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  • 1.
    Belelli, F.
    et al.
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 1, 20156, Milan, Italy.
    Casati, R.
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 1, 20156, Milan, Italy.
    Vedani, M.
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 1, 20156, Milan, Italy.
    Volpp, Jörg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Design and Characterization of Al–Mg–Si–Zr Alloys with Improved Laser Powder Bed Fusion Processability2022In: Metallurgical and Materials Transactions. A, ISSN 1073-5623, E-ISSN 1543-1940, Vol. 53, no 1, p. 331-343Article in journal (Refereed)
    Abstract [en]

    A key-factor for the industrial implementation of beam-based additive manufacturing technologies is the development of novel Al alloys characterized by enhanced hot-tearing resistance. Indeed, most of the standard Al alloys are susceptible to solidification cracking and can hardly be used to produce structural parts by laser-based additive manufacturing processes. In this study, we investigate the strategies to design high-strength Al alloys for Laser Powder Bed Fusion. The addition of Zr to the chemical composition of an Al–Mg–Si alloy (EN AW 6182) was carried out by following two different routes to promote the formation of equiaxed grains which are able to suppress hot cracking and enhance processability of the material. The first route is based on mechanical mixing of ZrH2 particles and gas-atomized Al alloy powder and on the in-situ reaction of the hydride to form Al3Zr nucleants. The second route relies on the use of pre-alloyed gas-atomized powders that feature Zr among the alloy elements. The specimens produced using pre-alloyed powder showed the best mechanical performance. After direct aging from the as-built condition, the alloy showed yield strength and ultimate tensile strength of 354 and 363 MPa, respectively, and elongation at fracture of 9.0 pct. The achieved properties are comparable to those of wrought 6182 alloy processed by conventional routes.

  • 2.
    Da Silva, Adrien
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Frostevarg, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Additive Manufacturing by laser-assisted drop deposition from a metal wire2021In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 209, article id 109987Article in journal (Refereed)
    Abstract [en]

    The subject of Additive Manufacturing includes numerous techniques, some of which have reached very high levels of development and are now used industrially. Other techniques such as Micro Droplet Deposition Manufacture are under development and present different manufacturing possibilities, but are employed only for low melting temperature metals. In this paper, the possibility of using a laser-based drop deposition technique for stainless-steel wire is investigated. This technique is expected to be a more flexible alternative to Laser Metal Wire Deposition. Laser Droplet Generation experiments were carried out in an attempt to accurately detach steel drops towards a desired position. High-speed imaging was used to observe drop generation and measure the direction of detachment of the drops. Two drop detachment techniques were investigated and the physical phenomena leading to the drop detachment are explained, wherein the drop weight, the surface tension and the recoil pressure play a major role. Optimised parameters for accurate single drop detachment were identified and then used to build multi-drop tracks. Tracks with an even geometry were produced, where the microstructure was influenced by the numerous drop depositions. The tracks showed a considerably higher hardness than the base wire, exhibiting a relatively homogeneous macro-hardness with a localised softening effect at the interfaces between drops.

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  • 3.
    Da Silva, Adrien
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Frostevarg, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander F.H
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Acceleration of metal drops in a laser beam2021In: Applied Physics A: Materials Science & Processing, ISSN 0947-8396, E-ISSN 1432-0630, Vol. 127, no 1, article id 4Article in journal (Refereed)
    Abstract [en]

    Different processes require the detachment of metal drops from a solid material using a laser beam as the heat source, for instance laser drop generation or cyclam. These techniques imply that the drops enter the laser beam, which might affect their trajectory. Also, many laser processes such as laser welding or additive manufacturing generate spatters that can be accelerated by the laser beam during flight and create defects on the material. This fundamental study aims at investigating the effects of a continuous power laser beam on the acceleration of intentionally detached drops and unintentionally detached spatters. Two materials were studied: 316L steel and AlSi5 aluminium alloy. High-speed imaging was used to measure the position of the drops and calculate their acceleration to compare it to theoretical models. Accelerations up to 11.2 g could be measured. The contributions of the vapor pressure, the recoil pressure, and the radiation pressure were investigated. The recoil pressure was found to be the main driving effect but other phenomena counteract this acceleration and reduce it by an order of magnitude of one to two. In addition, two different vaporization regimes were observed, resulting respectively in a vapor plume and in a vapor halo around the drop.

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  • 4.
    Da Silva, Adrien
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    The effects of laser irradiation on an aluminium powder stream in Directed Energy Deposition2021In: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810, Vol. 41, article id 101968Article in journal (Refereed)
    Abstract [en]

    Additive Manufacturing with aluminium alloys is a subject of increasing industrial interest. Directed Energy Deposition using high power lasers and a powder feed is a useful option but the interactions between the powder stream and the laser beam are not completely understood. It is well known that the powder particles heat up in the laser beam and some theoretical models predict that they can reach their vaporisation temperature and have their flight path altered by the associated recoil pressure. In order to learn more about these phenomena, powder streams were observed with a high-speed camera at different laser powers (up to 6 kW) and with three batches of powder (AlSi10Mg) of different particle sizes. The results showed an increase of powder focussing with increased laser power. In addition, some particles were found to disintegrate in the laser beam. It is demonstrated that particle disintegration is most likely to be caused by the momentum induced by the recoil pressure.

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  • 5.
    Da Silva, Adrien
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Wang, Sicong
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Vertical laser metal wire deposition of Al-Si alloys2020In: 11th CIRP Conference on Photonic Technologies [LANE 2020] / [ed] M. Schmidt, F. Vollertsen, E. Govekar, Elsevier, 2020, p. 341-345Conference paper (Refereed)
    Abstract [en]

    Additive Manufacturing of aluminium alloys has become crucial for lightweight applications. However, new materials and techniques need to be developed in order to achieve more advanced properties and higher efficiency. Therefore, a new energy-efficient wire deposition strategy was developed for processing aluminium-silicon alloys with Laser Metal Wire Deposition. Three alloys with different Si-contents were studied: AlSi5, AlSi10Mg and AlSi12. Different thicknesses of partially melted zones were observed and explained. The previous layer was partly remelted only by the heat conduction in the melt pool. It was found that the thickness of the partially melted zone depends on the difference of temperature between the liquidus and solidus.

  • 6.
    Dewi, Handika Sandra
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Fischer, Andreas
    University of Kassel, Institute of Materials Engineering – Metallic Materials.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Niendorf, Thomas
    University of Kassel, Institute of Materials Engineering – Metallic Materials.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Microstructure and mechanical properties of laser surface treated 44MnSiVS6 microalloyed steel2020In: Optics and Laser Technology, ISSN 0030-3992, E-ISSN 1879-2545, Vol. 127, article id 106139Article in journal (Refereed)
    Abstract [en]

    Fatigue property improvement for automotive components such as crankshafts can be achieved through material selection and tailored surface design. Microalloyed steels are of high interest for automotive applications due to their balanced properties, excellent hardenability and good machinability. Lasers facilitate efficient and precise surface processing and understanding the laser-material-property interrelationships is the key to process optimisation. This work examines microstructural development during laser surface treatment of 44MnSiVS6 microalloyed steel and the resulting mechanical properties. Laser beam shaping techniques are employed to evaluate the impact of beam shaping on the process. It revealed that ferrite structures remain in the treated area surrounded by martensite due to insufficient heating and dwell time of carbon diffusion.

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  • 7.
    Dewi, Handika Sandra
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Hullberg, Håkan
    Frostevarg, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Temperature distribution inside metal droplets influenced by tailored laser beam pulseManuscript (preprint) (Other academic)
  • 8.
    Dewi, Handika Sandra
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Pizarro Sanz, Rafael
    Sidenor, Bilbao, Spain.
    Fischer, Andreas
    University of Kassel, Institute of Materials Engineering –Metallic Materials, Kassel, Germany.
    Niendorf, Thomas
    University of Kassel, Institute of Materials Engineering –Metallic Materials, Kassel, Germany.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Carbon diffusion and martensitic transformation during laser surface treatmentIn: Article in journal (Other academic)
  • 9.
    Dewi, Handika Sandra
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Impact of laser beam oscillation strategies on surface treatment of microalloyed steel2020In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 32, no 4, article id 042006Article in journal (Refereed)
    Abstract [en]

    The depth homogeneity of laser-treated zones is one possible factor to define the quality and efficacy of altered mechanical properties in materials. For instance, half-rounded cross-sectional shapes of laser hardened zones using Gaussian beams provide dissimilar hardened depth in the edges and center of the treated area. This means that the in-depth distribution of compressive residual stress varies between the edges and the center of the hardened area. Nonhomogeneity of compressive residual stress distributions can inhibit fatigue properties and can lead to product failure. The utilization of oscillated laser beams has been proven to improve the welding efficiency and energy input distribution to the material, which promises achieving a homogeneous depth of laser-treated zones in hardening applications. Therefore, this work examines the influence of triangular, square, and circular beam oscillation strategies on the energy input distribution during the process and the geometry of the laser-treated zones on microalloyed steel. Laser beam pathways were assembled using a vector graphic editor to visualize the energy distribution from each oscillation strategy. Cross section images of the hardened tracks were taken and related to the thermal energy input profiles. It was revealed that each oscillation strategy demonstrates characteristic temporal and spatial thermal energy input distribution, influencing the geometry of the hardened zone. The circular oscillation strategy produced a widely constant depth in contrary to the triangular and square beam oscillation due to its characteristic energy distribution that allows homogeneous heat dissemination in the material. This confirms that the laser beam oscillation strategy can tailor the energy input distribution to optimize the processing outcome.

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  • 10.
    Dewi, Handika Sandra
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Frostevarg, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Siltanen, Jukka
    SSAB Europe Oy, Finland.
    Influence of mill scale on oxygen laser cutting processes2021In: IOP Conference Series: Materials Science and Engineering, Institute of Physics (IOP), 2021, Vol. 1135, article id 012008Conference paper (Refereed)
    Abstract [en]

    Mill scale formed on the surface of hot rolled steels consists of magnetite (Fe3O4), hematite (Fe2O3) and wustite (FeO) layers, which can protect the steels from corrosion and other atmospheric effects. Existence of mill scale on the specimens' surface has shown to be able to decrease the cut edge quality. Since the mechanism behind influence of mill scale on the laser cutting process is unknown, this work performs direct observation of oxygen laser cutting processes on specimens with and without removed mill scale layers. Oxygen laser cutting processes were carried out using Ytterbium fibre laser 1070 nm along the edge of 20-mm-thick-steel specimens which were attached to a borosilicate glass. Focal point of the laser beam was positioned to be 0.7 mm below the specimens' surface. A high speed imaging system was arranged to face the glass, recording the cut front and kerf dynamics during cutting processes. It was found that cut front inclination angle increase when the mill scale was removed from the specimens' surface. This implies that mill scale on the specimens' surface seem to contribute in increasing the exothermal energy during laser cutting processes.

  • 11.
    Dewi, Handika Sandra
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Short thermal cycle treatment with laser of vanadium microalloyed steels2020In: Journal of Manufacturing Processes, ISSN 1526-6125, Vol. 57, p. 543-551Article in journal (Refereed)
    Abstract [en]

    Improvement of crankshaft fatigue properties can be approached by altering its mechanical properties in the surface, such as laser surface treatment. Laser beam treatment offers efficient and precise surface hardening processing with possibility of reducing the production cost compared to the conventional hardening techniques. However, its characteristic of having short thermal cycle can be a challenge for the development of laser surface hardening techniques, such as inadequacy of literatures in phase transformation and resulting mechanical properties under rapid heating and cooling rate. Therefore, this work investigated the impact of short thermal cycles induced by the laser beam on the resulting microstructure and hardness properties in the surface of 38MnSiVS5 and 44MnSiVS6 microalloyed steels. Temperature cycles during the process were recorded and examined with the resulting microstructure along with microhardness values. 44MnSiVS6 microalloyed steel, which contains ca. double the amount of vanadium compared to 38MnSiVS5 steel, produces finer ferrite grains in the treated area for all investigated short thermal cycles. This fine-grained microstructure leads to steady hardness distributions in the treated area. The short thermal cycle was assumed to be unable to dissolve the vanadium precipitates that reside in the ferrite grains, which then initiate precipitation hardening.

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  • 12.
    Dewi, Handika Sandra
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Jörg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Influence of secondary-pass laser treatment on retained ferrite and martensite in 44MnSiVS6 microalloyed steel2022In: Materials Today Communications, ISSN 2352-4928, Vol. 31, article id 103282Article in journal (Refereed)
    Abstract [en]

    Overlapping regions of laser surface treatment are necessary features when processing large surface areas or cylindrical specimens. However, complex microstructural changes that appear in the regions with multiple heat treatment can affect their mechanical properties. Therefore, this study focuses on examining thermal cycle characteristics and resulting microstructures, particularly martensite and retained ferrite structures, to better understand the correlation between experienced thermal cycles and resulting microstructures. Laser surface hardening experiments on 44MnSiVS6 microalloyed steels together with thermal diffusion simulations were conducted to relate microstructures after the secondary pass of the laser treatment to the local thermal cycles experienced during the process. The amount of retained ferrite was calculated and compared to the respective thermal cycle characteristics. Regions which experienced thermal cycles below Ac3 temperature showed microstructures similar to those after tempering. The sizes of retained ferrite structures were found to decrease as the total holding time increases regardless of how the holding time is distributed in multiple laser treatments. However, the size of retained ferrite structures were constant in the region where tempering effect occurred. This shows that the amount of retained ferrite can be tailored by modifying the experienced total holding time and a reduction of retained ferrite structure happens only if the secondary thermal cycle is above Ac3 temperature.

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  • 13.
    Faue, Patrick
    et al.
    Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI 53706 USA.
    Rathmann, Lewin
    Bremer Institut für angewandte Strahltechnik GmbH, Bremen 28359, Germany.
    Möller, Marius
    Bremer Institut für angewandte Strahltechnik GmbH, Bremen 28359, Germany.
    Hassan, Mahmudul
    Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI 53706 USA.
    Clark, Samuel J.
    X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, USA.
    Fezzaa, Kamel
    X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, Lemont, IL 60439, USA.
    Klingbeil, Kevin
    Cross Product Solutions, LLC, Osceola, WI 54020, USA.
    Richter, Brodan
    Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI 53706 USA.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Radel, Tim
    Bremer Institut für angewandte Strahltechnik GmbH, Bremen 28359, Germany.
    Pfefferkorn, Frank E.
    Department of Mechanical Engineering, University of Wisconsin-Madison, Madison, WI 53706 USA.
    High-speed X-ray study of process dynamics caused by surface features during continuous-wave laser polishing2023In: CIRP annals, ISSN 0007-8506, E-ISSN 1726-0604, Vol. 72, no 1, p. 201-204Article in journal (Refereed)
    Abstract [en]

    During high-speed X-ray imaging of laser surface polishing experiments of specimens of 316L stainless steel at Argonne National Lab's Advanced Photon Source, it was discovered that the induced keyhole changes shape and dimensions while crossing an engineered surface feature without altering process parameters. It was observed that the post-surface feature keyhole was deeper than that of the pre surface feature keyhole. This work reports on the first in-situ observation of the effect of localized surface geometry on underlying melt pool behavior. This has implications for defect formation mechanisms during laser melting processes that rely on melt pool geometry.

  • 14.
    Fischer, Andreas
    et al.
    University of Kassel, Institute of Materials Engineering – Metallic Materials, Kassel, Germany.
    Dewi, Handika Sandra
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Otero Tranchero, Jacobo
    AIMEN Centro Tecnológico, Laser Applications Centre, Porriño, Spain.
    Arias Otero, Jorge Luis
    AIMEN Centro Tecnológico, Laser Applications Centre, Porriño, Spain.
    Souto Grela, Javier
    AIMEN Centro Tecnológico, Laser Applications Centre, Porriño, Spain.
    Köcher, David
    University of Kassel, Institute of Materials Engineering – Metallic Materials, Kassel, Germany.
    Krooß, Philipp
    University of Kassel, Institute of Materials Engineering – Metallic Materials, Kassel, Germany.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Niendorf, Thomas
    University of Kassel, Institute of Materials Engineering – Metallic Materials, Kassel, Germany.
    A pragmatic approach for assessment of laser-induced compressive residual stress profiles2021In: Journal of Manufacturing Processes, ISSN 1526-6125, Vol. 68, part A, p. 778-787Article in journal (Refereed)
    Abstract [en]

    Laser hardening is a very efficient technique for local surface treatment, however, in case of complex and large components robust processing is highly challenging due to limitations in terms of the absolute size of the overall heat-affected zone. As is shown in the present work, an increased in-depth effect can be achieved by tailoring the laser parameters without melting the surface layer. Optimization of process parameters leads to an elaborate test design demanding numerous verification measurements to determine essential material properties. In this context, the evaluation of compressive residual stress values in the surface layer is very important, e.g. in case of fatigue loaded components. However, residual stress profile measurements obtained by X-ray diffraction are very time-consuming and, thus, can significantly impair the laser parameter development cycle. For this reason, the present study introduces a novel pragmatic approach allowing for qualitative evaluation of laser-induced compressive residual stress states, in particular for multiple laser pass processes based on a Gaussian-like intensity profile. Based on straightforward analytical evaluation, several characteristic features of the affected surface layer, e.g. the position of the residual stress transition zone, can be correlated to a change of the local energy input. A novel parameter referred to as modified area energy is established in present work for this purpose. This novel energy approach provides for an essential contribution to the field of laser hardening to considerably shorten the experimental effort within the laser parameter search.

  • 15.
    Frostevarg, Jan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Jöerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Thompson, Cassidy
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Prasad, Himani Siva
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Fedina, Tatiana
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Brückner, Frank
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. Fraunhofer Institute for Material and Beam Technology, Dresden, Germany.
    Influence of the vapour channel on processing in laser powder bed fusion2019In: Procedia Manufacturing, E-ISSN 2351-9789, Vol. 36, p. 80-87Article in journal (Refereed)
    Abstract [en]

    Additive Manufacturing provides many opportunities to design and manufacture parts that are difficult or not possible to produce with conventional methods. In Selective Laser Melting (SLM) in powder bed fusion (PBF), melt pool dynamics and stability is dependent on a large number of factors, e.g. laser power output, power density, travel speed, reflectivity of powder bed, rapid heating and vaporization. Since travel speeds are often very fast and the laser interaction zone is small, the physical events become difficult to predict but also to observe. This work aims to describe the formation and geometrical characteristics of the vaporization zone during processing. Using a combination of theoretical descriptions, resulting material structures and a comprehensive analysis of high-speed images of the processing zone for different heat inputs and travel speeds, explanations for the dynamic melt pool behaviour are derived. The melting and pressures from processing involved moves powder particles next to it, changing the conditions for neighbouring tracks due to lack of material. These findings can provide a basis for creating more efficient and stable SLM processing, with fewer imperfections.

  • 16.
    Hauser, Tobias
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. Siemens AG, Otto-Hahn-Ring 6, 81739 München, Germany.
    Breese, Philipp Peter
    Siemens AG, München, Germany.
    Kamps, Tobias
    Siemens AG, München, Germany.
    Heinze, Christoph
    Siemens AG, Berlin, Germany.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Material transitions within multi-material laser deposited intermetallic Iron Aluminides2020In: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810, Vol. 34, article id 101242Article in journal (Refereed)
    Abstract [en]

    Laser Metal Deposition is a near-net-shape processing technology, which allows remarkable freedom in multi-material processing. In the present work, the multi-material processing of two intermetallic iron aluminides, Fe28Al(at.%) and Fe30Al5Ti0.7B(at.%), was investigated. It has been shown that multi-material processing of the two alloys via discrete as well as via gradual material transition is possible without any cracks for manufacturing small cubes. Cross-sections of manufactured parts and tracks showed that a preheating temperature of at least 400 °C is necessary to process crack free samples. EDX-analyses indicated that if a discrete material transition is required in multi-material processing, the material transition should be implemented in the vertical build-up direction because the mixing zone in this direction is significantly smaller than the mixing zone in the horizontal direction. Due to the stronger mixing effects in the horizontal direction, a gradual material transition by a linear progression should be implemented in this direction rather than in the vertical direction. The mixing effects are mainly caused by melt flow, while diffusion effects can be neglected.

  • 17.
    Hauser, Tobias
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. Corporate Technology, Siemens AG, D-81739 Munich, Germany.
    Da Silva, Adrien
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Reisch, Raven T.
    Chair of Robotics, Artificial Intelligence and Real-time Systems, Technical University of Munich, D-80333 Munich, Germany. Corporate Technology, Siemens AG, D-81739 Munich, Germany.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kamps, Tobias
    Corporate Technology, Siemens AG, D-81739 Munich, Germany.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Fluctuation effects in Wire Arc Additive Manufacturing of aluminium analysed by high-speed imaging2020In: Journal of Manufacturing Processes, ISSN 1526-6125, Vol. 56, no Part A, p. 1088-1098Article in journal (Refereed)
    Abstract [en]

    Wire Arc Additive Manufacturing is a near-net-shape processing technology which allows the cost-effective manufacturing of big and customized metal parts. In the present work the Wire Arc Additive Manufacturing of AW4043/AlSi5(wt.%) with different lead angles of the welding torch was investigated. It has been shown that for some lead angles fluctuation effects occur in the structures produced if the interlayer temperature is either too low or too high. All experiments were analysed by high-speed imaging whereby the welding phenomena could be observed. In the case of Wire Arc Additive Manufacturing with a lead angle above 10° at lower interlayer temperatures, the deposited track consists out of several, seperated WAAM globules and is no longer in a uniform track. In the case of the dragging and neutral Wire Arc Additive Manufacturing processes at higher interlayer temperatures, fluctuation effects occur. In addition, by evaluating the high-speed videos with computer vision, it was found that such fluctuation effects can be detected at the arc frequency of the process. To avoid fluctuation effects caused by too low or too high interlayer temperatures, a pushing Wire Arc Additive Manufacturing process with a slightly tilted lead angle should be used.

  • 18.
    Hauser, Tobias
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. Technology department, Siemens AG, D-81739 Munich, Germany.
    Reisch, Raven T.
    Chair of Robotics, Artificial Intelligence and Real-time Systems, Technical University of Munich, D-80333 Munich, Germany. Technology department, Siemens AG, D-81739 Munich, Germany.
    Breese, Philipp P.
    Coating Technology, Technical University Berlin, Pascalstr. 8–9, D-10587 Berlin, Germany. Technology department, Siemens AG, D-81739 Munich, Germany.
    Lutz, Benjamin S.
    Technology department, Siemens AG, D-81739 Munich, Germany.
    Pantano, Matteo
    Technology department, Siemens AG, D-81739 Munich, Germany.
    Nalam, Yogesh
    Technology department, Siemens AG, D-81739 Munich, Germany.
    Bela, Katharina
    Technology department, Siemens AG, D-81739 Munich, Germany.
    Kamps, Tobias
    Technology department, Siemens AG, D-81739 Munich, Germany.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander F.H
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Porosity in wire arc additive manufacturing of aluminium alloys2021In: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810, Vol. 41, article id 101993Article in journal (Refereed)
    Abstract [en]

    Wire Arc Additive Manufacturing is a near-net-shape processing technology which allows cost-effective manufacturing of large and customized metal parts. Processing of aluminium in Wire Arc Additive Manufacturing is quite challenging, especially in terms of porosity. In the present work, pore behaviour in Wire Arc Additive Manufacturing of AW4043/AlSi5(wt%) was investigated and a post-process monitoring approach was developed. It has been observed that as the shielding gas flow rate increases, the porosity in aluminium parts also increases due to the rapid solidification of the melt pool by forced convection. The higher convection rate seems to limit the escape of gas inclusions. Furthermore, gas inclusions escaping from the melt pool leave cavities on the surface of each deposited layer. Process camera imaging is used to monitor these cavities to acquire information about the porosity in the part. The observations were supported by Computational Fluid Dynamics simulations which show that the gas flow rate correlates with the porosity in aluminium parts manufactured by Wire Arc Additive Manufacturing. Since a lower gas flow rate leads to reduced convective cooling, the melt pool remains liquid for a longer period allowing pores to escape for a longer period and thus reducing porosity. Based on these investigations, a monitoring approach is presented.

  • 19.
    Hauser, Tobias
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. Technology, Siemens AG, D-81739 Munich, Germany.
    Reisch, Raven T.
    Chair of Robotics, Artificial Intelligence and Real-time Systems, Technical University of Munich, D-80333 Munich, Germany. Technology, Siemens AG, D-81739 Munich, Germany.
    Breese, Philipp P.
    Coating Technology, TU Berlin, Pascalstr. 8-9, 10587 Berlin, Germany. Technology, Siemens AG, D-81739 Munich, Germany.
    Nalam, Yogesh
    Technology, Siemens AG, D-81739 Munich, Germany.
    Joshi, Kaivalya S.
    Technology, Siemens AG, D-81739 Munich, Germany.
    Bela, Katharina
    Technology, Siemens AG, D-81739 Munich, Germany.
    Kamps, Tobias
    Technology, Siemens AG, D-81739 Munich, Germany.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Oxidation in wire arc additive manufacturing of aluminium alloys2021In: Additive Manufacturing, ISSN 2214-8604, E-ISSN 2214-7810, Vol. 41, article id 101958Article in journal (Refereed)
    Abstract [en]

    Wire Arc Additive Manufacturing is a near-net-shape machining technology that enables low-cost production of large and customised metal parts. In the present work, oxidation effects in Wire Arc Additive Manufacturing of the aluminium alloy AW4043/AlSi5(wt%) were investigated. Two main oxidation effects, the surface oxidation on aluminium parts and the oxidation anomalies in aluminium parts were observed and analysed. The surface oxidation on aluminium parts changed its colour during Wire Arc Additive Manufacturing from transparent to white. In the present work, it was shown by high-speed imaging that this change in the surface oxidation took place in the process zone, which was covered by inert gas. Since the white surface oxidation formed in an inert gas atmosphere, it was found that the arc interacts with the existing amorphous oxide layer of the previously deposited layer and turns it into a white duplex (crystalline and amorphous) oxide layer. In addition to the analysis of the white surface oxidation, oxidation anomalies, which occur at low shielding from the environment, were investigated. It was shown by physical experiments and Computational Fluid Dynamics simulations, that these oxidation anomalies occur at inadequate gas flow rates, too big nozzle-to-work distances, process modes with too high heat input, or too high wire feed rates. Finally, a monitoring method based on light emission spectroscopy was used to detect oxidation anomalies as they create peaks in the spectral emission when they occur.

  • 20.
    Hauser, Tobias
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. Technology Department, Siemens AG, 81739 Munich, Germany.
    Reisch, Raven T.
    Chair of Robotics, Artificial Intelligence and Real-Time Systems, Technical University of Munich, 80333, Munich, Germany; Technology Department, Siemens AG, 81739, Munich, Germany.
    Kamps, Tobias
    Technology department, Siemens AG, D-81739 Munich, Germany.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Acoustic emissions in directed energy deposition processes2022In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 119, no 5-6, p. 3517-3532Article in journal (Refereed)
    Abstract [en]

    Acoustic emissions in directed energy deposition processes such as wire arc additive manufacturing and directed energy deposition with laser beam/metal are investigated within this work, as many insights about the process can be gained from this. In both processes, experienced operators can hear whether a process is running stable or not. Therefore, different experiments for stable and unstable processes with common process anomalies were carried out, and the acoustic emissions as well as process camera images were captured. Thereby, it was found that stable processes show a consistent mean intensity in the acoustic emissions for both processes. For wire arc additive manufacturing, it was found that by the Mel spectrum, a specific spectrum adapted to human hearing, the occurrence of different process anomalies can be detected. The main acoustic source in wire arc additive manufacturing is the plasma expansion of the arc. The acoustic emissions and the occurring process anomalies are mainly correlating with the size of the arc because that is essentially the ionized volume leading to the air pressure which causes the acoustic emissions. For directed energy deposition with laser beam/metal, it was found that by the Mel spectrum, the occurrence of an unstable process can also be detected. The main acoustic emissions are created by the interaction between the powder and the laser beam because the powder particles create an air pressure through the expansion of the particles from the solid state to the liquid state when these particles are melted. These findings can be used to achieve an in situ quality assurance by an in-process analysis of the acoustic emissions.

  • 21.
    Hauser, Tobias
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. Technology Department, Siemens AG, D-81739 Munich, Germany.
    Reisch, Raven T.
    Artificial Intelligence and Real-time Systems, Technical University of Munich, D-80333 Munich, Germany; Technology Department, Siemens AG, D-81739 Munich, Germany.
    Seebauer, Stefan
    Technology Department, Siemens AG, D-81739 Munich, Germany; Institute of Solid State Physics, Technical University of Vienna, A-1040 Vienna, Austria.
    Parasar, Aashirwad
    Technology Department, Siemens AG, D-81739 Munich, Germany.
    Kamps, Tobias
    Technology Department, Siemens AG, D-81739 Munich, Germany.
    Casati, Riccardo
    Department of Mechanical Engineering, Politecnico di Milano, Via La Masa 34, 20156 Milano, Italy.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Multi-Material Wire Arc Additive Manufacturing of low and high alloyed aluminium alloys with in-situ material analysis2021In: Journal of Manufacturing Processes, E-ISSN 1526-6125, Vol. 69, p. 378-390Article in journal (Refereed)
    Abstract [en]

    In recent years, the interest in the improved functionalisation of Additive Manufacturing components through multi-material solutions has increased because of the new possibilities in product design. In this work, an advanced Wire Arc Additive Manufacturing process for fabrication of multi-material structures of different aluminium alloys was investigated. Mechanical properties such as tensile strength, yield strength, fracture elongation, and hardness were analysed for multi-material parts and compared with the mechanical properties of mono-material parts. It was found that the strength of multi-material components was limited by the properties of the individual aluminium alloys and not by those of the material transition zones. Microsections and EDX line scans revealed a smooth transition zone without any significant defects. Furthermore, process monitoring approaches for quality assurance of the correct material composition in such multi-material structures were investigated. Different sensor data were captured during multi-material Wire Arc Additive Manufacturing to identify and observe various characteristics of the process. It was shown that the voltage, current, acoustic, and spectral emission data can be used for in-situ monitoring to detect the chemical differences between the two aluminium alloys 6060 and 5087. Characteristic patterns in the frequency range were found, which can be attributed to a frequency shift that occurred due to the different material properties. Spectral analysis revealed changes in the ratios of green and blue light emission to red light emission, which was also due to the different magnesium contents.

  • 22.
    He, Hanbing
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Forouzan, Farnoosh
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Robertson, Stephanie M.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Vuorinen, Esa
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Microstructure and Mechanical Properties of Laser-Welded DP Steels Used in the Automotive Industry2021In: Materials, ISSN 1996-1944, E-ISSN 1996-1944, Vol. 14, no 2, article id 456Article in journal (Refereed)
    Abstract [en]

    The aim of this work was to investigate the microstructure and the mechanical properties of laser-welded joints combined of Dual Phase DP800 and DP1000 high strength thin steel sheets. Microstructural and hardness measurements as well as tensile and fatigue tests have been carried out. The welded joints (WJ) comprised of similar/dissimilar steels with similar/dissimilar thickness were consisted of different zones and exhibited similar microstructural characteristics. The trend of microhardness for all WJs was consistent, characterized by the highest value at hardening zone (HZ) and lowest at softening zone (SZ). The degree of softening was 20 and 8% for the DP1000 and DP800 WJ, respectively, and the size of SZ was wider in the WJ combinations of DP1000 than DP800. The tensile test fractures were located at the base material (BM) for all DP800 weldments, while the fractures occurred at the fusion zone (FZ) for the weldments with DP1000 and those with dissimilar sheet thicknesses. The DP800-DP1000 weldment presented similar yield strength (YS, 747 MPa) and ultimate tensile strength (UTS, 858 MPa) values but lower elongation (EI, 5.1%) in comparison with the DP800-DP800 weldment (YS 701 MPa, UTS 868 MPa, EI 7.9%), which showed similar strength properties as the BM of DP800. However, the EI of DP1000-DP1000 weldment was 1.9%, much lower in comparison with the BM of DP1000. The DP800-DP1000 weldment with dissimilar thicknesses showed the highest YS (955 MPa) and UTS (1075 MPa) values compared with the other weldments, but with the lowest EI (1.2%). The fatigue fractures occurred at the WJ for all types of weldments. The DP800-DP800 weldment had the highest fatigue limit (348 MPa) and DP800-DP1000 with dissimilar thicknesses had the lowest fatigue limit (<200 MPa). The fatigue crack initiated from the weld surface.

  • 23.
    Kaplan, Alexander F. H.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Höfemann, Matthias
    Salzgitter Mannesmann Forschung GmbH, 38239 Salzgitter, Germany.
    Vaamonde, Eva
    AIMEN Technology Center, 36418 Pontevedra, Spain.
    Ramasamy, Anandkumar
    Lincoln Electric Europe, 6534 AD Nijmegen, The Netherlands.
    Kalfsbeek, Bert
    Lincoln Electric Europe, 6534 AD Nijmegen, The Netherlands.
    Näsström, Jonas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Robertson, Stephanie M.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Frostevarg, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Jörg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Microstructures from wire-fed laser welding of high strength steel grades2020In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 32, no 2, article id 022050Article in journal (Refereed)
    Abstract [en]

    In welding, wire-feeding enables alteration of the resulting microstructure and, in turn, the mechanical behavior of the welded joint. For pipeline steel grades, very few commercial wires are matching at high strength and simultaneously ensure sufficient toughness. New wire chemistries need to be investigated. Promising consumable chemistries can be studied through metal cored wires. One promising concept is alloys that promote acicular ferrite instead of bainite. Interlocking instead of parallel laths can lead to higher toughness. In the gouge range of 15–19 mm, laser-arc hybrid welding has been studied for pipeline steel grades X80 and X100. For efficient mapping of various weld metal conditions, a simplifying “snapshot” method was developed. A pulse shaped laser beam melts wire pieces in a controlled manner, reproducing thermal cycles in welding. The weld metal tends to form bainite, but under certain conditions, complex microstructures with interlocking laths can be generated. Slow thermal cycles can lead to coalescence of the laths to coarser structures, while fast cycles favored finer structures and occasionally lath interlocking. The formation of acicular ferrite was difficult to achieve. Advanced wire chemistries lowered the hardness of the weld metal, as did preheating.

  • 24.
    Kaplan, Alexander F. H.
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Robertson, Stephanie M.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Frostevarg, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Melt pool forming a buttonhole in tailored blank welding with multiple laser spots2021In: IOP Conference Series: Materials Science and Engineering, Institute of Physics (IOP), 2021, Vol. 1135, article id 012022Conference paper (Refereed)
    Abstract [en]

    Laser beam welding of tailored blank butt joints of different sheet thickness generates asymmetric melt pool conditions. By employing two, three or four tailored laser beams, additional options for shaping the melt pool conditions can be offered. As observed by high speed imaging, in most multi-spot cases a large stable buttonhole was generated, by the trailing laser beams asymmetrically towards the thinner sheet. Correspondingly, the ablation pressure from the multiple boiling fronts has generated a fast melt jet, particularly along the thicker sheet. In many cases the boiling front kept open to the keyhole rear. The buttonhole differs from the Catenoid-like shape reported earlier. The walls are steeper and the horizontal shape can be asymmetric. The melt pool can switch between different stable modes. Inclined arrangement of three beams enabled even two separate, parallel boiling fronts and melt jets, combining behind the opening. Despite the large buttonhole, sound welds were achieved. Solely for four equal laser beams, arranged as a square, a melt pool without buttonhole was generated. Provided the driving forces from the ablation pressure along with the melt flow are sufficiently explored and understood, new opportunities to optimize the welding process are available.

  • 25.
    Kaplan, Alexander F.H
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Robertson, Stephanie M.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Frostevarg, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Ramasamy, Anandkumar
    Lincoln Electric Europe, Nijmegen, Netherlands.
    Kalfsbeek, Bert
    Lincoln Electric Europe, Nijmegen, Netherlands.
    Microstructure morphology characterization of welding consumables studied by pulse-shaped laser heating2019In: Procedia Manufacturing, E-ISSN 2351-9789, Vol. 36, p. 184-191Article in journal (Refereed)
    Abstract [en]

    During welding, wire consumables can essentially contribute to the resulting microstructures and mechanical properties. In order to maintain high toughness even for high strength steel, certain microstructures are desirable, particularly acicular ferrite. An efficient, controllable test method was developed during which the wire is molten and experiences a thermal cycle by a shaped laser pulse, or a sequence of pulses, which shall resemble continuous laser-arc hybrid welding or narrow gap multi-layer laser welding. Different thermal cycles and wire chemistries have led to manifold microstructures. The morphology of the microstructures can become complex. Therefore, more detailed characterization of essential morphology aspects was carried out, to distinguish different results. The thermal cycles from quenching have led to shorter, thicker laths with more random orientation. The latter can be favourable for high toughness. Short reheating cycles by about 200 K/s caused finer, longer and more parallel laths, as for bainite, in varying size of blocks. Other aspects considered were grain boundary ferrite and non-metallic inclusions. Systematic variation of the thermal cycle by the testing method along with systematic description of microstructure morphology in more detail is a promising method to identify and optimize favoured routes for wire chemistry and welding techniques.

  • 26.
    Laskin, Alexander
    et al.
    AdlOptica GmbH, Rudower Chaussee 29, 12489 Berlin, Germany.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Comparison of the thermal focus shift and aberration between the single-mode and multimode lasers2021In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 33, no 4, article id 042026Article in journal (Refereed)
    Abstract [en]

    Thermal lensing is a well-known but typically undesired effect during the use of laser optics. Nonuniform (gradient) heating due to absorption of high-power laser radiation in optical elements causes thermal lensing, paraxial focus shift, and aberration leading to changes in size and intensity profile of the focused spot in optics. Therefore, an analysis of primary physical effects of geometrical deformation of optical surfaces in the form of aspheric bulges and transformation of the material into a gradient refractive medium was conducted to quantitatively estimate the focus shift and aberrations. Since focus shift effects are different in the case of single-mode and multimode lasers, for both laser modes, the optimal relationships between the physical properties of optical materials for reduction in thermo-optical effects through compensating the material thermal expansion by the change in the refractive index—condition of self-compensation or athermalization were formulated. A comparison of the characteristics, namely, temperature coefficient of the optical pathlength and thermo-optical ratio allowed determination of the optimal materials for the optics for both single-mode and multimode high-power lasers: athermal crystalline quartz and specialty glasses, sapphire with extremely high thermal conductivity ensure minimal temperature gradients. Optics made of these materials exhibit a minimized thermal focus shift and aberration even during the absorption of laser energy in the bulk material and coatings by contamination, scratches, and other surface defects. Weak birefringence of crystalline quartz and sapphire does not prevent their successive use in laser optics.

  • 27.
    Laskin, Alexander
    et al.
    AdlOptica Optical Systems GmbH (Germany).
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    On the selection of materials for high-power laser optics with reduced thermal lensing2022In: High-Power Laser Materials Processing: Applications, Diagnostics, and Systems XI / [ed] Stefan Kaierle; Stefan W. Heinemann, SPIE - International Society for Optical Engineering, 2022, Vol. 11994, article id 1199408Conference paper (Refereed)
    Abstract [en]

    Thermal lensing is a well-known but undesired effect in high power laser optics for welding, 3D-printing and other technologies. Stability and performance of laser processing depend on the possibility to control and minimize the thermo-optical effects induced by non-uniform (gradient) heating due to absorption of laser energy in optical elements: paraxial focus shift and thermally induced aberration, which lead to a change in size and intensity profile of the focal spot. Analysis of primary physical effects: geometrical deformation of optical surfaces and the material transformation into a gradient refractive medium, allows the quantitative estimation of the wavefront beam distortion leading to focus shift and aberration. It also allows formulating an optimal relationship between the physical properties of optical materials to reduce the change in the wavefront through mutual compensation of thermo-optical effects induced by the thermal expansion and the refractive index change - athermalization condition. Athermal optics exhibit minimized thermal focus shift and aberration even when absorbing laser energy in the bulk material and coatings, by contamination or scratches. Considering physical characteristics the Temperature Coefficient of the Optical Pathlength and ThermoOptical Ratio allows determining the optimal materials for optics: athermal crystalline Quartz and specialty glasses, Sapphire with high thermal conductivity. Weak birefringence of Quartz and Sapphire doesn’t prevent their successive use in laser optics. The comparison of the theoretical analysis and experimental validation results of optics made of Fused Silica, N-BK7, crystalline Quartz and Sapphire confirm the theoretical method for reducing the thermal focus shift and effectiveness of the suggested approach.

  • 28.
    Laskin, Alexander
    et al.
    AdlOptica GmbH.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Laskin, Vadim A.
    AdlOptica GmbH.
    Ostrun, Aleksei B.
    St. Petersburg National Research University of Information Technologies, Mechanics and Optics.
    Beam shaping of focused radiation of multimode lasers2018In: High-Power Laser Materials Processing: Applications, Diagnostics, and Systems VII 2018 / [ed] Kaierle S.,Heinemann S.W., SPIE - International Society for Optical Engineering, 2018, Vol. 10525, article id 1052507Conference paper (Refereed)
    Abstract [en]

    Performance of various laser technologies like welding, metal sheet cutting based on the use of powerful multimode fiber lasers, fiber-coupled solid-state and diode lasers can be improved by manipulation of energy distribution perpendicular and along optical axis. Welding, cladding with 0.5-2 mm size working spots benefit from "inverse-Gauss" intensity profiles, and doubled or tripled spots perpendicular to axis are good solutions to provide more uniform temperature profiles on a workpiece. Thick metal sheet cutting, some types of welding get benefits from distributing the laser energy along the optical axis resulting in more efficient usage of laser energy, higher and more stable cutting edge quality, faster processing. Since the radiation of multimode lasers is of low spatial coherence, characterized by big Beam Parameter Products (BPP) or M2 values, it is difficult to control the intensity distribution by methods other than imaging of a fiber end using a collimator and focusing objective. A promising suggested solution is combining of imaging the fiber end and geometrical separation of focused spots either perpendicular to or along the optical axis using dedicated optical components. Thus, energy of high power lasers is distributed among multiple foci. To provide reliable operation with multi-kW lasers and avoid damages the multi-focus optical devices are designed as refractive elements with smooth optical surfaces. The paper presents descriptions of multi-focus optics as well as examples of intensity profile measurements of beam caustics and application results

  • 29.
    Laskin, Alexander
    et al.
    AdlOptica GmbH, Rudower Chaussee 29, 12489 Berlin, Germany.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Laskin, Vadim
    AdlOptica GmbH, Rudower Chaussee 29, 12489 Berlin, Germany.
    Nara, Takuji
    Profitet, 3-1-13, Tokiwa, Urawa-ku, Saitama-shi, Saitama-ken 330-0061, Japan.
    Jung, Seong Ryol
    ShinHoTek, #1306, 19 Gasan digital 1-ro, Geumcheon-gu, Seoul 08594, South Korea.
    Multispot optics for beam shaping of high-power single-mode and multimode lasers2021In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 33, no 4Article in journal (Refereed)
    Abstract [en]

    The performance of various laser technologies, such as welding, laser powder bed fusion (LPBF), brazing, cladding, and sheet metal cutting, based on the use of high-power multimode fiber lasers, fiber-coupled solid-state, and diode lasers, can be improved using the patent pending beam-shaping optics providing optimal energy distributions by splitting the laser beam into several separate spots in the working plane and variable energy sharing between these spots. Various patterns, such as square, line, and rhombus, consisting of four or nine separate spots, are expected to eliminate or reduce spatter and to realize optimum temperature distributions in the melt pool and stabilizing the processes in the welding of tailored blanks, copper and aluminum parts in the production of batteries, zinc-coated steel, cladding, and LPBF. Because multimode lasers have a comparably low spatial coherence characterized by large beam parameter products or beam quality (M²) values, it is difficult to control the intensity distribution by methods other than imaging the fiber end with a collimator and a focusing objective. The proposed solution is a combination of fiber end imaging and geometrical separation of focused spots perpendicular to the optical axis using special optical components and creating a working spot as a combination of several spots. Varying the energy portions in separate spots and the distances between them make it possible to optimize common spot intensity distributions for particular applications. To ensure reliable operation with multi-kW lasers, the refractive optical components of the multispot devices are implemented from athermal optical materials characterized by insignificant thermal lensing and, hence, negligible thermal focus shift and spherical aberration. The article presents descriptions of multispot optics and examples of intensity profile measurements and application results, while the reduction in spattering was observed using multispot laser welding. It is concluded that the melt pool flows homogenize when applying several laser spots compared to a single spot. The possibility of tailoring melt pool dimensions in LPBF was shown.

  • 30.
    Laskin, Alexander
    et al.
    AdlOptica Optical Systems GmbH, Berlin, Germany.
    Volpp, Joerg
    BIAS - Bremer Institut für angewandte Strahltechnik GmbH, Bremen, Germany.
    Laskin, Vadim
    AdlOptica Optical Systems GmbH, Berlin, Germany.
    Ostrun, Aleksei
    St. Petersburg National Research University of Information Technologies, Mechanics and Optics, St. Petersburg, Russia.
    Elongation depth of field by focusing radiation of multimode lasers using multi-focus beam shaping optics2017In: 2017 ICALEO Conference Proceedings (Electronic Access), Laser Institute of America , 2017, article id 603Conference paper (Refereed)
  • 31.
    Laskin, Alexander
    et al.
    AdlOptica GmbH (Germany) .
    Volpp, Joerg
    Bremer Institut für angewandte Strahltechnik GmbH (Germany) .
    Laskin, Vadim
    AdlOptica GmbH (Germany) .
    Ostrun, Aleksei
    St. Petersburg National Research Univ. of Information Technologies (Russian Federation).
    Multi-focus beam shaping of high power multimode lasers2017Conference paper (Refereed)
  • 32.
    Laskin, Alexander
    et al.
    AdlOptica GmbH.
    Volpp, Joerg
    Laskin, Vadim
    AdlOptica GmbH.
    Ostrun, Aleksei
    St Petersburg National Research Univ of Information Technologies.
    Refractive multi-focus optics for material processing2016Conference paper (Refereed)
  • 33.
    Lupi, Giorgia
    et al.
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 34, 20156 Milano (MI), Italy.
    de Menezes, João Teixeira Oliveira
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 34, 20156 Milano (MI), Italy.
    Belelli, Filippo
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 34, 20156 Milano (MI), Italy.
    Bruzzo, Francesco
    Fraunhofer, Institut für Werkstoff und Strahltechnik, Winterbergstraße 28, 01277 Dresden, Germany.
    López, Elena
    Fraunhofer, Institut für Werkstoff und Strahltechnik, Winterbergstraße 28, 01277 Dresden, Germany.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Castrodeza, Enrique Mariano
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 34, 20156 Milano (MI), Italy.
    Casati, Riccardo
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 34, 20156 Milano (MI), Italy.
    Fracture toughness of AlSi10Mg alloy produced by direct energy deposition with different crack plane orientations2023In: Materials Today Communications, ISSN 2352-4928, Vol. 37, article id 107460Article in journal (Refereed)
    Abstract [en]

    Three-point bending fracture toughness and tensile specimens were tested at room temperature along different crack plane orientations and loading directions. Before being machined and tested, the printed samples were subjected to heat treatment at 300 °C for 2 h to relieve the residual stresses. Microstructural and fractographic analyses were performed to investigate the fracture mechanisms and the crack propagation paths for each crack orientation. Significant differences in the fracture toughness were observed among the crack plane orientations. Specimens with cracks oriented in the X-Y direction featured the highest fracture toughness values (JIc = 11.96 kJ/m2), whereas the Z-Y crack orientation (perpendicular to the printing direction) performed the lowest fracture toughness values (JIc = 8.91 kJ/m2). The anisotropy in fracture toughness is mainly related to a preferential crack propagation path along the melt pool boundaries. At melt pool boundaries, pores are preferentially placed, coarsening of the microstructure occurs and there is higher Si content, leading to that area being less ductile and less resistant to crack propagation.

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  • 34.
    Lupi, Giorgia
    et al.
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 34, 20156 Milano (MI), Italy.
    de Menezes, João Teixeira Oliveira
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 34, 20156 Milano (MI), Italy.
    Belelli, Filippo
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 34, 20156 Milano (MI), Italy.
    Bruzzo, Francesco
    Fraunhofer, Institut für Werkstoff- und Strahltechnik, Winterbergstraße 28, 01277 Dresden, Germany.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Castrodeza, Enrique Mariano
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 34, 20156 Milano (MI), Italy.
    Casati, Riccardo
    Department of Mechanical Engineering, Politecnico di Milano, Via G. La Masa 34, 20156 Milano (MI), Italy.
    Fracture Toughness And Fatigue Properties Of Alsi10mg Alloy Produced By Direct Energy Deposition With Different Crack Plane Orientations2023In: Euro Powder Metallurgy 2023 (Euro PM2023) Proceedings, European Powder Metallurgy Association (EPMA) , 2023Conference paper (Refereed)
  • 35.
    Moradi, M.
    et al.
    Malayer Univ, Dept Mech Engn, Fac Engn, Malayer, Iran. Malayer Univ, Laser Mat Proc Res Ctr, Malayer, Iran.
    Khorram, A.
    KNToosi Univ Technol, Dept Mech Engn, Tehran, Iran.
    Fallah, M.M.
    Shahid Rajaee Teacher Training Univ, Tehran, Iran .
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Nd:YAG Laser Welding of Ti 6-Al-4V: Mechanical and Metallurgical Properties2019In: Lasers in engineering (Print), ISSN 0898-1507, E-ISSN 1029-029X, Vol. 43, no 1-3, p. 21-33Article in journal (Refereed)
    Abstract [en]

    Titanium and their alloys have high strength to weight ratio, good corrosion resistance and high melting point temperature lead to their application in engineering. Ti-6Al-4V alloy has been widely used in medical, chemistry and aerospace industries. In this research, laser welding of Ti-6Al-4V sheet with the thickness of 1.7 mm performed using a 400 W pulsed Nd:YAG laser. The effects of laser peak power, pulse duration and laser pulse energy on the weld bead profile, weld microhardness, tensile strength and microstructure were studied. Results showed that by increasing laser peak power, decreasing pulse duration and increasing laser pulse energy, the weld penetration increases. Metallurgical observations revealed that welded zone comprises of acicular αˊ phase within prior coarse β grains. Microhardness distribution is more uniform in welded zone. The microhardness value in the welded zone is the highest which is attributed to high martensitic phase content in the welded zone. The ultimate tensile strength (UTS) values show the variation from 649 to 998 MPa in the Nd:YAG laser welded samples.

  • 36.
    Nalam, Yogesh
    et al.
    Technology, Siemens AG, D-81739 Munich, Germany.
    Hauser, Tobias
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. Technology, Siemens AG, D-81739 Munich, Germany.
    Breese, Philipp Peter
    Technology, Siemens AG, D-81739 Munich, Germany.
    Kamps, Tobias
    Technology, Siemens AG, D-81739 Munich, Germany.
    Küsters, Yves
    Technology, Siemens AG, D-81739 Munich, Germany.
    Abdullah, Rebar Hama-Saleh
    Fraunhofer Institute for Laser Technology, Steinbachstrasse 15, 52074 Aachen, Germany.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander F H
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Gas flow study for development of a novel shielding gas nozzle for directed energy deposition processes using computational fluid dynamic simulations2021In: IOP Conference Series: Materials Science and Engineering, Institute of Physics (IOP), 2021, Vol. 1135, article id 012016Conference paper (Refereed)
    Abstract [en]

    Directed energy deposition (DED) enables the additive manufacturing of several materials such as molybdenum alloys that are very difficult to process by conventional methods. Some of these materials are highly reactive to gases in ambient atmosphere such as oxygen, and nitrogen. Oxidation during additive manufacturing significantly influences the mechanical properties of a part. In some cases, the shielding gas coverage of standard powder nozzles is not sufficient, and oxidation still takes place. A functional prototype of a compound multi flow path annular nozzle was developed using computational fluid dynamics simulations. Simulations were performed using multi-component miscible gas model. Prototypes were manufactured for several design iterations to test their functionality in cold flow conditions. In the end, an Inconel based prototype was built, using laser powder bed fusion. The volume of shielding gas cover over the substrate improved with the proposed design and the radial extent of 80 ppm oxygen concentration increased from 8 mm to 25 mm. Finally, Mo-Si-B alloy was deposited on a 1000 °C pre-heated substrate without significant oxidation or cracks.

  • 37.
    Prasad, Himani Siva
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Brueckner, Frank
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. Fraunhofer IWS, Winterbergstrasse 28, 01277, Dresden, Germany.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander F. H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Laser metal deposition of copper on diverse metals using green laser sources2020In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 107, no 3-4, p. 1559-1568Article in journal (Refereed)
    Abstract [en]

    Green laser sources are advantageous in the processing of copper due to the increase of absorptivity compared with more commonly available infrared lasers. Laser metal deposition of copper with a green laser onto various substrate metals namely copper, aluminium, steel and titanium alloy was carried out and observed through high-speed imaging. The effects of process parameters such as laser power, cladding speed and powder feed rate, and material attributes such as absorptivity, surface conditions and thermal conductivity are tied together to explain the size and geometry of the melt pool as well as the fraction of the power used for melting material. The copper substrate has the smallest melt pool with a high angle, followed by aluminium, steel and titanium alloy. The incorporation times for powder grains in the melt pools vary based on the substrate materials. Its dependency on material properties, including surface tension forces, melting temperatures and material density, is discussed. Oxide skins present on melt pools can affect powder incorporation, most significantly on the aluminium substrate. The lower limits of the fraction of power irradiated on the surface used purely for melting were calculated to be 0.73%, 2.94%, 5.95% and 9.78% for the copper, aluminium, steel and titanium alloy substrates, respectively, showing a strong dependence on thermal conductivity of the substrate material. For a copper wall built, the fraction was 2.66%, much higher than a single clad on a copper substrate, due to reduced workpiece heating. The results of this paper can be transferred to other metals with low absorptivity such as gold.

  • 38.
    Robertson, Stephanie
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Frostevarg, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Ramasamy, Anandkumar
    Lincoln Electric Europe, Nijmegen, Netherlands.
    Kalfsbeek, Bert
    Lincoln Electric Europe, Nijmegen, Netherlands.
    Volpp, Jörg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kaplan, Alexander F.H
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Microstructures of high strength steel welding consumables from directed thermal cycles by shaped laser pulses2020In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 109, no 9-12, p. 2653-2662Article in journal (Refereed)
    Abstract [en]

    Filler wire metallurgy was modified through temporally shaped laser pulses, controlling cooling cycles in a recently developed method. Trends were identified through efficient mapping while maintaining representative thermal cycles of welding processes. A primary pulse melted preplaced filler wires while a secondary, linearly ramped-down pulse elevated the nugget to re-austenization temperatures. Ramped-down pulses resulted in linear cooling rates comparable with and exceeding furnace-based methods, between 50 and 300∘C/s. The linear decay of laser output power guided the temperature through a regime to obtain desired microstructures. For three very high-strength steel filler wire chemistries, quenching resulted in smaller plates with cross-hatched microstructures, accompanied by grain boundary ferrite. Finer bainite microstructures started forming for fast linear temperature decay, about 250∘C/s. Slower decay or a weaker third cycle formed coarser microstructures with coalescent sheaves and less cross-hatching.

  • 39. Vollertsen, Frank
    et al.
    Volpp, Jörg
    Einfluss der Strahlformung mittels Bifokaloptik auf die Spritzerbildung beim Laserstrahltiefschweißen2017In: Schweissen & SchneidenArticle in journal (Other academic)
  • 40.
    Volpp, Joerg
    BIAS - Bremer Institut für angewandte Strahltechnik GmbH, Bremen, Germany.
    Analytisches Modell zur Vorhersage der Keyhole-Geometrie beim Laserstrahltiefschweißen2013In: Schweißen und Schneiden, ISSN 0720-9223, Vol. 65, no 3, p. 136-137Article in journal (Other academic)
  • 41.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Behavior of powder particles on melt pool surfaces2019In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 102, no 5-8, p. 2201-2210Article in journal (Refereed)
    Abstract [en]

    Additive Manufacturing is in progress to change the production and manufacturing environments and possibilities; however, thecomplex processes taking place are not completely understood yet. A better understanding of the incorporation mechanism of theparticles into the melt pool during blown powder processes could lead to methods to increase the powder and energy efficiency.Therefore, the incorporation mechanism was investigated in experiments and simulation. High-speed images made it possible toobserve the behavior of single particles on the melt pool. A model based on the temperature-dependent surface tension/energydifference between the particle and the melt pool calculated the time until particle incorporation. It was shown that the surfacetension characteristics during particle heating can even lift the particle from the melt pool. The calculated wetting behavior incombination with high kinetic energies should lead to an immediate immersion of the particle into the melt pool. Since observationsdo not show this behavior, it is concluded that the surface tension plays a role in the incorporation mechanism, but othereffects like the occurrence of oxide layers or additional particle heating by the laser beam seem to lead to different mechanismsand heat conductions to support an earlier incorporation.

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  • 42.
    Volpp, Joerg
    BIAS - Bremer Institut für angewandte Strahltechnik GmbH, Bremen, Germany.
    Einfluss des Strahlprofils auf die Porenbildung2015In: Schweissen und schneiden, Vol. 2015, no 4, p. 198-Article in journal (Other academic)
  • 43.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    High-temperature laser absorption of steel2023In: 2023 Conference on Lasers and Electro-Optics Europe and European Quantum Electronics Conference, CLEO/Europe-EQEC 2023, Institute of Electrical and Electronics Engineers Inc. , 2023Conference paper (Refereed)
  • 44.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Impact of Melt Flow and Surface Tension on Gap Bridging During Laser Beam Welding2023In: Lasers in Manufacturing and Materials Processing, ISSN 2196-7229Article in journal (Refereed)
    Abstract [en]

    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.

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  • 45. Volpp, Joerg
    Impact of process parameters on particle distribution and wear resistance during laser deep alloying processes2015Conference paper (Refereed)
  • 46. Volpp, Joerg
    Impacts on keyhole oscillations and process pores during laser deep penetration welding2015Conference paper (Refereed)
  • 47.
    Volpp, Joerg
    BIAS - Bremer Institut für angewandte Strahltechnik GmbH.
    Investigation on the influence of different laser beam intensity distributions on keyhole geometry during laser welding2012In: Physics Procedia, E-ISSN 1875-3892, Vol. 39, p. 17-26Article in journal (Refereed)
    Abstract [en]

    An analytical quasi-static model of the keyhole during laser deep penetration welding is introduced. This model is used to calculate the keyhole geometry depending on spatial laser beam intensity. Keyhole shapes can be found solving the energy and pressure equations. All necessary physical effects like Fresnel and plasma absorption, heat conduction and vaporization are implemented in the model. For evaluation a Gaussian and a top hat beam profile were used. Experimental measurements of the keyhole shape using copper inlays in aluminum base material show good agreement with the results of the modeling.

  • 48.
    Volpp, Joerg
    Bremen University.
    Keyhole oscillations during laser deep penetration welding at different spatial laser intensity distributions2014Conference paper (Refereed)
  • 49.
    Volpp, Joerg
    Bremer Institut für angewandte Strahltechnik GmbH, Bremen, Germany.
    Keyhole stability during laser welding – Part II: Process pores and spatters2017In: Production Engineering, ISSN 0944-6524, E-ISSN 1863-7353, Vol. 11, no 1, p. 9-18Article in journal (Refereed)
  • 50.
    Volpp, Joerg
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Laser beam absorption measurement at molten metal surfaces2023In: Measurement, ISSN 0263-2241, E-ISSN 1873-412X, Vol. 209, article id 112524Article in journal (Refereed)
    Abstract [en]

    Laser light absorption is one of the elementary effects of laser material processing. Absorption values are relevant to calculate the process efficiency and predict the impact on the material for the increasingly used laser processes. However, absorption measurement can be a complex task. At high temperatures of metals, only limited experimental data is available due to the dynamic surfaces and the often unknown emissivity needed for the temperature measurement. Models were created to predict the absorption at different temperatures, which are successful with assumptions in some regimes, but often fail in others. For improving the theoretical models, an experimental measurement of high-temperature metal surfaces is desired. Therefore, a radiometric measurement method is proposed in this work using a heating laser to create a metal melt pool, while measuring temperature and reflection of its surface by a second measuring laser beam. General tendencies known from literature could be confirmed by the measurements, while absorption values tend to scatter at increasing temperature. However, trends could be observed. Between melting and boiling temperature, a slight absorption increase was seen in the range between 35% and 38%. Those values indicate that both interband and intraband absorption must be considered to explain the absorption in this regime. At increased temperatures, the intraband absorption becomes the dominating absorption mechanism, reaching absorption values above 45% at very high temperatures.

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