Nd:YAG laser welding

by Paul Hilton

The Nd:YAG laser is one of the most versatile laser sources used in materials processing. The relative robustness and compactness of the laser and the possibility for the 1.06 micron light it produces to be transmitted to the workpiece via silica optical fibres, are two features which contribute to its success. Nd:YAG lasers were first commercialised operating mainly in pulsed mode, where the high peak powers which can be generated were found useful in applications such as drilling, cutting and marking. These pulsed lasers can also be utilised for welding a range of materials. More recently, high power (up to 10kW), continuous wave (CW) Nd:YAG lasers have become available. The Nd:YAG crystals in these lasers can be pumped either using white light flashlamps or, more efficiently, using laser diodes. The latter methods are used to produce high quality beams, which can be focused to smaller spots (and therefore produce higher power densities) than the flashlamp pumped lasers. Because of the possibility of using fibre optic beam delivery, these lasers are often used in conjunction with articulated arm robots, in order to work on components of complex shape.

Because of the wide range of applied power and power densities available from Nd:YAG lasers, different welding methods are possible. If the laser is in pulsed mode, and if the surface temperature is below the boiling point, heat transport is predominantly by conduction and a conduction limited weld is produced. If the applied power is higher (for a given speed), boiling begins in the weld pool and a deep penetration weld can be formed. After the pulse, the material flows back into the cavity and solidifies. Both these methods can be used to produce spot welds. A seam weld is produced by a sequence of overlapping deep penetration 'spot' welds or by the formation of a continuous molten weld pool. For the former, once the energy input is sufficient to ensure that the weld does not solidify between pulses, the 'keyhole' type weld normally associated with CO ₂ laser welding can be formed. Pulsed laser welding is normally used at thicknesses below about 3mm. Higher power 4-10kW CW Nd:YAG lasers are capable of keyhole type welding in materials from 0.8mm (car body steel) to 15mm (ship steel) thickness.

Nd:YAG laser welding is used commercially on a wide range of C-Mn steels, coated steels, stainless steels, aluminium alloys, titanium and molybdenum. The low heat input welding offered by Nd:YAG lasers is utilised in the electronics, packaging, domestic goods and automotive sectors, and significant interest has been shown more recently, particularly for the high power CW lasers, in the shipbuilding, oil and gas, aerospace and yellow goods sectors. Important R&D issues involve development of high power lasers of better beam quality, use of distributed energy in the beam focus, weld quality maintenance for both thick and thin sections and weld classification.

The principal risks involved in Nd:YAG laser welding are: optical (the beam can burn the skin or damage the retina if focused by the eye), electrical, and fume generation. A current application issue is safe use of Nd:YAG lasers in anything other than a fully opaque (to the Nd:YAG laser wavelength) enclosure, such as might be found in a shipyard for example. (twi.org)

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Use of excimer lasers for materials processing

by Dave Taylor

Excimer lasers are characterised by short wavelengths, high intensities and short pulse durations. These characteristics mean that a single photon is capable of breaking a chemical bond. The majority of laser materials processing techniques are essentially thermal processes in which absorption of a large number of photons heats the material to enable cutting, welding or surface modification operations to be performed.

Excimer lasers were first demonstrated in 1975, some time after many of the other laser sources, and are now fairly well established in their niche applications. The term 'excimer' stands for 'excited dimer', where 'dimer' refers to a diatomic molecule such as O ₂ or N ₂ . This is not strictly a correct term, as the two atoms that make up the molecules used in excimer lasers can be different. The most important molecules are rare gas halides such as F ₂ , ArF, KrCl, KrF, XeCl and XeF. These do not exist in nature but can be produced by passing an electrical discharge through a suitable gas mixture. This means that excimer lasers generate ultraviolet energy over a range of wavelengths, depending on the gas mixture used (e.g. 157nm for F ₂ and 351nm for XeF).

Typical average output powers are in the range from less than 1 watt up to around 200 watts. This is two orders of magnitude less than the more traditional Nd:YAG or CO ₂ lasers which operate in the infrared part of the spectrum. The high intensity beam of an excimer laser is the product of pulse energy (10 - 1000mJ), spot size (governed by focusing optics) and pulse duration (around 10ns).

Applications of excimer laser are primarily in machining of materials such as plastics, paper, ceramics, glasses, crystals, composites and biological tissue. When illuminated with an excimer laser, the relatively weak organic bonds are broken down. This creates a pressure rise and subsequent shock wave that removes material, with little heat transfer to the surrounding material, in a process called 'ablation'. This processing is usually most efficient when carried out using a mask with an image of the required feature. Excimer laser machining is used for its precision, producing features down to approximately 40µm in resolution, but with virtually no heat affected zone.

Excimer laser processing using step-and-repeat mask projection

Some research has been carried out in welding and cutting of sheet metals, but showed no significant advantages over CO ₂ or Nd:YAG laser which are available in much higher average powers. Excimer lasers do also lend themselves to more 'niche' applications, such as surface modification of metals and glass for strengthening adhesive bonding and smoothing of machined surfaces to increase wear resistance of components such as camshafts and pistons. As well as increasing wear resistance, this process is being studied as a means of increasing corrosion resistance through production of a thin, amorphous layer on the surface of the material.

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Diode lasers

by Paul Hilton

High power diode lasers (known as HPDL's) feature a very high electrical to optical power conversion efficiency coupled with a very compact size. With suitable 'focusing' optics, today's HPDL's are suitable for some materials processing applications. The laser diodes which drive the HPDL's are also being used to replace flashlamp pumping in solid state lasers. Diode lasers also exhibit very high wall plug efficiencies which can be greater than 30% on commercially available systems.

Diode lasers consist of a p-n 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.1). 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° perpendicular to the emitting line (known as the 'fast' axis) and about 10° along the emitting line (known as the slow axis).

Fig. 1 Laser diode schematic

Powers of the order 80W and higher 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) or InGaAlAs on a GaAs substrate (808nm). 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 manipulation to achieve the required power densities for material processing applications. It would appear that this is the area in which one 'diode laser' supplier may be distinguished from another.

A 3kW (highest currently available commercially is 6kW) 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 say 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 two cases. Two of these design configurations, suitable for material processing applications are described below. Lenses for beam shaping with diode lasers are usually manufactured from glass or fused silicon.

Individual Beam Shaping (IBS) Diode Laser

This system uses sophisticated optics to combine the beam from three individual diode bars mounted as can be seen in Fig.2a. In addition, special diodes are used where the emitting zone is confined to 5 areas 500µm wide with a centre to centre spacing of 1.5mm. This design is the basis of improved beam quality which permits the generation of focused spots about 0.25 x 0.6mm ². With its output power of about 150W, the power density ~10 ⁵W/cm ², is sufficient for conduction welding of metals.

Fig. 2a IBS diode laser schematic

Optical Fibre Delivered Diode Laser

This laser also uses a complex optical system designed to minimise the spot size from a single bar so that the beam can be launched down a silica optical fibre. After fast axis collimation with a micro lens, the slow axis is chopped into small beamlets by a special diamond machined mirror. A set of prisms then compresses these beamlets together before collimation with a cylindrical lens and final focusing via spherical optics. Fig.2b, shows how the combination of diamond machined mirror, prisms and lenses, produces a 0.8mm diameter beam for launching into the fibre. Using this configuration, a 35W single bar device can produce a power density of about 7 x 10 ³ W/cm ².

Fig. 2b Fibre delivered diode laser schematic

New Developments

Diode laser technology continues to develop at a fast pace. Its limitations continue, however, to be available spot size. Much effort has gone into this and most of the higher power (1kW+) diode lasers now use the technique of wavelength coupling in order to maximise power in a small spot.

Copyright ©2004 TWI Ltd

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Carbon dioxide laser

by Paul Hilton

The carbon dioxide (CO₂ ) gas laser, is one of the most versatile for materials processing applications, and emits infra red radiation with a wavelength between 9 and 11µm, although emission at 10.6µm is the most widely used. Of the several types of CO₂ laser that are available, the waveguide, the low power sealed tube and the transversely excited atmospheric (TEA) lasers are used for small scale materials processing applications. The fast axial flow CO₂ laser and the less widely used slow flow laser, are used for thick section cutting 1-15mm and deep penetration welding. While these lasers share the same active medium, they have important functional characteristics, which contribute to the wide range of CW (continuous wave) powers, pulse powers and pulse durations available from the CO₂ laser.

The active medium in a CO₂ laser is a mixture of carbon dioxide, nitrogen and (generally) helium. It is the carbon dioxide which produces the laser light, while the nitrogen molecules help excite the CO₂ molecules and increase the efficiency of the light generation processes. The helium plays a dual role in assisting heat transfer from the gas caused by the electric discharge used to excite the gas, and also helps the CO₂ molecules to return to the ground state.

Sealed Tube CO₂ Lasers

These lasers are operated as conventional gas discharge lasers in the form of long narrow glass tubes, filled with the lasing gas mixture. Electrodes at either end of the tube provide the discharge current. A totally reflecting and partially transmitting mirror, usually made from polished metal and coated zinc selenide respectively, form the resonant cavity. The tube is sealed using Brewster angled windows. Fig.1, shows a schematic drawing of a sealed tube CO₂ laser. As the electric discharge in the tube breaks down the CO₂ , an ordinary gas mixture would stop working very quickly and so methods are provided to cause the CO₂ to regenerate, either by addition of hydrogen or water or by the use of catalytic action. Several thousand hours of operation are possible with sealed tube CO₂ lasers before the tube has to be cleaned and re-filled or replaced. DC and sometimes RF discharges are used with these lasers. CW power up to about 200W is available from these lasers with good beam quality. Pulsed power supplies can produce laser pulses lasting 0.1 - 1msecs with peak powers 5-10 times the CW power level.

Fig. 1 Sealed tube CO 2 laser schematic

Waveguide CO₂ Lasers

The waveguide laser is an efficient way to produce a compact CO₂ laser. It consists of (see Fig.2), two transverse RF electrodes separated by insulating sections that form a bore region. The lateral dimensions of the bore are a few millimetres, which propagates the beam in 'waveguide mode'. The tube is normally sealed with a gas reservoir separate from the tube itself. The small bore allows high pressure operation and provides rapid heat removal; both of which lead to high gain and high power output from a compact unit.

Fig. 2 Waveguide CO 2 laser schematic

TEA CO₂ Lasers

Discharge instabilities prevent operation of CW CO₂ lasers at pressures above about 100mbar. Pulses in the nanosecond to microsecond duration range can be produced by passing a pulsed current transversely through the lasing gas. Such TEA (transversely excited atmospheric) lasers operate at gas pressures of one atmosphere and above in order to obtain high energy output per unit volume of gas. A transverse discharge from two long electrodes is employed (see Fig.3). Prior to application of the pulsed discharge, a form of pre-ionisation is used to ionise the space between the electrodes uniformly, thus allowing the discharge to proceed in a uniform fashion over the entire electrode assembly. The prime attractions of TEA lasers are their ability to generate short intense pulses and the extraction of high power per unit volume of laser gas. Pulse duration as low as a few tens of nanoseconds up to a few microseconds are possible. Pulse energies range from the millijoule region to 500Joules at pulse repetition rates from about 300Hz down to single shot.

Fig. 3 TEA CO 2 laser schematic

Optics for CO₂ Lasers

Reflective mirrors	- silicon with high reflectivity coatings, gold coated copper.
Lenses and windows	- gallium arsenide and germanium (not transparent in visible region) and coated zinc selenide (orange in the visible region).
Wallplug Efficiency	between 5% and 20%

Beam Diameter and Divergence

The shape and length of the laser cavity and nature of the resonator optics determine the beam diameter and divergence of the CO₂ laser. Typical ranges are:

	beam diameter (mm)	beam divergence (mrads)
Sealed tube:	1 - 7	2 - 6
Waveguide:	1 - 2	3 - 10
TEA:	4 - 12	0.5 - 3

Copyright TWI Ltd, 2000

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Laser Welding

Plastics are laser-welded by passing laser light through a (laser transparent) top part onto a (laser absorbent) bottom part. The absorbed laser energy softens and melts both parts. With externally applied clamping pressure, the parts are bonded upon cooling. Typically, diode lasers having a wavelength in the (infra-red) range of 800nm-1000nm are used in this process.

Advantages of laser welding:

• Joint design need only be surface to surface. There is no need for energy directors or collapse of the weld joint.
• Weld lines can be as narrow as 0.1mm (0.004 in.)
• Good welds can be achieved, even to a hermetic seal. Tensile strength is that of the unreinforced base resin
• There is no relative motion between the parts as happens with vibration welding. There are no vibrations that could damage electronic components
• Three-dimensional geometries can be welded
• There is no part marking or bleed through
• The joint has less flash than with other methods

The four main laser welding methods:

• Spot laser welding
• Line laser welding
• Mask laser welding
• Simultaneous through welding

In spot welding, a circular spot of laser energy traverses a pre-programmed contour path. The simultaneous line method creates a laser line for welding, while the mask method blocks the laser line in a predefined pattern. Simultaneous through welding delivers laser energy to the entire surface via a fiber optic head and typically runs a three to five second cycle. (ticona.com)

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Laser welding of plastics

Laser welding was first demonstrated on thermoplastics in the 1970's, but has only recently found a place in industrial scale situations. The technique, suitable for joining both sheet film and moulded thermoplastics, uses a laser beam to melt the plastic in the joint region. The laser generates an intense beam of radiation (usually in the infra red area of the electromagnetic spectrum) which is focussed onto the material to be joined. This excites a resonant frequency in the molecule, resulting in heating of the surrounding material. Two forms of laser welding exist; CO ₂ laser welding and transmission laser welding. CO ₂ laser radiation is readily absorbed by plastics, allowing quick joints to be made, but limiting the depth of penetration of the beam, restricting the technique to film applications. The radiation produced by Nd:YAG and diode lasers is less readily absorbed by plastics, but these lasers are suitable for performing transmission laser welding. In this operation, it is necessary for one of the plastics to be transmissive to laser light and the other to absorb the laser energy, to ensure that the beam focuses on the joint region. Alternatively, an opaque surface coating may be applied at the joint, to weld two transmissive plastics. Transmission laser welding is capable of welding thicker parts than CO ₂ welding, and since the heat affected zone is confined to the joint region no marking of the outer surfaces occurs.

The technique

CO ₂ laser welding

The CO ₂ laser is a well established materials processing tool, available in power outputs of up to 60kW, and most commonly used for metal cutting. The CO ₂ laser radiation (10.6µm) is rapidly absorbed in the surface layers of plastics. Absorption at these photon energies (0.12eV) is based on the vibration of molecular bonds. The plastics will heat up if the laser excites a resonant frequency in the molecule. In practice the absorption coefficients for the CO ₂ laser with most plastics is very high. Very rapid processing of thin plastic film is therefore possible, even with fairly modest laser powers (<1000w).> 2 laser beam cannot be transmitted down a silica fibre optic, but can be manipulated around a complex process path using mirrors and either gantry or robotic movement.

A CO 2 laser weld in 100µm polyethylene film at 100m/min with 100W laser power

Transmission laser welding - Nd:YAG laser

The Nd:YAG laser is also well established for material processing, and recent developments have led to increases in the power available to above 6kW and reduced the physical size of the laser. In general, the light from Nd:YAG lasers is absorbed far less readily in unpigmented plastics than CO ₂ laser light. The degree of energy absorption at the Nd:YAG laser wavelength (1.064µm, 1.2eV photon energy) depends largely on the presence of additives in the plastics. If no fillers or pigments are present in the plastics, the laser will penetrate a few millimetres into the material. The absorption coefficient can be increased by means of additives such as pigments or fillers, which absorb and resonate directly at this photon energy or scatter the radiation for more effective bulk absorption. The Nd:YAG laser may therefore be used for heating plastics to depths of a few millimetres or for heating a more highly absorbent medium (either metal or a plastic containing suitable additives) through or within the transmissive plastic part. The Nd:YAG laser beam can be transmitted down a silica fibre optic enabling easy flexible operation with gantry or robot manipulation.

Transmission laser welding - Diode laser

High power diode lasers (>100W) have been available since early 1997. They are now available up to 6kW and are competitively priced compared to CO ₂ and Nd:YAG lasers. The production of the diode laser light is a far more energy efficient process (30%) than CO ₂ (10%) or Nd:YAG (3%) lasers. The interaction with plastics is very similar to that of the Nd:YAG lasers, and applications overlap. The beam from a diode laser is typically rectangular in shape, which, while being preferential for some applications, limits the minimum spot size and maximum power density available. The diode laser source is small and light enough to be mounted on a gantry or robot for complex processing.

Diagram of transmission laser welding

Comparison of commercially available laser sources for plastics processing

Laser Type	CO 2	Nd:YAG	Diode
Wavelength (µm)	10.6	1.06	0.8-1.0
Max. power (W)	60,000	6,000	6,000
Efficiency	10%	3%	30%
Beam Transmission	Reflection off mirrors	Fibre optic and mirrors	Fibre optic and mirrors
Minimum spot size * (mm)	0.2-0.7 diam.	0.1-0.5 diam.	0.5x0.5
Capital Cost * (£k)	100W: £20k 1000W: £50k	100W: £40k 1000W: £80k	100W: £15-20k 1000W: £80-100k
Running Cost * (£/hr)	100W: £0.2-0.5 1000W: £2-4	100W: £0.1 1000W: £3-5	100W: £0.1-0.2 1000W: £1-2
Interaction with Plastics	Complete absorption at surface in <0.5mm	Transmission and bulk heating for 0.1-10mm	Transmission and bulk heating for 0.1-10mm

* Approximate figures for general case. Other equipment variants exist with different properties.

Scope

Laser welding is a high volume production process with the advantage of creating no vibrations and generating minimum weld flash. The technique relies on the initial outlay for a laser system, however, the benefits of a laser system include; a controllable beam power, reducing the risk of distortion or damage to components; precise focussing of the laser beam allowing accurate joints to be formed; and a non contact process which is both clean and hygienic. Laser welding may be performed in a single-shot or continuous manner, but the materials to be joined require clamping. Weld speeds depend on polymer absorption. It is possible to create joints in plastics over 1mm thick (with transmission laser welding) at up to at least 20m/min whilst rates of up to 750 m/min are achievable in the CO ₂ laser welding of films.

Adaptations of laser welding

Clearweld ^®

The Clearweld ^® process was invented, and has been patented, by TWI. It is being commercialised by Gentex Corporation. The process uses commercially available lasers in conjunction with infrared absorbing welding consumables.

The carbon black absorber traditionally used is replaced by a colourless, infrared absorbing medium thus expanding the applicability of the technique to clear plastics. The infrared absorbing medium is either printed/painted onto one surface of the joint, encompassed into the bulk plastic, or produced in the form of a film that can be inserted into the joint. It absorbs infra-red laser light allowing an almost invisible weld to be produced between materials that are required to be clear or have a predetermined colour. The process is especially suitable where the appearance of a product is important. In the case of fabrics joining, positioning of the infrared absorbing medium at the joint restricts melting to the interface rather than through the full thickness of the joint as occurs in other welding methods for fabrics. Consequently, flexible seams are produced making the process suitable for the joining of fabrics for clothing applications.

Additional information can be found on the Clearweld ^® website - www.clearweld.com.

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