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

Laser assisted electrodeposition of binary metallic alloys from water-based electrolytes: The case of palladium‐platinum

The present work investigates the applicability of laser assisted electrodeposition (LAE) to the plating of binary metallic alloys from aqueous solutions and in absence of any external polarization. PdPt was chosen as proto-typical system and the deposition was carried out from phosphate-based electrolytes. The effect of electrolyte formulation and laser parameters on the composition, thickness and physical properties of the alloys was studied. In order to provide more insight on the LAE deposition process of alloys, the electrochemical properties of the electrolytes employed were correlated with the properties of the materials obtained. In analogy with standard alloy plating processes, a clear link between the currents observed for the reduction of the two single metals and the composition/thickness of the coated PdPt layers could be established. Compositionally tunable and continuous PdPt coatings were obtained and their chemical, morphological and mechanical properties were determined. Finally, the possibility to selectively deposit the PdPt alloy was demonstrated by patterning a complex figure

Seam tracking and gap bridging during robotic laser beam welding via grayscale imaging and wobbling

The use of laser beam welding with robotic manipulators is expanding towards wider industrial applications as the system availability increases with reduced capital costs. Conventionally, laser welding requires high positioning and coupling accuracy. Due to the variability in the part geometry and positioning, as well as the thermal deformation that may occur during the process, joint position and fit-up are not always acceptable nor predictable a-priori if simple fixtures are used. This makes the passage from virtual CAD/CAM environment to real production site not trivial, limiting applications where short part preparations are a need like small-batch productions. Solutions that render the laser welding operations feasible for production series with non-stringent tolerances are required to serve a wider range of industrial applications. Such solutions should be able to track the seam as well as tolerating variable gaps formed between the parts to be joined. In this work, an online correction for robot trajectory based on a greyscale coaxial vision system with external illumination and an adaptive wobbling strategy are proposed as means to increase the overall flexibility of a manufacturing plant. The underlying vision algorithm and control architectures are presented; the robustness of the system to poor illumination conditions and variable reflection conditions is also discussed. The developed solution employed two control loops: the first is able to change the robot pose to follow varying trajectories; the second, able to vary the amplitude of circular wobbling as a function of the gap formed in butt-joint welds. Demonstrator cases on butt-joint welds with AISI 301 stainless steel with increased complexity were used to test the efficacy of the solution. The system was successfully tested on 2 mm thick, planar stainless-steel sheets at a maximum welding speed of 25 mm/s and yielded a maximum positioning and yaw-orientation errors of respectively 0.325 mm and 4.5°. Continuous welds could be achieved with up to 1 mm gaps and variable seam position with the developed control method. The acceptable weld quality could be maintained up to 0.6 mm gap in the employed autogenous welding configuration