Effect of an absorbent overlay on the residual stress field induced by laser shock processing on aluminum samples

Laser shock processing (LSP) or laser shock peening is a new technique for strengthening metals. This process induces a compressive residual stress field, which increases fatigue crack initiation life and reduces fatigue crack growth rate. Specimens of 6061-T6 aluminum alloy are used in this investigation. A convergent lens is used to deliver 2.5 J, 8 ns laser pulses by a Q-switch Nd:YAG laser, operating at 10 Hz. The pulses are focused to a diameter of 1.5 mm onto aluminum samples. Density of 2500 pulses/cm² with infrared (1064 nm) radiation was used. The effect of an absorbent overlay on the residual stress field using this LSP setup and this energy level is evaluated. Residual stress distribution as a function of depth is assessed by the hole drilling method. It is observed that the overlay makes the compressive residual stress profile move to the surface. This effect is explained on the basis of the vaporization of the coat layer suppressing thermal effects on the metallic substrate. The effect of coating the specimen surface before LSP treatment may have advantages on improving wear and contact fatigue properties of this aluminum alloy.     

Micromould based laser shock embossing of thin metal sheets for MEMS applications

Laser shock forming is a new material processing technology. Micro-channel with dimension of 260 μm × 59 μm was successfully fabricated on metallic foil surface using laser-generated shock wave. The work piece has a high spatial resolution at the micron-level. A series of experiments was conducted to validate the finite element model. An analysis procedure including dynamic analysis performed by ANSYS/LS-DYNA and static analysis performed by ANSYS is presented in detail to attain the simulation of laser shock embossing to predict the surface deformation. Micromould based laser shock embossing holds promise for achieving precise, well-controlled, low-cost, high efficiency of three-dimensional metallic microstructures. In addition, this technique can fabricate complex 3D microstructures directly by single pulse.     

Effect of plasma confinement on laser shock microforming of thin metal sheets

Laser shock forming is conceived as a non-thermal laser forming method of thin metal sheets using the shock wave induced by laser irradiation to modify the target curvature. The plastic deformation induced by the shock wave and the direct plasma pressure applied on the material generate a residual stress distribution in the material finally leading to its bending. Using water as a confinement medium for the plasma the pressure can be increased around 10 times and the final deformation has a dramatic increase.The effect can be made clearly apparent in thin specimens (up to 1 mm). In the present study thin (100 μm) stainless steel (AISI 316) strips (1 mm long and 300 μm wide) in single and double pinned configurations have been investigated.A Nd:YAG Laser (1064 nm) with 10 ns of pulse length (FWHM) and an energy of 21 mJ per pulse is focused in the strip (spot diameter of the spot = 500 μm).Experimental and numerical studies of the influence of plasma confinement in the process and number of applied pulses are presented.The study shows that the final bending of the specimens can be controlled on a relative wide range by a stable quasi-proportional relation to the number of applied pulses and, what is considered as of major importance, that plasma confinement increases the generated pressure and thus the bending in the target.Laser shock microforming in confined configuration is considered as a technique allowing the successful processing of components in a medium range of miniaturization.     

Laser shock wave consolidation of nanodiamond powders on aluminum 319

A novel coating approach, based on laser shock wave generation, was employed to induce compressive pressures up to 5 GPa and compact nanodiamond (ND) powders (4-8 nm) on aluminum 319 substrate. Raman scattering indicated that the coating consisted of amorphous carbon and nanocrystalline graphite with peaks at 1360 cm⁻¹ and 1600 cm⁻¹ respectively. Scanning electron microscopy revealed a wavy, non-uniform coating with an average thickness of 40 μm and absence of thermal effect on the surrounding material. The phase transition from nanodiamond to other phases of carbon is responsible for the increased coating thickness. Vicker's microhardness test showed hardness in excess of 1000 kg_f/mm² (10 GPa) while nanoindentation test indicated much lower hardness in the range of 20 MPa to 2 GPa. Optical surface profilometry traces displayed slightly uneven surfaces compared to the bare aluminum with an average surface roughness (R_a) in the range of 1.5-4 μm depending on the shock wave pressure and type of confining medium. Ball-on-disc tribometer tests showed that the coefficient of friction and wear rate were substantially lower than the smoother, bare aluminum sample. Laser shock wave process has thus aided in the generation of a strong, wear resistant, durable carbon composite coating on aluminum 319 substrate.     

Numerical modeling of laser shock peening with femtosecond laser pulses and comparisons to experiments

A physics-based model has been developed for laser shock peening (LSP) with femtosecond (fs) laser pulses (fs-LSP), which has never been reported in literature to the authors’ best knowledge. The model is tested by comparing simulations with measured plume/shock wave front transient propagations and the LSP-induced hardness enhancement layer thickness. Reasonably good agreements have been obtained. The model shows that fs-LSP can produce much higher pressure than LSP with nanosecond (ns) laser pulses (ns-LSP), and it can also generate very large compressive residual stress in the workpiece near-surface layer with a thickness up to ∼100 μm. The developed model provides a powerful guiding tool for the fundamental study and the practical applications of fs-LSP. This study, together with the recently reported work by Nakano et al. [Journal of Laser Micro/Nanoengineering 4(1) (2009) 35-38], has confirmed the feasibility of fs-LSP on both theoretical and experimental sides.     

强激光冲击铝合金改性处理研究

利用新型聚偏1.1-二氟乙烯（PVDF）压电传感器,实现了对激光引发的冲击波压力的实时测量,得到激光引发的冲击波峰压在铝中成指数型的衰减规律;观测了不同约束层材料在铝靶表面产生的激光冲击波,研究了不同约束层对冲击效果的影响;最后用激光冲击强化装置对7050-T7451航空铝合金结构材料进行了冲击强化处理,对试件激光冲击区存在的残余压应力及位错密度进行了测量。结果显示经激光冲击处理的试件表面具有极高的残余压应力,可达-200MPa以上。激光冲击处理后铝合金的位错密度得到显著的提高,疲劳寿命提高到175％～428％。这些重要结果对激光冲击改性处理技术的实际应用具有指导性作用。     

High level compressive residual stresses produced in aluminum alloys by laser shock processing

Laser shock processing (LSP) has been proposed as a competitive alternative technology to classical treatments for improving fatigue and wear resistance of metals. We present a configuration and results for metal surface treatments in underwater laser irradiation at 1064 nm. A convergent lens is used to deliver 1.2 J/cm² in a 8 ns laser FWHM pulse produced by 10 Hz Q-switched Nd:YAG, two laser spot diameters were used: 0.8 and 1.5 mm.Results using pulse densities of 2500 pulses/cm² in 6061-T6 aluminum samples and 5000 pulses/cm² in 2024 aluminum samples are presented. High level of compressive residual stresses are produced −1600 MPa for 6061-T6 Al alloy, and −1400 MPa for 2024 Al alloy. It has been shown that surface residual stress level is higher than that achieved by conventional shot peening and with greater depths. This method can be applied to surface treatment of final metal products.     

Laser Shock Processing of 6061-T6 Al alloy with 1064 nm and 532 nm wavelengths

Laser Shock Processing (LSP) has been proposed as a competitive alternative technology to classical treatments for improving fatigue and wear resistance of metals. We present a configuration and results in the LSP concept for metal surface treatments in underwater laser irradiation at 532 nm and 1064 nm. The purpose of the work is to compare the effect of both wavelengths on the same material. A convergent lens is used to deliver 1.2 J/pulse (1064 nm) and 0.9 J/pulse (532 nm) in a 8 ns laser FWHM pulse produced by 10 Hz Q-switched Nd:YAG laser with spots of a 1.5 mm in diameter moving forward along the work piece. A LSP configuration with experimental results using a pulse density of 2500 pulses/cm² and 5000 pulses/cm² in 6061-T6 aluminum samples are presented. High level compressive residual stresses are produced using both wavelengths. It has been shown that surface residual stress level is comparable to that achieved by conventional shot peening, but with greater depths. This method can be applied to surface treatment of final metal products.     

Effects of laser shock processing on 00Cr12 mechanical properties in the temperature range from 25 °C to 600 °C

Laser shock processing was performed on 00Cr12 standard tensile specimens to reveal its effect on fatigue properties. Mechanical properties of the specimens were tested at the temperatures of 25 °C, 400 °C, 500 °C and 600 °C respectively. The correlations between the fatigue times and the axial strain at different temperatures were explored. The results indicate that the anti-fatigue life of material is enhanced greatly at the room temperature after laser shock processing in which residual compressive stress is mechanically produced into the surface. The yield strength and the elasticity coefficient of 00Cr12 specimens are enhanced greatly after laser shock processing; the cycle times are obviously longer at the elevated temperature, and the laser-shocked samples exhibit lower plastic strain amplitudes compared with the non-treated ones.     

Patterning thin metallic film via laser structured weakly bound template

A weakly bound buffer material is structured on a surface by interfering low power laser beams, as a template for patterning metallic thin films deposited on top. The excess buffer material and metal layer are subsequently removed by a second uniform laser pulse. This laser pre-structured buffer layer assisted patterning procedure is demonstrated for gold layer forming a grating on a single crystal Ru(1 0 0) under UHV conditions, using Xe as the buffer material. Millimeters long, submicron (0.65 μm) wide wires can be obtained using laser wavelength of 1.064 μm with sharp edges of less than 30 nm, as determined by AFM. This method provides an all-in-vacuum metallic film patterning procedure at the submicron range, with the potential to be developed down to the nanometer scale upon decreasing the patterning laser wavelength down to the UV range.     

Diagnostics of picosecond laser pulse absorption in preformed plasma

We present results where highly supersonic plasma jets and accelerated plasma fragments are generated by interaction of an intense picosecond laser pulse with a metallic target (Al, Cu, W, and Ta) in gas atmosphere. The formation of jets and well-localized massive plasma fragments occurs when a strong forward shock from a main laser pulse and a reverse shock from a pre-pulse meet to. Interferometric and shadow graphic measurements with high temporal (100 ps) and spatial (1 μm) resolution yield information about the formation and evolution of plasma jets and plasma fragments. The excitation of the electric and self-generated magnetic field by ponderomotive force during propagation of the laser pulse in a gas atmosphere was investigated as well. It had been shown previously that under certain conditions a hollow current channel can be generated in laser-produced plasma. The azimuthal magnetic field in such a micro-channel was determined by Faraday rotation of a probing laser beam to be 7.6 MGauss (MG). Ion acceleration in a pinched annular current channel up to 8 MeV analogous to micro-“plasma focus” conditions, may be realized at lengths of 100 μm. Self-generated magnetic fields of 4-7 MG have also been measured in thin skin layers in front of shock waves, where well-collimated plasma blocks were separated and accelerated away from the plasma body. The velocity of dense plasma blocks reaches values of order of 3 × 10⁸ cm/s and they are stable during acceleration and propagation in gas.     

In situ monitoring the pulse CO2 laser interaction with 316-L stainless steel using acoustical signals and plasma analysis

In most laser material processing, material removal by different mechanisms is involved. Here, application of acoustic signals with thermoelastic (below threshold) and breakdown origin (above threshold) together with plasma plume analysis as a simple monitoring system of interaction process is suggested. In this research the interaction of pulse CO₂ laser with 200 ns duration and maximum energy of 1.3 J operating at 1 Hz with austenitic stainless steel (316-L) is reported. The results showed that the non-linear point of the curve can serve as a useful indicator of melting fluence threshold (in this case ≈830 J cm⁻²) with corresponding temperature calculated using plasma plume analysis. Higher acoustic amplitudes and larger plasma plume volume indicates more intense interaction. Also, analysis showed that a phase explosion process with material removal (ejecta) in the form of non-adiabatic (i.e., d_t ? α⁻¹) is at play after laser pulse is ended. Also, SEM photographs show different surface quality medication at different laser intensities, which indicates the importance of recoil momentum pressure and possibly electrons and ions densities in heat transfer. Finally, electrochemical test indicate an improved corrosion resistance for laser treated samples compared to untreated ones.     

Strength properties of an aluminum melt at extremely high tension rates under the action of femtosecond laser pulses

The dynamics of the melting of a surface nanolayer and the formation of thermal and shock waves in metals irradiated by femtosecond laser pulses has been investigated both experimentally and theoretically. A new experimental-computational method has been implemented to determine the parameters of laser-induced shock waves in metallic films. Data on the strength properties of the condensed phase in aluminum films at an extremely high strain rate ($ \dot V $ \dot V /V ∼ 10⁹ s⁻¹)under the action of a laser-induced shock wave have been obtained.     

Real time NDE of laser shock processing with time-of-flight of laser induced plasma shock wave in air by acoustic emission sensor

Acoustic emission sensor is used to research the time-of-flight of the shock wave induced by laser-plasma in air for real time nondestructive evaluation (NDE) of laser shock processing. The time-of-flight of the shock wave propagating from the source to the sensor declines nonlinearly and similarly at the different distances for different laser energies. The velocity of the shock wave at the distance of 30 mm increases faster than that of the distance of 35 mm. The relationship between the laser energy and the distance is almost linearly when the signal with distortion is measured by acoustic emission sensor. Finally, Taylor solution is used to analyze the experimental results, and the empirical formula between the energy of the shock wave and the laser energy is established, which will provide a theoretical basis for real time NDE of laser shock processing.     

Machining of transparent materials using an IR and UV nanosecond pulsed laser

Channels are traditionally machined in materials by drilling from the front side into the bulk. The processing rate can be increased by two orders of magnitude for transparent materials by growing the channel from the rear side. The process is demonstrated using nanosecond laser pulses to drill millimeter-sized channels through thick silica windows. Absorbing defects are introduced onto the rear surface to initiate the coupling of energy into the material. Laser drilling then takes place when the fluence exceeds a threshold. The drilling rate increases linearly with fluence above this threshold. While UV light drills about four times faster than IR light, the pulse length (in the nanosecond regime) and the pulse repetition rate (in the 0.1–10 Hz range) do not greatly influence the drilling rate per pulse. Drilling rates in excess of 100 μm per pulse are achieved by taking advantage of the propagation characteristics of the plasma created at the drilling front. The plasma during rear-side drilling generates a laser-supported detonation wave into the bulk material. The geometry also seems to increase the efficiency of the laser-induced plasma combustion and shock wave during the pulse by confining it in front of the channel tip. Received: 1 July 1999 / Accepted: 17 April 2000 / Published online: 20 September 2000     

Strengthening Effects of the Laser Light Impact on a TC17 Blade in View of Simulations and Experiments

Journal of Russian Laser Research - In this article, we study the strengthening effects of nanosecond laser shock processing (LSP) of a TC17 blade by simulations and experiments. A LSP model of the...     

Effect of laser shot peening on precipitation hardened aluminum alloy 6061-T6 using low energy laser

Mechanical properties of engineering material can be improved by introducing compressive residual stress on the material surface and refinement of their microstructure. Variety of mechanical process such as shot peening, water jet peening, ultrasonic peening, laser shot peening were developed in the last decades on this contrast. Among these, lasers shot peening emerged as a novel industrial treatment to improve the crack resistance of turbine blades and the stress corrosion cracking (SCC) of austenic stainless steel in power plants. In this study we successfully performed laser shot peening on precipitation hardened aluminum alloy 6061-T6 with low energy (300 mJ, 1064 nm) Nd:YAG laser using different pulse densities of 22 pulses/mm² and 32 pulses/mm². Residual stress evaluation based on X-ray diffraction sin² ψ method indicates a maximum of 190% percentage increase on surface compressive stress. Depth profile of micro-hardness shows the impact of laser generated shock wave up to 1.2 mm from the surface. Apart from that, the crystalline size and micro-strain on the laser shot peened surfaces have been investigated and compared with the unpeened surface using X-ray diffraction in conjunction with line broadening analysis through the Williamson-Hall plot.     

Simulations of grazing-incidence pumped Ni-like Sn X-ray laser at 11.9 nm

Grazing-incidence pumped Ni-like Sn X-ray laser media at 11.9 nm (4d-4p, J = 0-1) is modelled using code EHYBRID and a post-processor code. The required atomic data are obtained using the Cowan code. In this study the pre-formed plasma is pumped on longitudinal direction with a grazing angle. Detailed simulations were performed to optimize the driving laser configurations. Relatively high gain is produced for the Ni-like Sn X-ray laser at 11.9 nm with long pre-pulse and short main pulse drive energy of only 100 mJ on 4 mm slab targets. Using low intensity pre-pulse prior to long pulse decreases the electron density gradient. X-ray resonance lines between 13 and 25 Å emitted from tin plasma have been simulated using post-processor coupled with EHYBRID. The ratio of these resonance lines can be used to measure electron temperature of the laser produced Sn plasma.     

激光烧蚀硅过程中的元素沉积规律

激光在半导体加工行业(特别是硅材料)具有广阔的应用前景。激光与硅作用过程极其复杂,本文主要研究了紫外激光脉冲对硅进行烧蚀的形貌特征以及环境气体的影响。研究表明,紫外激光烧蚀硅产生激光等离子体的电离效应对烧蚀特性起了决定性的影响：气化、电离物的产生为材料的去除提供了条件,同时激光等离子体冲击波会把相变材料有效排出,激光等离子体光谱的电离效应则把空气中的氧元素有效电离并沉积到烧蚀产物中。     

Laser shock processing: a review of the physics and applications

Developed since the beginning of the 1970s in the United States (Battelle, Columbus), laser shock processing (LSP) is being extensively studied in France in order to improve the mechanical properties of metallic surfaces of dense or porous materials. This paper reviews the considerable data on LSP which has been obtained in recent years and provides an exhaustive account of current trends concerning the physics, the mechanics and the applications involved. After presenting some general and specific data regarding the physical principles of laser shock (laser system, plasma physics, pressure generation, physical limits) and mechanical effects induced (experimental and theoretical) on different materials, the efficiency of the process is illustrated through two potential industrial applications linked with modifications of surface states: fatigue and wear resistance of metals. Experience with LSP applications shows that, because it uses safe surface geometries and provides greater affected depths, LSP is about to emerge as a real alternative to classical treatments.