Materials processing tomorrow - new laser sources

TWI Bulletin, July - August 1998

Paul Hilton is Technology Manager - Lasers, where he has responsibility for the strategic development of laser materials processing. As such he has been instrumental in the setting up and management of several European collaborative research projects.

Dr Hilton has previously worked in the laser systems industry in the UK, and before that, was a researcher at an international scientific institute in France.

Recent commercial developments in two types of CO₂ gas laser, high power Nd:YAG lasers and diode lasers are presented here by Paul Hilton. Fast axial flow CO₂ lasers with output powers up to 20kW and smaller, lightweight, diffusion cooled slab geometry CO₂ lasers, with output powers up to 2.5kW, have recently appeared on the market. Advantages and applications of these lasers are described here. New, continuous wave, fibre delivered Nd:YAG lasers with output powers in the region of 4kW are also examined. Current and future applications, including remote processing, of these lasers are included. Lastly, the design and operation of diode lasers for materials processing applications and examples of the capabilities of these new lasers are illustrated.

Commercial application of lasers for materials processing activities effectively began in the early 1970s using gas and solid state lasers, both of which produce light in the invisible, infra red region of the spectrum. The carbon dioxide gas laser (CO₂) and the neodymium-yttrium aluminium garnet (Nd:YAG) laser, both invented around 1964, were quickly developed and commercialised. These two lasers continue to dominate sales of industrial lasers world wide, accounting for well over 90% of units shipped during 1997, for example. The main processes carried out with these lasers include cutting, welding and to a lesser extent heat treatment, of metallic materials. Until relatively recently CO₂ gas lasers could be categorised into three types - slow flow, fast axial flow and transverse flow, the separation being linked to output power and beam quality.

This position is now changing with, for example, fast axial flow lasers with output powers in the region of 20kW, and new diffusion cooled (or slab geometry) CO₂ lasers, with powers up to about 2.5kW, becoming available. The industrial Nd:YAG laser has also advanced in recent years with improvements to beam quality for the smaller pulsed lasers and the introduction of lasers with continuous wave output powers of around 4kW. These powers and the ability to pass the Nd:YAG laser beam through small diameter silica fibres make these new high power YAG lasers commercially very interesting.

Moving closer to the visible part of the spectrum, but still in the infra red, diode lasers are now beginning to be used for materials processing applications. It may come as a surprise to many that the type of laser commonly found in the compact disc player is also available with output powers up to 2kW! The diode laser is significantly more efficient than either the CO₂ or Nd:YAG laser and, in addition, can be made extremely small. As will be described later, focusing the beam of a diode laser needs special attention, but systems are available which can focus the beam to a small enough spot to pass it along an optical fibre. Alternative approaches prefer simply to mount the diode laser (because it is so small and light) onto the end of an articulated arm robot. Soldering, welding of metals and plastics, as well as various heat treatment activities are possible with diode lasers.

Although developed in the late sixties, only in the last few years have copper vapour lasers been used for materials processing. These lasers radiate in the visible part of the spectrum (yellow-green) at average output powers up to about 100W, but with very high pulse repetition rates (~20kHz). Pulse energies can be up to 20mJ corresponding to very high peak powers in each pulse, which can be above 100kW.

Copper vapour lasers are used mainly for scientific purposes but potential cost effective drilling and micromachining materials processing applications are emerging. Metals which respond well to copper vapour laser processing include aluminium, copper and brass, all traditionally difficult materials to cut and drill accurately to small dimensions. In addition, the heat affected zones produced by these lasers are very small.

In the ultra violet (and visible) end of the spectrum, the excimer laser dominates. Ablation and cold micromachining techniques are the hallmark of this laser. Very high pulse energies (up to 2J) are available, corresponding to peak powers up to 50mW. Plastics, ceramics and other non metals can be machined with the excimer laser with a resolution of the order 2µm obtainable.

This paper will concentrate on new developments in fast axial flow and diffusion cooled CO₂ lasers, high power Nd:YAG lasers and diode lasers. This will cover the power range from a few hundred to 20kW of output power. Welding of metallic materials is possible at both these limits.

High power fast axial flow CO₂ lasers

This type of laser, which is used on almost all the flatbed laser cutting systems employed throughout the world, was first developed at TWI in the early 1970s. A dc or rf current is used to excite a gas discharge from a mixture of CO₂, He and N₂. Cooling of the gas is achieved by convection through the discharge region, using a Rootes blower or (more recently) a turbo pump and a system of water cooled heat exchangers. Typically the gases flow at 300-500 m/sec through the discharges and control of the gas mixture and a leak light system provides for smooth plasmas.

The axial nature of the flow, discharge and optical oscillation favours an axially symmetric power distribution in the beam. The beam mode is of a low order and thus easily focussed to a small spot. Over 1 kW/m of discharge can be produced and although the original lasers were constructed of several discharges in line, optically folding the system has allowed small compact units to be designed and manufactured. Many fast axial flow lasers in the power range 1-6kW have been produced. Fewer high power units have been manufactured but today's highest powers are of the order 20kW, typified by the German manufacturer Trumpf with their model TLF20000t. This unit will produce 20kW of power with a stable resonator configuration, ie using a partially transmissive output window, and more power if an unstable resonator and aerodynamic output window is used.

As can be seen from Fig.1 the unit is very compact for the high output power available, some 2.3m x 3.6m x 2.7m. The discharge configuration uses a double fold, as can be seen in Fig.2, so providing eight separate discharges within the 3.6m length. The laser is excited by an rf discharge. This has several advantages, such as the electrodes can be positioned outside the discharge tubes and the laser can be energised and controlled electronically at high speed, thus making CNC control of the laser power simple. The rf power supplies also contribute to the wide range of stable powers available from the laser, and Trumpf specifies a 2% power stability through the range 1-20kW for this laser. Clearly deep penetration welding at high speeds is an obvious application for this type of laser. Trumpf claim a weld penetration of 20mm at a speed of 2 m/min on mild steel at a workpiece power of 19kW using this laser. App lications in this area include shipbuilding, pipe fabrication and general steel construction. As well as deep penetration welding, the high powers available also allow very high speed welding of thinner materials. Fig.3 for example, shows a 3mm weld in stainless steel produced using 18kW of power at a welding speed of 15 m/min. Applications in this area of weld penetration include transmission components and pipe fabrication. Such high power lasers would also appear to be ideally suited to surface engineering although commercial applications for lasers in this area are relatively few as the use of lasers is usually an expensive option. If one is considering purchase of a 20kW fast axial flow laser, then a budget of about £650,000 should be allowed for.

Diffusion cooled slab geometry CO₂ lasers

The efficiency of CO₂ lasers is not high, electrical consumption and waste heat contribute to the running cost, as does the relatively high cost of lasing gas. As a result, development effort has continued on other designs of CO₂ lasers with a view to improving the areas cited above. From the heat removal point of view, creating a laser discharge in a narrow gap between two flat metal plates (slab geometry), rather than along a cylindrical glass tube is very beneficial. This configuration, shown in Fig.4, offers attractive 'passive' gas cooling where the laser gas is static and heat removal is entirely based on thermal conduction through the gas (diffusion cooling). Such an approach depends on area scaling to achieve the discharge volume necessary for high power, in contrast to the length scaling which is characteristic of fast axial flow technology.

The key to creating slab discharges could not be found in conventional dc discharges but required the development of rf excitation techniques in the 1980s. Rofin Sinar has recently introduced a diffusion cooled slab geometry laser into the market place based on the arrangement depicted in Fig.4. The 'static' gas means of course that the large Rootes blowers or turbines and their electricity consumption are not required. The gas consumption in this type of laser is very low and Rofin claims a small cylinder of pre mixed gas in the laser head will last for 12 months of continuous operation. A beam shaping module is integrated into the laser head to produce a round symmetrical beam of high quality. The Rofin 'DC' range of lasers is available with powers up to 2.5kW. The low running costs, (typically 30kW electrical consumption) high beam quality and compact size of the laser head, only about 1.7m long, contribute to making this a very attractive laser in the highly comp etitive field of laser profile cutting, a market which uses the vast majority of CO₂ lasers produced in the 1-3kW range. Rofin Sinar has calculated approximate savings of £11,000 in running costs for a 2kW diffusion cooled laser, when compared to a similar output fast axial flow laser, over a 3000 hour operating period. Laser cutting machines (and some welding machines) are becoming larger and larger. Systems with axes over 30m long are being used, for example, in the shipbuilding sector. Because of the extended optical paths which would be required if a stationary laser was used, it is now common practice to move the laser on the cross beam of the cutting machine. The compact size and weight of the diffusion cooled CO₂ laser and minimal gas consumption, make this a good choice for such installations. The cost of a 2kW diffusion cooled laser would be approximately £95,000.

High power Nd:YAG lasers

Next to CO₂ lasers the solid state lasers, in particular the Nd:YAG laser, have been used extensively for many years in materials processing applications. In a solid state laser, a crystal rod is excited using flash lamps producing white light, and an optical resonator is produced by inclusion of a fully reflective rear mirror and a partially transmitting output window. Early versions of Nd:YAG lasers used for material processing were all pulsed devices, and the high peak pulse powers which were available quickly proved advantageous for drilling small holes.

The pulsed Nd:YAG lasers with average powers of a few hundred watts have also been used very effectively for the precision cutting and welding (usually conduction limited) applications using metals and other materials. The 1.06µm wavelength of the Nd:YAG laser is very suitable for transmission down small diameter, flexible optical fibres made of silica. These are similar to the fibres developed for telecommunication purposes but slightly larger in diameter, (a 600µm fibre will easily transmit 4kW of Nd:YAG power).

This fibre optic beam delivery is obviously a big advantage for the Nd:YAG laser over and above the CO₂ laser, which has to use mirrors to deliver its beam. This contributed to the incentive to develop a range of high power continuous wave Nd:YAG lasers to compete with the medium power CO₂ lasers. To date two manufacturers, Lumonics and Haas produce lasers with outputs in the region of 4kW. The schematic representation of such a laser is quite simple as can be seen from Fig.5. The system consists of several Nd:YAG cylindrical rods about 200mm long by 10mm diameter. Each of these is contained in a cooled, reflective cavity, in which one or more flash lamps are positioned. In this way, as much as possible of the flash lamp light is used to excite the lasing medium ( ie the crystal rod). Power is built up by adding rods in series, and the units are often folded in a similar manner to CO₂ systems to reduce the length. The diverging output from the laser is focused, using a glass lens, onto the end of the optical fibre delivery system. At this point a 'beam switch' unit can be positioned so that several fibres, leading to several workstations, can be used. In the workstation the beam exiting from the fibre, expands onto another lens, which is set to make the beam parallel before it is focussed by a third lens to a small spot. The last two lenses are usually housed together into a 'processing head' conveniently held in the arm of an articulated arm robot. The current applications for this type of laser are dominated by the automotive sector, for both tailored blank production and body in white assembly. With 4kW of power, 10mm of mild steel can be welded at about 0.5 m/min and thus other applications in the field of structural fabrication are actively being investigated. Figure 6 shows a macro section of a two pass T-joint weld in 12mm thick mi ld steel using a workpiece power of 3kW at a speed of 0.7 m/min, made with a fibre delivered Lumonics Nd:YAG laser. Long fibres offer the possibility of remote processing using this type of laser. In particular remote repair of pipework in the power generation and nuclear industry sectors is being fruitfully investigated. A continuous wave Nd:YAG laser capable of producing 4kW of power at the source would cost in the region of £250,000.

Diode lasers

High power diode lasers feature a high electrical to optical power conversion efficiency coupled with a very small size. Diode lasers consist of a pn junction within a multi-layer semiconductor structure. For powers greater than about 4W, the only commonly used manufacturing approach produces a diode laser bar about 10mm long, with emission of radiation confined to the narrow junction region (typically 1µm thick). Along the 10mm length, many thousands of single emitters, of the order 5µm wide, produce laser output with, because of diffraction, very large beam divergence. See Fig.7. The resulting beam with its large angular spread is characteristic of semiconductor lasers, and compared to other types of laser, presents a drawback in terms of focusability. The beam divergence is up to 90° along the emitting line (known as the slow axis). Powers of the order 50W can be achieved from one diode bar. For high power applications combining the power from several diode bars is required. For materials processing applications the semiconductor material is based on InGaAs on a GaAs substrate (940nm wavelength) or InGaA1As on a GaAs substrate (808nm wavelength). Both these wavelengths are invisible to the eye.

As a result of the rather unusual beam characteristics of the diode laser and the added complication of increasing power by adding diode bars, several different possibilities exist for beam combing to achieve the required power densities for material processing applications. It would appear that it is in this area that one 'diode laser' supplier is distinguished from another. A 2kW (highest currently available commercially) diode laser (including beam focusing) is smaller than a shoebox and its control, power supply and cooling system is the size of a two drawer filing cabinet, as can be seen in Fig.8. As a result, a clear division can be seen between those manufacturers who would place the laser directly on the arm of a robot and those who favour fibre optic beam delivery to a focusing head (the latter very similar to that required for a Nd:YAG laser).

The approach to beam shaping and focusing is therefore different for these cases. The highest power lasers take a relatively simple approach to beam shaping. 'Shaping' is used rather than 'focusing' as a small spot is difficult to achieve due to the way the light emerges from the diode. For 'focusing' of the fast axis, cylindrical microlenses are attached to the diode, and diode bars are stacked to produce an array of semi collimated stripes in a rectangular pattern. Clearly this cannot be reduced to a circular spot, so to use a square beam profile, (the best possible) the beam shape is adjusted with an anamorphic prism and the slow axis is collimated with a cylindrical lens before final focusing using a spherical optic. This approach is known as a stacked bar laser. Other more sophisticated approaches, such as individual beam shaping (IBS), produce a smaller spot but at the expense of a more complicated optical system. For low laser powers it is possible to focus the diode output to a spot small enough for transmission down an optical fibre, but once again a sophisticated optical system is required. The Table compares some of the significant parameters of the three laser types discussed. The figures relate to information available in 1997.

Table

Surface engineering, soldering, brazing, forming and conduction welding of thin materials have all been demonstrated with the diode laser. Figure 9 shows the section of a weld produced in polypropylene using approximately 500W of diode laser power. The lower polypropylene section has been doped black in order to make it absorb the diode laser light. This passes through the (transparent) top section and heats up the interface sufficiently to cause local melting. Plastics welding is regarded as a growth area for the diode lasers. High power diode lasers appear to be the low cost laser source of the future, but this will depend heavily on mass production of the semiconductors involved to keep the price sufficiently low in a very competitive market place. Even so, at least six manufacturers have brought diode lasers for materials processing applications onto the market during 1997, five of the above being European companies.

Conclusions

The fast axial flow laser, originally developed in the 1970s, continues to dominate sales of CO₂ lasers and recent developments have pushed the powers available with these units to 20kW and above. In the power range up to about 3kW, where the majority of lasers produced are destined for the very competitive cutting market, recent advances in CO₂ diffusion cooled slab geometry lasers, should produce economic benefits and increased efficiency. The high power Nd:YAG laser with its highly flexible fibre optic beam delivery capability is already being effectively used in the automotive industry sector and aerospace and structural fabrication applications are currently being investigated. Remote repair using these lasers is also an interesting possibility. The recently emerging diode lasers will have an important part to play in material processing but it would be wrong to assume that these lasers can automatically replace other laser sources with beams which are easily focused to a circular spot, due to the way in which laser diode light is generated. However, applications where a distributed or tailored energy distribution is better suited than a tight focused spot, will benefit from the diode laser developments.