Desarrollo de métodos para la evaluación integrada de propiedades mecánicas y superficiales inducidas en materiales metálicos mediante tratamiento superficial por ondas de choque generadas por láser

El tratamiento superficial por ondas de choque generadas por láser, LSP, es una técnica cuyo principal objetivo es el de la modificación del estado tensional de las primeras micras en profundidad de materiales metálicos. En sus comienzos está técnica fue empleada para inducir tensiones residuales de compresión en superficie, pero mientras se avanzaba en su desarrollo se empezaron a observar otros efectos. Profundizando en ellos se llega a la conclusión de que existe una fuerte relación entre todos, pero dependiendo de la aplicación a la que se vea sometido un componente tratado con LSP será necesario una serie de características que bien pueden ser ajustadas “a priori”. Para ello se ha de tener una buena caracterización del proceso láser y de las modificaciones que produce en las propiedades de un material determinado. Y es en este punto donde surge el problema: las modificaciones introducidas por el tratamiento láser son dependientes de la interacción de la energía del pulso láser con el material, es decir, para cada material es necesaria una caracterización previa de cómo sus propiedades son modificadas con las diferentes configuraciones del tratamiento LSP, encontrando para cada material un óptimo en los parámetros láser. En esta Tesis se pretende desarrollar una metodología para evaluar las modificaciones en las propiedades mecánicas y superficiales inducidas en materiales metálicos debido al tratamiento superficial por ondas de choque. De esta manera y avanzando de una manera lógica con la línea de investigación del grupo, se ha querido aplicar todo el conocimiento adquirido de la técnica para desarrollar esa metodología sobre un caso práctico: el empleo de dos configuraciones de tratamiento LSP sobre el acero inoxidable AISI 316L. Estas dos configuraciones elegidas se hacen en base a estudios previos, por parte del grupo de investigación, donde se han optimizados los parámetros para obtener el óptimo en lo que a perfil de tensiones residuales en profundidad se refiere. El material elegido como caso característico para llevar acabo la evaluación integrada del tratamiento LSP, de acuerdo con el propósito de esta Tesis, ha sido el acero inoxidable AISI 316L, debido a que este tipo de acero tiene una excelente resistencia a la corrosión en un amplio rango de atmosferas corrosivas, y es conocido como el grado estándar para un importante número de aplicaciones tecnológicas. La resistencia a la oxidación es buena incluso a altas temperaturas de servicio y la soldabilidad es excelente. Los aceros austeníticos son empleados en aplicaciones que soportan condiciones de alta temperatura y medios altamente corrosivos, como en reactores nucleares. Estos aceros resisten la corrosión en el agua de un reactor y procesos químicos en plantas que operan a temperaturas superiores a los 900 ˚C. En concreto el acero 316L se utiliza en la industria de equipamiento alimentario, en ambientes donde haya presencia de cloruros, en aplicaciones farmacéuticas, en la industria naval, en arquitectura, sector energético, centrales nucleares y en implantes médicos. Es decir, es un material ampliamente implantado en la industria, tanto en industrias tradicionales, como en industrias emergentes como la biomédica. El objetivo marcado para el desarrollo de la presente Tesis es caracterizar de forma precisa cómo el tratamiento superficial por ondas de choque generadas por láser es capaz de mejorar las propiedades de los materiales y cómo de estables son estas con la temperatura. Este punto es importante puesto que a la hora de introducir el proceso LSP en la industria no solo se tiene que tener en cuenta que las propiedades del material sean mejoradas, sino que también es necesario comprobar si esas mejoras se mantienen después de ser sometido el material a un tratamiento térmico ya que las condiciones de servicio de los materiales y componentes empleados no tienen por qué trabajar a temperatura ambiente. Para lograr el objetivo mencionado, el trabajo experimental realizado en la aleación seleccionada bajo todas las condiciones a estudio (material según fue recibido de fábrica, tratado con las dos configuraciones LSP y después de haber sido sometido al tratamiento térmico) ha consistido en lo siguiente: i) Estudios microestructural, morfológico y de composición química. ii) Medida de las tensiones residuales introducidas. iii) Caracterización superficial del material. iv) Estudio de las propiedades mecánicas: ensayos de tracción, ensayos de dureza, cálculo de la densidad de dislocaciones y ensayos de fatiga. v) Caracterización tribológica: ensayos de fricción y cálculo de la tasa de desgaste y volumen eliminado. vi) Caracterización electro-química para el material base y tratado con las dos configuraciones LSP. Se realizan medidas a circuito abierto, curvas de polarización (OCP), ensayos potenciostáticos y espectroscopia de impedancia electroquímica (EIS). El trabajo se ha llevado a cabo en los laboratorios del Centro Láser de la Universidad Politécnica de Madrid

Analysis of safety-based incentives in public private partnerships. The case of Spain

OUTLINE: • Introduction • Experimental Setup • Experimental Procedure • Experimental Results - Surface Roughness - Residual Stresses - Friction - Wear - EDX • Conclusion

Mechanical properties enhancement of high reliability metallic materials by laser shock processing

Laser shock processing (LSP) is being increasingly applied as an effective technology for the improvement of metallic materials 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) 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 and the evaluation of the corresponding induced properties as material specific volume reduction at the surface, microhardness and wear resistance. 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

Effect of Thermal Relaxation on LSP Induced Residual Stresses and Fatigue Life Enhancement of AISI 316L stainless steel

Effect of Thermal Relaxation on LSP Induced Residual Stresses and Fatigue Life Enhancement of AISI 316L stainless stee

Analysis of induced surface modifications effects on the electrochemical behaviour of LSP-treated metallic alloys

The systematic evaluation of the electrochemical changes induced in relevant materials by LSP treatment has been envisaged. Analysis of cyclic polarization curves with clear identification of corrosion parameters (Ecorr, Icorr, Mcorr, Ep, Ip) has been succesfully performed for Al2024-T351 and AISI 316L steel. Electrochemical Impedance Spectroscopy (EIS) has been used in order to correlate the observed electrochemical behaviour (even at long timescales) with surface modifications induced by the LSP treatment, especially roughness and elemental chemical composition. A good physico-chemical process understanding has been gained in the case of Al2024-T351 and AISI 316L steel. Work has been initiated for other key materials with a more complex behaviour (i.e. Ti6Al4V).The development of a systematic assessment capability for the corrosion susceptibility modification effects of the LSP technique is under way

Laser shock processing: an emerging technique for the mechanical and surface properties enhancement of metallic materials

Laser Shock Processing (LSP) is as an effective technology for the improvement of surface and mechanical properties of metallic alloys and is an emerging technology in its way to production engineering in direct competence with other well established technologies as, i.e. shot peening. The technique is based on the application of a high intensity pulsed laser beam on a metallic target forcing a sudden vaporization of its surface into a high temperature and density plasma that immediately develops inducing a shock wave propagating into the material. The main advantage of this 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 under cyclic load connected with improved wear and corrosion resistance. The laser shock effects achieved by this method are comparable to those of shot-peening: that is, a local material compression linked to the generation and displacement of defects, surface state modification and, most important, a compressing residual stress field whose magnitude and depth into the material is generally associated with large improvements in fatigue resistance. Along with a 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 under different LSP irradiation conditions in typical high strength materials will be presented in this paper

Effect of LSP treatment on the surface topography, friction and wear of Al2024 alloy

OUTLINE: •Introduction •Experimental Setup • Experimental Procedure • Experimental Results - Surface Roughness - Residual Stresses - Friction - Wear - EDX •Conclusion

Fatigue life enhancement of high reliability metallic components by laser shock processing

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 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 presente

Induction of through-thickness compressive residual stress fields in thin Al2024-T351 plates by laser shock processing

Purpose - With the aid of the calculational system developed by the authors, the analysis of the problem of laser shock processing (LSP) treatment for induction of residual stress (RS) fields for fatigue life enhancement in relatively thin sheets in a way compatible with reduced overall workpiece deformation due to spring-back self-equilibration has been envisaged. Numerical results directly tested against experimental results have been obtained confirming the critical influence of the laser energy and irradiation geometry parameters. The paper aims to discuss these issues. Design/methodology/approach - Plane rectangular specimens (160mm×100mm×2mm) of Al-cladded (-80µm) Al2024-T351 were considered both for LSP experimental treatment and for corresponding numerical simulation. The test piece is fixed on a holder and is driven along X and Y directions by means of an anthropomorphic robot. The predefined pulse overlapping strategy is used for the irradiation of extended areas of material. From the geometrical point of view, a full 3D configuration for the real geometry and for the sequential overlapping strategy of pulses has been considered. The FEM elements used for the simulation are an eight-node brick reduced integration with hourglass control in the treated area, namely C3D8RT, and a six-node trainer prism in the rest of the geometry, where there is no applied load, namely C3D6T, that ease meshing complex partitions. The element size in the nearest of the treated surface is 100×100×25µm, being the maximum element size which allows to maintain calculation convergence. Findings ? Numerical results directly tested against experimental results have been obtained confirming: first, the critical influence of the laser energy and irradiation geometry parameters on the possible thin sheets deformation, both at local and global scales. Second, the possibility of finding LSP treatment parameter regimes that, maintaining the requirements relative to in-depth RSs fields, are able to reduce the relative importance of sheet deformation. Third, the possibility of finding LSP treatment parameter regimes able to provide through-thickness compressive RSs fields at levels compatible with an effective fatigue life enhancement. Fourth, the possibility of improving this through-thickness compressive RSs fields by double-side treatments. Fifth, the capability of the experimental LSP treatment system at the authors site (CLUPM) of practically achieve the referred through-thickness compressive RSs fields in excellent agreement with the predictive assessment obtained by the used numerical code (SHOCKLAS®). Practical implications - The referred results provide a firm basis for the design of LSP treatments able to confer a broad range of RSs fields to thin components aiming the extension of their fatigue life, an enormously relevant field in which the authors are currently working. Originality/value - The LSP treatment of relatively thin specimens brings, as an additional consequence, the possible bending in a process of laser shock forming. This effect poses a new class of problems regarding the attainment of specified RS's depth profiles in the mentioned type of sheets, and, what can be more critical, an overall deformation of the treated component. The analysis of the problem of LSP treatment for induction of tentatively through-thickness RS's fields for fatigue life enhancement in relatively thin sheets in a way compatible with reduced overall workpiece deformation due to spring-back self-equilibration is envisaged in this paper for the first time to the authors knowledge. The coupled theoretical-experimental predictive approach developed by the authors has been applied to the specification of LSP treatments for achievement of RS's fields tentatively able to retard crack propagation on normalized specimens