Design and implementation of dynamic beam shaping in high power laser processing by means of a Deformable Mirror

Optimising the laser intensity distribution in high power laser processes, such as laser welding, laser cladding or laser hardening, can be used to tailor the local thermal fields and thermal cycles which, in turn, determine the final process results. Deformable Mirrors allow to dynamically shape the beam profile and previous studies showed their potential. However, only limited flexibility in achievable beam shapes is shown at higher power levels. In addition the relation between desired laser intensity profiles and required mirror surface profiles is nontrivial. In this work the design and implementation of a dynamic beam shaping system, capable of handling high laser powers (up to 1 kW), is presented and evaluated. To that end, several distinctly different laser intensity profiles are defined, corresponding mirror surfaces are determined and realised with the beam shaping system. Measurements of the laser intensity profiles were compared with laser intensity profiles simulated using a previously presented mathematical framework and showed a good agreement. From the measurements it was concluded that the setup is suitable for high laser powers (up to 1 kW) and is characterised by large depth of focus (&lt;14% change in dimensions at a distance of 100 mm from the focal plane).</p

Ejection Regimes in Picosecond Laser-Induced Forward Transfer of Metals

Laser-induced forward transfer (LIFT) is a 3D direct-write method suitable for precision printing of various materials, including pure metals. To understand the ejection mechanism and thereby improve deposition, here we present visualizations of ejection events at high-spatial (submicrometer) and high-temporal resolutions, for picosecond LIFT of copper and gold films with a thickness 50  nm≤d≤400  nm . For increasing fluences, these visualizations reveals the fluence threshold below which no ejection is observed, followed by the release of a metal cap (i.e., a hemisphere-shaped droplet), the formation of an elongated jet, and the release of a metal spray. For each ejection regime, the driving mechanisms are analyzed, aided by a two-temperature model. Cap ejection is driven by relaxation of thermal stresses induced by laser-induced heating, whereas jet and spray ejections are vapor driven (as the metal film is partly vaporized). We introduce energy balances that provide the ejection velocity in qualitative agreement with our velocity measurements. The threshold fluences separating the ejection regimes are determined. In addition, the fluence threshold below which no ejection is observed is quantitatively described using a balance between the surface energy and the inertia of the (locally melted) film. In conclusion, the ejection type can now be controlled, which allows for improved deposition of pure metal droplets and spray

Toward 3D Printing of Pure Metals by Laser-Induced Forward Transfer

3D printing of common metals is highly challenging because metals are generally solid at room conditions. Copper and gold pillars are manufactured with a resolution below 5 μm and a height up to 2 mm, using laser-induced forward transfer to create and eject liquid metal droplets. The solidified drop's shape is crucial for 3D printing and is discussed as a function of the laser energ

Revealing the effects of laser beam shaping on melt pool behaviour in conduction-mode laser melting

Laser beam shaping offers remarkable possibilities to control and optimise process stability and tailor material properties and structure in laser-based welding and additive manufacturing. However, little is known about the influence of laser beam shaping on the complex melt-pool behaviour, solidified melt-track bead profile and microstructural grain morphology in laser material processing. A simulation-based approach is utilised in the present work to study the effects of laser beam intensity profile and angle of incidence on the melt-pool behaviour in conduction-mode laser melting of stainless steel 316L plates. The present high-fidelity physics-based computational model accounts for crucial physical phenomena in laser material processing such as complex laser-matter interaction, solidification and melting, heat and fluid flow dynamics, and free-surface oscillations. Experiments were carried out using different laser beam shapes and the validity of the numerical predictions is demonstrated. The results indicate that for identical processing parameters, reshaping the laser beam leads to notable changes in the thermal and fluid flow fields in the melt pool, affecting the melt-track bead profile and solidification microstructure. The columnar-to-equiaxed transition is discussed for different laser-intensity profiles.Comment: 27 pages, 10 figure

Two-photon microscopy for microrobotics:Visualization of micro-agents below fixed tissue

Optical microscopy is frequently used to visualize microrobotic agents (i.e., micro-agents) and physical surroundings with a relatively high spatio-temporal resolution. However, the limited penetration depth of optical microscopy techniques used in microrobotics (in the order of 100 μm) reduces the capability of visualizing micro-agents below biological tissue. Two-photon microscopy is a technique that exploits the principle of two-photon absorption, permitting live tissue imaging with sub-micron resolution and optical penetration depths (over 500 μm). The two-photon absorption principle has been widely applied to fabricate sub-millimeter scale components via direct laser writing (DLW). Yet, its use as an imaging tool for microrobotics remains unexplored in the state-of-the-art. This study introduces and reports on two-photon microscopy as an alternative technique for visualizing micro-agents below biological tissue. In order to validate two-photon image acquisition for microrobotics, two-type micro-agents are fabricated and employed: (1) electrospun fibers stained with an exogenous fluorophore and (2) bio-inspired structure printed with autofluorescent resin via DLW. The experiments are devised and conducted to obtain three-dimensional reconstructions of both micro-agents, perform a qualitative study of laser-tissue interaction, and visualize micro-agents along with tissue using second-harmonic generation. We experimentally demonstrate two-photon microscopy of micro-agents below formalin-fixed tissue with a maximum penetration depth of 800 μm and continuous imaging of magnetic electrospun fibers with one frame per second acquisition rate (in a field of view of 135 × 135 μm2). Our results show that two-photon microscopy can be an alternative imaging technique for microrobotics by enabling visualization of micro-agents under in vitro and ex ovo conditions. Furthermore, bridging the gap between two-photon microscopy and the microrobotics field has the potential to facilitate in vivo visualization of micro-agents.</p

An Overview: Laser-Based Additive Manufacturing for High Temperature Tribology

Laser-based additive manufacturing (LBAM) is a versatile manufacturing technique, extensively adopted to fabricate metallic components of enhanced properties. The current review paper provides a critical assessment of the fabricated metallic coatings and parts through LBAM-processes [e.g., laser metal deposition (LMD) and selective laser melting (SLM)] for high temperature tribological applications. A succinct comparison of LBAM-fabrication and conventional manufacturing is given. The review provides an insight into the sophisticated application-driven material design for high temperature tribological contacts. The review highlights the major mechanisms behind the improvement in the tribology of the laser-deposits; properties evolving as a consequence of the microstructure, lamellar solid lubricants, sulfides, soft metals, lubricious oxides, and self-lubricating surfaces

Experimental investigation of the jet-on-jet physical phenomenon in laser-induced forward transfer (LIFT)

Understanding the physics behind the ejection dynamics in laser-induced forward transfer (LIFT) is of key importance in order to develop new printing techniques and overcome their limitations. In this work, a new jet-on-jet ejection phenomenon is presented and its physical origin is discussed. Time-resolved shadowgraphy imaging was employed to capture the ejection dynamics and is complemented with the photodiode intensity measurements in order to capture the light emitted by laser-induced plasma. A focus scan was conducted, which confirmed that the secondary jet is ejected due to laser-induced plasma generated at the center of the laser spot, where intensity is the highest. Five characteristic regions of the focus scan, with regards to laser fluence level and laser spot size, were distinguished. The study provides new insights in laser-induced jet dynamics and shows the possibility of overcoming the trade-off between the printing resolution and printing distance