P. Peyre

Surface modifications induced in 316L steel by laser peening and shot-peening. Influence on pitting corrosion resistance

The influence of laser peening (LP) on the electrochemical behavior of AISI type 316L stainless steel in a saline environment was evaluated. Surface modifications were investigated as they might have beneficial effects on the corrosion behaviour. Low residual stress and work hardening levels were found, when compared with a conventional shot-peening (SP) treatment, mainly because of the absence of martensite transformation in the case of LP. Surface changes were accompanied by small roughening effects and a global preservation of the surface chemistry after treatment. Therefore, electrochemical tests performed on samples after LP and SP treatments showed increases in rest potentials, reductions of passive current densities and anodic shifts of the pitting potentials evidenced by a stochastic approach of pitting. The better pitting resistance was observed after LP treatment, which seems to reflect a reduction or an elimination of active sites for pitting at lower potentials. Even though the deleterious surface state of shot peened surfaces possibly counterbalances the beneficial influence of residual stresses, a beneficial influence of mechanical surface treatments has been demonstrated regarding the localized corrosion properties.

Reduction of porosity content generated during Nd:YAG laser welding of A356 and AA5083 aluminium alloys

Abstract Porosity formation is greatly influenced in aluminium alloys by the low vaporisation point element (Mg, Zn) content, or by process instability such as key-hole closures that tend to entrap occluded gases during welding. Another important contribution comes from the hydrogen content, because of a very high solubility in molten aluminium that favours microporosity generation. In this paper, cw YAG laser welds on two aluminium alloys were carried out: a AA5083-O wrought alloy with a high Mg content (4.5%) and a A356 cast alloy with 7% Si and a cast oxide layer. The porosity content in laser beads was extensively studied, with the use of different experimental method (X-ray radiography+image analysis, tomography), in order to check the influence of mechanical surface preparation as well as process parameters (single or dual spot, different welding speeds). It was concluded that surface preparation as well as dual beam welding are adequate methods for reducing porosity formation tendency in laser assemblies.

Analytical and numerical modelling of the direct metal deposition laser process

The direct metal deposition (DMD) laser process is a novel technique, well adapted for aeronautical applications, that allows the building of complex 3D geometries through the interaction between a powder nozzle system and a continuous laser beam. A three-step analytical and numerical approach was carried out to predict the shapes of manufactured structures and thermal loadings induced by the DMD process. First, powder temperature was calculated using a recent analytical model, then the geometry of walls was predicted by a combined numerical + analytical modelling using a discretization of the physical interaction domain, and finally, a finite element calculation was carried out on COMSOL 3.3 Multiphysics software to describe thermal behaviour during DMD of a titanium alloy.Our thermal model takes into account the moving interface during metal deposition with a specific function κ (t, x, y, z) allowing the conductivity front to move simultaneously with the moving laser source (with an appropriate spatial energy distribution), thus representing rather precisely the DMD process. This allowed us to provide an adequate representation of temperatures near the melt-pool, and to reproduce with a good accuracy thermal cycles and melt-pool dimensions during the construction of up to 25-layer walls. This was confirmed by comparisons with experimental thermocouple data T = f(t), and fast camera melt-pool recording.

Laser-shock processing of aluminium-coated 55C1 steel in water-confinement regime, characterization and application to high-cycle fatigue behaviour

55C1 steel was irradiated with a high-power neodymium-glass laser with application to induce plastic shock waves within targets, through the expansion of a laser-induced surface plasma. Laser-shock processing experiments were conduced in the plasma-confined regime with water to increase the laser-induced peak stresses. Physical, mechanical and processings aspects were reviewed, such as the characterization of stress waves in coated steels with a VISAR velocimeter system, and the mechanical changes induced in 55Cl in terms of compressive residual stresses or work-hardening levels. With the use of convenient protective coatings, some 7-8 GPa peak stress levels could be achieved which authorized the generation of high compressive residual stress levels (nearly 80% of the compressive yield strength), but preserved the surface integrity from detrimental roughening. Surface modifications performed under different shock conditions were shown to display some 30% increase on the bending fatigue limits of 55C1 at R=0.1.

FEM calculation of residual stresses induced by laser shock processing in stainless steels

Laser shock processing, also known as laser shock peening, generates through a laser-induced plasma, plastic deformation and compressive residual stresses in materials for improved fatigue or stress corrosion cracking resistances. The calculation of mechanical effects is rather complex, due to the severity of the pressure loading imparted in a very short time period (in the ns regime). This produces very high strain rates (106 s−1), which necessitate a precise determination of dynamic properties.Finite element techniques have been applied to predict the residual stress fields induced in two different stainless steels, combining shock wave hydrodynamics and strain rate dependent mechanical behaviour. The predicted residual stress fields for single or multiple laser processes were correlated with those from experimental data, with a specific focus on the influence of process parameters such as pressure pulse amplitude and duration, laser spot size or sacrificial overlay.Among other results, simulations confirmed that the affected depths increased with pulse duration, peak pressure and cyclic deformations, thus reaching much deeper layers (> 0.5 mm) than with any other conventional surface processing. To improve simulations, the use of experimental VISAR determinations to determine pressure loadings and elastic limits under shock conditions (revealing different strain-rate dependences for the two stainless steels considered) was shown to be a key point.Finally, the influence of protective coatings and, more precisely, the simulation of a thermo-mechanical uncoated laser shock processing were addressed and successfully compared with experiments, exhibiting a large tensile surface stress peak affecting a few tenths of micrometres and a compressive sub-surface stress field.

Experimental determination by PVDF and EMV techniques of shock amplitudes induced by 0.6-3 ns laser pulses in a confined regime with water

With the objective to envisage short pulses for laser-shock hardening of materials, this paper reports experiments where laser-shock amplitudes P generated with 0.6-3 ns laser pulses at = 1.06 µm in a confined regime with water have been compared with those achieved with the usual 10-30 ns configuration. First, the experimental characterization of shock waves with polyvinylidene fluoride (PVDF) and electromagnetic (EMV) gauges shows that the short durations allow the generation of higher shock amplitudes than longer duration pulses (10 GPa versus 5 GPa) because of an increase of the pressure saturation intensity threshold Ith with short pulses (up to 100 GW cm-1 at 0.6 ns). Above Ith , a pressure pulse shortening accompanies the saturation. The P = f (I ) curves have been confirmed by surface deformation measurements induced on a Al12Si alloy. Second, the use of 10 µm aluminium coatings on 316L steel targets impacted at 40 GW cm-2 irradiance was shown to provoke a 25% maximum increase of the peak pressures by type mismatch acoustic impedance effects. Lastly, the PVDF technique is shown to be an accurate method to measure laser shock wave profiles in the 0-200 GW cm-2 regime, whereas the EMV technique is limited to intensity values of less than 20 GW cm-2 .

Laser peening processing effect on mechanical and tribological properties of rolling steel 100Cr6

This study is divided into two parts. In the first part, the influence of laser peening on the surface properties of a steel was studied. Investigative observations and comparative tests were effected on 100Cr6 rolling steel, untreated and treated by one and two passes of laser peening. Three principal parameters were analysed: surface morphology, mechanical state (residual stresses and hardness). These analyses showed that laser peening processing (LP) created a hardened layer. In the second part, the influence of the LP on the tribological behaviour of 100Cr6 rolling steel was studied. A linear rolling-sliding contact between 100Cr6 steel and a roller of 40CrMoV13 steel was used for different types of friction tests: firstly, a comparative study of the tribological behaviour of the steel untreated and treated by one and two passes of laser peening. Secondly, investigative tests were conducted under various contact pressures on steel which had been either untreated or treated by one laser peening pass. The experiments showed that there is a critical pressure which limits the protective properties of the treatment.

The generation of laser shock waves in a water-confinement regime with 50 ns and 150 ns XeCl excimer laser pulses

The generation of shock waves by laser-induced plasma in a water confinement regime has been investigated with 0.308 µm/50 ns and 0.308 µm/150 ns excimer laser pulses. Shock-wave profiles have been characterized by numerical simulations of rear surface velocity measurements behind Al foils with the use of a velocity interferometer system. The results are compared with those obtained with the third harmonic of a Nd:Yag. Above 1-2 GW cm-2, pressure levels saturate at 2-2.5 GPa and the pressure duration is much shortened by laser plasma breakdown which occurs in water. Therefore we suggest that this parasistic plasma generation is favoured by long pulse durations and a laser with short wavelengths.

Corrosion reactivity of laser-peened steel surfaces

Laser-shock processing or laser peening (LP) is a novel process used to reinforce surfaces by generating compressive residual stresses that has been investigated to change the surface mechanical state and modify the electrochemical properties of three commercial steels. The first part of this paper relates to experiments where LP has been applied to G10380 and G41400 steels for corrosion testing in an acid HKSO4-0.3 M solution. Only in the case of G41400 martensitic steel is a reduction of the corrosion current observed, depending on the degree of work hardening and the amplitude of compressive stresses. This indicates a small mechanochemical effect of LP, which seems to be restricted to martensitic structure. Second, the effect of LP on stress corrosion cracking (SCC) of AISI 316L stainless steel is demonstrated by static tests in MgCl2 44% – 153 °C solution. The results confirm the applicability of LP to suppress cracks on all the areas processed without occurrence of any problems in the treated-nontreated transitions zones.

Improving the Properties of Materials With Laser-Peening: An Overview on French Activities

Extensive work was carried out in France since 1986 on all the fields relevant to laser-shock processing (or laser-peening), including plasma physics, laser shock-waves detonics, surface modifications (residual stresses ...), and improvement of materials properties such as fatigue, corrosion or wear. The paper aims at presenting an overview on the most recent works about LP, with a special focus on FE modeling aspects for residual stress predictions, on pitting corrosion properties of a 316L steel after LP treatment, and on different fatigue applications, mainly on aluminium alloys. For most of the applications considered, the benefits of LP will be discussed, compared with usual mechanical treatments such as shot-peening.Copyright