An introduction to laser welding
Laser welding, also commonly known as laser beam welding (LBW), is the process by which materials such as metals and thermoplastics are fused together using either a continuous-wave or pulsed beam of amplified light emitted from a laser source.
The light is delivered to a workpiece through a system of glass lenses, mirrors and often a fibre optic cable.
The process has gained significant popularity over the years, developing a reputation for being a reliable joining technology with exceptional quality, precision, speed, flexibility and versatility.
This makes it an excellent choice for performing the standard lap, fillet, and butt welds commonly used in industries such as automotive, aerospace and shipbuilding. Laser welding is also well-suited to being automated on production lines, being easily integratable with a wide range of robotic technologies in remote welding applications. This has been particularly relevant in recent years with the gradual transition into the fourth industrial revolution: Industry 4.0.
Types of laser welding
Most laser welding techniques can be classified into two categories: heat conduction welding and keyhole welding.
In heat conduction welding, the energy is coupled into the workpiece solely through heat conduction. The material surface is heated above its melting point, but not so much to vaporise it, meaning the laser beam is absorbed only at the surface of the material rather than penetrating it. Such welds therefore typically exhibit a high width-to-depth ratio, with the depth being controllable by varying the duration of the pulse – heat conducts further down into the part the longer the pulse. Such welds are typically performed at energy densities of the order of 105W/cm2,using laser powers in the order of hundreds of watts. Due to the relatively low powers involved, weld depth typically ranges from only a few tenths of a millimetre to one millimetre. Heat conduction welding is generally used in applications where a particularly aesthetic weld is required or when particulates may be a cause for concern, for example in battery sealing.
Laser welding poses a number of advantages over conventional arc welding technologies. (Image: Shutterstock/Dizfoto)
Keyhole welding on the other hand is used where deeper, stronger welds are required. In this process, the laser beam heats up the material to the point where it vaporises, penetrating deep into the metal. This creates a cavity known as a keyhole, filled with either expanding vapour or plasma that prevents the collapse of the cavity walls, with temperatures rising well above 10,000K. Welding is achieved by traversing the keyhole along the joint to be welded, or moving the joint with respect to the laser beam. Surface tension then causes some of the molten material at the leading edge of the keyhole to flow around the cavity to the back, which then cools and solidifies to form the weld. Keyhole welding results in welds with a high depth-to-width ratio, reaching depths from millimetres to tens of millimetres in thickness. Such welds are typically performed at energy densities of the order of 106-107W/cm2 using laser powers typically in the order of kilowatts.
Benefits over alternative welding methods
Laser welding poses a number of advantages over conventional arc-based methods such as MIG,TIG and MAG welding. Such processes input large amounts of heat into the weld seams, which can lead to defects such as distortion and twisting, meaning post-processing techniques like grinding are required to achieve a more aesthetically pleasing weld. This can be very time consuming. Comparatively, laser welding delivers lower heat input, resulting in better looking welds with high repeatability, removing any need for post processing and thus improving the time efficiency of the whole process.
The controlled heat input of laser welding also makes it suitable for joining thin materials together – a task not typically achievable using arc-based welding methods. Arc welding is also somewhat restricted in the types of joint it can make, whereas laser welding can be used to perform a wide range of welds over complex geometries due to its light often being deliverable via a fibre optic cable, enabling it to access harder-to-reach areas of a workpiece. This ability to deliver the beam via a fibre optic cable is what also makes certain laser welding technologies easily integratable with robotics technologies.
Electron beam welding is an alternative remote joining method that involves an accelerated beam of electrons that can be magnetically deflected and focused to the workpiece. While the technique is generally suited to being able to perform thicker welds than laser-based methods, to prevent the electrons scattering or reacting with gas molecules, it must take place in a vacuum chamber. In addition to therefore having to wait for the chamber to be pumped down before welding can begin, this also limits the size of the parts that can be welded to the size of the vacuum chamber. Laser welding faces no such size limitations, however due to it taking place outside a vacuum chamber, a shielding gas is required to prevent the weld reacting with gases in the atmosphere.
Shielding gas required
In order to perform laser welding, shielding gas must also be delivered to the workpiece alongside the beam. This gas serves multiple purposes: it helps maintain a stable process and stable weld pool; keeps the weld material from reacting with the oxygen, nitrogen and hydrogen in the atmosphere (preventing weaker welds filled with defects such as voids); and minimises the formation of plasma above the weld, which would otherwise partially block and/or distort the incoming laser beam.
In general, the type of shielding gas used can influence the speed, microstructure, and overall shape of the weld. Shielding gases typically used are helium, carbon dioxide, nitrogen and argon, each with varying degrees of cost, plasma suppression, and oxidation prevention. Certain gases work better for certain materials. For example, nitrogen is known to react strongly with titanium (or austenitic stainless steels alloyed with titanium) to form titanium nitride compounds, which can lead to the resulting weld being brittle. Argon is therefore a suitable alternative for welding titanium-based alloys.
Using nitrogen during the welding of ferritic steels also has detrimental effects, leading to an increased quantity of martensite in the weld metal. This, in turn, can make the weld more brittle and more susceptible to hydrogen embrittlement.
Certain gases also work better for certain laser wavelengths. For example, helium is a suitable shield gas for CO2 laser welding due to the excellent plasma suppression it offers. The formation of plasma is more critical when welding with a CO2 laser compared to other lasers due to its relatively large wavelength, which is ten times that of widely used fibre lasers. The larger wavelength exhibits higher absorption in any plasma that does form, which as explained earlier can block and/or distort the beam.
Lasers used for welding
Laser sources often used for welding include solid-state lasers – such as Nd:YAG lasers, fibre lasers and direct diode lasers – and CO2 lasers.
Fibre lasers operate at around 1μm and offer excellent beam quality, small spot sizes, high-reliability and low maintenance. They offer good absorption in non-reflective metals and are a popular choice in modern laser welding applications. While CO2 lasers operating in the 9–11μm region of the spectrum have for many years been used for welding, in recent years they have seen ground increasingly taken from them – in this application at least – by fibre lasers, which due to their lower wavelength not only see better absorption in certain metals, but can also have their beam delivered via flexible optical fibre to the workpiece, increasing their versatility in welding applications. However, CO2 lasers are still sometimes used for creating thicker weld seams designed to resist large amounts of stress, for example when joining metal sheets in the shipbuilding industry, or when producing differential gears in the automotive industry. Nd:YAG lasers operate at the same wavelength as fibre lasers and offer high peak powers in small laser sizes, enabling welding with large optical spot size. This translates to maximised part fit-up and laser to joint alignment accommodation.
Direct diode lasers offer a larger spot size and higher wall-plug efficiency than fibre lasers, and are available over a broader range of wavelengths, from 780-1,060nm, 1,400-1,500nm, and more recently at the blue wavelength – 450nm. This new wavelength is particularly suited to copper welding due to its relatively high (65 per cent) absorption in the material, compared to standard infrared wavelengths, which only exhibit 5 per cent absorption in copper due to the material’s high reflectivity. Techniques such as wobble welding are therefore used with infrared lasers to overcome some of the challenges associated with this low absorption, reducing defects such as bubbles and spatter in the final weld.
Blue lasers are particularly suited to copper welding due to its relatively high (65 per cent) absorption in the material. (Image: Nuburu)
In more recent years technology has been released that can vary laser beam quality without the use of complex optics. Coherent, NLight, Trumpf, SPI Lasers, IPG, and Civan Lasers have all released such lasers, which are being shown to improve welding by reducing porosity, spatter, enable higher travel speeds and smoother bead surfaces. Trumpf’s BrightLine Weld technology, for example, alters the distribution of the laser power between an inner and outer core within the beam profile. Similar to the wobble welding technique previously mentioned, this can be used to improve copper welding.
New lasers that can vary laser beam quality without the use of complex optics can be used to dramatically reduced spatter. (Image: Coherent)
The BrightLine Weld laser is able to form a tiny spot, which melts the copper, while a larger spot keeps the weld keyhole open at the surface. Trumpf has shown this to produce copper contacts in e-mobility applications with weld seams free from pores and containing minimal spatter. Coherent meanwhile has demonstrated that its HighLight FL-Arm technology (pictured) delivers up to 80 per cent spatter reduction – compared with standard fibre laser technologies – when welding stainless steel for the tube or heat exchanger industry, and when welding powertrain components such as gears, clutches and axles.
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