In addition to conventional welding, laser-arc hybrid welding processes have recently been used in the maritime sector for existing joining tasks. When steel plate thicknesses exceed 12mm, however, no new welding processes have been established, leaving considerable need for innovative, efficient joining technologies.
The introduction of a high-power diode laser beam source with an optical output power of up to 60kW (LDF 60.000-200, Laserline) creates promise for using new, robust laser welding processes for high-quality joints in the thick sheet range up to 30mm at high speeds. Such processes are now being developed through close cooperation between industrial and scientific partners1.
In tandem with developing such processes, a concept for ensuring laser safety has to be determined that will adequately protect personnel in the vicinity of the operation. In particular, the strong scattered radiation from the laser process zone has to be considered2 to comply with the exposure limit values not only for the eye (ELVeye), but also for the skin (ELVskin), according to Directive 2006/25/EC.
The extent and spatial distribution of the scattered radiation emission has therefore been determined3, and the thermal load of the vicinity of the process zone estimated4 using a simple mathematical model, yielding distance-dependent average temperatures of the material of the local shielding. Here we have derived and provided recommendations for the design of the protective enclosure.
Welding and measurements
To analyse the radiation scattered from the process zone, a set-up with five photodiodes (measuring range 900-1,700nm) was used, enabling a systematic measurement as a function of the observation direction, as well as depending on the distance from the process zone (see figure 1).
Figure 1: Set-up with five photodiodes, represented by the blue objects, for scattered radiation measurements during welding2
Typically, an output power of 56kW was used to achieve optimised process conditions, corresponding to deep-penetration welding with a very large weld pool due to the large focal diameter of 4mm obtained with the optical configuration1.
Exemplarily, figure 2 shows the results of scattered radiation measurements for butt welding trials with filler wire (wire diameter 1.2mm) on 22mm-thick, surface-milled steel plates with the following process parameters: laser beam power 56kW, feed rate 1m/min, wire feed rate 20m/min, and focal position 0mm (top of plate). In this case, a ceramic rail with a 6mm-wide groove without a powder bed served as the weld pool backing. To reduce the number of measurements, a symmetry plane was assumed perpendicular to the feed direction, which in fact represents a rough approximation.
Figure 2: Polar contour diagram to visualise the irradiance E emitted from a butt welding process (weld pool backing: ceramic without powder), using filler wire (process parameters in the text)2
To determine the total power PR of the laser radiation scattered from the process zone into the hemisphere above the sample, the measurement data for the scattered radiation at a distance of 500mm (as shown in figure 2) were used. PR was obtained by integrating the irradiance E measured as a function of polar angle θ and azimuth angle φ. Since only discrete E values were measured, the integration is approximated by a weighted summation of the values. The total power PR for the selected process was calculated in this way to be 4,605W.
Temperature evolution of a potential shielding
With known PR values, the average temperature Tav of a hemispherical protective enclosure can be estimated as a function of the process time t for a defined protective shielding material. The basis of this estimation is the 3D heat equation, taking into account the radiation from a uniformly emitting point source, namely the welding process zone, in the centre of the sphere, as well as the temperature-dependent heat radiation emission from the two surfaces of the shielding with the emissivity values εin and εout. Ignoring heat conduction and heat transfer to the surrounding gas phase as energy dissipation mechanisms, the 3D heat equation can be solved analytically4.
Figure 3: Evolution of the temperature Tav of a hemispherical shielding as a function of the irradiation time t for four different powers PR, incident from a uniformly emitting point source in the centre of the sphere, representing the laser process zone2. Aluminium, thickness s = 2mm, melt temperature TM = 660°C, density ρ = 2.70g/cm³, specific heat capacity cs= 897J/(kg K), emissivity εin = εout = 0.2
For the calculation, a hemispherical shell made of 2mm-thick aluminium with an inner radius r = 500mm, corresponding to a diameter d = 1.0m was assumed. The result is shown in figure 3 for four values of the scattered radiative power PR. The dashed lines in the diagram illustrate the expected linear heating when all energy dissipation mechanisms are ignored – that is, also heat radiation. In this model, the averaged limit temperature Tmax is dependent only on the diameter of the shielding d, the scattered power PR, the emissivity values εin and εout, and the starting temperature T0, as given by the following equation:
The material properties’ specific heat capacity, density, surface emissivity values as well as material thickness determine the speed of the resulting temperature rise.
According to this model, the melting temperature of the aluminium is not reached by far. For PR = 4,605W, an averaged maximum temperature of Tmax = 154°C is obtained under the given boundary conditions. For instance, an increase of the dimensions of the protective shielding or the emissivity on the outside of the shielding, or a decrease of the emissivity on the inside of the shielding, can contribute to a reduction in the resulting thermal load.
Recommendations for safety measures
It is to be expected that temperatures critical to the components of the protective shielding can be reached during processing. Thus, the need for active cooling of the shielding – for example, water cooling – arises. Furthermore, sensitive parts within the enclosure – including the laser processing head – must be carefully selected and protected, for example, by special heat protection covers to avoid overheating and any resulting damages. In the end, an effective dissipation of the thermal energy introduced into the protective shielding can also serve to protect employees in the work area against thermally induced skin burns upon direct contact, or due to intense heat radiation.
Due to the size and weight of the steel plates to be welded, the manufacturing hardware containing the laser processing head, as well as the local shielding, has to be moved over the parts. Consequently, it is unavoidable that a residual gap remains below the shielding, which must be kept as small as technically possible. It is recommended to block any escaping scattered radiation by bristles made of carbon fibres, for example, which are thermally stable and can be drawn over the steel plates. In addition, the probability of stray radiation escaping from the enclosure can be reduced by a sheet metal skirt with high surface roughness at the lower end of the shielding.
In summary, the authors believe welding using high-power diode laser radiation, with up to 60kW of power, can be realised safely in terms of occupational health and safety by implementing the measures described. The required development work is currently being planned by the partners involved.
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Michael Hustedt and Oliver Seffer are research assistants at the LZH.
Alexander Hilck is the Niedersachsen Additiv project leader at the LZH.
This article was co-authored by Marius Lammers, Sarah Nothdurft, Jörg Hermsdorf and Stefan Kaierle, of the Laser Zentrum Hannover
Acknoweledgements
Main parts of the work were carried out in the German joint project DIOMAR ‘Dickblechschweißen mittels Höchstleistungs-Diodenlaser für maritime Anwendungen (Thick sheet-metal welding using high-power diode lasers for maritime applications)’. This project was funded by the Federal Ministry for Economic Affairs and Climate Action (BMWK) within the framework of the funding line, ‘Maritimes Forschungsprogramm (Maritime Research Programme)’ of the German Federal Government (reference no. 03SX452B), which is gratefully acknowledged. The authors wish to thank their collaboration partners Meyer Werft, Held Systems Deutschland and Laserline for their good cooperation and valuable support.
References
[1] O Seffer, S Nothdurft, A Hilck, M Hustedt, J Hermsdorf, S Kaierle, ‘Investigations on Laser Beam Welding of Thick Steel Plates Using a High-Power Diode Laser Beam Source,’ in ‘Proc. 41st annual International Congress on Applications of Lasers & Electro-Optics (ICALEO®),’ Orlando, FL/USA, 17–20 October 2022, to be published.
[2] A Hilck, M Hustedt, O Seffer, S Nothdurft, J Hermsdorf, S Kaierle, ‘Emission of scattered radiation from the process zone of welding processes using high power diode lasers,’ in ‘12th CIRP Conference on Photonic Technologies (LANE 2022),’ Procedia CIRP 2022; to be published.
[3] SRJ Braunreuther, ‘Untersuchungen zur Lasersicherheit für Materialbearbeitungsanwendungen mit brillanten Laserstrahlquellen,’ dissertation, Technical University of Munich, Forschungsberichte IWB, Vol. 283, Herbert Utz Verlag, Munich, 2014, ISBN 978-3-8316-4348-6.
[4] M Hustedt, V Niedens, A Brodeßer, T Bauche, J Hermsdorf, S Kaierle, ‘Assessment of steel shields for protection against laser radiation,’ Journal of Laser Applications 2021; 33:042053 (13 pages), doi: 10.2351/7.0000519
