Laser Material Processing Research Articles

Measurements at laser materials processing machines: spectrum deconvolution including uncertainties and model selection

Abstract. Laser materials processing of workpieces using ultra-short pulsed lasers can lead to unwanted X-ray emission. Their dose rate and spectral distribution have been precisely determined. The measurements were carried out using a thermoluminescence detector (TLD)-based spectrometer in which 30 TLD planes are arranged one behind the other, the first 10 layers made of polymethyl methacrylate, while the remaining 20 layers are interspaced by absorbers with, from the front to the back, increasing atomic charge and thickness. The penetration depth of the radiation into the spectrometer depends on its energy, so that the energy-resolved spectrum of the radiation can be calculated from the TLD dose values by means of mathematical methods (Bayesian deconvolution). The evaluation process also takes into account both the uncertainties of all input quantities and the possibility of adopting different models for the spectrum form. This allowed the resulting spectra to be associated with their realistic uncertainty. The measurements are traceable to the Système international d'unités (SI), i.e. the International System of Units. The results not only provide manufacturers and users of ultra-short pulsed lasers with important information on the design of the machines with regard to radiation protection, but were also included in the recently concluded legislative procedure in the field of radiation protection in Germany.

The formation mechanism of special globular surface grain during the solidification of laser surface remelted near β titanium alloys

During laser materials processing, the cooling rate (R) and thermal gradient (G) near the molten pool surface region are much higher than those in the traditional casting conditions, which may result in special solidification microstructures. In this study, the surface microstructure of near β titanium alloys was investigated by conducting both laser surface remelting experiments and phase-field simulations. It was found that the grains nucleated near the surface region of the molten pool exhibit near globular shape with 〈110〉-oriented side branches, which are very different from the ordinary dendrite morphology in casting conditions. Phase-field simulations were used to detect the formation mechanism of such special surface grain morphology. It was illustrated that the surface grain morphology is determined by the combined effect of high cooling rate (R) and high vertical thermal gradient (G). The high R results in the morphology transition from needle-like dendrite to near globular shape by increasing the undercooling and reducing the impact of anisotropy of interface energy. The high G promotes the grain contour to be more close to globular and facilitates the fins-like side branches to develop along 〈110〉 direction. Furthermore, a selection map of surface grain morphology at various thermal conditions (R and G) was established, which can be applied to predict the surface grain morphology.

Methods for the solid-state lasers generation modes control for the material laser processing efficiency improving

The areas of modern solid-state lasers application for solving technological problems of materials laser processing are described. The mod composition of laser radiation, which is used in various technological processes of materials laser processing, such as perforation, cutting and welding of materials, and others, are analyzed. The necessity of solid-state lasers functional improvement in terms of increasing the efficiency of their practical use is substantiated. A method for the prospective application of lasers for technological purposes is presented, based on the functional separation of the acting laser pulses of millisecond duration. The results of experimental testing of this method on the basis of a solid-state technological YAG: Nd3 + laser are presented. The results of studies of the efficiency of processing materials with different thermophysical properties by laser pulses with a complex temporal shape are presented. The experiments results are analyzed. Potential areas of promising use of this method are noted.

Laser processing over a large area by wavefront-controlled scanning

In the processing of laser materials, the scanning area is limited by the extent to which the laser focus can be maintained within a plane. In order to increase the processing area, one has to enlarge the size of the laser focal spot, and, therefore, reduce the peak intensity and spatial resolution. In this paper, the authors report on a method that extends the processing area by controlling the wavefront of a laser beam using adaptive optics. By adding Zernike modes to the laser beam wavefront, the axial (along the beam propagation direction) location of the focal spot can be tuned within a range larger than the confocal length (two times the Rayleigh length), while the lateral spot size (and, thus, peak intensity and resolution) is maintained throughout the tuning range. A wave propagation model that simulates the propagation of a wavefront-controlled beam is used to compare with the experimental results, which are also confirmed by Zemax calculation. Laser trepanning of circular holes with consistent size on a wide polypropylene sheet is demonstrated. This work shows the potential of tailoring the phase of a laser beam for improving the accuracy and throughput in large-area laser processing of materials.

Modelling of temperature fields in DP1000 steel during laser treatment using diffractive optical elements

Diffractive optical elements (DOEs) with their unique properties allow to form a predetermined beam intensity profile in the focal plane. The use of DOEs in laser material processing technologies reveals new possibilities for controlling the properties and operational characteristics of processed parts. The beam intensity profile formed by DOE is presented in the form of an analytical expression that was used to set a surface heat source for modelling thermal processes in DP 1000 steel. Experimental studies of samples under laser heating were performed. Simulation output results correlate well with the experimental data. The proposed simulation model, based on a precise heat input definition, is an intermediate step to the final goal, which is the prediction of structural changes in the zone of laser beam irradiation.

Multiphysics Modeling, Sensitivity Analysis, and Optical Performance Optimization for Optical Laser Head in Additive Manufacturing

Optical laser head is a key component used to shape the laser beam and to deliver higher power laser irradiation onto workpieces for material processing. A focused laser beam size and optical intensity need to be controlled to avoid decreasing beam quality and loss of intensity in laser material processing. This paper reports the multiphysics modeling of an in-house developed laser head for laser-aided additive manufacturing (LAAM) applications. The design of computer experiments (DoCE) combined with the response surface model was used as an efficient design approach to optimize the optical performance of a high power LAAM head. A coupled structural-thermal-optical-performance (STOP) model was developed to evaluate the influence of thermal effects on the optical performance. A number of experiments with different laser powers, laser beam focal plane positions, and environmental settings were designed and simulated using the STOP model for sensitivity analysis. The response models of the optical performance were constructed using DoCE and regression analysis. Based on the response models, optimal design settings were predicted and validated with the simulations. The results show that the proposed design approach is effective in obtaining optimal solutions for optical performance of the laser head in LAAM.

Machine learning for multi-dimensional optimisation and predictive visualisation of laser machining

Interactions between light and matter during short-pulse laser materials processing are highly nonlinear, and hence acutely sensitive to laser parameters such as the pulse energy, repetition rate, and number of pulses used. Due to this complexity, simulation approaches based on calculation of the underlying physical principles can often only provide a qualitative understanding of the inter-relationships between these parameters. An alternative approach such as parameter optimisation, often requires a systematic and hence time-consuming experimental exploration over the available parameter space. Here, we apply neural networks for parameter optimisation and for predictive visualisation of expected outcomes in laser surface texturing with blind vias for tribology control applications. Critically, this method greatly reduces the amount of experimental laser machining data that is needed and associated development time, without negatively impacting accuracy or performance. The techniques presented here could be applied in a wide range of fields and have the potential to significantly reduce the time, and the costs associated with laser process optimisation.

Wear of the PG-CP4 powder coating applied to HVG steel and subjected to laser treatment

The article considers the influence of the radiation power W, the longitudinal feed Spr of the laser beam and the distance L from the protective glass of the laser focusing head to the workpiece on the PG-CP4 powder coating absolute wear which was applied to HVG steel by plasma and subjected to laser treatment. A multi-factor model is established that relates the absolute wear of PG-CP4 coating to the input process factors and allows you to assign a laser treatment mode to provide the minimum wear of the processed surface layer. The greatest influence on coating absolute wear is provided by the longitudinal feed of laser beam, with which the wear increases. Compared to Spr, the influence of factor W on the coating absolute wear is 24% less, and the factor L influence is 4.4 times less. The minimum absolute wear of the coating occurs at W = 5 kW, Spr = 40 mm/s, L = 85 mm. The results of the research are recommended for use in enterprises that implement the processes of plasma and laser processing of materials, as well as in design organizations that develop modern technological laser systems.

Polarization Effects on Ablation Efficiency and Microstructure Symmetricity in Femtosecond Laser Processing of Materials—Developing a Pattern Generation Model for Laser Scanning

Polarization Effects on Ablation Efficiency and Microstructure Symmetricity in Femtosecond Laser Processing of Materials—Developing a Pattern Generation Model for Laser Scanning

Численное исследование динамики нагрева золота ультракороткими дихроматическими импульсами лазерного излучения

The interaction of femtosecond laser pulses of moderate intensity with metals by the example of gold is studied. The main attention is paid to dichromatic regimes of laser irradiation corresponding to a new trend in the field of laser pro-cessing of materials. Based on the two-temperature approximation, a new model is created which involves a dichro-matic source of the laser radiation energy and also takes into account the influence of the electron temperature on the reflection coefficient of the irradiated material and thermophysical properties of its electronic subsystem. For numerical implementation of the model, an implicit divergent finite-difference scheme is created with an improved method for describing the melting process in comparison with previous works. The model calculations have shown that the absorption of the laser energy by gold depends on the sequence of two pulses at different wavelengths and the delay time between them, and for correct description of the absorbed energy, it is necessary to take into account the change of the reflection coefficient of the surface during irradiation. The developed model may be of interest for optimizing the processing of materials with ultra-short dichromatic laser pulses.

Imaging Scatterometry with Extrapolation of Missing BRDF Data for Materials Used in Laser Material Processing.

Imaging scatterometry is a method for determining the reflection distribution based on bidirectional reflectance distribution function (BRDF) measurements. However, it has a well-known limitation that results obtained by imaging scatterometry for small illumination angles are practically useless. Therefore, we propose an approach for reconstruction of the reflection distribution based on a series of measurements at different illumination angles and extrapolation of the missing results to overcome this limitation. The developed algorithm was validated using bidirectional transmittance distribution function (BTDF) measurements. The BRDF measurements were carried out for materials that are commonly used in laser material processing, i.e. substrates and functional coatings. The obtained data were subsequently used to determine the total reflection intensity from all considered materials, which were characterized by reconstructed distributions.

Additive manufacturing technology: laser material processing and functionally graded materials

Professor Akinlabi’s research and her team has focused on the field of advanced and modern manufacturing processes like Laser Additive Manufacturing (AM), in particular laser material processing. Her other research work is focused on laser metal deposition and functionally graded materials of titanium-based alloys and other materials. Some of the studies she has been involved in focus on cladding titanium with titanium carbide for enhanced wear properties, the cladding of titanium alloy biological implants with hydroxyapatite (HAP) for improved osteo-integration, and the cladding of Grade 5 titanium alloy with copper for improved corrosion properties for marine applications. Akinlabi focuses her investigations on the development of advanced metallic coatings on Ti-6Al-4V substrate using additive manufacturing technology for improved surface performance; with targeted applications in the aerospace, automotive, and shipbuilding industries. This work makes a substantial contribution to knowledge by bringing the theoretical clarity and experimental studies required for the effective assessment of surface degradation mechanisms in additive manufactured Ti-6Al-4V alloy. This is ascribed to the elimination of high residual stresses and crack formation through the optimization of laser processing parameters, leading to enhanced quality of the coatings, surface adhesion between the substrate and the reinforcement materials, microstructural evolution and thus improved mechanical properties. Her research was developed to produce advanced innovative corrosion-wear resistant coatings with enhanced hardness, tribological property, and sustainable anti-corrosion performance thereby, consequently lengthening the lifespan and durability of titanium and its alloys, eliminating material loss and equipment damage, minimizing cost of maintenance, and reduced failure of this material. Despite all the benefits derived from AM technology, there are still a lot of unresolved issues with the technology that has hindered its performance and commercialisation thereby limiting its application to high tolerant utilizations. Professor Akinlabi research on additive manufacturing techniques had produced near-net-shape, light weight and high strength components which has gradually revolutionized the manufacturing sector. The use of the technology is now providing sustainable production benefits, as ability to repair and manufacture components can now be employed to increase product life circle. Against this background, the Additive Manufacturing technology is in itself referred to as a technology of the future despite its versatile applications in the industry. On the other hand, Functionally Graded Materials (FGMs) are advanced materials usually developed for specific and tailored applications. The FGMs also referred to as materials of the future as its applications are not yet fully explored for tailored applications. In this talk, Prof Akinlabi shared some of her research endeavours in the field of AM and FGMs, and also shared the scope on the primary objectives of the joint project which was to be undertaken on FGM of Titanium alloy and Titanium Carbide.

Determination of the Entire Stent Surface Area by a New Analytical Method

Stenting is a widely used treatment procedure for coronary artery disease around the world. Stents have a complex geometry, which makes the characterization of their corrosion difficult due to the absence of a mathematical model to calculate the entire stent surface area (ESSA). Therefore, corrosion experiments with stents are mostly based on qualitative analysis. Additionally, the quantitative analysis of corrosion is conducted with simpler samples made of stent material instead of stents, in most cases. At present, several methods are available to calculate the stent outer surface area (SOSA), whereas no model exists for the calculation of the ESSA. This paper presents a novel mathematical model for the calculation of the ESSA using the SOSA as one of the main parameters. The ESSA of seven magnesium alloy stents (MeKo Laser Material Processing GmbH, Sarstedt, Germany) were calculated using the developed model. The calculated SOSA and ESSA for all stents are 33.34%(±0.26%) and 111.86 mm (±0.85 mm), respectively. The model is validated by micro-computed tomography (micro-CT), with a difference of 12.34% (±0.46%). The value of corrosion rates calculated using the ESSA computed with the developed model will be 12.34% (±0.46%) less than that of using ESSA obtained by micro-CT.

Few-mode random fiber laser with a switchable oscillating spatial mode.

Random fiber lasers are of tremendous interest to diverse applications for optical fiber sensing, speckle-free imaging. To date, random fiber lasers with fundamental mode oscillation have been well developed. However, controllable oscillating spatial mode in random fiber lasers have not been reported yet. Here, we propose and demonstrate a few-mode random fiber laser with a switchable oscillating spatial mode based on mode injection locking. An external signal light is injected to realize the locking of transverse mode in this random fiber laser and the direct oscillations of the fundamental mode, hybrid mode, and high order mode can be realized, respectively. This random fiber laser operates in the high-order LP11 mode stably with a threshold of as low as 88 mW. High efficiency and high purity cylindrical vector beams can be obtained by removing the degeneracy of the LP11 mode. This work may pave a path towards random fiber lasers with controllable spatial modes for specific applications in mode division multiplexing, imaging, and laser material processing.

Fabrication of a 3D Multi-Depth Reservoir Micromodel in Borosilicate Glass Using Femtosecond Laser Material Processing.

Micromodels are ideal candidates for microfluidic transport investigations, and they have been used for many applications, including oil recovery and carbon dioxide storage. Conventional fabrication methods (e.g., photolithography and chemical etching) are beset with many issues, such as multiple wet processing steps and isotropic etching profiles, making them unsuitable to fabricate complex, multi-depth features. Here, we report a simpler approach, femtosecond laser material processing (FLMP), to fabricate a 3D reservoir micromodel featuring 4 different depths—35, 70, 140, and 280 µm, over a large surface area (20 mm × 15 mm) in a borosilicate glass substrate. The dependence of etch depth on major processing parameters of FLMP, i.e., average laser fluence (), and computer numerically controlled (CNC) processing speed (), was studied. A linear etch depth dependence on was determined while a three-phase exponential decay dependence was obtained for . The accuracy of the method was investigated by using the etch depth dependence on relation as a model to predict input parameters required to machine the micromodel. This study shows the capability and robustness of FLMP to machine 3D multi-depth features that will be essential for the development, control, and fabrication of complex microfluidic geometries.

Inhibition and enhancement of linear and nonlinear optical effects by conical phase front shaping for femtosecond laser material processing

The emergence of high-powered femtosecond lasers presents the opportunity for large volume processing inside of transparent materials, wherein a myriad of nonlinear optical and aberration effects typically convolves to distort the focused beam shape. In this paper, convex and concave conical phase fronts were imposed on femtosecond laser beams and focussed into wide-bandgap glass to generate a vortex beam with tuneable Gaussian-Bessel features offset from the focal plane. The influence of Kerr lensing, plasma defocussing, and surface aberration on the conical phase front shaping were examined over low to high pulse energy delivery and for shallow to deep processing tested to 2.5 mm focussing depth. By isolating the underlying processes, the results demonstrate how conical beams can systematically manipulate the degree of nonlinear interaction and surface aberration to facilitate a controllable inhibition or enhancement of Kerr lensing, plasma defocussing, and surface aberration effects. In this way, long and uniform filament tracks have been generated over shallow to deep focussing by harnessing surface aberration and conical beam shaping without the destabilizing Kerr lensing and plasma defocussing effects. A facile means for compressing and stretching of the focal interaction volume is presented for controlling the three-dimensional micro- and nano-structuring of transparent materials.

Analysis of Direct Optical Ablation and Sequent Thermal Ablation for the Ultrashort Pulsed Laser Photo-Thermal Micromachining

An ultra-fast pulsed laser for materials processing can obtain submicrometer- to nanometer-sized parts or patterns (precision or accuracy) because the heat cannot diffuse in time for an ultra-fast pulsed duration, and this causes a threshold of ablation in multi-photoabsorption. The optical and thermal effects significantly affect the processing quality of an ultrashort pulsed laser for materials. This study utilizes a Laplace transform method to display the optical and thermal effects on the temperature field and the ablated depth of an ultrashort pulsed laser processing of materials. The results reveal that If an ultrafast pulsed laser-induced heat can keep the irradiated region above the evaporated temperature until the thermal diffusion occurs in the lattice of materials, thermal ablation occurs. The optical ablation can get a better processing quality due to less thermal diffusion. This study theoretically elucidates that the depth of optical ablation approximates the product of an optical absorption length and the logarithm of the ratio of laser fluence to laser fluence threshold. It has also been shown that the optical and thermal ablation, respectively, occur in low and high laser fluence because the optical ablation depends directly on the main source of the incident ultrashort pulsed laser. However, the thermal ablation is determined by the residual heat directly from the incident ultrashort pulsed laser after the optical ablation. The increase rate of the ablated depth per pulse with laser fluence is actually lower at high laser fluences than that at low laser fluences because the thermal ablation using the residual heat directly from the incident ultrashort pulsed laser is governed at high laser fluences. This study will provide the closed-form of a solution that elucidate the direct optical ablation and sequent thermal ablation for the ultra-fast pulsed laser photo-thermal processing.

The effect of surface laser texturing on the corrosion performance of the biocompatible β-Ti12Mo6Zr2Fe alloy

Laser materials processing is a very attractive technology, due not only to the vast number of applications in a wide range of materials and industrial segments but also due to the constant progress in the development of laser devices in terms of power, control, and costs. In metals processing technology, a high and precisely controlled density of energy concentrated in a small region imposes specific characteristics for the surface, keeping bulk properties unchanged. Alternatively, laser surface modification can result in outstanding surface properties to ordinary/low-cost metals. In the present work, the corrosion behavior of the Ti12Mo6Zr2Fe β-metastable alloy has been evaluated after laser surface melting and re-solidification under an inert atmosphere. It was found that this processing route produced significant metallurgical and chemistry changes on the surface of the alloy, including grain refinement and oxide formation. It resulted in a significant improvement in the corrosion properties in comparison to the passivated metal, as observed in potentiodynamic polarization curves and by electrochemical impedance spectroscopy (EIS) in simulated body fluid (SBF). Electrical equivalent circuit (EEC) modeling of impedance data suggests that a duplex porous/dense passive layer present in the untreated alloy enables the occurrence of charge-transfer mechanisms at some extension while a thicker oxide layer present after laser texturing increases the capacitive behavior of the surface.

Freeform optics design for extended sources in paraxial approximation exploiting the expectation maximization algorithm.

Freeform optics generating specific irradiance distributions have been used in various applications for some time now. While most freeform optics design algorithms assume point sources or perfectly collimated light, the search for algorithms for non-idealized light sources with finite spatial as well as angular extent is still ongoing. In this work, such an approach is presented where the resulting irradiance distribution of a freeform optical surface is calculated as a superposition of pinhole images generated by points on the optical surface. To compute the required arrangement of the pinhole images for a prescribed irradiance pattern, the expectation maximization algorithm from statistics is applied. The result is then combined with a ray-targeting approach for finding the shape of the corresponding freeform optical surface. At its current state, the approach is applicable to Gaussian input irradiances, single-sided freeform optics and for the paraxial case. An example freeform optical surface for laser material processing is shown and discussed demonstrating the performance and the limitations of the presented approach.

Physical study of spatter and melt pool dynamics during millisecond laser metals drilling

Understanding spatter and melt pool dynamics during laser drilling remains a topic of considerable importance to determine the quality of processed materials. Due to metallic materials’ opacity to visible radiation, we studied the spatter dynamics outside of holes during laser drilling using shadow mapping. A 2D transient model for simulating laser material processing was implemented using COMSOL commercial Multiphysics software and the spatter phenomena and melt pool dynamics were assessed. The results can be used as a basis to explain the different processes involved in the material properties during laser processing, which is a powerful tool to understand spatter’ formation mechanism and provide theoretical references for optimizing processing parameters.