Increasing ultrafast laser texturing throughput using beam splitting optics

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Gwenn Pallier and Yoan Di Maio highlight the challenges of laser surface texturing and propose a strategy for increasing its efficiency

Surface functionalisation involves the manipulation of interfaces in order to optimise their properties, including: friction and wear coefficients; adhesion; hydrophobicity and heat transfer, etc. With the technique being applicable to both commercial and non-commercial products, it is gaining significant interest across a variety of applications and fields.

For example, in the medical sector, surfaces can be modified to better adhere to bone cells or repel bacteria, while in the energy sector the aerodynamic properties of wind turbine blades can be improved. In the defence sector, the optical properties of objects can even be modified for stealth purposes.

Laser surface texturing has demonstrated its ability to modify the properties of a wide range of surfaces on the micron scale, both without contact or the need for additional mechanical or chemical processes. This makes it a suitable, sustainable technology for use across a range of environments.

The challenges of laser surface texturing

Excellent quality surface texturing can be achieved using femtosecond lasers. Their ultrashort pulse duration means that the material is ablated almost instantaneously, thus greatly limiting the peripheral thermal effects of the machining process. While the texture quality achievable this way is considered sufficient by a large number of industrial manufacturers, delivering a practical level of throughput remains a significant challenge. Industrial applications require the texturing of very large surfaces extremely rapidly, which can not yet readily be addressed using the laser equipment currently available on the market. 

Figure 1: The setup used to split the beam of a 100W femtosecond laser in order to achieve parallel processing

Although increasingly powerful industrial lasers are becoming available, the use of higher-energy pulses alone does not improve texturing throughput performance. On the contrary, it either increases the thermally affected regions – thus degrading texturing quality – or reduces machining efficiency. Another strategy involves scanning the laser across surfaces much more quickly in order to spread the heat generated by the higher energy pulses, however in this case scanner speed becomes a limiting factor. 

One strategy that does look very promising involves splitting a high-energy laser beam into multiple, lower-energy sub-beams over a larger surface area in order to perform parallel machining. This enables all the available energy to be used during the process while maintaining an optimal energy level per sub-beam. This means that process quality can be preserved while exploiting the full potential of the higher-power ultrafast lasers becoming available.

Beam splitting on an industrial scale

As part of a collaboration between Cailabs and Manutech USD, a beam splitting system has been used to speed up an existing surface texturing process that produces surface cavities in stainless steel and nickel. These textures have previously demonstrated an increase of several orders of magnitude in the efficiency and lifetime of high-value parts by reducing their wear. 

Figure 2: Intensity profile in the split spot process plane

The beam from a 1,030nm, 100W Tangor laser from Amplitude, with a pulse duration of around 500fs, was split into five homogeneous beams by a Canunda-Split module from Cailabs. The five beams were then directed through a set of optics to a galvo scanner from Cambridge Technology (see Figure 1). The reflective operation of the Cailabs module enables it to optimally handle high power and energy while maintaining the duration of the femtosecond pulses. It can also be configured electronically to select several different beam splitting patterns.

Using an imaging system represented by lenses L3 and L4 in Figure 1, five homogeneous beams were generated with a diameter of 25µm and a pitch of 200µm in the machining plane (see Figure 2). The characterisation of the beams by an analyser in the working plane showed excellent beam uniformity, both in terms of energy (< 2.5%) and circularity (ellipticity < 5%). These qualities are essential for industrial applications, and in particular in the femtosecond regime where tolerances are even stricter as the pulse durations are used specifically for their precision at the micrometre and sub-micrometre scale. For this reason, beam mismatches are generally not tolerated and the degree of acceptance typically depends on the intended application, which is a major challenge for the market penetration of parallel processing modules such as Canunda-Split. 

Machining stainless-steel and nickel parts

Our first texturing experiment consisted of machining cavities on a 4 x 4mm stainless-steel surface (see Figure 3). The cavities were positioned 50µm apart, implying a pitch of the same value by the scanner, while the shaping points were spaced 200µm apart.

Figure 3: Microscopic view (left) and 2D section (right) of the textured stainless-steel sample

The texturing quality was well preserved in a multi-spot configuration, and so the homogeneity between the beams was largely sufficient and preserved in the addressed field of the F-Theta lens. This strategy provided a total time saving of a factor of 2.5 compared to single-spot machining in this application. The gain in terms of speed depends on the ratio between the time the laser is actually processing and the time the scanner is moving. For a given process, the longer the time the laser is drilling, the higher the speed gain will be with parallel processing. 

Figure 4: Nickel samples structured with grooves reconstructed digitally using optical microscopy. Top: Samples structured using a single spot with a 500x (left) and 1000x (right) objective lens. Bottom: Samples structured using multiple spots with a 500x (left) and 1000x (right) objective lens

Our second texturing experiment involved texturing grooves and cavities in nickel samples. For the grooves, the machined area was 1mm² with a constant pitch of 50µm. Once again the quality was well preserved by switching to a multi-spot configuration, as illustrated in Figure 4. For the cavities, the machining configuration was identical to that performed on stainless steel. Once again, excellent multi-spot uniformity was obtained, assessed by measuring the machining depths via confocal microscopy (see Figure 5).

Figure 5: Nickel samples structured with cavities reconstructed digitally using confocal microscopy. Top: Samples structured using a single spot with an average cavity depth of 4.98µm. Bottom: Samples structured using multiple spots with an average cavity depth of 3.98µm

It should be noted that with five-times more energy in the system to allow the beam to be split into five sub-beams, the machined depth was slightly shallower than with a single-spot configuration, which can be explained by additional losses along the optical chain. 

Conclusion

During this study a fully reflective beam shaping module was installed in an industrial set-up. It enabled powerful ultrafast laser pulses to be divided into sub-beams to conduct parallel processing, which enabled a 2.5x faster processing time than a single beam when texturing stainless-steel and nickel samples. Surface texturing quality was characterised using confocal microscopy by measuring the uniformity of the roughness generated. These results open up excellent opportunities for increasing the applicability of ultrafast laser surface texturing across a wide range of real industrial applications. 

Gwenn Pallier is a Product Line Manager for industrial applications at Cailabs.

Yoan Di Maio is a Research & Development Engineer at Manutech-USD.

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