Optimized Laser Processing Research Articles

Multiperformance optimization of tensile strength and heat input of laser transformation hardening of commercially pure titanium using Taguchi methodology and utility concept

Purpose of this paper: The main objective of this research paper is to investigate and evaluate the optimal values of laser process parameters: laser power, scanning speed, and focused position for the simultaneous minimization and maximization of heat input and tensile strength respectively by Taguchi method and utility concept approach in laser transformation hardening of commercially pure titanium sheet of 1.6mm thickness using continuous wave (CW) Nd:YAG laser beam. Design/methodology/approach: The effect of laser process parameters on the heat input and tensile strength properties of commercially pure titanium and subsequent optimal settings of the parameters have been obtained using Taguchi’s parametric design approach and utility concept. The response performance is analyzed based on signal-to-noise ratio and analysis of variance (ANOVA). The optimal levels of the laser process parameters were determined through the Analysis of Means (ANOM). Findings: The optimization results revealed that a combination of higher levels of scanning speed and focused position i.e., increase in defocus along with laser power in the lower level play important role in order to simultaneously minimize the heat input and to maximize the tensile strength (σ). The predicted optimal values of tensile strength and heat input of commercially pure titanium sheet produced by laser transformation hardening process are 464 N/mm 2 and 141.667 Watts, respectively, for the optimized laser process parameters with minimum LP=750 Watts, maximum SS=3000mm/min, and lower value of FP=-30mm (increase in defocus). Research limitations/implications: Research range is limited to investigation of optimization laser hardening process parameters for the simultaneous minimization and maximization of heat input and tensile strength respones.of commercially pure titanium. Hardness test responses are also be optimized in future research work. Practical implications: Laser transformation hardening is an innovative and advanced laser surface modification technique has been employed in aerospace, marine, chemical applications, heat exchangers, cryogenic vessels, components for chemical processing and desalination equipment, condenser tubing, airframe skin, and nonstructural components which introduces the advantageous residual stresses into the surface, improving the mechanical properties like wear, resistance to corrosion, tensile strength, and fatigue strength. Originality/value: Authors made an effort to optimize the laser process parameters in order to minmize the heat input and maximize tensile strength of laser transformation hardening of commercially pure titanium using Taguchi methodology and utility concept.

Optimized laser processing of calibration blocks for grinding burn detection with Barkhausen noise

Calibration samples for the Barkhausen noise (BN) method were produced with laser processing. A planet gear wheel used for production quality control was subjected to laser irradiation to verify the BN sensor output. Different samples were found to respond similarly to the laser processing although the laser parameters needed to be adjusted for different surface qualities separately. The surface optimization for laser processing was studied with different surface qualities of samples. The ground surface was compared with a sandblasted and vibratory ground surface. The ground and sandblasted surfaces were both amenable to the laser processing whereas the vibratory grinding process created inhomogeneous surfaces for laser beam absorption. Laser processing was found to produce uniform changes in the residual stress values in two perpendicular measuring directions. The root mean square value of the BN voltage signal exhibited linear correlations with the values of the residual stress and surface hardness.

Industrial PERL-type solar cells exceeding 19% with screen-printed contacts and homogeneous emitter

Aiming at the development of a cost-effective and industrially up-scalable process for the production of p-Si PERL-type solar cells, this work focuses on the selection and the development of an optimal rear-contacting method of screen-printed aluminum back electrodes through AlOx-based rear passivation stacks. Laser-Fired Contacts are first optimized by developing dedicated characterization protocols to define an optimal laser process. Laser parameters are systematically varied (e.g. pulse energy, power density, focus, and contact pitch) and their effect on the contacts quality is analyzed. Besides, a new contacting method, called Extended-Laser-Ablation (ELA), is presented, allowing even higher efficiencies to be attained thanks to better quality of local back-surface fields. Both contacting routes are then implemented and compared in the PERL pilot production process flow to investigate their effects on cells performances. Top efficiencies of 19.13% (AlOx/SiNx) and 19.5% (SiO2/AlOx/SiNx) are reached when the ELA process is used on monocrystalline Cz Si, using a close-to-standard process (75Ω/sq homogeneous emitter, screen-printed contacts, standard cleaning procedure), without any final forming gas annealing step required.

Electrically conductive ceramics and new joining technology for applications in HTR engineering

Ceramic constructional components are quite extensively required for operation of high-temperature nuclear reactors. Functional ceramics, in addition to constructional ceramics, are increasingly coming into the focus of research. Ceramic materials are predestined for use at high temperatures and in corrosive atmospheres. Modification of silicon carbide (SiC) by targeted doping, for instance, produces a suitable material for the production of heating conductors and thermoelectric generators. As a construction material, silicon carbide (SiC) is especially interesting due to its very good thermal, mechanical and radiological properties. SiC, furthermore, performs well when activated by neutron irradiation, with the induced activation subsiding after only a few hours (Hurtado, 1996). This property vector makes it an ideal starting material for use in a wide range of functional elements in high-temperature power engineering, particularly in high-temperature nuclear reactor engineering (e.g. V/HTR) including thermochemical plants for hydrogen generation or Synfuel production. In principle, it is possible to produce all-ceramic assemblies consisting of a thermoelectric generator and a sensor that can provide reliable measurement signals under extreme conditions in the high-temperature range without external power supply. This paper explains the feasibility of laser-joining such modified non-oxide ceramics, how to make electrically conductive joints, and thus, how to design complex assemblies. The parameters required for an optimal laser process to join ceramic materials were determined in extensive preliminary experiments. These investigations focused on the specific electrical resistances and optical properties. Specifically developed brazing fillers were fine-tuned so that the joints of the ceramics improved in terms of their physical interactions, chemical reactions and ability to bond or key chemically and mechanically with the ceramic surfaces. Thereby, the electrical conductivity required could be maintained up to temperatures above 950°C. Laser processing was carried out in a high-performance laser laboratory with diode lasers utilized in power ranges from 3kW to 10kW (continuous wave – cw). The measured four-point bending strength of the joined samples was between 120 and 170MPa (approx. 60% that of the starting material). No significant electrical conductivity drop was found in the joint area during resistance measurements. The results underline that the high-temperature laser joining technology used here is well-suited to provide electrically conductive ceramics with high-temperature resistant properties. This opens up new possibilities for the cost-effective and efficient manufacture of an entire range of high-temperature sensors as required in power engineering and, above all, in the field of high-temperature reactor engineering.

Evolution of laser-fired aluminum-silicon contact geometry in photovoltaic devices

The evolution of temperature and velocity fields during laser processing of solar cells to produce an ohmic contact between an aluminum thin film and a silicon wafer is studied using a transient numerical heat transfer and liquid metal flow model. Since small changes in pulse duration, power, and power density can result in significant damage to the substrate and, in extreme cases, expulsion of droplets from the molten zone, the selection of optimal laser processing parameters is critical. The model considers the unusually large heat of fusion of the Al-Si alloy formed during processing and the large composition-dependent two phase region. The calculated size and shape of the fusion zone were in good agreement with the corresponding experimental data, indicating the validity of the model and providing a basis for using the model to develop a better understanding of the laser-assisted fabrication of contacts for solar cell devices. The transient changes in the composition of the Al-Si molten region are found to have a major impact on the heat transfer during the formation of the contact. Consideration of the time-dependent concentration of Al in the molten region is also essential to achieve good agreement between the experimental and computed molten pool sizes. Process maps showing peak temperatures and the depth and width of the molten pool are presented in order to assist users in the selection of safe process parameters for the rapid fabrication of these silicon-based photovoltaic devices.

Automated overlays for laser peening

PurposeSacrificial opaque overlays used in laser peening provide optimal processing and protect the surface of the part being processed from thermal damage from the laser pulses. Traditional solid film overlays for laser peening often require several applications and the running of multiple partial laser peening sequences in order to completely process the desired surface. This paper aims to discuss an automated overlay system that eliminates these issues.Design/methodology/approachLSP Technologies' (LSPT') patented RapidCoater™ automated overlay system provides optimal laser processing and surface protection by providing a conformal opaque layer that is automatically refreshed between each laser pulses. PLC control provides precise timing of the application of the process overlays in synchronization with the laser pulse.FindingsUse of the RapidCoater system has been shown to reduce processing time by up to five times when compared to using tape overlays. Cost reductions of about 40 percent are also achieved.Originality/valueLSPT, Inc. invented and developed this proprietary technology to provide its laser peening customers with higher productivity and improved process affordability.

Application of the central composite design in optimization of laser transformation hardening parameters of commercially pure titanium using Nd:YAG laser

In this study, the application of response surface methodology (RSM) and central composite design for modeling, optimization, and an analysis of the influences of dominant laser-processing parameters namely: laser power (LP), scanning speed (SS), and focused position (FP) on heat input (HI) and hardened bead geometries such as hardened bead width (HBW), hardened depth (HD), angle of entry of hardened bead profile (AEHB) of laser transformation hardened surface quality of commercially pure titanium sheet of 1.6 mm in thickness using continuous wave (CW) 2-kW Nd:YAG laser is discussed. Experimental bead on trials of laser transformation hardening process were conducted on commercially pure titanium sheet. The design of experiments concept has been used to optimize the experimental conditions. The experimental data were collected based on a three-factor-five-level full central composite design. The multiple regression analysis using RSM was conducted to establish input–output relationships of the process. The mathematical models were developed and tested for adequacy using analysis of variance and other adequacy measures. The results show that models explain the heat input and hardened bead surface geometry successfully and satisfactorily. Using the developed models, the main and interaction effect of the laser process input variables on the heat input and hardened bead profile responses were investigated. It is found that all the investigated laser parameters are affecting the performance of the HI and hardened bead profile parameters significantly. The predicted results are compared with the experimental results and are good agreement with heat input and hardened bead profile parameters. The optimized laser process parameters obtained by numerical optimization using RSM, those ensuring the minimum HI = 181.33J/cm, maximum HBW = 2.292 mm, minimum HD = 0.162 mm, and minimum AEHB = 17.94° are LP = 760.08 W, SS = 2,460.3 mm/min and FP = −30 mm.

MEMS 소자의 비아 홀에 대한 레이저 공정변수의 최적화

MEMS 소자의 공정에서 가공된 비아 홀 품질은 소자의 성능에 가장 중요한 요소의 하나이다. Nd:YVO4 레이저로 가공한 비아 홀에 대한 레이저 미세가공의 일반적인 특징을 설명하고 그것의 측정에 대한 효율적인 최적화 방법을 소개한다. 본 논문의 최적화 방법은 직교다항식, 분산분석과 반응표면최적화는 최적 레이저 공정변수를 결정하고 주요 영향을 이해하는데 사용된다. 유의한 레이저 공정변수를 확인하고 이의 비아 홀 품질에 관한 영향을 고찰하였다. 레이저 공정변수의 최적 수준을 가지는 확인 실험은 최적화 방법의 유효성을 설명하기 위해 수행하였다.

Parameter optimization of laser transformation hardening by using Taguchi method and utility concept

This paper presents the application of Taguchi method and the utility concept for optimizing the laser process parameters in laser transformation hardening of commercially pure titanium using a continuous-wave 2-kW, Nd:YAG laser. In this study, a set of optimal laser process parameters were evaluated through the Taguchi method of orthogonal arrays and utility concept. Taguchi method and utility concept, a powerful tool to design optimization for quality, is used to find the set of optimal laser hardening parameters such as laser power (LP), scanning speed (SS), and focused position (FP) on two multiple performance characteristics of hardened bead geometries, namely, hardened bead width (HBW) and hardened depth (HD), developed for laser transformation hardening of commercially pure titanium. The utility concept has been employed for the multi-performance characteristics optimization using Taguchi design. The experiments were planned as per Taguchi’s L9 orthogonal array. A Taguchi L9 orthogonal array methodology was used to optimize the conditions for laser-hardened bead width and hardened depth. Taguchi tools such as analysis of variance (ANOVA), signal-to-noise ratio, and additive model have been used to analyze, obtain the significant parameters, and evaluate the optimum combination levels of laser transformation hardening process parameters. The optimal levels of the laser process parameters were determined through the analysis of means. The relative importance among the process parameters were identified through ANOVA. The ANOVA results indicated that the most significant process parameters are SS followed by LP and FP, which affect the optimization of multiple performance characteristics. The confirmation tests with optimal levels of laser process parameters were carried out to illustrate the effectiveness of Taguchi optimization method. The optimization results revealed that a combination of higher levels of SS and FP, i.e., increase in defocused beam with negative focal length along with LP in the lower level, is an essential laser hardening parameter to simultaneously minimize the HD and maximize the HBW.

Modelling and analysis on machining characteristics during pulsed Nd:YAG laser microgrooving of aluminium titanate (Al<SUB align=right>2TiO<SUB align=right>5)

Pulsed Nd:YAG laser beam is widely applied in micro-machining with different micro-scale geometries on engineering materials such as ceramics, composites, and diamond. The present research papers deals with the Response Surface Methodology (RSM) based mathematical modelling and analysis on machining characteristics of pulsed Nd:YAG laser during micro-grooving operation on flat surface of aluminium titanate. An attempt has been made to optimise laser parametric setting for achieving accurate taper angles of micro-grooves. Based on the modelling and parametric optimisation analysis, the optimal laser process parametric setting has been obtained as air pressure of 0.9 kg/cm², lamp current of 20.4 amp, pulse frequency of 2.8 kHz, pulse width of 2.7% of duty cycle and cutting speed 10 mm/s for achieving the optimum taper angle of 11.1°.

Erosion wear behavior of laser clad surfaces of low carbon austenitic steel

Austenitic steel surfaces are laser cladded using a 4 kW continuous wave CO 2 laser with coaxial powder feeding nozzle to investigate the improvement in slurry erosion characteristics. Colmonoy-6 and Inconel-625 are cladded on AISI 316L steel and AISI 304L steel, respectively by laser cladding. Initially, single-pass clad track is overlaid to optimize the laser processing parameters, namely scanning speed and powder feed rate to obtained a sound clad. Minimum cracks, porosity and distortion were found at scanning speed of 0.1 m/min and powder feed rate of 12 g/min. For these parameters, the dilution was 17.33% for Colmonoy-6 and 40% for Inconel-625. To clad large surface area, the optimized laser processing parameters were used to deposit the clad tracks with 60% overlap. Maximum surface hardness of 746 VHN is obtained in case of Colmonoy-6 clad on AISI 316L steel and is 352 VHN in case of Inconel-625 clad on AISI 304L steel. EDAX analysis shows higher degree of mixing of substrate material in the clad pool of Inconel-625 than Colmonoy-6. The results of slurry erosion test of Colmonoy-6 clad surface have shown improvement in erosion resistance of the order of 1.75–4.5 times of the substrate AISI 316L steel at all impact angles and the maximum wear angle has also increased which can be attributed to the increase in the surface hardness. However, Inconel-625 laser clad surface has shown little improvement in erosion resistance of the substrate AISI 304L steel at shallow impact angles with no significant improvement at normal impact condition. The SEM micrographs of worn out Colmonoy-6 clad surfaces at shallow impact angles show that the material is removed mainly by micro-cutting which increases with increase in the impact angle.

Comparison of Laser Chemical Processing and LaserMicroJet for structuring and cutting silicon substrates

This paper deals with the development of a new cutting method for thin silicon solar wafers with liquid-jet-guided lasers (LaserMicroJet®, LMJ, and Laser Chemical Processing, LCP). Several laser systems with different wavelengths were tested to find the optimum laser system and processing parameters in terms of efficient material removal and deep laser cutting. Water and potassium hydroxide were used as carrier liquids to enhance laser ablation. The ablation efficiency was defined as a target parameter and experimentally determined by performing single laser grooves. It is demonstrated that the ablation process of LMJ is mainly affected by silicon melting and then removing by the liquid-jet momentum for single laser grooves. Best result for deep laser grooves is achieved if evaporation dominates the ablation process. Better surface quality referred to laser-induced crystalline damage is presented for a cut wafer with LMJ in comparison to a standard multiwire slurry saw. This shows a great potential of wafering with liquid-jet-guided lasers although no optimal liquid media was used.

Laser surface alloying fabricated porous coating on NiTi shape memory alloy

Laser surface alloying technique was applied to fabricate a metallic porous coating on a solid NiTi shape memory alloy. By laser surface alloying a 40% TiH 2-60% NiTi powder mixture on the surface of NiTi alloy using optimized laser process parameters. a porous but crack-free NiTi layer can be fabricated on the NiTi substrate. The porous coating is metallurgically bonded to the substrate NiTi alloy. The pores are uniformly distributed and are interconnected with each other in the coating. An average pore size of less than 10 μm is achieved. The Ni content of the porous layer is much less than that of the original NiTi surface. The existence of the porous coating on the NiTi alloy causes a 37% reduction of the tensile strength and 55% reduction of the strain as compared with the NiTi alloy. Possible biomedical or other applications for this porous surface with good mechanical strength provided by the substrate are prospective.

Tailoring light pulse amplitudes for optimal laser processing and material modification

A laser processing technique has been developed for laser direct-write patterning tools that enable site-selective material modification processes on the scale of the laser spot size. The technique controls the delivery of laser energy for optimal laser processing by the dynamic regulation of individual laser pulses to form a prescribed sequence of amplitude-modulated laser pulses. This sequence of amplitude-modulated pulses, called a pulse-script, is then administered to each laser spot size area along a tool path regardless of the stage motion velocity. In our current configuration, up to 32 different pulse-scripts can be independently selected and applied during direct-write patterning at motion velocities exceeding 400 mm/s. The novel processing technique seamlessly merges information that depicts the tool path pattern in Cartesian space with the type of material modification and transformation processes that are desired at each laser spot location. This paper describes the fundamental and fun ctional aspects of this new technique. As a demonstration of the technique, a series of variable wavelength attenuators and embedded cutoff filters have been fabricated in a photo-structurable glass ceramic material.

The Symbiosis of Light and Matter: Laser-Engineered Materials for Photo-Functionality

AbstractCurrent technology trends imply that further miniaturization and integration of systems are anticipated, and one approach to realizing the next generation of systems integration is the development of materials that are protean, or capable of taking on various properties; in essence, the materials properties can be selectively altered via laser excitation. The objective is that a common substrate material can be used for multiple functions. Laser direct-write material processing, because of its maskless patterning flexibility, is inherently suitable for producing localized changes in materials properties and thereby fabricating linked, integrated devices on a common substrate. The enabling of this new form of materials processing requires a light-activated protean material along with a scheme for the dynamic regulation of the laser energy for optimum laser processing on the scale of the laser spot size and for patterning speeds on the order of meters per second. The “protean material” concept and a high-fidelity laser amplitude-modulation scheme are the focus of this article.

Zeolite LTA Nanoparticles Prepared by Laser-Induced Fracture of Zeolite Microcrystals

Zeolite LTA nanoparticles are prepared by laser-induced fragmentation of zeolite LTA microparticles using a pulsed laser. Zeolite nanoparticle formation is attributed to absorption of the laser at impurities or defects within the zeolite microcrystal generating thermoelastic stress that mechanically fractures the microparticle into smaller nanoparticle fragments. Experimentally, it is found that nanoparticles have a wide size and morphology distribution. Large nanoparticles (>200 nm) are typically irregularly shaped crystals of zeolite LTA, whereas small nanoparticles (<50 nm) tend to be spherical, dense, and amorphous, indicative of destruction of the original LTA crystal structure. Results of the fragmentation versus laser parameters show that shorter laser wavelengths are more efficient at producing zeolite nanoparticles, which is explained based on a larger cross section for optical absorption in the zeolite crystal. Increasing the laser energy density irradiating the sample was found to be a trade-off between increasing the amount of fragmentation and increasing the amount of structural damage to the zeolite crystal. It is suggested that in the presence of strongly absorbing defects, plasma formation is induced resulting in dramatically higher temperatures. On the basis of these results it is suggested the optimal laser processing conditions are 355 nm and 10 mJ/pulse laser energy for our LTA samples.

157?nmF_2-laser writing of silica optical waveguides in silicone rubber

Silica (SiO2) optical waveguides have been fabricated on the surface of silicone [(SiO(CH3)2)n] rubber by photochemical modification of silicone rubber into silica with 157 nm F2-laser radiation. The 2 mm thick silicone was exposed through a thin (approximately 0.2 mm) air layer to generate oxygen radicals that chemically assisted in the silica transformation. Silica waveguides were defined in 8-16 microm wide exposure strips by a proximity Cr-on-CaF2 photomask. Optimum laser processing conditions are presented for generating crack-free waveguides with good optical transparency at red (635 nm) and infrared (1550 nm) wavelengths. A propagation loss of approximately 6 dB/cm is reported at the 1550 nm wavelength.

Direct laser fabrication of nickel alloy samples

Direct laser fabrication (DLF) is an advanced manufacturing technology which can build full density metal components directly from CAD files without using any modules or tools. An open-loop controlled hardware system and associated control software for the DLF process of nickel alloy samples was constructed in our work. The hardware system is consisted of a CO 2 laser, a 4-axis CNC table, a coaxial powder nozzle and a powder recycler. In order to achieve the maximum flexibility and extensibility for the fabrication of metal parts, path plug-ins was introduced into the control software. The effect of the specific energy on the cross-section shape of nickel alloy cladding was studied by a single-track cladding experiment with different laser processing parameters. The comprehensive effect of the optimized laser processing parameters was also studied by an orthogonal experiment. The experimental results showed that the specific energy for laser processing is the most important factor, which controls the component qualities. There is an appropriate range for the specific energy in which the nickel alloy samples can be fabricated layer by layer with a uniform height. If the specific energy is too low, the inner height of a sample is lower than its contour height. Banding structure can be observed in the microstructure of nickel alloy samples. The gain size in the light zones of the bandings is much smaller than those in the dark zones.

A mathematical model of the removal of gold thin film on polymer surface by laser ablation

The removal effects of an Au thin film on polymer sheets by an ultrashort-pulsed Nd:YAG laser ablation was studied. A theoretical model to describe the laser energy required F 0 and optimal scanning velocity ν were presented and afterwards affirmed by experimental data. It is shown that the function of ν to ln F 0 follows a linear relationship; an increase in laser flux F 0 led to an initial rise in ablation rates R a followed by a decline. Namely there exists a maximum ablation rate or a best removal effect and it corresponds to an ascertainable laser energy and beam scanning velocity. Using the optimized laser processing parameters the Au thin film could be completely removed and satisfactory cleaning was achieved. The analytical method and the results were taken as a guide for accurately determining the laser parameters and proper conditions for the practical utilization of micro processing of the thin film by laser-ablation.

Flame-assisted laser surface treatment of refractory materials for crack-free densification

A study to improve the integrity and reliability of alumina-based refractory bricks, installed as linings in furnaces and incinerators, is being conducted. Owing to the inherent porosity of such heat-insulating materials, problems associated with molten slag penetration and chemical degradation are often encountered. It is known that denser refractories degrade less rapidly since such penetration is hindered. One aim for the current research is to seal the surfaces of these refractory materials by laser surface processing. In this way the bulk properties of the brick are retained, while the surface properties are improved. This paper reports on the results of an investigation to determine the optimum laser processing parameters that produce a zone of densified material containing few defects at the refractory surface. The high thermal stresses usually associated with laser treatment are often relieved by fracture of the laser-treated zone. However, by using a novel flame-assisted laser surface treatment, a reduction in thermal stress has been achieved, resulting in a crack-free, dense laser-treated zone. This treated zone should reduce or even eliminate the ingress of molten slags and gases, thus providing a reduced area for attack by corrosive species.