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Elefante, A., Nilsen, M., Sikström, F., Christiansson, A.-K., Maggipinto, T. & Ancona, A. (2019). Detecting beam offsets in laser welding of closed-square-butt joints by wavelet analysis of an optical process signal. Optics and Laser Technology, 109, 178-185
Open this publication in new window or tab >>Detecting beam offsets in laser welding of closed-square-butt joints by wavelet analysis of an optical process signal
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2019 (English)In: Optics and Laser Technology, ISSN 0030-3992, E-ISSN 1879-2545, Vol. 109, p. 178-185Article in journal (Refereed) Published
Abstract [en]

Robotized laser beam welding of closed-square-butt joints is sensitive to the positioning of the laser beam with respect to the joint since even a small offset may result in a detrimental lack of sidewall fusion. An evaluation of a system using a photodiode aligned coaxial to the processing laser beam confirms the ability to detect variations of the process conditions, such as when there is an evolution of an offset between the laser beam and the joint. Welding with different robot trajectories and with the processing laser operating in both continuous and pulsed mode provided data for this evaluation. The detection method uses wavelet analysis of the photodetector signal that carries information of the process condition revealed by the plasma plume optical emissions during welding. This experimental data have been evaluated offline. The results show the potential of this detection method that is clearly beneficial for the development of a system for welding joint tracking.

Place, publisher, year, edition, pages
Elsevier, 2019
Keywords
Laser beam welding, Joint tracking, Butt joints, Photodiode, Wavelet analysis
National Category
Manufacturing, Surface and Joining Technology
Research subject
Production Technology
Identifiers
urn:nbn:se:hv:diva-12832 (URN)10.1016/j.optlastec.2018.08.006 (DOI)000446949600023 ()2-s2.0-85051138319 (Scopus ID)
Funder
VINNOVA, 2016-03291
Available from: 2018-08-21 Created: 2018-08-21 Last updated: 2019-02-28Bibliographically approved
Tricarico, L., Ancona, A., Palumbo, G., Sorgente, D., Corizzo, O., Spina, R. & Lugara, P. M. (2019). Numerical and experimental investigation of the discrete spot laser hardening of a graphite-coated hypereutectoid steel using a fibre laser. The International Journal of Advanced Manufacturing Technology, 104(1-4), 1393-1402
Open this publication in new window or tab >>Numerical and experimental investigation of the discrete spot laser hardening of a graphite-coated hypereutectoid steel using a fibre laser
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2019 (English)In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 104, no 1-4, p. 1393-1402Article in journal (Refereed) Published
Abstract [en]

The single-pulse laser hardening of a hypereutectoid steel coated by a graphite layer was investigated using a numerical/experimental approach. Experimental tests were conducted on coated samples using a fibre laser source and without any gas shielding aiming to explore the effect of laser power, pulse energy and defocusing distance on the dimensions of the hardened region. The process operating window of the discrete spot laser hardening using the graphite layer was determined through a finite element model and compared with previous results obtained on uncoated samples. For the same laser power and interaction times, an enlargement of the hardened region was found when using the graphite coating, especially when operating at the lowest laser energy level. The process operating window remains similar in shape to the one of the uncoated steel but moves towards larger hardened diameters and much larger defocusing distances. Once the maximum temperature has been fixed, a linear relationship between the hardened diameter and the defocusing distance exists. No obvious surface oxidation occurs since the graphite coating acts as a protective layer. © 2019, Springer-Verlag London Ltd., part of Springer Nature.

Keywords
Coatings; Fiber lasers; Finite element method; Graphite; Steel fibers, Experimental investigations; Hypereutectoid steel; Laser energy levels; Laser hardening; Linear relationships; Maximum temperature; Surface oxidations; Surface structuring, Hardening
National Category
Manufacturing, Surface and Joining Technology Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-14480 (URN)10.1007/s00170-019-04014-4 (DOI)000483808200100 ()2-s2.0-85068736856 (Scopus ID)
Note

Funders: Italian Ministry of Education, Universities and Research Government  (grants PON01_02584, project acronym: SMATI; PON01_02238, project acronym: EURO6; and PON02_00576_3333604, project acronym: INNOVHEAD)

Available from: 2019-10-02 Created: 2019-10-02 Last updated: 2019-10-15
Nilsen, M., Sikström, F., Christiansson, A.-K. & Ancona, A. (2019). Robust vision-based joint tracking for laser welding of curved closed-square-butt joints. The International Journal of Advanced Manufacturing Technology, 101(5-8), 1867-1978
Open this publication in new window or tab >>Robust vision-based joint tracking for laser welding of curved closed-square-butt joints
2019 (English)In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 101, no 5-8, p. 1867-1978Article in journal (Refereed) Published
Abstract [en]

Robotized laser beam welding of closed-square-butt joints is sensitive to how the focused laser beam is positioned in relation to the joint, and existing joint tracking systems tend to fail in detecting the joint when the gap and misalignment between the work pieces are close to zero. A camera-based system is presented based on a high dynamic range camera operating with LED illumination at a specific wavelength and a matching optical filter. An image processing algorithm based on the Hough transform extracts the joint position from the camera images, and the joint position is then estimated using a Kalman filter. The filter handles situations, when the joint is not detectable in the image, e.g., when tack welds cover the joint. Surface scratches, which can be misinterpreted as being the joint, are handled by a joint curve prediction model based on known information about the nominal path defined by the robot program. The performance of the proposed system has been evaluated off line with image data obtained during several welding experiments.

Keywords
Laser beam welding, Joint tracking, Butt joints, Camera, Hough transform, Kalman filter
National Category
Materials Engineering Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-13211 (URN)10.1007/s00170-018-3044-0 (DOI)
Projects
VINNOVA project VarGa
Funder
Vinnova, 2016-03291
Note

First Online: 27 November 2018

Funders: SWE-DEMO MOTOR ; [2015-06047]

Available from: 2018-12-18 Created: 2018-12-18 Last updated: 2019-05-10Bibliographically approved
Mi, Y., Sikström, F., Nilsen, M. & Ancona, A. (2019). Vision based beam offset detection in laser stake welding of T-joints using a neural network. Paper presented at Conference of 17th Nordic Laser Materials Processing Conference, NOLAMP 2019 ; Conference Date: 27 August 2019 Through 29 August 2019. Procedia Manufacturing, 36, 42-49
Open this publication in new window or tab >>Vision based beam offset detection in laser stake welding of T-joints using a neural network
2019 (English)In: Procedia Manufacturing, E-ISSN 2351-9789, Vol. 36, p. 42-49Article in journal (Refereed) Published
Abstract [en]

This paper presents an experimental study where a vision camera integrates coaxially into a laser beam welding tool to monitor beam deviations (beam offset) in laser stake welding of T-joints. The aim is to obtain an early detection of deviations from the joint centreline in this type of welding where the joint is not visible from the top side. A polynomial surface fitting method is applied to extract features that can describe the behaviour of the melt pool. A nonlinear autoregressive with exogenous inputs neural network model is trained to relate eight image features to the laser beam offset. The performance of the presented model is evaluated offline by different welding samples. The results show that the proposed method can be used to guide post weld inspection and has the potential for on-line adaptive control. © 2019 The Author(s). Published by Elsevier B.V.

National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-14473 (URN)10.1016/j.promfg.2019.08.007 (DOI)2-s2.0-85072518360 (Scopus ID)
Conference
Conference of 17th Nordic Laser Materials Processing Conference, NOLAMP 2019 ; Conference Date: 27 August 2019 Through 29 August 2019
Funder
Knowledge Foundation
Available from: 2019-10-02 Created: 2019-10-02 Last updated: 2019-10-03
Gaudiuso, C., Giannuzzi, G., Volpe, A., Lugarà, P. M., Choquet, I. & Ancona, A. (2018). Incubation during laser ablation with bursts of femtosecond pulses with picosecond delays. Optics Express, 26(4), 3801-3813
Open this publication in new window or tab >>Incubation during laser ablation with bursts of femtosecond pulses with picosecond delays
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2018 (English)In: Optics Express, ISSN 1094-4087, E-ISSN 1094-4087, Vol. 26, no 4, p. 3801-3813Article in journal (Refereed) Published
Abstract [en]

Abstract: We report on an experimental investigation of the incubation effect during irradiation of stainless steel with bursts of ultrashort laser pulses. A series of birefringent crystals was used to split the pristine 650-fs pulses into bursts of up to 32 sub-pulses with time separations of 1.5 ps and 3 ps, respectively. The number of selected bursts was varied between 50 and 1600. The threshold fluence was measured in case of Burst Mode (BM) processing depending on the burst features, i.e. the number of sub-pulses and their separation time, and on the number of bursts. We found as many values of threshold fluence as the combinations of the number of bursts and of sub-pulses constituting the bursts set to give the same total number of impinging sub-pulses. However, existing incubation models developed for Normal Pulse Mode (NPM) return, for a given number of impinging pulses, a constant value of threshold fluence. Therefore, a dependence of the incubation coefficient with the burst features was hypothesized and experimentally investigated. Numerical solutions of the Two Temperature Model (TTM) in case of irradiation with single bursts of up to 4 sub-pulses have been performed to interpret the experimental results. © 2018 Optical Society of America.

Keywords
Electromagnetic pulse; Irradiation; Laser ablation; Stainless steel; Ultrafast lasers, Birefringent crystals; Experimental investigations; Incubation effects; Numerical solution; Separation time; Threshold fluences; Time separation; Two-temperature models, Ultrashort pulses
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-12193 (URN)10.1364/OE.26.003801 (DOI)000426268500008 ()2-s2.0-85042108738 (Scopus ID)
Available from: 2018-03-05 Created: 2018-03-05 Last updated: 2019-05-27Bibliographically approved
Gaudiuso, C., Giannuzzi, G., Choquet, I., Lugarà, P. M. & Ancona, A. (2018). Incubation effect in burst mode fs-laser ablation of stainless steel samples. Paper presented at Laser-Based Micro- and Nanoprocessing XII 2018; San Francisco; United States; 30 January 2018 through 1 February 2018. Proceedings of SPIE, the International Society for Optical Engineering, 10520, Article ID 105200A.
Open this publication in new window or tab >>Incubation effect in burst mode fs-laser ablation of stainless steel samples
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2018 (English)In: Proceedings of SPIE, the International Society for Optical Engineering, ISSN 0277-786X, E-ISSN 1996-756X, Vol. 10520, article id 105200AArticle in journal (Refereed) Published
Abstract [en]

We report on an experimental study of the incubation effect during irradiation of stainless steel targets with bursts of femtosecond laser pulses at 1030 nm wavelength and 100 kHz repetition rate. The bursts were generated by splitting the pristine 650-fs laser pulses using an array of birefringent crystals which provided time separations between sub-pulses in the range from 1.5 ps to 24 ps. We measured the threshold fluence in Burst Mode, finding that it strongly depends on the bursts features. The comparison with Normal Pulse Mode revealed that the existing models introduced to explain the incubation effect during irradiation with trains of undivided pulses has to be adapted to describe incubation during Burst Mode processing. In fact, those models assume that the threshold fluence has a unique value for each number of impinging pulses in NPM, while in case of BM we observed different values of threshold fluence for fixed amount of sub-pulses but different pulse splitting. Therefore, the incubation factor coefficient depends on the burst features. It was found that incubation effect is higher in BM than NPM and that it increases with the number of sub-pulses and for shorter time delays within the burst. Two-Temperature-Model simulations in case of single pulses and bursts of up to 4 sub-pulses were performed to understand the experimental results. © Copyright SPIE.

Keywords
Ablation; Irradiation; Laser ablation; Stainless steel; Ultrafast lasers, Birefringent crystals; Damage threshold; Incubation effects; Repetition rate; Threshold fluences; Time separation; Two Temperature Model; Ultrafast laser-matter interactions, Pulse repetition rate
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-12660 (URN)10.1117/12.2291612 (DOI)2-s2.0-85048541566 (Scopus ID)
Conference
Laser-Based Micro- and Nanoprocessing XII 2018; San Francisco; United States; 30 January 2018 through 1 February 2018
Available from: 2018-07-04 Created: 2018-07-04 Last updated: 2019-05-28Bibliographically approved
Nilsen, M., Sikström, F., Christiansson, A.-K. & Ancona, A. (2018). In-process Monitoring and Control of Robotized Laser Beam Welding of Closed Square Butt Joints. Paper presented at 8th Swedish Production Symposium (SPS 2018), Stockholm, Sweden, May 16-18, 2018. Procedia Manufacturing, 25, 511-516
Open this publication in new window or tab >>In-process Monitoring and Control of Robotized Laser Beam Welding of Closed Square Butt Joints
2018 (English)In: Procedia Manufacturing, E-ISSN 2351-9789, Vol. 25, p. 511-516Article in journal (Other academic) Published
Abstract [en]

In robotized laser welding of technical zero gap closed square butt joints it is critical to position the laser beam correct with regardsto the joint. Welding with an offset from the joint may cause lack of sidewall fusion, a serious defect that is hard to detect and gives a weak weld . When using machined parts with gap and misalignment between the parts that is close to zero, existing joint tracking systems will probably fail to track the joint. A camera based system using LED illumination and matching optical filters is proposed in this paper to address this issue. A high dynamic range CMOS camera and the LED illumination is integrated into the laser tool. The camera captures images of the area in front of the melt pool where the joint is visible and an algorithm based on the Hough transform and a Kalman filter estimates the offset between the laser spot and the joint position. Welding experiments, using a 6 kW fiber laser, have been conducted to evaluate the performance of the system. Promising results are obtained that can be used in the further development of a closed loop controlled joint tracking system.

Keywords
Laser welding, Butt joints, Joint tracking, Camera
National Category
Production Engineering, Human Work Science and Ergonomics Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-12828 (URN)10.1016/j.promfg.2018.06.123 (DOI)
Conference
8th Swedish Production Symposium (SPS 2018), Stockholm, Sweden, May 16-18, 2018
Projects
VINNOVA project VarGa (2016-03291)MoRE program project Hy-Las - Hybrid sensing for understanding of laser welding technology for process control, no 608473
Funder
Vinnova, 2016-03291EU, European Research Council, FP7/2007-2013
Note

Del av specialnumret Proceedings of the 8th Swedish Production Symposium (SPS 2018) Redaktörer: M. Onori, L. Wang, X. V. Wang och W. Ji

Available from: 2018-08-28 Created: 2018-08-28 Last updated: 2019-06-05Bibliographically approved
Kisielewicz, A., Sikström, F., Christiansson, A.-K. & Ancona, A. (2018). Spectroscopic monitoring of laser blown powder directed energy deposition of Alloy 718. Paper presented at 8th Swedish Production Symposium, SPS 2018, Stockholm, Sweden, 16-18 May, 2018. Procedia Manufacturing, 25, 418-425
Open this publication in new window or tab >>Spectroscopic monitoring of laser blown powder directed energy deposition of Alloy 718
2018 (English)In: Procedia Manufacturing, E-ISSN 2351-9789, Vol. 25, p. 418-425Article in journal (Refereed) Published
Abstract [en]

Experimental explorations of a spectrometer system used for in-process monitoring of the laser blown powder directed energy deposition of Alloy 718 is presented. Additive manufacturing of metals using this laser process experiences repeated heating and cooling cycles which will influence the final microstructure and chemical composition at every given point in the built. The spectrometer system disclosed, under certain process conditions, spectral lines that indicate vaporisation of chromium. Post process scanning electron microscope energy dispersive spectroscopy analysis of the deposited beads confirmed a reduction of chromium. Since the chromium concentration in Alloy 718 is correlated to corrosion resistance, this result encourages to further investigations including corrosion tests.

National Category
Metallurgy and Metallic Materials Production Engineering, Human Work Science and Ergonomics
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-13303 (URN)10.1016/j.promfg.2018.06.112 (DOI)2-s2.0-8506564084 (Scopus ID)
Conference
8th Swedish Production Symposium, SPS 2018, Stockholm, Sweden, 16-18 May, 2018
Note

Del av specialnumret Proceedings of the 8th Swedish Production Symposium (SPS 2018) Redaktörer: M. Onori, L. Wang, X. V. Wang och W. J

Funders: SUMANnext ; DigiAM

Available from: 2018-12-20 Created: 2018-12-20 Last updated: 2019-06-05Bibliographically approved
Tricarico, L., Ancona, A., Palumbo, G., Sorgente, D., Spina, R. & Lugarà, P. (2017). Discrete spot laser hardening and remelting with a high-brilliance source for surface structuring of a hypereutectoid steel. Materials & design, 115, 194-202
Open this publication in new window or tab >>Discrete spot laser hardening and remelting with a high-brilliance source for surface structuring of a hypereutectoid steel
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2017 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 115, p. 194-202Article in journal (Refereed) Published
Abstract [en]

In this work the single-pulse laser irradiation of a hypereutectoid steel was investigated using a fiber laser source, in a range of process parameters enabling surface hardening and remelting. Effects of laser power, pulse energy and defocusing distance were investigated using a numerical/experimental approach. Laser surface treatments were conducted on uncoated samples without any gas shielding, changing both the laser power and the pulse energy, and exploring a wide range of defocusing distances. Numerical simulations were conducted using a finite element model calibrated by means of an optimization procedure based on a specific calculation algorithm and using a subset of experimental data producing surface melting. Using both simulations and experiments, the process operating windows of the discrete spot laser treatment were determined: it was found that, when varying the laser power between 250 W and 750 W, melt-free hardened zones are produced with a maximum extension between 0.7 mm and 1.0 mm; on the contrary, in case of more tightly beam focusing conditions, surface melting occurred with a size of the re-melted areas ranging between 1.0 mm and 1.4 mm. Results further showed that a small change (generally 2–3 mm) of the defocusing distance suddenly brings the material from melting to a non-hardening condition. © 2016 Elsevier Ltd

Keywords
Fiber lasers; Finite element method; Hardening; Laser heating; Melting; Remelting; Steel fibers; Surface structure, Calculation algorithms; Hypereutectoid steel; Laser hardening; Laser re-melting; Laser surface treatment; Optimization procedures; Process parameters; Surface structuring, Laser windows
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering; Production Technology
Identifiers
urn:nbn:se:hv:diva-10268 (URN)10.1016/j.matdes.2016.10.079 (DOI)000390654500022 ()2-s2.0-84996878203 (Scopus ID)
Note

Funders:  Italian Ministry of Education, Universities and Research Government in the framework of the National Operative Programme (PON) for Research and Competitiveness (grants: PON01_02584 - project acronym: SMATI; PON01_02238 - project acronym: EURO6; PON02_00576_3333604 - project acronym: INNOVHEAD)

Available from: 2016-12-16 Created: 2016-12-14 Last updated: 2019-03-27Bibliographically approved
Volpe, A., Ancona, A., Trotta, G., Vázquez, R. M., Fassi, I. & Osellame, R. (2017). Fabrication and assembling of a microfluidic optical stretcher polymeric chip combining femtosecond laser and micro injection molding technologies. Paper presented at Laser-Based Micro- and Nanoprocessing XI 2017; San Francisco; United States; 31 January 2017 through 2 February 2017. Proceedings of SPIE, the International Society for Optical Engineering, Article ID 100920F.
Open this publication in new window or tab >>Fabrication and assembling of a microfluidic optical stretcher polymeric chip combining femtosecond laser and micro injection molding technologies
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2017 (English)In: Proceedings of SPIE, the International Society for Optical Engineering, ISSN 0277-786X, E-ISSN 1996-756X, article id 100920FArticle in journal (Refereed) Published
Abstract [en]

Microfluidic optical stretchers are valuable optofluidic devices for studying single cell mechanical properties. These usually consist of a single microfluidic channel where cells, with dimensions ranging from 5 to 20 Όm are trapped and manipulated through optical forces induced by two counter-propagating laser beams. Recently, monolithic optical stretchers have been directly fabricated in fused silica by femtosecond laser micromachining (FLM). Such a technology allows writing in a single step in the substrate volume both the microfluidic channel and the optical waveguides with a high degree of precision and flexibility. However, this method is very slow and cannot be applied to cheaper materials like polymers. Therefore, novel technological platforms are needed to boost the production of such devices on a mass scale. In this work, we propose integration of FLM with micro-injection moulding (ΌIM) as a novel route towards the cost-effective and flexible manufacturing of polymeric Lab-on-a-Chip (LOC) devices. In particular, we have fabricated and assembled a polymethylmethacrylate (PMMA) microfluidic optical stretcher by exploiting firstly FLM to manufacture a metallic mould prototype with reconfigurable inserts. Afterwards, such mould was employed for the production, through ΌIM, of the two PMMA thin plates composing the device. The microchannel with reservoirs and lodgings for the optical fibers delivering the laser radiation for cell trapping were reproduced on one plate, while the other included access holes to the channel. The device was assembled by direct fs-laser welding, ensuring sealing of the channel and avoiding thermal deformation and/or contamination. © 2017 SPIE.

Keywords
Biomechanics; Composite micromechanics; Cost effectiveness; Fabrication; Fluidic devices; Fused silica; Injection molding; Lab-on-a-chip; Laser beams; Manufacture; Microfluidics; Micromachining; Molding; Molds; Optical fiber fabrication; Optical fibers; Stretchers; Ultrashort pulses, Counter propagating lasers; Femtosecond laser micromachining; Flexible manufacturing; Lab-on-a-chip devices; Micro-injection molding; Micro-injection mouldings; Microfluidic optical stretcher; Technological platform, Polymers
National Category
Manufacturing, Surface and Joining Technology
Research subject
ENGINEERING, Manufacturing and materials engineering
Identifiers
urn:nbn:se:hv:diva-11620 (URN)10.1117/12.2251372 (DOI)2-s2.0-85019413106 (Scopus ID)
Conference
Laser-Based Micro- and Nanoprocessing XI 2017; San Francisco; United States; 31 January 2017 through 2 February 2017
Available from: 2017-09-21 Created: 2017-09-21 Last updated: 2019-05-23Bibliographically approved
Organisations
Identifiers
ORCID iD: ORCID iD iconorcid.org/0000-0002-6247-5429

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