Welding essentially means joining parts by heating their boundaries such that they melt (or at least get someone soft), and the subsequent solidification leads to fusion – the formation of a stable connection (a seam). In contrast to laser soldering, the so-called solidus temperature of the joined surfaces is exceeded by some amount. In many cases, no additional filler material is applied. Therefore, the pieces to be welded together must be fitted together quite closely. However, there are also welding processes using additional filler material, for example in the form of a powder or wire. Such a filler material allows one to work with a slightly wider initial gap and possibly with less precise preparation of the workpieces.
In case of laser welding (= laser beam welding), the heating is accomplished by absorption of laser light, which hits the workplace in the form of a laser beam. Frequently, the laser radiation (usually infrared light) is continuous while slowly moving the beam or the workpiece, but sometimes one uses laser pulses, e.g. from TEA CO2 lasers.
Laser welding is one of the most important techniques of industrial laser material processing. It can be applied to a variety of metals, although not all technologically important metals are well suited for welding; some materials have a tendency to form seams of poor quality, which is hard to improve even with optimized processes.
Laser welding processes can also be applied to various other materials such as polymers, mostly thermoplastics.
Various different welding geometries are possible, with specific advantages and disadvantages e.g. in terms of preparation efforts, accessibility and achieved mechanical stability. For example, one may simply join metal pieces face to face, or alternatively with some overlap, after bending the faces, or at a 90° angle. In other situations, welding is applied to cylindrical parts with a closed-path seam.
Applications of Laser Welding
Laser welding is very widely applied in modern industrial fabrication, particularly in larger settings. Some examples:
- It has become very important in automobile fabrication, were different kinds of steels, aluminum compounds and other metal pieces need to be joined, and different welding geometries can occur.
- Metal tubes are fabricated by bending sheets and joining the ends together with a linear weld seam. While this is mostly done with traditional techniques, lasers are apply e.g. where particularly high quality is needed.
- A wide range of industrial tools, household items etc. need to fabricated with welding techniques.
In various application areas, alternative welding processes would not be available. For example, laser processes allow one to join quite dissimilar materials, and realize unusual geometries.
Advantages of Laser Welding
Typical benefits of laser welding (in comparison with traditional welding techniques) are the following:
- One obtains clean and narrow seams, so that high-quality results are obtained with less post-processing.
- The heat-affected zone can be substantially narrower.
- High aspect ratios (ratio of depth to width) are possible.
- Laser welding can be applied to difficult cases with complicated contours or the combination of different materials, for example.
- Laser welding is well suited for integration into highly automated fabrication machinery.
Basically, the process of laser welding works such that one places the two parts to be joined in close proximity (with a narrow gap between them, or sometimes no gap at all) and then heats the contact area with a laser beam for a short while. That causes melting of the material, and the melt fills the gap; later on, it solidifies and forms the joint (seam). In most cases, the beam starts at one end and slowly moves along the whole interface, so that a continuous seam is formed. In some other cases, connections are only generated at certain points (point welding).
With moderately high optical intensities (of the order of 105 W/cm2), one can do conduction welding. Here, the absorption of laser light occurs mostly near the surface, and heat is transported to the surroundings through heat conduction. As a result, the width of the seam is then similar to its depth, or a bit wider. With that method, the welding depth is quite limited, but it is sufficient e.g. for joining not too thick metal sheets. Typically, the welding depth is between 0.3 mm and 3 mm for metals.
Besides melting of the metal, there is little vaporization, and therefore only weak generation of fumes and deposition of material on the input surface. Only, there is some heat affected zone around the seam, which means some degradation of the welding quality.
For conduction welding, one has substantial losses of laser power by reflection at the metal surface. For example, of the order of 30% of the incident power from a CO2 laser are absorbed on a steel surface, and of course only that part can be utilized for the welding process. In other situations, the absorption efficiency can be even lower. Substantially anomalous absorption, as occurs in other processes like laser cutting, can usually not be obtained because of the more moderate process conditions.
Keyhole Welding (Deep Welding)
By applying significantly higher laser intensities, one can realize keyhole welding, also called deep penetration laser welding, or in short deep welding. Typically, one applies an intensity above 106 W/cm2 = 10 kW/mm2. For example, one may focus a 1-kW beam to a spot with 0.12 mm beam radius, which leads to 22 kW/mm2. (The on-axis intensity is then even twice higher, if it is a Gaussian beam.) Under such conditions, the metal melt is heated so much that part of the material starts to be vaporized. Consequently, a vapor capillary (called keyhole) is formed, which can propagate downwards to a substantial depth, and a plume (cloud of hot fumes) appears above the welding location.
The keyhole may reach into the material by well more than 10 times (even of the order of 100 times) the beam diameter. Therefore, much thicker metal sheets can be welded than would be possible with conduction welding, which justifies the term deep welding. The keyhole can be kept open by the high vapor pressure, which works against gravitational forces and surface tension, which would tend to close it with melt. The whole process becomes additionally dynamic through the continuous movement of the welding head; this can cause the keyhole axis to be somewhat tilted against the beam direction.
When using a laser emitting in the 1-μm spectral region (e.g. a typical solid-state laser), its radiation can propagate along the keyhole with relatively weak optical losses. However, the process also works well with CO2 laser light, having a much longer wavelength which leads to substantial absorption in the vapor channel; the vapor then gets very hot (glowing intensely in blue color) and transports energy to the sides by thermal radiation and convection.
In any case, quite efficient absorption of laser radiation can be achieved – often of the order of 80% to 90%. There may still be essential reflection of laser light at the metal, but if it is reflection inside the keyhole, the radiation is directed to other metallic parts that can again be partly absorbed. Essentially, it is no more so easy for the radiation to escape into free space.
The physical processes involved in keyhole welding are quite complex. They involve aspects like absorption of the radiation (but with a special surface geometry, melted and vaporized material, anomalous absorption, partly due to inverse Bremsstrahlung absorption), melt and vapor flow driven by intense vapor pressure, surface tension, ionization, heat conduction, heat radiation of the plasma, etc.. Sophisticated multi-physics models (usually numerical models) are required for analyzing the details of such processes; careful observation of the welding zone alone does not provide sufficient insight. Various process details can be substantially different for welding with solid-state (or fiber) lasers in the 1-μm spectral region, as compared with processes based on CO2 lasers at 10.6 μ wavelength.
Normally, one applies a suitable process gas to protect the surfaces. In most cases, it is a chemically inert gas like nitrogen (N2), argon (Ar) or helium (He), which prevents unwanted oxidation of the surfaces. Sometimes, a process gas has additional functions beyond chemical protection, e.g. increasing carrier recombination in the plasma in order to increase its transparency (reduce shielding effects).
Generally, a slow flow of process gas is sufficient for welding (in contrast to laser cutting, for example, where a fast gas flow is often essential). However, another function of the gas can be to protect the optics of the laser processing head against depositions of debris and fumes. Often, an additional cross-jet within the welding head is used for additional protection; it may be mounted just below an optical window, protecting the focusing optics. One then needs to exchange that window less frequently.
In some situations, the seam obtained in deep welding is substantially wider at the top as a result of a fast convection flow in the melt. That can happen as a result of the Marangoni effect: a flow is induced by temperature-dependent and thus location-dependent surface tension, not just by density gradients. An additional driving force can be friction with the flow of hot vapor.
Remote Laser Welding
While laser welding is often done with the laser processing head being positioned quite close to the welding zone, there are also remote laser welding techniques, where the laser beam is sent over a substantially larger distance, e.g. of tens of centimeters or even more than one meter. This is called remote welding.
A typical advantage of remote welding is that using a laser scanner one substantially more rapidly position the welding spot, compared with cases where one needs to move the whole laser head and/or the workpiece. Particularly when a high laser power is available, one may then quickly make a long weld seam e.g. on an automobile door. Another advantage is that the risk of debris reaching critical parts of the processing had is strongly reduced.
On the other hand, a possible limitation is that the full advantages of remote welding can be realized only when no process gas needs to be applied, because otherwise one would still need to provide the relative movement of gas nozzle and workpiece.
In some cases, the workpieces need to be properly prepared. Often, it is important to have them clean enough, i.e., possibly apply some cleaning procedure, e.g. to remove oxidized layers or machine oils. In some cases, it is necessary to pre-heat parts before welding.
Single-beam and Multi-beam Methods
Usually, laser welding is done with a single laser beam. However, there are refined methods for improved quality where a few beams are used in combination:
- Dual-spot techniques use two closely spaced laser beams with equal power and diameter. The line through the two spots may be aligned along the movement direction or perpendicular to it.
- Trifocal welding is done with a main beam with large diameter, preceded by two more tightly focused beams.
For some applications, hybrid welding processes have been developed, where one uses a combination of two different heat sources – for example, a direct diode laser and a pulsed solid-state laser. It is also possible to combine some laser source with an electric arc. The latter can provide additional process energy in a relatively cheap way, while also supporting the absorption of laser light and possibly helping to transport filler material into the welding gap. Possible benefits are a higher processing speed despite a moderate laser power, while the processing quality may be higher than for a pure arc welding process.
Laser Welding of Polymers
The explanations above all apply to the routing of metals, which is the usual case. However, various laser-aided processes have also been developed for welding of polymers. Such materials differ a lot from metals in terms of solidus temperature, plasticity, vapor formation or chemical dissociation, and the absorption of light. Many material properties can be further modified by additives, sometimes intentionally to optimize welding processes.
In some situations, the laser welding of polymers is done in ways which are quite similar to metal welding: the parts to be joined are non-transparent to the laser light, absorb some optical power, get soft in the welding region and join. Joints can be generated at isolated points or along lines.
Quite different processes become possible through the fact that polymers can be quite transparent to laser radiation. For example, one may join a strongly absorbing polymers part with a transmitting one, sending the laser radiation to the interface through the transmitting material (transmission laser welding). Although the heat is originally generated only in the absorbing material, heat conduction can lead to the softening of both materials at the interface. A special aspect of transmission laser welding (in contrast to any other welding methods) is the easy access to the interface, e.g. to locations far away from any outer surface.
Given the very widespread use of polymers, for example as solid plastic parts or as foils, there is obviously a very wide range of applications of laser welding of polymers. This includes cheap mass products as well as highly specialized items. While laser welding as always competes with a variety of other methods, such as conventional welding and gluing methods, it can play out its specific advantages in many cases. Typical advantages are fast processing, no need for consumables, high flexibility and the suitability for automation.
Typical Welding Imperfections
The results of laser welding are generally not perfect, although often better than those of conventional methods like arc welding or resistance welding. Typical defects or imperfections are:
- Voids (e.g. small gas pores or larger voids) in the seam can be formed, e.g. if there is insufficient time for completely filling the volume with melt, which then solidifies. Also, cracks can occur in the seam. Such voids can substantially deteriorate the mechanical strength of the seam.
- There is some heat affected zone (HAZ) around the seam, e.g. with oxidation.
- Some of the vaporized material gets deposited on the neighbored surface (particularly for keyhole welding). This can also include droplets of molten material.
- The top and bottom surfaces may not be completely flat – they may be convex or concave. For example, a convex shape results when the groove is incompletely filled, because there was a too large gap. Such details can critically depend on how the parts were initially aligned.
- Incomplete penetration: The seam may not be consistently reaching the desired depth.
ISO standard 13919-1 provides ways to classify the welding quality. For a full assessment of the welding quality, one often needs to grind through the weld and inspect that surface with a microscope. That destructive method can of course not be regularly applied during production, but may be needed quite often for initially establishing the welding process.
The tendency for such imperfections can strongly depend on a number of process parameters, for example the beam parameters, the movement speed of the welding head, the alignment of parts, material details and the applied process gas flow.
The optimization of a laser welding process can aim e.g. at
- maximum welding speed,
- minimized requirements concerning laser power or energy per unit length (line energy),
- low gas consumption, and
- highest quality of the results in terms of mechanical stability, surface flatness and uniformity, size of the heat affected zone, etc.
What is the best solution can depend on the priorities for the particular application. Further, the situation can strongly depend on the given circumstances, such as the materials to be joined, their thickness and the needed seam geometry. Because so many variable parameters and points of interest are involved, it is not trivial to optimize the process; there are plenty of chances for improvements, but it can take a substantial time to identify the best combination of all the parameters and details.
Process diagnostics can play an important role in the optimization. Automated machines partly use process diagnostics directly to optimize parameters of the welding process. For example, the radiation of the metal plasma which occurs during keyhole welding provides useful information.
At least for conduction welding, the relevant physical processes are less complicated than for laser cutting and drilling, for example, thus also substantially easier to describe and predict with computer models. On the other hand, the involved joining geometries can exhibit a greater variety. For keyhole welding, the whole situation is rather complicated.
Lasers for Welding
Particularly for heavy machinery, high-power CO2 lasers are still widely used. However, diode-pumped solid-state lasers have become more and more important, first in the form of bulk lasers and later on also increasingly as fiber lasers.
A high laser beam quality is sometimes advantageous, particularly when stronger defocusing is required. Interestingly, however, best processing results are not always achieved with the best beam quality: there are cases where deep welding does not work ideally, since there is insufficient energy transport to the sidewalls of the keyhole. Apart from reducing the beam quality, there can then be other options such as using a laser scanner to rapidly rotate the beam along a small circle, effectively increasing the diameter of the average intensity profile.
Laser Welding Machines
A typical industrial laser welding machine contains the following:
- The core part is the laser source, e.g. a continuous-wave CO2 or solid-state laser system with an output power of several hundred watts or several kilowatts. It usually requires some auxiliary units, particularly for cooling the laser.
- A beam delivery system transports the laser light to the application area. For bridging large distances, one often uses high-power fiber cables.
- The welding head needs to be moved relative to the workpiece. That may be done by moving the workpieces only, by moving the laser head only (flying optics), or with hybrid solutions, assigning certain degrees of freedom to one or the other side. The welding head is usually equipped with a supply of processing gas.
- There can be additional facilities for automatically loading and removing workpieces, removing waste material, etc.
- Due to the substantial laser powers, laser safety is an important issue. It may be provided with very different means, e.g. by doing the welding only within a closed housing with an interlock, or by not having any personnel in the whole area during automated production.
While some welding machines are entirely optimized for one specific application, there are also multi-purpose machines with substantial flexibility to adapt to different scenarios.
Industrial welding machines are often equipped with sophisticated computer control for maximum productivity, i.e., also for efficiently utilizing an expensive laser system. Interfaces e.g. to CAD/CAM software and integration into a larger fabrication environment are often needed. The machines also contain a large number of sensors for monitoring the process as well as various internal parameters for quickly identifying possible faults.
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