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in Vol. 26 - September Issue - Year 2025
Improvement of Fatigue Properties in Additively Manufactured Metals by Submerged Laser Treatment Using a Fiber Laser
Fig. 1: Fatigue specimen of PBF-LS/AlSi10Mg: (a) Dimensions of as-built specimen, (b) Photograph

Fig. 1: Fatigue specimen of PBF-LS/AlSi10Mg: (a) Dimensions of as-built specimen, (b) Photograph

Fig. 2: Diagonal view of the SLP system using a fiber laser

Fig. 2: Diagonal view of the SLP system using a fiber laser

Fig. 3: Aspects of specimen observed by digital microscope

Fig. 3: Aspects of specimen observed by digital microscope

Fig. 4: Aspects of specimen observed by SEM

Fig. 4: Aspects of specimen observed by SEM

Fig. 5: Effect of post-processing on the fatigue life

Fig. 5: Effect of post-processing on the fatigue life

Fig. 6: Surface roughness of as-built, blast (B), blast (B) + laser treatment (LT) using fiber laser, blast (B) + laser treatment (LT) + laser cavitation peening (LCP) using Nd:YAG laser and LCP alone

Fig. 6: Surface roughness of as-built, blast (B), blast (B) + laser treatment (LT) using fiber laser, blast (B) + laser treatment (LT) + laser cavitation peening (LCP) using Nd:YAG laser and LCP alone

Fig. 7: Surface residual stress of as-built, blast (B), B+LT, B+LT+LCP and LCP alone

Fig. 7: Surface residual stress of as-built, blast (B), B+LT, B+LT+LCP and LCP alone

Fig. 8: Improvement of fatigue strength in as-built PBF–LS/AlSi10Mg specimens by LT and LCP

Fig. 8: Improvement of fatigue strength in as-built PBF–LS/AlSi10Mg specimens by LT and LCP

Introduction

Additively manufactured (AM) metals offer the advantage of being fabricated directly from CAD/CAM data, making it relatively easy to produce topology-optimized shapes that are difficult to achieve using conventional machining methods. However, as-built AM metals—such as those fabricated by powder bed fusion using laser sintering (PBF-LS)—exhibit significantly higher surface roughness due to the incomplete melting of metal particles during the AM process. These unmelted particles remain on the surface, resulting in poor fatigue properties [1, 2]. Therefore, post-processing is required to improve the fatigue properties of PBF-LS components.
It has been reported that shot peening (SP), laser peening (LP), cavitation peening (CP), and cavitation abrasive surface finishing (CASF) can improve the fatigue properties of AM metals [3–9]. In general, LP is classified into two types: one using a liquid film on the target surface, and the other, submerged laser peening (SLP), in which the target is placed in a water-filled chamber. In SLP, a bubble is generated after laser ablation (LA), behaving like a cavitation bubble. This laser-induced bubble is referred to as "laser cavitation (LC)." It has been reported that the impact induced by the collapse of LC is greater than that caused by LA alone when the transmitted impact is measured through the target [10]. In other words, SLP can be considered a type of CP that uses LC. Thus, it is referred to as "laser cavitation peening (LCP)" or "submerged laser peening (SLP)."
When comparing the fatigue strength of post-processed PBF-LS/Ti6Al4V, LCP demonstrated the greatest improvement [7]. In conventional LCP, an Nd:YAG laser with a Q-switch is used to generate nanosecond-order laser pulses. However, since the maximum repetition frequency is only a few dozen Hz, it takes considerable time to treat as-built PBF-LS/Ti6Al4V. As mentioned above, LA is also used for surface smoothing in LCP. If a fiber laser could be used for surface smoothing, the processing time might be drastically reduced, given that the maximum repetition frequency of a fiber laser is 50 kHz.
In the present study, to demonstrate the improvement of fatigue properties of additively manufactured metals by laser treatment (LT) using a fiber laser, PBF-LS/AlSi10Mg placed in a water-filled chamber was treated using the fiber laser. The fatigue properties of PBF-LS/AlSi10Mg were evaluated via a plane bending fatigue test. Surface properties such as surface roughness and residual stress were also measured.

Material and Methods

Figure 1 shows (a) the geometry and (b) a photograph of the PBF-LS/AlSi10Mg specimen. The PBF-LS conditions were as follows: the laser power was 400 W, the laser spot diameter was 100 μm, the preheating temperature of the build platform was 473 K, and the layer thickness was 30 μm. The particles used were 38–53 μm in diameter. After PBF-LS, the specimens were annealed at 473 K for 4.5 h to release residual stress and then cooled in air. To avoid crack initiation from the corners of the as-built specimen, the corners were rounded using a #180 rubber whetstone and #400 disk paper.
Figure 2 illustrates the LT system using the fiber laser, whose wavelength was 1080 ± 2 nm, maximum repetition frequency was 50 kHz, and maximum power was 500 W. The pulsed laser was reflected by mirrors and focused on the target in the water-filled chamber. For comparison with LT using the fiber laser, PBF-LS/AlSi10Mg was also treated by LCP using an Nd:YAG laser, whose maximum repetition frequency was 10 Hz, laser energy was 0.33 J, and wavelength was 1,064 nm.
The fatigue life at constant applied stress was evaluated using a moment-controlled plane bending fatigue test machine with a working frequency of 2 Hz, and the fatigue strength was evaluated using a displacement-controlled plane bending fatigue test machine with a maximum working frequency of 25 Hz. For both machines, the stress ratio R was set to −1 for all fatigue tests.
To investigate the effect of post-processing on residual stress, the surface residual stress (σR) was evaluated using the 2D method with an X-ray diffraction (XRD) system equipped with a two-dimensional (2D) position-sensitive proportional counter (PSPC). The X-rays used were Cu Kα rays operated at 40 kV and 40 mA. The lattice plane (h k l) used was Si (533), and the diffraction angle without strain was 136.9°. The diffracted X-rays were collected by scanning the specimen surface over an 8 mm × 8 mm area, using a 0.8 mm diameter collimator. A total of 24 diffraction rings from the specimen at various angles were detected, and the exposure time per frame at each position was 1 min. In calculating the residual stress, Young’s modulus and Poisson’s ratio were assumed to be 167.2 GPa and 0.221, respectively.
To elucidate the effects of surface characteristics on the fatigue properties, the surface roughness was also measured. The arithmetic mean surface roughness (Ra) was measured using a stylus-type profilometer.

Experimental Results

Figures 3 and 4 show the surface aspect of the specimens observed by a digital microscope and a scanning electron microscope (SEM) for (a) As-built, (b) Blast (B), (c) B + LT, (d) B + LT + LCP, and (e) LCP [9]. In the present experiment, blasting (B) was carried out at an air pressure of 0.7 MPa using garnet #150, based on the previous report. LT was conducted at a repetition frequency of 300 Hz, a pulse width of 300 μs, and a laser pulse density of 50 pulses/mm². The LCP conditions were a repetition frequency of 10 Hz and a laser pulse density of 4 pulses/mm².
As shown in Fig. 3(a) and Fig. 4(a), partially melted particles were present on the surface, and sharp valleys were observed. After blasting, the partially melted particles became dim, and the sharp valleys still remained. As shown in Fig. 3(c) and Fig. 4(c), circular defects were clearly observed, caused by melting from LT using the fiber laser. After B + LT + LCP, the circular defects became smaller. In the case of LCP alone, the surface aspect was similar to that of B + LT + LCP.
Figure 5 shows the fatigue life at σa = 110 MPa. Figures 6 and 7 present the surface roughness (Ra) and surface residual stress (σR), respectively. As the surface was smoothed and compressive residual stress was introduced by blasting (B), the fatigue life was extended. Laser treatment (LT) after blasting further smoothed the surface but introduced tensile residual stress, resulting in shorter fatigue life than the as-built condition. When LCP was applied after B + LT, the fatigue life was longer than with blasting or LCP alone, although the surface roughness increased slightly. This improvement was due to the introduction of large compressive residual stress.
Figure 8 shows the S–N curves for the as-built, B + LT + LCP, and LCP-alone specimens in comparison with casting [11]. When the fatigue strength at 107 cycles was obtained by Little’s method [12], it was 103 ± 12 MPa for B + LT + LCP and 85 ± 10 MPa for LCP, whereas it was 54 ± 9 MPa for the as-built specimen. As shown in Fig. 8, the fatigue properties of B + LT + LCP were better than those of casting and PBF-LS treated by LCP alone. This is because LT reduced surface roughness and produced a quenching-like effect. For example, the full width at half maximum was 0.734 ± 0.023° for the as-built specimen and 1.758 ± 0.001° after LT [9].
Namely, LT acted like a quenching process because the surface was heated to a high temperature by the pulsed laser and then quenched with water, as the LT in the present experiment was carried out in a water-filled chamber. In any case, although LT introduced tensile residual stress, applying LCP after LT was able to introduce larger compressive residual stress than that of LCP alone.

Conclusions

To demonstrate a post-processing method using a fiber laser for improving the fatigue properties of additively manufactured metals, as-built PBF-LS/AlSi10Mg was treated by blasting (B), laser treatment (LT) using a fiber laser, and laser cavitation peening (LCP) using an Nd:YAG laser. It was revealed that the fatigue strength of B + LT + LCP was higher than that of LCP alone, although LT introduced tensile residual stress. The main reasons are that LT smoothed the as-built surface and induced a quenching-like effect, while LCP subsequently generated greater compressive residual stress.

Acknowledgement

This work was partly supported by JSPS KAKENHI (22KK0050 and 23K25988) and JST CREST (JPMJCR2335).

References

[1] H. Soyama, and Y. Okura, “The use of various peening methods to improve the fatigue strength of titanium alloy Ti6Al4V manufactured by electron beam melting,” AIMS Materials Science, vol. 5, no. 5, pp. 1000-1015, 2018.
[2] H. Soyama, and A. M. Korsunsky, “A critical comparative review of cavitation peening and other surface peening methods,” Journal of Materials Processing Technology, vol. 305,  117586, 2022.
[3] H. Soyama, and D. Sanders, “Use of an abrasive water cavitating jet and peening process to improve the fatigue strength of titanium alloy 6Al-4V manufactured by the electron beam powder bed melting (EBPB) additive manufacturing method ” JOM, vol. 71, no. 12, pp. 4311–4318, 2019.
[4] H. Soyama, and F. Takeo, “Effect of various peening methods on the fatigue properties of Titanium alloy Ti6Al4V manufactured by direct metal laser sintering and electron beam melting,” Materials, vol. 13, no. 10, 2216, 2020.
[5] D. Sanders, H. Soyama, and C. De Silva, “Use of Cavitation Abrasive Surface Finishing to Improve the Fatigue Properties of Additive Manufactured Titanium Alloy Ti6Al4V,” SAE Technical Papers, 2021-01-0024, 2021.
[6] H. Soyama, and C. Kuji, “Improving effects of cavitation peening, using a pulsed laser or a cavitating jet, and shot peening on the fatigue properties of additively manufactured titanium alloy Ti6Al4V,” Surface and Coatings Technology, vol. 451, 129047, 2022.
[7] H. Soyama, K. L. Wong, D. Eakins, and A. M. Korsunsky, “The effects of submerged laser peening, cavitation peening, and shot peening on the improvement of the torsional fatigue strength of powder bed fused Ti6Al4V produced through laser sintering,” International Journal of Fatigue, vol. 185, 108348, 2024.
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[9] H. Soyama, “Improvement of Fatigue Strength in Additively Manufactured Aluminum Alloy AlSi10Mg via Submerged Laser Peening,” Coatings, vol. 14, no. 9, 1174, 2024.
[10] H. Soyama, “Comparison between the Improvements Made to the Fatigue Strength of Stainless Steel by Cavitation Peening, Water Jet Peening, Shot Peening and Laser Peening,” Journal of Materials Processing Technology, vol. 269, pp. 65-78, 2019.
[11]J. Linder, M. Axelsson, and H. Nilsson, “The influence of porosity on the fatigue life for sand and permanent mould cast aluminium,” International Journal of Fatigue, vol. 28, no. 12, pp. 1752-1758, 2006.
[12] R. E. Little, “Estimating the Median Fatigue Limit for Very Small Up-and-Down Quantal Response Tests and for S-N Data with Runouts,” ASTM STP, vol. 511, pp. 29-42, 1972.

Hitoshi Soyama 
Professor
Department of Finemechanics, 
Tohoku University 
6-6-01 Aoba, Aramaki, 
Aoba-ku, Sendai, 980-8579, Japan
E-mail: soyama@mm.mech.tohoku.ac.jp