Introduction 

Three dimensional additively manufactured metallic materials (3D-Metal) are attractive materials for medical implants and aviation components, as 3D-Metal is formed from CAD/CAM data directly and it can realize topology-optimized geometry. As the fatigue strength of 3D-Metal are nearly half of bulk metals, the improvement of fatigue strength of 3D-Metal is required, and mechanical surface treatment such as shot peening (SP) is one of those methods [1].

Recently, cavitation peening (CP), in which cavitation impact at bubble collapse is used, has been developed [2]. Normally, cavitation at CP is generated by injecting a high-speed water jet in water, i.e., a cavitating jet [3]. At submerged laser peening (LP), a bubble, which behaves like cavitation bubble, is generated after laser ablation, and the bubble collapse impact also utilizes for peening. Thus, it is called as “laser cavitation peening (LCP)” [4], and LCP can improve the fatigue strength of magnesium alloy AZ31 drastically [5]. In the case of LCP, laser ablation can smoothen the surface roughness of 3D-Metal, and it will assist the improvement of fatigue properties of 3D-Metal.

Normally, a rotating bending fatigue test is used to evaluate fatigue properties of peened metals. At the rotating bending fatigue test, high-precision centering (variance ≤ 50 μm) is required, then machining of specimens is necessary. Thus, the rotating bending fatigue test is not suitable for investigating the fatigue properties of as-built surface of 3D-Metal.

On the other hand, a plane bending fatigue test, in which the corners of specimens are rounded, has been used to evaluate the fatigue properties of 3D-Metal, then the effect of roundness of specimens could not be evaluated.

In the present paper, in order to evaluate fatigue properties of as-built 3D-Metal, a torsion fatigue tester was developed, and it was demonstrated that the improvement of the fatigue strength of 3D-Metal by LCP compared with that by SP.

Material and Methods 

Figure 1 shows a photograph of specimen of 3D-Metal and the geometry of specimen. The specimen was additively manufactured by direct metal laser sintering (DMLS) by using Ti6Al4V particles of 40 μm in diameter [6].

The condition of DMLS was as follows: the beam diameter, power, and scanning speed of the laser were 0.2 mm, 400 W, and 7 m/s, respectively, and the stacking pitch of layer was 60 µm. After DMLS, the specimens were annealed at 923 K for 3 h to release any residual stress, and then subjected to solution heat treatment at 1223 K under vacuum for 105 min before cooling under argon gas. Finally, each specimen was aged at 978 K under vacuum for 2 h before cooling under argon gas. In order to fix the specimen to a torsion fatigue tester, a chucking part was finished by a metallic file with a lathe.  

Figure 2 reveals schematic of laser cavitation peening system [6]. The used pulsed laser was Nd:YAG laser with Q-switch operated at the fundamental harmonic wavelength, i.e., 1,064 Nm. The pulsed laser was focused on a  specimen in a water-filled chamber made of hard glass using a convex lens. The diameter of the laser spot on the specimen surface was about 0.8 mm. The chamber was filled with degassed water of 5 L/min to remove the particles in the water produced by ablation of the specimen surface. The specimen was fixed to a holder, which moved in an axial direction with rotation using two stepping motors. The pulse density dL was defined by the horizontal interval dH and the vertical interval dV (see Figure 2), as shown in Eq. (1).

Figure 3 shows a torque-controlled torsion fatigue tester [6]. A special attachment of specimen for torsion test was placed on a load-controlled plane bending fatigue tester [7]. The attachment can be also used for a displacement-controlled plane bending fatigue testing machine. The misalignment and/or distortion due to DMLS process was corrected by the angle of the servomotor. The stress ratio R was −1 during the present fatigue test. The test frequency at the fatigue test was 2 Hz. The resolutions of the recorded angle and torque were 0.005° and 0.01 Nm, respectively. Both the torsion angle θ and torque Mt were monitored and recorded during the fatigue test. 

Experimental Results   

Figure 4 reveals the aspect of specimen during laser cavitation peening observed by a high-speed video camera [8]. At t = 0 ms, a bright spot due to laser ablation (LA) was observed. After LA, a bubble, i.e, a laser cavitation (LC) was initiated and developed then collapsed, i.e., 1st collapse, at t = 0.83 ms. After the 1st collapse, LC was developed again and then collapsed at t = 1.22 ms, which was the second collapse. Both of the first collapse and the second collapse were detected by a hydrophone [8].  

In order to show the surface of the specimens, Figures 5 and 6 reveal the aspects of the as-built specimen and the specimen treated by LCP observed by a digital microscope and a scanning microscope, respectively. As shown in Figs. 5 and 6, a lot of un-melted particles were observed on the surface of the as-built specimen. On the other hand, the surface treated by LCP appeared partially melted by laser ablation. Both of the arithmetical mean roughness and the maximum roughness height were decreased by LCP [6]. Also, LCP introduced the compressive residual stress [8] .

Figure 7 shows the amplitude relation of the torsion angle θ’ of specimens during torsion fatigue tests at τa = 450 MPa [6]. This is one of the advantages of the developed torsion fatigue tester. The torsion angle was normalized with respect to the initial amplitude of the torsion angle. The increase of θ’ revealed crack propagation, which was delayed by LCP and SP. Then, it can be said that the LCP and SP suppressed crack initiation rather than crack propagation compared with as-built one.

Figure 8 illustrates the result of the fatigue test for Ti6Al4V treated by LCP and SP compared with as-built specimen [9]. For LCP, dL = 40 pulse/mm2 was chosen. The fatigue strength at N = 107 was 217 MPa for as-built, 361 MPa for LCP and 285 MPa for SP. It was concluded that LCP improved the fatigue strength by 66% comparing with as-built one. 

Conclusions 

In order to investigate the improvement of the fatigue properties of as-built 3D-Metal modified by the mechanical surface treatments, the torsion fatigue tester was developed, and it was revealed that the developed fatigue tester could evaluate the fatigue life and strength of 3D-Metal of treated specimens compared with as-built specimen. In the present paper, Ti6Al4V manufactured by DMLS was tested, and it was concluded the laser cavitation peening improved the fatigue strength at 107 about 66 % comparing with as-built specimen.  The improvement of the fatigue properties by LCP was resulted from the suppression of the crack initiation rather than crack propagation.

Acknowledgement

This work was partly supported by JSPS KAKENHI Grant Number 23H01292 and 22KK0050, and the Amada Foundation AF-2021219-B3. 

Figures 1-3 and 7 are reprinted from Surface and Coatings Technology, vol. 451, 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,”. 2022, with permission from Elsevier, License Number 5584240855031.

References

[1] 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, 2216, 2020.

[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, “Cavitating jet: a review,” Applied Sciences, vol. 10, 2020.

[4] H. Soyama, and Y. Iga, “Laser cavitation peening: a review,” Applied Sciences, vol. 13, 6702, 2023.

[5] H. Soyama, C. Kuji, and Y. Liao, “Comparison of the effects of submerged laser peening, cavitation peening and shot peening on the improvement of the fatigue strength of magnesium alloy AZ31,” Journal of Magnesium and Alloys, vol. 11, pp. 1592-1607, 2023.

[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, “Evaluation of crack initiation and propagation of stainless steel treated by cavitating peening using a load controlled plate bending fatigue tester,” Metal Finishing News, vol. 15, no. 4, pp. 60-62, 2014.

[8] C. Kuji, and H. Soyama, “Mechanical surface teatment of titanium alloy Ti6Al4V manufactured by direct metal laser sintering using laser cavitation,” Metals, vol. 13, 181, 2023.

[9] H. Soyama, and C. Kuji, “Comparison on fatigue strength of additive manufactured titanium alloy by various peening methods,” Proceedings of Annual Meeting of Society of Materials Science, Japan, vol. 72, 2023. 

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

Assistant Professor, 

Department of Finemechanics, 

Tohoku University