Materials processing - the diode laser alternative

TWI Bulletin, September/October 2000

Chris Otter recently gained a BEng (Hons) Degree in Manufacturing Systems Engineering at the University of Hertfordshire. He has worked at TWI for 18 years in the areas of laser and microtechnology processes. As Project Leader in the Ceramics and Microtechnology group, he conducts R&D projects, production line troubleshooting/best practice audits and runs training courses in a variety of disciplines.

It may come as a surprise to many to find that the type of laser found in a CD player is now available for materials processing applications at powers up to 6kW. As Chris Otter explains when compared to the widely industrially used Nd:YAG and CO ₂ lasers, the efficiency of generation of diode laser light is high and, in addition, the diode devices are very compact. In fact, their only real drawback is that it is not so easy to focus diode laser light to a small circular spot to achieve the power densities (obtained relatively easily with Nd:YAG and CO ₂ lasers) required for materials processing. Nevertheless, applications using diode lasers in the areas of plastics welding, laser surface engineering and conduction-limited welding of metals are already in production and a deep penetration keyhole type weld in stainless steel, has been demonstrated in the laboratory.

What are laser diodes?

The first demonstration of a semiconductor laser was made (at approximately the same time) in 1962, by three separate groups in the USA. General Electric, IBM and MIT all demonstrated lasing action from gallium arsenide diodes which had to be cooled to 77K with liquid nitrogen. ^[1] Such operating conditions, coupled with output pulses that lasted only a few microseconds did not, at that time, inspire the use of these lasers for practical materials processing applications. The development of laser diodes was, in fact, fuelled by the simultaneous development of fibre-optic telecommunications and the need for low-cost stable light sources. Today's laser diodes are now capable of operating at room temperature and are conveniently packaged.

Laser diodes consist of a pn junction within a multi-layer semiconductor structure. In simple terms, light is emitted when a negative voltage is applied to the n material and a positive voltage to the p material (forward biasing of the pn junction). The result of forward biasing a semiconductor of any type is current flow, with energy being released at the junction. The particular p and n material combinations used in laser diodes emit energy in the form of light, as opposed to heat which is the case with silicon diodes. Emission of laser radiation is confined to the length of the narrow junction region (active layer) which is typically just 1µm thick. Reflective and semi-reflective facets form the familiar layout of a laser cavity. Figure 1 gives a view of a laser diode.

For materials processing applications, the semiconductor material is based on InGaAs on a GaAs substrate (940nm) or InGaAlAs on a GaAs substrate (808nm). Both these wavelengths are invisible to the human eye.

Diode lasers

High power laser diodes offer very high electrical to optical power conversion efficiency coupled with a very compact size when compared to either Nd:YAG or CO ₂ lasers. With suitable focussing optics the assembly becomes what is known as a high power diode laser (HPDL). Today's HPDLs can be applied to a number of materials processing applications. Amongst their key features are:

• High electrical efficiency: conversion of electrical power to light is around 30%.
• Compactness: the whole laser head can be mounted easily on a robot.
• No laser gas or excitation lamps are used reducing maintenance costs.
• Long life diodes give an estimated 5000-10000 hours of operation.

For powers greater than 4W, the only commonly used manufacturing approach produces a diode laser bar about 10mm long, similar to that shown in Fig.2. The bar has many thousands of single emitters, of the order 5µm wide, producing laser output with a very large beam divergence. The resulting beam is characteristic of semiconductor lasers and, compared to other types of laser, presents a drawback in terms of focusability. The beam spreads out at up to 30° included angle in the plane perpendicular to the emitting line (known as the 'fast' axis) and about 5° along the emitting line (known as the 'slow' axis ). ^[2]

Powers up to around 50W can be achieved from one diode bar. For higher power applications the output from several diode bars is required. For the highest power applications, the light from several stacks of bars is combined using optical polarisation and wavelength coupling elements. Beam transforming optics shape the output to either a round or rectangular spot. Many different methods are used by the manufacturers of diode lasers to focus the emerging beam, and it is this aspect which usually differentiates one manufacturer's laser from another.

A 3kW 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 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 being very similar to that required for a Nd:YAG laser).

Applications

• Cutting and welding of polymers
• Conduction-limited welding of metals
• Surface transformation of metals

Polymers

The ability of low power diode lasers to process polymers was demonstrated as long ago as 1993 . ^[3] A 25W diode laser was used with optical waveguides for beam manipulation, and a three lens focussing system with a focal length of 50mm. Table 1 shows the results of cutting trials for a variety of materials.

Table 1: Low power diode laser capabilities

Since 1993, the power output of commercial diode lasers has risen significantly (up to 6kW), offering greater production speeds for both cutting and welding of polymer based materials.

The wavelength of laser light determines the way in which its energy interacts with plastics. Carbon dioxide laser light at 10.6µm wavelength is rapidly absorbed in the material surface, whereas Nd:YAG (1.06µm) and diode (0.75-0.94µm) laser wavelengths are almost totally transmitted by some non-pigmented plastics. Novel techniques have been developed using dyes and pigments, which enable plastics to be welded in 'lap' configuration by both diode and Nd:YAG lasers.

Transmission laser welding can produce an apparently invisible weld between plastics by adding an infrared absorbing pigment, eg carbon black, to one of the components. In this approach, the laser beam is transmitted through the top layer and absorbed where it meets the absorbing surface of the second component, as can be seen in Fig.3. A 500W diode laser is capable of welding 2mm thick plastic using this technique at a rate of 5m/min, for example . ^[4]

The weld produced using this method does not affect the outer surfaces of either component, so the aesthetic quality of the parent material is maintained. Another development of this method enables two transparent components to be welded together in a similar manner.

ClearWeld ^TM is a welding process which utilises infrared light absorbing material to create a clear weld in thermoplastics. The process is generally carried out using a near infrared laser (e.g. diode laser) as the energy source. This selectively heats the interface between the plastics where the IR absorbing material is present, without melting the plastic on the outer surface of the joint. The result is a discreet weld, with melting only where it is needed, giving a rapid, effective and efficient method for joining plastics in the form of moulded components, sheet, film and even woven, non-woven and coated synthetic fabrics. High strength joints may be created without weld flash, without vibration during welding and with a very neat appearance. Welding of a wide range of different polymer types and colours has been demonstrated.

The ClearWeld process was invented, and is the subject of a patent application, by TWI. It is being commercialised by Gentex Corporation. The process uses lasers that are commercially available, and infrared absorbing welding consumables that are currently under development, but the intention is to create a product line of consumables compatible with commercially available delivery systems (e.g. inkjet inks, inks for liquid dispensing systems, thin films etc).

Metals

The potential applications for HPDLs in this area are as varied as they are for the other materials processing laser types. However, the characteristics of the beam make it better suited to some applications than others. The absorption of the more common diode laser wavelengths (808 and 940nm) by metals makes these attractive for laser soldering applications. Table 2 lists comparative reflection values as a percentage of incident laser energy, for typical materials found on a printed circuit board.

Table 2: Reflectivity comparison of typical circuit board materials ^[5]

Clearly, the diode laser wavelength is reflected to a much lesser degree than the CO ₂ wavelength and marginally better than Nd:YAG laser wavelength, thereby increasing the efficiency of the process and reducing the potential for damage to adjacent components or substrate. Other materials also show a marked increase in absorption at diode wavelengths when compared to the Nd:YAG wavelength, eg both tungsten and aluminium are 10% more absorptive.

Welding

Diode laser users and researchers have reported success with a variety of material types and applications, from simple butt welds in 0.8mm thick mild steel ( Fig.4) at 1 m/min with 1.4kW, to lap welds in dissimilar materials. For example, 1.2mm thick aluminium to 0.88mm thick mild steel, at 0.6 m/min with 0.7W. ^[6]

The domestic products manufacturing sector are already using diode lasers in production for the welding of thin section stainless steel sink tops. The excellent cosmetic appearance of the weld bead produced when welding stainless steel can be seen in Fig.5.

Surface modification

The large spot sizes available with the diode laser and the good absorption of the beam in metals has awakened an interest in laser surface engineering using diode lasers. Surface hardening can be performed with a diode laser without the need for pre-treatment, the rectangular beam profile providing a uniform intensity distribution, aiding in the production of more uniform hardened areas without centre line surface melting. Table 3 illustrates the benefits of diode lasers (in terms of beam width and speed) for surface hardening when compared with a similarly powered Nd:YAG laser.

Table 3: Processing parameters to achieve 1mm deep hardening ^[7]

Medical applications

Apart from their use in the production of sterile surgical packaging, diode lasers are being used by surgeons as a replacement for traditional surgical implements and also as a replacement for Nd:YAG lasers. A typical 60W surgical laser is about the size of a briefcase, is air cooled and runs from a standard single phase wall socket. Diode lasers are now widely used in Europe, and increasingly in the USA, for surgical procedures including general surgery, ophthalmic procedures, urology, gynaecology, plastic surgery and dermatology.

Conclusions

The ability to produce high quality welds in very thin materials, using both continuous wave and pulsed output, gives the diode laser potential to become an important tool in the production of ever smaller components for the burgeoning miniature/micro sensors and transducer markets. Continued development of beam shaping optics will also increase their versatility. Zoom type processing optics are already available which can vary spot size from 6-22mm in one axis, whilst maintaining a constant at 2.6mm width in the other axis. The resulting line shaped output provides a homogeneous beam highly suited to wide strip surface treatment operations.

The efficiency and compact size of the diode laser will contribute to its further application, particularly in the field of plastics joining and conduction welding of metals.

The worldwide production of laser diodes for all applications is increasing almost twofold each year. The net effect is a reduction in purchase price. Predictions on market growth for diode lasers for materials processing applications (wavelengths in the range 750-980nm, with output powers above 10W) show that total unit sales are set to rise from 2367 in 1999 to nearly 5500 for 2000. We will therefore see high power diode lasers becoming a more cost effective and commonly used tool for industry.

A 3mm deep penetration weld in stainless steel has been demonstrated using the diode laser. The CO ₂ laser first demonstrated a deep penetration weld, on similar thickness materials in 1970. It is interesting to wonder what the capabilities of the diode laser will be thirty years from now.

References