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AN ASMINTERNATIONAL PUBLICATIONAN ASMINTERNATIONAL PUBLICATION

METALS TECHNOLOGY FOR DESIGN, TESTING, ANDPROCESSINGMETALS TECHNOLOGY FOR DESIGN, TESTING, ANDPROCESSING

ADVANCED

MATERIALS&PROCESSES

ADVANCED

MATERIALS&PROCESSES

AUGUST 2003 VOLUME 161, NO. 8AUGUST 2003 VOLUME 161, NO. 8 wwwwww.as.asmiminternational.orgnternational.org/AM/AMPP

MATERIALSTESTING

SIKORSKYR-4 HELICOPTER

SIKORSKYR-4 HELICOPTER

MATERIALSTESTING

FEATURING

THE TESTING

BUYERSGUIDExFRACTOGRAPHY OF

POLYMERSxLASER PEENINGxMEDICAL DEVICES

CONFERENCE PREVIEW

xAUTOMOTIVE STEEL

BARS

FEATURING

THE TESTING

BUYERSGUIDExFRACTOGRAPHY OF

POLYMERS

xLASER PEENINGxMEDICAL DEVICES

CONFERENCE PREVIEW

xAUTOMOTIVE STEELBARS

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Laser peening is a rapidlydeveloping surface treatment that

puts the surface in a state ofcompressive residual stress

to improve resistance to SCC.

Michael R. Hill*, Adrian T. DeWald,Anne G. DemmaUniversity of California, Davis, California

Lloyd A. Hackel, Hao-Lin Chen,C. Brent DaneLawrence Livermore National Laboratory,Livermore, California

Robert C. Specht*, Fritz B. HarrisMetal Improvement Company, Livermore, California

Laser peening is a process in which a laser

beam is pulsed upon a metallic surface,producing a planar shockwave that travelsthrough the workpiece and plastically de-

forms a layer of material. The depth of plastic de-formation and resulting compressive residual stressare significantly deeper than possible with mostother surface treatments.

This article describes some recent developmentsand applications of laser peening. First we sum-

marize the history and process of laser peening. Wethen present test results from recent developmentefforts focused on new applications, describe re-cently commissioned commercial facilities, and dis-cuss some production-related advances.

Recent progressRecent dramatic progress in the commercializa-

tion of laser peening technology has been madepossible by the unique contributions of three dif-ferent entities: Metal Improvement Company(MIC), Lawrence Livermore National Laboratory(LLNL), and University of California, Davis (UCDavis). Chronologically, MIC funded a Coopera-tive Research and Development Agreement*Member of ASM International

(CRADA) with LLNL. The CRADA was focusedon applying to peening, the advanced solid statelaser technology that was originally developed for

military and other applications.The successful development of laser peening led

to the commercialization of the process by MIC.The LLNL laser technology provides a reliablepulsed laser source that operates at a pulse-repeti-tion rate more than ten times faster than previouslyavailable for laser peening. This increase in repeti-tion rate significantly reduces the time required toapply the laser peening treatment, and thereforehas increased throughput and reduced cost.

The group at UC Davis has developed key ad-vances in residual stress measurement, and has ap-plied this capability to better understand the effects

of a variety of laser peening parameters. UC Davishas also provided mechanical testing, microstruc-tural characterization, and structural modeling tothe joint research and development effort.

Laser peening systemLaser peening is a rapidly developing surface

treatment process that puts the surface in a state ofcompressive residual stress. Laser peening was firstdeveloped at Battelle Laboratory circa 1965, butwas not commercialized for years due to the lackof a reliable, high repetition rate, high average-power laser. To the authors knowledge, the firstcommercial application of laser peening was notuntil 1997 at GE Aircraft Engines (Cincinnati, Ohio)to mitigate foreign object damage on fan blade

LASER

PEENINGTECHNOLOGY~3-5 mm width

Shock wave

Workpiece Workpiece

(a) (b)

High-pressureplasma

Inertial tamping layer(water or glass)

Ablative layer (paint or tape)

Laser beam,5-15 GW/cm2,

10-30 ns pulse duration

Fig. 1 Description of laser peening process. (a) Workpiece is covered with aprotective ablative layer and an inertial tamping layer. (b) Laser pulse forms high-pressure plasma on the surface of the part, causing a shock wave to travel throughthe depth and plastically deforming material in its wake.

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leading edges for a military aircraft turbine engine.Production laser peening capability has grown

enormously in the past two years. MICs LaserPeening Division has recently commissioned two

commercial laser peening systems, which are beingused to treat turbine engine components.

The following description of laser peening hascertain details that are specific to the LLNL-MIClaser peening system, but other recent commercialand research laser peening systems are not signifi-cantly different. The LLNL-MIC laser peeningsystem is based on a novel Nd:glass slab, flashlamppumped laser (Fig 1.)

System parameters: The system has capabilityof 125 watts average power, a pulse width of 10 to100 ns, a pulse energy to 20 J, a repetition rate to 5Hz, and a nominally rectangular laser-spot profile.

In general, the typical laser peening parameters arepulse width of 10 to 30 ns, pulse energy of 10 to 20J, repetition rate of 3 Hz, and a laser spot size of 3to 5 mm square.

Surface prep: The laser source is directed at aprepared surface on a metallic workpiece. Considera planar section through the workpiece, at thepeening location. An opaque sacrificial ablativelayer of paint or tape is applied to this surface priorto processing. A transparent tamping layer oflaminar flowing water is also present, on top of theablative layer.

Laser pulse: A laser pulse is then directed at theworkpiece surface. The photons in the laser pulsepass through the transparent tamping layer, and areabsorbed by the ablative layer. This gives rise to a

rapidly expanding plasma cloud that is tamped tothe surface by the water layer. The tamped plasmaexpansion causes a pressure of 1 to 10 GPa to buildup on the workpiece surface over 10 to 100 ns.

Plastically deformed surface: A resulting planarshockwave then travels through the workpiece and

leaves plastically deformed material behind. Thehigh rate of deformation during laser peening pro-duces a layer of plastically deformed material thatis significantly deeper than possible with most othertechniques.

Residual stress in Alloy 22 weldsA major effort is currently under way to design

spent-fuel nuclear waste storage canisters capable ofsurviving thousands of years while interred atYucca Mountain. The current designs have an outercorrosion barrier of Alloy 22 (UNS N06022), anickel-based stainless steel that has excellent cor-rosion resistance.

Because the canisters will be sealed by welding,tensile weld residual stress will develop and mayprovide a driving force for stress corrosion cracking(SCC) over the extremely long service life of thesecanisters. The laser peening process is a leadingcandidate for mitigating the tensile weld residualstress in Yucca Mountain storage canisters, becauseour recent work has demonstrated that it producescompressive residual stress to great depths in Alloy22 welds. Here we summarize some of these meas-urements and provide graphic evidence of the ef-fect of laser peening on SCC.

Welded specimens: Residual stress was meas-

ured in as-welded and laser-peened Alloy 22 butt-welded samples. These samples were removedfrom a long, continuously butt-welded plate fabri-cated in 33 mm thick Alloy 22 plate with two-sided,multi-pass GTAW on a double-vee preparation.

The original welded plate was 812 mm long and200 mm wide, and was cut into four nominallyidentical 200 mm long specimens, two of whichwere tested during these experiments. An set of ef-fective laser peening parameters was developed,

based on parameter variations and residual stressmeasurements in small, 20-mm thick Alloy 22coupons. Following these preliminary studies, laserpeening was applied to a 100-mm square regionon one side of one of the Alloy 22 weld samples.

Stress maps: Two-dimensional maps of the lon-

Fig. 2 Longitudinal residual stress in a 33-mm thick Alloy 22 welded plate: (a) map of residual stress in unpeened weld; (b) map of residualstress in laser-peened weld; (c) line plot of residual stress versus depth at the center of the weld bead.

0 50 100 150 200

0 50 100 150 200

Laser peening

30

0

30

0

Dis

tancefromsurface,mm

(a)

(b)

400

300

200

100

0

100

200

300

400

MPa

(a) (b)

LP region

Fig. 3 Photograph of 316 stainless steel weld tested in boiling MgCl2 solution,showing (a) no SCC damage on the laser peened section, and (b) extensive SCCdamage on the unpeened section, with crack arrest near the boundary of laser peening(red dashed line).

400

200

0

200

400

600

Residualstress,MPa

5 10 15 20 25 30 35

Distance from surface, mm(c)

Longitudinal weldresidual stressCenter of weld bead

UnpeenedPeenedDifference

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gitudinal (weld-direction) component of residualstress in the peened and unpeened weld specimenswere measured by the recently developed contourmethod. Stress distributions for the peened and un-peened specimens are shown as contour plots inFig. 2a (unpeened) and Fig. 2b (laser peened), wherethe residual stress was determined in a singlemeasurement over the entire width and thickness ofthe welded sample. Laser peening produces a deep

layer of compressive stress throughout the entiretreated region (100-mm wide region on the bottomedge of Fig. 2b).

Quantitative comparison: To allow for quantita-tive comparison between the peened and the un-peened specimens, a line plot of the contour resultswas generated. A plot of residual stress versusdepth from the bottom surface at the center of theweld bead (x = 102 mm) is shown in Fig. 2c. Thisplot clearly demonstrates the ability of laser peeningto eliminate near-surface tensile weld residual stress.

At the center of the weld bead, laser peening pro-duced compressive residual stress to a depth of 4.0

mm. Additional data interpretation shows evengreater depths of compressive residual stress out-side the weld bead. For example, compressive stressis found to a depth of 6.8 mm at a location 30 mmfrom the weld center.

Additional experiments: Additional experimentshave been carried out to investigate the SCC be-havior of laser-peened welds, such as a partiallypeened 316 stainless steel butt-welded plate. Twosections of the plate were submerged in a boilingMgCl2 solution for an extended period. One sec-tion was completely laser-peened, and another sec-tion was only partially laser-peened. Although novisible cracking was observed in the laser-peened

region (Fig. 3a), extensive cracks developedthroughout the unpeened region (Fig. 3b). Cracksthat developed in the unpeened material were ar-rested when they reached the laser peened region.

Commercial applicationsIn May 2002, MIC commenced production laser

peening in a dedicated facility incorporating LLNLlaser technology. This facility was constructed ineight months, as MIC received a contract from anaerospace OEM to laser-peen a critical rotating ti-tanium component for a commercial turbine en-gine in September 2001. Within four months of

startup, the laser peening system was operating 24hours per day and five days per week. The laser iscurrently firing at high power settings more than100,000 times per day. Overall, the laser peeningsystem has proven to be robust and reliable,meeting the requirements of the turbine engine in-dustry and providing FAA and CAA certified laserpeening treatment.

In mid-2002, as the result of the successful startupof its first production laser peening system, thesame aerospace OEM committed to processing ofadditional rotating titanium components for otherturbine engine models, and MIC committed to con-struction of three additional laser peening systems.The second laser peening facility achieved fullpower operation in March 2003, and the remaining

laser peening systems are scheduled to be opera-tional by September 2003.

To further develop production capability for ti-tanium engine components, experiments have beenconducted to investigate the residual stresses in-troduced in titanium under a variety of conditions.One such study looked at the effect of the incidentangle of the laser beam on residual stress devel-oped by laser peening. Small coupons of 8.73 mmthick Ti-6Al-4V (45 x 45 mm in plane) were laser-peened with incident beam angles varying fromnormal to the surface (0 degrees) to 60 degrees fromnormal.

Near-surface residual stresses (measured by X-ray diffraction with layer removal) were largely in-dependent of the incident beam angle (Fig. 4). Be-cause the plasma cloud applies pressure normal tothe surface of the peened component (Fig. 1), notalong the laser incidence angle, the results are notunexpected. Nevertheless, the results verify thatpeening may be applied at high incident angles,and this provides significant flexibility whenpeening components with complex geometry. s

For more information:Lloyd A. Hackel: Program Leader,Laser Science and Technology Program, Lawrence Liv-ermore National Laboratory, L-482, 7000 East Avenue,Livermore, CA 94550; tel: 925/422-9009; e-mail: hackel1@llnl.gov; Web site: www.llnl.gov.

Michael R. Hill: Mechanical and Aeronautical Engi-neering Department, University of California, One ShieldsAvenue, Davis, CA 95616; tel: 530/754-6178; e-mail: mrhill@ucdavis.edu..

Rob Specht: N.A. Sales & Marketing, Metal Improve-ment Company, 10 Forest Avenue, Paramus, NJ 07652;tel: 610/688-7873; e-mail: rob_specht@metalimprovement.com; Web site: www.metalimprovement.com.

Fig. 4 Residual stress profiles in laser peened Ti-6Al-4V specimens for variouslaser incidence angles (X-ray diffraction measurements).

0 degrees48 degrees55 degrees60 degrees

0.2 0.4 0.6 0.8 1.0Depth, mm

0

200

400

600

800

1000

Residualstress,MPa

Reprinted from the August 2003 issue of Advanced Materials & Processes magazine