FEM analysis of the forming process of automotive suspension springs

Berti G. and Monti M. University of Padua, DTG, Stradella San Nicola 3, I-36100 Vicenza (Italy) guido.berti@unipd.it, manuel.monti@unipd.it.

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

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

conditions and different configurations of the coiling machine. The numerical simulations are

performed adopting the FEM software Simufact.Forming 9.0.1.

2. 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 14.50 mm. The formed spring has a

diameter Ø=90 mm and a pitch angle α=60° (Figure 3).

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.

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:

nf K εσ ⋅= (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):

05.02274 εσ ⋅=f (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/m3] Strain hardening exponent 0.05

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.

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]

3. Finite element analysis

The FEM code Simufact.Forming 9.0.1 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.

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 0.05 0.3

Cover plate friction factor 0.05 0.3

Guide friction factor 0.05 0.3

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 2k 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.

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

Friction factor FEM results

Sim

ula

tio

n

Shaped plate

Cover plate

Guide

Configuration

Maximum effective plastic strain

Maximum effective stress [MPa]

Maximum relative damage

1 0.05 0.05 0.05 A 0.333 2151 0.99 2 0.3 0.05 0.05 A 0.350 2157 0.99 3 0.05 0.3 0.05 A 0.337 2153 0.99 4 0.3 0.3 0.05 A 0.362 2161 0.99 5 0.05 0.05 0.3 A 0.335 2153 0.99 6 0.3 0.05 0.3 A 0.363 2161 0.99 7 0.05 0.3 0.3 A 0.354 2158 0.99 8 0.3 0.3 0.3 A 0.374 2164 0.99 9 0.05 0.05 0.05 B 0.182 2088 0

10 0.3 0.05 0.05 B 0.180 2087 0 11 0.05 0.3 0.05 B 0.179 2086 0 12 0.3 0.3 0.05 B 0.186 2090 0 13 0.05 0.05 0.3 B 0.220 2108 0.248 14 0.3 0.05 0.3 B 0.202 2094 0.248 15 0.05 0.3 0.3 B 0.217 2103 0.495 16 0.3 0.3 0.3 B 0.207 2096 0.248

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.

Table 5. Main effects

Friction factor

shaped plate cover plate guide

Configuration

Effective plastic strain 0.008 0.006 0.020 -0.154

Effective stress [MPa] 1 1.500 8.000 -63.250

Relative damage -0.031 0.031 0.155 -0.835

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.

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) 20. 1155 -

1174.

[2] Ardehali Barani, A., Li, F., Romano, P., Ponge, D., Raabe, D.: Design of high-

strength steels by microalloying and thermomechanical treatment. Materials Science

and Engineering, 463 (2007) 9. 138 - 146.

[3] Simufact Technical References: Crack prediction in massive forming via simulation,

2009.

[4] Lemaitre, J., Chaboche, J. L.: Mechanics of Solid Materials. Cambridge University

Press, 1990.

[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, 2009.

[6] Berti, G., Monti, M., Salmaso, L.: Introduzione alla metodologia DoE nella

sperimentazione meccanica. Padova, CLEUP Ed., 2002.