Numerical simulation of temperature field, equivalent stress field, velocity field, and stroke load curve of the upper die for the pre-forging and final forging process of aluminum alloy wheel hub was carried out by using Deform software, and microscopic grain simulation was carried out at three different points on the forging piece. The numerical simulation results were analyzed to provide a reference for optimizing the forging process.
At present, aluminum alloy wheels have been widely used domestically and internationally; with the development of science and technology and social improvement, aluminum alloy wheels will become the development trend of automobile wheels. The great advantages of aluminum alloy wheels: First, aluminum alloy wheels have high strength, are not easy to break, and have high fatigue and yield strength. Secondly, the density of the aluminum alloy wheel hub is small, and the quality of the wheel is light, reducing the quality of the car itself, which can increase the car’s speed. Third, the thermal conductivity of aluminum alloy is higher, while the thermal conductivity of steel is lower, so aluminum alloy wheels have better heat dissipation ability. Fourth, aluminum alloy has a higher modulus of elasticity and is not easy to deform. Therefore, aluminum alloy wheels are gradually replacing steel wheels. In recent years, the development of the automobile industry greatly promotes the forging and application of aluminum alloy wheel hubs in China. Therefore, it is very important to analyze and optimize the forging process of aluminum alloy wheel hubs. In the paper, through simulating the forging process, the forging process is analyzed, and suitable forging process and parameters of aluminum alloy wheels are derived, which provides designers with a reasonable design model, thus reducing the scrap rate of aluminum alloy wheel forging and creating more economic benefits.
1. Model Establishment
There is a national standard for wheel hubs of automobiles, so the wheel hub form is determined. Hub form is more complex; using direct one-time forging is more difficult, so the pre-forging and final forging are two steps to carry out. In the pre-forging process, the shape of the blank is changed to a cake shape to prepare for the final forging. The pre-forging parts obtained after pre-forging are shown in Fig. 1a. The final forging piece is shown in Fig. 1b.
Figure.1 Forging model
Aluminum alloy 6061 was selected as the billet material in the finite element analysis. The first use of SolidWorks software for aluminum alloy blanks, upper and lower die modeling, and saved as stl format export. Then the three-dimensional model of the upper and lower molds and blanks in the pre-forging process is imported into Deform software for numerical simulation.
2. Numerical simulation of the forging process
2.1 Load stroke curve
Pre-forging and forming process, the embryo preheating temperature is 480 ℃, the error is not more than 5 ℃, the mold preheating temperature is 350 ℃, the error is not more than 10 ℃, the upper mold downward speed of 20 mm / s, the friction factor is 0.3, the thermal conductivity of 11W/(mm·℃), up and down the mold divided into grids of 20,000, the workpiece is divided into grids of 12,000, the mold material is steel H13, the mold material 6061, the number of simulation steps is set to 120 steps, set the relationship between objects, then post-processing, simulation of the entire pre-forging process. The material is aluminum alloy 6061, and the number of simulation steps is set to 120; after setting the relationship between the objects, simulation is carried out, and then post-processing is carried out to simulate the whole pre-forging process.
Final forging in the production of a shorter time, but using Deform finite element software can be carefully observed to forge each step. In the final forging process, the preheating temperature of the embryo material is 480℃, with an error of no more than 5℃, the preheating temperature of the mold is 350℃, with an error of no more than 10℃, the downward speed of the mold is 8mm/s, the friction factor is 0.3, and the heat transfer rate is 11W/(mm·℃), the upper and lower molds are divided into grids of 20000. The workpieces are divided into grids of 12000; the molds’ materials are steel H13, and the molds are aluminum alloy 6061. For aluminum alloy 6061, the number of simulation steps is set to 100 steps, with each step downward pressure of 0.752mm, set the relationship between the object after the simulation, and then post-processing, simulation of the entire final forging process. Figure 2 shows the load-stroke curve of the forging process.
Figure.2 Load-stroke curve of the forging process
The figure shows that in the initial stage of pre-forging and final forging, the load rises rapidly with the contact between the upper die and the blank. The load increases steadily as the contact area between the upper die and the blank becomes larger. In the final forming stage, the load rises gradually faster, and the maximum load reaches 6.61×107N.
2.2 Temperature change
In the whole pre-forging process, the contact between the upper mold and the embryo material makes the embryo material deformed until the embryo material fills the mold cavity, which is carried out in a total of 120 steps. The simulation is stored every two steps, and each storage is done independently to view the simulation results of any stored simulation. The temperature variation of the aluminum bar blank profile during the forging process in the 20th and 120th steps is shown in Fig.3.
Fig.3 Temperature profile of aluminum alloy wheel forming process
From Fig. 3a and b, it can be seen that in the whole process of the upper die contacting with the blank until the metal fills the die cavity, the temperature of both ends of the aluminum bar starts to get lower after contacting with the die in the initial stage, which is due to the preheating temperature of the aluminum bar is 480°C. In contrast, the initial temperature of the die is 350°C. The aluminum bar will conduct heat to the upper and lower dies, which lowers the temperature.
The temperature distribution of the aluminum alloy wheel at step 20 and step 100 is obtained through the numerical simulation of the final forging process, as shown in Fig. 3c and d. In the whole process of the upper mold contacting with the blank until the metal overflows the mold cavity, the temperature of the upper and lower surfaces of the final forging blank starts to decrease after contacting with the mold at the initial stage, which is because the preheating temperature of the final forging blank is 480°C, while the initial temperature of the preheating of the mold is 350°C, and the final forging blank transfers heat to the upper and lower molds, which leads to a decrease in the temperature.
2.3 Metal flow law
After analyzing the metal velocity distribution graph in the forging process, the metal flow law can be obtained, the velocity distribution graph of pre-forging is shown in Figures 4a and b, and the velocity distribution graph of final forging is shown in Figures 4c and d. The metal flow law can be obtained by analyzing the metal velocity distribution graph in the forging process.
At the beginning of the pre-forging stage, the upper die and aluminum alloy embryo material upper contact, and the upper end of the first began to deform; the embryo material began to flow, so the upper end of the flow was faster than the lower end. After filling the mold cavity, the metal flow direction is mainly from the local start to the direction of the burr groove movement. The movement speed is larger because the late flow space is small; the maximum speed reaches 51.5mm/s. The metal flow speed is more average at the beginning of the final forging stage. At the 100th step, it can be observed that the metal flow is faster at the edge of the outer rim of the hub. In the final stage of forging, the flow rate at the partially unfilled gaps and fretting grooves is still relatively high, so defects such as fretting may be generated. In the final forming stage, the metal flow rate reaches a maximum of 3.5 mm/s.
Fig.4 Velocity distribution of the forging process
2.4 Equivalent stress field distribution
The equivalent stress field distribution during pre-forging and final forging is shown in Fig.5.
Figure.5 Equivalent stress field distribution in the forging process
As can be seen from the figure, in the early stage of pre-forging, the equivalent stress value is very uniform, the stress concentration is not obvious, and forging defects are not easy to appear. With the deepening of the forging process, the stress at the spoke and the stress at the hub fillet is greater than the stress at the hub, and the maximum stress reaches 69.3MPa, which is easy to produce defects in the forging process. In the initial stage of final forging, the stress change is small, overall, more uniform, not too obvious changes, stable in a range. As the forging process proceeds, the stress becomes significantly larger at the edge of the spoke and rim than in other places. In the final stage, the maximum stress reaches 52.5MPa. The places with large stress are prone to forging defects, so special attention should be paid to the spokes in the pre-forging process and the spoke and rim edges in the final design process.
3. Organizational simulation of the final forging process
Abstract lattice by meta cellular automata, DRX (dynamic recrystallization) extreme discrete type density is 0.02, the boundary movement speed of the grain is 1mm/s, the plastic shear modulus of the material is 260×109Pa, the initial discrete type density is 0.01, and the initial average size of the grains is set to 25 μm. The results of the simulation of the final forging process of aluminum alloy wheels with grain size are shown in Figure 6.
Figure.6 Aluminum alloy wheel hub final forging process grain size simulation
From the figure, it can be seen that the new grain is generated at the boundary of the initial grain and then continuously expanded; the initial grain is continuously compressed; in the forging process, the above process is repeated until the end of forging, and discontinuous dynamic recrystallization occurs during the forging process, at the end of the average size of the grains is about 4.0 μm. The deviation of the different parts is not large. The grain size of each point at the end of the final forging is shown in Figure 7.
The average grain size of point 2 is 4.13 μm, while the average grain sizes of point 1 and point 3 are 3.8 μm and 3.7 μm, respectively. The grains at point 2 are coarser, while the grains at point 1 and point 3 are finer, so the equivalent forces or temperatures applied to point 2 and its accessory parts are smaller than those of point 1 and point 3 during the forging process. The sizes of the majority of grains of point 2 are larger than those of the main sizes of point 1 and point 3. Most of the grains in point 2 are larger than the main size of point 1 and point 3. Coarse grain size will make the toughness and plasticity of forgings reduce fatigue performance decreased. Therefore, point 2 in the forging process is prone to defects such as coarse grains, further due to the lower temperature or stress caused by the smaller, so in the process design, to try to avoid defects.
Figure.7 Grain size and data of each point in the 100th step of the final forging
The simulation of the pre-forging and final forging of the aluminum alloy wheel forging process is carried out by Deform software. The load stroke curve, temperature change, metal motion rules, and equivalent stress field distribution of pre-forging and final forging are analyzed to understand the process feasibility of pre-forging and final forging and the possible defects and possible areas which provide theoretical support for the actual production. In this, by carrying out microscopic grain simulation based on the final forging macro process and analyzing the change of grain size, the simulation results of the final forging macro process are verified with each other to provide a theoretical basis that can be referred to for actual production.
Author: Zhou Zheng, Yang Sha