Showing papers in "Optics and Laser Technology in 2015"

Study on the microstructure, mechanical property and residual stress of SLM Inconel-718 alloy manufactured by differing island scanning strategy

Abstract: Inconel-718 has received an extensive using in mold industry. The selective laser melting (SLM) is providing an ideal means for manufacturing mold insert with complex geometrical features and internal architecture. During the manufacturing of high quality mold inserts with conformal cooling channel, the parameters play a vital role in the SLM process. In the study, the Inconel-718 alloys were manufactured by SLM with 2×2 mm 2 , 3×3 mm 2 , 5×5 mm 2 , and 7×7 mm 2 island scanning strategies. The microstructure, mechanical property, and residual stress were investigated by optical microscope, tensile test and Vickers micro-indentation, respectively. It can be found that the relative density increased with enlarging the island size; the results on the microstructure indicated that the cracks and more pores were detected in the 22-specimen; whilst the microstructures of all specimens were composed of fine dendritic grains, cellular, and columnar structures; the tensile testing suggested that the ultimate tensile strength and yield strength of all samples was similar; while the outcome of the residual stress showed that the value of residual stress was ranked in the following sequence: 22-specimen

Experimental investigation and statistical optimisation of the selective laser melting process of a maraging steel

Abstract: Selective Laser Melting (SLM) is an Additive Manufacturing process (AM) that built parts from powder using a layer-by-layer deposition technique. The control of the parameters that influence the melting and the amount of energy density involved in the process is paramount in order to get valuable parts. The objective of this paper is to perform an experimental investigation and a successive statistical optimization of the parameters of the selective laser melting process of the 18Ni300 maraging steel. The experimental investigation involved the study of the microstructure, the mechanical and surface properties of the laser maraging powder. The outcomes of experimental study demonstrated that the hardness, the mechanical strength and the surface roughness correlated positively to the part density. Parts with relative density higher than 99% had a very low porosity that presented closed and regular shaped pores. The statistical optimization determined that the best part properties were produced with the laser power bigger than 90 W and the velocity smaller than 220 mm/s.

Microstructure, microhardness and corrosion resistance of Stellite-6 coatings reinforced with WC particles using laser cladding

Abstract: The paper presents the method of preparation and study results of metal matrix composite coatings (MMC coating) in the system of Stellite-6 and tungsten carbides. Changes in microstructure, corrosion resistance, microhardness, phase and chemical composition as well as surface conditions were investigated. Stellite-6/WC MMC coatings were prepared by laser cladding technology using a 1 kW continuous wave Yb:YAG disk laser with a powder feeding system. Two different powder mixtures containing 30% and 60% of WC and three different values of laser beam power were used. It was found that increasing WC values caused an increase in microhardness on the cross-section of the produced coating in comparison to the substrate. Depending on the laser beam power, the coatings produced with 30% WC achieved microhardness in the range from about 350 HV0.05 (700 W) to about 680 HV0.05 (550 W). Twice as large amount of WC particles in the powder mixture resulted in increase of microhardness from about 700 HV0.05 (700 W) to about 1500 HV0.05 (550 W). In the coating M7C3, M6C and M23C6 carbides were identified by an X-ray diffraction method. Special attention was given to bondings between carbide particles and metal matrix, which had a characteristic microstructure. A reduction of corrosion resistance with increasing WC content in coating was also discovered.

Investigations on the influence of composition in the development of Ni-Ti shape memory alloy using laser based additive manufacturing

Abstract: Among the various shaped memory alloys (SMA), nitinol (Ni–Ti alloy) finds applications in automotive, aerospace, biomedical and robotics The conventional route of fabrication of SMA has several limitations, like formation of stable secondary phases, fabrication of simple geometries, etc This paper reports a novel method of fabricating SMA using a laser based additive manufacturing technique Three different compositions of Ni and Ti powders (Ni-45% Ti-55%; Ni-50% Ti-50%; Ni-55% Ti45%) were pre-mixed using ball-milling and laser based additive manufacturing system was employed to fabricate circular rings The material properties of fabricated rings were evaluated using Scanning Electron Microscopy (SEM), Differential scanning calorimeter (DSC), X-ray diffraction (XRD) system and micro-hardness test All the characterized results showed that SMA could be manufactured using the laser based additive manufacturing process The properties of laser additive manufactured SMA (Ni-50% Ti-50%) were found to be close to that of conventionally processed SMA

Effect of the scanning speed on microstructural evolution and wear behaviors of laser cladding NiCrBSi composite coatings

Abstract: Laser cladding composite coatings were fabricated on the surface of the Ti6Al4V substrate by fiber laser cladding the NiCrBSi alloy powder. The influences of scanning speed on the dilution rate and microstructure of the coatings were investigated in detail by X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). Combined with the analyses of microhardness and fracture toughness, the wear behaviors of the coatings obtained at different scanning speeds were revealed. Results indicated that the dilution rates of the coatings were similar (about 64.23%) with variations in scanning speed ranging from 5 mm/s to 15 mm/s. An abrupt decrease in dilution rate (37.06%) was observed at the scanning speed of 20 mm/s. Microstructural observation showed that the blocky TiB 2 and the cellular dendrite TiC particles were uniformly dispersed in the TiNi–Ti 2 Ni dual-phase intermetallic compound matrix at scanning speeds of 5–15 mm/s. When the scanning speed was further increased to 20 mm/s, the stripe-shaped CrB, gray irregular-shaped Cr 3 C 2 and black blocky TiC particles uniformly dispersed in the γ(Ni) matrix were synthesized in situ. The particles became finer with the increase in scanning speed. The average microhardness of the coating (1026.5 HV 0.2 ) at the scanning speed of 20 mm/s was enhanced significantly compared with that of the other three coatings (about 886.4 HV 0.2 ). The lowest average friction coefficient (about 0.371) was obtained at the scanning speed of 20 mm/s and was relatively stable with the change in sliding time. The lowest wear loss of the coating was also obtained at the scanning speed of 20 mm/s. Analyses of the worn surfaces showed that the coating prepared at the scanning speed of 20 mm/s was in good condition because of its excellent combination of resistance to micro-cutting and brittle debonding. Comparatively speaking, the coating produced at the scanning speed of 20 mm/s possessed excellent comprehensive mechanical properties.

Effect of power distribution on the weld quality during hybrid laser welding of an Al-Mg alloy

Abstract: This paper treats of the analysis of the effect of arc and laser powers on the quality of the arc assisted fiber laser welding of an Al–Mg alloy in the butt configuration. Grain size, weld geometry defects, porosity, and magnesium loss were measured. Magnesium content of the fused zone decreased as the laser power increased while the porosity increased with laser power. Microhardness profiles and tensile properties were explained on the basis of the joint microstructure and defects and related to the power distribution. The porosity level and Mg content in the fused zone affected both tensile strength and ductility. The power distribution that stabilized the welding process and minimize the weld porosity was defined.

Automatic detection of zebra crossings from mobile LiDAR data

Abstract: An algorithm for the automatic detection of zebra crossings from mobile LiDAR data is developed and tested to be applied for road management purposes. The algorithm consists of several subsequent processes starting with road segmentation by performing a curvature analysis for each laser cycle. Then, intensity images are created from the point cloud using rasterization techniques, in order to detect zebra crossing using the Standard Hough Transform and logical constrains. To optimize the results, image processing algorithms are applied to the intensity images from the point cloud. These algorithms include binarization to separate the painting area from the rest of the pavement, median filtering to avoid noisy points, and mathematical morphology to fill the gaps between the pixels in the border of white marks. Once the road marking is detected, its position is calculated. This information is valuable for inventorying purposes of road managers that use Geographic Information Systems. The performance of the algorithm has been evaluated over several mobile LiDAR strips accounting for a total of 30 zebra crossings. That test showed a completeness of 83%. Non-detected marks mainly come from painting deterioration of the zebra crossing or by occlusions in the point cloud produced by other vehicles on the road.

Random-type scanning patterns in laser shock peening without absorbing coating in 2024-T351 Al alloy: A solution to reduce residual stress anisotropy

Abstract: Laser Shock Peening (LSP) is considered as an alternative technology to shot peening (SP) for the induction of compressive residual stresses in metallic alloys in order to improve their fatigue, corrosion and wear resistance. Since laser pulses generated by high-intensity laser systems cover only a small area, laser pulses are generally overlapped and scanned in a zigzag-type pattern to cover completely the surface to be treated. However, zigzag-type scanning patterns induce residual stress anisotropy as collateral effect. The purpose of this paper is to describe and explain, for the first time and with the aid of the numerical model developed by the authors, the influence of the scanning pattern directionality on the residual stress tensor. As an effective solution, the authors propose the application of random-type scanning patterns instead of zigzag-type in order to reduce the mentioned residual stress anisotropy.

Raman optical fiber distributed temperature sensor using wavelet transform based simplified signal processing of Raman backscattered signals

Abstract: A Raman optical fiber distributed temperature sensor, using a wavelet transform based signal processing technique for backscattered anti-Stokes and Stokes signals, is presented. This technique mainly performs two functions. First, it equalizes the wavelength dependent optical fiber attenuation of two Raman backscattered signals and second, denoises these Raman signals without generating any appreciable spatial inaccuracy in locating the hot zones. The proposed technique enables automatic measurement of distributed temperature profile that has better temperature accuracy and very small spatial error in detecting the location of hot zones. The accuracy achieved in temperature measurement with processed Raman signals is much better than the accuracy obtained with unprocessed signals. Results show a maximum temperature error of ±3.5 °C in a temperature range of 25–295 °C and a maximum spatial error of ±3 cm in locating the hot zones over a sensing length of 205 m with spatial resolution of 1 m. The proposed technique has been used for the development of a prototype Raman optical fiber distributed temperature sensor (ROFDTS) system which employs 200/220 μm Polyimide coated Multimode optical fiber. The technique is much simpler compared to other complex techniques described in the literature and is suitable for general temperature sensing applications. The technique supports automatic and dynamic self-calibration of ROFDTS to take care of slow variations/drifts in the observed Raman signals which are due to fluctuations in laser power and laser–fiber coupling. A self-calibration setup has also been developed to track the changes in the temperature of calibration zone.

Microstructure and properties of Fe-based composite coating by laser cladding Fe–Ti–V–Cr–C–CeO2 powder

Abstract: In situ TiC–VC reinforced Fe-based cladding layer was obtained on low carbon steel surface by laser cladding with Fe–Ti–V–Cr–C–CeO 2 alloy powder. The microstructure, phases and properties of the cladding layer were investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), transmission electron microscopy (TEM), potentio-dynamic polarization and electro-chemical impedance spectroscopy (EIS). Results showed Fe–Ti–V–Cr–C–CeO 2 alloy powder formed a good cladding layer without defects such as cracks and pores. The phases of the cladding layer were α-Fe, γ-Fe, TiC, VC and TiVC 2 . The microstructures of the cladding layer matrix were lath martensite and retained austenite. The carbides were polygonal blocks with a size of 0.5–2 μm and distributed uniformly in the cladding layer. High resolution transmission electron microscopy showed the carbide was a complex matter composed of nano TiC, VC and TiVC 2 . The cladding layer with a hardness of 1030 HV 0.2 possessed good wear and corrosion resistance, which was about 16.85 and 9.06 times than that of the substrate respectively.

Formation quality optimization of laser hot wire cladding for repairing martensite precipitation hardening stainless steel

Abstract: Laser cladding is an advantaged repairing technology due to its low heat input and high flexibility. With preheating wire by resistance heat, laser hot wire cladding shows better process stability and higher deposition efficiency compared to laser cold wire/powder cladding. Multi-pass layer were cladded on the surface of martensite precipitation hardening stainless steel FV520B by fiber laser with ER410NiMo wire. Wire feed rate and preheat current were optimized to obtain stable wire transfer, which guaranteed good formation quality of single pass cladding. Response surface methodology (RSM) was used to optimize processing parameters and predict formation quality of multi-pass cladding. Laser power P, scanning speed Vs, wire feed rate Vf and overlap ratio η were selected as the input variables, while flatness ratio, dilution and incomplete fusion value as the responses. Optimal clad layer with flat surface, low dilution and no incomplete fusion was obtained by appropriately reducing Vf, and increasing P, Vs and η. No defect like pore or crack was found. The tensile strength and impact toughness of the clad layer is respectively 96% and 86% of those of the substrate. The clad layer showed nonuniform microstructure and was divided into quenched areas with coarse lath martensite and tempered areas with tempered martensite due to different thermal cycles in adjacent areas. The tempered areas showed similar hardness to the substrate.

Application of laser ultrasonic technique for non-contact detection of structural surface-breaking cracks

Abstract: Based on the finite element method (FEM), the surface-breaking cracks have been investigated by using the laser-generated Rayleigh wave. The features of laser-generated Rayleigh wave interaction with cracks are analyzed in time and frequency domain. The simulation results show that the surface acoustic wave induced by the pulsed laser is sensitive to the surface-breaking cracks. As the crack depth increases, the transmission coefficients almost linearly decrease and the reflection coefficients show a dip. The corresponding experimental results have verified the feasibility of numerical calculation and reached a good agreement with simulation results. The research findings would provide a potential application for testing surface-breaking cracks of aircraft parts.

Experimental study of porosity reduction in high deposition-rate Laser Material Deposition

Abstract: For several years, the interest in Additive Manufacturing (AM) is continuously expanding, owing to the paradigm shift that new production processes, such as Laser Material Deposition (LMD), provide over conventional manufacturing technologies. With LMD, three-dimensional, complex components out of a wide range of materials can be manufactured consecutively layer-by-layer. Despite the technological advantages of the LMD process, currently achieved deposition-rates of approx. 0.5 kg/h for Inconel 718 (IN 718) remain a major concern in regards to processing times and economic feasibility. Moreover, processing conditions need to be chosen carefully or else material defects can be systematically formed either at the interface separating two adjacent clad layers, at the bonding zone or within the bulk of the layer. In this respect, the effects of powder humidity, laser power, nominal powder particle size, powder morphology and shielding gas flow rate on the porosity in laser deposited single tracks at an increased deposition-rate of approx. 2 kg/h was investigated through experiments. Based on experimental results, several approaches of reducing porosity in high deposition-rate LMD are proposed in this paper.

Effects of post-heat treatment on microstructure and properties of laser cladded composite coatings on titanium alloy substrate

Abstract: The composite coatings were produced on the Ti6Al4V alloy substrate by laser cladding. Subsequently, the coatings were heated at 500 °C for 1 h and 2 h and then cooled in air. Effects of post-heat treatment on microstructure, microhardness and fracture toughness of the coatings were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), optical microscopy (OM). Wear resistance of the coatings was evaluated under the dry sliding reciprocating friction condition at room temperature. The results indicated that the coatings mainly consist of a certain amount of coarse white equiaxed WC particles surrounded by the white-bright W 2 C, a great deal of fine dark spherical TiC particles and the matrix composed of the α (Ti), Ti 2 Ni and TiNi phases. Effects of the post-heat treatment on phase constituents and microstructure of the coatings were almost negligible due to the low temperature. However, the post-heat treatment could decrease the residual stress and increase fracture toughness of the coatings, and fracture toughness of the coatings was improved from 2.77 MPa m 1/2 to 3.80 MPa m 1/2 and 4.43 MPa m 1/2 with the heat treatment for 1 h and 2 h, respectively. The mutual role would contribute to the reduction in cracking susceptibility. Accompanied with the increase in fracture toughness, microhardness of the coatings was reduced slightly. The dominant wear mechanism for all the coatings was abrasive wear, characterized by micro-cutting or micro-plowing. The heat treatment could significantly decrease the average friction coefficient and reduce the fluctuation of the friction coefficient with the change in sliding time. The appropriate heat treatment time (approximately 1 h) had a minimal effect on wear mass loss and volume loss. Moreover, the improvement in fracture toughness will also be beneficial to wear resistance of the coatings under the long service.

Laser surface forming of AlCoCrCuFeNi particle reinforced AZ91D matrix composites

Abstract: Traditionally, the laser melt injection (LMI) technique can only be used for forming ceramic particles reinforced metal matrix composites (MMCs) for enhancing surface properties of lightweight engineering materials. In this research, the LMI method was employed to form metal particles reinforced MMCs on AZ91D instead. This was viable because of the unique properties of the AlCoCrCuFeNi high-entropy alloy (HEA) metal particles used. The large difference in melting point between the HEA and the substrate material (AZ91D), and the limited reaction and the lack of fusion between the HEA and Mg have made it possible that a metal particles reinforced AZ91D composite material was produced. The reason of limited reaction was considered mainly due to the relatively high mixing enthalpy between the HEA constituent elements and Mg. Although there was some melting occurred at the particles surface with some solute segregation found in the vicinity close to the surface, intermetallic compounds were not observed. With regard to the wear resistance of the MMCs, it was found that when the volume fraction of the reinforcement phase, i.e. the HEA particles, reached about 0.4, the wear volume loss of the coating was only one-seventh of that of the substrate material.

Understanding laser beam brightness: A review and new prospective in material processing

Abstract: This paper details the importance of brightness in relation to laser beams. The ‘brightness’ of lasers is a term that is generally not given much attention in laser applications or in any published literature. With this said, it is theoretically and practically an important parameter in laser-material processing. This study is first of a kind which emphasizes in-depth, the concept of brightness of lasers by firstly reviewing the existing literature and the progress with high brightness laser-material processes. Secondly, the techniques used to enhance the laser beam brightness are also reviewed. In addition, we review the brightness fundamentals and rationalize why brightness of lasers is an important concept. Moreover, an update on the analytical technique to determine brightness using the current empirical equations is also provided. A modified equation to determine the laser beam brightness is introduced thereafter. work The modified equation in turn is a new parameter called “Radiance Density”. Furthermore, research studies previously conducted to modify laser design to affect laser beam brightness are also discussed. The paper not only involves a review of the techniques used to improve laser beam brightness but also reviews how bright lasers can be employed to enhance/improve laser process capabilities leading to cost reduction of the laser assisted processes in areas such as manufacturing.

Ablation behaviors of carbon reinforced polymer composites by laser of different operation modes

Abstract: Laser ablation mechanism of Carbon Fiber Reinforced Polymer (CFRP) composite is of critical meaning for the laser machining process. The ablation behaviors are investigated on the CFRP laminates subject to continuous wave, long duration pulsed wave and short duration pulsed wave lasers. Distinctive ablation phenomena have been observed and the effects of laser operation modes are discussed. The typical temperature patterns resulted from laser irradiation are computed by finite element analysis and thereby the different ablation mechanisms are interpreted. (C) 2015 Elsevier Ltd. All rights reserved.

Invariant quaternion radial harmonic Fourier moments for color image retrieval

Abstract: Moments and moment invariants have become a powerful tool in image processing owing to their image description capability and invariance property. But, conventional methods are mainly introduced to deal with the binary or gray-scale images, and the only approaches for color image always have poor color image description capability. Based on radial harmonic Fourier moments (RHFMs) and quaternion, we introduced the quaternion radial harmonic Fourier moments (QRHFMs) for representing color images in this paper, which can be seen as the generalization of RHFMs for gray-level images. It is shown that the QRHFMs can be obtained from the RHFMs of each color channel. We derived and analyzed the rotation, scaling, and translation (RST) invariant property of QRHFMs. We also discussed the problem of color image retrieval using invariant QRHFMs. Experimental results are provided to illustrate the efficiency of the proposed color image representation.

Graphene saturable absorber for diode pumped Yb:Sc2SiO5 mode-locked laser

Abstract: High-quality monolayer graphene was successfully fabricated by chemical vapor deposition technology. By using the graphene as a saturable absorber, the Yb:Sc 2 SiO 5 crystal passively mode-locked laser was demonstrated for the first time. Stable mode-locked laser pulses were obtained with a repetition rate of 90.7 MHz and an average output power of 480 mW at the center wavelength of 1062.8 nm. The maximum single pulse energy and the maximum peak power were 5.3 nJ and 378 W, respectively.

Microstructure and mechanical properties of hot wire laser clad layers for repairing precipitation hardening martensitic stainless steel

Abstract: Precipitation hardening martensitic stainless steel (PH-MSS) is widely used as load-bearing parts because of its excellent overall properties. It is economical and flexible to repair the failure parts instead of changing new ones. However, it is difficult to keep properties of repaired part as good as those of the substrate. With preheating wire by resistance heat, hot wire laser cladding owns both merits of low heat input and high deposition efficiency, thus is regarded as an advantaged repairing technology for damaged parts of high value. Multi-pass layers were cladded on the surface of FV520B by hot wire laser cladding. The microstructure and mechanical properties were compared and analyzed for the substrate and the clad layer. For the as-cladded layer, microstructure was found non-uniform and divided into quenched and tempered regions. Tensile strength was almost equivalent to that of the substrate, while ductility and impact toughness deteriorated much. With using laser scanning layer by layer during laser cladding, microstructure of the clad layers was tempered to fine martensite uniformly. The ductility and toughness of the clad layer were improved to be equivalent to those of the substrate, while the tensile strength was a little lower than that of the substrate. By adding TiC nanoparticles as well as laser scanning, the precipitation strengthening effect was improved and the structure was refined in the clad layer. The strength, ductility and toughness were all improved further. Finally, high quality clad layers were obtained with equivalent or even superior mechanical properties to the substrate, offering a valuable technique to repair PH-MSS.

Generation of surface plasmon resonance and lossy mode resonance by thermal treatment of ITO thin-films

Abstract: Silicon wafers coated with Indium Tin Oxide (ITO) by application of sputtering technique have been characterized after different post-annealing techniques, showing that this last factor is critical for the quality of the thin-film and for the creation and tuning of both surface plasmon resonances and lossy mode resonances. By adequate selection of the ITO thin-film thickness both resonances can be tracked in the same spectrum, which can be used in sensor and optical communications fields.

Laser fusion–brazing of aluminum alloy to galvanized steel with pure Al filler powder

Abstract: The fusion–brazing connection of the dissimilar metal 5052 aluminum alloy/ST07Z steel was achieved by using the Nd:YAG laser with pure Al filler powder, and the effects of the laser power and powder feeding speed on the formation and mechanical properties of the resultant joints were investigated. The experimental results show that melting–brazing connection of 5052 aluminum alloy/galvanized steel can be successfully achieved, and the zinc plating layer has played the role of flux, assuring the brazing properties. The intermetallic compound layer was generated on the welded brazing interface. The joint exhibited a shear strength of 174 N/mm if the thickness of the intermetallic layer at the interface is about 6–7 μm.

Magnetic field and temperature sensor based on a no-core fiber combined with a fiber Bragg grating

Abstract: An optical fiber magnetic field sensor based on the no-core fiber (NCF) and magnetic fluid (MF) is proposed and demonstrated. By using the temperature sensing property of fiber Bragg grating (FBG), the magnetic field sensor with dynamic temperature compensation can be achieved. The transmittance of the NCF that is sealed in a capillary with MF is highly sensitive to the surrounding magnetic field. Experimental results show that the sensor has the magnetic field sensitivity of 9.2 pm/mT ranging from 0 to 9 mT and the temperature sensitivity of 11.9 pm/°C ranging from 15 to 45 °C. The prototype might be attractive due to its low fabrication cost and simple configuration, which are desirable features in magnetic field measurement.

Operation of continuous wave and Q-switching on diode-pumped Nd,Y:CaF2 disordered crystal

Abstract: The properties of continuous wave (CW) and Q-switching on the diode-pumped Nd,Y:CaF2 disordered crystal were demonstrated around 1 μm in this paper. Using the different cavity lengths, we measured the output properties of CW of Nd,Y:CaF2 disordered crystal. Then based on Cr4+:YAG as the saturable absorber, the Q-switched pulse characteristics of this novel disordered crystal were researched for the first time. The maximum average output power of 120 mW and the shortest pulse width of 132 ns were obtained at the pulse repetition rate of 6.2 kHz.

Diode-pumped Yb,Y:CaF2 laser mode-locked by monolayer graphene

Abstract: The large-area and high-quality monolayer graphene saturable absorber with a sandwich structure is prepared by the chemical vapor deposition technique. Using graphene saturable absorber, the mode locking operation of a diode-pumped Yb,Y:CaF2 laser is demonstrated. Without extra negative dispersion elements, 4.8 ps pulses are yielded at 1051 nm. The pulse repetition rate is 60 MHz.

Ab-initio study of L-Tartaric Acid (LTA) single crystal for NLO application

Abstract: Single crystals of LTA were grown by slow evaporation solution technique (SEST). Powder X-ray diffractometry (PXRD) confirmed the monoclinic crystal structure, whereas the high resolution X-ray diffractometry (HRXRD) ascertained the good crystalline perfection (FWHM=25 arcsec) of the as-grown single crystal. Optical study demonstrated the high optical transparency (85%). Photoluminescence (PL) spectroscopy illustrated the PL-emission at λ=388 nm and optical band gap ~3.2 eV. The SHG efficiency of LTA single crystal (6.61) is comparatively greater than the referenced (KDP) single crystal (6.08). Thermal analysis revealed that the crystal is thermally stable up to ~170 °C. The calculated value of the activation energy (ΔE) and corrected hardness (H0) for the crystal were found to be equal to 1.16 eV and 46.66 kg/mm2, respectively. The as-grown LTA single crystals can be used as a potential candidate for NLO material as well as in electronic and optoelectronic devices.

Formation of cylindrical micro-lens array on fused silica glass surface using CO2 laser assisted reshaping technique

Abstract: This paper demonstrates the laser assisted formation of plano-convex cylindrical and flat-top curved micro-lens array on fused silica glass surface. Initially, femtosecond laser pulses are irradiated on the sample glass to fabricate periodic linear micro-gratings on the glass surface. Afterwards, we reshape the micro-gratings by several times irradiation of a CO 2 laser beam by focusing the laser beam on top of the micro-gratings. As a consequence, plano-convex cylindrical micro-lens array with a period varying from 20 to 40 µm are formed on fused silica glass surface. However, flat-top curved gratings’ array is observed on the glass surface for a gratings’ period of 50 µm. The fabricated micro-lenses show great consistency in size and shape throughout the sample area. Furthermore, we analyze the formation mechanism of micro-lens array on glass surface using the CO 2 laser assisted reshaping technique. The proposed reshaping technique exhibits great potential for forming a large variety of micro-lens arrays on the surface of various transparent materials.

Experimental investigation of laser forming of a saddle shape with spiral irradiating scheme

Abstract: In this work, spiral irradiating scheme is proposed and investigated experimentally for laser forming of a saddle shape It is proved by experimental results that this new irradiating scheme is a suitable and powerful method for production of saddle shapes In addition, the effects of various process parameters such as pitch of spiral path, number of spiral paths, in-to-out spiral path and reversely out-to-in spiral path on the obtained saddle shape are investigated The results show with decreasing the pitch of spiral path, due to increase in the induced heat flux into the plate, deformations in other words curvatures of obtained saddle shape are considerably increased Also it is shown that curvatures associated with out-to-in spiral path are larger than curvatures of in-to-out spiral path due to change in geometrical constraints It is seen from results that with increasing the number of irradiation passes, curvatures of obtained saddle shape are increased considerably

Procedure for quality inspection of welds based on macro-photogrammetric three-dimensional reconstruction

Abstract: The results of visual inspection of welds depend on the visual ability of inspector. With the optical macro-photogrammetric low-cost procedure proposed in this paper only a digital single lens reflex camera with a macro-lens and a photogrammetric reconstruction software developed by the authors are needed for the generation of accurate and scaled 3D models of welds directly from images taken by a non-expert operator. This result eliminates the need of an in-situ assessment by the inspector, since it can be done directly using the 3D models generated, which the inspector can consult for the performance of all the measurements required by international standards.

Influence of the per pulse laser fluence on the optical properties of carbon nanoparticles synthesized by laser ablation of solids in liquids

Abstract: In this work we present experimental results on the optical characterization of carbon-nanoparticles (CNPs) synthesized by the laser ablation of solids in liquids technique (LASL). A pulsed Nd-YAG laser, a graphite disk and acetone were used in the laser ablation experiments. The per pulse laser fluence was varied, while all the other irradiation parameters (irradiation time, repetition rate, etc.) were kept constant. Both the graphite target and the obtained CNPs were characterized by Raman micro-spectroscopy. The colloidal solutions were characterized by UV–vis and photoluminescence (PL) spectroscopies. Additionally, the CNPs were also characterized by TEM and HRTEM. Our results show that spherical nanoparticles in the range of 4–20 nm in diameter were obtained. UV–vis and PL results for the obtained CNPs colloidal solutions showed that the optical absorption and PL intensity are dependent on the per pulse laser fluence. We also found that the PL spectral emission of the CNPs can be tuned from blue to yellow by varying the excitation wavelength.