Laser Shock Processing (LSP) has been demonstrated as an effective technology for improving surface and mechanical properties of metals. The main recognized advantages of the technique consist in its capability of inducing a relatively deep compression residual stresses field allowing an improved mechanical behaviour against fatigue crack initiation and growth, mechanical wear and stress corrosion. Although significant experimental work has been performed in order to explore the optimum conditions for the application of the treatments and to assess their capability to provide enhanced mechanical properties, only limited attempts have been developed in the way of physical understanding of the characteristic processes and transformations taking place under the LSP regime. In the present paper, the integrated numerical-experimental approach to LSP processes design developed by the authors is presented, the incorporation of increasingly more accurate models for the characterization of metallic materials behaviour under LSP conditions being an always present objective. Different practical results at laboratory scale on the application of the LSP technique to different materials with different irradiation parameters are presented along with physical interpretations of the induced mechanical effects. Concrete issues as laser-plasma interaction in the ns, GW/cm2 regime, material behaviour description for cyclically compressed matter, numerical simulation methods for the coupled plasma-thermomechanic analysis, practical implementation of the technique according to different approaches, etc. are discussed in the view of the last developments contributed by the authors and, finally, a tentative summary of the still open questions for the better knowledge and control of the process is presented.
The use of pulsed laser irradiation techniques has proven to be a clearly effective procedure for the achievement of surface properties modification via micro-/nano-structuration, different conceptual approaches having been the subject of research and extensively reported in the literature. Beyond the broad spectrum of applications developed for the generation of structured surfaces of metallic materials with specific contact, friction and wear functionalities, the application of laser sources to the surface structuration of metal surfaces for the modification of their wettability and corrosion resistance properties is considered. Multi-scaled hierarchical surface microstructures fabricated on characteristic alloys (the concrete case of Ti6Al4V alloy is considered as example) by the combination of two complementary laser micro/nano-structuring techniques (Direct Laser Writing and Direct Laser Interference Patterning) are reported. Static contact angle measurements show a clearly hydrophobicity enhancement for both types of processing options and a clear improvement on the corrosion resistance of patterned samples of either type is observed. A discussion of the reported features in view of the applicability of the technique to industrial-scope problems is provided.
Strengthening techniques allows enhance metal physical properties. Laser shock peening (LSP) technique consist in a surface treatment which a high power laser pulse induces a compressive residual stress field through mechanical shock waves, increasing hardness, corrosion resistance, fatigue resistance. In comparison with the shot peening technique, LSP is a method that allows precision controlling the laser incidence on the surface under treatment increasing the surface quality in the surface under treatment. In this work, mechanical shock waves are induced in aluminum and measure using two different experimental approaches. First, using a PVDZ sensors and secondly, strain gauges are used. Experimental results are presented.
Laser shock processing (LSP) is increasingly applied as an effective technology for the improvement of metallic materials mechanical properties in different types of components as a means of enhancement of their mechanical behavior. As reported in the literature, a main effect resulting from the application of the LSP technique consists on the generation of relatively deep compression residual stresses field into metallic alloy pieces allowing the life improvement of the treated specimens against wear, crack growth and stress corrosion cracking. Additional results accomplished by the authors in the line of practical development of the LSP technique at an experimental level (aiming its integral assessment from an interrelated theoretical and experimental point of view) are presented in this paper. Concretely, experimental results on the residual stress profiles and associated mechanical properties modification successfully reached in typical materials under different LSP irradiation conditions are presented along with a practical correlated analysis on the protective character of the residual stress profiles obtained under different irradiation strategies. In this case, the specific behavior of a widely used material in high reliability components (especially in nuclear and biomedical applications) as AISI 316L is analyzed, the effect of possible “in-service” thermal conditions on the relaxation of the LSP effects being specifically characterized.
Laser peening has recently emerged as a useful technique to overcome detrimental effects associated with other well-known surface modification processes such as shot peening or grit blasting used in the biomedical field. It is worthwhile to notice that besides the primary residual stress effect, thermally induced effects might also cause subtle surface and subsurface microstructural changes that might influence corrosion resistance and fatigue strength of structural components. In this work, plates of Ti-6Al-4V alloy of 7 mm in thickness were modified by laser peening without using a sacrificial outer layer. Irradiation by a Q-switched Nd-YAG laser (9.4-ns pulse length) working at the fundamental 1064-nm wavelength at 2.8 J/pulse and with water as a confining medium was used. Laser pulses with a 1.5-mm diameter at an equivalent overlapping density of 5000 cm−2 were applied. Attempts to analyze the global-induced effects after laser peening were addressed by using the contacting and noncontacting thermoelectric power techniques.
Laser peening has recently emerged as a useful technique to overcome detrimental effects associated to another wellknown surface modification processes such as shot peening or grit blasting used in the biomedical field. It is worth to notice that besides the primary residual stress effect, thermally induced effects might also cause subtle surface and subsurface microstructural changes that might influence corrosion resistance. Moreover, since maximum loads use to occur at the surface, they could also play a critical role in the fatigue strength. In this work, plates of Ti-6Al-4V alloy of 7 mm in thickness were modified by laser peening without using a sacrificial outer layer. Irradiation by a Q-switched Nd-YAG laser (9.4 ns pulse length) working in fundamental harmonic at 2.8 J/pulse and with water as confining medium was used. Laser pulses with a 1.5 mm diameter at an equivalent overlapping density (EOD) of 5000 cm-2 were applied. Attempts to analyze the global induced effects after laser peening were addressed by using the contacting and non-contacting thermoelectric power (TEP) techniques. It was demonstrated that the thermoelectric method is entirely insensitive to surface topography while it is uniquely sensitive to subtle variations in thermoelectric properties, which are associated with the different material effects induced by different surface modification treatments. These results indicate that the stress-dependence of the thermoelectric power in metals produces sufficient contrast to detect and quantitatively characterize regions under compressive residual stress based on their thermoelectric power contrast with respect to the surrounding intact material. However, further research is needed to better separate residual stress effects from secondary material effects, especially in the case of low-conductivity engineering materials like titanium alloys.
Laser shock processing (LSP) is being increasingly applied as an effective technology for the improvement of metallic materials mechanical and surface properties in different types of components as a means of enhancement of their corrosion and fatigue life behavior. As reported in previous contributions by the authors, a main effect resulting from the application of the LSP technique consists on the generation of relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical behaviour, explicitly the life improvement of the treated specimens against wear, crack growth and stress corrosion cracking. Additional results accomplished by the authors in the line of practical development of the LSP technique at an experimental level (aiming its integral assessment from an interrelated theoretical and experimental point of view) are presented in this paper. Concretely, follow-on experimental results on the residual stress profiles and associated surface properties modification successfully reached in typical materials (especially Al and Ti alloys characteristic of high reliability components in the aerospace, nuclear and biomedical sectors) under different LSP irradiation conditions are presented along with a practical correlated analysis on the protective character of the residual stress profiles obtained under different irradiation strategies. Additional remarks on the improved character of the LSP technique over the traditional “shot peening” technique in what concerns depth of induced compressive residual stresses fields are also made through the paper.
Profiting by the increasing availability of laser sources delivering intensities above 109 W/cm2 with pulse energies in the
range of several Joules and pulse widths in the range of nanoseconds, laser shock processing (LSP) is being
consolidating as an effective technology for the improvement of surface mechanical and corrosion resistance properties
of metals and is being developed as a practical process amenable to production engineering.
The main acknowledged advantage of the laser shock processing technique consists on its capability of inducing a
relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical
behaviour, explicitly, the life improvement of the treated specimens against wear, crack growth and stress corrosion
cracking.
Following a short description of the theoretical/computational and experimental methods developed by the authors for
the predictive assessment and experimental implementation of LSP treatments, experimental results on the residual stress
profiles and associated surface properties modification successfully reached in typical materials (specifically Al and Ti
alloys) under different LSP irradiation conditions are presented. In particular, the analysis of the residual stress profiles
obtained under different irradiation parameters and the evaluation of the corresponding induced surface properties as
roughness and wear resistance are presented.
The paper provides a thorough analysis of a surface layer prior to and after Laser Shock Processing under different
parameter conditions. Optimum conditions were determined using a factorial design aided with an analysis of variance
(ANOVA). As the first experimental factor, pulse density with 900 and 2500 pulse/cm2 was chosen. The second factor
was the type of material, i.e. aluminium alloys ENAW 6012 and ENAW 6082. Evaluation of different conditions of
specimen treatment was made using three response variables, i.e. surface roughness, magnitude of residual stresses, and
corrosion resistance given by pitting potential (Epitt). A thorough analysis of the modified surface layer was made with
measurement of microhardness, and with images obtained in optical and electronic microscopy (SEM) and energy
dispersion spectroscopy (EDS) to determine corrosion products around the pits formed after corrosion testing. The
experiments confirmed characteristic influences of individual LSP parameters on properties of the treated material
surfaces. ENAW 6012 aluminium alloy showed better corrosion resistance than ENAW 6082 alloy with lower pitting
attack at its surface. Corrosion tests showed that a higher pulse density produces shifting of the pitting potential to more
positive values, which ensures higher corrosion resistance from the viewpoint of preserving a passive protecting film on
the surface layer.
In this paper we present an original approach to estimate the heat affected zone in laser scribing processes for photovoltaic applications. We used high resolution IR-VIS Fourier transform spectrometry at micro-scale level for measuring the refractive index variations at different distances from the scribed line, and discussing then the results obtained for a-Si:H layers irradiated in different conditions that reproduce standard interconnection parameters. In order to properly assess the induced damage by the laser process, these results are compared with measurements of the crystalline state of the material using micro-Raman techniques. Additionally, the authors give details about how this technique could be used to feedback the laser process parametrization in monolithic interconnection of thin film photovoltaic devices based on a-Si:H.
Laser shock processing (LSP) or laser shock peening has been proposed as a competitive alternative technology to
classical treatments for improving fatigue and wear resistance of metals. This process induces a compressive residual
stress field which increases fatigue crack initiation life and reduce fatigue crack growth rate. We present a configuration
and results in the LSP concept for metal surface treatments in underwater laser irradiation at 1064 nm with and without a
thin surface paint layer. A convergent lens is used to deliver 1, 2 J/cm2 with a 8 ns laser FWHM pulse produced by a 10
Hz, Q-switched Nd:YAG laser with a spot diameter of a 1,5 mm moving forward along the workpiece. A LSP
configuration with experimental results using a pulse density of 5000 pulses/cm2 in 6061-T6 aluminum samples are presented.
Following its first developments in the 1970's, and profiting by the increasing availability of laser sources delivering intensities above 109 W/cm2 with pulse energies in the range of several Joules and pulse widths in the range of nanoseconds, laser shock processing (LSP) is being consolidating as an effective technology for the improvement of surface mechanical and corrosion resistance properties of metals and is being developed as a practical process amenable to production engineering. The main acknowledged advantage of the laser shock processing technique consists on its capability of inducing a relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical behaviour, explicitly, the life improvement of the treated specimens against wear, crack growth and stress corrosion cracking.
Although significant work from the experimental side has been contributed by several authors to explore the optimum conditions of application of the treatments and to assess their ultimate capability to provide enhanced mechanical behaviour to work-pieces of typical materials, only limited attempts have been developed, however, in the way of full comprehension and predictive assessment of the characteristic physical processes and material transformations with a specific consideration of real material properties.
In the present paper, a review on the physical issues dominating the development of LSP processes from a high intensity laser-matter interaction point of view is presented along with the theoretical and computational methods developed by the authors for their predictive assessment, and practical results at laboratory scale on the application of the technique to different materials are shown along with corresponding results on the mechanical properties improvement induced by LSP treatments.
Although significant work has been conducted in order to explore the optimum conditions of application of LSP
treatments and to assess their capability to provide enhanced mechanical properties, only limited attempts have been
developed in the way of predictive assessment of the characteristic physical processes and transformations with a specific
consideration of the associated laser interaction dynamics. For this reason, additionally to the authors' efforts in the line
of the numerical predictive assessment of the effects induced by the LSP technique from a predominantly mechanical
point of view, the observation and analysis of the plasma dynamics following laser interaction have been envisaged as a
means for the proper assessment of the shocking process relative to the material target and also as a way of plasma
dynamics control in view of process optimization. In the present paper, the basis for the plasma spectroscopic observation
of LSP experiments in combination to numerical dynamics simulations are proposed as a means for the extraction of
relevant guidelines for process design optimization.
Hydrogenated amorphous silicon has been widely studied last years, both from the basic research and industrial points of view, due to the important set of potential applications that this material offers, ranging from Thin Films Transistors (TFTs) to solar cells technologies. In different fabrication steps of a-Si:H based devices, laser sources have been used as appropriate tools for cutting, crystallising, contacting, patterning, etc., and more recent research lines are undertaking the problem of a-Si:H selective laser ablation for different applications.
The controlled ablation of photovoltaic materials with minimum debris and small heat affected zone with low processing costs, is one of the main difficulties for the successful implementation of laser micromachining as competitive technology in this field. This work presents a detailed study of a-Si:H laser ablation in the ns regime. Ablation curves are measured and fluence thresholds are determined. Additionally, and due to the improved performance in optolectronic properties associated to the nanocrystalline silicon (nc-Si:H), some samples of this material have been also studied.
Position detectors are useful for alignment and orientation sensing. Charge-coupled devices (CCDs) are used in small-area systems. Four-quadrant diodes are a low-cost, limited-accuracy alternative. In cases where either large area or reliability under harsh conditions are required, thin-film-silicon sensors may become the only reasonable choice. The paper proposes a simple structure for making such devices, describes the first experiments and discusses the key issues faced, with emphasis on laser scribing.
Laser shock processing (LSP) has been proposed as a competitive alternative technology to classical treatments for improving fatigue and wear resistance of metals. We present a configuration and results in the LSP concept for metal surfaces treatments in underwater laser irradiation at 1064 nm. A convergent lens is used to deliver 2.5 J/cm2 in a 8 ns laser FWHM pulse produced by a Q-switch Nd:YAG Laser. Experimental results using a pulse density of 5 000 pulses/cm2 and spots of 0.8 mm in diameter in 1040 steel samples are presented. Compressive residual stress distribution as a function of depth is assessed by the hole drilling method. High level compressive residual stresses are produced using 1064 nm wavelength. This method can be applied for surface treatment of final metal products.
Laser micromachining of semiconductor and Transparent Conductive Oxides (TCO) materials is very important for the practical applications in photovoltaic industry. In particular, a problem of controlled ablation of those materials with minimum of debris and small heat affected zone is one of the most vital for the successful implementation of laser micromachining.
In particular, selective ablation of thin films for the development of new photovoltaic panels and sensoring devices based on amorphous silicon (a-Si) is an emerging field, in which laser micromachining systems appear as appropriate tools for process development and device fabrication. In particular, a promising application is the development of purely photovoltaic position sensors. Standard p-i-n or Schottky configurations using Transparent Conductive Oxides (TCO), a-Si and metals are especially well suited for these applications, appearing selective laser ablation as an ideal process for controlled material patterning and isolation.
In this work a detailed study of laser ablation of a widely used TCO, Indium-tin-oxide (ITO), and a-Si thin films of different thicknesses is presented, with special emphasis on the morphological analysis of the generated grooves. The profiles of ablated grooves have been studied in order to determine the best
processing conditions, i.e. laser pulse energy and wavelength, and to asses this technology as potentially competitive to standard photolithographic processes.
The encouraging results obtained, with well defined ablation grooves having thicknesses in the order of 10 μm both in ITO and a-Si, open up the possibility of developing a high-performance double Schottky photovoltaic matrix position sensor.
Laser micromachining techniques are among the most promising fabrication processes in strategic industrial fields. Although lasers systems have been widely applied last twenty years in semiconductor industry for microfabrication process development, the current availability of new excimer and Diode Pumped Solid State Lasers (DPSS) sources are extending the applications fields of laser microprocessing. Nowadays MEMs, fluidic devices, advanced sensors and biomedical devices and instruments are among the more promising developments of this technology. Nevertheless the fast progress of this technology has brought as a consequence the building up of specific laser based machines for each process of interest (most of them until now strictly 2D), and an important gap has been generated, from the fabrication point of view, in fully 3D potential applications. In this work, the conception, design and first results of a fully automatized 3D laser micromachining workstation, based on the main concept of flexibility, is presented. This system integrates two UV laser sources, excimer and DPSS in ns pulse regime, and an advanced positioning system (with six degrees of freedom) for complex parts machining. Several examples of first results obtained with this system, including processing of semiconductors for sensoring and photovoltaic applications, organic materials for biomedical devices and metallic materials for different strategic industrial sectors are presented.
Laser shock processing (LSP) has been proposed as a competitive alternative technology to classical treatments for improving fatigue and wear resistance of metals. We present a configuration and results in the LSP concept for metal surfaces treatments in underwater laser irradiation at 535 nm. A convergent lens is used to deliver 0.9 J/cm2 in an 8 ns laser FWHM pulse produced by 10 Hz Q-switched Nd:YAG Laser with spots of a 1.5 mm in diameter moving forward along the workpiece. A LSP configuration with experimental results using a pulse density of 2 500 pulses/cm2 and 5 000 pulses/cm2 in 6061-T6 aluminum samples are presented by 532 nm. High level of compressive residual stresses are produced (600 MPa-750 MPa) with a depth of 1.2 mm. It has been shown that surface residual stress level is comparable to that achieved by conventional shot peening, but with greater depths. This method can be applied to surface treatment of final metal products.
Although valuable experimental work has been performed in order to explore the optimum conditions of application of the treatments and to assess their capability to provide enhanced mechanical properties, there is little work done on the theoretical prediction of these optimum parameters. In the present paper, a model is presented to provide an estimation of the residual stresses and surface deformation in order to see the influence of the different parameters in the process. The influence of pulse duration, pulse pressure peak, spot radius, number of shots, overlapped shots and material properties are studied. The great influence of 3D deformation effects in the process is clearly shown as one of the most important limiting factors of the process traditionally neglected in previous literature. Additionally, from the experimental point of view, in the present paper a summary is provided of different results obtained from the most recent LSP experiments carried out by the authors along with some conclusions for the assessment of LSP technology as a profitable method for the extension of fatigue life in critical heavy duty components.
In the present paper, a model is presented aiming to provide a physical basis for the theoretical estimation of both the plasma expansion dynamics following laser irradiation and the shock wave propagation into the treated material with a specific consideration of its constitutional properties (i.e. taking into account its real mechanical and E.O.S. parameters). Although initially limited to a 1D description, the proposed model aims to overcome the difficulties existing for an accurate theoretical estimation of the process due to the treated material behavior not directly amenable to analytical solutions and sometimes modeled through empirical approaches, and to provide a detailed treatment of the plasma behavior (i.e. ionization, breakdown, etc.). Additionally, and as a direct consequence of its analytical-numerical character, the model can provide a fully time dependent representation of the processes, including the laser pulse temporal profile, what is a real advance over previous theoretical studies.
The interaction in air of high intensity excimer lasers (KrF) with metals (Aluminum and aluminum alloys) and ceramics (Al2O3, ZrO2, AlN, SiC) has been investigated. Results concern the dynamics of the generated plasma and include the visualization of the luminous plasma front and the developed shock waves by means of an ICCD camera. At the same time, a shadowgraphy optical device has allowed to observe simultaneously the formation and expansion of plasma and shock wave fronts propagating into the surrounding gas during and after the irradiation pulse (20 ns). Complex structures inside the plasma plume have been observed inducing turbulence phenomena after irradiation that could be detrimental for high repetition rates and need further study. From a theoretical point of view, numerical simulations of the described irradiation experiments have been attempted trying to predict the observed plasma dynamics and, at the same time, to provide a macroscopic estimate of the mechanical transformations induced in the treated material. A 1D thermofluiddynamic code with detailed atomic and EOS parameters has been used for the simulation of the plasma dynamics and a full 3D finite element code provided with temperature dependent material data has been used for the macroscopic assessment.
The unique properties of a UV laser beam (high energy, short pulse duration) allow to transform the surface of ultrahard materials such as ceramics. In this way, a KrF excimer laser was used in this study in order to modify in selected zones, the surface of metals (aluminum alloy, titanium alloy and stainless steel) and oxides (Al2O3, ZrO2), carbide (SiC) and nitride (AlN). These ceramics possess good mechanical and thermal properties but exhibit a brittle behavior due to the granular structure. In a suitable range where the irradiated zone is melted and defects are removed, initial properties are modified (roughness, porosity, hardness, chemical composition). A cleaned and smoothed surface can be obtained without pores and cracks. These sites where corrosion attack starts are minimized and can lead to improve functions in potential industrial applications. The results presented in this work have been obtained by different analysis techniques such as scanning electron microscopy (SEM) to examine morphology, Auger spectroscopy (AES) to give chemical composition and depths profiles, mechanical tests to show roughness and hardness, grazing X-ray diffraction (XRD) to find structure.
On the basis of previous work, the development of a model-based monitoring and on-line automatic regulation system applicable to the real-time control of laser welding and surface heat treatment has been undertaken. In both kinds of applications, the main diagnosis system considered in the control loop is a thermal camera either used to provide a 2D temperature map or, more suitably according to typical response time parameters, used as a line scanner sweeping the appropriate diagnosis zone. From the automatic regulation point of view, the developed control system is defined to correct typical perturbations in incoming laser power, material thickness or piece motion speed within the appropriate time scale in order to assure a practical validity of the in-course weld seam or heat treatment track, respectively. The resulting control module, grounded on reasonably reliable calculations of the processes parameters performed with own-generated models provides a valuable tool for the practical implementation of the referred applications with special regard to quality assurance purposes.
On the basis of previous experience in the field of theoretical and numerical characterization of the laser-plasma interaction at very high intensities, a computational scheme has been set up for the analysis of these processes and applied to the characterization of some of the most representative among them, with special emphasis in providing results directly suitable for experimental model validation and for use both in a predictive way for the a priori process design and with process control purposes.
A set of numerical models for the characterization of laser processing applications is developed. The main physical and calculational features of these models are given along with some results on their comparison to experimental data and other well established theoretical models. Special emphasis is made on the suitability of the set of models for applications design and practical implementation.
In this paper, a short conceptual review (avoiding details en physico-mathematical formulation for the sake of brevity) of the fundamental energy transport mechanisms in the laser matter interaction at high intensities will be presented along with the associate calculational tools normally used for their simulation, and, as a sample of this kind of calculational SchemeS, a description will be provided on a particular calculaticnal rrdel able to simulate the plasma behaviour under a coupled schen relating the radiative and fluiddynamic rrdium evolution.
Traditionally, the application of laser beams to the processing of material workpieces has been restricted to relatively low intensity regines, mainly because of the linited power of the available laser sources and the potentially bad laser-matter coupling properties introduced by the weak plasna generated at sonewhat higher intensity regimes (I>10 W/cm2).
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.