An analysis of the forging processes for 6061 aluminum-alloy wheels

Y.H. Kim, T.K. Ryou, H.J. Choi, B.B. Hwang*

Department of Industrial Automation Engineering, Inha University, 253 Yonghyun-dong, Nam-ku, Inchon 402-751, South Korea

Received 15 June 2001; accepted 22 January 2002

Abstract

The metal forming processes of aluminum-alloy wheel forging at elevated temperatures are analyzed by the finite element method. A

coupled thermo-mechanical model for the analysis of plastic deformation and heat transfer is adapted in the finite element formulation. In

order to consider the strain-rate effects on material properties and the flow stress dependence on temperatures, the rigid visco-plasticity is

applied to the simulation. Several process conditions were applied to the simulation such as punch speed, rim thickness, and the depth of die

cavity. Experiment for a simplified small-scale model is carried out and compared with the simulation in terms of forging load to verify the

validity of the formulation adapted in this study. Then, various processes with full-scale model for a 6061 aluminum-alloy wheel are

simulated. Material flow, pressure distributions exerted on the die wall, temperature distributions and forging loads are summarized as basic

data for process design and selection of a proper press equipment. # 2002 Published by Elsevier Science B.V.

Keywords: Thermo-viscoplastic finite element analysis; 6061 Aluminum-alloy wheel; Strain-rate effect; Punch speed; Rim thickness; Forging load

1. Introduction

Large plastic deformations, without a failure of formed

parts, are possible in hot forging processes since flow

stresses are dramatically reduced and work hardening does

not occur very much at elevated temperature. Therefore, hot

forging processes are often used in the case of large deforma-

tions and limited capacity of press equipment because the

flow stresses and forming energies are decreased at evaluated

temperature. Aluminum alloys are widely utilized in auto-

motive and aircraft industry due to their various advantages

such as lightness, good forgeability, high wear resistance, etc.

[1] The 6061 aluminum alloys applied for the wheel forging

processes in the present study have more desirable character-

istics in those aspects than other materials.

Quantitative analysis for material flow, die chilling, form-

ing loads and temperature distributions, etc., is required for a

successful design of optimized process in hot forging opera-

tions. Also, forming conditions such as material properties,

friction and lubrication, cooling effect, punch speed, and the

initial temperature of die and workpiece, etc., which affect

to flow stress and forgeability, are needed to be carefully

reviewed [2,3].

Recently, the finite element analysis of a forming process

is regarded as a useful technique for process design and

selection of a proper working condition. Because of the

strain-rate sensitivity of a material at the elevated tempera-

ture, the visco-plastic formulation is required for a proper

process analysis in hot forging. These formulations were

introduced and generalized by Perzyna [4] and Cristescu [5],

respectively. The current study in this paper is performed by

introducing the coupled analysis model of deformation and

heat transfer suggested by Rebelo and Kobayashi [6,7].

The simulation and experiment for a simplified small-

scale model is carried out and the results of them are

compared in terms of forging load to verify validity of

the formulation adapted in this study. Then, various hot

forging processes for the full-scale model of an automotive

aluminum wheel are simulated and summarized in terms of

forging loads, temperature distributions, etc. The results

from simulations could offer basic data for the selection

of a proper press equipment and process design especially

for forging speed. Also, it is simulated by means by ALPID

(analysis of large plastic incremental deformation) simula-

tion [8], which is finite element code to analyze the forging

loads, temperature distributions, etc.

2. Experiment and analysis

The product geometry of an aluminum-alloy wheel as

forged before machining is shown in Fig. 1. As seen in the

figure, the structure of wheel is composed of rim and

web. The lengthy rim of wheel is formed by forward and

Journal of Materials Processing Technology 123 (2002) 270–276

* Corresponding author.

0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V.

PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 0 0 8 7 - 0

backward extrusion, and then cut by machining to a proper

size. Finally, rim is roll-formed to a complete product shape.

The main purpose of this study is to select proper press

equipment for forging 6061 aluminum-alloy wheel and to

review the effects of process variables such as punch speed

and rim thickness, etc. on forging load. In the current

analysis, simulation of a simplified small-scale model is

implemented and the forming load obtained from simulation

is compared with that from experiment. Then, various

processes with a real model for a 6061 aluminum-alloy

wheel are simulated to get extensive results.

Fig. 2 shows the geometry of initial stock and tooling for

a simplified small-scale model and a full-scale model,

respectively. Punch nose is designed to decrease forging

load and to remove the workpiece from punch conveniently

after forging (Fig. 2(a)). As seen from the ratio of the forged

rim length to the initial workpiece height, the material

undergoes very severe deformation and the process is per-

formed in one-step forming operation.

A hydraulic press of 800 tonf pressing capacity with an

accumulator drive is used in the experiment with a simplified

small-scale model. Though punch speed is not exactly

measured during the forging process in experiment, the

punch speed is assumed to be 0.1 m/s at initial forging stage

and 0.03 m/s near the end of operation within the speed

range expressed in Ref. [1]. Thus, both constant and variant

punch speeds through forging strokes are applied to simula-

tions with simplified and real model, respectively. In the case

of an accumulator driven system, the punch speed becomes

slow as the stroke proceeds [1]. Therefore, four distin-

guished average speeds between the start and end of forging

are employed in simulations and the average speeds in each

step are shown in Table 1 [1]. As also seen from the table, the

average speed in each step is presented as a percentage of the

initial deformation speed.

The process conditions used in the current analysis are

shown in Table 2. In order to compare the forging load from

simulation with that from experiment, the pre-analyses with

a simplified small-scale model are performed under the

process conditions such as rim thickness of 6 mm, constant

punch speed of 0.03 m/s, and variant punch speed as referred

in Table 2. Six different cases with full-scale model are

conducted in the simulation with two constant punch speeds,

viz. 0.1 and 0.2 m/s, for rim thickness of 7 and 8.5 mm,

respectively, and with variant punch speed (0.03–0.2 m/s

during stroke) for rim thickness of 7 and 8.5 mm, respec-

tively. The results obtained from simulation are compared

with each case in terms of forging loads, etc.

In order to see the deformation behavior as a function of

time in hot forging processes, the visco-plastic flow stress is

Fig. 1. Wheel configuration just after forging.

Fig. 2. Geometry of initial stock and tooling: (a) simplified small-scale model; (b) full-scale model.

Table 1

Punch speed during stroke

Total

stroke (mm)

20% of total stroke 40% of total stroke 34% of total stroke 6% of total stroke

Stroke

(mm)

Average speed

(mm/s) (%)

Stroke

(mm)

Average speed

(mm/s) (%)

Stroke

(mm)

Average speed

(mm/s) (%)

Stroke

(mm)

Average speed

(mm/s) (%)

Ref. [1] 76.2 15.3 220 (100) 30.4 160 (72.7) 26.1 78 (35.5) 5.1 13 (5.9)

Simplified model 30 6 300 (100) 12 200 (66.7) 10 100 (33.3) 2 20 (6.7)

Real model 21 4 300 (100) 8 200 (66.7) 7 100 (33.3) 2 20 (6.7)

Y.H. Kim et al. / Journal of Materials Processing Technology 123 (2002) 270–276 271

introduced and expressed as in Eq. (1) [9]. The rate-sensitivity

of the flow stress is given in Eq. (2) [9].

s¼sinh�1f½expð1:852� 104=TðKÞÞ�6:803�10�11 _e�1=4:940g

0:011

�ðMPaÞ (1)

ds

d_e¼ 18:403K

_e0:798

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 þ K2 _e

0:405q ðMN=m

2sÞ (2)

where

K ¼ exp1:852 � 104

TðKÞ

� �� 6:803 � 10�11

� �1=4:940

The thermal properties of workpiece and dies are given in

Table 3 [10].

3. Results and discussion

3.1. Comparison with experimental data

(small-scale model)

The forging load obtained from pre-analysis is compared

with that from experiment. As shown in Fig. 3, the maximum

forging load from pre-analysis with constant punch speed of

0.03 m/s for full stroke is in good agreement with that from

experiment. Fig. 4 represents the load–stroke relationships

from simulation with a variant punch speed in the simplified

model. The punch speed increases stepwise at points ‘‘B’’,

‘‘D’’, and ‘‘E’’ in the figure as the stroke proceeds. At point

‘‘C’’, forward and backward extrusions start at the same time

and the punch load steeply increases. At point ‘‘E’’, the

punch load rapidly decreases due to a sudden change of

punch speed in simulation. The maximum forging load in

this case is also in good agreement with that from experi-

ment. Therefore, it can be concluded that the forging loads

from simulations could be utilized as a useful information

for selecting press equipment with proper load capacity.

3.2. Analysis for full-scale model

Several simulations of hot forging processes with a full-

scale model of an automotive 6061 aluminum-alloy wheel

were performed to provide information in selecting a proper

press equipment in terms of load capacity, etc. Results from

simulation are discussed as follows.

Fig. 5 shows the pressure distribution exerted on the die

wall for constant punch speeds of 0.1 and 0.2 m/s with rim

thickness of 7 and 8.5 mm, respectively. As shown in the

figure, the maximum pressure exerted on the die is about

1000 MPa, which tells that the tooling could be operated

safely. The maximum pressure exerted on the die wall is

recommended to be less than about 4000 MPa for the safe

tooling [11]. It can be concluded from the simulation results

that the pressure distribution exerted on the die is very much

Table 2

Process conditions

Simplified pre-analysis Main analysis

Friction factor 0.2 0.2

Initial billet temperature (8C) 450 450

Initial punch and die temperature (8C) 200 200

Environment temperature 27 27

Punch speed (m/s) 0.03 Variant 0.2 0.1 Variant

Rim thickness (mm) 6 6 7 8.5 7 8.5 7 8.5

Table 3

Thermal properties

Workpiece (6061) Dies (H13)

Conductivity (N/s/K) 240 28.4

Density � heat capacity (N/mm2/K) 2.8 3.676

Heat transfer coefficient to the

environment (N/s/mm/K)

0.007

Heat transfer coefficient of

the lubricant (N/s/mm/K)

35.02

Radiation coefficient�Boltzmann constant (N/s/mm/K4)

85 � 10�13

Fig. 3. Comparison of punch load between experiment and simulation.

272 Y.H. Kim et al. / Journal of Materials Processing Technology 123 (2002) 270–276

similar for all cases. The maximum values of pressure

exerted on the die for each case of simulation are summar-

ized in Table 4.

In final forming stage, nodal velocity field for punch speed

of 0.2 m/s and rim thickness of 7 mm is presented in Fig. 6.

As seen in the figure, flow velocities are relatively large in

the top region of rim, compared with the velocities distrib-

uted in the web. On the other hand, the flow velocities in

the bottom region of rim are practically negligible. The

results show that forward extrusion is completed earlier than

backward one in the forming processes of a 6061 aluminum-

alloy wheel. In the cases of different punch speed and rim

thickness condition, nodal velocity field turned out to be

quite similar.

Just after forging, the temperature distributions within

the workpiece, the punch and the die are shown in Fig. 7 for

a constant punch speed of 0.1 m/s for a rim thickness of

7 mm and for a constant punch speed of 0.2 m/s with a rim

thickness of 8.5 mm, respectively. A steep temperature

gradient can be seen near the contact region between the

Fig. 4. Load–stroke curve with variant punch speed in simplified model.

Fig. 5. Pressure distributions. (a) Constant punch speed: 0.1 m/s, r.t.: 7 mm (left), 8.5 mm (right). (b) Constant punch speed: 0.2 m/s, r.t.: 7 mm (left), 8.5 mm

(right).

Table 4

Summary of die pressures, temperatures, punch loads and consumed energies

Punch speed (m/s) 0.2 0.2 0.1 0.1 Variant

Rim thickness (mm) 7 8.5 7 8.5 7 8.5

Maximum die pressures (MPa) 970 841 924 932 941 938

Extreme temperature (8C)

Workpiece (minimum value) 382 377 361 363 341 358

Punch (maximum value) 225 222 241 243 237 249

Die (maximum value) 236 234 257 255 257 261

Maximum press loads (tonf) 7196.20 5870.32 5961.69 5628.30 7381.60 5703.27

Forging energies (kJ) 829.99 732.32 757.09 680.97 781.01 625.83

Y.H. Kim et al. / Journal of Materials Processing Technology 123 (2002) 270–276 273

die and the workpiece. This is assumed to be due to heat

conduction from the workpiece to the punch and die during

the forging operation. The minimum temperature within the

workpiece appears near the bottom of the rim and the

maximum temperature at the die is shown near the branch

region between the top and the bottom of the rim. Through-

out the deformation period, the temperature changes within

the workpiece and the die are much larger for punch speed of

0.1 m/s than for that of 0.2 m/s. This is assumed to be the

reason for the forging with punch speed of 0.1 m/s to take

more time than for 0.2 m/s. It can also be concluded from the

simulation results that the temperature distributions for

different punch speeds in combination with different rim

thickness are very close to those discussed earlier. The

extreme values of temperature within the workpiece, the

punch and the die are summarized in Table 4.

Fig. 8 shows the load–stroke curves for four different

cases of simulation, namely, two different punch speeds

(0.2 and 0.1 m/s) and rim thicknesses (7 and 8.5 mm),

respectively. As shown in the figure, punch loads gradually

increase until the stroke reaches the line ‘‘A’’, which indi-

cates that the forward and backward extrusions are about to

start at this time. After the stroke passes through the line ‘‘A’’

in the figure, the punch loads stay more or less constant,

which means that the forging is under typical extrusion

condition and at steady state. As also seen in the figure,

the punch load abruptly increases in the case of constant

punch speed of 0.2 m/s and rim thickness of 7 mm (see point

‘‘B’’). This is why the workpiece near the bottom of rim

meets the die at this time in this case. Thus, for three other

simulations, the die depth for rim is adjusted to be deeper to

avoid the peak load. With the same rim thickness, forging

load is greater by 200–500 tonf with the higher punch speed

than with the lower one. This is due to the strain-rate

sensitivity on the flow stress of the material at elevated

temperature. With the same punch speed, forging load is

greater by 500–1000 tonf with thinner rim than with thicker

one. This explains why the flow resistance is greater with

smaller gap. On the whole, maximum punch load in each

case is shown to be between 5600 and 7200 tonf when

Fig. 6. Velocity fields for punch speed of 0.2 m/s.

Fig. 7. Temperature distributions: (a) punch speed 0.1 m/s, rim thickness 7 mm; (b) punch speed 0.2 m/s, rim thickness 8.5 mm.

Fig. 8. Load–stroke curves for punch speed of 0.1 and 0.2 m/s.

274 Y.H. Kim et al. / Journal of Materials Processing Technology 123 (2002) 270–276

constant punch speeds are applied. The figure demonstrates

that the effect of the rim thickness on forging load is larger

than that of the punch speed.

The simulated load–stroke curves (say, accumulator dri-

ven) with varying punch speed during the stroke are shown

in Fig. 9 with different rim thickness (7 and 8.5 mm). The

trend of the curves is very much similar to that in Fig. 8. At

points ‘‘A’’, ‘‘C’’ and ‘‘D’’, the punch speed decreases as

much as described before. Forward and backward extrusions

are about to start in the neighborhood of point ‘‘B’’ and

finally get to the steady state near the point ‘‘C’’, where the

punch load starts to level off. It is also shown in the figure

that forging load is greater with the thinner rim than with the

thicker one. At point ‘‘D’’, the punch load decreases abruptly

due to a sudden drop of punch speed. The maximum loads

during operations and consumed energies obtained from the

simulation results are summarized in Table 4. Considering

the cost of press equipment, it is desirable that the press

capacity for an automotive wheel forming process used in

the current study be selected as 6000 tonf (as referred to in

Table 4).

4. Conclusions

The finite element analyses for the hot forging processes

of an automotive aluminum-alloy wheel are performed

in this study. The results obtained from the real model

analysis are reviewed with each case, and provided basic

data for process design and selection of press equipment.

Conclusions are summarized as follows:

(1) The robustness of numerical solutions is verified by the

comparison between the finite element analysis results

and the experimental ones in terms of forging load for a

simplified small-scale model.

(2) In given conditions, tooling is safe in terms of pressure

exerted on the die wall.

(3) Usually, the bottom rim section is formed earlier than

the top section.

(4) During the forging operation, the slower the punch

speed, the larger is chilling effect near the contact

region between the die and the workpiece.

(5) The bottom rim section of die is desirable to be open in

terms of low load requirement.

(6) Rim thickness has a key effect on the forging load.

Thus, the 6000 tonf press of pressure capacity for rim

thickness of 8.5 mm and the 8000 tonf press for rim

thickness of 7 mm are recommended for the forging

equipment.

Acknowledgements

This work was supported by the development program for

the exemplary schools in information and communication

from the Ministry of Information and Communication

(MIC) and RA research grant from Inha University.

Fig. 9. Load–stroke curves with variant punch speed.

Y.H. Kim et al. / Journal of Materials Processing Technology 123 (2002) 270–276 275

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