Microstructure Evolution of 5052 Aluminum Sheet in Electromagnetic High-Speed Impact

: Electromagnetic forming (EMF) is a high-speed forming technology, which can not only improve the formability of hard-to-form materials but also reduce springback. Electromagnetic high-speed impact can further improve the formability compared with electromagnetic free forming. The microscopic deformation mechanism of electromagnetic high-speed impact of aluminum alloy is discussed in this paper. The microstructures of electromagnetic high-speed impact of an aluminum alloy sheet were characterized. The microscopic deformation mechanisms of electromagnetic forming and electromagnetic high-speed impact were shown, respectively. The research results showed that electromagnetic high-speed impact could signiﬁcantly improve the microhardness of the workpiece. The grains broke up and then became small subgrains during electromagnetic high-speed impact. The deformation mechanism was dominated by dislocation cross slip under electromagnetic high-speed impact.


Introduction
Aluminum alloy sheets are widely used in vehicles and airplanes because of the advantage of fuel economy and reduced environmental pollution [1,2]. However, aluminum alloy sheets are prone to cracking due to their low formability at room temperature in traditional stamp forming processes. Additionally, large springback tends to occur, which affects forming precision [3]. Therefore, fracture and large rebound seriously restrict the wide application of aluminum alloy sheets in the manufacturing industry.
Electromagnetic forming (EMF), a high-speed forming technology, can not only improve formability but also reduce springback [4,5]. The formability of aluminum alloy sheets in high speed forming had been studied by many researchers. The electromagnetic ring expansion process of AA6061-T4 and pure copper was investigated by Tamhane et al. [6], and a doubled formability was reported, comparing it with the formability in quasi-static (QS) forming. By comparing the forming limits of Ti-6Al-4V and AA5052-O under QS and EMF, Li et al. [7] observed a formability increase by 24.37% and 10.97%, respectively, in EMF. It was reported by Vohnout [8] that the formability of 6111-T4 and 5754-O aluminum alloy was improved by QS preforming followed by high-speed forming.  Figure 1 shows the EMF equipment, which consisted of a pulse generator and a capacitor device. The pulse generator was mainly composed of a power supply and a control system, as shown in Figure 1a. The capacitor device mainly consisted of a series of capacitor banks, as shown in Figure 1b The experimental setup of electromagnetic high-speed impact of 5052 aluminum alloy was shown in Figure 2. The sheet was placed on the top of the conical die, and the flange region of the sheet was restrained by the blank holder. A flat spiral coil was placed above the sheet. The experimental setup of the electromagnetic high-speed impact is schematically shown in Figure 3. When the stored energy was discharged, the pulse current passed through the coil and induced an eddy in the sheet, generating a dynamic Lorentz's force, which pushed the sheet deformation downwards. The diameter of the flat spiral coil was 106 mm. The inner diameter of the coil was 10 mm. The coil had a six-turn rectangular cross-section shape, and the cross-section size was 3 × 10 mm. The distance between every two turns was 5 mm. A conical die was used in the electromagnetic highspeed impact experiment. The cone base diameter of the conical die was 120 mm, with an entry radius of 10 mm, a tip radius of 5 mm, and a cone angle of 40°.  The experimental setup of electromagnetic high-speed impact of 5052 aluminum alloy was shown in Figure 2. The sheet was placed on the top of the conical die, and the flange region of the sheet was restrained by the blank holder. A flat spiral coil was placed above the sheet. The experimental setup of the electromagnetic high-speed impact is schematically shown in Figure 3. When the stored energy was discharged, the pulse current passed through the coil and induced an eddy in the sheet, generating a dynamic Lorentz's force, which pushed the sheet deformation downwards. The diameter of the flat spiral coil was 106 mm. The inner diameter of the coil was 10 mm. The coil had a six-turn rectangular cross-section shape, and the cross-section size was 3 × 10 mm. The distance between every two turns was 5 mm. A conical die was used in the electromagnetic high-speed impact experiment. The cone base diameter of the conical die was 120 mm, with an entry radius of 10 mm, a tip radius of 5 mm, and a cone angle of 40 • . The experimental setup of electromagnetic high-speed impact of 5052 aluminum alloy was shown in Figure 2. The sheet was placed on the top of the conical die, and the flange region of the sheet was restrained by the blank holder. A flat spiral coil was placed above the sheet. The experimental setup of the electromagnetic high-speed impact is schematically shown in Figure 3. When the stored energy was discharged, the pulse current passed through the coil and induced an eddy in the sheet, generating a dynamic Lorentz's force, which pushed the sheet deformation downwards. The diameter of the flat spiral coil was 106 mm. The inner diameter of the coil was 10 mm. The coil had a six-turn rectangular cross-section shape, and the cross-section size was 3 × 10 mm. The distance between every two turns was 5 mm. A conical die was used in the electromagnetic highspeed impact experiment. The cone base diameter of the conical die was 120 mm, with an entry radius of 10 mm, a tip radius of 5 mm, and a cone angle of 40°.    The sidewall and center region of the specimen impacted to the conical die at a high speed when the coil discharged. In order to consider the impact characteristics in EMF, the deformed workpiece was divided into four parts (region A, B, C, and D). Region A, which was considered to be the initial material zone, was located at the flange area of the workpiece, as shown in Figure 4. Region B was located at the half radius of the coil. Region D was located at the workpiece center, and region C was between region B and region D. The high-speed impact process of electromagnetic forming is schematically shown in Figure 5. Region B firstly impacted to the sidewall of the die due to the maximum electromagnetic force that appeared at the half radius of the coil. Region D impacted to the tip of the conical die at high speed under the inertia force. However, region C did not impact to the conical die at high speed because there was no magnetic pressure existing at this region. Region C gradually deformed along with the deformation of region B [15]. The electromagnetic formed specimen during high-speed impact was shown in Figure 4. The specimens were split for microstructure characterization after electromagnetic high-speed impact.  The sidewall and center region of the specimen impacted to the conical die at a high speed when the coil discharged. In order to consider the impact characteristics in EMF, the deformed workpiece was divided into four parts (region A, B, C, and D). Region A, which was considered to be the initial material zone, was located at the flange area of the workpiece, as shown in Figure 4. Region B was located at the half radius of the coil. Region D was located at the workpiece center, and region C was between region B and region D. The high-speed impact process of electromagnetic forming is schematically shown in Figure 5. Region B firstly impacted to the sidewall of the die due to the maximum electromagnetic force that appeared at the half radius of the coil. Region D impacted to the tip of the conical die at high speed under the inertia force. However, region C did not impact to the conical die at high speed because there was no magnetic pressure existing at this region. Region C gradually deformed along with the deformation of region B [15]. The electromagnetic formed specimen during high-speed impact was shown in Figure 4. The specimens were split for microstructure characterization after electromagnetic high-speed impact. The sidewall and center region of the specimen impacted to the conical die at a high speed when the coil discharged. In order to consider the impact characteristics in EMF, the deformed workpiece was divided into four parts (region A, B, C, and D). Region A, which was considered to be the initial material zone, was located at the flange area of the workpiece, as shown in Figure 4. Region B was located at the half radius of the coil. Region D was located at the workpiece center, and region C was between region B and region D. The high-speed impact process of electromagnetic forming is schematically shown in Figure 5. Region B firstly impacted to the sidewall of the die due to the maximum electromagnetic force that appeared at the half radius of the coil. Region D impacted to the tip of the conical die at high speed under the inertia force. However, region C did not impact to the conical die at high speed because there was no magnetic pressure existing at this region. Region C gradually deformed along with the deformation of region B [15]. The electromagnetic formed specimen during high-speed impact was shown in Figure 4. The specimens were split for microstructure characterization after electromagnetic high-speed impact.

Micro Hardness Test
Samples with the size of 6 mm × 5 mm were cut from the electromagnetic high-speed impact workpiece. The samples were inlaid and then were ground with abrasive papers of #400, #800, #1200, and #2000 sequentially. Finally, the samples were mechanically polished. The Vickers hardness values of samples were obtained by the digital micro hardness tester (430SVD, Buehler Co., Lake Bluff, IL, USA). The positions of the micro hardness test were along the center line of the sample cross section. The load and duration time were set at 9.8 N and 15 s, respectively. The micro hardness was calculated by the average value of six points on each sample section. The error of hardness measurement was claimed to be ±0.75%.

Electron Back Scattering Diffraction Test
The test samples for the electron back scattering diffraction (EBSD) test were ground with silicon carbide abrasive papers of #400, #800, #1200, #2000, and #4000 sequentially. Then, the samples were polished with W1.0 diamond grinding paste. The sample microstructures were characterized by EBSD with a MAIA3 XMH field emission gun scanning electron microscope (FEG-SEM, TESCAN, MAIA3 XMH, Brno, Czech Republic). This experiment setup was equipped with an EBSD system. A 20 kV operating voltage and 12 mm working distance was used in low vacuum. A scan step of 0.8 µm was used for the measurements.

Transmission Electron Microscopy Test
The samples were burnished into 40 µm thickness foils with #400, #1000, and #2000 abrasive paper. Discs 3 mm in diameter were obtained by punching. Then, the oxide layer and surface residue were removed by an ion thinning apparatus (Gatan model 695, San Diego, CA, USA) to meet the requirements of the TEM test. Finally, a TEM (FEI Talos F200, Hillsboro, OR, USA) was used to observe the morphological characteristics of the samples.

Micro Hardness
The micro hardness of different regions of the 5052 aluminum alloy sheet after electromagnetic high-speed impact forming is shown in Figure 6, and the micro hardness values are shown in Table  3. The micro hardness of the initial material (region A) was 62.0 HV. Region B was the first to be highspeed impacted to the conical die because region B was located at half the radius of the coil, which distributed the largest electromagnetic force; the micro hardness of region B increased to 85.0 HV.

Micro Hardness Test
Samples with the size of 6 mm × 5 mm were cut from the electromagnetic high-speed impact workpiece. The samples were inlaid and then were ground with abrasive papers of #400, #800, #1200, and #2000 sequentially. Finally, the samples were mechanically polished. The Vickers hardness values of samples were obtained by the digital micro hardness tester (430SVD, Buehler Co., Lake Bluff, IL, USA). The positions of the micro hardness test were along the center line of the sample cross section. The load and duration time were set at 9.8 N and 15 s, respectively. The micro hardness was calculated by the average value of six points on each sample section. The error of hardness measurement was claimed to be ±0.75%.

Electron Back Scattering Diffraction Test
The test samples for the electron back scattering diffraction (EBSD) test were ground with silicon carbide abrasive papers of #400, #800, #1200, #2000, and #4000 sequentially. Then, the samples were polished with W1.0 diamond grinding paste. The sample microstructures were characterized by EBSD with a MAIA3 XMH field emission gun scanning electron microscope (FEG-SEM, TESCAN, MAIA3 XMH, Brno, Czech Republic). This experiment setup was equipped with an EBSD system. A 20 kV operating voltage and 12 mm working distance was used in low vacuum. A scan step of 0.8 µm was used for the measurements.

Transmission Electron Microscopy Test
The samples were burnished into 40 µm thickness foils with #400, #1000, and #2000 abrasive paper. Discs 3 mm in diameter were obtained by punching. Then, the oxide layer and surface residue were removed by an ion thinning apparatus (Gatan model 695, San Diego, CA, USA) to meet the requirements of the TEM test. Finally, a TEM (FEI Talos F200, Hillsboro, OR, USA) was used to observe the morphological characteristics of the samples.

Micro Hardness
The micro hardness of different regions of the 5052 aluminum alloy sheet after electromagnetic high-speed impact forming is shown in Figure 6, and the micro hardness values are shown in Table 3. The micro hardness of the initial material (region A) was 62.0 HV. Region B was the first to be high-speed impacted to the conical die because region B was located at half the radius of the coil, which distributed the largest electromagnetic force; the micro hardness of region B increased to 85.0 HV. Although the plastic deformation amount of region C was larger than region B, it could be seen that the average micro hardness (73.5 HV) was lower. This could be because the sheet in region C did not experience high-speed impact to the conical die due to low magnetic pressure in region C [15]; the sheet in region C gradually adhered to the die surface along with the surrounding material. Thus, work hardening caused by high-speed impact would been occurred, as was found in region B. Region D was the center region of the sheet, which was high-speed impacted to the conical die under inertia forces. The micro hardness of region D increased significantly to 87.0 HV due to high-speed impact and large plastic deformation. The thicknesses of deformation regions B, C, and D were 1.82 mm, 1.76 mm, and 1.68 mm, respectively. To sum up, the results shown in Table 3 indicated that electromagnetic high-speed impact significantly improved the micro hardness of the workpiece.
Metals 2020, 10, x FOR PEER REVIEW 6 of 10 Although the plastic deformation amount of region C was larger than region B, it could be seen that the average micro hardness (73.5 HV) was lower. This could be because the sheet in region C did not experience high-speed impact to the conical die due to low magnetic pressure in region C [15]; the sheet in region C gradually adhered to the die surface along with the surrounding material. Thus, work hardening caused by high-speed impact would been occurred, as was found in region B. Region D was the center region of the sheet, which was high-speed impacted to the conical die under inertia forces. The micro hardness of region D increased significantly to 87.0 HV due to high-speed impact and large plastic deformation. The thicknesses of deformation regions B, C, and D were 1.82 mm, 1.76 mm, and 1.68 mm, respectively. To sum up, the results shown in Table 3 indicated that electromagnetic high-speed impact significantly improved the micro hardness of the workpiece.

Discussion of the EBSD Results
The grain morphologies were obtained by the EBSD test in the three typical regions of the electromagnetic high-speed impact samples, as shown in Figure 7. It showed that the grain size in Figure 7b-d decreased in the process of electromagnetic forming compared with the initial grain size shown in Figure 7a. In addition, the grain morphology and the average size were changed significantly during electromagnetic high-speed impact compared with region A. Region B of the sheet impacted to the sidewall of the conical die at a high speed during electromagnetic discharge, and region B was subjected to severe compressive stress. Therefore, the grains appeared to be significantly elongated, and the grain size decreased significantly, as shown in Figure 7b. The grain morphology of region C was changed greatly compared with the initial state due to large plastic deformation. However, although region C was subjected to greater plastic deformation, the grain size did not decrease compared to region B. Therefore, high-speed impact would significantly reduce the grain sizes. There was no electromagnetic force in region D, and a serious impact between this region and the top of the conical die occurred due to the effect of inertia action. As seen in Figure 7d, the grains became flatter and smaller compared to region B, because the center region of the specimen impacted to the conical die more severely. Therefore, the center region of the workpiece was subjected to higher impact compressive stress, and the large contact stress led to the significant increase in hardness.
The percentage of average grain diameter and the number of grains are presented in Figure 8. The proportion of small-diameter grains obviously increased when the sheet high-speed impacted to

Discussion of the EBSD Results
The grain morphologies were obtained by the EBSD test in the three typical regions of the electromagnetic high-speed impact samples, as shown in Figure 7. It showed that the grain size in Figure 7b-d decreased in the process of electromagnetic forming compared with the initial grain size shown in Figure 7a. In addition, the grain morphology and the average size were changed significantly during electromagnetic high-speed impact compared with region A. Region B of the sheet impacted to the sidewall of the conical die at a high speed during electromagnetic discharge, and region B was subjected to severe compressive stress. Therefore, the grains appeared to be significantly elongated, and the grain size decreased significantly, as shown in Figure 7b. The grain morphology of region C was changed greatly compared with the initial state due to large plastic deformation. However, although region C was subjected to greater plastic deformation, the grain size did not decrease compared to region B. Therefore, high-speed impact would significantly reduce the grain sizes. There was no electromagnetic force in region D, and a serious impact between this region and the top of the conical die occurred due to the effect of inertia action. As seen in Figure 7d, the grains became flatter and smaller compared to region B, because the center region of the specimen impacted to the conical die more severely. Therefore, the center region of the workpiece was subjected to higher impact compressive stress, and the large contact stress led to the significant increase in hardness. the conical die. The grain size decreased substantially during the electromagnetic high-speed impact, as shown in Figure 8d, and more sliding systems were activated in this region.
It can be seen in Figure 7 that when the specimen impacted to the die at high speed, not only the grain morphology and size was changed, but also sub-grains were generated. The grains broke into small subgrains when the center of the sheet impacted to the conical die under inertial action, and a large number of small grains were formed during electromagnetic high-speed impact. The percentage of average grain diameter and the number of grains are presented in Figure 8. The proportion of small-diameter grains obviously increased when the sheet high-speed impacted to the conical die. The grain size decreased substantially during the electromagnetic high-speed impact, as shown in Figure 8d, and more sliding systems were activated in this region.
Metals 2020, 10, x FOR PEER REVIEW 7 of 10 the conical die. The grain size decreased substantially during the electromagnetic high-speed impact, as shown in Figure 8d, and more sliding systems were activated in this region. It can be seen in Figure 7 that when the specimen impacted to the die at high speed, not only the grain morphology and size was changed, but also sub-grains were generated. The grains broke into small subgrains when the center of the sheet impacted to the conical die under inertial action, and a large number of small grains were formed during electromagnetic high-speed impact.

Discussion of the TEM Results
In order to understand the plastic deformation mechanism of the 5052 aluminum alloy sheet, it was necessary to analyze the microstructure evolution in electromagnetic high-speed impact. The TEM photograph of the undeformed specimen is presented in Figure 9a,b. The dislocation density was low in the original material. The TEM photographs of region C are shown in Figure 9c,d. Dislocation density of region C increased significantly compared with region A. The microstructure of the electromagnetic forming specimen mainly included dislocation bands, as shown in Figure 9d. Thus, the deformation mechanism was plane slip during electromagnetic forming.
As seen in Figure 9e, trigeminal grains appeared at region D. The dislocation density of region D increased significantly compared with region C. It can be seen in Figure 9f that the dislocation wall and dislocation cell were found in region D. Thus, at the microscopic level, dislocation cross slip was the deformation mechanism in electromagnetic high-speed impact forming. Therefore, the main deformation mechanisms in electromagnetic forming and in electromagnetic high-speed impact were both dislocation slip mechanisms, but the difference was that the deformation mechanism in electromagnetic forming was dislocation plane slip, and the deformation mechanism in electromagnetic high-speed impact was dislocation cross slip. The deformation mechanism in the present research was different from the results in the literature [13,14]. With increasing dislocation density, the orientation difference increased within deformed grain, and the increase of dislocation density led to the increase of micro hardness.  It can be seen in Figure 7 that when the specimen impacted to the die at high speed, not only the grain morphology and size was changed, but also sub-grains were generated. The grains broke into small subgrains when the center of the sheet impacted to the conical die under inertial action, and a large number of small grains were formed during electromagnetic high-speed impact.

Discussion of the TEM Results
In order to understand the plastic deformation mechanism of the 5052 aluminum alloy sheet, it was necessary to analyze the microstructure evolution in electromagnetic high-speed impact. The TEM photograph of the undeformed specimen is presented in Figure 9a,b. The dislocation density was low in the original material. The TEM photographs of region C are shown in Figure 9c,d. Dislocation density of region C increased significantly compared with region A. The microstructure of the electromagnetic forming specimen mainly included dislocation bands, as shown in Figure 9d. Thus, the deformation mechanism was plane slip during electromagnetic forming.
As seen in Figure 9e, trigeminal grains appeared at region D. The dislocation density of region D increased significantly compared with region C. It can be seen in Figure 9f that the dislocation wall and dislocation cell were found in region D. Thus, at the microscopic level, dislocation cross slip was the deformation mechanism in electromagnetic high-speed impact forming. Therefore, the main deformation mechanisms in electromagnetic forming and in electromagnetic high-speed impact were both dislocation slip mechanisms, but the difference was that the deformation mechanism in electromagnetic forming was dislocation plane slip, and the deformation mechanism in electromagnetic high-speed impact was dislocation cross slip. The deformation mechanism in the present research was different from the results in the literature [13,14]. With increasing dislocation density, the orientation difference increased within deformed grain, and the increase of dislocation density led to the increase of micro hardness. present research was different from the results in the literature [13,14]. With increasing dislocation density, the orientation difference increased within deformed grain, and the increase of dislocation density led to the increase of micro hardness.

Conclusion
The micro hardness and microstructures of 5052 aluminum alloys were characterized after electromagnetic high-speed impact. The results of this work are summarized below.
Although the plastic deformation amount in region C was larger than that of region B, the average micro hardness was lower. Therefore, electromagnetic high-speed impact could significantly improve the micro hardness of the sheet.

Conclusion
The micro hardness and microstructures of 5052 aluminum alloys were characterized after electromagnetic high-speed impact. The results of this work are summarized below.
Although the plastic deformation amount in region C was larger than that of region B, the average micro hardness was lower. Therefore, electromagnetic high-speed impact could significantly improve the micro hardness of the sheet.
When the specimen impacted to the die at a high speed, not only the grain morphology and size was changed, but also sub-grains were generated. The grains broke into small subgrains when the center of the sheet impacted to the conical die under inertial action Large plastic deformation occurred in region C during electromagnetic forming, and the microstructure of region C was mainly dislocation bands. The deformation mechanism was dominated by a planar slip under electromagnetic forming. The microstructure of region D was mainly dislocation wall and dislocation cell after electromagnetic high-speed impact. Thus, the microscopic deformation mechanism was dislocation cross slip in electromagnetic high-speed impact.
Electromagnetic high-speed impact could improve the formability of hard-to-deform material. Therefore, it was of great significance to expand the application of electromagnetic high-speed impact in manufacturing of hard-to-deform parts in automotive and airplane applications. The microscopic mechanism, deformation behavior, and performance of the formed part of 5052 aluminum alloy by electromagnetic high-speed impact will be studied deeper in our future work.