New refractive beam shaping methods for laser materials processing

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Joerg Volpp is an associate professor and Adrien da Silva is a PhD student at the Luleå University of Technology.

Joerg Volpp and Adrien da Silvaat the Luleå University of Technology, investigate beam shaping possibilities for high-power processing

Co-authored by Alexander Laskin, from AdlOptica

Beam shaping technologies open up new possibilities and processing options for controlling heat input during laser materials processing. For high-power applications, beam shaping is usually done through either beam oscillation or by manipulating the spatial intensity distribution of the beam in one layer to create, for example, several laser spots, tophat profiles or donut profiles.

Refractive optics for high-power laser applications

Refractive beam shaping, which uses either mirrors or lenses, enables nearly lossless laser energy transfer through refractive optics, with a high flexibility of energy distribution in one plane or along the beam axis, for example between single laser spots, which has been shown to improve process quality.

Recently AdlOptica’s QuattroXX and FoXXus refractive optics were installed and tested at the laser laboratory at Luleå University to explore the effects of beam shaping on laser material processing at high power. Both optics are capable of splitting a laser beam into up to four separate beams. The QuattroXX produces spots beside each other, while the FoXXus enables manipulation of the beam waist distribution along the beam axis (see figure 1). For both optics, the distance between the spots is defined by the optical setup, while the intensity in each spot can be manually changed by rotating the optical elements relative to each other.

Figure 1: Optics and beam measurements of up to four beams next to each other (top, QuattroXX) and along the beam axis (bottom, FoXXus)

Bridging the gap

A widely known challenge in laser welding is gap bridgeability. Due to its typically small dimensions, a high intensity laser beam can transmit through the gap between the joining partners. This leads to a loss of energy and a reduced energy input for the welding process, which can be interrupted or lead to a reduced penetration depth, lack of fusion and thereby reduced mechanical properties.

Using the beam shapes enabled by the QuattroXX, it was possible to increase the bridgeable gap width from 0.1 to > 0.4mm in the used setup. Two dual-phase steel sheets (DP800 and DP1000 at 1.1mm thickness) were joined in butt joint configuration at a laser power of 3kW (15kW IPG fibre laser) and a processing speed of 3m/min, welded in the focal position of the beam.

As the energy input can be shifted to the joining partners without gap transmission losses, the optic’s additional feature of varying energy densities between individual laser spots also opens up new possibilities in joining dissimilar 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 the depth of the vapour channel leads to a more stable process (figure 2).

Figure 2: High-speed images (top) and vapour channel appearance (bottom) at different beam shapes produced with FoXXus with several beam waists along the optical axis: a) two distant beam waists, b) four beam waists and c) two close beam waists.

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.

Joerg Volpp is an associate professor and Adrien da Silva is a PhD student at the Luleå University of Technology. 

The gap bridging experiment in this article was conducted by Stephanie Robertson and Alexander Kaplan.

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