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Abstract
In this paper, residual stress and plastic deformation of TC4 titanium alloys and AA7075 aluminum alloys after laser shock peening (LSP) with the laser pulses that have the same energy and peak intensity but different time profiles have been studied. The results show that the time profile of the laser pulse has a significant influence on LSP. The difference between the results of LSP with varying laser input mode has been contributed to the shock wave caused by different laser pulse. In LSP, the laser pulse with a positive-slope triangular time profile could induce a more intense and deeper residual stress distribution in metal targets. Residual stress distribution changing with laser time profiles suggests that shaping the laser time profile is a potential residual stress control strategy for LSP. This paper comprises the first step of this strategy.
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1. Introduction
Laser shock peening (LSP) is an improved technology that relies on the stress effect. A laser pulse is used to induce plasma below the confinement layer, and a shock wave generated by a plasma explosion is loaded onto the target’s surface. When the shock wave traverses the target, plastic deformation occurs, leaving residual compressive stress in the target. Residual compressive stress plays a significant role in extending the fatigue life of metal targets [1].
An important goal of LSP is to distribute residual compressive stress deep and evenly (at the same depth), primarily determined by the spatiotemporal distribution and the peak pressure of the shock load. Therefore, increasing the intensity or loading time of the shock wave with limited laser energy can improve the processing potential and application range of LSP.
The peak pressure of laser-induced shock waves is proportional to the square of the laser’s intensity, according to Fabbro’s widely accepted analytical equation [2]. However, increasing laser intensity by downsizing laser spots does not always increase the depth of residual stress layers: increasing laser intensity increases the risk of a breakdown in the confinement layer [3], which reduces shock wave peak pressure.
According to the experimental results, the loading time of the shock waves is several times the width of the laser pulse [4]. As a result, a long pulse laser is preferable in LSP. However, the laser pulse width should not exceed 30 ns in most cases, as too long a laser pulse can cause thermal damage and other issues. Berthe demonstrated that parasitic plasma is easier to generate in confinement layers using a long pulse laser, and parasitic plasma reduces loading time [4]. Furthermore, Hu proposed that the residual stress on the center of laser spots is significantly changed by relaxation waves in the case of long laser pulses, resulting in the residual stress holes phenomenon [5]. The size of the laser spots is another factor influencing the loading time. Alexandre pointed out that downsizing laser spots causes radial plasma leakage, which shortens the pressure pulse duration [6].
For the above reasons, laser pulse configurations seem very limited, with little research focusing on laser pulse structure control. Most research about LSP focuses on improving properties of the shocked target, such as: corrosion resistance [7,8], mechanical properties [9,10], and etc. And others focus on the evolution of the target microstructure after LSP [11,12]. For experimental parameters, some researchers have focused on the effect of laser intensity [13], impact number [14], or overlap ratio [15] in LSP. Gaussian-like pulses of Q-switched lasers are usually employed as the default laser time profile [13,14]. Only a few studies employ laser pulse structure (such as time profile and spatial distribution) as research objects [16], and even fewer attempt to design or modify the laser pulse shape to obtain the desired shock load and improve energy efficiency in LSP.
The question then becomes, can shock waves be affected by adjusting the laser time profile with only 10-30 ns laser pulse width? In other words, the laser-induced shock wave is a “slow” process compared to the width of the laser pulse. Can the shock wave respond to microscopic changes in the laser time profile and subsequently alter the residual stress distribution in the target? This will be an extremely important and interesting investigation.
To answer above questions, in this paper, laser pulses with different shapes but the same peak power density, pulse width and single pulse energy are constructed and used for LSP. Firstly, shock waves induced by the two different time profile laser pulses are compared via PVDF measurement. Then, laser pulses with the two time profiles are used for LSP. And the residual stress and plastic deformation of the metal target in LSP are discovered.
2. Experiment setup
2.1 Laser pulse shaping
A 100 J-level nanosecond laser system at the Harbin Institute of Technology is used to obtain varying time profile laser pulses.
This laser system is based on an all-fiber front-end system, a preamplifier system and a main amplifier system. The key process of laser time profile shaping is performed in the front-end system. The all-fiber front-end system contains a continuous fiber oscillator, an arbitrary waveform generator (AWG) and a fiber intensity modulator (IM). Based on the intensity modulation function of IM, a continuous laser output from the continuous fiber oscillator could be turned to a laser pulse. And the time profile of this laser pulse is controlled by IM’s driving signal. IM’s driving signal is generated by the AWG. Therefore, by changing the input file of the AWG, the driving signal of IM is changed and the time profile of the laser pulse outputting from the IM is changed. The shaped laser pulse then amplifies by fiber amplifiers in the front-end system and output.
The custom designed laser pulse output from the front-end system is about 10 nJ for each pulse. Then, after the laser pulse passes through the preamplifier system, the output energy increases to about 2.5 mJ. Finally, the laser pulse is amplified to the required energy by the main amplifier system (four-level MOPA structure amplifier system). The maximum output energy of this nanosecond laser system could be up to 100 J @ 1053 nm and 5 ns rectangular pulses. This system works on a single trigger mode, and the working frequency is one pulse per 15 minutes. More details about this laser system could be seen in Ref. [17].
2.2 Laser shock peening
Laser pulses (1053 nm) output from the laser system are directed to the target area. As shown in Fig. 1(a), in the target area, the laser beam converged by a lens (f = 1200 mm) irradiates vertically on a horizontally placed target. The size of the laser radiation spot on the target is controlled by adjusting the distance from the target to the lens. The targets are TC4 titanium alloy and AA7075 aluminum alloy, which have received much attention in LSP. Table 1 shows the mechanical properties of TC4 and AA7075. The top surface area of the target is 40 mm × 80 mm and is polished to a mirror finish. In the LSP experiment, the top surface is covered with 100-µm aluminum foil as the ablation layer and 4 mm still waters as the confinement layer. The aluminum foil uses a normal commercial aluminum foil tape, aimed to prevent laser thermal ablation of the target.
Fig. 1. Experiment setup. a) Setup of the target area in LSP. b) Time profile of DT and UT pulse. c) Impact sites on the target surface.
In laser-induced plasma explosions and driving shock waves, irrespective of the initial plasma ignition in the early stage or the inverse bremsstrahlung absorption in the later stage, their behaviors all depend on the instantaneous laser intensity and laser energy pre-deposition. In this experiment, two types of laser pulses with time profiles symmetrical to each other are designed to investigate the influence of laser energy-temporal distribution (energy is concentrated in the front or the end) on shock waves and then how to affect the residual stress distribution in the target. As shown in Fig. 1(b), the first is a negative-slope triangular profile with monotonically decreasing power after a steep front (referred to as a DT pulse in this article), and the second is a positive-slope triangular profile with monotonically increasing power to its peak (called UT pulse). The full width of the DT and UT pulses at half maximum is ∼5 ns, and the full width at the bottom is 20 ns. The energy of the laser pulses used in LSP experiment is approximately equivalent (around 40 J), and the peak intensity of all laser pulses is guaranteed to be about 10 GW/cm2 to avoid laser-induced parasitic plasma in confinement layers [2]. The only difference between DT and UT pulses in this experiment is their time profiles.
One TC4 target and four AA7075 targets were used in this experiment. Figure 1(c) depicts four impact sites on their top surface. For each target, the impact sites 1 and 2 were shocked by DT pulse (DT-LSP) once and three times, respectively, and sites 3 and 4 were shocked by UT pulse (UT-LSP) thrice and once, respectively. After each shock, the ablation layer and confinement layer are both replaced with new materials.
In LSP experiment, the target distance from the lens about 1040 mm. The laser radiation spot on the target is a 7-8 mm diameter hat-like spot. Figure 2 shows the spatial profile of the spot. Spatial uniformity of the spots is represented by the ratio of the average intensity to maximum intensity. The calculated value of this ratio is ∼0.44.
3. Results
3.1 Shock waves induced by varying laser pulse
A PVDF piezoelectric film with a 3 mm active area diameter (Jinzhou Kexin, China, JYC03-3B) is used to measure shock waves induced by laser pulse. The PVDF film is connected in parallel with 5 Ω resistors to ensure a response time < 1 ns. A PMMA block (40 mm × 80 mm × 10 mm) is used as a substrate to avoid reflection of shock waves. The PVDF sensor and PMMA substrate replace the metal target of the experiment setup introduced in Fig. 1(a). And active area of the PVDF film is centered at the laser irradiation spot.
Figure 3(a) gives two types of laser’s time profile that all has a bottom width of 20 ns. When the two types laser pulses have the same energy of single pulse, they could have the similar peak intensity. Figure 3(b) shows the peak pressure of the shock wave induced by the two types laser pulses that changes with the laser’s peak intensity. Each scatter point shown in Fig. 3(b) represents at least three measurements based on PVDF sensors. The lines in Fig. 3(b) are calculated values based on Fabbro’s mode [20].
Fig. 3. Peak pressure of shock waves changing with laser intensity under varying laser pulse input. a) Normalized time profile of laser input pulses. b) Peak pressure measured by PVDF sensor and fitting line of peak pressure data.
As shown in Fig. 3, for different laser pulse shape, the peak pressure has different increasing trend line. When the input intensity is weak, the peak pressure induced by the two different laser pulse is close. With the increase of laser intensity, the difference in peak pressure induced by the two types of laser pulse increases. The result indicates that the laser’s time profile affects the increasing mode of laser induced shock waves. And in the case of a strong laser pulse, UT-pulse creases a stronger laser induced shock wave.
3.2 Plastic deformation—the macrocrater depth
In this study, the depth at the center of the macrocrater is used as a scale to evaluate the degree of plastic deformation caused by laser pulses with varying time profiles. Due to the existence of ablation film on the metal target, there is no ablation effect on the target. The macrocrater on the target surface is formed only by stress effect. Based on the assumptions of Su [21] and Ballard [22], the macrocrater depth increases with the peak pressure and pulse width of the shock wave applied to the target.
There are no obvious depressions on the surface of the TC4 target after LSP, whereas macrocraters visible with the naked eye are induced on AA7075 targets. A Zygo NewView 8200 optical profiler is used to measure the morphology of the macrocraters on the surface of the AA7075 targets and to compare the effects of the laser time profile on the plastic deformation in LSP. Figure 4(a) depicts the morphology of the macrocraters using four different LSP modes, with the texts in brackets representing the label for each target. Figure 4(b) shows the depth at the center of the craters depicted in Fig. 4(a).
Fig. 4. Plastic deformation for each spot of AA7075 targets. a) The morphology of macrocraters. b) The center depth of craters.
According to Fig. 4, no matter three times shock or single shock, UT-LSP has a deeper macrocrater. It means that UT-LSP could induce more intense plastic deformation. This result can be attributed to a higher peak pressure that can be induced by UT-pulse as shown in Fig. 3. On this basis, an inference can be made: the laser time profile could alter the plastic deformation process in LSP.
3.3 Characteristics of residual stress distribution—depth and uniformity
The residual stress of a TC4 target and an AA7075 target was measured along the depth direction using an iXRD stress measurement system. Figure 5 depicts the diff-residual stress values for the two targets at various depth positions (the signs of the values indicate the directions of stresses). The diff-residual stress is the difference between the residual stresses at each test position and the corresponding reference position. The reference positions are located away from the laser irradiation area and at the same depth as the test positions. Given that pretreatment, including mechanical polishing and electrochemical polishing is required when detecting residual stress below the target top surface, diff-residual stress is the true residual stress caused by LSP because it eliminates the potential influence of pretreatment on the residual stress. When considering the effect of the laser time profile on the residual stress distribution in the target body after LSP, diff-residual stress is equivalent to residual stress.
Fig. 5. Diff-residual stress distribution of TC4 target and AA7075 target. The blue dots correspond to the centers of each shock site, while the red dots correspond to 2 mm offset positions from the centers.
According to the experimental results, the residual stress distributions of TC4 and AA7075 targets are very different, even though they were all processed using the same scheme described in Section 2.
The main difference between the AA7075 and the TC4 targets is the position where the maximum residual compressive stress appears. For the TC4 target, the maximum residual compressive stress was observed on the top surface of the target regardless of DT-LSP or UT-LSP, and residual compressive stress decreases with depth. While for the AA7075 target, the maximum residual compressive stress was observed in the depth range of 300–500 µm (DT-LSP and UT-LSP). This can be explained as follows: because the Hugniot limit ($\textrm{HEL} = \frac{{1 - \nu }}{{1 - 2\nu }}\textrm{Ys}$) of AA7075 is less than that of TC4 (HEL = 1.06 GPa for AA7075 and HEL = 1.72 GPa for TC4), the plastic deformation of the AA7075 target is more intense when subjected to the same shock load.
The uniformity of residual stress is also considered. The stress difference between the centers of the shocked sites and the 2 mm offset positions at the same depth (denoted as Δσ) is used to characterize the uniformity. Figure 6 shows the uniformity of residual stress that caused by different LSP modes in AA7075 target and TC4 target.
Fig. 6. The uniformity of residual stress caused by different LSP modes. a) AA7075 target with a single LSP, b) AA7075 target with three times LSP, c) TC4 target with a single LSP, d) TC4 target with three times LSP.
For the TC4 target with a single UT-LSP shock, the Δσ on the top surface layer is about 100 MPa and gradually tends to zero at a depth of 280 µm (Fig. 6(c)). And Δσ is significant from the top surface to a depth of 550 µm for the TC4 target with three UT-LSP shock (Fig. 6(d)). In contrast to the TC4 target, the Δσ for the AA7075 target lacks regularity. At each depth, the Δσ for the AA7075 target remains a relatively large value (Fig. 6(a) and Fig. 6(b)). Δσ is generated by the stress wave reflected from the edge of laser spots. With the increase of depth, the intensity of shock waves decreases. At the same time, the intensity of the reflected waves decreases with the increase of depth. Thereby, in TC4 target, Δσ trends to decrease with the depth. However, for AA7075 target, AA7075 alloy has a lower HEL and therefore more susceptible to the reflected stress wave. This is the reason why the AA7075 target remains a relatively large Δσ at each depth.
Although the residual stress distributions of the TC4 target and the AA7075 target are different, there are some interesting similarities when focusing on the influence of LSP modes on the residual stress.
UT-LSP usually leads to a greater depth of the residual compressive stress layer than DT-LSP. For the TC4 target, the depth is 280 µm for a single DT-LSP shock and increase to 550 µm after three DT-LSP shocks. While the UT-LSP could reach a depth of 800 µm with only one shock, which is deeper than the depth caused by three DT-LSP shocks. This trend also exists in the AA7075 target. For the AA7075 target, both single and three UT-LSP shocks, the depth can reach about 2000µm, which is as same as the depth cause by three DT-LSP shocks and deeper than the depth caused by a single DT-LSP (∼1000 µm).
Compared to DT-LSP, UT-LSP often has a higher maximum residual compressive stress. For the TC4 target, the maximum residual compressive stress caused by a single DT-LSP is only −140 MPa which is less than the stress caused by a single UT-LSP shock (−350 MPa). And the maximum value of three DT-LSP shocks (−360 MPa) is also less than the value of three UT-LSP shocks (−450 MPa). As same as the TC4 target, in the AA7075 target, UT-LSP has a higher maximum residual compressive stress for both single and three shocks.
DT-LSP seems could get a better uniformity of residual stress. For the TC4 target, Δσ gradually decreases to zero with the increase of depth in UT-LSP. While in DT-LSP, Δσ is not visible in the entire range of the residual compressive stress depth, whether single or three shocks.
Above results imply that the UT pulse can produce a deep and intense residual compressive stress, but will destroy residual stress uniformity.
4. Discussion
4.1 Pressure loading with different laser time profiles
In Fabbro’s mode, the pressure of the laser induced shock wave is connected with the input laser pulse and the development of plasma’s thickness [20]. The laser energy is absorbed by the plasma and used to increase plasma’s internal energy and as the pressure work. According to energy conservation, Fabbro gives:
In addition, based on the shock-wave relation, there is a relationship between pressure P and plasma’s thickness L as:
where, Z is the composite shock-wave impedance. Equation (2) and Eq. (3) describe the development of plasma controlled by laser time profile. By putting intensity distribution I(t) of DT-pulse or UT-pulse into Eq. (2) and Eq. (3), and fitting the calculated peak pressure with the measurement data, the initial plasma thickness L0 = L(0) and the fraction α of different laser pulse induced plasma could be validated.Initial plasma thickness L0 represents the size of the gas bubble formed by laser ablation at the beginning of laser input. Fraction α reflects the energy partition during the plasma develop process. Through fitting measured pressure data into this model, as shown in Fig. 3(b), the initial plasma thickness and the fraction for UT or DT laser pulse are obtained. Based on this model, L0 = 119 µm and α = 0.36 are obtained for UT laser pulses and L0 = 0.7 µm and α = 0.02 for DT laser pulses.
A larger initial plasma thickness L0 of UT pulses means that UT pulses heat and evaporate more ablation material at the laser’s front edge. And a large fraction α of thermal energy to internal energy of UT pulses means that there is a less fraction of internal energy, (1-α)Ei, used to ionize the gas.
Thereby, the temporal distribution of input laser energy (laser time profile) affects the pressure loading by adjusting the energy partition ratio of the generation and evolution process of laser induced plasma. Vaporizing the ablation layer with low initial energy at the beginning and then heating and ionize the plasma following, such as the UT pulse, will generate a high-amplitude laser-induced shock wave. Therefore, there is a possibility of modifying pressure loading by changing the time profile of the laser pulse.
In addition, the parasitic plasma is another factor that will affect the pressure loading. When parasitic plasma is generated, the peak pressure of laser induced shock waves will decrease [2]. However, with the increase of laser intensity, there is no significant reduces of peak pressure as shown in Fig. 3. So, the parasitic plasma is not the reason why the peak pressure induced by the UT pulse is higher than that of the DT pulse.
4.2 Potential strategies to control residual stress
According to the results of LSP, UT-LSP could result a more intense residual compressive stress distribution, however, the residual distribution of DT-LSP seems to be more uniform (as shown in Fig. 5 and Fig. 6). To illustrate these differences, a finite element model (FEM) is produced to investigate effects of the pressure loading time profile on the residual stress distribution. Figure 7(a) shows the part of the FEM model (16 × 16 × 4 mm3 AA7075 aluminum alloy part) and the pressure loading area on this part (4 mm diameter circle). The time profile of pressure loading to simulate UT-LSP and DT-LSP are obtained by sampling the measurement pressure profile, as shown in Fig. 7(b).
Fig. 7. FEM model for laser shock peening. a) Spatial distribution of pressure loading: an 4 mm diameter circle area on a 16 × 16 × 4 mm3 aluminum alloy part. b) Time profile of pressure loading: marked lines for FEM calculation and dash lines for experiment data.
Based on this model, residual stress along the surface direction and the maximum residual compressive stress along the depth direction are respectively compared for two different loading profile. Figure 8(a) shows the surface residual distribution. It can be seen that UT pressure loading creases more intense residual stress on the surface. But there is a decrease of residual compressive stress in the center of the pressure loading area (residual stress hole) for UT pressure loading. The maximum residual compressive stress along the depth direction given in Fig. 8(b) also proves higher residual compressive stress distribution for UT pressure loading.
Fig. 8. Residual distribution simulated by FEM model. a) Residual stress along the surface direction. b) the maximum residual compressive stress along the depth direction.
Above simulation results show that UT-laser pulse induced pressure loading could create more intense but less uniform residual stress distribution, which are similar to experimental results. According to these results, the residual stress distribution of LSP could be affected by adjusting the time profile of input laser pulses. Moreover, adjusting laser time profile may be a potential strategy to eliminate residual stress holes problem in LSP.
The UT pulse can generate a large and deep residual compressive stress, and the DT pulse may suppress the generation of residual stress holes. However, it is challenging to achieve both. How can a suitable laser pulse time profile be designed to achieve the desired compressive residual stress distribution in LSP? This is an interesting topic that should be researched further to obtain some quantitative rules. This article is only the first step in this investigation.
5. Conclusion
This study demonstrates that changing the time profile of input laser pulses can alter the shock loading and affect the residual stress distribution. It is found that initial plasma size and the partition of plasma’s internal energy are affected by the energy distribution of input laser pulse. A bigger part of plasma’s internal energy that used to increase plasma’s thermal energy could increase a higher peak pressure. And the energy partition is affected by initial plasma size. In this paper, two types of laser pulse with symmetrical temporal structure (UT pulse and DT pulse) are constructed. LSP processes with UT or DT pulse causes different residual stress distributions. Thereby, a potential strategy to control the distribution of residual stress in LSP by laser pulse shaping is proved. This paper is the first step toward developing LSP residual stress optimization or control technology based on this strategy. In the future, the most important topics in LSP, such as maximum residual compressive stress, plastic-affected depth, and residual stress uniformity, will be addressed by this technology.
Funding
National Natural Science Foundation of China (No. 61622501).
Disclosures
The authors have no conflicts to disclose.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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