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  • 51.
    Karlsson, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Two laser welding cases and suitable documentation methods2010Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [sv]

    Avhandlingen behandlar lasersvetsning, speciellt dess geometriska defekter och hur dokumentering av genererad kunskap kan förbättras. Lasersvetsning är en viktig och avancerad tillverkningsmetod inom industrin. En variant av lasersvetsning som börjat användas är laserhybridsvetsning med MAG. Det är väsentligt inom svetsning att kunna kontrollera svetsprocessen för att få önskad kvalitet, speciellt med avseende på undertryckande av defekter och ogynnsamma ytgeometrier. De mekaniska egenskaperna för en produkt i drift kan lida av små och knappt synliga svetsdefekter, exempelvis svetsdiken. Processen som avgör den resulterande kvaliteten är multifysikalisk och ickelinjär. Dokumentering av parametrarna, processvillkor och kvalitetskriterier är svårt och i dagsläget otillfredsställande. Därför behandlar artikel 1 mekanismer och utmaningar för effektiv dokumentering av kunskap inom lasersvetsning. Enligt livscykelteorin beror inte dessa problem enbart på när kunskapen genereras, identifieras, fångas och lagrad, utan även hur den hämtas, delas och används. I artikel 2 studeras resulterande geometrier för en hörnfog med varierande parametrar. Bl.a. svetsdiken undersöks. Resultaten kategoriseras, kartläggs och dokumenteras. I ett försök att bättra kunskaps- formalisering, förvaring och åtkomst som diskuteras i artikel 1, används en ny metod kallad Matrix Flow Chart, MFC. Svetsdiken orsakas av olika villkor hos de metalliska ytorna i laserhybridsvetsning, vilket i artikel 3 och 4 två olika sorters svetsdiken undersöks med hjälp av optisk analys, SEM, kemisk analys och höghastighetsfilmning. I artikel 3 dokumenteras de observerade fynden i ett vanligt letter-format, som är en extremt kortfattad dokumenteringsmetod. I artikel 4 förklaras de fysiska mekanismerna för svetsprocessen som skapar de två typerna av svetsdiken i ett vanligt manuskriptformat. Ett sekventiellt flödesschema visas passa dokumentering av fynden.

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  • 52. Karlsson, Jan
    et al.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Fibre laser welding for lightweight design2009Ingår i: Congress proceedings: ICALEO, 28th International Congress on Applications of Lasers & Electro-Optics : November 2 - 5, 2009 - Orlando, FL : Laser Materials Processing Conference, Laser Microprocessing Conference, Nanomanufacturing Conference, poster presentation, gallery, Orlando, Fla.: Laser institute of America , 2009, Vol. 102, s. 1548-1557Konferensbidrag (Refereegranskat)
    Abstract [en]

    Two different high strength steel grades have been welded by a fibre laser as a fillet corner joint. The purpose of the experiments is to use thinner materials of higher strength as a weight reduction strategy. The sensitivity of the welding process to the beam inclination angle, the beam positioning and the focal plane position were studied. High-speed imaging, process emission monitoring, measurement of the weld surface topography, mechanical testing and FE-analysis have been used to analyze the process and the resulting weld. The combined study by these methods gives a more comprehensive picture for studying the weld quality. The Bifurcation Flow Chart (BFC) as a new method has been applied to standardize and generalize the findings.

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  • 53.
    Karlsson, Jan
    et al.
    Luleå tekniska universitet, Institutionen för tillämpad fysik, maskin- och materialteknik.
    Kaplan, Alexander F. H.
    Luleå tekniska universitet, Institutionen för tillämpad fysik, maskin- och materialteknik.
    Analysis of a fibre laser welding case study, utilising a matrix flow chart2011Ingår i: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 257, nr 9, s. 4113-4122Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    For fibre laser welding of an eccentric corner joint, the quality of the resulting weld cross section was studied with respect to the dependence on process parameters like lateral laser beam alignment, beam inclination, focal plane position or welding speed. The complex load situation of the support beamer was simplified to bending of one corner. Due to fatigue load, the weld properties causing the peak stress are essential, in particular the top and root shape of the weld cross section. For the parameters varied, the resulting shapes were categorized into different top and root classes, determined by certain key dimensions, considering also welding defects like undercuts. The shapes are boundary conditions for Finite Element Analysis of the joint under load for quantitative comparative analysis of the maximum stress. As two high strength steel grades were joined, the hardness transition across the weld was of interest, too. High speed imaging of the weld pool surface shape provided additional information on the relation between the parameter input and quality output. The different trends identified were discussed and guidelines were derived. As the systematic documentation of results is unsatisfactory in welding, a new method was developed and applied for the first time, called the Matrix Flow Chart. It enables an illustrative view on the resulting welding trends in a combined manner and is extendable by other researchers.

  • 54.
    Karlsson, Jan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Markmann, Christoph
    Luleå tekniska universitet.
    Alam, Md. Minhaj
    Luleå tekniska universitet.
    Kaplan, Alexander F. H.
    Luleå tekniska universitet.
    Parameter influence on the laser weld geometry documented by the matrix flow chart2010Ingår i: Physics Procedia, E-ISSN 1875-3892, Vol. 5, nr 2, s. 183-192Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Three variants of fibre laser welding of corner joints where studied. In service a welded piece experiences fatigue load in a complex manner. The peak stress responsible for the fatigue life of the product is mainly determined by positioning and the geometry of the resulting weld. Some top and root shapes where identified for the joints studied. While parameter documentation is straightforward, generalization and combination of knowledge is a challenge. A new documentation method, the Matrix Flow Chart, MFC, turned out to be a powerful solution for large scale documentation, combination and generalization. Three variants of fibre laser welding of corner joints where studied. The peak stress responsible for the fatigue life of the product is mainly determined by the geometry of the resulting weld. Different top and root shape classes were identified for the joints studied. The shapes mainly depend on geometrical laser beam parameters and govern the peak stress. While parameter documentation used to be straightforward, generalization and combination of knowledge is a challenge. A new documentation method, the Matrix Flow Chart, MFC, turned out to be a promising solution for large scale documentation.

  • 55.
    Karlsson, Jan
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Norman, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Rubin, Per
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Materialvetenskap.
    Lamas, Javier
    Centro Tecnolóxico do Naval Galego, Ferrol.
    Yañez, Armando
    Universidade da Coruña, Ctr Invest Tecnol.
    Observation of the mechanisms causing two kinds of undercut during laser hybrid arc welding2011Ingår i: Applied Surface Science, ISSN 0169-4332, E-ISSN 1873-5584, Vol. 257, nr 17, s. 7501-7506Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Two different kinds of undercut were identified when laser hybrid welding hot rolled HSLA-steel in either the as-rolled condition or with the top surface mill scale removed. The presence of mill scale on the steel surface was found to give a sharp angled undercut combined with a sharp oxide inclusion at the edge of the weld which would have the same mechanical effect as a crack in this position. Removal of the surface oxides before welding resulted in the elimination of the oxide inclusions and a more rounded undercut geometry indicative of superior mechanical properties, particularly fatigue life. The mechanisms of the formation of both types of undercut have been analysed by high speed photography and SEM.

  • 56.
    Lamas, Javier
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Gap bridging for two modes of laser arc hybrid welding2015Ingår i: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 224, s. 73-79Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The topology of welds is an important quality criterion for which the mechanisms behind are only partially understood. The impact of gap width on the resulting weld shape produced by laser-arc hybrid welding was studied for two arc modes: pulsed arc and cold metal transfer. The weld bead and root topology were scanned and analysed. For widening gaps below a limiting width, top reinforcement turns into underfill. Above a certain gap width (0.8 mm in this case), a bridge of melt is created across the weld zone. Although high speed imaging shows very different drop transfer and weld pool formation for the two arc modes, the bridging behaviour shows the same trend for both modes. This bridging effect is important from an industrial point of view because, when viewed from the top, it gives the misleading impression that a good weld has been produced. For even wider gaps the melt collapses into the gap. These different regimes are analysed and discussed for both modes.

  • 57.
    Lamas, Javier
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling. Centro Tecnolóxico do Naval Galego, Ferrol, Spain.
    Karlsson, Jan Erik
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Norman, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander F. H.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Yañez, Armando
    Centro de Investigacións Tecnolóxicas, Universidade da Coruña, Ferrol 15403 (A Coruña), Spain.
    The effect of fit-up geometry on melt flow and weld quality in laser hybrid welding2013Ingår i: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 25, nr 3Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Hybrid laser-arc welding has a good tolerance to poor fit up as compared to simple laser welding. For a butt joint, the joint fit-up variations can be reduced to two local properties: the gap width and the vertical edge mismatch. The impact of these two properties on the resulting weld quality has been studied systematically in this paper. The original edges as well as the resulting weld surface topography have been scanned in three dimensions in order to study trends. During hybrid welding, the melt flow and the electric arc were observed at the top surface by high speed imaging to analyze the complex fluid flow phenomena.

  • 58.
    Moradi, Mahmoud
    et al.
    Faculty of Mechanical Engineering, K.N. Toosi University of Technology, Tehran.
    Ghoreishi, Majid
    Faculty of Mechanical Engineering, K.N. Toosi University of Technology, Tehran.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    An investigation on stability of laser hybrid arc welding2013Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 51, nr 4, s. 481-487Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The stability of the weld surface quality resulting from laser–arc hybrid welding of 4 mm thick steel was studied. The trends of stability in terms of top weld width variation were estimated by using design of experiments, where different types of unstable welds were distinguished. High speed imaging of the process supported the interpretation of the trends. High arc voltage and short distance laser–arc has destabilized the process. For a stable process the applied spray mode has caused a short arc and symmetric central drop transfer while for high voltage the arc became long and wide and the drops traveled sideward. The potential and limits of the design of experiments method for such kinds of applications were discussed.

  • 59.
    Moradi, Mahmoud
    et al.
    Department of Mechanical Engineering, Faculty of Engineering, Malayer University.
    Salimi, Nahid
    Department of Computer Engineering, Malayer Branch, Islamic Azad University.
    Ghoreishi, M.
    Faculty of Mechanical Engineering, K.N. Toosi University of Technology, Tehran.
    Abdollahi, Hadi
    Department of Mechanical Engineering, Urmia University of Technology.
    Shamsborhan, Mahmoud
    Department of Engineering, Mahabad Branch, Islamic Azad University.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Ilar, Torbjörn
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Parameter dependencies in laser hybrid arc welding by design of experiments and by a mass balance2014Ingår i: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 26, nr 2, artikel-id 22004Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Reinforcement, undercut, and root drop-through during laser hybrid arc welding of steel were studied in dependence of gap width, welding speed, and wire feeding rate. Generalized trends were obtained through design of experiments. Most of the trends could be explained by a mass balance while some parameter impacts relied on more complex mechanisms. In particular, different levels of complexity of parameter dependencies were distinguished, ranging from monotonous behaviour to maxima and to changing signs of the trends. The findings are of high practical relevance to optimize the process with respect to the weld quality. Moreover, the potential and limits of the design of experiments method, of a mass balance, and of the matrix flow chart method are discussed.

  • 60.
    Norman, Peter
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet.
    Karlsson, Jan
    Luleå tekniska universitet.
    Classification and generalization of data from a fibre-laser hybrid welding case2010Ingår i: Physics Procedia, E-ISSN 1875-3892, Vol. 5, nr 2, s. 69-76Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Compared to autonomous laser welding, the amount of parameters is higher for laser hybrid welding. Consequently, empirical optimisation of these parameters is a challenge. Handling and evaluation at a higher systematic level is desired in order to enhance the ability to build research on previous knowledge. Such new approach is studied on a case with 10 mm high strength steel sheets, fibre-laser hybrid welded. Aim of the paper is to provide a method of documenting and handling data, transferable to other disciplines, to continuously build knowledge, to simplify repetition of experiments and to facilitate the start-up phase of new trials. Starting from 30 experimental results depending on 23 parameters, by the Matrix Flow Chart a guideline has been developed that filters the information through combination, priorities and quality categorization. A chart resulted where five categories of poor quality are graphically related to a high quality category which can be achieved when following the guidelines for eight main arc- or laser-parameters. The chart is a guideline suitable for extension and for exploring the limits of its validity.

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  • 61.
    Norman, Peter
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Karlsson, Jan
    Kaplan, Alexander F. H.
    Monitoring undercut, blowouts and root sagging during laser beam welding2009Ingår i: Lasers in Manufacturing 2009: proceedings of the Fifth International WLT-Conference Lasers in Manufacturing, LIM 2009 : Munich, Germany, June 15th - 18th, 2009 / [ed] Andreas Ostendorf, Stuttgart: Gabler Verlag/GWV Fachverlage GmbH, 2009Konferensbidrag (Refereegranskat)
    Abstract [en]

    Using a 14 kW CO2-laser, 12mm thick cold-formed steel S420MC has been welded to a machined shaft pivot made of 25CrMo4 steel as part of a truck rear axel. A photodiode-based, on-line process monitoring system has been applied for detecting defects. However, the occurrence of certain defects, namely undercut, blowouts and root sagging is often not detectable from the sensor signal. The time dependent signal is collected from emissions from the melt pool surface as well as from the plasma plume. Based on the evaluation of high speed images, an explanation of the potential and limitations of detection of these defects has been generated. Although preliminary conclusions have been drawn, uncertainties regarding emissivity and the keyhole and plasma radiation characteristics require further studies.

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  • 62.
    Norman, Peter M.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Karlsson, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Kaplan, Alexander F.H.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Mechanisms forming undercuts during laser hybrid arc welding2011Ingår i: Physics Procedia, E-ISSN 1875-3892, Vol. 12, nr 1, s. 201-207Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    That the final quality of a welded joint, using a laser hybrid arc process, is sensitive not only to the geometrical joint conditions, but also to the content of elements in the surface itself, is already known. However the description of the mechanisms forming undercut defects is still vague. In the following study two kinds of undercuts were identified. It is shown how the mill scales affects the melt motion and what mechanisms actuate the creation of the defects. The conclusion that surface oxides affect the severity of weld undercuts is straight forward but here also the mechanism is explained.

  • 63.
    Näsström, Jonas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Arc formation in narrow gap hot wire laser welding2018Ingår i: Welding Journal, ISSN 0043-2296, Vol. 97, nr 6, s. 171S-178SArtikel i tidskrift (Refereegranskat)
    Abstract [en]

    Many heavy industrial applications, e.g. shipbuilding and offshore, rely on thick-section, high-quality welds. Unfortunately, traditional arc-based techniques are often found wanting due to a limited penetration depth and excessive heat-affected zone. The former is typically solved by having a wide groove filled by multiple weld passes, which is both costly and time consuming. Other processes such as autonomous laser or electron beams can join thick materials, but have disadvantages such as increased hardness and solidification cracks inside the welds. A promising in-between technique to join thick sheets is narrow gap multi layer laser welding (NGMLW), using less filler material while also offering more control of weld properties. This technique is often used with laser scanning optics and cold wire, or a defocused laser and electrically heated wire. This paper investigates the limitations of the latter during NGMLW, mainly using high-speed imaging to directly observe and explain process behavior. Increased deposition rates are wanted, but heating also consequently needs to be increased for proper bead fusion. Arc occurrences are found to be the cause of instabilities. They are observed occasionally even at low voltages, but more frequently at higher outputs, and then are also more disruptive to the process.

  • 64.
    Näsström, Jonas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Multipass laser hot-wire welding: Morphology and process robustness2017Ingår i: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 29, nr 2, artikel-id 022014lArtikel i tidskrift (Refereegranskat)
    Abstract [en]

    There are great prospects for utilizing multipass laser hot-wire welding to join thick steel sheets, especially for techniques commonly performed in single passes, e.g., laser arc hybrid welding, fall short, presenting great opportunities for vehicle industries and offshore applications. Many modern approaches for applying these techniques rely on customized wire feeding nozzles or special scanner optics to ensure proper laser–wire interactions and, in turn, robust process behavior, making them less accessible to many industries. Here, we present a modified adaption of laser hot-wire welding, utilizing more readily available equipment, including an unmodified welding source and a nozzle, presented and evaluated through means of, e.g., high speed imaging and macroscopy. This technique was found to have high process robustness, especially for sealing passes, if wire resistance heating is kept within suitable levels. It is able to both maintain proper laser–wire interaction and produce close to net-shape weld caps. Also, recommended process parameters are presented, together with a description of a potential method for suppressing solidification cracking.

  • 65.
    Näsström, Jonas
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Silver, Tore
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Hot-wire Laser Welding of Deep and Wide Gaps2015Ingår i: Physics Procedia, E-ISSN 1875-3892, Vol. 78, s. 247-254Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Heavy section Gas Metal Arc Welding (GMAW) usually requires special edge preparation and several passes. One alternative for increased performance is Laser Arc Hybrid Welding (LAHW). For very thick sheets however, imperfections like root drops or solidification cracks can occur. In this study, other techniques are also studied, including multi-pass filling of deep gaps with wire deposition. A laser is then used to melt the filler and base material. The hot- and cold wire laser welding processes are highly sensitive to wire-laser positioning, where controlled melting of the wire is essential. Apart from a comprehensive literature survey, preliminary experiments were also performed in order to find a novel method variant that can successfully fill deep and wide gaps. The method applied uses a defocused laser that generates the melt pool. A resistance heated wire is fed into the melt pool front in a leading position. This is similar to additive manufacturing techniques such as laser direct metal deposition with wire. A layer height of several millimeters can be achieved and rather low laser power can be chosen. The preliminary experiments were observed using high speed imaging and briefly evaluated by visual examination of the resulting beads. Using a defocused laser beam turned out to have two major advantages; 1. It adds heat to the melt pool in a manner that properly fuses the bottom and walls of the base material. 2. It counteracts difficulties due to an irregularly oscillating filler wire. These early results show that this can be a promising technique for joining thick steels with wide gaps.

  • 66.
    Olsson, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling. Laser Nova AB, SE-831 48 Östersund, Sweden.
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander F. H.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Normal vector distribution as a classification tool for convoluted rough surfaces with overhanging features2020Ingår i: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 32, nr 2, artikel-id 022030Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Convoluted rough surfaces involving overhanging features can be a natural consequence of laser additive manufacturing and other spray techniques or can be generated deliberately by laser surface texturing, e.g., to aid osseointegration. Overhanging features add an extra level of complexity to the topography of a rough surface and can have a substantial effect on wettability, etc. However, features of this type are invisible to traditional surface roughness measurement techniques. This work presents a computer-based surface analysis method that gives useful information about the presence and nature of overhanging features on rough technical surfaces. The technique uses micro-computer tomography to generate a typical cross section of the surface under investigation. The angles of the vectors normal to the surface can then be analyzed to reveal the presence of overhanging features, which can also be indicated by the standard deviation of the normal vector distribution. Titanium surfaces generated by six different techniques were compared. The characteristics of these surfaces varied strongly, as did the shapes of the overhangs involved. These variations are reflected by different statistical distributions of the normal vectors.

  • 67.
    Olsson, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling. Laser Nova AB, SE-831 48 Östersund, Sweden .
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander F.H
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Topographical evaluation of laser generated surfaces using statistical analysis of surface-normal vector distributions2021Ingår i: IOP Conference Series: Materials Science and Engineering / [ed] Jan Frostevarg, Adrien Da Silva, Marie-Louise Palmblad, Lars Andersson, Institute of Physics (IOP), 2021, Vol. 1135, artikel-id 012023Konferensbidrag (Refereegranskat)
    Abstract [en]

    Surfaces generated by Additive Manufacturing or laser texturing can involve the solidification of droplets of liquid, which can give rise to overhanging features on the solidified surface. Overhanging features add a layer of complexity to the surface topography and are undetectable by standard surface roughness measurement techniques such as profilometry. Such features are important because they can have a considerable effect on surface properties such as wettability. New techniques and algorithms are therefore required to analyse and quantify convoluted surfaces with overhanging (re-entrant) features. Earlier work by the authors introduced the concept of using X-ray micro-computed tomography (Micro-CT) to identify the directions of vectors normal to the surface at any point and thus indicate the presence or absence of overhanging features. This paper divides overhanging features into two types; simple and compound, and introduces new, size independent, analysis techniques which measure what proportion of each type is on the surface. Another extension of the analysis is the comparison of surface profiles taken in different directions in order to identify any surface roughness anisotropies.

  • 68.
    Olsson, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling. Laser Nova AB.
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Palmquist, A.
    Department of Biomaterials, University of Gothenburg.
    Brånemark, R.
    Department of Orthopaedic Surgery, University of California, San Francisco.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    The production of osseointegrating (bone bonding) surfaces on titanium screws by laser melt disruption.2018Ingår i: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 30, nr 4, artikel-id 042009Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Several techniques can be used to modify implant surfaces in order to accelerate bone growth around titanium implants. One method is to generate a surface structure which stimulates bone growth and remodelling. This paper describes and explains a non-ablative method for producing osseointegrating (structural and functional bone bonding) surfaces on titanium implants using laser processing. The focus is especially on surface texturing of dental implant screws, where the ability of a Nd:YAG laser to generate ‘splashy’ surfaces covered in resolidified micro scale droplets coated with nano-scale surface oxides is assessed. The surfaces produced were analysed by scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDS). It is concluded that laser processing using Q-pulsed Nd:YAG lasers can generate surfaces which match the demands set by clinical experience. One important characteristic of the surfaces discussed here is that they involve overhanging features which are suitable for trapping red blood cells and which cannot be created by mechanical or chemical roughening techniques.

  • 69.
    Olsson, Rickard
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling. Laser Nova AB, Östersund, Sweden.
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Palmquist, Anders
    Department of Biomaterials, University of Gothenburg, Gothenburg, Sweden.
    Brånemark, Rickard
    Department of Orthopaedic Surgery, University of California, San Francisco, CA, USA.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Formation of osseointegrating (bone integrating) surfaces on titanium by laser irradiation2019Ingår i: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 31, nr 2, artikel-id 022508Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Pulsed lasers can be used to modify the surface of medical implants in order to accelerate bone growth (osseointegration). A surface covered in attached droplets with diameters between 1 and 20 μm is a beneficial surface for rapid osseointegration. This paper presents the results of an experimental program in which a broad range of laser parameters and different atmospheres were used to create different surface textures on titanium substrates, including the desired "attached droplet" topology. The resulting surfaces were analyzed by scanning electron microscopy and micro-computer tomography. The paper explains how different types of surfaces are created by the laser-material interaction under different conditions and focus characteristics. It is shown that optimization of the laser parameters results in a robust process, which produces a surface that is fundamentally different from those created by nonlaser methods.

  • 70.
    Pocorni, Jetro
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling. Laser Expertise Ltd., Acorn Park Industrial Estate.
    Deichsel, Eckard
    Bystronic Laser AG, Industriestrasse 21, CH-3362 Niederönz.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Fibre laser cutting stainless steel: Fluid dynamics and cut front morphology2017Ingår i: Optics and Laser Technology, ISSN 0030-3992, E-ISSN 1879-2545, Vol. 87, s. 87-93Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper the morphology of the laser cut front generated by fibre lasers was investigated by observation of the ‘frozen’ cut front, additionally high speed imaging (HSI) was employed to study the fluid dynamics on the cut front while cutting. During laser cutting the morphology and flow properties of the melt film on the cut front affect cut quality parameters such as cut edge roughness and dross (residual melt attached to the bottom of the cut edge). HSI observation of melt flow down a laser cutting front using standard cutting parameters is experimentally problematic because the cut front is narrow and surrounded by the kerf walls. To compensate for this, artificial parameters are usually chosen to obtain wide cut fronts which are unrepresentative of the actual industrial process. This paper presents a new experimental cutting geometry which permits HSI of the laser cut front using standard, commercial parameters. These results suggest that the cut front produced when cutting medium section (10 mm thick) stainless steel with a fibre laser and a nitrogen assist gas is covered in humps which themselves are covered by a thin layer of liquid. HSI observation and theoretical analysis reveal that under these conditions the humps move down the cut front at an average speed of approximately 0.4 m/s while the covering liquid flows at an average speed of approximately 1.1 m/s, with an average melt depth at the bottom of the cut zone of approximately 0.17 mm.

  • 71.
    Pocorni, Jetro
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling. Laser Expertise Ltd., Acorn Park Industrial Estate.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Investigation of the Piercing Process in Laser Cutting of Stainless Steel2017Ingår i: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 29, nr 2, artikel-id 022201Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper investigates the laser piercing process which precedes nearly every laser cutting operation. The two most important aspects of the piercing process are: a) How long does it take to pierce the material? And b) How wide is the pierced hole? If the hole is no wider than the cut line, the material can be pierced on the line to be cut. In this paper 10 mm thick stainless steel was pierced by a multikilowatt fibre laser to compare efficiency and quality when piercing with a continuous wave (cw) output and a selected range of power modulation parameters. The different processes were observed by high speed imaging and subsequently examined by visual observation. High speed imaging is used to time the penetration event and to study the laser-material interactions involved in drilling the pierced holes. The results show that appropriate laser power modulation settings can considerably reduce both the piercing time and the required energy to generate any piercing hole required for the subsequent cutting process. This pulse-pierce technique and the differences between piercing with a continuous and a power modulated laser beam are further explained and discussed. Also the effect on the size of the entrance to the pierced hole depending on power modulation regimes was investigated in this paper.

  • 72.
    Pocorni, Jetro
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander F. H.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Dynamic laser piercing of thick section metals2018Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 100, s. 82-89Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Before a contour can be laser cut the laser first needs to pierce the material. The time taken to achieve piercing should be minimised to optimise productivity. One important aspect of laser piercing is the reliability of the process because industrial laser cutting machines are programmed for the minimum reliable pierce time. In this work piercing experiments were carried out in 15 mm thick stainless steel sheets comparing a stationary laser and a laser which moves along a circular trajectory with varying processing speeds. Results show that circular piercing can decrease the pierce duration by almost half compared to stationary piercing. High speed imaging (HSI) was employed during the piercing process to understand melt behaviour inside the pierce hole. HSI videos show that circular rotation of the laser beam forces melt to eject in opposite direction of the beam movement, while in stationary piercing the melt ejects less efficiently in random directions out of the hole.

  • 73.
    Pocorni, Jetro
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Powell, John
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling. Laser Expertise Ltd., Acorn Park Industrial Estate.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander F.H.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    The geometry of the cutting front created by Fibre and CO2 lasers when profiling stainless steel under standard commercial conditions2018Ingår i: Optics and Laser Technology, ISSN 0030-3992, E-ISSN 1879-2545, Vol. 103, s. 318-326Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Cutting fronts created by CO2 and fibre lasers in stainless steel at thicknesses between 2 mm and 10 mm have been ‘frozen’ and their geometry has been measured. Standard commercial cutting parameters were used to generate the cuts for both types of laser. The resulting three-dimensional cutting front shapes have been curve fitted as polynomials and semicircles. Various features of the cutting front geometry are discussed including the lack of correlation of the cut front inclination with either the relevant Brewster angle or the inclination of the striations on the cut edge.

  • 74.
    Powell, John
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Ilar, Torbjörn
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Torkamany, Mohammad J.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Na, Suck-Joo
    Department of Mechanical Engineering, KAIST, Daejeon 305-701, South Korea.
    Petring, Dirk
    Fraunhofer ILT, D-52074, Aachen, Germany.
    Zhang, Linjie
    Department of Physics, Xi'an Jiaotong University, Xi'an 710049, China.
    Kaplan, Alexander F. H.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Weld root instabilities in fiber laser welding2015Ingår i: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 27, nr Suppl. 2, artikel-id S29008Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Fiber laser welding and fiber laser-arc hybrid welding have been developed into very useful industrial processes over the past few years. However, the use of fiber lasers can be associated with quality problems at the weld root such as intermittent penetration, melt ejection, and humping. This paper explains the mechanisms which create these instabilities and suggests a repair technique which would alleviate the problems in some cases. The main difference between fiber and CO2 laser welding is that during fiber laser welding there is a strong downward thrust on the melt in the keyhole.(This is not the case during CO2 laser welding.) The downward thrust on the melt is generated as a result of the fiber laser evaporating the top faces of bumps on the melt surface. The downward flow has two main effects;(a) the melt can achieve velocities which result in melt ejection from the root of the weld-which can result in intermittent penetration,(b) the supply of hot metal to the bottom of the weld makes the process much more thermally uniform at the top and bottom of the weld. This uniformity means that the melt is extended backward several millimeters on the top and bottom surfaces-which can result in melt humping in the weld root as well as on the top surface. This paper examines these weld root instabilities and also describes a technique of weld root remelting which has been shown to be effective in smoothing out the root of the weld to improve its tensile and fatigue properties

  • 75. Powell, John
    et al.
    Lamas, J.
    Centro Tecnolóxico do Naval Galego, Ferrol.
    Karlsson, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Norman, Peter
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Yañez, A.
    Centro de Investigacións Tecnolóxicas, Universidade da Coruña, Ferrol.
    The sensitivity of hybrid laser welding to variations in workpiece position2011Ingår i: Physics Procedia, E-ISSN 1875-3892, Vol. 12, nr 1, s. 188-193Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    High speed imaging has been used to analyze the sensitivity of the Hybrid laser welding process to variations in the laser-arcworkpiece geometry along the axis of the laser beam. The welding process was found to be stable within a certain range of workpiece positions. Outside of this range the process became unstable. If the workpiece was too close to the laser/arc combination, the two energy sources did not supplement each other sufficiently. If the workpiece was too far away the droplets from the arc interfered with the laser-keyhole interaction.

  • 76.
    Robertson, Stephanie
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Näsström, Jonas
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Berndtsson, Therese
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Evaluation of pre-determined dilution of high strength steels by the Snapshot method2021Ingår i: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 139, artikel-id 106512Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Dilution is an unavoidable consequence of multi-material fusion processing, i.e. welding, cladding etc. In this paper we propose a novel method for controlled dilution experiments, analyzing microstructural trends of steel filler wire diluted with steel base metal. The highlight of this method is the control of processing conditions used to melt a pre-determined dilution of two high strength steels. The materials involved are S960QL base metal machining chips and a chopped under-matched wire consumable, which is used to increase the toughness of welded joints. These materials were combined in specific mass ratios in a prepared cavity and then melted by a pulsed laser beam. A high-speed RGB camera evaluated the relative spatial temperature of the melt surface. The molten mass then solidified into a uniform nugget, confirmed by energy dispersive x-ray spectrometry (EDS) to have a homogenous chemical composition (a ‘Snapshot’ nugget). Hardness values obtained for different dilution levels were compared to a narrow gap multi-layer laser weld (NGMLW), with a decreased dilution rate yielding a decreased hardness. The Snapshot microstructures created are similar to the different regions of the NGMLW, in the weld cap and in the body of the weld. Snapshot nuggets were also evaluated for non-metallic inclusion (NMI) size distributions relating to the dilution levels (NMIs are important indicators for acicular ferrite, which has been shown to increase impact toughness).

  • 77.
    Robertson, Stephanie
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Ramasamy, Anandkumar
    Lincoln Electric Europe, Nijmegen, Netherlands.
    Kalfsbeek, Bert
    Lincoln Electric Europe, Nijmegen, Netherlands.
    Volpp, Jörg
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander F.H
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Microstructures of high strength steel welding consumables from directed thermal cycles by shaped laser pulses2020Ingår i: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 109, nr 9-12, s. 2653-2662Artikel i tidskrift (Refereegranskat)
    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.

  • 78.
    Robertson, Stephanie
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Volpp, Jörg
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander F. H.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Ramasamy, Anandkumar
    Lincoln Electric Europe, Nieuwe Dukenburgseweg 20, Nijmegen 6534, Netherlands.
    Kalfsbeek, Bert
    Lincoln Electric Europe, Nieuwe Dukenburgseweg 20, Nijmegen 6534, Netherlands.
    Microstructural Effects of Controlled Dilution of High Strength Steel Wire into S960QL2019Ingår i: Procedia Manufacturing, E-ISSN 2351-9789, Vol. 36, s. 146-153Artikel i tidskrift (Refereegranskat)
    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.

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  • 79.
    Robertson, Stephanie M.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    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 cycles2019Ingår i: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 31, nr 2, artikel-id 022605Artikel i tidskrift (Refereegranskat)
    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.

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  • 80.
    Robertson, Stephanie M.
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander F.H.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Material ejection attempts during laser keyhole welding2021Ingår i: Journal of Manufacturing Processes, ISSN 1526-6125, Vol. 67, s. 91-100Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Material loss during keyhole mode laser welding often leads to surface imperfections that can negatively impact component performance. High-speed imaging identified four types of material ejection attempts: classical spatter, re-captured spatter, protuberances, and scalloping. The momentum attributed to the melt body, which is influenced by the keyhole properties, dictates whether the ejection attempt is successful or if the material is re-captured. The relationship between the dynamics of the keyhole and melt pool was elaborated in an extended systematic description of melt ejection attempts, which were then classified. Ejection attempts were often observed to be proceeded by a co-current swelling of the melt, adjacent to the keyhole, followed by a melt depression. The melt swell would elongate into a melt column with a concentration of momentum, where excessive momentum causes the melt to neck and separate into spatter. Trajectory determines if the spatter becomes a permanent fixture or re-incorporates into the melt body, with the latter having the possibility to cause further melt body disturbances leading to more ejection attempts. If the melt column fails to neck and separate, or an additional force acts upon the column, a protuberance or a scallop could then form. Keyhole and melt pool fluctuations were sometimes observed to be accommodated, avoiding material ejection. In these cases, a stable weld could be obtained with large variations in the dimensions of the melt pool and the keyhole.

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  • 81.
    Siva Prasad, Himani
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Kaplan, Alexander
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    The stability of laser welding with an off-axis wire feed2019Ingår i: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 264, s. 84-90Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The concept using an off-axis filler wire during laser welding is introduced here in order to provide added process robustness considering gap width variations. Its stability is investigated with respect to gap width, welding speeds and powers. Geometry of the welds is analysed by tracing of weld cap edges and joint cross sections, connecting trends to weld parameters. High speed imaging and streak images are used to further study and describe sequences of events, including undercut formation. Formation of imperfections are found to be mainly correlated to wire feed position variations at the surface due to irregular melting of the wire tip.

  • 82.
    Volpp, Joerg
    et al.
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Frostevarg, Jan
    Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Produkt- och produktionsutveckling.
    Elongated cavities during keyhole laser welding2021Ingår i: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 206, s. 109835-, artikel-id 109835Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    During laser beam deep penetration welding, the characteristic keyhole occurs that enables efficient energy absorption due to multiple reflections. Since this welding mode usually shows high dynamic behaviour, often a high amount of weld imperfections like spattering or porosity occur. Elongated keyholes during buttonhole, a.k.a donut welding, have shown the possibility to create smooth welding tracks when controlled variations of energy input were induced, e.g. beam oscillation. It was observed that the elongated keyholes can maintain themselves due to the balancing effects of the melt pool surface tension, which can form a catenoid shape that does not require any additional forces to be stable. Basic theoretical descriptions of the catenoid shape keyholes were derived. However, the complex system requires a more detailed investigation of the effect of the metal plume and the melt flows in order to explain the opening and maintenance effects of the elongated keyholes.

    Therefore, this work examined different modes of elongated keyholes, comparing traditional non-oscillating keyholes to keyholes formed by different beam scanning strategies. Analysis was made mainly by observations with high-speed-imaging and theoretical considerations to explain the phenomena of elongated keyholes. The observations showed that extended keyholes are not necessarily related to a reduced occurrence of imperfections and not either to the formation of perfect catenoid shapes. The keyhole front is deformed where the laser beam impacts and the rear wall is impacted where the vapour plume strikes. The opening of an elongated keyhole is likely related to the keyhole vapour plume that pushes the keyhole rear wall rearwards, while the non-affected rear wall parts do not collapse the keyhole due to the balancing nature of the second curvature of the rear wall.

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