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  • 1.
    Eriksson, Ingemar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    High speed imaging analysis of laser welding2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Laser welding is often considered a new and exotic manufacturing method even though it has been used in industrial applications for nearly fifty years. In the early years only a few special applications justified the high investment cost involved, but as the price of the laser sources has reduced, industrial interest in laser welding has increased. As different weld situations have appeared, involving new materials etc. there is an increasing need to understand the weld process on a fundamental level, especially for the newer, high power and high quality 1μm laser sources (Disk laser, Fiber laser). Laser welding sometimes involves production limitations that are caused by the process itself, not the laser source. Weld defects such as humping or severe spattering can make the weld quality unacceptable and more knowledge of the physics involved in defect generation is needed. In this thesis high speed imaging is used as a method of acquiring fundamental knowledge about laser welding. Modern digital high speed cameras in combination with powerful laser illumination provide a clear and detailed view of the actual weld process. The information in these high speed videos provides a possibility to see how the process behaves. Just as a slow motion goal camera helps the referees to rule accurately in an athletic event, the high speed cameras can help laser welding researchers to improve their fundamental understanding. This thesis is composed of seven publications in scientific journals which are thematically linked by their focus on high speed imaging analysis of laser welding. In two shorter letters, new measurement methods are presented. In the first case a streak image method is utilized to measure the fluid flow velocity on the keyhole front, and in the second a pulsed digital holography method was employed to measure deformation during laser spot welding. The streak image method is further developed in two subsequent papers to confirm and quantify the downward flow on the keyhole front during high speed welding. In the three additional papers both new and previously known laser welding phenomena are analyzed by high speed imaging. The first of these papers discusses the correlation between the size of the vapor plume above the keyhole and the signal acquired by a commercial “laser weld monitoring” system. The next paper gives practical guidelines on how to choose parameters in a laser hybrid welding system, and the final paper discusses conditions under which surface tension effects can produce a self-sustaining hole in the melt pool that might produce defects in the weld.

  • 2.
    Eriksson, Ingemar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Laser welding in slow motion2011In: LIA Today, Vol. 19, no 2, p. 12-Article in journal (Other academic)
  • 3.
    Eriksson, Ingemar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Optical monitoring and analysis of laser welding2011Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    After the laser was invented in 1960, it was not long until someone started using this powerful source of light to weld parts together. Laser welding became an industrial application during the 1970's and the field has developed ever since. In 2008 a new 15kW fibre laser was installed at Luleå University of Technology, and at the same time a considerable investment was made in new digital high speed cameras. This combination of equipment enabled research on a new level, and a first step towards a more general understanding of laser welding.This thesis presents the results acquired by analyzing high speed videos of laser welding.Qualitative and quantitative results from these high speed videos have revealed a considerable amount of information about the physics which underlies the laser welding process, including direct measurements of fluid flow within the melt pool and the interpretation of electromagnetic signals which emanate from the welding process.The thesis comprises three papers which are thematically linked by their concentration on the analysis of high speed imaging of the laser welding process.Paper A concerns the quantitative evaluation of high speed imaging of the time-dependent metal vapour jet that streams out of the laser welding vapour capillary. This work has revealed an important correlation. The output of commercially available process monitoring photodiodes (used for detecting infrared radiation) correlates with the fluctuating vapour jet above the weld, instead of, as was previously assumed, with the radiation from the molten and solid surface.In paper B, for the first time, ultra-high speed images of the surface of the laser welding vapour capillary have been obtained, (at a rate of 180 000 frames per second) with good spatial resolution and contrast. In addition, a streak technique was developed that measures and the time-dependent melt flow velocity along a selected line. Wave-like patterns that flow down the capillary have been directly observed. These phenomena are of essential importance for a basic understanding of the laser welding process and are a very powerful support for future research, e.g. for modelling and simulation.Using the above method, the velocity of the flowing vapour capillary waves was quantitatively evaluated in Paper C for varying process parameters, like laser power, focus position or welding speed, revealing clear, important trends of the laser welding process.

  • 4.
    Eriksson, Ingemar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Powell, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    New high-speed photography technique for observation of fluid flow in laser welding2010In: Optical Engineering: The Journal of SPIE, ISSN 0091-3286, E-ISSN 1560-2303, Vol. 49, no 10Article in journal (Refereed)
    Abstract [en]

    Recent developments in digital high-speed photography allow us to directly observe the surface topology and flow conditions of the melt surface inside a laser evaporated capillary. Such capillaries (known as keyholes) are a central feature of deep penetration laser welding. For the first time, it can be confirmed that the liquid capillary surface has a rippled, complex topology, indicative of subsurface turbulent flow. Manipulation of the raw data also provides quantitative measurements of the vertical fluid flow from the top to the bottom of the keyhole.

  • 5.
    Eriksson, Ingemar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Haglund, Peter
    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.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Holographic measurement of thermal distortion during laser spot welding2012In: Optical Engineering: The Journal of SPIE, ISSN 0091-3286, E-ISSN 1560-2303, Vol. 51, no 3Article in journal (Refereed)
    Abstract [en]

    Welding distortion is an important engineering topic for simulation and modeling, and there is a need for experimental verification of such models by experimental studies. High-speed pulsed digital holography is proposed as a measurement technique for out-of-plane welding distortion. To demonstrate the capability of this technique, measurements from a laser spot weld are presented. A complete twodimensional deformation map with submicrometer accuracy was acquired at a rate of 1000 measurements per second. From this map, particular points of interest can be extracted for analysis of the temporal development of the final distortion geometry.

  • 6. Eriksson, Ingemar
    et al.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Evaluation of laser weld monitoring: a case study2009In: 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, p. 1419-1425Conference paper (Refereed)
    Abstract [en]

    On-line monitoring of the quality of laser welding is of interest for many industrial applications. For photodiodes the monitoring strategy usually aims at observing whether the signal exceeds a threshold. This well known technique is mainly based on empirical values, thus the monitoring has to be trained for each application. For improved understanding of the context between the physics of a welding defect generated and the resulting voltage signal, the experiments were observed by high speed imaging, followed by evaluation and modelling. A commercial system with three detector wavelength windows was studied for nine different industrial welding applications with distinct defects. We here present selected cases for which we try to generalise the findings and to draw conclusions for the applicability of commercial monitoring systems. For example humping was clearly detected by the signal, spatter and crater formation for overlapping Zn-coated steel surprisingly not. An interesting cause for welding defects is contamination of the joint prior to welding by oil, grease or detergents. The corresponding experiments showed unexpected signal responses, which can be partially explained by the analysis. We summarise that improved understanding of the signal causes facilitates to apply monitoring systems in a reliable manner.

  • 7.
    Eriksson, Ingemar
    et al.
    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.
    Powell, John
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Melt behavior on the keyhole front during high speed laser welding2013In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 51, no 6, p. 735-740Article in journal (Refereed)
    Abstract [en]

    The flow of molten metal on the front wall of a laser generated welding keyhole has been observed by high speed photography, optically measured by mapping the flow of ripples on the liquid surface and theoretically calculated. A clear downward flow can be observed and measured by a Particle Image Velocimetry algorithm. A theoretical calculation of the melt thickness on the keyhole front is also presented. Results indicate that the thickness of the liquid on the keyhole front is similar to that of the resolidified layer found in micrographs of the front if the laser is suddenly turned off. The measured surface ripple flow speeds are between two and four times as high as the theoretical average fluid flow rate.

  • 8. Eriksson, Ingemar
    et al.
    Norman, Peter
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Basic study of photodiode signals from laser welding emissions2009In: 12th NOLAMP proceeding 2009: Nordic Laser Materials Processing Conference ; 24th - 26th August 2009 in Copenhagen, Kgs. Lyngby: ATV-SEMAPP , 2009Conference paper (Refereed)
    Abstract [en]

    Photodiodes are commonly used to monitor laser welding as a cheap, rugged, online method to get an indication of changes in the process, particularly the occurrence of defects. However, the correlations between the signal characteristics and the process are usually empirical and not fully understood. In this basic study we try to obtain a better understanding of the generation of the signals from the laser welding process. By synchronising high speed imaging with photodiode signals in three wavelength spectra, we attempt to identify the contributions from different geometrical domains and temporal events, particularly from the weld pool surface, the keyhole opening and the escaping metal vapour flow. It is particularly valuable to study dynamic behaviour during pulsed laser welding. A quantitative estimation is difficult, but , with the help of high speed photography and modelling, clear correlations between different dynamic events and signal changes can be identified.

  • 9.
    Eriksson, Ingemar
    et al.
    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.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Guidelines in the choice of parameters for hybrid laser arc welding with fiber lasers2013In: Physics Procedia, ISSN 1875-3892, E-ISSN 1875-3892, Vol. 41, p. 119-127Article in journal (Refereed)
    Abstract [en]

    Laser arc hybrid welding has been a promising technology for three decades and laser welding in combination with gas metal arc welding (GMAW) has shown that it is an extremely promising technique. On the other hand the process is often considered complicated and difficult to set up correctly. An important factor in setting up the hybrid welding process is an understanding of the GMAW process. It is especially important to understand how the wire feed rate and the arc voltage (the two main parameters) affect the process. In this paper the authors show that laser hybrid welding with a 1 μm laser is similar to ordinary GMAW, and several guidelines are therefore inherited by the laser hybrid process.

  • 10.
    Eriksson, Ingemar
    et al.
    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.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    High speed video analysis of melt flow inside fiber laser welding keyholes2011In: Congress proceedings ICALEO: 30th International Congress on Applications of Lasers & Electro-Optics : October 23-27, 2011, Orlando, Fl., Laser institute of America , 2011, p. 221-226Conference paper (Refereed)
    Abstract [en]

    A high speed video survey of melt flow within fiber laser welding keyholes has revealed a number of interesting observations about the mechanics of weld formation. Using Streak Images and FFT (Fast Fourier Transform) techniques to interpret the video results, quantitative values of fluid flow velocities down the keyhole front have been established. The results have lead to a phenomenological understanding of some of the quality problems which arise at high welding speeds -such as undercut and humping.

  • 11.
    Eriksson, Ingemar
    et al.
    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.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Measurements of fluid flow on keyhole front during laser welding2011In: Science and technology of welding and joining, ISSN 1362-1718, E-ISSN 1743-2936, Vol. 16, no 7, p. 636-641Article in journal (Refereed)
    Abstract [en]

    This paper presents the results of a high speed video survey of melt flow on the front face of a keyhole created during fibre laser welding. Using fast Fourier transform techniques, quantitative values of fluid flow velocities down the keyhole front have been established. The results have led to a phenomenological understanding of some of the quality problems which arise at excess welding speeds. The downward flow velocity on the keyhole front is found to be generally independent of welding speed, and proportional to laser power

  • 12.
    Eriksson, Ingemar
    et al.
    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.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Melt flow measurement inside the keyhole during laser welding2011In: 13th NOLAMP Conference: 13th Conference on Laser Materials Proce Nordic Countries 27 - 29 June 2011 / [ed] Einar Halmøy, Trondheim: Department of Geography, Norwegian University of Science and Technology, 2011Conference paper (Refereed)
    Abstract [en]

    Laser keyhole welding has been in use for decades, but many of the complex mechanisms which take place within the keyhole remain poorly understood. Thispaper describes a streak image technique based on videos from high speed digital cameras. Itis similar to the technique used in goal cameras to visualize time-dependent events. Thecamera’s ability to acquire high speed image patterns with clear grayscale contrast hasenabled us to see the melt flow on the keyhole surface. In this paper the measured flowvelocity down the front of the keyhole is presented, showing a clear vertical downwardmotion on the keyhole front with speeds of the order of meters per second.

  • 13.
    Eriksson, Ingemar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Powell, John
    Kaplan, Alexander
    Signal overlap in the monitoring of laser welding2010In: Measurement science and technology, ISSN 0957-0233, E-ISSN 1361-6501, Vol. 21, no 10Article in journal (Refereed)
    Abstract [en]

    Laser weld monitoring is usually based on the feedback from three photodiodes which are intended to provide independent information about the thermal condition of the melt (the T signal), the radiation from the plume of a heated gas above the melt (the P signal) and the amount of reflected laser light (the R signal). This work demonstrates that, in fact, the plume of the hot gas above the weld pool contributes a large part of the thermal signal, which has hitherto been assumed to come only from the melt itself. It is suggested that the correlation between the T and P signals is so strong that a T-P signal would be more useful than the raw T signal in identifying the fluctuations in infrared radiation from the melt pool

  • 14.
    Eriksson, Ingemar
    et al.
    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.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Surface tension generated defects in full penetration laser keyhole welding2014In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 26, no 1, article id 12006Article in journal (Refereed)
    Abstract [en]

    During laser keyhole welding of thin plates the melt pool is relatively wide compared to the plate thickness. Under certain conditions an elongated keyhole can be created and a permanent hole is sometimes left in the weld seam. The generation of such holes is determined by surface tension effects in the melt which can generate a self sustaining geometry at the rear of the melt pool. The geometry of the shape is known as a catenoid and has clear geometrical limits.

  • 15.
    Eriksson, Ingemar
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Powell, John
    Kaplan, Alexander
    Ultra high speed camera investigations of laser beam welding2010In: Congress proceedings ICALEO: 29th International Congress on Applications of Lasers & Electro-Optics : September 26 - 30, 2010, Anaheim, CA, USA, Anaheim Marriot ; [including] Laser Materials Processing Conference, Laser Microprocessing Conference, Nanomanufacturing Conference / [ed] Xinbing Liu, Anaheim, CA: Laser institute of America , 2010, p. 172-178Conference paper (Other academic)
    Abstract [en]

    With the help of high speed camera equipment, researchers can now directly observe melt flow phenomena which only could be imagined or simulated in the past. In this paper the motion of the molten metal inside the keyhole has been observed at frame rates as high as 180000 fps to provide useful information about the laser beam welding process. This paper describes two different cases where high speed imaging has given new insights into the laser beam welding process: A. In pulsed welding the melt pool oscillations can create ripples on the solidified weld surface. With the help of a high speed camera this phenomenon was analyzed, and the pulse shape was adjusted to reduce the ripple amplitude to a minimum. B. In Zn-coated steel edge welding it could be seen that blowouts were sometimes created in the melt pool at some distance behind the keyhole. Direct slowmotion observation has led to the formulation of a phenomenological model of this effect.

  • 16.
    Haglund, Peter
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Eriksson, Ingemar
    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.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Surface tension stabilized laser welding (donut laser welding): A new laser welding technique2013In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 25, no 3Article in journal (Refereed)
    Abstract [en]

    A new laser welding process is presented which delivers a porosity free, spatter free weld. The process involves a donut shaped melt with a central hole of the same order of magnitude as the material thickness. The laser illuminates only the leading section of this melt. The hole is kept open by the melt surface tension.

  • 17.
    Ilar, Torbjörn
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Eriksson, Ingemar
    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.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Root humping in laser welding: an investigation based on high speed imaging2012In: Physics Procedia, ISSN 1875-3892, E-ISSN 1875-3892, Vol. 39, p. 27-32Article in journal (Refereed)
    Abstract [en]

    Regular drop formation (humping) along the weld root during laser welding was studied by high speed imaging of the melt pool underneath the workpiece. The formation of droplets towards the rear of this weld pool was seen to be primarily caused by the pumping of melt from the bottom of the keyhole and the influence of gravity drawing melt into a sagging hump. This is a different process from the one which creates humps on the top surface of welds.

  • 18.
    Kaplan, Alexander
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Norman, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Eriksson, Ingemar
    Analysis of the keyhole and weld pool dynamics by imaging evaluation and photodiode monitoring2009In: Proceedings of LAMP2009: The 5th International Congress on Laser Advanced Materials Processing ; LPM2009, the 10th International Symposium on Laser Precision Microfabrication ; HPL2009, the 5th International Symposium on High Power Laser Processing ; Kobe, June 29 - July 2, 2009 ; [congress program & technical digest], Tokyo: Sumitomo Corp. , 2009Conference paper (Refereed)
    Abstract [en]

    Process monitoring of laser welding defects by commercial systems based on photodiodes has high potential, but improved understanding is desirable for more systematic industrial use. The signal is generated from spectrally filtered emissions or reflections from the dynamic welding process. Improved knowledge on the cause of the signal was achieved by accompanying high speed imaging. Numerical evaluation of the images and subsequent modelling provided additional information to judge the potential and limits of commercial photodiode sensors. For welding with a 3 kW Nd:YAG-laser, a 15 kW fibre laser or a 14 kW CO2-laser, various joint cases were studied with respect to defects like craters, spatter, blowholes, underfill, root drop-out, lack of penetration, lack of fusion or pores. From the findings a more general theoretical description of the links between the welding process, the defect mechanism and the generated signal was initiated.

  • 19.
    Kaplan, Alexander
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Soldatov, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Noël, Maxime
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Norman, Peter
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Eriksson, Ingemar
    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.
    Fang, Shaoli
    Baughman, Ray
    Incorporation of CNT-yarns into metals by laser melting of powder2012In: 31st International Congress on Applications of Lasers and Electro-Optics (ICALEO) Proceedings, Laser institute of America , 2012, p. 1239-1246Conference paper (Refereed)
  • 20.
    Keskitalo, M.
    et al.
    University of Oulu, Oulu Southern Institute.
    Mentyjärvi, K.
    University of Oulu, Oulu Southern Institute.
    Sundqvist, Jesper
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Eriksson, Ingemar
    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.
    The influence of shielding gas on the properties of laser welded stainless steel2013In: 14th NOLAMP Conference: The 14th Nordic Laser Materials Processing Conference, August 26th – 28th 2013, Gothenburg, Sweden / [ed] Alexander Kaplan; Hans Engström, Luleå: Luleå tekniska universitet, 2013, p. 155-161Conference paper (Other academic)
    Abstract [en]

    Argon is generally used as shielding gas for laser welding. As Argon is an inert gas it doesnot have influence on the microstructure of the weld material or on the heat input of the weld,whereas nitrogen has solubility to austenite. Therefore nitrogen as an interstitial atomincreases the hardness of the weld. This has been detected when comparing the hardnessprofiles of nitrogen shielded welds of austenitic stainless steel with Argon shielded welds.Nitrogen as the shielding gas can compensate the softer structure of the weld in workhardenedand nitrogen-alloyed steels.For the duplex stainless steel grades the nitrogen additions promote formation ofaustenite in the weld, which decreases the risk of lowered toughness. In this study theinfluence of nitrogen shielding gas on the strength of the laser weld has been examined. Thestrength of laser welds of nitrogen alloyed work-hardened stainless steel seems to be slightlybetter when using nitrogen shielding gas compared to welds for argon shielding gas. It hasalso been verified that nitrogen as shielding gas decreases the risk of weak toughness of laserwelded duplex stainless steel.

  • 21. Norman, Peter
    et al.
    Eriksson, Ingemar
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Monitoring laser beam welding of zinc coated sheet metal to analyze the defects occurring2009In: 12th NOLAMP proceeding 2009: Nordic Laser Materials Processing Conference ; 24th - 26th August 2009 in Copenhagen / [ed] Erling Dam Mortensen, Kgs. Lyngby: ATV-SEMAPP , 2009Conference paper (Refereed)
    Abstract [en]

    During laser welding of overlapping zinc-coated steel sheets typically blowouts as a defect can occur. They can be crucial to the mechanical performance and to the optical appearance of the product. In the present research approved regions are compared to regions where the blowouts are in majority. Time-dependent radiation is captured with a commercial photodiode monitoring system in three different wavelengths ranges, 400-600 nm, 1100-1800 nm and 1064 nm, corresponding to metal vapour emissions, surface temperature emissions and reflected laser light, respectively. Accompanying high speed imaging as well as mathematical modelling of the process emissions was carried out, enabling combined analysis. Different gap widths and laser beam positions were studied. From imaging, surface elevations of the melt indicate a Zn-bubble formation that travels to the rear part of the melt pool before it breaks through, often completely separated from the keyhole. The escaping vapour ejects melt and causes a significant crater, if not flooded with melt again. Although this violent surface event is clearly visible, it was hardly found in the temperature sensor signal. The separate mechanisms that can be responsible for the lack of contribution of the dynamic blow-out formation to the emissions generating the photodiode signal are discussed, e.g. melt surface geometry changes, drops, temperature and emissivity changes, the role of the Znvapour and the detection area of the sensor. This study demonstrates that obvious defects can sometimes be hard to detect on-line.

  • 22.
    Olsson, Rickard
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Eriksson, Ingemar
    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.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Advances in pulsed laser weld monitoring by the statistical analysis of reflected light2011In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 49, no 11, p. 1352-1359Article in journal (Refereed)
    Abstract [en]

    This paper describes two new techniques for monitoring the quality of laser welds by statistical analysis of the reflected light signal from the weld surface. The first technique involves an algorithm that analyses the variance of the peak values of the reflected signal as a measure of the stability of the surface during pulsed Nd:YAG laser welding in the heat conduction mode. Kalman filtering is used to separate a useful signal from the background noise. A good correlation between weld disruption and signal fluctuation has been identified. This technique could be used in tandem with the present practice of simply using the peak values of reflected (or emitted) light as an indicator of weld quality. The second technique investigated involves an assessment of the temporal shape of the power distribution of individual reflected pulses in comparison with an average of the results from a high quality weld. Once again a high correlation between a poor signal match and inferior quality welding was discovered, which may pave the way towards a new generation of optical weld monitoring devices.

  • 23. Olsson, Rickard
    et al.
    Eriksson, Ingemar
    Powell, John
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Pulsed laser weld quality monitoring by the statistical analysis of reflected light2009In: 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: AT-Fachverlag , 2009, p. 369-374Conference paper (Refereed)
    Abstract [en]

    This paper describes a technique for monitoring the quality of laser welds by statistical analysis of the reflected light signal from the weld surface. An algorithm is used which analyses the variance of the peak values of the reflected signal as a measure of surface weld dynamics during pulsed Nd:YAG laser welding in the heat conduction mode. Kalman filtering is used to separate a useful signal from the background noise. A good correlation between weld disruption and signal fluctuation has been identified. This technique could be used in tandem with the present practice of simply using the peak values of the reflected (or emitted) light as an indicator of weld quality.

  • 24.
    Olsson, Rickard
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Eriksson, Ingemar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Powell, John
    Langtry, A.V.
    GE Healthcare, Oxford.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Challenges to the interpretation of the electromagnetic feedback from laser welding2011In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 49, no 2, p. 188-194Article in journal (Refereed)
    Abstract [en]

    This paper considers the point that it is not possible to interpret individual weld perturbations from the raw electromagnetic feedback collected from laser weld zones. The presentation of electromagnetic data as a 3D cloud is presented as a new, useful tool in the analysis of this feedback. It is shown that there is a very low correlation between the plasma or thermal signals and the reflected light signal from the weld zone, and that a strong correlation exists between the plasma and thermal signals. It is also demonstrated that data points from a weld perturbation form a different 3D cluster to those from the stable welding process. A strategy for future real time data analysis is presented utilising a suitably shaped data cloud envelope. The rates of data fit to the various segments of such an envelope could be correlated with specific weld anomalies.

  • 25.
    Olsson, Rickard
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Eriksson, Ingemar
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Powell, John
    Langtry, A.V.
    GE Healthcare, Oxford.
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Studies in the interpretation of the reflected feedback from laser welding2010In: Congress proceedings ICALEO: 29th International Congress on Applications of Lasers & Electro-Optics : September 26 - 30, 2010, Anaheim, CA, USA, [including] Laser Materials Processing Conference, Laser Microprocessing Conference, Nanomanufacturing Conference / [ed] Xinbing Liu, Anaheim, CA: Laser institute of America , 2010, p. 191-199Conference paper (Refereed)
    Abstract [en]

    This paper investigates two methods of monitoring the laser welding process by statistical analysis of the reflected laser light signal. It has been discovered that statistical data analysis reveals more about perturbations in the welding process than the simple upper and lower threshold limits which are in common use today. This work indicates the way forward for the development of the next generation of laser processing monitoring equipment

  • 26.
    Sundqvist, Jesper
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Eriksson, Ingemar
    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.
    Keskitalo, M.
    University of Oulu.
    Mentyjärvi, K.
    University of Oulu.
    Influence of the metallurgy on fatigue crack propagation in welded high strength steel joints2013In: 14th NOLAMP Conference: The 14th Nordic Laser Materials Processing Conference, August 26th – 28th 2013, Gothenburg, Sweden / [ed] Alexander Kaplan; Hans Engström, Luleå: Luleå tekniska universitet, 2013, p. 25-35Conference paper (Refereed)
    Abstract [en]

    A literature study of high strength steels, fatigue and fatigue assessment of welds has beenconducted and is briefly presented in this paper together with experiments on fatigue crackgrowth rates of laser welded high strength steel. It is well-known that the fatigue life ofwelded joints is heavily dependent upon the surface geometry and welding defects because ofcrack initiation from the high stress concentrations associated with these types of weldingflaws. However, the crack propagation through different weld zones of laser-welded highstrength steels and the corresponding impact from the metallurgy is not fully understood.The experiments comprise three-point bending fatigue tests on laser-welded highstrength steel with machined surfaces. Measurement of the fatigue crack propagation ratetransverse the weld and hence through the different metallurgy and hardness of the heataffected zone and of the weld can contain information on the impact of the metallurgy on thecrack propagation speed. The influence of different high strength steel grades and of differentwelding parameters on the crack propagation and fatigue life is discussed

  • 27.
    Sundqvist, Jesper
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Eriksson, Ingemar
    Kaplan, Alexander
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development.
    Keskitalo, Markku
    University of Oulu, University of Oulu, Oulu Southern Institute.
    Mäntyjärvi, Kari
    University of Oulu.
    Granström, Jan
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mechanics of Solid Materials.
    Sundin, Karl-Gustaf
    Measuring the influence of laser welding on fatigue crack propagation in high strength steel2013In: ICALEO, 32nd International Congress on Applications of Lasers & Electro-Optics: October 6-10, 2013, Hyatt Regency Miami, Miami, FL USA, Orlando, Fl.: Laser institute of America , 2013Conference paper (Refereed)
1 - 27 of 27
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