By: Andreas Brenner, Fraunhofer-Institute for Laser Technology and Fabian Kurzidim, Volkswagen AG
Increasing demand for surface functionality
Surface functionality is an increasing and crucial factor for the success and acceptance of a product. Through structured surfaces products can gain additional functions. In the automotive industry for example microstructures enable friction reduction in combustion engines or optimize lighting efficiency in LED lights. A glance into a vehicle’s interior is enough to see that detailed structures convey the feeling of exclusivity. For a long time, leather was the dominant look. Now the preference is for fine, more technical structures.
Able to offer virtually unlimited precision, lasers are the right tool for the job. To ensure that productivity matches precision, development is under way on a machine that will be able to efficiently process even large surfaces thanks to a combination of two different pulse types.
Ultrashort pulse lasers have for many years been the tool of choice for processing microstructures. No matter what the material, ultrashort pulse lasers can ablate even in the micrometer range with high precision (cf. picture 1). The only catch is that it takes plenty of time concerning the industrial application.
Up to now, the answer has either been more laser power, faster scanning or splitting the laser beams into multiple beams. A new research and industry consortium is taking a different approach, with partners developing a laser machine that uses an ultrashort pulse laser only for the finest details. The rest of the work is previously done using a productive nanosecond laser.
State of the art versus new approach
The most common way to create surface functionality at 3D parts in automotive interior are replication processes via structured mold tools. Often used manufacturing processes like photochemical etching are limited in precision and in flexibility. The individual stages are repeated over and over, requiring processors with a feeling for finesse since they are not reproducible.
This work can also be done using lasers; especially nanosecond lasers achieve similar throughput rates to etching processes. But they tend to reach their limits when it comes to precision – they begin to melt the material leading to rough contours, especially for intricate structures.
Ultrashort pulse lasers emitting picosecond pulses might not achieve the required throughput rates, but they can perform ultra-precise ablation. Finding a way to combine picosecond and nanosecond pulses is the goal of the eVerest research project, funded by the German Federal Ministry of Education and Research.
Experimental tests are carried out on hot-working steel blanks 1.2738 using two different laser systems. The ns-machined structures are prepared by the project partner VW. A Lasertec 125 from DMG Mori is used, equipped with a 100 W IPG fiber laser at a wavelength of 1064 nm, a characteristically pulse duration of 400 ns and a repetition rate of 100 kHz. The ps-experiments are carried out in-house at Fraunhofer ILT with a Time-Bandwidth Duetto laser system (integrated in a DML 40 SI laser structuring machine) that provides ultrashort laser pulses with a pulse duration of 10 ps. The laser source offers an average power of 12 W at a wavelength of 1064 nm and an adjustable repetition rate between 100 kHz and 8.2 MHz.
For the underlying experimental setup a structure that is recreated of a carbon texture (cf. picture 2 right) are carried out on a two dimensionally probe. The microstructures contained herein are 60 µm width and 50 µm deep. The two laser sources are combined sequentially. The larger amount and the basis of the structure is ablated by the ns laser process until a depth of 100 µm where the microstructures begin. In the remaining volume until a depth of 150 µm is reached microstructures can be generated by highly precise ultrashort laser pulses (cf. picture 3). So 67 % of the depth will be ablated by ns-laser pulses and 33 % by ps-laser pulses.
Combined process saves time and quality
With the approach followed here to combine ns and ps laser processes their individual weaknesses should be overcome and their individual strength should be exploited instead. The results for combined processing show that regarding processing time the advantages of single ns laser ablation could be used while creating the basic structure with the biggest volume. Regarding quality the hybrid process utilize the benefits of ps laser ablation. Surface roughness is almost identical to a single ps process. Furthermore the most valuable asset is the microstructure quality without melt protrusions equal to a single ps process. This kind of hybrid process enables almost unlimited design textures containing microstructures with high definition (cf. picture 4).
In the end it is a question of what weighs more – productivity or quality. Using a combined process strategy a significant better surface quality without melt protrusions was achieved. However, in comparison to a single ns process the processing time for the hybrid process was 4.5 to 8.8 times. The processing time is currently the biggest compromise to make. “However, it is important to consider that with a combined process strategy the processing time of a single ps process could be more than halved”, underlines Andreas Brenner, researcher at Fraunhofer ILT. Focusing on this fact the promised advantage of the ns process, about saving processing time with a combined process strategy in comparison to a single ps process, could be exploited.
Understanding the process is the key
Although the process itself is being developed with partners at Volkswagen, its areas of application extend far beyond the automotive industry. No matter whether they are for embossing rollers for the printing industry or large bearings for the rotor shafts of wind turbines, functional surfaces are in demand in any number of sectors.
These issues are also the focus of this year’s AKL- International Laser Technology Congress, which will be held on Mai 02 until 04, 2018 in Aachen. This will be the twelfth year for the event that brings together specialists in laser development, process technology and the industry at large to discuss about topics in macro and micro processing, laser source developing and additive manufacturing.