Adrian H. A. Lutey

Modeling of Thin-Film Single and Multilayer Nanosecond Pulsed Laser Processing

A complete model of nanosecond pulsed laser scribing of arbitrary thin multi-layer structures is presented. The chain of events is separated according to time-scale; an initial simulation considers material response during the pulse; another combines this result with the much slower effects of heat flow away from the laser axis. The former considers heating, vaporisation and phase explosion of metals in the course of a single pulse, accounting for variations in thermal conductivity and optical absorption as the material becomes superheated and approaches its critical temperature. The latter calculates the bidimensional heat flow in a complete multi-layer structure over the course of a scribing operation, combining material properties and considering removal by both short-pulse ablation and long-term heating of the work piece. Simulation results for the single pulse ablation of an aluminium target align well with published experimental data both in terms of phase explosion threshold and ablation depth as a function of fluence. Bidimensional heat flow simulations of a polypropylene–aluminium–polypropylene triplex structure reveal the progression of events towards steady state behaviour; aluminium ejected due to short-pulse ablation and plastic removed due to conduction.© 2013 ASME

Towards Laser-Textured Antibacterial Surfaces

Escherichia coli and Staphylococcus aureus bacterial retention on mirror-polished and ultrashort pulse laser-textured surfaces is quantified with a new approach based on ISO standards for measurement of antibacterial performance. It is shown that both wettability and surface morphology influence antibacterial behavior, with neither superhydrophobicity nor low surface roughness alone sufficient for reducing initial retention of either tested cell type. Surface structures comprising spikes, laser-induced periodic surface structures (LIPSS) and nano-pillars are produced with 1030 nm wavelength 350 fs laser pulses of energy 19.1 μJ, 1.01 μJ and 1.46 μJ, respectively. SEM analysis, optical profilometry, shear force microscopy and wettability analysis reveal surface structures with peak separations of 20–40 μm, 0.5–0.9 μm and 0.8–1.3 μm, average areal surface roughness of 8.6 μm, 90 nm and 60 nm and static water contact angles of 160°, 119° and 140°, respectively. E. coli retention is highest for mirror-polished specimens and spikes whose characteristic dimensions are much larger than the cell size. S. aureus retention is instead found to be inhibited under the same conditions due to low surface roughness for mirror-polished samples (Sa: 30 nm) and low wettability for spikes. LIPSS and nano-pillars are found to reduce E. coli retention by 99.8% and 99.2%, respectively, and S. aureus retention by 84.7% and 79.9% in terms of viable colony forming units after two hours of immersion in bacterial broth due to both low wettability and fine surface features that limit the number of available attachment points. The ability to tailor both wettability and surface morphology via ultrashort pulsed laser processing confirms this approach as an important tool for producing the next generation of antibacterial surfaces.

An improved model for cold metal transfer welding of aluminium alloys

An improved model is proposed for automated cold metal transfer (CMT) welding based on a time-dependent double-ellipsoidal volumetric heat flux distribution. Equations are evaluated numerically within COMSOL Multiphysics for CMT welding of a 3-mm-thick AA5754 Al–Mg alloy plate. The simulation calculates transient and steady-state temperature distributions within the weld seam and heat-affected zone (HAZ). Validation of the model is achieved by comparing simulated temperatures with measured values from thermocouples in the HAZ during welding experiments, as well as through comparison of the calculated fusion zone and microscope images of the weld seam. Under steady-state conditions, large differences between the peak and average calculated temperatures in the weld pool highlight the underlying phenomenon responsible for improvements in weld quality for thin sheets with CMT compared to conventional joining processes. The developed simulation provides opportunities for process optimisation and sensitivity analysis in many applications.

Laser Profiling of Aluminum Oxide Grinding Wheels

Laser profiling experiments are performed at normal incidence on fine grain medium density aluminum oxide grinding wheels with a pulsed nanosecond 1064nm fiber laser source with maximum pulse fluence 369J/cm2. In order to determine the incision depth and ideal laser pass separation distance, laser exposures are first performed on high purity, low porosity aluminum oxide blocks and subsequently analyzed with an optical profiler operating in confocal mode. This ablation data is then applied to path planning for grinding wheel profiling experiments, with division of the necessary removal depth according to the measured incision depth and ideal pass separation distance. X-ray computed tomography is utilized to determine the resulting profile accuracy as a function of process parameters. Test results indicate a maximum profile accuracy in the order of 200μm; however, in order to approach the accuracy of diamond dressing, some two orders of magnitude lower, it is likely that tangential laser incidence is necessary.Copyright

Laser processing of thin films for industrial packaging

Single layer thin-film materials such as aluminum, polyethylene, polypropylene, and their multi-layer combinations such as aluminum-paper have been exposed to different laser radiation. A wide number of samples have been processed with 10 - 12.5 ns IR and Green, and 500 - 800 ps IR laser radiation at different translating speeds ranging from 50 mm/s to 1 m/s. High quality incisions have been obtained for all tested materials within the experimental conditions. The presented results provide the necessary parameters for an efficient cut and processing of the tested materials, for the employment of pulsed laser sources in the packaging industry, allowing the laser to prevail in lieu of more costly and energy intensive methods.

Picosecond and nanosecond pulsed laser ablation of aluminium, polypropylene, polyethylene, and their thin-film combinations

The pico- and nano-second ablation thresholds and subsequent pulse-energy cut-depth and width relationships of aluminium, polypropylene (PP), polyethylene (PE) and their various thin-film combinations have been determined at 515 nm and 1064 nm. High quality incisions were obtained for all structures within certain parameter ranges. All ablation thresholds were found to be functions of the temporal pulse-width. Numerical simulations revealed the underlying mechanisms as phase explosion and thermal conduction. The presented results provide necessary parameters for the efficient cut and scribe of such materials, allowing the laser to prevail in lieu of more costly and energy intensive methods.

Characterization of Lattice Structures for Additive Manufacturing of Lightweight Mechanical Components

Tensile and compression test specimens comprising lattice structures with simple cubic, crossing-rod and body-centered cubic (BCC) unit cells are produced via SLM additive manufacturing (AM) of AISI 316L stainless steel and CoCr powder. Equivalent stress-elongation curves are obtained, with equivalent strength, specific strength, stiffness modulus and specific stiffness calculated based on specimen density and sample cross-section. The obtained results highlight the fact that analogous structures can behave very differently depending on the chosen material. While large differences are obtained in strength and stiffness between the different unit cell types, specific strength and specific stiffness vary to a lesser extent. Two case studies are presented, including a porous structure suitable for bone implants in the field of biomedical engineering and an AISI 316L food packaging machine component. The results obtained in this study provide useful guidelines and equivalent properties for designers wishing to exploit the advantages of internal lattice structures in AM.Copyright

Laser Assisted Cold Bending of High Strength Steels

This paper presents the feasibility of an innovative application of laser-assisted bending process. The high strength steel sheets bending, carried out after a laser heat treatment, is studied. Several strategies aimed at obtaining a ductile structure along the bending line, suitable for cold forming, are investigated. The influence of laser processing parameters on the microstructure, hardness and strength of the sheets are discussed and analyzed. In order to predict the temperature and ensure the repeatability and reliability of the process, a model for heat treatment simulation is developed. The study of the experimental data and the integration with the simulation of the heating phase lead to the definition of specific process parameters suitable for achieving a crack-free cold bending of high strength steels.Copyright

A Multi-Axis Deep Drawing Servo Press With Non-Overconstrained Architecture

Servo actuated presses can provide maximum pressing force at any slide position in the same manner that hydraulic presses do, while offering several benefits in terms of precision, energy conversion efficiency and simplicity due to their lack of hydraulic circuitry and oil. Several press builders have developed electric-spindle actuated presses; however, issues relating to multi-axis architecture have been neglected. The present study proposes an innovative method of avoiding overconstrained architecture by implementing a kinematic mechanism that connects multiple servo axes to one slide. Servo axis design is developed by creating a dynamic model of a kinematic chain composed of a servo-motor, gearbox reducer and ball screw transmission. A study of a biaxial industrial servo press prototype with non-overconstrained architecture, currently under construction, is presented as proof of concept. It is shown that such a non-overconstrained multi-axis press can be constructed from commercially available components, achieving high energy efficiency at high load with relatively simple construction.Copyright

Pulsed Laser Ablation of Lithium Ion Battery Electrodes

Lithium ion battery electrodes have been exposed to 1064nm nanosecond pulsed laser irradiation with pulse energy in the range 8μJ – 1mJ and fluence in the range 3.2 – 395J/cm2. Experiments have been executed at translational velocities of 100mm/s and 1m/s, allowing individual characterization of the graphite and lithium metal oxide coatings of the copper anode and aluminum cathode, respectively, as well as that of the complete multi-layer structures. A 3D optical profiler has been utilized to measure the incision depth of all samples and allow observation of the process quality. At high velocity, partial or complete removal of the upper coating layers was achieved with little or no impact on the underlying metallic layers. At low velocity, complete cuts were possible under certain conditions, with process efficiency found to be almost entirely governed by the response of the metallic layers. While the coating layers of each electrode exhibited different responses than the metallic layer, the influence of the latter was found to be dominant for cutting operations. Shorter pulses with fluence in the range 30 – 60J/cm2 were found to lead to optimum process outcomes with the employed laser source.Copyright