Defocused Laser Beam Research Articles

An approach to modelling defocusing and keyhole reflectivity in keyhole laser processes

Keyhole laser-based processes have been adopted widely due to the high-quality welds and cuts that they produce and the accurately controlled dimensions of the end part. Fast-running simulation tools are required in order to estimate the dependence of process outputs, efficiency, and stability on the process variables. This work presents a fast-running simulation tool for keyhole laser-based processes with inert or no assisting gas that takes into account the effect of laser beam defocusing and the alteration of the effective reflectivity of the material due to the keyhole cavity. The model is built upgrading a successful model for the thermal phenomena based on finite differences and the enthalpy method. The presented model is compared to experimental results for the laser cutting process with both CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} and fiber laser sources and inert assisting gas. The model accurately predicts cutting depth and kerf, heat-affected zone width, and the dependence of the cutting depth on the position of the focal plane. The model manages to achieve reduced computational time enabling its utilization in process digital twins.

The effect mechanism of laser beam defocusing on the surface quality of IN718 alloy prepared by laser powder bed fusion

The surface quality issue of the laser powder bed fused (LPBF-ed) parts deteriorates the mechanical properties. Laser beam defocusing (LBD) can effectively improve the surface quality. To reveal the effect mechanism of LBD on the surface quality of the LPBF-ed parts, a laser powder bed fusion (LPBF) numerical model is established. Based on the calculated results, the formation process of the surface defects such as bulges, depressions, partially melted powders and spatters and the corresponding melt pool dynamic behaviors are investigated. The effects of LBD on the melt pool dynamic behaviors during the surface defects formation and the track width are discussed. The results show that LBD can improve the surface quality by reducing Marangoni force and recoil pressure to reduce bulge height and depression depth, increasing track width to decrease partially melted powders, and increasing surface tension and reducing kinetic energy to decrease spatters.

Healing of keyhole porosity by means of defocused laser beam remelting: Operando observation by X-ray imaging and acoustic emission-based detection

One of the remaining challenges in Laser Powder Bed Fusion (LPBF) of metals is the control of the formation of keyhole pores, resulting from a local excessive energy input during processing. Such defects can lead to degraded mechanical properties and are typically detected and/or removed after the process through non-destructive quality-inspection procedures and porosity-removal treatments. Monitoring and controlling the formation of defects during the LPBF process can allow circumventing such time-consuming and costly post-process stages. This paper develops a new approach to perform in-situ healing of deep keyhole pores, using a positively defocused laser beam with finely tuned laser remelting process parameters. Synchrotron radiographic images of the process zone are acquired during laser remelting. The use of operando imaging enables the visualization of pore removal during processing, and unveils the effect of various remelting conditions on the healing efficiency. The acoustic signals generated during laser remelting are recorded using a high-sensitivity optical microphone, and analyzed in parallel with the X-ray images, allowing the acoustic signature of defect healing to be identified. The present paper demonstrates for the first time that an airborne acoustic sensor can be used to monitor the healing of keyhole pores during LPBF.

Butt welding of SUS304 and Inconel718 tubes by using defocused laser beam

Joining stainless steel to superalloy is currently of extensive interest for applications in aviation and automotive industries. However, conventional welding is prone to encounter defects such as cracks and austenite grain coarsening in the fusion zone. In the present study, laser welding was applied to join SUS304 stainless steel and Inconel718 superalloy circular tubes due to their precise local heat input and accuracy. The effects of defocusing distance, welding speed, and laser power on welding characteristics were studied by changing the values of the mentioned parameters, which manifested that different process parameters exerted a tremendous impact on the cross section morphology and shape of the weld seam. In addition, finite element simulation software was used to simulate temperature field distribution. The results revealed that there would be a buffering region on the temperature field once the laser power gradually decreased, which remarkably reflected the effect of the laser power descending on eliminating weld craters. Therefore, the crater defects caused by laser beam accelerating and decelerating at the start and end of welding could be effectively eliminated through synchronously regulating laser power in the real welding process.

Computer-aided process planning system for laser forming: from CAD to part

Laser forming is a highly flexible and iterative contactless thermomechanical forming process that utilises a defocused laser beam to induce material shortening and bending. There is a high potential for the use of laser forming in rapid prototyping, small and unique batch production; however, to achieve that potential a certain level of automation is required. The presented work seeks to address this need of automation by proposing an end-to-end sheet-metal manufacturing system utilising laser forming and cutting, where the input for the system is a 3D CAD model and the output is a manufactured product. The contribution of the presented work is interconnected laser forming and cutting framework. The presented framework consists of subsystems for automatic extraction of geometrical information from a CAD model, process planning and generation of manufacturing information, scheduling of tasks and a feedback control loop for laser forming. The framework is experimentally validated and has shown consistent forming and cutting behaviour on two types of parts with variation in cutting features, bend angle and lengths of the bend's free edge.

Laser beam remelting of stainless steel plate for cladding and comparison with conventional CMT process

Progressing towards circular economy requires smarter and more efficient use of energy and resources. Laser beam can be efficient and flexible tool for melting different metals, commonly used in cladding and additive manufacturing (AM) with a wire and powder feedstock. As an alternative, feedstock in the form of plates and sheets can be used for cladding to achieve corrosion resistant surfaces. Compared to powder or wire, plates are easier to process, less costly to use, and may come as scrap metal. This leads to smarter and more efficient resource utilization. However, processing plates in such way is not mature and requires more in-depth investigation to be competitive with well-established methods. In this work, 2.0 mm thick 316L stainless steel plates were remelted by a high-power fibre laser beam for cladding on carbon steel substrates. It was compared to the conventional cold metal transfer (CMT) welding-based arc cladding which is frequently used due to a low heat input. In the first phase, different defocusing distances were studied to understand the laser remelting process capabilities to optimize the productivity. It was found that a highly defocused laser beam provided unstable melt pool conditions with low track quality. Compared to CMT, the laser remelting provided enhanced productivity, reduced heat input by 50% per pass, and lower distortions. Microhardness testing showed an increase in hardness in the intermediate layer towards the fusion line due to carbon diffusion. Despite a higher delta ferrite formation in laser-remelted tracks, a comparable corrosion protection to CMT was observed. The proposed method is promising for reducing CO2 emissions with respect to reusing scrap metal in the form of plates or use of ordinary plates instead of filler wires which opens possibilities for further enhancements.

Precipitation hardening stainless steel samples processed by additive manufacturing: Process parameters and thermo mechanical treatments effects on microstructure and corrosion resistance

The effects of different process parameters and thermo mechanical treatments on 17–4 PH samples processed by Selective Laser Melting have been investigated in this study. Overlap, scanning radius and laser beam defocusing were varied while all the other process parameters were kept constant. So, the role of process parameters was analysed in terms of microstructure, defectiveness and response to heat treatment. Specifically, the as built samples were subjected to stress relaxation heat treatment (650 °C-2 h) and H 900 heat treatment (solution treatment at 1100 °C 1 h + 480 °C-1 h). On the as built, stress relieved and H 900 heat treated samples the corrosion resistance, microstructural evolution and hardness were studied. Moreover, Ultrasonic Peening treatment (UPT) was applied and its effect corrosion resistance was evaluated. The study was carried out by optical, scanning and transmission electron microscopes, Vickers hardness, DRX and Image analysis software. It was found that the amount of austenitic phase increases with the decreasing of the scanning radius from 5000 µm to 1250 µm; with positive defocusing of laser beam (+1mm) respect to in focus processed sample and with increasing of the overlap from 25 µm to 50 µm.Each samples exhibit pores lower than 0,3% in terms of area percentage and mostly circular in morphology. In the process variability window, no significant difference in defects occurrence were observed. In the as built state, different values of hardness were found according to the different microstructure and average grain size. After the stress relaxation heat treatments at 650 °C-2 h all the samples exhibit similar hardness values while the most austenitic and finer grained as built samples exhibit higher increasing in hardness after H900 heat treatment. Corrosion resistance changes as function of the different process parameter and heat treatment and is improved by Ultrasonic Peening mechanical treatment.

Interface detection by picosecond Laser-Induced Breakdown Spectroscopy (LIBS): Application to a physical vapor deposited tungsten layer on a copper-chromium-zirconium substrate

The Laser-Induced Breakdown Spectroscopy (LIBS) technique is tested to control the ablation of a tungsten layer having a dozen micrometers thickness obtained by physical vapor deposition (PVD) on a CuCrZr substrate figuring plasma facing components used in the WEST tokamak. In the conditions of a Gaussian laser source with a wavelength of 532 nm, 30 ps as laser time duration and irradiance of ∼1017 W m−2 in air at atmospheric pressure, the mean ablation rate of the PVD tungsten is 90 nm per pulse, which is much lower than the one of bulk tungsten measured at 500 nm per pulse. The ablation process can lead to the formation of secondary craters not directly driven by the laser irradiance distribution.The ablation rate on CuCrZr is measured at 600 nm per pulse. Measuring the deposited layer thickness requires to spectroscopically monitor the CuCrZr components to avoid any damage of the substrate. We show that this can be performed in real time. A more secure process based on the use of a defocused laser beam is tested and leads to significantly lower ablation rate with the same precision for the thickness measurement. The increase in the ablation radius may be used to clean substrates polluted by tungsten layers.

Self-thermophoresis of laser-heated spherical Janus particles

An analytic framework is presented for calculating the self-induced thermophoretic velocity of a laser-heated Janus metamaterial micro-particle, consisting of two conducting hemispheres of different thermal and electric conductivities. The spherical Janus is embedded in a quiescent fluid of infinite expanse and is exposed to a continuous light irradiation by a defocused laser beam. The analysis is carried under the electrostatic (Rayleigh) approximation (radius small compared to wavelength). The linear scheme for evaluating the temperature field in the three phases is based on employing a Fourier–Legendre approach, which renders rather simple semi-analytic expressions in terms of the relevant physical parameters of the titled symmetry-breaking problem. In addition to an explicit solution for the self-thermophoretic mobility of the heated Janus, we also provide analytic expressions for the slip-induced Joule heating streamlines and vorticity field in the surrounding fluid, for a non-uniform (surface dependent) Soret coefficient. For a ‘symmetric’ (homogeneous) spherical particle, the surface temperature gradient vanishes and thus there is no self-induced thermophoretic velocity field. The ‘inner’ temperature field in this case reduces to the well-known solution for a laser-heated spherical conducting colloid. In the case of a constant Soret phoretic mobility, the analysis is compared against numerical simulations, based on a tailored collocation method for some selected values of the physical parameters. Also presented are some typical temperature field contours and heat flux vectors prevailing in the two-phase Janus as well as light-induced velocity and vorticity fields in the ambient solute and a new practical estimate for the self-propelling velocity.Graphic abstract

Integrated laser based pre‐tempering at laser welding of AISI 1045 steel by using 3D‐scanner optics

AbstractLaser‐based pre‐heating of laser beam welding with a 3D scanning optics, applied to AISI 1045 steel, is studied. Laser beam welding of heat‐treatable steel is challenging due to martensitic hardening in combination with defects. Pre‐tempering aims the reduction of the cooling rates and martensitic microstructure within the weld seam. An oscillating defocused laser beam was guided over the surface for pre‐heating by means of a 3D scanner optics. During pre‐heating, the laser power, the scanning speed and the number of cycles were varied. Welding with 4000 W and 2 m/min with a focused laser beam was executed. Thus the resulting temperature profile behind the ongoing laser beam and cooling time T8|5 between 800 °C–500 °C was significantly extended. Two parameter combinations (15 cycles|600 W|50 mm/s(2) and 10 cycles|800 W|50 mm/s2) succeeded in a microstructure of bainite and martensite. By extending the cooling time T8|5 to 3.11 s(2) and 4.17 s2. Thus, average hardness for laser based pre‐tempering of 487 HV 0.5(2) and 455 HV 0.52 was achieved. As a reference, global pre‐heating at 400 °C using a heating plate can reduce the average hardness of the weld zone from 729 HV 0.5 at room temperature to 304 HV 0.5 at a cooling time T8|5 of 5.63 s.

An autonomous laser kirigami method with low-cost real-time vision-based surface deformation feedback system

We introduce an autonomous laser kirigami technique, a novel custom manufacturing machine system which functions somewhat similar to a photocopier. This technique is capable of creating functional freeform shell structures using cutting and folding (kirigami) operations on sheet precursors. Conventional laser kirigami techniques are operated manually and rely heavily on precise calibrations. However, it is unrealistic to design and plan out the process (open loop) to realize arbitrary geometric features from a wide variety of materials. In our work, we develop and demonstrate a completely autonomous system, which is composed of a laser system, a 4-axis robotic arm, a real-time vision-based surface deformation monitoring system, and an associated control system. The laser system is based on the Lasersaur, which is a 120-Watt CO2 open-source laser cutter. The robotic arm is employed to precisely adjust the distance between a workpiece and the laser lens so that a focused and defocused laser beam can be used to cut and fold the workpiece respectively. The four-axis robotic arm provides flexibility for expanding the limits of possible shapes, compared to conventional laser machine setups where the workpiece is fixed on rigid holders. The real-time vision-based surface deformation monitoring system is composed of four low-cost cameras, an integrated AI-assisted algorithm, and the sensors (detachable planar markers) mounted on the polymer-based sheet precursors, and allows real-time monitoring of the sheet forming process with a geometric feature estimation error less than 5% and delay time around 100 ms. The developed control system manages the laser power, the laser scanning speed, the motion of the robotic arm based on the designed plan, and the close-loop feedback provided by the vision-based surface deformation monitoring system. This cyber-physical kirigami platform can operate a sequence of cutting and folding processes in order to create kirigami objects. Hence, complicated kirigami design products with various different polygonal structures can be realized by undergoing sequential designed laser cuts and bends (at any folding angles within designed geometric tolerance) using this autonomous kirigami platform.

Low-Cost Laser Micromachining Super Hydrophilic-Super Hydrophobic Microgrooves for Robotic Capillary Micromanipulation of Microfibers.

Capillary self-alignment technique can achieve highly accurate and fast alignment of micro components. Capillary self-alignment technique relies on the confinement of liquid droplets at receptor sites where hydrophobic–hydrophilic patterns are widely used. This paper reports a low-cost microsecond pulse laser micromachining method for fabrication of super hydrophilic–super hydrophobic grooves as receptor sites for capillary self-alignment of microfibers. We investigated the influence of major manufacturing parameters on groove sizes and wetting properties. The effects of the width (20 µm–100 µm) and depth (8 µm–36 µm) of the groove on the volume of water droplet contained inside the groove were also investigated. We show that by altering scanning speed, using a de-focused laser beam, we can modify the wetting properties of the microgrooves from 10° to 120° in terms of the contact angle. We demonstrated that different types of microfibers including natural and artificial microfibers can self-align to the size matching super hydrophilic–super hydrophobic microgrooves. The results show that super hydrophilic–super hydrophobic microgrooves have great potential in microfiber micromanipulation applications such as natural microfiber categorization, fiber-based microsensor construction, and fiber-enforced material development.

Residual stress reduction of LPBF-processed CM247LC samples via multi laser beam strategies

Based on SLM parameters from previous works, which guarantee fully dense and crack free CM247LC samples, multi laser beam strategies have been pursued to reduce residual stresses or rather distortion during LPBF processing. By using a second post heating and non-melting laser source with a defocused laser beam and lateral offset, cantilever distortion is reduced more than 7.5%, compared to the reference. Based on pre-tests with 9 different offset parameters, the optimum offset has been identified. Also, an upper limit for the laser power of 65 W is identified for the second heat laser beam with a spot diameter of 380 μm, to avoid re-melting and creating new defects. A theoretical “two bar model,” to explain the residual stress behavior and reduction with multi laser beam offset strategy during the LPBF process, is presented. Furthermore, re-melting cracks, defects, and microstructure are analyzed in conjunction with the second defocused offset laser, in case of a 200 W laser power, an increased scan speed of 1300 mms/s, and a reduced hatch distance. Secondary electron signal (SE) images of re-melting cracks are analyzed and compared to SE-image of hot cracks (solidification cracks). Based on electron backscatter diffraction (EBSD), the results of the microstructure from the last mentioned multi laser beam approach, which creates re-melting cracks, are presented and analyzed.

Effects of laser beam defocusing on high-strain-rate tensile behavior of press-hardened Zn-coated 22MnB5 steel welds

Zn-based coatings have emerged as viable alternatives to the conventional Al-Si-coating for 22MnB5 press-hardening steel. However, excessive weld concavity associated with laser beam welding of Zn-coated 22MnB5 imposes an adverse effect on dynamic tensile behavior of the welds after press-hardening. In the present study, laser beam defocusing is used to mitigate excessive weld concavity during laser welding of Zn-coated 22MnB5 steel. The high-speed digital image correlation (DIC) technique during high strain rate tensile testing confirmed that the modified laser weld concavity by beam defocusing can successfully shift the failure location from the fusion zone to the base material. Laser beam defocusing reverses the negative strain rate sensitivity of the welds which drastically improves the dynamic loading performance of Zn-coated 22MnB5 laser joints in automotive crash situations.

Atomic structure of carbon clusters laser-produced diamond-like carbon films

Structural state of thin diamond-like carbon films has been studied by high-resolution transmission microscopy. Diamond-like carbon films have been obtained by vapor-gas phase carbon concentration on glass substrates, using defocused NTS 300 laser beam for graphite targets evaporation in vacuum. Defocused laser beam provided an area of 3 mm in diameter on the target for carbon direct evaporation. It has been established that diffraction of electrons identifies diamond reflections (111) and (220), and this corresponds to interplane spacing d111 = 0.207 nm, d220 = 0.119 nm of the diamond lattice. The obtained values of interplanar distances are close to the values of interplanar distances of carbon macroscopic crystals with d111 = 0.205 nm and d220 = 0.125 nm. The research of the film atomic structure shows that sp3-bonding generally participate in assembling carbon atoms into diamond clusters having the size less than 1 nm. The ratio of graphite clusters formed by sp2-bonds within the structure of diamond-like carbon films is negligible. In spite of the pronounced cluster pattern of the structure, the film is not an analogue of the polycrystalline one with graded junction from one crystalline domain to another observed. As distinguished from polycrystals, there is no structure stepwise changing on the mating clusters boundaries. It is shown that clusters are linked with each other into homogeneous cluster structure by sp3- and sp2-bonds. These bonds are well-identified in the diamond-like carbon film structure, as the length of sp2-bonds exceeds considerably the length of sp3- bonds. Carbon film properties are determined by diamond clusters due to domination of clusters with tetragonal diamond lattice, for instance, the film is a good dielectric.

Pre-heating by defocusation of the CO2-Laser polishing beam: an experimental report from the lab-floor -INVITED

The laser beam polishing for glass and plastics is a purely thermal process and melts the ground or lapped structures to a depth of limited extent. This results in a smoothing of the surface, whereby the 1st - 4th order shape deviations can be corrected very well and transparent surfaces are created. The process is excellently suited for quartz glasses and other optical glasses with a low coefficient of expansion α. Furthermore, thermoplastics or metallic molds for injection molding and precision molding applications can also be polished with the laser beam. On the other hand, special measures are required for glasses with a high α, e.g. preheating of the component. For the investigations, a defocused laser beam was used for the defined preheating of glasses with high linear expansion coefficients. After reaching the material-specific preheating temperature, the laser beam was focused and the polishing process started. A defined cooling process follows again with a defocused beam. In this way, a ground biconvex lens made of boron crown glass was successfully polished. The laser-polished surfaces have an RMS value of 1- 3 nm. The polishing process can be used very flexibly. Likewise, very differently shaped optical components can be polished. The newly developed polishing regime is transferable to other optical glasses with high linear expansion coefficients.

Opto-thermoelectric microswimmers

Inspired by the “run-and-tumble” behaviours of Escherichia coli (E. coli) cells, we develop opto-thermoelectric microswimmers. The microswimmers are based on dielectric-Au Janus particles driven by a self-sustained electrical field that arises from the asymmetric optothermal response of the particles. Upon illumination by a defocused laser beam, the Janus particles exhibit an optically generated temperature gradient along the particle surfaces, leading to an opto-thermoelectrical field that propels the particles. We further discover that the swimming direction is determined by the particle orientation. To enable navigation of the swimmers, we propose a new optomechanical approach to drive the in-plane rotation of Janus particles under a temperature-gradient-induced electrical field using a focused laser beam. Timing the rotation laser beam allows us to position the particles at any desired orientation and thus to actively control the swimming direction with high efficiency. By incorporating dark-field optical imaging and a feedback control algorithm, we achieve automated propelling and navigation of the microswimmers. Our opto-thermoelectric microswimmers could find applications in the study of opto-thermoelectrical coupling in dynamic colloidal systems, active matter, biomedical sensing, and targeted drug delivery.

Homogenization of temperature distribution at metal-polymer interface during Laser Direct Joining

The strength of joints produced by Laser Direct Joining depends on several factors and it is greatly influenced by the temperature at the metal-polymer interface. The temperature should be kept within a certain processing window to increase the overall strength of the joint to enable enough polymer melting and avoid detrimental phenomena such as polymer degradation. The influence of laser beam defocusing, and laser beam position has been investigated. To this end, a FE model of the process was developed to predict the thermal field during the joining process. Measurements of experimental tests were used to calibrate and validate the numerical model. Particularly, the peak temperature measurements were used for determination of absorption coefficient by means of inverse analysis technique. On the other hand, the model validation was performed by comparing the thermal field predicted by the model with the morphology of the joints. Once validated, the numerical model was used to understand how laser beam defocusing and laser beam position affect the temperature gradients at the metal-polymer interface. In addition, energy efficiency issue was also investigated. The results indicated that laser beam defocusing can be mainly exploited to reduce the temperature peaks within a narrow region (beam spot). On the other hand, the laser beam position enabled to modify the thermal field over a wider region, but it involved lower energy efficiency.

Creation of corrosion resistant coatings on ductile iron by re-melting flame sprayed layers using laser and electron beam

The paper presents the results of the research of the formation of thin surface corrosion resistant layers on ductile iron with spheroidal graphite. Surface layers were created by a two-step technological process. In a first step, the surface layer material was applied to the surface of the ductile iron by means of a flame-powder coating in a thickness of 0.8 to 1 mm. The treatment of the layer was carried out in the second step by means of a defocused laser beam or a programmed-deflected electron beam. The influence of the laser beam defocusing and the influence of the energy distribution of the deflected electron beam on the characteristics of the formed thin surface layer was studied. 40 mm thick GGG 40 ductile iron samples were used as the substrate experimental material. The Ni-B-Si-type nickel powder with the designation NP22 and Ni-Cr-B-Si-type nickel powders of the type NP52 were used to form the surface layer. The quality of layers was assessed on the basis of surface forming, metallographic evaluation of selected properties of the layer and measurement of micro-hardness at the interface of the layer and the substrate material.

Laser-assisted narrow gap arc welding of an 18MND5 steel thick plate

Narrow gap arc welding is a common solution for the welding of thick structures. In this study, a defocused laser beam is used to pre-melt the narrow gap walls in front of an arc-welding bath. Such a welding configuration can be referred to a hybrid welding configuration. In the present work, a particular attention is given to evaluation of the interaction between an arc plasma and a defocused laser beam. High-speed imaging of the metal transfer through arc plasma is achieved thanks to a diode laser illumination system. Electrical arc parameters are logged, synchronously, in order to perform a correlation analysis and to make a diagnosis of the interaction level between laser beam and arc plasma.