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  • 1.
    Frostevarg, Jan
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Robertson, Stephanie
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Embedding Carbon Fibre Structures in Metal Matrixes for Additive Manufacturing2017In: Physics Procedia, ISSN 1875-3892, E-ISSN 1875-3892, Vol. 89, p. 39-48Article in journal (Refereed)
    Abstract [en]

    It is possible to reinforce structures and components using carbon fibres for applications in electronics and medicine, but most commonly used in reinforcing resin fibre composites for personal protection equipment and light weight constructions. Carbon fibres act as stress redistributors while having increased electrical and thermal conductivities. These properties could also be utilized in metal matrixes, if the fibres are properly fused to the metal and the structure remains intact. Another recently developed high potential carbon structure, carbon nanotube- (CNT) yarns, has similar but even greater mechanical properties than common carbon fibres. Via laser cladding, these reinforcing materials could be used in a plethora of applications, either locally (or globally) as surface treatments or as structural reinforcements using multi-layer laser cladding (additive manufacturing). The challenges of embedding carbon fibres or CNT-yarns in a CuAl mixture and SnPb solder wire using lasers are here investigated using high speed imaging and SEM. It is revealed that the carbon fibres have very high buoyancy in the molten metal and quickly degrades when irradiated by the laser. Wetting of the fibres is shown to be improved by a Tungsten coating and embedding of the structures after processing are evaluated using SEM and Raman spectroscopy.

  • 2.
    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.

  • 3.
    Robertson, Stephanie
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Microstructural Effects of Controlled Dilution of High Strength Steel Wire into S960QL2019In: Procedia Manufacturing, E-ISSN 2351-9789, Vol. 36, p. 146-153Article in journal (Refereed)
    Abstract [en]

    Controlled dilution experiments were conducted in a novel manner that allowed for precise dilution of base material into the wire consumables, enabling for a prompt analysis of microstructural trends as dilution is altered. Different heat cycles and cooling rates of the wire material, without base metal additions, were shown to cause different microstructures, varying from parallel plates to random or interlocking[SR1]  orientation, with varying size of packets. The proposed method enables more controlled conditions with a known dilution value from mass percentages. Chopped filler wire is weighed and added to the base metal crucible, base metal chips are also weighed and added to the filler wire in specific mass percentages. A pulsed laser irradiates the metal, melting the mixture into a sample nugget. Lack of fusion is a benefit in this method as contamination from the base plate is negligible. The cooling rate is influenced by the pulse shape, and can be used to reheat the nugget, demonstrating the microstructural evolution in a complex thermal cycle. This method is demonstrated for S960QL steel with under-matched wire consumable, generally used for laser-arc hybrid processes to obtain high toughness, where a representative thermal cycle is needed. The thermal cycle is measured via a remote process, Dualscope, to evaluate the spacial temperature of the surface. The microstructures found using the snapshot method are similar to those found in the narrow gap multi-layer weld, different only in the size of the grains and packets.

  • 4.
    Robertson, Stephanie
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Microstructural manipulation by laser irradiation of prepared samples: The ’Snapshot Method’2019Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Various metallurgical microstructures and their formation are studied in this thesis by using a laser beam to melt a variety of materials with different chemical compositions over a range of thermal cycles. The laser beam was used conventionally in a narrow gap multi-layer weld, used for welding large depths with filler wire additions, as well as a non-traditional simulated welding approach labelled here as the Snapshot method.

    In laser beam welding, materials go through rapid heating and cooling cycles that are difficult to mimic by other techniques. In welding, any microstructural development depends on complex combinations of chemistry and thermal cycles but is also influenced by melt flow behavior. In turn, microstructural morphologies influence the mechanical behavior which can suffer due to inappropriate microstructural constituents. The Snapshot method, through control of thermal cycling and material composition, can achieve the same rates while guiding microstructural development to form tailored properties.

    The tunable laser beam properties can be exploited to develop an experimental welding simulation (Snapshot method), which enables the complex interlinked chemical and thermal events which take place during welding to be studied in a controlled manner. Exploring the microstructural relationships to their thermal history provides a greater knowledge into tailoring microstructural compositions to obtain various required mechanical properties for laser welding, additive manufacturing and also non-laser welding techniques.

    The feasibility of the Snapshot method is investigated in the three appended journal publications. High speed imaging and thermal recording have proved to be essential tools in this work, with analysis from optical microscopy and EDX/EDS to provide additional support. The Snapshot method is introduced as a concept in Papers I and II, demonstrating successfully guided thermal histories after obtaining molten material. Application of a second and third heating cycle, reheating the structure without melting, yielding altered microstructures. Reaching the austenitisation temperature range allowed for the simulation of complex multi-layer welding thermal histories. Geometrically non-uniform material additions are utilized in Paper III, which investigated the formation of microstructures through the chemical composition route. New chemical compositions were obtained by different degrees of dilution of the weld filler wire by the base material.

  • 5.
    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.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Hong, Seong Min
    Department of Welding and Joining Science Engineering, Chosun University, Gwangju, Republic of Korea.
    Kim, Jong-Hee
    Department of Welding and Joining Science Engineering, Chosun University, Gwangju, Republic of Korea.
    Bang, Hee-Song
    Department of Welding and Joining Science Engineering, Chosun University, Gwangju, Republic of Korea.
    Tailored laser pulse method to manipulate filler wire melt metallurgy from thermal cycles2019In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 31, no 2, article id 022605Article in journal (Refereed)
    Abstract [en]

    A robust method is introduced to simulate and study the filler wire metallurgy for controlled cooling conditions after melting, enabling efficient mapping with prompt analysis of trends. Proposed is a reduced, though representative, process with more controllable conditions. Short lengths of filler wires are preplaced in a cavity, drilled into a base metal sheet. Irradiation by a pulsed laser beam melts the wire to generate a sample nugget. Pulse shaping influences the cooling rate, granting the ability to tailor weldament microstructures. The method is demonstrated for S1100QL steel and undermatched filler wire, to obtain high toughness for processes like laser-arc hybrid welding, where a representative thermal cycle is needed. For high toughness, a controlled amount of acicular ferrite and, in turn, nonmetallic inclusions is desirable. This “snapshot” method has revealed a characteristic histogram of inclusion sizes, for different pulse shapes. Additional information on the thermal cycle can be acquired by employing thermocouples, a pyrometer, or advanced methods like high speed imaging or modeling. The method offers a wide spectrum of variants and applications.

  • 6.
    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.
    Kalfsbeek, Bert
    Lincoln electric europe.
    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 pulsesIn: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774Article 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 in a cavity, forming a nugget. A secondary ramp-down pulse elevated the nugget temperatures to re-austenisation followed by linear temperature decay, guiding the temperature through a regime to a desired microstructure. For three very high strength steel wire chemistries, quenching yielded 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 led to less cross-hatching, coarser microstructure and coalescent sheaves.

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