Objective Laser shock imprinting (LSI) is a manufacturing technique for material strengthening and forming using high-pressure plasma shock waves induced by laser pulses. It has been widely used in many fields. Warm laser shock peening (WLSP) combines the advantages of laser shock peening, dynamic strain aging, and dynamic precipitation and can produce microstructures with high stability. The LSI technology can produce regular large-area periodic microstructures with different shapes from hundreds of microns to nanometers on a metal foil surface. Corresponding to WLSP, the temperature-assisted LSI technology changes the forming process, forming quality, and forming mechanism of an aluminum foil. Therefore, it is important to conduct a detailed investigation on warm laser shock imprinting (WLSI) and reveal the mechanism of high strain rate plastic deformation hardening and dynamic recovery softening during multiple WLSI.
Methods WLSI of an aluminum foil at different imprinting temperatures and imprinting times was conducted using the WLSI experimental devices. The imprinting temperature was controlled using an electric heating plate. We tested the forming height, surface quality, surface hardness, and microstructure of the aluminum foil using an optical profilometer, scanning electron microscope, nano-indenter, and transmission electron microscope, respectively. The ABAQUS/Explicit module was used to analyze the transient mechanical effect of the aluminum foil during the WLSI process, in which the residual stress and deformation speed of the forming parts were also analyzed.
Results and Discussions For WLSI at different temperatures, when the imprinting temperature was 25 ℃, the forming height was about 8.2 μm. When the imprinting temperatures were 150 ℃ and 225 ℃, the forming height was increased to 9.3 μm, and the microstructure on the aluminum foil surface had a good forming quality. When the imprinting temperature was 300 ℃, the forming height was dropped to about 8.35 μm, and the formed part surface had a poor oxidation phenomenon (Fig. 3). The simulation results by the ABAQUS/Explicit module showed that the maximum deformation speed of the aluminum foil at 300 ℃ was about 41.8 m/s, 10.6% higher than that at 25 ℃ (37.8 m/s). Furthermore, the WLSI introduced high residual stress at the top of the microstructure. With the increase of imprinting temperature, the area of the high residual stress was increased, the maximum residual stress was decreased, and the difference between the maximum and the minimum internal stresses was decreased gradually (Fig. 5). In the WLSI at different imprinting times, compared with the forming height after single imprinting (3.8 μm), those after two (5.8 μm) and three (7.4 μm) successive imprintings were increased by 52.6% and 94.7%, respectively (Fig. 6). It should be noted that slip and twinning are the main deformation mechanisms of materials. Aluminum is a high-fault-energy metal with a small expansion dislocation width but does not easily form twin. In this study, the laser-induced shock pressure was 3.8 GPa, and the strain rate was greater than 10 4s -1 during the WLSI shock hardening and softening process. The WLSI process triggered dislocation slip in different slip planes and formed dense dislocation. The grains’ dislocation entanglement and chaotic dislocation entanglement separated the high- and low-density dislocations to form cellular substructure and sub-grains. The high-density dislocation, small cellular substructure, and sub-grains made the strength and hardness of the aluminum foil increased, which led to the second and third WLSI deformation increments. We divided the softening process at multiple WLSI into two processes, namely, dynamic recovery and dynamic recrystallization, based on the deformation conditions. Furthermore, we compared the refinement methods for high- and low-fault-energy materials with that for the medium-fault-energy materials. Our results show that the refinement methods for high- and low-fault-energy materials are more simple than that for the medium-fault-energy materials. Owing to the high-fault-energy material of the aluminum foil, dislocation slip and dynamic recrystallization result in grain refinement. However, owing to the low temperature, short deformation time, small deformation degree, and high-fault-energy, only dynamic recovery occurred in our experiment. Moreover, no dynamic recrystallization and grain refinement occurred, and a large number of cellular substructures and high-density dislocation were retained in the grains (Fig. 9). Therefore, compared with dynamic recovery softening, shock hardening plays a dominant role in multiple WLSI.
Conclusions This study demonstrates that an increase in imprinting temperature reduces the flow stress of aluminum foil and makes its formation easy. WLSI leads to a high forming height and good surface quality at 150 ℃ when the imprinting temperature is 300 ℃. Furthermore, the springback and shrinkage of the aluminum foil lead to a small forming height, whereas the oxidation leads to bad surface quality. With the increase of imprinting times, the forming height of the aluminum foil gradually increases, whereas that of each imprinting decreases. After three imprinting times at 200 ℃, the forming part surfaces maintain good oxidation state and surface quality. Multiple WLSI can enhance the deformation resistance of the formed parts and strengthen the mechanical properties of the aluminum foil. Thus, the foil is subjected to the dual effects of shock hardening and recovery softening. The shock hardening plays an important role in the experiment, which ultimately leads to the successive increment of the hardness and decrement of the forming height of the aluminum foil.
