This document summarizes research on laser-generated stress waves. Key points:
1) Pulsed lasers can generate high-amplitude stress waves by rapidly heating a material's surface. Covering the surface with a transparent material enhances peak pressures up to 10 GPa.
2) Stress waves modify materials' properties similarly to shock waves. This process has increased strength and hardness in various metals.
3) Experiments measured stress wave pressures using piezoelectric transducers. Theoretical modeling agreed well with measurements. Stress wave rise time depends on heated material temperature while decay is slower, governed by heat transfer.
4) Research is further exploring how in-depth stress fields and material deformation vary with boundaries
The document discusses the effects of laser shock processing on the fatigue properties of 2024-T3 aluminum. It describes experiments using different laser beam geometries and process conditions to determine their impact on residual stress distributions and fatigue life. Key findings include that lower laser power densities produced higher compressive residual stresses near the surface, and the use of a momentum trap when shocking from one side helped minimize distortions from the reflected tensile wave.
The document summarizes an experiment investigating plastic deformation in an Fe-3 wt pct Si alloy caused by stress waves generated using a pulsed laser. Key findings:
1) Combining a transparent quartz overlay with a lead foil overlay on the alloy's surface was most effective at generating deformation, due to the lead's lower vaporization energy.
2) Intermediate laser power densities around 5 x 108 w/cm2 caused the most deformation. Higher power led to plasma formation reducing pressure.
3) Thinner alloy samples exhibited more uniform deformation across their thickness, with the thinnest samples (0.02 cm) showing about 1% tensile strain hardening throughout.
4) Both
This document discusses applications of laser peening to titanium alloys, including using it to increase fretting fatigue resistance. It describes how laser peening was used to enhance fatigue life of integrally bladed rotors for aircraft engines. Small spot laser peening allowed treatment of interior surfaces like dovetail slots. Testing found laser peening increased fretting fatigue life by at least 25 times for Ti-6Al-4V samples under different contact pressures. New technologies like the RapidCoater system automated the laser peening process to reduce costs.
The document discusses a new material removal process called arc ablation. Preliminary results show it can remove materials like hardened steel, Inconel, and titanium at rates far exceeding plasma cutting for the same power levels. Arc ablation uses an electric arc to melt material, which is then removed by a rotating copper tool. Tests achieved removal rates up to 97 mm3/sec at 4kW on steel. Challenges include extending it to hole making and achieving removal rates of 1000 mm3/sec at 40kW. The document describes experimental setup, parameters tested, and results obtained, finding arc ablation has potential for fast, low-cost material removal.
The document discusses various non-destructive testing techniques used to inspect materials and components without damaging them. It describes liquid penetrant testing which uses dyes to detect surface defects, magnetic particle testing which uses magnetic fields to find surface and subsurface flaws in ferromagnetic materials, ultrasonic testing which uses ultrasonic pulses to characterize materials and locate internal defects, and radiographic testing which uses x-rays or gamma rays to create images of internal structures and defects. The document provides details on the equipment, procedures, advantages and limitations of each non-destructive testing method.
This document summarizes research on the elastic-plastic properties of Zr-Cu based bulk metallic glasses (BMGs) through indentation and numerical studies. Key findings include:
1) Wedge indentation experiments were conducted on Zr48Cu36Al8Ag8 BMG samples to analyze shear band formation and propagation underneath the indenter.
2) Finite element modeling using cohesive zone elements was able to simulate shear band initiation and damage propagation based on experimental results.
3) Preliminary results show that the deformability of the indenter can affect shear band initiation and propagation patterns in the BMG material. Further modeling is being done to better capture shear band nucleation and effects.
Microstructural and Torsional Fatigue Characteristics of Singleshot and Scan ...Fluxtrol Inc.
Scan and single-shot induction hardening were used to harden 1045 and 10V45 steels. Torsional fatigue testing found that while case hardness was higher in 10V45, 1045 exhibited greater ductility during crack propagation, leading to improved fatigue life. Scan hardening produced a finer prior austenite grain size and lower residual stresses than single-shot hardening. However, fatigue performance was similar between the two processes at 550 MPa stress amplitude, with single-shot showing 70% higher life at 650 MPa.
Laser Beam Machining and it's full Criteria.
By- Engr. Md Abu Bakar Siddique
Industrial and Production Engineer,
Rajshahi University of Engineering and Technology,
Bangladesh
A Review on optimization of Dry Electro Discharge Machining Process Parametersijsrd.com
This document reviews optimization of dry electric discharge machining (EDM) process parameters. It discusses how dry EDM replaces liquid dielectric with gas, improving environmental friendliness. The literature review examines studies on how factors like voltage, current, and gas pressure affect material removal rate, surface roughness, and tool wear rate. Response surface methodology and algorithms like NSGA-II and genetic algorithms have been used to develop models and optimize the conflicting objectives of maximizing material removal rate while minimizing surface roughness. Overall, the document provides an overview of dry EDM optimization research aimed at improving process performance and efficiency.
The document describes the synthesis and morphology of silicon nanoparticles deposited on a silicon dioxide substrate using low pressure chemical vapor deposition with varying deposition times. Atomic force microscopy and image analysis software were used to characterize the nanoparticles and found that their height, density, and size varied with deposition time, with heights between 1-3 nm, densities from 2x1011 to 3.5x1011 particles/cm2, and sizes of 2-10 nm. The goal was to study how the morphological and electrical characteristics of the nanoparticles changed with different deposition parameters.
This document is a seminar report on laser beam welding of plastics submitted by Deepa Ram. The report provides an overview of laser plastic welding, including the fundamentals of the process, common laser sources used, welding of similar and dissimilar plastics, advantages, applications in automotive and electronics industries, and quality control methods. It also discusses hybrid laser welding technologies and fiber laser welding assisted by a solid heat sink. The report was submitted in fulfillment of course requirements and provides a comprehensive review of the topic of laser plastic welding.
The document summarizes research into designing a piezoelectric-actuated mirror mount with a bandwidth greater than 100kHz. Key factors influencing the first mechanical resonance were identified as the thickness and application of adhesive. An optimal design was determined to be a 0.25" diameter tungsten-carbide filled brass mount. This design achieved a measured bandwidth of 392kHz, exceeding the target of 100kHz. Future work may standardize adhesive application and investigate alternative adhesives.
PARAMETRIC OPTIMIZATION OF ELECTROCHEMICAL MACHININGAnmol Mangat
1. The document discusses parametric optimization of electrochemical machining (ECM). ECM is a non-traditional machining process that removes metal through reverse electroplating. It can machine hard metals and complex geometries.
2. The author designs and fabricates an ECM machine to surface finish cylindrical workpieces. Experiments are conducted to analyze the effect of voltage, time, and revolutions per minute on surface finish improvement percentage.
3. A relationship between process parameters and output characteristics is developed using a design of experiment approach to optimize the ECM process for improving surface finish.
The term laser is an acronym for Light Amplification by Stimulate Emission of Radiation.
A laser beam is a powerful, narrow, monochromatic and directional beam of electromagnetic radiation.
Often , these beams are within the visible spectrum of light.
A laser device excites the atom in a lasing medium. The electrons of these atom move to a higher orbit, then release photons, creating a laser beam.
This document describes a novel method of drilling into hard non-conductive materials using localized microwave energy. The microwave drill works by concentrating microwave radiation into a small volume under the material surface using a near-field concentrator, creating a hot spot. The concentrator then penetrates the hot spot as it undergoes rapid thermal runaway. Experimental tests showed the microwave drill can drill holes in materials like concrete, silicon, ceramics, rocks, glass, plastic and wood. A theoretical model is also proposed to simulate the coupled electromagnetic and thermal effects during drilling.
Measurement of residual stresses in weldmentsN.Prakasan
The document discusses various techniques for measuring residual stresses in weldments. There are two main categories of techniques - stress relaxation techniques and diffraction techniques. Stress relaxation techniques like sectioning, drilling and hole-drilling determine residual stress by measuring strain release when material is removed. Diffraction techniques like X-ray and neutron diffraction measure strain in crystal lattices to determine stress. Ultrasonic techniques also measure wave velocity changes through materials under stress. Each technique has advantages and limitations for measuring surface, subsurface or internal residual stresses in welded structures.
This document discusses the structural-acoustic coupling effects on a non-vacuum packaged vibratory cylinder gyroscope. A finite element model is used to analyze how factors like the gap size between the resonant shell and sealing cap, and the degree of vacuum, affect the vibration of the resonant shell. Both the vibration amplitude and operating frequency of the shell are found to change under the influence of structural-acoustic coupling. Experiments also show that varying the radial gap size affects the gyroscope's mechanical sensitivity. The results provide insight into the performance of such gyroscopes without vacuum packaging.
Torsional Fatigue Performance of Induction Hardened 1045 and 10V45 SteelsFluxtrol Inc.
Microalloying of medium carbon bar steels is a common
practice for a number of traditional components; however, use
of vanadium microalloyed steels is expanding into
applications beyond their original designed use as controlled
cooled forged and hot rolled products and into heat treated
components. As a result, there is uncertainty regarding the
influence of vanadium on the properties of heat treated
components, specifically the effect of rapid heat treating such
as induction hardening. In the current study, the torsional
fatigue behavior of hot rolled and scan induction hardened
1045 and 10V45 bars are examined and evaluated at effective
case depths of 25, 32, and 44% of the radius. Torsional fatigue
tests were conducted at a stress ratio of 0.1 and shear stress
amplitudes of 550, 600, and 650 MPa. Cycles to failure are
compared to an empirical model, which accounts for case
depth as well as carbon content.
Use of Laser Generated Shocks to Improve Metals & AlloysLSP Technologies
This document discusses how lasers can be used to generate high-pressure shock waves in metals in order to improve their mechanical properties. Specifically:
1) Lasers with energies up to 500 Joules are used to generate stress waves exceeding 5 GPa in pressure in metals covered with transparent overlays like quartz or water.
2) These high pressures, above the elastic limit of most metals, cause dislocation networks that strengthen the metal.
3) Experiments showed strengthening of aluminum alloys and stainless steel through increased hardness and strength as well as improved fatigue properties.
This study investigated the impact of a nickel interlayer on the electrical resistance of a tin-tin interface under fretting loading conditions. Two coating systems were tested: bronze-tin and bronze-nickel-tin. Using variable displacement amplitude testing, the transition amplitude from partial slip to gross slip was determined. Constant displacement amplitude tests then evaluated the influence of the nickel interlayer on electrical endurance. The results showed that the nickel interlayer did not influence endurance in gross slip but eliminated copper diffusion through the tin coating, preventing copper oxide formation and extending the domain of partial slip. This increased the reliability of the electrical contact.
Shock wave compression of condensed matterSpringer
This document provides an introduction and overview of shock wave physics in condensed matter. It discusses the assumptions made in treating one-dimensional plane shock waves in fluids and solids. It briefly outlines the history of the field in the United States, noting that accurate measurements of phase transitions from shock experiments established shock physics as a discipline and allowed development of a pressure calibration scale for static high pressure work. It describes some of the practical applications of shock wave experiments for providing high-pressure thermodynamic data, understanding explosive detonations, calibrating pressure scales, and enabling studies of materials under extreme conditions.
The document discusses various characterization techniques used to analyze nanomaterials. It begins by providing historical context on the origins of nanotechnology and then describes several microscopy and spectroscopy methods. Scanning electron microscopy, transmission electron microscopy, X-ray diffraction, small angle X-ray scattering, and scanning probe microscopy are some of the key techniques explained in the document.
This document summarizes the review and response to a manuscript on controllable coloration of metals using pulsed laser radiation. The reviewer raises several questions and concerns about the manuscript. The authors provide detailed responses to each question, clarifying the mechanism of coloration, temperature measurements, compositional analysis methods, and limitations of their thermodynamic modeling approach. They agree to some suggested changes and clarifications to better support their results and address the reviewer's feedback.
1) Laser shock hardening was used to increase the strength of weld zones in 5086-H32 and 6061-T6 aluminum alloys. Shocking both sides simultaneously was more effective than shocking one side.
2) Tensile testing found that laser shocking increased the yield strength of 5086-H32 to the bulk level and increased 6061-T6 yield strength midway between welded and bulk levels.
3) Microstructural analysis showed laser shocking introduced heavy dislocation tangles indicative of cold working in both alloys.
Structural Changes in the Surface Layer of Deep Rolled Samples Due to Thermal...IJERA Editor
Deep rolling processes initiate plastic deformations in the surface layer. The local characteristics of deformation are dependent on the induced stress expressed by the local stress tensor. Equivalent stresses above yield strength cause plastic deformation. Additionally the intrinsic energy, e. g. the dislocation density, is enhanced and the residual stress state is changed. The effects to a deep rolled surface from an increase in temperature are mainly dependent on the material, the microstructure, the initial residual stress state, the inclusion density, the distribution of soluted alloying elements and the plastic deformation. In the described experiments the interactions between deformation and temperature of the steel grade AISI 4140 (42 CrMo 4) used for all further experiments in a transregional Collaborative Research Center (CRC) were to be examined. The most simple investigation methods were chosen deliberately to allow a better statistical support of correlations between introduced strains and material reactions for a wide variation of process parameters. Since the visual effects by light microscopy in AISI 4140 were very small, the experiments were repeated with german grade 18 CrNiMo 7-6 (comparable to AISI 4820). This paper focuses on the micro structural changes in defined deep rolled surface regions due to an increase in temperature. The work described is part of the Collaborative Research Center “Process Signatures”, collaboration between Bremen University, Technical University Aachen, Germany and Oklahoma State University Stillwater, USA.
Lattice Energy LLC - Two Facets of W-L Theorys LENR-active Sites Supported b...Lewis Larsen
“Spatial coherence and stability in a disordered organic polariton condensate”
K. Daskalakis et al. Physical Review Letters 115 pp. 035301 - 06 (2015)
Inside a laser-pumped microcavity, they demonstrated the formation of spatially localized, entangled plasmon condensates in 100 nm layer of organic TDAF molecules at room temperature in a disordered system. Created plasmon condensates have spatial dimensions that seem to max-out at diameters of ~100 μ; beyond this critical size limit they destabilize. First-order temporal coherence of condensates = 0.8 picoseconds (ps); this is in reasonable agreement with coherence decay time estimate of 1 ps which is calculated from the observed emission linewidth.
According to Widom-Larsen theory of LENRs, many-body collective quantum and electromagnetic effects are crucial and enabling to the operation of electroweak nuclear catalysis at ambient temperatures; quantum entanglement amongst protons and plasmons at LENR sites is inferred; 1 ps lifetime of plasmon condensate is very ample time for LENRs. In 2006 EPJC paper (Widom & Larsen) we originally estimated the size of many-body coherence domains in LENR sites on metallic hydride surfaces to be ~ 1 - 10 μ. As discussed in this document, in 2009 Larsen extended Widom-Larsen theory to cover occurrence of LENRs on organic aromatic molecules; at that time, maximum size of W-L coherence domains was re-estimated and increased up to ~ 100 μ. It is not known whether this striking similarity to Daskalakis et al.’s apparent size limit of 100 μ is coincidental. W-L active site functions like a microcavity; thus seems reasonable to speculate that the surface plasmons in LENR-active sites form condensates similar to what Daskalakis et al. observed.
Surface plasmon resonance (SPR) is a phenomenon that occurs when light strikes a metal surface like gold or silver at a particular angle. SPR results in a reduction in the intensity of reflected light, and is highly sensitive to changes in the refractive index of the medium near the metal surface. This makes SPR useful for measuring molecular adsorption and interactions on the metal surface. Common applications of SPR include gas detection, electrochemistry, and life science applications like measuring protein-ligand binding.
This document summarizes molecular dynamics simulations of radiation damage in zirconia (ZrO2) at energies ranging from 0.1-0.5 MeV. The simulations find that while zirconia is highly resistant to amorphization, there is still a large number of point defects and small defect clusters created by the radiation. However, these defects are isolated from each other, resulting in dilute damage that does not disrupt the long-range crystalline structure. The simulations quantify the number of displacements and defects over time and find that electronic energy losses play an important role in the damage evolution. The findings have implications for using zirconia in nuclear waste storage by suggesting radiation can create many point defects even while
Lattice Energy LLC-Synopses of Selected WLT Technical Papers-Jan 30 2012Lewis Larsen
This document provides summaries of several technical papers published by A. Widom and L. Larsen of Lattice Energy LLC regarding their theory of low-energy nuclear reactions (LENRs). The papers explain the physics behind how LENRs may occur at metallic hydride surfaces and how this could explain experimental results without requiring new physics. Key aspects of the theory include the production of ultra-low momentum neutrons via weak interactions of electrons and protons/deuterons, and the ability of heavy electrons to absorb and downconvert gamma rays into lower energy photons. The summaries provide an overview of the theoretical work and how it relates to experimental observations of LENR phenomena.
Characteristics of shock reflection in the dual solution domainSaif al-din ali
This document summarizes a numerical study that investigates the use of laser energy deposition to induce transitions between regular and Mach shock reflections in supersonic flow over dual wedge configurations. The study validated its numerical approach by comparing results to an experiment involving laser deposition in front of a sphere. Simulations then examined how varying the position and amount of laser energy deposition could influence transition characteristics in the dual solution domain over wedge configurations. Key findings included how transition time and occurrence depended on deposition parameters and position relative to the shock waves and wedges.
Characteristics of shock reflection in the dual solution domainSaif al-din ali
This document summarizes a numerical study that investigated the effects of laser energy deposition on shock reflection transitions in supersonic flow. The study used computational fluid dynamics simulations to model laser energy being deposited in front of symmetrical wedges, creating a dual solution domain where different shock reflection patterns can occur. The results showed that laser energy deposition could induce transitions between regular and Mach shock reflections, and that the transition characteristics depended on the location and amount of energy deposited. Depositing more energy required more time for transition, and transition did not occur above a certain energy level or when depositing on the centerline.
This document summarizes a study on measuring corrosion of metals in contact with wood. It discusses how corrosion occurs due to certain conditions like the presence of water and oxygen. The study examines different test methods used to measure corrosion rates of bolts, nuts and other fasteners when in contact with treated wood. Electrochemical impedance spectroscopy was used to analyze corrosion, and results were presented as Nyquist plots and tables. Higher moisture in wood leads to faster corrosion as it improves conductivity. While corrosion kinetics increase with temperature, thermodynamic tendency for corrosion decreases as it is an exothermic reaction.
A Review: Effect of Laser Peening Treatment on Properties And Life Cycle of D...IOSRJMCE
- In this review, the effect of laser peening process with and without protective coating is discussed over the different material and it is observed that the residual stress are induced in material surface up to some depth according to process parameters of LSP. Fatigue strength and micro-hardness of material are enhance by inducing residual stresses which further depends on process parameters and material properties.
1. Laser pulses were used to generate shock waves in solids with peak pressures ranging from 5 to 120 kilobars. Peak pressures of around 35 kilobars were achieved at a fluence of 1000 J/cm2, while applying a transparent overlay increased the peak pressure to 120 kilobars.
2. Measurements found that plasma initiation occurred around 8 nanoseconds into a laser pulse when exposing a graphite coating. The resulting shock wave rose sharply at 37 nanoseconds, with only a small precursor pressure seen earlier.
3. Precursor pressures seen with carbon-resin coatings but not with pure graphite coatings, supporting the idea that precursors are due to surface ablation pressures before plasma formation
12-2-Publication-Experimental Analysis of Explosive FormingSaeed Jabalamelian
This study numerically simulates and experimentally tests the explosive hydroforming process used to form torispherical heads made of aluminum alloy AA5083. Finite element models using LS-DYNA software were developed to simulate the process, applying the Johnson-Cook and Modified Zerilli-Armstrong constitutive models. The models were verified against experimental tests. The simulation captured most material behaviors under different stress states but did not fully describe the transient zone between tension and compression. The predicted width of the transient rim was smaller than seen experimentally. Overall the blast loading simulation showed good agreement with Cole's relation for underwater detonation of small charges, with 95% accuracy.
Ryan Stillwell is a materials scientist who conducts research on quantum materials such as superconductors and topological insulators under extreme conditions. He received his PhD from Florida State University in 2013, studying the Fermi surface reconstruction of chromium at high pressure and magnetic fields. He is currently a postdoctoral researcher at Lawrence Livermore National Laboratory, where he investigates actinide and lanthanide systems using high pressure and magnetic field techniques to explore quantum interactions in these materials.
It describes how different properties of materials changes when reduced to nano. Property includes electrical, optical, mechanical, magnetic, thermal etc.
Physics of shock waves and high temperature hydrodynamic, chap xiRobert Weinheimer
This document discusses shock waves in solids. It begins by introducing the importance of studying how shock waves propagate in condensed substances like metals and water, both theoretically and practically. It then discusses how experimental investigations of solids under high pressures, which can be achieved through powerful shock waves, provide information about materials' properties. The rest of the document analyzes the thermodynamic properties of solids at high pressures and temperatures, focusing on how the elastic and thermal components of pressure and energy change with compression and expansion. It describes how interatomic forces lead to different compressibility in solids compared to gases.
This document outlines quality requirements that suppliers must meet when providing materials or services to LSP Technologies. It defines the scope, purpose, and users of the document. Key sections require suppliers to maintain quality systems based on ISO 9001, communicate issues and improvements, and meet requirements for design, purchasing, inspection, calibration, nonconforming materials, corrective actions, and record keeping. The document aims to ensure high quality, on-time delivery to meet aerospace industry demands.
This document summarizes research on using laser shock processing to treat fastener holes in aging aircraft structures. Laser shock processing uses high-intensity laser pulses to generate deep compressive residual stresses near the surface of metals. These compressive stresses can significantly increase fatigue life by inhibiting crack initiation and growth. The document describes how laser shock processing works and provides examples showing it dramatically improves fatigue life in aluminum alloys. It proposes applying the technique to fastener holes, which are high-stress locations prone to fatigue cracks. This could help extend the lifespan of aging aircraft by reducing failures originating at fastener holes.
LSP Technologies offers innovative laser technologies that are revolutionizing several industries. Founded in 1995 by former Battelle scientist Jeff Dulaney, LSPT has experienced rapid growth under his leadership through developing new laser technologies and expanding their Ohio facilities. LSPT has been awarded over 60 patents and prestigious awards for groundbreaking laser technologies. They are applying lasers to metal finishing, composites, defense systems, and developing new applications like inspecting bonded joints in aircraft.
Effect of Laser Peening on the life of failure mode of a cold pilger dieLSP Technologies
The laser shock peening (LSP) process was used to impart compressive residual stresses to pilger dies made of A2 tool steel, increasing their life. X-ray diffraction analysis found LSP generated compressive stresses over 1 mm deep in treated dies. An LSP-treated die produced 300% more tubing than standard dies before failure, exhibiting altered failure from crack propagation to plastic deformation and flaking. While pitting from LSP may have influenced earlier cracking, LSP significantly extended die life through deep compressive stresses that impeded crack growth.
Laser Shock Peening Brochure by LSP TechnologiesLSP Technologies
The LaserPeen process improves fatigue resistance by inducing deep compressive residual stresses in metal parts. Laser peened titanium alloy fan blades with simulated damage showed equal or better fatigue life than undamaged blades. The deep compressive stresses from laser peening provide much greater improvements in fatigue life and strength than conventional shot peening. LSP Technologies has developed high throughput laser peening technologies for aerospace, automotive, medical, and other fatigue-critical metal parts and components.
Extensive testing has demonstrated that laser-generated stress waves can be used to test the strength of bonds in composite materials. Previous methods could only fully test bonds through destructive proof testing. Laser bond inspection uses shock waves from high-intensity lasers to nondestructively evaluate bond strength locally. Testing of various bond strengths in composite samples showed that laser bond inspection can detect weak bonds caused by issues like poor mixing or contamination. A prototype portable laser device has been designed for implementation in factories to inspect composite assembly bonds.
1. Reproduced with permission from B. P. Fairand and A. H. Clauer, “Laser Generated Stress Waves: Their
Characteristics and Their Effects to Materials,” Conference Proceedings #50: Laser-Solid Interactions and
Laser Processing - 1978, S. D. Ferris, N. J. Leamy and J. M. Poate (eds.) 27-42. Copyright 1979,
American Institute of Physics. If you wish to find out more about the American Institute of Physics, you
can visit them on the Web at http://www.aip.org/.
LASER GENERATED STRESS WAVES: THEIR CHARACTERISTICS AND
THEIR EFFECTS TO MATERIALS
B. P. Fairand and A. H. Clauer
Battelle Memorial Institute, 505 King Avenue, Columbus, Ohio 43201
ABSTRACT
When the energy from a powerful pulsed laser is trained on the surface of an absorbent material, a
high-amplitude stress wave is generated. If the surface is covered with a material which is transparent to
the incident laser light, peak pressure environments are significantly enhanced compared to free surface
conditions. Experimental pressure measurements using piezoelectric pressure transducers have
demonstrated that peak pressures up to approximately 10 GPa can be generated in this manner. The rise
time of these pressure waves, which is controlled by the temperature of the laser heated absorbent material,
approximates the shape of the incident laser pulse. The decay time of the pressure waves is slower than the
laser pulse because it is governed by the rate at which work is done on surrounding materials and the rate at
which heat is conducted out of the heated vapor into colder adjacent materials. Theoretical calculations of
the pressure environments using a one-dimensional radiation hydrodynamic computer code are in good
agreement with the experimental measurements.
The stress wave propagates into the material and modifies the material's substructure and properties
in a way similar to shock waves generated by other means. This process has been successfully used to
increase the strength and hardness of several aluminum alloys, titanium, and the hardness of stainless steel.
The yield strength of the heat-affected zones in welded aluminum structures has been increased to values up
to the strength of the parent material. Recent studies have demonstrated that laser shock processing can
also be used to improve the fatigue life in an aluminum alloy.
INTRODUCTION
The potential of using pulsed lasers to generate high intensity stress waves in materials was first
recognized and explored by the early nineteen sixties.1'2 Later work established that a major enhancement
1
2. in the amplitude of the laser generated stress waves occurred if the absorbent surface was covered with a
material transparent to the incident laser light.3-8 Stress waves generated under these conditions were
found to be sufficiently intense to plastically deform metals and alloys even when the experiments were
conducted in a gas environment such as air at standard conditions.9-11 The ability to generate high
intensity pressure environments in materials without imposing the constraint of conducting the experiments
in vacuum stimulated interest in using these laser induced pressure waves to alter the properties of
materials in a manner similar to high explosive and flyer plate shock deformation of metals and alloys. The
laser also offered attractive characteristics as a source of high intensity pressure environments which
provided added incentive for investigating the properties and applications of laser generated stress waves.
This paper examines the types of high amplitude pressure environments one can generate with a
pulsed laser and defines important parameters governing the interaction mechanisms and their impact on
the resultant pressures. This analysis is confined to a pulsed neodymium-glass laser because it was the
experimental facility used in essentially all of our investigations; however, other lasers with wave-lengths
ranging from the infrared to the visible and near ultra-violet have the potential of producing similar
environments.
The effects of these stress waves to the surface and in-depth-properties of materials also is
investigated in this paper. These effects range from increasing the surface hardness to improvements in
yield strength and increases in the fatigue life of other metals.
PROPERTIES OF LASER GENERATED STRESS WAVES
An understanding of the effects to material properties by the laser induced stress waves first requires
a quantitative description of the pressure environments generated at the surface of the material and how
changes in the laser parameters and surface conditions of the target affect these pressure environments.
These properties of laser generated stress waves are described in this section of the paper. A second
important consideration which is required to complete our understanding of the effects of these stress waves
to materials is the propagation characteristics of the stress waves and modifications of the in-depth stress
fields introduced by physical and geometrical boundaries. Work published in this area for laser induced
stress wave environments and flyer plate experiments clearly demonstrate that boundaries affect the
resultant in-depth stress wave environments and modes of deformation.12,13 Work is presently underway
to explore in detail the in-depth properties of laser induced stress waves and the material deformation
introduced by these environments. This work will be reported at a later date.
EXPERIMENTAL AND THEORETICAL ANALYSIS OF SOURCE PRESSURE ENVIRONMENTS
Most of our laser generated stress wave studies utilized a high power CGE VD-640 Q-switched
neodymium-glass laser, which consists of an oscillator followed by six amplifier stages. This system is
able to emit pulses with full widths at one-half maximum (FWHM) ranging from 1 nanosecond to about
100 nanoseconds and energies up to 500 J. In addition to the CGE laser, some work was conducted with a
2
3. 5 joule A.O. Model 30 Q-switched neodymium-glass laser which has pulse width (FWHM) of
approximately 40 nanoseconds. The shapes of the laser pulses emitted by these systems were measured
with photodiode detectors whose outputs were fed to fast oscilloscopes.
Carbon calorimeters were used to monitor the output energies of the two laser systems. The
procedure was to measure the energy arriving at the experimental site and then normalize this energy to an
on-line calorimeter which detected a small fraction of the incident beam emanating from a beam splitter.
Dielectric mirrors were used to direct the laser radiation to the target material and simple convex
convergent lenses were used to focus the laser radiation to the desired spot size.
The pressures were measured with commercially available X-cut quartz crystal transducers of a
shorted guard-ring design. Metal targets to measure the pressure environments produced on metallic
surfaces were prepared by vacuum depositing 3-µm-thick films directly onto the front electrode surface of
the quartz transducers. In some of the pressure measurements, a layer of black paint was sprayed onto the
metal surface or directly onto the transducer electrode before application of the transparent overlay
material. Transparent materials with widely differing acoustic impedances, e.g. water, plastic, and glass
(quartz) were used to evaluate the effect of this parameter on the amplitude and time history of the stress
waves. Metal targets with different thermal properties also were investigated to determine the effect of these
parameters on the pressure environments. The experimental pressure measurements were compared to
theoretical predictions obtained from a one-dimensional code called LILA. This code which is based on the
method of finite differences was used to simulate the thermal and hydrodynamic response of materials
irradiated by a laser beam. A description of the models contained in this code is presented in another
publication.14 Basically, absorption of laser light in metal targets is first treated by classical interaction
of electromagnetic radiation with a metallic conductor and then in the heated plasma, the absorptance is
handled via an inverse Bremsstrahlung process. An analytical equation of state is used to describe the
behavior of the metal by a superposition of terms including zero temperature behavior, thermal motion of
heavy particles, and the thermal excitation and ionization of electrons. Thermal transport processes include
radiation diffusion and conduction by the metal atoms and ionized electrons in the plasma.
RESULTS OF ANALYSIS
The shape and amplitude of the resultant stress wave depends on the temperature history of the
heated vapor. This in turn depends on the laser power density at the interaction surface and the laser
energy deposition time. The laser energy must be deposited in a relatively short time to avoid the diffusion
of energy away from the interaction zone and the effect of hydrodynamic processes, both of which reduce
the amplitude of the stress wave. The thermal conductivity and heat of vaporization of the absorbent
material can also affect the stress wave environment, particularly as one goes to lower power densities.
3
4. The measured and predicted peak pressure environments generated in different target materials
covered with different transparent overlays is shown in Figure 1 as a function of the incident laser power
density, e.g. energy per unit area, divided by the width of the laser pulse. The peak pressures calculated by
the LILA code, which are shown as curves through the data, are in good agreement with the experimental
data. As seen from Figure 1, the use of different target absorbers has little effect on peak pressure once the
incident laser power densities are increased above 1-2 x 109 W/cm2. At these higher laser power densities,
calculations predict most of the absorbed energy initially goes into heating of the vapor. For this reason,
the shape of the stress wave closely follows the shape of the laser pulse until the laser pulse begins to decay
The decay time of the stress waves is much slower because it is governed by the rate at which work is done
on surrounding materials and the rate at which heat is conducted out of the vapor into the colder adjacent
materials. Experimental measurements of pressure confirm this type of behavior. This is illustrated in
Figure 2 which compares the measured time history of the laser pulse with the measured pressure pulse for
the case of a transparent water overlay on aluminum and quartz over aluminum.
When laser power densities are decreased below approximately 1 x 109 W/cm2, thermal properties
of the absorbent material begin to have a significant effect on the pressure environments. This is the reason
the two peak pressure curves shown in Figure 1 for quartz overlays begin to diverge as the value of the
laser power density approaches 1 x 109 W/cm2. It has been determined that materials with low thermal
conductivities tend to confine the absorbed energy to the interaction zone for longer times, thus maintaining
the high temperatures needed to generate high amplitude pressures. With low heat of vaporization material,
more energy is available for heating purposes and less energy is lost to internal energy of the phase change.
Figure 3 shows the effect to the pressure environments and temperatures in the interaction zone from
changing the thermal properties of the absorber. These predicted curves demonstrate that zinc with its
lower thermal conductivity and heat of vaporization is able to generate higher amplitude and longer
duration pressure pulses than aluminum given the same incident laser environment.
The change in pressure resulting from the use of different transparent overlays also is illustrated in
Figure 1. The controlling factor in this case is the acoustic impedance of the overlay material. Water has
an acoustic impedance about 1/10 that of quartz, which is the reason the pressures generated with water
overlays are lower than quartz. It is important to note, however, that water still provides an effective
method of generating high amplitude stress waves needed to shock process materials. From a practical
standpoint, when using laser shocking as a materials processing method, liquids such as water have obvious
advantages over solid overlays.
4
7. MATERIAL EFFECTS
The response of materials to laser shocking was first investigated by looking at changes in hardness
and tensile properties of alloys and studying the modified microstructures.9 In some cases, these results
could be compared to the large body of information generated from high explosive and flyer plate shocking
of materials. Later work was directed more toward possible applications of the laser shock process. For
example, increases in the strength properties of welded structures were investigated and other properties
such as fatigue and corrosion resistance were examined.
Iron base alloys, several aluminum alloys and a titanium alloy have been included in our studies.
The iron alloys consisted of stainless steel and an Fe 3wt% Si alloy which was selected for research
purposes because it can be etch-pitted for visual examination of the laser-shock-induced strain field. In the
case of aluminum, both non-heat-treatable and heat-treatable alloys in the peak and overaged conditions
have been laser shocked. Results of this work are discussed in the following sections.
7
8. ALUMINUM ALLOYS
The first alloys to be laser shocked were a 7075 T73 aluminum in the overaged condition and a 7075
T6 aluminum in the peak aged condition.9 Based on these experiments, it was determined that the
mechanical properties, e.g., strength and hardness, of the overaged alloy responded favorably to the laser
shock treatment. For example, in 7075 T73, tensile tests showed that the 0.2% offset yield strength was
increased as much as 30% over unshocked values and the ultimate strength was increased more than 10%.
Laser shocking of the peak aged alloy under similar laser conditions resulted in little change in the
mechanical properties. These changes in the mechanical properties were interpreted in terms of the
microstructural changes induced by laser shocking. Transmission electron micrographs of the unshocked
and shocked microstructures of these two alloys are shown in Figure 4. As seen from this figure, the
shocking process introduced a very dense tangled dislocation substructure. These shocked microstructures
were similar to those produced by high explosive shocking.15 The lack of shock strengthening in the peak
aged condition is understandable because strengthening due to fine precipitates
present in the unshocked substructure mitigates any possible strength contribution due to the shock-induced
dislocation substructure. Later work with peak aged and underaged 2024 aluminum alloys exhibited the
same type of behavior. The surface hardness of an underaged 2024 T3 was increased by more than 20%
over its unshocked value which was approximately the hardness increase caused by heavy cold working of
the alloy. The peak aged 2024 T8 alloy, on the other hand, showed little change in surface hardness.
Recent work at higher incident power densities and pressures shows that even the peak aged alloys will
respond favorably to the laser shock treatment. The higher pressure requirements for improving the
properties of peak aged alloys are due to the higher yield strength and lower strain hardening rates in these
alloys. These results are consistent with flyer plate shocking of the same alloys.16
There are several possible applications where laser shock treatment of aluminum alloys may offer a
new and important processing procedure. One example might be the in situ treatment of "soft" weld zones.
In many welded aluminum structures, the weld and its adjacent heat affected zone (HAZ) are a region of
weakness having a lower strength than the rest of the structure. The strength of this region can be
increased by a post-weld heat treatment or by mechanical working, such as rolling the weld bead or
explosive shocking. These approaches, however, are often either not practicable or are undesirable. Laser
shocking offers an alternative technique for increasing the strength properties of weld zones without intro-
ducing the undesirable aspects of other post weld treatment processes. To test the laser's ability to shock
process weld zones, tensile specimens were cut from welded plates of 5086 H32 and 6061 T6 aluminum
and laser shocked. The 5086 H32 aluminum is a solid-solution strengthened alloy and 6061 T6 is a peak
aged age-hardenable alloy. Tensile tests were run on shocked and unshocked specimens. Results of these
tests are shown in Figure 5. As seen in this figure, after laser shocking, the tensile yield strength of 5086
H32 was raised to the bulk value and the strength of 6061 T6 was raised midway between the welded and
bulk levels. The change in microstructure which is responsible for this change in strength properties is
illustrated in Figure 6. The shocked microstructure shows heavy dislocation ranges typical of cold
working.
8
10. Laser shocking of weld zones has potential application in several areas of industry. A shock
10
11. treatment could be beneficial wherever welded aluminum structures are used and the structure, or part, is
designed to accommodate the mechanical properties of the weld. For example, in seam-welded aluminum
pipe, a postweld laser shock treatment would increase the strength properties in the welded area and thus
reduce the wall thickness required for safe operation. A laser shock process also may find application in
welded rail structures or in the welded aluminum hulls of high-speed surface ships.
Another application area presently being investigated is the use of laser generated shocks to improve
the fatigue properties of regions around fasteners in airplane structures. Tests conducted to date show large
improvements in fatigue life are produced by the laser shock process. Both crack growth and fastened joint
specimens in plate thicknesses up to 0.25 inches have been tested. For example, in a 7075 T6 aluminum
alloy, the high cycle fatigue life of laser treated fastened joint specimens was approximately 100 times
greater than untreated specimens and the rate of crack growth in laser treated crack growth specimens was
reduced by an even greater factor.
STEEL ALLOYS
As noted earlier in this paper, the Fe 3wt% Si alloy was studied because it can be readily etch pitted
to show the distribution and approximate magnitude of plastic deformation. The type of deformation
generated in this alloy from laser shocking is shown in Figure 7. These transverse sections were taken from
discs of different thicknesses which were laser shocked on the top surface. The darker regions correspond
to areas which have undergone higher deformation than the lighter zones. As shown in Figure 7, the
amount of deformation at the front and back surface was about the same. Also, the lightly deformed
central layer decreased in thickness with decreasing specimen thickness. The thinner specimens showed
shock induced deformation over most of the thickness. Uniform shock hardening corresponding to about 1
percent tensile strain was observed in the 0.02 cm thick specimen. Both slip and twinning were present in
the shocked specimens.
In addition to the iron alloy, stainless steel also has been laser shocked. This material exhibits a
cumulative improvement in material properties from repeated laser shocks., For example, the surface
hardness of 304 SS showed little increase after one shock. As shown in Figure 8, five shocks were able to
increase surface hardness by about 40 percent. This increase in hardness with multiple shocks can be
attributed to an increase in dislocation density as shown by the transmission electron micrographs in Figure
9. The ability to multiple shock a material and improve properties in cumulative way is an important
aspect of laser shock processing because multiple shocking with a laser is relatively straightforward. In
this way, significant improvements in material properties can be realized at peak pressures too low to cause
significant hardening with a single pulse.
11
15. TITANIUM ALLOYS
Titanium and titanium-vanadium alloys containing up to 20 wt percent vanadium were laser
shocked. The titanium-vanadium alloys were selected to investigate the possibility of inducing an ω phase
transformation by the laser generated pressure waves. Surface and bulk microhardness measurements
showed hardness increases of up to 20 percent which were relatively independent of composition. There
were modest increases in tensile strength in some of the alloys. The presence of a shock induced phase
transformation was not detected in magnetic susceptibility measurements and examination by transmission
electron micrographs. Effects of laser induced shock waves to these alloys are still under investigation.
CONCLUSIONS
It has been determined that pressures up to approximately 10 GPa can be generated in metals and
alloys with short duration laser pulses. These pressures can be generated in an air or other gas environment
if the surface is covered with a transparent material. The duration of the pressure waves, which is dictated
by the laser energy deposition time, is typically less than a few tenths of a microsecond. These pressure
environments are able to modify the microstructures and mechanical properties, e.g., strength, hardness,
and fatigue, of several aluminum, iron, and titanium alloys. In all of the cases examined, the shock
microstructures show the presence of tangled networks of dislocations which are similar in their appearance
to high explosive and flyer plate shocked material.
ACKNOWLEDGEMENTS
The help of B. Campbell and M. Cantin in assisting with the laser experiments and material analyses
is hereby acknowledged. R. Jung is also acknowledged for his assistance in the computer modeling. These
studies were supported in part by the National Science Foundation, National Aeronautics and Space
Administration, and the Army Research Office.
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