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Effects of laser shock peening on microstructures and properties of 2195 Al-Li alloy
Journal of Alloys and Compounds 781 (2019) 330e336
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effects of laser shock peening on microstructures and properties of 2195 Al-Li alloy Yang Yang a, *, Xiaolong Lian a, Kai Zhou a, Guojie Li b a b
School of Material Science and Engineering, Central South University, Changsha, 410083, China Xi'an Tianruida Photoelectric Technology Development Co., Ltd, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2018 Received in revised form 7 December 2018 Accepted 8 December 2018 Available online 10 December 2018
The effects of laser shock peening (LSP) on microstructures and properties of 2195 Al-Li alloy were studied in this paper. The top surface microstructure of this alloy after LSP was observed using transmission electron microscopy (TEM). The change of stress corrosion resistance after LSP was studied by slow strain rate tensile (SSRT) test. Moreover the effect of LSP on the mechanical properties was characterized by the microhardness and the residual stress test. The microstructure observation results suggested that the surface grain of this alloy was refined to the nanometer with size of 93 nm after single LSP and it can be further refined to 70 nm by 3 times LSP, the grain refinement during LSP should be considered as a dislocation evolution process based on the mechanism of rotating dynamic recrystallization (RDR). The results of mechanical properties test showed that LSP could improve the hardness and introduce the residual compressive stress in the surface of target alloy, and the 3 times LSP can further improve the properties on the base of single LSP. Furthermore, the experimental results of SSRT indicated that LSP was an effective method to improve the resistance to stress corrosion of 2195 Al-Li alloy, because the residual compressive stress and refined grains in the surface can prevent the initiation and growth of cracks, while retard the stress corrosion cracking. © 2018 Elsevier B.V. All rights reserved.
Keywords: LSP Grain refinement Residual compressive stress Stress corrosion cracking resistance 2195 Al-Li alloy
1. Introduction Laser shock peening (LSP) is a surface processing technology that offers life extension of metallic structural components for aerospace, automotive, and power generation industries [1]. It can improve the property of materials, and it has been widely applied and studied in recent years. Tong [2] investigated the effect of laser shock peening on microstructure and hot corrosion of TC11 alloy and found that a large number of crystal defects produced by LSP added element diffusion paths to the surface to form oxide protective layer in a very short time. And these oxide protective layers can improve the corrosion resistance of TC11 alloy. Sun [3] found that the fatigue life of the LSPed sample was increased up to 2.4 times that of the unpeened counterpart. Luo [4] found that LSPinduced surface nanostructure increased the efficiency of the aluminizing process and the fatigue resistance, because the thermal stability of surface nanostructure and high-density dislocation
* Corresponding author. School of Materials Science and Engineering, Central South University, Changsha, 410083, China. E-mail address: [email protected] (Y. Yang). https://doi.org/10.1016/j.jallcom.2018.12.118 0925-8388/© 2018 Elsevier B.V. All rights reserved.
induced by LSP increased its surface activity and made more channels available for element diffusion during the high temperature. Park [5] investigated the effects of laser shock peening on friction characteristics of JIS-AC8A aluminum-silicon alloy and found the friction coefficient of laser peened surface was significantly lower than that of unpeened samples. Wang [6] investigated the effect of laser shock peening on creep properties of 7075 aluminum alloy and found that LSP can effectively enhance the creep resistance of 7075 aluminum at the elevated temperature and working stress. Luo's work [7] indicated that LSP generated deeper compressive residual stress and effectively decreased the corrosion fatigue crack growth rate, and increased coverage layers further improved the corrosion fatigue life of notched specimens. Tang [8] investigated the effect of LSP on plasma nitriding and found that LSP increased the surface roughness and formed a pre-hardening layer of about 200 mm. The previous studies on LSP confirmed that LSP could refine the grains [9e11]. However, the nanocrystallization of the surface layer induced LSP is still a problem to be solved at present. The TC11 titanium alloy was treated at different LSP times by Nie [12], and the experimental results showed that only dislocation cells with
Y. Yang et al. / Journal of Alloys and Compounds 781 (2019) 330e336
size of about 1 mm appeared in the surface of this alloy after single LSP and the appearance of nanocrystallines needed the LSP treatment for 3 times at least. The research of Lu [13] suggested that the grains in the surface of the LY12 aluminum alloy was refined from 10 mm to 3e5 mm after single LSP, and it was refined to submicron (100e200 nm) level after 5 times LSP, there were no nanocrystallines appeared in LY12 aluminum during LSP. Ge [14] studied the effect of laser energy on microstructure of Mg-3Al-1Zn alloy treated by LSP and found the grain size in top surface decreased with laser pulse energy increasing from 1.19 to 2.3 mm for 6.5 J to 0.04e0.27 mm for 8.5 J. Sun [15] found the average grain size decreased from 59.7 mm to 46.7 mm in 2319 aluminum alloy after LSP. Our previous study also found that the surface grain of TC17 titanium alloy was only refined to submicron level after single LSP [16,17]. As we all know, nanomaterials have superior performance incomparable with ordinary materials; it will bring the improvement of strength, hardness and other performance when the grain size of metal materials reached the nanometer level. The nanocrystallization in the surface of metals was always a hot research direction in the field of material science [18]. 2195 Al-Li alloy was always used in the aerospace field because of its low density and high strength, but the extreme application environment of aerospace put forward more demanding requirements for Al-Li alloy [19]. The surface nanocrystallization material of 2195 Al-Li alloy with uniform structure was prepared by LSP for the first time. The microstructure in the surface of LSPed materials was observed using transmission electron microscopy (TEM), the surface hardness, residual surface stress of 2195 Al-Li alloy after single LSP and 3 times LSP treatment were compared, and the property of stress corrosion resistance of this LSPed alloy was also investigated in the present work. 2. Material and methods 2.1. Sample preparation The forged 2195 Al-Li alloy was used in this study, and its composition was shown in Table 1 The aged state alloy was treated by single LSP and 3 times LSP treatment under the same LSP parameters. The sample states were: a. solid solution þ aging þ single LSP, b. solid solution þ aging þ3 times LSP. The solid solution experiment was carried out in a salt bath furnace with a temperature of 510 C for 30min, and the age treatment was at the temperature of 180 C for 12 h. 2.2. LSP experiment When the shortwave laser beam with high energy density was irradiated on the surface of the metal material, the ablation layer material will transfer instantaneously to plasma, and the rapid expansion of the plasma can produce an impact pressure with GPa level at the surface of the target material. The continuous impact pressure will cause conduct of stress wave in the surface of the material, thus bring the change of structure and the improvement of performance. The LSP experiment was carried out on a high repetition rate
Table 1 Chemical composition of 2195 Al-Li alloy. Component
Cu
Li
Mg
Ag
Zr
wt/%
4.0
1.0
0.4
0.4
0.12
331
Nd: glass laser. The determined LSP parameters are shown in Table 2. Black tape was used as the thermo-protective layer, and flowing water with a thickness of 1 mm was taken as the confining layer. 2.3. Microstructure observation The microstructures of 2195 Al-Li alloy after LSP was analyzed by Tecnai G2 20AEM transmission electron microscope (TEM) and the operating voltage was 200 KV. The thin film samples of TEM which were prepared by mechanical thinning, and single side thinning of ion beam was used to observe the surface microstructures after LSP. TEM samples were prepared as follows: Firstly cutting two strips perpendicular to the treated surface with dimensions of 1.4 mm 1.4 mm 15 mm, adhered the treated surfaces face to face. Then inserted into a copper tube with a diameter of 3 mm and bonded them together with glue. Slices with thickness of 1 mm were cut down perpendicular to the axis of tube, then thinned the slices to about 50 mm by mechanical grinding. The final TEM specimens were prepared by ion-beam thinning with ion energy of 4.5 kV and the incident angle of 7 for perforation and 3.5 kV/2 for 25min to refine the thin zone. The XRD experiment was conducted by D/Max 2500 X-ray diffractometer (operated at 40 kV and 40 mA) to analyse the surface information of 2195 aluminum-lithium alloy after LSP; the scanning scope was from 10 to 90 . 2.4. Mechanical property characterization The surface hardness after LSP treatment was measured by HVS1000 microhardness test machine with a 100 g load and a 10 s holding time. Several measurements along depth direction perpendicular to the treated surface were taken at intervals of 100 mm, and the mean of 3 measurements at every depths was used to be the microhardness value of this layer. X-ray residual stress tester was used to measure the residual surface stress of 2195 Al-Li alloy after LSP. The test area was on the LSPed surface, the working voltage was 40 KV and the working current was 40 mA. The scanning step and counting time was 0.02 and 5S respectively. The measuring error was 25 MPa or less, and it will be retested if the results were beyond the range. Before the test, the samples were ultrasonically cleaned with ethanol. 2.5. Test of resistance to stress corrosion The experimental methods of stress corrosion test can be divided into three types: constant deformation, constant load and slow strain rate test method, and the slow strain rate tensile has great advantages as compared to other two methods. Firstly, the slow strain rate method has a higher sensitivity to the stress corrosion cracking; secondly, many information can be obtained by this method, such as fracture time ft, maximum load Lmax and reduction in cross-sectional area Z, etc; furthermore, it can simulate the actual medium environment and get more accurate results.
Table 2 Processing parameters used in LSP. Type
Value
Spot diameter/mm Pulse energy/J Pulse width/ns Laser wavelength/nm Energy stability/% Overlapping ratio/%
3 5 20 1064 ±1.5 50
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The effect of LSP on stress corrosion property of 2195 Al-Li alloy was investigated by SSRT in this paper. The specimens were placed in a corrosive medium and inert medium (air) for slow strain rate tensile test and the strain rate was set to 106s1 [20]. In order to simulate the environment of high concentration of chloride ion, the solution used in stress corrosion test was 3.5% NaCl solution at room temperature and its pH value was 6.8 according to the standard of ASTM G44-2005. The sample size was designed by reference to the standard of GBT 15970.7e2000 and the specific dimension was shown in Fig. 1. The thickness of the specimen is 5 mm, and the width is 5 mm at the center 20 mm of the standard distance segment. The sample was polished by sandpaper and cleaned with acetone, then cleaned with distilled water and dried it. Double-sided LSP treatment at the center 20 mm of the standard distance segment was carried out. 3. Results and discussion Fig. 2. The XRD analysis results of different impact times.
3.1. Microstructural characteristics after LSP 3.1.1. Analysis results of XRD The X-ray diffraction theory stated that the grain refinement could be reflected in the width of spectral peaks. According to the Scherrer Equation [21]:
D¼
Kl B cos q
(1)
where K is a constant, D is the average thickness of a grain perpendicular to the crystal face, B is FWHM of diffraction peaks, q is the diffraction angle, and l is X-ray wavelength. It is ready to conclude that the width of spectral peaks at the same diffraction angle increased with the refinement of grain gradually. Fig. 2 is the XRD analysis results at the surface of the sample without LSP and the samples after different times LSP. The main peak at 60 e70 was selected and the a-Al diffraction peak at 65 was used to analyse the information after LSP. It can be clearly seen that the a-Al diffraction peak after LSP was wider than that of the specimen without LSP, which indicated that the grain size of the surface layer after LSP refined than the substrate. Regarding the 3 times LSPed sample, the diffraction peak of the a-Al was slightly widened but not obvious as compared to single LSP, which suggested that the 3 times LSP can further refine the surface grains on the base of single LSP, but the subsequent refinement was limited. 3.1.2. Observation results of TEM The observation results of microstructures in Fig. 3 showed that the major precipitate of 2195 aluminum-lithium alloy before LSP was T1(Al2CuLi) phase, and there was also a small amount of acicular globular d’ (Al3Li) phase and bean-shaped b0 (Al3Zr) phase. The original grain size was about 10 mm. Fig. 4 is the TEM observation results after single LSP, Fig. 4(a) shows a bright field image in the top surface. It can be clearly seen from the figure that a large number of fine grains appeared in the
Fig. 1. The dimensions of SSRT sample.
Fig. 3. The observation result of TEM before LSP.
top surface after LSP, and the size of grains was at the range of 40e150 nm. It can be seen the equiaxed grains with clear high angle grain boundaries and regular shapes from the enlarged image of the part of Fig. 4(a) shown in Fig. 4(b). Fig. 4(c) was the selected area electron diffraction (SAED) corresponding to Fig. 4(a), and the diffraction patterns shown in the image were ring-shaped, which indicated that the originally coarse grain had fully refined and the orientation was randomly distributed. The 15 different fields were selected for statistical analysis and the results were shown in Fig. 4(d), which suggested that the average grain size was about 90 nm, it reached the nanometer level. Fig. 5 is the results of TEM observation at the top surface after LSP treatment for 3 times. It was seen from the bright field images shown in Fig. 5(a) and (b) that there were a lot of refined grains with size in the range of 30e100 nm. The grain statistics results shown in Fig. 5(d) suggested that the average grain size was about 70 nm. The SAED results of Fig. 5(c) shows more continuous annular patterns which indicated that a large number of randomly oriented nanocrystals are distributed in the surface of the alloy. Furthermore, the results of the microstructural observation showed that the T1 phase in the surface of 2195 Al-Li alloy dissolved after
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Fig. 4. The observation result of TEM after single LSP: (a) Bright field image with low magnification at the surface; (b) Bright field image with high magnification at the part of (a); (c) corresponding selected area electron diffraction; (d) histogram of grain size distribution.
LSP, which was consistent with the phenomenon of precipitates dissolution during high strain rate loading conditions we observed before. As an ultra-high strain rate surface treatment technology, the peak impact pressure of GPa level can be generated instantaneously by LSP when it acted on the metal surface, and the strain rate reached the magnitude of 107s1 [16]. As a kind of metal with high stacking fault energy, the plastic deformation of 2195 Al-Li alloy was mainly under the dislocation slip mechanism. And it is reported that the time of twin atoms to rearrangement along twin direction was not enough under the ultra-high strain rate condition of 107s1 magnitude, therefore, the twin deformation was difficult to occur [22,23]. Consequently, the grain refinement during LSP should be a dynamic process based on dislocation evolution. The observation results of Figs. 4 and 5 showed that the surface grains of 2195 Al-Li alloy after LSP had obvious recrystallization characteristics, therefore, the formation of refined grains must undergo a special recrystallization process. The classical recrystallization mechanism cannot explain the recrystallization of LSP during ultra-high strain rate deformation. The dynamic rotation recrystallization (RDR) mechanism proposed by Meyers [24] suggested that the subgrains can rotate to form recrystallized grain under the action of the minimum driving force. The deformation at the strain rate of 107s1 could be considered as an adiabatic process, and the adiabatic temperature rise provided thermodynamic conditions for the mechanism of RDR. Therefore, the grain refinement of 2195 Al-Li alloy during LSP could be considered as a dynamic process that the dislocation transformed to dislocation
entanglement, dislocation cell, subgrain, and evolved into grain with high angle boundary through the mechanism of RDR finally. The dislocation density of 3 times LSPed specimen was higher than that of single LSP, so the size of dislocation cells and subgrains was also smaller. According to the kinetics equation [25] of the RDR mechanism shown in Formula 2, the smaller the size of the subgrain is, the shorter the time needed to complete the recrystallization will be, it's more conducive to the operation of the RDR mechanism with small grain size. Consequently, the 3 times LSP treatment can further refine the nanocrystallized grains at the base of single LSP.
t¼
L1 kTf ðqÞ 4shDb0 expðQb =RTÞ
(2)
where t is time; s is the grain boundary thickness; h is the grain boundary energy; Db0 is a constant referred to grain boundary diffusion; L1 is subgrain diameter; Qb is the activation energy for grain boundary diffusion; f(q) is the function about disorientation of subboundaries. The microstructure evolution mechanism was discussed in detail in our previous work [26]. 3.2. Resistance to stress corrosion The samples without LSP and after single LSP were placed in the air and 3.5% Nacl solution respectively for the SSRT experiment, The strain rate was set to 106s1, and the stress-strain curve was shown in Fig. 6. It can be seen that the elongation and tensile
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Fig. 5. The observation result of TEM after 3 times LSP: (a) Bright field image with low magnification at the surface; (b) Bright field image with high magnification at the part of (a); (c) corresponding selected area electron diffraction; (d) histogram of grain size distribution.
Fig. 6. Stress-strain curves after the SSRT test.
strength of the samples in the NaCl solution are lower than that in the air. Therefore, the 2195 Al-Li alloy had the stress corrosion sensitivity. The parameters of every SSRT curve was shown in Table 3, which indicated that the tensile strength and elongation of the sample after LSP was 362.15 MPa and 12.79% respectively when it placed in the air, and it decreased to 326.34 MPa and 11.41% correspondingly placed in the NaCl solution. The tensile strength and elongation of the samples without LSP were significantly lower than those of the LSPed samples both in the air and solution, the corresponding values are 350.07 MPa, 12.79% and 304.03 MPa, 10.51%, respectively. It is found that the specimen after LSP had a larger reduction in cross-sectional area after analyzing the tensile fracture, which was 13.11% and 12.02% correspondingly in the air and solution, the reduction in cross-sectional area of the sample without LSP was 11.82% and 10.51%, respectively. The stress corrosion index ISSRT can be calculated by the various parameters obtained in the SSRT test, which can reflect the stress
Table 3 The experimental results of SSRT. Test state
Tensile strength (MPa)
Elongation (%)
Area reduction (%)
ISSRT (%)
With LSP in the air With LSP in the solution Without LSP in the air Without LSP in the solution
362.15 326.34 350.07 304.03
12.79 11.41 11.82 10.51
13.11 12.02 12.23 10.94
10.99 14.16
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corrosion cracking sensitivity of the alloy more accurately than the single mechanical property index and it was usually used as an important criterion for the resistance to stress corrosion. A high ISSRT value indicates a high sensitivity to the stress corrosion cracking and the formula ISSRT was expressed as follows [27]:
ISSRT ¼ 1 ½ss ð1 þ ds Þ=½sA ð1 þ dA Þ
(3)
where ss and ds indicate tensile strength and elongation at tensile in the solution, and sA and dA is strength and elongation in the air respectively. The calculated results showed that the stress corrosion factor ISSRT of the LSPed specimen was 10.99%, while the index of the sample without LSP was 14.16%. The greater value of ISSRT indicated that the material had a higher stress corrosion cracking (SCC) tendency. Consequently, the experimental results of SSRT suggested that the LSP treatment can effectively improve the stress corrosion resistance of 2195 Al-Li alloy. The failure mode of materials was SCC in the stress corrosion media and the failure process underwent the stages of SCC initiation, growth and failure. The existence of the residual compressive stress increased the threshold of crack initiation in the top surface, which made the crack difficult to initiate, and the residual compressive stress also formed a continuous resistance in the stage of SCC growth. Therefore, the surface residual compressive stress induced by LSP was an important factor for the improvement of the resistance to stress corrosion. In the stage of SCC growth, the grain refinement layer formed during LSP also played a significant role. Due to the grain boundary ratio after refinement was greatly increased, a great number of grain boundaries caused the crack growth to encounter much resistance whether it was intergranular propagation or transgranular propagation, which made the crack hard to grow and run through, while it was difficult to occur SCC. 3.3. Mechanical properties Fig. 7 shows the relationship between the microhardness and the depth below top surface of the 2195 Al-Li alloy after different times LSP. It was clear that the microhardness value decreased continuously with the increase of the depth, the maximum hardness value appeared at the top surface, and the matrix hardness of all the samples was equal to about 95 HV. The surface hardness after 3 times LSP was 130 HV and increased about 36.8% as compared to the matrix, it was 120 HV after single LSP and
Fig. 8. Tested results of residual stress at the surface.
increased ratio was about 26.3%. The tested results suggested that the 3 LSPed sample had greater hardness than the single LSPed one, and the hardened depth was about 1.8 mm and 1.5 mm respectively. The residual surface stress of the samples after LSP was measured and the results shown in Fig. 8 indicated that the residual compressive stress could be introduced on the surface of 2195 Al-Li alloy by LSP. After single LSPed treatment, the surface residual stress was about 199 MPa, and it reached 266 MPa after 3 times of LSP. The surface grains were refined after LSP, and the grain refinement inevitably brought the increase of hardness according to the Hall-Petch relation. The gradient grain refinement layer caused the gradient change of hardness, and the maximum of hardness also appeared at the top surface where there were the most deformation degree and the finest grains. At the same time, the plastic deformation induced by LSP also introduced the crystal defects in the surface layer with a gradient feature, which also played an important role in the improvement of the surface hardness. The affected depth of plastic deformation of the 3 times LSPed samples was higher than the single LSPed one, therefore, the depth of the hardened layer was higher. Moreover, the surface grain size was smaller, and the crystal defect density was higher after the 3 times LSP. Consequently, the effect of hardening of 3 times LSPed specimen was more significant. The plastic deformation induced by LSP showed the lattice distortion at the microstructure, and the lattice distortion will produce the residual stress, the residual stress that compressed the alloy was the residual compressive stress. The existence of this residual compressive stress prevented crack initiation and reduced crack growth rate and effectively improved material properties [28]. The degree of lattice distortion in the surface of the 3 LSPed samples is greater compared with single LSPed samples. Therefore, it had a higher value of residual compressive stress. 4. Conclusions
Fig. 7. Gradient distributions of microhardness.
(1) The surface nanocrystallization material of 2195 Al-Li alloy with uniform microstructure was prepared by LSP. The surface grain of this alloy was refined to the nanometer with size of about 93 nm after single LSP and it can be further refined to about 70 nm by 3 times LSP. (2) The stress corrosion index ISSRT of the specimens with single LSP and without LSP were 10.99% and 14.16% respectively, which proved the LSP treatment can effectively improve the
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stress corrosion resistance of 2195 Al-Li alloy. The residual compressive stress and grain refinement at the surface can hinder the initiation and growth of cracks and retard the occurrence of stress corrosion cracking. (3) The improvement ratio of surface hardness after the 3 times LSP and single LSP were 36.8% and 16.3% respectively. The residual compressive stress value after single LSP was 199 MPa, while it reached 266 MPa after the 3 times LSP. The grain size became finer and the density of defects became higher after 3 times LSP as compared to single LSP, which were the main reasons for the improvement of its mechanical properties. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51574290, 51871243,51274245), NSAF (No. U1330126). References [1] S. Zabeen, K. Langer, M.E. Fitzpatrick, Effect of alloy temper on surface modification of aluminium 2624 by laser shock peening, Surf. Coat. Technol. 347 (2018) 123e135. [2] Z.P. Tong, X.D. Ren, Y.P. Ren, F.Z. Dai, Y.X. Ye, W.F. Zhou, L. Chen, Z. Ye, Effect of laser shock peening on microstructure and hot corrosion of TC11 alloy, Surf. Coat. Technol. 335 (2018) 32e40. [3] R.J. Sun, L.H. Li, W. Guo, P. Peng, T.G. Zhai, Z.G. Che, B. Li, C. Guo, Y. Zhu, Laser shock peening induced fatigue crack retardation in Ti-17 titanium alloy, Mater. Sci. Eng. A 737 (2018) 91e104. [4] S.H. Luo, W.F. He, L.C. Zhou, X.F. Nie, Y.H. Li, Aluminizing mechanism on a nickel-based alloy with surface nanostructure produced by laser shock peening and its effect on fatigue strength, Surf. Coat. Technol. 342 (2018) 29e36. [5] J.S. Park, I. Yeo, I. Jang, S.H. Jeong, Improvement of friction characteristics of cast aluminum-silicon alloy by laser shock peening, J. Mater. Process. Technol. 266 (2019) 283e291. [6] J.T. Wang, L. Xie, K.Y. Luo, W.S. Tan, L. Cheng, J.F. Chen, Y.L. Lu, X.P. Li, M.Z. Ge, Improving creep properties of 7075 aluminum alloy by laser shock peening, Surf. Coat. Technol. 349 (2018) 725e727. [7] K.Y. Luo, Y.F. Yin, C.Y. Wang, Q.F. Chai, J. Cai, J.Z. Lu, Y.F. Lu, Effects of laser shock peening with different coverage layers on fatigue behaviour and fractural morphology of Fe-Cr alloy in NaCl solution, J. Alloys Compd. 773 (2019) 168e179. [8] L. Tang, W.J. Jia, J. Hu, An enhanced rapid plasma nitriding by laser shock peening, Mater. Lett. 231 (2018) 91e93. [9] J.M. Yang, Y.C. Her, N.L. Han, A. Clauer, Laser shock peening on fatigue behavior of 2024-T3 Al alloy with fastener holes and stop holes, Mater. Sci.
Eng. A-Struct. 298 (2001) 296e299. [10] B.S. Yilbas, S.Z. Shuja, A. Arif, M.A. Gondal, Laser-shock processing of steel, J. Mater. Process. Technol. 135 (2003) 6e17. [11] Y.L. Liao, S. Suslov, C. Ye, J.C. Cheng, The mechanisms of thermal engineered laser shock peening for enhanced fatigue performance, Acta Mater. 60 (2012) 4997e5009. [12] X.F. Nie, W.F. He, S.L. Zang, X.D. Wang, J. Zhao, Effect study and application to improve high cycle fatigue resistance of TC11 titanium alloy by laser shock peening with multiple impacts, Surf. Coat. Technol. 253 (2014) 68e75. [13] J.Z. Lu, K.Y. Luo, Y.K. Zhang, Grain refinement of LY2 aluminum alloy induced by ultra-high plastic strain during multiple laser shock processing impacts, Acta Mater. 58 (2010) 3984e3994. [14] M.Z. Ge, J.Y. Xiang, Z. Fan, Y.L. Lu, W.N. Lei, Effect of laser energy on microstructure of Mg-3Al-1Zn alloy treated by LSP, J. Alloys Compd. 734 (2018) 266e274. [15] R.J. Sun, L.H. Li, Y. Zhu, W. Guo, P. Peng, B.Q. Cong, J.F. Sun, Z.G. Che, B. Li, C. Guo, L. Liu, Microstructure, residual stress and tensile properties control of wire arc additive manufactured 2319 aluminum alloy with laser shock peening, J. Alloys Compd. 747 (2018) 255e265. [16] Y. Yang, H. Zhang, H.C. Qiao, Microstructure characteristics and formation mechanism of TC17 titanium alloy induced by laser shock processing, J. Alloys Compd. 722 (2017) 509e516. [17] Y. Yang, K. Zhou, H. Zhang, H.B. Hu, H.C. Qiao, Thermal stability of microstructures induced by laser shock peening in TC17 titanium alloy, J. Alloys Compd. 767 (2018) 253e258. [18] K. Lu, J. Lv, Surface nanocrystallization of metallic materials-presentation of concept being a new approach, J. Mater. Sci. Technol. 15 (1999) 193e196. [19] R.K. Gupta, N. Nayan, G. Nagasireesha, Development and characterization of Al-Li alloys, Mater. Sci. Eng. A 420 (2006) 228e234. [20] Z.M. Lu, L.M. Shi, S.J. Zhu, Z.D. Tang, Y.Z. Jiang, Effect of high energy shot peening pressures on stress corrosion cracking of the welded joint of 304 austenitic stainless steel, Mater. Sci. Eng. A 637 (2015) 170e174. [21] A.L. Patterson, The scherrer formula for X-Ray particle size determination, Phys. Rev. 56 (1939) 978e982. [22] G.T. Gray, High-strain-rate deformation: mechanical behavior and deformation substructures induced, Mater. Res. 42 (2012) 285e303. , K.M. Knowles, P.J. Doorbar, R.D. Cutts, D. Rugg, Microstructural [23] S.J. Laine characterisation of metallic shot peened and laser shock peened Ti-6Al-4V, Acta Mater. 123 (2017) 350e361. [24] M.A. Meyers, Y.B. Xu, Q. Xue, Microstructural evolution in adiabatic shear localization in stainless steel, Acta Mater. 51 (2003) 1307e1325. [25] Y. Yang, B.F. Wang, J. Xiong, Y. Zeng, Z.P. Chen, X.Y. Yang, Adiabatic shear bands on the titanium side in the titanium/mild steel explosive cladding interface: experiments, numerical simulation and microstructure evolution, Metall. Mater. Trans. A 37 (2006) 3131e3137. [26] Y. Yang, K. Zhou, G.J. Li, Surface gradient microstructural characteristics and evolution mechanism of 2195 aluminum lithium alloy induced by laser shock peening, Opt. Laser. Technol. 109 (2019) 1e7. [27] J.T. Wang, Y.K. Zhang, Effects of laser shock peening on stress corrosion behavior of 7075 aluminum alloy laser welded joints, Mater. Sci. Eng. A 647 (2015) 7e14. [28] S.M. Charles, W. Tao, Ye Lin, C. Graham, Y.W. Mai, Laser shock processing and its effects on microstructure and properties of metal alloys: a review, Int. J. Fatig. 24 (2012) 1021e1036.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Effects of laser shock peening on microstructures and properties of 2195 Al-Li alloy Yang Yang a, *, Xiaolong Lian a, Kai Zhou a, Guojie Li b a b
School of Material Science and Engineering, Central South University, Changsha, 410083, China Xi'an Tianruida Photoelectric Technology Development Co., Ltd, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 June 2018 Received in revised form 7 December 2018 Accepted 8 December 2018 Available online 10 December 2018
The effects of laser shock peening (LSP) on microstructures and properties of 2195 Al-Li alloy were studied in this paper. The top surface microstructure of this alloy after LSP was observed using transmission electron microscopy (TEM). The change of stress corrosion resistance after LSP was studied by slow strain rate tensile (SSRT) test. Moreover the effect of LSP on the mechanical properties was characterized by the microhardness and the residual stress test. The microstructure observation results suggested that the surface grain of this alloy was refined to the nanometer with size of 93 nm after single LSP and it can be further refined to 70 nm by 3 times LSP, the grain refinement during LSP should be considered as a dislocation evolution process based on the mechanism of rotating dynamic recrystallization (RDR). The results of mechanical properties test showed that LSP could improve the hardness and introduce the residual compressive stress in the surface of target alloy, and the 3 times LSP can further improve the properties on the base of single LSP. Furthermore, the experimental results of SSRT indicated that LSP was an effective method to improve the resistance to stress corrosion of 2195 Al-Li alloy, because the residual compressive stress and refined grains in the surface can prevent the initiation and growth of cracks, while retard the stress corrosion cracking. © 2018 Elsevier B.V. All rights reserved.
Keywords: LSP Grain refinement Residual compressive stress Stress corrosion cracking resistance 2195 Al-Li alloy
1. Introduction Laser shock peening (LSP) is a surface processing technology that offers life extension of metallic structural components for aerospace, automotive, and power generation industries [1]. It can improve the property of materials, and it has been widely applied and studied in recent years. Tong [2] investigated the effect of laser shock peening on microstructure and hot corrosion of TC11 alloy and found that a large number of crystal defects produced by LSP added element diffusion paths to the surface to form oxide protective layer in a very short time. And these oxide protective layers can improve the corrosion resistance of TC11 alloy. Sun [3] found that the fatigue life of the LSPed sample was increased up to 2.4 times that of the unpeened counterpart. Luo [4] found that LSPinduced surface nanostructure increased the efficiency of the aluminizing process and the fatigue resistance, because the thermal stability of surface nanostructure and high-density dislocation
* Corresponding author. School of Materials Science and Engineering, Central South University, Changsha, 410083, China. E-mail address: [email protected] (Y. Yang). https://doi.org/10.1016/j.jallcom.2018.12.118 0925-8388/© 2018 Elsevier B.V. All rights reserved.
induced by LSP increased its surface activity and made more channels available for element diffusion during the high temperature. Park [5] investigated the effects of laser shock peening on friction characteristics of JIS-AC8A aluminum-silicon alloy and found the friction coefficient of laser peened surface was significantly lower than that of unpeened samples. Wang [6] investigated the effect of laser shock peening on creep properties of 7075 aluminum alloy and found that LSP can effectively enhance the creep resistance of 7075 aluminum at the elevated temperature and working stress. Luo's work [7] indicated that LSP generated deeper compressive residual stress and effectively decreased the corrosion fatigue crack growth rate, and increased coverage layers further improved the corrosion fatigue life of notched specimens. Tang [8] investigated the effect of LSP on plasma nitriding and found that LSP increased the surface roughness and formed a pre-hardening layer of about 200 mm. The previous studies on LSP confirmed that LSP could refine the grains [9e11]. However, the nanocrystallization of the surface layer induced LSP is still a problem to be solved at present. The TC11 titanium alloy was treated at different LSP times by Nie [12], and the experimental results showed that only dislocation cells with
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size of about 1 mm appeared in the surface of this alloy after single LSP and the appearance of nanocrystallines needed the LSP treatment for 3 times at least. The research of Lu [13] suggested that the grains in the surface of the LY12 aluminum alloy was refined from 10 mm to 3e5 mm after single LSP, and it was refined to submicron (100e200 nm) level after 5 times LSP, there were no nanocrystallines appeared in LY12 aluminum during LSP. Ge [14] studied the effect of laser energy on microstructure of Mg-3Al-1Zn alloy treated by LSP and found the grain size in top surface decreased with laser pulse energy increasing from 1.19 to 2.3 mm for 6.5 J to 0.04e0.27 mm for 8.5 J. Sun [15] found the average grain size decreased from 59.7 mm to 46.7 mm in 2319 aluminum alloy after LSP. Our previous study also found that the surface grain of TC17 titanium alloy was only refined to submicron level after single LSP [16,17]. As we all know, nanomaterials have superior performance incomparable with ordinary materials; it will bring the improvement of strength, hardness and other performance when the grain size of metal materials reached the nanometer level. The nanocrystallization in the surface of metals was always a hot research direction in the field of material science [18]. 2195 Al-Li alloy was always used in the aerospace field because of its low density and high strength, but the extreme application environment of aerospace put forward more demanding requirements for Al-Li alloy [19]. The surface nanocrystallization material of 2195 Al-Li alloy with uniform structure was prepared by LSP for the first time. The microstructure in the surface of LSPed materials was observed using transmission electron microscopy (TEM), the surface hardness, residual surface stress of 2195 Al-Li alloy after single LSP and 3 times LSP treatment were compared, and the property of stress corrosion resistance of this LSPed alloy was also investigated in the present work. 2. Material and methods 2.1. Sample preparation The forged 2195 Al-Li alloy was used in this study, and its composition was shown in Table 1 The aged state alloy was treated by single LSP and 3 times LSP treatment under the same LSP parameters. The sample states were: a. solid solution þ aging þ single LSP, b. solid solution þ aging þ3 times LSP. The solid solution experiment was carried out in a salt bath furnace with a temperature of 510 C for 30min, and the age treatment was at the temperature of 180 C for 12 h. 2.2. LSP experiment When the shortwave laser beam with high energy density was irradiated on the surface of the metal material, the ablation layer material will transfer instantaneously to plasma, and the rapid expansion of the plasma can produce an impact pressure with GPa level at the surface of the target material. The continuous impact pressure will cause conduct of stress wave in the surface of the material, thus bring the change of structure and the improvement of performance. The LSP experiment was carried out on a high repetition rate
Table 1 Chemical composition of 2195 Al-Li alloy. Component
Cu
Li
Mg
Ag
Zr
wt/%
4.0
1.0
0.4
0.4
0.12
331
Nd: glass laser. The determined LSP parameters are shown in Table 2. Black tape was used as the thermo-protective layer, and flowing water with a thickness of 1 mm was taken as the confining layer. 2.3. Microstructure observation The microstructures of 2195 Al-Li alloy after LSP was analyzed by Tecnai G2 20AEM transmission electron microscope (TEM) and the operating voltage was 200 KV. The thin film samples of TEM which were prepared by mechanical thinning, and single side thinning of ion beam was used to observe the surface microstructures after LSP. TEM samples were prepared as follows: Firstly cutting two strips perpendicular to the treated surface with dimensions of 1.4 mm 1.4 mm 15 mm, adhered the treated surfaces face to face. Then inserted into a copper tube with a diameter of 3 mm and bonded them together with glue. Slices with thickness of 1 mm were cut down perpendicular to the axis of tube, then thinned the slices to about 50 mm by mechanical grinding. The final TEM specimens were prepared by ion-beam thinning with ion energy of 4.5 kV and the incident angle of 7 for perforation and 3.5 kV/2 for 25min to refine the thin zone. The XRD experiment was conducted by D/Max 2500 X-ray diffractometer (operated at 40 kV and 40 mA) to analyse the surface information of 2195 aluminum-lithium alloy after LSP; the scanning scope was from 10 to 90 . 2.4. Mechanical property characterization The surface hardness after LSP treatment was measured by HVS1000 microhardness test machine with a 100 g load and a 10 s holding time. Several measurements along depth direction perpendicular to the treated surface were taken at intervals of 100 mm, and the mean of 3 measurements at every depths was used to be the microhardness value of this layer. X-ray residual stress tester was used to measure the residual surface stress of 2195 Al-Li alloy after LSP. The test area was on the LSPed surface, the working voltage was 40 KV and the working current was 40 mA. The scanning step and counting time was 0.02 and 5S respectively. The measuring error was 25 MPa or less, and it will be retested if the results were beyond the range. Before the test, the samples were ultrasonically cleaned with ethanol. 2.5. Test of resistance to stress corrosion The experimental methods of stress corrosion test can be divided into three types: constant deformation, constant load and slow strain rate test method, and the slow strain rate tensile has great advantages as compared to other two methods. Firstly, the slow strain rate method has a higher sensitivity to the stress corrosion cracking; secondly, many information can be obtained by this method, such as fracture time ft, maximum load Lmax and reduction in cross-sectional area Z, etc; furthermore, it can simulate the actual medium environment and get more accurate results.
Table 2 Processing parameters used in LSP. Type
Value
Spot diameter/mm Pulse energy/J Pulse width/ns Laser wavelength/nm Energy stability/% Overlapping ratio/%
3 5 20 1064 ±1.5 50
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The effect of LSP on stress corrosion property of 2195 Al-Li alloy was investigated by SSRT in this paper. The specimens were placed in a corrosive medium and inert medium (air) for slow strain rate tensile test and the strain rate was set to 106s1 [20]. In order to simulate the environment of high concentration of chloride ion, the solution used in stress corrosion test was 3.5% NaCl solution at room temperature and its pH value was 6.8 according to the standard of ASTM G44-2005. The sample size was designed by reference to the standard of GBT 15970.7e2000 and the specific dimension was shown in Fig. 1. The thickness of the specimen is 5 mm, and the width is 5 mm at the center 20 mm of the standard distance segment. The sample was polished by sandpaper and cleaned with acetone, then cleaned with distilled water and dried it. Double-sided LSP treatment at the center 20 mm of the standard distance segment was carried out. 3. Results and discussion Fig. 2. The XRD analysis results of different impact times.
3.1. Microstructural characteristics after LSP 3.1.1. Analysis results of XRD The X-ray diffraction theory stated that the grain refinement could be reflected in the width of spectral peaks. According to the Scherrer Equation [21]:
D¼
Kl B cos q
(1)
where K is a constant, D is the average thickness of a grain perpendicular to the crystal face, B is FWHM of diffraction peaks, q is the diffraction angle, and l is X-ray wavelength. It is ready to conclude that the width of spectral peaks at the same diffraction angle increased with the refinement of grain gradually. Fig. 2 is the XRD analysis results at the surface of the sample without LSP and the samples after different times LSP. The main peak at 60 e70 was selected and the a-Al diffraction peak at 65 was used to analyse the information after LSP. It can be clearly seen that the a-Al diffraction peak after LSP was wider than that of the specimen without LSP, which indicated that the grain size of the surface layer after LSP refined than the substrate. Regarding the 3 times LSPed sample, the diffraction peak of the a-Al was slightly widened but not obvious as compared to single LSP, which suggested that the 3 times LSP can further refine the surface grains on the base of single LSP, but the subsequent refinement was limited. 3.1.2. Observation results of TEM The observation results of microstructures in Fig. 3 showed that the major precipitate of 2195 aluminum-lithium alloy before LSP was T1(Al2CuLi) phase, and there was also a small amount of acicular globular d’ (Al3Li) phase and bean-shaped b0 (Al3Zr) phase. The original grain size was about 10 mm. Fig. 4 is the TEM observation results after single LSP, Fig. 4(a) shows a bright field image in the top surface. It can be clearly seen from the figure that a large number of fine grains appeared in the
Fig. 1. The dimensions of SSRT sample.
Fig. 3. The observation result of TEM before LSP.
top surface after LSP, and the size of grains was at the range of 40e150 nm. It can be seen the equiaxed grains with clear high angle grain boundaries and regular shapes from the enlarged image of the part of Fig. 4(a) shown in Fig. 4(b). Fig. 4(c) was the selected area electron diffraction (SAED) corresponding to Fig. 4(a), and the diffraction patterns shown in the image were ring-shaped, which indicated that the originally coarse grain had fully refined and the orientation was randomly distributed. The 15 different fields were selected for statistical analysis and the results were shown in Fig. 4(d), which suggested that the average grain size was about 90 nm, it reached the nanometer level. Fig. 5 is the results of TEM observation at the top surface after LSP treatment for 3 times. It was seen from the bright field images shown in Fig. 5(a) and (b) that there were a lot of refined grains with size in the range of 30e100 nm. The grain statistics results shown in Fig. 5(d) suggested that the average grain size was about 70 nm. The SAED results of Fig. 5(c) shows more continuous annular patterns which indicated that a large number of randomly oriented nanocrystals are distributed in the surface of the alloy. Furthermore, the results of the microstructural observation showed that the T1 phase in the surface of 2195 Al-Li alloy dissolved after
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Fig. 4. The observation result of TEM after single LSP: (a) Bright field image with low magnification at the surface; (b) Bright field image with high magnification at the part of (a); (c) corresponding selected area electron diffraction; (d) histogram of grain size distribution.
LSP, which was consistent with the phenomenon of precipitates dissolution during high strain rate loading conditions we observed before. As an ultra-high strain rate surface treatment technology, the peak impact pressure of GPa level can be generated instantaneously by LSP when it acted on the metal surface, and the strain rate reached the magnitude of 107s1 [16]. As a kind of metal with high stacking fault energy, the plastic deformation of 2195 Al-Li alloy was mainly under the dislocation slip mechanism. And it is reported that the time of twin atoms to rearrangement along twin direction was not enough under the ultra-high strain rate condition of 107s1 magnitude, therefore, the twin deformation was difficult to occur [22,23]. Consequently, the grain refinement during LSP should be a dynamic process based on dislocation evolution. The observation results of Figs. 4 and 5 showed that the surface grains of 2195 Al-Li alloy after LSP had obvious recrystallization characteristics, therefore, the formation of refined grains must undergo a special recrystallization process. The classical recrystallization mechanism cannot explain the recrystallization of LSP during ultra-high strain rate deformation. The dynamic rotation recrystallization (RDR) mechanism proposed by Meyers [24] suggested that the subgrains can rotate to form recrystallized grain under the action of the minimum driving force. The deformation at the strain rate of 107s1 could be considered as an adiabatic process, and the adiabatic temperature rise provided thermodynamic conditions for the mechanism of RDR. Therefore, the grain refinement of 2195 Al-Li alloy during LSP could be considered as a dynamic process that the dislocation transformed to dislocation
entanglement, dislocation cell, subgrain, and evolved into grain with high angle boundary through the mechanism of RDR finally. The dislocation density of 3 times LSPed specimen was higher than that of single LSP, so the size of dislocation cells and subgrains was also smaller. According to the kinetics equation [25] of the RDR mechanism shown in Formula 2, the smaller the size of the subgrain is, the shorter the time needed to complete the recrystallization will be, it's more conducive to the operation of the RDR mechanism with small grain size. Consequently, the 3 times LSP treatment can further refine the nanocrystallized grains at the base of single LSP.
t¼
L1 kTf ðqÞ 4shDb0 expðQb =RTÞ
(2)
where t is time; s is the grain boundary thickness; h is the grain boundary energy; Db0 is a constant referred to grain boundary diffusion; L1 is subgrain diameter; Qb is the activation energy for grain boundary diffusion; f(q) is the function about disorientation of subboundaries. The microstructure evolution mechanism was discussed in detail in our previous work [26]. 3.2. Resistance to stress corrosion The samples without LSP and after single LSP were placed in the air and 3.5% Nacl solution respectively for the SSRT experiment, The strain rate was set to 106s1, and the stress-strain curve was shown in Fig. 6. It can be seen that the elongation and tensile
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Fig. 5. The observation result of TEM after 3 times LSP: (a) Bright field image with low magnification at the surface; (b) Bright field image with high magnification at the part of (a); (c) corresponding selected area electron diffraction; (d) histogram of grain size distribution.
Fig. 6. Stress-strain curves after the SSRT test.
strength of the samples in the NaCl solution are lower than that in the air. Therefore, the 2195 Al-Li alloy had the stress corrosion sensitivity. The parameters of every SSRT curve was shown in Table 3, which indicated that the tensile strength and elongation of the sample after LSP was 362.15 MPa and 12.79% respectively when it placed in the air, and it decreased to 326.34 MPa and 11.41% correspondingly placed in the NaCl solution. The tensile strength and elongation of the samples without LSP were significantly lower than those of the LSPed samples both in the air and solution, the corresponding values are 350.07 MPa, 12.79% and 304.03 MPa, 10.51%, respectively. It is found that the specimen after LSP had a larger reduction in cross-sectional area after analyzing the tensile fracture, which was 13.11% and 12.02% correspondingly in the air and solution, the reduction in cross-sectional area of the sample without LSP was 11.82% and 10.51%, respectively. The stress corrosion index ISSRT can be calculated by the various parameters obtained in the SSRT test, which can reflect the stress
Table 3 The experimental results of SSRT. Test state
Tensile strength (MPa)
Elongation (%)
Area reduction (%)
ISSRT (%)
With LSP in the air With LSP in the solution Without LSP in the air Without LSP in the solution
362.15 326.34 350.07 304.03
12.79 11.41 11.82 10.51
13.11 12.02 12.23 10.94
10.99 14.16
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corrosion cracking sensitivity of the alloy more accurately than the single mechanical property index and it was usually used as an important criterion for the resistance to stress corrosion. A high ISSRT value indicates a high sensitivity to the stress corrosion cracking and the formula ISSRT was expressed as follows [27]:
ISSRT ¼ 1 ½ss ð1 þ ds Þ=½sA ð1 þ dA Þ
(3)
where ss and ds indicate tensile strength and elongation at tensile in the solution, and sA and dA is strength and elongation in the air respectively. The calculated results showed that the stress corrosion factor ISSRT of the LSPed specimen was 10.99%, while the index of the sample without LSP was 14.16%. The greater value of ISSRT indicated that the material had a higher stress corrosion cracking (SCC) tendency. Consequently, the experimental results of SSRT suggested that the LSP treatment can effectively improve the stress corrosion resistance of 2195 Al-Li alloy. The failure mode of materials was SCC in the stress corrosion media and the failure process underwent the stages of SCC initiation, growth and failure. The existence of the residual compressive stress increased the threshold of crack initiation in the top surface, which made the crack difficult to initiate, and the residual compressive stress also formed a continuous resistance in the stage of SCC growth. Therefore, the surface residual compressive stress induced by LSP was an important factor for the improvement of the resistance to stress corrosion. In the stage of SCC growth, the grain refinement layer formed during LSP also played a significant role. Due to the grain boundary ratio after refinement was greatly increased, a great number of grain boundaries caused the crack growth to encounter much resistance whether it was intergranular propagation or transgranular propagation, which made the crack hard to grow and run through, while it was difficult to occur SCC. 3.3. Mechanical properties Fig. 7 shows the relationship between the microhardness and the depth below top surface of the 2195 Al-Li alloy after different times LSP. It was clear that the microhardness value decreased continuously with the increase of the depth, the maximum hardness value appeared at the top surface, and the matrix hardness of all the samples was equal to about 95 HV. The surface hardness after 3 times LSP was 130 HV and increased about 36.8% as compared to the matrix, it was 120 HV after single LSP and
Fig. 8. Tested results of residual stress at the surface.
increased ratio was about 26.3%. The tested results suggested that the 3 LSPed sample had greater hardness than the single LSPed one, and the hardened depth was about 1.8 mm and 1.5 mm respectively. The residual surface stress of the samples after LSP was measured and the results shown in Fig. 8 indicated that the residual compressive stress could be introduced on the surface of 2195 Al-Li alloy by LSP. After single LSPed treatment, the surface residual stress was about 199 MPa, and it reached 266 MPa after 3 times of LSP. The surface grains were refined after LSP, and the grain refinement inevitably brought the increase of hardness according to the Hall-Petch relation. The gradient grain refinement layer caused the gradient change of hardness, and the maximum of hardness also appeared at the top surface where there were the most deformation degree and the finest grains. At the same time, the plastic deformation induced by LSP also introduced the crystal defects in the surface layer with a gradient feature, which also played an important role in the improvement of the surface hardness. The affected depth of plastic deformation of the 3 times LSPed samples was higher than the single LSPed one, therefore, the depth of the hardened layer was higher. Moreover, the surface grain size was smaller, and the crystal defect density was higher after the 3 times LSP. Consequently, the effect of hardening of 3 times LSPed specimen was more significant. The plastic deformation induced by LSP showed the lattice distortion at the microstructure, and the lattice distortion will produce the residual stress, the residual stress that compressed the alloy was the residual compressive stress. The existence of this residual compressive stress prevented crack initiation and reduced crack growth rate and effectively improved material properties [28]. The degree of lattice distortion in the surface of the 3 LSPed samples is greater compared with single LSPed samples. Therefore, it had a higher value of residual compressive stress. 4. Conclusions
Fig. 7. Gradient distributions of microhardness.
(1) The surface nanocrystallization material of 2195 Al-Li alloy with uniform microstructure was prepared by LSP. The surface grain of this alloy was refined to the nanometer with size of about 93 nm after single LSP and it can be further refined to about 70 nm by 3 times LSP. (2) The stress corrosion index ISSRT of the specimens with single LSP and without LSP were 10.99% and 14.16% respectively, which proved the LSP treatment can effectively improve the
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stress corrosion resistance of 2195 Al-Li alloy. The residual compressive stress and grain refinement at the surface can hinder the initiation and growth of cracks and retard the occurrence of stress corrosion cracking. (3) The improvement ratio of surface hardness after the 3 times LSP and single LSP were 36.8% and 16.3% respectively. The residual compressive stress value after single LSP was 199 MPa, while it reached 266 MPa after the 3 times LSP. The grain size became finer and the density of defects became higher after 3 times LSP as compared to single LSP, which were the main reasons for the improvement of its mechanical properties. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51574290, 51871243,51274245), NSAF (No. U1330126). References [1] S. Zabeen, K. Langer, M.E. Fitzpatrick, Effect of alloy temper on surface modification of aluminium 2624 by laser shock peening, Surf. Coat. Technol. 347 (2018) 123e135. [2] Z.P. Tong, X.D. Ren, Y.P. Ren, F.Z. Dai, Y.X. Ye, W.F. Zhou, L. Chen, Z. Ye, Effect of laser shock peening on microstructure and hot corrosion of TC11 alloy, Surf. Coat. Technol. 335 (2018) 32e40. [3] R.J. Sun, L.H. Li, W. Guo, P. Peng, T.G. Zhai, Z.G. Che, B. Li, C. Guo, Y. Zhu, Laser shock peening induced fatigue crack retardation in Ti-17 titanium alloy, Mater. Sci. Eng. A 737 (2018) 91e104. [4] S.H. Luo, W.F. He, L.C. Zhou, X.F. Nie, Y.H. Li, Aluminizing mechanism on a nickel-based alloy with surface nanostructure produced by laser shock peening and its effect on fatigue strength, Surf. Coat. Technol. 342 (2018) 29e36. [5] J.S. Park, I. Yeo, I. Jang, S.H. Jeong, Improvement of friction characteristics of cast aluminum-silicon alloy by laser shock peening, J. Mater. Process. Technol. 266 (2019) 283e291. [6] J.T. Wang, L. Xie, K.Y. Luo, W.S. Tan, L. Cheng, J.F. Chen, Y.L. Lu, X.P. Li, M.Z. Ge, Improving creep properties of 7075 aluminum alloy by laser shock peening, Surf. Coat. Technol. 349 (2018) 725e727. [7] K.Y. Luo, Y.F. Yin, C.Y. Wang, Q.F. Chai, J. Cai, J.Z. Lu, Y.F. Lu, Effects of laser shock peening with different coverage layers on fatigue behaviour and fractural morphology of Fe-Cr alloy in NaCl solution, J. Alloys Compd. 773 (2019) 168e179. [8] L. Tang, W.J. Jia, J. Hu, An enhanced rapid plasma nitriding by laser shock peening, Mater. Lett. 231 (2018) 91e93. [9] J.M. Yang, Y.C. Her, N.L. Han, A. Clauer, Laser shock peening on fatigue behavior of 2024-T3 Al alloy with fastener holes and stop holes, Mater. Sci.
Eng. A-Struct. 298 (2001) 296e299. [10] B.S. Yilbas, S.Z. Shuja, A. Arif, M.A. Gondal, Laser-shock processing of steel, J. Mater. Process. Technol. 135 (2003) 6e17. [11] Y.L. Liao, S. Suslov, C. Ye, J.C. Cheng, The mechanisms of thermal engineered laser shock peening for enhanced fatigue performance, Acta Mater. 60 (2012) 4997e5009. [12] X.F. Nie, W.F. He, S.L. Zang, X.D. Wang, J. Zhao, Effect study and application to improve high cycle fatigue resistance of TC11 titanium alloy by laser shock peening with multiple impacts, Surf. Coat. Technol. 253 (2014) 68e75. [13] J.Z. Lu, K.Y. Luo, Y.K. Zhang, Grain refinement of LY2 aluminum alloy induced by ultra-high plastic strain during multiple laser shock processing impacts, Acta Mater. 58 (2010) 3984e3994. [14] M.Z. Ge, J.Y. Xiang, Z. Fan, Y.L. Lu, W.N. Lei, Effect of laser energy on microstructure of Mg-3Al-1Zn alloy treated by LSP, J. Alloys Compd. 734 (2018) 266e274. [15] R.J. Sun, L.H. Li, Y. Zhu, W. Guo, P. Peng, B.Q. Cong, J.F. Sun, Z.G. Che, B. Li, C. Guo, L. Liu, Microstructure, residual stress and tensile properties control of wire arc additive manufactured 2319 aluminum alloy with laser shock peening, J. Alloys Compd. 747 (2018) 255e265. [16] Y. Yang, H. Zhang, H.C. Qiao, Microstructure characteristics and formation mechanism of TC17 titanium alloy induced by laser shock processing, J. Alloys Compd. 722 (2017) 509e516. [17] Y. Yang, K. Zhou, H. Zhang, H.B. Hu, H.C. Qiao, Thermal stability of microstructures induced by laser shock peening in TC17 titanium alloy, J. Alloys Compd. 767 (2018) 253e258. [18] K. Lu, J. Lv, Surface nanocrystallization of metallic materials-presentation of concept being a new approach, J. Mater. Sci. Technol. 15 (1999) 193e196. [19] R.K. Gupta, N. Nayan, G. Nagasireesha, Development and characterization of Al-Li alloys, Mater. Sci. Eng. A 420 (2006) 228e234. [20] Z.M. Lu, L.M. Shi, S.J. Zhu, Z.D. Tang, Y.Z. Jiang, Effect of high energy shot peening pressures on stress corrosion cracking of the welded joint of 304 austenitic stainless steel, Mater. Sci. Eng. A 637 (2015) 170e174. [21] A.L. Patterson, The scherrer formula for X-Ray particle size determination, Phys. Rev. 56 (1939) 978e982. [22] G.T. Gray, High-strain-rate deformation: mechanical behavior and deformation substructures induced, Mater. Res. 42 (2012) 285e303. , K.M. Knowles, P.J. Doorbar, R.D. Cutts, D. Rugg, Microstructural [23] S.J. Laine characterisation of metallic shot peened and laser shock peened Ti-6Al-4V, Acta Mater. 123 (2017) 350e361. [24] M.A. Meyers, Y.B. Xu, Q. Xue, Microstructural evolution in adiabatic shear localization in stainless steel, Acta Mater. 51 (2003) 1307e1325. [25] Y. Yang, B.F. Wang, J. Xiong, Y. Zeng, Z.P. Chen, X.Y. Yang, Adiabatic shear bands on the titanium side in the titanium/mild steel explosive cladding interface: experiments, numerical simulation and microstructure evolution, Metall. Mater. Trans. A 37 (2006) 3131e3137. [26] Y. Yang, K. Zhou, G.J. Li, Surface gradient microstructural characteristics and evolution mechanism of 2195 aluminum lithium alloy induced by laser shock peening, Opt. Laser. Technol. 109 (2019) 1e7. [27] J.T. Wang, Y.K. Zhang, Effects of laser shock peening on stress corrosion behavior of 7075 aluminum alloy laser welded joints, Mater. Sci. Eng. A 647 (2015) 7e14. [28] S.M. Charles, W. Tao, Ye Lin, C. Graham, Y.W. Mai, Laser shock processing and its effects on microstructure and properties of metal alloys: a review, Int. J. Fatig. 24 (2012) 1021e1036.