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Influence of oscillation frequency and focal diameter on weld pool geometry and temperature field in laser beam welding of high strength steels
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Procedia CIRP 00 (2017) 000–000 Procedia CIRP 74 (2018) 470–474 www.elsevier.com/locate/procedia th 10 CIRP 10th CIRPConference Conferenceon onPhotonic PhotonicTechnologies Technologies [LANE [LANE 2018] 2018]
Influence of oscillation frequency diameter 28th CIRP Design Conference,and Mayfocal 2018, Nantes, Franceon weld pool geometry and temperature field in laser beam welding of high strength A new methodology to analyze the functional and physical architecture of steels existing products for an assembly oriented product family identification Vincent Manna,*, Konstantin Hofmanna, Kerstin Schaumbergera, Tobias Weigerta, Stephan a,b Alain Etienne, Paul Stief *,a,bJean-Yves Dantan, SiadatRotha,c, Michael Schustera, Jan Hafenecker , Simon Hübner , Lisa Lipinskia,b,Ali Stephan a,b,c Schmidt École Nationale Supérieure d’Arts et Métiers, Arts et Métiers ParisTech, LCFC EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France
a Bayerisches Laserzentrum GmbH (blz), Konrad-Zuse-Straße 2-6, 91052 Erlangen, Germany b * Corresponding Tel.:Technologies +33 3 87 37 (LPT), 54 30; Friedrich-Alexander-Universität E-mail address: [email protected] Institute ofauthor. Photonic Erlangen-Nürnberg, Konrad-Zuse-Straße 3-5, 91052 Erlangen, Germany c
Erlangen Graduate School in Advanced Optical Technologies (SAOT), Paul-Gordan-Straße 6, 91052 Erlangen, Germany
* Corresponding author. Tel.: +49-9131-97790-16 ; fax: +49-9131-97790-11. E-mail address: [email protected]
Abstract In today’s business environment, the trend towards more product variety and customization is unbroken. Due to this development, the need of Abstract agile and reconfigurable production systems emerged to cope with various products and product families. To design and optimize production systems as well as to choose the optimal product matches, product analysis methods are needed. Indeed, most of the known methods aim to As weld pool geometry and thermomechanical strains are known to affect formation of solidification cracks, the influence of a superimposed analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and beam oscillation on these characteristics is investigated for this paper. In this context the effects of the oscillation frequency and focal diameter nature of components. This fact impedes an efficient comparison and choice of appropriate product family combinations for the production on the weld pool and temperature field are determined by means of infrared thermography. As a result an increase of the weld pool size for system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to cluster larger focal diameters and a more even shape of the seam edges for higher frequencies can be identified. these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable © 2018 2018 The The Authors. Authors. Published Published by by Elsevier Ltd. Ltd. This This is is an an open open access access article article under under the CC BY-NC-ND BY-NC-ND license license © assembly systems. Based on Datum Elsevier Flow Chain, the physical structure of the productstheis CC analyzed. Functional subassemblies are identified, and (http://creativecommons.org/licenses/by-nc-nd/3.0/) (https://creativecommons.org/licenses/by-nc-nd/4.0/) a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the Bayerisches Laserzentrum Peer-review under under responsibility responsibility of of the Bayerisches Laserzentrum GmbH. GmbH. Peer-review similarity between product families the by providing design support to both, production system planners and product designers. An illustrative example of a nail-clipper is used to explain the proposed methodology. An industrial case study on two product families of steering columns of Keywords: laser beam welding; high strength steels; beam oscillation; weld pool geometry; infrared thermography; solidification conditions; temperature field thyssenkrupp Presta France is then carried out to give a first industrial evaluation of the proposed approach. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 28th CIRP Design Conference 2018.
1. Introduction
Keywords: Assembly; Design method; Family identification
Nowadays declining limits for the emission of CO2 and NOX, increasing costs for energy and the high demand for vehicles with a greater range [1] strengthen the need 1.electric Introduction for lightweight constructions in automotive manufacturing [2,3]. their fast high tensile strength in and the ductility and their DueDue to to the development domain of relatively low costs, high strength steels are suitable materials communication and an ongoing trend of digitization and for these applications. [3] During the welding process of these digitalization, manufacturing enterprises are facing important materials, the risk for the appearance of weld seam defects challenges in today’s market environments: a continuing such as pores and cracks is significantly higher [4] due the tendency towards reduction of product development timestoand higher alloying content in comparison to low strength steels. shortened product lifecycles. In addition, there is an increasing Especially the risk of hotbeing cracks is increased larger demand of customization, at the same timebyinthe a global temperature interval between solidus and liquidus temperature competition with competitors all over the world. This trend, [5] caused by thethedifferent meltingfrom temperatures the which is inducing development macro to of micro alloying elements. In contrast to laser beam welding of markets, results in diminished lot sizes due to augmenting aluminum, steels are often welded without filler wire and product varieties (high-volume to low-volume production) [1].
shielding gas by means of fast scanner optics, which enable shorter positioning times. [6] Hence a reduction of hot cracking susceptibility by the use of filler wires is undesirable. As the risk of hot cracking is also influenced by the solidification such as shape manufactured and size of theand/or melt of the productconditions, range and characteristics pool and the temperature field, a targeted modification of assembled in this system. In this context, the main challenge in these properties reduces hot cracking susceptibility without modelling and analysis is now not only to cope with single changing athe alloying composition use ofproduct filler materials. products, limited product range or by existing families, The underlying mechanism for this is a change in thetogrowth but also to be able to analyze and to compare products define direction of families. dendritesItduring solidification process in the new product can bethe observed that classical existing rear area of the melt pool. According to this theory, dendrites product families are regrouped in function of clients or features. which areassembly directed oriented to the middle offamilies the weldareseam induce an However, product hardly to find. agglomeration of low melting alloying elements in the center On the product family level, products differ mainly in two of thecharacteristics: weld seam and, to the shrinkage of the the main (i) in theconsequence number of components and (ii) cooling material, center line cracks. In contrast, dendrites type of components (e.g. mechanical, electrical, electronical). which are oriented in welding directionmainly enable single a backfilling of Classical methodologies considering products appearing cavities by the melt from the melt pool. In this or solitary, already existing product families analyze the
To cope with this augmenting variety as well as to be able to product structure on a physical level (components level) which 2212-8271 possible © 2018 Theoptimization Authors. Published by Elsevier is an opencauses access article under theregarding CC BY-NC-ND license identify potentials in Ltd. the This existing difficulties an efficient definition and (http://creativecommons.org/licenses/by-nc-nd/3.0/) production system, it is important to have a precise knowledge comparison of different product families. Addressing this Peer-review under responsibility of the Bayerisches Laserzentrum GmbH.
2212-8271 © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 2212-8271 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of scientific the Bayerisches Laserzentrum GmbH. Peer-review under responsibility of the committee of the 28th CIRP Design Conference 2018. 10.1016/j.procir.2018.08.148
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Vincent Mann et al. / Procedia CIRP 74 (2018) 470–474 Author name / Procedia CIRP 00 (2018) 000–000
context a decrease of welding velocity is known to significantly change the melt pool from a tear drop shaped melt pool with an acute angle of the solidification fronts to an oval shaped melt pool with an obtuse angle of solidification fronts. [7] Since a lower welding velocity decreases the productivity of the welding process, there is a need for other strategies for manipulating the shape of the melt pool without changing properties of the weld seam. For this purpose a superimposed beam oscillation provided by the combination of brilliant beam sources and fast scanner optics seems to be an appropriate tool for a manipulation of the melt pool shape. In this context it is already proven that beam oscillation influences the width of the weld seam and the penetration depth in welding aluminum to copper in lap joint configuration [8], as well as the gap bridgeability [9] and surface roughness of the weld seam in laser beam welding of aluminum alloys. [10] Furthermore a sinusoidal oscillation at very low oscillation frequencies (< 5 Hz) can reduce hot cracking susceptibility, if the melt pool is small enough to follow the oscillation pattern. In this case, the main directions of shrinkage and solidification are not parallel as for laser beam welding without beam oscillation, but perpendicular which decreases crack initiation strains. [11] In contrast, shape and size of the keyhole can be modified by very high oscillation frequencies (> 2 kHz) [12] in order to reduce spatters [13]. Nevertheless there are only few results [14] for laser beam welding with smaller oscillation frequencies (< 100 Hz), for which a modification of the weld pool geometry is possible while keeping similar properties of the weld seam. Moreover the effect of the focal diameter is not specified up to now. Thus the effect of these parameters on melt pool properties and temperature field is presented within this paper. 2. Experimental setup The experiments for this paper were carried out with a TruDisk 6001 with a maximum laser power of 6 kW, a wavelength (λL) of 1,030 nm and a beam parameter product of 4 mm x mrad. The focusing and movement of the beam is realized by a PFO33-2-scanner optics from Trumpf with a collimation length of 150 mm and a focusing length of 255 mm leading to a magnification factor of 1.7. In order to gain different focal diameters, optical fibers with three core diameters (dFB) of 100 µm, 150 µm and 200 µm are used subsequently yielding focal diameters (dF) of 170 µm, 255 µm and 340 µm in the focal plane of the beam. As no additional axle was used, the linear movement of the beam and the superimposed beam oscillation were realized by the scanner optics solely. The figure overlap was chosen to be zero and a circular oscillation pattern (Pat) was used for all experiments. For process monitoring a thermographic camera from IRCAM was installed in a lateral position next to the scanner optics. The experimental setup is shown in Fig. 1. The sensor of the thermographic camera detects IRradiation in the spectra from 3 µm to 4.8 µm and has a maximum spatial resolution of 320 x 256 pixels at a pixel length of 78 µm. The maximum frame rate (fC) was 1 kHz at a integration time (tI) of 0.01 ms.
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Fig. 1. Experimental setup for welding experiments and thermographic process monitoring
For the experiments, the focal plane of the laser beam was positioned on the surface of the specimens and the energy per unit length (E) was kept constant on a level of 90 J/mm. The weld seams are positioned with a lateral distance (a) of 1.5 mm from the symmetry plane of the specimens in order to gain a non-symmetric temperature field. Further process parameters are listed in Table 1. Table 1. Welding and oscillation parameters used for the experiments Parameter
Symbol
Tested range
Unit
Laser power
PL
0.90; 3,15; 5.40
kW
Linear welding velocity
vL
10; 35; 60
mm/s
Oscillation frequency
f
40; 60;80
Hz
Oscillation amplitude
A
0.5
mm
Beam velocity
vB
86 – 222
mm/s
The material used for investigations was a high strength low alloy steel with a tensile strength of 700 N/mm² and an elongation at break of 10 %. The thickness (d) of the sheets was 1.8 mm. The specimens were laser cut to Double-FanShaped specimens [14] which were derived from the FanShaped test according to [15]. All welds were carried out as full-penetration bead-on-plate welds.
Fig. 2. Geometry of the self-restrained hot cracking test specimens and schematic drawing of the measured values of the melt pool geometry
By means of the thermographic videos the melt pool was characterized by the melt pool length, width and area as well as the angle of solidification fronts. The values were determined as the mean value of five pictures from each of the
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three repetition of every experiment. Fig. 2 shows the geometry of the specimens and area which was observed by the thermographic camera as well as a schematic drawing of the measured values from the melt pool geometry. 3. Results and Discussion 3.1. Influence of oscillation frequency Besides the oscillation amplitude and the oscillation pattern, the beam velocity along the oscillation paths is determined by the oscillation frequency. As the energy per unit length is kept constant for the experiments, the oscillation frequency influences mainly the distribution of the laser energy over the process zone. Consequently the variation of the melt pool shape is reduced, which is indicated by a smaller indention of the melt pool within the circles in Fig. 3. Accordingly, for a lower frequency of 40 Hz, the shape of the front area of the melt pool is significantly influenced by the position of the laser beam, whereas the shape of the melt pool is stabilized by a more homogeneous energy input at higher frequencies. Moreover the size of the melt pool increases while the temperature gradient is reduced (recognizable by larger lighter areas behind the melt pool) with increasing oscillation frequency. Thus, for the examined case of a full penetration weld, the proportion of absorbed energy is increased by a direct irradiation of the surface at higher frequencies compared to a heat transport to the non-irradiated areas within the oscillation path by heat conduction at lower frequencies.
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higher linear welding velocities, as the total amount of irradiated areas and absorbed energy is increased. Consequently the area of the melt pool surface in Fig. 4 c) also increases for higher oscillation frequencies and linear welding velocities. Within the heat affected zone, these effects also induce higher strains as well as greater strain rates for higher oscillation frequencies and linear welding velocities. In contrast, the angle of solidification fronts in the rear area of the melt pool in Fig. 4 d) decreases with increasing linear welding velocities as a consequence of the elongation and narrowing of the melt pool. Similar to the melt pool length, the effect is increasing with oscillation frequencies. Besides the melt pool geometry, also other process characteristics are influenced by the oscillation frequency. As shown by [16] higher welding velocities result in an enhanced spatter formation due to a higher inclination angle of the keyhole as shown in Fig. 5. This theory can be confirmed for laser beam welding with beam oscillation. In this case besides the linear welding velocity also the oscillation frequency influences the formation of spatters. Accordingly, during the experiments a higher beam velocity at higher frequencies also increases the amount and size of spatters.
Fig. 4. Measured geometry next to the waist of the specimens for three different oscillation frequencies. Parameters: PL: 0.90 kW, 3.15 kW, 5.40 kW; Pat: circle; A: 0.5 mm; a: 1.5 mm; dFB: 100 μm; dF: 170 μm; λL: 1,030 nm; Material: High-strength-low-alloy steel; d: 1.8 mm; Double-Fan-Shaped specimens; recording parameters: tI: 0.01 ms; fC: 1 kHz Fig. 3. Weld pool shape next to the waist of the specimens at three different oscillation frequencies. Parameters: PL: 3.15 kW; vL: 35 mm/s; Pat: circle; A: 0.5 mm; a: 1.5 mm; dFB: 100 μm; dF: 170 μm; λL: 1,030 nm; Material: Highstrength-low-alloy steel; d: 1.8 mm; Double-Fan-Shaped specimens; recording parameters: tI: 0.01 ms; fC: 1 kHz
Since for a constant oscillation frequency the non-irradiated areas become larger with increasing linear welding velocities, the effect of oscillation frequency is stronger for higher linear welding velocities. This is confirmed by measurement results for the melt pool geometry shown in Fig. 4. Here the decrease of the width of the melt pool in Fig. 4 a), which is caused by a higher temperature gradient and less time for heat conduction at higher linear welding velocities, is reduced in case of higher oscillation frequencies. Furthermore higher oscillation frequencies extend the length of the melt pool in Fig. 4 b) at
Fig. 5. Schematic drawing of spatter formation at different welding velocities according to [16]
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Vincent Mann et al. / Procedia CIRP 74 (2018) 470–474 Author name / Procedia CIRP 00 (2018) 000–000
3.2. Influence of focal diameter The intensity of the laser beam is related to the square of the focal diameter. Thus process characteristics at a constant energy per unit length significantly depend on the focal diameter. Therefore larger focal diameters usually result in reduced penetration depths, higher inclination angles of the keyhole and keyhole diameters as well as shallow and wide weld seams. In the investigated case of full penetration welds in thin sheets, higher inclination angles and larger keyhole diameters can increase the amount of absorbed energy up to a certain limit, as the proportion of transmitted energy is reduced. This is confirmed by the melt pool geometries shown in Fig. 6. Here a comparison between focal diameters of 170 µm and 255 µm reveals an increase of melt pool length and width. Besides this, the length of the parallel flanks in the center area of the melt pool is reduced, whereas the angle of solidification fronts stays constant. A further increase of the focal diameter leads to a melt pool, which is even smaller than the melt pool with a focal diameter of 170 µm. As this behavior is confirmed by several experiments also for other frequencies, there seems to be a fundamental change in absorption conditions, as soon as the focal diameter is increased from 255 µm to 340 µm. A reason for this is the higher transmission for larger focal diameters, as the ratio of keyhole diameter and the inclination angle of the keyhole is changed. According to that, a constant level of absorption for a larger keyhole diameter can only be achieved, if the inclination angle increases similarly to the same extent, which was not the case for the change from 255 µm to 340 µm.
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angle of solidification fronts at higher welding velocities is stopped for the focal diameters of 255 µm and 340 µm, whereas the decrease continues for 170 µm focal diameter. As a result the strains in the heat affected zone will be highest for a focal diameter of 255 µm, so that in combination with a larger angle of solidification fronts a focal diameter of 340 µm seems to be beneficial with regard to the formation of hot cracks.
Fig. 7. Measured geometry next to the waist of the specimens for three different focal diameters. Parameters: PL: 0.90 kW, 3.15 kW, 5.40 kW; Pat: circle; A: 0.5 mm; f: 60 Hz; a: 1.5 mm; λL: 1,030 nm; Material: Highstrength-low-alloy steel; d: 1.8 mm; Double-Fan-Shaped specimens; recording parameters: tI: 0.01 ms; fC: 1 kHz; a): melt pool width; b): melt pool length; c): melt pool area; d): angle of solidification fronts
4. Conclusion
Fig. 6. Weld pool shape next to the waist of the specimens for three different focal diameters. Parameters: PL: 0.90 kW; vL: 35 mm/s; Pat: circle; A: 0.5 mm; f: 60 Hz; a: 1.5 mm; λL: 1,030 nm; Material: High-strength-low-alloy steel; d: 1.8 mm; Double-Fan-Shaped specimens; recording parameters: tI: 0.01 ms; fC: 1 kHz
As shown in Fig. 7, the influence of focal diameter is decreasing with increasing linear welding velocity. Accordingly, only the melt pool width of the experiments with a focal diameter of 170 µm and linear welding velocity 60 mm/s in Fig. 7 a) differs in comparison to the melt pool width of the other focal diameters. This is caused by a lower inclination angle of the keyhole and the accompanying lower amount of energy which is lost by spatters and is also applicable for the melt pool length in Fig. 7 b) and the melt pool area in Fig. 7 c). In consequence the decrease of the
The aim of this paper was the investigation of the influence of the focal diameter and the oscillation frequency on melt pool geometry and temperature field during laser beam welding of high strength steels with beam oscillation. Here, the variation of the melt pool geometry during the process is compensated by means of higher oscillation frequencies. Moreover increasing oscillation frequency also supports the formation of spatters. Simultaneously for higher frequencies the melt pool geometry is changed towards longer and narrower melt pools, which can be critical with regard to the hot cracking susceptibility. The effect of the focal diameter depends significantly on the linear welding velocity and the oscillation frequency. Here the ratio of the keyhole diameter and the inclination angle of the keyhole determine the portion of absorbed energy and consequently the melt pool geometry. Within the investigated parameter field, the middle focal diameter of 255 µm seems to be disadvantageous with regard to the hot cracking susceptibility. Acknowledgements The presented results are accomplished with the project SQLaP, which is funded by the Bavarian Research Foundation. The authors want to thank the Foundation and the Bavarian State Government for funding.
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References [1] Chan C C. The State of the Art of Electric, Hybrid, and Fuel Cell Vehicles. Proceedings of the IEEE. Vol. 95, No. 4, April 2007. p. 704 – 718. [2] Kiani M, Gandikota I, Rais-Rohani M, Motoyama K. Design of lightweight magnesium car body structure under crash and vibration constraints. Journal of Magnesium and Alloys 2, 2014. p. 99 – 108. [3] Sutar V, Dharankar C S, Thirupathi Raju B. High Strength Steel for Automotive Applications. International Research Journal of Engineering and Technology (IRJET), Volume: 03 Issue: 05. May 2016. p. 966 – 968. [4] Weidinger P, Günther K, Fitzel M, Logvinov R, Ilin A, Ploshikhin V, Hugger F, Mann V, Roth S, Schmidt M. Testing of new materials and computer aided optimization of process parameters and clamping device during predevelopment of laser welding processes. 8th International Conference on Photonic Technologies LANE 2014. Physics Procedia 56. 2014. p. 487 – 496 [5] Cross C E. On the Origin of Weld Solidification Cracking, Hot cracking phenomena in welds (Böllinghaus, T., Herold, H.), Springer Verlag, Berlin, Heidelberg. 2005. p. 3. [6] Stritt P. Prozessstrategien zur Vermeidung von Heißrissen beim RemoteLaserstrahlschweißen von AlMgSi 6016: Doctoral Thesis. University of Stuttgart. Institut für Strahlwerkzeuge (IFSW) 2016. [7] Katayama S. Solidification phenomena of weld metals (1st report). Characteristic solidification morphologies, microstructures and solidification theory. Welding International. 2000. p. 939. [8] Fetzer F, Jarwitz M, Stritt P, Weber R, Graf T. Fine-tuned remote laser welding of aluminum to copper with local beam oscillation. 9th International Conference on Photonic Technologies, Physics Procedia 83, 2016. p. 455.
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[9] Müller A, Goecke S-F, Albert F, Sievi P, Rethmeier M. Laser beam oscillation strategies for fillet welds in lap joints. 8th International Conference on Photonic Technologies LANE 2014. Physics Procedia 56. 2014. p. 458 – 466 [10] Seefeld T, Schultz V. New developments in filler wire assited laser beam joining of aluminum. Proceedings of LAMP2013 - the 6th International Congress on Laser Advanced Materials Processing. 2013. p. 1 – 7. [11] Choi K-D, Ahn Y-N, Kim C. Weld strength improvement for Al alloy by using laser weaving method. Journal of Laser Applications (22/2010). 2010. p.116-119 [12] Kraetzsch M, Standfuss J, Klotzbach A, Kaspar J, Brenner B, Beyer E. Laser Beam Welding with High-Frequency Beam Oscillation:Welding of Dissimilar Materials with Brilliant Fiber Lasers. Lasers in Manufacturing Conference 2011. Physics Procedia 12. 2011. p. 142–149 [13] Schweier M, Heins J F, Haubold M W, Zaeh M F. Spatter formation in laser welding with beam oscillation. Lasers in Manufacturing Conference 2013. Physics Procedia 41. 2013. p. 20 – 30. [14] Mann V, Holzer M, Hofmann K, Korbacher A, Roth S, Weidinger P, Schmidt M. Influence of oscillation parameters on melt pool geometry and hot cracking susceptibility during laser beam welding of high strength steels. Lasers in Manufacturing Conference 2017. Contribution 95. 2017. p. 1 – 11. [15] Matsuda F, Nakata K. A New Test Specimen for Self Restraint Solidification Crack Susceptibility Test of Electron Beam Welding Bead, Welding Research Institute of Osaka University, Osaka. 1982. [16] Weberpals J P. Nutzen und Grenzen guter Fokussierbarkeit beim Laserschweißen. Doctoral Thesis. University of Stuttgart. Institut für Strahlwerkzeuge (IFSW) 2010.
ScienceDirect
Available online atonline www.sciencedirect.com Available at www.sciencedirect.com
ScienceDirect ScienceDirect
Procedia CIRP 00 (2018) 000–000
www.elsevier.com/locate/procedia
Procedia CIRP 00 (2017) 000–000 Procedia CIRP 74 (2018) 470–474 www.elsevier.com/locate/procedia th 10 CIRP 10th CIRPConference Conferenceon onPhotonic PhotonicTechnologies Technologies [LANE [LANE 2018] 2018]
Influence of oscillation frequency diameter 28th CIRP Design Conference,and Mayfocal 2018, Nantes, Franceon weld pool geometry and temperature field in laser beam welding of high strength A new methodology to analyze the functional and physical architecture of steels existing products for an assembly oriented product family identification Vincent Manna,*, Konstantin Hofmanna, Kerstin Schaumbergera, Tobias Weigerta, Stephan a,b Alain Etienne, Paul Stief *,a,bJean-Yves Dantan, SiadatRotha,c, Michael Schustera, Jan Hafenecker , Simon Hübner , Lisa Lipinskia,b,Ali Stephan a,b,c Schmidt École Nationale Supérieure d’Arts et Métiers, Arts et Métiers ParisTech, LCFC EA 4495, 4 Rue Augustin Fresnel, Metz 57078, France
a Bayerisches Laserzentrum GmbH (blz), Konrad-Zuse-Straße 2-6, 91052 Erlangen, Germany b * Corresponding Tel.:Technologies +33 3 87 37 (LPT), 54 30; Friedrich-Alexander-Universität E-mail address: [email protected] Institute ofauthor. Photonic Erlangen-Nürnberg, Konrad-Zuse-Straße 3-5, 91052 Erlangen, Germany c
Erlangen Graduate School in Advanced Optical Technologies (SAOT), Paul-Gordan-Straße 6, 91052 Erlangen, Germany
* Corresponding author. Tel.: +49-9131-97790-16 ; fax: +49-9131-97790-11. E-mail address: [email protected]
Abstract In today’s business environment, the trend towards more product variety and customization is unbroken. Due to this development, the need of Abstract agile and reconfigurable production systems emerged to cope with various products and product families. To design and optimize production systems as well as to choose the optimal product matches, product analysis methods are needed. Indeed, most of the known methods aim to As weld pool geometry and thermomechanical strains are known to affect formation of solidification cracks, the influence of a superimposed analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and beam oscillation on these characteristics is investigated for this paper. In this context the effects of the oscillation frequency and focal diameter nature of components. This fact impedes an efficient comparison and choice of appropriate product family combinations for the production on the weld pool and temperature field are determined by means of infrared thermography. As a result an increase of the weld pool size for system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to cluster larger focal diameters and a more even shape of the seam edges for higher frequencies can be identified. these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable © 2018 2018 The The Authors. Authors. Published Published by by Elsevier Ltd. Ltd. This This is is an an open open access access article article under under the CC BY-NC-ND BY-NC-ND license license © assembly systems. Based on Datum Elsevier Flow Chain, the physical structure of the productstheis CC analyzed. Functional subassemblies are identified, and (http://creativecommons.org/licenses/by-nc-nd/3.0/) (https://creativecommons.org/licenses/by-nc-nd/4.0/) a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the Bayerisches Laserzentrum Peer-review under under responsibility responsibility of of the Bayerisches Laserzentrum GmbH. GmbH. Peer-review similarity between product families the by providing design support to both, production system planners and product designers. An illustrative example of a nail-clipper is used to explain the proposed methodology. An industrial case study on two product families of steering columns of Keywords: laser beam welding; high strength steels; beam oscillation; weld pool geometry; infrared thermography; solidification conditions; temperature field thyssenkrupp Presta France is then carried out to give a first industrial evaluation of the proposed approach. © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the 28th CIRP Design Conference 2018.
1. Introduction
Keywords: Assembly; Design method; Family identification
Nowadays declining limits for the emission of CO2 and NOX, increasing costs for energy and the high demand for vehicles with a greater range [1] strengthen the need 1.electric Introduction for lightweight constructions in automotive manufacturing [2,3]. their fast high tensile strength in and the ductility and their DueDue to to the development domain of relatively low costs, high strength steels are suitable materials communication and an ongoing trend of digitization and for these applications. [3] During the welding process of these digitalization, manufacturing enterprises are facing important materials, the risk for the appearance of weld seam defects challenges in today’s market environments: a continuing such as pores and cracks is significantly higher [4] due the tendency towards reduction of product development timestoand higher alloying content in comparison to low strength steels. shortened product lifecycles. In addition, there is an increasing Especially the risk of hotbeing cracks is increased larger demand of customization, at the same timebyinthe a global temperature interval between solidus and liquidus temperature competition with competitors all over the world. This trend, [5] caused by thethedifferent meltingfrom temperatures the which is inducing development macro to of micro alloying elements. In contrast to laser beam welding of markets, results in diminished lot sizes due to augmenting aluminum, steels are often welded without filler wire and product varieties (high-volume to low-volume production) [1].
shielding gas by means of fast scanner optics, which enable shorter positioning times. [6] Hence a reduction of hot cracking susceptibility by the use of filler wires is undesirable. As the risk of hot cracking is also influenced by the solidification such as shape manufactured and size of theand/or melt of the productconditions, range and characteristics pool and the temperature field, a targeted modification of assembled in this system. In this context, the main challenge in these properties reduces hot cracking susceptibility without modelling and analysis is now not only to cope with single changing athe alloying composition use ofproduct filler materials. products, limited product range or by existing families, The underlying mechanism for this is a change in thetogrowth but also to be able to analyze and to compare products define direction of families. dendritesItduring solidification process in the new product can bethe observed that classical existing rear area of the melt pool. According to this theory, dendrites product families are regrouped in function of clients or features. which areassembly directed oriented to the middle offamilies the weldareseam induce an However, product hardly to find. agglomeration of low melting alloying elements in the center On the product family level, products differ mainly in two of thecharacteristics: weld seam and, to the shrinkage of the the main (i) in theconsequence number of components and (ii) cooling material, center line cracks. In contrast, dendrites type of components (e.g. mechanical, electrical, electronical). which are oriented in welding directionmainly enable single a backfilling of Classical methodologies considering products appearing cavities by the melt from the melt pool. In this or solitary, already existing product families analyze the
To cope with this augmenting variety as well as to be able to product structure on a physical level (components level) which 2212-8271 possible © 2018 Theoptimization Authors. Published by Elsevier is an opencauses access article under theregarding CC BY-NC-ND license identify potentials in Ltd. the This existing difficulties an efficient definition and (http://creativecommons.org/licenses/by-nc-nd/3.0/) production system, it is important to have a precise knowledge comparison of different product families. Addressing this Peer-review under responsibility of the Bayerisches Laserzentrum GmbH.
2212-8271 © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0/) 2212-8271 © 2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of scientific the Bayerisches Laserzentrum GmbH. Peer-review under responsibility of the committee of the 28th CIRP Design Conference 2018. 10.1016/j.procir.2018.08.148
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context a decrease of welding velocity is known to significantly change the melt pool from a tear drop shaped melt pool with an acute angle of the solidification fronts to an oval shaped melt pool with an obtuse angle of solidification fronts. [7] Since a lower welding velocity decreases the productivity of the welding process, there is a need for other strategies for manipulating the shape of the melt pool without changing properties of the weld seam. For this purpose a superimposed beam oscillation provided by the combination of brilliant beam sources and fast scanner optics seems to be an appropriate tool for a manipulation of the melt pool shape. In this context it is already proven that beam oscillation influences the width of the weld seam and the penetration depth in welding aluminum to copper in lap joint configuration [8], as well as the gap bridgeability [9] and surface roughness of the weld seam in laser beam welding of aluminum alloys. [10] Furthermore a sinusoidal oscillation at very low oscillation frequencies (< 5 Hz) can reduce hot cracking susceptibility, if the melt pool is small enough to follow the oscillation pattern. In this case, the main directions of shrinkage and solidification are not parallel as for laser beam welding without beam oscillation, but perpendicular which decreases crack initiation strains. [11] In contrast, shape and size of the keyhole can be modified by very high oscillation frequencies (> 2 kHz) [12] in order to reduce spatters [13]. Nevertheless there are only few results [14] for laser beam welding with smaller oscillation frequencies (< 100 Hz), for which a modification of the weld pool geometry is possible while keeping similar properties of the weld seam. Moreover the effect of the focal diameter is not specified up to now. Thus the effect of these parameters on melt pool properties and temperature field is presented within this paper. 2. Experimental setup The experiments for this paper were carried out with a TruDisk 6001 with a maximum laser power of 6 kW, a wavelength (λL) of 1,030 nm and a beam parameter product of 4 mm x mrad. The focusing and movement of the beam is realized by a PFO33-2-scanner optics from Trumpf with a collimation length of 150 mm and a focusing length of 255 mm leading to a magnification factor of 1.7. In order to gain different focal diameters, optical fibers with three core diameters (dFB) of 100 µm, 150 µm and 200 µm are used subsequently yielding focal diameters (dF) of 170 µm, 255 µm and 340 µm in the focal plane of the beam. As no additional axle was used, the linear movement of the beam and the superimposed beam oscillation were realized by the scanner optics solely. The figure overlap was chosen to be zero and a circular oscillation pattern (Pat) was used for all experiments. For process monitoring a thermographic camera from IRCAM was installed in a lateral position next to the scanner optics. The experimental setup is shown in Fig. 1. The sensor of the thermographic camera detects IRradiation in the spectra from 3 µm to 4.8 µm and has a maximum spatial resolution of 320 x 256 pixels at a pixel length of 78 µm. The maximum frame rate (fC) was 1 kHz at a integration time (tI) of 0.01 ms.
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Fig. 1. Experimental setup for welding experiments and thermographic process monitoring
For the experiments, the focal plane of the laser beam was positioned on the surface of the specimens and the energy per unit length (E) was kept constant on a level of 90 J/mm. The weld seams are positioned with a lateral distance (a) of 1.5 mm from the symmetry plane of the specimens in order to gain a non-symmetric temperature field. Further process parameters are listed in Table 1. Table 1. Welding and oscillation parameters used for the experiments Parameter
Symbol
Tested range
Unit
Laser power
PL
0.90; 3,15; 5.40
kW
Linear welding velocity
vL
10; 35; 60
mm/s
Oscillation frequency
f
40; 60;80
Hz
Oscillation amplitude
A
0.5
mm
Beam velocity
vB
86 – 222
mm/s
The material used for investigations was a high strength low alloy steel with a tensile strength of 700 N/mm² and an elongation at break of 10 %. The thickness (d) of the sheets was 1.8 mm. The specimens were laser cut to Double-FanShaped specimens [14] which were derived from the FanShaped test according to [15]. All welds were carried out as full-penetration bead-on-plate welds.
Fig. 2. Geometry of the self-restrained hot cracking test specimens and schematic drawing of the measured values of the melt pool geometry
By means of the thermographic videos the melt pool was characterized by the melt pool length, width and area as well as the angle of solidification fronts. The values were determined as the mean value of five pictures from each of the
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three repetition of every experiment. Fig. 2 shows the geometry of the specimens and area which was observed by the thermographic camera as well as a schematic drawing of the measured values from the melt pool geometry. 3. Results and Discussion 3.1. Influence of oscillation frequency Besides the oscillation amplitude and the oscillation pattern, the beam velocity along the oscillation paths is determined by the oscillation frequency. As the energy per unit length is kept constant for the experiments, the oscillation frequency influences mainly the distribution of the laser energy over the process zone. Consequently the variation of the melt pool shape is reduced, which is indicated by a smaller indention of the melt pool within the circles in Fig. 3. Accordingly, for a lower frequency of 40 Hz, the shape of the front area of the melt pool is significantly influenced by the position of the laser beam, whereas the shape of the melt pool is stabilized by a more homogeneous energy input at higher frequencies. Moreover the size of the melt pool increases while the temperature gradient is reduced (recognizable by larger lighter areas behind the melt pool) with increasing oscillation frequency. Thus, for the examined case of a full penetration weld, the proportion of absorbed energy is increased by a direct irradiation of the surface at higher frequencies compared to a heat transport to the non-irradiated areas within the oscillation path by heat conduction at lower frequencies.
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higher linear welding velocities, as the total amount of irradiated areas and absorbed energy is increased. Consequently the area of the melt pool surface in Fig. 4 c) also increases for higher oscillation frequencies and linear welding velocities. Within the heat affected zone, these effects also induce higher strains as well as greater strain rates for higher oscillation frequencies and linear welding velocities. In contrast, the angle of solidification fronts in the rear area of the melt pool in Fig. 4 d) decreases with increasing linear welding velocities as a consequence of the elongation and narrowing of the melt pool. Similar to the melt pool length, the effect is increasing with oscillation frequencies. Besides the melt pool geometry, also other process characteristics are influenced by the oscillation frequency. As shown by [16] higher welding velocities result in an enhanced spatter formation due to a higher inclination angle of the keyhole as shown in Fig. 5. This theory can be confirmed for laser beam welding with beam oscillation. In this case besides the linear welding velocity also the oscillation frequency influences the formation of spatters. Accordingly, during the experiments a higher beam velocity at higher frequencies also increases the amount and size of spatters.
Fig. 4. Measured geometry next to the waist of the specimens for three different oscillation frequencies. Parameters: PL: 0.90 kW, 3.15 kW, 5.40 kW; Pat: circle; A: 0.5 mm; a: 1.5 mm; dFB: 100 μm; dF: 170 μm; λL: 1,030 nm; Material: High-strength-low-alloy steel; d: 1.8 mm; Double-Fan-Shaped specimens; recording parameters: tI: 0.01 ms; fC: 1 kHz Fig. 3. Weld pool shape next to the waist of the specimens at three different oscillation frequencies. Parameters: PL: 3.15 kW; vL: 35 mm/s; Pat: circle; A: 0.5 mm; a: 1.5 mm; dFB: 100 μm; dF: 170 μm; λL: 1,030 nm; Material: Highstrength-low-alloy steel; d: 1.8 mm; Double-Fan-Shaped specimens; recording parameters: tI: 0.01 ms; fC: 1 kHz
Since for a constant oscillation frequency the non-irradiated areas become larger with increasing linear welding velocities, the effect of oscillation frequency is stronger for higher linear welding velocities. This is confirmed by measurement results for the melt pool geometry shown in Fig. 4. Here the decrease of the width of the melt pool in Fig. 4 a), which is caused by a higher temperature gradient and less time for heat conduction at higher linear welding velocities, is reduced in case of higher oscillation frequencies. Furthermore higher oscillation frequencies extend the length of the melt pool in Fig. 4 b) at
Fig. 5. Schematic drawing of spatter formation at different welding velocities according to [16]
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3.2. Influence of focal diameter The intensity of the laser beam is related to the square of the focal diameter. Thus process characteristics at a constant energy per unit length significantly depend on the focal diameter. Therefore larger focal diameters usually result in reduced penetration depths, higher inclination angles of the keyhole and keyhole diameters as well as shallow and wide weld seams. In the investigated case of full penetration welds in thin sheets, higher inclination angles and larger keyhole diameters can increase the amount of absorbed energy up to a certain limit, as the proportion of transmitted energy is reduced. This is confirmed by the melt pool geometries shown in Fig. 6. Here a comparison between focal diameters of 170 µm and 255 µm reveals an increase of melt pool length and width. Besides this, the length of the parallel flanks in the center area of the melt pool is reduced, whereas the angle of solidification fronts stays constant. A further increase of the focal diameter leads to a melt pool, which is even smaller than the melt pool with a focal diameter of 170 µm. As this behavior is confirmed by several experiments also for other frequencies, there seems to be a fundamental change in absorption conditions, as soon as the focal diameter is increased from 255 µm to 340 µm. A reason for this is the higher transmission for larger focal diameters, as the ratio of keyhole diameter and the inclination angle of the keyhole is changed. According to that, a constant level of absorption for a larger keyhole diameter can only be achieved, if the inclination angle increases similarly to the same extent, which was not the case for the change from 255 µm to 340 µm.
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angle of solidification fronts at higher welding velocities is stopped for the focal diameters of 255 µm and 340 µm, whereas the decrease continues for 170 µm focal diameter. As a result the strains in the heat affected zone will be highest for a focal diameter of 255 µm, so that in combination with a larger angle of solidification fronts a focal diameter of 340 µm seems to be beneficial with regard to the formation of hot cracks.
Fig. 7. Measured geometry next to the waist of the specimens for three different focal diameters. Parameters: PL: 0.90 kW, 3.15 kW, 5.40 kW; Pat: circle; A: 0.5 mm; f: 60 Hz; a: 1.5 mm; λL: 1,030 nm; Material: Highstrength-low-alloy steel; d: 1.8 mm; Double-Fan-Shaped specimens; recording parameters: tI: 0.01 ms; fC: 1 kHz; a): melt pool width; b): melt pool length; c): melt pool area; d): angle of solidification fronts
4. Conclusion
Fig. 6. Weld pool shape next to the waist of the specimens for three different focal diameters. Parameters: PL: 0.90 kW; vL: 35 mm/s; Pat: circle; A: 0.5 mm; f: 60 Hz; a: 1.5 mm; λL: 1,030 nm; Material: High-strength-low-alloy steel; d: 1.8 mm; Double-Fan-Shaped specimens; recording parameters: tI: 0.01 ms; fC: 1 kHz
As shown in Fig. 7, the influence of focal diameter is decreasing with increasing linear welding velocity. Accordingly, only the melt pool width of the experiments with a focal diameter of 170 µm and linear welding velocity 60 mm/s in Fig. 7 a) differs in comparison to the melt pool width of the other focal diameters. This is caused by a lower inclination angle of the keyhole and the accompanying lower amount of energy which is lost by spatters and is also applicable for the melt pool length in Fig. 7 b) and the melt pool area in Fig. 7 c). In consequence the decrease of the
The aim of this paper was the investigation of the influence of the focal diameter and the oscillation frequency on melt pool geometry and temperature field during laser beam welding of high strength steels with beam oscillation. Here, the variation of the melt pool geometry during the process is compensated by means of higher oscillation frequencies. Moreover increasing oscillation frequency also supports the formation of spatters. Simultaneously for higher frequencies the melt pool geometry is changed towards longer and narrower melt pools, which can be critical with regard to the hot cracking susceptibility. The effect of the focal diameter depends significantly on the linear welding velocity and the oscillation frequency. Here the ratio of the keyhole diameter and the inclination angle of the keyhole determine the portion of absorbed energy and consequently the melt pool geometry. Within the investigated parameter field, the middle focal diameter of 255 µm seems to be disadvantageous with regard to the hot cracking susceptibility. Acknowledgements The presented results are accomplished with the project SQLaP, which is funded by the Bavarian Research Foundation. The authors want to thank the Foundation and the Bavarian State Government for funding.
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