Effects of Laser Peening with a Pulse Energy of 1.7 mJ on the Residual Stress and Fatigue Properties of A7075 Aluminum Alloy

Abstract

Laser peening without coating (LPwC) using a palmtop-sized microchip laser has improved the residual stresses (RSs) and fatigue properties of A7075 aluminum alloy. Laser pulses with a wavelength of 1.06 μm and duration of 1.3 ns from a Q-switched Nd:YAG microchip laser were focused onto A7075 aluminum alloy samples covered with water. X-ray diffraction revealed compressive RSs on the surface after irradiation using laser pulses with an energy of 1.7 mJ, spot diameter of 0.3 mm, and density of 100–1600 pulse/mm². The effects were evident to depths of a few hundred micrometers and the maximum compressive RS was close to the yield strength. Rotation-bending fatigue experiments revealed that LPwC with a pulse energy of 1.7 mJ significantly prolonged the fatigue life and increased the fatigue strength by about 100 MPa with 10⁷ fatigue cycles. The microchip laser used in this study is small enough to fit in the hand or be mounted on a robot arm. The results may lead to the development of tools that extend the service life of various metal parts and structures, especially outdoors where conventional lasers are difficult to apply.

1. Introduction

Laser shock peening (LSP), or laser peening (LP), is a well-qualified technique to induce compressive RSs in the near-surface layer of metals [1,2,3,4,5,6]. LSP prolongs the service life of metal components by retarding fatigue crack initiation and propagation and has been applied to the fan and turbine blades of jet engines to prevent foreign object damage (FOD) since the 1990s [7,8,9]. In the early stage of LSP development, high-power lasers such as 100 J-class Nd:glass lasers were used [1]; recently, 10 J-class Nd:YAG or Nd:YLF lasers have been commercially employed [10,11]. When using such high-power lasers, a surface coating (an adherent metal tape, polymer tape, or paint) is necessary to prevent surface melting and damage [12,13].

Another type of LSP that does not use such a coating, namely LPwC, was invented in the mid-1990s [14,15] to reduce stress corrosion cracking (SCC) susceptibility and has been used to treat nuclear power reactor components since 1999 [6,16,17,18]. Reactor components are immersed in water to shield gamma radiation; as such, they cannot be coated. However, even without coating, surface compression was achieved by reducing the pulse energy to about 200 mJ or less and increasing the pulse density [14,19]. In the 2000s, it was found that LPwC significantly enhanced fatigue properties despite increasing surface roughness [20,21,22,23,24,25]. LPwC is a simple technique whereby successive laser pulses are irradiated to water-covered metal materials. The LPwC pulse energy is lower than that of LSP; thus, the laser is smaller. However, the total system remains large, as it includes laser-delivering optics, a power supply, a chiller, and a controller; LPwC has thus not been widely used.

To deal with this issue, Taira et al. started to develop a monolithic high-power microchip laser with an Nd:YAG laser medium pumped by laser diodes and a Cr:YAG passive Q-switch in the late 1990s [26,27]. They also recently reported direct bonding of transparent optical elements at room temperature, achieving a revolutionary laser architecture called distributed face cooling (DFC) [28]. Heat is removed efficiently, permitting pulse energies of up to 20 mJ [29]. The pulse duration is about 1 ns or less, which is one-tenth that of conventional Q-switched Nd:YAG lasers. Therefore, the peak laser intensity of a 20 mJ-class microchip laser is equivalent to that of a laser of 200 mJ pulse energy.

In this study, we employed a commercial prototype of palmtop-sized microchip lasers (3 mJ) [30,31,32], and subjected A7075 aluminum alloys to LPwC with a pulse energy of 1.7 mJ to investigate whether such low-energy treatment could induce compressive RSs and enhance fatigue properties. As a result, compressive RS close to yield strength was confirmed by X-ray diffraction (XRD), and rotation-bending experiments revealed improved fatigue properties.

2. Materials and Methods

2.1. Sample Preparation

We used A7075-T73 plates with a thickness of 3.175 mm (1/8 inch) to study RSs induced by LPwC. Fatigue samples were prepared by lathing from an A7075BE-T6511 rod with a diameter of 15 mm. The chemical compositions and mechanical properties of both materials are listed in

Table 1. Chemical compositions of the materials used in the experiments (wt%).

Material	Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti	Al
A7075-T73	0.07	0.17	1.5	0.02	2.5	0.19	5.7	0.03	Bal.
A7075BE-T6511	0.08	0.19	1.8	0.04	2.6	0.20	5.8	0.01	Bal.

and

Table 2. Mechanical properties of the materials used in the experiments.

Material	0.2% Proof Stress	Tensile Strength	Elongation
A7075-T73	425 MPa	497 MPa	12.0%
A7075BE-T6511	627 MPa	660 MPa	9.2%

. The shape and dimensions of the fatigue samples are described in Figure 1. The stress concentration area had a radius of curvature of 10 mm and minimum diameter of 5 mm. The resulting stress concentration factor, Kt, was 1.074.

2.2. LPwC Setup and Conditions

The LPwC setup used to explore RS development in A7075-T73 plates is shown in Figure 2. Laser pulses from a palmtop-sized microchip laser impacted the samples via a plano-convex lens mounted in a coaxial nozzle. Water from the nozzle removed ablation products, thus keeping the laser path clean to prevent absorption or scattering. During laser irradiation, the plate was driven in two dimensions using a linear X–Y stage. We tested two LPwC modes; an underwater mode and a water spray mode (Figure 2, left and right, respectively). The only difference between the modes was the water level. Sakino et al. [33] confirmed that both modes were identical in terms of the ability to create RSs in HT780 high-strength steel. We used both LPwC modes to treat flat A7075-T73 samples and assessed whether the RSs differed.

During LPwC of round rod samples, each sample was placed on a rotating stage and simultaneously driven up or down by a linear stage in the direction of the sample axis; thus, the sample was treated in a spiral manner. A photograph of the setup is shown in Figure 3; we employed the underwater mode to reduce splashing and peening noise.

Table 3. The LPwC conditions.

Parameter	RS Sample	Fatigue Sample
Laser wavelength (μm)	1.06	1.06
Pulse energy (mJ)	1.7	1.7
Pulse duration (ns)	1.3	1.3
Spot diameter (mm)	0.30	0.30
Peak power density (GW/cm2)	1.9	1.9
Pulse repetition rate (Hz)	50	50
Pulse density (pulse/mm2)	100–1600	800

summarizes the laser irradiation conditions. The pulse energy was calculated from the energy at the laser outlet considering energy absorption over the water path of the laser beam. The pulse irradiation density ranged from 100 to 1600 pulse/mm² for RS evaluation but was fixed at 800 pulse/mm² for the rod samples. Despite the low pulse energy, the pulse duration was as short as 1.3 ns. Thus, the peak laser power density was quite high, comparable to that of LSP with pulses of a few tens of nanoseconds in duration.

The processing speed can be calculated by dividing the laser pulse repetition rate by the pulse irradiation density. At a pulse repetition rate of 100 Hz and a pulse irradiation density of 800 pulse/mm², it takes about 13.3 min to process a unit square centimeter.

2.3. Measurement of RSs

Surface RSs were measured in two directions (X and Y), X is the laser sweep direction and Y is perpendicular to X, using the μ-X360s device (Pulstec Industrial Co. Ltd., Shizuoka, Japan) [34]; this employs the “cosine α” XRD method [35]. The characteristic Cr-kα X-ray was used for irradiation via a collimator of diameter 1.0 mm, and X-rays diffracted by the Al 311 planes were collected by an area detector. The sample was rocked through 10° to obtain a smooth diffraction pattern and reduce the standard deviation of the regression line. The RS depth profiles were estimated via alternating XRD with electrolytic polishing. RS redistribution after removal of a strained area is not large when the removed volume is small. Therefore, the RS values obtained via XRD were not corrected.

2.4. Fatigue

Rotation-bending fatigue was applied to rod samples after LPwC and reference samples without LPwC in ambient conditions; the stress ratio was −1.0, the frequency was 66.7 Hz (4000 rpm), and the run-out was set to 2 × 10⁷ cycles. A cantilever-type fatigue machine was employed (Figure 4).

3. Results and Discussion

3.1. Surface RSs on A7075-T73

LPwC in both the underwater and water spray modes (Figure 2) was applied to A7075-T73 plates with a thickness of 3.175 mm (1/8 inch). As described in Section 2.2, laser pulses with a wavelength of 1.06 μm, pulse energy of 1.7 mJ, pulse duration of 1.3 ns, and repetition rate of 50 Hz were focused on 0.30-mm-diameter spots. The peak power density (laser fluence) was 1.9 GW/cm². The irradiated pulse density ranged from 100 to 1600 pulse/mm².

Figure 5 shows the surface RSs on A7075-T73 plates; σ_x and σ_y denote the RS components parallel (X) and perpendicular (Y) to the laser sweep direction, respectively. The standard deviation of the RSs was about ±10 MPa. We confirmed that the absolute value of σ_y tends to be larger than that of σ_x, as obtained in our initial study [14]. The RSs afforded by both LPwC modes (underwater and water spray modes) did not differ. The RSs tended to saturate at a pulse density of around 400 pulse/mm².

3.2. RS Depth Profiles

The RS depth profiles are depicted in Figure 6 for A7075-T73 subjected and not subjected to LPwC. Compression reached a depth of about 0.25 mm at pulse densities of 400 and 800 pulse/mm². We expected that the depth would increase as the pulse density increased [6], but this was not the case; probably because the compression induced by LPwC saturated at about 0.25 mm. The maximum compressive RS in the Y-direction was about 400 MPa, which is close to the yield strength (the 0.2% proof stress in Table 2) of the material.

The sample thickness was 3.175 mm, which was much greater than the depth of compression. Therefore, the samples after LPwC have no or only a little distortion. Even if there is some distortion, the RSs will release only slightly and the effects of LPwC would likely be marginally underestimated. Further experiments or simulations might be useful to confirm this.

3.3. Fatigue Properties

A7075-T6511 rods were subjected to underwater LPwC (Figure 3). The conditions were the same as for the plate samples, except that the pulse density was fixed at 800 pulse/mm². Then, rotation-bending fatigue loading was applied to samples that had or had not undergone LPwC using the fatigue machine as shown in Figure 4. The results are summarized in Figure 7 as S–N curves. The data with arrows reached the run-out. The regression curves were obtained using the Strohmeyer’s expression, and the curves for the reference samples and the samples with LPwC are shown in red and blue, respectively. The notation of 10%, 50%, and 90% in the figure are the fracture probabilities predicted by the regression analysis. For the reference samples, the 50% probability of fracture is expressed by

log (σ_a − 149.2) = −0.1987 log (N) + 3.178

(1)

with the sum of squared residuals of 0.00297 and the standard deviation of 0.0273. For the samples with LPwC, it is expressed by

log σ_a = −0.05113 log (N) + 2.857

(2)

with a sum of squared residuals of 0.00285 and a standard deviation of 0.0239.

LPwC at a low pulse energy of 1.7 mJ significantly improved the fatigue characteristics of A7075BE-T6511; the fatigue strength was enhanced about 1.5-fold and the fatigue life about 100-fold. Sakino et al. investigated in detail the effects of LPwC on the fatigue properties of welded joints of 490 MPa grade rolled steel plates and concluded that the main factor in the fatigue properties’ improvement was the compressive RSs introduced into the surface around the weld toe by LPwC [25]. We attempted XRD on A7075BE-T6511 but failed to measure RSs because the material was heavily textured, probably due to fabrication via extrusion. However, the authors expect that LPwC introduced compressive RSs close to the yield strength, as in the case of A7075-T73 (Figure 5 and Figure 6), which resulted in improved fatigue properties.

Figure 8 shows the fracture surfaces of selected samples. The stresses in parentheses are the stress amplitudes applied during the fatigue cycles. In the reference sample without LPwC (250 MPa) and a sample that underwent LPwC (450 MPa), the fracture surfaces are relatively flat and nearly perpendicular to the sample axes. However, LPwC samples at stress amplitudes of 350 and 400 MPa exhibited complicated fracture surfaces with undulations. The original material was extruded and then subjected to tensile pre-straining during stress-relief; the crystal grains would be expected to be strongly oriented and a material textured in this manner may exhibit various fracture surfaces when fatigue loading is combined with the RSs caused by LPwC [36]. When the fatigue loading is sufficiently high, the fatigue crack grows perpendicular to the loading direction, yielding a simple fracture surface. However, if the fatigue loading is smaller than (or competes with) the RSs and effects attributable to the mechanical anisotropy of a textured material, the resulting fracture surface is complicated, reflecting the inherent crystal structure.

4. Conclusions

We subjected A7075-T73 and A7075BE-T6511 aluminum alloys to LPwC at a pulse energy of 1.7 mJ, which is orders of magnitude smaller than the pulse energies of current LSP [1,2,3,4,5] and LPwC [6,14,15,16,17,18] devices. XRD revealed that significant compressive RSs (close to the yield strength) were generated on A7075-T73, and the affected depth was about 0.25 mm from the surface. The rotation-bending fatigue experiments of A7075BE-T6511 showed that LPwC at a pulse energy of 1.7 mJ enhanced the fatigue strength by about 1.5-fold and prolonged the fatigue life by about 100-fold. Thus, even when a palmtop-sized microchip laser was used, LPwC was highly effective, significantly improving the RSs and fatigue properties of A7075 aluminum alloys.

Current LPwC/LSP lasers are bulky and are placed on vibration isolation tables in clean rooms or under ambient temperature control. Their application has therefore been limited to indoor use for production. In contrast, palmtop-sized high-power microchip lasers are portable, extending LPwC applications to various objects not only indoors but also outdoors, such as infrastructure maintenance [6,37,38].