metals

Review Effect of Laser Peening on the Mechanical Properties of Aluminum Alloys Probed by Synchrotron Radiation and X-Ray Free Electron Laser

Yuji Sano 1,2,3,* , Kiyotaka Masaki 4, Koichi Akita 5, Kentaro Kajiwara 6 and Tomokazu Sano 3,7

1 Energy Systems Research and Development Center, Toshiba Energy Systems & Solutions Corporation, 8 Shinsugita-cho, Isogo-ku, Yokohama, Kanagawa 235-8523, Japan 2 Division of Research Innovation and Collaboration, Institute for Molecular Science, National Institutes of Natural Sciences, 38 Nishigo-Naka, Myodaiji, Okazaki, Aichi 444-8585, Japan 3 Department of Quantum Beam Physics, Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan; [email protected] 4 Department of Mechanical Systems Engineering, National Institute of Technology, Okinawa College, 905 Henoko, Nago, Okinawa 905-2192, Japan; [email protected] 5 Department of Mechanical Systems Engineering, Faculty of Science and Engineering, Tokyo City University, 1-28-1 Tmamazutsumi, Setagaya-ku, Tokyo 158-8557, Japan; [email protected] 6 Japan Synchrotron Radiation Research Institute, 1-1-1, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan; [email protected] 7 Division of Materials and Manufacturing Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan * Correspondence: [email protected]; Tel.: +81-564-55-7246



Received: 13 October 2020; Accepted: 6 November 2020; Published: 9 November 2020


Abstract: Synchrotron radiation (SR) and X-ray free electron laser (XFEL) are indispensable tools not only for the exploration of science but also for the evolution of industry. We used SR and XFEL to elucidate the mechanism and the effects of laser peening without coating (LPwC) which enhances the durability of metallic materials. X-ray diffraction (XRD) employing SR revealed that the residual stress (RS) in the top surface became compressive as the laser pulse irradiation density increased with appropriate overlapping of adjacent laser pulses. SR-based computed tomography (CT) was used to nondestructively reconstruct three-dimensional (3D) images of fatigue cracks in aluminum alloy, revealing that LPwC retarded crack propagation on the surface and inside of the sample. SR-based computed laminography (CL) was applied to friction stir welded (FSWed) aluminum alloy plates to visualize fatigue cracks propagating along the welds. The fatigue crack had complicated shape; however, it became a semi-ellipsoid once projected onto a plane perpendicular to the fatigue loading direction. Ultra-fast XRD using an XFEL was conducted to investigate the dynamic response of aluminum alloy to an impulsive pressure wave simulating the LPwC condition. The diffraction pattern changed from spotty to smooth, implying grain refinement or subgrain formation. Shifts in diffraction angles were also observed, coinciding with the pressure history of laser irradiation. The durations of the dynamic phenomena were less than 1 µs; it may be possible to use high-repetition lasers at frequencies greater than kHz to reduce LPwC processing times.

Keywords: laser shock peening; X-ray diffraction; residual stress; computed tomography; fatigue crack; grain refinement; aluminum alloy; friction stir welding

Metals 2020, 10, 1490; doi:10.3390/met10111490 www.mdpi.com/journal/metals Metals 2020, 10, 1490 2 of 19

1. Introduction Structural materials in actual service are subjected to external loads in corrosive environments. Defects or cracks almost always commence at the surface because of stress concentration attributable to bending, torsion, roughness, or pitting. Therefore, surface enhancement is beneficial to extend the service life [1]. Laser shock peening (LSP) is a well-established method that induces compressive RSs on the surface, thus preventing crack initiation and propagation [2–9]. Since the mid-1990s, LSP has been used as a countermeasure against so-called foreign object damage (FOD) to prolong the high-cycle fatigue (HCF) life of jet-engine fan blades [10–12]. Recently, a sophisticated LSP system with a pulse energy of 10 J and a repetition rate of 20 Hz was commercialized [3]. Some researchers are applying LSP to new areas such as engineering ceramics [13] and medical implant [14]. Meanwhile, the authors previously described a laser peening process using low-energy laser pulses (0.1 J or less) [15–19]; no surface preparation (pasting of a “sacrificial overlay”) was required. We initially termed this process laser peening (LP) [19] and later laser peening without coating (LPwC) [20,21] to emphasize that no sacrificial overlay or coating was necessary. In this context, a novel technology of LPwC without using any confining medium named “Dry-LP” was developed using a femtosecond laser and the effects to prolong fatigue life [22] and reduce corrosion rate [23] were confirmed for A2024 aluminum alloy. In LPwC, laser pulses directly impinge on the surface, heat it, and evaporate the surface to create a laser-induced high-pressure plasma, which induces impulsive pressure waves that propagate internally to generate permanent strain and a beneficial compressive RS in the near-surface layer. However, this may also induce a negative heat effect on the surface, creating an undesirable tensile RS that competes with the favorable effect of the compressive RS [24,25]. Recently, we summarized the results of a series of XRD experiments using SR and concluded that LPwC induced compressive RSs on top surfaces with the exception of the final laser spot when the irradiating laser pulse density was high and the adjacent laser pulses overlapped appropriately [26]. We took full advantage of valuable SR characteristics (high brilliance and spatial coherence) when precisely mapping RS distributions on top surfaces after single, line, and areal irradiations. The tensile component of RSs created by a laser pulse can be wiped out by succeeding laser pulses that appropriately overlap the prior pulse. To explore fatigue crack propagation, we employed SR-based computed tomography (CT) and computed laminography (CL) to obtain 3D images of fatigue cracks; we utilized phase-contrast to enhance the edge of crack images. We succeeded in imaging a casting defect and a fatigue crack commencing from that defect in a rod-type fatigued sample of AC4CH cast aluminum alloy [27–29]. Intermittent CT imaging during fatigue loading showed that LPwC reduced crack propagation. CL was employed to reconstruct images of fatigue cracks propagating in friction stir welded (FSWed) joints of A6061 aluminum alloy [30]. Friction stir welding (FSW) imparts a unique structure, reflecting material flow during welding [31–36]. In general, fusion welding creates a fused zone and an adjoining heat-affected zone (HAZ), the mechanical properties of which are inferior to those of the base material. Therefore, fatigue cracks usually commence in this region and propagate along the weld line or the HAZ [35,37]. In FSWed materials, a thermomechanically affected zone (TMAZ) exists between the stir zone (SZ; corresponding to the fused zone of fusion welding) and the HAZ, rendering crack propagation complicated and unpredictable. However, SR-based CL successfully reconstructed 3D images of fatigue cracks in FSWed joints and showed that the projected images of cracks on the plane perpendicular to the fatigue-loading direction were semi-ellipsoidal, as expected given the base material. Although the fatigue cracks propagated in an unpredictable manner, the projected images grew smoothly as the number of fatigue loading cycles increased, reducing the unpredictability of crack propagation in FSWed joints. XFEL affords much higher brightness than SR [38–42]. A single XFEL pulse contains sufficient photons to study the dynamic response of materials to laser pulses using time-resolved XRD [43–45]. Immediately after completion of the SPring-8 angstrom compact free electron laser (SACLA; the XFEL facility constructed by RIKEN; Harima, Japan [38,39]) in 2011, we commenced “pump-probe” experiments to explore the microscopic phenomena and fundamental mechanism of material plastic Metals 2020, 10, 1490 3 of 19 deformation under LPwC conditions. Prior to the availability of XFEL, the effects of LPwC had been studied statically thus via evaluation of RSs and fatigue. A finite element method (FEM) based on continuum mechanics well-reproduced the RS state after LPwC, except that of the top surface subjected to a thermomechanical effect [5,25,46,47]. These approaches do not deal directly with microscopic phenomena, such as dislocation, grain refinement, phase transformation, or precipitation; such data are essential when seeking to understand plasticity, material hardening, and RS stability. Here, we present the results of SR-based CT and CL on aluminum alloys in Sections2 and3, respectively; we nondestructively visualized fatigue cracks in a 3D manner and evaluated the propagation. The results of ultra-fast XRD using an XFEL are described in Section4; these revealed the dynamic responses of an aluminum alloy under an impulsive pressure wave that simulated the LPwC condition.

2. Imaging of Fatigue Cracks and Propagation Using SR-Based CT Fatigue is a major cause of component failure. It is essential to understand how fatigue cracks propagate with reference to the critical safety points. Fatigue cracks are initially small; it is difficult to obtain 3D images of them using conventional X-ray CT, fluoroscopy, or other nondestructive methods. We sought to use an SR-based CT technique with a phase-contrast effect to image tiny fatigue cracks in macroscopic engineering objects [27–30,48,49].

2.1. Sample Preparation Fatigue test samples were prepared from a cast Al-Si-Mg alloy (JIS AC4CH). The shape and dimensions are shown in Figure1[ 50]. The chemical composition of AC4CH is given in Table1. A minute hole (diameter 0.3 mm) was drilled in the center of each sample. Rotating-bending fatigue was used to trigger a crack commencing at the drill hole and extending to a length of 2.5 mm on the surface. Next, three samples out of eight were subjected to LPwC to induce compressive surface RSs that covered the crack. Then, an additional 1 105 loading cycles were applied to all eight samples to × confirmMetals 2020 the, 10, e xff FORect ofPEER LPwC REVIEW oncrack propagation. 4 of 19

Figure 1.1. The shape and dimensions of AC4CH samples used for rotating-bending fatigue testing. TheThe dimensionsdimensions areare expressedexpressed inin millimeters.millimeters.

Table 1. The chemical composition of thethe AC4CH cast aluminum alloy.

ElementElement CuCu SiSi MgMg ZnZn Fe Fe Mn Mn Al Al CompositionComposition (wt.%) (wt.%) 0.032 0.032 6.92 6.92 0.3070.307 0.009 0.158 0.158 0.007 0.007 Bal. Bal.

The crack length was measured by the replication method. A thin film of acetyl cellulose was attached to the crack surface using methyl acetate, the texture of the surface was transferred to the film, and the film was observed using a microscope to determine the crack length. Regarding LPwC, laser pulses with a wavelength of 532 nm and an energy of 100 mJ were irradiated to the sample immersed in water with a spot diameter 0.6 mm and a pulse density of 27 pulse/mm2, affording a peak

Figure 2. Crack growth behavior of AC4CH fatigue samples. Each curve is shifted so that the fatigue cycle at the crack length of 2.5 mm is the origin of the horizontal axis. Laser peening without coating (LPwC) was applied, and fatigue loading continued. Computed tomography (CT) was performed after an additional 105 loading cycles at SPring-8.

2.2. Experimental Setup for SR-based CT CT imaging of fatigue cracks used beamline BL19B2 of SPring-8 as shown in Figure 3. The X-ray energy was adjusted to 28 keV using a double-crystal monochromator. The sample was placed about 110 m distant from the X-ray source (a bending magnet). The X-ray area detector (a cooled CCD camera) was positioned 0.8 m behind the sample to allow a phase-contrast effect. During sample rotation, projected data of 1,024 × 1,024 pixels were recorded every 0.5° from 0 to 180°. The detector pixel size corresponded to about 6 µm, given its optical magnification of a relay lens after an X-ray- to-visible converter. Each slice was reconstructed using a filtered-back projection algorithm. The distance between the X-ray source and the sample was determined by considering the blur of the image and the resolution of the detector [48]. The X-ray source size was 0.3 mm wide and 0.1 mm high. Since the magnification of the experimental system (D2/D1) was 7.3 × 10−3, the horizontal blur was 2.2 µm and the vertical was 0.7 µm, which are sufficiently small compared to the resolution of the X-ray area detector of about 6 µm.

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Metals 2020, 10, 1490 4 of 19 power density of about 4 GW/cm2 [48]. RSs of about 100 to 200 MPa appeared on the surface after − − LPwC [50]. Typical depth profiles of RS are reported elsewhere [48]. Figure 1. The shape and dimensions of AC4CH samples used for rotating-bending fatigue testing. The crack propagation curves are plotted in Figure2 for samples that did and did not undergo The dimensions are expressed in millimeters. LPwC. Each curve is shifted so that the fatigue loading cycle at a crack length of 2.5 mm on the surface is the origin of the horizontal axis. This compensates for crack initiation and propagation variations Table 1. The chemical composition of the AC4CH cast aluminum alloy. in the fatigue loading cycles until the crack length of 2.5 mm was attained. In the additional loading cycles, the cracks continuouslyElement grew inCu samples Si not subjectedMg Zn to LPwC,Fe but notMn in samplesAl subjected to LPwC which thusComposition prohibited (wt.%) crack propagation.0.032 6.92 0.307 0.009 0.158 0.007 Bal.

Figure 2. Crack growth behavior of AC4CH fatigue samples. Each curve is shifted so that the fatigue Figure 2. Crack growth behavior of AC4CH fatigue samples. Each curve is shifted so that the fatigue cycle at the crack length of 2.5 mm is the origin of the horizontal axis. Laser peening without coating cycle at the crack length of 2.5 mm is the origin of the horizontal axis. Laser peening without coating (LPwC) was applied, and fatigue loading continued. Computed tomography (CT) was performed after (LPwC) was applied, and fatigue loading continued. Computed tomography (CT) was performed an additional 105 loading cycles at SPring-8. after an additional 105 loading cycles at SPring-8. 2.2. Experimental Setup for SR-Based CT 2.2. Experimental Setup for SR-based CT CT imaging of fatigue cracks used beamline BL19B2 of SPring-8 as shown in Figure3. The X-ray energyCT wasimaging adjusted of fatigue to 28 keVcracks using used a double-crystalbeamline BL19B2 monochromator. of SPring-8 as Theshown sample in Figure was placed3. The X-ray about energy110 m distant was adjusted from the to X-ray 28 keV source using (a a bending double-cryst magnet).al monochromator. The X-ray area detector The sample (a cooled was placed CCD camera) about 110was m positioned distant from 0.8 mthe behind X-ray thesource sample (a bending to allow magnet). a phase-contrast The X-ray eff areaect. detector During sample(a cooled rotation, CCD camera)projected was data positioned of 1024 10240.8 m pixels behind were the recorded sample to every allow 0.5 a fromphase-contrast 0 to 180 . Theeffect. detector During pixel sample size × ◦ ◦ rotation,corresponded projected to about data 6ofµ 1,024m, given × 1,024 its opticalpixels were magnification recorded every of a relay 0.5° lensfrom after 0 to an180°. X-ray-to-visible The detector pixel size corresponded to about 6 µm, given its optical magnification of a relay lens after an X-ray- converter.Metals 2020, 10 Each, x FOR slice PEER was REVIEW reconstructed using a filtered-back projection algorithm. 5 of 19 to-visible converter. Each slice was reconstructed using a filtered-back projection algorithm. The distance between the X-ray source and the sample was determined by considering the blur of the image and the resolution of the detector [48]. The X-ray source size was 0.3 mm wide and 0.1 mm high. Since the magnification of the experimental system (D2/D1) was 7.3 × 10−3, the horizontal blur was 2.2 µm and the vertical was 0.7 µm, which are sufficiently small compared to the resolution of the X-ray area detector of about 6 µm.

Figure 3. The CT setup with a phase-contrast eeffectffect at BL19B2 of SPring-8.

2.3. TheThe Phase-Contrast distance between Effect the X-ray source and the sample was determined by considering the blur of the imageBefore and experiment, the resolution the phase-contrast of the detector effect [48]. Thewas X-rayevaluated source via size ray-trace was 0.3 calculation mm wide [51–54]. and 0.1 mmThe high. Since the magnification of the experimental system (D /D ) was 7.3 10 3, the horizontal blur sample serves as a concave lens because the refractive index2 of1 a hard X-ray× − within a material is was 2.2 µm and the vertical was 0.7 µm, which are sufficiently small compared to the resolution of the slightly smaller than unity (in this case, by 6.8 × 10−7) [48,53]. The calculated edge enhancement µ X-rayattributable area detector to phase-contrast of about 6 ism. shown in Figure 4. A pair of bright and dark lines (a fringe) must be evident in the projected image. The extent of edge enhancement is several tens of micrometers, thus much greater than the resolution of the X-ray area detector (about 6 µm).

(a) (b)

Figure 4. Edge enhancement calculation of the projected X-ray image afforded by phase-contrast: (a) The calculative geometry. The sample serves as a concave lens; (b) Edge enhancement with bright and dark areas at the boundary.

2.4. Reconstruction of Crack Images

CT was used to image fatigue cracks after the additional 105 loading cycles in samples subjected or not to LPwC. Figure 5 (a) shows images at the levels of the drilled holes of such samples. The hole is notably distorted after LPwC, implying that LPwC induced significant plastic surface flow [28]. The contrasting area near the hole is the crack opening. As predicted by Figure 4, the boundaries between the objects and air are enhanced by the pair of bright and dark lines. The linear attenuation coefficients along the line A-A’ in the sample not subjected to LPwC are plotted in Figure 5 (b). The edge enhancement attributable to phase-contrast is clear.

Metals 2020, 10, x FOR PEER REVIEW 5 of 19

Metals 2020, 10, 1490 5 of 19 Figure 3. The CT setup with a phase-contrast effect at BL19B2 of SPring-8.

2.3.2.3. The The Phase-Contrast Phase-Contrast Effect Effect BeforeBefore experiment, experiment, the the phase-contrast phase-contrast effect eff ectwas was evaluated evaluated via ray-trace via ray-trace calculation calculation [51–54]. [51 The–54 ]. sampleThe sample serves serves as a concave as a concave lens lensbecause because the refractive the refractive index index of a ofhard a hard X-ray X-ray within within a material a material is 7 is slightly smaller than unity (in this case, by 6.8 10−7 )[48,53]. The calculated edge enhancement slightly smaller than unity (in this case, by 6.8 ×× 10 −) [48,53]. The calculated edge enhancement attributableattributable to to phase-contrast phase-contrast is isshown shown in inFigure Figure 4.4 A. Apair pair of ofbright bright and and dark dark lines lines (a fringe) (a fringe) must must be evidentbe evident in the in projected the projected image. image. The extent The extent of edge of edgeenhancement enhancement is several is several tens of tens micrometers, of micrometers, thus muchthus muchgreater greater than the than resolution the resolution of the ofX-ray the X-rayarea detector area detector (about (about 6 µm). 6 µm).

(a) (b)

FigureFigure 4. 4. EdgeEdge enhancement enhancement calculation calculation of the of theprojected projected X-ray X-ray image image afforded afforded by phase-contrast: by phase-contrast: (a) The(a) Thecalculative calculative geometry. geometry. The Thesample sample serves serves as a as concave a concave lens; lens; (b) ( bEdge) edge enhancement enhancement with with bright bright andand dark dark areas areas at at the the boundary. boundary.

2.4.2.4. Reconstruction Reconstruction of of Crack Crack Images Images 5 CTCT was was used used to to image image fatigue fatigue cracks cracks after after the the additional additional 10 105 loadingloading cycles cycles in in samples samples subjected subjected oror not not to to LPwC. LPwC. Figure Figure 55 (a)a shows shows images images at at the the levelslevels ofof thethe drilleddrilled holesholes ofof such samples. The The hole hole isis notably notably distorted distorted after after LPwC, LPwC, implying implying that that LP LPwCwC induced induced significant significant plastic plastic surface surface flow flow [28]. [28 ]. TheThe contrasting contrasting area area near near the the hole hole is is the the crack crack op opening.ening. As As predicted predicted by by Figure Figure 4,4, the the boundaries betweenbetween the the objects objects and and air air are are enhanced enhanced by by the the pair pair of of bright bright and and dark dark lines. lines. The The linear linear attenuation attenuation coefficientscoefficients along along the the line line A-A’ in thethe samplesample notnot subjectedsubjected to to LPwC LPwC are are plotted plotted in in Figure Figure5b. 5 The(b). edgeThe Metals 2020, 10, x FOR PEER REVIEW 6 of 19 edgeenhancement enhancement attributable attributable to phase-contrast to phase-contrast is clear. is clear.

(a) (b) FigureFigure 5.5. TheThe eeffectffect ofof phase-contrast:phase-contrast: ((aa)) ReconstructedReconstructed sliceslice imagesimages atat thethe levellevel ofof thethe drilleddrilled hole;hole; (b(b)) a A plot plot of of the the linear linear attenuation attenuation coe coefficientsfficients along along the the line line A-A’ A-A’ of of Figure Figure 5a 5. The(a). The border border between between the objectthe object (AC4CH) (AC4CH) and airand is air enhanced. is enhanced.

TheThe crackcrack image image of of each each slice slice was was extracted extracted and sequentiallyand sequentially stacked stacked to build to 3Dbuild images 3D images of fatigue of samplesfatigue samples that did andthat diddid notand undergo did not LPwCundergo (Figure LPwC6)[ (Figure28]. The 6) white[28]. The shadows white areshadows the crack are openings. the crack Theopenings. upper imagesThe upper are viewsimages parallel are views to the parallel sample axis;to the the sample lower imagesaxis; the are lower views images perpendicular are views to perpendicular to that axis. The drilled holes are in the centers of the shadows; the cracks propagated from the holes. A comparison shows that fatigue crack propagation was retarded by LPwC not only on the surface but also internally.

(a) (b)

Figure 6. Three-dimensional (3D) crack images of AC4CH fatigue samples: (a) The crack in the reference sample (no LPwC); (b) The crack in the sample that underwent LPwC. A comparison shows that LPwC retarded crack propagation.

3. Imaging of Fatigue Cracks in FSWed Joints Using SR-Based CL FSW is an emerging solid-state joining process and serves as an alternative to fusion welding of aluminum alloys [55–57]. Heat input is less than that associated with fusion welding; FSW thus creates less distortion, reducing time and labor for post-processing. However, the stirring creates inhomogeneous anisotropic microstructures reflecting material flow. These influence the mechanical properties, especially fatigue, which must be assessed prior to any application. We earlier showed that LPwC enhanced the HCF properties of FSWed A6061-T6 aluminum alloy plates under plane- bending fatigue, as shown in Figure 7 [58,59]. LPwC was applied at first to the top surface (crown side) with a 60 mJ pulse energy, a 0.7 mm spot diameter and an 18 pulse/mm2 pulse density, then applied to the bottom surface (root side) with a 27 pulse/mm2 pulse density. LPwC increased the fatigue strength of the FSWed material beyond that of the base material under an HCF regime.

Metals 2020, 10, x FOR PEER REVIEW 6 of 19

(a) (b)

Figure 5. The effect of phase-contrast: (a) Reconstructed slice images at the level of the drilled hole; (b) A plot of the linear attenuation coefficients along the line A-A’ of Figure 5 (a). The border between the object (AC4CH) and air is enhanced.

The crack image of each slice was extracted and sequentially stacked to build 3D images of Metalsfatigue2020 samples, 10, 1490 that did and did not undergo LPwC (Figure 6) [28]. The white shadows are the 6crack of 19 openings. The upper images are views parallel to the sample axis; the lower images are views perpendicular to that axis. The drilled holes are in the centers of the shadows; the cracks propagated that axis. The drilled holes are in the centers of the shadows; the cracks propagated from the holes. from the holes. A comparison shows that fatigue crack propagation was retarded by LPwC not only A comparison shows that fatigue crack propagation was retarded by LPwC not only on the surface but on the surface but also internally. also internally.

(a) (b)

FigureFigure 6.6. Three-dimensional (3D) crack images of AC4CHAC4CH fatiguefatigue samples:samples: ( a) The crackcrack inin thethe referencereference samplesample (no(no LPwC);LPwC); ((bb)) TheThe crackcrack inin thethe samplesample thatthat underwentunderwent LPwC.LPwC. AA comparison showsshows thatthat LPwCLPwC retardedretarded crackcrack propagation.propagation.

3.3. Imaging of Fatigue Cracks in FSWed JointsJoints UsingUsing SR-BasedSR-Based CLCL FSWFSW is an emerging solid-state solid-state joining joining process process and and serves serves as as an an alternative alternative toto fusion fusion welding welding of ofaluminum aluminum alloys alloys [55–57]. [55–57 ].Heat Heat input input is isless less than than that that associated associated with with fusion welding; FSW thusthus createscreates lessless distortion,distortion, reducingreducing timetime andand laborlabor forfor post-processing.post-processing. However, thethe stirringstirring createscreates inhomogeneousinhomogeneous anisotropicanisotropic microstructuresmicrostructures reflectingreflecting materialmaterial flow.flow. TheseThese influenceinfluence thethe mechanicalmechanical properties,properties, especiallyespecially fatigue, fatigue, which which must must be be assessed assessed prior prior to anyto any application. application. We earlierWe earlier showed showed that LPwCthat LPwC enhanced enhanced the HCF the propertiesHCF properties of FSWed of FSW A6061-T6ed A6061-T6 aluminum aluminum alloy plates alloy under plates plane-bending under plane- fatigue,bending as fatigue, shown as in shown Figure7 in[ 58Figure,59]. LPwC7 [58,59]. was LPwC applied was at firstapplied to the at topfirst surface to the (crowntop surface side) (crown with a 2 60Metalsside) mJ 2020with pulse, 10 a,energy, x60 FOR mJ PEER pulse a 0.7 REVIEW mmenergy, spot a diameter 0.7 mm spot and andiameter 18 pulse and/mm an pulse18 pulse/mm density,2 thenpulse applied density, to7 thenof the 19 2 bottomapplied surface to the (rootbottom side) surface with a(root 27 pulse side)/mm withpulse a 27 density.pulse/mm LPwC2 pulse increased density. the LPwC fatigue increased strength the of However, the propagation behaviors of fatigue cracks in FSWed joints differed greatly from those in thefatigue FSWed strength material of beyondthe FSWed that ofma theterial base beyond material that under of anthe HCF base regime. material However, under an the HCF propagation regime. behaviorsthe base material of fatigue [60–65]. cracks in FSWed joints differed greatly from those in the base material [60–65].

Figure 7. S-NS-N curves curves of of the the A6061-T6 A6061-T6 base base material material (BM), (BM), thethe friction friction stir stirwelded welded material material (FSW), (FSW), and andboth both materials materials after after LPwC. LPwC. The The fatigue fatigue strength strength of ofthe the FSWed FSWed material material was was enhanced enhanced by by LPwC to beyond that of the BM.

As described in Section 2, CT was used to reconstruct 3D images of fatigue cracks in rod-shaped AC4CH samples. However, it is difficult to apply CT to laterally extended objects such as FSWed joints because X-ray attenuation drastically varies during the rotational scan. CL can be used to visualize the internal structures of objects such as printed circuit boards [66–71]. The CL setup is similar to that of CT; the tomographic axis is inclined as shown in Figure 8. In this configuration, the X-ray attenuation does not change significantly during the rotation. We performed SR-based CL to nondestructively reconstruct 3D images of fatigue cracks inside the FSWed joints at BL19B2 in SPring- 8. The reconstructed 3D images were compared to macroscopic photographs of the fracture surface, demonstrating the effectiveness of SR-based CL.

Figure 8. The sample arrangement for synchrotron radiation (SR)-based computed laminography (CL).

3.1. Preparation of FSWed Joints The base material (BM) was a 3 mm thick A6061-T6 aluminum alloy plate with the chemical composition described in Table 2. The FSW procedure has been described elsewhere [59]. The mechanical properties of BM and the joint are listed in Table 3.

Table 2. The chemical composition of the A6061-T6 base material (BM).

Element Si Fe Cu Mn Mg Cr Zn Ti Al Composition (wt.%) 0.65 0.2 0.30 0.06 1.04 0.13 0.04 0.02 Bal.

Metals 2020, 10, x FOR PEER REVIEW 7 of 19

However, the propagation behaviors of fatigue cracks in FSWed joints differed greatly from those in the base material [60–65].

Figure 7. S-N curves of the A6061-T6 base material (BM), the friction stir welded material (FSW), and Metals both2020, 10materials, 1490 after LPwC. The fatigue strength of the FSWed material was enhanced by LPwC to7 of 19 beyond that of the BM.

AsAs describeddescribed inin SectionSection2 2,, CT CT was was used used to to reconstruct reconstruct 3D 3D images images of of fatigue fatigue cracks cracks in in rod-shaped rod-shaped AC4CHAC4CH samples.samples. However, it is didifficultfficult toto applyapply CTCT toto laterallylaterally extendedextended objectsobjects suchsuch asas FSWedFSWed jointsjoints becausebecause X-rayX-ray attenuationattenuation drasticallydrastically variesvaries duringduring thethe rotationalrotational scan.scan. CLCL cancan bebe usedused toto visualizevisualize thethe internalinternal structuresstructures ofof objectsobjects suchsuch asas printedprinted circuitcircuit boardsboards [66[66–71].–71]. TheThe CLCL setupsetup isis similarsimilar toto that of CT; CT; the the tomographic tomographic axis axis is is inclined inclined as as shown shown in inFigure Figure 8.8 In. Inthis this configuration, configuration, the theX-ray X-ray attenuation attenuation does does not not change change significantly significantly du duringring the the rotation. rotation. We We performed performed SR-based CLCL toto nondestructivelynondestructively reconstruct reconstruct 3D 3D images images of of fatigue fatigue cracks cracks inside inside the the FSWed FSWed joints joints at at BL19B2 BL19B2 in in SPring-8. SPring- The8. The reconstructed reconstructed 3D 3D images images were were compared compared to to macroscopic macroscopic photographs photographs of ofthe the fracturefracture surface,surface, demonstratingdemonstrating thethe eeffectivenessffectiveness ofof SR-based SR-based CL. CL.

Figure 8. The sample arrangement for synchrotron radiation (SR)-based computed laminography (CL). Figure 8. The sample arrangement for synchrotron radiation (SR)-based computed laminography 3.1. Preparation of FSWed Joints (CL).

3.1. PreparationThe base material of FSWed (BM) Joints was a 3 mm thick A6061-T6 aluminum alloy plate with the chemical composition described in Table2. The FSW procedure has been described elsewhere [ 59]. The mechanical propertiesThe base of BM material and the (BM) joint arewas listed a 3 mm in Table thick3. A6061-T6 aluminum alloy plate with the chemical composition described in Table 2. The FSW procedure has been described elsewhere [59]. The mechanical propertiesTable of 2. BMThe chemicaland the joint composition are listed of thein Table A6061-T6 3. base material (BM).

ElementTable 2. The chemical Si Fecomposition Cu of Mn the A6061-T6 Mg base Cr material Zn (BM). Ti Al Composition (wt.%) 0.65 0.2 0.30 0.06 1.04 0.13 0.04 0.02 Bal. Element Si Fe Cu Mn Mg Cr Zn Ti Al Composition (wt.%) 0.65 0.2 0.30 0.06 1.04 0.13 0.04 0.02 Bal. Table 3. The mechanical properties of the A6061-T6 BM and FSWed material.

Material Tensile Strength (MPa) 0.2% Proof Strength (MPa) Elongation (%) Base material 336 318 15.7 FSWed material 195 - 4.8

Fatigue samples were cut from FSWed plates via wire electric discharge machining. A sample is shown in Figure9. The hatched area is the weld zone (WZ) and the red circles are the fields of view in the CL setup. The top (crown) and bottom (root) surfaces were skimmed with an end mill to remove burrs or irregularities formed during FSW. The top, bottom, and side surfaces of each sample were polished. A hole 0.3 mm in diameter and depth was drilled in the center of the WZ to initiate a fatigue crack. Cyclic bending-loading was applied in a direction perpendicular to the weld line, at a constant strain amplitude, a stress ratio R = 1, and a frequency of 22 Hz under ambient conditions. − MetalsMetals 2020 2020, ,10 10, ,x x FOR FOR PEER PEER REVIEW REVIEW 88 of of 19 19

TableTable 3. 3. The The mechanical mechanical properties properties of of th thee A6061-T6 A6061-T6 BM BM and and FSWed FSWed material. material.

MaterialMaterial TensileTensile Strength Strength (MPa) (MPa) 0.2%0.2% Proof Proof Strength Strength (MPa) (MPa) Elongation Elongation (%) (%) BaseBase material material 336 336 318 318 15.7 15.7 FSWedFSWed material material 195 195 - - 4.8 4.8

FatigueFatigue samples samples were were cut cut from from FSWed FSWed plates plates via via wi wirere electric electric discharge discharge machining. machining. A A sample sample is is shownshown inin FigureFigure 9.9. TheThe hatchedhatched areaarea isis thethe weldweld zonezone (WZ)(WZ) andand thethe redred circlescircles areare thethe fieldsfields ofof viewview inin thethe CLCL setup.setup. TheThe toptop (crown)(crown) andand bottombottom (root(root)) surfacessurfaces werewere skimmedskimmed withwith anan endend millmill toto removeremove burrs burrs or or irregularities irregularities formed formed during during FSW. FSW. The The top, top, bottom, bottom, and and side side surfaces surfaces of of each each sample sample werewere polished. polished. A A hole hole 0.3 0.3 mm mm in in diameter diameter and and depth depth wa wass drilled drilled in in the the center center of of the the WZ WZ to to initiate initiate a a fatigue crack. Cyclic bending-loading was applied in a direction perpendicular to the weld line, at a fatigueMetals 2020 crack., 10, 1490 Cyclic bending-loading was applied in a direction perpendicular to the weld line,8 at of 19a constantconstant strain strain amplit amplitude,ude, a a stress stress ratio ratio R R = = − −1,1, and and a a frequency frequency of of 22 22 Hz Hz under under ambient ambient conditions. conditions.

FigureFigure 9. 9. The The shape shape shape and and and dimensions dimensions dimensions of of of FSWed FSWed FSWed samples. samples. The The The dimensions dimensions dimensions are are are expressed expressed expressed in in in millimeters. millimeters. millimeters. TheThe red red circles circles are areare the thethe fields fieldsfields of of view view of of CL. CL.

3.2.3.2. Experimental Experimental Se Se Setuptuptup of of SR-based SR-based SR-Based CL CL CL AA schematic schematicschematic ofof thethethe setup setupsetup is isis shown shownshown in inin Figure FigureFigure 10 10..10. The TheThe tomographic tomographictomographic axis axisaxis was waswas inclined inclinedinclined by 30byby◦ 30°(30°ϕ = ((φφ60 ==◦ 60°in60° Figure inin FigureFigure8). The 8).8). X-rayTheThe X-rayX-ray energy energyenergy was adjusted waswas adjustedadjusted to 28 keV toto 2828 using keVkeV a usingusing monochromator. aa monochromator.monochromator. The X-ray TheThe area X-rayX-ray detector areaarea detector(adetector cooled (a (a CCD cooled cooled camera) CCD CCD camera) camera) captured captur captur projectioneded projection projection data every data data 0.5 every every◦ over 0.5° 0.5° 360 over over◦ with 360° 360° an with with exposure an an exposure exposure time of time time 400 ofmsof 400400 per msms flame. perper flame.flame. Every EveryEvery 20 exposures, 2020 exposures,exposures, the sample thethe sampsamp waslele shifted waswas shiftedshifted downward downwarddownward and the andand projection thethe projectionprojection data of datadata the ofacrylicof thethe acrylicacrylic pipe (without pipepipe (without(without the sample) thethe sample)sample) were collected werewere collectedcollected to allow toto compensationallowallow compensationcompensation for X-ray forfor attenuationX-rayX-ray attenuationattenuation by the pipe.by the The pipe. window The window size of the size projection of the projection was set to was 1984 set 7680to 1,984 pixels × 7,680 after 2pixels2 binning, after 2 representing× 2 binning, by the pipe. The window size of the projection was set× to 1,984 × 7,680 pixels× after 2 × 2 binning, representingarepresenting tradeoff between a a tradeoff tradeoff image between between resolution image image andresolution resolution easy data-handling. an andd easy easy data-handling. data-handling. The effective The The detectoreffective effective pixeldetector detector size pixel pixel was size5.7size µ waswasm after 5.75.7 µm theµm after binning,after thethe binning, includingbinning, includingincluding the optical thethe magnification. opticaloptical magnification.magnification. The size The ofThe the sizesize field ofof the ofthe view fieldfield was ofof viewview 11.3 mmwas (horizontal)11.3 mm (horizontal)4.4 mm (vertical).× 4.4 mm The(vertical). distance The between distance the between bending the magnet bending and magnet the sample and wasthe was 11.3 mm (horizontal)× × 4.4 mm (vertical). The distance between the bending magnet and the sample52sample m, rendering waswas 5252 m,m, the renderingrendering X-ray beam thethe X-rayX-ray parallel. beambeam The parallparall detectorel.el. TheThe was detectordetector positioned waswas positionedpositioned 0.8 m behind 0.80.8 mthem behindbehind sample thethe to sampleincorporatesample to to incorporate incorporate the phase-contrast the the phase-contrast phase-contrast effect [29 effect ,effect30,48 [29,30,48].]. [29,30,48]. Slice images Slice Slice images wereimages reconstructed were were reconstructed reconstructed from a from from set of a a 720set set ofangularlyof 720720 angularlyangularly equidistant equidistantequidistant 2D projection 2D2D projectionprojection data using datadata a filtered-backusingusing aa filtered-backfiltered-back projection projectionprojection algorithm algorithmalgorithm and 3D images andand 3D3D of imagestheimages fatigue of of the the cracks fatigue fatigue obtained cracks cracks byobtained obtained stacking by by the stacking stacking 2D sliced the the images.2D 2D sliced sliced images. images.

FigureFigure 10. 10. The The SR-based SR-based CL CL setup setup at at BL19B2 BL19B2 of of SPring-8. SPring-8.

To obtain clear images of fatigue cracks, a specially designed sample holder was placed on the

rotational stage of beamline BL19B2 at SPring-8. As shown in Figure 11, the holder retains the sample using screwed bolts and controls the bending strain by adjusting the bolts so that the fatigue crack opens slightly. This enhances the contrast of crack images. The effective size of the field of view was several millimeters; it was thus necessary to combine the reconstructed 3D images for each field of view to obtain an entire fatigue crack image that ran through several fields of view. Metals 2020, 10, x FOR PEER REVIEW 9 of 19

To obtain clear images of fatigue cracks, a specially designed sample holder was placed on the rotational stage of beamline BL19B2 at SPring-8. As shown in Figure 11, the holder retains the sample using screwed bolts and controls the bending strain by adjusting the bolts so that the fatigue crack Metals 2020, 10, x FOR PEER REVIEW 9 of 19 opens slightly. This enhances the contrast of crack images. The effective size of the field of view was severalTo obtainmillimeters; clear imagesit was thusof fatigue necessary cracks, to comba speciallyine the designed reconstructed sample 3D holder images was for placed each fieldon the of rotationalview to obtain stage an of beamlineentire fatigue BL19B2 crack at imagSPring-8.e that As ran shown through in Figure several 11, fields the holder of view. retains the sample using screwed bolts and controls the bending strain by adjusting the bolts so that the fatigue crack opens slightly. This enhances the contrast of crack images. The effective size of the field of view was severalMetals 2020 millimeters;, 10, 1490 it was thus necessary to combine the reconstructed 3D images for each field9 of of 19 view to obtain an entire fatigue crack image that ran through several fields of view.

Figure 11. The scanning equipment of SR-based CL and the sample holder.

3.3. Surface Observations on Samples with Fatigue Cracks

A crack was created and propagated in an FSWed sample via fatigue loading over 5.80 × 104 cycles with an initialFigure stress 11. TheThe amplitude scanning scanning equipment equipment of 160 MPa. of of SR-bas SR-based CL wased CL performed and the sample for several holder. fields of view at 3.3.intervals Surface ofObservations 2 mm to cover on Samplesthe whole with crack. Fatigue The Cracks entire image was built by connecting the five fields 3.3.of view Surface at theObservations same elevation. on Samples Figure with 12 Fatigue compares Cracks the reconstructed image of the fatigue crack at the A crack was created and propagated in an FSWed sample via fatigue loading over 5.80 104 sampleA crack surface was and created the corresponding and propagated photograph. in an FSWed Ring sampleartifacts via remain fatigue in the loading reconstructed over 5.80 image; ×× 104 cycles with an initial stress amplitude of 160 MPa. CL was performed for several fields of view at cycleshowever, with the an crack initial shape stress is amplitudewell-reproduced of 160 byMPa. CL. CL was performed for several fields of view at intervals of 2 mm to cover the whole crack. The entire image was built by connecting the five fields of intervalsIn general, of 2 mm a tofatigue cover thecrack whole in a crack.homogeneous The entire material image waspropagates built by alongconnecting a near-straight the five fields line view at the same elevation. Figure 12 compares the reconstructed image of the fatigue crack at the ofperpendicular view at the same to the elevation. loading Figure direction. 12 compares However, the in reconstructed the FSWed imagesample of of the Figure fatigue 12, crack the atcrack the sample surface and the corresponding photograph. Ring artifacts remain in the reconstructed image; samplepropagates surface with and a largethe corresponding deviation from photograph. the straight Ri line.ng artifacts This may remain be attributable in the reconstructed to the distinctive image; however, the crack shape is well-reproduced by CL. however,microstructure the crack formed shape during is well-reproduced FSW [62–65]. by CL. In general, a fatigue crack in a homogeneous material propagates along a near-straight line perpendicular to the loading direction. However, in the FSWed sample of Figure 12, the crack propagates with a large deviation from the straight line. This may be attributable to the distinctive microstructure formed during FSW [62–65].

FigureFigure 12.12. TheThe fatiguefatigue crackcrack onon thethe samplesample surface.surface.

In general, a fatigue crack in a homogeneous material propagates along a near-straight line 3.4. Three-Dimensional Imaging of a Fatigue Crack in an FSWed Joint perpendicular to the loading direction. However, in the FSWed sample of Figure 12, the crack propagatesA fatigue with crack a large was deviationcreated in fromanother the sample straight over line. 3.37 This × 10 may5 loading be attributable cycles using to thean initial distinctive stress microstructureamplitude of 120 formed MPa. during TheFigure reconstructed FSW 12. [ 62The–65 fatigue]. 3D image crack on is theshown sample in surface.Figure 13, reproducing the shape,

3.4. Three-Dimensional Three-Dimensional Imaging of a Fatigue Crack in an FSWed Joint

A fatigue crack was created in another sample overover 3.373.37 × 105 loadingloading cycles cycles using using an an initial initial stress stress × amplitude of 120 MPa. The The reconstructed reconstructed 3D 3D image image is is shown shown in in Figure Figure 13,13, reproducing reproducing the the shape, morphology, and texture of the fracture surface very well. The initially drilled hole is at the center of the image. Three cracks can be identified in Figure 13; the main crack initially propagated from the drilled hole as expected; further two cracks then commenced at the corners between the top and both side surfaces. Finally, the sample separated into two pieces when the three cracks combined. The 2D Metals 2020, 10, x FOR PEER REVIEW 10 of 19 Metals 2020, 10, x FOR PEER REVIEW 10 of 19 morphology,morphology, and and texture texture of of the the fracture fracture surfacesurface very well. TheThe initiallyinitially drilled drilled hole hole is is at at the the center center of of the image. Three cracks can be identified in Figure 13; the main crack initially propagated from the Metalsthe 2020image., 10, 1490Three cracks can be identified in Figure 13; the main crack initially propagated from10 the of 19 drilleddrilled hole hole as as expected; expected; further further two two cracks cracks thenthen commenced atat the the corners corners between between the the top top and and both both sideside surfaces. surfaces. Finally, Finally, the the sample sample separated separated intointo two pieces whenwhen the the three three cracks cracks combined. combined. The The 2D 2D representationsrepresentations shown shown in in red, red, green, green, andand blueblue areare cutaways of of the the 3D 3D image image at at the the planes planes shown shown by by thethe rectangular rectangular dotteddotted dotted lines.lines. lines. The The arrows arrows indicateindicate the di directionsrectionsrections of ofof the thethe visual visualvisual angles anglesangles of ofof the thethe 2D 2D2D cracks. cracks.cracks.

Figure 13. The reconstructed SR-based CL 3D image and a photograph of the fatigue crack. Figure 13. The reconstructed SR-based CL 3D image and a photograph of the fatigue crack. FigureFigure 14 14 shows shows the the 2D 2D projection projection images images of theof the main main crack crack and and the crackthe crack length, length, depth, depth, and aspectand Figure 14 shows the 2D projection images of the main crack and the crack length, depth, and ratio.aspect The ratio. images The are images viewed are fromviewed the from direction the directio of the redn of arrow the red of arrow Figure of 13 Figure at 2.00, 13 3.00, at 2.00, and 3.00, 3.37 and10 5 aspect3.37 ×ratio. 105 fatigue The images cycles. are Surprisingly, viewed from once the the directio crack nimage of the projected red arrow onto of Figurea plane 13 perpendicular at 2.00, 3.00,× toand fatigue cycles. Surprisingly, once the crack image projected onto a plane perpendicular to the fatigue 3.37the × fatigue 105 fatigue loading cycles. direction, Surprisingly, the crack once shape the became crack image a clear projected semi-ellipsoid onto (Figurea plane 14perpendicular (a)); also, the to loading direction, the crack shape became a clear semi-ellipsoid (Figure 14a); also, the crack propagation thecrack fatigue propagation loading direction,curve was the smooth crack (Figure shape 14became (b)) even a clear though semi-ellipsoid the crack exhibited (Figure 14a complicated (a)); also, the curve was smooth (Figure 14b) even though the crack exhibited a complicated 3D structure. The crack crack3D structure.propagation The curve crack waslength smooth on the (Figure surface 14es timated(b)) even by though CL agreed the crackwell with exhibited that derived a complicated using length on the surface estimated by CL agreed well with that derived using the replication method. 3Dthe structure. replication The method. crack length The crack on the depth surface tended estimated to plateau by CL as agreedthe crack well propagated, with that asderived would using be The crack depth tended to plateau as the crack propagated, as would be expected during plane bending theexpected replication during method. plane bendingThe crack fatigue, depth decreasing tended to the plateau aspect ratio.as the The crack results propagated, thus imply as that would crack be fatigue, decreasing the aspect ratio. The results thus imply that crack propagation in FSWed joints is expectedpropagation during in FSWedplane bending joints is fatigue,well-characterized decreasing by the SR-based aspect ratio. CL. The The aspect results ratio thus yielded imply bythat CL crack is well-characterized by SR-based CL. The aspect ratio yielded by CL is very useful; this is necessary propagationvery useful; in this FSWed is necessary joints is when well-characterized calculating the stressby SR-based intensity CL. factor, The aspectbut is difficult ratio yielded to determine by CL is when calculating the stress intensity factor, but is difficult to determine nondestructively. verynondestructively. useful; this is necessary when calculating the stress intensity factor, but is difficult to determine nondestructively.

(a) (b)

Figure 14. a Crack propagation(a) behavior: ( ) Images of the main crack viewed(b from) the fatigue loading direction; (b) crack propagation curves. The arrows point to the reference axes.

Metals 2020, 10, x FOR PEER REVIEW 11 of 19

Figure 14. Crack propagation behavior: (a) Images of the main crack viewed from the fatigue loading direction; (b) Crack propagation curves. The arrows point to the reference axes.

4. Ultra-Fast XRD of an Aluminum Alloy under Impulsive Stress Wave The macroscopic phenomena in LPwC, except those of the top surface, are well-understood by precision experiments and time-dependent elastoplastic FEM simulations [5,46,47]. However, the microscopic aspects (dislocation, grain refinement, dynamic precipitation, and phase transformation) are not well-characterized because observational tools affording adequate space-time resolution were unavailable prior to XFEL. We thus studied the microscopic dynamics of an A6061 aluminum alloy by ultra-fast XRD during plastic deformation after intense laser pulse irradiation simulating LPwC at the Japanese XFEL facility, SACLA [72,73].

4.1. Experimental Procedure The sample material was that used in Section 3, thus the A6061 aluminum alloy. Foils 0.1 mm in thickness were solution heat-treated and stuck to acrylic plates using vacuum grease. The plate confines the plasma generated by ablative interaction of the laser pulse with the sample surface, magnifyingMetals 2020, 10 ,the 1490 plasma pressure up to several gigapascals when simulating LPwC conditions [18,74].11 of 19 The setup of a diffractometer is shown in Figure 15, and the sample arrangement in Figure 16 together with4. Ultra-Fast a sample XRD photograph of an Aluminum taken after Alloy a series under of pump-probe Impulsive Stress experiments. Wave Laser pulses from a frequency-doubled Nd:YAG laser (λ = 532 nm) were irradiated onto the sampleThe through macroscopic the 10 phenomenamm thick acrylic in LPwC, plate via except a planoconvex those of the lens top (f surface,= 150 mm). are The well-understood pulse energy, pulseby precision duration experiments and focusing and spot time-dependent diameter were elastoplastic about 400 mJ, FEM 6–8 simulations ns (the FWHM), [5,46,47 ].and However, 1.5 mm respectively,the microscopic affording aspects (dislocation,a peak power grain density refinement, of 3–4 GW/cm dynamic2 and precipitation, a plasma andpressure phase of transformation) 3–4 GPa. The impulsiveare not well-characterized stress wave generated because by observational the high-pressure tools aplasmaffording propagated adequate space-timeinto the sample resolution as a quasi- were planeunavailable wave priorbecause to XFEL.of the Weone-dimensional thus studied the (1D) microscopic sample geometry. dynamics When of an the A6061 stress aluminum wave arrived alloy byat theultra-fast opposite XRD free during sample plastic surface, deformation the sample after intense began laserto macroscopically pulse irradiation deform simulating (Figure LPwC 16 at (b)). the JapaneseTherefore, XFEL diffraction facility, data SACLA must [ 72be, 73collected]. before the deformation. In general, the material exposed to LPwC (or LSP) should be thick enough to hold the inducing 4.1. Experimental Procedure compressive RS; a thickness of 10 times or more the compression depth is desirable to avoid the macroscopicThe sample deformation material waswhich that results used inin Sectionrelieving3, thus the RS. the A6061Nonetheless, aluminum in this alloy. experiment, Foils 0.1 mm a thin in foilthickness (0.1 mm were thick) solution was heat-treatedused to observe and stuckdynamic to acrylic phenomena plates using on the vacuum opposite grease. free Thesurface plate of confines the foil withoutthe plasma significantly generated byattenuating ablative interaction the impulsive of the pre laserssure pulse wave with and the samplethe thermal surface, effect magnifying of the laser the plasmapulse irradiation. pressure up Therefore, to several the gigapascals interpretation when of simulating the experiment LPwC is conditionscomplicated [18 compared,74]. The setupto that of in a thediff ractometeractual LPwC is shownbecause in of Figure the sample 15, and deformation, the sample arrangement the reflection in Figureof the pressure16 together wave with at a the sample free photographsurface and the taken subsequent after a series propagation of pump-probe of a tensile experiments. stress wave in the sample.

(a) (b)

Figure 15. ConfigurationConfiguration of of the the laser laser pump pump and and the the XFEL XFEL probe probe experiment experiment performed performed at SACLA: at SACLA: (a) (Thea) The setup setup around around the the sample sample and and the the diffractometer diffractometer in in preparation preparation for for ultra-fast ultra-fast X-ray X-ray diffraction diffraction Metals(XRD); 2020, (10b), atxAt FOR the the PEER moment moment REVIEW of of laser la ser pulse pulse irradiation. irradiation. 12 of 19

(a) (b)

FigureFigure 16. 16.The The sample sample configuration: configuration: (a ()a The) The acrylic acrylic plate plate was was placed placed on on the the di diffractometerffractometer with with a tilta tilt angleangle of of 16 16°.;◦.; (b ()b a) magnifiedA magnified view view of of the the sample sample foil foil after after pump-probe pump-probe experiments. experiments. Each Each laser laser pulse pulse locallylocally deforms deforms the the sample, sample, creating creating a conea cone with with a roundeda rounded head. head.

LaserThe XFEL pulses photon from a energy frequency-doubled was adjusted to Nd:YAG 10 keV laser(λ = 0.124 (λ = nm)532 nm)with werean energy irradiated spread onto of 50 the eV. sampleThe X-ray through penetration the 10 mm depth thick into acrylic the sample plate viaalumin a planoconvexum alloy was lens about (f = 15020 µm, mm). considering The pulse the energy, XFEL pulseincident duration angle andof 74°. focusing The XFEL spot foot diameterprint on were the sample about 400was mJ,an ellips 6–8 nse with (the major FWHM), and andminor 1.5 axes mm of 2 respectively,0.7 and 0.2 mm, affording respectively, a peak thus power small density compared of 3–4 to GWthe /lasercm spotand diameter a plasma of pressure 1.5 mm. ofThis 3–4 allowed GPa. collection of diffracted X-ray from the laser-irradiated area only; the data were not contaminated by X-ray from uncompressed area. The diffracted X-ray was recorded using a 2D detector; the MPCCD covered diffraction angles (2θ) from 25 to 40°. After recording, the foil and acrylic plate were shifted by 6 mm; the next laser pulse was to a fresh foil region with different delay time.

4.2. Comparison of Diffraction Patterns before and after Laser Irradiation Figure 17 compares the diffraction patterns of solution heat-treated A6061 aluminum alloy before (pre-shock) and after (post-shock) laser pulse irradiation. Pre-shock diffraction data were acquired for the pristine sample setup shown in Figure 16 (a). Then, a backing plate was set on the sample to avoid the deformation and the Nd:YAG laser was fired, which allowed the pressure wave passed through the foil without reflection at the opposite surface of the foil. Post-shock diffraction data were collected for the same sample after removal of the backing plate. We acquired 100 sets of diffraction data to reduce statistical noise and create a clear image of the post-shock. The diffraction patterns of Al (111) and Al (200) changed from spotty to near-continuous rings, reflecting refinement of the crystal grains or subgrain formation by the impulsive stress wave imparted by the laser pulse [72,73]. The diffraction angles (2θ) did not change significantly, thus matching the angles of the stress- free thin foil. We superimposed the pre-shock (red) and post-shock (green) images so that changes in the diffraction pattern can be easily recognized.

Figure 17. Diffraction pattern of the solution heat-treated A6061 aluminum alloy. The spotty pattern prior to laser irradiation (pre-shock) changed to a quasi-ring pattern (post-shock).

Metals 2020, 10, x FOR PEER REVIEW 12 of 19

Metals 2020, 10, 1490 12 of 19

The impulsive stress wave generated by the high-pressure plasma propagated into the sample as a quasi-plane wave because of the one-dimensional (1D) sample geometry. When the stress wave arrived at the opposite free sample surface, the sample began to macroscopically deform (Figure 16b). Therefore, diffraction data must be collected before the deformation. In general, the material exposed to LPwC (or LSP) should be thick enough to hold the inducing compressive RS; a thickness of 10 times or more the compression depth is desirable to avoid the macroscopic deformation which results in relieving the RS. Nonetheless, in this experiment, a thin foil (0.1 mm thick) was used(a) to observe dynamic phenomena on the opposite(b) free surface of the foil withoutFigure significantly 16. The sample attenuating configuration: the impulsive (a) The acrylic pressure plate wave was placed and the on thermalthe diffractometer effect ofthe with laser a tilt pulse irradiation.angle of 16°.; Therefore, (b) A magnified the interpretation view of the sample of the experimentfoil after pump-probe is complicated experiments. compared Each laser to that pulse in the actuallocally LPwC deforms because the of sample, the sample creating deformation, a cone with thea rounded reflection head. of the pressure wave at the free surface and the subsequent propagation of a tensile stress wave in the sample. The XFEL photon energy was adjusted to 10 keV keV ( (λλ = 0.1240.124 nm) nm) with with an an energy energy spread spread of of 50 50 eV. eV. The X-ray penetration depth into the sample alumin aluminumum alloy was about 20 µm,µm, considering the XFEL incident angle of 7474°.◦. The The XFEL XFEL foot footprintprint on the sample was an ellips ellipsee with major and minor axes of 0.7 and 0.2 mm, respectively, thusthus small compared toto thethe laserlaser spotspot diameter ofof 1.5 mm. This allowed collection of diffracted diffracted X-ray from the laser-irradiatedlaser-irradiated area only; the data were not contaminated by X-ray from uncompressed area. The diffracted diffracted X-ra X-rayy was recorded using a 2D detector; the MPCCD covered didiffractionffraction angles (2θ)) fromfrom 2525 toto 4040°.◦. After recording, the foil and acrylic plate were shifted by 6 mm; the next laser pulse was to a fresh foil region withwith didifferentfferent delaydelay time.time.

4.2. Comparison Comparison of Diffraction Diffraction Patt Patternserns before and after Laser Irradiation Figure 1717 compares compares the the di ffdiffractionraction patterns patterns of solutionof solution heat-treated heat-treated A6061 A6061 aluminum aluminum alloy beforealloy (pre-shock)before (pre-shock) and after and (post-shock) after (post-shock) laser pulse laser irradiation. pulse irradiation. Pre-shock Pre-shock diffraction diffraction data were data acquired were acquiredfor the pristine for the sample pristine setup sample shown setup in shown Figure in 16 Figurea. Then, 16 a(a). backing Then, platea backing was setplate on was the sampleset on the to sampleavoid the to avoid deformation the deformation and the Nd:YAG and the Nd:YAG laser was laser fired, was which fired, allowed which allowed the pressure the pressure wave passed wave passedthrough through the foil withoutthe foil without reflection reflection at the opposite at the oppo surfacesite of surface the foil. of Post-shock the foil. Post-shock diffraction diffraction data were datacollected were for collected the same for sample the same after sample removal after of remo the backingval of the plate. backing We acquiredplate. We 100 acquired sets of di100ff ractionsets of diffractiondata to reduce data statisticalto reduce noisestatistical and createnoise and a clear create image a clear of theimage post-shock. of the post-shock. The diffraction The diffraction patterns patternsof Al (111) of Al and (111) Al (200)and Al changed (200) changed from spotty from tospotty near-continuous to near-continuous rings, reflectingrings, reflecting refinement refinement of the ofcrystal the crystal grains grains or subgrain or subgrain formation formation by the by impulsive the impulsive stress wavestress imparted wave imparted by the laserby the pulse laser [ 72pulse,73]. The[72,73]. diff Theraction diffraction angles angles (2θ) did (2θ not) did change not change significantly, significantly, thus matchingthus matching the angles the angles of the of stress-freethe stress- freethin thin foil. foil. We superimposedWe superimposed the pre-shockthe pre-shock (red) (red) and and post-shock post-shock (green) (green) images images so that so that changes changes in the in thediff ractiondiffraction pattern pattern can can be easily be easily recognized. recognized.

Figure 17. DiffractionDiffraction pattern of the solution heat-treat heat-treateded A6061 aluminum alloy. The spotty pattern prior to laser irradiation (pre-shock) changedchanged toto aa quasi-ringquasi-ring patternpattern (post-shock).(post-shock).

Metals 2020, 10, x FOR PEER REVIEW 13 of 19

4.3. Time-Resolved Ultra-fast XRD at Elapsed Time of 12 and 16 ns The transient diffraction pattern at 12 ns after laser irradiation is shown in Figure 18. The stress distribution in the sample at this time is schematically depicted in Figure 18 (a). It was assumed that the sample surface was exposed to a compressive stress from 0 to 8 ns (the laser pulse duration); the stress wave was generated instantaneously and propagated into the sample at the longitudinal sound velocity in aluminum alloy (6.3 km/s). The diffraction pattern begins to change at 12 ns because the compressive stress wave has just attained a depth of about 20 µm (the X-ray penetration depth) from the opposite free sample surface. Figure 18 (b) compares the initial diffraction pattern and the pattern 12 ns after laser irradiation. The dotted lines indicate the normal diffraction lines for the stress-free sample. The laser irradiation induced a diffuse spot indicated by arrow “S1” at a high angle compared to the normal Al (111) diffraction line. It seems that the impulsive stress wave compressed grains, inducing grain refinement or subgrain formation, and as a result, the diffraction condition of “S1” was satisfied. A superimposition of spots “S2” and “S3” before (red) and after (green) laser irradiation is shown in Figure 18 (c). If a spot is unaffected by laser irradiation, the superimposition of red and green renders the spot yellow. Spot “S2” shifted from an original stress-free position to a higher diffraction angle and became diffuse; the adjacent spot “S3” seemed to be unaffected. This means that the stress wave attained the grain causing spot “S2”and compressed that grain, possibly triggering grain refinement or subgrain formation. The grain corresponding to spot “S3” may have been close to the surface; the compressive stress wave had not arrived there by 12 ns. Assuming that plastic deformation commences at a strain of 0.2%, the corresponding shift in the diffractionMetals 2020, 10 angle, 1490 would be about 0.1°. The observed shift of spot “S2” was about 0.4°, thus much13 of 19 larger than the 0.1° response to the yield stress. This implies that the grain was over-compressed by the impulsive stress wave at 12 ns and relaxed via grain refinement or subgrain formation, creating diffraction4.3. Time-Resolved spot diffusion. Ultra-Fast The XRD shift at of Elapsed “S1” was Time also of 12 about and 160.4°, ns reflecting a similar situation. TheThe diffraction transient di patternffraction at pattern 16 ns after at 12 laser ns after irra laserdiation irradiation is shown is in shown Figure in 19. Figure The 18compressive. The stress stressdistribution wave arrives in the sampleat the free at this sample time surface is schematically at around depicted 16 ns (Figure in Figure 19 (a)).18a. A It typical was assumed diffraction that patternthe sample is shown surface in Figure was exposed 19 (b). toThe a compressivespots indicated stress by arrows from 0 shift to 8 nsto (thehigher laser diffraction pulse duration); angles and the becomestress wave diffuse was at generated 16 ns, similar instantaneously to the changes and note propagatedd in Figure into 18. the Note sample that at the the diffraction longitudinal intensity sound ofvelocity spot in“S5” aluminum decreases, alloy and (6.3 the km /s).intensity The di ffofraction “S6” pattern increases, begins implying to change possible at 12 ns changes because thein misorientationscompressive stress between wave adjacent has just attainedgrains. The a depth shifts of of about “S4”, 20“S5”,µm and (the “S6” X-ray are penetration about 0.3°, depth) thus much from largerthe opposite than the free 0.1° sample response surface. to the 0.2% strain.

(a) (b) (c)

FigureFigure 18. 18. DiffractedDiffracted XFEL XFEL patterns patterns of of the the solution solution heat heat-treated-treated A6061 A6061 aluminum aluminum alloy alloy at at 12 12 ns ns after after laserlaser pulse pulse irradiation: ((aa)) TheThe positionposition of of the the stress stress wave; wave; (b )(b di) ffDiffractionraction patterns patterns before before and and after after laser laserirradiation; irradiation; (c) superimposition (c) Superimposition of di ffofraction diffraction spots spots “S2” and“S2” “S3”. and “S3”.

Figure 18b compares the initial diffraction pattern and the pattern 12 ns after laser irradiation. The dotted lines indicate the normal diffraction lines for the stress-free sample. The laser irradiation

induced a diffuse spot indicated by arrow “S1” at a high angle compared to the normal Al (111) diffraction line. It seems that the impulsive stress wave compressed grains, inducing grain refinement or subgrain formation, and as a result, the diffraction condition of “S1” was satisfied. A superimposition of spots “S2” and “S3” before (red) and after (green) laser irradiation is shown in Figure 18c. If a spot is unaffected by laser irradiation, the superimposition of red and green renders the spot yellow. Spot “S2” shifted from an original stress-free position to a higher diffraction angle and became diffuse; the adjacent spot “S3” seemed to be unaffected. This means that the stress wave attained the grain causing spot “S2”and compressed that grain, possibly triggering grain refinement or subgrain formation. The grain corresponding to spot “S3” may have been close to the surface; the compressive stress wave had not arrived there by 12 ns. Assuming that plastic deformation commences at a strain of 0.2%, the corresponding shift in the diffraction angle would be about 0.1◦. The observed shift of spot “S2” was about 0.4◦, thus much larger than the 0.1◦ response to the yield stress. This implies that the grain was over-compressed by the impulsive stress wave at 12 ns and relaxed via grain refinement or subgrain formation, creating diffraction spot diffusion. The shift of “S1” was also about 0.4◦, reflecting a similar situation. The diffraction pattern at 16 ns after laser irradiation is shown in Figure 19. The compressive stress wave arrives at the free sample surface at around 16 ns (Figure 19a). A typical diffraction pattern is shown in Figure 19b. A superimposition of spots “S5” and “S6” before and after laser irradiation is shown in Figure 19c. The spots indicated by arrows shift to higher diffraction angles and become diffuse at 16 ns, similar to the changes noted in Figure 18. Note that the diffraction intensity of spot “S5” decreases, and the intensity of “S6” increases, implying possible changes in misorientations between Metals 2020, 10, 1490 14 of 19

adjacent grains. The shifts of “S4”, “S5”, and “S6” are about 0.3 , thus much larger than the 0.1 Metals 2020, 10, x FOR PEER REVIEW ◦ 14 of 19◦ response to the 0.2% strain.

(a) (b) (c)

Figure 19. Diffracted XFEL patterns of the solution heat-treated A6061 aluminum alloy at 16 ns after Figure 19. Diffracted XFEL patterns of the solution heat-treated A6061 aluminum alloy at 16 ns. laser pulse irradiation: (a) The position of the stress wave; (b) diffraction patterns before and after laser c 4.4. Effectirradiation; of Laser ( Irradiation) superimposition on Diffraction of diffraction Patterns spots to “S5”1 µs and “S6”. 4.4.The Effect typical of Laser diffraction Irradiation patterns on Diffraction to 1 µs Patterns are summarized to 1 µs in Figure 20, which shows the changes after laserThe typicalirradiation. diffraction The insets patterns for 12, to 16, 1 µ 20,s are and summarized 30 ns show the in Figurepositions 20, of which the laser-induced shows the changes stress wave.after laser The irradiation.enlarged images The insets show for certain 12, 16, diffraction 20, and 30 sp nsots show that the clearly positions reveal of the the changes. laser-induced The stress stress wavewave. arrives The enlarged at the sample images surface show certainat around diff 16raction ns, reflects, spots that and clearly then travels reveal as the a changes.tensile stress The wave stress towardwave arrives the laser-irradiated at the sample surface. surface The at around tensile 16 co ns,mponent reflects, traveling and then downward travels as is a tensilepartially stress canceled wave bytoward the initial the laser-irradiated compressive surface.stress wave The tensile traveling component upward traveling to about downward 20 ns. Then, is partially the stress canceled wave by propagatesthe initial compressive downward stressas a tensile wave traveling wave. Therefore, upward to the about diffraction 20 ns. Then, angle the shifts stress become wave propagates small or negligibledownward at as about a tensile 20 ns wave. and later. Therefore, the diffraction angle shifts become small or negligible at about 20 nsThe and overall later. change in the diffraction pattern seems to commence at about 30 ns and to be completeThe overallwithin 1 change µs, probably in the di atff ractionabout 100 pattern ns, implying seems to commencethat the durations at about of 30 dynamic ns and to phenomena be complete arewithin typically 1 µs, probablyless than at1 µs. about Therefore, 100 ns, implyingthe plasti thatc deformation the durations induced of dynamic by the phenomena succeeding arelaser typically pulse seemsless than to be 1 µindependents. Therefore, to the that plastic of the deformation previous laser induced pulse even by the at succeedingthe pulse repetition laser pulse rate seems of 1 MHz, to be whichindependent is much to thathigher of thethan previous the maximum laser pulse pulse even repeti at thetion pulse rate repetition of 300 Hz rate [75] of 1 ever MHz, used which in isactual much LPwChigher applications; than the maximum this suggests pulse repetitionthe potential rate to of significantly 300 Hz [75] everincrease used the in actualpulse repetition LPwC applications; rate and throughputthis suggests of the LPwC. potential to significantly increase the pulse repetition rate and throughput of LPwC. TheThe Al Al (200) (200) and and Al Al (111) (111) diffraction diffraction angles angles at at 1 1 µsµs seem seem to to be be much much greater greater than than those those of of the the stress-freestress-free sample. sample. This This is is probably probably attributable attributable to the macroscopic deformationdeformation shownshown inin FigureFigure 16 16b. (b).Such Such sample sample deformation deformation can can shift shift the ditheffraction diffract centerion center upstream upstream of the of XFEL the XFEL by a maximumby a maximum of about of about 1 mm, resulting in a false shift of about 0.2 to 0.3° in the diffraction angle, corresponding well 1 mm, resulting in a false shift of about 0.2 to 0.3◦ in the diffraction angle, corresponding well with the withobserved the observed shifts at shifts 1 µs. at 1 µs. A spot corresponding to Mg5Si6 is evident at 20 ns. This precipitate is to be expected in A6061 A spot corresponding to Mg5Si6 is evident at 20 ns. This precipitate is to be expected in A6061 aluminumaluminum alloy. alloy. It It is is possible possible that that the the impulsive impulsive stress stress waves waves induced induced the the dynamic dynamic precipitation precipitation of of Mg5Si6, but other possibilities include rotation of a crystal grain to satisfy the diffraction conditions. Mg5Si6, but other possibilities include rotation of a crystal grain to satisfy the diffraction conditions. DetailedDetailed experiments experiments are are required required for for further further discussion. discussion.

Metals 2020, 10, 1490 15 of 19 Metals 2020, 10, x FOR PEER REVIEW 15 of 19

Figure 20. The diffraction patterns of solution heat-treated A6061 aluminum alloy to 1 µs. The insets Figure 20. The diffraction patterns of solution heat-treated A6061 aluminum alloy to 1 µs. The insets for 12, 16, 20, and 30 ns schematically show the position of the laser-induced stress wave in the 0.1 mm for 12, 16, 20, and 30 ns schematically show the position of the laser-induced stress wave in the 0.1 thick sample. The enlarged images show the changes in the diffraction spots. mm thick sample. The enlarged images show the changes in the diffraction spots. 5. Conclusions 5. Conclusions LPwC is a surface enhancement technology; SR and XFEL are essential for microscopic analysis, and wereLPwC indispensable is a surface enhancement over the course technology; of LPwC SR development and XFEL are becauseessential they for microscopic explain the analysis, relevant phenomenaand were indispensable precisely. In over particular, the course XFEL of renders LPwC development it possible to observebecause thethey dynamic explain behaviorsthe relevant of phenomenamaterials under precisely. LPwC In conditions, particular, revealing XFEL renders the mechanism it possible thereof to observe and facilitating the dynamic the developmentbehaviors of materialsof novel processes. under LPwC We conditions, have used revealing SR and XFEL the me tochanism explore thethereof mechanism and facilitating and eff ectsthe development of LPwC for ofnearly novel 20 processes. years [76 ].We XRD have experiments used SR and employing XFEL to explore SR spatially the mechanism resolved the and strain effects and of allowed LPwC for RS nearlymapping 20 showingyears [76]. that, XRD after experiments LPwC, the topemploying surface wasSR spatially compressive resolved [28]. the Using strain these and results allowed and RS an mappingempirical showing model, we that, explained after LPwC, how the LPwC top surface induced was compressive compressive RSs [28]. on theUsing top these surface results [26]. and Here, an weempirical describe model, how we we explained visualized how fatigue LPwC cracks induced with compressive the aid of SR-based RSs on the CT top and surface CL, and [26]. how Here, we performedwe describe time-resolved how we visualized XRD using fatigue an XFEL.cracks The with results the aid can of be SR-based summarized CT asand follows. CL, and how we performedThe eff time-resolvedect of LPwC on XRD fatigue using crack an XFEL. growth The in results AC4CH can cast be aluminumsummarized alloy as follows. was studied using SR-basedThe effect CT at of SPring-8. LPwC on Edge fatigue enhancement crack growth created in AC4CH by phase-contrast cast aluminum was apparentalloy was and studied 3D images using SR-basedof the fatigue CT at cracks SPring-8. were Edge reconstructed. enhancement The created fatigue by cracks phase-contrast were visualized was apparent non-destructively and 3D images [27]. ofCrack the fatigue growth cracks retardation were recons after LPwCtructed. was The evident fatigue not cracks only were on the visualized sample surface,non-destructively but also in [27]. the Crackinterior. growth The results retardation agreed after well LPwC with surface was evident observations not only employing on the sample optical surface, microscopy but [also28]. in the interior.SR-based The results CL at agreed SPring-8 well was with used surface to visualize observations fatigue cracksemploying in FSWed optical A6061-T6 microscopy aluminum [28]. alloy joints.SR-based We thus CL confirmed at SPring-8 that was CL canused be to used visualize to evaluate fatigue laterally cracks extendedin FSWed objects. A6061-T6 Crack aluminum images werealloy successfullyjoints. We thus reconstructed confirmed andthat agreed CL can well be withused theto evaluate actual fracture laterally surfaces. extended CL canobjects. reproduce Crack imagesthe 3D shapeswere successfully of fatigue cracks reconstructed in FSWed and joints agreed in a nondestructive well with the manner.actual fracture The propagation surfaces. CL of such can reproduce the 3D shapes of fatigue cracks in FSWed joints in a nondestructive manner. The propagation of such cracks can be evaluated via intermittent CL during fatigue testing. This provides

Metals 2020, 10, 1490 16 of 19 cracks can be evaluated via intermittent CL during fatigue testing. This provides crucial information for optimizing materials and processing of FSW, yielding higher-quality FSWed joints that are durable. We performed time-resolved XRD to observe the time evolution of strain and microstructural changes in the A6061 aluminum alloy. We delivered laser pulse irradiation during pump-probe experiments in the Japanese XFEL facility, SACLA, and observed phenomena suggesting grain refinement, subgrain formation, and precipitation [73]; these are closely related to the mechanism of LPwC. We used thin foil samples and collected diffracted X-rays of XFEL single pulses at the free surface opposite the laser irradiation. Physical interpretation of the results was not easy because of stress wave reflection at the free surface and sample deformation. However, we observed over-compression and the diffusion of diffraction spots caused by the impulsive stress wave. We estimate that the duration of the dynamic phenomenon is less than 1 µs, which may render it possible to use high-repetition lasers beyond kHz to reduce LPwC processing time. This would reinvigorate LPwC by removing the main obstacle to extension of LPwC applications.

Author Contributions: Conceptualization, Y.S.; Methodology, Y.S., K.M., K.A., K.K. and T.S.; Investigation, Y.S., K.M., K.A., K.K. and T.S.; Resources, Y.S., K.M., K.A., K.K. and T.S.; Writing—Original Draft Preparation, Y.S.; Writing—Review and Editing, Y.S., K.M., K.A., K.K. and T.S.; Supervision, Y.S. and T.S. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: The authors thank the members of the SACLA engineering team especially Y. Inubushi (JASRI) and T. Sato (RIKEN, currently at SLAC) for their vital support of our XFEL experiments. The synchrotron radiation experiments were performed at the BL19B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2011A1685, 2011B1861, 2012A1274, 2012B1740, 2013B1863, 2014A1700, and 2014B1927). The XFEL experiments were performed at the BL3 [EH2] and [EH3] of SACLA with the approval of JASRI (Proposal Nos. 2012A8012, 2012B8011, 2013A8025, 2013B8029, 2014A8015, and 2014B8016). Conflicts of Interest: The authors declare no conflict of interest.

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