Laser Scanning Velocity Research Articles

Surface Treatment of AISI M2 Tool Steel by Laser Melting

Attempt was made to improve the surface hardness and wear properties of AISI M2 high speed tool steel. Laser surface melting (LSM) of tool steel was conducted with 2.2 KW Nd:YAG laser as heating source. Conventional hardening of the tool steel was applied. Characterizing the LSM, with optical and field emission scanning electron microscopy and surface hardness technique was used to evaluate the micro-hardness and mechanical behaviour of different regions of melting pool. AISI M2 tool steel is approximately HV 260, hardness after conventional treatment was 850 HV and the hardness after laser surface heat treatment is around 900 HV. It was found that there is a considerable influence of the laser power density and scanning velocity on the melted zone dimensions and the re-solidified structure. Increasing laser energy and reducing the laser scanning rate results in deeper and wider melt pool formation.

Process modelling in laser forming of doubly-curved sheets from cylinder shapes

3D laser forming Laser forming becomes a more practical process in industrial applications depending on the inverse problem for a desired shape, in which the process planning (heating line and heating condition) is one of the most important parameters. The forming is more efficient for the doubly curved plate thermally formed from the rolled cylinder plate than from the flat plate in industry. Therefore, this paper presents a planning strategy for the heating line and the heating condition in laser forming of doubly curved shapes from singly curved shapes. A finite element model accompanied with strain energy optimization is established to develop the saddle sheet to the cylinder shape, which can determine the strain distribution that is required in the thermal forming process. The heating paths on the cylinder surface are automatically generated by the vector transferred from the synthesized strain. The heating condition including laser power and scanning velocity are determined by geometrical difference along the corresponding heating lines between the initial cylinder and target saddle surfaces. The overall methodology is validated by experiments.

Modeling and simulation of thermal field and solidification in laser powder bed fusion of nickel alloy IN625

Finite element modeling and simulation of laser powder bed fusion process (L-PBF) provides physical insight about the laser processing that is often not possible or highly difficult by in-situ monitoring or in-process measurements. A three-dimensional (3D) thermal field especially into depth direction that is not visible by a thermal camera can be obtained by solving the 3D heat transfer problem. Furthermore, microstructure growth can be modelled to predict the direction of solidification in the fabricated part. This paper presents 3D Finite Element Method (FEM) based simulation models developed for laser processing of single- and multi-tracks with different energy density levels dependent upon process parameters such as laser power and scan velocity. This 3D L-PBF process model is validated with in-situ thermal measurements and further improved by utilizing predicted spattering of powder material which is in turn included as a stochastic heat loss in the 3D FEM model. Thermal gradients extracted from these simulations utilized to compute the directions in the resulting solidification which are found to be in reasonable agreements with experimental observations. Finally, a phase field method based post processing is applied on the thermal solution to simulate the nucleation phenomenon and columnar dendrite formation during solidification.

Study of Femtosecond Laser Ablation Effect on Micro-Processing for 4H-SiC Substrate

4H-SiC substrate was ablated by linearly polarized femtosecond (fs) laser in three direct write methods at different parameters, such as repetition rate, scanning velocity and fluence, etc. Two processing modes, transverse scanning mode (TSM) and cross irradiation mode (CIM), were introduced. The surface morphologies were observed by scanning electron microscopy (SEM) for detailed investigation. It was found that the surface morphologies differed remarkably at different processing parameters. Firstly, the shapes of micro craters fabricated at different repetition rates and ablation time duration were respectively investigated. The shape of fs laser spot was demonstrated to play an important role for the generation of micro craters. Secondly, the effect of scanning velocity on the formation of nanoripples and micro grooves were investigated. It was found that the spatial ripples could be refabricated during repeated fs laser ablation; periodic ripples and micro grooves depended on fs laser scanning velocity. Agglomerative substance was fabricated especially at slow scanning velocity. Furthermore, rippled surfaces induced at different fluence were achieved and exhibited. Regular and uniform surfaces with periodic ripples were fabricated at the fluence of 0.31∼0.38 J/cm2. Finally, CMP was carried out to study the effect of fs laser ablation on polishing.

Femtosecond laser inscribed cladding waveguide lasers in Nd:LiYF4 crystals

Depressed circular cladding, buried waveguides were fabricated in Nd:LiYF4 crystals with an ultrafast Yb-doped fiber master-oscillator power amplifier laser. Waveguides were optimized by varying the laser writing conditions, such as pulse energy, focus depth, femtosecond laser polarization and scanning velocity. Under optical pump at 799 nm, cladding waveguides showed continuous-wave laser oscillation at 1047 nm. Single- and multi-transverse modes waveguide laser were realized by varying the waveguide diameter. The maximum output power in the 40 μm waveguide is ∼195 mW with a slope efficiency of 34.3%. The waveguide lasers with hexagonal and cubic cladding geometry were also realized.

Evaluation of the surface finish and airborne particles generated by nanosecond pulsed laser rastering processes

The ambient air pollution of nano- and micrometric sized particles produced during a rastering process is measured and analyzed together with the topographic measurements of the rastered surface on sheets of stainless steel samples. The rastering process performed consisted of carrying out ablation with consecutive nanosecond infrared laser pulses along parallel lines. The topographic measurements and the measurements of the air concentration of the total active surface of the nanoparticles as well as the number of micrometric particles generated during 1 min of the rastering process were measured for different adjustments of the laser power, pulse frequency, and scan velocity. Exposure to very high nanoparticulate air concentration was measured, which is a health risk that should be avoided. The laser power should be reduced as much as possible to minimize the air pollution, and by analyzing the surface finish of the rastered surfaces, it was observed that the main factor that provided the best finish qu...

Forming of closed-cell aluminum foams under thermal loadings: experimental investigation

Metal foams are a new category of materials that in the last decade have been introduced thanks to their good physical and mechanical properties such as high stiffness and low density. Metal foam sheets cannot be formed or bent using the conventional mechanical forming processes because of their low ductility. To overcome this difficulty, a thermal approach was followed in this study, namely, the laser forming process was used to achieve higher bending angles in aluminum closed-cell foam sheets by using the irradiation of a CO2 laser beam on the surface of the sheets to produce a temperature gradient across the sheet thickness. The effect of the process parameters, including the laser power, scan velocity, beam diameter, and number of the scan passes on the obtainable bending angle, was investigated. Results showed that using concentrated thermal loadings significantly enhanced the formability of the metal foam sheets, compared to applying mechanical bending forces. Finally, an equation was derived to predict the bending angle as a function of the input process parameters.

A study on the effect of energy input on spatter particles creation during selective laser melting process

Selective laser melting (SLM) is a promising manufacturing technique for the production of complex metallic components. One of the crucial factors influencing the mechanical properties of the final product is spatter particles formation during the process. In this study, high- speed photography is utilized to record the formation mechanisms and the dynamic behavior of spatter particles. An image processing analysis framework is utilized to assess the distribution of spatter particles under various energy inputs. It is found that changing the laser scan velocity has more influences on spatter formation in comparison with the energy input. The relationship between the numbers of created spatter particles, induced unmelted regions and density variability are interpreted and discussed based on other observations, such as microscopic examination and density analysis of SLM parts. The obtained results could be used to enhance the current manufacturing process parameters optimization methods in SLM process.

Theoretical and experimental analysis of the impact on ablation depth of microchannel milling using femtosecond laser

The plasma brightness cannot be used as a direct indicator of ablation depth detection by femtosecond laser was experimentally demonstrated, which led to the difficulty of depth measurement in the maching process. The tests of microchannel milling on the silicon wafer were carried out in the micromachining center in order to obtain the influences of parameters on the ablation depth. The test results showed that the defocusing distance had no significant impact on ablation depth in LAV effective range. Meanwhile, the reason of this was explained in this paper based on the theoretical analysis and simulation calculation. Then it was proven that the ablation depth mainly depends on laser fluence, step distance and scanning velocity. Finally, a research was further carried out to study the laser parameters which relate with the microchannel ablation depth inside the quartz glass for more efficiency and less cost in processing by femtosecond laser.

Design and characterization of textured surfaces for applications in the food industry

The aim of this work is to design, manufacture and characterize surface morphologies on AISI 316L stainless steel produced by a custom designed laser-texturing strategy. Surface textures were characterized at a micrometric dimension in terms of areal parameters compliant with ISO 25178, and correlations between these parameters and processing parameters (e.g. laser energy dose supplied to the material, repetition rate of the laser pulses and scanning velocity) were investigated. Preliminary efforts were devoted to the research of special requirements for surface morphology that, according to the commonly accepted research on the influence of surface roughness on cellular adhesion on surfaces, should discourage the formation of biofilms. The topographical characterization of the surfaces was performed with a coherence scanning interferometer. This approach showed that increasing doses of energy to the surfaces increased the global level of roughness as well as the surface complexity. Moreover, the behavior of the parameters Spk, Svk also indicates that, due to the ablation process, an increase in the energy dose causes an average increase in the height of the highest peaks and in the depth of the deepest dales. The study of the density of peaks Spd showed that none of the surfaces analyzed here seem to perfectly match the conditions dictated by the theories on cellular adhesion to confer anti-biofouling properties. However, this result seems to be mainly due to the limits of the resolving power of coherence scanning interferometry, which does not allow the resolution of sub-micrometric features which could be crucial in the prevention of cellular attachment.

Geometry characteristics modeling and process optimization in coaxial laser inside wire cladding

Coaxial laser inside wire cladding method is very promising as it has a very high efficiency and a consistent interaction between the laser and wire. In this paper, the energy and mass conservation law, and the regression algorithm are used together for establishing the mathematical models to study the relationship between the layer geometry characteristics (width, height and cross section area) and process parameters (laser power, scanning velocity and wire feeding speed). At the selected parameter ranges, the predicted values from the models are compared with the experimental measured results, and there is minor error existing, but they reflect the same regularity. From the models, it is seen the width of the cladding layer is proportional to both the laser power and wire feeding speed, while it firstly increases and then decreases with the increasing of the scanning velocity. The height of the cladding layer is proportional to the scanning velocity and feeding speed and inversely proportional to the laser power. The cross section area increases with the increasing of feeding speed and decreasing of scanning velocity. By using the mathematical models, the geometry characteristics of the cladding layer can be predicted by the known process parameters. Conversely, the process parameters can be calculated by the targeted geometry characteristics. The models are also suitable for multi-layer forming process. By using the optimized process parameters calculated from the models, a 45 mm-high thin-wall part is formed with smooth side surfaces.

Evaluation of microstructures and properties of laser-annealed electroless Ni–P/Ni–Mo–P duplex coatings

Duplex Ni–P (inner)/Ni–Mo–P (outer) coatings on carbon steel were prepared by electroless deposition using dual baths, and then subjected to laser annealing to achieve crystallization. The microstructures in terms of degrees of crystallization and crystalline sizes were characterized using a quantitative XRD method. The evaluation of the properties of the Ni–P/Ni–Mo–P coatings, including hardness and corrosion behavior, was carried out using nanoindentation, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) in 10% HCl solution. It was found that the microstructures of the duplex coatings laser-annealed were determined by the laser scanning velocities when the power and beam dimension of the laser beam were kept constant. The hardness of the duplex coatings was dependent on the crystallization degree, relative sizes between Ni3P and Ni/Ni-Mo phases. The results of the electrochemical measurements revealed that the duplex coatings provided better corrosion resistance than the single Ni–P coating. The observation of the corrosion morphologies by SEM/EDS showed that the duplex coatings exhibited a combined corrosion attack of uniform corrosion and pitting corrosion. A loss of protection ability of the duplex coatings to substrate was found when the pitting corrosion reached the steel surface, leading to spallation of the coatings. The best corrosion protection of the duplex coatings was achieved when the coatings were laser-annealed at the scanning velocity of 14mm/s.

Material removal effect of microchannel processing by femtosecond laser

Material processing using ultra-short-pulse laser is widely used in the field of micromachining, especially for the precision processing of hard and brittle materials. This paper reports a theoretical and experimental study of the ablation characteristics of a silicon wafer under micromachining using a femtosecond laser. The ablation morphology of the silicon wafer surface is surveyed by a detection test with an optical microscope. First, according to the relationship between the diameter of the ablation holes and the incident laser power, the ablation threshold of the silicon wafer is found to be 0.227J/cm2. Second, the influence of various laser parameters on the size of the ablation microstructure is studied and the ablation morphology is analyzed. Furthermore, a mathematical model is proposed that can calculate the ablation depth per time for a given laser fluence and scanning velocity. Finally, a microchannel milling test is carried out on the micromachining center. The effectiveness and accuracy of the proposed models are verified by comparing the estimated depth to the actual measured results.

A high precision scheme for micro‐channel processing in quartz glass using femtosecond laser

AbstractUltra‐short pulse laser with unique advantages in micromachining, especially on the hard brittle material becomes the focus of study on micro‐structure processing. Theoretical and experimental researches are taken from the ablation properties of micro‐channel in the quartz glass by femtosecond laser. As the ablation depth of quartz glass has not relationship obviously with defocusing distance within LAV effective range, a new way to control precisely the ablation depth of quartz glass is present in the study. With fixed laser power, the ablation depth could be known easily by the length of the ablation streak while moving the femtosecond laser focus along the straight line with a 45‐degree angle from the quartz glass surface within the LAV range. Research is carried out to study laser pulse power, scanning velocity and step distance to do something with micro‐channel ablation quality. According to the parameters of test, the micro‐channel with rectangle cross section of the quartz glass is processed, and then we analyze the ablation morphology and processing efficiency. The results provided a important suggestion to process micro‐structure precisely for quartz glass by femtosecond laser.

Effect of laser treatment parameters on surface modification and tribological behavior of AISI 8620 steel

In this study, the effect of different shielding gas and laser scanning velocity on the resulting retained austenite (RA) content and tribomechanical properties of AISI 8620 steel were investigated. RA generation was comparatively lower at higher scanning velocity and with argon as the shielding gas due to lower beam interaction time and inert environment. Varied heat input and differing reaction volume resulted in different hardness levels on these samples. Air shielded samples showed slightly higher friction coefficient than the other samples due to higher surface roughness and shearing of oxide layers. The nitrogen shielded samples exhibited the best wear resistance among all samples, which was attributed to nitride formation on surface and to high hardness resulting from the laser treatment.

The Fabrication of Porous Si with Interconnected Micro-Sized Dendrites and Tunable Morphology through the Dealloying of a Laser Remelted Al-Si Alloy.

Coral-like porous Si was fabricated through the dealloying of a laser remelted as-cast AlSi12 alloy(Al-12 wt % Si). The porous Si was composed of interconnected micro-sized Si dendrites and micro/nanopores, and compared to flaky Si, which is fabricated by direct dealloying of the as-cast AlSi12 alloy. The structure of the porous Si was attributed to the dendritic solidification microstructure formed during the laser remelting process. The micropore size of the porous Si decreased from 4.2 μm to 1.6 µm with the increase in laser scanning velocity, indicating that the morphology of porous Si could be easily altered by simply controlling the laser remelting parameters. The coral-like porous Si provided enough space, making it promising for high-performance Si-based composite anode materials in lithium-ion batteries. The proposed hybrid method provides a straightforward way of tuning the porous structure in the dealloyed material.

Laser surface modification of 316L stainless steel.

Medical grade 316L stainless steel was laser surface melted (LSM) using continuous wave Nd-YAG laser in argon atmosphere at 1 and 5 mm/s. The treated surfaces were characterized using electron backscatter diffraction to study the influence of top surface crystallographic orientation and type of grain boundaries on corrosion resistance, wettability, and biocompatibility. The laser scan velocity was found to have a marginal influence on the surface roughness and the type of grain boundaries. However, the crystal orientation density was found to be relatively high in 1 mm/s samples. The LSM samples showed a higher concentration of {101} and {123} planes parallel to the sample surface as well as a higher fraction of low-angle grain boundaries. The LSM samples were found to exhibit better surface wettability and enhanced the viability and proliferation of human fetal osteoblast cells in vitro when compared to the untreated samples. Further, the corrosion protection efficiency of 316L stainless steel was improved up to 70% by LSM in as-processed condition. The increased concentration of {101} and {123} planes on surfaces of LSM samples increases their surface energy, which is believed to be responsible for the improved in vitro cell proliferation. Further, the increased lattice spacing of these planes and high concentration of low-energy grain boundaries in LSM samples would have contributed to the better in vitro corrosion resistance than untreated 316L stainless steel. Our results indicate that LSM can be a potential treatment option for 316L stainless steel-based biomedical devices to improve biocompatibility and corrosion resistance. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 569-577, 2018.

Heat transfer and diffusion-controlled kinetics of liquid–solid phase in titanium matrix composite during selective laser melting

This study describes a self-consistent theoretical model of simulating diffusion-controlled kinetics on the liquid–solid phase boundary during high-speed solidification in the melt pool after the selective laser melting (SLM) process for titanium matrix composite based on Ti–TiC system. The model includes the heat transfer equation to estimate the temperature distribution in the melt pool and during crystallization process for some deposited layers. The temperature field is used in a micro region next to solid–liquid boundary, where solute micro segregation and dendrite growth are calculated by special approach based on transient liquid phase bonding. The effect of the SLM process parameters (laser power, scanning velocity, layer thickness and substrate size) on the microstructure solidification is being discussed.

Optimization of CO2laser welding process parameters of PP/EPDM/Clay nanocomposite using response surface methodology

In recent years, polymer – based nanocomposites have found wide applications in various industries. Polypropylene / ethylene – propylene - diene monomer /Nanoclay (PP/EPDM/Nanoclay) nanocomposite is one of these materials that has many applications in automotive and aircraft industries. Welding as a fabrication process has attracted the attention of researchers for joining these materials. Among welding processes, laser welding because of its advantages can be a choice for joining of PP/EPDM/Nanoclay nanocomposites. In this paper, the effect of CO2 laser power, scan velocity, stand-off distance and clay content on impact strength of butt-welded PP/EPDM/Nanoclay 3.2 mm sheets is investigated. The response surface methodology (RSM) is used to develop a statistical model relating the above parameters to the impact strength of the weld joint. The results indicated that increase in clay content and scan velocity decreased the impact strength whereas stand-off distance increased it. The maximum impact strength of 70 J/m is achieved at laser power of about 103 W, velocity of 400 mm.min-1, stand-off-distance 8 mm and clay content of about 1.3% wt.

The Influence of Laser Surface Remelting on the Microstructure of EN AC-48000 Cast Alloy

Abstract Paper present a thermal analysis of laser heating and remelting of EN AC-48000 (EN AC-AlSi12CuNiMg) cast alloy used mainly for casting pistons of internal combustion engines. Laser optics were arranged such that the impingement spot size on the material was a circular with beam radius rb changes from 7 to 1500 μm. The laser surface remelting was performed under argon flow. The resulting temperature distribution, cooling rate distribution, temperature gradients and the depth of remelting are related to the laser power density and scanning velocity. The formation of microstructure during solidification after laser surface remelting of tested alloy was explained. Laser treatment of alloy tests were perform by changing the three parameters: the power of the laser beam, radius and crystallization rate. The laser surface remelting needs the selection such selection of the parameters, which leads to a significant disintegration of the structure. This method is able to increase surface hardness, for example in layered castings used for pistons in automotive engines.