Laser Material Processing
Laser material processing is one of the main areas of laser applications (and more generally of harnessing light), having a particularly strong economical impact. It is nowadays used in a very wide and diverse range of industrial fabrication techniques, involving mass production of common goods as well as very specialized applications. Such processes can be applied to a wide range of materials, including many different metals (from thin foils to thick sheets), ceramics, glasses, polymers (plastics), textiles, leather, paper and wood. A wide range of laser sources and processing methods is employed, adapted to the strong differences concerning various material properties (e.g. strength of light absorption, hardness, melting and evaporation temperature, thermal conductivity, tendency to oxidize, etc.) and the intended processing results.
The term laser-aided manufacturing may be regarded as slightly more general, also including alignment lasers and lasers for optical metrology e.g. with interferometers. However, the term is often used just as a synonym for laser material processing.
Laser beam machining denotes those methods of laser material processing which are subtractive (removing material) – e.g. laser cutting, drilling, milling or scribing, but neither additive manufacturing nor surface modification.
Laser material processing competes with a wide range of other fabrication techniques, and generally exhibits a number of attractive features:
- In some areas, it allows for fabrication steps and results for which no alternative methods are available. That applies to various applications in micromachining with ultimate spatial precision. Another example is processing within transparent materials, where access with conventional tools would be impossible. Also, materials can be processed with would be very hard to process with other means.
- In many cases, it allows for very rapid and efficient fabrication, which is particularly important for mass production.
- There are often substantial advantages in terms of the quality of processing results. For example, miniature parts can be fabricated with good surface quality, and the contactless processing avoids contamination with materials from mechanical tools.
- The contactless operation also avoids the wear-off of mechanical tools. The laser source and other parts of the system may still need some maintenance and repairs, but often much less than traditional mechanical processing systems.
- Great flexibility results from the possibility to modify and optimize various parameters, such as the optical wavelength, the applied optical power or pulse energy, pulse duration and temporal shape, focusing conditions and polarization. Some machines allow one to modify several such parameters easily with computer control.
- Laser technology is well suited for integration into modern systems, e.g. unmanned product environments involving robotics and automation, CAD and CAM (computer-aided design and manufacturing) – a part of industry 4.0.
On the other hand, laser-based fabrication methods have certain typical problems and limitations:
- Typical negative side effects of laser processes occur, such as heat affected zones (e.g. with oxidization of material) and the generation of toxic fumes (e.g. in polymer ablation).
- A frequently encountered problem is the relatively high cost of the required machinery, which however may be offset by rapid production (at least for cases with large production volumes) and the flexibility for adaptation to new processes (not so often requiring new machinery).
- For some processes, the energy consumption is quite high, and a powerful electric installation is needed, also often special water cooling facilities, chillers and the like. However, substantial electricity savings are possible in other cases.
Purposes of Processing
Generally, laser material processing is based on some kind of interaction of laser light (or more generally laser radiation) with some kind of usually solid material. Such interactions can be utilized for many different purposes:
- Laser machining utilizes subtractive processes, i.e., the controlled removal of material in order to modify the shape of parts or to separate parts. Examples are laser cutting, drilling and milling.
- Other subtractive processes are laser cleaning and the removal of isolation materials.
- Some methods are used for joining materials, e.g. laser welding and soldering.
- The production of visible structures is the purpose of laser marking and engraving.
- One can protect surfaces with laser cladding or coating.
- Various methods are used for laser surface modification, e.g. laser hardening.
- There are methods of laser additive manufacturing for generating 2D or 3D structures, e.g. with laser sintering.
Some methods are denoted based on the involved physical processes, rather than on the application. For example, some kind of laser ablation is involved in many methods, e.g. in laser engraving, or with a completely different purpose in laser drilling. Similarly, laser sintering may be performed with the purpose of producing a cladding.
Various kinds of physical processes can be involved in laser material processing; the most important ones are the following:
- The (normally only partial) absorption of laser radiation leads to heating of the material. In the most moderate cases, the material remains solid, but certain modifications are facilitated by the elevated temperature – for example, oxidation of the surface or stress-induced bending.
- For stronger heating, the material may melt, i.e., go into its liquid state. The melt may accumulate in the same volume which was originally occupied by solid material, and re-solidify when the beam is switched off or moved away. In other cases, the melt may flow away or be blown away. Even when the melt remains in place, it may exhibit substantial convection currents (driven by temperature gradients and the Marangoni effect, related to surface tension) which contribute to heat removal.
- For still stronger heating, at least some of the hit material can be transformed into vapor. That may lead to a substantial vapor pressure, which may also act on the melt – for example, lead to expulsion of the melt. That can be important for drilling and cutting processes, for example. The transformation into vapor is sometimes called sublimation (somewhat inaccurately), when there is little accumulation of a liquid phase.
- Some materials such as certain polymers or wood cannot be melted, but rather removed only by photodissociation (photolysis) of the molecular structure.
- In various ways, the absorption of laser radiation can be modified – sometimes even dramatically – as a result of the induced changes in the workpiece. For example, a metal melt may exhibit somewhat stronger absorption than the solid metal, and the absorption may steeply rise further when vapor is formed. Besides the change of material properties, the created geometry can also have additional effects on the distribution of radiation and its absorption. For example, in laser drilling and welding operations one often obtains a vapor channel through which the radiation can get to the bottom without excessive power losses; this is a kind of waveguide. That phenomenon is exploited for keyhole welding (deep welding).
- Laser radiation can also be absorbed in vapor, and the vapor may become ionized through various physical processes – not only heating, but also direct interaction of laser radiation with carriers in the formed plasma. Created plasma plumes may effectively shield the workpiece against further laser radiation, making processes ineffective at least for the residual duration of the currently applied laser pulse.
- A melt is often notoriously unstable in its surface shape and undergoes complicated changes of geometry, for example the formation of droplets with different sizes. These substantially reflect laser light and thus modify the intensity distribution in complicated and not perfectly reproducible ways. Such interactions can also cause process instabilities.
- In some cases, and injected process gas plays an important role. For example, one may use an oxygen gas jet in a metal cutting process, which helps not only to more effectively remove material, but also to generate additional process heat by oxidation of some of the material. In other cases, inert gases are used for suppressing certain unwanted chemical reactions such as oxidation, or for increasing the transparency of a vapor for laser radiation.
The involved absorption processes often differ quite substantially from ordinary linear absorption of the original material. It was already mentioned that the absorption properties of a metal can change quite substantially when it is melted and particularly when a vapor and plasma is formed. Also, multiphoton absorption processes become very important e.g. in the processing of glasses with our short pulses: such processes can generate some initial excitation, and further absorption can result from the resulting high free carrier density.
In many cases, the combination of various interaction processes leads to a rather complicated situation. That applies not only to processes with highest intensities (e.g. “cold ablation”, where purely thermal processes are not dominant), but to a large part of all used laser-based techniques. For accurately analyzing the processes, sophisticated multi-physics models are required, which take into account a variety of aspects such as phase transitions, heat capacity and thermal conduction, vapor and plasma formation, movement of materials, possibly also chemical reactions etc. Such work can be difficult, but can contribute a lot to the understanding of the processes and then help to better optimize them for specific purposes. When fundamental limitations are identified, the resulting conclusions may nevertheless help to make progress e.g. by not trying further what cannot work and possibly developing new technical approaches.
For some of the mentioned physical processes to get started, certain intensity levels must be reached of a sufficiently long time. The requirements for the different processes are very different:
- For processes like laser hardening, soldering and tempering, relatively low intensity levels (usually below 105 W/cm2) are required, but over a relatively long times, sometimes longer than one second.
- For laser welding, higher intensities of the order of 106 W/cm2 to 107 W/cm2 are required, again over relatively long times.
- For cutting and drilling, still higher intensities of the order of 108 W/cm2 are needed, but often over substantially shorter times.
- Various kinds of laser ablation processes take place at intensity levels typically between 109 W/cm2 to 1012 W/cm2, but on very much shorter time scales between 100 fs and 100 ns.
Because the higher-intensity processes are intrinsically faster, so that the high intensities are required over shorter times, the applied levels of laser fluence (optical energy per unit area) vary much less than the intensities, although they still vary by several orders of magnitude. It is remarkable that ablation processes, for example, work at similar fluence levels as laser hardening, only with the energy delivery being spread over far longer times. Note, however, that subsequent pulses used for ablation may hit the same spot on the workpiece many times; the total interaction time can thus be many orders of magnitude longer than the pulse duration, and the total fluence can be much higher.
Another dimension is related to the pulse repetition rate. In many cases, the processes can essentially be used in the same way, just at a higher speed, simply by using a higher pulse repetition rate, which is associated with a correspondingly higher laser average power. The situation only profoundly changes once the repetition rate is so high that e.g. the plume created by one pulse cannot vanish before the next pulse arrives.
Micro- and Macroprocessing
A particularly important application area is laser microprocessing, our somewhat more specifically laser micromachining (for subtractive processes only). These terms apply when the workpieces are at least structures created on those get down to dimensions of well below a millimeter. For example, it is possible to drill holes with diameters of only some tens of microns in stainless steel parts, even with a substantial aspect ratio; such results would be very hard to obtain with any traditional machining techniques. At the same time, specific limitations of laser-based processes such as the limited processing speed with limited laser powers become less relevant, because only rather small amounts of material needs to be ablated.
Ultrafast lasers are particularly important in such application areas; they allow for finest processing results, apart from applications where sufficient absorption of the laser light would not be achieved without the extremely high peak intensities arranged by such laser systems. A downside remains the substantial cost of such systems. Therefore, longer-pulse laser systems (typically in the low nanosecond regime) may be preferred where such pulse parameters are sufficient.
The opposite of microprocessing/machining is macroprocessing and macromachining, respectively. Some suppliers focus on one of those areas only, while others cover both.
Generally, it is important to understand the processing efficiency, including its limits and methods of optimizing it. Depending on the process, it may be quantified in different ways. For laser welding, for example, one may consider the required optical energy per unit length, while for operations on areas one may take the required total fluence (energy per unit area), of course taking into account that many laser pulses may need to be sent to the same area to obtain the required results.
The process efficiency can depend on many details of the involved physical processes. Some examples:
- A high efficiency of absorption of the laser radiation is desirable – reflected light essentially means lost energy. It may be acceptable that absorption is initially weak, if the process changes the conditions such that the absorption is rapidly increased.
- A significant loss of energy can also result from low-intensity parts of the spatial beam profile, in which the threshold for the required processes is not reached. Therefore, the transverse beam shape can influence the processing efficiency.
- Heat conduction (particularly in metals) leads to a loss of energy into the volume of material, where it may even be detrimental, for example by creating a heat-affected zone. That problem can often be strongly reduced by using shorter laser pulses.
- Heat radiation (thermal radiation) is another possible loss channel.
- A generated plasma can in some cases shield the workpiece against the laser radiation, strongly reducing the processing efficiency, while in other cases it even helps to couple in the energy.
- Vaporization and ionization of material consumes substantial amounts of energy. It can be substantially more efficient to expel material in liquid form, possibly assisted with a fast flowing process gas. There are trade-offs with processing quality, however. One may thus use combined processes, involving both efficient liquid expulsion and high-intensity post-processing.
Processes based on ultrashort laser pulses (with picosecond or even femtosecond durations) have the advantage that energy losses by heat conduction and heat radiation are largely eliminated. On the other hand, the vaporization and ionization consumes a lot of energy. Therefore and because of the tentatively more limited available laser average power, the processing speed and efficiency are often not that high. Also considering the substantially higher cost per watt of average power, one finds that ultrafast laser processing is problematic in competition with alternative processes with longer pulses, as far as those are available and working well enough. However, there many cases where the required processing results (e.g. in terms of quality) are only possible with ultrafast laser methods.
Important Laser Material Processing Methods
In the following, some of the most important laser material processing methods are briefly described. Most of them are explained more in depth in separate encyclopedia articles.
Most of the processes can be applied either on very tiny length scales (micro-processing) or on larger length scales (macro-processing).
Various kinds of materials which are important in industrial fabrication can be ablated, mostly using short or ultrashort laser pulses. A large number of pulses is applied, while moving the laser processing head (or the workpiece), often systematically along a predefined pattern with a certain overlap of the zones affected by single pulses.
Here, the most important parameter is the very high intensity level, as can be achieved in conjunction with very short pulse durations; of lower importance is the wavelength of the radiation. Methods of thermal laser ablation require nanosecond pulses in case of metals, while much longer pulse durations are suitable for ceramic materials, for example, because those exhibit a lower thermal conductivity.
One may ablate some depths of a homogeneous material, sometimes for forming certain surface structures. In other cases, some kind of film on another material is removed – for example, an oxide layer or a previously applied coating or paint (devarnishing). Conveniently, the film to be removed often exhibits much stronger absorption, so that it is easy to let the underlying material unaffected (principle of selective absorption).
Some examples for mostly industrial applications of laser ablation:
- Machine parts are processed to get microtextured surfaces, e.g. in order to reduce friction of lubricated parts. An example are cylinders and pistons of combustion engines.
- Thin-film photovoltaic panels need to be insulated at their boundaries. Efficient laser processes are nowadays used for that purpose.
- Ablation processes are also important for laser cleaning. They can be applied, for example, for regenerating industrial tools and for restoring artworks.
- Seals for water-tight housings are often made by first depositing a continuous layer of some elastomer and then removing all of that except for the wanted seal stripe with a laser.
- Insulated electrical conductors (e.g. ribbon cables) frequently need to be connected at certain locations after locally removing an insulating polymer with a laser. The process must be done such that oxidation of the conductor is avoided.
- Some laser marking (engraving) processes are also based on material ablation.
Laser drilling means the generation of (mostly small-diameter) holes, which either go to a limited (and hopefully well-defined) depth (blind holes) or through the full thickness of some metal plates, for example. A wide range of materials can be processed, including metals (even quite hard ones), ceramics, glasses, semiconductors and other crystals.
Drilling a hole may be done with a single pulse (in sufficiently thin materials, e.g. metal foils) or with a sequence of many pulses (percussion drilling).
Larger holes are efficiently generated with trepanning, i.e., with cutting out the contour of the hole. A modified method is helical drilling, where the beam is also moved in z direction such as to obtain a helical path of the focus.
Some examples for applications of laser drilling:
- Very small diameter holes are required for certain machine parts, e.g. for injection nozzles as used for fuel injectors in combustion engines.
- Large numbers of tiny holes are needed for some types of filter sieves.
- Larger holes are required for air cooling of turbine blades, for venting purposes, for instrumentation and various other purposes.
- In laser cutting processes (see below), the process often needs to start with the generation of an initial hole, from which the actual cutting process can continue.
- In electronic manufacturing, many small holes for contacting components on printed circuits boards need to be fabricated quickly.
Laser drilling is particularly suitable when very thin holes with large aspect ratio (ratio of length to diameter) need to be generated, which is hard with conventional mechanical methods. Also, it is often the only choice for fragile materials, which would break when applying mechanical processes.
Laser drilling processes are often carefully optimized to expand the performance (e.g. concerning hole diameter, hole depth and aspect ratio) and the processing quality, e.g. concerning consistent hole diameters (low conicity), nicely circular cross sections (low ellipticity) and low deposition of material around the holes.
See the article on laser drilling for more details.
Laser cutting is in some respects similar to drilling, but aimed at separating parts over some length. It often begins with drilling (piercing) to get some initial hole, from where the cutting process can continue by the smooth movement of the laser processing head and/or the workpiece. A defined gap (kerf) needs to be obtained in order to achieve the separation. For that, some amount of material has to be removed, either in liquid form (as melt) or by vaporization. The latter generally leads to higher processing quality, but also to a lower process efficiency. In some cases, a substantial part of the process heat is generated by oxidation of the metal, achieved by injecting purified oxygen. In other cases, an inert process gas is applied for improved quality.
Efficient laser cutting methods and machine systems have been developed for a wide range of industrial applications, ranging from the cutting of metal sheets in shipbuilding to precision machining, even micro-machining. Various types of metals can be cut, from thin foils to thick sheets, also a wide range of polymers (plastics), and even brittle materials such as ceramics, glasses and semiconductors.
In some cases, excellent cutting quality is achieved, while the quality is just satisfactory in other situations.
See the article on laser cutting for more details.
Welding means joining parts by heating their boundaries, letting them melt and combine. The heating can conveniently be done by absorption of laser light.
Different laser welding processes have been developed. For seams with a small or moderate depth, conduction welding with moderate optical intensities can be done, while deep welding with a much higher aspect ratio (seam depth to width) can be achieved with deep welding, applying substantially higher optical intensities. Such processes have a wide range of applications in industrial manufacturing.
See the article on laser welding for more details.
Lasers can be used in various ways to mark materials. One possibility is laser engraving, i.e., removing some depth of material from a homogeneous surface. In other cases, one removes a thin coating, e.g. an anodized layer from an aluminum part, or some paint layer; in such cases, removal of only a thin layer can be sufficient for obtaining a strong visual contrast. Other methods are based on surface modifications, which can again result from a variety of physical effects of thermal or non-thermal origin.
There is a wide range of applications of laser marking in industrial fabrication. It is applied to many machine tools, printed circuit boards, integrated circuits and other electronic components, cables, keyboard patterns, credit cards and food packages. Some of those labels need to be readable for consumers, while others are used in quality monitoring and error tracing. In comparison with other (non-laser) marketing techniques such as ink jet printing and stamping, laser marking has substantial advantages in terms of flexibility, processing speed, quality and operation cost.
Pulsed solid-state lasers of moderate average power are mostly used for marking of metals, while for ceramics, paper, cardboard and wood CO2 lasers are common, and excimer lasers are used in some special cases, e.g. for glasses.
See the article on laser marking for details.
Laser Surface Modification
There is a wide range of methods for modifying (improving) surfaces, for example of machine parts. A prominent example is laser hardening, which is applied mostly to carbon-rich steels and to cast iron. Other important methods are laser remelting, varnishing, annealing, honing, alloying and coating.
See the article on laser surface modification for details.
Soldering methods are used for joining parts while limiting the required heating. In contrast to welding, they are based on melting not the parts to be joined, but some soldering agent (solder), which forms a solid connection of the parts when re-solidifying.
With moderately focused laser beams (spot diameter well below 1 mm), very controlled heating can be achieved, so that soldering of very fine structures is possible. That is used for attaching mainsprings in mechanical watches, and in many other areas with fine mechanical parts.
Laser soldering is also very important in micro-electronics, where it is used mostly for making electrical contacts.
Besides, fluxless hard soldering processes with high-power lasers are common in automotive manufacturing; they are not subject to some of the limitations of welding.
See the article on laser soldering for details.
Laser Cladding and Coating
Cladding and coating both mean that a layer of distinct material is formed on some base material. The difference is that cladding means making a metallic layer on a metallic base please, while for coating it can be a different combination of materials, and also coatings are typically much thinner than claddings. The common purpose of both methods is usually to provide some kind of protection to a surface, for example against abrasion or corrosion.
Laser Additive Manufacturing
Laser additive manufacturing processes are those where additional material is added to workpieces with the help of a laser beam. That is in a way the opposite of laser machining, which is always subtractive.
The purpose of a laser additive manufacturing method can be to build more or less complex objects, for example in the context of rapid prototyping and tooling, or just to produce a layer of some material (see above, laser cladding and coating).
See the article on laser additive manufacturing for details.
Various kinds of unwanted materials can be removed from surfaces by applying sufficiently intense laser light. For example, artworks exposed to polluted air, which created dark depositions, can be cleaned with lasers while preserving the original material. More frequently, such methods are applied to efficiently cleaning parts in industrial manufacturing processes.
See the article on laser cleaning for details.
Technical Aspects of a Laser Material Processing System
A typical laser material processing system comprises the following parts:
- The central part is some type of laser source, which in addition to the actual laser may contain one or more optical amplifiers, a frequency doubler, an optical parametric oscillator or other means for nonlinear frequency conversion. There may also be additional components of the system for pulse picking, for pulse shaping or for conditioning the laser beam.
- The laser source often requires auxiliary systems such as a laser power supply, a laser cooling unit, an interlock system for laser safety, and a supply of certain gases (e.g. in gas lasers).
- There is usually some kind of beam delivery system for delivering the generated laser beam to the application area. (In some cases, multiple beams, perhaps even at different wavelengths and/or with different focusing conditions, are superimposed.) The final beam conditioning is often made in some laser processing head, which may have additional means e.g. for injecting a processing gas and for monitoring the process (e.g. with a camera). The laser processing head is often mounted on a robot arm.
- One usually require some means for supplying and removing the workpieces, and for moving the beam focus relative to the workpieces. It is common to use computer-controlled motorized means for moving the workpiece and/or the laser processing head (“flying optics”) – often realizing some degrees of freedom with one of those and other degrees with the other one. Different types of laser scanners allow one to rapidly scan a laser beam over some area (e.g. in one or two dimensions), either in a regular fashion or with each movement specifically controlled through a computer.
In addition, one may require installations for purposes like insuring laser safety and safely removing toxic fumes and accumulating debris.
See the article on laser processing heads with many more details of practical importance.
Used Laser Sources
A wide range of different laser sources is used for laser material processing, with specific advantages and limitations depending on the specific application requirements:
- Some gas lasers, particularly CO2 lasers are still widely used because of their specific advantages for certain processes. The combination of high output power and high beam quality is often relevant, also the long emission wavelength of typically 10.6 μm, which leads to superior absorption in various materials (e.g. polymers, wood, ceramics, but usually not in metals). The operation mode can be continuous-wave or pulsed.
- Excimer lasers still play an important role in the area of ultraviolet lasers, despite their limitations in terms of wall-plug efficiency.
- Solid-state lasers, nowadays mostly in diode-pumped versions, are most widely used. While the vast majority of such lasers operates in the spectral region between 1 μm and 1.1 μm, where best laser performance is possible, various other wavelengths can be reached with methods of nonlinear frequency conversion. In particular, there are many green lasers based on frequency doubling, also ultraviolet lasers based on frequency tripling and quadrupling.
- Fiber lasers (often in conjunction with high-power fiber amplifiers) are a specific type of solid-state laser which has become more and more important. They typically feature a high wall-plug efficiencies, high beam quality and (particularly for continuous-wave lasers) high output power. However, compared with bulk lasers they are much more limited in terms of pulse energy.
- The direct application of diode lasers (→ direct diode lasers), e.g. in the form of diode bars, diode stacks or VCSEL arrays, is very attractive in terms of laser cost and wall-plug efficiency. It was initially difficult because of the quite limited beam quality, but substantial advances in beam shaping and the increasing use of spectral beam combining have changed that; even demanding applications like laser cutting and welding can now sometimes be done with such lasers. Therefore, direct diode lasers now more and more replace some diode-pumped solid-state lasers – but only in the domain of continuous-wave operation, because their potential for pulse generation is highly limited.
In any case, a well engineered industrial laser system is highly desirable in various practical respects, such as reliability and lifetime, quick availability of possibly needed replacement parts etc. Other practically important aspects are of course the installation cost (for making the laser apparatus, transporting it to the factory and installing it) and the running expenses (electricity consumption, gas consumption, maintenance, repairs).
For more details, see the article on lasers for material processing.
Important Parameters of the Used Laser Sources
In a laser material processing, quite a number of different parameters characterizing the laser radiation can be very relevant, i.e., part of the laser specifications:
- Optical wavelength: In most cases, this is important because light absorption is essential for the used processes, and the strength of absorption can critically depend on the wavelength. For example, polymers can be easily ablated with intense ultraviolet pulses, while they would be hard to process with visible or near-infrared light.
- Optical power and intensity: An appropriately high optical intensity (power per unit area) is needed for many processes. That can in principle be achieved even with a low-power by sufficiently focusing (concentrating) the radiation to a spot on the workpiece, but the optical power is also often very important for getting the process to work at all or for achieving an economically viable processing speed.
- Beam quality, quantified e.g. with the M2 factor: that determines how easily the radiation can be focused to a small spot, and how small the working distance (between the laser processing head and the beam focus) can be. Besides, it has an influence on how smooth and consistent the intensity profile is. In some cases, a very clean Gaussian beam or a highly uniform flat-top beam profile is required.
- Radiance (often inaccurately called brightness or brilliance): this results from the combination of power and beam quality, and determines what optical intensity level can be achieved on a given spot size and with a limited beam divergence.
- Pulse parameters: In many cases, one uses pulsed lasers, where the pulse durations can differ a lot between different applications – one uses femtosecond lasers, picosecond lasers, nanosecond lasers and in some cases free-running pulsed lasers with microsecond pulse durations. Particularly relevant parameters are the pulse energy and pulse duration, and the peak power is essentially the energy divided by the duration, but with some influence of the pulse shape. The pulse shape itself is important in some of the applications. In many cases, one applies regular pulse trains, or sometimes bursts of pulses.
Often quite irrelevant for material processes is the spectral bandwidth (linewidth) of the laser source, because the absorption properties of workpieces normally do not vary significantly within the optical bandwidth of a laser, even if it is e.g. several nanometers wide.
Different kinds of beam delivery systems can be used. For solid-state lasers, mostly operating in the 1-μm wavelength region, fiber cables are often quite convenient, because one can move the output and without caring about the detailed position of the middle part of the fiber cable. However, such cables need to be highly multimode for transmission of high optical powers, and that means that a high beam quality of the laser source cannot be utilized for the process. In that respect, articulated arm delivery systems are clearly superior, since they can more or less preserve even diffraction-limited beam quality. Here, however, one has to deal with more delicate mechanics and issues like mechanical vibrations, contamination of mirrors etc.
Apart from beam quality, polarization of the laser light can also be important. That may or may not be preserved in an articulated arm system, and it is usually not preserved in a high-power fiber cable.
It is generally important to accurately position the laser beam focus on a workpiece, and often also to appropriately move that position during the processing operation. For example, in laser welding one needs to move the laser spot with an appropriate velocity to obtain both a good weld quality and a reasonable processing speed.
Depending on various circumstances of the application, one may move only the workpiece with a fixed laser beam, move the laser beam over a fixed workpiece, or moved those items in a coordinated fashion. Sometimes, not only the laser processing head but even the whole laser source is moved; that is done, for example, with robust and not too heavy CO2 lasers, where the beam delivery system cannot be arbitrarily flexible. The laser may then be mounted on a movable robot arm, for example.
The working distance (between the processing head and the workpiece) needs to be accurately controlled, since the beam focus usually should be at the workpiece surface. Sometimes, that distance needs to be actively controlled with a distance sensor and an automatic feedback system.
In some cases, one prefers a large working distance, for example in order to do fast remote welding on large workpieces while moving only the beam direction and focus position, but not the heavy workpiece or the robot arm. The large distance also reduces the risk of debris damaging the optical window of the processing head, so that the window does not need to be replaced that often. On the other hand, this requires a higher beam quality, putting another possibly serious restriction on the choice of laser source. Also, larger optical elements may then be required, increasing both cost and weight. One may also need to work without a process gas, which introduces additional limitations.
Protection Against Back-reflections
Particularly metallic workpieces may cause strong back-reflections of light, when hit perpendicularly by a beam. When a substantial amount of the laser light is sent back to the source, that can cause serious problems, from the destabilization of laser operation to complete destruction of the laser source. Some amplified systems (containing a laser amplifier, as usually required in fiber-based systems) are particularly sensitive. However, there are kinds of amplifiers which are quite insensitive to reflections despite their high gain – for example, regenerative amplifiers for ultrashort pulses.
Processing with non-normal beam incidence, leading to a sufficiently good separation of the reflected beam, may be a solution in some cases, but cannot always be applied – for example not for drilling holes perpendicular to a metal surface. One may then have to use additional means to protect the laser source against back-reflected light, or use a less sensitive source. In principle, one can use Faraday isolators, but these are available only for limited power levels. The same applies for a working principle applied to some CO2 lasers: using an absorbing thin-film reflector in conjunction with a phase-retarding mirror, where the latter effectively rotates the polarization direction by 90° in a double pass and the former mirror then mostly absorbs the resulting p-polarized light.
Not only for basic research, but also in industrial applications it is often highly desirable to obtain as much information on the process conditions as possible. For example, this can be used in a welding process to carefully control the gap between the two pieces to be joined. One may also monitor temperature conditions and various other aspects either in order to automatically control the process (e.g. for regulating the speed of movement or the laser power) or to interrupt it if any significant malfunction is detected.
Various kinds of facilities can be used for such purposes, some of which are often integrated into a laser processing head:
- A camera can directly provide images of the processing region. The resulting data may be simply displayed on a monitor, recorded for later inspection, or even automatically processed with appropriate image recognition software.
- A simple photodetector, possibly equipped with a suitable kind of optical filter, can give some indications on the process. For example, it can monitor the glow of a laser-induced plasma and give warnings when it is too strong or unusually weak.
- In some cases, a spectrometer may be used to obtain more detailed information.
- The combination of imaging with spectroscopy is hyperspectral imaging, which has the highest potential for revealing a lot of details, but requires a substantially larger effort to install and use it.
- Another powerful method is high-speed X-ray imaging, which can provide valuable insights on the inner processes in the context of scientific research.
Modern industrial laser machining systems contain a computer which collects a substantial number of signals from the laser system and the processing head. It may continuously record such data and compare them in real time with reference data in order to quickly identify possible problems. Besides, the data may be stored in case that they are needed for later troubleshooting, for example if quality issues on the process parts are discovered.
Note that there can be many different reasons for degradation of performance, for example aging of the laser system, contamination of critical optical components, misalignment due to vibrations or mechanical shocks, overheating of optics, a lack of gas supply or variable properties of workpieces. Therefore, it could be rather hard and time-consuming to identify problems without extensive monitoring of various details.
Analysis of Processing Results
Apart from monitoring the process, one often needs to carefully analyze the results. Some of the often used methods are:
- Surfaces and various features of fabricated parts are inspected with a microscope. Even tiny surface structures can be revealed with suitable methods of microscopy.
- Optical profilometers are suitable for precisely measuring surface reliefs, assessing roughness of surfaces etc.
- Some important inner features may not be readily accessible for microscopic inspection. One sometimes requires destructive methods for creating such access. For example, welds are often laid open by grinding throughout the whole structure; the created surface can then be analyzed in great detail. Unfortunately, such processes are not only destructive, but also time-consuming. However, they are sometimes required for checking the quality of results and possibly optimizing the process.
- There are also non-destructive imaging methods which are suitable for revealing enough features. Examples are X-ray imaging and ultrasound imaging. For example, unwanted voids in fabricated structures can reliably be revealed that way. Obtaining a high spatial resolution, however, requires sophisticated imaging technology.
- Soldering joints often have the purpose to create a reliable electrical connection, and in such cases it may be appropriate to measure the electrical resistance.
- Results of laser surface modification can often be checked by spectroscopy based on reflected light.
Various types of safety hazards are encountered in laser material processing. The most obvious aspect is laser safety, since the involved laser powers are often very high – orders of magnitude higher than what is needed to blind people, and also often by far high enough to cause serious injury to the skin. What is sufficient to cut thick metal sheets, for example, is obviously also sufficient for inducing deadly or mutilating injuries within a very short time. Depending on the widely different circumstances in industrial settings, different solutions are required to ensure laser safety. There is usually no other choice than keeping any personal out of the dangerous areas during operation. However, there are different ways of achieving that.
Smaller working areas are often enclosed in a housing, equipped with an interlock to make sure that the machine can operate only when everything is closed so that no dangerous radiation can possibly exit the machine. Operators can then still stand directly in front of the machine, controlling it and monitoring the process through windows. With such provisions, a machine can be in laser safety class I even if it contains a kilowatt laser.
In some cases, one can to some extent rely on laser safety glasses for eye protection, which however should then be consistently used. As that is hard to reliably guarantee, and the protection against intense beams is anyway limited, they can usually not be the primary safety measure, but only one of several measures, giving additional safety.
Usually, a laser safety officer with well defined competences needs to be assigned to a laser manufacturing area. Valuable advice may also be obtained from the system suppliers, often having substantial experience concerning safety hazards and appropriate countermeasures.
Laser machining operations can also be associated with various other types of hazards. Some examples:
- Many lasers operate with quite high voltages, sufficient for causing fatal electric shocks. Such risks can be increased by the possibility of damaging electric cables in harsh industrial environments. Therefore, such cables should be properly protected.
- Debris from laser cutting operations, for example, can cause serious burning injuries, partly due to its high temperature. Proper eye protection is particularly important.
- Toxic, possibly even carcinogenic fumes are generated, for example when evaporating polymers. These must be effectively removed from the processing area and treated with appropriate filters or exhaust systems.
- There are also often substantial injury risks created by fast moving mechanical parts, if there is a chance that operators get in contact with them during operation.
Factors for Further Progress
Substantial further progress in the field of laser material processing is expected, and can result from a range of different developments:
- Some types of laser sources are already highly optimized, with limited potential for further improvements; they can still benefit from further improved parts (e.g. diode bars) with lower cost. Other laser types can still be improved substantially in terms of performance while also lowering the cost. This is particularly the case in the area of ultrafast laser material processing.
- Scientific research on the details of the involved physical processes can identify remaining problems and suitable solutions. In simpler cases, that can mean the adaptation of parameters like laser wavelength, pulse duration and fluence. However, methods are sometimes modified quite profoundly, e.g. by employing a tailored burst mode scheme for best results with maximum efficiency. Generally, one is dealing with a complex multi-parameter optimization, where trial-and-error approaches are often not sufficient for getting the best results.
- A crucial aspect is a detailed understanding of manufacturing requirements. The most successful suppliers of laser processing machinery are not automatically those who are best in developing lasers. They also need to identify opportunities for new applications, remaining problems in existing applications and the most viable strategies for all sorts of improvements.
- Software for machine control and efficient integration into a larger manufacturing environment is also of crucial importance. Beyond software, the organization of complex working processes has profound effects on the cost, efficiency and reliability of results.
An intense and fruitful cooperation of people with different perspectives is essential for further progress – involving those developing lasers, others investigating machining processes and still others understanding a wider context in industrial fabrication, including technologies like high-precision mechanics, sensing, automation and robotics.
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See also: laser applications, laser machining, laser hardening, laser drilling, laser cutting, laser welding, laser cladding, laser coating, laser marking, laser soldering, laser cleaning, laser processing heads
and other articles in the categories laser devices and laser physics, laser material processing