thixoforming characteristics of thermo-mechanically treated aa 6061 alloy for suspension parts of...

Thixoforming characteristics of thermo-mechanically treatedAA 6061 alloy for suspension parts of electric vehicles

Sang-Yong Lee*, Se-Il OhDepartment of Materials Processing, Korea Institute of Machinery and Materials,

66 Sangnam Changwon, Kyungnam 641-010, South Korea

Abstract

Thixoforming trials of AA 6061 alloy, which is a wrought aluminum alloy, were undertaken to manufacture the steering knuckles designed

for suspension parts of low speed electric vehicles. The billets used as a starting material were thermo-mechanically treated to induce

thixoformable fine globular microstructures during subsequent reheating process. After a slug was reheated up to the working temperature

(�645 8C) using the horizontal induction heating device, the part was formed by vertical injection of a heated slug into the die cavity.

The well-developed globular microstructures could be observed in most sections of the thixoformed parts except few surface regions close

to gate where occasional unrecrystallized large grains are present. However, liquid segregation between near and far gate region could not be

avoided, which was probably resulted from high injection speed. The reason for that will be explained using semi-solid characteristic flow

behaviors and the method to reduce liquid segregation will be suggested.

In order to evaluate the mechanical properties of AA 6061 thixoformed parts, a series of test specimens carefully classified by their cutting

location in a part were used for tensile test. From tensile test results, it could be revealed that tensile properties were strongly affected by liquid

segregation. Especially, variation of tensile properties was more dominant when the part was heat treated by T6 condition. These results may

be attributed to the segregation of alloying elements such as Mg and Si caused by the segregation of liquid phase involving much alloying

element. Therefore, it is necessary to reduce liquid phase segregation in order to obtain more uniform tensile properties.

By comparing tensile properties of AA 6061 thixoformed parts to those of previously thixoformed parts made from cast grade A357 alloy, it

was found that AA 6061 parts showed far better ductility, even though strength level is slightly lower, than A357 parts at both as-formed and

T6 condition. In this study, it was confirmed that wrought aluminum alloy, AA 6061 has the reasonable thixoformability and mechanical

properties for thixoforming of real parts such as steering knuckles.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Thixoforming; AA 6061; Steering knuckle; Low speed electric vehicle; Tensile properties

1. Introduction

For last few decades, thixoforming technologies have

rapidly progressed for industrial application and are now

being successfully used to produce high volume and safety

critical parts such as fuel rails and master brake cylinders

[1]. In particular, this technology showed outstanding

achievements in automotive industry. However, only few

cast grade Al–Si alloys such as A356 and A357 are being

used for high volume production due to their excellent

thixoformability and workability. In the viewpoint of such

limitation, it is evident that the application scope of thix-

oforming technology is considerably narrowed [2].

In order to overcome this limitation, a lot of researches

have been done to develop thixoforming process for various

wrought aluminum alloys [3–5]. However, when thixoform-

ing of wrought aluminum alloys is considered, it should be

noted that there are several problems to solve. First, unlike

cast Al–Si aluminum alloys, feedstocks for thixoforming of

wrought aluminum alloys are not existed in commercial

market. Even though several production routes such as strain

induced melt activated (SIMA) process, recrystallization

and partial melting processes (RAP) and magnetohydrody-

namic stirring (MHD) have been considered and developed

[6–8], it seems that the most effective methods are not

determined until now. Second, because liquid fraction of

wrought aluminum alloys is very rapidly changed with

temperature change, the careful attention, when a slug is

reheated, must be given to obtain the reasonable liquid

fraction ensuring good thixoformability. Third, at semi-solid

state, wrought aluminum alloys have higher viscosity and

hot cracking tendency than cast grade aluminum alloys [9].

Journal of Materials Processing Technology 130–131 (2002) 587–593

* Corresponding author. Fax: þ82-55-280-3498.

0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.

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

Therefore, great deals of systematic researches associated

with not only starting billets but also optimum process

parameters are needed to solve these problems and to

successfully commercialize these technologies.

Up to date, most of the studies on the thixoforming of

wrought aluminum alloys have been done by reheating

thermo-mechanically treated billets, which was known as

SIMA process. In SIMA, desirable globular microstructures

surrounded by lower melting phase can be developed

through recrystallizaion of heavily deformed structures

and subsequent partial melting at grain boundary region.

Since various kinds of aluminum billets having different

extrusion ratios can be easily provided by commercial

market, it is believed that the use of extrusion billets as a

starting material is the most effective approach for thixo-

forming of wrought aluminum alloys even though several

other methods such as upsetting and swaging can be used for

thermo-mechanical treatment of billets.

On the basis of previous researches [3–5], where thix-

oforming possibility of wrought aluminum alloys were

reported, a wrought aluminum alloy, AA 6061 was used

to form the real parts—the steering knuckles whose design

was provided by courtesy of an electric vehicle manufactur-

ing company. Since steering knuckles, which support both of

the car body and wheels, have to endure several dynamic

loads including high impact forces, these parts need both

high strength and toughness. Therefore, AA 6061 alloy,

which has higher ductility and toughness than cast grade

aluminum such as A356 or A357, may become more suitable

materials for steering knuckles. That is the motivation to

challenge thixoforming of wrought aluminum alloys for

steering knuckles.

In this paper, the detail thixoforming processes and results

was described and the tensile properties of this part has been

compared to those of the same part formed using A357 alloy.

2. Experimental

2.1. Materials

Hot extrusion billets having the extrusion ratio of 13:1

were selected as a starting material since the heated samples

of these billets showed small grain size (�70 mm) to guar-

antee good thixoformability (Fig. 1). However, it should be

noted that despite high extrusion ratio of 13:1, uncrystallized

large grains (Fig. 2(a)) are retained over all surface layers of

which thickness approximately measured 5–6 mm. It is

believed that lack of deformation on surface regions due

to friction between die surface and extruded materials may

become one reason for abnormal microstructures of surface

layer. Therefore, in this study, surface layer of starting

materials was removed by 5 mm in depth to avoid involving

large grains of surface region in the thixoformed part. In

addition, few elongated large grains were occasionally dis-

covered at other regions, as shown in Fig. 2(b). These grains

will be retained in thixoformed parts. One emphasis is

placed on this study: Unlike conventional SIMA, all billets

Fig. 1. Comparison of microstructures between as-extrusion billet and reheated billet. (a) As-extruded state (extrusion ratio 13:1), (b) after reheated state. In the

case of as-extruded sample (a), only heavily deformed structures were observed instead of well-defined globular grain structures shown in the reheated sample (b).

Fig. 2. Occasional large grains discovered at the reheated samples. (a) Surface region (transverse section), (b) interior region (longitudinal section). Most of

the large grains were observed at the surface region which was due to the intrinsic low deformation during extrusion process.

588 S.-Y. Lee, S.-I. Oh / Journal of Materials Processing Technology 130–131 (2002) 587–593

used for this study were processed by only hot extrusion

without any subsequent cold working treatment.

2.2. Basic tool design and forming process concept

Fig. 3 shows basic layout of the device used for thixo-

forming of steering knuckles. After a reheated slug was

placed on the top of the injection piston, this slug was

vertically injected through the gate located in the middle

region of the bottom die. The tool design used for this study

is somewhat different from the tool design for general die

casting because this tool does not have a separate gate and

runner region, i.e. a slug is directly entered into the die cavity

having the part shape. Two advantages, in the commercial

view point, can be taken from this new tool design concept:

(1) without a separate gate and runner structure, the size of

the tool could be considerably reduced, (2) the additional

hydraulic ejecting system was not needed because part

ejection can be done using the injection piston for shot after

opening upper die. Additionally, since the die filling distance

and time could be effectively reduced by use of this tool

design, the slow injection ensuring non-turbulent laminar

flow may be easily realized without the problems related to

incomplete filling. This point has more important meaning

when wrought aluminum alloys are used for thixoforming.

Generally, wrought aluminum alloy is heated up to the

temperature corresponding to 20–30% liquid fraction

because it is very hard to safely maintain higher liquid

fraction during heating. Accordingly, the cooling of wrought

aluminum alloys occurs far more rapidly during die filling

process than that of cast grade aluminum alloys having

higher liquid fraction for thixoforming. Therefore, in the

case of wrought aluminum alloys, this new tool design

concept can make thixoforming of large and complex shapes

much easier.

2.3. Thixoforming trials for steering knuckles

Cylindrically hot extruded AA 6061 billets were cut and

machined to a rod shape having 130 mm in length and

65 mm in diameter. These rods are generally called a slug

in thixoforming technology. After a slug was put on the slug

container (Fig. 4), the slug was inductively heated with

power input controlled by the power–time curve previously

determined by several heating trials. Temperature variation

during heating was monitored using two K-type thermo-

couples inserted in the near surface and center region of a

slug. Through temperature measurement, it was confirmed

that the temperature difference between surface and center

region was less than 2 8C when the working temperature was

reached. The detailed power–time curve and temperature

data were shown in Fig. 5. After a slug was reheated up to the

working temperature, 645 8C, the reheated slug was manu-

ally moved to the top of the injection piston and then injected

into the die cavity preheated to around 200 8C. The ram

speed and maximum ram force used for injection were

600 mm/s and 60 t, respectively. After thixoforming of

steering knuckles, the formed parts were ejected using

injection cylinder without any help of special hydraulic

ejecting device.

3. Results and discussion

3.1. External appearance of thixoformed parts

From the external view of thixoformed parts (Fig. 6), it

was known that complete die filled shape could be obtained.

Surface of these parts also showed very good condition.

Especially, it is important to note that sticking tendency of a

part toward die surface is lower than that of a part formed

using A357 despite higher working temperature. This fea-

ture made part ejection very easy and good surface without

any scratches possible.

3.2. Microstructural investigation

Through microstructural examination of thixoformed

parts, fine globular microstructures, which are characteristic

microstructures of thixoformed parts, were observed in most

Fig. 3. Basic layout for thixoforming device (vertical injection type). Fig. 4. Slug container operated by pneumatic system.

S.-Y. Lee, S.-I. Oh / Journal of Materials Processing Technology 130–131 (2002) 587–593 589

of the cross-sectional area of a part. However, liquid phase

segregation between near and far gate region and occasional

large grains were also discovered. Fig. 7 represents the

microstructural variations along cross-section A–A0 of

Fig. 6. The clear difference of liquid phase (dark region)

fraction between near (Fig. 7(e)) and far (Fig. 7(a) and (f))

gate region can be confirmed from this figure. Large elon-

gated grains, which are regarded as unrecrystallized grains

retained after reheating of a slug, were also observed at some

regions (Fig. 7(c) and (d)). These large grains were fre-

quently discovered in the surface of near gate region, which

is due to poor flowing characteristics of these microstruc-

tures, i.e. the region involving these large grains was not

injected far away but trapped near gate region. The reason

for liquid phase segregation can be explained by sponge

effect [10]. During very fast injection, liquid phase is

quickly squeezed out along the mold cavity and then solid

phases are followed. Thus, large liquid fraction may be

accumulated in far gate regions. Therefore, this problem

may be significantly improved by reducing the injection

speed. In addition, the reason why large elongated grains are

frequently discovered near gate region may be demonstrated

as follows: the region involving large grains has poor flow

characteristics so that other regions having fine grain struc-

ture could be easily penetrated through the large grain

regions. After then, when injection force reaches above

the critical stress, at which large grain regions can start to

move, large grain regions are filled into the die. These flow

behaviors are schematically illustrated by Fig. 8. Generally,

liquid phase segregation is likely to result in alloying ele-

ment segregation because liquid phase in semi-solid state

has higher content of alloying elements than solid phase, as

shown in Fig. 10. In order to make sure macroscopic

segregation of alloying elements, chemical analysis with

respect to four different cross-sectional locations were car-

ried out using atomic absorption spectroscopy. The chemical

analysis results were summarized in Table 1. As expected,

the concentration of alloying elements such as Cu, Mg and

Si is much lower in the near gate regions (Region #3) than in

the other regions.

3.3. Tensile test results

Tensile test specimens obtained from several locations

shown in Fig. 9 were used to evaluate mechanical properties

of thixoformed parts. All test results were described in

Table 2 including the test results [11] previously obtained

from A357 thixoformed knuckles. Like A357 parts, both

yield stress and ultimate tensile stress at as-formed condition

show comparatively similar value for all specimens while

those values after T6 heat-treatment show large differences

between near gate region (Region E) and the other regions.

These results well correspond to the chemical composition

variation caused by liquid phase segregation. Since the

content of alloying elements contributed to age hardening

is the lowest in the Region E, it is evident that this region

showed the lowest strength level after T6 heat-treatment. In

Fig. 5. Detailed power–temperature–time curves and data for AA 6061 reheating process. Induction heating was performed by gradually decreasing the

power with three steps and the elapsed time needed to reach working temperature is 440 s (7 min 20 s).

Fig. 6. External appearance of thixoformed steering knuckle.


Fig. 7. Microstructures along cross-section A–A0 of a part shown in Fig. 6. Far higher solid fraction in near gate region (e) was observed compared to the

other regions (in semi-solid state, globular white phases represent solid whereas dark phases represent liquid).

Fig. 8. Schematic diagram showing semi-solid flow behavior during injection. As a slug is injected into the cavity, most of liquid phases are squeezed out far

away whereas solid grains including large grains are piled up in near gate region.

Fig. 9. Line analysis of reheated AA 6061 sample (surface large grain region) using SEM–EDX. (a) SEM micrograph of line analysis region, (b) chemical

composition profiles along the line. Note: high concentration of Si, Fe and Cu was detected at grain boundary region, where liquid phase is formed during

reheating and a coarse particle was present at grain interior.


addition, it is important to note that the difference of strength

level after T6 heat-treatment is more dominant in AA 6061

than in A357. That is because AA 6061 is more sensitive to

age hardening than A357. Except Region E, it is not easy to

explain the reason for mechanical property variations among

the other regions (from A to D regions) with only liquid

segregation phenomena because the difference of liquid

fraction among these regions is not clear while the difference

of liquid fraction between Region E and the other regions is

very clear. Those may be affected by the presence of casting

defects such as porosity and shrinkage as well as micro-

structural difference such as liquid phase fraction so that the

mechanical values indicate the combined effect on both

microstructures and casting defects.

Elongation of AA 6061 parts is generally higher than that

of A357 parts at both as-formed and T6 condition. Espe-

cially, when the part was heat-treated by T6 condition, AA

6061 showed much higher elongation than A357. Thus,

when considering good ductility and toughness of AA

6061 at high strength level (T6 condition), it is said that

AA 6061 is more suitable as material for suspension parts,

which should endure frequent dynamic impact during ser-

vice than A357.

3.4. Discussion

When the direct injection tool type, which is similar to the

tool used for this study, is employed to effectively shorten

Table 1

Chemical analysis results (unit: wt.%)

Positiona Al Cu Mg Si Fe

Specification Balance 0.15–0.4 0.8–1.2 0.4–0.8 0.7 (maximum)

Region #1 Balance 0.27 0.88 0.34 –

Region #2 Balance 0.28 0.92 0.58 –

Region #3 Balance 0.17 0.66 0.043 –

Region #4 Balance 0.28 0.88 0.37 –

a Region #3 (near gate region) corresponding to the lowest liquid fraction shows the lowest concentration of alloying elements such as Cu, Mg and Si.

Fig. 10. Tensile test specimens and their taken positions: (a) dimension of tensile test specimens, (b) locations of tensile test specimens.

Table 2

Tensile properties of several locations in AA 6061 thixoformed knucklesa

Specimen position Yield stress (MPa) Ultimate tensile stress (MPa) Elongation (%)

As-formed T6 As-formed T6 As-formed T6

Region A 92 (95) 263 (261) 173 (270) 320 (342) 18.1 (13.7) 15.2 (2.8)

Region B 85 (94) 270 (280) 189 (261) 313 (346) 11.8 (13.4) 10.2 (4.0)

Region C 90 (102) 265 (267) 195 (266) 314 (347) 20.4 (13.6) 8.7 (4.6)

Region D 88 (111) 266 (261) 193 (262) 314 (352) 20.9 (14.6) 10.6 (7.2)

Region E 92 (97) 193 (220) 161 (235) 235 (337) 28.4 (21.7) 11.5 (4.2)

a The values described in the parentheses indicate tensile properties of A357 thixoformed knuckles.


the die-filling distance and time, it is important to note that

the liquid phase segregation cannot be completely avoided,

although it can be reduced by controlling injection speed, as

described in the previous section. Especially, the region

close to the gate, in most cases, will show the lowest liquid

phase, which results in the clearly different mechanical

values with regard to the other regions. Therefore, it is

necessary that the matter associated with liquid phase

segregation must be considered when the direct injection

tool is chosen to manufacture a part. Generally, liquid phase

segregation must be reduced to guarantee the uniform

quality of the part. However, if the function and the struc-

tural response of the part are well known, this problem in the

view of ideal quality may be compromised with real situa-

tion in the view of the application. For instance, if trivial or

low stress is acted, while part is in operation, on the weak

region having the lowest strength value, this may not lead to

serious problem for application. In the case of the steering

knuckle, Region E which is the weak region over all part is

placed in very low stress during operation. Therefore, it is

said that the current tool design and manufacturing concept

is reasonable for application.

4. Conclusion

This study was mainly focused on the experimental results

obtained from thixoforming trials to build a steering knuckle

with thermo-mechanically treated wrought aluminum alloy,

AA 6061 and these experimental results can be described

below:

(1) Steering knuckles could be successfully thixoformed

using a specially designed tool with which die filling

time was effectively reduced by canceling the separate

gate and runner structure. Thus, tool design concept

used for this study was proven to be effective for

thixoforming of wrought aluminum alloy having

relatively low fluidity and poor thixoformability.

(2) Severe liquid phase segregation between near gate and

the other regions could be observed via microstructural

examination and chemical composition segregation

resulted from liquid phase segregation was also

confirmed via chemical analysis. By considering

characteristic flow behavior in semi-solid state, it can

be demonstrated that liquid phase segregation is mainly

due to high injection speed. Therefore, it is suggested

that this problem may be considerably improved by

determining the optimum injection speed.

(3) A series of tensile tests were conducted to investigate

the relationship between liquid phase segregation and

mechanical properties. As expected, it could be

revealed that near gate region corresponding to the

lowest liquid phase showed the lowest strength level

compared to the other regions after T6 heat treatment.

These results well explained the effect of mechanical

properties on alloying element segregation since the

strength of AA 6061 can be remarkably increased by

the presence of alloying elements after T6 heat

treatment.

(4) This study showed great potential for thixoforming

application of wrought aluminum alloy, AA 6061 since

its high ductility could be maintained at not only as-

formed condition but also after T6 heat-treatment.

In future, the study on thixoforming of wrought aluminum

alloys will be continued to reduce the liquid phase segrega-

tion and achieve uniform quality.

Acknowledgements

The research described in this paper was sponsored by

Korea National Cleaner Production Center. The authors

would also like to thank ATT R&D for providing the design

for steering knuckles.

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thixoforming characteristics of thermo-mechanically treated aa 6061 alloy for suspension parts of...

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