FEATURE

In a material world

William Payne assesses the impact of photonics on materials processing

The use of lasers in materials processing continues to accelerate. Quality improvements in existing laser techniques and the development of new laser technologies is driving the development of rapid manufacturing – closing the gap with conventional production techniques on volume, cost and range of materials. Lasers for materials processing are becoming cheaper – especially CO2-based models – and more flexible: industrial materials processing is seeing the emergence of fibre laser systems, which are able to combine a wide range of industrial applications with notable cost efficiencies.

Lasers are replacing conventional chemical and mechanical processing techniques with safer, faster, more flexible and cheaper technologies in manufacturing applications that range from the latest high-tech flat panel displays and semiconductor chips through to ancient wood conservation and DIY wood products.

Transforming production

Changes in western manufacturing are accelerating the uptake of lasers. As high volume production shifts increasingly off-shore, western firms are turning to agile solutions. Close-to-market and fast-to-market, agile western firms can build market share and value if they can achieve the right levels on cost, volume and quality. That means not just rapid prototyping, but rapid manufacturing, too.

Lasers are central to both approaches – from the original prototyping technologies, such as stereolithography (SLA), to the latest developments. The early rapid manufacturing technologies are derived from prototyping. As a result, they have often suffered quality and cost issues – and limitations in the range of materials they can process – when compared to conventional high volume, lowcost manufacturing. But recently, new laser-based technologies are broadening the range of materials that rapid manufacturing can handle, improving quality and bringing cost down. Established rapid materials processing technologies have also undergone significant improvements that are expanding the scope of rapid manufacturing and improving volume and cost factors.

According to US manufacturing research consultancy Wohlers Associates, the rapid manufacturing market is growing on average by around 15 per cent a year. Uptake is keenest in typically high-tech areas, such as medical, defence, automotive and aerospace, but is fast growing in consumer and retail as well. As the cost-to-volume ratio comes down, the focus of the market is likely to broaden. SLA is the longest-established rapid manufacturing technology. It is limited to resins, and resulting products can be brittle, and sometimes difficult to machine. On the plus side, the technology is well understood, simple and relatively inexpensive, and both fine detail and high-quality finishes are possible. One of the key issues with SLA has been ‘trapped volumes’, where pools of uncured resin form, weakening the structure and sometimes causing subsequent collapse. However, researchers at the University of Cardiff announced a solution to this problem at the end of last year.

Selective laser sintering (SLS) is another – somewhat later – technology that has made the transition out of rapid prototyping into rapid manufacturing. Its underlying characteristics have adapted well to agile production and incremental improvements have helped drive the wider adoption of rapid manufacturing. SLS is structurally stronger than SLA, and tends to last longer. A wider range of materials can be processed, including metal, and bonding is easier. SLS has been adopted across a range of applications, including Formula One racing and the International Space Station.

Both these technologies are used principally for rapid plastics manufacture. New technologies that are widening the range of materials and types of structures that can be processed include laser engineered net shaping (LENS) and direct metal laser sintering (DMLS). LENS is a technique developed at America’s Sandia National Laboratories. Metal powder is melted by laser using a deposition head. It can produce parts from a wide range of special alloys including aluminium, titanium and stainless steel. LENS can equal conventional manufacturing in engineering quality, and has found applications in repair and in rapid manufacturing of high-performance parts.

DMLS is a sintering process developed by EOS of Munich. Applications include machining and tool making. One client of EOS, CPM Fastools of Maryland, which makes moulds for medical devices, replaced conventional CNC milling with direct metal laser sintering.

Their move to rapid manufacturing took eight weeks or more out of their time to market, cut costs and led to the company doubling its revenues.

Water jet guided lasers from Synova are used for delicate materials processing.

Cutting costs

Such laser-based techniques are not restricted to the high-tech worlds of aerospace, defence or automotive. A British firm and Loughborough University employed laser sintering to develop the world’s first football boot designed for the individual player. Prior 2 Lever is a specialist in podiatry and highperformance athletic footwear. They created a new approach, called ‘corrective geometry’, which would allow outsoles to be individualised for players to reduce the risk of injury. They needed a sophisticated technique for producing their customised products that would also be cost-effective enough to produce a small number of boots for each individual player in a 12-man team.

They turned to the Rapid Manufacturing Research Group at Loughborough University. Loughborough’s answer was an SLSbased solution. The outcome of this cooperation is Assassin, P2L’s individualised football boots, which have been adopted by Premiership teams in England.

Another example of how laser sintering allows smaller firms to bring innovative new products to market without crippling set-up costs or long over-runs is provided by manufacturer Product Partners and its FireRay fire detection system. The FireRay system is designed for smoke and fire detection in large commercial spaces, such as warehouses. However, changes in such environments often cause expansion and contraction of the buildings, resulting in misalignment of the detector beam and potential inaccuracy. Product Partners developed an automatic gimbal that would correct the beam’s alignment.

Its design was highly complicated, however, and threatened to increase the cost and complexity of producing the detector. Product Partners took the design to Ogle Prototypes, an industrial design and prototyping firm based in Letchworth that had designed the Reliant Scimitar and the Raleigh Chopper. Their solution was SLS: since it can produce parts with very complex geometries and parts can be built within other parts, it was ideal for FireRay’s automatic gimbal. Moreover, the mechanism could be produced immediately from existing CAD files with low cost of set-up.

The rise of fibre lasers

Conventional industrial materials processing is broadly dominated by Nd:YAG and CO2 lasers. The latter is growing fast in lighter industry, mainly through the increasing availability of low cost units. But the fastest growth in materials processing lasers is in the fibre laser sector. Typical applications include metal cutting, welding, silicon cutting, ceramic scribing, spot welding, bending, powder deposition, surface modification and marking.

Fibre lasers are typically employed in high-power applications, such as highperformance alloy engineering, where the power of the laser is key.  At the other end of the performance spectrum, electronics manufacturers are turning to water jet guided lasers – such as manufactured by Swiss company Synova – for delicate materials processing on ultra-thin (50-100μm) wafers and compound semiconductors such as GaAs and GaN.

A fine jet of water guides the laser beam, acting like a conventional optical fibre, by means of total reflection. The water jet also acts to cool the wafer at the point of laser cutting, and cleans the surface, preventing any particles from sticking to the semiconductor.

Another solid state laser now widely used for materials processing is the Q-switched diode pumped solid state laser (DPSSL). These lasers are compact, low-maintenance and robust. One emerging application is as a replacement for lithographic etching in the production of solar panels and flatpanel displays. Etching is employed to create the transparent semiconductor circuitry that controls the displays. Lithographic etching is costly, complex and – due to the toxic chemicals employed – potentially dangerous.

High-powered DPSSL lasers offer a cheaper, simpler and safer alternative for thin film ablation on flat-panel glass, according to DPSSL specialist Powerlase. The system is around 50 per cent cheaper than conventional technology, and has been shown to produce significantly higher yields as the technique is better suited for very large panel thin film ablation than conventional wetetch lithography.

A more unusual application for laser ablation – this time involving an excimer laser – but  requiring no less delicacy than thin film flat panel etching, is ancient wood restoration. Excimer lasers have recently been employed to clean and protect a site intimately connected with the Reformation and the development of eastern German culture. It is the first time that lasers have been used in the conservation of historical wood.

The conclusion by researchers studying the process is that lasers offer a conservation technique that is significantly superior to existing methods.  Pirna, a small town near Dresden in Saxony, in east Germany, has undergone a far-reaching restoration since 1991. The work has transformed the town into one of the most important historically preserved sites in Germany. A key part of the town is the Tetzelhaus, once home to Johann Tetzel, one of the most infamous peddlers of indulgences in Germany and a contemporary of Martin Luther.

Restoring Tetzel’s former home was a major challenge for conservators. The 600-year-old wooden building was in a poor state, and much was heavily damaged. Abrasive cleaning  methods, such as dry blasting would have severely damaged the wood further. Applying a scalpel – a tried and tested method for cleaning wood – would be too timeconsuming and inefficient. Using water vapour carried the risk of infecting the wood with rot, and would have stained it.

Instead the conservators applied an excimer Nd:YAG laser (Siemens  XP2020) operating at 308nm. Laser cleaning of large areas of historically significant wood had never been  tried before; there was no previous experience of laser cleaning wood. A team from the Scientific University of Dresden documented the process and the results. Their conclusion was that laser cleaning led to strikingly good results, with layers coming off easily. Remains could be removed from holes and depressions in the wood without any visible damage to the underlying structure.

Carbonisation of the surface wood was less than was expected, and the ablated wood had improved preparation and penetration for specially adapted adhesives and coatings better than alternatively prepared wood surfaces.

A hardwood MDF

The mediaeval wood panelling in Tetzel’s house is a long way from Medium Density Fibreboard. MDF is an ‘engineered wood’ made of reconstituted softwood. It is made from recycled wood waste, and also from fast-growing softwood trees – so it can be environmentally kinder than slow-growing hardwoods, which typically come from virgin forests that are stripped and left bare. It is also cheaper, which is a key factor in many of its potential applications.

But MDF has a dull appearance, and has a fibrous, absorbant surface. Both these factors have restricted the range of retail applications to which it can be applied. It does not take well to many of the new finishes being applied to interior doors, and it does not have a suitable finish for double glazing frameworks. Now a consortium of wood trade bodies and manufacturers have collaborated with the University of Warwick in the LaserCoat project, which is funded by the UK Department of Business, Enterprise and Regulatory Reform.

Led by Warwick’s Dr Ken Young, the LaserCoat project has devised ways of using lasers that transform the appearance of MDF, giving it a surface finish similar to some of the most expensive hardwood grains. The technology can mimic real wood grains, and can also be used to produce logos, and even coloured and shaped decorative surfaces.

LaserCoat has the potential to widen greatly the commercial applications of MDF. It produces very hardwearing surfaces, and makes the wood suitable for flooring and wood panelling, where it had not been used before. It also makes it suitable for use in double glazing and other external applications.

In almost every field of industrial materials processing, lasers are proving themselves more adaptable, faster, less expensive and safer than conventional processing technologies. Both in manufacturing processes and in product processing, lasers are effecting wide-reaching, disruptive changes that combine reduced environmental impact and greater safety with improved cost, adaptability and time to market.

Fabric welding by Greg Blackman

Ian Jones, Aurélie Brun and Jo Lewis of TWI in Cambridge, UK, have developed a technique for laser welding fabric seams. The technique produces stronger, more watertight seams with a minimal mark weld line and is something they hope will be taken up by the textiles industry as a whole.

Laser-welded seams were five times more watertight than conventional seamsams.

Working with Monarch Textiles, a protective clothing manufacturer based in Nottinghamshire and other companies as part of a European collaborative project, the team at TWI has manufactured a waterproof jacket using a fibre delivered  diode laser to weld seams.

‘[TWI’s] process is much faster than standard [textile product manufacturing] techniques. It takes 10 minutes to weld all the seams on the jacket,’ Brun explains. Laser welded seams tested almost four times stronger than the industry standard for the fabric type and were five times more watertight than the required standard for water penetration. Usually, it is the seams of waterproof jackets that are the least robust to general wear-and-tear and the first part to lose their waterproof properties. The technique has the potential for improving the quality of the seams and the quality of the jacket.

TWI has also been working alongside Carmarthenshire College in Wales to develop a unique manual laser welding system in the style of a sewing machine. In place of the needle there is a fixed laser and fabric can be manually run through it to create the weld.

The first step of fabric welding using a laser is to spray the material with an infrared absorber along the weld line. The two materials are then clamped together and a laser beam is passed through the upper material layer onto the absorber. This then generates heat and melts the fabric from the joint interface outwards, creating the weld without affecting the surface appearance or feel. ‘The process is relatively precise,’ says Jones, ‘and one can easily control the depth [of the weld] and hence the seam properties.’

The project’s success rested upon the implementation of Clearweld, a technique, developed and patented by TWI in 1998 in collaboration with Gentex Corporation, which avoids the use of carbon black as an infrared absorber. Instead, a compound that is clear in the visible spectrum but absorbs heavily at wavelengths in the infrared region is used.

The jacket was manufactured from commercial laminated fabric, although Brun is clear that not all textiles can be processed in this way. One of the limitations for choice of fabric is that at least one part must be infrared transparent to allow transmission welding. Also, once welded, seams cannot be reworked.

There would also be an initial capital cost, as manufacturers would have to invest in new laser equipment to replace the traditional mechanical devices used now. ‘This process is already used widely in plastics welding,’ explains Jones, ‘but it will be a large step [for the textiles industry] to make the transition from a manual process to a fully automated system.’

Surface hardening with diode lasers

Case hardening of metal parts refers to the modification of their surface characteristics (e.g. wear and friction) without altering the bulk metal properties (e.g. tensile strength, flexibility). CO2 lasers have been used for case hardening steel for more than 30 years. However, the case hardening properties of CO2 lasers, together with their cost of ownership characteristics, have limited their success in this application. But according to Sri Venkat, director of marketing at Coherent, a new approach to case hardening based on the high-power, direct diode laser is gaining popularity. He claims it is one of the fastestgrowing applications for the company’s HighLight line of highpower diode lasers.

Saw tooth selectively hardened by a Highlight, high power diode laser system.

During case hardening, the laser illuminates a localised area of the work piece. This light is absorbed near the surface, and causes rapid heating limited to just a few hundreds of microns depth from the surface. This produces the desired metallurgical and chemical changes to the outermost layer of the part. The bulk heat capacity of the material is sufficient to act as a eat sink for the extraction of heat from the surface, therefore enabling self quenching.

Compared to flame hardening and induction hardening, laser processing offers several advantages, including rapid processing, precise control over case depth and minimal part distortion. In particular, part distortion is typically low enough that subsequent post-processing steps to restore dimensional accuracy, such as grinding or machining, are not necessary.

The primary drawback with CO2 laser hardening is that the 10.6μm output is not well absorbed by virtually any metal. As a result, surfaces must first be ‘painted’ with an absorptive coating. In contrast, the 805nm output of a high-power diode laser system is well absorbed, eliminating the need for surface preparation, and all the costs associated with the painting process. Furthermore, the highpower diode laser outputs a beam shape that is well matched in size to many hardening tasks, and which can be re-shaped to match the dimensions of a specific task.

Diode laser processing is most advantageous when the part has a specific, limited surface area that needs to be hardened, or if the part is so large that it is costprohibitive to heat treat with conventional means. As a result, laser diode hardening is currently being employed with components such as pistons, cams and crankshafts.