E-Archive

Science Update

in Vol. 25 - May Issue - Year 2024
Diffraction Study Reveals Mechanism of Laser Peening without Ablative Layer
Yuji Sano

Yuji Sano

Koich Akita

Koich Akita

Fig. 1: Schematic of laser peening with ablative layer

Fig. 1: Schematic of laser peening with ablative layer

Fig. 2: Residual stress profile on and around laser-irradiated spots

Fig. 2: Residual stress profile on and around laser-irradiated spots

Table 1: Single laser pulse irradiation conditions

Table 1: Single laser pulse irradiation conditions

Fig. 3: Strategy of laser pulse irradiation for surface compression in LPwC

Fig. 3: Strategy of laser pulse irradiation for surface compression in LPwC

Fig. 4: Surface residual stress on laser-irradiated lines

Fig. 4: Surface residual stress on laser-irradiated lines

Fig. 5: Overlap effect on surface residual stress and depth profile

Fig. 5: Overlap effect on surface residual stress and depth profile

Fig. 6: LPwC on the inside wall of a pipe specimen with an inner diameter of 9.5 mm

Fig. 6: LPwC on the inside wall of a pipe specimen with an inner diameter of 9.5 mm

Fig. 7: Lattice strain of SK3 pipe specimens with 9.5 mm inner diameter

Fig. 7: Lattice strain of SK3 pipe specimens with 9.5 mm inner diameter

Introduction

Laser peening (LP) or laser shock peening (LSP) is a surface engineering technique that produces beneficial compressive residual stresses (RS) on the surface of metals and alloys by irradiating with successive laser pulses at peak power densities of approximately 1 to 10 GW/cm2 through a layer of water as shown in Fig. 1 [1]. Since intense laser pulses inevitably damage the top surface, an ablative layer is formed to protect the surface prior to laser irradiation. This process began to be used in practical applications in the early 1990s, such as reducing foreign object damage (FOD) in jet engine turbine blades by significantly improving the fatigue properties of metal parts [2]. However, due to the difficulty of forming ablative layers in complex shapes, applications have been limited to near-planar surfaces.

In the mid-1990s, another type of LP, called LPwC (Laser Peening without Coating), which does not use any ablative layer, was invented [3, 4] and its effectiveness in preventing fatigue and stress corrosion cracking (SCC) was confirmed [5]. LPwC uses a small laser spot, typically 0.2 to 1.5 mm, allowing it to flexibly follow complex structures [6]. However, since heated surfaces shrink after laser pulse irradiation, it was considered inevitable that tensile RS would remain on the top surface of parts to which LPwC was applied [7]. A major challenge in realizing LPwC was identifying ways to mitigate the thermal effects of direct laser pulse irradiation. Therefore, in the 30 years of LPwC development, we have lowered the pulse energy and shortened the pulse duration to reduce the heat input to the surface, and increased the overlap of successive laser pulses to eliminate the tension on the spot of the previous laser pulse [4, 8]. Here we present a detailed RS distribution around the laser-irradiated spot using synchrotron radiation and show how compressive RS is built up on the surface by LPwC [9]. 


Single pulse irradiation

Reference specimens were prepared by annealing HT1000 high-strength steel at 973 K for 3 hours, followed by furnace cooling to relieve RSs caused by machining. After annealing, the RSs on the top surface were close to zero (0 ~ 50 MPa). Next, a single laser pulse from a Q-switched Nd:YAG laser with a wavelength of 532 nm and a pulse duration of 7 ns was irradiated under the conditions as shown in Table 1. Since the peak power density was constant at approximately 4 GW/cm2, the peak plasma pressure was considered to be the same for all irradiations.

The surface RS distribution over the laser spots was precisely measured by X-ray diffraction (XRD) using synchrotron radiation at BL3A of KEK-PF (Photon Factory of High Energy Accelerator Research Organization) and BL46XU of SPring-8. The details of the experiments can be found in [9, 10]. Fig. 2 shows the RS distribution on the specimen surface. The abscissa is normalized by the radius of the laser spot so that the edge position of the laser spot is unity.

This result indicates the presence of a tensile component at the irradiated spot due to the thermal effect of laser irradiation. However, outside the irradiated spot, a compressive component was observed, which is due to plastic strain caused by the impulse of the laser pulse through the Poisson’s effect. The tensile component decreases rapidly as the spot diameter decreases. This is believed to be due to the three-dimensional cooling effect resulting from the reduction of the laser spot diameter, even if the laser power density remains constant.


Line and area irradiation

The result in the previous section suggests that successive laser pulse irradiation with high overlap results in overall surface compression, except for the last laser spot [9]. Fig. 3 schematically shows the surface RS state after single pulse, line and area irradiations. The red area corresponds to the laser spot with tension, while the blue area is compressive and surrounds the laser spot. By increasing the overlap, the RS state would become compressive. To confirm this assumption, JIS SHY685 structural steel specimens were irradiated in line with a pulse energy of 215 mJ, a spot diameter of 1.0 mm, and a power density of 4 GW/cm2. The pulse density was varied at 1, 5, 10, and 100 pulses/mm to investigate the effect of overlap on RS [11, 12]. 

The RS in the x-direction (laser sweep direction) on the laser-irradiated lines is shown in Fig. 4 together with the RS measurement positions. The X-ray irradiation area is 0.5 mm in diameter. The horizontal axis is the distance from the last laser irradiation spot, and the origin is the end of the laser irradiation lines. By increasing the overlap, the RSs on the line decreased and became compressive at 100 pulses/mm as expected. At 1 pulse/mm, there is no overlap, so the geometric effects alternate between the center and edge of the laser pulses.

To investigate the overlap effect on the surface RS and depth profile in area irradiation (e.g., LPwC treatment), the RS depth distribution of SUS304 austenitic stainless steel specimens with different overlaps was measured by XRD using Mn-Kα characteristic X-rays and electrolytic polishing. The results are shown in Fig. 5. Here, the overlap (Cv) is defined as Cv = (πD2/4) × Nd, where D is the laser spot diameter and Nd is the irradiated pulse density which is the number of laser spots in a unit area. In the reference specimen, the tensile RS exists down to a depth of 0.1 mm due to surface grinding. The area irradiation of laser pulses induced compressive RS which increased with increasing overlap. When the overlap is relatively small, such as 100% or 200%, the RS improvement at the top surface is not sufficient. This is believed to be due to the tensile component remaining on the surface, which cannot be completely eliminated at small overlap, as shown in Fig. 3 (top right). 

Roughly speaking, the surface RS is determined by the balance between the compressive component from the impulse of the laser pulse irradiation and the tensile component from the subsequent shrinkage. Therefore, it is important to reduce the pulse energy and shorten the pulse duration (interaction time) while maintaining the peak power intensity, which is directly related to the plasma pressure. On the material side, the surface of materials with lower CTE (coefficient of thermal expansion) is more likely to be compressive than those with higher CTE [13]. Another unexplored parameter is the thermal conductivity, which definitely affects the surface RS after LPwC. Materials with high thermal conductivity (e.g., aluminum alloys) are more easily compressed than materials with low thermal conductivity (e.g., austenitic stainless steels). 


Distinctive application of LPwC

One of the major advantages of LPwC is its applicability to complicated geometries such as inner holes. We developed a pencil-type optical head for LPwC, as shown in Fig. 6 (left) [14], and applied it to a pipe specimen, as shown in Fig. 6 (right) [15]. The specimen was made of an extruded carbon tool steel rod (JIS SK3), and the central area along the axis was drilled to produce pipe-shaped specimens of 150 mm in length with an outer diameter of 17 mm and an inner diameter of 9.5 mm. The wall thickness is 3.75 mm. LPwC was applied to the 50 mm long central section with a pulse energy of 70 mJ, a spot diameter of 0.7 mm, and a pulse density of 100 pulses/mm2, corresponding to a peak power density of 4.2 GW/cm2 and an overlap of 3800%.

Neutron diffraction was used to evaluate the effect of LPwC on the pipe specimen. The lattice strain was measured with a neutron diffractometer at JRR-3 (Japan Research Reactor-3) of JAEA (Japan Atomic Energy Agency). Strains in three directions were necessary to derive the RS strictly. However, in this study, the strain was measured in the circumferential (θ) direction through the entire pipe wall and partially in the radial (r) direction, taking into account the available machine time. Fig. 7 shows the strain distribution in the θ direction (εθ) across the wall thickness after LPwC compared to that of an unpeened specimen for the reference. LPwC introduced compressive strain to the depth of nearly 1 mm from the inner surface. On the contrary, the strain on the outer side tended to become tensiled so as to compensate for the compressive strain near the inner surface. The radial strain (εr) was also measured at the distances of 0.4 and 1.9 mm from the inner surface for the specimen after LPwC. By assuming the plane stress, it was made possible to deduce that the RS near the inner surface would be compressive due to LPwC, while those in the middle and outer regions would be tensile.


Conclusions

To clarify the mechanism of compressive RS generation by LPwC without using any ablative layer, the RS distributions after single and line irradiation of laser pulses were precisely measured using synchrotron radiation. As a result, it was found that the RS within the laser irradiation spot was tensile, but there was a compressive region around it, and it was found that the whole surface in the area irradiation could be made compressive by irradiating the laser pulse with sufficient overlap. LPwC was performed on the inside of a thin pipe to demonstrate the advantage of LPwC. Neutron diffraction analysis non-destructively confirmed that LPwC induced compressive residual stress on the inner surface of the pipe.


Acknowledgements

This work was supported in part by the JST-MIRAI Program [Grant No. JPMJMI17A1] and MEXT KAKENHI [Grant No. JP20K04185]. The synchrotron radiation experiment at BL3A in Photon Factory (PF) was performed with the approval of the Photon Factory Program Advisory Committee of the High-Energy Accelerator Research Organization (KEK, Proposal No. 2003G032 and 2007G082), and at BL46XU in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI, Proposal No. 2003B0678). The neutron diffraction experiment at Japan Research Reactor-3 (JRR-3) was performed under the approval (E4026) of the Japan Atomic Energy Agency (JAEA). 


References

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[3] Y. Sano, et al., “Residual stress improvement in metal surface by underwater laser irradiation”, Nucl. Instrum. Methods Phys. Res. B Vol. 121 (1-4), 1997, pp. 432–436.

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[7] P. Peyre, et al., “FEM calculation of residual stresses induced by laser shock processing in stainless steels”, Model. Simul. Mater. Sci. Eng. Vol. 15, 2007, pp. 205–221.

[8] Y. Sano, et al., “Effects of laser peening with a pulse energy of 1.7 mJ on the residual stress and fatigue properties of A7075 aluminum alloy”, Metals Vol. 11, 2021, p. 1716.

[9] Sano, et al., “A mechanism for inducing compressive residual stresses on a surface by laser peening without coating”, Metals Vol. 10, 2020, p. 816.

[10] Y. Sano et al., “Measurement of three-dimensional residual stress distribution in single laser pulse irradiated area”, SPring-8 User Experiment Report No.12 (2003B), July 2004, JASRI.

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[12] H. Tanaka, “Experimental study on compressive residual stress evolution by pulse laser irradiation”, J Soc. Mater. Sci. Japan Vol.55 (7), 2006, pp. 641-646 (in Japanese).

[13] Y. Sano, K. Akita, “Dependence of surface residual stress on the coefficient of thermal expansion for materials subjected to laser peening without coating”, Opt. Laser Technol. Vol. 156, 2022, p. 108528.

[14] M. Yoda, et al, “Development and application of laser peening system for PWR power plants”, Proc. 14th International Conference on Nuclear Engineering (ICONE-14), Miami, FL, 17–20 July 2006, ICONE14-89228.

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Yuji Sano, (Ph.D. in Eng.)

Specially-appointed Professor

SANKEN, Osaka University

Mihogaoka 8-1, Ibaraki,

Osaka 567-0047, Japan

E-mail: yuji-sano@sanken.osaka-u.ac.jp

 

Koich Akita

(Ph.D. in Eng.)

Professor

Department of Mechanical Systems Engineering, Tokyo City University

Tamazutsumi 1-28-1, Setagaya-ku, Tokyo 158-8557, Japan

E-mail: akitak@tcu.ac.jp