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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.
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