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  • 1.
    Brandau, Benedikt
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. JENOPTIK Optical Systems GmbH, System Development Advanced Manufacturing, Göschwitzerstraße 25, 07745 Jena, Germany.
    Da Silva, Adrien
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Wilsnack, Christoph
    Fraunhofer, Institute for Material and Beam Technology IWS, Winterbergstraße 28, 01277 Dresden, Germany.
    Brueckner, Frank
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. Fraunhofer, Institute for Material and Beam Technology IWS, Winterbergstraße 28, 01277 Dresden, Germany.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Absorbance study of powder conditions for laser additive manufacturing2022In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 216, article id 110591Article in journal (Refereed)
    Abstract [en]

    Absorbance is often used for simulations or validation of process parameters for powder-based laser materials processing. In this work, the absorbance of 39 metal powders for additive manufacturing is determined at 20 laser wavelengths. Different grain sizes and aging states for: steels, aluminum alloys, titanium alloys, Nitinol, high entropy alloy, chromium, copper, brass and iron ore were analyzed. For this purpose, the absorbance spectrum of the powders was determined via a dual-beam spectrometer in the range of λ = 330 - 1560 nm. At the laser wavelengths of λ = 450 nm, 633 nm and 650 nm, the absorbance averaged over all materials was found to increase by a factor of 2.4 up to 3.3 compared to the usual wavelength of λ = 1070 nm, with minimal variations in absorbance between materials. In the investigation of the aged or used powders, a loss of absorbance was detectable. Almost no changes from the point of view of processing aged and new AlSi10Mg powders, is expected for laser sources with λ = 450 nm. The resulting measurements provide a good basis for process parameters for a variety of laser wavelengths and materials, as well as a data set for improved absorbance simulations.

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  • 2.
    Da Silva, Adrien
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Aspects of material and heat transfer in drop- and powder-based laser additive manufacturing2023Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Additive Manufacturing became a major research topic and part of industrial production in the past years. Numerous techniques now allow to build 3D structures with a wide choice of materials. When it comes to processing of metals, a laser beam is often used as a heat source to melt either a wire or powder. Novel approaches of material deposition are also developed, such as Laser Droplet Generation, which could potentially be applied to Additive Manufacturing. During the process, the laser beam light is partly absorbed by the material, and is then converted to heat, which can induce melting and even vaporization. Additive Manufacturing presents several processing challenges, such as the recoil pressure acting on the drops and powder particles that affects their trajectory. Storage and recycling of the powders is also an important aspect since the powder properties are changed through aging. Another challenge is the adjustment of process parameters according to varying deposition conditions, where the use of process monitoring techniques is crucial.

    Therefore, this thesis aims at better understanding (i) the effects of recoil pressureon metal drops and powder particles, (ii) powder aging and its effects on the process, and (iii) process optimisation and stability via monitoring. In the six adjoined papers, high-speed imaging and thermal imaging were used to observe laser Additive Manufacturing processes involving both metal drops and powders. The videos enabled to observe drop detachments, measure trajectories, plot powder density maps, quantify powder catchment in the melt pool, measure themelt pool geometry, detect oxides, and extract cooling rates. The experimental results were supplemented with material analysis and theoretical calculations of thermodynamics, recoil pressure and surface tension.

    These studies allowed to conclude that the recoil pressure induced by laser irradiation on a drop or a powder particle can have some significant effect such as acceleration, change of trajectory, or disintegration. However, these effects seem to be considerably lower than what theoretical models predict. It was also found that the recoil pressure can be used to accurately detach drops from a wire, which was utilised as a new material deposition method for Additive Manufacturing. In Directed Energy deposition, it was showed that aging of the aluminium powder feedstock should be avoided since it induces high porosity, high dilution and decreased mechanical properties. Finally, to guarantee a defect-free deposition during the whole process, it was demonstrated that a thermal camera can be used to monitor the melt pool size, which allows to apply appropriate laser power adjustments to compensate for changing building conditions.  

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  • 3.
    Da Silva, Adrien
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Laser-induced recoil pressure on metal drops and powder particles2021Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Additive Manufacturing became a major research topic and part of industrial production in the past years. Numerous techniques now allow to build 3D structures with a wide choice of materials. When it comes to processing of metals, a laser beam is often used as a heat source to melt either a wire or powder, where the trajectory of spatters and powder particles can be affected by the laser beam radiation. Laser beam irradiation is partly absorbed by the material, and is then converted to heat, which can cause melting and even vaporization. The vaporization of material induces a recoil pressure on the melt pool, which affects its geometry and dynamics. However, the effects of the recoil pressure on airborne objects such as drops and powder particles are still relatively unknown. Their different sizes and boundary conditions compared to a melt pool might affect their behaviour under high laser beam radiation. 

    Therefore, this thesis aims at better understanding the effects of the recoil pressure on metal drops and powder particles, as well as their impacts on Additive Manufacturing processes, especially Directed Energy Deposition and Laser Metal Wire Deposition. In the three adjoined papers, high-speed imaging was used to observe (i) powder blown through a laser beam, (ii) drops falling in a laser beam, and (iii) drops detaching from a wire in a laser beam. The videos enabled to calculate the acceleration of powder particles and drops of different sizes, the density map of the powder stream, and the detachment direction of the drops. The experimental results were completed with theoretical calculations of thermodynamics, recoil pressure and surface tension. 

    These studies allowed to conclude that the acceleration induced by the recoil pressure on a drop or a powder particle increases with decreased size. Moreover, the recoil pressure causes a slight deviation of the powder stream in Directed Energy Deposition that can induce a better powder focusing. The recoil pressure can also cause the disintegration of powder particles in the laser beam. Finally, it was shown that the recoil pressure can be used to detach drops on demand from a wire and accelerate them towards the substrate where they can be strategically deposited for building additive structures.

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  • 4.
    Da Silva, Adrien
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Belelli, Filippo
    Department of Mechanical Engineering, Politecnico di Milano, 1 Via Privata Giuseppe La Masa, 20156 Milano, Italy.
    Lupi, Giorgia
    Department of Mechanical Engineering, Politecnico di Milano, 1 Via Privata Giuseppe La Masa, 20156 Milano, Italy.
    Bruzzo, Francesco
    Fraunhofer, Institut für Werkstoff und Strahltechnik, Winterbergstraße 28, 01277 Dresden, Germany.
    Brandau, Benedikt
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. System Development Advanced Manufacturing, JENOPTIK Optical Systems GmbH, Göschwitzerstraße 25, 07745 Jena, Germany.
    Maier, Lukas
    IMR – Metal Powder Technologies GmbH, Jessenigstraße 4, 9220 Lind ob Velden, Austria.
    Pesl, Alexander
    IMR – Metal Powder Technologies GmbH, Jessenigstraße 4, 9220 Lind ob Velden, Austria.
    Frostevarg, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Casati, Riccardo
    Department of Mechanical Engineering, Politecnico di Milano, 1 Via Privata Giuseppe La Masa, 20156 Milano, Italy.
    Lopez, Elena
    Fraunhofer, Institut für Werkstoff und Strahltechnik, Winterbergstraße 28, 01277 Dresden, Germany.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Influence of aluminium powder aging on Directed Energy deposition2022In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 218, article id 110677Article in journal (Refereed)
    Abstract [en]

    The use of aluminium alloys for Additive Manufacturing is of high interest for advanced geometries and lightweight applications. In Directed Energy Deposition, a powder stock is processed with a laser beam, which offers a high process flexibility. However, aging of the powder feedstock during storage or after recycling remains fundamentally challenging for aluminium alloys because of their sensitivity to oxida-tion and porosity. In order to investigate these effects, AlSi10Mg powder batches were aged in different conditions and processed by Directed Energy Deposition. The results showed that powder aging does not significantly change the particle size or morphology, but it introduces more oxygen and hydrogen in the powder. The oxidation of the particles reduces the laser beam absorbance of the powder and increases wetting of the melt pool, which affects the track geometry. A 3.5 to 4.2 times higher porosity was observed in the material deposited from aged powder, which are most likely hydrogen pores causedby the increased hydrogen content in the aged powder. The tensile properties of the parts built with aged powder showed 19.0% lower yield strength, 14.2% lower ultimate strength and 99.2% higher elongation, which are most likely the results of the coarser microstructure and increased porosity.

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  • 5.
    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.
    Kaplan, Alexander F. H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Melt pool monitoring and process optimisation of directed energy deposition via coaxial thermal imaging2023In: Journal of Manufacturing Processes, ISSN 1526-6125, Vol. 107, p. 126-133Article in journal (Refereed)
    Abstract [en]

    In Laser-based Directed Energy Deposition of metal powder, the use of optimised parameters allows the deposition of defect-free material, while diverging from these optimised parameters can typically result in high porosity, high dilution or different track geometry. One of the main challenges when building complex geometries is that the geometrical and thermal conditions of the deposition are constantly changing, which requires to adjust the process parameters during the production. In order to facilitate this process, sensors such as thermal cameras can be used to extract data from the process and adapt the parameters to keep the process stable despite external disturbances. In this research, different signals extracted from a coaxial thermal camera are investigated and compared for process optimisation. To investigate such possibilities, five overlapped tracks are deposited at constant laser powers in order to extract average pixel values as well as the melt pool area, length, width and orientation. The behaviour of each track deposition is modelled as a function of the laser power, and these models are used to calculate and test laser power reduction strategies based on different signals. The results show that the melt pool area is the most relevant signal to use for an efficient process control, resulting in a stable process with only ±1.6 % of signal variation from track to track.

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  • 6.
    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.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    The influence of laser-induced recoil pressure on particles speed in Directed Energy Deposition2022In: 12th CIRP Conference on Photonic Technologies [LANE 2022] / [ed] M. Schmidt, F. Vollertsen, B.M. Colosimo, Elsevier, 2022, Vol. 111, p. 381-384Conference paper (Refereed)
    Abstract [en]

    Directed Energy Deposition is a common Additive Manufacturing technique used for its high deposition rate, but the interactions between the powder stream and the laser beam are still not completely understood. It is known that the powder particles heat up in the laser beam and some theoretical models predict that they can reach vaporization temperature and are significantly accelerated by the recoil pressure. In order to learn more about these phenomena, AlSi10Mg powder streams were observed with a high-speed camera at different laser powers and a particle-tracking software was used to measure the speed of the particles. The results show no significant increase of particle speed near the powder focus where the powder is processed, nor further below the powder focus. The high initial speed of the particles results in a short travelling time through the laser beam and in a very small effect of the recoil pressure.

  • 7.
    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.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Thermal monitoring for directed energy deposition of stainless steel, bronze, and cobalt-based alloy2022In: Surface & Coatings Technology, ISSN 0257-8972, E-ISSN 1879-3347, Vol. 451, article id 129078Article in journal (Refereed)
    Abstract [en]

    Laser cladding and Directed Energy Deposition are two related processes that allow the deposition of specific surface coatings and the production of additively manufactured parts. In both processes, the selection of optimised parameters results in the deposition of high-density material with low dilution. However, the thermal and geometrical conditions constantly change during the process and the parameters need to be continually adapted in order to avoid defects or poor properties. In this context, the development of closed-loop monitoring systems is crucial in order to widen the field of possible applications towards more complexity, with a more stable process and higher materials properties. In this research, the possibility of thermal monitoring with middle-wave and long-wave infra-red cameras is investigated for Directed Energy Deposition of 316L, Stellite 21 and CuSn10. The melt pool length and the cooling rate are extracted from thermal imaging while the laser power was varied, and these results are compared to the materials properties of the deposited tracks. The main results show that an increase of melt pool length results in a decrease of porosity and an increase of dilution, which induces a change of hardness. The melt pool length can be regulated by adjusting the laser power in order to keep both the porosity and the dilution within acceptable values.

  • 8.
    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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  • 9.
    Da Silva, Adrien
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Kasvayee, Keivan Amiri
    R&D filler metals, ESAB AB, Lindholmsallén 9, 402 77 Göteborg, Sweden.
    Frostevarg, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Zachrisson, Jan
    R&D filler metals, ESAB AB, Lindholmsallén 9, 402 77 Göteborg, Sweden.
    Kaplan, Alexander F.H.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Laser Metal Wire drop-by-drop Deposition: a material and dilution investigation2021In: IOP Conference Series: Materials Science and Engineering, Institute of Physics (IOP), 2021, Vol. 1135, article id 012001Conference paper (Refereed)
    Abstract [en]

    Additive Manufacturing has become a field of high interest in the industry, mostly due to its strong freedom of design and its flexibility. Numerous Additive Manufacturing techniques exist and present different advantages and disadvantages. The technique investigated in this research is a drop-by-drop deposition alternative to Laser Metal Wire Deposition. This technique is expected to induce a better control over the power input in the material, resulting in a better power efficiency and tailorable material properties. The aim of this research is to investigate selected material properties of the structures produced with the drop-by-drop deposition technique. Multi-drops structures were deposited from 316L, Inconel 625 (NW6625) and AlSi5 (AW4043) wires. Two drop deposition methods were investigated: (i) a contactless recoil pressure driven detachment for 316L and Inconel 625, (ii) a contact-based surface tension driven detachment for AlSi5. A material characterization including optical microscopy, EDS and hardness measurements was performed in transverse and longitudinal cross-sections. The microstructure of the deposited material, the dilution with the substrate and the heat affected zone were analysed. The contactless detachment showed a higher dilution than the contact-based technique due to the laser irradiating the substrate between two drop detachments, which melts the substrate that then mixes with the deposited drops.

  • 10.
    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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  • 11.
    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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  • 12.
    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.

  • 13.
    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.

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