**6. Laser shock peening of aluminium alloys**

Laser shock peening (LSP) is an innovative surface treatment technique, which is successfully applied to improve fatigue performance of metallic components. After the treatment, the fatigue strength and fatigue life of a metallic material can be increased remarkably owing to the presence of compressive residual stresses in the material. The increase in hardness and yield strength of metallic materials is attributed to high density arrays of dislocations and formation of other phases or twins, generated by the shock wave.

The ability of a high energy laser pulse to generate shock waves and plastic deformation in metallic materials was first recognised and explored in 1963 in the USA (Ding & Ye, 2006). A schematic configuration of an LSP process on a workpiece is shown in figure 36 (Dubourg et al., 2005).

When shooting an intense laser beam on to a metal surface for a very short period of time (around 30 ns), the heated zone is vaporised to reach temperatures in excess of 10 000°C and then is transformed to plasma by ionisation. The plasma continues to absorb the laser energy until the end of the deposition time. The pressure generated by the plasma is transmitted to the material through shock waves (Ding & Ye, 2006). Although metals can be highly reflective of light, keeping the constant laser power density and decreasing the wavelength from IR to UV can increase the photon–metal interaction enhancing shock wave generation. However, the peak plasma pressure may decrease because decreasing the wavelength decreases the critical power density threshold for a dielectric breakdown, which in turn limits the peak plasma pressure. The dielectric breakdown is the generation of plasma not on the material surface, which absorbs the incoming laser pulse, limiting the energy to generate a shock wave. In Figure 37, the decrease in the wavelength from IR to green reduces the dielectric breakdown threshold from 10–6GW/cm2, resulting in maximum peak pressures of approximately 5.5 and 4.5GPa, respectively (Ding & Ye, 2006).

Fig. 36. Schematically principle of laser shock processing (Ding & Ye, 2006)

Laser shock peening (LSP) is an innovative surface treatment technique, which is successfully applied to improve fatigue performance of metallic components. After the treatment, the fatigue strength and fatigue life of a metallic material can be increased remarkably owing to the presence of compressive residual stresses in the material. The increase in hardness and yield strength of metallic materials is attributed to high density arrays of dislocations and

The ability of a high energy laser pulse to generate shock waves and plastic deformation in metallic materials was first recognised and explored in 1963 in the USA (Ding & Ye, 2006). A schematic configuration of an LSP process on a workpiece is shown in figure 36 (Dubourg et

When shooting an intense laser beam on to a metal surface for a very short period of time (around 30 ns), the heated zone is vaporised to reach temperatures in excess of 10 000°C and then is transformed to plasma by ionisation. The plasma continues to absorb the laser energy until the end of the deposition time. The pressure generated by the plasma is transmitted to the material through shock waves (Ding & Ye, 2006). Although metals can be highly reflective of light, keeping the constant laser power density and decreasing the wavelength from IR to UV can increase the photon–metal interaction enhancing shock wave generation. However, the peak plasma pressure may decrease because decreasing the wavelength decreases the critical power density threshold for a dielectric breakdown, which in turn limits the peak plasma pressure. The dielectric breakdown is the generation of plasma not on the material surface, which absorbs the incoming laser pulse, limiting the energy to generate a shock wave. In Figure 37, the decrease in the wavelength from IR to green reduces the dielectric breakdown threshold from 10–6GW/cm2, resulting in maximum peak pressures of approximately 5.5 and 4.5GPa, respectively (Ding & Ye, 2006).

**6. Laser shock peening of aluminium alloys** 

al., 2005).

formation of other phases or twins, generated by the shock wave.

*Plasma*

*Laser pulse*

*Shockwave deforms surface*

Fig. 36. Schematically principle of laser shock processing (Ding & Ye, 2006)

*Water Confining Layer*

*Workpiece*

*Absorbing Layer*

The transmission of an incident laser pulse throughout a water layer is expected to be controlled significantly by its pulse duration and / or to its rise time. Indeed, the faster energy deposition may generate the better laser-target coupling in plasma confined regime with water (Peyre et al., 2005).

Payer et al. (Peyre et al., 2005) studies the influence of laser intensity, wavelength, and pulse duration on the pressures generated in plasma. Results are presented in figures 38, 39.

Fig. 37. Peak plasma pressures obtained in WCM as a function of laser power density at 1.064mm, 0.532mm and 0.355mm laser wavelength (Ding & Ye, 2006)

Fig. 38. Influence of laser intensity and pulse durations on the pressures generated in plasma confined with water regime (λ=1.06 μm)-compaison with the analytical model of confinement (25 ns) (Peyre et al., 2005)

Laser Surface Treatments of Aluminum Alloys 147

Residual stresses increase with increasing laser induced pressures until a given pressure level called Psat where a plastic saturation occurs and above which CRS remain nearly constant. Below HEL (Hugoniot Elastic Limit), no plastic deformation occurs and in turn no residual stresses. Maximum RS levels induced by LSP are close to -0.5 σY for one local

Fig. 40. Influence of the mechanical properties of the targets on the residual stress levels achievable by LSP - Results taken from (Aluminium alloys), (Astroloy Ni superalloy),

Many recent studies have evidenced the beneficial influence of LSP on mechanical cyclic properties. On cast and wrought aluminium alloys (Al-7Si and Al12Si, 7075), some 25 to 40 % fatigue limit increases were displayed on notched specimens submitted to R=0.1 bending loadings. These results, superior to shot-peening (+25 % versus +12 % on 7075) were shown to be due to some large improvements in the fatigue crack initiation stage (Peyre et al.,

Lu et al. (Lu et al., 2010) studied the effect of laser shock peening on properties of aluminum alloys. In their report, the residual stress profiles of the treated samples after multiple LSP impacts with the impact time as functions of the distance from the top surface are shown in Figure 41. The substrates are approximately in the zero-stress state, indicating that the effect of initial stress on the shock waves may be ignored (Tan et al., 2004). It can be noted from Figure 41 that the significant compressive residual stresses mainly exist in near-surface regions for all cases and the top surfaces have the maximum values of compressive residual

The peak surface compressive residual stress and the depth of compressive residual stress are significantly increased to 116 MPa and to 0.79 mm, respectively, as a result of 3 LSP impacts on the sample surface. After 4 LSP impacts, the peak value of surface compressive residual stress is increased to 123 MPa, and the depth of compressive residual stress reaches about 0.80 mm. It can be seen that the surface compressive residual stress is increased by

(X100CrMo17) , (316L and X12CrNiMo12-2-2) (Ding & Ye, 2006)

1996).

stresses (Lu et al., 2010).

deformation and -0.7 σY for numerous ones (Figure 40) (Ding & Ye, 2006).

Fig. 39. Influence of laser wavelength on the pressures generated in plasma confined regime with water (all measurements performed at 25 ns pulse durations except 0.308 μm at 50 ns) (Peyre et al., 2005)

It can be seen that the maximum available pressure was saturated to nearly 5-6 GPa above 8- 10 GW/cm2 laser intensity (Fig.1). This saturation was shown to occur because of a parasitic breakdown plasma at the surface of the water which effect was to limit the energy reaching the target and cut temporally the incident laser pulse, thus reducing its effective duration 8. These pressure levels are usually sufficient to harden all the metallic materials but, most of times, impact sizes need to be reduced to reach the convenient power densities (Peyre et al., 2005). Also, the first conclusion to draw from these results is that pressure saturation levels increase with shorter laser pulse durations: from 5-5.5 GPa at 25 ns to 6 GPa at 10 ns and 9.5- 10 GPa at 0.6 ns. At shorter durations, the pressure saturation occurs at much higher laser intensity (Ith = around 100 GW/cm2 versus 10 GW/cm2 at 10-30 ns). This clearly indicates that energy transmissions through the water thickness are improved and that deleterious effects from breakdown plasmas are reduced by the use of shorter durations (Peyre et al., 2005). As can be seen from figure 39 Maximum output pressures Pmax and intensity thresholds Ith tend to be reduced with decreasing wavelengths. At the same pulse duration, maximum pressures decrease from 5.5 GPa at 1.06 µm to 5 GPa at 0.532 µm and 3.5 GPa at 0.355 µm. Intensity thresholds in the UV regime are also reduced to nearly 4 GW/cm2 versus 10 GW/cm2 at 1.06 µm. Moreover, the pressure durations (and in turn the transmitted laser pulse durations) decrease much more drastically above the intensity thresholds at lower wavelength. Also, at low intensity (1-4 GW/cm2) the efficiency of the pressure generation is shown to be improved at 0.532 µm and 0.355 µm. Indeed, according to the analytical model of confinement, the ''α'' coefficient gives a good fitting with experimental measurement with α = 0.45 versus α = 0.3 in the IR configuration). This could be due to a better target-plasma absorption in the UV range (Peyre et al., 2005). LSP generates compressive residual stresses (CRS) which are known to be the key to enhanced surface Properties (Ding & Ye, 2006).

Fig. 39. Influence of laser wavelength on the pressures generated in plasma confined regime with water (all measurements performed at 25 ns pulse durations except 0.308 μm at 50 ns)

It can be seen that the maximum available pressure was saturated to nearly 5-6 GPa above 8- 10 GW/cm2 laser intensity (Fig.1). This saturation was shown to occur because of a parasitic breakdown plasma at the surface of the water which effect was to limit the energy reaching the target and cut temporally the incident laser pulse, thus reducing its effective duration 8. These pressure levels are usually sufficient to harden all the metallic materials but, most of times, impact sizes need to be reduced to reach the convenient power densities (Peyre et al., 2005). Also, the first conclusion to draw from these results is that pressure saturation levels increase with shorter laser pulse durations: from 5-5.5 GPa at 25 ns to 6 GPa at 10 ns and 9.5- 10 GPa at 0.6 ns. At shorter durations, the pressure saturation occurs at much higher laser intensity (Ith = around 100 GW/cm2 versus 10 GW/cm2 at 10-30 ns). This clearly indicates that energy transmissions through the water thickness are improved and that deleterious effects from breakdown plasmas are reduced by the use of shorter durations (Peyre et al., 2005). As can be seen from figure 39 Maximum output pressures Pmax and intensity thresholds Ith tend to be reduced with decreasing wavelengths. At the same pulse duration, maximum pressures decrease from 5.5 GPa at 1.06 µm to 5 GPa at 0.532 µm and 3.5 GPa at 0.355 µm. Intensity thresholds in the UV regime are also reduced to nearly 4 GW/cm2 versus 10 GW/cm2 at 1.06 µm. Moreover, the pressure durations (and in turn the transmitted laser pulse durations) decrease much more drastically above the intensity thresholds at lower wavelength. Also, at low intensity (1-4 GW/cm2) the efficiency of the pressure generation is shown to be improved at 0.532 µm and 0.355 µm. Indeed, according to the analytical model of confinement, the ''α'' coefficient gives a good fitting with experimental measurement with α = 0.45 versus α = 0.3 in the IR configuration). This could be due to a better target-plasma absorption in the UV range (Peyre et al., 2005). LSP generates compressive residual stresses (CRS) which are known to be the key to enhanced

(Peyre et al., 2005)

surface Properties (Ding & Ye, 2006).

Residual stresses increase with increasing laser induced pressures until a given pressure level called Psat where a plastic saturation occurs and above which CRS remain nearly constant. Below HEL (Hugoniot Elastic Limit), no plastic deformation occurs and in turn no residual stresses. Maximum RS levels induced by LSP are close to -0.5 σY for one local deformation and -0.7 σY for numerous ones (Figure 40) (Ding & Ye, 2006).

Fig. 40. Influence of the mechanical properties of the targets on the residual stress levels achievable by LSP - Results taken from (Aluminium alloys), (Astroloy Ni superalloy), (X100CrMo17) , (316L and X12CrNiMo12-2-2) (Ding & Ye, 2006)

Many recent studies have evidenced the beneficial influence of LSP on mechanical cyclic properties. On cast and wrought aluminium alloys (Al-7Si and Al12Si, 7075), some 25 to 40 % fatigue limit increases were displayed on notched specimens submitted to R=0.1 bending loadings. These results, superior to shot-peening (+25 % versus +12 % on 7075) were shown to be due to some large improvements in the fatigue crack initiation stage (Peyre et al., 1996).

Lu et al. (Lu et al., 2010) studied the effect of laser shock peening on properties of aluminum alloys. In their report, the residual stress profiles of the treated samples after multiple LSP impacts with the impact time as functions of the distance from the top surface are shown in Figure 41. The substrates are approximately in the zero-stress state, indicating that the effect of initial stress on the shock waves may be ignored (Tan et al., 2004). It can be noted from Figure 41 that the significant compressive residual stresses mainly exist in near-surface regions for all cases and the top surfaces have the maximum values of compressive residual stresses (Lu et al., 2010).

The peak surface compressive residual stress and the depth of compressive residual stress are significantly increased to 116 MPa and to 0.79 mm, respectively, as a result of 3 LSP impacts on the sample surface. After 4 LSP impacts, the peak value of surface compressive residual stress is increased to 123 MPa, and the depth of compressive residual stress reaches about 0.80 mm. It can be seen that the surface compressive residual stress is increased by

Laser Surface Treatments of Aluminum Alloys 149

Fig. 43. Schematic illustrations of micro-structure characteristics along depth direction in the

It is well known that residual stresses in metal materials are often the result of micro-plastic deformation accompanying the micro-structure changes (Yilbas & Arif, 2007). As a result, it is reasonable to assume that the LSP induced strengthening in metal materials is due to the generation of dislocations. The schematic illustrations of the micro-structure characteristics of the hardening layer subjected to 3 LSP impacts are shown in Figure 43. After 3 LSP impacts, the change of dislocation structure can be also clearly seen at different layers, i.e., it varies from DLs to DTs and DDWs, to subgrains or refined grains as functions of the distance from the top surface. After multiple LSP impacts, the grains in the SPD layer are clearly refined and there are plenty of DLs and DTs with high density in the SPD layer. As a result of the grain refinement, the shocked area is strengthened according to the classical

2 *<sup>N</sup>*

2 *<sup>p</sup>*

Here µ is the shear modulus (~35 GPa for Al alloy), γ is the stacking fault energy (104–142 mJ m–2 for Al alloy (Lu et al., 2010)), D is the grain size, and bN and bP are the magnitudes of the Burgers vectors of the perfect dislocation and the Shockley partial dislocation, respectively. The parameter α reflects the character of the dislocation and contains the

The grain boundaries are taken as dislocation sources, as predicted by computer simulations for subgrains or refined grains. When the grain size becomes smaller than a critical value,

2 ( *<sup>N</sup> p p* )

γ

The generation of subgrain interfaces and stacking faults offers an alternative interpretation to dislocation pile-up at grain boundaries to explain the continuous grain-size

αμ

*b bb*

*b D b* αμ

*b D* αμ

*p*

γ

= (13)

= + (14)

<sup>−</sup> <sup>=</sup> (15)

*N*

τ

*p*

τ

scaling factor between the length of the dislocation source and the grain size.

*c*

*D*

hardening layer subjected to 3 LSP impacts (Lu et al., 2010)

dislocation theory (Chen et al., 2003), where

DC, determined by equating Eqs. (13) and (14),

25.93% and 13.73% when the impact time increases from 1 to 2 and from 2 to 3, whereas the surface compressive residual stress is increased by 6.89% when the impact time increases from 3 to 4, but the surface compressive residual stress is kept to about 123 MPa after the multiple LSP with 4 and 5 LSP impacts (Lu et al., 2010). It can be seen from Figure 42 that the increasing rate of surface compressive residual stress decreases almost linearly with the impact time, but the increase of surface residual stresses gradually reaches the saturated state when the impact time exceeds 4. The similar results can be seen elsewhere (Masse & Barreau, 1995; Ding & Ye, 2003).

Fig. 41. Residual stress profiles of the hardening layer after multiple LSP impacts with the impact time (Lu et al., 2010)

Fig. 42. The comparison between the increasing rate of surface residual stress and the impact time (Lu et al., 2010)

25.93% and 13.73% when the impact time increases from 1 to 2 and from 2 to 3, whereas the surface compressive residual stress is increased by 6.89% when the impact time increases from 3 to 4, but the surface compressive residual stress is kept to about 123 MPa after the multiple LSP with 4 and 5 LSP impacts (Lu et al., 2010). It can be seen from Figure 42 that the increasing rate of surface compressive residual stress decreases almost linearly with the impact time, but the increase of surface residual stresses gradually reaches the saturated state when the impact time exceeds 4. The similar results can be seen elsewhere (Masse &

> 10 5 0 -5 -10

**5 impacts** 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

from 1 to 2 from 2 to 3 from 3 to 4 from 4 to 5

Fig. 42. The comparison between the increasing rate of surface residual stress and the impact

**Depth (mm)**

Fig. 41. Residual stress profiles of the hardening layer after multiple LSP impacts with the


**1 impact 2 impacts 3 impacts 4 impacts**

Barreau, 1995; Ding & Ye, 2003).

 15 0 -15 -30 -45 -60 -75 -90 -105 -120 -135

**Residual stress (MPa)**

impact time (Lu et al., 2010)

time (Lu et al., 2010)

30

25

20

15

The increasing rate (%)

10

5

0

Fig. 43. Schematic illustrations of micro-structure characteristics along depth direction in the hardening layer subjected to 3 LSP impacts (Lu et al., 2010)

It is well known that residual stresses in metal materials are often the result of micro-plastic deformation accompanying the micro-structure changes (Yilbas & Arif, 2007). As a result, it is reasonable to assume that the LSP induced strengthening in metal materials is due to the generation of dislocations. The schematic illustrations of the micro-structure characteristics of the hardening layer subjected to 3 LSP impacts are shown in Figure 43. After 3 LSP impacts, the change of dislocation structure can be also clearly seen at different layers, i.e., it varies from DLs to DTs and DDWs, to subgrains or refined grains as functions of the distance from the top surface. After multiple LSP impacts, the grains in the SPD layer are clearly refined and there are plenty of DLs and DTs with high density in the SPD layer. As a result of the grain refinement, the shocked area is strengthened according to the classical dislocation theory (Chen et al., 2003), where

$$
\tau\_N = \frac{2a\mu b\_N}{D} \tag{13}
$$

$$
\tau\_p = \frac{2a\mu b\_p}{D} + \frac{\mathcal{V}}{b\_p} \tag{14}
$$

Here µ is the shear modulus (~35 GPa for Al alloy), γ is the stacking fault energy (104–142 mJ m–2 for Al alloy (Lu et al., 2010)), D is the grain size, and bN and bP are the magnitudes of the Burgers vectors of the perfect dislocation and the Shockley partial dislocation, respectively. The parameter α reflects the character of the dislocation and contains the scaling factor between the length of the dislocation source and the grain size.

The grain boundaries are taken as dislocation sources, as predicted by computer simulations for subgrains or refined grains. When the grain size becomes smaller than a critical value, DC, determined by equating Eqs. (13) and (14),

$$D\_c = \frac{2a\mu \left(b\_N - b\_p\right)b\_p}{\mathcal{Y}} \tag{15}$$

The generation of subgrain interfaces and stacking faults offers an alternative interpretation to dislocation pile-up at grain boundaries to explain the continuous grain-size

Laser Surface Treatments of Aluminum Alloys 151

The compressive residual stress distribution along surface layer for laser-peened and shot-

Table 2. Fatigue lives of specimens and FLPF under 300MPa stress (Gao, 2011)

Fig. 45. Compressive residual stress field caused by shot peening (Gao, 2011)

Fig. 46. Compressive residual stress field caused by laser peening (Gao, 2011)

peened specimens under different regimes are shown in Figures 45, 46.

strengthening, as suggested by Eq. (14), and the strain hardening of the metal materials. The reaction between the laser shock wave and the sample will be generated near the sample surface, leading to the generation of the dislocation and the micro-structural deformation near the surface, which can be explained by the fact that the compressive residual stresses are generated in the PD layer, and the magnitude of the compressive residual stress decreases away from the top surface.

The grain refinement mechanism is schematically illustrated in figure 44. Based on the micro-structure features observed in various layers with different strains in the hardening layer, the following elemental states are involved in the grain refinement process: (1) development of DLs in original grains (state (I) in figure 44); (2) the formation of DTs and DDWs due to the pile-up of DLs (state (II) in figure 44); (3) transformation of DTs and DDWs into subgrain boundaries (state (III) in figure 44); and (4) evolution of the continuous dynamic recrystallization (DRX) in subgrain boundaries to refined grain boundaries (states (IV) and (V) in figure 44).

Fig. 44. Schematic illustration showing micro-structural evolution process of LY2 Al alloy induced by multiple LSP impacts (Lu et al., 2010)

The comparison of effect sot peening and laser sock peening on fatigue behavior of Al- alloy was investigated by Gao (Gao, 2011). To determine the effect of surface enhancement on fatigue property and get the optimum parameters, the FLPF analysis under the same stress load or strain load conditions is usually employed. The FLPF is calculated as:

$$FLPF = \frac{N\_{\text{mod}\,ifedspecium}}{N\_{\text{baselinespecium}}} - 1\tag{16}$$

For the different surface conditions, the fatigue lives of specimens and FLPF are listed in Table 2.

strengthening, as suggested by Eq. (14), and the strain hardening of the metal materials. The reaction between the laser shock wave and the sample will be generated near the sample surface, leading to the generation of the dislocation and the micro-structural deformation near the surface, which can be explained by the fact that the compressive residual stresses are generated in the PD layer, and the magnitude of the compressive residual stress

The grain refinement mechanism is schematically illustrated in figure 44. Based on the micro-structure features observed in various layers with different strains in the hardening layer, the following elemental states are involved in the grain refinement process: (1) development of DLs in original grains (state (I) in figure 44); (2) the formation of DTs and DDWs due to the pile-up of DLs (state (II) in figure 44); (3) transformation of DTs and DDWs into subgrain boundaries (state (III) in figure 44); and (4) evolution of the continuous dynamic recrystallization (DRX) in subgrain boundaries to refined grain boundaries (states

> Subgrain boundary

*<sup>N</sup>* <sup>=</sup> <sup>−</sup> (16)

state (II)

*(c) Dislocation wallls and dislocation tangles*

state (III)

decreases away from the top surface.

*(a) Dislocation (b) Dislocation lines*

Boundary

induced by multiple LSP impacts (Lu et al., 2010)

Table 2.

*(f) Refined grains (e) DRX nucleation (d) Subgrains*

state (V) state (IV)

Fig. 44. Schematic illustration showing micro-structural evolution process of LY2 Al alloy

load or strain load conditions is usually employed. The FLPF is calculated as:

*FLPF*

*N*

The comparison of effect sot peening and laser sock peening on fatigue behavior of Al- alloy was investigated by Gao (Gao, 2011). To determine the effect of surface enhancement on fatigue property and get the optimum parameters, the FLPF analysis under the same stress

For the different surface conditions, the fatigue lives of specimens and FLPF are listed in

mod 1 *ifiedspecimen baselinespecimen*

state (I)

(IV) and (V) in figure 44).


Table 2. Fatigue lives of specimens and FLPF under 300MPa stress (Gao, 2011)

The compressive residual stress distribution along surface layer for laser-peened and shotpeened specimens under different regimes are shown in Figures 45, 46.

Fig. 45. Compressive residual stress field caused by shot peening (Gao, 2011)

Fig. 46. Compressive residual stress field caused by laser peening (Gao, 2011)

Laser Surface Treatments of Aluminum Alloys 153

Guillaumin, V., & Mankowski, G. (1999). Localized corrosion of 2024 T351 aluminium alloy

Hu, C., Xin, H., & Baker, T.N. (1996). Formation of continuous surface Al-SiCp metal matrix

Huang, Y.L., Liang, G.Y., Su, J. Y., & Li, J.G. (2005). Interaction between laser beam and

Jouvard, J.M., Grevey, D.F., Lemoine, F., & Vannes, A.B. (1997). Continuous Wave Nd:YAG

Leech, P. W. (1989). The Laser Surface Melting of Aluminium-Silicon-Based Alloys. *Thin* 

Li, R., Ferreira, M.G.S., Almeida, A., Vilar, R., Watkins, K. G., McMahon, M.A., & Steen,

Lu, J.Z., Luo, K.Y., Zhang, Y.K., Cui, C.Y., Sun, G.F., Zhou, J.Z., Zhang, L., You, J., Chen,

Majumdar, D. J. & Manna, I. (2002). A Theoretical Model for Predicting Microstructure during Laser Surface Alloying. *Lasers in Engineering*, Vol. 12, pp. 171-190. Masse, J. E., & Barreau, G. (1995). Surface modification by laser induced shock waves.

Munitz, A. (1985). Microstructure of rapidly solidified laser-molten Al– 4.5 wt % Cu

Nagarathnam, K. & Taminger, K.M.B. ( 2001). Technology Assessment of Laser-Assisted

Peyre, P., Fabbro, R., Berthe, L., Scherpereel, X., & Bartnicki, E. (2005). Laser shock

Peyre, P., Fabbro, R., Merrien, P., & Lieurade, H.P. (1996). Laser shock processing of

Pinto, M. A., Cheung, N., Ierardi, M.C.F., Garcia, A. (2003). Microstructural and hardness

Rams, J., Padro, A., Urena, A., Arrabal, R., Viejo, F., & Lopez, A.J. (2007). Surface treatment

Renk, T. J., Buchheit, R. G., Sorensen, N. R., Cowell Senft, D., Thompson, M. O., &

alloying with high-power ion beams", *Phys. Plasmas*, Vol. 5, pp. 2144-2150. Sallamand, P., & Pelletier, J. M. (1993). Laser cladding on aluminium-base alloys:

Materials Processing in Space. CP 552, *Space Technology and Applications International* 

processing of materials and related measurements", CLFA/LALP, 94114 Arcueil,

aluminium alloys. Application to high cycle fatigue behavior. *Materials Science &* 

investigation of an aluminum–copper alloy processed by laser surface melting.

of *aluminium matrix* composites using a high power diod laser. Surface and Coatings Technology,

Grabowski, K. S. (1998). Improvement of surface properties by modification and

microstructural features", *Materials Science and Engineering*, Vol. A 171, pp. 263-270.

composite by overlapping laser tracks on AA6061 alloy. Materials Science and

powder stream in the process of laser cladding with powder feeding. Model Simul.

Laser Cladding Modeling: A Physical Study of Track Creation During Low Power

W.M. (1996). Localized corrosion of laser surface melted 2024-T351 aluminium

K.M., & Zhong, J.W. (2010). Grain refinement of LY2 aluminum alloy induced by ultra-high plastic strain during multiple laser shock processing impacts. *Acta* 

in chloride media. *Corrosion Science*, Vol. 41, pp.421-438.

Processing. Journal of Laser Application, Vol. 9, pp. 43-50.

alloy. *Surface and coatings technology*, Vol. 81, pp.290-296.

Technology, Vol. 12, pp. 227-232.

*Solid Films*, Vol. 177, pp. 133 140.

*Materialia*, Vol. 58, pp. 3984–3994.

France.

Surface Engineering, Vol. 11, p.131-132.

*Engineering,* Vol. A210, pp.102-113.

Vol. 202, pp. 1199-1203

surfaces. *Metall Trans B,* Vol. 16, pp.149– 161.

*Forum*, paper edited by M.S. El-Genk, pp. 153-160.

*Materials Characterization*, Vol. 50, pp. 249– 253.

Mater. Sci. Vol. 13, pp.47-56.

The fatigue strength for 1×107 cycles of 7050–T7451 aluminum alloy was increased by shot peening and laser peening. Fatigue strength of the best-laser-peened specimens is 42% higher than as-machined specimens and the fatigue strength of the best shot-peened specimens is 35% higher than as-machined (Gao, 2011).
