Laser Peening For Improving Metallic Components

Suitable variation of residual stress profiles, fatigue strength and frequently also corrosion resistance of a material, are key requirements to be fulfilled for usability and long life of a vital machine component. Laser Peening (LP) is an innovative surface treatment which was initially developed for the aeronautic industry as the method for the improvement of the fatigue cracking resistance of the turbine spades of an aircraft, such as Falcon F-16 and Rockwell F-22. LP is based on plasma generation, which produces shock impact waves, which, in turn, produce elastoplastic shifts of atomic planes and generation of compressive residual stresses of high gradient [1]. LP has been applied to different types of steels, aluminium and titanium alloys [2]. In practice, technologists and engineers frequently require the introduction of compressive residual stresses since it improves fatigue resistance of a material [3, 4]. In the study [5] of the effects of LP parameters such as laser-beam power density, pulse duration, and pulse density it was found that a correct choice of the processing parameters enabled desired variations of micro-hardness and residual stresses in the surface of machine components. Such achieved condition provided improved material resistance to fatigue. The applicability of laser peening was also confirmed to improve stress-corrosion cracking [6]

Residual stress analysis of friction stir processed AA6061

Currently, after appropriate heat treatments, the 6xxx aluminium alloys are used in a variety of applications including aircraft fuselage skins and automobile body panels and bumpers, instead of more expensive 2xxx or 7xxx alloys. The paper presents the results of the residual stress analysis in case of Friction Stir Processing - FSP of AA6061-T4 plates, 10 mm thickness, demonstrating the process ability to locally modify the base metal properties. The residual stresses, DRX depicted, are correlated with the microstructure and hardness results

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

Thermoelectric assessment of laser peening induced effects on a metallic biomedical Ti6Al4V

Laser peening has recently emerged as a useful technique to overcome detrimental effects associated to another well-known 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

Análisis de la Influencia de los Parámetros Operacionales en Recubrimientos Superficiales Asistidos por Láser de Acero AISI 431 sobre Acero al Carbono

En el presente artículo se muestran los resultados obtenidos de un estudio basado en una serie de recubrimientos de acero inoxidable martensítico AISI 431 sobre acero al carbono. El objetivo principal de este trabajo es valorar la influencia de los principales parámetros operacionales que intervienen en el proceso. This paper presents the main results of a study based on laser cladding of AISI 431 stainless steel powder on low carbon steel experiments. The objective of this work is to show the influence of the processing parameters in the clad

Predictive assessment of plasma dynamics effects on the shock transformation of metallic alloys by laser shock processing

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

Analysis of plasma thermal surface effects on the residual stress field induced by LSP in Al2024-t351

In Laser Shock Processing (LSP) a high intensity pulsed laser beam is focused at the interface between a metallic target and a transparent confining material (normally water) that induces a residual stress distribution in the target material. Without a protective coating thermal effects are present near the target surface. A calculational model has been developed, able to systematically study LSP processes, starting from laser-plasma interaction and coupled thermo-mechanical target behavior. We present results obtained in LSP treatments without coating. In particular the relative influence of thermal/mechanical effects shows that: each effect has a different temporal scale and thermal effects are limited to a small region near the surface; repeated pulses increase maximum compressive residual stress and the depth of the compressive residual stress region; compressive residual stresses very close to the surface level can be induced even without any protective coating through the application of adjacent pulses

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 presente

Scientific and Technological Issues on the Application of High Intensity Lasers to Material Properties Modification: The case of Laser Shock Processing of Metallic Alloys

Laser Shock Processing (LSP) has been practically demonstrated as a technique allowing the effective induction of residual stresses fields in metallic materials allowing a high degree of surface material protection. Experimental results obtained with commercial Q-switched lasers prove complete feasibility at laboratory scale. Depending on initial material mechanicla properties, the remaining residual stresses fields can can reach depths and maximun values providing an effectively enhanced behaviour of materials against fatigue crack propagation, abrasive wear, chemical corrosion and other failure conditions. This makes the technique specially suitable and competitive with presently use techniques for the treatment of heavy duty components in the aeronautical, nuclear an automotive industries. However, according to the inherent difficulty for prediction of the shock waves generation (plasma) and evolution in treatedmaterials, the practical implementation of LSP processes needs an effective predictive assessment capability. A physically comprehensive calculational tool (SHOCKLAS) has been developed able to sistematically study LSP processe

Análisis de Tensiones Residuales Inducidas en Aleaciones Metálicas por Tratamientos Superficiales mediante Ondas de Choque Generadas por Láser

En el presente artículo se plantea un estudio sobre la medida de tensiones residuales mediante el método del taladro en banda extensométrica (o agujero ciego) aplicado en probetas de aluminio 2024-T351, previamente tratadas superficialmente mediante laser shock processing (método de tratamiento superficial por ondas de choque generadas por láser, conocido como LSP). Finalmente, se realiza una comparación entre los resultados experimentales obtenidos al medir las tensiones residuales obtenidas en probetas tratadas mediante LSP y los valores obtenidos mediante un modelo de simulación