ANALYSIS: BEAM SHAPING
materials by tailoring the beam to their boundary conditions. Such as by applying a higher energy input into the material with a higher melting point. These first tests using the same processing parameters, as described above, indicate that tailoring the heat input even improves the joining of sheets with different thicknesses by guiding the melt and enabling robust bonding. This effect was shown for a DP800 (2mm thickness) and a DP1000 (1.1mm thickness) configuration, while the focal position of the laser beam was positioned on the material surface of the DP800 sheet. In addition, the distribution of laser energy into several spots next to each other was found to enable a reduction of spatter in the deep penetration welding of steel sheets. Spattering can occur when the metal vapour exiting from the vapour channel detaches melt drops from the surrounding melt pool. The wider opening of the vapour
channel using multiple beams can lead to a decreased melt detachment probability, and therefore a higher quality weld seam and less solidified spatters attached on the welded parts. The creation of up to four beam waists along the optical axis using the FoXXus optic denotes a quasi-elongated depth of field. The experimental setup consisted of a beam created by a 15kw IPG fibre laser, guided by a 100µm diameter fibre to create the several foci with a focusing lens of 200mm focal length. A laser power of 5kW at a processing speed of 7.5m/min, including argon gas shielding, was applied. Experimenting with both titanium and steel alloy sheets of 1mm thickness indicated the positioning of the laser beam waists influences spattering, while the created wider keyhole opening supports spatter suppression. It is assumed the more homogeneous energy input into
“Beam shaping offers a new promising parameter for laser process optimisation”
the depth of the vapour channel leads to a more stable process (figure 2).
An additional feature of
the FoXXus laser beam is the creation of multiple different spatial intensity distributions along the beam axis. In the beam waists, the fibre end is visible as a tophat profile. However, the central tophat profile is superpositioned by the other beams, which leads to additional intensity around the central spot. Such beam intensity distributions are assumed to support welding, cladding and additive processes in the future.
Outlook Refractive beam shaping offers the possibility to produce several tailored beam shapes at low cost. We have identified fields of applications for refractive beam shaping at high laser power, which offers a promising parameter for laser process optimisation. High potential applications are seen in additive manufacturing, to reduce process dynamics in powder bed processes or increase the process efficiency in directed energy deposition by tailoring the melt pool dimensions. Spatter reduction and increased gap bridgeability were achieved when welding steel and titanium sheets at high laser power. However, the findings indicate that the effects can be reproduced and transferred to further materials and process dimensions. l
Joerg Volpp is an associate professor and Adrien da Silva is a PhD student at the Luleå University of Technology
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