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Lasers have been employed in a variety of welding applications for many years. And, as laser technology further develops and diversifies, its uses in welding continue to expand. This article by Coherent provides an overview of high power lasers in keyhole welding.

Most traditional (non-laser) welding techniques currently in use are variations of arc welding. In these methods, two pieces of metal are first brought into contact or close physical proximity. The edges of the pieces may have been shaped to facilitate their joining. A high voltage is established between an electrode and the contact region, creating an arc which melts the material (or, in some cases an additional filler material or the electrode itself). The melted material fills any gap between the workpieces, or overlays them, and then solidifies to join the parts.

The primary advantage of most arc welding methods is their relatively low cost, particularly in terms of the capital equipment expenditure. Furthermore, arc welding techniques are well understood and widely employed, and standards for producing and testing them are well established, so there’s not much of a learning curve in bringing these processes on line.

The major disadvantages of arc welding mostly derive from subjecting parts to high heat. This can result in microstructures in the melted material that yield poor strength in the weld joint, and a relatively large heat affected zone in the material adjacent to the weld. Additionally, the parameters of the arc are influenced by the local electric field, and can therefore not be set independently.

Laser Keyhole Welding

Most laser welding techniques can be classified into two basic categories, “keyhole” and “conduction mode” welding. Both of these welding modes are capable of being performed autogenously, that is, without filler metal, as well as with filler, if so desired.

Keyhole, or deep penetration welding, is commonly encountered when welding thicker materials at high laser powers. In keyhole welding, the laser is focused so as to achieve a very high power density at the work piece. At the focus of the laser beam, the metal actually vaporises, opening up a blind hole (the keyhole) within the molten metal pool. Vapour pressure holds back the surrounding molten metal and keeps this hole open during the process. The laser power is mainly absorbed at the vapour melt boundary and the keyhole walls. The focused laser beam and the keyhole continuously move along the welding path. At the front of the keyhole, new material is molten, and at the back, it resolidifies to become the welded joint.

The small size of the keyhole region results in a precise, narrow fusion zone, with a high aspect ratio (depth to width) as compared to arc welding methods. Furthermore, the highly localised application of heat means that bulk of the work piece acts as an effective heat sink so the weld region heats up and cools down rapidly. This minimises the size of the heat affected zone, and reduces grain growth. Thus, the laser can generally produce stronger joints than arc welding, which is one of its primary benefits.

Laser welding also offers greater flexibility than arc welding, since it is compatible with an extremely broad range of materials, including carbon steel, high strength steel, stainless steel, titanium, aluminium, and precious metals. It can also be used to join dissimilar materials, as differences in material melting temperatures and heat conduction are of minor importance in the process.

In addition, laser welding delivers significant cost advantages over traditional methods, when all the process steps are considered. In particular, the precise application of heat minimises distortion in the weld and overall part, thus eliminating the need for post processing in many cases. Plus, the ability to project the laser beam over relatively long distances with essentially no power loss makes it easy to integrate laser welding with other production processes, and lends itself well to integration with manufacturing robotics. Last, but not least, new product configurations with reduced flange sizes can be realized, which is critical for light weight vehicles in the automotive industry.

Fibre Lasers for Welding

Modern CO2 and fibre lasers easily deliver the beam parameters and power requirements for keyhole welding. Since almost all metals become increasingly absorptive at shorter wavelengths, process efficiency is enhanced at the shorter fibre laser wavelength of ~1 μm, as compared to CO2 laser wavelength of 10.6 μm.

Fibre lasers, in particular, match the requirements of keyhole welding extremely well. They typically offer output powers in the range of 500 W to 10 kW, and can readily achieve focused spot sizes in the necessary range between 40 μm and 800 μm, even at relatively large working distances. From a practical standpoint, the use of beam delivery fibre expands integration options and facilitates the use of the laser in the manufacturing environment. Finally, the high reliability, excellent uptime and favourable cost of ownership characteristics of fiber lasers make them an economically viable and attractive choice for production welding applications.

There are currently several manufacturers of high power fibre lasers for welding and other materials processing applications. Coherent | Rofin fibre lasers, for example, can deliver a combination of performance, reliability, ease of integration and cost characteristics that is optimum for welding and other materials processing applications. To understand how this is achieved, it’s useful to examine some of the design and construction details of these lasers.

The drawing shows the main elements of the fibre laser oscillator employed by Coherent | Rofin. The laser resonator is formed by a large mode area (LMA), Yb-doped, double clad optical fibre and fibre Bragg gratings for resonator mirrors. This is pumped from each end by a series of diode laser pump modules, whose outputs are fibre coupled into the gain fibre.

Coherent | Rofin fibre laser oscillator schematic, including 12 diode laser pump modules, and 6×1 fibre coupling modules which inject pump light into the gain fibre, and allow efficient extraction of the laser output.

Based on this design, one set of pumps and gain fibre can produce output powers of up to 3kW. The output from up to four of these single mode fibre laser units can then be combined into one multimode fibre to achieve powers of up to 10kW. Alternately, the “standard” cabinet supports splitting the output from a single fibre laser into four separate fibres through the use of the integrated fibre-to-fibre switches.

Thus, this modular construction approach allows Coherent | Rofin to offer several options in terms of output power, delivery fibre diameter, and beam parameter product. The benefit is the ability to readily adapt the laser beam characteristics to precisely match the exact requirements of a specific process.

Some users have experienced fibre laser damage or process inconsistencies caused by back reflections when processing highly reflective metals, such as copper and brass. Coherent | Rofin lasers utilise an optimised power generation and delivery technology, as well as sensors at different positions within the system, to protect laser components from such damage. These safeguards eliminate the problem of back reflections, and allow reliable welding of brass, aluminium and copper without any concern for damaging the laser.

Of course, the fibre laser is just one part of the entire welding system, which also includes a beam focusing welding head, as well as control electronics. In addition to fibre lasers, Coherent | Rofin also offers beam delivery components which mount into customers’ machines. These can be fixed optics or complete, integrated scanning solutions, which include control of all relevant laser parameters, to fully optimize the welding process. Moreover, these integrated solutions often feature fast and flexible beam scanning technology which allows rapid beam movement from one welding contour to the next. This increases the productivity of a laser processing system enormously.

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