Al/Cu bimetal components (BMCs) possess higher thermal conductivity, better wear resistance and corrosion resistance than Al and Al alloys. It is a big challenging task to fabricate a high-performance Al/Cu BMC by a conventional infrared (IR) laser-directed energy deposition (L-DED) process due to the crack formation and high IR laser reflectivity. A novel high-speed L-DED process was proposed to successfully fabricate a crack-free Al/Cu BMC by controlling the residual stress distribution and brittle intermetallic compounds (IMCs) formation. The damaging effect of the reflective IR laser rays on the laser equipment can also be suppressed. Compared with the Al base metal, the microhardness, wear and electrochemical corrosion performances of the Al/Cu BMC were significantly improved. The residual stress distribution was predicted and measured. The IMC formation mechanism was revealed based on element distributions, the formation energy and the effective heat of formation of each IMC. The study would provide the basis for manufacturing crack-free and high-performance multi-material components in many applications in different industries.

Significant performance improvement of modern laser technologies such as welding, additive manufacturing, brazing, cladding, sheet metal cutting, based on the use of multi-kW multimode fiber lasers, fiber-coupled solid-state and diode lasers, can be improved using beam shaping optics providing optimal energy distribution by splitting the laser beam into several beamlets creating by further focusing separate multiple spots in the working plane and variable sharing energy between these spots. Various multi-spot patterns, such as square, line, rhombus, consisting of 4 or 9 separate spots, allow eliminating or reducing spatter, realizing optimum temperature distribution in the melt pool and stabilizing the processes in welding of tailored blanks, copper and aluminium parts in the production of batteries, zinc coated steel, cladding. Multimode lasers are characterized by low spatial coherence (large BPP or M2 values), therefore the most reliable optical approach to control the intensity distribution is imaging the fiber end with a collimator and a focusing objective. The proposed multi-spot beam shaping method presents a combination of fiber end imaging and geometrical separation of focused spots perpendicular to the optical axis, thus creating a compound working spot, called as quattroXX-spot or peaXXus-spot, as a combination of several spots. Varying the energy portions in separate spots and the distances between them make it possible to optimize for a particular application common intensity distribution of the compound spot. To ensure reliable operation with multi-kW lasers and to avoid optics damage the multi-focus optical devices are designed as refractive elements with smooth optical surfaces made of optical materials self-compensating thermo-optical effects that provides insignificant thermal lensing and, hence, negligible thermal focus shift and spherical aberration. The paper presents the proposed multi-spot optics, shows intensity profile measurements and application results.

During laser processing, complex effects can occur regarding the laser-material interactions. A high laser energy input leads to surface melting and even boiling. The resulting recoil pressure can create the so-called keyhole, a vapor channel existing during welding and called cut front during laser cutting. On the keyhole front wall, the induced recoil pressure pushes the melt downwards and can ejects melt drops. Usually, those melt ejections are seen as undesired spattering or necessary waste to enable the cutting. However, outflow characteristics can tell more about the complex process behavior. Therefore, this work aimed to relate melt ejection formation effects to keyhole behavior in order to get a better understanding of the complex laser-matter-interactions and fluid flows. Axial beam shaping was used to create different energy inputs into the keyhole front walls. Beam shaping was done with an optic that can superposition up to four laser beams in axial direction, leading to varying intensity distributions on the inclined keyhole front walls. Based on high-speed image analysis, it was seen that different outflow characteristics occur depending on the beam shapes. A high intensity on the front keyhole wall could be related to high temperatures on the keyhole wall. The outflow mechanism was shown to be able to move from corrugating to atomizing drop generation at increasing temperature due to temperature-dependent material properties. The main influencing factors are assumed to be the vapor speed and the keyhole/drop diameters that define the outflow mechanism.

This book focuses on the mechanisms of how laser light is produced, guided, and focused for materials processing, and these are explained in an easy-to-understand language for practical use. It emphasizes a basic understanding of the principles necessary to run lasers in a safe and efficient way and provides information for quick access to laser materials processing for laser users. The book exhibits the following features:

• Provides simple explanations and descriptions of complex laser material interaction mechanisms to help readers understand relevant effects during laser beam irradiation of materials.

• Explains the main high-power laser materials processing methods, giving hints to get started with the processing and how to avoid imperfections.

• Focuses on high-power laser applications that are explained in an accessible, descriptive way with practical explanations and minimal formulas.

• Teaches how to measure laser beam characteristics and how to install and handle laser equipment correctly.

• Gives practical advice on typical equipment arrangements and parameter ranges.

This practical handbook serves as a guide for students studying production technologies to learn about laser processes, and for engineers who want to start working with laser processes safely and quickly.

Laser beam welding is an essential technology to enable the transformation to enforce e-mobility. When manufacturing light weight structures like the chassis, precision, speed, quality and low deformation can be expected when using the laser beam as a welding heat source. However, the laser beam is typically used at small dimensions and can fail to transfer its energy to the joining partners when the gap between them becomes large. Beam shaping technologies have developed in the last years to be flexibly used for high-power processes and provide an opportunity to alter the energy input and thereby improve the welding quality and gap bridgability. In this work, multi-spot beam shaping was analyzed using up to nine spots. Experiments were performed using different beam shapes in order to redistribute the energy input, recording the process using high-speed imaging for detection of melt pool dimensions. Those were used as input for a simplified analytical model predicting the process collapse based on the available melt material. Several beam shapes created melt pools that support the material availability behind the keyhole(s). Numerical simulations showed that directed melt flows induced by the keyhole(s) can increase the gap bridgability.

High-power laser interaction with a metal material can induce melting and even evaporation. However, the origin and content of vapor emissions based on temperature, material alloy elements, and other external conditions are not fully understood yet. Therefore, in this study, the content of laser-induced vapor was systematically examined during directed energy deposition processes. Single tracks of aluminum bronze were deposited with both continuous and periodically modulated laser powers. The duration and laser power of the modulations were set to achieve the same total line energy input. With the aid of those laser power modulations, controlled emissions were temporarily excited and observed. Optical emissions were captured with a spectrometer and a high-speed camera and related to the melt pool temperature signals and surface dynamics. The intensity of the emissions as well as the impact on the local chemical composition depend on the modulation parameters. Tracks deposited with short, high-power peaks in the modulation pattern showed chemical compositions comparable to those tracks that were continuously welded, whereby the intensity of spectral emissions was significantly increased. It can be concluded that the intensity of the measured spectral emissions correlates with the measured melt pool temperature signal and the dynamic movement of the vapor plume.

Surface tension is an important characteristic of materials. In particular at high temperatures, surface tension values are often unknown. However, for metals, these values are highly relevant in order to enable efficient industrial processing or simulation of material behavior. Plasma, electron or laser beam processes can induce such high energy inputs, which increase the metal temperatures to, and even above, boiling temperatures, e.g., during deep penetration welding or remote cutting. Unfortunately, both theoretical and experimental methods experience challenges in deriving surface tension values at high temperatures. Material models of metals have limitations in explaining complex ion interactions, and experimentally measuring temperature and surface tension at high temperatures is a challenge for methods and equipment. Therefore, surface wave analysis was conducted in this work to derive surface tension values around the boiling temperature of steel and identify trends. In addition, a simple ion interaction calculation was used to simulate the impacting parameters that define the surface tension. Since both the experimental values and simulation results indicate an increasing trend in surface tension above the boiling temperature, it is concluded that the dominating attractive forces above this temperature should increase with increasing temperature and lead to increasing surface tension forces in the surface layers of liquid metal.

Surface tension is an essential parameter that defines many aspects of materials processing. In particular, at high temperatures, surface tension data of metals is missing. Due to the challenges during high temperature measurements, mainly extrapolated or theoretical data are available. Therefore, an adaption of the falling oscillating droplet method is suggested to derive surface tension values of liquid steel surfaces. A spherical droplet was pre-positioned on plastic foil to be melted by a laser beam. During falling, high-speed imaging could record the oscillations and related frequency spectra were derived. Based on extracted characteristic frequencies, surface tension values were obtained comparing different theoretical models and adaptions to impacts of gravity, asphericity and viscosity of the material. The method was shown to give reasonable values of surface tension when accounting for gravity impacts.

Material properties of metals and metal alloys at high temperatures are often unknown, but necessary to understand physical mechanisms for prediction and improvement of high temperature processes, such as laser beam technologies. Surface tension is an elementary property that was measured in this study above the boiling temperature of steel using a laser-induced vapor channel in a steel substrate and the extraction of the vapor channel diameter from in-situ X-ray observations. The measurement principle is based on the pressure balance inside the keyhole, where the recoil pressure from keyhole wall vaporization works against the surface tension pressure from the surrounding melt pool. An increase in surface tension at increasing temperatures above the boiling point was measured against theoretical expectations. In order to create the keyhole shapes measured, the surface tension must increase to counterbalance the increasing recoil pressure.