- Email: [email protected]
Ultrashort pulse laser micro-welding of cyclo-olefin copolymers
Optics and Lasers in Engineering 93 (2017) 178–181
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Ultrashort pulse laser micro-welding of cyclo-olefin copolymers ⁎
Gian-Luca Roth , Stefan Rung, Ralf Hellmann
MARK
University of Applied Sciences Aschaffenburg,Wuerzburger Str. 45, 63743 Aschaffenburg, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Ultrashort pulse Femtosecond laser Laser welding Cyclo-olefin copolymers Transparent polymers Laser bonding
We report on the joining of transparent thermoplastic polymers using infrared femtosecond laser pulses. Due to nonlinear absorption, the developed micro-welding process for cyclo-olefin copolymers does not require any intermediate absorbing layers or any surface pre-processing of the welding partners. In view of an optimized and stable micro-welding process, the influence of the welding speed and focal position on both, the quality and shear force strength are investigated. We highlight that welding seam widths of down to 65 µm are feasible for welding speeds of up to 75 mm/s. However, a variation of the welding speed affects the required focal position for a successful joining process. The shear force strength of the welding seam is determined to 37 MPa, which corresponds to 64% of the shear strength of the bulk material and is not affected by the welding speed.
1. Introduction During the last decade ultrashort pulse lasers have become a reliable tool for processing a wide range of transparent materials, e.g. glass and polymers [1]. This trend has been supported by the ongoing improvement of femtosecond laser technology including high pulse energies and repetition rates in combination with stable laser sources. Processing transparent materials using femtosecond laser pulses is based on nonlinear absorption mechanisms, such as, e.g., multi-photon and tunnel absorption or avalanche ionization, which in turn are triggered by the high peak intensities inside the focal volume [2]. Reported femtosecond processes include surface functionalization [3,4], drilling and cutting [5], dicing [6] and volume interactions as, e.g., waveguide direct writing [7] and 3D-structuring of photosensitive glasses [8]. In this context, particular attention is put on the joining of transparent materials. This approach has been applied to different glasses e.g. fused silica [9] and borosilicate glass [10] using pulse durations from 85 fs to 10 ps [11,12] and repetition rates from 1 kHz to 9.4 MHz [10,13]. This novel process has also been applied for PMMA by Volpe et al. [14,15] using near infrared (NIR) ultrashort pulse laser systems with pulse duration of 650 fs and a repetition rate of 5 MHz, resulting in a welding speed of 0.1 mm/s in a multi scanning technique. A different approach to achieve a laser welding of transparent polymers without any additional absorbing layers has been proposed by Mingareev et al. [16] and Routsalainen et al. [17] employing a Thulium fiber laser with an emission wavelength of about 2 µm. At this wavelength, polymers like polymethylmethacrylate (PMMA) are semitransparent, enabling sufficient linear absorption. However, as for
⁎
linear absorption the laser radiation is absorbed all along its propagation length through the material, this process is associated with a largely spread heat-affected zone and is highly influenced by the thickness of the upper joining partner. A popular potential application of laser based joining techniques is the welding of transparent lab-on-chip-devices including microfluidic structures for medical use. In contrast to alternative joining techniques, e.g. thermal or solvent bonding, a process using ultrashort laser pulses exhibits distinct advantages. Firstly, it prevents any potential channel collapse and heat influence to the microfluidic systems. Secondly, compared to conventional laser bonding by continuous wave NIR diode or fiber laser based on linear absorption, it circumvents the application of an absorbing layer at the interface of the joining partners and thereby avoids potential sources of contamination [18]. In this study, we report on the laser joining of transparent cyclo-olefin copolymers using femtosecond laser pulses. These materials have become a promising alternative over other transparent polymers, e.g. PMMA, due to their high glass transition temperature, a high chemical resistance, a low water absorption and a high transparency down to the UV spectral range [19,20]. These attractive properties enable a broad range of sensing applications [21] and optofluidic devices. A reliable, high quality welding process can foster these trends. 2. Experimental 2.1. The laser system We used an ultrashort pulse laser (Light Conversion, Pharos-10–
Corresponding author. E-mail address: [email protected] (G.-L. Roth).
http://dx.doi.org/10.1016/j.optlaseng.2017.02.006 Received 18 October 2016; Received in revised form 11 January 2017; Accepted 13 February 2017 0143-8166/ © 2017 Elsevier Ltd. All rights reserved.
Optics and Lasers in Engineering 93 (2017) 178–181
G.-L. Roth et al.
600) at a wavelength of 1028 nm having an adjustable pulse duration from 220 fs to 15 ps. The chirped pulse amplification system generates a maximum energy per pulse of 200 μJ at a repetition rate of 50 kHz. The laser beam with a diameter of 10 mm (1/e2) is positioned by a galvanometer scanner (Newson Engineering, Rothor AR800) in combination with a telecentric f-Theta lens with a focal length of 100 mm (Qioptiq, Linos F-Theta Ronar) resulting in a focal spot diameter of 14 µm and an accessible field of 54×54 mm2. The focal height is controlled by a linear z-stage (Aerotech, Pro 165). 2.2. Materials and characterization We processed cyclo-olefin copolymer (COC) plates (size of 11×22 mm) from Ticona GmbH (brand name TOPAS 6015). The material is characterized by a glass transition temperature of 150 °C and a tensile strength of 58 MPa. The plates had a thickness of 1.1 mm and a surface roughness Ra of 25 nm. At room temperature, the refractive index of the material at 1030 nm has been determined to 1.52 using an Abbe refractometer. Based on transmission and reflection measurements, the linear absorption coefficient at the laser wavelength is calculated to 17.44 m−1. The layout of the platelets for the tensile shear tests consists of 2 parallel welding seams with a length of 10 mm and a lateral distance of 500 µm. Welding seam dimensions were selected to get a robust chip design and to minimize the impact of single welding defects. The specimen and testing setup are shown in Fig. 1 (a). The spatial extension of the welding seam is characterized by transmission light microscopy (Nikon, Eclipse LVDIA-N). The mechanical quality of the seam is evaluated by the separating force measured by a tensile tester (Shimadzu, EZ-TEST LX). In order to calculate the joining strength the size of the welded area is determined by a laser scanning confocal microscope (Keyence, VK X-200). A scanning electron microscope (Phenom, X Pro) is used to observe cracked welding seams.
Fig. 2. Required focal position ▵z for a successful laser welding at different scanning speeds.
interface into the propagation direction (shift denoted as ▵z ) of the impinging laser [22]. 3. Results and discussion Initially, the influence and the parameter interaction of i) the laser welding speed and ii) the focal position on the welding performance are determined. In order to achieve highest nonlinear absorption, the shortest pulse duration of our laser system of 220 fs is used. The repetition rate is set to the maximum value of 612 kHz to ensure sufficient heat generation to enable adequate joining. In accordance to our previous studies [22] a suitable laser power of 1500 mW is chosen. The influence of the welding speed is determined between velocities of 10 mm/s and 75 mm/s. These limits are defined by a poor welding quality due to a large heat influence at the lower velocity limit and by an irregular and interrupted welding seam at the upper limit. These welding speeds are significantly higher as compared to both Thulium laser based bonding and previously reported ultrashort pulse welding of PMMA using 5 mm/s [16] and 0.1 mm/s [14], respectively. Fig. 2 summarizes the focal offsets ▵z with respect to the interface of the welding partners for different welding speeds. Apparently, at low welding speeds a higher focal correction is necessary to generate a welding seam. At higher processing speeds the required focal position remains at a constant level, i.e. ▵z is constant at a value about −0.56 mm. The shift for decreasing welding speed can be attributed to an increased heat accumulation. In addition, the larger heat accumulation results in a larger process window, which is illustrated by a broader spread of ▵z values. Comparable results have been obtained by Miyamoto et al. [23] for processing borosilicate glass.
2.3. Process setup A successful laser welding requires a minimized air gap between the unprocessed joining partners. A sufficient small spacing among them is indicated by the appearance of Newtons rings upon white light illumination. The required pressure to minimize the gap was applied by a pneumatic short stroke cylinder (Festo, AEVC-63-10-I-P) with a maximal force of 1870 N. Taking the platelet size of 242 mm2 into account, a pressure of 2.58 MPa has been applied. To stabilize the process setup during welding, the clamping was realized by a 20 mm fused silica plate above and a PMMA board below the chips. The complete process setup is presented in Fig. 1 (b). The focal position of the laser is synchronized with the focal plane of a camera vision system. By focusing the camera vision on the interface between the joining partners and taking dispersion effects of glass and TOPAS into account the initial position of the focal spot of the laser is defined. As shown previously, to guarantee an efficient and reliable welding process, the focal position has to be shifted in respect to the
3.1. Welding seam width In general, the width and the quality of welding seams between two transparent plates are influenced by the heat input to the interface during the process. In our study, the heat input is determined by the focal position and the scanning speed. In Fig. 3 (top) the average welding seam width, determined by twenty measuring points along a 2 mm seam length with a 100 µm measurement interval, versus ▵z at a fixed scanning speed of 25 mm/s is shown. Within a focal position range between −0.68 µm and −0.56 µm (indicated by vertical dashed line in Fig. 3 (top)) the welding seam width varies only slightly, highlighting a good quality and stable welding process. Outside this regime, the welding seam width exhibits a higher standard deviation, i.e. the process becomes less stable. In Fig. 3 (bottom) the high quality process windows for different welding speeds are summarized showing a similar trend compared to the larger general process window for joining shown in Fig. 2. The influence of the welding speed on the welding seam width within this high quality process window is summarized in Fig. 4. While the average welding width remains almost constant in this regime, the standard deviation increases with increas-
Fig. 1. Lap shear test setup (a) and scheme of the cross-section of the laser processing (b). Please note the offset of the focal position ▵z .
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G.-L. Roth et al.
Fig. 4. Average welding seam width using the high quality process window for each applied scanning speed.
reported by Volpe et al. [14] for fs-welding of PMMA and Richter et al. [24] for volume processing of glass and have been identified as voids. As our images show, at higher welding speeds voids inside the welding seam create a continuous centerline with a melted subsurface. In addition, a deliquescent border is observed that contributes to the larger scattering of the welding seam width as shown in Fig. 4. At a higher welding speed the input laser power is not sufficient to achieve a uniform temperature profile inside the melting pool. Melted polymer from the center of the welding seam runs inside the gap between unmelted areas and solidifies. Colored areas around the welding seams are based on interference effects. Furthermore, interference fringes are only visible around of the melted trace and not within the actual joining area, indicating that full joining of the two parts is achieved. Scanning electron microscope pictures of again separated welding partners shown in the right hand side of Fig. 5 confirm these statements of a complete welding at lower scanning speeds and a deliquescent border
Fig. 3. Averaged welding seam width at different focal positions at a scanning speed of 25 mm/s (top) and required focal position ▵z of the low scattering process window at different scanning speeds (bottom).
ing welding speed. In Fig. 5 top view images of two welding seams created at different welding speeds are shown (left hand side of Fig. 5), revealing dark spots inside the welding seam. Similar spots have been
Fig. 5. Welding seams generated at 20 mm/s (top) and 65 mm/s (bottom) in joined state using an optical microscope for the connected seams (left hand side) and top faces of the bottom laminates in separated state using a scanning electron microscope (right hand side).
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does not affect the surrounding material, it has the potential to foster transparent microfluidic and optofluidic applications. References [1] Gattass RR, Mazur E. Femtosecond laser micromachining in transparent materials. Nat Photonics 2008;2(4):219225. [2] Stuart BC, Feit MD, Herman S, Shore BW, Perry MD. Nanosecond-to-femtosecond laser-induced breakdown in dielectrics. Phys Rev B 1996;53(4):1749. [3] Bulushev E, Bessmeltsev V, Dostovalov A, Goloshevsky N, Wolf A. High-speed and crack-free direct-writing of microchannels on glass by an IR femtosecond laser. Opt Lasers Eng 2016;79:3947. [4] Schwarz S, Rung S, Hellmann R. Generation of laser-induced periodic surface structures on transparent material-fused silica. Appl Phys Lett 2016;108(18):181607. [5] Butkus S, Paipulas D, Sirutkaitis R, Gaizauskas E, Sirutkaitis V. Rapid cutting and drilling of transparent materials via femtosecond laser filamentation. J Laser Micro Nanoeng 2014;9(3):213. [6] Tamhankar A, Patel R. Optimization of UV laser scribing process for light emitting diode sapphire wafers. J Laser Appl 2011;23(3):32001. [7] Nolte S, Will M, Burghoff J, Tuennermann A. Femtosecond waveguide writing: a new avenue to three-dimensional integrated optics. Appl Phys A 2003;77(1):109111. [8] Sugioka K, Cheng Y. Femtosecond laser processing for optofluidic fabrication. Lab on a Chip 2012;12(19):35763589. [9] Huang H, Lih-Mei Y, Jian L. Direct welding of fused silica with femtosecond fiber laser. SPIE LASE. International Society for Optics and Photonics; 9; 824403; 2012. [10] Kongsuwan P, Satoh G, Yao YL. Transmission welding of glass by femtosecond laser: mechanism and fracture strength. J Manuf Sci Eng 2012;134(1):11004. [11] Watanabe W, Onda S, Tamaki T, Itoh K, Nishii J. Space-selective laser joining of dissimilar transparent materials using femtosecond laser pulses. Appl Phys Lett 2006;89(2):21106. [12] Miyamoto I, Horn A, Gottmann J. Local melting of glass material and its application to direct fusion welding by ps-laser pulses. J Laser Micro Nanoeng 2007;2(1):714. [13] Richter S, Nolte S, Tuennermann A. Ultrashort pulse laser welding - a new approach for high- stability bonding of different glasses. Phys Procedia 2012;39:556562. [14] Volpe A, di Niso F, Gaudiuso C, de Rosa A, Vzquez R, Ancona A, et al. Welding of PMMA by a femtosecond fiber laser. Opt Express 2015;23:4. [15] Volpe A, Di Niso F, Gaudiuso C, de Rosa A, Vzquez R, Ancona A, et al. Femtosecond fiber laser welding of PMMA. Laser-based Micro- and Nanoprocessing IX Proceedings of SPIE 2015; 9351. [16] Mingareev I, Weirauch F, Olowinsky A, Shah L, Kadwani P, Richardson M. Welding of polymers using a 2 μm thulium fiber laser. Opt Laser Technol 2012;44:7. [17] Ruotsalainen S, Laakso P, Kujanp V. Laser welding of transparent polymers by using quasi-simultaneous beam off-setting. Scanning Tech Phys Procedia 2015;78:272284. [18] Jiang X, Chandrasekar S, Wang C. A laser microwelding method for assembly of polymer based microfluidic devices. Opt Lasers Eng 2015;66:98–104. [19] Castillo-Len J, Svendsen WE. Lab-on-a-chip devices and micro-total analysis systems. A practical guide. Springer; 2015. [20] Nunes P, Ohlsson P, Ordeig O, Kutter J. Cyclic olefin polymers emerging materials for lab-on-a-chip applications. Microfluid Nanofluid 2010;9:2–3. [21] Rosenberger M, Schmauss B, Hellmann R. Influence of the UV dosage on planar bragg gratings in cyclo-olefin copolymer substrates. Opt Mater Express 2016;6(6):2118–27. [22] Roth G, Rung S, Hellmann R. Welding of transparent polymers using femtosecond laser. Appl Phys A 2016;122(2):1–4. [23] Miyamoto I, Horn A, Gottmann J, Wortmann D, Yoshino F. Fusion welding of glass using femtosecond laser pulses with high-repetition rates. JLMN-J Laser Micro/ Nanoeng 2007;2(1):57–63. [24] Richter S, Dring S, Burmeister F, Zimmermann F, Tuennermann A, Nolte S. Formation of periodic disruptions induced by heat accumulation of femtosecond laser pulses. Opt Express 2013;22(13):1545215463. [25] Montgomery DC. Design and analysis of experiments. Hoboken, NJ; John Wiley Sons, Inc., 2013.
Fig. 6. Shear strength of weldings seams created at different welding speeds.
at higher welding speeds. Furthermore, in the inset of the SEM picture at a welding speed of 20 mm/s an unstructured melted zone around the welded area is identified, indicating that the strength of the welding zone is not constant over the cross-section of the seam. 3.2. Welding strength The strength of the welding seam is measured by a tensile-shear strength test. The shear force was applied parallel to the specimen as shown in Fig. 1 (a) and spread uniformly over the joined plates. Experiments were performed for 7 welding speeds using 8 samples per parameter. In Fig. 6, mean values and standard deviations for the measured welding shear strengths are shown for each welding speed. Obviously, the shear strength and its deviation are not affected by the applied welding speed in the observed interval, which is supported by a single factor analysis of variances (ANOVA) [25]. The average shear strength can be calculated to 37 MPa, corresponding to 64% of the bulk material shear strength. This is in good agreement to previously reported results for TOPAS 6017 and a similar process setup using a laser power of 1700 mW and a welding speed of 20 mm/s [22]. Compared to Thulium laser bonding of transparent materials with a strength of 13 MPa for PMMA [16] (19% of the shear strength of bulk material), using an ultrashort-pulse laser leads to a significantly higher bonding strength. 4. Conclusions Transparent cyclo-olefin copolymers are welded using a 220 fs ultrashort pulse laser system at 1500 mW. The influence and interaction of the welding speed and the focal position on the welding seam width and shear force strength are studied. Welding seam widths down to 65 µm are achieved for a welding speed of up to 75 mm/s, in this regime a shear force strength of up to 37 MPa, i.e. 64% of the bulk material shear strength, is feasible. With respect to process development, the focal position turns out to be a significant influencing factor that has to be adjusted for different welding speeds. Both, welding speed and welding strength of the ultrashort pulse micro-welding are superior to comparable laser based joining processes of transparent polymers. As the ultrashort pulse laser based micro-welding approach
181
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Ultrashort pulse laser micro-welding of cyclo-olefin copolymers ⁎
Gian-Luca Roth , Stefan Rung, Ralf Hellmann
MARK
University of Applied Sciences Aschaffenburg,Wuerzburger Str. 45, 63743 Aschaffenburg, Germany
A R T I C L E I N F O
A B S T R A C T
Keywords: Ultrashort pulse Femtosecond laser Laser welding Cyclo-olefin copolymers Transparent polymers Laser bonding
We report on the joining of transparent thermoplastic polymers using infrared femtosecond laser pulses. Due to nonlinear absorption, the developed micro-welding process for cyclo-olefin copolymers does not require any intermediate absorbing layers or any surface pre-processing of the welding partners. In view of an optimized and stable micro-welding process, the influence of the welding speed and focal position on both, the quality and shear force strength are investigated. We highlight that welding seam widths of down to 65 µm are feasible for welding speeds of up to 75 mm/s. However, a variation of the welding speed affects the required focal position for a successful joining process. The shear force strength of the welding seam is determined to 37 MPa, which corresponds to 64% of the shear strength of the bulk material and is not affected by the welding speed.
1. Introduction During the last decade ultrashort pulse lasers have become a reliable tool for processing a wide range of transparent materials, e.g. glass and polymers [1]. This trend has been supported by the ongoing improvement of femtosecond laser technology including high pulse energies and repetition rates in combination with stable laser sources. Processing transparent materials using femtosecond laser pulses is based on nonlinear absorption mechanisms, such as, e.g., multi-photon and tunnel absorption or avalanche ionization, which in turn are triggered by the high peak intensities inside the focal volume [2]. Reported femtosecond processes include surface functionalization [3,4], drilling and cutting [5], dicing [6] and volume interactions as, e.g., waveguide direct writing [7] and 3D-structuring of photosensitive glasses [8]. In this context, particular attention is put on the joining of transparent materials. This approach has been applied to different glasses e.g. fused silica [9] and borosilicate glass [10] using pulse durations from 85 fs to 10 ps [11,12] and repetition rates from 1 kHz to 9.4 MHz [10,13]. This novel process has also been applied for PMMA by Volpe et al. [14,15] using near infrared (NIR) ultrashort pulse laser systems with pulse duration of 650 fs and a repetition rate of 5 MHz, resulting in a welding speed of 0.1 mm/s in a multi scanning technique. A different approach to achieve a laser welding of transparent polymers without any additional absorbing layers has been proposed by Mingareev et al. [16] and Routsalainen et al. [17] employing a Thulium fiber laser with an emission wavelength of about 2 µm. At this wavelength, polymers like polymethylmethacrylate (PMMA) are semitransparent, enabling sufficient linear absorption. However, as for
⁎
linear absorption the laser radiation is absorbed all along its propagation length through the material, this process is associated with a largely spread heat-affected zone and is highly influenced by the thickness of the upper joining partner. A popular potential application of laser based joining techniques is the welding of transparent lab-on-chip-devices including microfluidic structures for medical use. In contrast to alternative joining techniques, e.g. thermal or solvent bonding, a process using ultrashort laser pulses exhibits distinct advantages. Firstly, it prevents any potential channel collapse and heat influence to the microfluidic systems. Secondly, compared to conventional laser bonding by continuous wave NIR diode or fiber laser based on linear absorption, it circumvents the application of an absorbing layer at the interface of the joining partners and thereby avoids potential sources of contamination [18]. In this study, we report on the laser joining of transparent cyclo-olefin copolymers using femtosecond laser pulses. These materials have become a promising alternative over other transparent polymers, e.g. PMMA, due to their high glass transition temperature, a high chemical resistance, a low water absorption and a high transparency down to the UV spectral range [19,20]. These attractive properties enable a broad range of sensing applications [21] and optofluidic devices. A reliable, high quality welding process can foster these trends. 2. Experimental 2.1. The laser system We used an ultrashort pulse laser (Light Conversion, Pharos-10–
Corresponding author. E-mail address: [email protected] (G.-L. Roth).
http://dx.doi.org/10.1016/j.optlaseng.2017.02.006 Received 18 October 2016; Received in revised form 11 January 2017; Accepted 13 February 2017 0143-8166/ © 2017 Elsevier Ltd. All rights reserved.
Optics and Lasers in Engineering 93 (2017) 178–181
G.-L. Roth et al.
600) at a wavelength of 1028 nm having an adjustable pulse duration from 220 fs to 15 ps. The chirped pulse amplification system generates a maximum energy per pulse of 200 μJ at a repetition rate of 50 kHz. The laser beam with a diameter of 10 mm (1/e2) is positioned by a galvanometer scanner (Newson Engineering, Rothor AR800) in combination with a telecentric f-Theta lens with a focal length of 100 mm (Qioptiq, Linos F-Theta Ronar) resulting in a focal spot diameter of 14 µm and an accessible field of 54×54 mm2. The focal height is controlled by a linear z-stage (Aerotech, Pro 165). 2.2. Materials and characterization We processed cyclo-olefin copolymer (COC) plates (size of 11×22 mm) from Ticona GmbH (brand name TOPAS 6015). The material is characterized by a glass transition temperature of 150 °C and a tensile strength of 58 MPa. The plates had a thickness of 1.1 mm and a surface roughness Ra of 25 nm. At room temperature, the refractive index of the material at 1030 nm has been determined to 1.52 using an Abbe refractometer. Based on transmission and reflection measurements, the linear absorption coefficient at the laser wavelength is calculated to 17.44 m−1. The layout of the platelets for the tensile shear tests consists of 2 parallel welding seams with a length of 10 mm and a lateral distance of 500 µm. Welding seam dimensions were selected to get a robust chip design and to minimize the impact of single welding defects. The specimen and testing setup are shown in Fig. 1 (a). The spatial extension of the welding seam is characterized by transmission light microscopy (Nikon, Eclipse LVDIA-N). The mechanical quality of the seam is evaluated by the separating force measured by a tensile tester (Shimadzu, EZ-TEST LX). In order to calculate the joining strength the size of the welded area is determined by a laser scanning confocal microscope (Keyence, VK X-200). A scanning electron microscope (Phenom, X Pro) is used to observe cracked welding seams.
Fig. 2. Required focal position ▵z for a successful laser welding at different scanning speeds.
interface into the propagation direction (shift denoted as ▵z ) of the impinging laser [22]. 3. Results and discussion Initially, the influence and the parameter interaction of i) the laser welding speed and ii) the focal position on the welding performance are determined. In order to achieve highest nonlinear absorption, the shortest pulse duration of our laser system of 220 fs is used. The repetition rate is set to the maximum value of 612 kHz to ensure sufficient heat generation to enable adequate joining. In accordance to our previous studies [22] a suitable laser power of 1500 mW is chosen. The influence of the welding speed is determined between velocities of 10 mm/s and 75 mm/s. These limits are defined by a poor welding quality due to a large heat influence at the lower velocity limit and by an irregular and interrupted welding seam at the upper limit. These welding speeds are significantly higher as compared to both Thulium laser based bonding and previously reported ultrashort pulse welding of PMMA using 5 mm/s [16] and 0.1 mm/s [14], respectively. Fig. 2 summarizes the focal offsets ▵z with respect to the interface of the welding partners for different welding speeds. Apparently, at low welding speeds a higher focal correction is necessary to generate a welding seam. At higher processing speeds the required focal position remains at a constant level, i.e. ▵z is constant at a value about −0.56 mm. The shift for decreasing welding speed can be attributed to an increased heat accumulation. In addition, the larger heat accumulation results in a larger process window, which is illustrated by a broader spread of ▵z values. Comparable results have been obtained by Miyamoto et al. [23] for processing borosilicate glass.
2.3. Process setup A successful laser welding requires a minimized air gap between the unprocessed joining partners. A sufficient small spacing among them is indicated by the appearance of Newtons rings upon white light illumination. The required pressure to minimize the gap was applied by a pneumatic short stroke cylinder (Festo, AEVC-63-10-I-P) with a maximal force of 1870 N. Taking the platelet size of 242 mm2 into account, a pressure of 2.58 MPa has been applied. To stabilize the process setup during welding, the clamping was realized by a 20 mm fused silica plate above and a PMMA board below the chips. The complete process setup is presented in Fig. 1 (b). The focal position of the laser is synchronized with the focal plane of a camera vision system. By focusing the camera vision on the interface between the joining partners and taking dispersion effects of glass and TOPAS into account the initial position of the focal spot of the laser is defined. As shown previously, to guarantee an efficient and reliable welding process, the focal position has to be shifted in respect to the
3.1. Welding seam width In general, the width and the quality of welding seams between two transparent plates are influenced by the heat input to the interface during the process. In our study, the heat input is determined by the focal position and the scanning speed. In Fig. 3 (top) the average welding seam width, determined by twenty measuring points along a 2 mm seam length with a 100 µm measurement interval, versus ▵z at a fixed scanning speed of 25 mm/s is shown. Within a focal position range between −0.68 µm and −0.56 µm (indicated by vertical dashed line in Fig. 3 (top)) the welding seam width varies only slightly, highlighting a good quality and stable welding process. Outside this regime, the welding seam width exhibits a higher standard deviation, i.e. the process becomes less stable. In Fig. 3 (bottom) the high quality process windows for different welding speeds are summarized showing a similar trend compared to the larger general process window for joining shown in Fig. 2. The influence of the welding speed on the welding seam width within this high quality process window is summarized in Fig. 4. While the average welding width remains almost constant in this regime, the standard deviation increases with increas-
Fig. 1. Lap shear test setup (a) and scheme of the cross-section of the laser processing (b). Please note the offset of the focal position ▵z .
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G.-L. Roth et al.
Fig. 4. Average welding seam width using the high quality process window for each applied scanning speed.
reported by Volpe et al. [14] for fs-welding of PMMA and Richter et al. [24] for volume processing of glass and have been identified as voids. As our images show, at higher welding speeds voids inside the welding seam create a continuous centerline with a melted subsurface. In addition, a deliquescent border is observed that contributes to the larger scattering of the welding seam width as shown in Fig. 4. At a higher welding speed the input laser power is not sufficient to achieve a uniform temperature profile inside the melting pool. Melted polymer from the center of the welding seam runs inside the gap between unmelted areas and solidifies. Colored areas around the welding seams are based on interference effects. Furthermore, interference fringes are only visible around of the melted trace and not within the actual joining area, indicating that full joining of the two parts is achieved. Scanning electron microscope pictures of again separated welding partners shown in the right hand side of Fig. 5 confirm these statements of a complete welding at lower scanning speeds and a deliquescent border
Fig. 3. Averaged welding seam width at different focal positions at a scanning speed of 25 mm/s (top) and required focal position ▵z of the low scattering process window at different scanning speeds (bottom).
ing welding speed. In Fig. 5 top view images of two welding seams created at different welding speeds are shown (left hand side of Fig. 5), revealing dark spots inside the welding seam. Similar spots have been
Fig. 5. Welding seams generated at 20 mm/s (top) and 65 mm/s (bottom) in joined state using an optical microscope for the connected seams (left hand side) and top faces of the bottom laminates in separated state using a scanning electron microscope (right hand side).
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Fig. 6. Shear strength of weldings seams created at different welding speeds.
at higher welding speeds. Furthermore, in the inset of the SEM picture at a welding speed of 20 mm/s an unstructured melted zone around the welded area is identified, indicating that the strength of the welding zone is not constant over the cross-section of the seam. 3.2. Welding strength The strength of the welding seam is measured by a tensile-shear strength test. The shear force was applied parallel to the specimen as shown in Fig. 1 (a) and spread uniformly over the joined plates. Experiments were performed for 7 welding speeds using 8 samples per parameter. In Fig. 6, mean values and standard deviations for the measured welding shear strengths are shown for each welding speed. Obviously, the shear strength and its deviation are not affected by the applied welding speed in the observed interval, which is supported by a single factor analysis of variances (ANOVA) [25]. The average shear strength can be calculated to 37 MPa, corresponding to 64% of the bulk material shear strength. This is in good agreement to previously reported results for TOPAS 6017 and a similar process setup using a laser power of 1700 mW and a welding speed of 20 mm/s [22]. Compared to Thulium laser bonding of transparent materials with a strength of 13 MPa for PMMA [16] (19% of the shear strength of bulk material), using an ultrashort-pulse laser leads to a significantly higher bonding strength. 4. Conclusions Transparent cyclo-olefin copolymers are welded using a 220 fs ultrashort pulse laser system at 1500 mW. The influence and interaction of the welding speed and the focal position on the welding seam width and shear force strength are studied. Welding seam widths down to 65 µm are achieved for a welding speed of up to 75 mm/s, in this regime a shear force strength of up to 37 MPa, i.e. 64% of the bulk material shear strength, is feasible. With respect to process development, the focal position turns out to be a significant influencing factor that has to be adjusted for different welding speeds. Both, welding speed and welding strength of the ultrashort pulse micro-welding are superior to comparable laser based joining processes of transparent polymers. As the ultrashort pulse laser based micro-welding approach
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