Laser Cavitation Peening and Its Application for Improving the Fatigue Strength of Welded Parts

metals

Article Laser Cavitation Peening and Its Application for Improving the Fatigue Strength of Welded Parts

Hitoshi Soyama

Department of Finemechanics, Tohoku University, Sendai 980-8579, Japan; [email protected]; Tel.: +81-22-795-6891; Fax: +81-22-795-3758

Abstract: During conventional submerged laser peening, the impact force induced by laser ablation is used to produce local plastic deformation pits to enhance metallic material properties, such as fatigue performance. However, a bubble, which behaves like a cavitation, is generated after laser ablation, known as “laser cavitation.” On the contrary, in conventional cavitation peening, cavitation is generated by injecting a high-speed water jet into the water, and the impacts of cavitation collapses are utilized for mechanical surface treatment. In the present paper, a mechanical surface treatment mechanism using laser cavitation impact, i.e., “laser cavitation peening”, was investigated, and an improvement in fatigue strength from laser cavitation peening was demonstrated. The impact forces induced by laser ablation and laser cavitation collapse were evaluated with a polyvinylidene fluoride (PVDF) sensor and a submerged shockwave sensor, and the diameter of the laser cavitation was measured by observing a high-speed video taken with a camera. It was revealed that the impact of laser cavitation collapse was larger than that of laser ablation, and the peening effect was closely related to the volume of laser cavitation. Laser cavitation peening improved the fatigue strength of stainless-steel welds. Keywords: surface modification; laser peening; cavitation peening; welding; fatigue; stainless steel Citation: Soyama, H. Laser Cavitation Peening and Its Application for Improving the Fatigue Strength of Welded Parts. Metals 2021, 11, 531. https://doi.org/ 1. Introduction 10.3390/met11040531 The weld points between metallic material components are the weakest in terms of fatigue properties [1], as welding causes residual tensile stress [2–4]. It has been reported Academic Editor: Krzysztof Rokosz that the fatigue strength of the heat-affected zone (HAZ) is higher than that of base metal in some cases; however, the effect not only occurs at the microstructure of the HAZ, but causes Received: 25 February 2021 welding defect and stress concentration [5]. Mechanical surface treatment such as peening Accepted: 22 March 2021 can improve the fatigue properties by introducing compressive residual stress and welding Published: 24 March 2021 line deformation. Thus, it is worthwhile developing a new peening technique, such as laser peening, which can control peening area, to enhance the fatigue life and strength of welds, Publisher’s Note: MDPI stays neutral as the peening area required is limited to near the HAZ region. with regard to jurisdictional claims in Ultrasonic impact peening [6,7], hammer peening, and burnishing [8] are applicable published maps and institutional affil- for the post-processing of welds to reduce tensile residual stress and notch effects in iations. welds. Water jet peening also reduces the residual stress of 304 stainless steel welds [9]. Improvement in stainless steel weld fatigue strength using a water jet peening method has been reported [10]. The fatigue life of 316L stainless steel welds treated by water jet peening using a 300 MPa water jet, cavitation peening using a 30 MPa submerged Copyright: © 2021 by the author. jet, and shot peening using shots accelerated by an 8 MPa water jet was compared in a Licensee MDPI, Basel, Switzerland. previous paper [10]. Water jet peening refers to a peening method using a water column’s This article is an open access article impingement impact using a water jet, and cavitation peening refers to a peening method distributed under the terms and using the cavitation bubble collapse impact [11,12]. Note that cavitation is still a big conditions of the Creative Commons problem for hydraulic machinery, as cavitation impacts produce erosion [13]. However, Attribution (CC BY) license (https:// cavitation impacts can be utilized for peening in the same way as shot peening. The great creativecommons.org/licenses/by/ advantage of cavitation peening is lower surface roughness, as no shots are used [14]. 4.0/).

Metals 2021, 11, 531. https://doi.org/10.3390/met11040531 https://www.mdpi.com/journal/metals Metals 2021, 11, 531 2 of 14

During conventional cavitation peening, a submerged high-speed water jet with cavitation, i.e., a cavitating jet [15,16], has been used to generate cavitation, a kind of “hydrodynamic cavitation.” Soyama created a cavitating jet in air by injecting a high-speed water jet into a low-speed water jet injected into air, using a concentric nozzle [17,18]; research on the cavitating jet in air was followed by Marcon et al. [19,20]. It has been reported that the cavitating jet in air improves the fatigue properties of friction stir welded aluminum alloy [21]. Information about peening using ultrasonic cavitation can be found in the review by Soyama [12]. There are three types of laser peening; peening methods using a pulse laser: water film types [22–25], in which a target is covered with a water film, a pulse laser is irradiated, and a shockwave induced by laser ablation is trapped by the water’s inertial force, caus- ing plastic deformation of the target; submerged laser peening [26,27], in which the target is placed in water, which has been applied to mitigate stress corrosion cracking in nuclear power plants [28]; a laser peening method without a sacrificial overlay under atmospheric conditions using a femtosecond laser [29]. Submerged laser peening produces a bubble, which behaves like a cavitation bubble, after laser ablation and produces a bubble collapse impact [11,30]. In the present paper, the bubble caused by the pulse laser is called “laser cavitation.” In the case of submerged laser peening, it is believed that the peening process occurs due to laser ablation [27], as the amplitude of the pressure wave induced by laser ablation is bigger than that of laser cavitation, when the pressure wave in water is measured by a submerged shockwave sensor [30]. However, the impact generated by laser cavitation is larger than that of laser ablation [11], when the impact through in the target is measured by a polyvinylidene fluoride (PVDF) sensor, originally developed to evaluate cavitation impacts [31,32]. Namely, in the case of submerged laser peening, one pulse laser can hit twice, i.e., laser ablation and laser cavitation. When conditions are optimized, the impact of laser cavitation is larger than that of laser ablation [11]. As the impact force direction of cavitation can be controlled by the boundary condition [33], the impact of laser cavitation at submerged laser peening can be removed by setting a boundary wall around laser cavitation. When the peening intensity of the submerged laser peening was measured in a previous study, the peening intensity of the submerged laser peening without laser cavitation impact was nearly half of that with laser cavitation impact [34]. On the contrary, submerged laser peening can use laser cavitation impact without laser ablation by focusing water near the target [35]. It has been reported that laser cavitation without laser ablation introduces compressive residual stress into austenitic stainless steel SUS316L [36]. Thus, optimized submerged laser peening is a kind of cavitation peening that uses laser cavitation. In the present paper, cavitation peening using laser cavitation is referred to as “laser cavitation peening”. During conventional submerged laser peening, the second harmonic wavelength of a Q-switched Nd:YAG laser, i.e., 532 nm, is used to minimize laser absorption by water. An Nd:YAG laser’s fundamental harmonic wavelength, i.e., 1064 nm, has been used to investigate bubble behavior during cavitation generation [37], as the heat is more concentrated. Note that 40% of the pulse energy at 1064 nm is lost to obtain 532 nm during wavelength conversion. When using the impact of laser cavitation, it should be considered which wavelength is better, 532 nm or 1064 nm, by considering the absorption of the laser by water and the energy loss due to wavelength conversion. This paper consists of two parts. The first demonstrates the importance of laser cavitation during submerged laser peening, i.e., laser cavitation peening, considering the relation between the size of the laser cavitation and the peening intensity using wavelengths of 532 and 1064 nm of the Nd:YAG laser. The second demonstrates the improvement of weld fatigue properties by laser cavitation peening. Metals 2021, 11, x FOR PEER REVIEW 3 of 15 Metals 2021, 11, 531 3 of 14

2. Laser Cavitation Peening 2. Laser Cavitation Peening 2.1. Experimental Apparatus and Procedures to Compare Laser Ablation and Laser Cavitation 2.1. Experimental Apparatus and Procedures to Compare Laser Ablation and Laser Cavitation Figure 1 illustrates a schematic diagram of the submerged laser peening system used to investigate peeningFigure intensity.1 illustrates The a schematicused laser diagram pulse source of the (Continuum, submerged laser San peeningJose, CA, system used USA) was a toQ-switched investigate Nd:YAG peening laser intensity. with Thewavelength used laser conversion. pulse source By (Continuum,controlling the San Jose, CA, USA) was a Q-switched Nd:YAG laser with wavelength conversion. By controlling the wavelength conversion, both the 532 nm and 1064 nm wavelengths could be obtained. wavelength conversion, both the 532 nm and 1064 nm wavelengths could be obtained. The pulse width, the beam diameter, and the laser pulse’s repetition frequency were 6 ns, The pulse width, the beam diameter, and the laser pulse’s repetition frequency were 6 ns, 6 mm, and 10 Hz, respectively. The maximum energy was 0.2 J at 532 nm and 0.35 J at 6 mm, and 10 Hz, respectively. The maximum energy was 0.2 J at 532 nm and 0.35 J at 1064 nm. Using reflective mirrors, the pulse laser was expanded by a concave lens and 1064 nm. Using reflective mirrors, the pulse laser was expanded by a concave lens and focused onto the specimen placed in a water-filled chamber made of 3 mm thick hard focused onto the specimen placed in a water-filled chamber made of 3 mm thick hard glass using convex lenses to avoid damage to the glass of the chamber. The focal length of glass using convex lenses to avoid damage to the glass of the chamber. The focal length the final convex lens was 100 mm. The distance in air (sa) from the final convex lens to the of the final convex lens was 100 mm. The distance in air (sa) from the final convex lens glass and the distance in water (sw) from the glass to the target were optimized to enhance to the glass and the distance in water (sw) from the glass to the target were optimized to the peening intensity. A handmade PVDF sensor [31,32] was installed between the target enhance the peening intensity. A handmade PVDF sensor [31,32] was installed between the and the holder to measure the impact passing through the specimen. The specimen was target and the holder to measure the impact passing through the specimen. The specimen tightened towas the holder tightened with to the the PVDF holder sensor with by the screws. PVDF The sensor PVDF by screws.sensor was The not PVDF cali- sensor was brated to revealnot calibratedthe impact to between reveal the laser impact ablation between and laser laser cavitation ablation and collapse. laser cavitationA sub- collapse. merged shockwaveA submerged sensor shockwave (Dr. Müller sensor Instruments (Dr. Müller GmbH Instruments & Co. KG, GmbH Oberursel, & Co. Ger- KG, Oberursel, many) was alsoGermany) placed wasin the also chamber placed to in eval the chamberuate the pressure to evaluate wave the propagation pressure wave in the propagation in water. A high-speedthe water. video A high-speed camera observed video th camerae laser observed ablation and the laser laser cavitation ablation and aspects laser cavitation induced by theaspects pulse induced laser at by230,000 the pulse frames laser per atsecond. 230,000 The frames laser percavitation’s second. maximum The laser cavitation’s diameter wasmaximum evaluated diameter by measuring was evaluated the bubbl bye size’s measuring image therecorded bubble by size’s the high-speed image recorded by the video camera. The developing time of laser cavitation (tD) was measured using the sen- high-speed video camera. The developing time of laser cavitation (tD) was measured using sor’s signal recordedthe sensor’s by signala digital recorded oscilloscope by a digital (Tektronix, oscilloscope Inc., Beaverton, (Tektronix, OR, Inc., USA). Beaverton, OR, USA).

Figure 1. SchematicFigure diagram 1. Schematic of the diagram submerged of the laser submerged peening system.laser peening PVDF, system. polyvinylidene PVDF, polyvinylidene ﬂuoride; sa, distance fluo- in air; sw, distance in water.ride; sa, distance in air; sw, distance in water.

An Almen gageAn Almen of N strip gage [38] of Nand strip a plat [38]e andspecimen a plate made specimen of Japanese made ofIndustrial Japanese Industrial Standards JISStandards SUS316L JISstainless SUS316L steel stainless and A201 steel7-T3 and duralumin A2017-T3 were duralumin treated were and had treated their and had their arc height measuredarc height by measured a test strip by gage a test [3 strip8], and gage were [38 ],then and used were to then evaluate used topeening evaluate peening × × intensity by intensitylaser cavitation by laser peening. cavitation The peening. size of the The tested size of materials the tested was materials 50 × 20 was × 3 50mm 20 3 mm for stainless steel, 50 × 20 × 5 mm for duralumin, and 76 × 19 × 0.8 mm for the N strip. for stainless steel, 50 × 20 × 5 mm for duralumin, and 76 × 19 × 0.8 mm for the N strip. At At the impact measurement, one or two specimens with changing laser powers were used the impact measurement, one or two specimens with changing laser powers were used for each material with a moving specimen so that the pulses did not overlap. At the arc for each material with a moving specimen so that the pulses did not overlap. At the arc height measurement, six to seven specimens were used for each material with changing height measurement, six to seven specimens were used for each material with changing laser power. The N strip or the plate specimen was placed on a stage with stepping laser power. The N strip or the plate specimen was placed on a stage with stepping motors motors and moved orthogonally with respect to the laser beam. The pulse density (dL) was

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and moved orthogonally with respect to the laser beam. The pulse density (dL) was calcu- calculatedlated from fromthe speed the speed (vs) and (vs )stepwise and stepwise distance distance (dS) of ( dtheS) ofstepping the stepping motor, motor, and the and repeti- the repetitiontion frequency frequency was 10 was Hz, 10 as Hz, seen as in seen Equation in Equation (1). (1). 10 = 10 n (1) dL = ∙ (1) vs·dS where n is the number of scans. where n is the number of scans.

2.2.2.2. ResultsResults andand DiscussionsDiscussions ComparingComparing withwith LaserLaser AblationAblation andand LaserLaser CavitationCavitation FiguresFigures2 2and and3 3show show laser laser ablation ablation (LA) (LA) and and laser laser cavitation cavitation (LC) (LC) produced produced by by pulse pulse laserslasers atat λ = 532 532 nm nm and and 1064 1064 nm. nm. Each Each figure ﬁgure show showss (a) (a)LA LAand and LC changing LC changing withwith time time(t), (b) (t ),signal (b) signal from fromthe PVDF the PVDF sensor, sensor, and (c) and signal (c) signalfrom the from submerged the submerged shockwave shockwave sensor. sensor.At both Atλ = both 532 λand= 5321064 and nm, 1064 LC developed nm, LC developed after LA afterand reached LA and maximum reached maximum size at t = size0.48 atmst for= 0.48 λ =ms 532for nmλ and= 532 t = nm0.50 and ms fort = λ 0.50 =1064 ms nm, for λthen=1064 it collapsed nm, then at it t collapsed= 0.96 ms for at tλ= = 0.96 532 msnm forandλ t= = 5321.00 nm ms andfor λt = 1064 1.00 msnm, for respectively.λ = 1064 nm, At respectively.both λ = 532 and At both1064 λnm,= 532 the andamplitude1064 nm of, the the PVDF amplitude sensor of signal the PVDF and th sensore submerged signal and shockwave the submerged sensor had shockwave peaks at sensorLA and had the peaks first atLC LA collapse. and the As ﬁrst shown LC collapse. in Figures As shown2b and in 3b, Figures the PVDF2b and sensor3b, the detected PVDF sensorthe second detected LC collapse the second at t LC = 1.49 collapse ms for at tλ= = 1.49 532 msnm for andλ =t 532= 1.52 nm ms and fort =λ 1.52= 1064 ms nm, for λrespectively,= 1064 nm, respectively,although the although submerged the shockwav submergede sensor shockwave could sensor not detect could the not second detect thecol- secondlapse of collapse LC. It can of LC.therefore It can be therefore concluded be concluded that the PVDF that sensor the PVDF could sensor evaluate could the evaluate impact themore impact accurately more accuratelythan the submerged than the submerged shockwave shockwave sensor. sensor.

FigureFigure 2.2.Laser Laser ablationablation (LA)(LA) andand laserlaser cavitationcavitation (LC)(LC) producedproduced byby pulsepulse laserlaser ofofλ λ= =532 532nm. nm. ((aa)) LALA andand LC LC observed observed throughthrough neutral neutral density density filter;filter; ((bb)) signalsignal fromfrom aa polyvinylidenepolyvinylidene fluoridefluoride (PVDF)(PVDF) sensor;sensor; ((cc)) signalsignal fromfrom submergedsubmerged shock-shock- wavewave sensor. sensor. Note that the noise levels of λ = 532 and 1064 nm were slightly different, as the specimen and PVDF sensor were tightened to the holder by screws. The other important result was that the amplitude of the signal from the PVDF sensor at the first LC collapse was larger than that of LA, although the amplitude of the signal from the submerged shockwave sensor at LA was larger than that of the first LC collapse, in agreement with [30]. As mentioned above, as the PVDF sensor could detect the impact more accurately than the submerged shockwave sensor, the first LC collapse was larger than that of LA. Namely, with optimized submerged laser peening, the impact induced by the first LC collapse was more effective than that of LA. The vibration induced by the bubble collapse did not reduce the peening intensity, as the vibration frequency induced by the bubble collapse was much higher than the repetition of the laser.

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Figure 3. LA and LC produced by a pulse laser at λ = 1064 nm. (a) LA and LC; (b) signal from a PVDF sensor; (c) signal Figure 3. LA and LC produced by a pulse laser at λ = 1064 nm. (a) LA and LC; (b) signal from a PVDF sensor; (c) signal from submerged shockwave sensor. from submerged shockwave sensor. Revealing a key parameter of LC, Figure4 illustrates the relationship between the Note that the noise levels of λ = 532 and 1064 nm were slightly different, as the spec- developing time of LC (tD)(µs), and the maximum diameter of LC (dmax) (mm) induced imen and PVDF sensor were tightened to the holder by screws. The other important result by the pulse laser. As the bubble was developed and then shrunk, dmax was deﬁned by thewas diameter that the amplitude which had of a maximumthe signal withfrom thethe time.PVDF In sensor Figure at4 ,the “hemispherical first LC collapse bubble” was referslarger tothan the that hemispherical of LA, although bubble the induced amplit byude the of pulse the signal laser thatfrom irradiated the submerged the target shock- and developed,wave sensor as at shown LA was in larger Figures than2a andthat3 a.of Thethe first other LC symbols collapse, show in agreement the spherical with bubbles [30]. As producedmentioned by above, the pulse as the laser, PVDF which sensor was could focused detect in water the impact without more the target accurately changing than with the watersubmerged depth. shockwave Ion-exchanged sensor, water the wasfirst used.LC coll Theapse air was content larger ratio than and that the of temperature LA. Namely, of Metals 2021, 11, x FOR PEER REVIEWthewith test optimized water were submerged controlled laser by peening, degassing the membrane impact induced and a chillerby the tofirst examine LC collapse the6 waterof was 15

quality.more effective All symbols than that were of almost LA. The on vibration the line, as induced calculated by the by Equationbubble collapse (2). did not reduce the peening intensity, as the vibration frequency induced by the bubble collapse was much higher than the repetition of thedmax laser.= 0.0103 tD (2) Revealing a key parameter of LC, Figure 4 illustrates the relationship between the developing time of LC (tD) (μs), and the maximum diameter of LC (dmax) (mm) induced by the pulse laser. As the bubble was developed and then shrunk, dmax was defined by the diameter which had a maximum with the time. In Figure 4, “hemispherical bubble” refers to the hemispherical bubble induced by the pulse laser that irradiated the target and developed, as shown in Figures 2a and 3a. The other symbols show the spherical bubbles produced by the pulse laser, which was focused in water without the target changing with water depth. Ion-exchanged water was used. The air content ratio and the temperature of the test water were controlled by degassing membrane and a chiller to examine the water quality. All symbols were almost on the line, as calculated by Equation (2).

= 0.0103 (2) It could be concluded that the maximum diameter of LC was proportional to the LC development time (tD), which was defined by the time from laser ablation to the first LC collapse. Thus, in the present paper, tD was used as the main parameter of LC.

FigureFigure 4. 4. RelationshipRelationship between between the the developing developing time time ttDD andand the the maximum maximum diameter diameter ddmaxmax ofof LC. LC. It could be concluded that the maximum diameter of LC was proportional to the LC In order to compare capability to generate the laser cavitation of the pulse laser using development time (tD), which was deﬁned by the time from laser ablation to the ﬁrst LC λ = 532 nm and 1064 nm, Figure 5 shows tD as a function of the relative energy of the collapse. Thus, in the present paper, tD was used as the main parameter of LC. source laser (EL); the tD of λ = 1064 nm was larger than that of λ = 532 nm. Namely, the pulse laser using λ = 1064 nm could generate a larger LC, as dmax can be calculated by tD. As larger cavitations generate larger impacts [39–42], a wavelength of 1064 nm should be better than 532 nm from an LC generation perspective. Thus, λ = 1064 nm was chosen for the following experiment. At EL > 0.8, tD of λ = 1064 nm was slightly lower than that of λ = 532 nm, as the small bubble was generated near the focal point due to heat convergence, and it could be reduced by optimizing the focal length of the final convex lens.

Figure 5. Developing time of laser cavitation as a function of the relative energy of the source laser.

The effects of the signals from the PVDF sensor and the submerged shockwave sensor on peening intensity were investigated. Figure 6 illustrates the relationship between arc height h and amplitude of the signal from (a) the PVDF sensor and (b) the submerged shockwave sensor. Table 1 shows the correlation coefficient between the arc height and output signal from sensors. The specimens were treated with 4 pulses/mm2, and the tape of the N strip refers to black aluminum tape with a thickness of 80 μm, as black aluminum tape has been used for conventional laser peening with a water film to protect the surface. As shown in Figure 6a, both the LA and LC signals from the PVDF sensor were proportional to the arc height. As shown in Table 1, the correlation coefficient of the signal from the PVDF sensor was larger than that of the submerged shockwave sensor, except for the

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Figure 4. Relationship between the developing time tD and the maximum diameter dmax of LC.

InIn order order to to compare compare capability capability to generate to generate the thelaser laser cavitation cavitation of the of pulse the pulse laser laserusing using λ = 532 nm and 1064 nm, Figure 5 shows tD as a function of the relative energy of the λ = 532 nm and 1064 nm, Figure5 shows tD as a function of the relative energy of the source source laser (EL); the tD of λ = 1064 nm was larger than that of λ = 532 nm. Namely, the laser (EL); the tD of λ = 1064 nm was larger than that of λ = 532 nm. Namely, the pulse laser pulse laser using λ = 1064 nm could generate a larger LC, as dmax can be calculated by tD. using λ = 1064 nm could generate a larger LC, as dmax can be calculated by t . As larger As larger cavitations generate larger impacts [39–42], a wavelength of 1064 nm shouldD be cavitations generate larger impacts [39–42], a wavelength of 1064 nm should be better than better than 532 nm from an LC generation perspective. Thus, λ = 1064 nm was chosen for 532 nm from an LC generation perspective. Thus, λ = 1064 nm was chosen for the following the following experiment. At EL > 0.8, tD of λ = 1064 nm was slightly lower than that of λ = experiment. At E > 0.8, t of λ = 1064 nm was slightly lower than that of λ = 532 nm, 532 nm, as the smallL bubble wasD generated near the focal point due to heat convergence, as the small bubble was generated near the focal point due to heat convergence, and it and it could be reduced by optimizing the focal length of the final convex lens. could be reduced by optimizing the focal length of the ﬁnal convex lens.

FigureFigure 5. 5. DevelopingDeveloping time time of oflaser laser cavitation cavitation as a as func a functiontion of the of relative the relative energy energy of the of source the source la- laser. ser. The effects of the signals from the PVDF sensor and the submerged shockwave sensor on peeningThe effects intensity of the signals were investigated.from the PVDF Figure sensor6 illustratesand the submerged the relationship shockwave between sen- arc sorheight on peeningh and amplitudeintensity were of theinvestigated. signal from Figure (a) the6 illustrates PVDF sensor the relationship and (b) the between submerged arcshockwave height h and sensor. amplitude Table 1of shows the signal the from correlation (a) the PVDF coefficient sensor between and (b) the the submerged arc height and shockwaveoutput signal sensor. from Table sensors. 1 shows The the specimens correlation were coefficient treated between with 4 the pulses/mm arc height2 ,and and the outputtape of signal the Nfrom strip sensors. refers The to blackspecimens aluminum were treated tape withwith a4 pulses/mm thickness of2, and 80 µthem, tape asblack ofaluminum the N strip tape refers has to beenblack usedaluminum for conventional tape with a thickness laser peening of 80 μ withm, as a black water aluminum film to protect tapethe surface.has been Asused shown for conventional in Figure6a, laser both peening the LA andwith LC a water signals film from to protect the PVDF the surface. sensorwere Asproportional shown in Figure to the 6a, arc both height. the AsLA shownand LC in signals Table 1from, the the correlation PVDF sensor coefficient were ofpropor- the signal tionalfrom to the the PVDF arc height. sensor As was shown larger in than Table that 1, the of thecorrelation submerged coefficient shockwave of the sensor, signal from except for thethe PVDF N strip sensor with was tape, larger and than the that averaged of the submerged value of the shockwave PVDF sensor sensor, was except larger for than the that of the submerged shockwave sensor. It can be concluded that the signal from the PVDF sensor reveals the peening effect.

Table 1. Correlation coefﬁcient between the arc height and output signal from the sensors.

PVDF Sensor Submerge Shockwave Sensor Materials LA LC LA LC Duralumin 0.961 0.982 0.913 0.299 Stainless steel 0.925 0.947 0.881 0.381 N strip with tape 0.927 0.892 0.933 0.130 N strip without tape 0.983 0.963 0.937 0.146 Averaged value 0.95 ± 0.03 0.95 ± 0.04 0.92 ± 0.03 0.24 ± 0.12

In order to investigate the effect of LC on the peening intensity, Figure7a shows the relationship between the developing time of LC (tD) and the arc height (h). As calculated by Equation (2), the tD was proportional to the diameter of the LC. The specimens were treated with 4 pulses/mm2. When the following power law was assumed as the relationship between tD and h as shown in Equation (3), the exponent n was 3.3 for duralumin, 4.1 for stainless steel, 6.4 for the N strip without tape, and 7.0 for the N strip with tape.

n h ∝ tD (3) Metals 2021, 11, x FOR PEER REVIEW 7 of 15

Metals 2021, 11, 531 N strip with tape, and the averaged value of the PVDF sensor was larger than that of7 ofthe 14 submerged shockwave sensor. It can be concluded that the signal from the PVDF sensor reveals the peening effect.

Figure 6. 6. RelationshipRelationship between between the the arc arc he heightight and and amplitude amplitude of the of theoutp outputut signal signal from from sensors. sensors. (a) PVDF(a) PVDF sensor; sensor; (b) (submergedb) submerged shockwave shockwave sensor. sensor. It has been reported that the aggressive intensity of cavitation is proportional to the Table 1. Correlation coefficient between the arc height and output signal from the sensors. volume of cavitation [15,40,42] and the threshold level of the erosion caused by cavitation impact [43]. Therefore, EquationPVDF (4) was Sensor assumed. Submerge Shockwave Sensor Materials LA LC LA LC h ∝ (t − t )3 (4) Duralumin 0.961 D 0.982th 0.913 0.299 Stainless steel 0.925 0.947 0.881 0.381 where tth was the threshold level, obtained using the least square method (Figure7a). EquationN strip (4) withdemonstrates tape that 0.927 weak cavitation 0.892 with a diameter 0.933 smaller than 0.130 0.0103 tth in mm,N strip considering without Equation tape (2), 0.983 would scarcely 0.963deform a material. 0.937 The tth is shown 0.146 in Table 2 andAveraged Figure7b (dashed-dottedvalue 0.95 line ± 0.03 in the same 0.95 color± 0.04 as the 0.92 symbol). ± 0.03 There 0.24 was ± a 0.12 correlation between surface Vickers hardness without peening and threshold level (Figure7a,b) ; the correlationIn order to coefﬁcients investigate are the shown effect of in LC Table on2 the. peening intensity, Figure 7a shows the relationshipThe threshold between diameter the developingdth (mm) time was of calculatedLC (tD) and from the arctth height(µs) by (h the). As relationship calculated bybetween Equation the (2), developing the tD was time proportional of LC and theto the diameter diameter of LCof the to investigateLC. The specimens the threshold were treatedlevel, as with described 4 pulses/mm in Equation2. When (2), the and following the threshold power levels law in wastth µ assumeds and dth asmm the are relation- shown shipin Table between2. The tD correlation and h as shown was improvedin Equation by (3), considering the exponent the n threshold was 3.3 for level duralumin,(Figure7 4.1b) . forAs thestainless number steel, used 6.4 in for the the dataset N strip was without six or tape, seven and in the 7.0 presentfor the N study, strip the with probability tape. of a non-correlation was less than 0.005% for duralumin, 0.046% for stainless steel, 0.003% for the N strip with tape, and 0.141% for the N strip without tape. As the probability of a non-correlation was less than 1%, it can be concluded that the relationship was highly

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∝ (3) It has been reported that the aggressive intensity of cavitation is proportional to the volume of cavitation [15,40,42] and the threshold level of the erosion caused by cavitation impact [43]. Therefore, Equation (4) was assumed.

∝ (4)

where tth was the threshold level, obtained using the least square method (Figure 7a). Equation (4) demonstrates that weak cavitation with a diameter smaller than 0.0103 tth in mm, considering Equation (2), would scarcely deform a material. The tth is shown in Table 2 and Figure 7b (dashed-dotted line in the same color as the symbol). There was a correlation between surface Vickers hardness without peening and threshold level (Figure 7a,b); the correlation coefficients are shown in Table 2. The threshold diameter dth (mm) was calculated from tth (μs) by the relationship between the developing time of LC and the diameter of LC to investigate the threshold level, as described in Equation (2), and the threshold levels in tth μs and dth mm are shown in Table 2. The correlation was improved by considering the threshold level (Figure 7b). As the number used in the dataset was six or seven in the present study, the probability of a Metals 2021, 11, 531 8 of 14 non-correlation was less than 0.005% for duralumin, 0.046% for stainless steel, 0.003% for the N strip with tape, and 0.141% for the N strip without tape. As the probability of a non- correlation was less than 1%, it can be concluded that the relationship was highly significant.signiﬁcant. Thus, it Thus,can be it concluded can be concluded that the that rela thetionship relationship between between the developing the developing time of time LC of consideringLC considering the threshold the threshold level leveland the and arc the height arc height was highly was highly significant. signiﬁcant.

(a)

(b)

FigureFigure 7. 7.RelationshipRelationship between between the the developing developing time time of laser of laser cavitation cavitation and and the thearc archeight. height. (a) The (a) The powerpower law law without without considering considering the the threshold threshold level; level; ( b)) the power lawlaw consideringconsidering the the threshold threshold level. level. Table 2. Relationship between surface Vickers hardness Hv and threshold level.

Correlation Correlation Probability of Coefﬁcient without Coefﬁcient Non-Correlation Surface Vickers Threshold Level Materials Considering Considering of Considering Hardness Hv Threshold Level Threshold Level Threshold Level tth µs dth mm Duralumin 0.995 0.995 0.005% 129.4 ± 3.0 64 0.66 Stainless steel 0.982 0.983 0.046% 175.2 ± 1.3 198 2.04 N strip with tape 0.992 0.996 0.003% 424.4 ± 4.7 384 3.96 N strip without tape 0.924 0.944 0.141% 424.4 ± 4.7 478 4.92

As shown in Table2, thresh level increases the surface Vickers harness without peening. In order to investigate the tendency between the surface Vickers hardness and the threshold level, Figure8 illustrates the relation between the surface Vickers harness without peening and the threshold level. It is known that Vickers hardness corresponds well to the yield stress of the material. Therefore, it is reasonable that the threshold level that deforms the material increases with Vickers hardness. As the number in the dataset was four and the correlation coefficient was 0.962, the probability of a non-correlation was 3.8% (Figure8 ). When the probability of a non-correlation is less than 5%, it can be concluded that the relationship is significant. Thus, it can be concluded that the relationship between the surface Vickers harness without peening and the threshold level was significant. When an N strip with black aluminum tape was used, a larger LC was produced by a larger LA Metals 2021, 11, x FOR PEER REVIEW 9 of 15

Table 2. Relationship between surface Vickers hardness Hv and threshold level.

Correlation Coeffi- Probability of Non- Threshold Level Correlation Coeffi- cient without Con- Correlation Surface Vickers Materials cient Considering sidering Threshold of Considering Hardness Hv tth µs dth mm Threshold Level Level Threshold Level Duralumin 0.995 0.995 0.005% 129.4 ± 3.0 64 0.66 Stainless steel 0.982 0.983 0.046% 175.2 ± 1.3 198 2.04 N strip with tape 0.992 0.996 0.003% 424.4 ± 4.7 384 3.96 N strip without tape 0.924 0.944 0.141% 424.4 ± 4.7 478 4.92

As shown in Table 2, thresh level increases the surface Vickers harness without peening. In order to investigate the tendency between the surface Vickers hardness and the threshold level, Figure 8 illustrates the relation between the surface Vickers harness without peening and the threshold level. It is known that Vickers hardness corresponds well to the yield stress of the material. Therefore, it is reasonable that the threshold level that deforms the material increases with Vickers hardness. As the number in the dataset was

Metals 2021, 11, x FOR PEER REVIEWfour and the correlation coefficient was 0.962, the probability of a non-correlation10 ofwas 15 3.8% (Figure 8). When the probability of a non-correlation is less than 5%, it can be con- Metals 2021, 11, 531 cluded that the relationship is significant. Thus, it can be concluded that the relationship9 of 14 between the surface Vickers harness without peening and the threshold level was signifi- motors,cant. When considering an N strip the with previous black reportaluminum [11]. tapeAs the was repetition used, a larger frequency LC was of theproduced laser was by 10 Hz, the specimen was treated widthwise at vs. = 5 mm/s and moved in dS = 0.5 mm adue larger to theLA blackdue to aluminum the black aluminum tape, as the tape, tape as notthe onlytape attenuatednot only attenuated the impact the but impact also stepsbut also in theincreased longitudinal LA. direction of the specimen, as shown in Figure 9 (red arrow). increasedThe fatigue LA. properties were examined using a conventional Schenk-type displacement-controlled plane-bending fatigue tester at R = −1. During plane-bending fatigue testing, displacement was produced by an eccentric wheel, and the applied stress was calculated from the bending moment, measured by a load cell [44]. The span length at a fixed point was 65 mm, and the test frequency was 12 Hz (Figure 9). The applied stress (σa) during the test was calculated from the bending moment (M), the width of the specimen (b) (i.e., 20 mm), and the thickness (δ) measured by a digital caliper, with an accuracy of 0.01 mm, as shown in Equation (5). 6 = (5) As one of the major fatigue property factors in welded parts is residual stress, the residual stress near the welding line was evaluated using 2D X-ray diffraction with a two- dimensional detector [45]. The optimal conditions for residual stress measurement using 2D X-ray diffraction with specimen oscillation, considering the error, have been reported previouslyFigureFigure 8. 8. RelationshipRelationship [46]. The betweenX-ray between tube the the surfaceused surface was Vicker Vickers Cr sK hardness hardnessα and was and and operatedthe the threshold threshold at 35 level. level. kV and 40 mA. The diameter of the collimator was 0.146 mm. The used lattice plane, (h k l), was the γ-Fe 3. Improvement of the Fatigue Strength of Welded Part by Laser Cavitation Peening (23. 2Improvement 0) plane and theof thediffraction Fatigue angle Strength without of Welded strain was Part 128 by degrees. Laser Cavitation The diffraction Peening from 3.1. Experimental Apparatus and Procedures for the Fatigue Test of Welds the3.1. surfaceExperimental was obtained Apparatus by and osci Procellatingdures the for specimen the Fatigue in theTest ω of-direction Welds by 8 degrees during theThe measurement. welded specimen The initial was ω treated angle bywas laser 110 cavitationdegrees. The peening 24 diffraction to demonstrate rings from the The welded specimen was treated by laser cavitation peening to demonstrate the im- theimprovement specimen at in various the fatigue angles strength were detected, of welded and metals the exposure by the proposed time per laser frame cavitation at each provement in the fatigue strength of welded metals by the proposed laser cavitation peen- positionpeening, was as shown20 min. in It Figurewas reported9, which that also the illustrates residual thestress coordinates at the surface for residualtreated by stress the ing, as shown in Figure 9, which also illustrates the coordinates for residual stress meas- submergedmeasurement laser (blue peening arrows). revealed The stainless-steelthe tension, and plate the (Japaneseresidual stress Industrial at 30 μ Standardsm below the JIS urement (blue arrows). The stainless-steel plate (Japanese Industrial Standards JIS surfaceSUS316L), well with corresponded a U-notch andto the a radius fatigue of proper 1.5 mm,ties was ofmachined the stainless and steel welded [11]. using Thus, single- the SUS316L), with a U-notch and a radius of 1.5 mm, was machined× and welded using single- residualpass Tungsten stresses Inert at z Gas= 0 μ TIGm and welding. 30 μm Thewere plate measured was 95 using180 electrochemical mm and had a thicknesspolishing. of pass3 mm. Tungsten The ﬁve Inert specimens Gas TIG for welding. the plane-bending The plate fatiguewas 95 test× 180 were mm machined and had froma thickness one plate. of 3 mm. The five specimens for the plane-bending fatigue test were machined from one plate. The specimen was treated by the proposed laser cavitation peening system, as shown in Figure 1. The wavelength of the Nd:YAG laser was 1064 nm, considering the above- mentioned results. The welded side was treated with a dL = 4 pulses/mm2 using stepping

Figure 9. Geometry of the welded specimen for displacement-controlled plane-bending fatigue test, Figure 9. Geometry of the welded specimen for displacement-controlled plane-bending fatigue coordinates for residual stress measurement, and the direction of laser cavitation peening. test, coordinates for residual stress measurement, and the direction of laser cavitation peening. The specimen was treated by the proposed laser cavitation peening system, as shown 3.2.in FigureResults1 .of Thethe Welded wavelength Part Treated of the by Nd:YAG Laser Cavitation laser was Peening 1064 nm, considering the above- 2 mentionedFigure 10 results. shows The the welded welded side part was of treatedthe specimen with a withdL = 4and pulses/mm without laserusing cavitation stepping peeningmotors, after considering the fatigue the test. previous In both report cases, [11 the]. As specimen the repetition fractured frequency at the HAZ. of the The laser color was of10 the Hz, surface the specimen treated by was the treated laser cavitation widthwise peening at vs. became = 5 mm/s black and due moved to oxidation in dS = caused 0.5 mm bysteps LA, inand the each longitudinal spot deformed direction by LA of theand specimen, LC was revealed as shown (Figure in Figure 10b).9 The(red deformation arrow). of theThe welding fatigue line properties was difficult were examinedto observe. using The a improvement conventional Schenk-typein the fatigue displacement- properties couldcontrolled have been plane-bending caused by fatigueintroducing tester residual at R = − stress1. During into plane-bendingthe HAZ and reducing fatigue testing,weld- ingdisplacement defects. was produced by an eccentric wheel, and the applied stress was calculated from the bending moment, measured by a load cell [44]. The span length at a ﬁxed point was 65 mm, and the test frequency was 12 Hz (Figure9). The applied stress ( σa) during the test was calculated from the bending moment (M), the width of the specimen (b) (i.e.,

Metals 2021, 11, 531 10 of 14

20 mm), and the thickness (δ) measured by a digital caliper, with an accuracy of 0.01 mm, as shown in Equation (5). 6 M σ = (5) a b δ As one of the major fatigue property factors in welded parts is residual stress, the residual stress near the welding line was evaluated using 2D X-ray diffraction with a two- dimensional detector [45]. The optimal conditions for residual stress measurement using 2D X-ray diffraction with specimen oscillation, considering the error, have been reported previously [46]. The X-ray tube used was Cr Kα and was operated at 35 kV and 40 mA. The diameter of the collimator was 0.146 mm. The used lattice plane, (h k l), was the γ-Fe (2 2 0) plane and the diffraction angle without strain was 128 degrees. The diffraction from the surface was obtained by oscillating the specimen in the ω-direction by 8 degrees during the measurement. The initial ω angle was 110 degrees. The 24 diffraction rings from the specimen at various angles were detected, and the exposure time per frame at each position was 20 min. It was reported that the residual stress at the surface treated by the submerged laser peening revealed the tension, and the residual stress at 30 µm below the surface well corresponded to the fatigue properties of the stainless steel [11]. Thus, the residual stresses at z = 0 µm and 30 µm were measured using electrochemical polishing.

3.2. Results of the Welded Part Treated by Laser Cavitation Peening Figure 10 shows the welded part of the specimen with and without laser cavitation peening after the fatigue test. In both cases, the specimen fractured at the HAZ. The color of the surface treated by the laser cavitation peening became black due to oxidation caused by LA, and each spot deformed by LA and LC was revealed (Figure 10b).

Metals 2021, 11, x FOR PEER REVIEWThe deformation of the welding line was difﬁcult to observe. The improvement11 in of the 15 fatigue properties could have been caused by introducing residual stress into the HAZ and reducing welding defects.

Figure 10.10. Fractured surfacesurface ofof aa weldedwelded part.part. ((aa)) Non-peenedNon-peened specimenspecimen ((σσaa == 229 MPa, N = 688,800); (b) specimen treatedtreated by laserlaser cavitationcavitation peeningpeening ((σσaa = 319 MPa, N == 516,300). 516,300).

The residual stress σ as a function of the distance from the welding line x at z = 0 and The residual stress σRR as a function of the distance from the welding line x at z = 0 and µ 30 μm in the x- and y-directions were calculated to investigate the effect of laser cavitation peening on on the the residual residual stress stress in in the the HAZ HAZ (Figure (Figure 11). 11 The). Themeasurement measurement location location was mid- was mid-width,width, i.e., y i.e., = 0y (Figure= 0 (Figure 9). The9). The tensile tensile residual residual stress stress was was introduced introduced by by laser laser ablation ablation at z µ atz = 0= μ 0m,m, and and compressive compressive residual residual stress stress was was introduced introduced by by laser laser cavitation cavitation peening peening at at z z = 30 µm (Figure 11). It can be concluded that the tensile residual stress was introduced = 30 μm (Figure 11). It can be concluded that the tensile residual stress was introduced by by laser ablation, but the compressive residual stress was introduced under the surface by laser ablation, but the compressive residual stress was introduced under the surface by the proposed laser cavitation peening. The distribution of the residual stress treated by the proposed laser cavitation peening. The distribution of the residual stress treated by laser cavitation peening changing with depth, compared with cavitation peening and shot laser cavitation peening changing with depth, compared with cavitation peening and shot peening, was revealed in [47]. peening, was revealed in [47].

(a) (b) Figure 11. Residual stress at the surface z = 0 μm and z = 30 μm of non-peened (NP) and laser cavitation-peened (LCP) specimens. (a) x-direction; (b) y-direction.

S-N curves were obtained from the plane-bending fatigue test for non-peened (NP) and laser cavitation-peened (LCP) specimens to investigate the improvement in the fatigue properties of welded stainless steel with laser cavitation peening (Figure 12). The results of shot peening (SP), water jet peening (WJP), and cavitating jet peening (CJP), i.e., cavitation peening using cavitating jet, from reference [10] are also shown. The S-N of LCP was located on the upper right-hand side of NP (Figure 12); therefore, the fatigue life and strength were improved by LCP. When the fatigue strength was calculated using Little’s

Metals 2021, 11, x FOR PEER REVIEW 11 of 15

Figure 10. Fractured surface of a welded part. (a) Non-peened specimen (σa = 229 MPa, N = 688,800); (b) specimen treated by laser cavitation peening (σa = 319 MPa, N = 516,300).

The residual stress σR as a function of the distance from the welding line x at z = 0 and 30 μm in the x- and y-directions were calculated to investigate the effect of laser cavitation peening on the residual stress in the HAZ (Figure 11). The measurement location was mid- width, i.e., y = 0 (Figure 9). The tensile residual stress was introduced by laser ablation at z = 0 μm, and compressive residual stress was introduced by laser cavitation peening at z = 30 μm (Figure 11). It can be concluded that the tensile residual stress was introduced by laser ablation, but the compressive residual stress was introduced under the surface by the proposed laser cavitation peening. The distribution of the residual stress treated by Metals 2021, 11, 531 11 of 14 laser cavitation peening changing with depth, compared with cavitation peening and shot peening, was revealed in [47].

(a) (b)

Figure 11. Residual stress at the surface z = 0 0 μµm and z = 30 μµm of non-peened (NP) and laser cavitation-peened (LCP) specimens. ( (a)) xx-direction;-direction; ( b) y-direction.

S-NS-N curvescurves were were obtained obtained from from the the plane-be plane-bendingnding fatigue fatigue test test for for non-peened (NP) (NP) andand laser cavitation-peenedcavitation-peened (LCP)(LCP) specimens specimens to to investigate investigate the the improvement improvement inthe in fatiguethe fa- tigueproperties properties of welded of welded stainless stainless steel with steel laser with cavitation laser cavitation peening peening (Figure (Figure 12). The 12). results The Metals 2021, 11, x FOR PEER REVIEWresultsof shot of peening shot peening (SP), water (SP), water jet peening jet peening (WJP), (WJP), and cavitating and cavitating jet peening jet peening (CJP), (CJP), i.e.,12 of cav-i.e., 15 cavitationitation peening peening using using cavitating cavitating jet, jet, from from reference reference [10 [10]] are are also also shown. shown. TheThe S-NS-N of LCP waswas located located on on the the upper upper right-hand right-hand side side of of NP NP (Figure (Figure 12);12); therefore, the fatigue life and strengthstrength were were improved improved by by LCP. LCP. When the fatigue strength was calculated using Little’s method [48], the fatigue strength was 159 ± 20 MPa for NP and 244 ± 12 MPa for LCP. method [48], the fatigue strength was 159 ± 20 MPa for NP and 244 ± 12 MPa for LCP. Namely,Namely, LCP improved the fatigue strength ofof weldedwelded stainlessstainless steelsteel byby 53%.53%.

FigureFigure 12. 12. S-NS-N diagramdiagram of of welded welded specimens specimens with with and and wi withoutthout laser laser cavitation cavitation peening peening compared compared with other peening methods. NP, NP, non-peened; LCP, laserlaser cavitationcavitation peening;peening; SP,SP, shotshot peening;peening; WJP, WJP,water water jet peening; jet peening; CJP, cavitatingCJP, cavitating jet peening jet peen (cavitationing (cavitation peening peening using using cavitating cavitating jet). Thejet). SP,The WJP, SP,and WJP, CJP dataand CJP were data taken were from taken [10]. from [10].

RegardingRegarding the the welding welding line line observations observations and and the the residual residual stress stress results, results, the the improve- improve- mentment in in the the fatigue fatigue strength strength mechanism mechanism was was no nott from from the the decrease decrease in stress concentration duedue to to the the deformation deformation of of the the welding welding line, line, but but from from the the introduction introduction of compressive of compressive re- sidualresidual stress stress from from LCP. LCP. The The fatigue fatigue properti propertieses of of LCP LCP were were better better than than the the other other peening methods,methods, i.e., SP, WCP, andand CJPCJP (Figure(Figure 1212).). Figure 13 shows the fractured surface of (a) a non-peened specimen and (b) a specimen treated by laser cavitation peening; in both cases, a crack was initiated at the void or a defect of welding near the surface, as the maximum tensile stress was applied at the surface during the bending fatigue test. For the non-peened specimen, the crack initiation point exhibited a wavy pattern at each weld, which would be a weak region; such defects are easily included during welding.

(a) (b)

Figure 13. Aspect of the fractured surface observed by SEM. (a) Non-peened specimen (σa = 229 MPa, N = 688,800); (b) specimen treated by laser cavitation peening (σa = 319 MPa, N = 516,300).

Metals 2021, 11, x FOR PEER REVIEW 12 of 15

method [48], the fatigue strength was 159 ± 20 MPa for NP and 244 ± 12 MPa for LCP. Namely, LCP improved the fatigue strength of welded stainless steel by 53%.

Figure 12. S-N diagram of welded specimens with and without laser cavitation peening compared with other peening methods. NP, non-peened; LCP, laser cavitation peening; SP, shot peening; WJP, water jet peening; CJP, cavitating jet peening (cavitation peening using cavitating jet). The SP, WJP, and CJP data were taken from [10].

Regarding the welding line observations and the residual stress results, the improvement in the fatigue strength mechanism was not from the decrease in stress concentration Metals 2021, 11, 531 due to the deformation of the welding line, but from the introduction of compressive12 of re- 14 sidual stress from LCP. The fatigue properties of LCP were better than the other peening methods, i.e., SP, WCP, and CJP (Figure 12). Figure 13 showsshows thethe fractured fractured surface surface of of (a) (a) a non-peeneda non-peened specimen specimen and and (b) (b) a specimen a speci- mentreated treated by laser by laser cavitation cavitation peening; peening; in both in both cases, cases, a crack a crack was was initiated initiated at at the the void void or or a adefect defect of of welding welding near near the the surface, surface, as the as maximumthe maximum tensile tensile stress stress was applied was applied at the surface at the surfaceduring theduring bending the bending fatigue fati test.gue For test. the For non-peened the non-peened specimen, specimen, the crack the initiationcrack initiation point pointexhibited exhibited a wavy a wavy pattern pattern at each at each weld, weld, which which would would be a be weak a weak region; region; such such defects defects are areeasily easily included included during during welding. welding.

(a) (b)

Figure 13. AspectAspect of of the the fractured fractured surface surface observed observed by by SEM. SEM. (a ()a Non-peened) Non-peened specimen specimen (σa ( σ=a 229= 229 MPa, MPa, N =N 688,800);= 688,800); (b) specimen treated by laser cavitation peening (σa = 319 MPa, N = 516,300). (b) specimen treated by laser cavitation peening (σa = 319 MPa, N = 516,300).

4. Conclusions In order to investigate the effect of laser cavitation LC, which was generated after laser ablation LA, on submerged laser peening, impact and pressure wave were measured by using the handmade PVDF sensor and the conventional submerged shock wave sensor. The peening effect of the wave length of the Q-switched Nd:YAG laser was also investigated by comparing with the fundamental harmonic wave length, i.e., 1064 nm, which is normally used to generate cavitation, and the second harmonic wave length, i.e., 532 nm, which is normally used in conventional submerged laser peening to avoid absorption by water. After investigating LC, an improvement in the stainless-steel weld fatigue properties was demonstrated with the now named laser cavitation peening LCP. The results obtained can be summarized as follows: (1) In order to investigate the peening effect of LA and LC, the impact measurement using the PVDF sensor was found to be better than that of the conventional submerged shockwave sensor, as the PVDF sensor could detect the second collapse of LC and the shockwave sensor could not. Both the LA and LC signals from the PVDF sensor were proportional to the arc height, and the correlation with the signal from the PVDF sensor was larger than that of the submerged shockwave sensor. (2) The impact of LC measured by the PVDF sensor was larger than the impact of LC, although the pressure wave of LA measured by the submerged shockwave sensor was larger than that of LC. Namely, during optimized submerged laser peening, the impact induced by LC was more effective than that of LA, called laser cavitation peening (LCP). (3) LCP, using a wavelength of 1064 nm, was more efﬁcient than using 532 nm, as 40% of the pulse energy at 1064 nm was lost during wavelength conversion to 532 nm. (4) The peening intensity was proportional to the volume of LC during LCP, considering the target metals’ threshold level. The threshold level was roughly proportional to the surface Vickers hardness of the target metals without LCP. (5) LCP could improve the fatigue properties of JIS SUS316L stainless steel welds by 53%. Metals 2021, 11, 531 13 of 14

Funding: This research was partly supported by the JSPS KAKENHI, grant numbers 18KK0103 and 20H02021. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the author. Conﬂicts of Interest: The author declares no conﬂict of interest.

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