Iron GH2036 alloy residual stress thermal relaxation behavior in laser shock processing

Optics & Laser Technology 74 (2015) 29–35

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Iron GH2036 alloy residual stress thermal relaxation behavior in laser shock processing X.D. Ren a,b,n, W.F. Zhou a, S.D. Xu a, S.Q. Yuan b, N.F. Ren a, Y. Wang b, Q.B. Zhan a a b

Department of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, PR China Research Center of Fluid Machinery Engineering and Technical, Jiangsu University, Zhenjiang 212013, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 17 March 2015 Received in revised form 15 May 2015 Accepted 16 May 2015 Available online 3 June 2015

Laser shock processing (LSP) has a significant effect on the fatigue performance of Iron GH2036 alloy by inducing a deep compressive residual stress field. The effects of isothermal annealing treatments on Iron GH2036 alloy compressive residual stress after laser shock processing at various temperatures ranging from 200 °C to 650 °C were studied comprehensively using experimental and simulation methods. The thermal stability of compressive residual stress induced by LSP was investigated. The residual stress distributions of original and processed specimens were measured by the X-ray diffraction method. The results showed that the experimental measurements were well consistent with the simulation data. Residual stress of laser shock processing Iron GH2036 alloy had released but not completely released at the selected temperatures and during the initial exposure period (less than 30 min); the thermal relaxation magnitude was large and increased with the rise of applied temperature. We infer that the main mechanism of thermal relaxation is the mechanism involving rearrangement and annihilation of dislocation. & 2015 Elsevier Ltd. All rights reserved.

Keywords: Laser shock processing Residual stress Thermal relaxation

1. Introduction Mechanical surface treatments, such as deep rolling (DR) [1], laser shock processing (LSP) [2,3], shot peening (SP) [4–6], low plasticity burnishing (LSB) [7], glass bead peening (GBP) [8] and water peening (WP) [8] are known to induce several beneficial effects into metallic surfaces, which serve to enhance fatigue properties by improving the resistance against fatigue crack initiation and propagation. These beneficial effects prevail only if the near-surface compressive residual stresses or work hardening remain stable during mechanical loading or exposure to elevated temperature. Compared with the traditional process, LSP induces deeper compressive stress layer with relatively low cold hardening. Researchers have found that temperature, exposure time and initial cold hardening are the primary influence factors for residual stress relaxation. Aghdam et al. [9] studied the time independent thermo-mechanical residual stress relaxation around cold expanded fastener holes in aluminum alloy 7075-T6 by finite element simulation and experiment, which suggested that higher temperatures caused more relaxation of residual stress. Nikitin et al. [10] investigated thermal relaxation of LSP and DR AISI304 at n Corresponding author at: Department of Mechanical Engineering, Jiangsu University, Zhenjiang 212013, PR China. Tel./fax: þ86 511 88780352. E-mail address: [email protected] (X.D. Ren).

http://dx.doi.org/10.1016/j.optlastec.2015.05.009 0030-3992/& 2015 Elsevier Ltd. All rights reserved.

elevated temperatures. Results showed that increasing temperature lead to more stress relaxation. Leverant et al. [11] found that only a small relaxation of residual stress occurred in surface and subsurface layer of SP Ti–6Al–4V when exposed at temperatures from 25 °C to 310 °C. Nalla et al. [12] compared thermal relaxation of residual stresses in Ti–6Al–4V induced by DR and LSP at 450 °C, and residual stress released completely at 450 °C. Khadhraoui et al. [13] studied the effects of exposure time and applied temperature on SP Inconel 718. A significant decrease of the initial residual stress occurred in the first period of exposure time, followed by slowing down and then stabilized. Masmoudi et al. [14] found that significant compressive residual stress was retained near the surface in SP IN100 after exposure at 750 °C. Feng et al. [15] found that the residual stress relaxation in SP TC4-DT was influenced by annealing temperature and time. Hyukjae et al. [16] found about 10% of surface residual stress in SP Ti–6Al–4V released at 100 °C after exposure for 24 h, about 30% at 260 °C and 95% at 370 °C. Zhong et al. [17,18] found that residual stress released mainly during the initial period of exposure and the relaxation magnitude increased with the increase of applied temperature in LSP IN718 and Ti–6Al–4V. Ren et al. [19] found that the formation of high density dislocation structure and the pinning effect at the high temperature induced lower residual stress relaxation of LSP steel alloy in the temperature range 25–600 °C. GH2036 is mainly used in aircraft engine components which serve in high temperature environment and always bear fluctuating

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loads. The damage of aero engine components is generally owing to the surface micro-cracks. The compressive residual stress induced by LSP can improve the fatigue performance of metallic materials on the surface, which would restrain the formation and propagation of crack. However, compressive residual stress would release at elevated temperatures. To the best of our knowledge, the influences of LSP on mechanical properties of GH2036 at elevated temperatures have never been reported in literatures. Therefore, it is of great significance to study the thermal stability of LSP GH2036. In this paper, the process of residual stress relaxation of GH2036 at elevated temperatures was investigated by experimental method and finite element method (FEM). According to the behaviors of thermal relaxation in LSP GH2036, a proper processing technique of laser shock processing can be chosen, and a considerable reference about the application of laser shock processing technology in high temperature components of engine would be provided.

2. LSP and material model determination The surface of the material to be processed was coated with ablative material and a thin film of water was used as the transparent confining layer over the surface. Fig. 1(a) shows the laser shock processing process [3,18]. When the laser beam irradiates the workpiece, the ablative layer vaporizes and converts into plasma after absorbing energy from the high intensity laser. As the plasma expands rapidly, high pressure and short duration shock wave generates and propagates into the metal material. The presence of the water layer tends to confine the energy and increase the intensity of the pressure pulse of shock wave. The shock wave can be much larger than the dynamic yield strength of the metal material and induce plastic deformation at the surface and subsurface layer of workpiece. Associated with the plastic deformation is the generation of a deep compressive residual stress layer extending from the surface to depths up to 1 mm or more, primarily depending on the energy density of laser and material properties. To obtain the proper loading conditions, a pressure model based on previous study [20] was involved in this paper. A quarter of the configuration of the model is employed for the model is symmetric and subjected to a symmetric uniform load. To accurately acquire the residual stress field, it is necessary to have sufficient mesh density within the finite element (FE) model and a finer mesh around loading area, which is shown in Fig. 2(c). The metal material would experience extreme high strain rate (106/s–108/s) plastic deformation during LSP [3]. For reflecting the high strain rate behavior of material, lots of material models have been developed to account for the effect of strain hardening, strain rate hardening and thermal softening. These models include the Johnson–Cook (JC) [3,23], Zerilli–Armstrong (ZA) [20], modified ZA

[18], Khan–Huang–Liang (KHL) [17] and others. All of them have been successively implemented to simulate the distribution of residual stress in metal material induced by LSP. The JC model was chosen as material model since the material constants of JC model determined by high strain rate and high temperature experimental data were suitable in describing the effect of temperature on flow stress, and LS-DYNA was chosen in this study. According to Johnson–Cook model, Von mises stress of material is given as [3,17,18]

⎡ ⎛ T − T ⎞m⎤ ⎛ ε′ ⎞⎤⎡ 0 σY = (A + Bεn)⎢1 + C ln⎜ ⎟⎥⎢1 − ⎜ ⎟ ⎥ ⎝ Tm − T0 ⎠ ⎥⎦ ⎝ ε0′ ⎠⎦⎢⎣ ⎣

(1)

where ε is the effective plastic strain, ε0 is the effective plastic strain rate, and ε′0 ¼ 1.0 s  1. T0 and Tm are the room temperature and melting temperature respectively. A, B, n, c and m are experimentally determined constants. The complete JC model parameters for GH2036 calibrated in this paper are thus given as A¼900 MPa, B ¼1200 MPa, n ¼0.6, c¼ 0.0092 and m ¼1.27 [18]. The pressure waveform of laser shock wave is similar to the laser wave profile tested by oscilloscope according to Fabbro et al.'s findings [20,24], which reveals that the response time of laser inducing shock wave is about 3 times of the laser pulse width, or even longer. In this study, as the laser pulse width is 25 ns, the response time is set as 100 ns. The temporal pressure pulse profile is plotted in Fig. 1(b). Thermal loading conditions of 200 °C, 300 °C, 400 °C, 500 °C, 600 °C and 650 °C are applied on the FE model with the residual stress distribution from LSP as the initial condition to simulate the influence of different temperatures on LSP GH2036 alloy. The purely thermal loading condition is simulated by a convection boundary condition q = h(T − T∞) [17,18], where q is the heat flux across the boundary, h is the heat transfer coefficient of air, which is 110 W/(m2 °C) for natural convection in this simulation, T is the initial temperature of specimen, which is taken to be room temperature, and T∞ is the imposed heating temperature. The heat capacity of GH2036 is 440 J/(kg °C), and the thermal conductivity is 17.14 W/(m °C) [25].

3. Material and experimental details 3.1. Experimental material The chemical compositions and morphology of GH2036 are shown in Table 1 and Fig. 2, respectively. The specimens were cut into a cuboid shape, and then polished with sanding and polishing. The oil and impurity on specimens were cleaned by using anhydrous alcohol or acetone solution. Specimens were processed by using the Q switched Nd:Glass laser system with aluminum foil on a single side. The primary processing parameters were laser beam energy of 10 J, spot diameter of 3.5 mm and pulse width of 25 ns

Fig. 1. (a) Schematic representation of laser shock processing and (b) the temporal pressure pulse profile.

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Fig. 2. Morphology of the specimen: (a) before LSP; (b) after LSP; (c) FE model of 10 mm  10 mm  5 mm for LSP and thermal relaxation simulation.

Table 1 Chemical compositions of Iron GH2036 alloy (wt%). C

Cr

Ni

Mo

V

Mn

Si

Fe

0.34–0.40

11.5–13.5

7.0–9.0

1.10–1.40

1.25–1.55

7.50–9.50

0.30–0.80

Remaining

for every shot. Water flow was maintained at the same rate to ensure the same water film thickness in each shot. 3.2. Thermal relaxation experimental After LSP, the processed specimens were exposed to elevated temperatures of 200 °C, 300 °C, 400 °C, 500 °C, 600 °C and 650 °C in a vacuum furnace for 60 min, and finally air cooled 1 h. We select 650 °C as one of the temperatures for thermal relaxation study of GH2036 since it is a typical service temperature of Iron base alloy in aerospace engine applications. Surface residual stress fields and the depth profiles of the residual stress with single impacts after LSP were measured by using XRD with sin2 ψ method. To measure residual stress along the depth and avoid the influence of external stress induced by peeling off material on compressive residual stress, the electropolishing material removal method was involved. Electro-polishing was performed in the solution of 3.5% saturated sodium chloride solution, and material removal was controlled by the applied voltage and time. The area surrounding the polished region was masked to maintain uniformity and consistency during the material removal.

4. Results and discussions 4.1. Analysis of experimental results and simulation results The surface residual stress and residual stress in depth obtained by experiments and simulation are shown in Fig. 3. The maximum compressive residual stress is observed at the surface layer, which is about 480 MPa, and the simulated data is about 510 MPa. The depths of compressive residual stress layer induced

by LSP with experimental and simulation methods are approximately 0.55 mm and 0.62 mm, respectively. The experimental data are well consistent with the simulation results in the range of allowable error. Researchers [21,22,26] have found that stress decreases as the style of index attenuation with the distance from top surface increases. Moreover, tensile stress will replace compressive stress at a certain depth. The residual stress acquired by experiment and simulation of LSP specimens after annealing at the temperatures of 200 °C, 300 °C, 400 °C, 500 °C, 600 °C and 650 °C for 60 min are shown in Fig. 3. It can be observed that the residual stress in LSP GH2036 will release at various temperatures ranging from 200 °C to 650 °C, but will not completely release at 650 °C. The maximum compressive residual stress obtained by experiment decreased from  480 MPa to 385 MPa, 355 MPa,  310 MPa,  256 MPa, 160 MPa and  135 MPa after exposure for 60 min at heating temperatures of 200 °C, 300 °C, 400 °C, 500 °C, 600 °C and 650 °C, reducing 19.8%, 26.0%, 35.4%, 46.7%, 66.7% and 71.8%, respectively. Residual stress releases significantly and the thermal relaxation magnitude increases with the rise of temperature, which means the effect of temperature on thermal relaxation of residual stress is remarkable. With temperature rising from 200 °C to 650 °C, the maximal thermal relaxation occurred at the surface layer and the depth of compressive residual stress layer decreases correspondingly. It could be observed that the distribution of residual stress becomes much more uniform after thermal relaxation. The maximum compressive residual stress obtained by simulation decreased from 510 MPa to  401 MPa,  370 MPa, 330 MPa, 270 MPa,  170 MPa and  145 MPa after exposure for 60 min at heating temperature of 200 °C, 300 °C, 400 °C, 500 °C, 600 °C and 650 °C. The results show that residual stress decreased by 21.4%, 27.5%, 35.3%, 47.1%, 66.6% and 71.5% respectively, which agree with

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Fig. 3. The residual stress at elevated temperatures: (a) surface residual stress obtained by experiment; (b) surface residual stress obtained by simulation; (c) residual stress distribution along with the depth by experiment; and (d) residual stress distribution along with the depth by simulation.

Fig. 4. (a) LSP GH2036 thermal relaxation behaviors at surface and different layers of 0.1 mm, 0.2 mm, 0.35 mm and 0.5 mm by experiment; (b) thermal relaxation behaviors at surface and different layers of 0.1 mm, 0.2 mm, 0.35 mm and 0.5 mm by simulation.

experimental data. Therefore this model is applied extensively in the numerical study of the thermal relaxation behavior in LSP GH2036. The strengthening effect by LSP is still significant at elevated temperatures, which is beneficial to improve the fatigue life of components. The effect of applied temperature on residual stress induced by LSP obtained from experiment and simulation is shown in Fig. 4, which indicates that residual stress at surface and subsurface of LSP GH2036 releases obviously but not longer significantly as the depth exceeds a certain value (0.5 mm) with the increase of exposure temperature.

The relationship between exposure time and residual stress induced by LSP obtained from experiment and simulation is plotted in Fig. 5. The residual stress in the surface of LSP GH2036 decreases with the increase of exposure time, and releases quickly during the initial exposure period at the selected temperature. The rate of thermal relaxation decreases and tends to zero gradually while the variation trend of residual stress stays the same at different exposure temperatures. The magnitude of thermal relaxation increases with the rising of exposure temperature at selected time. However, residual stress induced by LSP did not release completely at selected annealing times and temperatures.

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Fig. 5. Stress relaxation at different exposure times with annealing temperatures of 200 °C, 300 °C, 400 °C, 500 °C, 600 °C and 650 °C: (a) obtained by experiment and (b) obtained by simulation.

Fig. 6. Influence of exposure time on the thermal relaxation of LSP GH2036 at 200 °C, 300 °C, 400 °C, 500 °C, 600 °C and 650 °C: (a) experimental data and (b) simulated results.

In terms of the temporal characteristic, compressive residual stress releases mainly in the initial 30 min of heat treatment. Residual stress tends to stabilize after a longer exposure time as the thermal relaxation stops. According to the previous studies [13,16,18], a large proportion of residual stress is released in the initial exposure period, normally between 3 min and 1 h, followed by a stabilization of stress at last. The residual stress relaxation is thought to occur due to two main reasons: material softening at elevated temperatures and non-uniform distribution of compressive residual stress. The intrinsic yield stress of GH2036 becomes lower when the temperature increases. As a result, the initial value of residual stress, which is further above the decreased yield stress, will accelerate the visco-plastic flow, leading to the relaxation of residual stress in turn. It is expected that much more thermal relaxation occurs when the applied temperature is higher because higher temperature results in a more significant reduction of yield stress, which can be seen from the comparison among Fig. 5. At the same time, areas with very low residual stress are almost not subjected to relaxation. Therefore, it can be inferred that higher temperature leads to more stress relaxation, lower initial stress experiences less relaxation and residual stress exhibits more uniform spatial distribution after releasing. 4.2. Discussion on residual stress relaxation at elevated temperatures The effects of applied temperature and exposure time on thermal relaxation are controlled by thermally activated mechanism. Thermal

relaxation of residual stress can be described by using Zener–Wert– Avrami function [17,18], namely

σ RS σ0RS

= exp[ − (Ata)m]

(2)

where σ0RS is the initial stress before annealing, σ RS is the residual stress at a given time ta and temperature Ta, m is the numerical parameter depending on the dominant relaxation mechanism, and A is the function associated with material properties and temperature. The function is given as [21,22]

⎡ ΔH ⎤ A = Bexp⎢− ⎥ ⎣ kTa ⎦

(3)

where B is the material constant with the value of 2.05 × 1012min−1, k is the Boltzmann constant and the value is 8.617343 × 10−5eV/K , ΔH is the activation enthalpy of material for the relaxation process, and Ta is the annealing temperature. Eq. (2) can be deduced as

ln [ ln(σ0RS /σ RS )] = m ln A + m ln ta

(4)

Fig. 6 shows the plot of ln [ ln(σ0RS /σ RS )] versus ln ta at a given annealing temperature Ta according to experimental data and simulation results. As revealed in Fig. 6, the relationship between ln [ ln(σ0RS /σ RS )] and ln ta is an approximate linear relationship at a constant annealing temperature and the slope of straight line is mi. The value of m is determined by the mean value of these slopes.

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According to Eq. (3), the activation enthalpy ΔH of the relaxation process for LSP GH2036 can be determined. According to the experimental data, when the exposure temperature is 200 °C, 300 °C, 400 °C, 500 °C, 600 °C and 650 °C, the value of m is 0.40, 0.407, 0.399, 0.471, 0.563 and 0.519 respectively. Therefore, the mean value of m is 0.46, and the calculated activation enthalpy ΔH is about 1.86 eV. Based on the simulated data, the value of m is 0.22, 0.30, 0.311, 0.412, 0.512 and 0.513 at 200 °C,

Fig. 7. Influence of annealing temperature on LSP GH2036 residual stress with exposure time of 60 min.

300 °C, 400 °C, 500 °C, 600 °C and 650 °C respectively. The mean value of m is 0.38, and the value of ΔH is about 1.88 eV. Fig. 7 shows the relationship of residual stress and annealing temperature at a given exposure time ta (ta ¼60 min) according to the measured and simulated data. It is observed that stress changes smoothly with the rising of temperature between 200 °C and 450 °C and the slopes of σ RS –Ta curves increase significantly after 450 °C but tend to be smooth again after 600 °C, which indicates that temperatures ranging from 450 °C to 600 °C have greater impacts on thermal relaxation of LSP GH2036 alloy compared with lower or higher temperature. As mentioned above, when the annealing temperature is high enough to reduce the intrinsic yield stress, the residual stress releases significantly. The intrinsic yield stress of GH2036 decreases more remarkably with temperature rising in the range from 450 °C to 600 °C, which means the magnitude of stress relaxation in this temperature interval is bigger compared to others. From the discussion above, it could be concluded that the effect of applied temperature and exposure time on thermal relaxation of LSP iron base alloy GH2036 can be well described by Zener–Wert–Avrami function. Grain refinement, high dislocation density and the low cold hardening rates induced by laser shock processing have significant influence on thermal stability of LSP GH2036 at elevated temperatures. Researchers [10,27] found that dynamic thermal recovery and recrystallization is the main mechanism causing thermal relaxation of residual stress at elevated temperatures. Essentially, dynamic recovery is caused by dislocation glide,

Fig. 8. Schematic of grain and dislocation evolutionary during isothermal annealing: (a) high density dislocation induced by LSP; (b) dislocation slip and climb; (c) dislocation counterbalance in cells; (d) sub-grain boundary forming; (e) sub-grain boundary moving; and (f) sub-grain growth and coalescence.

X.D. Ren et al. / Optics & Laser Technology 74 (2015) 29–35

dislocation climb and dislocation density decrease. We infer that the main mechanism of thermal relaxation is the mechanism involving rearrangement and annihilation of dislocation. A presumed mechanism of stress thermal relaxation is proposed, which involves the evolution of dislocation density and morphology. As shown in Fig. 8(a), LSP produces a high density of dislocations in the specimen, and the formation of dislocations occupies the vast majority of storage energy obtained from LSP in the deformed metal. This part of the energy induces the deformed metal into a thermodynamic instability state, making the metal have a spontaneous tendency to return to the steady state of lowest free enthalpy. In the isothermal process, dislocations are activated and start to slip and climb. During this time, unlike dislocations meet and counterbalance, so that the dislocation density in the sample decreases. On the other hand, the movement of dislocations is impeded due to the pinning effect of precipitated phases, forming dislocation walls and dislocation cells, as revealed in Fig. 8(b). As shown in Fig. 8(c), unlike dislocations in the dislocation cells continue to counterbalance and the dislocation density keeps declining. In Fig. 8(d), edge dislocations gather at the cell walls so the cell walls become thinner and clear sub-grain boundaries form gradually. Dislocation walls of polygonization constitute small-angle sub-grain boundaries, dividing the original crystal into many sub-grains. Meanwhile, dislocations on the subgrain boundaries transfer to the adjacent boundaries through climbing and slipping, so the primary grain boundaries disappear and a large-angle sub-grain is formed, which is illustrated in Fig. 8 (e) and (f). The entire process consumes a large amount of dislocations density, which means residual stresses release quickly in this period. In the subsequent process, sub-grains in the crystals rotate and move inside the crystals to release the storage energy in the metal, and residual stresses continue to release but slowly.

5. Conclusions Laser shock processing has unique advantages in improving the mechanical properties and fatigue performance of materials by inducing high compressive residual stress. The influences of thermal treatment on residual stress of LSP GH2036 alloy at the temperatures ranging from 200 °C to 650 °C were investigated by comprehensively using experimental and simulation methods. The magnitude of thermal relaxation is influenced by the applied temperature and exposure time of annealing. The thermal relaxation magnitude increases with the increase of temperature between 200 °C and 650 °C. The surface residual stress releases much more easily than the subsurface. Moreover, maximal relaxation occurs at the depth of maximum compressive residual stress, and lower initial residual stress is subjected to less stress relaxation. With the applied temperature rising, the rate of thermal relaxation increases but decreases with the increase of exposure time. The distribution of residual stress becomes uniform after thermal relaxation for a long time. We infer that the primary mechanism of thermal relaxation is the mechanism involving rearrangement and annihilation of dislocation. The present work has only focused on the regulation of thermal relaxation influenced by exposure time and applied temperature. Further work should be carried out to study the influences of coupling mechanical and thermal loads which are close to the actual serving environment. In addition, the metallographic structure and micro-structure evolution of LSP GH2036 at elevated temperatures will be studied in the further work.

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Acknowledgments The authors are grateful to the Project supported by the National Natural Science Foundation of China (Grant nos. 51275556 and 51239005), the Project Funded by the Six Major Talent Peak of Jiangsu Province (2012-ZBZZ-025).

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