As one of the most important parts in the chassis system, the control arm plays a role of force transmission, guidance and support in the suspension, and its comprehensive mechanical properties have a great impact on the maneuverability and stability of the vehicle during driving [1-2]. With the development of lightweight technology, the research and production of lightweight materials such as aluminum and magnesium instead of steel have gradually increased, and the forming methods have become more diversified [3-4]. Compared with other processes, the finished parts obtained by forging have better properties. In this paper, the forging of the upper right control arm of an automobile aluminum alloy is taken as the research object. Combining with forming process theory and Computer Aided Engineering (CAE), the forming process routes of two-pass roll forging, two-station bending, pre-forging and final forging are formulated, and Solidworks is used The software designed the corresponding mold of each process. By using the finite element analysis software Deform, the forging process of the control arm was simulated under certain process parameters, and the rationality of the forming process and die design was verified by the analysis and simulation results. In addition, on the basis of the above research, the preforging sequence was selected and the single factor method was used to study the influence law of blank initial forging temperature and friction factor on forging forming, which provided a reference for the selection of die forging forming process parameters of 6082 aluminum alloy control arm. At last, the experiment is verified with actual production.

Design of forging process of control arm

1.1 Process analysis of control arm

The forging of the aluminum alloy control arm is shown in Figure 1, and its overall shape is U-shaped. In addition to the nearly cylindrical ends, the forging is perpendicular to the bending axis of the I-shaped section, the upper and lower asymmetry, the thickness of the middle web is small, and the local structure of the bar on the edge of the two arms is complex. The forging material is 6082 aluminum alloy. As a typical 6000 series aluminum alloy, it is widely used in automotive structural parts due to its excellent comprehensive properties such as high specific strength, excellent thermoplastic forming ability and strong corrosion resistance [5-6]. By analyzing the structure of the forging, it can be seen that the forming difficulties are as follows: (1) the cross section area of the forging along the bending axis changes dramatically, and the material volume needs to be allocated reasonably; (2) The cross section area of the connecting flange at both ends, the middle and the big arm side of the forging is large, and it is easy to fill defects; (3) In the process of forming I-section, folding defects are easy to occur. Therefore, the control arm studied in this paper adopts the following forming scheme: blanking - heating - forming - pre-forging - final forging - cutting, in which the forming process includes roll forging and bending.

1.2 Finite element model establishment

After completing the billet and mold modeling of each process through the 3D modeling software Solidworks, the billet and mold were exported into STL format and imported into Deform. Parameters of each process are set according to production experience, as shown in Table 1. In order to better fit the deformation in actual production, the billet shape at the end of the previous process is simulated as the beginning of the next process. In order to improve the simulation efficiency, the number of grids is 60000, and the volume compensation of billets is carried out. The pre-forging forming process is more complicated, and the mesh is redivided into 160,000 with a size ratio of 5. The friction model is shear friction which is more suitable for volume forming. Taking into account the biting of billet, the friction factor of roll forging sequence is 0.7, and that of bending, pre-forging and final forging sequence is 0.4. The heat transfer coefficient between billet and die is 11 N·(s·mm·℃)-1, and the convective heat transfer coefficient between billet and air is 0.02 N·(s·mm·℃)-1.

Table 1 Setting of finite element simulation parameters

Numerical simulation of preforming process

2.1 Roll Forging

During roll forging, the metal billet is subjected to continuous pressure from a pair of counter-rotating dies and is subjected to local plastic deformation through contact with the roll forging groove. This forging method can effectively redistribute the volume of the metal billet along the length direction and improve the utilization rate of the material. According to formula (1), the calculation blank drawing shown in Figure 2 is obtained, and the roll forging blank drawing is simplified and used as the basis for the design of roll forging dies. The maximum cross-sectional area of roll forging blank is the cross-sectional area of blanking blank. According to the principle of constant volume, extrusion bar material with size of 55 mm×395 mm is used for blanking.

Fig.2 Calculated billet chart

Where: FA is to calculate the cross-sectional area of the blank; Fd is the cross-sectional area of the die forging; η is the flash filling coefficient; Fm is the cross-sectional area of the flash.

Too many roll forging passes will affect the production efficiency. Too few roll forging passes and too much reduction may lead to flash, folding and other defects [7]. Therefore, it is necessary to reasonably determine the number of roll forging passes, and its calculation formula is shown in equation (2).

Where: n is the number of roll forging passes; λz is the total elongation coefficient of roll forging. λp is the average elongation coefficient. Calculated n=2.18, using two roll forging.

After the cross section selection and longitudinal size calculation are completed, the roll forging die model is obtained by using the stretching, lofting, scanning and combination operations in Solidworks software [8]. FIG. 3 shows the two-pass roll forging die obtained after multiple simulation analysis and mold structure optimization. When pre-processing in Deform, the flow constraint is set on one end to simulate the clamping effect of the fixture on the billet. As can be seen from FIG. 4a, during the first roll forging pass, the equivalent strain in the deformation section when the thick end of the billet is in front is significantly smaller than that when the thin end is in front. According to the results of the second pass, the equivalent strain is larger in the region with large deformation. FIG. 4b shows the velocity field distribution at the late stage of the two-pass deformation. The metal materials flow axially along the forging, with good flow line shape and no interweaving and turbulence, indicating that the billet is stressed evenly in the upper and lower groove.

Fig.3 Roll forging dies
(a) The first pass (b) The second pass

Fig.4 Roll forging simulation results

(a) Equivalent strain fields (b) Velocity fields

2.2 Bending

Bending is to make the forging better filled in the die forging, the billet through bending deformation to make its shape closer to the final forging. Therefore, the shape of the bending die is mainly determined by the contour shape of the control arm forging. Due to the limitation of the shape of the forging, the cross-sectional area of the two ends of the roll forging is much larger than the cross-sectional area of the deformation section, that is, the rebate is formed at the U-shaped opening, and the bending can not be formed at one time. Therefore, a bending process is added, two-step bending is adopted, the two arms are prebent at a certain Angle, and then the bending of the bottom of the U-shaped forging is completed. Figure 5 shows the equivalent stress distribution after each pass bending. The analysis shows that both passes bending are local deformation, mainly distributed in the part in contact with the die, and the overall equivalent stress is small. After two times of bending and deformation, the bending parts conform to the shape of pre-forging.

Fig.5 Bending simulation results
(a) The first pass (b) The second pass

Numerical simulation of preforging process

3.1 Thermodynamic coupling analysis of forging forming

Preforging is an important preforming step to create better conditions for the final forging, and it is also a process with the largest deformation degree in the forming process of the control arm forging. Therefore, the quality of pre-forging will directly affect the quality of forming forgings. Figure 6 is the model of the pre-forging die, and the numerical simulation results are shown in Figure 7. The pre-forging is well filled and there is no folding defect in the forging area. Figure 7a shows the temperature distribution law of the pre-forging. The lowest temperature area is at the two ends and the high bar that is inside the two connecting ends, while the highest temperature part appears at the connection between the edge of the pre-forging and the bridge part of the flash. Combined with the analysis of equivalent stress field (FIG. 7b) and velocity field of A-A cross-section (FIG. 7d), we can see: In the process of die pressing, the large cross section area at both ends and in the middle preferously contacts with the upper die and starts filling. Due to the small deformation degree, the equivalent stress in these areas is small, and the heat generated by deformation is less than the heat lost in contact with the die. Therefore, the temperature here is low, and surface coarse crystals may be generated. As the forging die continues to press down, excess metal is extruding from the die to the flyside. Due to the minimum height of the bridge, the equivalent stress here is greater than 107 MPa, indicating that the metal at the flyside bridge is subjected to greater flow resistance, intense deformation and faster flow speed, and heat is generated by deformation and friction, resulting in increased temperature at the bridge connection [9]. As can be seen from FIG. 7c, the equivalent strain distribution during preforging is similar to the equivalent stress distribution, and the overall strain is uniform, indicating that no local severe deformation occurs during preforging. By observing the velocity field at the A-A section of the middle boss of the forging, it is found that the metal flow velocity in the forging area is small and uniform, and there is no turbulence and interweaving.

Fig.6 Pre-forging dies
(a) Upper die (b) Lower die

Fig.7 Pre-forging simulation results
(a) Temperature field (b) Equivalent stress field (c) Equivalent strain field (b) Velocity field of A-A cross-section

3.2 Influence of blank initial forging temperature on pre-forging

Different from steel billet, aluminum alloy has a narrow forging temperature range. Relevant studies [10-11] show that forging aluminum alloy within a reasonable forging temperature range and appropriately increasing the initial forging temperature of billet can help improve metal flow, reduce deformation resistance, improve metal filling effect and eliminate coarse crystals. In order to get the forging temperature more suitable for this forging, the initial forging temperature of the blank is changed when other process parameters are set unchanged. As can be seen from FIG. 8, with the increase of the initial forging temperature of the blank, both the minimum and maximum temperatures of the final pre-forging increase linearly. The average equivalent stress decreases with the increase of initial forging temperature. Under the initial forging temperature of each blank, the final forging temperature did not reach the overburning temperature of 590 ℃. According to literature [12], when the temperature reaches 570 ℃, the comprehensive performance will be decreased. When the initial forging temperature is 430 ℃ and below, the minimum temperature is less than 400 ℃; When the initial forging temperature is 510 ℃, the maximum temperature is close to 570 ℃, there is a risk of overall performance decline. To sum up, the reasonable range of initial forging temperature of blank during pre-forging is 450-490 ℃.

Fig.8 Effect of billet intitial forging temperature on pre-forging

3.3 Influence of friction factor on pre-forging forming

In the process of aluminum alloy forging, the fluidity of aluminum alloy is poor and the viscosity is large. Lubrication of the die during forging can effectively improve metal flow and avoid mold sticking, which will also affect the forming quality of the forging [13]. FIG. 9 shows the load-travel curve of the upper die under different friction factors. As the friction factor decreases, the load decreases significantly. At the early stage of forging, when the die is in contact with the billet with a high cross-section height, the load difference is not obvious because of the local deformation; As the die travel continues, the load gap gradually increases, when the web and ribs begin to fill, the load value difference is large. When the friction factor is 0.1, the preforging forming load is the smallest and the maximum load is 6.45×106N.

Fig.9 Effect curves of friction factor on upper die load

FIG. 10 shows the influence of different friction factors on the maximum equivalent strain. In forging, the area with large equivalent strain is often the difficult part of forging. With the increase of friction factor, the maximum equivalent strain of preforging increases. Therefore, it is of practical significance to improve the metal filling of the hard-to-form part of the forging and to increase the life of the die.

Fig.10 Effect curve of friction factor on maximum equivalent strain

Simulation and test of final forging process
 

The final forging die is designed according to the final forging by adding 0.8% of the hot and cold scaling ratio and taking into account the flash edge. The blank is the pre-forging die obtained when the initial forging temperature of the blank in Section 3 is 450 ℃ and the friction factor is 0.4. The shape of the final forging is similar to that of the pre-forging, and its deformation is mainly the finishing of the local structure. It can be seen from FIG. 11 that the temperature distribution of the final forging is basically the same as that of the pre-forging. After the final forging, the lowest temperature of the forging increases somewhat, no folding defects occur in the forging area, and the filling condition is good.

Fig.11 Simulation result of final forgings
 

Based on the numerical simulation results, a pilot production test was conducted on the sample, and the forgings after trimming were shown in Figure 12. The key parts of the trial forging, such as the two ends, the middle and the big arm flange, are well filled, which is consistent with the shape of the simulation forging, indicating the rationality of the forming process design and the reliability of the finite element simulation results.

Fig.12 Forgings after trimming

conclusion

(1) The structural characteristics and forming difficulties of automobile aluminum alloy control arm are analyzed, and the forming processes of two-pass roll forging, two-station bending, pre-forging and final forging are formulated, and the corresponding dies of each process are designed. Deform software is used to simulate the forging process of aluminum alloy control arm. The results show that each process is in good shape, the final forging is basically complete and there is no folding in the forging area, and the rationality of the process plan and die design is also explained. (2) The influence of blank initial forging temperature and friction factor on pre-forging forming was studied by single factor method. The results show that with the increase of blank forging temperature, the temperature of forming preforging increases, and the average equivalent stress decreases. When the initial forging temperature of blank is 450-490 ℃, the temperature distribution of preforging is more reasonable. With the decrease of friction factor, the load value of preforging die decreases, the maximum equivalent strain decreases, the lubrication is improved, the metal flow forming is beneficial, and the die life is prolonged. (3) According to the simulation results to guide the production test, the control arm forgings with good filling and no defects are obtained.