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  • 1.
    Amer, Eynas
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Shock wave generation in laser ablation studied using pulsed digital holographic interferometry2008In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 41, no 21Article in journal (Refereed)
    Abstract [en]

    Pulsed digital holographic interferometry has been used to study the shock wave induced by a Q-switched Nd-YAG laser (λ = 1064 nm and pulse duration 12 ns) on a polycrystalline boron nitride (PCBN) ceramic target under atmospheric air pressure. A special setup based on using two synchronized wavelengths from the same laser for processing and measurement simultaneously has been introduced. Collimated laser light (λ = 532 nm) passed through the volume along the target and digital holograms were recorded for different time delays after processing starts. Numerical data of the integrated refractive index field were calculated and presented as phase maps showing the propagation of the shock wave generated by the process. The location of the induced shock wave front was observed for different focusing and time delays. The amount of released energy, i.e. the part of the incident energy of the laser pulse that is eventually converted to a shock wave has been estimated using the point explosion model. The released energy is normalized by the incident laser pulse energy and the energy conversion efficiency between the laser pulse and PCBN target has been calculated at different power densities. The results show that the energy conversion efficiency seems to be constant around 80% at high power densities.

  • 2. Kaplan, Alexander
    A model of deep penetration laser welding based on calculation of the keyhole profile1994In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 27, no 8, p. 1805-1814Article in journal (Refereed)
    Abstract [en]

    A model describing the process of deep-penetration laser welding has been developed by calculating the keyhole profile using a point-by-point determination of the energy balance at the keyhole wall. A formula for heat conduction was derived from the model of a moving line source of heat. The various absorption mechanisms were modelled. The corresponding absorbed power transferred to the keyhole wall balances the conduction losses, which yields the local inclination of the wall. The thermodynamics and the flow of metal vapour inside the keyhole have been calculated. Accordingly, beam damping due to the plasma plume above the workpiece and the mean plasma absorption coefficient in the keyhole could be estimated. The keyhole profile tends to a geometry that distributes the major part of the beam to the front wall owing to higher conduction losses at the upstream side. The reasons for decreasing energy absorption with increasing welding speed are discussed.

  • 3. Kaplan, Alexander
    et al.
    Baeva, Margarita
    Department of Physics, Technical University of Varna.
    Baev, Plamen
    Institute of Astronomy, Bulgarian Academy of Sciences.
    An analysis of the heat transfer from a moving elliptical cylinder1997In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 30, p. 1190-1196Article in journal (Refereed)
    Abstract [en]

    A theoretical analysis of the heat transfer from a moving elliptical cylinder is carried out and general solutions in elliptical coordinates are obtained for aspect ratios β, both β and β. The heat-flux distribution and total heat-flow rate are calculated, investigated for a wide range of parameters and presented graphically. The results obtained are useful for applications concerning thermal processes (laser cutting, welding, heat treatment and so on). A comparison of a moving circular cylinder with a numerical model of laser cutting is considered and discussed.

  • 4.
    Kaplan, Alexander
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Mizutani, Masami
    Osaka University.
    Katayama, Sieji
    Osaka University.
    Matsunawa, Akira
    Osaka University.
    Unbounded keyhole collapse and bubble formation during pulsed laser interaction with liquid zinc2002In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 35, no 11, p. 1218-1228Article in journal (Refereed)
    Abstract [en]

    Suppression of pore defects in keyhole laser spot welding demands for a theoretical description of the fundamental process. Investigating the unbounded keyhole collapse in liquid Zn instead of a solid provided a simplified situation offering several advantages. Improved high speed x-ray transmission imaging due to an enlarged keyhole in the absence of violent melt motion was enabled, which also facilitated the development of a semi-analytical mathematical model. Good correspondence between the experimentally and theoretically obtained transient keyhole and bubble shape permitted physical analysis by the model. Characteristic timescales were identified for post-vaporization, vapour relaxation, cooling, collapse, bubble contraction, oscillations and buoyancy. Recondensation due to rapid cooling turns out to be responsible for shielding gas flow into the keyhole, finally maintaining a spherical bubble. Creation of a convergent keyhole is a possibility to avoid bubbles.

  • 5.
    Lampa, Conny
    et al.
    Luleå tekniska universitet.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Powell, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Magnusson, Claes
    An analytical thermodynamic model of laser welding1997In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 30, no 9, p. 1293-1299Article in journal (Refereed)
    Abstract [en]

    An earlier model of deep-penetration laser welding has been simplified in order to provide a useful model of process analysis. This work involves the modelling of the various energy-absorption mechanisms which determine the keyhole shape and thus the dimensions of the melt pool. The penetration depth and weld width (top and bottom) predicted by the model are shown to be in close agreement with experimental results. The widening of the top of the weld seam as a result of Marangoni flow is accurately modelled by introducing an artificially enhanced value for the workpiece's thermal conductivity towards the top of the weld. The model allows analysis of the dependence of the weld profile on the process parameters

  • 6.
    Nordlund, Michael
    et al.
    Department of Chemistry - BMC, Uppsala University.
    Bhandary, Sumanta
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Sanyal, Biplab
    Uppsala universitet, Department of Physics and Astronomy, Uppsala University.
    Almqvist, Nils
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Löfqvist, Torbjörn
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Grennberg, Helena
    Department of Chemistry - BMC, Uppsala University.
    Side-selective self-assembly of graphene and FLG on piezoelectric PVDF from suspension2016In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 49, no 7, article id 07LT01Article in journal (Refereed)
    Abstract [en]

    The deposition of few-layer graphene by self-assembly from suspension onto a piezoelectric polymer substrate is presented. The graphene self-assembles with negligible overlap between flakes, and with high selectivity for one of the faces of the substrate, an observation which is discussed and rationalized. A computational study on a model system further confirms the theory and supports the experimental results. The highest obtained degree of surface coverage was estimated to 77%

  • 7. Norman, Peter
    et al.
    Engström, Hans
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Theoretical analysis of photodiode monitoring of laser welding defects by imaging combined with modelling2008In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 41, no 19Article in journal (Refereed)
    Abstract [en]

    On-line process monitoring of laser welding defects by detecting characteristic changes of the process emissions via photodiodes has high potential but, due to the complex process, lack of directly interpretable and thus controllable correlations. Deep analysis of the process by high speed imaging in combination with modelling enables one to discuss the correlations between the signal and the process, as was demonstrated for quasi-steady state conditions and for transient phenomena. Despite improved knowledge through the study, several uncertainties like the emissivity, the keyhole radiation characteristics and the temperature field need to be identified more accurately for a complete theoretical description of the signal causes.

  • 8. Powell, John
    et al.
    Petring, D.
    Fraunhofer-Institute for Laser Technology, Aachen.
    Kumar, R.V.
    University of Cambridge.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Voisey, K.T.
    University of Notttingham.
    Laser-oxygen cutting of mild steel: the thermodynamics of the oxidation reaction2009In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 42, no 1Article in journal (Refereed)
    Abstract [en]

    In a considerable proportion of the published work on the subject of laser-oxygen cutting of mild steel, the details of the oxidation reaction are overlooked or confused. For example, it is not uncommon for the oxidized material to be attributed with the physical characteristics of iron rather than iron oxide. Also, the fact that the oxidation reaction cannot take place above a certain temperature limit is usually overlooked. This paper presents, for the first time, an in-depth analysis of the Iron to FeO oxidation reaction in the context of laser-oxygen cutting of mild steel. The paper concludes by presenting a number of guidelines for future theoretical models.

  • 9.
    Råsander, Mikael
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science. Department of Materials, Imperial College London.
    Moram, M.A.
    Department of Physics, Cavendish Laboratory, University of Cambridge.
    Elastic constants of the II–IV nitride semiconductors MgSiN2, MgGeN2 and MgSnN22018In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 51, article id 375101Article in journal (Refereed)
    Abstract [en]

    The single crystal elastic constants, polycrystalline elastic moduli and related properties of orthorhombic MgSiN2, MgGeN2 and MgSnN2 have been calculated using density functional theory and compared to the related wurtzite structured AlN, GaN and InN. Since there are no experimental studies of single crystal elastic properties of neither MgSiN2, MgGeN2 or MgSnN2, we have established the accuracy of the calculations by comparison with experimental data for AlN, GaN and InN. The calculated polycrystalline elastic moduli of MgSiN2 are found to be in good agreement with available experimental elastic moduli. It will be shown that MgSiN2 and MgGeN2 have a small xy-plane lattice mismatch with AlN and GaN, respectively, while at the same time being significantly softer than both AlN and GaN. This shows that MgSiN2 and MgGeN2 should be possible to be grown on AlN and GaN without significant lattice mismatch or strain.

  • 10.
    Tokarev, Vladimir
    et al.
    Technical University of Vienna.
    Kaplan, Alexander
    Suppression of melt flows in laser ablation: application to clean laser processing1999In: Journal of Physics D: Applied Physics, ISSN 0022-3727, E-ISSN 1361-6463, Vol. 32, no 13, p. 1526-1538Article in journal (Refereed)
    Abstract [en]

    It is shown that in laser ablation of materials with large Prandtl numbers (mainly ceramics and polymers) a motion of the melt along the surface caused by the vapour plume pressure is essentially retarded for thin enough melt layers due to the onset of viscous friction. For polymers in nanosecond laser ablation this melt displacement can be obtained to be less than the ablation depth per pulse (which is typically 0.2-1 µm for nanosecond irradiation), when the absorption coefficient, α, and the kinematic viscosity, v, satisfy the condition α2v > 108 s-1. Thus, clean precise laser ablation for such polymers can be explained simply in terms of the thermal mechanism, without invoking the concept of photochemical decomposition, in terms of absorption coefficient, melt viscosity and pressure of the ablation plume on the irradiated surface. From this point of view, several factors facilitating clean laser ablation in multipulse irradiation are discussed. However, for metals (usually having a very small Prandtl number) the viscous friction has no significant effect on the retardation lateral melt flow to the periphery. The quality of the laser spot border can still be improved by using laser pulses shorter than 1 ps. In this case, in a shallow spot, the alternative (explosive) melt expulsion mechanism becomes predominant, producing material removal mainly transverse to the spot surface. Thus, the lateral (along the surface) component of melt expulsion appears to be strongly suppressed, having no chance to spoil the border of the spot.

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