Research on temperature-assisted laser shock imprinting and forming stability

Research on temperature-assisted laser shock imprinting and forming stability

Optics and Lasers in Engineering 114 (2019) 95–103 Contents lists available at ScienceDirect Optics and Lasers in Engineering journal homepage: www...

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Optics and Lasers in Engineering 114 (2019) 95–103

Contents lists available at ScienceDirect

Optics and Lasers in Engineering journal homepage:

Research on temperature-assisted laser shock imprinting and forming stability Yang Haifeng a,b, Xiong Fei a,∗, Liu Kun a, Man Jiaxiang a, Chen Haoxue a, Liu Hao a, Hao Jingbin a a b

School of Mechatronic Engineering, China University of Mining and Technology, XuZhou 221116, China Jiangsu Key Laboratory of Mine Mechanical and Electrical Equipment, China University of Mining and Technology, XuZhou 221116, China

a r t i c l e

i n f o

Keywords: Temperature-assisted laser shock imprinting Warm laser shock imprinting Forming mechanism Forming law High temperature stability

a b s t r a c t Demand for forming precision and the high-temperature stability of micro-structures is growing quickly in the fields of MEMS, optoelectronic devices, aerospace and biomedical engineering. In order to improve the precision and stability of micro-structures, and to achieve the rapid manufacture of formed parts, the temperature-assisted laser shock imprinting (TALSI) technique is proposed for fabricating large-area micro-structures on the surface of metal foil. In this study, nanosecond lasers with low energy and high frequency were used to carry out laser shock imprinting (LSI) experiments on aluminium foil at different temperatures, and recovery experiments were also implemented at different temperatures. By analysing the surface morphology, profile curve and forming depth of the micro-structures, we obtain the forming law and the optimum imprinting temperature. The influence of temperature on the plasticity, flow stress, dynamic yield strength, and deformation resilience of aluminium foil was experimentally determined, and the stress distribution at imprinting temperatures of 25 °C and 200 °C were investigated by ABAQUS software. The results indicate that warm laser shock imprinting (WLSI) can improve the forming depth, forming accuracy, high temperature stability and residual stress homogenisation of formed parts.

1. Introduction The rapid development of scientific and industrial technology has put forward more stringent requirements for precise, multi-functional micro devices [1,2]. Currently, micro devices ware widely used in the fields of micro electronic systems, micro sensors, communication, aerospace, and biomedical [3-5]. Due to the low efficiency, poor precision, and high cost of traditional manufacturing methods, it is difficult to satisfy the requirements of industrial production, which has made laser shock imprinting (LSI) technology attractive for many interests [6,7]. In 2006, the research team of Cheng GJ first put forward the LSI technique and manufactured formed parts at a 300 𝜇m scale [8]. Their results indicated that the technology was suitable for the plastic forming of most metals. Then, the Shen ZB research team and others initially established the theoretical system of LSI, and analysed the characteristics and failure forms in the forming process [1,9]. In 2010, Gao H had fabricated the 100 𝜇m array structure using LSI, which indicated that the LSI technology can effectively improve the mechanical properties of formed parts [10,11]. Using this technology, researchers at Purdue University successfully produced ordered particle micro-structures (of 10 nm diameter) on a thin aluminium foil in 2016, and analysed the microcosmic evolution process of nanocrystals in the forming process. In

addition, they also concluded that this method could effectively adjust the geometry of metal nanostructures [12]. Compared with the traditional manufacturing methods of micro devices, LSI is a non-touch machining technology. Because the laser can be precisely controlled, formed parts with complex shapes can be manufactured by LSI, which has advantages such as fast forming speed, high efficiency, low-cost, and widespread application [13]. The hardness, wear resistance, corrosion resistance, and fatigue life of parts can be improved because the technology combines the advantages of laser shock peening (LSP) and plastic forming technology [14–16]. However, when the formed parts fabricated by LSI are used in a high temperature environment, the stability of the structure and residual stress is poor, and the formed parts recover their initial states easily [17,18]. Therefore, in this research, a method of temperature-assisted laser shock imprinting (TALSI) was put forward to solve these problems. In addition, the technology also can be called warm laser shock imprinting (WLSI) when LSI is implemented at high temperature. WLSI is developed from warm laser shock peening (WLSP) and LSI, and some research on WLSP has been carried out. In 2009, the research team of Cheng GJ [19] proposed WLSP, and the experimental results showed that WLSP could induce nanoscale precipitation and high-density dislocation, and produce higher surface strength and lower surface roughness than LSP, which were conducive to improving fatigue life. Subsequently,

Corresponding author. E-mail address: [email protected] (X. Fei). Received 9 June 2018; Received in revised form 31 October 2018; Accepted 2 November 2018 Available online 7 November 2018 0143-8166/© 2018 Elsevier Ltd. All rights reserved.

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Fig. 1. TALSI process: (a) the schematic diagram of TALSI process, (b) the SEM image of micro mould, (c) the laser spot trajectory and lap effect.

Liao et al. [20] and Tani et al. [21] mentioned that this technique played an important role in improving the residual stress and the stability of micro-structures, and Fortunato et al. [22] pointed out the difference between LSP and WLSP. Zhou JZ [23,24] of Jiangsu University also carried out in-depth study of WLSP, and found that treated specimens of nickelbased alloy remained at different temperatures for a certain period of time, and the release rate of surface residual stress of the specimens fabricated by WLSP was slower than that of specimens fabricated by LSP. In addition, in a study of Ti6Al4V titanium alloy, he found that the influence depth of residual compressive stress obtained by WLSP was nearly 1.4 times as deep as that by LSP at room temperature [25]. WLSI integrated the advantages of LSI, dynamic strain ageing (DSA), and dynamic precipitation (DP). DSA, a strengthening mechanism, usually results in more uniform and highly dense dislocation structures. As a thermo mechanical precipitation effect, DP can produce the nucleation of precipitates during the deformation process [26]. The action of above three mechanisms gives the formed parts better stability and better adaptability to high temperature environments for an extended time [27]. In this paper, TALSI of aluminium foil at different temperatures was studied. The forming law and stress distribution characteristics of aluminium foil at different temperatures were studied by experiments and numerical simulations. Through recovery experiments at different temperatures, the stability of formed parts was tested and the optimum imprinting temperature was derived. Additionally, the forming mechanism of TALSI was emphatically discussed and determined. Ultimately, these results show that WLSI (imprinting temperature 150 °C) has a better effect on the stability of formed parts.

temperature and high pressure formed, and continued to absorb laser energy, which caused rapid heating and expansion. Then, a high intensity shock wave induced by a plasma explosion acting on the surface of the metal layer. When the shock wave pressure exceeded the dynamic yield strength of the target material, plastic deformation of the target material occurred according to the shape of the micro mould under different temperatures, which resulted in the work hardening and residual stress in the metal layer [28,29]. In the TALSI experiments, an ultraviolet laser (model DSH-355-10, wavelength 355 nm, pulse duration 10 ns) with the characteristics of high frequency (ranging from 1 Hz to 150 kHz) and low pulse energy (the maximum pulse energy was 1mJ) was chosen as the source of exposure. In subsequent experiments, the pulse frequency was 1000 Hz, the single pulse energy was 109 𝜇J, the diameter of the laser focused spot was 20 𝜇m, and the laser power density was approximately 3.47 GW/cm2 . The K9 glass with a 2 mm thickness was selected as the confinement layer. Compared with water, K9 glass has a higher shock impedance and bigger breakdown threshold, and it can produce a shock wave with a larger peak pressure. Because aluminium has excellent mechanical properties and widespread application, two layers of aluminium foil of 10 𝜇m thickness served separately as ablative layer and metal layer. In order to ensure the quality, homogeneity, and repeatability of the micro-structures on the metal layer surface, two layers of aluminium foil must be firmly pressed against the micro mould surface and closely fitted on the K9 glass [19]. According to Fabbro’s model, the peak pressure of the shock wave generated by the laser pulse was calculated as approximately 2.65 GPa [30]. The strain rate ranged from 106 to 107 s−1 during the LSI process, and it depended on the pulse duration as well as the laser intensity [19]. In the experiment, the micro mould of copper material had many square holes, and its structure and size are shown in Fig. 1(b). In this figure, the side length, rib width, and depth of the square holes are 90 𝜇m, 35 𝜇m, and 15 𝜇m respectively. A laser scanner was used to achieve movement and overlap of the laser spots to fabricate large area microstructures on the surface of the aluminium foil, as shown in Fig. 1(c). In addition, the moving speed of the spot was 15 mm/s, the spacing between two near paths of the moving laser was 15 𝜇m, and so the degree of overlap of the laser spot was 25%. Finally, the temperature of the aluminium foil was adjusted with a heating or cooling device. The imprinting temperatures ranged from −25 °C to 300 °C, and the temperature interval was set to 25 °C.

2. Experiments and characterisation 2.1. TALSI experimental process TALSI originates from WLSP and LSI, and its schematic diagram is shown in Fig. 1(a). The experimental system for TALSI mainly includes pulse laser, confinement layer, ablative layer, metal layer, micro mould, and heating or cooling device. During the TALSI process, a laser beam with high power density and short pulse width passed through the confinement layer (water or K9 glass) and irradiated the ablative layer (black lacquer or aluminium foil). Laser energy was absorbed by the ablative layer, which gasified quickly. Meanwhile, plasma with high 96

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2.2. Recovery experiments at different temperatures In the recovery experiment, the formed parts were kept at a certain temperature for a period of time to simulate the work setting of formed parts at different ambient temperatures. The high temperature stability of formed parts was judged according to the measurement of the forming depth for recovered formed parts. From the TALSI experimental results, 25 °C and 150 °C were selected as the imprinting temperatures for investigating the high temperature stability of formed parts. The process could also be called laser shock imprinting at room temperature (RTLSI) or WLSI when separate imprinting temperatures are 25 °C and 150 °C, respectively. The recovery experiments of the formed parts were then carried out at 100 °C, 200 °C, and 300 °C in a muffle furnace for 3 h. 2.3. Measurements and characterisation Fig. 2. Pressure curve of shock wave over time.

The surface morphology of the aluminium foil was observed after TALSI and the recovery experiments by scanning electron microscope (SEM) (Quanta 250) and optical profiler (DVM5000, Leica, Germany). The profile curve and forming depth of formed micro-structures were measured after TALSI and the recovery experiments by optical profiler. The surface of the micro mould, original aluminium foil, and formed parts imprinted by TALSI were scanned using an atomic force microscope (AFM) (CSPM5500, Benyuan Nano-Instrument, China). To reduce measurement error and improve the measurement accuracy, each sample was scanned at three different places, and an average of the three measurements was used to represent the measured value of the whole sample.

25 °C; c 150 °C; d 300 °C). The micro-structures were convex, and their shape was consistent with the micro mould. From the SEM images, we can conclude that the grid structure of the micro mould was accurately copied onto the surface of aluminium foil, and the micro-structures of the formed parts were uniform. Additionally, as seen from the enlarged images, the edges and sharp corners of the grid were copied well. From Fig. 3(a) to Fig. 3(c), with the increase in the imprinting temperature, the micro-structures became clearer and the imprinting effect is also better. Apparently, the quality of the formed part is best at an imprinting temperature of 150 °C. However, when the imprinting temperature is 300 °C, the effect of imprinting obviously decreases, as shown in Fig. 3(d). As shown in the enlarged images of Fig. 3(d), the surface of the aluminium foil has serious oxidation. The width of the concave area of the formed part is obviously increased, to approximately 41 𝜇m. Compared with the rib width (35 𝜇m), the width of the concave area of the formed part increased by 17.1%. The main reason for this phenomenon was the increase in temperature. We made the following two observations: (1) the deformation of the micro mould resulted in an increase in rib width during the RTLSI process under 300 °C; (2) because the thickness of the aluminium foil was thin, with cooling, the deformation of microstructures was restored after TALSI, and the side wall retraction of the square bulge widened the concave area. The surface morphology of formed parts, the profile curve, and the forming depth of the micro-structures are shown in Fig. 4 (a −25 °C; b 25 °C; c 150 °C; d 300 °C). According to the profile curve from Fig. 4(a) to Fig. 4(c), it can be seen that the forming depth increased with the increase in imprinting temperature, and the forming depth was the deepest (9.317 𝜇m) at 150 °C. Compared with the forming depth (8.213 𝜇m) at room temperature (25 °C), the forming depth at 150 °C increased by 13.4%. However, when the temperature was 300 °C, the forming depth decreased to 8.351 𝜇m, which was equivalent to the forming depth at room temperature. As shown in Fig. 4, when the imprinting temperature was −25 °C, the profile curve of the micro-structure was an arc; when the imprinting temperature was 25 °C, the top of the profile curve was horizontal; when the imprinting temperature was 150 °C, the horizontal section of the top of the profile curve was longer; when the temperature raised to 300 °C, the horizontal section of the profile curve was shorter and the forming effect was worse. In this experiment, with an increase in temperature, all the strength indexes of aluminium foil decreased, the plasticity improved and the flow stress was reduced. As a result, micro-structures were more easily formed, and the side walls of the formed micro-structures were closer to the ribs of the micro mould, and the top horizontal section of the profile curve grew longer. However, with further increase in imprinting temperature, the deformation resilience of the formed parts increased during cooling. Therefore, when the temperature was too high, the pre-

2.4. Numerical simulation In order to simulate the transient plastic deformation process of aluminium foil during the TALSI process, a 3-D FE model of TALSI was built by ABAQUS software. According to the simulation results, the distribution and magnitude of residual stress and the whole dynamic process of plastic deformation could be observed. The Johnson-Cook (J-C) model was adopted in this FE model, and the model was established with the following assumptions: (1) the micro mould is a rigid body; (2) because the laser spot diameter is larger than its ablative coating thickness, laser irradiation could be considered uniform; (3) the horizontal dimension of aluminium foil is much larger than its thickness, and the particles cannot move laterally, so the strain state of aluminium foil could be considered one-dimensional, and the laser shock wave could be regarded as a onedimensional strain plane wave when it propagates in the aluminium foil; (4) the deformation of aluminium foil is isotropic under the plasma shock wave; (5) the flow stress and elastic properties include temperature dependence; (6) frictionless contact is assumed between aluminium foil and micro mould [19,31]. It is important to note that the confinement layer has the effect of maintaining pressure, so the pulse width of the shock wave was broadened. It was generally believed that the action time of the shock wave was 3 times that of the pulse duration, so the action time of the force was 30 ns in the simulation [32]. In addition, the amplitude of f(t) changed with time (Pmax was the peak pressure of the shock wave, and tp was the pulse duration), and the key points (Special time points; they correspond to the points in Fig. 2.) were used to fit the loading curve of the shock wave in the LSI process [33], as shown in Fig. 2. On the basis of the interpolation algorithm, loads at any time were calculated, and then the corresponding loads were introduced into the model. 3. Results and discussion 3.1. The TALSI experiments at different temperatures Regular micro-structures were formed on the aluminium foil surface after TALSI at different temperatures, as shown in Fig. 3 (a −25 °C; b 97

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Fig. 3. SEM images of formed parts at different imprinting temperatures. Fig. 4. Surface morphology and profile curve of formed parts at different imprinting temperatures.


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Table 1 Parameters of aluminium foil in J-C model. A











where 𝜎 is the equivalent dynamic yield strength, 𝜀 is the equivalent plastic strain, 𝜀̇ is the plastic strain rate, and 𝜀̇ 0 is the reference strain rate, Tr is the room temperature (25 °C), T is the imprinting temperature, and Tm is the melting point of metal (660 °C). A, B, C, m, and n are material constants. The parameters for aluminium foil in this model are listed in Table 1 [34]. Additionally, formed parts showed elastic strain, and the deformation produced a certain amount of resilience after unloading. With the increase in temperature, the elastic modulus of aluminium foil decreased, and the elastic deformation of the formed part increased. Therefore, the higher the imprinting temperature, the greater the resilience to deformation in the subsequent cooling process, and the smaller the forming depth of the micro-structures. Meanwhile, the rise in temperature also added a degree of friction between the metal and the micro mould, increasing the resistance in the TALSI process and reducing the forming depth [35,36]. In short, the effect of temperature on the forming depth is multifaceted. The increase in temperature not only increased the plasticity of the aluminium foil and reduced the dynamic yield strength and flow stress, but also added frictional resistance and resilience to deformation after unloading during the imprinting process. Thus, the forming depth exhibited the following characteristics in response to the influence of temperature: (1) the forming depth was shallow at low temperature; (2) the forming depth reached a maximum at 150 °C; 3) there were two stable stages of forming depth across the entire temperature range from −25 °C to 300 °C; 4) the forming depth decreased when the temperature was too high. As a general rule, the uneven surface of the workpiece tended to cause stress concentration and micro cracks under cyclic loading, which had a negative effect on its fatigue resistance. An increase in roughness would aggravate the degree of concavity and convexity on the workpiece surface, which would lead to a reduction in workpiece useful life [37,38]. The roughness of the concave area of the formed parts obtained by TALSI showed no regularity across the temperature range, and the roughness hovered at 120 nm. Nevertheless, compared with the original surface of the aluminium foil, the surface roughness of the formed parts was greatly reduced from 207 nm to 120 nm, which represents a drop of 42.0%, as shown in Fig. 6(a). Moreover, Fig. 6(b)–(d) present the atomic force scanning images of the original aluminium foil, micro mould, and formed parts, respectively. The reduction in the surface roughness was related to the extrusion deformation of the aluminium foil during the imprinting process. Owing to the higher shock pressure, the micro protrusion height of the aluminium foil surface was greatly reduced during the extrusion process, which effectively reduced the surface roughness. Moreover, the aluminium foil was in close contact with the micro mould during the imprinting process, so the surface roughness of the micro mould had a certain effect on the surface roughness of the formed parts. In this work, the plastic deformation process of aluminium foil under an ultra-high strain rate was numerically simulated by ABAQUS software. The plastic deformation of aluminium foil by WLSI (imprinting temperature 150 °C) is shown in Fig. 7(a), and the stress distribution at different times is also presented. The maximum of residual stress is concentrated mainly at the root of the convex micro-structures on the surface of the aluminium foil. Fig. 7(b) shows the profile curve and forming depth of the micro-structures at various times. With the increase in plastic deformation, the two sides of the profile curve are closer to vertical, which indicates that the side walls of the square micro-structures gradually move toward the ribs of the micro mould. The forming depth

Fig. 5. Curve of forming depth vs. imprinting temperature.

cision of the formed parts was worse, and the top horizontal section of the profile curve was also shortened. Fig. 5 shows the change in the forming depth curve with the change in imprinting temperature, in which each point is an average value of three measured values of micro-structure depth. This image shows that the temperature rose from −25 °C to 25 °C, and the forming depth increased rapidly during this interval. However, as the temperature continued to rise up to 125 °C, the forming depth didn’t change significantly, and then remained at approximately 8.2 𝜇m. When the temperature rose to 150 °C, the forming depth increased again. The forming depth became stable again from 150 °C to 225 °C, at approximately 9.2 𝜇m. As the imprinting temperature continued to rise, the forming depth decreased, and its value was finally stable and closed to 8.2 𝜇m. At the lowest imprinting temperature within the entire temperature range of −25 °C to 300 °C, that is, the imprinting temperature of −25 °C, the forming depth was smallest and the average depth was only 5.314 𝜇m. This is 35.2% less than the average forming depth (8.203 𝜇m) at room temperature. When the imprinting temperature was 150 °C, the forming depth was largest, at approximately 9.290 𝜇m. As the temperature increased, the kinetic energy from atomic activity increased and the critical shear stress due to atomic interaction weakened. Meanwhile, the diffusion of various point defects also accelerated, and the dislocation motion due to diffusion was more likely, which resulted in a decrease in dynamic yield strength in the aluminium foil. In addition, as the temperature increased, the effect of thermal activation energy was enhanced, and the dislocation motion due to the decrease in effective stress resulted in a decrease in flow stress. Finally, the increase in the imprinting temperature enhanced the softening action of the dynamic recovery and dynamic recrystallisation, which reduced or eliminated the process hardening caused by plastic deformation and allowed the aluminium foil to form easily, thus the precision of the formed parts was improved [26]. Conversely, when the temperature dropped below room temperature, the plasticity of the aluminium foil was worse, the dynamic yield strength increased, and the forming depth decreased under the same shock pressure. According to the J-C strain sensitivity plasticity model [30], the exact relationship between the dynamic yield strength and imprinting temperature of aluminium foil can be derived from the TALSI process. The following formula (1) shows the decrease in dynamic yield strength with the increase in imprinting temperature. ( )[ ( ) ] 𝑇 − 𝑇𝑟 m 𝜀̇ 𝜎 = (𝐴 + 𝐵 𝜀n ) 1 + 𝐶 ln 1− 𝜀̇ 0 𝑇𝑚 − 𝑇𝑟


(1) 99

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Fig. 6. Curve of roughness vs. temperature and scanning images of the atomic force.

Fig. 7. (a) Plastic deformation process of aluminium foil by WLSI, (b) profile curve and forming depth of micro-structures imprinted by WLSI at various times.

Fig. 8. Overall distribution of residual stress and the residual stress curve of the micro-structures at imprinting temperatures of 25 °C and 150 °C.

increased with loading time of shock wave pressure, and the final forming depth was approximately 10 𝜇m, which was larger than the experimental value of 9.29 𝜇m. This difference was mainly attributed to the unavoidable energy loss in the actual experiment, for instance, the loss of energy during transmission or the uncertain absorption efficiency of energy by the metal layer, which was not considered in the simulation process. Another possible cause that cannot be ignored is that in the process of numerical simulation, the friction between the micro mould and the aluminium foil was not considered in the model, and the existence of friction would reduce the forming depth of the formed parts. Fig. 8 shows the overall distribution of residual stress and the residual stress curve of micro-structures imprinted by RTLSI and WLSI. It can

be seen from the image of the overall residual stress distribution that the maximum residual stress and the range of the residual stress distribution (the difference between the maximum residual stress and the minimum residual stress) for WLSI were 354.8 MPa and 311.1 MPa. Compared with the formed part imprinted by RTLSI (the maximum residual stress was 379.7 MPa; the range of residual stress distribution was 356.7 MPa), the decreases with WLSI were 6.56% and 12.78% respectively. In the residual stress curve of the micro-structures section, the residual stress of the formed parts imprinted by WLSI was less than that of the formed parts imprinted by RTLSI. In addition, the residual stress distribution of the formed parts imprinted by WLSI was more uniform and tended to be stable in the middle area of the micro-structures, but the formed 100

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Fig. 9. Surface morphology, profile curve of micro-structures and the SEM images after recovery experiments at 300 °C.

parts imprinted by RTLSI varied greatly. These indicate that temperature not only reduces the maximum residual stress, but also makes the distribution of residual stress tend toward homogenisation. During the work process, especially in high temperature environments, the residual stress of the formed parts decreased constantly. Different speeds of stress relaxation can gradually change the shape of formed parts (such as warping deformation), thus reducing their original accuracy and producing installation errors. However, the residual stress in formed parts fabricated by WLSI show uniform distribution, the speed of stress relaxation is similar at the same temperature, and the installation surface of the parts produces mainly translational motion, so the surface has little influence on installation error. The above results may be attributable to the following two points: (1) high temperature made the material flow tend toward homogenisation, and the effect of work hardening was weakened. The deformation combined with DSA made the dislocation in aluminium foil more uniform. Because the lattice distortion was largely derived from dislocation, grain boundaries and sub-grain boundaries, and the lattice distortion also led to the formation of residual stress, the homogenisation of dislocation made the residual stress more uniform [39]; (2) high temperature caused partial residual stress of formed parts to release during the cooling process after WLSI.

Among them, the change in forming depth for WLSI (imprinting temperature 150 °C) was less than that for RTLSI (imprinting temperature 25 °C) in recovery experiments, which indicated that the stability of the formed parts imprinted by WLSI was higher. The relationship trend between change in forming depth and recovery temperature is shown in Fig. 10(a). Compared with the no-recovery formed parts, all decrements in forming depth for WLSI were less than for RTLSI, as shown in Fig. 10(b). The recovery rates were at a maximum for RTLSI and WLSI when the recovery temperature was 300 °C. The forming depth decrement in formed parts imprinted by RTLSI was approximately 2.5 𝜇m, and the recovery rate of the forming depth was 34.2%. However, the forming depth decrement in parts imprinted by WLSI was only 1.7 𝜇m, and the recovery rate of the forming depth was 21.4%. In this investigation, the experimental results showed that WLSI was not only better than RTLSI on the forming effect, but also the WLSI had better high temperature stability. Compared with RTLSI, the forming depth of WLSI was deeper, and the high temperature stability of the micro-structure surface was better. The reason for this was that WLSI produced the formation of nanoscale precipitates through dynamic ageing and a high-density dislocation structure, and more nanoscale precipitates were able to nucleate and grow with the dynamic ageing [26,19]. The high density nanoprecipitates were very effective in dislocations pinning, which led to better high temperature stability and higher dislocation density [26,40]. Moreover, because the nonequilibrium defects caused by the ultrahigh strain rate and ultra-high energy would induce grain differentiation, a nanocrystalline layer can be produced by RTLSI and WLSI in aluminium foil, as shown in Fig. 11. The nanocrystalline layer has been showen to greatly improve the comprehensive properties of the formed parts [41,42]. In the process of studying nano-scale forming by LSI, Cheng GJ et al. [26,28] also found that LSI with high power density and ultra-high strain rate could produce grain refinement and a nanocrystalline layer on the surface of metal. In the WLSI process, the dislocations pinning points by solute atoms served as a new dislocation source

3.2. Recovery experiments of formed parts imprinted by WLSI and RTLSI With the aim of investigating the structural stability of formed parts at different working temperatures, the formed parts were heated to a certain temperature (100 °C, 200 °C, and 300 °C) and then the three-hour deformation recovery experiment was carried out. The formed parts (imprinting temperatures: 25 °C and 150 °C) were recovered at 300 °C, and the surface morphology, profile curve, and SEM images after recovery are shown in Fig. 9. It was found that the surface morphology and profile curve of the micro-structures were almost unchanged, but the forming depth in all formed parts decreased by varying degrees after recovery. 101

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Fig. 10. Laws of change in formed parts with recovery temperature: (a) forming depth after recovery, (b) decrement in forming depth. Fig. 11. Schematic diagram of amorphous layer and nanocrystalline layer formed on the surface of aluminium foil by RTLSI and WLSI.

when the strain increased to a certain value at the DSA temperature, and some new dislocations were generated from the dislocations pinning points under ultra-high pressure [20,40,43]. Ye YX [44,45] and Meng XK [46] also mentioned that dislocation was one of the main deformation mechanisms of LSI in their paper. Thus, the dislocation density was higher and the distribution of dislocation structure was more uniform, which contributed to the formation of an amorphous layer on the aluminium foil. This amorphous layer would improve the strength, toughness, wear resistance and corrosion resistance of formed parts.

perature on dynamic yield strength, plasticity, flow stress, deformation resilience, and the friction coefficient of aluminium foil. The recovery experiments indicated that the WLSI has many advantages that can make high temperature stability more reliable, and the residual stress more homogeneous. The increase in temperature caused WLSI to produce an amorphous layer on the surface of the aluminium foil. Meanwhile, the nanocrystalline layer, the high-density dislocation, and the dislocations pinning effect caused by solute atoms were generated in the metal layer. These were the main reasons for improvement in the forming quality of formed parts. All experimental results indicate that WLSI technology can reduce the anisotropy of metal materials and make the flow more homogenous, so as to improve forming depth and precision. Owing to the combined effect of DSA and DP, WLSI can also improve the high temperature stability of micro-structures, expand the application scope of formed parts, and promote the development of the LSI technology.

4. Conclusion In this work, TALSI experiments were carried out on aluminium foil. By analysing the surface morphology, micro-structure profile curve, forming quality, and forming depth of the formed parts at different imprinting temperatures, the law of change in forming depth with temperature was derived, and the optimum imprinting temperature was determined to be 150 °C. These results were attributed to the influence of tem102

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