FEM analysis of the forming process of automotive suspension springs

Transcription

1 FEM analysis of the forming process of automotive suspension springs Berti G. and Monti M. University of Padua, DTG, Stradella San Nicola 3, I Vicenza (Italy) Abstract This paper deals with the FEM analysis of the forming process of automotive suspension coil springs. Nowadays in automotive industry great efforts are spent in achieving weight reduction of cars components. The coil springs are not exempt. For this component weight reduction can be obtained by reducing the spring wire diameter. However, to assure that springs maintain the required mechanical properties, it is necessary to adopt material with high strength [1]. Concerning the manufacturing process of coil springs, the trend is to produce springs by cold forming of strain hardened wires. In this case the wires are subjected to heat treatments of hardening and tempering before the coiling process. This leads to an improvement of productivity since it is not required an heat treatment after forming. However forming a high strength material already hardened and tempered is critical. The material has a very low ductility that can lead to coil's failure during forming process. The process is sensitive both to the process conditions (such as friction), and to the set up of the spring making machine. Aim of this work is the numerical investigation of the coiling process. Different friction conditions and different configurations of the coiling machine are considered. The main target is the determination of a satisfactory lubricant condition and a configuration of the coiling machine which allows the correct production of the spring without any breakage of the wire. Keywords: Finite element method, Lemaitre damage model, process optimization, spring forming. 1. Introduction The first automotive coil spring was on the model-t (Ford) in 1910 (top speed 25 miles/hr). The earliest coil spring material used had approximately a 500 MPa design stress level. Coil spring materials have developed to the point where today it is common to have a coil spring

2 with a design stress of around 1200 MPa. Springs are made using new materials that provide excellent structural performance while reducing weight and the cost of manufacturing. For these requirements high-strength martensitic steels are widely used. Those steels are used today also in airframes including landing gear components, shafts, gears and in automotive structures as stabilizers [1, 2]. Concerning the manufacturing process of coil springs, the trend is to improve the productivity. To do this the production of springs is obtained by cold forming of strain hardened wires (produced by drawing operations). In this case, before the coiling process is performed, the wires are subjected to heat treatments of hardening and tempering. Since it is not required an heat treatment after forming, this kind of manufacturing process leads to an improvement of productivity. However forming an high strength material already hardened and tempered is critical. The material has a very low ductility that can lead to coil's failure during the forming process. This is sensitive both to the process conditions (such as friction), and to the set up of the spring making machine. Requirements of lubricants for spring forming operations are more severe than for most other metalworking operations. The high pressures that may be reached require special lubricants to prevent galling, seizure, or fracture of the wire, as well as excessive tool wear. Improper lubricating oils or compounds interfere with close-tolerance work and cause variations in the finished parts. The most usual lubricant is the one that comes on the oil-tempered grade of spring wire. During heat treatment, oxidation of the surface is permitted under carefully controlled conditions. The oxide layer thus formed acts as a lubricant during coiling. Its characteristics must be carefully controlled with respect to thickness, adherence and flakiness. Considering the coiling machine, low lubrication is required in order to allow that the feeding rolls perform the wire's feed. On the other hand, high friction in the forming zone leads to high forming forces and therefore to high normal and tangential loads on the forming tools. For this reason idle rolls are adopted as forming tools; this kind of tools leads to rolling friction condition. The final geometry of the spring depends on the geometry and on the configuration of the forming rolls and therefore can not be considered a variable in the optimization of the forming process. The aim of this work is the numerical investigation of the coiling process. In the first part the paper details the industrial case; it consists of the spring forming of a strain hardened wire by means of a Wafios machine. The Company producer of the springs evidenced some breakage of the wire during the production. In the second part a satisfactory lubricant condition and a configuration of the coiling machine which allows the correct production of the spring without any breakage of the wire is determined taking into account different friction

3 conditions and different configurations of the coiling machine. The numerical simulations are performed adopting the FEM software Simufact.Forming The industrial case The industrial case consists in the manufacturing process of coil springs produced by cold forming of a strain hardened wires. The spring forming machine (Wafios) adopted by the Company producer of the springs is shown in Figure 1. Figure 1. The spring making machine. The main parts of the spring forming machine are: a set of feeding rolls. The wire's feed speed is 0.67 m/s, a couple of idle rolls are adopted as forming tools, a shaped plate is used to direct the wire toward the forming rolls, a cover plate is adopted to assure contact between the wire and the shaped plate. An example of the springs manufactured by the Company is shown in Figure 2. The spring case of study is produced from a wire having diameter of mm. The formed spring has a diameter Ø=90 mm and a pitch angle α=60 (Figure 3).

4 Figure 2. Examples of springs produced by the Company. Figure 3. Spring case of study (Ø=90 mm, α=60, d=14.50 mm). Spring is made of a high-strength martensitic steel (54SiCr6). The wire to be formed is obtained by cold drawing operations. After drawing the wire is subjected to heat treatments of hardening and tempering. During heat treatment, oxidation of the surface is permitted; the resulting oxide on the wire surface will act as a lubricant during subsequently coiling operation. The drawn and heat treated wire was characterized by tensile test. The relevant nominal stress strain curve is shown in Figure 4. Figure 4. Nominal stress strain diagram of the drawn and heat treated wire. The true stress true strain curves pertaining to drawn and heat treated wire is shown in Figure 5.

5 Figure 5. True stress true strain diagram of the drawn and heat treated wire. The curve relevant to the wire material was approximated by means of the Hollomon constitutive law: σ f n = K ε (1) where ε is the deformation (total strain), K is the strength coefficient and n is the strain hardening exponent. Fitting of the experimental data led to the following true stress-true strain curve (which is also shown in Figure 5): 0.05 σf = 2274 ε (2) The elasto-plastic constants of the material are summarized in Table 1. Table 1. Elasto-plastic constants of the material. Basic material constants Plastic material constants Young's Modulus 200 [GPa] Minimum yield stress 1663 [MPa] Poisson's ratio 0.28 Yield constant 2274 [MPa] Density 8027 [kg/m 3 ] Strain hardening exponent 0.05

6 The data obtained from tensile test are also used to determine the parameters of the Lemaitre damage model [3] according to the damage mechanics theory of Chaboche and Lemaitre [4]. The damage parameters are summarized in Table 2. Table 2. Parameters of the Lemaitre damage model. Critical damage 0.34 Maximum stress tensile test [MPa] 1867 Damage resistance parameter 1.76 Equivalent strain at maximum stress 0.05 During coiling process the Company evidenced some breakages of the wire. Some examples of coil's failure are reported in Figure 6. Figure 6. Examples of coil's failure during forming process. The investigation performed to detect the causes of coil's breakage [5] indicated that the possible causes of failure during coiling process are: i) configuration of the coiling machine, ii) low ductility of wire's material and, iii) friction conditions.

7 Concerning the coiling machine, the analysis of the forming process indicated that: i) geometry and configuration of the forming rolls determine the final geometry of the spring and therefore can not be considered in the optimization of the forming process and, ii) the position of the shaped plate respect to the forming rolls affect the formability of the coils. Springs are made using new materials that provide excellent structural performance while reducing the cost of manufacturing. For these requirements high-strength martensitic steels are widely used. Springs are produced by cold forming of wires which are subjected to heat treatments of hardening and tempering before the coiling process. Forming an high strength material already hardened and tempered is critical. The material has a very low ductility that can lead to coil's failure during forming process. However, unless a redesign of the whole production cycle, material can not be considered for the optimization of the coiling process. Regarding friction, during coiling process, the Company adopts as lubricant the oxide formed during heat treatment after drawing operations. Therefore thickness and adherence of oxide play an important role in determining friction conditions between wire and the part of coiling machine where sliding contacts are present (shaped plate, cover plate, guide). Different wires presenting both coil s breakages and no breakages have been analysed in the metallurgical laboratory. The oxide thickness was measured analyzing the images of the cross section of the wire samples acquired from a digital microscope. The comparison indicates that the wires with good formability (no breakages) present an oxide layer more thick and adherent (Figure 7) than the others (Figure 8). Figure 7. Wire with good formability. Oxide thickness: 14 [µm] Figure 8. Wire which presented coil's failure during forming process. Oxide thickness: 2 [µm]

8 3. Finite element analysis The FEM code Simufact.Forming is used to perform the 3D mechanical analysis of the coiling process. The wire is fed at the constant velocity of 0.67 m/s by means of an hydraulic press imposed to the pulling system (in order to simplify the FE model, feeding rolls are used only as support for the wire). A rotation axis in y direction is imposed to the forming rolls. Hexahedral elements are adopted to mesh the wire with an element edge size of 3.5 mm. The adopted 3D model is shown in Figure 9. The wire is assumed elastoplastic and relevant constants are reported in Table 1. The parameters of the Lemaitre damage model (Table 2) are also introduced in the material definition. Figure 9. FE model of the forming process of automotive suspension springs. In order to determine a satisfactory lubricant condition and a configuration of the coiling machine which allows the correct production of the spring without any breakage of the wire, different friction conditions as well as different configurations of the coiling machine are considered. Concerning friction condition, the presence/absence of oxide is simulated adopting low/high friction factor at the interface between wire and the part of coiling machine where sliding contacts are present (shaped plate, cover plate, guide). Tresca law is used to model the friction stress τ. Levels of friction factor to be considered in the optimization of the coiling process are reported in Table 3.

9 Table 3. Friction factor at the interface between wire and the part of coiling machine adopted in the simulations. Low High Shaped plate friction factor Cover plate friction factor Guide friction factor Regarding the position of the shaped plate respect to the forming rolls, two different configurations have been explored (Configuration A and Configuration B in Figure 10). Figure 10. Configurations of the coiling machine adopted in the simulations. Design of Experiments (DoE) techniques are used to define the simulation plan. A 2 k factorial design is chosen [6]. Four factors (k=4) are considered and two levels are assigned to each of them. The maximum effective plastic strain, the maximum effective stress and the maximum relative damage are observed as responses. 4. Results and discussion The design matrix and relevant results of FEM simulations is reported in Table 4. Some images relevant to numerical results of coiling process simulations are shown in Figure 11.

10 Figure 11. Numerical results of coiling process simulations. Simulation 8. Relative damage Simulation 11. Effective stress Table 4. Design matrix and results of FEM simulations Simulation Shaped plate Friction factor Cover plate Guide Configuration Maximum effective plastic strain FEM results Maximum effective stress [MPa] Maximum relative damage A A A A A A A A B B B B B B B B The obtained FEM results allow the calculation of the main effects. They represent the means of the responses variables for each level of the four factors. The results are reported in Table 5 and relevant graphs (main effects plots) are shown in Figure 12, 13 and 14.

11 Table 5. Main effects Friction factor Configuration shaped plate cover plate guide Effective plastic strain Effective stress [MPa] Relative damage Figure 12. Main effects plots of effective plastic strain. Figure 13. Main effects plots of effective effective stress. Figure 14. Main effects plots of relative damage. It is possible to remark that the main effects for shaped plate friction factor, cover plate friction factor and guide friction factor are much smaller than the main effect for configuration. Moreover all responses increases when friction factor increases and decreases when configuration B is adopted.

12 On the basis of these results it is possible to conclude that low friction conditions at the interface between wire and the part of coiling machine where sliding contacts are present (shaped plate, cover plate, guide) as well as the configuration of the coiling machine shown in Figure 10 (Configuration A) allow the correct production of the spring without any breakage of the wire. The obtained results are confirmed by actual industrial production: wires having an oxide thick and adherent evidenced good formability respect to wire with an oxide thin and not adherent. 5. Conclusions In this paper the optimization of the forming process of automotive suspension coil springs is presented. By means of the FEM software Simufact.Forming 9.0.1, different friction conditions and different configurations of the coiling machine are considered. The conclusions is that low friction conditions at the interface between wire and the part of coiling machine where sliding contacts are present (shaped plate, cover plate, guide) as well as a configuration of the coiling machine allow the production of the spring with low levels of stress and damage on formed wire. 6. References [1] Prawoto, Y., Ikeda, M., Manville, S.K., Nishikawa, A.: Design and failure modes of automotive suspension springs. Engineering failure analysis, 15 (2008) [2] Ardehali Barani, A., Li, F., Romano, P., Ponge, D., Raabe, D.: Design of highstrength steels by microalloying and thermomechanical treatment. Materials Science and Engineering, 463 (2007) [3] Simufact Technical References: Crack prediction in massive forming via simulation, [4] Lemaitre, J., Chaboche, J. L.: Mechanics of Solid Materials. Cambridge University Press, [5] Berti, G., Monti, M.: Indagine comparativa dell influenza di rugosità e morfologia della superficie esterna sulla formabilità di fili trafilati pretemprati. Vicenza, DTG Internal report, [6] Berti, G., Monti, M., Salmaso, L.: Introduzione alla metodologia DoE nella sperimentazione meccanica. Padova, CLEUP Ed., 2002.