Resistance modification of diamond through silicon incorporation
Prieske, Markus; Vollertsen, Frank

The application of polycrystalline diamond coatings in the industrial sector is rising due to its outstanding properties.
They are for example deployed for heat sinks in electrical circuits because of the high thermal conductivity. Diamond is
an electrical insulator with a resistance higher than 1010 Ω. For applications as in semiconductor technologies or to avoid
static electricity in components induced by friction, diamond coatings with a lower electrical resistance are necessary.
The aim of this study was to modify the resistance of diamond coatings through silicon incorporation. The polycrystalline
diamond coatings were deposited by a laser-based plasma chemical vapour deposition (CVD) process without a chamber
at atmospheric pressure. The in situ silicon incorporation was realised by the supply of a solid silicon carbide precursor
into the plasma flame during the CVD process. The evaporation of silicon during diamond growth induced impurities into
the diamond layer. The crystal structure of the diamond films was verified by Raman spectroscopy and scanning electron
microscopy (SEM). To analyse the distribution of the incorporated silicon, focused ion beam profiles were produced and
recorded by scanning electron microscopy. Hereby it was shown that the silicon is incorporated underneath as well as in
between the polycrystalline diamonds. Furthermore, a doping of the diamond crystals was archived which is proved by a
sharp peak at 738 nm in the luminescence spectra. That peak is characteristic for the existence of silicon-vacancy
centres. Using backscattered electron microscopy (BSE), it was detected that the silicon content in the centre of the
process position is lower than at the edges. This led to a variation of the resistance from kΩ at the edges to higher than
60 MΩ in the centre of the coating. A sheet resistance of 2.43 MΩ ± 1.98 MΩ was measured by a 4-point probe van der
Pauw resistivity measurement on a single diamond crystal in the centre of the coating.

Keywords: polycrystalline CVD-diamond; coatings; resistance modification;

_____

Laser softening of ultra-high strength steels for self-piercing riveting process
Shin, Dongsig; Ahn, Sanghoon; Lee, Jaehoon; Kim, Jengo; Jang, Junho; Cho, Shinhu; Lee, Jongkook

Ultra-high-strength steels have recently attracted attention in the automotive industry because they can be used to
fabricate lightweight structures and can improve crash test results. However, due to the poor formability of these steels,
their application to car bodies is limited. A possible solution to this limitation is a localized heat treatment using a laser
softening process. Here, a laser heat treatment was conducted using a 2.5 kW high-power diode laser. The output power
was controlled to achieve a constant temperature. As a result, the transformation of cementite in martensite was
observed by non-isothermal tempering below a temperature of 500 °C. In the end, a self-piercing riveting process which
is capable of attaching laser-heat-treated ultra-high-strength steels to an aluminum plate was successfully realized. This
outcome shows that non-isothermal tempering by means of a laser heat treatment can improve the formability of ultra-
high-strength steels.

Keywords: Heat treatment; laser softening; ultra-high-strength steels; self-piercing riveting; high-power diode laser; non-isothermal
tempering

_____

Laser hardening of thin walled parts with cryogenic cooling
Klocke, Fritz; Gräfe, Stefan; Schulz, Martin; Zheng, Gaoyuan

The surface properties of functional components have a significant influence on the wear resistance or fatigue behavior and therefore on the lifetime of the part. To improve these surface properties, one common industrial process is laser hardening. The aim of the process is to heat the material locally over the Ac3 temperature staying below the melting temperature and to get martensite by self-quenching. However, in case of thin walled parts, the volume of the material is not large enough to transport the heat fast enough out of the process zone for self-quenching. To realize a sufficient cooling and to get simultaneous a high depth of hardening, the approach of using an active cryogenic cooling for laser hardening of thin walled parts was investigated. As a result of the cryogenic cooling with liquid CO2, the temperature decreased fast enough under the martensite start temperature that the microstructure changed to martensite. Due to the position of the localized cooling point in relation
to the position and feed rate of the laser spot, parts with a thickness of 1 mm and 3 mm have been hardened. As a consequence of the process parameters and the cryogenic cooling, the achieved depth of hardening of the thin walled parts are in range between 0.2 mm and 1 mm. This paper will present in detail this innovative approach and the results.

Keywords: Macro Processing; Surface Treatment; Laser Hardening; Cryogenic Cooling

_____

Influence of alloying elements on the mechanical properties and defect formation at wire based laser beam alloying of hot-working tool steel
Hofmann, Konstantin; Holzer, Matthias; Mann, Vincent; Meyer, Ann-Christin; Roth, Stephan; Schmidt, Michael

In order to reduce wear at hot stamping processes, the mechanical properties of highly stressed tool surfaces can be modified by applying laser beam alloying with filler wire and beam oscillation. By means of increasing the carbon content as well as the content of carbide forming alloying elements, the formation of carbides, featuring a high microhardness, can be induced. Since the use o f filler materials containing carbon leads to brittle microstructural properties, the general processability of such filler wires has to be analyzed with regard to the formation of imperfections in the microstructure. This paper discusses the influence of different alloying concepts which are suitable to increase the microhardness. Thereby the influence of the carbon content as well as the content of carbide forming alloying elements is analyzed regarding the microhardness and the crack formation. Based on the results of the investigations, recommendations for the quantitative and qualitative selection of the alloying composition as well as a process strategy are derived.

Keywords: laser beam alloying, beam oscillation, filler wire, wear resistance, hot stamping;

_____

Laser surface texturing of natural stones
Chantada, Adolfo; Penide, Joaquín; Pou, Pablo; Riveiro, Antonio; del Val, Jesús; Quintero, Félix; Soto, Ramón; Lusquiños, Fernando; Pou, Juan

In recent years, controlling the wetting features of materials is attracting much interest in applications related to surface cleaning. The phenomenon of surface cleaning is closely related to morphology, and chemistry of surfaces. These characteristics can be modified by subjecting materials to surface treatments. One of the most promising techniques aiming to modify surface features is the laser texturing. In such a way, it is possible to achieve the surface properties leading to the desired wettability in an accurate manner, and with minor contamination. In this work, the wetting features of Zimbabwe black granite, a middle-to-fine-grained natural stone commonly employed as countertops in kitchens and bathrooms, are modified by laser surface texturing. The main aim is to increase its hydrophobic degree so as to reduce the attachment of contaminants on the surface. For such purpose, two laser sources ( = 1064 and 532 nm) were employed. The effect of the laser wavelength, along with other processing parameters, were evaluated. In this way, those parameters resulting in the highest hydrophobic degree were identified. It was found that the 532 nm laser wavelength was the most effective one in this regard. Furthermore, the morphology resulted from the laser surface texturing was found to be the principal phenomenon governing the wettability modifications, as the chemical composition remained virtually unaltered after treatment.

Keywords: laser surface texturing, wettability, roughness, natural stone, design of experiments

_____

Surface oxidation of titanium by cw-Nd:YAG laser
Rodríguez, Ángel; N. Montero, Javier; Amado, José M.; Tobar, María J.; Yáñez, Armando

Titanium oxide coatings present interesting properties, as high chemical stability, biocompatibility, very good adhesion and excellent mechanical properties, which makes them very attractive for several applications, as medical implants or tools used in corrosive and aggressive environments. Moreover, due to the generation of the oxide layer, the surface is colored, which makes the coatings useful for decorative applications. One of the techniques that can be used to achieve the oxidation of titanium is the laser oxidation treatment. The properties and color of the oxide layers obtained will depend on the laser beam parameters and those of the environment. In this work surface oxidation of Titanium was performed by means of a Nd:YAG laser (λ = 1064 nm) in continuous wave mode under air environment. The effect of the process parameters on the oxide color and on the layer thickness growth was studied. A three dimensional heat FEM model coupled with an oxidation kinetics law was used to predict the oxide growth and the resultant color. The experimental and the numerical results obtained were compared in order to verify the model.

Keywords: Surface treatment; Laser oxidation; Titanium; Numerical simulation.

_____

Quasi-simultaneous local hardness reduction via Remote Laser Scanner for cost-effective mechanical joining of press-hardened high-strength steel 22MnB5
Surrey, Philipp; Emmelmann, Claus; Ivanov, Dmitry

The diversification of materials is an effective way to decrease car body weight and CO2 emission of modern automobiles. Hot stamped steel structures are already broadly used because of their high strength and significant weight reduction potential. In order to create multi material car bodies mechanical joining technics such as clinching and self-pierce riveting are used since they allow bimetallic joints in combination with adhesives. However, the joinability by clinching and self-pierce riveting of high-strength steels such as 22MnB5 in the press-hardened state is limited due to the tremendous hardness. A thermal hardness reduction in the joint area enables the production of multi-material joints including martensitic high-strength steels with classical joining technologies, as it is investigated thoroughly by others.
The produced joints showed sufficient load-bearing capabilities for automotive applications, but the present heat treatment strategies cannot be implemented in industrial production chains economically due to their long cycle times. In this paper an innovative approach is presented to decrease the process time of the local heat treatment using a Remote Laser Scanner. It is proposed to irradiate several joint areas periodically by a high power laser source. Furthermore, it is proposed that the fast periodic energy input leads to a hardness reduction of the joint areas quasi-simultaneously and therefore decreases the overall process time drastically. In this paper the process is described more detailed and the results of feasibility studies are presented.

Keywords: Remote Laser Scanner; heat treatment; press-hardned steel; mechanical joingin; automotive

_____

Surface finish using laser-thermochemical machining
Eckert, Sandro; Köhnsen, André; Vollertsen, Frank

The surface integrity of metallic workpieces after laser chemical machining (LCM) is mainly determined by different corrosion mechanisms such as intergranular, pitting, uniform and high temperature corrosion. The corrosive surface attack is the consequence of a subtractive manufacturing process based on chemical etching in an electrolyte media enhanced by laser heating. By using a scanner-based LCM-setup uniform corrosion can be used for an effective surface smoothing of titanium workpieces. Independent from the initial surface roughness the smoothing can achieve optical quality (Sa < 0.1 μm) and result in polished surface finish with a macroscopic shiny effect taking into account the relevant laser and scanning parameters. Furthermore, ripple-like surface structures are observed with a periodicity close to the laser wavelength and an orientation perpendicular to the polarization of the laser radiation. While scanning multiple times the ripple structures can be continued coherently over a macroscopic large scale to a high periodic reflection grating. In addition, electron scanning microscopic study of the material and electrolyte specific characteristics is performed. This reveals microstructure-related intergranular corrosion during the laser chemical polishing. At high laser power pitting and high temperature corrosion dominates the chemical removal process that leads to an increased surface roughness
and limits the process time.

Keywords: Polishing; Laser micro machining; Surface integrity; Nano structure; Topography

_____

Feasibility study for the automation of a selective Laser Deburring Process
Conrad, Christian; Möller, Mauritz; Jereczek, Mateusz; Emmelmann, Claus

In the field of deburring processes a totally automated solution is not available. Selective Laser Deburring (SLD) is a laser based edge-refinement process for sheet-metal parts which is being developed at the LZN. It is wear-free and one laser source is used to remelt burrs and edges of different materials to a defined edge radius. This study analyses the feasibility of the laser-based deburring in automated industrial plants. The aim is to clarify if a deburring of complex sheet metal parts is possible. Preceding studies examined the SLD process parameters and a thermographic quality assurance with short and linear edges. Based on these studies the deburring quality over long edges, different orientations of the deburred edge and the transition between deburred edges are investigated in this paper. This experimental study leads to recommendations for automated SLD process chain solutions.

Keywords: Selective Laser Deburring; laser cutting