Applied multi-pulsed laser in surface treatment and numerical-experimental analysis

This paper presents a comparison between simulation and experimental results of the melting process of metallic material by a pulsed laser source Nd-YAG. The simulations of temperature and velocity fields of melted material were done by solving the transient heat transfer and fluid-flow equations. Variations of the thermophysical properties were considered. Furthermore, the model included the effects of the surface-tension gradient on the fluid surface and the buoyancy force. The simulation was useful in improving our understanding of the phenomena occurring in the treated material. Using a laser triangulation sensor, an experimental study was also conducted on the surface profile of the melted zones to seek a relationship between the so-called keyhole effect and the laser triangulation measurements. The keyhole effect induced strong surface deformations and often formed cavities, which were undesirable in the surface treatment process. The laser power, energy density, and treatment duration could be optimized to prevent the keyhole effect. The predicted laser melted zone (LMZ) morphology was in good agreement with the corresponding experimental measurements for various irradiation conditions, as long as the keyhole effect did not occur.     

Microstructure, hardness and stress in melted zone of 42CrMo steel by wide-band laser surface melting

Using laser surface melting (LSM) of a roller, to obtain the desired distribution of the microstructure, hardness and residual stresses with minimum distortion, is essential in order to improve machining efficiency and to achieve reliable service performance. In this study, a 3D finite element model has been developed to simulate the wide-band LSM process and predict the thermal and mechanical properties in the melted zone. The microstructure evolution, hardness distribution and stress field in the melted zone with different laser power were simulated. With the increase of the laser power from 3000 to 3800 W, the width and the depth of the laser melted layer increase, while the laser power has a little effect on the martensite contents, which exceed 90% in the melt-hardened zone. It greatly affects the mechanical properties in the melt-hardened zone with its volumetric expansion effect and the hardness increases by 2-3 times. The residual stress distributed within the melt-hardened zone is always of the compressive type. The amplitude of compressive stress exists in the transition region, and the amplitude of von Mises stress within the heat affected-zone (HAZ) decreases with the increase in laser power. The accuracy of the developed finite element simulation strategy is validated for phase proportion and hardness distributions through the wide-band LSM on roller steel with proper instrumentation for data measurement. This agreement is encouraging.     

Formation of periodic surface structures by ultrashort laser pulses

The formation of periodic surface structures by ultrashort laser pulses was observed experimentally and explained theoretically. The experiments were performed on graphite with picosecond laser pulses. The spatial period of the structures is of the order of the wavelength of the incident radiation, and the orientation of the structures is correlated with the direction of polarization of the light. The key point of the theoretical model proposed is resonance excitation of surface electromagnetic waves, which under conditions such that the temperature of the electronic subsystem is decoupled from the temperature of the crystal lattice causes a “temperature grating” to be written on the flat solid surface of the sample while the laser pulse is being applied on account of the temperature dependence of the surface impedance. The formation of a periodic surface profile from the temperature grating occurs by the volume expansion of a melted layer near the surface of the material. For typical values of the surface tension and viscosity for metals, there is not enough time for the periodic profile to be resorbed before the liquid layer solidifies. The formation of periodic surface structures is delayed in time relative to the laser pulse. Zh. éksp. Teor. Fiz. 115, 675–688 (February 1999)     

Structure and properties of the bronze laser alloyed with titanium

The paper describes the microstructure and properties (microhardness and wear resistance) of the bronze laser alloyed with titanium. The laser alloying was done using a pulsed Nd:YAG laser with a generated beam energy of 25-35 J. A very fine microstructure was formed under such rapid solidification conditions like laser treatment. The high chemical homogeneity and fine structure of the melted zone were attributed to high cooling rates due to the short interaction time with Nd:YAG pulsed laser radiation and relatively small volume of the melted material. The structure obtained in the surface layer after laser alloying permits to get a high level of hardness and an improved wear resistance.     

Microstructure and corrosion behavior of austenitic stainless steel treated with laser

Surface modification of AISI316 stainless steel by laser melting was investigated experimentally using 2 and 4 kW laser power emitted from a continuous wave CO₂ laser at different specimen scanning speeds ranged from 300 to 1500 mm/min. Also, an investigation is reported of the introduction of carbon into the same material by means of laser surface alloying, which involves pre-coating the specimen surfaces with graphite powder followed by laser melting. The aim of these treatments is to enhance corrosion resistance by the rapid solidification associated with laser melting and also to increase surface hardness without affecting the bulk properties by increasing the carbon concentration near the surface. Different metallurgical techniques such as optical microscopy, scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the microstructure of the treated zone. The microstructures of the laser melted zones exhibited a dendritic morphology with a very fine scale with a slight increase in hardness from 200 to 230 Hv. However, the laser alloyed samples with carbon showed microstructure consisting of γ dendrite surrounded by a network of eutectic structures (γ+carbide). A significant increase in hardness from 200 to 500 Hv is obtained. Corrosion resistance was improved after laser melting, especially in the samples processed at high laser power (4 kW). There was shift in I_corr and E_corr toward more noble values and a lower passive current density than that of the untreated materials. These improvements in corrosion resistance were attributed to the fine and homogeneous dendritic structure, which was found throughout the melted zones. The corrosion resistance of the carburized sample was lower than the laser melted sample.     

Effect of pulsed laser parameters on in-situ TiC synthesis in laser surface treatment

Commercial titanium sheets pre-coated with 300-μm thick graphite layer were treated by employing a pulsed Nd:YAG laser in order to enhance surface properties such as wear and erosion resistance. Laser in-situ alloying method produced a composite layer by melting the titanium substrate and dissolution of graphite in the melt pool. Correlations between pulsed laser parameters, microstructure and microhardness of the synthesized composite coatings were investigated. Effects of pulse duration and overlapping factor on the microstructure and hardness of the alloyed layer were deduced from Vickers micro-indentation tests, XRD, SEM and metallographic analyses of cross sections of the generated layer. Results show that the composite cladding layer was constituted with TiC intermetallic phase between the titanium matrix in particle and dendrite forms. The dendritic morphology of composite layer was changed to cellular grain structure by increasing laser pulse duration and irradiated energy. High values of the measured hardness indicate that deposited titanium carbide increases in the conditions with more pulse duration and low process speed. This occurs due to more dissolution of carbon into liquid Ti by heat input increasing and positive influence of the Marangoni flow in the melted zone.     

Microstructure of nodular cast iron after excimer laser surface processing

Nodular iron of martensitic structure was treated by means of a XeCl laser prototype. The energy density varied from 0.3 to 5 J/cm² and the number of shots from 4 to 40. Conversion electron Mössbauer spectroscopy, conversion X-ray Mössbauer spectroscopy and grazing incidence X-ray diffraction were used to characterize the irradiated surface. Some Rutherford backscattering spectrometry measurements were performed to control surface oxidation and carbon distribution. It is shown that after irradiation austenite formed in a rather deep heat affected zone (10–20 m) compared to the thickness of the melted zone ( 1 m). The austenite amount as well as its carbon content increase with energy density and number of shots up to a threshold of carbide formation. Beyond the threshold Fe₂C, Fe₃C and Fe₅C₂ formed only in the melted zone. The carbon content as a function of depth is constant in the melted layer, then decreases quickly from the melted layer-heat affected zone interface down its initial value. The carbon content is shown to govern the evolution of phases content in the melted layer depending on the laser treatment conditions.     

激光在烟幕中传输的蒙特卡罗模拟

建立蒙特卡罗计算模型,模拟计算1.06μm脉冲激光在石墨烟幕中的传输.分析透过率与石墨粒子粒径、烟幕浓度、烟幕厚度的关系,用卷积方法计算一定宽度的脉冲激光穿过烟幕后的波形,数值仿真δ脉冲和矩形脉冲激光在各种石墨烟幕中的时间展宽特性.模拟结果表明:石墨粒子存在一最佳半径(0.16μm),该半径的石墨粒子烟幕不仅使激光能量衰减最强,而且产生的时间展宽也最显著;激光脉冲宽度越小,其波形变化和时间展宽越显著,微秒量级脉宽的激光脉冲穿过烟幕后波形变化和时间展宽效应不明显.     

Numerical and experimental study of the Ti6Al4V macrostructure obtained by Nd:YAG laser

Titanium and its alloys (Ti6Al4V) have been widely used in the biomedical field; nevertheless, they should be subject to specific surface treatments, before being implanted, in order to improve bio-integration. Although laser processing is a useful technique for this purpose, different aspects of the basic mechanisms of this process are still in progress, with special emphasis on the modeling structure formation on the irradiated surface. For this research, the finite element method was used to study the generation of a macrostructure on the Ti6Al4V surface using a Nd:YAG laser. The temperature profiles, estimated during the extremely high heating and cooling rates caused by the output power of the laser beam, allowed us to analyze, among other things, the melting depth and the heat affected zone, in order to optimize the process. Moreover, the experimental results (SEM data) were positively compared with the numerical model, and a relationship of the crater profile formation (depth to width ratio) was determined.     

Microstructure and corrosion properties of diode laser melted friction stir weld of aluminum alloy 2024 T351

Friction stir welding is a promising solid state joining process for high strength aluminum alloys. Though friction stir welding eliminates the problems of fusion welding as it is performed below melting temperature (T_m), it creates severe plastic deformation. Friction stir welds of some aluminum alloys exhibit relatively poor corrosion resistance. This research enhanced the corrosion properties of such welds through diode laser surface melting.A friction stir weld of aluminum alloy 2024 T351 was laser melted using a 1 kW diode laser. The melt-depth and microstructure were investigated using optical and scanning electron microscopy. The melt zone exhibited epitaxially grown columnar grains. At the interface between the melted and the un-melted zone, a thick planar boundary was observed. Energy dispersive spectroscopy analyzed the redistribution of elemental composition. The corrosion properties of the laser melted and native welds were studied in aqueous 0.5 M sodium chloride solution using open circuit potential and cyclic potentiodynamic polarization. The results show noticeable increase in the pit nucleation resistance (390 mV) after the laser surface treatment. The repassivation potential was nobler to the corrosion potential after the laser treatment, which confirmed that the resistance to pit growth was improved.     

Microstructure evolution of WC/steel composite by laser surface re-melting

WC/steel composites fabricated by electro-slag melting and casting were re-melted by transverse flow CO₂ laser. Optical microscopy, scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and transmission electron microscopy were used to analyze the microstructure evolution in laser melted layer. It was found that the laser-affected zone has three distinguished zones, the melted, transient and heat affected zone. The phases of the melted zone were composed of WC, Fe₃W₃C, (Cr,Fe)₇C₃, martensite and retained austenite. The microstructure evolution in the melted zone was represented by the transformation of three parts including the steel matrix, WC particles cluster and dispersed carbides. A significant reactant was herringbone eutectic carbide of Fe₃W₃C. The effect of laser scanning rate was mainly behaved in affecting the melt depth, microstructure of transient zone and dissolution of medium carbides. In comparison with the substrate, the melted zone has much higher microhardness.     

考虑非傅立叶效应的固液相变分子动力学模拟

采用耦合双温度模型的分子动力学方法对飞秒激光照射金箔的固液相变过程进行了模拟研究,利用序参数法对固液原子进行判定从而确定了金箔发生相变时的固液界面位置和温度,对基于傅立叶定律的抛物线模型和考虑非傅立叶效应的双曲线模型模拟得到的结果进行对比分析,在此基础上采用耦合双曲线模型的分子动力学方法研究了激光能量密度和脉冲宽度对金箔相变过程的影响.结果表明,当激光作用于金箔时,金箔上表面首先熔化,固液界面随时间不断向金箔底部移动,并且在相同条件下,双曲线模型下的金箔熔化深度和固液界面温度均大于抛物线模型的结果.当考虑非傅里叶效应时,激光能量密度越大,固液界面温度越高,金箔熔化时间越短;激光脉冲宽度越小,固液界面温度越大,金箔熔化速度越快.     

Surface relief diffraction gratings fabricated in ZnSe by frequency doubled Nd:YAG laser micromachining

A novel and rapid laser ablation method for the fabrication of diffractive optical elements (DOE) in ZnSe that takes advantage of the relatively low intensity damage threshold of the material is presented. The structures were characterized in terms of their shape and diffraction efficiency at normal incidence under 10.6 μm radiation for TE and TM polarizations. Sample surface polishing as well as the possible effect of the melted zone and structural modification of the material around the ablated region on the power transmission capability of the grating are also discussed.     

Effect of medium on friction and wear properties of compacted graphite cast iron processed by biomimetic coupling laser remelting process

Stimulated by the cuticles of soil animals, an attempt to improve the wear resistance of compact graphite cast iron (CGI) with biomimetic units on the surface was made by using a biomimetic coupled laser remelting process in air and various thicknesses water film, respectively. The microstructures of biomimetic units were examined by scanning electron microscope and X-ray diffraction was used to describe the microstructure and identify the phases in the melted zone. Microhardness was measured and the wear behaviors of biomimetic specimens as functions of different mediums as well as various water film thicknesses were investigated under dry sliding condition, respectively. The results indicated that the microstructure zones in the biomimetic specimens processed with water film are refined compared with that processed in air and had better wear resistance increased by 60%, the microhardness of biomimetic units has been improved significantly. The application of water film provided finer microstructures and much more regular grain shape in biomimetic units, which played a key role in improving the friction properties and wear resistance of CGI.     

Computational and experimental study of a melt-hardened zone on a roller modified by wide-band laser treatment

A three-dimensional finite element model has been developed to simulate the wide-band laser remelting process and predict the thermal and mechanical properties in the melt-hardened zone. The simulation of the laser remelting process was performed using the nonlinear thermo-mechanical properties, based on a wide-band heat source model. The temperature fields, phase transformations, hardness and residual stress distributions in the melt-hardened zone were analyzed. In the remelting zone, the transformed volumetric percentage of martensite is beyond 80% and the excessive transformed martensitic structure greatly affects the mechanical properties in the melt-hardened zone with its volumetric expansion effect. After remelting, the hardness can be improved and the residual stress distributed within the melted zone is mainly of the compressive type, while the tensile stress in the heat-affected zone (HAZ) may cause the initiation of cracks. The computational results are in good agreement with experimental measurements.     

Applications of excimer laser in nanofabrication

This paper addresses novel applications of an excimer laser (308 nm wavelength, 20 ns pulse duration) in nanofabrication. Specifically, laser assisted nanoimprint lithography (LAN), self-perfection by liquefaction (SPEL), fabrication of metal nanoparticle arrays, and the fabrication of sub-10-nm nanofluidic channels are covered. In LAN, a polymeric resist is melted by the laser pulse, and then imprinted with a fused silica mold within 200 ns. LAN has been demonstrated in patterning various polymer nanostructures on different substrates with high fidelity and uniformity, and negligible heat effect on both the mold and the substrate. SPEL is a novel technology that uses selective melting to remove fabrication defects in nanostructures post fabrication. Depending on the boundary conditions, SPEL is categorized into three basic types: Open-SPEL that takes place with surface open, Capped-SPEL where a cap plate holds the top surface of the nanostructures and Guided-SPEL where a plate held a distance above the structure guides the molten materials to rise and form a new structure with better profile. Using SPEL (in less than 200 ns), we have achieved a reduction of line edge roughness (LER) of Cr lines to 1.5 nm (3σ) (560% improvement from the original), which is well below what the previous technologies permit, and a dramatic increase of the aspect ratio of a nanostructure. We have used SPEL to make sub-25-nm smooth cylindrical NIL pillar molds and smoothing Si waveguides. Excimer laser is also used to make metal nanoparticles. Monolayers of particles are fabricated on various substrates (silicon, fused silica and plastics) by exposing thin metal films to a single laser pulse. Periodic nanoparticle arrays have been fabricated by fragmentation of metal grating lines. The periodicity of these nanoparticles can be regulated by surface topography such as shallow trenches. Finally, an excimer laser pulse has been used to melt the top portion of 1D and 2D Si gratings to seal off the top surface, forming enclosed nanofluidic channel arrays. The channel width has been further reduced to 9 nm using self-limited thermal oxidation. DNA stretching using 20 nm wide self-sealed channels is also demonstrated.     

Microstructure and properties of high chrome steel roller after laser surface melting

Laser surface melting of high chrome steels was achieved by a 5 kW continuous wave CO₂ laser. The microstructure of the laser surface-melted steels was investigated by scanning electron microscopy, transmission electron microscopy and X-ray diffractometry, and the hardness profiles were determined by a Vickers hardness tester. The corrosion behavior in 3.5% NaCl solution was studied by electrochemical corrosion equipment. The large carbides of high chrome steels are completely dissolved and ultrafine dendrites of austenite with submicroscopic M₂₃C₆ carbides precipitation are formed in the melted zone. The austenite in the melted zone has a high tempering stability. The corrosion resistance of the laser surface-melted steels is significantly improved due to the dissolution of carbides and the increase of the alloying elements in the solid solution as well as the large amount of austenite.     

Excimer Laser Processing of Grey Cast Iron: Microstructural and Tribological Characterization

Pearlitic grey iron was treated by means of a XeCl laser to remelt only a shallow layer. This process allows to modify the heteregeneous surfaces of cast irons in order to improve their tribological behaviour. The energy density ranged from 0.3 to 3 J/cm² and the number of shots from 4 to 40. It is shown that after laser irradiation austenite is formed in the melted layer; compressive stresses at the surface were exhibited as well as the deposition of graphite and the formation of carbides. It is shown that excimer laser processing highly decreases the coefficient of friction and the wear rate. This revised version was published online in August 2006 with corrections to the Cover Date.     

Experimental investigation of spatiotemporal evolution of femtosecond laser pulses during small-scale self-focusing

Small-scale self-focusing (SF) causes rapid increase in the partial spatial intensity, breaking up the spatial profile of the beam into an intensity increasing zone (IZ) and a non-increasing zone (NIZ). We measure the evolutions of the pulse width at the IZ and NIZ with small-scale SF, and our results show that the pulse width at the IZ becomes narrower with increasing partial spatial intensity. The pulse width at the NIZ is almost unchanged with increasing laser power. We find that our experimental results are in good agreement with an approximate theoretical analysis.     

Effect of Pulse Width and Fluence of Femtosecond Laser on Electron--Phonon Relaxation Time

The electron phonon relaxation time as functions of pulse width and fluence of femtosecond laser is studied based on the two-temperature model. The two-temperature model is solved using a finite difference method for copper target. The temperature distribution of the electron and the lattice along with space and time for a certain laser fluence is presented. The time-dependence of lattice and electron temperature of the surface for different pulse width and different laser fluence are also performed, respectively. Moreover, the variation of heat-affected zone per pulse with laser Auence is obtained. The satisfactory agreement between our numerical results and experimental data indicates that the electron-phonon relaxation time is reasonably accurate with the influences of pulse width and Auence of femtosecond laser.