Micro Processing: Surface Functionalization

Laser based surface structuring for lightweight design
Max Kahmann, Ulf Quentin, Marc Kirchhoff, Rüdiger Brockmann, Klaus Löffler

One of the most important issues in automotive industry is lightweight design, especially since the CO 2 emission of new
cars has to be reduced by 2020. Plastic and fiber reinforced plastics (e.g. CFRP and GFRP) receive besides new
manufacturing methods and the employment of high-strength steels or non-ferrous metals increasing interest.
Especially the combination of different materials such as metals and plastics to single components exhausts the entire
potential on weight reduction. This article presents an approach based on short laser pulses to join such dissimilar
materials in industrial applications.

Keywords: lightweight design, short pulse lasers, surface structuring, micro processing, metal-plastic joining


Investigation and application of laser induced surface functionalization with pulse delays between 40ns and 50µs on silicon and steel foils
Viktor Schütz, Jürgen Koch, Oliver Suttmann, Ludger Overmeyer

Laser induced quasi-periodic cone-like surface structures contribute to the functionalization of material surfaces. The
resulting topography depends on manifold laser and ambient process parameters. In this study, the fundamental
influence of a pulse delay between 40ns and 50μs has been investigated for the laser processing of silicon. During the
processing, parameters of laser fluence and pulse overlap were set at constant condition used in the three laser systems.
As the results, laser induced surface topographies are visualized by microscopy and analyzed with self-developed image
processing tools to determine cone distance and cone density. The obtained knowledge from laser-induced quasi-
periodic cone-like structuring helps to develop laser processes for various applications and materials, e.g. for
photovoltaics or in medicine. In an exemplary case the surface functionalization is demonstrated for cathodes on steel

Keywords: surface functionalization; quasi-periodic structures; pulse delay variation


One-Step Generation of Ultrahydrophobic Aluminum Surface Patterns by Nanosecond Lasers
Radhakrishnan Jagdheesh, Juan J. García-Ballesteros, José L. Ocaña

In recent years, metal surfaces that could mimic the water repellence properties of some natural surfaces have
been the area of an intense research due to their potential applications such as self-cleaning, anti-contaminating and
anti-sticking applications. Ultra short laser machining/structuring is a promising technique to obtain the dual scale
roughness on the metal surface, which promotes the complex interfaces between solid-liquid-air, thus improves the
wetting property of the surface. An attempt was made to study the improvement of wetting properties of aluminum by
nanosecond laser source in one step process. Flat aluminum sheets of thickness 100 μm were laser machined with
ultraviolet laser pulses of 30 ns with different laser parameters to optimize the process parameters. The samples
produced at the optimum conditions with respect to contact angle measurement were subjected to microstructure and
chemical analysis. The wetting properties were evaluated by static contact angle measurements on the laser patterned
surface. The laser patterned microstructures exhibited ultrahydrophobic surface with a maximum contact angle of 180°
for the droplet volumes of 8μL.

Keywords Surface: Microstructuring, Ultrahydrophobic, Nanosecond Lasers.


Structuring of injection moulding tools with ultrashort laser pulses for surface functionalization after casting
Sebastian Wächter, Felix Dreisow, Sabine Sändig

We structured injection molding tools and subsequently imprint the micro-structured surface onto the work piece
during the molding. We studied the influence of the laser parameters for the fabrication of micro- and nanostructures.
They are characterized using laser scanning microscopy and scanning electron microscopy. Finally, the tools are applied
for two different purposes. We achieved vanishing adherence to blood in small capillaries of microfluidic devices. In a
second example, we show that the forces to remove the tool from the workpiece after casting are reduced.


Laser beam propagation and energy deposition in particulate PEEK layers
Hendrik Sändker, Jochen Stollenwerk, Johannes Hofmann, Peter Loosen

The usage of high-temperature resisting thermoplastic polymers like PEEK (polyether ether ketone) as coating material
represents an alternative to conventional corrosion protection layers. One promising approach to manufacture these
coatings is to deposit particulate PEEK and, subsequently, melt the PEEK particles by laser radiation in order to create a
dense, pore-free layer. Understanding the laser beam propagation and the energy deposition in the particulate PEEK
layer is essential to simulate (time-dependent) temperature distributions within the layer system. These simulations
allow an efficient identification of suitable process parameters for laser melting of the PEEK layer. Therefore, the
absorbance of the layer system composed of substrate and PEEK powder with admixed laser-absorbing particles is
determined experimentally. Subsequently, a model-based simulation of the optical properties is conducted and fitted to
the measured data. Hence, the complex refractive indices of the bulk materials, particle-size distribution, powder
density, multiple reflection, various scattering effects as well as layer thickness are taken into account. Based on the
simulation results the spatial energy deposition in the layer system is extracted.

Keywords: Surface Functionalization; Fundamentals and Process Simulation


Use of high-energy laser radiation for surface preparation of magnesium for adhesive applications
Norbert Schneider, Christian Wrobel, Jens Dr. Holtmannspoetter, Guenther Prof. Dr. Loewisch

This paper is intended to demonstrate how the parameters for the surface preparation of magnesium alloys for adhesive
bonding can be optimized. The effects of different laser parameters are analyzed using a combination of advanced
sample preparation and ultra-high resolution scanning electron microscopy on the nanoscale level and a specific
combination of mechanical tests on the macroscopic level. This data allows a discussion of the physical principles and
the key parameters influencing the interaction of laser radiation with the magnesium surface.

Keywords: Micro Processing, Surface Functionalization


Nanostructures fabricated by laser interference lithography and their potential applications
Evaldas Stankevicius, Mantas Garliauskas, Gediminas Raciukaitis

We introduce a rapid and flexible method for polymeric nanostructure fabrication by using four-beam
interference lithography. The influence of the laser processing parameters (peak pulse intensity, the number
of laser beams, etc.) and photopolymer thickness to the shape of these nanostructures are analyzed, and the
shape formation of nanostructures is explained. Also, the potential applications of the structures fabricated
by interference lithography are discussed.

Keywords: Nanostructures, multi-photon polymerization; interference lithography, photonics, micro-optics, scaffolds.


Investigation of the influence of laser surface modifications on the adhesive wear behavior in dry cold extrusion of aluminum
Ingo Roß, André Temmler, Edgar Willenborg, Marco Teller

One of the main wear mechanisms in cold extrusion of aluminum is adhesion. While this can be prevented by excessive
usage of lubrication, due to environmental and economic reasons a surface modification which allows dry metal
forming, i.e. processing without lubrication is highly desired. In this paper first results concerning the effect of the
spectral surface roughness characteristics of laser polished specimens made from AISI D2 cold work steel on their
tendency to adhesive wear with aluminum are presented. By using macro polishing as well as micro polishing different
spatial wavelength ranges of the surface roughness are modified, resulting in surfaces with a unique spectral roughness
distribution. The laser polished specimens are tested in a compression-torsion tribometer under conditions adapted
from cold extrusion. Before and after testing the topography of each specimen is measured via white light
interferometry. By comparing both topographies the volume of adherent aluminum is determined. The influence of the
laser surface modification is investigated on basis of the spectral roughness dis tribution and the characterization of the
adhesive wear behavior.

Keywords: laser polishing; adhesion; wear; cold extrusion; compression-torsion tribometer


Fabrication of graphene-chitosan electrodes for sensing applications by laser induced modification of the composite film
Romualdas Trusovas, Raimonda Celiešiute, Rasa Pauliukaite, Gediminas Raciukaitis

Graphene based electrodes have already shown an advantage in electro-catalytic activity and macroscopic scale
conductivity. Formation of uniform graphene structure remains a significant challenge. Lasers have been already shown
as useful tool for formation and modification of graphene structures.

In this research, we present our results on formation graphene-chitosan electrodes by a picosecond laser irradiation of
the active film. Graphene was casted by a spin-coating from the weak acidic dispersion of the chitosan solution on the
ITO electrode surface, and the laser treatment was applied for regular nanostructure formation.

Modified electrodes were investigated applying the cyclic voltammetry (CV) and electrochemical impedance
spectroscopy (EIS). The same characterization was performed after the laser treatment of the samples. Experimental
setup of laser treatment included picosecond laser (Atlantic, 10 ps, 100 kHz, Ekspla) working at the 1064 nm and 532 nm
wavelength and the galvanometric scanner with a focusing objective (F=80 mm). Picosecond laser parameters and GO
concentration were optimized according to the capacitance and the resistance changes, calculated from the EIS data of
the laser-modified electrodes. The Raman spectroscopy showed that laser irradiation can cut out graphene sheets in the
graphene-chitosan composites into smaller pieces inducing more edge defects. Moreover, laser irradiation with the
average power higher than 150 mW, caused an increase in capacitance at the electrode surface due to the formation of
nanocrystals of graphene.

Keywords: graphene; chitosan; picosecond laser; electrode;


Influence of laser marking on stainless steel surface and corrosion resistance
Martin Kucera

Laser marking is the modern industrial application for non-contact surface modification. An incidence of a laser beam on
the marked surface causes material and structural changes, which lead to optical changes of the surface. The processes
during the laser-surface interaction can also affect other surface properties, especially corrosion properties in the case of
stainless steel. Laser marking of stainless steel using the pulsed fiber laser SPI G3 is described in the contribution.
Possibilities and limitations of steel laser marking are discussed. Some of the latest results of the material analysis and
corrosion tests of the laser treated material are presented. Examples of laser marking influence on steel surface,
structure and corrosion properties are shown.

Keywords: fiber; laser; marking; stainless steel; corrosion


High velocity laser printing of conductive tracks
Daniel Puerto, Emeric Biver, Catalin Constantinescu, Dimitris Karnakis, Anne-Patricia Alloncle, Philippe Delaporte

Printing micrometer size conductive structures is a key challenge for the development of printed
electronics. Inkjet technology is the main digital process currently used for this application, but it
suffers from some drawbacks like head clogging and limitation to low viscosity inks. We use the laser-
induced forward transfer (LIFT) technique to print at high-velocity long lines of metallic nanoparticle
ink. A picosecond laser emitting at 343 nm with a repetition rate of 1MHz is used to realize 2D
conductive tracks at velocity as high as 10m/s. The control of process parameters allows the
fabrication of 20μm widthlines with various thicknesses and a resolution of few micrometers.

The physics of laser printing was studied by means of time-resolved imaging technique and these
results areused to discuss the potential and limitations of this technology. Sensor electrodes were
realized on flexible substrates as well as passive components like resistors and capacitors, by printing
high viscosity inks. These applications will be presented to illustrate the feasibility of using high
repetition rate laser for the fast and reliable printing of conductive structures.


Adjustment of surface energy on steel surfaces due to CLP generation by picosecond laser processing
Tom Häfner, Johannes Heberle, Daniel Holder, Michael Schmidt

With regard to mechanical and chemical resistivity metal surfaces can benefit from hydrophobic behavior. Against this
background, results of laser-induced generation of hydrophobic surfaces on three different steel alloys without any
additional coating are presented. Therefore, a dual-scale structure - consisting of micro cones and nano-ripples and
droplets - is generated by picosecond laser structuring in ambient air.

Firstly, the dependencies of the formation of these micro cones, called cone-like protrusions (CLP), on different
parameters of the laser ablation process - the peak fluence, the spot diameter, the effective pulse number per unit area and
the material - are investigated. Secondly, the hydrophobicity and surface energy of the resulting surface topography of the
irradiated substrate are evaluated by means of contact angle measurements. For that purpose identical topographies are
generated on stainless steel, hot- and cold-working steel. The time-dependency of the contact angle is investigated
depending on the material and the morphology.

For CLP formation a high number of scans is preferred to provide a high effective pulse number per unit area, which is
advantageous for homogeneous generation of larger micro cones. By measuring the contact angle a change from an
initially hydrophilic to a hydrophobic behavior of the surfaces can be observed due to a change of surface chemistry over
time. Thus, contact angles higher than 90° can be measured on laser treated surfaces of different steel alloys.

Keywords: Micro processing; ablation; surface functionalization; steel alloy


Tribological surface functionalization via femtosecond laser-induced periodic surface structures on metals
Jörn Bonse, Sandra Höhm, Robert Koter, Manfred Hartelt, Dirk Spaltmann, Simone Pentzien, Stephan Marschner, Alexandre Mermillod-Blondin, Arkadi Rosenfeld, Jörg Krüger

Laser-induced periodic surface structures (LIPSS, ripples) were generated on titanium and steel
surfaces upon irradiation with multiple linear polarized femtosecond laser pulses in air environment
(pulse duration 30 fs, central wavelength 790 nm, pulse repetition rate 1 kHz, Gaussian beam shape).
The conditions (laser fluence, spatial spot overlap) were optimized in a sample-scanning geometry
for the processing of large surface areas covered homogeneously by two different types of
nanostructures, i.e., low-spatial frequency LIPSS (LSFL) with periods around 600 nm and high-spatial
frequency LIPSS (HSFL) having periods around 100 nm only.
The tribological performance of both types of nanostructured surfaces was characterized under reciprocating sliding condition against a ball of hardened steel at 1 Hz using different lubricants. After
1,000 cycles, the corresponding wear tracks were characterized by optical and scanning electron
microscopy. For specific conditions, the wear was strongly reduced and the laser-generated
nanostructures (LSFL) endured the tribological treatment. Simultaneously, a significant reduction of
the friction coefficient was observed in the laser-irradiated LIPSS-covered areas when compared to
the non-irradiated surface, indicating the potential benefit of laser surface structuring for tribological
applications. For optimization, the spatially Gaussian shaped beam used for the laser processing was transformed
into a “Top-Hat” distribution at the surface of the samples. This was experimentally realized by using
a spatial light modulator (SLM). The tribological performance of samples processed with a Top-Hat
beam is compared to the one generated with a Gaussian shaped laser beam.


Studies on Laser Surface Texturing of Titanium Alloy (Ti-6Al-4V)
Jyotsna Dutta Majumdar, Renu Kumary, Heino Besser, Tim Scharnweber, Wilhelm Pfleging

In the present study, a detailed characterization of laser-assisted surface textured titanium alloy (Ti6Al4V) has been
undertaken. Laser surface texturing with line and dimple geometry has been carried out using ArF excimer las er
operating at a wavelength of 193 nm with a pulse length of 5 ns. Following surface texturing, an extensive
characterization of the textured surface has been carried out by scanning electron microscopy, electron back scattered
diffraction (EBSD) and X-ray diffraction technique. There is refinement of microstructure along with a higher mass

-titanium phase and oxides of titanium (rutile, anatase and few Ti2O3 phase) in the textured surface as
compared to as-received one. Furthermore, in order to investigate the impact of laser surface texturing on surface
energy; wettability studies have been carried out before and after laser modification. The area fractions of linear texture
and dimple texture measured by image analysis software were 45 % and 20 %, respectively. The surface energy (and
hence, wettability) was increased due to linear (45.6 mN/m ) and dimple ( 39.4 mN/m) texturing as compared to as -
received Ti-6Al-4V (37 mN/m ).

Key words: Laser surface texturing,Ti-6Al-4V, nanoindentation.


Non-uniform micro-texturing of tribological steel surfaces by femtosecond laser ablation
Antonio Ancona, Giuseppe Carbone, Michele Scaraggi, Annalisa Volpe, Michele De Filippis, Pietro Mario Lugarà

Femtosecond laser ablation (fsLA) allows fabricating surface micro-textures of complex shape and geometry with a
micrometer precision. In this work, we exploit the intrinsic flexibility of fsLA technology to realize non-uniform micro-
textures on steel thrust bearing (un-tapered) pad surfaces, which have been shown (within the recent Bruggeman
Texture Hydrodynamics theory, BTH) to optimize the tribological characteristics of the bearing. Moreover, a complete
tribological characterization of all the sample surfaces was performed. We found, in agreement with the BTH
predictions, that the micro-fluid dynamics, occurring locally at the scale of the structural defect and induced by the
same, is integrated out in term of a macro hydrodynamic regime in the friction curve, which would otherwise not exist
for the macro-geometry of the contact.

Keywords: Ultrafast lasers, Laser Ablation, Laser Surface Texturing, Bruggeman Texture Hydrodynamics


Generation of low-spatial frequency Laser Induced Periodic Surface Structures Driven by Surface Finish
Stefan Rung, Florian Preusch, Ralf Hellmann

We investigate the influence of different angles between the polishing direction of linearly polished surfaces and the
electrical field of the impinging laser on the generation of low spatial frequency LIPSS on stainless steel. The electrical
field is rotated with respect to the polishing direction and its effect on the orientation and homogeneity of the LIPSS is
determined. In addition, the influences of the initial surface roughness and laser parameters such as the laser fluence on
the generation of LIPSS are investigated. It can be shown, that the formation of LIPSS is driven by the initial surface
roughness. The experimental results lead to the assumption that LIPSS are attracted by the linear grooves caused by
polishing. Depending on the used parameter set, the orientation of the generated LSFL formation deviate up to a value
of 45° against the common predictions. Furthermore, a dependence of the required fluence for LSFL on surface
roughness and polishing direction is demonstrated. In particular, LSFL generated with a low fluence are more attracted
by the surface polishing. Continuatively, the results may contribute to a further understanding of the underlying
mechanisms involved in the generation of LIPSS. In addition, the results can be useful for producing LIPSS on large-scale
for possible applications.

Keywords: LIPSS; ripples; LSFL; surface structure; surface influence


Selective Femtosecond-Laser Structuring of Dielectric Thin Films with Different Band Gaps: A Time-Resolved Study of Ablation Mechanisms
Stephan Rapp, Gerrit Heinrich, Heinz P. Huber, Michael Schmidt

In the production process of silicon microelectronic devices and high efficiency silicon solar cells,
local openings in thin dielectric layers are required. In the latter case, silicon nitride (SiNx) thin films
are used as anti-reflection layers and silicon oxide (SiO2) films as passivation layers. The openings can
be selectively structured with ultra-short laser pulses by confined laser ablation processes in a fast
and efficient single-pulse production step.
The aim of this work is to obtain a deeper understanding of the physical laser-material interaction
during the laser ablation processes. For this purpose, two dielectric thin films with different band gap
energies, SiO2 (Egap = 8 eV) and SiNx (Egap = 2.5 eV), on planar silicon (Si) wafers are structured with
infrared fs-laser pulses (E = 1.2 eV). Pure Si is laser processed in comparison to study the role of bare
substrate. The results show, that SiO2 layers are selectively structured by confined laser ablation. SiNx
layers, however, are ablated by a combination of direct laser ablation and confined laser ablation at
fluences well above the ablation threshold (factor 2.5). Then, SiNx islands remain in the spot center.
The applied Gaussian shaped laser pulses cause a nonlinear multi-photon absorption in the spot
center leading to a local direct laser ablation process of the SiNx film. Due to the larger band gap of
SiO2 films direct ablation is not observed here.
Pump-probe investigations are performed to investigate and to compare the temporal course of the
different ablation types. The direct ablation process is observed on the Si sample, the pure confined
ablation on the SiO2 sample. The combination of both ablation types is observed during the
structuring. By comparing the temporal ablation process of a substrate and two thin film systems with different
band gaps the corresponding physical ablation mechanisms can clearly be identified. Absorption
leads to direct laser ablation in the bare Si as well as in the center of the irradiated
SiNxwhereas low
absorption in the rim of the irradiated SiNx and in the SiO2 film leads to absorption in the underlying
Si substrate leads to confined ablation.


Improving a Finite Element thermal simulation of the nanosecond laser ablation on silicon targets
Germano Galasso

In semiconductor manufacturing, as in several other industrial sectors, lasers are massively used in
many machining processes. In particular, nanosecond laser ablation is a valid alternative for the
dicing of ultra-thin silicon wafers, providing better cut quality and lower production costs with
respect to the more traditional mechanical sawing. Despite the broad application of lasers in
industrial machining, much remains to be understood about the mechanisms underlying laser
ablation. Numerical modeling is a useful tool in the understanding of the complicated mixture of
tightly interconnected physics involved and in the optimization of the dicing process, which is
nowadays still performed by a trial and error approach. Nevertheless, the complex multi-physical
nature of laser ablation poses serious challenges in the simulation of the process and in the
development of a common modeling framework. A gap exists in particular between pure and applied
researchers. The former usually analyze in detail the fundamental mechanisms involved in the
interaction of laser with matter, using analytical or one-dimensional models which are generally not
suited for the simulation of real applications. The latter, while attempting to describe the overall
industrial process typically by means of simulation packages, often approximate the problem by
neglecting or over simplifying important aspects, as the effect of the laser-induced plasma or the
target response at critical temperature. This work is aimed to bridge this gap. A thermal transient
finite element model is developed using a commercial package. Additionally, theoretical and
numerical techniques are described and implemented in order to account in a simplified, yet
physically consistent manner, for the most relevant mechanisms underlying nanosecond laser
ablation. The improved finite element model, among several other aspects, considers the initial non-
equilibrium plasma formation governed by collisional and radiative processes. This description is
essential, as the laser-induced plasma shields the target from the incoming laser beam and reduces
both the efficiency and the controllability of the process. Further, the main mass removal
mechanisms are qualitatively accounted for by an element deactivation technique. In particular the
transition from evaporative to volumetric removal occurring at the target critical temperature is
implemented. The numerical results are finally compared with experimental data.


Improvement of the adhesion between CoCr and dental ceramics by laser surface structuring
Sina Hallmann, René Nodop, Christian Daniel, Martin Weppler, Jügen Geis-Gerstorfer, Claus Emmelmann

The preparation of the interface between dental ceramics and non-precious metal alloy cobalt-chromium (CoCr) at
veneered dental restorations is crucial to the durability of the bonding strength and thus to the lifetime of the
prosthesis. Conventionally the surface finishing is carried out by manual sandblasting, which is a highly subjective
process. Due to this fact, the reproducibility of the required surface roughness for the bonding is limited. In addition,
embedded residues of the blasting material can result in failure of the bonding.

Laser based surface finishing represents a promising approach to condition dental prostheses. Through its reproducible
working principle while avoiding the use of foreign particles, laser processing offers advantages over the conventional
process. With laser ablation the creation of a determined surface roughness as well as the functionalization of surfaces
by defined structures becomes feasible.
Therefore surface structures that enhance the adhesion between CoCr and
dental ceramics have been developed in the present study. The subject was to improve the metal-ceramic bonding
strength by conditioning the CoCr surface. First, the structures were derived from natural models, abstracted to
technical application and then implemented with laser ablation. The structures were ablated on CoCr test specimen with
a nanosecond (ns) pulsed Yb fiber laser with maximal pulse energy of 1 mJ. After ceramic veneering, a Schwickerath test
was performed. Overall, the laser structured surfaces show bonding strengths up to 57 MPa. These adhesive forces are
significantly higher than those of conventionally treated surfaces. Thus, laser ablation appears to be an attractive
technology for surface finishing of dental implants for reasons of reproducibility and enhanced surface properties.

Keywords: laser ablation; surface structures; adhesion of metal and ceramics; dental prosthesis; CoCr


Pulsed Laser Surface Pre-Treatment of Aluminium to Join Aluminium-Thermoplastic Hybrid Parts
Andre Heckert, Christian Singer, Michael F. Zaeh

Thermal joining of aluminium and thermoplastics by laser radiation is promising. To obtain a high joint strength, the metal
surface is pre-treated prior to bonding. Laser surface pre-treatment was used to pre-treat aluminium specimens in this study.
The laser process allows a flexible processing of surface structures in the range from nanoscopic to macroscopic scale. An
infrared pulsed single-mode fibre laser with a pulse width of 100 ns was used to form a microscopic structure on the
specimens. The surface was overlain by a porous oxide layer. This oxide layer differs significantly from the natural
passivation layer. It is assumed that the oxide layer contributes to a great extent to the cohesion between the aluminium and
the thermoplastic material. This study focuses on the generation of the microscopic structure with an oxide layer and its
influence on the resulting joint strength between aluminium and thermoplastics. To create a variety of surface topologies,
different cumulative energies were applied to the specimens. The resulting surface topologies were investigated by scanning
electron microscopy, the surface enlargement was characterized using gas adsorption, the surface roughness was quantified
by laser scanning microscopy, and the surface chemistry was analysed by X-ray photoelectron microscopy. The oxide layer
was removed from the structured specimens by an etching process to compare both samples, with and without a porous
oxide layer. Subsequently, joints of the laser structured aluminium and the thermoplastics were prepared by thermal joining.
The joint strength was measured by tensile shear tests and correlated to the surface areas, roughness values , and laser
energies for etched and non-etched aluminium specimens to identify the influence of the microscopic structure and the oxide

Keywords: Laser, Surface Pre-Treatment, Aluminium-Thermoplastic Hybrid Joint


Surface conditioning of copper to improve the continuous wave laser micro welding
Nerea Otero, Pablo Romero, Christian Hoff, Jörg Hermsdorf

Copper is used in almost all electronic components. The contacts are laser welded if a low electrical
resistance, a high mechanical and thermal long-term stability is required. For example these requests
occur in modern motor vehicles, where the power electronics are installed in the engine room to
save weight and cost of electric lines. One of the main problems in laser welding of copper is the low
and locally varying absorption of the infrared radiation of established laser systems, which currently
limits the stability of laser welding. This limits the efficiency of conventional laser welding, as well as
its reliability.

Studies have shown that the irradiation of the copper surface with a green laser (532 nm) adduced a
significantly higher absorptivity of infrared laser radiation. Figure 1 shows welding results with and
without preconditioning. It can be seen that for preconditioning only a fraction of the energy is
required. With the combination of green and IR radiation a 100% weld probability and an energy
saving of 20 40 % was detected.

This work analyses the surface conditioning of copper, by irradiation of the copper surface with a 532
nm nanosecond laser, to improve the welding quality of the copper with an infrared continuous wave
fiber laser source. The irradiation under well-defined conditions produces a durable preconditioning,
which improves welding quality when irradiated on the preconditioned area.

This paper is focused on the analysis of the copper surface, after the laser preconditioning. In this
presentation, the process conditions and laser parameters for surface conditioning of copper are
analysed, to demonstrate the relationship between: the laser parameters and the surface conditions
after the irradiation. Furthermore the influence of surface conditioning to the subsequent welding
process with continuous wave infrared radiation will be shown.


Femtosecond laser manufacturing of highly hydrophobic hierarchical structures fabricated by combining surface microstructures and LIPSS
Miguel Martínez-Calderon, Ainara Rodríguez, Antonio Dias, Mikel Gómez-Aranzadi, Santiago Miguel Olaizola

The manufacturing of metal surfaces with highly controllable wetting properties is becoming a very active and promising
area in research and engineering. Based on nature examples like rose petals or lotus effect, an effective approach for this
purpose is the combination of micro- and nano-structures into hierarchical structures. Traditionally these structures are
manufactured by using different types of coatings or by processes which involve too many steps and techniques to become
commercial solutions. However, this can be achieved by only using femtosecond laser ablation in air atmosphere. In this
work we have developed hierarchical structures that consist of micro-patterned surfaces covered by nanostructures with
this technique.

The first part of this work is a complete study to determine the microscale modifications produced on a stainless steel
alloy (AISI304) surface at high pulse energy, different velocities, and focal distance in order to obtain microstructures with
a selected depth of around 10 μm and line widths of 20 μm. The second part of the work is focused on finding the optimal
irradiation parameters to obtain the nanostructure pattern. Nanostructures have been defined by means of Laser Induced
Periodical Surface Structures (LIPSS) of around 250 nm high and a period of 600 nm, which constitutes the nanostructure
pattern. Finally, dual scale gratings of 50 mm2 were fabricated for a range of irradiation parameters (line period, fluence,
velocity, ablated depth or focal distance) and their effect on the measured contact angle. Combining the micro-pattern
with the LIPSS nano-pattern, highly hydrophobic surfaces have been developed with measured static contact angles higher
than 150º starting from an initial contact angle of 75º.


Comparison of UV- to M-IR laser for surface pre-treatment based on the ILSS-test
David Blass, Stefan Kreling, Klaus Dilger

Carbon fiber-reinforced plastics (CFRP) offer a great potential for any lightweight construction. Besides there are also
some challenges especially the joining technology and the repair process. Adhesive bonding is a key factor to solve
both challenges, namely and based on the common CFRP fabrication processes it requires a surface pre-treatment
because of residues of release agents. For the removal of those contaminations and also for the ablation of whole fiber
plies for the repair process, laser radiation is a suitable tool. However due to the thermal interaction of laser radiation
and CFRP there is a tendency to cause delamination during laser treatment. This paper describes the approach of
predicting the delamination tendency in bonded CFRP joints with a very simple mechanical test. It is shown that the
results gained out of the ILSS-test are correlating with the amount of delamination in adhesively bonded and previously
laser ablated CFRP-joints.

Keywords: CFRP; pre-treatent; ILSS; delamaniation