Stress relaxation ageing behaviour and constitutive modelling of a 2219 aluminium alloy under the effect of an electric pulse

Accepted Manuscript Stress relaxation ageing behaviour and constitutive modelling of a 2219 aluminium alloy under the effect of an electric pulse Zhan Lihua, Ma Ziyao, Zhang Jiao, Tan Jingsheng, Yang Zhan, Li Heng PII:

S0925-8388(16)31002-7

DOI:

10.1016/j.jallcom.2016.04.051

Reference:

JALCOM 37245

To appear in:

Journal of Alloys and Compounds

Received Date: 22 October 2015 Revised Date:

3 April 2016

Accepted Date: 5 April 2016

Please cite this article as: Z. Lihua, M. Ziyao, Z. Jiao, T. Jingsheng, Y. Zhan, L. Heng, Stress relaxation ageing behaviour and constitutive modelling of a 2219 aluminium alloy under the effect of an electric pulse, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.04.051. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Stress relaxation ageing behaviour and constitutive modelling of a 2219 aluminium alloy under the effect of an electric pulse Zhan Lihuaa, b,*, Ma Ziyaoa, b, Zhang Jiaoa, b, Tan Jingshenga, b, Yang Zhana, b, Li Hengc a

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State Key Laboratory of High Performance Complex Manufacturing, Central South University, Changsha 410083 China; b School of Mechanical and Electrical Engineering, Central South University, Changsha 410083 China; c State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China

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Abstract: In creep age forming (CAF), stress relaxation and the creep process occur during the ageing heat treatment. However, there is a remarkable difference between the deformation and phase transformation activation energies in aluminium alloys. To achieve high-performance and high-precision collaborative manufacturing of large-scale complex panels with high levels of reinforcement, the difference in the energy barriers between the stress relaxation process and the ageing precipitation process needs to be coordinated. An electric pulsed current (EPC) was first introduced into the CAF process for 2219 aluminium. It was found that the EPC can effectively regulate the stress relaxation behaviour during the initial ageing stage. That is, the application of EPC during the first 1 h of the ageing treatment process decreases the stress gradients for different initial stress levels, which makes it possible to reduce the deformation and phase transformation inhomogeneities resulting from stress differences inside a component under a bending load. Meanwhile, the apparent deformation activation energy decreases with the EPC, which decreases the energy barrier difference between the stress relaxation and ageing precipitation from 47 kJ/mol to 18 kJ/mol. Furthermore, the action mechanism for EPC on the stress relaxation process is discussed, and a constitutive model for stress relaxation age forming considering the effect of the EPC is established. Key words: 2219 aluminium alloy; stress relaxation ageing; electric pulse; creep mechanism; constitutive model Introduction To manufacture the complex panels of a rocket fuel tank, research on age forming with creep/stress relaxation has been conducted in many developed countries. CAF refers to a combined creep and ageing treatment process in which creep during ageing is the mechanism used to promote the formation and retention of the shape of a metal component. Additionally, ageing treatments can improve a metal’s mechanical properties[1]. This process possesses a number of advantages, such as homogeneous deformation, fine forming precision, good repeatability, high efficiency, low residual

*Corresponding author. Tel.:+8673188830254; Email address: [email protected]

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stress and size stability. The synergy of the forming process’s focus on precision and the heat treatment’s focus on the mechanical properties enables the manufacture of complex panels [2, 3]. CAF has been applied in the manufacture of the skin components of airplanes with small curvatures (the wing panels), which have a low stress state and a small stress gradient along the thickness direction. The influence of stress on the precision and properties is relatively small. However, for large-scale complex panels with high-degrees of reinforcement (the fuselage panels), the internal stress state is very complex: the highest tensile stress is on the die side and the highest compressive stress is on the opposite side. Studies have shown that the difference in stress clearly results in a difference in the precipitation of the creep/stress relaxation ageing process and further results in inhomogeneities in both the deformation and phase transformation. Meanwhile, there is a remarkable difference between the deformation activation energy and the phase transformation activation energy of aluminium alloys [4, 5]. Thus, achieving the collaborative manufacture of large-scale, high-performance, high-precision and complex panels is a challenge for the industrial application of CAF. EPC is considered to be a useful auxiliary energy field that can be employed for metal forming process. Stashenko [6] found that EPC can lower the required load during the creep process. Troitskii [7] found that EPC promotes stress relaxation processes. Wang Jingpeng of CAS [8] and Tang Fei of Tsinghua University [9] found that EPC helps decrease the residual stress. Although there have been a few studies on the influence of EPC on age forming or stress relaxation, its influence on CAF processes and a constitutive model have not been developed. Thus, the successful introduction of EPC to the CAF process would be a breakthrough for dealing with the challenges mentioned in the introduction. Because of its excellent welding properties, corrosion resistance and high temperature mechanical properties, 2219 aluminium is used as the main material for the panels of rocket fuel tanks [10,11]. EPC is applied during the creep/stress relaxation age forming process. Therefore, the possibility of the collaborative manufacture of large complex components using multiple EPC fields and its effects on temperature and stress are studied in this work. Constitutive models for 2219 aluminium fabricated with and without EPC are established, respectively. 2. Material and method 2.1 Material The material used here was 2219 aluminium rolled plate with pre-deformation provided by the China Academy of Launch Vehicle Technology. The chemical composition is shown in Table 1. Based on GB/T 2039-2012, the plates were cut into 2-mm-thick standard creep specimens with WEDM, as shown in Fig 1. Table 1 The main chemical composition of the 2219 aluminium alloy (mass%) Element Content

wt%

Cu

Mg

Mn

Fe

Si

Zn

Zr

Al

5.8-6.8

0.02

0.2-0.4

0.3

0.2

0.1

0.1-0.25

Bal.

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2 - φ8++00..21

Fig 1 Dimensions of the samples (mm)

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2.2 Test method The stress relaxation ageing tests were conducted on an RMT-D10 electronic testing machine used to determine high-temperature creep rupture strength. The precision is ±2 for the temperature and ±3 N for stress. The pulsed power supply was switched on after the specimen was fixed in the machine so that the temperature and stress were increased to the desired value. During the entire test, thermocouples were placed on the specimen to detect the temperature of the top, middle and bottom parts, respectively. The temperature of the middle part was taken as the temperature of the specimen. The tests used a home-made insulation device. The specimen was insulated from the thermocouples using mica sheets. EPC was applied lengthwise along the specimen. The parameters of the one-directional pulse are: a maximum current density of

jmax = 80 A / cm 2 , a

first 1 h of the test.

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frequency of f = 1000 Hz and a duty ratio of D = 50% . EPC is applied for only the

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3. Results and discussion 3.1 Influence of the EPC on the stress relaxation ageing behaviour of 2219 aluminium The tests are conducted at 165 for 11 h. The conventional/EPC stress relaxation curves are shown in Figs 2 and 3 for different initial stresses (120~225 MPa), respectively. As shown in Fig 2, there are three stages in the stress relaxation process for 2219 aluminium: the rapid relaxation stage, the transit relaxation stage with a decreasing relaxation rate and the steady relaxation stage. Because of the high effective stress during the primary period of stress relaxation, abundant mobile dislocations and vacancies interact with a low-resistance, short-range barrier, causing the stress to decrease rapidly. With the activation effect of the stress and temperature, the dislocations glide or climb, the elastic deformation becomes plastic, and the effective stress decreases. Meanwhile, the precipitation of the second-phase particles increases the short-range barrier, which slows the relaxation rate, and, subsequently, the steady relaxation stage begins [12]. When the initial stress range is between 120 and 195 MPa, the higher the initial stress is, the higher the remaining stress after the same time of stress relaxation is. However, when the initial stress exceeds 195 MPa and the stress relaxation time exceeds 1 h, the trend is reversed.

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Fig 3 EPC stress relaxation curves The EPC decreases the sensitivity of the stress relaxation to the initial stress (as shown in Fig 3). The EPC is only applied during the first 1 h of the test. For different initial stresses, the remaining stress after 11 h of EPC stress relaxation approaches the

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same value (the difference of the remaining stress after 11 h of the conventional stress relaxation is 73 MPa, whereas that for the EPC stress relaxation is 29 MPa). Therefore, by using EPC, the homogeneity of the stress increases, the difference in deformation and phase transformation is reduced and the precision and comprehensive properties of the components are improved. Comparisons of the stress relaxation curves for different initial stress states are shown in Fig 4.

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Fig 4. Comparisons of the stress relaxation curves for different initial stresses: (a) 120 MPa, (b) 150 MPa, (c) 180 MPa, (d) 195 MPa, (e) 210 MPa and (f) 225 MPa.

As shown in Fig 4, the influence of the EPC on the stress relaxation behaviour depends on the initial stress. First, during the EPC application period (1 h), stress relaxation is promoted, and as the initial stress increases, the degree of promotion increases and reaches a peak at 195 MPa. However, after removing the EPC, the follow-up effects vary: (1) at 120 MPa, the previous EPC retards stress relaxation, (2) at 150 MPa, the EPC rarely produces any response, and (3) at 180 MPa or higher, the EPC promotes stress relaxation, which increases first and then decreases with the increase of the initial stress (peak at 195 MPa). Meanwhile, the remaining stress increases first and then decreases with the increase in the initial stress, reaching a peak at 195 MPa. Therefore, we can conclude that the conventional stress relaxation cannot decrease the stress gradients effectively, whereas the application of EPC can

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significantly decrease the stress gradients. Furthermore, the inhomogeneities in the deformation and phase transformation can be decreased and the window for CAF can be expanded. 3.2 The establishment of a stress relaxation ageing constitutive model for 2219 aluminium based on the creep theory 3.2.1 A theoretical analysis of the stress relaxation ageing constitutive model based on the creep theory During the stress relaxation process, the total strain εs is equal to the sum of the elastic strain εe and the plastic strain εp: 1

ε& & where ε e and p stand for the elastic deformation and plastic deformation rates,

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respectively. Meanwhile, the elastic deformation rate can be defined using Young’s modulus E (73.8 GPa for 2219 aluminium[13]) so that the stress relaxation rate is defined as [14]: 1 ε&e = σ& r 2 E Thus: 1 ε&p = − σ& r 3 E Stress relaxation is a special form of the creep phenomenon where the elastic strain in metals gradually transforms into creep strain, resulting in a decrease in the stress. Meanwhile, the creep behaviour of 2219 aluminium can be described using a hyperbolic sine constitutive model[15]: . Q n ε p = A [sinh(ασ )] exp(− ) 4 RT where A and α are material constants, σ is the test stress, n is the stress exponent, R is the molar gas constant, T is the thermodynamic temperature and Q is the apparent deformation activation energy. The creep deformation in stress relaxation processes mainly results from two forms of dislocation movement: glide and climb [16]. Two types of resistance need to be overcome: the short-range resistance that mainly results from the Peierls-Nabarro force, which applies to extremely fine precipitate particles, solution atoms gathering around active dislocations and the dislocation intersections, whose power levers are shown in Table 2 and long-range internal stress mainly resulting from the elastic stress field emerging from the grain boundaries, the precipitate particles and the dislocations[17]. The short-range resistance can be overcome by thermal activation, whereas the long-range internal stress can only be overcome by an external force. Table 2 Features of short-range resistance The power lever High

∆F

τˆ

Examples

Gb3

>Gb/λ

dispersively distributed second phase

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long-range internal stress ( τ in ) and the effective stress needed to overcome the short-range resistance along with thermal activation ( τ e ). The thermal activation

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energy is a thermodynamic parameter that represents the energy required to overcome the short-range resistance. In this stress-related thermal activation process, the dislocations overcome the short-range resistance via the effects of both the effective stress and the thermal activation. Therefore, the apparent deformation activation energy can be represented by the thermal activation energy and the energy supplied by the effective stress to overcome the short-range resistance [14]:

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activation volume. Meanwhile, for aluminium alloys that can be age strengthened, the second precipitation produces extra short-range resistance, which increases the apparent deformation activation energy. Thus, the apparent deformation activation energy can be modified to: Q = Q 0 + Q P - τ eV *

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deformation activation energy. Thus, based on the creep theory, the stress relaxation constitutive model for 2219 aluminium is: .

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Q0 +QP -τ eV * ) RT

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3.2.2 The apparent deformation activation energy of the stress relaxation ageing process The essence of stress relaxation is the thermal-mechanical coupled creep processes. The creep rate’s relation to temperature can be defined using the Arrhenius function [14]: Q & & 8 ε = ε 0 exp(− ) RT & is a structural parameter related to the microstructure and the stress. where ε 0 Because stress relaxation under constant strain (creep under constant stress) can be considered a monodrome function being modified by temperature, the apparent deformation activation energy can be represented as [14]:

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Stress(MPa)～Q(kJ/mol) ～145～～～～～～～～～～44 ～140～～～～～～～～～～56 ～135～～～～～～～～～～86 ～130～～～～～～～～～～89 ～125～～～～～～～～～～98 ～120～～～～～～～～～101 ～115～～～～～～～～～103 ～110～～～～～～～～～～94 ～100～～～～～～～～～100 ～90～～～～～～～～～～～～97 ～80～～～～～～～～～～～～96 ～70～～～～～～～～～～～～98 ～60～～～～～～～～～～～102 ～50～～～～～～～～～～～～99

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Fig 6 Apparent deformation activation energy for the conventional stress relaxation ageing process

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Fig 6 shows that the apparent deformation activation energy increases with the decrease in the effective stress. When the effective stress decreases to 125 MPa, the apparent deformation activation energy fluctuates around 99 kJ/mol. The trend for the apparent deformation activation energy matches that for the stress relaxation of 1Cr-Mo-V[14] and the creep of Al-5wt%Zn[18]. The steady apparent deformation activation for the stress relaxation of 2219 aluminium ranges from the grain boundary diffusion activation energy (84 kJ/mol) to the self-diffusion activation energy (142 kJ/mol)[19]. During the primary stress relaxation period, the density of the second phase is low, which has the highest resistance against the dislocations. Therefore, the short-range resistance is small, and the thermal activation energy is low. At the same time, because of the relatively high effective stress and active dislocations that result from the pre-deformation, the average activation volume of the dislocations is small, and the apparent activation energy is low. When the stress decreases and precipitation occurs, the effective stress decreases. However, during the later period of stress relaxation, the growth of the dislocation movement resistance, the apparent thermal activation energy and the average activation volume of the dislocations lead to an increase in the apparent deformation activation energy. The effective stress and the apparent deformation activation energy reach a balance. The steady apparent deformation activation energy is 99 kJ/mol. 3.2.3 The regression of the parameters in the stress relaxation ageing constitutive model based on the creep theory

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Fig 7 Fitting curves for the conventional stress relaxation at 120~180 MPa

Fig 8 Fitting curves for the conventional stress relaxation at 195~225 MPa The remaining stress and stress relaxation rates are combined in Fig 2 into a data pair based on the ageing time. Nonlinear fitting is performed with Origin using Formula (7) (Figs 7 and 8) to obtain the material constants for the stress relaxation constitutive model, as shown in Table 3.

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Table 3 Material constants for the conventional stress relaxation constitutive model at different initial stresses The initial stress/MPa Q/KJ.mol-1 n α A R2 120 2.53 1.03E-12 1.16E+28 0.957 150 3.22 1.22E-11 1.80E+31 0.987 180 3.25 1.18E-11 1.62E+31 0.962 99 195 3.69 3.48E-11 1.08E+33 0.989 210 3.58 2.51E-11 5.55E+32 0.971 225 2.99 4.63E-12 1.20E+30 0.980

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According to the correlation coefficient R2, the stress relaxation constitutive model based on creep theory can accurately describe the stress relaxation for 2219 aluminium. The apparent stress exponent, which represents the stress relaxation mechanism (Table 4), first increases and then decreases with increases in the initial stress [20].

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Table 4 Relationship between the apparent stress exponent and the mechanism behind it

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The apparent stress exponent n The mechanism n=1 the diffusion creep mechanism n=2 the grain boundary sliding mechanism n=3 the dislocation glide mechanism n>4 the dislocation climb mechanism When the initial stress is 120 MPa, the apparent stress exponent n is between 2 and 3, meaning that the creep behaviour is affected by both grain boundary sliding and the dislocation glide. When the initial stress is 150 MPa, 180 MPa, 195 MPa and 210 MPa, n is between 3 and 4, and the creep behaviour is affected by both dislocation glide and climb. When the initial stress is 225 MPa, n is approximately 3, and the creep behaviour is mainly affected by dislocation glide. Dislocation movement is the main cause behind the creep deformation and the external stress is essential to the dislocation movement. The increase in the apparent stress exponent indicates an increase in the sensitivity to the stress and an increase in the distribution of the dislocation movement related to creep deformation. When the initial stress is 120 MPa, the effective stress is relatively low and grain boundary sliding is relevant compared to creep deformation from dislocation glide. From 120 MPa to 195 MPa, the increase in the effective stress promotes dislocation glide and climb, which increases the proportion of the dislocation movement and the apparent stress exponent. From 195 MPa to 225 MPa, the effective stress decreases with an increase in the initial stress after 1 h of stress relaxation ageing. Thus, the dislocation movement is slowed and the apparent stress exponent decreases. Table 5 Driving mechanism for different initial stresses The initial stress/MPa The mechanism 120

the grain boundary sliding mechanism

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3.3 The establishment of the EPC stress relaxation ageing constitutive model for 2219 aluminium 3.3.1 The influence of EPC on the apparent deformation activation energy for the stress relaxation ageing process The EPC stress relaxation curves at 150 MPa and for 11 h at different temperatures are shown in Fig 9. The higher the temperature is, the higher the stress relaxation rate is, which is indicative of thermal activation in the stress relaxation process. However, the influence of the temperature is much weaker, which means that the EPC reduces the apparent deformation activation energy for the stress relaxation process. The apparent deformation activation energy for different actual (effective) stresses is shown in Fig 10.

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～Stress(MPa)～Q(kJ/mol) ～145～～～～～～～～44.9 ～140～～～～～～～～58.7 ～135～～～～～～～～70.2 ～130～～～～～～～～65.9 ～125～～～～～～～～71.9 ～120～～～～～～～～70.4 ～115～～～～～～～～70.9 ～110～～～～～～～～71.6 ～100～～～～～～～～84.0 ～～90～～～～～～～～97.6 ～～80～～～～～～～～100.3 ～～70～～～～～～～～81.7 ～～60～～～～～～～～72.5 ～～50～～～～～～～～72.1 ～～40～～～～～～～～67.6

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Fig 10 Apparent deformation activation energies for the stress relaxation ageing process In Fig 10, the apparent deformation activation energy increases with the decrease in the effective stress. When the effective stress decreases to 125 MPa, the apparent deformation activation energy fluctuates around 74 kJ/mol, which matches the trends for the conventional stress relaxation. However, there are two special points at 90 MPa and 80 MPa that are ignored because the EPC is only applied during the first 1 h of the test and the EPC condition for each sample is different. For example, when the ageing time is more than 1 h and no EPC is used, the effective stress in the sample at 175 reaches 80 MPa. However, when the ageing time is less than 1 h and the EPC is used, the effective stress in the sample at 155 and 165 is at 80 MPa. The application of the EPC significantly decreases the apparent deformation activation energy for stress relaxation in 2219 aluminium (from 99 kJ/mol to 74 kJ/mol, a reduction of 25.3%). From the author’s previous study, the conventional ageing activation energy is 52 kJ/mol, and the EPC ageing activation energy is 56 kJ/mol[21]. The difference in the energy barriers between the creep deformation, and the phase transformation decreases from 47 kJ/mol to 18 kJ/mol as a result of the EPC. 3.3.2 The action mechanism behind the EPC on the stress relaxation ageing process and the establishment of the constitutive model The dislocation movement is the main cause behind the creep deformation in the stress relaxation process. The EPC’s influence on the dislocation movement is described as follows: 1 The electromigration effect of the EPC is anticipated to be important when considering the influence of EPC on the diffusion-controlled phase transformations [22] . The introduction of the EPC will produce a high-speed electron flow, which

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[22, 25]

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vacancies and solute atoms weakens the drag effect of the solute atoms and reduces the resistance to dislocation movement. Thus, dislocation glide is accelerated. Meanwhile, the extra vacancy flux will significantly reduce the dislocation climb resistance and produce extra dislocation climb. The dislocations’ speed towards the sub-boundary vd can be represented as[16]:

ΦΩ 10 b where Ω is an atom’s volume and b is the Burger’s Vector. The extra particle flux will produce an extra directed dislocation flux. Thus, dislocation movement and stress relaxation processes are promoted. Meanwhile, the EPC weakens the particle and dislocation reverse movement[26] and reduces the apparent deformation activation energy. However, the electromigration effect from the EPC will significantly promote precipitation during the second phase. On the one hand, precipitation will increase the short-range resistance. On the other hand, vacancies will be consumed and dislocation climb is slowed. The influence of the EPC results in a competition between the two mechanisms and varies from having a positive or negative influence depending on the initial stress. 2 The electron-wind-force [27] caused by the EPC is the interaction between

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to eliminate dislocation pinning and open up entanglement, helps dislocations overcome short-range resistance and finally reduces the apparent deformation activation energy of the stress relaxation process. Meanwhile, the short-range resistance is smaller and the dislocations are more evenly distributed. Thus, the remaining stress is reduced. The EPC stress relaxation constitutive model can be defined as: . Q n σ = − AE [sinh(ασ ) ] i +CZ*eρfjmτ p exp(− i ) 11 kT

{

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}

*

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where ni is the EPC stress exponent; C is the electromigration effect influence *

coefficient; Qi is the EPC apparent deformation activation energy; Vi is the EPC activation volume; Z* is an effective valence (its value is normally 10[22]); e is the charge on an electron; ρ is the resistivity (at 165℃, this is 73.5 nΩ.m for 2219 aluminium[13]); and f, jm and τp are the frequency, the maximum electric current density and the pulse duration of the EPC, respectively. The EPC mainly promotes stress relaxation during the first stage and the influence of the EPC varies with the initial stress. Thus, the stress relaxation constitutive model using EPC can be modified

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3.3.3 The regression of the parameters in the EPC stress relaxation ageing constitutive

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Fig 11 Fitting curves for the EPC stress relaxation at 120~180 MPa

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Fig 12 Fitting curves for the EPC stress relaxation at 195~225 MPa The remaining stress and stress relaxation rate are combined in Fig 3 into a data pair based on the ageing time. Nonlinear fitting is performed with Origin using Formula (13) (Figs 11 and 12) to obtain the material constants for the stress relaxation constitutive model, as shown in Table 6. Table 6 Material constants for the EPC stress relaxation constitutive model at different initial stresses

Qi/KJ.mol-1

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74

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-12.46 68.60 30.77 47.14 45.82 68.79

0.940 0.950 0.940 0.936 0.932 0.955

According to the correlation coefficient R2, the EPC stress relaxation constitutive model can accurately describe the stress relaxation for 2219 aluminium fabricated using EPC. Comparing Tables 6 and 3, the stress exponent is increased, which indicates that the EPC promotes dislocation movement and increases the dislocation climb during creep deformation. The reasons for this include the following: the effective stress required to overcome the short-range resistance is increased, and the apparent deformation activation energy is decreased so that dislocation movement is promoted. Meanwhile, the extra vacancy flux caused by the EPC decreases the dislocation climb resistance and increases the proportion of dislocation climb during

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electromigration effect. When the initial stress is 120 MPa, the electromigration effect (only representing the extra particle flux’s influence on the stress relaxation) retards stress relaxation during the entire test. When the initial stress is 150 MPa, the electromigration effect retards stress relaxation during the early stage. However, when the external stress is smaller than 30.87 MPa, the electromigration effect promotes stress relaxation. When the initial stress is 180 MPa, 195 MPa, 210 MPa or 225 MPa, the electromigration effect promotes the stress relaxation first and then retards it. The threshold values are 30.77 MPa, 47.14 MPa, 45.82 MPa and 68.78 MPa, respectively. On the one hand, the EPC decreases the apparent deformation activation energy and promotes the stress relaxation process. On the other hand, the EPC retards (120 MPa and 150 MPa) or promotes the process through the extra particle flux. Comprehensively, when the initial stress is 120 MPa, the EPC retards the stress relaxation process. When the initial stress is 150 MPa, the EPC neither retards the process nor promotes it. For the other initial stresses, the EPC promotes the process and the promotion effect increases before decreasing with the increase in the initial stress. The selectivity of the EPC helps realise the homogeneity of the properties and the residual stress after the ageing treatment.

Conclusions 1 The stress relaxation behaviour of 2219 aluminium at 165℃ was studied. We found that the apparent activation energy of the stress relaxation for 2219 aluminium is 99 kJ/mol, and the stress exponent first increases and then decreases with the increase in the initial stress. 2 The extra particle flux that resulted from the electromigration effect decreases the sensitivity of stress relaxation to the initial stress and significantly decreases the stress gradients of the material during the 1 h ageing treatment. 3 The effective stress that resulted from the electron-wind-force evidently decreases the apparent deformation activation energy (from 99 kJ/mol to 74 kJ/mol), which indicates that the EPC can make it possible to realize collaborative manufacturing by regulating the energy barrier difference between the creep deformation and the phase transformation. 4 An EPC stress relaxation constitutive model of 2219 aluminium at 165℃ has been established and can accurately describe its EPC stress relaxation behaviour.

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Acknowledgements The authors would like to thank the Key Program of the National Natural Science Foundation of China (no. 51235010) and the National Key Basic Research Development Plan Funded Project of China (no. 2014cb046602) for their financial support. References [1] Zeng Yuansong, Huang Xia, Huang Shuo, The research situation and the developing

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Highlights EPC (electric pulse current) is applied in the first 1h of stress relaxation process. The sensitivity of stress relaxation to the initial stress is decreased by EPC. The gap between energy barriers of deformation and phase transformation is narrowed. The creep mechanism under the effect of EPC is discussed. The EPC stress relaxation constitutive model is established.