Search

In the present study, hybrid metal matrix composites, Al2024/10Al2O3, Al2024/10Al2O3/3MgO, Al2024/10Al2O3/6MgO, Al2024/10Al2O3/3MgO/1.5 Gr, Al2024/10Al2O3/3MgO/3Gr, and reinforcement samples (AA 2024) produced with powder metallurgy process. AA 2024 and reinforcement powders were determined mixture rations and separately mixed during 30 minutes in a three-dimensional Turbula mixer. The mixed compositions were pressed at 300 MPa and sintered at 550°C during 1 h. After that, three materials were extruded at the same temperature. Experimental results show that hybrid metal matrix composites (HMMCs) a better wear resistance than the reinforcement samples because of higher hardness. Gr behave as a lubricant during wear process. The wear resistance of HMMCs can be optimized with controlling of the reinforcement content and type.

This paper reports on combining non-diffracting Bessel beams and ultra-short pulsed laser ablation for the generation of micro-channels. The small focal spot and the long focus depth make Bessel beams suitable as an ideal tool for fabricating sharp-edged channels. In this context, we present a study on the generation of rectangular micro-channels that are highly interesting for future investigations on material properties of additive manufactured and cast aluminum samples, e.g., using them as initial notches or defects, respectively, for fatigue and damage tolerance assessment. In order to achieve the highest quality of rectangular microchannels, optimal processing conditions were achieved by analyzing the influence of the laser power, the number of exposure cycles, and the axial position of the sample on the ablation process. Similar experiments using a Gaussian Beam were performed to compare the different beam shapes regarding the resulting channel geometry and ablation efficiency. Nevertheless, the walls of the Bessel beam processed channels are almost perpendicular to the surface and the bottom of the channel is almost flat. Thus, the Bessel beam generated micro-channels are more rectangular than the ones processed with the Gaussian beam. [ABSTRACT FROM AUTHOR]

The facile manufacture of PA12 MWCNT/silica (50/50 by weight) nanocomposite powders through a high energy mixing process is presented, which are useful to generate 3D objects by a novel Laser Polymer Deposition (LPD) process. The mixing as well as the LPD process led to no discernible changes in the material properties (DSC, SEM, LD) of the core‐shell nanocomposites, enabling the recycling of unconverted powder. The built parts yield ultimate tensile stresses and Young's modulus at 10%–20% of the bulk material. Partially unmolten particles and voids were identified as the main mechanical failure mechanism in the built parts. The mechanical properties are better with low additive content (Young's modulus: 89.8 ± 5.4 MPa; UTS: 12.9 ± 5.3 MPa with 0.25 wt% additives). Electronic conductivity up to the region of moderate conductivity could be achieved by multiwalled carbon nanotube (MWCNT) network formation (8 × 10−4 S cm−1 at 1.25 wt% of additives). A variant of the processing strategy revealed that a higher mechanical strength can be achieved by a laser induced remelting of the traces following their initial construction. [ABSTRACT FROM AUTHOR]

In this study, we report on the integration of microfluidic channels and optical components by focusing femtosecond laser radiation inside transparent cyclic olefin copolymer (COC) bulk material. An internal localized material modification is triggered based on nonlinear absorption of the laser radiation's high intensities inside the focal volume. The size and shape of the three-dimensional internal modification are controlled by using an adaptive beam shaping setup. The irradiated areas show a positive refractive index shift and can be used as Type I internal optical waveguides. Furthermore, precise control of the spatial pulse-to-pulse distance in combination with a suitable beam profile enables the integration of functional photonic elements, e.g., Bragg gratings, into the waveguide. Thus, it also enables the generation of integrated photonic sensors. In addition, internal fs laser-induced modifications are characterized by a lower thermal stability as compared to the pristine polymer material. By performing a post-annealing process step, internal hollow microstructures are created by gaseous degradation of the exposed areas. Circular microchannels can be generated in a deliberately chosen layout by employing motorized 3D stages. In comparison to etching-based fabrication methods the proposed technology facilitates unlimited channel lengths, as it omits restrictions arising from an etching selectivity and duration. Altogether, this contribution paves the way towards the fabrication of internal three-dimensional optofluidic devices, equipped with a photonic sensor. In contrast, microfluidic and photonic structures are both created by femtosecond laser direct writing inside of transparent polymers. [ABSTRACT FROM AUTHOR]

Laser turning with ultra-short laser pulses provides the possibility of manufacturing rotationally symmetric geometries, even of hard to machine materials as fused silica, down to a micrometer scale. In this laser micro machining approach, the laser beam is oriented perpendicularly to the rotational axis and tangentially to the material, whereas the target geometry is realized by moving the constantly rotating specimen according to the specified geometry under the focused laser spot. Due to the friction-less laser ablation it is possible to realize a turning process in fused silica without the use of a contacting tool and therefore without causing mechanical tensions in the work piece. The processing head focuses the laser to a calculated spot size of 18 µm in diameter and the applied processing gas pressure assists in removing the ablated material. In this study, femto-second laser turning of fused silica with a diameter of 3.7 mm is investigated. The influence of the applied pulse energy is stated and the use of a trepanning optic during the turning process is characterized. Furthermore, comprehensive parameter studies in order to optimize roughness and to find the maximum achievable ablation rate are shown. Limited by the mechanical speed of the rotational axis, the highest feasible ablation rate is 19.5 mm³/min. A roughness of Ra 0.65 µm are demonstrated. [ABSTRACT FROM AUTHOR]

We report on the effect of different transversal beam shapes on the efficiency of ablation processes and the resulting surface characteristics. A possibility to efficiently apply ultrashort pulsed lasers with high average power is beam shaping. By using a cooled reflective based liquid crystal spatial light modulator to generate different spot distributions, it is possible to spatially allocate the available power to avoid excessive high fluences. In our experiments, we determine the optimal fluence to ablate the maximum volume per watt to be in the range of 0.2-0.4 J/cm². Based on this fluence, we increase the number of spots from one to a maximum of twenty to ablate steel in a multilayer scanning-based process. In this context, we examine the influence of different separation distances between the spots on the ablation efficiency and roughness. Subsequently to these investigations, we develop an efficient roughing process with higher ablation rates and a nearly constant roughness. [ABSTRACT FROM AUTHOR]

We report on the fabrication of rotationally symmetrical geometries with ultrashort laser pulses impinging the constant rotating workpiece tangentially. This particular micro machining of geometry is referred to as laser turning, upon which the laser beam is, in general, oriented perpendicularly to the rotational axis and tangentially to the work piece. The target geometry is realized by moving the constantly rotating specimen according to the specified geometry along the focused laser spot. In contrast to mechanical turning, laser turning induces a minimized amount of lateral forces through friction that might deteriorate the geometry. In this study, laser turning is examined for stellite as a function of pulse energy and pulse length in the range between 240 fs and 10 ps. Shorter pulse lengths and higher pulse energies increase the achievable ablation rate, while in the pulse length regime of below 1 ps lower roughness is achieved. With a maximum ablation rate of 1.27 mm?/min and a minimum roughness of 0.17 µm, laser turning combines both, fast processing and high surface quality. [ABSTRACT FROM AUTHOR]

The authors report on the rapid fabrication of an axicon array, consisting of 37 individual axicons in a hexagonal arrangement, made of fused silica by an all laser-based manufacturing method within only 23 min. Therefore, a two-step process is used, first to ablate the silica substrate in a layer-by-layer process, generating the predefined geometry of the axicon array with a femtosecond laser, and second to polish the rough optical element by applying a CO 2 laser to reach smooth surfaces. Here, the roughness is reduced from 0.36 μ m before to 48 nm after the polishing step, thus reaching optical quality. The finalized axicon array was placed into a femtosecond laser machine for a detailed evaluation of the resulting quasi-Bessel beams. It is found that all sub-beams exhibit the typical zeroth-order Bessel beam intensity distribution, in turn confirming that the manufacturing process used here is well suitable for the fabrication of complex optical geometries. Cross sections of the sub-beams in both x- and y-directions show an almost identical intensity profile, indicating the high contour accuracy of the axicon array. The diameter of the quasi-Bessel beams is measured to be in the range of 9.4–10.3 μ m [full width at half-maximum (FWHM)], and the Bessel range in propagation direction amounts to between 8.0 mm and 8.5 mm (FWHM). [ABSTRACT FROM AUTHOR]