Abstract
In this study, the structural optimization, the direct-reverse superplastic forming (SPF) process the thickness distribution and the tensile property of the industrial 5083 aluminum alloy pallet with multi-deep cavities were investigated. The tensile property was carried out at room temperature and was introduced into the finite element method (FEM) to investigate the influence of the different structures with reinforcing ribs on the mechanical property of the industrial 5083 aluminum alloy. The results showed that the structure with radial reinforcing ribs possessed good load-bearing performance. Then the split forming process was introduced and the pallet was divided into the main body and the bottom supports. The FEM was conducted to predict the direct-reverse forming process of the main body, and the results showed that the minimum thickness occurred at the bottom corner. The argon gas pressure loading path was modified and the main body of the pallet was successfully manufactured by the direct-reverse SPF process. Finally, the thickness distribution was measured and the maximum thickness difference between the FEM and experimental measurement was 0.1 mm. The tensile property of post-forming material was also investigated, and the tensile strength, yield strength and the elongation were decreased slightly.
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Introduction
Aluminum alloys have been widely used as structural materials in the aircraft industry for several decades1,2,3. Compared with other structural materials, such as high-strength steel alloys, most aluminum alloys have a better strength-to-weight ratio4,5. The Al-Mg-Mn alloys are known for their excellent mechanical properties, such as high strength, high thermal stability at elevated temperatures, good corrosion resistance, and good ductility6,7,8,9.Owing to its low density, excellent corrosion resistance and mechanical properties, AA5083 has been widely applied in the railway vehicles, shipbuilding, and other industries. And it is considered to be an ideal aluminum alloy material for SPF process.
SPF is an advanced sheet metal forming process adopted mainly in the aerospace and automotive industries, as well as for biomedical applications10,11. In recent years, the demand of energy efficiency make the need of lightweight structural metals growing, which led to the increasing need of aluminum alloys12. The SPF process provided a viable solution that could guarantee to fabricate the structure with complex geometries and high shape accuracy, and reached one-step manufacturing step11,13,14. While the lack of SPF process was that the thickness was prone to reduce seriously at the local area.
At present, 5xxx aluminum alloy used in SPF could be divided into two categories. One is the typical fine-grained 5xxx aluminum alloy with grain size less than 10 µ m. The other one is coarse-grained 5xxx aluminum alloy with grain size greater than 10 µ m. Park et al.15 obtained the maximum elongation to failure of 740% in a 5083 Al alloy by introducing a ultrafine grained structure of 0.3 μm through severe plastic deformation and by adding a dilute amount of scandium (Sc) as a microstructure stabilizer. Fakhar et al.16examined the superplastic behavior of fine and ultra fine-grained AA5083 achieved by double equal channel lateral extrusion (DECLE) and the strain rate sensitivity of 0.43 were evaluated. In general, fine-grained aluminum alloy are mainly obtained through large plastic deformation technologies such as equal channel angular extrusion (ECAP), repeated extrusion (CEC), and thermomechanical treatment (TMP), and the process was complex and the cost was high17,18,19,20,21.
Coarse-grained superplasticity based on solute drag creep does not depend on grain size, and large elongation can be obtained22,23,24 .The cost reduction could be effectively achieved by using coarse-grained material to replace the fine-grained material. Therefore, it was of great significance to investigate the superplastic forming process of coarse-grained 5083 aluminum alloy to manufacture large-size structure. The direct-reverse superplastic forming has the advantages of improve the thickness uniformity25. Jiang et al.26 successfully fabricated the TC4 deep cylinder whose wall thickness accuracy was demanded in the range of 1.6 ± 0.2 mm and the thickness distribution is uniform. Thus, the direct-reverse superplastic forming was applied to the fabrication of the pallet.
The aim of this work was to manufacture the aluminum alloy pallet with good mechanical property by the direct-reverse SPF process. With the help of FEM for structural optimization, the pallet with radial reinforcement ribs was selected. The split forming of the main body and the bottom supports was designed. Then the FEM of the direct-reverse SPF process for the main body of the pallet was conducted. And the main body was successfully fabricated. The thickness distribution of the main body was measured and the tensile behavior of the as-received material and post-forming material at room temperature was investigated.
Experimental
The chemical composition of industrial 5083 aluminum alloy sheet with the thickness of 2.0 mm was shown in Table 1. The tensile property of the as-received material and post-forming material was investigated at room temperature. Tensile specimens prepared with dimensions of 15 mm × 4 mm × 2 mm. These samples were mechanically ground by the SiC sanding papers to remove any surface scratches and oxide skin. The tensile testing at room temperature was carried out on a Shimadzu AG-X testing machine with the speed of 1 mm/min.
In order to promote the mechanical properties of the pallet, the three structures with different reinforcing ribs were designed. The FEM was carried out to evaluate the mechanical properties of the three structures by ANSYS software. The FE models were simplified to improve computation efficiency due to the symmetry of the structure. The schematic diagrams of simplified finite element model at compressive performance test, bending test and forklift test were illustrated in Fig. 1. At these tests, the lower platforms and forklift dentures were fixed and load surfaces were applied normal loads. In Fig. 1a, the sizes of the lower platform and load surface were 600 mm × 600 mm. In Fig. 1b-c, the sizes of the lower platform, forklift dentures and load surfaces were 50 mm × 600 mm, 100 mm × 600 mm, and 600 mm × 1200 mm, respectively. The pallet was two dimensional surface model. The element size of the lower platforms, forklift dentures, and load surfaces was set to 10 mm, and that of the pallet was set to 6 mm.
The simplified model of FEM: (a) Compressive performance test; (b) Bending test; (c) Forklift test (This figures were partially created with WPS 365 Education Edition 12.1, https://365.wps.cn/edu/home/).
The high temperature uni-axial tensile tests were conducted on the Instron 5500 testing machine with a heating furnace at 400℃−520℃ with an interval of 40℃, and the initial strain rate was 0.0005 s−1, 0.001 s−1, 0.005 s−1. The temperature difference was controlled within 2 °C along the gauge length with the help of three individual thermocouples.
The true strain-true stress curves of AA5083 at different strain rates and temperatures were shown in Fig. 2. It was seen that the tensile strength and the elongation were sensitive to the temperatures and the strain rates. The tensile strength was positively correlated with the strain rates and negatively correlated with the temperatures. At the strain rate of 0.005 s−1 and 0.001 s−1, the maximum elongations were obtained at 480 ℃. When the strain rate was 0.0005 s-1.the maximum elongation was obtained at 520 ℃. The elongations were more than 200% at some temperatures and strain rates. At all the deformation condition, the elongation of 242% was achieved at 480 ℃/0.001 s−1 and that of 230% at 520 ℃/0.0005 s−1. The elevated temperature could effectively decrease the flow stress of the material and promote the forming quality of the structure. Thus, the forming temperature could be selected as 520 °C.
The true strain-true stress curves of AA5083 at: (a) 0.005 s−1; (b) 0.001 s−1 (c) 0.0005 s−1.
In order to predict the forming process, the FEM was applied to investigate the direct-reverse SPF process of the main body. The forming process of the main body could be divided into three stages: the drawing forming, the reverse SPF and the direct SPF. The MSC.MARC software was applied for the finite element method. The initial finite element model of the main body was shown in Fig. 2. Due to the symmetry of the main body and the dies, a quarter model was constructed to refine the mesh and reduce the amount of calculation.
The size of the sheet in Fig. 3 was 700 mm×700 mm×2 mm and the shell element size was 5 mm×5 mm. The friction coefficient between the dies and the sheet was set to be 0.3 and the coulomb friction model was selected. In General, the effect of the strain hardening on stress was ignored during the superplastic forming process, thus the constitutive equation of the rigid-plastic model following the power law relationship was applied during SPF process. During the drawing stage, the temperature was same with that of SPF process and the drawing rate was still low, thus the rigid-plastic model was applied in all the three stages for FEM. Figure 3(b) illustrated the boundary condition of FEM and the fixed displacement of the nodes at the symmetry planes was set at three stages. In addition, the fixed displacement of the nodes at the suppressing border areas and the face load for superplastic forming controlling on the selected faces were set at the last two stages. The target strain rate was set to be 0.0005 s−1. The matrix solver with symmetric multifrontal sparse type was selected and the maximum duration of numerical operation can reach 3000 s.
The FEM: (a) initial simplified model; (b) the boundary condition (This figures were partially created with WPS 365 Education Edition 12.1, https://365.wps.cn/edu/home/).
The main body of the pallet with direct-reverse SPF process was conducted on the 5000 kN SPF machine. The forming temperature was 520 °C. The argon gas was used to shape the structure and three different gas pressure loading paths were applied. The thickness gauge was used to measure the thickness of the structure. In order to make the thickness accurate, the thickness measurement of each point was conducted at least three times and the average value of thickness was considered as the final value.
Results and discussion
The structural design
The initially designed aluminum alloy pallet with nine deep cavities was illustrated in Fig. 4. The shape is rectangular and the dimensions were 1165 mm × 1165 mm × 110 mm. The pallet contains nine equidistant stepped deep cavities with the depth of 110 mm, and the maximum diameter of deep cavity was 220 mm and the minimum diameter of was 90 mm. The thickness of the pallet was 2 mm.
The initial structure of aluminum alloy pallet with multi deep cavities (This figure was created with SolidWorks 2020, https://www.solidworks.com/).
It was well known that the bending resistance of flat surfaces was poor, thus the reinforcing ribs should be drawn in the pallet so as to improve the load-bearing performance of the pallet. The distribution and shape of reinforcing ribs would affect the bending resistance of the structure. Therefore, the pallets with different arrangements of reinforcing ribs had been designed. The structural optimization of the aluminum alloy pallets with reinforcing ribs of radial shape, annular staggered shape and straight staggered shape was shown in Fig. 5.
Structural design of aluminum alloy pallet with reinforcing ribs of different shapes: (a) Radial; (b) Annular staggered; (c) Straight staggered (This figures were partially created with Solidworks 2020, https://www.solidworks.com/).
The compressive performance test with the load of 80 kN was simulated, and the deformation distribution of the pallets with different structures was shown in Fig. 6. It was shown that the maximum displacements of the pallets with the radial reinforcement ribs, the annular staggered reinforcement ribs and the straight staggered reinforcement ribs were 4.065 mm, 5.124 mm and 6.78 mm, respectively. There were two large deformation zones occurring. One was located at the edge of the pallet and between the external deep cavities. The other one located at the middle area of the adjacent four deep cavities. The structure of these two zones affected the compressive performance. The deformation of pallet with radial reinforcement ribs was belong to the one type and that of the other two pallets was belong to the other one type. It was seen that the edge zone structure of three different pallets and the displacement of this zone were basically similar, indicating that the compressive performance of three pallets was consistent with each other at the edge zone. At the middle area of the adjacent four deep cavities, the displacement of the pallets with staggered reinforcement ribs was larger than that of the pallet with radial reinforcement ribs. It could be indicated that the staggered reinforcement ribs in this area reduced the compressive performance of the pallet. The radially continuous reinforcement ribs were conducive to improving the stiffness of this area.
The displacement distribution of different pallet at compressive performance test: (a) Radial; (b) Annular staggered; (c) Straight staggered (This figures were partially created with WPS 365 Education Edition 12.1, https://365.wps.cn/edu/home/).
The load of 24 kN was applied at the forklift lifting test. The displacement distribution of the pallets with different structures was shown in Fig. 7. The deformation of pallets presented an “M” shape and the maximum displacement occurred at the outer edge of the pallet parallel to the fork teeth. The maximum displacements of the pallets with the radial reinforcement ribs, the annular staggered reinforcement ribs and the straight staggered reinforcement ribs were 14.36 mm, 22.83 mm and 19.76 mm, respectively. The pallet with the radial reinforcement ribs had best forklift bending resistance than that of the other two pallets.
The displacement distribution of the different pallet at the forklift lifting condition. (a) Radial; (b) Annular staggered; (c) Straight staggered. (This figures were partially created with WPS 365 Education Edition 12.1, https://365.wps.cn/edu/home/).
Figure 8 showed the displacement distribution of different structural pallets at bending test with a load of 40 KN. The pallet was placed on the ground and loaded at the middle area of the pallet. The deformation of the pallets exhibited a “W” shaped with the maximum deformation zone located near the loading bars. The maximum displacements of the pallets with the radial reinforcement ribs, the annular staggered reinforcement ribs and the straight staggered reinforcement ribs at the bending test were 12.88 mm, 20.89 mm and 17.88 mm, respectively. The deflection of pallet with the radial reinforcement ribs was smallest at the same load.
The displacement distribution of the different pallet at bending test. (a) Radial; (b) Annular staggered; (c) Straight staggered (This figures were partially created with WPS 365 Education Edition 12.1, https://365.wps.cn/edu/home/).
The deformation of the pallet at three tests was considered comprehensively, it can be concluded that the pallet with radial reinforcement ribs has the good load bearing performance. Thus, the pallet with radial reinforcement ribs would be fabricated subsequently.
The forming design
Due to the large depth of the nine deep cavities of the pallets, it was prone to cause thinning or cracking of the cavity by one-time integrated forming, which would result in the reduction or the failure of the load bearing performance. Therefore, the split forming process was introduced to fabricate the aluminum alloy pallet. The schematic diagram of split forming was illustrated in Fig. 9. The pallet was divided into two parts: the main body and the bottom supports. The main body is located above the horizontal line, and the bottom supports were located below the horizontal line. The main body consisted of the compressive surface, the reinforcement ribs, and the upper portion of the stepped deep cavities. And the bottom supports were the lower portion of the cavities. In order to guarantee the thickness distribution more uniform, which could be prepared by the direct-reverse SPF process. The bottom supports were prepared by cold stamping process. Finally, the two parts are connected by riveting to form the overall pallet. In this paper, only the fabrication of the main body was designed and investigated.
The schematic diagram of split forming (This figures were partially created with WPS 365 Education Edition 12.1, https://365.wps.cn/edu/home/).
The FEM of the direct-reverse SPF process
The results of the FEM at the three stages during the forming process were shown in Fig. 10. The thickness distribution after the drawing was shown in Fig. 10(a), and the minimum thickness was 1.973 mm. It was shown that there was no obvious thickness reduction at the drawing stage. Then the reverse SPF was conducted. Figure 10(b) showed the thickness distribution of the structure after reverse SPF and the thickness curve at the symmetry line with the arc length increasing from 0 to 700 mm. In order to keep the thickness enough for the direct SPF, the sheet incompletely contacted with the reverse die and the minimum thickness in this stage was limited to be more than 1.4 mm. Thus it was seen that the minimum thickness was 1.44 mm and it took place at the bottom of the inner cavity. And it was slightly less than that of the outer ring. Finally, the direct SPF was simulated. The thickness distribution was illustrated in Fig. 10(c). It can be seen that the minimum thickness was 1.14 mm and it occurred at the corner of the cavities. The maximum thinning ratio was 43%. Based on the results, it could be concluded that this process was feasible for manufacturing the main body of the pallet.
Thickness distribution of the structure at different stage: (a) drawing process; (b) reverse SPF; (c) direct SPF (This figures were partially created with WPS 365 Education Edition 12.1, https://365.wps.cn/edu/home/).
The direct-reverse SPF process
The forming temperature is maintained at 520 °C for about 0.5 h to make the temperature uniform, then the drawing begins. In order to maintain the target strain rate of 0.0005 s−1, the time-pressure curves of the reverse-direct SPF process for the pallet were generated through FEM and were shown in Fig. 11. Then the time-pressure curves were optimized to make the gas pressure more suitable for actual control. The optimized time-pressure curve was applied at the beginning of the two SPF stages.
The time-pressure curves of the FEM: (a) the reverse SPF; (b) the direct SPF.
The time-pressure curves for the direct-reverse SPF of the main body were illustrated in Fig. 12, and the left side of black dashed line expressed the reverse bulging gas pressure and the right side expressed the direct bulging gas pressure. The obtained main body of the pallet after direct-reverse SPF with the different time-pressure curves was shown in Fig. 12.
The different gas pressure loading path of SPF process: (a) Plan A; (b) Plan B; (c) Plan C.
Firstly, the gas pressure was loaded with plan A, and the reverse bulging time was 9 min and the maximum gas pressure was 0.4 MPa for 2 min. After the reverse bulging, the direct bulging was carried out. When the gas pressure increased to 0.6 MPa, the local rupture occurred at the sidewall of deep cavity and was shown in Fig. 13(a). The rupture resulted from the high gas pressure and long reverse bulging time, these caused the severe local thinning.
The main body fabricated by different plans: (a) Plan A; (b) Plan B; (c) Plan C (This figures were partially created with WPS 365 Education Edition 12.1, https://365.wps.cn/edu/home/).
Then, the plan B was applied. In plan B, the time and maximum pressure of reverse bulging decreased to 7 min and 0.3 MPa. And these of direct bulging were 26 min and 1.4 MPa for 3 min. The main body was successfully fabricated in Fig. 13(b). It had no dents or cracks, and the quality is good with only edge warping present. The plan B was modified to plan C with the maximum pressure and time of direct bulging decreasing to 1.2 MPa and 27 min. Figure 13(c) showed the fabricated main body by plan C. It was seen that the forming quality is good with no obvious warping at the edges, good filling of the rounded corners and good flatness of the bottom for the nine deep cavities.
The thickness distribution
Thickness uniformity is an important factor that will influence the mechanical properties of a structure27 and is consequently important to control. Measurements to guarantee the thickness were carried out with an ultrasonic thickness gauge to investigate thickness distribution and optimize the process. In order to guarantee the measuring accurancy, the thickness for one point was measured at least three times and the average value was set as the final value. Due to the symmetry of the pallet, the thickness distribution was measured along the 90° and 135° lines. The thickness of the rounded corners cannot be accurately measured by the thickness gauge, thus the measurement points were located at the flat area. Five measurement points were selected at the bottom of the deep cavity. The sidewall of deep cavity was divided into the straight sidewall at the lower and the inclined sidewall at the upper. Two measurement points were selected on the straight sidewall. One or two points were selected on the inclined sidewall. Measurement points were uniformly selected on the reinforcement zone. Finally, a total of 68 measurement points were selected on the two paths, as shown in Fig. 14.
The measurement points (135° line: 1–38; 90° line: 38–68) (This figures were partially created with WPS 365 Education Edition 12.1, https://365.wps.cn/edu/home/).
The comparison of thickness distribution at the corresponding points between the FEM and experimental measurement along the direction of 90° and 135° was shown in Fig. 15. The results of experimental measurement showed that the thickness of the bottom plane area of the deep cavity varied from 1.1 mm to 1.4 mm and it was less than that of other areas. The thinnest thickness of 1.13 mm took place at the point 13, 36 and 40, and they were close to the corner of deep cavity. The maximum thinning ratio was 43.5%. The thickness of the straight sidewall was less than that of the inclined sidewall and it is between 1.3 mm and 1.7 mm. The thickness of the two reinforcement ribs is uniform with the value almost varying from 1.70 mm to 1.80 mm. The thickness of the ribs along 135° and 90° was close to 1.77 mm and 1.72 mm, respectively. From Fig. 15, it was seen that the thickness of FEM and experimental measurement at the same point was almost equal. The maximum thickness difference at point 10 and point 43 was 0.09 mm and 0.1 mm, respectively, which was far less than the thickness. It showed that the direct-reverse SPF process and thickness variation for the main body of the pallet could be well predicted with finite element simulation.
Thickness distribution comparison of finite element simulation and experiment along the different direction.
The tensile property
The tensile specimens were also cut from the main body of the pallet and the tensile test was carried out at room temperature. In order to obtain the elongation and the strength accurately, at least three tensile specimens were tested and the average value was selected as the final value.
The tensile property of the as-received material and post-forming material at room temperature was represented in Fig. 16. The tensile strength and elongation of as-received material were 235 MPa and 29.5%, respectively. The tensile strength and elongation decreased slightly. The reduction of tensile strength was 8 MPa and that of elongation was 3.2%. The reduction of tensile property resulted from the microstructure evolution. The thermal exposure at high temperature and the deformation caused the grain coarsen.
Tensile property of the as-received material and the post-forming material: (a) tensile strength and yield strength; (b) elongation.
Conclusions
The structural optimization of the pallet and the direct-revise SPF process of the main body were investigated, and the following conclusions could be drawn:
(1) The pallet with radial reinforcement ribs has the good load bearing performance, and the split forming process was an appropriate way to fabricate the pallet with deep cavities.
(2) The main body of the pallet for industrial 5083 aluminum alloy was successfully manufactured by the direct-revise SPF process, and the maximum thickness reduction at the bottom corner was about 43.5%.
(3) The maximum thickness difference between FEM and experiment was 0.1 mm, which indicated that the forming process for the main body could be well predicted.
(4) Compared with the as-received material, the tensile strength, yield strength and elongation of the post-forming material reduced by 8 MPa, 5 MPa and 3.2%, respectively.
Data availability
All data generated or analysed during this study are included in this published article and its supplementary information files.
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Acknowledgements
This work was supported by Henan Provincial Department of Science and Technology Research Project (Grant No. 252102220067), National Natural Science Foundation of China (Grant No. 52375318), China National University Student Innovation and Entrepreneurship Development Program (Grant No. 202410481001), Young Backbone Teachers’ Project of Nanyang Normal University (Grant No. 2023-QNGG-8)，Natural Science Foundation Cultivation project of Nanyang Normal University (Grant No. 2025PY039).
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Zhihao Du：Conceptualization，Writing - Original Draft，Writing - Original DraftXinhua Gao：Methodology，Validation，InvestigationXiangxiang Dai ：Formal analysis，Investigation，VisualizationGuofeng Wang： Project administration，Funding acquisition，Writing - Original DraftShuobing Chen ：ResourcesCongzheng Zhang：Resources，Formal analysisLiang Li ：Resources，Formal analysis.
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Du, Z., Gao, X., Dai, X. et al. Structural optimization and direct reverse superplastic forming process for aluminum alloy multi cavities pallet. Sci Rep 15, 15508 (2025). https://doi.org/10.1038/s41598-025-00250-9
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DOI: https://doi.org/10.1038/s41598-025-00250-9
