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  • 1. Pocorni, Jetro
    Experimental and theoretical investigation of the laser cutting process2015Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis concerns experimental investigations of laser cutting with theoretical and practical discussions of the results. The thesis is made up of three papers which are linked in such a way that each of them studies a different aspect of laser cutting: In paper I the two major laser types in cutting, namely CO2 and fiber lasers, are compared to each other by a self-defined cut efficiency. Next in paper II the laser cutting process is observed with a high speed imaging, HSI, camera to give information about the melt flow in the cut zone. In paper III the initiation of the laser cutting process, called piercing, is studied. Paper I is about investigating the effect of material type, material thickness, laser wavelength, and laser power on the efficiency of the cutting process for industrial state-of-the-art cutting machines. Here the cutting efficiency is defined in its most fundamental terms: as the area of cut edge created per Joule of laser energy. This paper presents phenomenological explanations for the relative cutting efficiencies of fiber lasers and CO2 lasers and the mechanisms affecting these efficiencies for stainless steels and mild steel over a range of thicknesses. The paper also involves a discussion of both theoretical and practical engineering issues.In Paper II a new experimental technique has been developed which enables High Speed Imaging of laser cut fronts produced using standard, commercial parameters. The results presented here suggest that the cut front produced when cutting 10 mm thick medium section stainless steel with a fibre laser and a nitrogen assist gas is covered in humps which themselves are covered in a thin layer of liquid. A combination of HSI results and theoretical analysis has revealed that these humps move down the cut front at an average speed which is a factor three less than the liquid flow speed. Paper III addresses a specific topic: Before any cut is started the laser needs to pierce the material. 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 the laser piercing process is investigated using a wide range of laser pulse parameters, for stainless steel using a fibre laser, to discover their influence on pierce time and pierced hole diameter. A high speed imaging camera is used to time the penetration event and to study the laser-material interactions involved in drilling the pierced holes. Optimum parameters have been identified for both pierce time and pierce hole width.

  • 2. Pocorni, Jetro
    Laser cutting and piercing: Experimental and theoretical investigation2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis concerns experimental investigations of laser cutting and piercing, with theoretical and practical discussions of the results. The thesis is made up of an introduction to laser cutting and six scientific Papers. These Papers are linked in such a way that each of them studies a different aspect of laser cutting: process efficiency in Paper I, morphology and melt flow on the laser cut front in Papers II, III and IV and laser piercing in Papers V and VI. Paper I investigates the effect of material type, material thickness, laser wavelength, and laser power on the efficiency of the cutting process for industrial state-of-the-art CO2 and fibre laser cutting machines. Here the cutting efficiency is defined in its most fundamental terms: as the area of cut edge created per Joule of laser energy. In Paper II a new experimental technique is presented which has been developed to enable high speed imaging of laser cut fronts produced using standard, commercial parameters. The results presented here suggest that the cut front produced when cutting 10 mm thick medium section stainless steel with a fibre laser and a nitrogen assist gas is covered in humps which themselves are covered in a thin layer of liquid. Paper III presents numerical simulations of the melt flow on a fibre laser ablation-driven processing front during remote fusion cutting, RFC. The simulations were validated with high speed imaging observations of the processing front. The simulation results provide explanations of the main liquid transport mechanisms on the processing front, based on information on the temperature, velocity and pressure fields involved. The results are of fundamental relevance for any process governed by a laser ablation induced front. In Paper IV 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. The resulting three-dimensional shapes have been curve fitted as ninth order polynomials. 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. In this paper, mathematical descriptions of the cutting fronts are obtained, which can be used as input parameters by any researcher in the field of laser cutting simulations. Paper V investigates the subject of laser piercing. Before any cut is started the laser needs to pierce the material. In this paper the laser piercing process is investigated using a wide range of laser pulse parameters, for stainless steel using a fibre laser. The results reveal the influence of pulse parameters on pierce time and pierced hole diameter. A high speed imaging camera was used to time the penetration event and to study the laser-material interactions involved in drilling the pierced holes. In Paper VI a ‘dynamic’ or ‘moving beam’, laser piercing technique is introduced for processing 15 mm thick stainless steel. 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 a comparison was made between a stationary laser and a laser which moves along a circular trajectory with varying processing speeds. High speed imaging was employed during the piercing process to understand melt behavior inside the pierce hole. Throughout this work experimental techniques, including advanced high speed imaging, have been used in conjunction with simulations and theoretical analysis, to provide new knowledge for understanding and improving laser beam cutting and its associated piercing process.

  • 3. Pocorni, Jetro
    et al.
    Han, Sang-Woo
    Cheon, Jason
    Na, Suck-Joo
    Kaplan, Alexander
    Bang, Hee-Seon
    Numerical simulation of laser ablation driven melt waves2017In: Journal of Manufacturing Processes, Vol. 30, p. 303-312Article in journal (Refereed)
    Abstract [en]

    Numerical simulations on the melt flow down the keyhole front during fibre laser welding are presented here. The calculations confirm the existence of melt waves previously observed by high speed imaging, with velocities ranging between 4-10 m/s. The simulations provide spatial and temporal information on the temperature and flow fields, particularly within the melt film volume, which cannot be observed by high speed imaging. The ablation pressure achieves high values around wave-peaks and at the bottom of the front, just before droplets are sheared off. The simulation results provide explanations on the main liquid transport mechanisms within the keyhole based on information on the temperature, velocity and pressure field and on the geometrical front conditions.

  • 4. Pocorni, Jetro
    et al.
    Petring, Dirk
    Powell, John
    Deichsel, Eckard
    Kaplan, Alexander
    Differences in Cutting Efficiency between CO2 and Fiber Lasers when Cutting Mild and Stainless Steels2014Conference paper (Refereed)
  • 5. Pocorni, Jetro
    et al.
    Petring, Dirk
    Powell, John
    Deichsel, Eckard
    Kaplan, Alexander
    Measuring the Melt Flow on the Laser Cut Front2015In: Physics Procedia, Vol. 78, p. 99-109Article in journal (Refereed)
    Abstract [en]

    The flow characteristics on the laser cut front for 10 mm stainless steel AISI 304 (EN 1.4301) are studied in this paper using High Speed Imaging (HSI). The laser cut samples were produced with a 6 kW fiber laser with nitrogen gas assist. Previous work in this field has used unusual cutting parameters to make the experimentation easier. This work presents, for the first time, HSI results from standard commercially viable cutting parameters. This was made possible by the development of a new experimental technique. The results presented here suggest that the cut front produced when cutting stainless steel with a fiber laser and a nitrogen assist gas is covered in bumps which themselves are covered in a thin layer of liquid. Under the conditions shown here the bumps move down the cut front at an average speed of approximately 0.4m/s. The liquid flows at an average speed of approximately 1.1m/s. The average melt depth at the bottom of the cut zone under these conditions is approximately 0.17 mm.

  • 6. Pocorni, Jetro
    et al.
    Petring, Dirk
    Powell, John
    Deichsel, Eckard
    Kaplan, Alexander
    The Effect of Laser Type and Power on the Efficiency of Industrial Cutting of Mild and Stainless Steels2016In: Journal of manufacturing science and engineering, Vol. 138, no 3Article in journal (Refereed)
    Abstract [en]

    This paper investigates the effect of material type, material thickness, laser wavelength and laser power on the efficiency of the cutting process for industrial state-of-the-art cutting machines. The cutting efficiency is defined in its most basic terms: as the area of cut edge created per Joule of laser energy. This fundamental measure is useful in producing a direct comparison between the efficiency of fiber and CO2 lasers when cutting any material. It is well known that the efficiency of the laser cutting process generally reduces as the material thickness increases, because conductive losses from the cut zone are higher at the lower speeds associated with thicker section material. However, there is an efficiency dip at the thinnest sections. This paper explains this dip in terms of a change in laser-material interaction at high cutting speeds. Fiber lasers have a higher cutting efficiency at thin sections than their CO2 counterparts, but the efficiency of fiber laser cutting falls faster than that of CO2 lasers as material thickness is increased. This is the result of a number of factors including changes in cut zone absorptivity and kerf width. This paper presents phenomenological explanations for the relative cutting efficiencies of fiber lasers and CO2 lasers, and the mechanisms affecting these efficiencies for stainless steels (cut with nitrogen) and mild steel (cut with oxygen or nitrogen) over a range of thicknesses. The paper involves a discussion of both theoretical and practical engineering issues. Key Words; Laser Cutting, Fiber Laser, CO2 Laser, Efficiency.

  • 7. Pocorni, Jetro
    et al.
    Powell, John
    Deichsel, Eckard
    Frostevarg, Jan
    Kaplan, Alexander
    Fibre laser cutting stainless steel: Fluid dynamics and cut front morphology2017In: Optics and Laser Technology, Vol. 87, p. 87-93Article in journal (Refereed)
    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.

  • 8. Pocorni, Jetro
    et al.
    Powell, John
    Frostevarg, Jan
    Kaplan, Alexander
    Investigation of the Piercing Process in Laser Cutting of Stainless Steel2017In: Journal of laser applications, Vol. 29, no 2Article in journal (Refereed)
    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.

  • 9. Pocorni, Jetro
    et al.
    Powell, John
    Frostevarg, Jan
    Kaplan, Alexander F. H.
    Dynamic laser piercing of thick section metals2018In: Optics and lasers in engineering, Vol. 100, p. 82-89Article in journal (Refereed)
    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.

  • 10. Pocorni, Jetro
    et al.
    Powell, John
    Frostevarg, Jan
    Kaplan, Alexander F. H.
    The geometry of the cutting front in Fibre and CO2 laser cutting of stainless steelIn: Article in journal (Refereed)
    Abstract [en]

    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. The resulting three dimensional shapes have been curve fitted as ninth order polynomials in the xz and yz planes and as a semicircular geometry in the xy plane. 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

  • 11. Pocorni, Jetro
    et al.
    Powell, John
    Ilar, Torbjörn
    Schwarz, A.
    Kaplan, Alexander
    Measuring the State-of-the-Art in Laser Cut Quality2013In: 14th NOLAMP Conference: The 14th Nordic Laser Materials Processing Conference, August 26th – 28th 2013, Gothenburg, Sweden, 2013, p. 101-108Conference paper (Refereed)
    Abstract [en]

    This paper discusses the strategy appropriate to investigating the state of the art of laser cutting from an industrial point of view. The importance of creating the samples in a high quality industrial environment is emphasised and preliminary results are presented.

1 - 11 of 11
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