'swipes' (R)
Fig.4. Surfi-Sculpt process operation; after two 'swipes' (L) and after four 'swipes' (R)

The first such Surfi-Sculpt protrusions were of low aspect ratio, as were the accompanying holes. Even so, they comfortably exceeded what was possible using EBT, and thus encouraged further work. Soon we were making protrusions and holes of dizzying aspect ratio, up to 30:1 in some cases ( Figure 5).

Fig.5. High aspect ratio features; from the bottom of the hole to the top of the protrusion is almost 6mm
Fig.5. High aspect ratio features; from the bottom of the hole to the top of the protrusion is almost 6mm

However, to implement either EBT or Surfi-Sculpt processes consistently, we needed to do everything possible to ensure that we could both generate and maintain the correct electron beam quality and motion. This meant we needed improved equipment.

2.4 A challenge - Improved EB deflection capability.

The first EB deflection system used for EBT development consisted of a 256-point digital programmable system, driving supplementary coils that were fitted to an otherwise conventional Hamilton-Standard (Zeiss) type W2 150kV EB machine.

It was soon found that we could usefully improve the EBT process by including a so-called 'secondary' or 'motif' beam manipulation. We employed a second unsynchronized beam deflection within a 'nesting' coil for this purpose and immediately gained significant benefit.

When developing equipment for improved EBT, with an eye to subsequent industrial use, an improved beam deflection system was required. This would need to be sufficiently flexible for development work, whilst being robust and reliable enough for at least pilot production work. Preferably the system should also allow the possibility of a CNC interface.

Accordingly a system comprising five programmable arbitrary function generators (AFGs), amplifiers, and coils was constructed. This system was programmable via a standard PC, for flexibility, but once the system was initialised, the PC would be out of the control path. This was to allow reliable operation ( Figure 6).

Fig.6. Beam deflection system (schematic)
Fig.6. Beam deflection system (schematic)

The chosen AFGs allowed up to 16000 coordinates each on 'primary' (X/Y) 'secondary' (U/V) and 'Z' axes, the latter being a 'fast focus' control. By employing both X/Y and U/V sets, a total of 256 x 106 co-ordinates are possible, each in theory with a further 16000 focus possibilities via the 'Z' axis.

Pattern programming is achieved via specially written software, containing suitable functionality for programming of EBT and Surfi-Sculpt patterns.

This system has since been further improved in series manufacture under license, with increased numbers of programmed co-ordinates available, as well as a CNC control interface for automated production work.

2.5 Another challenge - improved beam quality and consistency

At the start of the EBT work, TWI's 'HS1' EB machine's only other significant upgrade was an improved electron gun design, including a refined electrode geometry and a ribbon cathode system.

Over a period of time, in which many thousands of welding and EBT experiments were carried out, it became clear that there was scope for further improvement of the electron gun system. This was of a conventional type in which a directly heated cathode in a triode configuration would be built into an exchangeable 'gun' comprising the entire bias electrode.

Specifically, there were the following problems:

  1. The cathode mountings were not sufficiently stable.
  2. Different bias electrodes, nominally identical, gave different beam qualities and bias voltages.
  3. Different bias electrodes, nominally identical, required different beam alignment settings.
  4. There was little scope for changes of gun geometry, (e.g. to produce beams of different power density etc.) without further gross changes to alignment settings.
  5. Over time, gradual wear of a 'favoured' bias electrode would cause changes in beam quality.
  6. Replacement guns would frequently require a lengthy 'conditioning' period before stable operation would be assured.
  7. Routine maintenance (eg cleaning) of gun bodies was awkward and time-consuming.
  8. Deterioration of the cathode could cause changes in beam quality.

The first of these problems was addressed by the design and manufacture of a high stability cathode mounting system. This was fitted to the original gun system, and immediately produced a more consistent beam quality. However, problems 2 - 7 required another solution.

After a lengthy period of design and testing, an improved gun system was developed, incorporating the improved cathode mountings, as well as further changes designed to enhance the functionality and versatility of the gun system.

The improved gun system allows gun changes with little or no re-alignment required. Pre-conditioned assemblies may be rapidly installed giving a typical 'gun change' time of five minutes or so. The gun change operation is significantly de-skilled. To change gun geometries is now as easy as to change guns. Part-worn gun assemblies can be stored and re-used. Routine maintenance of the gun system is both eased, and carried out almost entirely off-line. All the wearing parts of the gun system can be exchanged at low cost. Long-term consistency of beam quality is assured.

TWI's improved gun system is licensed for commercial manufacture, and can be adapted for retro-fitment to a very wide range of extant EB machines.

Of the original list of problems, only problem 8 cathode deterioration remained unaddressed.

2.6 Another challenge - how to assure beam quality, despite cathode deterioration?

With a conventional electron gun design, cathode deterioration is more or less inevitable. Careful gun design can help to mitigate its worst effects, but it cannot be entirely prevented. Under extremely aggressive conditions, the use of a 'magnetic trap' system or an 'angled gun column' to help slow cathode deterioration is beneficial. However, these systems are not always applicable, as they may distort the beam, or offer other operational disadvantages. Given that cathode deterioration is extremely rapid with some materials and processes, and rather slower with others, even frequent changes of cathode may not offer a good solution to the problem of cathode deterioration.

It was decided to implement a flexible 'beam probing system' to not only prove beam quality improvements via changes to gun design, but to allow rapid assessment of beam quality even in production conditions.

Although comprehensive beam quality data can be obtained via 'pinhole' probing, obtaining such data can be slow, and its correct interpretation may be complex. By contrast, obtaining and interpreting less comprehensive 'slit probe' data is swift and relatively straightforward. All that is required is to be able to scan the beam at a speed of ~100m/s in a controlled fashion.

Fig.7. (L to R) Probe unit with low-power heat sink, typical single slit probe signal, typical 'through focus' composite probe data
Fig.7. (L to R) Probe unit with low-power heat sink, typical single slit probe signal, typical 'through focus' composite probe data

TWI's probe system ( Figure 7) is therefore normally configured as a slit system, although configuration as a pinhole device is also possible. The in-chamber hardware comprises a simple probe head that can be configured for a variety of tasks. In the event of a beam strike, the probe head can be replaced very quickly. Repair to the damaged head is carried out off-line, and typically requires a minimum of new components.

The probe system can be fitted to most EB machines. Where a suitable beam deflection system exists, this may be used to throw the beam over the probe device. Where coils exist, but a suitable amplifier does not, a so-called' universal deflection amplifier' may be used for probing. The system will cope with coils that vary in sensitivity, inductance, and resistance over a very wide range (x10 2 for most parameters). After probing, the coils then resume their normal duties.

Slit data sufficient to characterize the beam quality for a typical welding process can be acquired in just a few seconds. One of the possible calibration problems with this type of system concerns the speed of the beam over the slit; without this information, measurement of beam diameter is not possible. The TWI probe system employs a novel strategy to generate a suitable 'calibration signal' within the data signal itself, without adding appreciably to the complexity of the probe system.

A version of the TWI probe system and the 'universal deflection amplifier' are now commercially available under license.

2.7 Surfi-Sculpt - new qualities, new possibilities

The technologies described above were primarily developed to enable the transition from laboratory to production of the EBT process. However, they also helped to enable development of the Surfi-Sculpt process. With the beam probing system, trialling and refinement of new gun designs was readily achieved, and their consistency could be tested and proven. With the improved electron gun system, practical implementation of revised gun design prototypes became both easy and precise. Finally, the improved beam deflection systems and associated software allowed complex processes to be programmed and implemented without difficulty.

Each of the above improvements, taken alone, would be a significant benefit to process development. Together, they were more than additively beneficial to further progress; each compounded the value of the other.

With the development of improved electron gun geometries, it was possible to generate beams with approximately twice the intensity of the original EBT gun. Using this improved system, great strides were made with the Surfi-Sculpt process; we could now see boundless possibilities. This lead to an increase in both the depth and the breadth of our experimental work in this field.

2.8 Surfi-Sculpt - multiple development paths, myriad opportunities

When the Surfi-Sculpt process was first realized, just one EB machine ('HS2') at TWI was equipped to carry out the process.

To give an idea of the breadth of effort in this field; at the time of writing, experimental work of this type has been carried out on a further five power beam systems at TWI. In addition, two dedicated EB Surfi-Sculpt development machines are under construction, and a further three EB machines are to be upgraded to allow implementation of this technology ( Figure 8).

Fig.8. Surfi-Sculpt development is branching out to meet R&D and future production needs
Fig.8. Surfi-Sculpt development is branching out to meet R&D and future production needs

A good impression of the depth of the effort is gained by examining the number of tests and procedures that has now been carried out using the 'HS2' machine, which has been a 'workhorse' for this development; it is difficult to be certain of this number, but it is thought presently to be in excess of six thousand.

Notable highlights of these studies include:

  1. Macro- Surfi-Sculpt . Features of at least 10mm in height have been made, using higher power beams (~5kW and more).
Fig.9. Macro Surfi-Sculpt in C-Mn steel. Each feature is over 10mm in height
Fig.9. Macro Surfi-Sculpt in C-Mn steel. Each feature is over 10mm in height

Micro- Surfi-Sculpt . Features down to ~30µm width have been made using our experimental 'high intensity' EB equipment.

Fig.10. Micro Surfi-Sculpt in stainless steel. Each 'fin' structure is less than 50µm in width
Fig.10. Micro Surfi-Sculpt in stainless steel. Each 'fin' structure is less than 50µm in width

Laser Surfi-Sculpt . The Surfi-Sculpt process has been shown to be feasible using a laser beam. Progress to date has been limited primarily by available laser beam manipulation equipment. The full potential of the process variant, and if it might be limited by the fundamentally different beam/metal interaction are both unknown at present. Further work is in progress.

Fig.11. A laser Surfi-Sculpt feature ~1mm height, in Stainless steel, produced using a CW Nd:YAG laser
Fig.11. A laser Surfi-Sculpt feature ~1mm height, in Stainless steel, produced using a CW Nd:YAG laser
Low voltage Surfi-Sculpt . The process has primarily been deployed at accelerating potentials between 60 and 130kV to date. However, developments are underway to produce EB machines which are designed to operate at 20-60kV for further processing studies.

Sequential indexed multiplex (SIM) Surfi-Sculpt . Use of this strategy allows the complexity and sequencing of the Surfi-Sculpt action to be increased. This permits the formation of more complex features, whilst retaining the desired interval between visits to overlapping or adjacent locations.

Fig.12. Honeycomb treatment made using SIM Surfi-Sculpt , in Titanium alloy
Fig.12. Honeycomb treatment made using SIM Surfi-Sculpt , in Titanium alloy

2.9 Surfi-Sculpt applications development

The possibilities and implications of Surfi-Sculpt technology are wide-ranging. Some of the possible applications are as follows:

  • Composite to metal bonding, Comeld TM .
  • Promotion of adhesive bonding.
  • Bonding and direct moulding of polymers to metal parts.
  • Manufacture of aerodynamically enhanced surfaces.
  • Manufacture of hydrodynamically enhanced surfaces.
  • Manufacture of mechanically keyed surfaces.
  • Manufacture of filters, and other applications requiring shaped slots and holes, e.g. for mixing of gases and/or liquids.
  • Manufacture of surfaces for direct bonding of materials via interlocking/deformation.
  • Manufacture of surfaces with enhanced thermal properties.
  • Preparation of surfaces prior to coating application for improved coating performance.
  • Preparation of joint surfaces for enhanced bonding and controlled/graded interfacial properties.
  • Manufacture of tailored surfaces with specific wave interaction, absorption, emission and/or propagation properties.
  • Manufacture of surfaces with enhanced biocompatibility and bio-functionality.
  • Manufacture of 'Aerosheet' - an internally lightened series of materials with excellent specific modulus and other properties.
  • Manufacture of locally alloyed functional surfaces with specific mechanical, electrical, magnetic, thermal, chemical, or other properties.

The Surfi-Sculpt process is presently under development for a number of the above, some of which are detailed below.

2.10 Surfi-Sculpt for heat transfer and drag reduction

Heat transfer and drag coefficient are two opposing sides of the same coin; control of one implies that there is some control of the other.

Surfi-Sculpt can be used to create shapes on surfaces that are difficult or even impossible to manufacture via other routes. For heat transfer applications, a simplistic analysis would indicate that for any given application, the preferred design would lie within a triangular space with nodes representing cost, drag and efficiency.

Fig.13. Simple analysis of design and performance of heat sinks
Fig.13. Simple analysis of design and performance of heat sinks

Short length (stream-wise), fin-like shapes allow the highest thermal efficiencies, but typically create the most drag as well. Short length fins keep the boundary layer thickness down for good heat transfer, but typically have high drag coefficients with conventional manufacturing technologies.

High stagnation pressures at the leading edges may well be combined with poor flow attachment at the trailing edges. A 'pin' type surface is the logical extension of the 'short fin' idea, but creates the highest drag, as typical manufacturing technologies cannot make streamlined shapes. However, by using Surfi-Sculpt , complex 3-D shapes with good streamlining in the principal flow direction are possible. This should offer enhanced heat transfer combined with low drag.

Protrusion features can be varied in height, width, density etc. over a heat exchange surface using Surfi-Sculpt , thus offering ideal conditions at every position.

Other design possibilities include controlled vorticity; not all turbulence is created equal, and certain types of vorticity (swirling, tumbling, and stream-wise) may be beneficial within the flow. By manufacturing curved fins as required, Surfi-Sculpt offers the potential to optimise vorticity for heat transfer applications.

Fig.14. Streamlined fins should allow heat transfer with less drag; curved features may be used to promote vorticity
Fig.14. Streamlined fins should allow heat transfer with less drag; curved features may be used to promote vorticity

Controlled vorticity may offer the key to drag reduction surfaces and treatments for other applications, also. Much of the work carried out on drag reduction for items such as aircraft and ships has concentrated on two main principles; textured surfaces, and perforated surfaces.

Textured surfaces can work via two routes; first, it may be possible to delay or inhibit the transition from laminar to turbulent flow in the boundary layer. Second, the nature of the boundary layer turbulence can be controlled, by promoting vorticity that can lower overall drag. In nature, 'shark skin' is thought to have some benefit in this regard.

The latter route is certainly a contentious issue; its proponents suggest that the transition to turbulent flow is dictated by an energy balance; at some point the shearing within a laminar flow absorbs enough energy to support other alternative flow patterns, ie turbulent flow. Normally, this turbulent flow is essentially random in nature, but it need not be.

Thus turbulence which contains directed or controlled vorticity modes may 'take energy' and thus suppress other modes of turbulence. This means that if 'low drag' turbulence modes can be enhanced, then 'high drag' modes will be suppressed, and overall drag may be reduced.

Perforated surfaces for drag reduction may operate in three distinct ways. The most common method for airfoils is to use suction through micro-perforations to draw the boundary layer inside the wing, thus creating a laminar flow over the entire surface. However both outflow and pulsed flow through perforations have also been claimed as viable drag reduction techniques.

These latter techniques do not suffer in the same way from practical difficulties such as hole blockage. For marine applications, there is the possibility of outflow of gas through perforations, thus reducing the viscous drag on a ship's hull.

Whether via texture or perforation, all these drag reduction strategies have some common features:

  1. Modelling is difficult; only now are Computational Fluid Dynamics (CFD) systems capable of good predictions.
  2. Experimental work is also difficult and expensive.
  3. Good results from either CFD or practical experiment may be disputed or are only applicable over a narrow range of conditions.
  4. Active systems may require an energy input that negates any benefit.
  5. There are practical difficulties in maintaining the surface in the correct condition to be beneficial at all times.

All the above considerations increase the difficulty and expense of development work in this field. However, there is another common feature to much work carried out to date. This is in relation to the design of such surfaces; for modelling or experimental work, it has to be possible to make or at least imagine the correct surface.

It is entirely possible that some of the features that can be made via Surfi-Sculpt may not be made using other processes, and have not yet been imagined, either. There may therefore be new possibilities for solutions using this manufacturing technology.

TWI is presently actively seeking potential collaborators for a proposed project to develop improved solutions for enhanced heat transfer.

2.11 Surfi-Sculpt for composite to metal bonding - 'Comeld TM '

Bonding of dissimilar materials is inherently problematic. Differing thermal and mechanical properties invariably result in stress concentrations at the interface between the materials. When bonding composites to metals this is exacerbated by the typical anisotropy of the composite material. In many composite to metal joints there exists the real possibility that there is a joint failure mode that, although it occurs at a high load, absorbs little energy. Partial joint failures of this type may be difficult to detect, since there is already a discontinuity in the materials properties at the interface between the composite and the metal.

By pre-treating the metallic joint component using Surfi-Sculpt , there exists the possibility of making composite to metal 'Comeld' joints with radically altered properties. Essentially the joint can be improved in some or all of the following ways:

  • Increased joint surface area
  • Increased joint strength
  • Better stress distribution near the joint interface
  • Better matching of materials modulii
  • Increased energy absorption to failure
  • Improved load carrying in damaged joints
  • Easier detection of partially failed or damaged joints

It should be noted that many of the characteristics of these joints arise from the intertwining of metal 'fibres' with the fibres in the composite material. However, the technique also allows more subtle design concepts. For example, those concerning the bending properties of the Surfi-Sculpt protrusions, and how they match the shear modulus of the composite. Alternatively how the material is taken from the bulk of the substrate in order to manufacture the protrusions; this can affect the modulus of the substrate, making it anisotropic and potentially a better match for the composite.

Fig.15. A typical Surfi-Sculpt treatment for 'Comeld' composite bonding, in Ti 6Al 4V
Fig.15. A typical Surfi-Sculpt treatment for 'Comeld' composite bonding, in Ti 6Al 4V

Mechanical testing of joints prepared using Comeld bonding has given remarkable results, some of which are depicted in Figures 16-20. Improvements in both strength and absorbed energy have been demonstrated. Since failure in the composite cannot readily occur near the interface unless a significant volume fraction of material is damaged in the process, the energy absorbed can be substantial. Even if the joint design is compromised so as to promote failure in the metal, this usually results in a higher energy to failure ( Figures 19 and 20). Further improved Ti/CRFP Comeld joints have exhibited 50% greater strength and up to eight times the absorbed energy during testing.

Fig.16. GFRP/Stainless steel test joints before and after tensile testing
Fig.16. GFRP/Stainless steel test joints before and after tensile testing
Fig.17. Tensile test data: Comeld joint (Stainless Steel 316L/glass-reinforced polyester) compared with control
Fig.17. Tensile test data: Comeld joint (Stainless Steel 316L/glass-reinforced polyester) compared with control
Fig.18. Comeld joint bend test data vs. control. GFRP/Stainless steel
Fig.18. Comeld joint bend test data vs. control. GFRP/Stainless steel
Fig.19. CFRP (0,90 UD epoxy pre-preg)/ Titanium 6Al 4V tensile test data
Fig.19. CFRP (0,90 UD epoxy pre-preg)/ Titanium 6Al 4V tensile test data
Fig.20. Failed CFRP/Ti test joint, as per data in Figure 19. This joint design failed in the metal
Fig.20. Failed CFRP/Ti test joint, as per data in Figure 19. This joint design failed in the metal

2.12 Surfi-Sculpt for biocompatibility

This technology has clear potential for improved bio-functionality and bio-compatibility, either by direct processing of metal implants and devices, or by treatment of moulds and other manufacturing devices. TWI presently has an arrangement with Symmetry Medical Ltd regarding the application of these processes for orthopaedic bone implants, and is presently engaged in development work to produce optimised treatments for such implants.

The use of these productive and cost-effective technologies opens up several new design opportunities. Improved implant fixation in both long and short terms is sought, together with improved product consistency and bone/implant integration.

Fig.21. Surfi-Sculpt treatments requiring multiple stages may be required for biomedical applications
Fig.21. Surfi-Sculpt treatments requiring multiple stages may be required for biomedical applications

Other medical applications of these technologies are also under consideration.

This technology has clear potential for improved bio-functionality and bio-compatibility, either by direct processing of metal implants and devices, or by treatment of moulds and other manufacturing devices.

2.13 Pre-treatment for surface coatings

Promise has been shown in the use of Surfi-Sculpt to promote adhesion between a substrate and a coating. Figure 22 shows a titanium surface coated by a thermally sprayed alumina coating.

Fig.22. Alumina coated titanium, prepared using Surfi-Sculpt
Fig.22. Alumina coated titanium, prepared using Surfi-Sculpt

In this case, the coating follows the profile of the surface to some extent. However, there was evidence that the coating was preferentially filling the cavities and it is expected that a finer scale surface profile could allow a smoother surface to be created.

Other work carried out recently [4] used Surfi-Sculpt to prepare material prior to direct deposition of metal by vacuum plasma spraying. With some Surfi-Sculpt pre-treatment's, approximately twice the usual thickness of sprayed material could be deposited without the usual problems experienced with delamination arising from thermal expansion coefficient mismatch. These results have stimulated ongoing studies into the type of Surfi-Sculpt pre-treatment that will help to give the optimal 'functionally graded' interface allowing improved adhesion of difficult or inherently mismatched coatings.

Regardless of the coating method or surface treatment method used, it seems the two will interact. The pre-treatment will affect the adhesion of the coating, and its successful deposition may require specific changes to the coating process itself. Further research in this area is ongoing.

2.14 'Aerosheet' lightweight functional structures

It has long been desirable to manufacture lightweight materials for use in panels and other applications in which the specific bending stiffness is an important property. One method by which this may be achieved is to make' honeycomb' structures, with welded, adhesively bonded or brazed facing sheets. The 'missing material' at the centre of the section thickness normally contributes to the weight but not the bending stiffness; thus a massive improvement in some properties is obtained. Such materials are widely used but suffer from some disadvantages, mainly regarding cost, joining, and durability.

Another group of 'functional structures' comprises materials with some similar characteristics, but the internal cavities may be linked together, or to perforations in one or both facing sheets. Such structures may be employed for cooling, heating, fluid mixing purposes etc. Again, the manufacture, joining and durability of such structures is often problematic.

Fig.23. Reverse face of Ti material, perforated using Surfi-Sculpt . Note absence of burrs
Fig.23. Reverse face of Ti material, perforated using Surfi-Sculpt . Note absence of burrs

2.15 Process efficiency

Tests to date have exhibited a range of process efficiencies. It is estimated that typically, only 10% of the available beam energy is used to move material during the Surfi-Sculpt process. Direct losses (through backscatter etc) are estimated to be in the region of 30%. The remaining heat input into the work may help the process to function, but not all of it may be necessary. Work continues on development of the most efficient Surfi-Sculpt treatments.

2.16 Surfi-Sculpt - new equipment challenges

A good many potential Surfi-Sculpt applications are likely to require a combination of fine-scale treatment and large area coverage. It is postulated that the process only works efficiently in any given material at a specific swipe speed and beam power density. Given this restriction, it is clear that a single beam will always have a limited potential to treat a given area. Whenever more power is applied with a single beam, either the power density or the beam diameter will slip out of the preferred range.

One obvious solution to this is to have many beams working simultaneously. Multi-beam EB systems fall into two distinct categories:

  1. 'Multi-cathode'. In which several 'beamlets' are projected through a common set of electron optical components.
  2. 'Multi-column'. In which each beam has its own electron optical system.

A multi-cathode system must inevitably present the beamlets off-axis to either the lens or the deflection coil. Normally this beam presentation results in significant aberration, compromising either the focused beam quality, or the quality of the beam deflection pattern shape. Although multi-cathode systems can be made to work for some applications, it is thought that beam quality issues alone will make their use in EB machines designed for Surfi-Sculpt somewhat problematic. Multi-column machines have the attraction that each beamlet may not be compromised in quality. However, conventional electron gun columns are typically expensive to build, comprising ~10-20% of the cost of a typical EB machine.

Although there are potential economies to be had by sharing power supplies and vacuum components between gun columns, adding conventional gun columns to an EB machine has a high marginal cost at present. It is therefore intended to devise novel means of engineering low-cost methods of electron gun column manufacture for this purpose. If this development is to be undertaken successfully, it will be of most benefit if power supplies can be shared without difficulty. This 'sharing' requirement places an extra constraint on the system; each gun must perform identically.

A further constraint becomes significant when a large number of similar guns is envisaged. Each must be as reliable as possible. For example, even if each gun is 99% reliable, a ten-gun system will only be 90% available.

To be able to cover entire areas via the process, it is essential that the area that may be processed by each gun column is sufficiently large that it may slightly overlap that area covered by its neighbour. In the case of a simple linear array of gun columns, this 'process footprint' must be at least as large as the pitch of the gun columns ( Figure 24).

Fig.24. Multi-gun column process schematic, linear array
Fig.24. Multi-gun column process schematic, linear array

This arrangement is thought to be very difficult to achieve in practice, as it requires very compact electron optics.

It may be that an easier route is to create staggered rows of gun columns ( Figure 25) thus easing the 'process footprint'/gun column diameter requirement.

Fig.25. Multi-gun column process schematic, staggered array
Fig.25. Multi-gun column process schematic, staggered array

Finally, each gun must be quickly and economically serviced; if (say) the cathode assembly is renewed, this must be quickly and reliably achieved, at low cost. It may be that an unconventional solution to these challenges is required.

2.17 Summary

Unique and innovative Electron Beam Texturing (EBT) and Surfi-Sculpt processes have been developed, and continue to be refined and adapted as opportunities arise for a wide range of new applications.

Several EB generation, analysis and control technologies have been developed and deployed in response to the challenges that have arisen during the development process. Several of these technologies are now commercially available from licensees such as CVE Ltd and licensed jobbing shops. These technologies have been used successfully on multiple EB machines for both R&Dmp;D and production work.

Diverse applications of both EBT and Surfi-Sculpt technologies are either presently entering production or are under active development.

2.18 Acknowledgements

Special thanks are due to many colleagues at TWI, including Dr Paul Hilton for the laser Surfi-Sculpt work, Dr Carrie Spence for the Comeld work, and Sheila Stevens for the bulk of the SEM work.

3 Bibliography

  1. International Patent Publication Number WO 2002/094497 A3. 'Modulated Surface Modification.' Applicant: The Welding Institute. Inventors: Bruce Guy Irvine Dance.
  2. International Patent Publication Number WO 2004/028731 A1. 'Workpiece Structure Modification.' Applicant: The Welding Institute. Inventors: Bruce Guy Irvine Dance and Ewen James Crawford Kellar.
  3. J. Barrett, EB texturing locks trickiest surfaces, Eureka; Sept 1994, pp26,27.
  4. G. Thomas, R.G. Castro, G. Matthews, P. Coad, P S Grant; 'Vacuum Plasma Spraying of Thick W coatings onto surface sculptures for fusion armour applications', International Thermal Spray Conference May 2006, Seattle.
  5. B.G.I. Dance, A.L. Buxton; ' Surfi-Sculpt A New Electron Beam Processing Technology', 8th International Conference on Electron Beam technologies, June 2006, Varna.

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