Lasers in the manufacturing of cardiovascular metallic stents: Subtractive and additive processes with a digital tool

Laser beams can be manipulated to achieve different types of interaction mechanisms with metals allowing them to heat, melt, vaporize, or ablate them. Today's laser sources are robust, fast-addressable optoelectronic devices, easily integrated into automation systems along with sophisticated CAD/CAM solutions. Being a photonic digital tool, the laser beam is a fundamental tool for Industry 4.0 and is already widely exploited in the manufacturing of metallic stents. The conventional manufacturing method of laser cutting employs a subtractive method to cut the stent mesh on tubular feedstock. On the other hand, laser beams can be exploited to melt metallic powders to produce stent geometries in a layer-by-layer fashion. The present work provides a short state of the art review concerning the works focusing on the two laser-based manufacturing processes underlining the evolution of the laser source types and used materials. The work provides insights into the future opportunities and challenges that should be faced by the manufacturing research communities in the light of improving the biomedical device performance by exploiting the possibilities provided by the digital tool

Densification mechanism for different types of stainless steel powders in Selective Laser Melting

Selective laser melting is a powder based additive manufacturing process where the metallic powder particles are melted by a high power laser beam. Different types of stainless steel powders made by gas and water atomization were analyzed before processing, in particular regarding their particle size distributions and morphology. Particle analysis was carried out using laser diffraction technologies and digital image analysis. A suitable designed experiment has been carried out and the specimen density has been measured and linked to the properties of the powders. Eventually the possibility to reach high density specimen by adjusting process parameters is discussed

Feasibility of using bio-mimicking fish scale textures in LPBF for water drag-reducing surfaces

In this work, bio-mimicking fish scale textures are produced by LPBF and AlSi7Mg0.6 powder to reduce drag forces on nautical components. For this purpose, a surface texture inspired by the European bass skin was modelled and parametrized. Textures were applied over the external surface of purpose-designed specimens. Additive manufacturing quality of textures was assessed using focus variation microscopy to examine surface roughness as well as geometrical errors. Once the feasibility of producing the desired bio-mimicking surfaces was confirmed, the designed surface patterns were analysed in the computation fluid dynamics modelling environment. The behaviour of the surfaces was characterized in terms of drag force generated over a fixed dimension plate model. The most promising configuration was further investigated in a sensitivity analysis where variations in main stream velocity and in surface roughness are applied. Drag reduction was related to the lowering of the viscous component and was found to be in the order of 1–2%, with respect to a smooth surface, for free stream velocity of 2.5–5 m s−1 and average roughness smaller than the as-built condition. The results confirm that the modelled surfaces can be reproduced with sufficient geometrical fidelity, showing great promise for drag-reducing metallic components produced by additive manufacturing

Investigation of pulse shape characteristics on the laser ablation dynamics of TiN coatings in the ns regime

n this work, the ablation dynamics of TiN coating with a ns-pulsed fibre laser in a wide range of pulse durations were studied. Critical time instances within the pulse duration were assessed by reflected pulse analysis. Digital holography was employed to investigate the shock wave expansion dynamics within and beyond the pulse duration. The results depict that the absorption behaviour changes as a function of the pulse rise time. Moreover, planar expansion of the shock wave is observed, which is generally linked to higher machining quality and absence of excessive plasma. The results of the study are interpreted to depict the required characteristics of optimized pulse shapes in the ns-region for improved micromachining performance

Evaluation of Self-Mixing Interferometry Performance in the Measurement of Ablation Depth

This paper studies self-mixing interferometry (SMI) for measuring ablation depth during laser percussion drilling of TiAlN ceramic coating. The measurement performance of SMI was investigated in a large processing range producing blind microholes with depths below and beyond the average coating thickness. Signal characteristics of the measurement system were evaluated indicating sources of disturbance. The SMI measurements were compared with a conventional measurement device based on focus variation microscopy to evaluate the measurement error. The measurement error classes were defined, as well as defining the related error sources. The results depict that the measurement error was independent of the processing condition, hence the hole geometry and ablation rate. For 76% of cases, measurement error was below the intrinsic device resolution obtainable by simple fringe counting of half a wavelength (λ/2 = 0.393 μm)

Coordination of spatial and temporal laser beam profile towards ultra-fine feature fabrication in laser powder bed fusion

Laser powder bed fusion (LPBF) is a metal additive manufacturing technology that provides high shape and application flexibilities. Although dimensional flexibility is high in theory thanks to the non-contactless micro range processing tool (i.e., laser beam) and powder, the fabrication robustness of thin and ultra-thin features (dnominal?200 ?m) is still a challenge for the technology. In particular, geometrical fidelity and dimensional accuracy problems have been raising towards the ultra-thin fabrication segment. Although there were different studies that presented solutions for robust and sustainable fabrication strategy in ultra-thin segment in the literature, the vast majority of them focused on process itself directly, and the technological feasibility of the LPBF systems was not considered. However, without considering technological feasibility of the LPBF systems, the presented solutions in the literature are far from providing solid basis and they may mislead the users in the case of direct application. In this sense, this study mainly focused on the scanning capability of the systems for features under 200 µm dimensional range with temporal and spatial laser beam management in the case of conventional scanning strategy (contour and hatch). For this purpose, the fabrication process reduced to the two dimensions (scanning region) via the laser marking tests, and custom laser parameters, which are provided by the industrial grade open architecture LPBF system, has been used. Here, it has been reported that two different errors related to scanning performance and process parameters for continuous and pulsed wave laser emission modes, separation, and compensation of these two errors, and investigation methodology for technological feasibility of the LPBF machines. Moreover, in the pulsed wave laser emission mode, two linear parameters have been presented to optimize spatial energy distribution. Considering the results coming from the practical observations and measurements, it is possible to indicate that the technological feasibility of the utilized LPBF system should be key concern before laser or scan related parameters optimization. The results show that if correct scanning parameters have been selected in the technological feasibility window of the system, scan trajectories based on conventional hatching method can be carried out with sufficient geometrical accuracy

Laser weldability of laser powder bed fused AlSi7Mg0.6

Laser Powder Bed Fusion (LPBF) allows to manufacture components with lightweight and near net shape suited to aerospace and aviation applications employing Al-alloys. The process is highly suited to one-of-a-kind or small batch production of small to medium sized parts. As the maturity of the process and its end-users increase, the demand for larger components becomes more relevant. The increase of part size by increasing the size of the LPBF machine inevitably increases the cost and the complexity of the employed system. Moreover, using multiple lasers in a large powder bed to produce larger parts may bring residual stresses, part deformation and a higher chance of process failure. In the light of these, the use of joining operations, in particular welding, appears as a suitable option for the production of large components via LPBF. Indeed, the process lends itself well to also producing dedicated joint edge preparations, thickness and section variation within the location of the welded joint. Amongst different processes, laser welding stands out as a viable option as it can provide narrower weld seam and heat affected zone, produce less deformation on the parts and be automated with cartesian or robotic manipulators. This work discusses the influence of different laser welding strategies on the LPBF produced AlSi7Mg0.6

Application of self-mixing interferometry for depth monitoring in the ablation of TiN coatings

Among possible monitoring techniques, self-mixing interferometry stands out as an appealing option for online ablation depth measurements. The method uses a simple laser diode, interference is detected inside the diode cavity and measured as the optical power fluctuation by the photodiode encased in the laser diode itself. This way, self-mixing interferometry combines the advantages of a high resolution point displacement measurement technique, with high compactness and easiness of operation. For a proper adaptation of self-mixing interferometry use in laser micromachining to monitor ablation depth, certain optical, electronical, and mechanical limitations need to be overcome. In laser surface texturing of thin ceramic coatings, the ablation depth control is critically important to avoid damage by substrate contamination. In this work, self-mixing interferometry was applied in the laser percussion drilling of TiN coatings. The ∼4 μm thick TiN coatings were drilled with a 1 ns green fiber laser, while the self-mixing monitoring was applied with a 785 nm laser diode. The limitations regarding the presence of process plasma are discussed. The design criteria for the monitoring device are explained. Finally, the self-mixing measurements were compared to a conventional optical measurement device. The concept was validated as the measurements were statistically the same

Comprehensive benchmarking of laser welding technologies including novel beam shapes and wavelengths for e-drive copper hairpins

Laser welding is the industrially accepted method for the joining of Cu hairpin windings in the production of electric drives. High brilliance laser beams are scanned over the bare ends of the Cu wires producing a rapid connection through deep penetration remote welding. Despite being an accepted manufacturing method, laser welding of Cu hairpins still requires detailed studies concerning manufacturing productivity and quality. As the availability of novel laser sources with higher power levels, new wavelengths, and beam shaping capabilities increase, the need for benchmarking studies emerges. In this work, six different laser welding systems were compared in terms of process productivity and quality during the welding of Cu hairpins used for automotive traction. The different solutions presented power levels from 3 to 6 kW, with wavelengths from near infrared (NIR) to visible, including in source dynamic beam shaping. The weld bead formation was observed through high-speed imaging. The welds were analyzed in terms of their geometry, internal defects, and most relevantly for their mechanical strength. The results showed advantages of each of the employed system while the laser systems providing the highest irradiance profile produced the fastest weld with more elevated mechanical strength independently from the wavelength