Laser Welding

LASER WELDING

Aluminium

The advent of higher laser powers, improved beam focusing systems and better beam quality has led to power densities sufficient to overcome the high surface reflectivity of aluminium. Some alloys are prone to cracking, but optimisation of the welding conditions and use of filler wire can eliminate this problem. Wire feed is also used for improving weld metal properties and tolerance to joint fit-up.

5000 series alloys - good weldability with or without filler wire.

6000 series alloys - good weldability with or without filler wire.

2000 series alloys - parameter development is continuing.

7000 series alloys - parameter development is continuing.

Porosity can also occur when laser welding aluminium, predominantly due to hydrogen entrapment in the molten pool. However, this can be minimised by correct cleaning and adequate shielding during welding.

Applications for Aluminium

Current and future industrial applications of aluminium laser welding includes fast ferries (catamarans), aluminium car components (e.g. Audi A2) and airframe structures.

Benefits of Laser Welding

High processing speed

Deep penetration welds

Low heat input, minimal distortion

Readily automated

Typical ParametersThickness

mm CO 2 laser Solid state laser

Power

kW Speed

m/min Power

kW Speed

m/min

2 5 6 2 1

2 4 6

6 5 1 4 1.0

Titanium

Titanium is used in many industrial sectors for its light weight and high strength (aerospace sector, sports, oil and gas), excellent corrosion properties (chemical industry, oil and gas) and bio-compatibility (medical).

Weld Quality

Titanium is highly reactive with both oxygen and nitrogen. To ensure welds are of good quality it is essential to prevent absorption of gases into the molten pool, as this leads to poor ductility and porosity. This requires: correct choice of shielding gas

adequate shielding methods

pre-cleaning (de-greasing)

good joint surface quality

Titanium and its alloys are readily welded with lasers observing these precautions. In practice, a specially prepared trailing shoe delivering inert gas is used to prevent oxidation of the solidifying and cooling weld metal.

Benefits of Laser WeldingHigh productivity (nearly 10 times faster than TIG).

Low heat input and therefore low distortion.

Ease of automation for repeatability.

No need for filler wire, thus reducing costs.

Nd:YAG laser weld in

6 mm thick Ti-6Al-4V

Typical Parameters for Ti-6Al-4VThickness

mm Laser Power

kW Speed

m/min

4 CO 2 5 2.0

8 Fibre 7 1.5

6 Nd:YAG 4 1.0

3 Nd:YAG 4 2.5

Thick Sections

A laser beam can be easily focused to very high power densities, ca 10 6 W/cm 2 . At these intensities, when laser welding metal, a column of ionised metal vapour which forms below the beam impingement point, absorbs the incoming laser energy, producing narrow, deep weld profiles. This 'keyhole' welding process is more efficient than a process where the weld shape is governed by thermal conduction. CO 2 , Nd:YAG and fibre lasers can achieve such power densities, with the most powerful lasers capable of welding 25mm C-Mn steel in a single pass.

Benefits and Application of Laser WeldingHeavy construction, shipbuilding, oil and gas, power, process plant and aerospace industry sectors have the need for welding thick sections and are potential markets for laser welding.

There are many benefits :

Deep narrow welds

Low heat input

Minimal distortion

High joint completion rates

Joint design flexibility

Minimal use of consumable

Ease of automation

TWI is working on welding parameter and process development of CO 2 , Nd:YAG and fibre lasers at powers up to 7kW. At these powers welding of thick section material is possible at high speeds, for example 8mm thick C/Mn steel can be welded in a single pass at speeds up to 1.5m/min.

Thin Sections

A laser beam can be focused to a very small spot diameter, creating a powerful, precise heat source suitable for welding. These high power densities mean that laser welding is generally fast with minimal amounts of heat and distortion. For example, welding speeds of over 10m/min are achievable in steel 1mm thick. Moreover, virtually any weldable material may be processed, making laser welding an ideal tool for high volume manufacturing. Lasers can be used for continuous welding by manipulation of either the laser or the component. The technique is applicable to 2D and 3D components, rotary welding, and even spot welding, where completion rates of 120,000 welds per hour are achieved in some industrial applications.

Benefits and Applications of Laser Welding

Lasers are ideal for high volume manufacturing as they have high welding speeds and level of automation allowing 24 hours a day operation. A laser welding cell can be very cost effective for an appropriate application. For example, a cell working a two shift system could cost around £60 per hour, including capital investment and running costs. At a welding speed of 10m/min this would be approximately £0.15 per metre of weld. Other benefits include low distortion, high accuracy and aesthetically appealing welds.Lasers are capable of welding:

C-Mn steels

Stainless steels

Aluminium alloys

Nickel alloys

Titanium alloys

Plastics

Tailored Blanks

Joining materials of different thickness, strength or coating type can produce a 'tailored blank' ready for pressing. Lasers are ideal for this application as they both cut individual blanks and weld the component parts to form a finished tailored blank.

Advantages of Laser Welding

High processing speed for high productivity

Flexibility gives multi-tasking capability

Low heat input gives minimal distortion

Ideal for automation

Benefits of Tailored Blanks

Cost saving.

Weight reduction.

Increased material utilisation.

Improved corrosion resistance.

Less press tooling required.

Tailored blanks are of prime importance in weight and cost savings for automotive body and structural components. The majority of car manufacturers utilise them and annual production is approximately 30 million and growing rapidly.

Laser welding has many benefits for tailored blank production including non-linear welds, welding of dissimilar thicknesses, the ability to weld aluminium tailored blanks, narrow weld and HAZ areas and proven industrial technology.

Laser welding of plastics

Introduction

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).>

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 processingLaser 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.

Clearweld ®: Upper left sample shows conventional transmission laser welding with carbon black as the laser absorbing material. Lower right sample shows the use of a novel infrared absorbing medium to create a low visibility joint

Cut/seal

In the cut/seal process, careful control of the laser beam profile allows film to be both welded and cut in a simultaneous operation. This development is of particular use in the production of packaging items and plastic bags.

Fibre lasers

A recent advancement in laser welding is the development of fibre lasers. These operate at a wavelength of between 1 and 2 µm and offer improved beam quailty compared with diode and Nd:YAG lasers. Their use in plastics welding is under evaluation.

Applications

Laser welding has proved to be especially effective in the welding of thermoplastic films in a lap joint configuration. The speeds attainable with laser welding make it especially suitable for use in the packaging industry, whilst biomedical applications exploit the cleanliness of the process. The Clearweld ® process extends its applicability to circumstances where final appearance is important. Applications in the areas of food packaging, medical devices and packaging, electronic displays and fabrics are being developed.

Infrared absorbing media can also be used to weld fabrics. These are samples of welds in waterproof laminated fabric

Automation

Laser welding is generally carried out under CNC control. The system must maintain beam-to-joint alignment and stand-off distance to within less than 1mm. The processing head for CO 2 lasers is often mounted on a 3-axis gantry. Alternatively, the workpiece may be moved on a table or traverse.

The wavelength of Nd:YAG and fibre laser light allows it to pass along an optical fibre, increasing its flexibility and making it easy to use the processing head with a multi-axis robot.

Good component fit-up is essential for successful automation, requiring control over the accuracy and repeatability of the components to be welded. Seam tracking and adaptive control of the process can benefit when welding large or complex structures.

Benefits of Automated Processing

Automation yields fast, accurate and repeatable production. In many cases, CAD-drawings can be used, simplifying programming of the machine. Robot mounted processing heads make manufacture of 3-dimensional products relatively easy. The introduction of easy-to-use adaptive control systems will markedly increase the practical application of laser welding in industry.