J.L. Ocaña

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.

Optical characterization of extremely small volumes of liquid in sub-micro-holes by simultaneous reflectivity, ellipsometry and spectrometry

We have fabricated and characterized a lattice of submicron cone-shaped holes on a SiO(2)/Si wafer. Reflectivity profiles as a function of angle of incidence and polarization, phase shift and spectrometry are obtained for several fluids with different refractive indexes filling the holes. The optical setup allows measuring in the center of a single hole and collecting all data simultaneously, which can be applied for measuring extremely low volumes of fluid (in the order of 0.1 femtolitres) and label-free immunoassays, as it works as a refractive index sensor. A three layer film stack model is defined to perform theoretical calculations.

Laser ablation modelling of aluminium, silver and crystalline silicon for applications in photovoltaic technologies

Abstract Laser material processing is being extensively used in photovoltaic applications for both the fabrication of thin film modules and the enhancement of the crystalline silicon solar cells. The two temperature model for thermal diffusion was numerically solved in this paper. Laser pulses of 1064, 532 or 248 nm with duration of 35, 26 or 10 ns were considered as the thermal source leading to the material ablation. Considering high irradiance levels (108–109 W cm−2), a total absorption of the energy during the ablation process was assumed in the model. The materials analysed in the simulation were aluminium (Al) and silver (Ag), which are commonly used as metallic electrodes in photovoltaic devices. Moreover, thermal diffusion was also simulated for crystalline silicon (c-Si). A similar trend of temperature as a function of depth and time was found for both metals and c-Si regardless of the employed wavelength. For each material, the ablation depth dependence on laser pulse parameters was determined by means of an ablation criterion. Thus, after the laser pulse, the maximum depth for which the total energy stored in the material is equal to the vaporisation enthalpy was considered as the ablation depth. For all cases, the ablation depth increased with the laser pulse fluence and did not exhibit a clear correlation with the radiation wavelength. Finally, the experimental validation of the simulation results was carried out and the ability of the model with the initial hypothesis of total energy absorption to closely fit experimental results was confirmed.

Bio inspired self-cleaning ultrahydrophobic aluminium surface by laser processing

Micro channels and pillars were fabricated by a nanosecond laser source on thin aluminum foil of 100 μm thickness. The wettability of the laser processed μ-patterns were evaluated by the static water contact angle and found to be at the Cassie–Baxter state. The roll-off and static contact angle are 5° and 180° respectively for the μ-pillar structures. The μ-pillar structure which has a well-formed μ-cell structure demonstrated water droplet rebounding and a self-cleaning effect with edible sugar on the laser patterned surface. The elemental analysis suggests that the pulse width, frequency and power density could greatly influence the nature of the new aluminum oxide surfaces formed after laser processing, particularly in relation to the activation of those surfaces against hydroxylation and further chemisorption of organic molecules from air that transforms the surface free energy leading to a transition from a hydrophilic to ultrahydrophobic surface in a short interval of time.

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.

Laser Shock Processing of the Maraging Steel Surface

Laser Shock Processing (LSP) is a process of laser treating of a surface with a pulsed beam of high power density. The process enables hardening of a thin surface layer; therefore, it is suitable for the improvement of fatigue strength of quality materials. Locally directed mechanical waves produce a considerably increased dislocation density in the thin surface layer, which affects the variations of microhardness and residual stresses. The magnitude and variation of the residual compressive stresses in the surface layer are favourable, which ensures higher fatigue strength. Laser shock processing (LSP) is more exacting than conventional shot peening, but it shows certain advantages such as better control of the surface state, processing of locally limited surfaces and a possibility to produce different transitions between the processed surface and the non-processed one. LSP has so far been tested and efficiently applied to various materials, including maraging steels. Relevant publications often deal with LSP mechanisms and the influence of the process on the dynamic strength of maraging steel, but less frequently the influence of individual characteristics such as the microstructure of matrix and of precipitated phases or residual stresses. The present paper deals with LSP of 12% Ni maraging steel. The material chosen is suitable for the production of complex structural parts and dies for die casting, which require high resistance of the material to thermo-mechanical loads. By means of measurement of the state before and after LSP, the value of the mean roughness Ra, surface defects and the variation of residual stresses in the thin surface layer were determined. After LSP of the surface, the influence of processing parameters such as laser-beam diameter and pulse density per unit of area was established.

On the fatigue behavior of medical Ti6Al4V roughened by grit blasting and abrasiveless waterjet peening

Flat fatigue specimens of biomedical Ti6Al4V ELI alloy were surface-processed by high pressure waterjet peening (WJP) without abrasive particles using moderate to severe conditions that yield roughness values in the range of those obtained by commercial grit blasting (BL) with alumina particles. Fatigue behavior of WJP and BL specimens was characterized under cyclical uniaxial tension tests (R=0.1). The emphasis was put on a comparative analysis of the surface and subsurface induced effects and in their relevance on fatigue behavior. Within the experimental setup of this investigation it resulted that blasting with alumina particles was less harmful for fatigue resistance than abrasiveless WJP. BL specimens resulted in higher subsurface hardening and compressive residual stresses. Specimens treated with more severe WJP parameters presented much higher mass loss and lower compressive residual stresses. From the analysis performed in this work, it follows that, in addition to roughness, waviness emerges as another important topographic parameter to be taken into account to try to predict fatigue behavior. It is envisaged that optimization of WJP parameters with the aim of reducing waviness and mass loss should lead to an improvement of fatigue resistance.

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

FEM model of butt cold welding

Abstract Butt cold pressure welding is a solid state manufacturing process with several important applications, but with a gap in its fundamentals. This paper presents a new approach of the research in the field, bringing both theoretical and practical original contributions to the knowledge and creating the bases for the development of new processes addressing modern materials. First, an overview of finite element method (FEM) used in butt cold welding is presented. Correlation between stresses and material deformation is further addressed. Original indicators related to cold welding process initiation are introduced: critical deformation, welding critical stress and welding critical radius. The paper brings an original interpretation of cold welding process based on FEM model and on microstructural considerations. Results of mechanical tests and of macro- and microscopic analysis are provided to validate the model.