Sculpted surfaces produced by beams

Tom Shelley reports on progress with a process for producing metal surfaces with protuberances and pits that is now finding commercial application.

Laser and electron beam sculpting of metal surfaces allows them to interlock with other materials with which they would not naturally bond

This is an advance of considerable relevance to makers of surgical implants, while the surfaces can also be used to generate structures that alter fluid flow over a surface, enabling improved efficiency in heat transfer while minimising fluid flow resistance.

A set of techniques called 'Surfi Sculpt' has been under development for some years at TWI in Abington, Cambridgeshire. They started out using electron beams, and then additionally used laser beams to produce structures in a wide variety of materials, including stainless steel and titanium. The laser techniques can also be applied to specially pigmented plastics.

The basic technique is to use a fine, high-intensity beam of electrons or light to melt a small area of surface and then quickly move the beam onwards. Owing to vapour pressure and surface tension effects, this leaves an intrusion or depression where the beam impacts and a mound or protrusion behind it. In this way, it is possible to make repeated structures tens of microns in size at one end of the scale, or up to tens of millimetres wide and high at the other.

Electron beams are scanned using magnetic fields in a vacuum, but laser beams are scanned using galvanometer mirrors and can be used in air or inert gas. There is a problem in that laser beams need to be focused and distance to the target varies with angle, but flat field lenses have been developed that have a wide working range without the need to refocus. Machines have also been developed in which the lens can be moved longitudinally in order to maintain fineness of spot.

As an example, a research paper reports that a 5mm high protrusion in titanium plate was made in about 5s using 1kW of laser power and eight radially sequential arms, each of which was swiped 60 times at 16m/min. The time delay between swipes was 0.5ms.

Most of the work done at TWI to date, however has been with electron beams. Because the electrons penetrate a small distance into the material, they both heat and melt a small volume of material all at the same time, whereas a laser beam applies heat only to the immediate surface. About 10% of the available beam energy is used to move material and losses through backscatter and other effects are thought to be about 30%. Using an electron beam, it is possible to make textures of 500 to 10,000 hole/protrusion pairs per second, with just one visit of the beam to each location.

Special electron guns have been optimized for Surfi-Sculpt processing, which produce smaller and more intense beams than are normally available for electron beam welding. Cambridge Vacuum Engineering (CVE) has a licence to produce these and has built several EB machines for those processes that are in current production use.

Commercially, Thermacore Europe is exploring the use of the process for a number of different heat exchanger products and is in the process of testing prototype liquid cold plate heat exchangers. The customised surfaces are designed to improve the performance of the heat exchanger without increasing the size of the product. Because the process is completely flexible, it is possible to produce fins that maximize heat exchanged and minimise fluid drag on a micro or mm scale. It is also possible to produce surfaces that minimise air or fluid drag, using the shark skin effect, as well as making novel structures for filters, fluid mixing and to enhance the adhesion of coatings.

The process has received significant interest from manufacturers of orthopaedic implants where the surfaces have the potential to improve the joint between implant and bone. Other uses include TWI's patented 'Comeld' process for improving the joining of metals to composites. Such bonds show much greater integrity than using fasteners or adhesives. The processes are not restricted to flat surfaces but may be applied to a variety of 3D shapes.

Tom Shelley

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