J.A. Porro

Predictive assessment and experimental characterization of the influence of irradiation parameters on surface deformation and residual stresses in laser-shock-processed metallic alloys

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.

Laser Shock Processing as a Method for the Improvement of Metallic Materials Surface Properties: A Discussion on the Influence of Combined Mechanical and Thermal Effects

Laser shock processing (LSP) has been presented as an effective technology for improving surface mechanical and corrosion properties of metals, and is being developed as a practical process amenable to production engineering. The main acknowledged advantages of the laser shock processing technique consist 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. In the present paper, practical results at laboratory scale on the application of Laser Shock Processing are presented showing the obtained tensile residual stresses relaxation along with corresponding preliminary results about the resulting mechanical properties improvement induced by the treatment. Additionally, the influence of different irradiation parameters will be presented along with a physical interpretation of the mechanical effects induced in the materials by the characteristic fast laser-plasma interaction regime occurring in the process and model based assessments on the real possibilities of the technique as a substitutive of traditional techniques as, for example, shot peening. From a specific point of view, a critical analysis of the relative influences of coupled thermal and mechanical stress and deformation effects during LSP is presented.

Thermomechanical modelling of stress fields in metallic targets subject to laser shock processing

Purpose – Laser shock peening (LSP) is mainly a mechanical process, but in some cases, it is performed without a protective coating and thermal effects are present near the surface. The numerical study of thermo‐mechanical effects and process parameter influence in realistic conditions can be used to better understand the process.Design/methodology/approach – A physically comprehensive numerical model (SHOCKLAS) has been developed to systematically study LSP processes with or without coatings starting from laser‐plasma interaction and coupled thermo‐mechanical target behavior. Several typical results of the developed SHOCKLAS numerical system are presented. In particular, the application of the model to the realistic simulation (full 3D dependence, non‐linear material behavior, thermal and mechanical effects, treatment over extended surfaces) of LSP treatments in the experimental conditions of the irradiation facility used by the authors is presented.Findings – Target clamping has some influence on the re...

Laser Shock Processing: an emerging technique for the enhancement of surface properties and fatigue life of high-strength metal alloys

Profiting by the increasing availability of laser sources delivering intensities above 10 9 W/cm 2 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 steels and Al and Ti alloys) under different LSP irradiation conditions are presented

Induction of Engineered Residual Stresses Fields and Associate Surface Properties Modification by Short Pulse Laser Shock Processing

Laser shock processing (LSP) is consolidating as an effective technology for the improvement of metallic materials surface properties involving their fatigue life. The main acknowledged advantage of the LSP 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. Progress 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, is presented in this paper. Concretely, experimental results on the residual stress profiles and associated surface properties modification successfully reached in typical materials (especially Al and Ti alloys) under different LSP irradiation conditions are presented, a correlated analysis of the residual stress profiles obtained under different irradiation strategies and the evaluation of the corresponding induced surface properties as roughness and wear resistance being also presented. Through a coupled theoretical- experimental analysis the real possibilities of the LSP technique as a possible substitutive of related traditional surface modification techniques as, for example, shot peening.

Numerical Thermo-Mechanical Modelling of Stress Fields and Residual Constraints in Metallic Targets Subject to Laser Shock Processing

In the analysis of the thermomechanical behaviour of the target material subject to Laser Shock Processing (LSP), most of the simplified models used for the analysis of its residual shocked state rely on rather simple estimations or material response equations that rarely take into account a detailed description of the material subject to a simultaneous dynamic compression and either deformation-induced or plasma-driven thermal heating. The calculational system developed by the authors (SHOCKLAS) includes a coupled analysis of the pressure wave applied to the target material as a result of the plasma buildup following laser interaction and the shock wave propagation into the solid material with specific consideration of the material response to thermal and mechanical alterations induced by the propagating wave itself (i.e. effects as elastic-plastic deformation, changes in elastic constants, etc.). The model is applicable to the typical behaviour shown by the different materials through their dynamic strain-stress relations. In the present paper, the key features and several typical results of the developed SHOCKLAS calculational system are presented. In particular, the application of the model to the realistic simulation (full 3D dependence, non linear material behaviour, thermal and mechanical effects, treatment over extended surfaces) of LSP treatments in the experimental conditions of the irradiation facility used by the authors is presented

Laser shock processing as a method of decreasing fatigue of a die-casting die made of maraging steel

Laser Shock Processing (LSP) is a process of laser treatment with a pulsed beam of high-power density. The process generates locally limited mechanical waves that increase the through-depth density of dislocations. This entails a change in mechanical properties, particularly at the workpiece surface. The treatment with laser-induced shock waves is suitable for structural parts and machine elements subjected to high thermo-dynamical loads. LSP can substantially improve the wear resistance, which is of exceptional significance to die-casting tools made of maraging steel. The paper describes the effects of LSP on chosen 12% Ni maraging steel, which is suitable also for the manufacture of tools for die casting of aluminium alloys. After laser treatment, measurements of residual stresses and microhardness and other properties, including surface defects at the micro level, were carried out. The results of the study confirmed that exceptionally favourable residual stresses and microhardness could be obtained.

Characterization of laser peening-induced effects on a biomedical Ti6Al4V alloy by thermoelectric means

Abstract. 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 2.8u2009u2009J/pulse and with water as a confining medium was used. Laser pulses with a 1.5-mm diameter at an equivalent overlapping density of 5000u2009u2009cm−2 were applied. Attempts to analyze the global-induced effects after laser peening were addressed by using the contacting and noncontacting thermoelectric power techniques.

Induction of engineered residual stresses fields and enhancement of fatigue life of high reliability metallic components by laser shock processing

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.

Characterisation of Fatigue and Crack Propagation in Laser Shock Peened Open Hole 7075-T73 Aluminium Specimens

The goal of this research activity is to evaluate the capability of Laser Shock Peening (LSP) technology to improve fatigue life in open-hole aluminium specimens. Thin, dog-bone specimens were LSP treated in direct ablation mode and subsequently tested. The obtained results have not proven the advantage of LSP technology over traditional residual stress insertion techniques around open-holes, such as cold working. Therefore, the focus of the activity was moved towards understanding the causes of the observed fatigue life reduction.