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Laser cutting and piercing: Experimental and theoretical investigation
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. (Produktionsutveckling)ORCID iD: 0000-0001-6261-2019
2017 (English)Doctoral 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 front during remote fusion cutting, RFC. The simulations were carried out at the edge of a sheet to obtain a processing front with an open view for high speed imaging validation experiments. 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. 

Paper IV addresses the macro geometry of the CO2 and fibre laser cut front for a range of thicknesses. The cut front was ‘frozen’ by turning off the laser in the middle of a cut line. The macro geometry of the cut front was measured by considering the kerf shape and cut front inclination angles. Mathematical formulations of the cut front geometry were obtained by applying curve-fitting techniques to these measurements.  The resultant mathematical description of the cut front geometry can be used for theoretical calculations of the beam absorptivity in laser cutting. Scanning electron microscopy, SEM, is used to observe the morphology of the melt on the ‘frozen’ cut front. Standard and commercial laser cutting parameters were used to produce the laser cut samples. 

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.

 

Place, publisher, year, edition, pages
Luleå: Luleå tekniska universitet, 2017.
Keyword [en]
Laser cutting, laser piercing, efficiency, fibre laser, CO2 laser, high speed imaging
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
URN: urn:nbn:se:ltu:diva-63498ISBN: 978-91-7583-916-5 (print)ISBN: 978-91-7583-917-2 (electronic)OAI: oai:DiVA.org:ltu-63498DiVA: diva2:1097828
Public defence
2017-09-29, E 231, Luleå tekniska universitet/ 971 87 Luleå , Luleå, 09:30 (English)
Opponent
Supervisors
Available from: 2017-05-24 Created: 2017-05-23 Last updated: 2017-05-24Bibliographically approved
List of papers
1. The Effect of Laser Type and Power on the Efficiency of Industrial Cutting of Mild and Stainless Steels
Open this publication in new window or tab >>The Effect of Laser Type and Power on the Efficiency of Industrial Cutting of Mild and Stainless Steels
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2016 (English)In: Journal of manufacturing science and engineering, ISSN 1087-1357, E-ISSN 1528-8935, Vol. 138, no 3, 31012Article in journal (Refereed) Published
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.

National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-8003 (URN)10.1115/1.4031190 (DOI)67180056-91cc-4c93-abd0-5dc50bc39299 (Local ID)67180056-91cc-4c93-abd0-5dc50bc39299 (Archive number)67180056-91cc-4c93-abd0-5dc50bc39299 (OAI)
Note

Validerad; 2015; Nivå 2; 20150807 (jetpoc)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-06-19Bibliographically approved
2. Fibre laser cutting stainless steel: Fluid dynamics and cut front morphology
Open this publication in new window or tab >>Fibre laser cutting stainless steel: Fluid dynamics and cut front morphology
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2017 (English)In: Optics and Laser Technology, ISSN 0030-3992, E-ISSN 1879-2545, Vol. 87, 87-93 p.Article in journal (Refereed) Published
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.

National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-9223 (URN)10.1016/j.optlastec.2016.08.002 (DOI)000384867900012 ()7c9617db-b7dc-4cd4-bfde-993e46efa21a (Local ID)7c9617db-b7dc-4cd4-bfde-993e46efa21a (Archive number)7c9617db-b7dc-4cd4-bfde-993e46efa21a (OAI)
Note

Validerad; 2016; Nivå 2; 20160817 (andbra)

Available from: 2016-09-29 Created: 2016-09-29 Last updated: 2017-06-19Bibliographically approved
3. Investigation of the Piercing Process in Laser Cutting of Stainless Steel
Open this publication in new window or tab >>Investigation of the Piercing Process in Laser Cutting of Stainless Steel
2017 (English)In: Journal of laser applications, ISSN 1042-346X, E-ISSN 1938-1387, Vol. 29, no 2, 022201Article in journal (Refereed) Published
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.

Keyword
Laser cutting, laser piercing, efficiency, fibre laser, high speed imaging
National Category
Production Engineering, Human Work Science and Ergonomics
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-60387 (URN)10.2351/1.4983260 (DOI)2-s2.0-85019497656 (ScopusID)
Projects
HALO project
Funder
EU, FP7, Seventh Framework Programme, 314410
Note

Validerad; 2017; Nivå 2; 2017-05-17 (andbra)

Available from: 2016-11-14 Created: 2016-11-14 Last updated: 2017-06-19Bibliographically approved
4. Numerical simulation of laser ablation driven melt waves
Open this publication in new window or tab >>Numerical simulation of laser ablation driven melt waves
Show others...
(English)Article in journal (Refereed) Submitted
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.

Keyword
Computational fluid dynamics, Laser ablation, Wavy surface, Molten metal, High speed imaging
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-63491 (URN)
Available from: 2017-05-23 Created: 2017-05-23 Last updated: 2017-05-23
5. Dynamic laser piercing of thick section metals
Open this publication in new window or tab >>Dynamic laser piercing of thick section metals
(English)Article in journal (Refereed) Submitted
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.

Keyword
Laser cutting, laser piercing, reliability, efficiency, fibre laser, high speed imaging
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-63492 (URN)
Available from: 2017-05-23 Created: 2017-05-23 Last updated: 2017-06-19
6. The geometry of the cut front in Fibre and CO2 laser cutting of stainless steel
Open this publication in new window or tab >>The geometry of the cut front in Fibre and CO2 laser cutting of stainless steel
(English)Manuscript (preprint) (Other academic)
Abstract [en]

This paper addresses the macro geometry of the CO2 and fibre laser cut front for a range of thicknesses. The cut front was ‘frozen’ by turning off the laser in the middle of a cut line. The macro geometry of the cut front was measured by considering the kerf shape and cut front inclination angles. Mathematical formulations of the cut front geometry were obtained by applying curve-fitting techniques to these measurements. The resultant mathematical description of the cut front geometry can be used for theoretical calculations of the beam absorptivity in laser cutting. Scanning electron microscopy, SEM, is used to observe the morphology of the melt on the ‘frozen’ cut front. Standard and commercial laser cutting parameters were used to produce the laser cut samples.

Keyword
Laser cutting, Fibre laser, Melt flow, curve-fitting, cut front inclination
National Category
Manufacturing, Surface and Joining Technology
Research subject
Manufacturing Systems Engineering
Identifiers
urn:nbn:se:ltu:diva-63493 (URN)
Available from: 2017-05-23 Created: 2017-05-23 Last updated: 2017-06-19

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Citation style
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Output format
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