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Numerical simulation of laser ablation driven melt waves
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Product and Production Development. (Produktionsutveckling)ORCID iD: 0000-0001-6261-2019
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(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 [en]
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: urn:nbn:se:ltu:diva-63491OAI: oai:DiVA.org:ltu-63491DiVA: diva2:1097826
Available from: 2017-05-23 Created: 2017-05-23 Last updated: 2017-05-23
In thesis
1. Laser cutting and piercing: Experimental and theoretical investigation
Open this publication in new window or tab >>Laser cutting and piercing: Experimental and theoretical investigation
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
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:nbn:se:ltu:diva-63498 (URN)978-91-7583-916-5 (ISBN)978-91-7583-917-2 (ISBN)
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

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