Laser Scan Rate Research Articles

Laser Bending of 5005 Aluminum Alloy Sheets

The laser bendability of thick aluminum sheets has been scarcely investigated previously; in this article this property was evaluated for sheets of 5005 aluminum alloy by means of a high power diode laser. In the experimentation, the width of the sample to bend was changed as well as laser power and scan rate. The effect of other process parameters was also investigated; the surface aspect and the sample orientation with regard to the rolling direction. Mechanical tests and surface analyses were performed before and after laser testing so as to deepen the laser-material interaction. A thermal numerical model was also implemented to study the effect of the laser power on the laser beam absorption during processing. As a result, a good bendability of the 5005 alloy sheets was observed though many parameters need to be considered for the process control. A simple process map for line bending can be extracted from experimental data only if some of those parameters are fixed; above all the sample orientation. These results are found to be significant in better understanding the bending behavior of thick aluminum sheets with high density laser power when a complex interaction between process and material parameters is present.

Ultra-sensitive carbon monoxide detection by using EC-QCL based quartz-enhanced photoacoustic spectroscopy

A quartz-enhanced photoacoustic spectroscopy (QEPAS) based sensor for carbon monoxide detection at ppbv levels was developed with a 4.65 μm external-cavity quantum cascade laser operating both in continuous wave (cw) and pulsed modes. A 23-fold enhancement of the measured CO signal amplitude was obtained when water vapor, acting as a catalyst for vibrational energy transfer, was added to the targeted analyte mixture. In the cw mode, a noise-equivalent sensitivity (NES, 1σ) of 2 ppbv was achieved at a gas pressure of 100 Torr, for 1-s lock-in amplifier (LIA) time constant (TC), which corresponds to a normalized noise equivalent absorption coefficient (NNEA) of $1.48\times 10^{-8}~\mathrm{cm}^{-1}\,\mathrm{W}/\sqrt{\mathrm{Hz}}$ . In the pulsed mode, the determined NES and NNEA were 46 ppbv and $1.07\times 10^{-8}~\mathrm{cm}^{-1}\,\mathrm{W}/\sqrt{\mathrm{Hz}}$ , respectively, for a 3-ms LIA TC at atmospheric pressure with a laser scan rate of 18 cm-1/s and a 50 % duty cycle. An intercomparison between cw and pulsed QEPAS-based CO detection is also reported.

Laser Surface Modification of Borochromizing C45 Steel

Laser Surface Modification of Borochromizing C45 SteelIn this study the test results for borochromized C45 steel after laser surface modification were presented. Influence of laser heat treatment on the microstructure and microhardness of surface layer was investigated. The process of borochromizing consisted of chromium plating followed by diffusion boronizing. The laser heat treatment (LHT) of multiple tracks in the helical line was carried out with CO2laser beam. The technological laser TRUMPF TLF 2600 Turbo CO2of the nominal power 2.6 kW was applied. Borochromizing was carried out with laser power density q = 41.40 kW/cm2and at laser beam scanning rate v = 0.67 m/min and v = 2.016 m/min. Measurements of microhardness were conducted using the Vickers' method and Zwick 3212 B hardness tester. Microstructure observations were performed by means of an optical microscope Metaval Carl Zeiss Jena and scanning electron microscope Tescan VEGA 5135. After laser heat treatment with re-melting a three-zone layer was obtained, which included: re-melted zone, heat affected zone and a core. Influence of laser treatment parameters on thickness of melted zone and microstructure of the surface layer was tested. The microhardness tested along the axis of track of the surface layer after laser modification was about 800-850 HV. The results of tests showed influence of laser power density and scanning rate on microstructure and properties of borochromized layers.

Microstructural Modification of Laser-Bent Open-Cell Aluminum Foams

Laser forming tests have been performed on open-cell aluminum alloy foams with different pore size. Laser power was fixed at 150 W, a total of 150 laser scans led to a bending angle up to 60°, depending on the laser scan rate. At the end of the laser bending, the foams were left to cool and samples were extracted for analysis by means of an optic microscope. The alloy microstructure was investigated in different points of the samples and correlated with the processing conditions. Image analysis was also carried out to extract the percentage of melted area due to laser heating.

Influence of Laser Processing Parameters on Microstructure and Corrosion of Laser Surface Melted ZE41 Magnesium Alloy

In the present study, surface melting of a magnesium alloy, ZE41, was performed with an Nd:YAG laser, using different laser parameters (scan rate and beam type). The microstructure of the laser treated and untreated specimens were characterised by optical and scanning electron microscopy. The degree of microstructural refinement and melt depth was found to be a function of the laser scan rate. Electrochemical characterisation of the different laser treated specimens along with the untreated alloy was performed using electrochemical impedance spectroscopy. The laser treated specimens and untreated alloy showed similar corrosion resistance.

Dimensional analyses and surface quality of pulsed UV laser micro-machining of STAVAX stainless steel mold inserts

Laser micro-machining is a new, precise, and very flexible process in micro-mold manufacturing, especially for difficult to machine material, i.e., hardened steel. The aim of the work reported in this paper was to utilize response surface methodology to optimize the dimensional accuracy and surface finish for STAVAX stainless steel mold inserts in the pulsed UV laser micro-machining. The influence of laser machining parameters on the ablated depth and surface roughness of the machined mold inserts have been experimentally investigated. The parameters of insert quality are analyzed under varying laser power, pulse frequency, hatched spacing, scan rate, and number of passes. The settings of the laser micro-machining parameters are determined by using design of experiments method. The analysis of variance, and regression analyses are employed to find the optimal levels and to analyze the effect of the parameters on the depth accuracy values and surface finish. Confirmation experiments with the optimal levels of micro-machining parameters are carried out in order to illustrate the effectiveness of the multi-optimization method. The validity of regression approach to process optimization is well established.

연료전지용 금속 분리판 제작을 위한 DLM공정 특성 연구

Manufacturing of the bipolar plate of a direct methanol fuel cell (DMFC) by direct laser melting technology (DLM) was attempted. The DLM technology is highly influenced by process parameters such as laser power, scan rate and layering height. Therefore, an analysis of the DLM technology was performed under various conditions. The bipolar plates were fabricated using the DLM process with 316L stainless steel (STS 316L) plates and powder. Powder melting trials at various energy density were performed in order to select a feasible melting range for a given laser power. The melting line height increases and eventually saturates when the energy density increases, but decreases when the laser power increases at a given energy density. For the estimation of the potential performance of the bipolar plate, the surface roughness and contact resistance of the DLM layer were also analyzed. The changes of line height and thickness are useful information to report when manufacturing bipolar plate of fuel cell through the DLM process.

Growth characteristics of directionally solidified Al2O3/YAG/ZrO2 ternary hypereutectic in situ composites under ultra-high temperature gradient

Oxide eutectic ceramic in situ composites have attracted significant interest in the application of high-temperature structural materials because of their excellent high-temperature strength, oxidation and creep resistance, as well as outstanding microstructural stability. The directionally solidified ternary Al2O3/YAG/ZrO2 hypereutectic in situ composite was successfully prepared by a laser zone remelting method, aiming to investigate the growth characteristic under ultra-high temperature gradient. The microstructures and phase composition of the as-solidified hypereutectic were characterized by using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The results show that the composite presents a typical hypereutectic lamellar microstructure consisting of fine Al2O3 and YAG phases, and the enriched ZrO2 phases with smaller sizes are randomly distributed at the Al2O3/YAG interface and in Al2O3 phases. Laser power and scanning rate strongly affect the sample quality and microstructure characteristic. Additionally, coarse colony microstructures were also observed, and their formation and the effect of temperature gradient on the microstructure were discussed.

Application of direct laser melting to restore damaged steel dies

Direct laser melting (DLM) technology can be applied to restore damaged steel dies. To understand the effects of DLM process parameters such as the laser power and scan rate, a series of experiments was conducted to determine the optimal operating parameters. To investigate the laser melting characteristics, the depth/height ratio, depth/width ratio and micro-hardness as a function of the laser energy density were analyzed. Fe-Cr and Fe-Ni layers were deposited on a steel die with 11.38 J/mm2 of energy input. The wear-resistance and the friction coefficient of the deposited layer were investigated by a pin-on-disk test. The penetration depth decreased as the scan rate increased as a consequence of the shorter interaction time. The depth/height ratio of the deposited layer decreased with an increase in the scan rate. The depth/width ratio increased as laser power increased and the scan rate decreased. The deposition shape of the Fe-Ni powder was relatively shallow and wide compared with that of the Fe-Cr powder. The scan rate had a substantial effect upon the deposition height, with the Fe-Cr powder melting more than the Fe-Ni powder. The micro-hardness of the layer melted from the powders is higher than that of the substrate, and the hardness of the laser-surface-melted layer without any metal powder is higher compared to that of the metal-powder-melted layer. The direct laser melting process with Fe-Ni powder represents a superior method when restoring a steel die when the bead shape and hardness of the restored surface are important outcome considerations.

Directional solidification and microstructural development of Al 2O 3/GdAlO 3 eutectic ceramic in situ composite under rapid growth conditions

Oxide eutectic ceramic in situ composites prepared by directional solidification from the melt in the Al 2O 3–Ln 2O 3 systems are promising candidates for the manufacture of turbine blades because of their excellent mechanical properties. With high temperature gradient and rapid cooling rate, directionally solidified (DS) Al 2O 3/GdAlO 3(GAP) eutectic ceramics having smooth surface and full density are successfully prepared by laser zone remelting at the scanning rate of 240–720 mm/h. The microstructure development and solidification behaviour under rapid growth rate are investigated. Fine typical DS irregular eutectic structure of “Chinese script” consisting of interpenetrating α-Al 2O 3 and GAP phases is obtained at the scanning rate below 360 mm/h. Further increasing the scanning rate, the as-solidified Al 2O 3/GAP eutectic presents a combination of “Chinese script” and elongated colony microstructure with complex regular structure. Inside the colonies, the rod-type or lamellar-type eutectic microstructures with ultra-fine GAP surrounded by the Al 2O 3 matrix are observed. The interphase spacing is strongly dependent on the laser scanning rate, rapidly decreasing to the sub-micron range for the samples grown at the highest rate. Moreover, the formation condition and solidification mechanism of the particular microstructure of the ceramic composite during rapid solidification are discussed.

선택적 레이저 용융 공정시 용융 거동에 대한 공정 분석

Abstract Selective laser melting (SLM) is emerged as a new manufacturing technique to directly fabricate precise parts using metallic materials. The final characteristics of a component fabricated through the SLM process are strongly dependent upon various parameters such as laser power, scan rate and pulse duration, etc. This paper, therefore, focuses on the dimensional characteristics of melted 20㎛ Fe-Cr-Ni powder by fiber laser for the selective laser melting process. With energy density decrease, the height and depth were decreased. Although the conditions are of the same energy density, the shape is different by laser power and scan rate. The shapes at various laser parameters were divided into 3 groups based on depth over height. The smooth regular shape is obtained under the conditions of 50㎛ of powder height and 15-20㎲ of pulse duration. And the laser power influenced the variation of shape more significantly than the scan rate. Key Words : Selective Laser Melting(SLM), Fiber Laser, 20㎛ Fe-Cr-Ni Powder, Processing Parameters, Melted Shape

Fabrication and Characterization of Al<sub>2</sub>O<sub>3</sub>/GdAlO<sub>3</sub> Eutectic Ceramic <i>In Situ</i> Composite by Laser Zone Remelting

In situ composite of Al2O3/GdAlO3(GAP) ceramic eutectic prepared by directional solidification is an interesting candidate for the manufacture of turbine blades because of its excellent mechanical properties. In the present paper, directionally solidified Al2O3/GAP eutectic in situ composite ceramics are manufactured by the laser zone remelting technique to investigate the rapid solidification process. The laser power and scanning rate necessary to carry out the ceramic melt growth is determined. The characteristic microstructure is investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD). The as-solidified Al2O3/GAP eutectic presents an elongated colony structure consisting of only -Al2O3 and GAP phases with an oriented growth array. The eutectic spacing is strongly dependent on the laser scanning rate, rapidly decreasing to the sub-micron range for the samples grown at the highest rate. Besides, the formation condition and evolution of the particular microstructure of the composite during rapid solidification are discussed.

Applications of midinfrared quantum cascade lasers to spectroscopy

We review the use of both pulsed and continuous wave quantum cascade lasers in high-resolution spectroscopic studies of gas phase species. In particular, the application of pulsed systems for probing kinetic processes and the inherent rapid passage structure that accompanies observations of low-pressure samples using these rapidly chirped devices are highlighted. Broadband absorber spectroscopy and time-resolved concentration measurements of short-lived species, respectively exploiting the wide intrapulse tuning range and the pulse temporal resolution, are also mentioned. For comparison, we also present recent sub-Doppler Lamb-dip measurements on a low-pressure sample of NO, using a continuous wave external cavity quantum cascade laser system. Using this methodology the stability and resolution of this source is quantified. We find that the laser linewidth as measured via the Lamb-dip is ca. 2.7 MHz as the laser is tuned at comparably slow rates, but decreases to 1.3 MHz as the laser scan rate is increased such that the transition is observed at 30 kHz. Using this source, wavelength modulation spectroscopy of NO is presented.

Selective laser melting of aluminium components

Previous work has shown that the processing of aluminium alloys by selective laser melting (SLM) is difficult, with reasonable components only being produced with high laser powers (minimum 150 W) and slow laser scanning speeds. The high laser power is a significant problem as it is higher than that used in many SLM machines. Also, the combination of high power and low speed creates a large melt pool that is difficult to control, leading to balling of the melt and possible damage to the powder distribution system. Even when processing is carried out successfully, the high power and slow scan speed significantly increase build time and the manufacturing costs. This paper considers the changes that can be made to the SLM process so as to reduce the laser power required and increase the laser scanning rates, while still producing components with a high relative density. It also considers why aluminium and its alloys are much more difficult to process than stainless steels and commercially pure titanium. Two MCP Realizer machines were used to process 6061 and AlSi12 alloys, one processing at 50 W and the other 100 W laser power. Even with an optimum combination of process parameters a maximum relative density of only 89.5% was possible (achieved with 100 W). The major confounding factor for processing aluminium and its alloys was found to be oxidation due to the presence of oxygen within the build chamber. This formed thin oxide films on both the solid and molten materials. It was observed that the oxide on the top of the melt pool vaporised under the laser creating a fume of oxide particles, while melt pool stirring, probably due to Marangoni forces, tended to break the oxide at the base of the melt pool allowing fusion to the underlying tracks. However, the oxides at the sides of the melt pool remained intact creating regions of weakness and porosity, as the melt pool failed to wet the surrounding material. Therefore, if 100% dense aluminium components are to be produced by SLM, using low laser powers, methods need to be developed that can either disrupt these oxide films or avoid their formation.

Growth and characterization of nanostructured Al 2O 3/YAG/ZrO 2 hypereutectics with large surfaces under laser rapid solidification

The preparation of large bulk oxide eutectics with homogeneous and dense structure in nano-scale by melt growth method is a difficult challenge. Fully dense, homogeneous and crack-free ternary nanostructured Al 2O 3/YAG/ZrO 2 hypereutectic plate with large surface is successfully obtained by laser remelting. The hypereutectic in selected composition presents an ultra-fine eutectic-like microstructure consisting of alternating interpenetrating Al 2O 3, YAG and ZrO 2 lamellae with mean interphase spacing of about 150 nm, which is much smaller than the ternary eutectic composition grown at the same growth conditions. With the increase of laser scanning rate, the lamellar spacing is rapidly decreased. The minimum value obtained is 50 nm. The analysis indicates that the strong faceted growth behavior and cooperative branching of the component phases related with high entropies of fusion and large kinetic undercooling during laser rapid solidification are the primary formation reasons for the irregular eutectic growth morphology. Furthermore, the unique cellular microstructure with complex structure is also observed at high growth rate, and their formation mechanism and effect of the composition on the microstructure are discussed.

Investigation of the solidification behavior of Al 2O 3/YAG/YSZ ceramic in situ composite with off-eutectic composition

Directionally solidified Al 2O 3/YAG/YSZ ceramic in situ composite is an interesting candidate for the manufacture of turbine blade because of its excellent mechanical property. In the present study, two directionally solidified hypoeutectic and hypereutectic Al 2O 3/YAG/YSZ ceramic in situ composites are prepared by laser zone remelting, aiming to investigate the solidification behavior of the ternary composite with off-eutectic composition under high-temperature gradient. The results show that the composition and laser scanning rate significantly influence the solidification microstructure. The ternary in situ composite presents ultra-fine microstructure, and the eutectic interspacing is refined with the increase of the scanning rate. The Al 2O 3/YAG/YSZ hypoeutectic ceramic displays an irregular hypoeutectic network structure consisting of a primary Al 2O 3/YAG binary eutectic and fine Al 2O 3/YAG/YSZ ternary eutectic. Only at low scanning rate, homogeneous ternary eutectic-like microstructures are obtained in the hypoeutectic composition. Meanwhile, the Al 2O 3/YAG/YSZ hypereutectic ceramic shows homogeneous eutectic-like microstructure in most cases and the eutectic interspacing is finer than the ternary eutectic. Furthermore, the formation and evolution mechanism of the off-eutectic microstructure of the ternary composite are discussed.

Microstructure and hardness scaling in laser-processed B 4C–TiB 2 eutectic ceramics

Surface layers of the pseudo-binary eutectic comprised of boron carbide (B 4C) and titanium diboride (TiB 2) were directionally solidified via direct laser irradiation in an argon atmosphere. The resulting surface eutectic layers had highly oriented lamellar microstructures, whose scale (i.e. interlamellar spacing) was controlled directly by the laser scan rate, following an inverse square root dependence for lower solidification velocities. Higher velocities (>∼4.2 mm/s) departed from this relationship, although well-ordered microstructures were still achieved. A concomitant increase in the Vickers hardness with decreasing interlamellar spacing was observed, although the trend did not correspond to traditional Hall–Petch behavior. The hardness of the eutectic composites became load-independent at indenter loads greater than 9.81 N, indicating a potential transition from plastic to fractural deformation during indentation. A Vickers hardness of 32 GPa was achieved in the highest solidification velocity samples (42 mm/s) which had interlamellar spacings of 180 nm.

Jet-cooled infrared spectra of molecules and complexes with a cw mode-hop-free external-cavity QCL and a distributed-feedback QCL

The coupling of quantum cascade lasers (QCLs) with an off-axis cavity enhanced absorption (CEA) spectrometer and an astigmatic multiple pass absorption (MPA) spectrometer are described in this paper. A continuous wave (cw) liquid nitrogen cooled distributed-feedback QCL at 5.7 μm and a cw room temperature mode-hop-free external-cavity QCL at 6.1 μm were employed as the light source. For the CEA spectrometer, the effects of mirror size and laser scan rate were evaluated. For the MPA spectrometer, a pair of astigmatic mirrors with a 55 cm mirror distance was aligned to the 366-pass configuration. The jet-cooled samples were generated using a homemade pulsed slit jet nozzle assembly. Two LabVIEW programs were written to automate and synchronize the timing of the laser scan, the pulsed slit jet molecular expansion, and the data acquisition. Infrared spectra of jet-cooled methyl lactate and the Ar–H2O complex and room temperature N2O and NH3 samples were measured using both the rapid scan and the wavelength modulation methods to evaluate the sensitivity and resolution of the CEA and MPA spectrometers. The combination of the MPA spectrometer with the external-cavity QCL using the rapid scan method was found to be the best suited combination to measure high resolution jet-cooled infrared spectra.

Directional Solidification and Characterization of Al<sub>2</sub>O<sub>3</sub>/Er<sub>3</sub>Al<sub>5</sub>O<sub>12</sub> Eutectic <i>In Situ</i> Composite by Laser Zone Remelting

Directionally solidified (DS) oxide eutectic in situ composites are attracting increasing attention because of their unique properties and potential applications to high temperature structural materials, optical or electronic devices. Among the alumina-based eutectic composites, DS Al2O3/Er3Al5O12(EAG) eutectic is considered to be promising candidate for use as selective emitter at high temperature. In this work, eutectic in situ composites of Al2O3/EAG rods having smooth surface and full density are successfully prepared by directional solidification using the laser zone remelting method, aiming to investigate the growth characteristic of this novel binary eutectic under high temperature gradient. The microstructure is investigated by scanning electron microscopy (SEM), energy disperse spectroscopy (EDS) and X-ray diffraction (XRD). The Al2O3/EAG eutectic presents a very fine irregular network structure consisting of only -Al2O3 and Er3Al5O12 phases without grain boundaries and amorphous phases between interfaces. The eutectic interphase spacing is strongly dependent on the laser scanning rate, rapidly decreasing at the sub-micron levels for the samples grown at high rate. Furthermore, the microstructural formation and evolution of the composite are analyzed and discussed.

Effect of Laser Parameters on Sintered Powder Morphology

Selective laser sintering is a kind of rapid prototyping process whereby a three-dimensional part is built layerwise by laser scanning a powder. This process is highly influenced by powder and laser parameters such as laser power, scan rate, spot size and layer thickness. Therefore a study on fabricating a line with Fe-Ni-Cr powder on AISI H13 tool steel has been performed by selective laser sintering. In this study, fabrication was performed by experimental facilities consisting of a 200W fiber laser which can be focused to 0.08 mm and atmospheric chamber which can control atmospheric pressure with argon. The line was fabricated with various laser power, scan rate and layer thickness. Line width and surface quality were investigated. With power increase or layer thickness decrease, line width was decreased and line surface quality was improved with scan rate optimization.