[China Aluminum Industry Net] With the rapid development of the automotive industry and the growing awareness of environmental protection of human beings, the requirements for vehicle safety and fuel efficiency are getting higher and higher, making automotive panels gradually develop toward lightweight materials. Aluminum alloy has the advantages of high specific strength and good corrosion resistance. Under the premise of not reducing the original safety performance of the automobile, it obviously reduces the weight of the automobile and achieves the goals of energy saving and environmental protection. However, the aluminum alloy sheet has low plasticity at room temperature, poor normal temperature drawing performance, cracking and wrinkling, and the dimensional accuracy is difficult to control, and a body covering member having a complicated shape cannot be successfully machined.
Studies have shown that in the warm forming conditions (200 °C - 350 °C), aluminum alloy sheet plasticity is greatly improved, and the flow stress is reduced. Compared with normal temperature deep drawing, under the condition of warm forming, the deep drawing performance of the sheet can be significantly improved, and the springback of the formed part is small, and the surface quality of the part is good. Therefore, the use of warm forming technology to produce aluminum alloy cover can greatly promote its application in complex body parts.
In this paper, the commercial finite element software ABAQUS is used to numerically simulate the deep drawing process of aluminum alloy plates for automotive use. Through experimental design methods, the influence of temperature distribution on the deep drawing performance of aluminum alloy plates is discussed. Medium temperature field settings provide a reference.
1ã€Finite element modeling
Due to the symmetry, the molds and plates are simplified to 2D axisymmetric models, as shown in Figure 1. The finite element software used was commercial finite element software ABAQUS/standard, and the finite element model was a thermocoupled four-node biaxial axisymmetric cell CAX4RT. The thickness of the sheet was divided into 5 layers, and a total of 360 elements were divided into slabs and tools. The thermal conduction is included in the thermo-mechanical coupling finite element analysis. The material density is 2700kg/m3, the specific heat is 920J/(kg·K), the thermal conductivity is 121W/(m·K), and the heat transfer coefficient between the slab and the tool is 1400W/(m2·K). In the simulation, the aluminum alloy plate 5083-O is an isotropic material. The material parameters under the warm forming conditions are Naka's test data, the thickness is 1mm, and the yield criterion is the von Mises criterion. In the simulation, the punch speed is 2.5 mm/s, the constant blank holder force is 2 MPa, and the friction coefficient between the sheet and the tool is assumed to be 0.1.
2, research methods
In this study, the tool is divided into three temperature zones, as shown in Figure 1. Zone A represents the bottom of the punch, B represents the flange, C represents the radius of the die, and each temperature zone is assumed to be independent; In order to set the initial temperature of the slab, it is considered that the whole is a temperature zone D, and the temperature field is set to two levels of normal temperature (25° C.) and heating (250° C.).
The experimental design method—partial factor analysis method was used to design the scheme. The experimental factors were four temperature zones A to D in Figure 1, with levels of 25°C and 250°C. Table 1 test program, a total of 8 sets of simulation calculations.
The drawing performance is evaluated by the critical punch stroke CPS. The larger the value, the better the drawing ability. In the simulation, if the sheet thickness reduction rate reaches 30%, the failure is considered to occur. At this time, the punch stroke is the critical punch stroke CPS.
3, results and analysis
The ABAQUS runs the test column No. 8 in Table 1. The critical punch stroke CPS of cylinder drawing under various temperature conditions is listed in Table 1. From Table 1, it can be seen that the initial temperature arrangement has an important influence on the CPS value. After statistical analysis of the experimental design, the influence of various factors and reasonable temperature distribution are listed in Figure 2 and Table 2. The factor that has a greater influence on the drawing performance is the temperature in zone A, followed by the temperature in flange B zone. When the punch is kept at a lower temperature level (eg, room temperature 25° C.), the flange is heated to a higher temperature (eg, 250° C.), which further contributes to the ability of the aluminum alloy sheet to be drawn. At the same time, the calculation results in Table 1 show that the lower the temperature at the corner of the die, the better the drawing ability, but the degree of influence is not strong; and the effect of the initial temperature of the slab on the drawing ability is worth noting. Heating to the same temperature as the flange will reduce the CPS value.
From the analysis results in Table 2, it can be seen that the preferred temperature distribution is that only the flange is heated to 250° C., and its drawing ability is better. Under this condition, the deep drawing process was simulated, and the calculation results showed that the deep drawing was successfully completed.
In deep drawing, the slab at the flange undergoes compressive deformation and then enters the concave mold cavity. At this time, the deformed area is transformed into the force-transmitting area. When the flange is in a high temperature condition, the deformation resistance of the slab in the flange deformation area is reduced, and when the bottom of the punch is at a lower temperature, the sheet material has a high tensile strength, and the reinforcement of the sidewall, especially the fillet of the punch The carrying capacity. If the die is at a low temperature near the fillet, the slab will flow out of the high-temperature flange area and will decrease its temperature when passing through the die radius. This will further enhance the load-bearing capacity of the sidewalls, which is more conducive to improving the aluminum alloy plate pull. Deep ability. It can be seen that in the temperature draw of aluminum alloy plates, a reasonable temperature setting is the key to improving the drawing ability. The differential temperature deep drawing mode, that is, heating at the die flange and the punch at a lower temperature, is a better process for improving the drawing performance of the aluminum alloy sheet.
(a) Cracks near the punch fillet
(b) Cracks near the die fillet
Fig. 3 shows two types of failures in aluminum alloy plate during temperature draw. The temperature conditions during molding are shown in Table 3. Fig. 3(a) is a deep-frequent form of cracking at normal temperature, that is, cracks occur near the fillet of the punch. When the flange is heated to 250°C, the failure form of Fig. 3(b) occurs. Near the corner of the die, this is a defect that rarely occurs at room temperature. These forms of failure are consistent with previous experimental observations. In the deep drawing of this mode, although the sheet is thinned at the fillet of the punch, it is at a low temperature, the material has high tensile strength, and the sheet near the die fillet flows out of the flange. In the high temperature state, the material has low tensile strength. From the viewpoint of the overall carrying capacity, the sheet material near the corner of the die is weak at this time, so that the rupture occurs here.
4 Conclusion
Using the thermo-mechanical coupling finite element method and the experimental design method, a reasonable distribution of the initial temperature in the temperature deep drawing of the aluminum alloy plate cylinder parts was realized.
(1) The temperature of the bottom of the punch and the die flange determines the drawing ability of the aluminum alloy plate. When the die flange is at a higher temperature and the bottom of the punch is at a room temperature, the differential temperature drawing mode is more conducive to the drawing of the material. Deep ability.
(2) In the differential temperature drawing of the cylinder, the crack may appear near the fillet of the punch or near the fillet of the die.
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