Showing papers in "Journal of Laser Applications in 2016"

Investigations on ultra-high-speed laser material deposition as alternative for hard chrome plating and thermal spraying

Abstract: The development of new coating technologies for the wear and corrosion protection of large, high-quality components in the manufacturing industry is becoming more significant, not only from an economic but also from an ecological perspective. In various branches such as aerospace, oil and gas, automotive, papermaking, and others, hard chrome plating (HCP) is a widespread, standard process. However, the implementations of EU directives such as 1999/13/EC (VOC), 2011/65/EU (RoHS), and 2012/19/EU (WEEE) as well as the EU regulation EC 1907/2006 (REACH) to environmental protection, to environmental protection, CO2-reduction, and energy efficiency are leading to considerable market upheavals in the future. In HCP, toxic and carcinogenic hexavalent chromium (Cr6+) is used, which can only be used with authorization after the so-called sunset date in September 2017. Similar applies to the use of nickel in electroplating have been classified as dangerous for the environment and toxic by the World Health Organizati...

Simulation of transport phenomena and melt pool shape for multiple layer additive manufacturing

Abstract: A three-dimensional (3D) numerical simulation of transport phenomena using volume of fluid method is conducted for multiple-layer single track laser additive manufacturing (LAM). It predicts the temperature and fluid flow velocity distributions, transient variation of the melt pool fluid boundary shape and remelting, and solidified build geometry during deposition of successive LAM layers. The prediction showed reasonable accuracy in predicting peak temperature and deposit geometry. The prediction error of peak temperature is less than 2.5%, and prediction error of deposit height and width are less than 12%. Correlations between dimensionless process/material parameters (Pe, Pr, and Ma) and melt pool 3D fluid flow patterns and liquid boundary shape were studied. The analysis showed the hemispherical melt pool free surface in LAM causes the mechanisms that determine melt pool liquid-solid boundary shape to be different from melt pools formed on a flat surface. Although the maximum surface velocity decreased from 8.59 cm s−1 on the first layer to 5.06 cm s−1 on the fifth layer, the outward Marangoni flow is redirected from outward to downward due to the increase of surface curvature from 39.5° on the first layer to 75.9° on the fifth layer. Consequently, the penetration into the solid substrate at the outward edges becomes deeper and the pool bottom becomes more convex. These detailed physical insights provided by process simulations facilitate prediction of localized dimensional variations in LAM builds.

High power disk laser-metal active gas arc hybrid welding of thick high tensile strength steel plates

Abstract: In this study, welding with hybrid heat sources combined with a high power disk laser and a metal active gas (MAG) arc was carried out on 780 MPa high strength steel plates of 12 mm in thickness. The effects of respective welding conditions, such as laser power, root gap and welding speed on the penetration, geometry, and defects of weld beads, were investigated. The results showed that the process window of welding conditions for the production of good welded joints was narrow. Also, the laser keyhole behavior, the molten pool geometry, and the melt flows inside the molten pool were observed by the high-speed x-ray transmission real-time imaging system. It was confirmed that the melt flows inside the molten pool during hybrid welding were different between humping and good weld beads. The melt flowed from the bottom tip of a keyhole to the humping portion and did not flow forward to the keyhole bottom tip, resulting in the formation of the humping defect. On the other hand, in the case of good weld beads, the tungsten particle flowed to the back part of the molten pool but then flowed to the forward keyhole. It was clarified that a good weld bead without humping nor underfilling could be produced due to the forward melt flow.

Effect of plume on weld penetration during high-power fiber laser welding

Abstract: To clarify the effect of the laser-induced plume on weld penetration during high-power fiber laser welding, this paper investigates the change in weld penetration, plume behavior and temperature, particles present in the plume, and the attenuation of the horizontal probe laser. Results show that the weld penetration had an improvement of about 20% as the plume was blown away by using a supersonic cross jet. The plume temperature at a height of 5 mm was approximately 5078 K. The weld penetration, plume temperature, and plume attenuation to a probe laser decreased as plume height increased. A plume height of approximately 80 mm resulted in a shape similar to that of a focused laser beam; a cone of white smoke composed of numerous particles appeared around the plume. Calculated and measured results indicate that the interaction of laser particles in the plume accounts for the primary impact on laser-plume interaction. It is found that, if the plume height increases, the weld penetration decreases correspondi...

Microstructures and tensile properties of Inconel 718 formed by high deposition-rate laser metal deposition

Abstract: The aim of this study is to characterize microstructures and tensile properties of Inconel 718 (IN718) formed by high deposition-rate laser metal deposition (LMD), and furthermore to verify that the properties of the material are equivalent to those obtained by conventional manufacturing processes, such as casting and forging, and therefore satisfy the specifications for industrial applications. Initially, the powdery additive was characterized in terms of chemical composition, morphology, and porosity. Afterward, blocks for producing tensile specimens were deposited by applying the newly developed high deposition-rate LMD process that has a deposition rate of approximately 2 kg/h. Finally, microstructures and tensile properties of directly deposited and heat-treated material were analyzed, respectively. From the results, precipitation of an irregular shaped phase, which is believed to be Laves phase, and segregation of Nb and Mo were found at interdendritic regions of the directly deposited material. The...

Full penetration laser beam welding of thick duplex steel plates with electromagnetic weld pool support

Abstract: Full penetration high power bead-on-plate laser beam welding tests of up to 20 mm thick 2205 duplex steel plates were performed in PA position. A contactless inductive electromagnetic (EM) weld pool support system was used to prevent gravity drop-out of the melt. Welding experiments with 15 mm thick plates were carried out using IPG fiber laser YLR 20000 and Yb:YAG thin disk laser TruDisk 16002. The laser power needed to achieve a full penetration was found to be 10.9 and 8.56 kW for welding velocity of 1.0 and 0.5 m min−1, respectively. Reference welds without weld pool support demonstrate excessive root sag. The optimal value of the alternating current(AC) power needed to completely compensate the sagging on the root side was found to be ≈1.6 kW for both values of the welding velocity. The same EM weld pool support system was used in welding tests with 20 mm thick plates. The laser beam power (TRUMPF Yb:YAG thin disk laser TruDisk 16002) needed to reach a full penetration for 0.5 m min−1 was found to be 13.9 kW. Full penetration welding without EM weld pool support is not possible—the surface tension cannot stop the gravity drop-out of the melt. The AC power needed to completely compensate the gravity was found to be 2 kW.

Analysis of the physical processes occurring during deep penetration laser welding under reduced pressure

Abstract: Recent published experimental results obtained on deep penetration laser welding realized under reduced ambient pressure have shown very interesting results: resulting weld seams have geometrical characteristics that are similar to those obtained with electron beams. They show an increased penetration depth that can reach a factor two compared to atmospheric experiments, a larger aspect ratio with narrow and parallel sides of the weld seam. Also some humps around the rim of the keyhole appear. Of course, these modifications depend on the ambient pressure, but one also observes that these interesting improvements become independent of the ambient pressure below some critical pressure and also disappear at high welding speeds. Moreover, it is also observed that this critical pressure, below which these improvements do not vary, increases with the welding speed. In these previous publications, all these different characteristic results have not been explained. It is therefore the purpose of this paper to explain these different results. This has been obtained by using 3D numerical simulations of deep penetration laser welding and by studying the corresponding variation of physical parameters inside the keyhole (temperature, recoil pressure, and hydrodynamics of the vapor plume). An explanation for the evolution of these resulting weld seam geometries, as a function of the main operating parameters is proposed: ambient pressure, welding speed, and laser beam parameters (power and beam spot diameter). It is then possible to estimate this characteristic critical pressure, which is compared favorably with the corresponding previous experimental results. As a consequence, this analysis allows to define the optimum conditions for the improvement of the weld seam characteristics realized under reduced ambient pressure for an industrial environment.

High performance corrosion resistant coatings by novel coaxial cold- and hot-wire laser cladding methods

Abstract: In the last few years, coaxial laser heads have been developed with centric wire feeding equipment, which enables the laser processing of complex-shaped objects in various applications. These newly developed laser heads are being used particularly in laser brazing experiments in the automotive industry. This study presents experimental results of using a coaxial laser head for cold- and hot-wire cladding application. The coaxial wire cladding method has significant improvements compared with the off-axis wire cladding method such as independence of the travel direction, alignment of the wire to the laser beam, and a reduced number of controlling parameters. These features are important to achieve high quality coatings. Cladding tests were conducted on mild steel with a coaxial laser wire welding head using Ni-based Inconel 625 and Thermanit 2509 super duplex stainless steel solid wires in order to determine the properties of the cladding process and the coatings deposited. The corrosion resistance of the ...

Laser impact welding application in joining aluminum to titanium

Abstract: Thin metal foil joining has wide applications in medical device and microelectronics. In this paper, laser impact welding was implemented to join aluminum foil to titanium sheet. The velocity of Al flyer was measured with photonic Doppler velocimetry. The maximum velocity reached up to 1000 m/s within 0.2 μs. Varied thickness (25–250 μm) Al flyer was successfully welded with Ti target. Weld strength was measured with peel test. Weld area was estimated with resistance measurement method. The effect of laser spot size, flyer thickness, standoff distance on weld strength, weld area, and microstructure was analyzed. The microstructure was studied with scanning electron microscopy (SEM). By comparing the amplitude and wavelength of the waves at the bonding interface, it is suggested that the wave formation was related to the impact velocity. SEM back scattered electron image did not show apparent diffusion across the weld interface. Both twinning and severe plastic deformation were observed at Ti side along th...

Parameter optimization for high speed remote laser cutting of electrodes for lithium-ion batteries

Abstract: To reduce carbon emission, transportation sector has adapted lithium-ion battery-based hybridization of gasoline and diesel engines due to its efficiency, the availability of technologies, and nation-wide infrastructures. To cut prismatic and cylindrical electrodes for lithium-ion batteries, die cutting and rotary knife slitting have been used. Both techniques have disadvantages such as tool wear, process instability, inconsistency of cut quality, and redesign of mechanical cutting processes due to various battery sizes. High speed remote laser cutting overcomes these disadvantages with characteristics such as contact-free process, high energy concentration, low noise level, fast processing speed, very narrow heat affected zone, applicability to nearly all materials, and flexibility of laser power. Optimization of key parameters, or power and scanning speed, has been presented for laser cutting of electrodes for lithium-ion batteries. An acceptable clearance width is observed. The line energy is defined a...

Hardware-in-the-loop control of additive manufacturing processes using temperature feedback

Abstract: Laser-based additive manufacturing is a technology for the production of freeform metallic parts. In order to produce parts with high quality, it is important for the manufacturing processes to be controllable with a high degree of precision. Current additive manufacturing systems attempt to reach this goal by carefully tuning the operational parameters, often in combination with a feedback control system. These systems are based on low order, empirical models of the process, which may limit the performance that can be achieved. This paper introduces a control system based on a high order physical heat conduction model of the melt pool dynamics. The control system serves as a framework which can be applied to many laser material processes in which high precision is required such as laser cladding and selective laser melting. The controller is able to regulate the melt pool size by modulating the laser power using a number of surface temperature measurements as the feedback signal. A hardware-in-the-loop (...

Effects of laser beam spatial distribution on laser-material interaction

Abstract: The efficiency of any laser materials processing depends on the efficient energy transfer from the laser to a substrate. One of the critical factors in the process is the spatial distribution or the “mode” of the laser beam. Although an inverse Bremsstrahlung initiates the photon to electron energy transfer, a major part of the process is transport phenomena created by the absorbed energy. A mathematical model for transport phenomena during laser materials interaction is developed that includes multiple reflections, capillary and thermocapillary forces, recoil pressure, temperature distribution, melt pool flow, and phase changes. This model simulates interaction between a CO2 laser (λ = 10.6 μm) with four different spatial distributions and iron. The results of a simulation for fundamental understanding of this laser-material interaction are presented in this paper. First, overall keyhole behaviors are compared in terms of response time, penetration hole geometry, and absorptivity. Furthermore, a dimensionless parameter is developed to examine keyhole collapse quantitatively. Finally, velocity, temperature, and, intensity fields are analyzed.

Laser surface structuring with 100 W of average power and sub-ps pulses

Abstract: High throughput still represents a key factor for industrial use of ultrashort pulses in the field of surface structuring. Reliable systems with average powers up to 100 W are today available. It has already been proved that metals, especially steel having a low threshold fluence, can be machined with excellent surface quality at average powers of more than 40 W and a spot radius of about 25 μm, if a polygon line scanner, offering fast scanning speeds, is used. A further scale-up into the 100 W regime should be possible for metals showing a threshold fluence of about 0.2 J/cm2 or higher. But, it will lead to problems with heat accumulation in the case of steel and a straight forward scale-up is not possible. In order to keep a good surface quality, the machining strategy has to be adapted. A maximum flexibility can be obtained with an “interlaced” mode by using very high marking speeds of several 100 m/s and repetition rates of several tenths of MHz. As this is at the edge of today available technologies,...

Laser cutting of composites—Two approaches toward an industrial establishment

Abstract: Carbon fiber reinforced plastics (CFRP) are of high interest as a lightweight material due to their high strength-to-weight ratio. Within industrial fields with a requirement on lightweight design and energy efficiency, the demand of CFRP increases continuously. Up to now, the cutting of CFRP, as a basic production step within the production of composite parts, is mainly performed by conventional cutting techniques such as milling and abrasive water jet cutting. These techniques are known to have drawbacks, e.g., force input, handling of auxiliaries, and tool wear. The laser cutting of CFRP has already shown high potential to be a practical alternative due to the wear-free and contactless processing. However, as a thermal technique, laser cutting has to overcome the challenge of heat input into the material. Two different approaches concerning deterioration free cutting will be examined and compared within this research. For that purpose, two different laser sources were used. Both laser sources have a maximum average laser power of PL = 1500 W, one emitting continuous wave and the other emitting pulsed wave. The first approach investigated is the laser cutting with a single mode fiber laser emitting continuous wave at high beam quality. A small focal diameter was achieved by the optical setup used, enabling narrow cutting kerfs and finely machined cutting contours. The relative movement of laser beam and material was realized by a combination of scanning optic and three precise linear stages. The second laser source is a high power thin-disk laser emitting nanosecond pulses. The optical setup consists of a fiber with a diameter of d = 600 μm and a galvanometer scanner by TRUMPF, called 3D programmable focusing optic (PFO-3D), leading to a focal diameter bigger than that of the single mode fiber laser. The relative movement was realized by the PFO-3D and a six-axis robot system. The required process strategies, differences, benefits, and drawbacks of both setups will be reviewed. The review will be completed by an examination of the complexity of the setups and introduction of efficient processing solutions. Comparative investigations with an identical material for both described setups were performed as a basis for discussion. Differences exist in appearance of the cuts and the choice of best parameters, while comparable qualities and efficiencies can be achieved for both setups.

Laser beam welding of electrical contacts for the application in stationary energy storage devices

Abstract: The energy sector has been changing in the past few years, driven by the transition toward renewable energy. This affects the technologies, as well as the structure of energy production by means of a decentralized and time-dependent energy generation. The resulting effects on the power grid require local storage systems to store the surplus energy and to limit the feed-in power. For these energy storage systems, the use of commercial 26650 LiFePO4 battery cells is highly promising. Since the capacity of these cells is comparatively low, a large quantity of cells is needed to match the storage requirements. For this reason, the interconnection between individual battery cells is the basic prerequisite for the production of energy storage systems. Recent research has shown that laser beam welding is suitable for the welding of small electrical contacts. However, the welding process of 26650 cells with contacts made of nickel plated steel is very complex. The requirements regarding the heat input during the ...

Laser surface modification for the prevention of biofouling by infection causing Escherichia Coli

Abstract: Bacteria have evolved to become proficient at adapting to both extracellular and environmental conditions, which has made it possible for them to attach and subsequently form biofilms on varying surfaces. This has resulted in major health concerns and economic burden in both hospital and industrial environments. Surfaces which prevent this bacterial fouling through their physical structure represent a key area of research for the development of antibacterial surfaces for many different environments. Laser surface treatment provides a potential candidate for the production of antibiofouling surfaces for wide ranging surface applications within healthcare and industrial disciplines. In the present study, a KrF 248 nm Excimer laser was utilized to surface pattern polyethylene terephthalate (PET). The surface topography and roughness were determined with the use of a Micromeasure 2, 3D profiler. Escherichia coli (E. coli) growth was analyzed at high shear flow using a CDC Biofilm reactor for 48 h, scanning el...

Evaluating femtosecond laser ablation of graphene on SiO2/Si substrate

Abstract: We demonstrate a uniform single layer micropattern of graphene on 300 nm thick SiO2 on a Si substrate using a 1030 nm, 280 fs laser. The cutting process was conducted in air, the pattern defined through the motion of a high-precision translation stage. Approximately 1.6 μm wide graphene microchannels were cut with uniform widths and well defined edges. The ablation threshold of graphene was determined to be 66–120 mJ/cm2, at which the selective removal of graphene was achieved without damage to the SiO2/Si substrate. Scanning electron microscopy images revealed high quality cuts (standard deviation 40 nm) with little damage or re-deposition. Raman maps showed no discernible laser induced damage in the graphene within the ablation zone. Atomic force microscopy revealed an edge step height ranging from less than 2 to 10 nm, suggesting little removal of SiO2 and no damage to the silicon (the central path showed sub ablation threshold swelling). The effect of the ultrafast laser on the surface potential at the cut edge has been measured and it showed a distinguishable boundary.

Dross-free submerged laser cutting of AZ31 Mg alloy for biodegradable stents

Abstract: Submerged cutting of AZ31 Mg alloy was studied with ns-pulsed green fiber laser and three different submersion liquids, namely, water, alcohol-water solution, and paraffin based oil. Compared to conventional laser cutting with coaxial process gas, differences due to optical, chemical, and mechanical effects were identified. An analytical solution was introduced to assess the fluence decay due to beam enlargement and absorption in the submersion liquid. The chemical reactions between the Mg alloy and submersion liquid were defined, and weight loss due to chemical effects was studied in static immersion tests. The mechanical instability related to liquid breakdown was studied to reveal the threshold levels for the liquids. The interaction of these mechanisms was studied within process. The results showed dross-free cutting is achievable by submerging the Mg alloy in 0.5 mm alcohol-water solution, which shows a processing condition that enables chemical dissolution of dross without excessive fluence loss or liquid instability. The cut quality of the optimized conditions was comparable to fs-pulsed laser cutting of the same material.

Laser adhesion test for thermal sprayed coatings on textured surface by laser

Abstract: The laser shock adhesion test (LASAT) is a technique allowing the generation of high tensile stresses in materials. The LASAT consists in focusing a pulsed laser beam on a water-confined target. The laser pulse crosses the water transparent layer and is absorbed by the target. High energetic plasma is created at the surface of the sample. As a response to the expansion of the plasma, a shock wave is generated and propagates through the sample. This shock wave leads to the generation of high tensile stresses in the sample. These stresses allow the interface solicitation in order to evaluate the dynamic adhesive bond strength of coated systems. In order to determine interface strengths, this technique has already proven its feasibility. In this paper, the adhesion strength of coated system was evaluated using LASAT for two surface pretreatments of substrates obtained by grit-blasting and laser surface texturing techniques. The generation of the high-intensity shock wave by laser plasma in the water-confinem...

Laser cleaning of commercial Al alloy surface for tungsten inert gas welding

Abstract: Surface preparation is known to have critical effect on welding quality of engineering materials, for instance, oxidation layer may reduce the weld performance significantly due to porosity issue. Laser cleaning is considered as a promising technique to do surface preparation for welding process. This paper presents the effect of laser cleaning on surface preparation for tungsten inert gas (TIG) welding of commercial aluminum alloy. We demonstrated the feasibility of using laser cleaning method to minimize the oxides layer of Al alloy for TIG welding. This one-step process without use of chemicals or any other additives is environmentally friendly. Comparison of the performance between conversional cleaning method and laser cleaning method was examined carefully to identify the changes in microstructure and chemical analysis of Al alloy welds after TIG welding processing. Special attention was paid to porosity distribution in the fusion zone after laser cleaning. In addition, some thermal melting was found to occur and refined microstructure took place between the fusion zone and the substrate after rapid solidification. It is further proposed that laser cleaning can refine microstructure of the interface between filter wire and the substrate during TIG welding, which may play a significant role in affecting the weld quality.

Finite element modeling of an alternating current electromagnetic weld pool support in full penetration laser beam welding of thick duplex stainless steel plates

Abstract: An electromagnetic weld pool support system for 20 mm thick duplex stainless steel AISI 2205 was investigated numerically and compared to experiments. In our former publications, it was shown how an alternating current (AC) magnetic field below the process zone directed perpendicular to the welding direction can induce vertically directed Lorentz forces. These can counteract the gravitational forces and allow for a suppression of material drop-out for austenitic stainless steels and aluminum alloys. In this investigation, we additionally adopted a steady-state complex magnetic permeability model for the consideration of the magnetic hysteresis behavior due to the ferritic characteristics of the material. The model was calibrated against the Jiles–Atherton model. The material model was also successfully tested against an experimental configuration before welding with a 30 mm diameter cylinder of austenitic stainless steel surrounded by duplex stainless steel. Thereby, the effects of the Curie temperature o...

Effects of laser-induced oxidation on the corrosion resistance of AISI 304 stainless steel

Abstract: The paper presents an analysis of the impact of oxide layers, manufactured by means of laser radiation (with varying energy densities), on the resistance of AISI 304 stainless steel to corrosion. Corrosion resistance tests have been carried out in a weakly acidic environment (H2SO4, pH = 3.5) and a neutral environment (NaCl, pH = 7). The experiment was performed with the use of a laser system equipped with the Ytterbium Yb3+ fiber laser with a pulse duration of 230 ns. It has been shown that the thermal-oriented laser process significantly affects the course and nature of cathode-anode processes occurring on the surface of irradiated materials. Compared to the unstructured reference samples, there has been a significant increase in corrosion resistance of AISI 304 steel in the range of laser fluence between 50 and 70 J/cm2. The Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses of the chemical composition of the surface of the samples as well as the change of their morpholo...

Comparison of processing window and porosity distribution in laser welding of 10 mm thick 30CrMnSiA ultrahigh strength between flat (1G) and horizontal (2G) positions

Abstract: With the development of high brightness and high power laser, it is possible to weld thick sections using single pass autogenous laser welding. However, it often suffers from very narrow processing parameter windows when welding thick sections. Furthermore, porosity, back sagging, and undercut are the main defects for laser welding of thick section high strength steel plates. A comparison of flat welding position (1G) and horizontal welding position (2G) in laser welded 10 mm thick 30CrMnSiA high strength steel plates was investigated in this paper. The results show that the 1G position has a very narrow processing window, while the 2G position has a much wider processing window. The porosity in the 2G position laser welded joint distributed in the upper part of welds, while that in the 1G position laser welded joint was distributed along the weld center. Welded joints with acceptable porosity in the 2G position laser welding can be obtained for an X-shaped weld profile and wider weld bead. The peak longitudinal residual stresses of the 2G position laser welded 30CrMnSiA steel were located in the heat-affected zone (HAZ), and the lower residual stresses were presented in the weld center. The peak transverse residual stress was presented in the weld center with the lower stresses located in the two sides HAZ.

Role of CO2 laser cutting conditions on anisotropic properties of nanocomposite contain carbon nanotubes

Abstract: This study attempts to investigate different CO2 laser conditions on electrical conductivity and surface properties of multiwalled carbon nanotubes/poly methyl methacrylate (MWCNT/PMMA) nanocomposite. In order to reach this aim, outputs like morphology of surface, electrical conductivity of laser cut surface, heat affected zone (HAZ), burr, and surface roughness were studied. The nanocomposite, which contains carbon nanotubes, exhibits anisotropic behavior, therefore, the laser cutting outputs are investigated within the in-flow direction and perpendicular to the flow direction. Morphology of the nanocomposites and cut surfaces are investigated using scanning electron microscopy that showed the contacts between nanotubes are increased after laser cutting. Findings obviously show increasing the laser power and decreasing the feed rate improve the electrical conductivity. As a prominent result, the minimum surface electrical resistivity was obtained from over 2 × 1014 to 886 Ω by using a power of 120 W, a c...

Three dimensional simulation of high speed remote laser cutting of cathode for lithium-ion batteries

Abstract: Cut surface quality of electrodes' affect lithium-ion battery performance. Current uses of mechanical cutting require expensive tooling. Furthermore, tool wears out over time so that it results in process instability and poor cut quality. This may cause an internal short circuit and significant heat generation. These problems can be solved by using a laser cutting technique since it has many advantages, such as no tool wear, high energy concentration, fast processing speed, very narrow heat affected zone, applicability to nearly all materials, and flexibility of laser power. Understanding physical phenomena provides significant advantages to fully utilize the remote laser cutting of electrodes for lithium-ion batteries. In this paper, a 3D mathematical model of three-dimensional self-consistent remote laser cutting is developed for cathode (LiCoO2-coated aluminum) of lithium-ion batteries. Characteristics of the remote laser cutting of cathodes, liquid/vapor (L/V) interface geometry, and melt pool flow ar...

Use of a diffractive optic for high power laser cutting

Abstract: There is a current interest in using laser cutting for nuclear decommissioning applications. The benefits of using lasers for this application include the high speeds available, the tolerance of the process, the lightness of the cutting head, the lack of a reaction force with the part being cut, and the ease of automation of the laser cutting process. Of course laser cutting is a thermal process and a potential detriment, is that the residual laser beam, passing through the kerf, might damage or indeed set fire to something positioned behind the part being cut. This paper describes the use of a diffractive optical element in the laser beam forming optics, designed to extend the depth of focus of the system, without increasing the focal length of the focusing optic. In this way, for cutting thick materials, the goal is to achieve the cutting performance of a long focal length lens, with the beam divergence of a short focal length lens. A design of diffractive optical element is presented, which when used w...

Influences on incubation in ps laser micromachining of steel alloys

Abstract: Efficient and accurate generation of micro holes and feature geometries is one object of investigation in ultrashort pulse laser processing. Different analytical models exist to describe the relationship between energy input and removed volume. These models relate to the ablation threshold and the energy penetration depth representing material-dependent parameters. Both parameters are influenced by incubation. Against this background, incubation effects depending on the number of pulses applied on two steel alloys are presented in the paper. The ablation threshold and the energy penetration depth are analyzed by the zero-damage method and an analytical model based on Beer's law describing the crater depth. The pulse frequency is chosen to fP = 100 Hz to investigate incubation excluding the influence of temporal effects between subsequent pulses. The used ultrashort pulsed laser has a pulse duration of 10 ps and a wavelength of 1064 nm. Craters are generated with defined pulse numbers between N = 1 and N = 1000. Two commonly used steel alloys—the cold working steel 1.2379 and the austenitic stainless steel 1.4301—are investigated and compared. Additionally, the first one is investigated at two levels of hardness. All sample surfaces have the same initial average roughness Sa ≈ 0.02 μm which was realized by manual polishing. The comparison of two incubation models shows higher coincidence of the experimental results and the model proposed by Ashkenasi than for the model published by Jee, especially for pulse numbers N > 100. The linear optical penetration depths which are determined by ellipsometry measurements are larger than the energy penetration depths which are analyzed for the ablation regimes of the gentle phase. The main reasons for the pulse-number dependent decrease of the ablation threshold are investigated. Absorption changes due to surface modifications such as oxidation and generation of different surface morphologies which are observed by scanning electron microscope. The obtained results are the basis for further studies on incubation including temporal multipulse effects.

Modeling of weld pool phenomena in tungsten inert gas, CO2-laser and hybrid (TIG+CO2-laser) welding

Abstract: The efficiency of the welding process in terms of weld penetration and weld width is greatly determined by the heat, mass, and charge transfer phenomena in the weld pool. These phenomena, in turn, depend on the thermal and electromagnetic interaction of the heat source used with the metal being welded. The most adequate models of the welding processes should consider the interaction of the phenomena in the heat source, on the base metal surface and inside its volume by a self-consistent way. This paper is devoted to the development of a self-consistent model of weld pool dynamics in tungsten inert gas (TIG), laser and hybrid (laser + TIG) spot welding without a keyhole formation. The proposed model allows simulation of processes taking place in the weld pool and on its surface. The model takes into account free surface deformation, influence of plasma shear stress, thermocapillary Marangoni effect, and Lorentz forces on the weld pool, as well as of the processes in the arc plasma including laser-arc interaction. For this purpose, the model of the weld pool is combined with a model of arc plasma column where the interaction processes between Gaussian beam radiation emitted by a continuous-wave CO2 laser and the argon arc plasma are described. The equations of the model proposed are solved numerically by means of finite element method. The simulation results are compared with real welding experiments performed with steel S-235JR. A good accordance between simulation and experimental results is observed.

Optimizing the processing of sapphire with ultrashort laser pulses

Abstract: The ability to rapidly and precisely generate high-quality features with small dimensions in sapphire is paramount for broadening its appeal and expanding its utilization for consumer electronics applications. Intrinsic properties of sapphire, including high scratch resistance, make it an attractive option for these purposes, but the ability to machine fine features in sapphire substrates with common mechanical and laser-based methods has proved elusive to this point. In this study, we present results from a series of systematic trials to determine the optimum laser processing parameters for drilling 400 μm diameter holes with no cracks or chips and <5° taper in 430 μm thick sapphire wafers with a 0.8 ps 1030 nm source. Holes are drilled at repetition rates from 21 to 1042 kHz, overlaps from 70% to 98%, and translation of the beam waist through the sample at rates from 10 to 200 μm/s. We present qualitative and quantitative results generated from laser scanning microscopy demonstrating that holes with <5° taper and no cracks or chips can be drilled at repetition rates of 260 kHz with 90% and 95% overlap and 521 kHz with 95% overlap. We find that the optimum processing parameters for drilling holes with <5° taper correlates well with the conditions necessary for avoiding chipping, cracking, and back-side damage rings. Holes with <5° taper can be drilled in as short as 4–6 s per holes, and holes with <2° taper can be drilled in 10–12 s per hole.

Vertical-cavity surface emitting laser-diodes arrays expanding the range of high-power laser systems and applications

Abstract: Thermal treatment may be by far the most frequent process used in manufacturing, but only at a few places lasers could make an inroad. For thermal treatment, homogeneous illumination of large areas at a lower brightness, and accurate temporal as well as spatial control of the power is required. This is complicated for conventional high-power lasers, while vertical-cavity surface emitting laser-diode (VCSEL) arrays inherently have these capabilities. Because of their fast switching capability and low power dissipation, VCSELs have been widely used for datacom and sensing applications. By forming large-area arrays with hundreds of VCSELs per mm2, their use can be further expanded to high-power applications. In this way, power densities of several W/mm2 are achieved, making the VCEL arrays an ideal solution for many heating applications, ranging from melting and welding of plastics and laminates to curing, drying, and sintering of coatings. A turn-key system concept has been developed allowing fast and easy configuring systems to the specifications of the applications. The compact and robust system can be built directly into the manufacturing equipment, thus making expensive fibers and homogenizing optics superfluous. These systems are now finding their first inroads into industrial applications and have been designed-in into commercially available production machines.