Advances in macro-scale laser processing

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Advances in macro-scale laser processing Michael Schmidt (2)a,b,*, Michael Zäh (2)c, Lin Li (1)d, Joost Duflou (1)e, Ludger Overmeyer (2)f, Frank Vollertsen (1)g a

Institute of Photonic Technologies, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany Graduate School in Advanced Optical Technologies, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany Institute for Machine Tools and Manufacturing Technology, Technical University of Munich, Munich, Germany d Laser Processing Research Centre, University of Manchester, Manchester, UK e Centre for Industrial Management, Katholieke Universiteit Leuven, Leuven, Belgium f Laser Zentrum Hannover e.V., Hannover, Germany g Bremen Institute for Applied Beam Technology and University of Bremen, Bremen, Germany b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Laser Processing Laser welding Cutting Surface modification

Laser material processing is a technology with an increasing number of industrial applications, especially for aviation and automotive. The present keynote paper focusses on the advances in macro-scale laser processing, meaning that the laser generated structures are large enough to be visible with the naked eye. Next to that, also the reasons for the advances, especially the improvements regarding the power and the beam quality of the laser sources, the higher capabilities of the systems technologies and an increasing process understanding are shown. Nevertheless, challenges like a holistic understanding of the physical phenomena and the evolution of defects in some processing methods remain to be solved in future. © 2018 Published by Elsevier Ltd on behalf of CIRP.

1. Introduction to advanced macro-scale laser processing Due to its high flexibility and potentially high processing speed, the laser is used as a tool in an increasing number of industrial applications, which will lead to a doubling of the world market for laser material processing from 2011 to 2020 to a total amount of 43 Billion Euros [15]. Moreover, using the laser as a tool contributes to the goal of a waste-free production by reducing the use of resources and increases production efficiency [15]. Thus, laser material processing was a topic of prior CIRP keynote papers, focusing on additive manufacturing [223] or micro material processing [141]. In contrast, this keynote paper focusses on the advances in macro-scale laser processing. For confining the subject the macro-scale scale is defined as the length scale on which laser generated structures are large enough to be visible with the naked eye and without magnifying optical instruments [100]. In recent years, an exponential increase in possible applications for macro-scale laser processing techniques can be observed. The present keynote paper aims to summarize the reasons for this increase, shows specific examples for new applications for different processes and gives an outlook on emerging technologies.

industrial sectors where new potential applications for the laser as a tool for material processing evolve. However, each market has its own requirements and regulations regarding e.g. the size of the machined structures and quality definitions up to safety regulations. These specific demands, which act as driving forces for the development of new laser machining processes, are shown in the following section. 2.1. Markets Macro-scale laser processing includes cutting and joining processes as well as surface treatment. A large number of application areas can be identified, e.g. laser cutting or laser welding, drilling, cleaning and structuring [200]. Revenues of the laser material processing market show a strong dominance of macro material processing with >1 kW power sources, accounting for 59% of total sales. This area also recorded a strong growth rate of 9%. These revenues are mainly attributable to the cutting of metal, especially sheet metal, as displayed in Fig. 1. Laser

2. Driving forces for advances in laser machining processes As the major driving forces for advances in laser machining new markets, materials and systems technology can been identified. Especially aviation, automotive and heavy industries are innovative * Corresponding author at: Institute of Photonic Technologies, FriedrichAlexander-Universität Erlangen-Nürnberg, Erlangen, Germany. E-mail addresses: [email protected], [email protected] (M. Schmidt).

Fig. 1. Laser revenues by application [24].

https://doi.org/10.1016/j.cirp.2018.05.006 0007-8506/© 2018 Published by Elsevier Ltd on behalf of CIRP.

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beam welding recorded a very high growth rate (17%) in 2015, due to the strongly increased sales figures of the global automotive industry. The most important markets for laser material processing are presented in the following sub-sections [24]. 2.1.1. Aviation Lightweight components are of particular interest to the aviation industry. The goal of weight reduction must be accompanied by high production efficiency and component performance. Laser beam welding and brazing offers the possibility to join light metals (e.g. aluminum, titanium, magnesium) and their combinations. Especially reinforced titanium structures for aerospace applications show the potential of a combination of laser welding and straightening for the precision manufacturing. However, laser welding of those light metals still suffers from statistically occurring weld seam irregularities such as notches or holes, which reduce the mechanical properties of the joint.

radiation on large workpieces such as those used in shipbuilding. In this area of application, the workpiece is no longer transported to a special workstation, but the beam source is used at different locations. In addition, the machining station does not have to be larger than the workpiece [218]. 2.1.4. Electronics industry Macro-scale laser material processing is increasingly applied in other markets due to technological and scientific progress. An example is the electronics industry, where wire bonding is often used to produce electrical contacts. Applications include chip-onboard technology, power module technology, high-frequency technology and the assembly of microsystems. Wire bonding is a welding process in which an automatically fed wire is joined by ultrasonic welding on two or more substrates [76]. To increase the process stability and to weld larger wire cross-sections, a laserbased wire bonding system might be used. However, highly brilliant laser radiation and beam oscillation are needed for this application to increase the joining area and to regulate the welding depth [157].

2.1.2. Automotive Increased product customization and decreased time-tomarket cause intensified levels of competition among car manufacturers. A high performance and sustainable production becomes an essential feature to address the growing consumer demand for greater variety of goods and services [35]. In the future, automotive manufacturing will be characterized by an increased automation, a higher flexibility, and a modular architecture [40]. The need for higher flexibility might be met by a larger operating distance. For decreasing the amount of CO2 emissions, lightweight construction is of particular importance [235]. The use of new materials such as aluminum and fiber reinforced plastics (FRP) requires new production processes for these materials. For example, the cutting process of carbon fiber reinforced plastics (CFRP) is a challenging task due to the abrasive character of the carbon fiber. Also new production processes for the connection of different materials are necessary. A joint of FRP and aluminum can be realized by thermal joining, whereby the polymer is melted by an external heat input and then pressed with the aluminum. However, in order to meet the requirements of the automotive industry regarding the joint strength, the metal surface must be pretreated. Depending on the polymer, a pulsed or a cw laser beam source can be used. The required process speeds for inline production pose high demands on the laser system with regard to the maximum output power of the beam source and the deflection speed of the optics [80–82]. Another approach to achieve the emission targets is the increasing electrification of the vehicle fleet. The production of the essential energy storage systems provides completely new areas of applications for laser material processing. In particular, the production of the battery modules and the corresponding electrical contacting of the battery cells might be executed by laser radiation [137,139,224,225]. Lasers can also be used throughout the entire process chain for the production of battery cells, e.g. for cutting electrodes [128,135], contacting the battery stacks, or welding the battery housing [114]. Another challenge associated with electro-mobility is the processing of highly conductive materials such as copper or aluminum. The high reflectivity complicates the laser process, so that beam sources with high intensities or wavelengths in the visible range must be used [55,144].

The advance in systems technology is another important reason for new possible applications. The ongoing change from CO2 to solid state lasers with a better beam quality, so called highbrightness lasers, and new direct diode lasers with higher wallplug efficiency opens new fields of applications. Especially the beam quality is an important feature of a laser source since a better beam quality (i.e. a smaller beam parameter product) allows a smaller laser beam focus on the work piece, smaller processing optics or a larger working distance between the processing optic and the work piece. Particularly the latter one increases processing speed and thus efficiency. On the other hand, an increased process understanding, supported by new process observation and modelling methods, allows a better use of available process windows and the development of new application scenarios. The defined driving forces and their effects are described in detail in the following sections.

2.1.3. Heavy industries For heavy industries, new products and production possibilities are developed to machine the large structures used, such as thick profile welding in shipyards, steel structures under permanent stress and large steel structures for pressure vessels [218]. These areas require welding with large welding depths. Flexible beam guidance and the development of compact and robust beam sources are needed to make it possible to use laser

2.2.1. From CO2 to solid state Typical macro-scale laser processing is characterized by the conversion of photonic into thermal energy by radiation absorption and the resulting well-defined, spatially limited heating of the material to be processed. In this respect, the laser wavelength is a decisive factor for the interaction with the material, in particular, as industrial laser processing can only be realized economically if processing velocity and quality are high enough. The absorption of

2.1.5. Requirements due to Industry 4.0 Industry 4.0 is one of the biggest trends in production engineering. Recent advances in the manufacturing industry have led to the use of cyber-physical systems (CPS), in which information from all perspectives is recorded, monitored and exchanged between the physical factory floor and the cyber computational space. In addition, networked machines can work more efficiently, cooperatively, and robustly by using advanced information analysis [136]. Within laser material processing, the progress of suitable sensor technology is the enabler to fully exploit the potential of Industry 4.0 at the manufacturing level. Examples are flexible and reconfigurable systems that adapt to changing requirements or self-optimizing processes that can independently acquire or derive process parameters for a new task [14,164]. The requirements cannot be fulfilled at present, or only partially. For this reason, intensive work is required in the field of manufacturing technology in order to develop new solutions such as predictive models or inline process monitoring. Overall, the extensive data collection combined with Industry 4.0 can lead to a further continuous growth across all markets presented. 2.2. Systems technology

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laser radiation by different materials depends strongly on the wavelength of the radiation. This leads to different application fields for laser beam sources with their characteristic wavelengths. While metals generally show a high absorption for shorter wavelengths around 1 mm and below, e.g. glasses require longer wavelengths for an appropriate absorption [145]. Today, highpower laser systems exist for most of the interesting wavelength regimes, so that there is a good chance that the optimal laser wavelength for processing a specific material can be found. One of the most common industrial laser systems is still the CO2 laser with its wavelength in the middle-infrared (MIR) region [182], achieving wall-plug efficiencies between 15% and 20% [116]. It shows a relatively high degree of absorption for a lot of natural and synthetic materials [286], especially for polymers and polymer composites. Consequently, CO2 lasers are mainly used in high-throughput laser cutting processes, e.g. for plastics, wood etc., but also for cutting and welding metals such as steel, using laser powers of 20 kW, such as the Trumpf TruFlow [31], shown in Fig. 2.

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Recently, there have been several laser systems developed which are actually replacing the conventional solid state lasers. The first of these new laser types is the disk laser [64,187,268], in which the active laser medium, i.e. the laser crystal, is a thin crystalline, mostly disk shaped layer made e.g. from yttrium aluminum garnet highly doped with ytterbium (Yb:YAG, wavelength 1030 nm). The thickness of the active layer typically amounts from 100 mm to 200 mm [268]. The back side of this layer is coated with a highly reflecting material and serves as resonator mirror. The laser radiation is generated by multiple passes of the diode laser pump radiation through the active layer. On the back side, there is also a heat sink which enables an effective cooling of the disk, taking into account the rather large beam diameter compared to the layer thickness. This construction helps to avoid thermal lens effects and thus is a prerequisite for the high beam quality and the high wall-plug efficiency of more than 25% [25,77]. The maximum output power currently available for industrial application is 16 kW combining several laser disks [265], as shown in Fig. 3.

Fig. 3. One beam source of a 16 kW disk laser providing 4 kW from a single disk; the black body in the center covers the laser active medium [262]. Fig. 2. Beam source of a 20 kW CO2 laser with folded resonator [263].

Despite established several decades ago, the CO2 laser technology was still advancing by further developments leading to laser powers of 100 kW in 2004 [231]. The main wavelength of these laser systems is 10.6 mm. However, the amplification of other MIR wavelengths such as 9.4 mm is possible with band-selective resonator optics. A widespread industrial high-power CO2 laser system, providing high efficiency and beam quality, is the so-called slab laser (e.g. Coherent/Rofin DC 080 with 8 kW optical output power [42]). A short-pulsed CO2 laser with pulse durations in the upper nanosecond range is the so-called TEA (transversal excited atmospheric pressure) laser. These lasers are used e.g. for marking applications. Apart from this, it is possible to actively Q-switch a CO2 laser by means of a rotating mirror or an electro-optic switch, achieving pulse durations in the lower microsecond range. In this way, peak powers up to gigawatts can be reached [282]. In recent years, laser systems emitting in the near-infrared (NIR) wavelength range have become increasingly important in the field of materials processing. A main reason for this development is the possibility to guide the NIR radiation through optical waveguides, thus providing a maximal processing flexibility. Initially, solid state lasers with yttrium aluminum garnet (YAG) crystals, doped e.g. with neodymium ions (Nd:YAG lasers, wavelength 1064 nm [63]), as active laser medium were the most prominent lasers in this wavelength range. The wall-plug efficiency of this laser type is rather low considering the classical pumping method with flash lamps (2–5% [70]). Using laser diodes as narrow-banded pumping sources, the wall-plug efficiency can be enhanced considerably up to more than 10% [25]. Due to thermal lens effects, Nd:YAG lasers allow only for limited beam qualities. Such lasers, providing optical output powers of up to several kW in continuous-wave mode, have been used extensively e.g. for cutting and welding applications with metallic materials. In pulsed mode, mostly generated by Q-switching [118], Nd:YAG lasers are often used for marking applications.

This kind of laser source can be used for several processes in the field of laser material processing such as e.g. high power welding processes [22,23,236] and laser-arc hybrid welding processes [131,233,234]. The high beam quality described by a small beam parameter product (BPP 2 mm mrad for TruDisk 2000 with an output power of 2 kW [265]), allows for the usage of an optical waveguide with a minimal diameter of 50 mm and thus a beam focus diameter on the work piece in the same size. Disk lasers can also be pulsed by means of Q-switching or mode coupling. For instance, a fiber-coupled disk laser with an average output power of 1.5 kW at a wavelength of 1030 nm with a maximum pulse energy of 80 mJ has been developed [28,250]. In addition, disk lasers exist as frequency-doubled lasers, i.e. at an emission wavelength of 515 nm generated by second harmonic generation (SHG), with a laser output power of up to a few hundred watts [264] and even as frequency-tripled laser systems emitting in the ultraviolet (UV) wavelength range at 343 nm, generated by third harmonic generation (THG) [264]. SHG and THG are typically performed by irradiation of specific non-linear crystalline materials (e.g. potassium dihydrogenphosphate), positioned subsequent to the fiber resonator, with intense radiation of the fundamental wavelength (e.g. 1030 nm) [27,58,116,160]. The second new laser type operating in the NIR wavelength range with high output power and beam quality is the fiber laser. Here, the active laser medium is the core of a thin glass fiber doped with rare earth elements such as erbium, ytterbium, neodymium or thulium [60,78,186,278]. The most common fiber laser wavelength for industrial laser materials processing is 1070 nm (Yb-doped fiber) due to the high gain [214]. The excitation energy can be coupled into the glass fiber parallel to the fiber core by using the radiation of fiber-coupled laser diodes, which are spliced to the fiber laser fiber [281]. A more efficient pumping can be achieved by using so-called double-clad fibers [102,197], consisting of three layers (core, inner cladding and outer cladding with decreasing refractive indices). Here, the inner core acts as a single-mode fiber

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for the laser emission, while the inner cladding and core together guide the pump radiation. The laser resonator is typically defined by fiber Bragg gratings (FBGs) which are generated by means of the radiation e.g. of an UV laser [158]. Apart from the high beam quality, fiber lasers are characterized by high radiation conversion efficiency, an effective cooling through the surface of the fiber, thus avoiding thermal lens effects, a compact and maintenance-free design and an effective production technology with fiberintegrated optical components. To achieve high optical output powers in the kW range, several fiber lasers with advanced pumping laser diodes are combined. Accordingly, laser output powers of more than 100 kW in continuous wave mode with wallplug efficiencies of more than 40% are available for industrial laser material processing, as shown in Fig. 4 [104].

Fig. 4. 100 kW laser used in [104].

On the other hand, single-mode lasers, i.e. laser systems with highest beam qualities (TEM00 mode) can be purchased with output powers of up to 10 kW [181,240]. Due to their high beam quality, such lasers are ideal tools for remote processing. Meanwhile, high-power fiber laser systems with wavelengths of 1567 nm [95] and in the range between 1,900 nm and 2040 nm [96] are available as well. Furthermore, there are new fiber laser systems emitting in the visible (VIS) range at 532 nm [97], where the laser wavelength is equal to the second harmonic of the fundamental wavelength of a Nd:YAG solid state laser (1064 nm). Pulsed operation of fiber lasers is possible by Q-switching, mode coupling (for ultrashort pulses) or usage as master oscillator power amplifier (MOPA). The latter variant offers the advantage that the pulse duration is defined by the seed laser, which can be a tunable diode laser for example. However, the laser power density transportable in the fiber is limited due to the small core diameter, i.e. too high power densities destroy the fiber core material. Consequently, the pulse peak power is limited as well. A typical ratio between achievable peak power and average power of pulsed fiber lasers is about 10 [79]. Moreover, ultrashort-pulsed fiber lasers are available [264]. The third type of new promising laser systems emitting radiation in the NIR wavelength range is represented by highbrightness direct diode lasers (semiconductor lasers) with high beam quality. As mentioned before, laser diodes are used to pump modern solid state (disk and fiber) lasers. However, these radiation-emitting semiconductor elements can also be used directly to build up laser systems for materials processing [134]. Such systems are of increasing interest as they are robust, compact and efficient (wall-plug efficiencies up to more than 60% due to the direct conversion of electric energy to laser radiation [172]). To achieve higher output powers required for macro-scale laser processing, several single-emitting diodes with output powers of few watts (this value is enhanced permanently) are combined electrically and optically to laser bars with output powers in the order of 100 W or more, mounted on an adequate heat sink [11,48]. The optical combination is complex, as the single diodes show strong divergence and asymmetric radiation characteristics parallel and perpendicular to the diode junction plane

(slow axis and fast axis), and requires a complex set of micro-optic components. As a result of this construction, the beam quality is rather low. Thus, this type of laser system, typically providing laser wavelengths between 808 nm and 980 nm, which decreases the material processing efficiency. Thus, these systems are suited only for welding applications with polymers (laser transmission welding). To reach higher power levels, diode laser bars, in turn, have been combined to stacks with output powers of up to more than 1 kW [11,185]. Due to the high packing density and power flux density, specific micro-channel heat sinks with water cooling are required to remove the excessive energy. Further laser power increase up to the multi-kilowatt range is performed by combining different polarization directions and wavelengths. For instance, two diode laser stacks, each of them emitting linearly polarized light, can be combined almost without quality and power loss by mounting them in an orthogonal configuration. Using these multikilowatt diode lasers it is even possible to weld high reflective metals such as aluminum and copper [242]. The most powerful diode laser systems currently available for industrial laser processing provide output powers of more than 20 kW in continuous-wave mode [132]. An exemplary application of those lasers is the welding of steel with 40 kW [154]. However, those systems show a low beam quality and emit a broad wavelength range from 900 to 1080 nm. Thus, the processing efficiency is also quite low. To obtain diode lasers with high power together with high beam quality, suitable for remote operation or cutting processes, recent advances have been made by using dense wavelength division multiplexing to combine narrow-banded laser light (973– 979 nm) of different diodes [86,279]. This technology has already been used to build high-brilliance diode lasers with an output power of 4 kW, as presented by Trumpf [93] and shown in Fig. 5.

Fig. 5. Beam source of a diode laser with two beam exits (left) and multiple diode stacks (right) [261].

2.2.2. Systems: advances in beam delivery and deflection In order to enable to process material using the optimal beam source, the beam has to be delivered from the laser to the handling system and/or processing head including the focusing unit. Delivering laser radiation can be performed in free space using mirrors or coupled into flexible fibers. Using CO2 lasers is limiting the beam delivery to mirrors, resulting in a mechanical and thermal coupling of beam source and processing head. This leads to the necessity to design the laser processing system both mechanically and thermally robust and increases the investment costs. When using solid-state laser sources or direct diode lasers with a wavelength in the range of 1 mm fibers, which are often referred to as light cables, can be used to transport the light. This results in a mechanical and thermal decoupling of beam source and processing head and allows some significant advantages: (i) decreasing the cost of machines by reducing the rigidity, (ii) completely

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different approaches for the machine concepts. Fiber guiding is the main driver to enable robot based machining systems which are used e.g. for welding. Still, it has to be kept in mind, that there are mirror based beam guiding systems for robot integration available on the market for special applications. For technical uses the fiber core and cladding have to be protected by a jacket to protect the delicate fiber. The jacket often contains water cooling to avoid overheating due to optical losses. Fibers are currently available for wavelengths ranging from 800 nm to 1100 nm for macro processing. The maximum length of optical fibers for high power transmission is in the range of up to 100 m, enabling a centralized beam source unit to deliver laser radiation to wide spread processing systems. Depending on the core diameter fibers can be categorized in single-mode and multi-mode fibers. Currently, mostly multi-mode fibers with minimal available core diameters in the range of 30 mm are used in industrial scenarios. The maximum available fiber diameters are around 2 mm [87], but these larger core diameters are restricting the flexibility of the optical cables. The necessity to use large core diameter fibers is caused by possible damage of the fiber facet when coupling highly intensive light into or out of the fiber [155]. Those high intensities are a result from focusing high average powers in the range of 20 kW to small areas or applying pulsed beam sources with pulse durations in the nanosecond regime with moderate average powers of about 1 kW. Multi-mode fibers are easy to use in industrial application since light coupling into a fiber is quite easy. The largest disadvantage of multi-mode fibers is the fact that the beam profile changes throughout the fiber length. Thus, a Gaussian shaped beam with high intensity in the center will change to a so-called flat-top beam profile, where the intensity is quite equally distributed over the beam diameter, as shown in Fig. 6.

Fig. 6. A Gaussian beam profile is transferred to a flat-top beam profile after transfer through a multi-mode fiber [133].

In contrast, single-mode fibers have a core diameter of smaller than 10 mm which allows the transport of only a single propagation mode per polarization direction per wavelength. This allows that the transverse intensity profile at the fiber output has a fixed shape, thus a Gaussian beam generated from a laser source will also have a Gaussian shape at the fiber exit. However, there is a need for laser beams with a low BPP and coupling of a laser beam into the fiber is challenging. Moreover, when using high intensity lasers stimulated Brillouin scattering and Rayleigh backscattering can occur [220], leading to an increase of losses in the fiber. Thus, large mode area (LMA) fibers with a larger core diameter are used for decreasing the laser beam intensity inside the fiber. However, for avoiding the evolution of multiple modes inside the fiber, the change of the transversal refractive index in the fiber core should be minimized. Thus, photonic crystal fibers [117] might be used in future applications. Summarizing, beam delivery systems are preferably fibers which depict numerous benefits in comparison to mirrors. However, for CO2 lasers mirror based beam delivery systems have to be used, because there are no fibers available to transport light in the wavelength range of 10 mm with high average power. Processing heads and handling systems are essential for achieving the processing tasks. Processing heads have to solve the task of focusing laser radiation to the desired spot size and, in some cases, to shape the intensity distribution, e.g. to a so-called top-hat. Today, they often also have to fulfill monitoring and sensor tasks for quality assurance and closed loop control of the process.

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Classical processing heads are often used for metal cutting, welding and laser cladding. They rely on handling systems to create the relative movement between work piece and laser focus and can be integrated into classical flat-bed machines as well as for robot based processing systems. For a lot of applications scanning the laser beam across the work piece surface and in some cases into it (i.e. a movement of the focal spot in three dimensions) is demanded. With the advances made in developing new high-brightness laser sources with a good beam quality the laser beams can be focused over a higher distance and thus the distance between the focusing optics and the work piece can be increased. This development lead to different types of mirror systems which allow one-, two- and (principally) threedimensional deflection of the focal spot, while essentially only one- and two-dimensional scanning is commercially available, see Table 1. The scanner systems always increase the speed of the laser spot compared to the conventional movement, which is the welding speed. The speed of the laser spot relative to the work piece is called travel speed, e.g. it is the absolute value of the velocity between laser spot and work piece. It is defined by the sum of the components of the velocities in x-, y- and z-direction generated by the scanner and the welding speed. The welding speed is the macro-scale processing speed, e.g. the relative speed between the work piece and the processing zone. Table 1 Scanner systems. The maximum power and maximum amplitude can be applied only alternatively, not in combination. Scanner type

Dimension

Max. frequency

Max. laser power

Max. angle/ amplitude

See e.g.

Spring pendulum Galvo Galvo Galvo Galvo

1D

1 kHz

30 kW

20 mm

[7,10]

1D 2D 2D 2  2D

0.5 kHz 4 kHz 0.5 kHz 1.2 kHz

15 kW 3 kW 12 kW 8 kW

4 mm 3.1 mrad 3 mm 1 mm

[18] [277] [72,98] [165,210]

Tasks that benefit from fast beam deflection often use galvo scanners. They consist of galvanometric driven mirrors to realize deflection velocities of the laser focus of up to 10 m/s and are often used for remote processing [283]. In contrast to large field scanner welding systems developed for 2D or 3D remote welding (i.e. welding with a large distance between the focusing optics and the work piece) which primarily provide for a programmable welding path independent of or combined with the movement of robotic axis, the scanners considered in this section are typically designed to provide a high scan frequency at small scan amplitude. It should be noted, however, that the scan dynamics depend largely on the size of the systems, i.e. a high scan frequency will require small and lightweight mirrors, thus limiting the permissible laser beam diameter and power level. Moreover, a high scan frequency will only allow for limited scan amplitude. The working field of a galvo scanner relies on the chosen focusing length. A typical focusing length in macro-scale processing is 450 mm which results in a working field of 2000 mm. Therefore, most applications in macro-scale processing rely on an additional handling system for the scan head, such as axes or robots. A recent approach (dimension ‘2  2D’ in Table 1) introduces a set of four galvanometer scanners to superpose large field 2D remote welding with local high frequency 2D beam oscillation, thus allowing for both a higher precision by active seam tracking, and a higher weld seam quality due to stabilization of keyhole and melt pool [165]. Higher scanning speeds for random-access beam deflection can be achieved by acousto-optic deflectors (AOD) but at a reduced working field [21]. Such devices are especially important for high repetition laser systems to increase productivity and quality by avoiding thermal effects like debris [33].

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Fig. 7. Different process emissions for the exemplary process of laser metal deep penetration welding; own work.

Observation by means of high speed imaging makes the process behavior accessible [57]. Due to the increased frame rates of modern high speed cameras process features with high fluctuations can be observed with high accuracy and reliability [257] and correlations between the different process features can be quantified [256]. In cutting processes flow characteristics of the melt at the cut front can be identified [198]. Furthermore, emissions in the VIS can be used to determine keyhole length in deep penetration welding which allows the adjustment of the laser power in-situ in order to realize a defined penetration depth [2]. By adaption of the illumination, the morphology of parts of the process zone can be measured and correlated to process stability [59]. By using cameras working in the IR range, temperature information can be derived, whereas the emissivity of the observed materials has to be taken into account. Usually InGaAs-Chips are used to detect electromagnetic emissions in the range of 1.1– 1.9 mm. It has been shown, that thermographic imaging can be used to detect lack of fusion in welding technologies [85]. In order to gain absolute temperature measurements, the emitted spectrum has to be analyzed. Therefore, spectroscopy can be used. A combination of a spatially and spectrally resolving technique can be seen in hyperspectral imaging [243], whereby absolute temperature information might be derived. Common imaging technologies can only capture information from directly accessible features of the process zone. In order to get information from interfaces of liquid and solid material and their geometry, X-ray imaging technologies can be applied [8]. It has been shown, that the highly inaccessible shape of the keyhole and the melt pool boundary to the solid base material in deep penetration welding can be imaged by using X-ray imaging technology [1]. Moreover, the formation of melt spatter when welding copper could by analyzed by the use of this technique [83]. Optical coherence tomography (OCT) can be used to determine geometrical data of the interaction zone between laser and material without the need for an elaborate x-ray setup. By the use of OCT, topographic information of surfaces or the penetration depth in welding technologies [19] can be derived. There, it was shown that fluctuations in the depth can be linked to pore formation [30]. Fluctuations of the pressure inside the process zone lead to acoustic emissions which can be detected in form of air- or structure-borne noise. Especially a change from one stable process state to another can lead to characteristic acoustic emissions [17]. Moreover, it has been shown that changing connections states of joining partners in welding [255] and brazing [258] leads to changes in the structure-borne noise. By combining data from multiple sensors, additional insights can be gained. It has been shown that X-ray imaging and high speed imaging can be combined to access the three dimensional shape of the melt pool-material interface [29]. Due to the high sample rates of used sensing techniques and the combination of multiple measurements, a structured processing of the acquired data is crucial. One example is the use of image processing algorithms to quantify melt and gas flows in laser metal welding [254]. Most of the mentioned approaches lead to an increased process understanding and the ability to widen existing process windows or even to open new applications. Another tool for reaching these goals is modelling, which is often used in connection to process observations. The state of the art of process modelling is shown in the next section.

Most important is the evaluation of parts of the emitted electromagnetic spectrum. Radiation from the visible (VIS) to infrared spectrum (IR) can be used to derive process information [239]. High speed imaging is used to capture a representation of the highly dynamic process zone [266]. Illumination systems might be used to make all parts of the accessible process zone visible. Other sensing techniques use properties of reflections of structured incident electromagnetic radiation for measurements.

2.3.2. Modelling First numerical process models of laser material processing date back more than 40 years ago [125,167,244]. Since then, many models and approaches from complex multiphysical process models mainly for fundamental research to computing time optimized tools for specific application in industry have evolved. Most attention has been paid to the simulation of laser beam welding. Typically, three subtypes are defined: process simulations, material simulations and structural simulations.

Another possibility to achieve this is beam shaping where the laser energy is distributed into the desired shape or process parallelization is made possible, which becomes more and more important as the average power of laser system increases. While static beam shaping can be achieved by fixed refractive or diffractive optical elements, dynamic beam shaping enables tailored energy distribution at hand. Two AODs can be used for this purpose [20,74] having the advantage over established beam shaping systems like liquid-crystal-on-silicon devices of sustaining higher powers up to several kW. Tailored energy distribution directly influences the temperature distribution in the work piece, but finding an optimum distribution experimentally without understanding the specific process properties is very costly in terms of time and money. For gaining an increased process understanding a lot of different approaches for process observation and modelling have been developed in recent years which are summarized in the next section. 2.3. Process understanding 2.3.1. Process observation The evolution of process defects is still limiting the number of possible macro-scale laser applications. One key factor to assure high quality in various process states is the identification of relevant operating parameters. Thus, different process observation techniques are used to gain new insights. By deriving characteristic relations between process emissions and process states, new control mechanisms can be determined to overcome respective limitations. Process emissions can be caused by a variety of phenomena. In most laser-based processes matter is present in multiple states. If the melting temperature of a solid material is exceeded, the solid material transits into the liquid phase and a melt pool can be formed. Above the melt pool, vaporized material might reach the plasma state. In all four cases different mechanisms of measureable process emissions occur, as exemplary shown in Fig. 7 for laser metal deep penetration welding.

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Process simulations are used to gain information about quantities and characteristics during the welding process itself, e.g. temperature fields, weld pool geometry and process stability. In basic models, an equivalent heat source, mostly the Goldak double ellipsoid, is employed to calculate the energy input [68]. More complex multiphysical models, however, calculate the energy input through angle-dependent absorption and the formation of the keyhole directly. They mostly employ a FiniteVolume (FVM) and multiphase Volume-of-Fluid (VoF) description and have become increasingly accurate and have proven to be able to deepen process understanding and explaining the interaction of the different physical aspects of the process [119,179,183,252,285,287]. Material models aim at reproducing the microstructure, the resulting material properties and associated phenomena such as hot cracks or hydrogen pores [159,193]. Especially for the prediction of hot cracking, these models are often coupled with structural simulations [50,196]. Structural simulations are conducted to find the resulting residual stresses and distortion in the welded part and mostly use a Finite-Element description (FEM). Many models partially or fully decouple the thermo-mechanical problem, which turned out to be in good agreements with experimental values [168,189]. In some cases only the mechanics of the process is calculated, based on techniques such as plasticity-based distortion analysis or inherent strain method [103,195]. Due to the similarity of the underlining physics, the models for laser bending are also very similar to those of the structural welding simulations. Again, mostly FEM approaches are used and the focus is often put on computationally efficient calculations [92,123,194]. Only very few works exist on laser brazing, which either focus only on a single process aspect, for example wetting, or extend a process model originally developed for laser welding [49,217,248]. Multiple simulation approaches exist for the various laser cutting processes. Regarding gas assisted fusion cutting, several fluid-dynamical FVM models were developed investigating the cut front angle, melt expulsion and striation structure and the influence of beam properties [4,56,120,121,126]. Also, approaches transforming the physics involved into a free boundary problem were developed, making the problem computationally efficiently solvable [171,190,201]. Only very few models, however, were developed or applied onto the relatively new processes remote fusion cutting (RFC) and remote ablation cutting (RAC), mainly investigating the role of the vapor pressure, the processes’ main driving force [121,156,178]. Although most modeling effort on drilling and ablation focuses on ultrashort laser pulses, there are also several models on laser drilling with short pulses. Similarly to laser cutting, a model based on the free boundary formulation of the physics was published, reducing the dimensionality and therefore the computational complexity of the problem [124,229]. But also FVM and FEM models set up similar to process simulation models for laser welding were employed to explain melt and vapor dynamics and the processing results in dependence of various process parameters such as the pulse length [3,138,178,216]. The described driving forces due to new systems technologies and an increased process understanding lead to new technologies and applications for macro-scale laser processes, which are shown in the next chapter. 3. Technologies and applications The advances in systems technology, process observation methods and modeling approaches lead to new applications in laser macro-scale processing. In the following, an overview on those new applications is given. This overview is restricted to cutting, joining, changing material properties, alloying and drilling since these are the most relevant applications for macro-scale laser processing [221].

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3.1. Cutting As illustrated in Section 2.1, cutting is the most relevant industrial application in the context of macro-scale laser processing by far. Also, a substantial growth in metal cutting could be observed for the last years. This is usually ascribed to a higher quality and productivity that can be achieved through the use of lasers compared to conventional cutting methods. In terms of cut quality, not only smooth and clean, but also square and welldefined cut edges can potentially be produced. The cut edges even can be directly rewelded without further cleaning or treatment necessary. As for the process characteristics, laser cutting excels by its high flexibility and machining rates. This mainly results from the fact that it is a non-contact cutting process. Also, system technologies capable of guiding the laser beam or positioning the component in respect to the laser beam highly dynamically became readily available. Thus, complex trajectories, both twoand three-dimensional, can be cut and adapted individually with little effort. This allows for economical manufacturing particularly for small batch sizes [246]. As a result of the intrinsic characteristics of laser radiation, energy can be provided in a very precise and well-defined manner. This is considered advantageous, in particular for cutting applications. With kerf widths in the order of 0.1 mm, fine and profilecutting can be performed. High quality cuts with low thermal stresses and small heat-affected zones can be fabricated because the heat input is confined to a small area. In addition, laser radiation can be used to process a huge variety of materials including polymers, metals, wood, glass and even difficult-tomachine materials such as advanced ceramics [44]. For a more detailed discussion of the interactions during laser cutting, the machining is subsequently divided into processes with and without process gas. In gas-assisted techniques, the material is heated locally, molten and subsequently blown out of the cut kerf. In processes without assist gas, in contrast, the material removal is based on vaporization of the material rather than it being ejected by the assist gas. 3.1.1. Gas-assisted cutting One reason for the broad applicability of laser cutting to a large variety of materials can be found in the different methods of cutting that can be achieved. These are essentially fusion cutting using a jet of inert gas, reactive fusion cutting using an oxygen gas jet as additional energy input, evaporative cutting especially suited for low enthalpy of vaporization materials like polymers and organic materials and the controlled fracture technique, as a rather mechanical process suited particularly for brittle materials like ceramics [44]. Since fusion cutting represents the most common laser-based process, most scientific and industrial attention was focused on this process compared to the other gas-assisted cutting processes, therefore most advances were made regarding this process [121]. During fusion cutting, the material is heated just above the liquidus temperature. Parallel to the laser beam, an inert process gas is applied at high pressure typically by means of a conical nozzle. Consequently, the assist gas induces a momentum to the material. As soon as the material is molten and the momentum of the assist gas on the melt film exceeds the surface tension forces, it is blown out of the kerf vertically. Thus, from a thermodynamic point of view this is a very efficient method to remove material. The most drastic upheaval in laser fusion cutting was caused by the change from 10 mm radiation using CO2 lasers to 1 mm radiation using solid state lasers. Due to the higher absorptivity of metals at the wavelength of 1 mm and the general economic benefits of solid state lasers, solid state lasers soon replaced CO2 lasers as the most widespread beam source for laser fusion cutting [201]. However, this triumph has two limitations. First of all, it turned out that for sheets thicker than 3–5 mm, the wavelength of 10 mm remains more beneficial for the process due to the larger Brewster angle, leading to increased absorption. A second cause,

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often attributed to this phenomenon, concerns multiple reflections within the cut kerf, so-called waveguiding. This obviously beneficial and important effect for cutting of thick section sheets, is often assigned to 10 mm radiation, but as simulations revealed, multiple reflections are more prominent for 1 mm and this waveguiding effect cannot be referred to as another reason for this effect [191]. The second major drawback of laser fusion cutting with 1 mm radiation concerns the quality. Cuts with a thickness larger than 2 mm produced with this wavelength suffer from large surface roughness, which correlates quite well with the onset of the influence of multiple reflections, which can destabilize the cutting zone, leading to coarse striations [191]. Striation refers to periodic lines appearing on the cut surfaces, which have been associated with laser cutting for the last 40 years. They have been investigated ever since then, in the beginning only for CO2 lasers, but since the problem aggravated so much with the upheaval of solid state lasers, they have come more to the fore of research. Striation results in rough cut surfaces and stress concentrators. There have been several suggested mechanisms for striation formation including separate melting fronts between the laser generated melt pool and oxygen gas induced sideways burning [9], self-limiting of oxygen diffusion due to surface oxidization [99], fluctuation of laser power and gas flow that leads to hydrodynamic instability [230] and melt removal instabilities caused by pressure gradient driven melt flow and varying melt droplet size within the cut kerf [38,153,270]. To avoid striation during laser cutting of mild steels an approach with a slightly defocused laser beam, a lower assist gas pressure (1–2 bar) than used for standard laser cutting, a lower beam divergence across the cut depth, and cutting at a specific speed was reported [143,241]. In order to improve the surface quality further, an equation for optimized cutting speeds for striation-reduced laser cutting of mild steel was developed [143]. The improvement regarding the surface roughness can be taken from Fig. 8. Although laser cutting is faster than mechanical cutting, due to the differences in material properties between carbon fibers and the epoxy resin, laser cutting of CFRP composites can often result in undesirable heat affected zones, exposing fibers, and cut kerf tapering. Due to the higher absorptivity of a wavelength in the range of 10 mm, CO2 laser cutting of CFRP would be more effective.

However, as described in Section 2.2.2, CO2 laser light cannot be delivered through optical fibers and is thus less easy to provide to the specific work piece. Thus, ultrafast lasers such as fs and ps lasers have emerged for cutting of CFRP, since they can cut CFRP by multiple pass, rapid vaporization and raster scanning ablation with almost no heat affected zones. However, the material removal rate, and thus the productivity, is too low for cutting large sheets of thicknesses beyond 1 mm. Therefore, fiber laser cutting of CFRP has been studied intensively within the last years. The use of N2 assist gas and focusing the laser beam slightly below the target material surface have been found to give a better cut quality with minimum resin matrix recession and kerf width [169]. 3.1.2. Remote cutting The already high productivity of laser machining processes can be further enhanced by increasing the distance between the laser optics and the component to be machined as well as by using highly dynamic scanning optics. According to laser welding processes, methods are typically referred to as remote processes for working distances larger than 300 mm [259]. This in turn implicates that no gas can be supplied to the process zone by nozzles that are attached to the laser optics, as is usually the case in the gas-assisted methods discussed above. The material removal, therefore, is governed by ablation processes. Remote ablation cutting is a method to gradually create a cutting kerf by means of a high intensity laser beam. The ablation depth after a single pass typically ranges from 30 mm to 100 mm [53,150]. For higher cutting depths, multiple passes are required accordingly. In contrast, gas-assisted methods are performed within a single pass. High-brightness laser sources are used to provide intensities in the order of 108 W/cm2 [53,152]. The threshold intensity at which a keyhole starts to form is typically estimated to be around 106 W/cm2, for comparison [199]. Therefore, it can be assumed that both liquid and vapor state of the material exist in the process zone. In fact, evidence was found for molten material [166] as well as for vaporized material exhibiting a significant recoil pressure [149]. The subsequent expansion of the metal vapor expels the molten material from the kerf ground toward the direction of the laser beam. The advantage of the applicability to a broad range of materials of laser cutting in general and of remote ablation cutting in particular can be seen in the good machinability of CFRP. A high-speed capture of the process and the excellent quality of the untreated cut edges of an exemplary component are illustrated in Fig. 9.

Fig. 9. Cutting of CFRP; based on Ref. [249].

Fig. 8. Surface scanned images using a white light interferometer showing (a) striation, (b) without striation in laser cutting mild steel sheets [143].

Also, the beam guidance by means of a scanning optics allows for easy adaption of the cutting strategy. This can be used to control the heat affected zone in the material and to significantly improve the mechanical characteristics and the fatigue resistance of machined components. This can be seen as an additional degree of freedom for the machining of CFRP components compared to conventional processes such as water jet cutting. It could be shown that the controlled heat input can be used to enhance the fatigue resistance compared to water jet cutting where no heat affected zone occurs [249]. However, the applicability of remote ablation cutting is restricted by the sheet thickness. One downside of this process is that the effective cutting speed drops drastically for high sheet thicknesses as the cutting path has to be scanned multiple times.

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For this reason, remote fusion cutting as an innovative approach to accomplish high cutting depths is the subject of current research. Remote fusion cutting is closely related to deep penetration welding, as the process essentially starts with a keyhole that is formed due to material vaporization. The cutting depth in this process is achieved in a single pass and is equal to the component thickness. Consequently, the keyhole that was initially formed opens, and the characteristic cutting front as shown in Fig. 10 is established. The molten material is accelerated downward due to the vapor pressure and humps occur on the cutting front.

Fig. 10. Schematic process illustration of remote fusion cutting; based on Ref. [226].

As the cutting kerf is open on both sides, a fraction of the laser radiation can exit the kerf at the bottom side without contributing to the material removal. The energy efficiency is therefore slightly below that of remote ablation cutting. Also, the cut edges are typically oxidized in remote fusion cutting whereas ablation cutting induces only minimal oxidization. As the material removal rate during ablation cutting drops drastically for high sheet thicknesses, research is concentrated on remote fusion cutting as a method to cut metal sheets up to several millimeters in thickness. A thorough understanding of the mechanisms involved is needed to enhance the suitability for industrial applications with complex component geometries. The first investigations on the cutting process were carried out on cutting steel sheets using CO2 lasers [6]. The expulsion of the molten material was found to be caused by a hydrostatic pressure in the melt. A two-dimensional thermal model was used to enhance the process understanding for cutting with pulsed CO2 lasers [65]. In further studies, the findings on the removal mechanisms and the boundary conditions of the process parameters were transferred and enhanced to enable cutting with highly brilliant laser sources [32]. Internal processes within the melt and the resulting melt pressure were identified as predominant effects for the material removal [232]. These findings could be confirmed by a numerical model that is capable of simulating both remote ablation and remote fusion cutting [180]. Also, it could be shown that existing models for laser welding can be adapted to simulate remote fusion cutting, as there are similar challenges in computing the geometry of the boundary surface between the liquid and the vapor state of the material for example. Further insights into the complex mechanisms can be gained by multiphysical simulations. Taking into account the dominant physical effects such as beam propagation, absorption, heat transport and fluid flow, a consistent simulation of the process is feasible. These models allow for the analysis of the process factors that are not observable by conventional experimental methods. An exemplary output of a two-phase model that calculates the free surface during remote fusion cutting is shown in Fig. 11. The model considers the effects mentioned above by means of a ray tracer and a computational fluid dynamics simulation based on the finite volume method [121].

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The simulation results are in good agreement with the process dynamics evaluated in experiments. Accordingly, the initial penetration as the material starts to evaporate can be seen (a) as well as the subsequent formation of a keyhole (b, c) and the establishment of the cutting front (d). Still, systematic correlations of simulative studies and experimental results are missing and are the subject of research in order to enable a broad industrial applicability of remote fusion cutting. In conclusion, the development of laser sources and beam guidance systems facilitates a highly dynamically and flexible energy deposition. For this reason, the laser became an advantageous tool in particular for cutting applications and could substitute the majority of conventional processes. Since a variety of beam characteristics are available and different process strategies can be implemented with little effort, a multitude of cutting methods can be utilized to machine almost every material. Therefore, there is a broad range of applications ranging from shipbuilding, where 15 mm thick plates have to be cut, to electronics applications, where new manufacturing processes have been developed, such as resistance trimming of circuits [246]. A holistic understanding of the diverse cutting methods is hindered by the existence of a multitude of process regimes, in which different physical effects dominate the process output. Such models help to increase the use of the laser in industrial applications, especially when complex components are considered. 3.2. Welding 3.2.1. Tailored energy distribution 3.2.1.1. Beam scanning. The achievements in beam quality have been the enabler for the efficient use of technologies, which are based on tailored energy distributions in laser beam processing, as described in Section 2.2.2. In the following, new applications which have been enabled by this development are discussed. A high-frequency 1D-scanner was used to realize a new type of welding, the so-called ‘buttonhole welding’ [271]. Therefore, a lateral scanning (in-plane perpendicular to the weld seam) was used during deep penetration welding of aluminum alloys. Within a certain processing window [227] a very smooth weld seam is achieved, having the surface quality of a brazing seam, see Fig. 12 for comparison.

Fig. 12. Comparison of weld seam appearance from standard laser beam welding and buttonhole welding [227].

Fig. 11. Simulation of remote fusion cutting; based on Ref. [121].

Simulation pointed out the influence of the wire on the formation of the buttonhole, which is initiated by the keyhole [39]. The buttonhole formation is affected by the shape of molten wire such as wire tip length and melting angle, which changes with the oscillation frequency of laser beam. Mathematically spoken, the buttonhole is a catenoid. It separates the melt pool into a

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turbulent part, comprising the fast moving keyhole and turbulent melt, and a smooth part, where solidification takes place. While most of the research using scanning devices for a tailored energy input focus on lateral or circular scanning, also longitudinal scanning (i.e. back and forth scanning parallel to the welding direction) can have beneficial effects. The use of such longitudinal beam oscillation can significantly decrease pore formation in deeppenetration laser welding of multiple-sheet specimens having extensive polymeric contaminations, see Fig. 13. The welding speed was maintained at 3 m/min [280].

Fig. 13. Amount of center porosity after welding of severely contaminated high alloyed steel layers as a function of the longitudinal oscillation width [280].

3.2.1.2. Beam shaping. With the introduction of efficient high power solid state lasers welding was more and more conducted with disc and fiber lasers instead of CO2-lasers. This change brought up problems with spatter and pores. Understanding the influence of the energy distribution on the porosity or spatter could yield a measure for its reduction. By analyzing the keyhole dynamics a variable called ‘spring parameter’ could be derived, which was useful to describe the keyhole properties, which in turn proved to determine the amount of pores and spatter [273]. The higher effective spring parameter of the top hat beam compared to a Gaussian beam leads to higher frequencies and lower amplitudes occurring in the keyhole oscillations. It is concluded that the beam profile can significantly affect the keyhole dynamics during deep penetration welding [275]. 3.2.1.3. Modelling. Especially for laser welding the modelling of heat input is challenging. While the Goldak source [68] is often used for deep penetration welding, a double-point source was proposed for the transition range between heat conduction welding and deep penetration welding. It was demonstrated that the double-point model can be used for the prediction of the welding depth and seam cross section [211]. An example for a special model, applicable to laser heat conduction welding, is given in Ref. [112]. This model can be used to predict the welding depth in heat conduction welding in overlap configuration, if some model parameters are determined experimentally for the actual case. 3.2.2. Delicate materials 3.2.2.1. Welding of copper. Welding of copper became more and more important during recent years due to electro mobility and energy storage systems. The high reflectivity, high thermal conductivity, and the steep changes of the absorption coefficient especially at temperatures where a change in the state of matter occurs are main factors for the difficulties in laser joining of copper and its alloys. The main problem beside the back reflection of laser irradiation into the laser (which may result in severe damage of the laser) is the formation of pinholes due to melt ejection. It has been

shown that the formation of a bubble at the keyhole root is the driving force for the melt ejections. Consequently, these ejections as well as pores can be avoided by a proper modulation of the laser power, which counteracts the formation and stability of bubbles [83]. Other investigations confirmed the influence of the laser power and welding speed, while an important influence of the shielding gas (Helium and no shielding gas were compared) was shown [144]. The negative influence of ambient air instead of Helium can be understood qualitatively from the changes in the melt pool cross section with a narrow throat and the increased weld depth, which both increase the probability for melt ejection. Due to the important role of bubbles with a certain size for the mechanism of melt ejection, pinholes do not occur in heat conduction welding of thin sheets. Using a shorter wavelength (515 nm) also increases the Fresnel absorption at the copper surface significantly, enabling heat conduction welding without any pores for thin copper sheets having a thickness of 0.3 mm [115]. At the same time, the laser power could be reduced without changing the welding speed. Others adapted the assumption that the boiling point of pure copper – and in turn the abrupt change of the state of matter – instead of a boiling range (as common for alloys) is the reason for seam defects like pores and pinholes. Reducing the ambient pressure was suitable to enhance the result, but for a sound weld seam, the combination with beam scanning had to be used [208]. The combination of a low ambient pressure (1 hPa) and slight scanning (1 mm scanning width) yield sound weld seams even for thick section welding [209]. It was shown that scanning alone can avoid pores and pinholes [72]. Above a minimum speed of approx. 5 m/min sound welds are observed. The result is unchanged, whether this speed is achieved by the welding speed (i.e. the linear feed forward) alone or the travel speed during scanning. A welding depth of up to 1.3 mm was achieved using a 1.5 kW single mode fiber laser [72]. All in all, the problem of pinholes might be understood as a result of overheating of the welding root, which will depend on the material due to differences in absorption. For a given reflectivity R the number of reflections n can be calculated for a residual beam intensity In from In = I0 Rn, where I0 is the initial intensity. If e.g. 99% of the power have to be absorbed, the residual relative intensity is In/I0 = 0.01 and the number of reflections is given by n = ln (In/I0)/ln (R). As the absorption of the laser beam even on molten copper is low, much more multiple reflections are necessary for absorption of 99% of the beam, compared to other materials. The rough estimate leads to some 75 reflections for copper, while for Aluminum already 55 reflections and for steel less than 12 reflections are sufficient. As soon as a cavity is built at the weld root, the laser beam might be ‘caught’ in this cavity, i.e. the remaining energy is absorbed by multiple reflections inside this cavity. This stabilizes the cavity, enabling a permeation of shielding gas, which is heated further and in turn produces the melt ejection. In vacuum welding, the permeation of shielding gas cannot occur, which reduces the ejections, as only the copper vapor is further heated. In aluminum alloys, the residual energy is much less and therefore not sufficient in most cases to produce ejections, but in bead on plate welding process pores remain. In full penetration welding, the residual energy is transmitted and no process pores occur due to this mechanism. In steel the absorption of the energy occurs in much less reflections, which results in less residual energy and overheating. Thus, less process pores are observed. 3.2.2.2. Welding of press-hardened steel. Press-hardened steels are used for light weight design of cars due to their very high tensile strength of more than 1000 MPa. An AlSi-coating is applied to avoid oxidation during the sheet hot forming process. During welding, this coating can be converted to a brittle layer of intermetallic phases parallel to the fusion line. This is a metallurgical notch. Fracture can be shifted from the heat affected zone to that brittle zone [113] both during static or cyclic testing

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[54], reducing the strength of welded joints. There are different options to avoid the loss of strength by this metallurgical notch. One option is the design approach: joint positions are placed in low loaded regions of the subassembly, where the reduced strength of the joint is not critical. One other possibility to avoid the notch effect is to remove the AlSi-coating before welding. This is an additional step, which might be done by using short pulse lasers [271]. It was shown that the embrittlement of the steel weld seam by the aluminum contamination can be avoided by an appropriate decoating using short pulse lasers [54]. The third method to avoid both the reduction of strength and the additional processing step is to produce multiple parallel small welds, which add to an appropriate joint area and high joint strength [129]. Due to the smaller intersection line of the AlSicoated surface and the weld seam, the amount of Aluminum is small enough to avoid inclusions of a critical size. This improved the performance of the welds [129]. The high strength of press hardened steel increases also the springback. The scatter in springback becomes more difficult to be controlled; therefore sufficient gap bridgeability is demanded. Using a lateral scanned single mode laser in GMA hybrid welding gap bridging is feasible up to 1 mm. This can be achieved with a high welding velocity of 6 m/min, see Fig. 14 [130].

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Fig. 15. A cross section view of laser lap welded zinc coated steel on an aluminum alloy sheet [37].

Fig. 16. A cross section view of laser butt welded Ti alloy and nickel alloy sheets [36].

Fig. 14. Proof of gap bridgeability up to 1 mm in welding of 22MnB5 steel.

3.2.2.3. Hybrid joints. There is already a long history of research on laser welded hybrid joints from dissimilar metals like titanium to aluminum with applications to aircraft manufacture [127] or copper to aluminum with applications to Li-ion batteries [276]. Due to the different material properties (e.g. melting point, thermal conductivity, density, laser beam absorptivity) of the two different materials to be welded, their welding is very challenging. Often brittle intermetallic phases are generated in the mixed material weld zones. Fig. 15 gives an example of successful welding of zinc-coated steel with an Al alloy for automotive manufacture application without leaving gaps [37]. Also successful welding of Ni alloy and Ti alloy has been demonstrated by the same research group, as shown in Fig. 16 [36]. The key to the success for welding dissimilar materials has been in both applications to tailor the energy distributions and thermal history such that brittle intermetallic phases can be minimized or eliminated. Combining metals and polymers came up more recently. The strength of metal to polymer joints made by laser irradiation is strongly dependent on the chemical and micro-geometrical structure of the interface. Despite the fact that undercuts and other microstructural features enhance the bond strength, the roughness value alone is not suited to predict the bond strength of a joint between the steel DC01 and polymer PA6.6 [26]. For metal to

polymer welding of the polymer COP to high alloyed steel SUS304 the formation of oxygen functional groups were found to be essential. The pre-treatment of COP lead to such groups which interfered with the oxide layer on the steel surface, improving the joint strength [7]. Sandwich plates from steel and polymer were welded using a CO2-Laser. It was shown that the attenuation behavior of the composite is slightly deteriorated, but welding of tailored blanks by the method was feasible [219]. Of course pores and spatter arise due to evaporation of the polymer core. This can be avoided by a thermal pre-treatment of the edges of the sheet. By laser heating to temperature just below the melting point of the metal plates, polymer is evaporated, which leads to defect free joints afterwards [219]. A similar problem arises in joining of the aluminum-titaniuminterface element from aluminum-CFRP-hybrid joints, as was already mentioned above. 3.2.3. Augmented welding There are different approaches to enhance the welding results or widen the process limits in laser welding to weld thicker sheets. The penetration depth of laser welding is strongly dependent on laser power density and welding speed. A common knowledge of weld penetration depth is typically 1 mm/kW laser power. For a 10 kW laser, the maximum welding penetration depth in a single pass is around 10 mm for most engineering materials and even less for aluminum alloys. Also as the laser power density increases to achieve higher penetration depth, excessive plasma formation and higher vapor pressure would lead to blocking of the laser beam and the formation of various weld defects such as spatter and partial weld dropout. 3.2.3.1. Hybrid welding. Hybrid welding was introduced in 1979 by Steen [245] in order to augment the laser energy for thick plate welding due to the lack of stronger lasers at that time. Since then, mechanisms are discussed about the beam-arc-interaction, as the hybrid process proved to be more stable than the sub-processes by its own. It was shown by experiments that even a low laser power

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of 300 W can take stabilizing influence on TIG arc [251]. In that case, the interpretation of the effect was intensively discussed. Other effects were more clearly, like the stabilizing effect on the arc at the moment of appearance of a laser induced keyhole. The stabilization could be seen from a reduction of the arc voltage and the more straight trace of the arc on the work piece [162]. Hybrid welding combines the beneficial features of both processes. Therefore, it is often used for enhancement of gap bridge ability. While the target of the first approach using hybrid welding was thick plate welding, advance is made in thin plate welding [260]. Using a scanning beam of a 1 kW single mode laser and a MIG process, low alloyed steel was joined in butt weld configuration having a gap of 50% of the sheet thickness s0 = 1.4 mm. 3.2.3.2. Electromagnetic forces. Another method to augment the laser welding process is the application of magnetic fields; both constant or alternating magnetic fields were evaluated. The key issue is the Lorentz force, which is induced in the melt. One aim was to support the melt especially in thick plate welding against sagging; another aim was to enhance the homogenization in welding with filler wire. Melt sagging in thick plate welding in down-hand position can be explained by the hydrostatic pressure of the melt, which is on its own sufficient to exceed the Laplace pressure [73]. Magnetic fields can be used as a contactless support (comparable to a backing plate) to avoid seam sagging and drop out in thick plate welding of high alloyed steel [13]. A FEM simulation of the relevant forces which determine the fluid flow during welding has showed that a magnetic flux of only 70 mT is sufficient in welding of aluminum plates of 20 mm in thickness to compensate the gravitation forces [12]. The influence of constant alternating magnetic fields on homogenization of the melt was investigated by in-process high speed X-ray evaluation of the melt flow. Tungsten and tin tracer particles were used to determine the influence of the flow speed. It was shown that both the mean velocity and the maximum velocity of the melt flow can be increased by the magnetic field, while static fields had the strongest effect, see Fig. 17 [62].

Ambient pressures down to 1 mbar at high laser powers up to 26 kW were investigated [105]. Experimental results show a strong influence of the welding speed and ambient pressure on the penetration depth (see Fig. 18) and the weld shape. For welding of 304 type steel the occurrence of humping was seen in some cases. It is assumed that the interaction of the welding plume above the keyhole and the laser beam is detrimental at atmospheric pressure. The measurement of the beam deflection by the welding plume using a cross probe technique showed a significant reduction of this laser-plume-interaction at reduced pressure [105].

Fig. 18. Influence of welding speed and ambient pressure on welding depth [105].

The higher process stability in welding under vacuum was attributed to the different influence of the ambient pressure on the melting and boiling temperature by the following mechanism. While the boiling temperature is strongly decreased with decreasing pressure, the melting point is essentially unchanged. It is argued that this leads to thinner liquid films between the solid material and the metal vapor in the keyhole. The thinner film in turn is less prone to instabilities [206]. Another advantage of laser welding in vacuum is important for welding of delicate materials like titanium alloys. In laser vacuum welding at 0.1 hPa, no problems occurred with oxidation during welding of titanium. Butt joints with a plate thickness (TiAl4V) of 40 mm were performed without defects using a 16 kW disk laser [207]. 3.3. Brazing

Fig. 17. Maximum and mean value of the local velocities of the tungsten tracer particles for different flux densities and frequencies [62].

3.2.3.3. Welding under reduced pressure. Despite the fact that one of the advantages of laser welding compared to electron beam welding is the unconfined use at ambient atmospheric pressure, welding under reduced pressure was investigated to clarify potential benefits. Increased welding depth and process stability as well as reduced problems with shielding against oxidation are the benefits of the application of reduced ambient pressure.

3.3.1. Energy efficiency One of the problems in laser brazing is the interaction of wire feed and energy supply to the seam surface. Trailing wire feed, which is common for brazing, results in shadowing of the gap between the work pieces; in turn, this reduces the wetting and penetration depth and finally the strength of the brazing seam. These effects in comparison to twin beam brazing are explained in Ref. [88]. Like in early attempts (see e.g. [75]) two separate beams from different laser sources are used, but the high investment costs proved to be the show stopper. The size of the brazing head was a further problem. This was addressed in Ref. [71], where a twin spot brazing head with reduced size is shown. By the use of this system, sound wetting depth was achieved even at high brazing speed of 8 m/min [71]. One solution for an energy efficient and fast brazing process is proposed in Ref. [161]. Instead of a twin beam, a single beam is used, but due to a modified setup, the laser light reflected from the wire can be used for preheating of the surfaces inside the gap. Sound welds having a high wetting length at high brazing speed can be achieved, see Fig. 19.

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Fig. 19. Top surface and cross section of a brazing seam using a setup which harnesses the reflected light [161].

Another approach to increase the efficiency of laser brazing by making use of two-dimensional tailored energy input by lateral scanning is demonstrated in Ref. [204]. There, the feasibility of keyhole brazing is shown. In this process a single mode laser is used at low focal diameter and high scanning speed, while the lateral scanning was done with a width less than the wire diameter. The wire absorbed the laser energy very well due to the keyhole process and the heating of the substrate is done by heat conduction from the overheated wire material. Thus, sound brazing seams are achieved (see Fig. 20). A low wetting angle and no melting of the base material can be seen. Future process enhancements shall increase the brazing speed and the wetting width.

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An alternative to the hybrid brazing head is the use of active brazing wire. In Ref. [203] the feasibility of fluxless brazing of aluminium by using a lithium-alloyed Al-Si-wire was demonstrated the first time. The wire was manufactured by casting and rotary swaging. Despite the fact, that only standard shielding gas was used during laser brazing, sound results could be achieved [203]. In the case of zinc coated steel, wetting may not come to an equilibrium state due to fast cooling, which is influenced by the evaporation of zinc. It was shown that more than 20% of the energy during solidification of the AlSi12 brazing wire can be dissipated by the cooling effect of the evaporation of the zinc layer. On the other hand, when welding aluminum to steel, the evaporation is of less importance, if the filler material comes into contact not only with the zinc coated steel surface, but also with the aluminum alloy. In that case the heat conduction into aluminum becomes dominant [61]. Scanning in feed direction was used to enhance wetting when brazing zinc coated steel using copper based filler wire for body in white-production [84]. The experiments showed a more stable brazing process with scanning, which resulted in improved appearance of the seam. 3.3.3. Reliability Brazing of zinc coated steel using CuSi3 filler wire was analyzed concerning the sensitivity of the alignment of the laser beam and the seam. By using a thoroughly validated FEM model it could be shown that a small lateral misalignment of up to 0.4 mm is not critical, but above that value the seam becomes very asymmetric due to the asymmetry in heat input [49]. This results in less or even no bonding at one side. Brazing is often used for joints with high strength and thermal stability (compared to e.g. gluing) and materials which must not be affected too much by heating or even melting. In these cases, welding is very often not feasible. Thin foils are an example for such delicate materials. Thus, it was suggested to use droplet joining as a measure for such tasks [69]. By the use of this technique, thin silver foils (thickness below 30 mm) could be joined, see Fig. 22. The advantage of that method is the very good control of energy input into the work piece, which is indirectly done via overheating of the droplet material and not directly by irradiation of the work piece [69].

Fig. 20. Brazing seam from deep penetration brazing [204].

3.3.2. Wetting efficiency A further environmental issue of brazing is how to achieve sound wetting results. Fluxes are commonly used especially for laser brazing of aluminum alloys, but due to different disadvantages concerning environment, processing costs, and corrosion behavior they should be abandoned. This is feasible, if the cleaning effect is derived by an alternative method. It was shown that a combined laser and plasma arc process leads to sound wetting [202], see Fig. 21. By an in-process monitoring of the treatment using the hybrid brazing head the advance of the cleaned zone was measured. The differences between AW-5083 and AW-1050 were explained by the evaporation of magnesium in case of AW-5083, leading to a stabilization of the plasma arc [205].

Fig. 22. Joint made by droplet welding. Base metal and wire for the droplets are made of silver [69].

3.4. Changing material properties

Fig. 21. Wetting from brazing on aluminium with flux (left) and fluxless with a laserplasma arc brazing head [163].

The properties of a surface without intended change of geometry can be changed by laser hardening, remelting, disperging or alloying. One common problem of these methods occurs during treatment of larger surfaces by parallel traces. In that case, parts of the previous (already cooled) trace can be annealed by lateral heat flow. This can result e.g. in unintended reduction of hardness, as it was modeled in Ref. [253]. However, due to the gained advances regarding the beam quality of laser sources smaller beam foci are possible. Moreover, the possibility to deflect the beam with high

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speed and in arbitrary shapes allows a tailored energy distribution and thus a tailored treatment of the work pieces. Therefore, laser hardening can be done by laser heating in traces up to 100 mm in width [215], which helps to avoid the problem of overlap in heating at least for flat samples. Another approach was proposed in Ref. [177]. Cylindrical work pieces are usually laser hardened by irradiation of a helical path on the mantle surface. By increasing the rotational speed, a nearly homogeneous temperature at an annular ring at the surface is derived. This ring is shifted along the cylinder, yielding a homogeneous hardening [177]. While the hardening depth is limited to 2 mm [215], specific advantages of laser hardening compared to other hardening processes are the reduced or even vanishing amount of post processing and the cost reduction by reduced energy consumption and the elimination of external cooling processes by e.g. water spray [215]. Moreover, increasing the surface temperature above the melting point leads to surface remelting, which can be used for the reduction of the surface roughness and surface hardening [247]. Depending on the thickness of the molten layer, cooling rates may exceed 104 K/s, leaving a white layer behind. In the case of 42CrMo4 (EN10083-3) this layer consists of martensite and showed a Vickers hardness of 550. The surface quality was deteriorated by craters which built up during remelting by evaporation of residual contaminations, which result from the processing route during steel making. The difference between disperging and alloying is given by the behavior of added material during remelting, if it stays solid (disperging) or dissolutes in the melt. Usually, remelting, disperging and alloying are done as a heat conduction process, i.e. the energy is absorbed only at the surface of the work piece and transfers into the depth by heat conduction and – after melting – by convection. In Ref. [272] a deep penetration alloying by using a keyhole process for the alloying and disperging process is proposed. The difference to heat conduction alloying is twofold: first, a smaller spot diameter is used, which leads to the formation of a keyhole instead of Fresnel absorption at the surface. Second, the forward feed motion of the beam at the surface is overlayed by a scanning action. The latter lead to a kind of stirring in the melt pool and is intended to improve the homogeneity of the distribution of the added material. This new process (called ‘deep alloying’ both for alloying and disperging) has the advantage, that nearly rectangular traces of much higher depth are derived, instead of the shallow ones of the circle segment, which is observed after heat conduction alloying. In Ref. [274] the influence of the particle material on the distribution after deep penetration disperging in

Fig. 23. Particle distribution along the depth of deep penetration disperged traces [274].

aluminum is investigated. While the influence of the scanning path was of minor importance TiB2 showed the best homogeneity along the depth, see Fig. 23. The effects were attributed to the material properties, which have an influence on wetting and the kinetic energy at the moment of entry into the melt [274]. The deep alloying can be used for different purposes. In Ref. [269] the use of the process in a process chain, which also comprises selective laser melting and/or thin film coating for the predeposition of the alloying components is shown. By changing the thickness of the layers and/or the melting depth during the deep alloying process, a high number of different alloy compositions can be generated. These are used within a high throughput method for material development [269], which was proposed by Ref. [151] and is rather new to be used for high-performance structural materials. The handling of powder material might be a safety issue in industrial application. In Ref. [91] a variant of deep alloying with wire instead of powder as additional material is proposed. However, this might lead to an unequal distribution of the alloying elements. Thus, highly dynamic beam scanning technologies are used and the influence of different beam oscillation frequencies on the mixing behavior of the alloys is analyzed. It was shown that a scanning frequency of 200 Hz is useful for proper homogenization of the added elements, which were carbon and some carbide forming elements like tungsten, molybdenum and vanadium. The concentration of the elements after deep alloying shows local deviations of less than 0.2% [89]. Despite the fact, that the rectangular cross sections may be added easier to a larger volume with constant depth than circle segments, there may be an unwanted heat treatment of the previous trace, when scanning along parallel traces. This problem was solved taking advantage from FEM-simulation [90]. The simulation helped to determine the processing parameters (line energy and process distance), where no influence of a trace on the previous trace is given. This was possible due to the fact, that the critical transition temperature was as low as 100  C. As long as the temperature of one area does not drop below this temperature, reheating by the heat from the following track does not affect the hardness [90]. 3.5. Surface cladding and alloying The laser cladding process has been subject of continuous investigations over the past 30 years [288]. Due to these ongoing research activities, nowadays, this technology is not only used for cladding applications, but also for repair works of investment goods and in additive manufacturing to generate near net shape parts. One of the major advantages of the laser cladding process is the strong interface bond [38]. However, due to further process specifics like the possibilities of modifying free form surfaces, locally tailoring surface qualities and processing an extensive material portfolio, laser cladding became a versatile process for various applications. Not only the starting material, which is often powder, but also the vast number of processing parameters are responsible for the high complexity of this technology. Hence, selecting the right parameters for a stable and reproducible cladding process becomes difficult and resource intensive. A design of experiments method was used already 15 year ago [41], to cope this challenge. While that work was focused experimentally on base parameter like travel speed, powder feed rate etc., other work used analytical models to understand the meaning of processing parameters, for the process efficiency regarding the powder usage [184]. More recent work addresses further parameters like the angles of the set-up, i.e. the tilt angle of the work piece out of the horizontal position and the angle between the normal vector of the work piece and the laser beam [222]. Latest research activities within the field of laser cladding are focusing on the usage of high power laser sources or scanning optics in order to increase deposition rate or to manipulate the coating process. By using scanner optics for guiding the focused laser beam on the substrate surface and adjusting the scanning

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amplitude and the laser powder the overall geometry of the generated clad can be adjusted to specific needs and hence, the laser cladding process becomes more versatile [188]. For coating applications high deposition rates are of major interest. Hence, high power solid state lasers with an output power >10 kW are used in order to generate corrosion resistant coatings [267]. Also direct diode lasers became a very useful power source for cladding applications [16]. Due to output powers of more than 6 kW and a rectangular beam profile, this laser source can be seen as good alternative to widely used CO2 and Yd:YAG lasers for coating applications. Despite the important information about the process delivered by the mentioned investigations, a further important parameter when cladding parts in industry is the geometry of the work piece, as it determines the actual heat flow. This may lead to unwanted changes of the cladding properties. Trial and error approaches or FEM simulation for the evaluation of the right processing parameters are not feasible, when treating single parts, e.g. for regeneration. Therefore a closed loop control is implemented. A two channel thermal camera with the benefit of compensation of emissivity was applied in a laser cladding process, which gives access to much more disturbance insensitive temperature field information than other available sensors. Thermal information recorded during laser cladding neither showed signs of falsifications due to powder, gas or fume, nor due to varying emissivity in the measured area, which enabled to clearly resolve the melt pool edges as well as the melt trail. At the end, the results proved to have much better quality in respect to thickness and dilution [122].

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actually prepared for real flight testing, as the structures passed the according readiness levels successfully. 3.6.1. Drilling of metallic materials with ceramic thermal barrier coatings Thermal barrier coated metallic materials are widely used in the aero-engines and gas turbine systems to enable higher operating temperatures. Cooling holes are often needed that require drilling through thermal barrier coatings (TBCs) and metal substrates at low drilling angles. Conventional electro-discharge machining is difficult to apply, as ceramic coatings are non-conductive. Typical problems in millisecond pulsed laser drilling of metallic materials with TBCs is the delamination of the TBCs on the leading edge when drilling at a lower angle. A study revealed that the main mechanisms in the delamination of TBC in low angle laser drilling are the melt/vapor ejection [237]. This problem was overcome by introducing a twin-gas jet laser drilling to re-direct the angle of melt/vapor ejection and delamination-free laser drilling of thermal barrier coated metallic materials was achieved [238]. The twin-gas jet laser drilling set up is shown in Fig. 25 and the results of laser drilling in comparison to standard laser drilling is shown in Fig. 26.

3.6. Drilling Laser drilling is widely applied in the aerospace industry (e.g. for cooling holes), automobile industry (e.g. for injection nozzles) and circuit manufacture, due to very high productivity compared with conventional mechanical and electro discharge machining. A great advantage is the capability of drilling through multiple layers of materials including conducting and non-conducting materials. Considerable studies had been carried out in understanding the heat and mass flows in laser drilling [146,147,170], prevention of spatter formation on the top surface [148] and hole taper control [142]. More recent developments in laser drilling include the drilling of metallic materials with ceramic thermal barrier coatings, and drilling of CFRP composites. One recent development of an industrial use of laser drilling is the high lift technology in aircraft manufacture. Panels with a high density of holes are produced by drilling 450 holes per second [228] which is only possible by the use of high-brilliance lasers and fast beam scanning. Structures like shown in Fig. 24 are produced by laser drilling, laser welding and laser straightening and are

Fig. 25. Illustration of twin gas jet laser drilling process [238].

Fig. 26. Comparison of laser drilling of nickel alloy with a thermal barrier coating (a) standard drilling leading edge, (b) standard drilling trailing edge, (c) twin gas jet drilling leading edge, (d) twin gas jet trailing edge cross sections [238].

Fig. 24. Laser drilled perforated structures to provide a possibility for Hybrid Laminar Flow Control (HLFC).

3.6.2. Ultrashort pulsed laser drilling In drilling strategies with cw and even short pulsed (1 n) lasers, a molten phase occurs during the ablation process leading to residues and even burrs at the edges of entrance and exit holes. Even larger regions surrounding the processed zone might be affected by thermal diffusion. This so-called heat affected zones

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(HAZ) are characterized by changed morphology and physical properties of the material, often visible due to coloring. Ultrashort pulsed (USP) lasers (typically <10 ps) offer the possibility to overcome this issue as with well-chosen processing parameters little to no heat is accumulated in the material, where the ablation depth per pulse corresponds to the optical penetration depth [173]. Therefore this processing technique is sometimes labelled as “cold ablation”, where the material is mostly vaporized and re-solidified debris is easily removable. Therefore also the inner surface of a hole is much smoother compared to common production techniques like micro-EDM [212]. Additionally much smaller structures compared to conventional laser drilling can be produced opening the possibility to have a more precise control of the final shape of a hole. In helical drilling the beam inclination angle and the position on the material can be freely chosen which allows for precisely controlling the taper geometry of the drilled holes [101]. Several different practical implementations for a helical drilling optic have been proposed in recent years [47,284,289]. While drilling durations are typically significantly higher for USP lasers as shown in Fig. 27, the higher quality and flexibility of the process can outweigh this drawback. USP drilling in industrial applications is currently used for diesel injector nozzles, shown in Fig. 28, where the geometry of each nozzle has to be controlled precisely to achieve the highest possible efficiency for combustion.

Fig. 27. Definition of the different drilling technologies and evaluation regarding drilling duration and precision [289].

Combining thermal and USP laser drilling, where in a first step a through-hole is drilled and in a second step the roughness of the surface and the HAZ is removed, can improve the productivity while still maintaining a high shape-accuracy [213]. Emerging high average power, high repetition rate USP lasers allow for combining both in a single laser set-up, where high fluences and high repetition rates lead to a thermal ablation while changing the parameters can yield a “cold ablation”. 3.7. Assisted machining Laser assisted machining refers to the use of a laser beam as a heat source to facilitate conventional machining for the improved performance, quality and cost. In recent years, due to the newly available high power lasers especially laser assisted milling and turning have been investigated. These processes involve the use of a laser beam for heating up the work piece locally prior to cutting to reduce hardness and brittleness of the material so that it can be more easily removed by cutting tools. It is mostly applied to difficult-to-machine materials such as ceramics, titanium alloys, nickel alloys, tool steels and metal matrix composites. Laser assisted machining of titanium alloys has shown a 170% increase in tool life due to the reduction in cutting forces [46]. Laser assisted machining of nickel alloys has shown a 200– 300% increase in tool life and an improvement of surface roughness by a factor of two to three when the material was locally heated up to 650  C [5]. In laser assisted machining of ceramics such as partially stabled zirconia it has shown an increase of tool life by 40 times when the material was heated to 900–1100  C [192]. In laser assisted machining of hardened tool steel in a turning process, a 100% increase of tool life was demonstrated [52]. In laser machining of particle reinforced metal matrix composite (MMC, A359 Al with 20% SiC), it has shown a 27% reduction in surface roughness and a 170–235% increase in tool life when the material was heated to 300  C [45]. The lasers used for laser assisted milling and turning are typically continuous wave (CW) diode lasers, CO2 lasers, and fiber lasers with powers of 200 W–5 kW. More recently a different type laser assisted machining was demonstrated. There, a pulsed laser was used to produce a pilot hole followed by either electrical discharge machining (EDM) or mechanical drilling [140,175,176]. Summarizing this chapter, manifold new applications have been evolved in recent years due to the advances in laser sources and beam deformation and deflection. However, next to the advances in the number of possible applications and the process quality, the ecological footprint of macro-scale laser processes is a key factor for industrial application. Thus, the following chapter shows how to calculate the ecological footprint in general and gives an overview on the ecological footprint of different laser material processes. 4. Ecological footprint

Fig. 28. Section of a diesel engine with glow plug and diesel injector [174].

Due to the energy intensity of laser systems as applied for manufacturing purposes, it could be assumed that the associated environmental impact of laser processing would mainly be related to the electricity generation. However, in order to have a more refined image of the different causes of environmental impact in major laser based manufacturing systems, all in- and outputs to such systems have to be considered and different modes in which the systems can be used have to be taken into account according to their probability of occurrence as observed in industrial practice. For a systematic description of environmental impact quantification methods underlying the results presented in the remainder of this section, the reader is referred to Refs. [107,108] and the emerging ISO 14955 standard [94]. For a full calculation of the ecological footprint not only the energy input, but also system requirements, such as assist gas, laser gas and other consumables have to be quantified. Also

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emissions to the environment and material waste streams are taken into account, with anticipation of the most likely waste treatment methods for recyclable fractions. The results of such inventarisation exercises allow to perform unit process Life Cycle Assessments (LCAs) for laser based processes. As result of an LCA study the environmental impact according to the different considered impact categories can be expressed in Ecopoints (Pts) for a functional unit that typically corresponds to an effective processing period (unit flow), e.g. an hour of effective operation. An Ecopoint is defined as a normalized impact score, with 1000 Pts corresponding to the impact of the average European over a full year [66]. The next section contains a summary of results obtained using this approach for a number of representative processes. 4.1. Impact assessment of laser use in manufacturing Position and control systems as well as heating and cooling systems often require considerable energy flows that could be considered as fixed system requirements when analyzing the system efficiency of a machine system for laser processing. Since the range of required laser powers is rather broad, from intensive processes like laser cutting and welding, that typically require multi-kW sources, over additive manufacturing processes consuming tens to hundreds of Watts, to pulsed microprocessing lasers consuming equivalent average powers of just a few Watt, these overhead power needs are not necessarily proportional. In order to illustrate this, the environmental impact of two rather extreme laser applications is documented below in Figs. 29– 32 laser cutting with multi-kW laser power and selective laser sintering of polymers with two 50 W CO2 sources. Fig. 30 shows the breakdown of the impact over different system aspects. Although not the only cause of impact, electricity consumption does prove to be an important factor for CO2 laser cutting, which also implies that the local electricity mix used in the region of use of the system will significantly influence the environmental effects. A more detailed analysis of the environmental impact of laser cutting can be found in Ref. [106]. Similarly Fig. 31 contains results for analysis of Selective Laser Sintering (SLS) on an EOSINT P760 [EOS, 2012] machine equipped with two 50W CO2 lasers. As can be seen in Fig. 32, in contrast with the laser cutting process, the material waste stream proves to be dominant in this process, which is directly related to the fact that about half of the non-sintered powder cannot be reused in the process after sieving the residual material. In comparison with other energy needs, the laser related energy consumption is only of secondary importance. A more detailed study of the environmental impact of SLS can be found in Ref. [110].

Fig. 30. Environmental impact [Pts] for 1 h of CO2-laser cutting at maximum output power according to the ReCiPe Endpoint (H) V1.04/Europe ReCiPe H/A [67] method: aggregated impact and impact categories scores for the average European energy mix [106].

Fig. 31. Environmental impact [Pts] for 1 h of SLS processing for different polyamide materials and coating thicknesses [106].

Fig. 32. Environmental impact distribution of 1 h of SLS processing on an EOSINT P760 machine tool using PA2200 polyamide with 0.12 mm layer thickness [109].

Fig. 29. Environmental impact [Pts] for 1 h of CO2-laser cutting at maximum output power according to the ReCiPe Endpoint (H) V1.04/Europe ReCiPe H/A [67] method: aggregated impact and impact categories scores for the average European energy mix [106].

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4.2. Influence of technological development For processes for which the environmental impact is dominated by the energy demand of the laser source, a shift towards more energy efficient technologies implies a significant impact reduction. The rather fast transition from CO2 to solid state fiber lasers for heavy duty industrial applications is a major observation in this context. In Fig. 33 measured source efficiencies are compared for CO2, fiber and disc lasers, with an obvious advantage for the solid state sources [111].

Fig. 34. Machine tool input power consumption as a function of the laser output power level for eight different CO2-laser cutting machine tools [111].

5. Emerging technologies

Fig. 33. Input power demand at maximum capacity for 53 different commercial available laser cutting machine tools for different source technologies [111].

Direct Diode Lasers also show potential for a significant efficiency improvement compared to CO2 technology. But the efficiency of individual source modules, typically around 40%, is affected by the beam coupling techniques used in higher power sources that in most cases require combining multiple wavelengths in a single optical system. Resulting absorptive heat losses in the optical path were found to reduce the observed laser system efficiencies to around 25% [43]. Recent measurements conducted by KU Leuven on commercial Fiber laser systems show laser source efficiencies of between 38% and 45% for 3 kW and 6 kW lasers, and machine tool efficiencies as high as 28% for laser cutting systems operating at full load. When assessing the full consequence of the ongoing technology shift, a substantial impact reduction can be observed, in line with the importance of the laser source energy consumption as part of the total impact. However, the effect of the technology shift on energy reuse, e.g. for workshop heating purposes in colder climates, needs to be taken into account, implying a partial rebound effect on the achievable improvement. 4.3. Planning and processing strategies minimizing impact When analyzing the opportunities for minimizing the environmental impact associated with laser processing, the substantial overhead in fixed energy consumption is relevant to consider. This is obvious in Fig. 34 for a range of laser cutting machine tools with multi-kW sources. Direct consequence of this observation is that utilizing a laser system at reduced power level corresponds to decreasing its system level efficiency. Formulated differently, maximizing the power output and correspondingly the feed rate of the laser processing system is typically the advisable optimization strategy. When multiple options for the maximum output of laser sources within a specific technology are available, opting for the source with the lowest nominal power that can successfully perform a task, and running it at maximum load is advisable. This strategy is compatible with the typical economic optimization target that also favors fast processing since this allows to minimize both the investment and operational costs [51].

Currently, only in 10–20% of possible applications lasers are used [15]. Accordingly already known laser applications have a big potential to enter the market by being cheaper or easier to handle in future. This chapter deals with such laser applications and those not known today. These can either evolve through technology push or market pull. In the following section, examples are given for both of these reasons for innovation to allow an outlook into the future of macroscale laser processing.

Fig. 35. Overview of the average leverage impact measure (LIM) on photonic technologies of enabled manufacturing industries, comparing the scores on 2010 and 20201.

5.1. Technology push Innovation created from technology normally is created from fundamental research and long-term Often disruptive Technologies are technology push created and themselves create new demand. An important factor for quantifying the technology push created by laser technology is the leverage effect [34]: “The proportional contribution photonics makes to the value of an end product or service, either by enabling/enhancing the productivity of the manufacturing process, or by providing/enhancing functionality in the end device, without which the end product would not be competitive.” (Definition from Ref. [34]). In other words: If 1 The choices made by the respondents to the survey are semi-quantitative and thus can be translated into LIMs. Based on further expert consultation we assigned typical fixed percentages as benchmarks to the values of the scores. “Crucial” should be interpreted as at least a 90% LIM and this is represented by a value score of 4. The other benchmark values were chosen similarly with score value 1 = 1% and score value 2 = 10% and a score value of 3 = 50% – thus giving a roughly logarithmic scale which agreed well with independent LIM estimates made by the project team. Once converted into LIM percentages the scores were averaged between respondents in the usual way.

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photonics technologies were not available, what proportion of the functionality of the end product would be reduced? EU did study different technology areas: Scanning, sensing & imaging; Information, communication and networks; Screens & Displays; Laser systems; Advanced Lighting; Photonic Energy Systems. The conclusion from EU [34] reads as follows. “The future dependency of enabled manufacturing industries and final markets will increase significantly (Fig. 35). This means that photonics is a highly dynamic and increasing technology and will [still be a] “key technology” in the next decade. Therefore, it can be seen as an important driver for innovation, also with new research based opportunities.” 5.2. Market pull Despite the fact that fundamental research is never driven by market pull, approaches found in fundamental research can be further developed by applied research which can in turn generate a market pull for the specific development of an end-product. Innovation is reasoned by specific needs of customers – marketing department triggers innovation. The innovation can be described as incremental where the application is known, as short-term development. Here the definition of the “end user” of a market pull driven development can be seen as the applicator of the laser process, e.g. a car manufacturer and not his customers. Megatrends can be identified as drivers for new technological solutions. Especially laser technologies are very promising to find such solutions [15]. Market pull can be quantified by the photonics market leverage (PML) which is “the size of the market affected i.e. the economic market value of photonics enabled manufacturing industries and final markets, that are impacted by photonic technologies, either by improving its productivity or creating new or better products or services with additional functionality” [34]. According to Ref. [34] the future situation looks as follows: Increase e.g. from 62 to 72% in electronics and optical equipment and 59–73% for manufacture of vehicles and large machinery (from 2010 to 2020). Key drivers for market pull driven innovation in laser technology are the miniaturization of products and processes, the increasing intelligence of systems, ubiquitous networks (IoT), robotization, virtual modeling and testing, convergence of technologies as well as new organizational business models to research (Open Innovation). The development of EUV technology is an example for massive leverage, whereas Mobility, especially electric vehicles is one of the main drivers of development in photonics, as can be seen by the increasing PML 59–73% for manufacture of vehicles and large machinery (2010–2020). The use of laser technology in battery manufacturing and cell phone manufacturing are further examples stemming from market pull. A next step will be intelligent systems technology created by a fusion of semi-conductors and laser systems technology to intelligent production systems that are flexible, agile and allow a high degree of individualization. As well with industry 4.0 the enhanced interconnect between supplier and OEM and as well the connection between supplier and client will allow even more and faster individualization. This is enabled by laser technology but will in return pull new innovation on the laser manufacturing side. Summing up the technological examples in this paper, the next big market pull for laser technology can already be identified and this one is looking for a technology push as enabler. In macroscale joining, may it be welding with high brightness cw lasers or brazing as well as in surface structuring with ultrashort pulsed lasers, the demand for a very fast beam deflection – comparable to electron beam movement – clearly shows up. The process related reasons are of course different but all deal with tailoring the heat input into the workpiece on a very fine level. This is only one example for the still unsolved and but partially unthought potential lying in the usage of light as a tool that will make this technology not only a driver in the decade but in this century.

19

Acknowledgements The authors thank the members of the CWG “Lasers in Production” for supporting this keynote. Moreover, the authors are thankful to Eric Eschner, Stefanie Kohl, Phil Koshy, Johannes Schilp, Volker Schulze, Thomas Seefeld, Panagiotis Stavropoulos, Felix Tenner and Konrad Wegener for their valuable support to this keynote.

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