J. Mater. Sci. Technol., 2011, 27(1), 8-14.
Microstructural Evolution of 6061 Alloy
during Isothermal Heat Treatment
†
Na Wang1) , Zhimin Zhou1) and Guimin Lu2)
1) School of Sciences, Northeastern University, Shenyang 110004, China
2) School of Resources and Environmental Engineering, East China University of Science and Technology, Shanghai
200237, China
[Manuscript received October 13, 2009, in revised form January 1, 2010]
The semi-solid billet of 6061 aluminum alloy was prepared by the near-liquidus semi-continuous casting (LSC)
with rosette or near-spheroide grains. The pre-deformation processing was applied before partial remelting to
further improve the microstructure and properties of the semi-solid alloy. The effects of different processing
parameters, such as holding temperature and holding time, on the semisolid microstructures during partial
remelting have been investigated. It was found that the optimal partial remelting parameters should be
630◦ C and 10–15 min for 6061 alloy cold rolled with 60% reduction in height of pre-deformation. The
coarsening rates were anasysed by Lifshitz-Slyozov-Wagner (LSW) theory. The pre-deformed 6061 alloy exhibits
lower coarsening rate constants than that of the as-cast one, and also lower than other alloys processed by
different method found in previous literature. It is because the coarsening rate is associated with the initial
microstructure and composition of the alloy. The secondary phases in the alloy inhibit the migration of the
liquid film grain boundaries. The microstructure obtained by using the combination of near-liquidus semicontinuous casting and pre-deformation treatment is better than that without pre-deformation processing,
which demonstrates that the used method is promising for fabricating high quality semi-solid alloys.
KEY WORDS: Semi-solid forming; Near-liquidus semi-continuous casting; 6061 alloy;
Pre-deformation; Partial remelting; Coarsening
1. Introduction
Al-Mg-Si based aluminum alloys, e.g. 6061 alloy,
etc., are widely used in automotive and aerospace
applications because of their high properties such as
good strength, formability, weldability and corrosion
resistance[1–6] . A weight saving is expected in application of automotive components by replacing steels
with aluminum alloys, which will result in great improvements in energy saving. To improve the strength
and the formability of lightweight aluminum alloys
for further industrial applications[7] , semi-solid forming (SSF)[8–9] technique is used as an alternative to
† Corresponding author. Prof.; Tel.: +86 13840094271; E-mail
address: zmzhou@imp.neu.edu.cn (Z.M. Zhou).
traditional casting and forging processes. SSF is a
method that can produce complex shape products.
The process has advantages of productions of high
quality and performance, and low cost. SSF is now a
commercially manufacturing route producing millions
of near net-shape parts per annum for the automotive
industry.
As a key branch of semi-solid technology, thixoforming attracted much attention due to its technical and economic advantages in processing alloys,
which comprises of preparation, partial remelting and
thixoforming of semi-solid billets. The preparation
of semi-solid billet is fundamental to thixoforming
process. The near-liquidus semi-continuous casting
(LSC)[10,11] based upon pouring temperature control-
N. Wang et al.: J. Mater. Sci. Technol., 2011, 27(1), 8–14
ling for the control of solid particle morphology is a
semi-solid preparation technology with high efficiency,
low investment and extensive alloy application scope.
The partial remelting of semi-solid billet is a critical procedure in the thixoforming process. Its purpose
is not only to obtain a desirable nominal liquid fraction, but also to ensure the transformation of the solid
phase to a spheroidal morphology with fine grain size.
During partial remelting, the alloy was heated up to a
temperature at which the solid and liquid phases coexist in equilibrium[12] . Its features lie in obtaining the
desirable nominal liquid fraction through the control
of temperature, and realizing long-time holding to ensure complete transition from dendritic or rosette to
spherical. However, a long-time holding often results
in the coarsening of grains[13] , which is detrimental
to the thixotropic properties of semi-solid billet and
the mechanical properties of thixoformed parts[14–16] .
Therefore, the rate of microstructural coarsening in
the semi-solid state during reheating then determines
whether a material is suitable or not.
In general, coarsening is regarded as diffusion controlled process and can be described by the Lifshitz,
Slyozov and Wagner (LSW) theory[17–19] which gives
a simple form as follows.
d3 − d30 = Kt
(1)
where d is the average particle diameter at time t,
d0 at time t=0 and K is the coarsening rate. Many
researchers have studied the coarsening behavior of
different semisolid alloys by using the LSW theory.
Ji et al.[20] examined the coarsening of solid particles
of Mg-9Al-1Zn prepared by using a twin-screw slurry
maker at 593◦ C, and found that the coarsening exponents in the LSW equation are 8.2 and 12.7 for 300
and 800 r/min, respectively. The previous studies on
isothermal coarsening support the conclusion that the
value of K is lower at lower temperature. MansonWhitton et al.[23] discussed the inhibition of coarsening by grains with grain boundary liquid films to account for slower rates of coarsening in alloy AA2618
than that in Al-4 wt. pct Cu alloy. Freitas et al.[24]
compared coarsening in alloy 2024 with Al-4 wt. pct
Cu in the semi-solid state and discussed the retarding
effect that intermetallics exert on particle growth.
In this paper, the semi-solid billet is cast by LSC
to obtain the near-spheroide and rosette microstructure. Subsequently, some samples of the billet are
reheated into semisolid state for partial remelting,
some others are cold rolled before heat treatment.
The effect of pre-deformation and holding temperature and time on the semisolid microstructure was
experimentally studied, and the coarsening rate was
calculated by using Eq. (1). It is found that the alloy processed by LSC method and pre-deformation
has lower coarsening rate during the process of partial remelting compared with the results obtained by
previous researchers.
9
2. Experimental
2.1 Material
The experimental 6061 alloy was prepared by using commercial purity aluminum (99.7%), Al-22%Si
alloy and magnesium etc. The raw material alloys
were melted at 760◦ C in a 2RZ30 induction furnace.
After degassing, holding and deslagging, the melt was
purified in a temperature precisely controlled furnace,
and then cooled to the destined temperature and kept
for a desired time interval. With cooling intensity of
0.05 m3 ·min−1 , the casting velocity of 150 mm·min−1
and the pouring temperatures of 657◦ C, 6061 alloy
is semi-continuously cast into ingot with 120 mm in
diameter and 1600 mm in length.
2.2 Heat treatment
The chemical composition of 6061 alloy in this
paper is 0.63 wt% Si, 1.09 wt% Mg, 0.10 wt% Fe,
0.07 wt% Ti, 0.25 wt% Cu, 0.21 wt% Cr, and Al balance. The differential scanning calorimetric analysis
(DSC) for the semi-solid 6061 alloy was conducted on
a NETZSCH STA449C integrated thermal analyzer.
The measured solidus and liquidus temperatures are
582.8◦ C and 652◦ C, respectively. To study the effects
of temperature, the samples were heated at the temperatures of 610, 620, 630 and 640◦ Cfor 15 min. And
to investigate the effects of time, the samples were
held at 630◦ C for 5, 10, 15 and 20 min, respectively.
The billet was machined into 20 mm×20 mm×
20 mm for partial remelting experiments. In order
to investigate the influence of pre-deformation on the
partial remelting microstructures, some samples were
subjected to one pass cold rolling with 60% reduction. Both as-cast and pre-deformed samples were
placed in the electric resistance furnace for the further semi-solid heat treatment. The heating rate and
fluctuation of the temperature were controlled within
10◦ C·min−1 and ±1◦ C, respectively. The specimens
were wrapped by aluminum foil in advance to keep
it a correct shape during the reheating process. After
the semi-solid heat treatment, the samples were taken
out immediately for water quenching, polished, etched
with the mixed acid solution of 2 ml HF, 3 ml HCl,
5 ml HNO3 , and 190 ml H2 O. The microstructure was
observed on an optical microscope (Leica DMR), and
the grain size (d=(4A/π)1/2 , where A is the area of
the grain) and the grain roundness (P 2 /(4πA), where
P is the perimeter of grain/rosette) were automatically calculated by the Image-Pro Plus software.
3. Results and Discussion
The microstructural evolution of 6061 alloy in the
process of partial remelting was experimentally studied. Figure 1 shows the microstructures of the as-cast
10
N. Wang et al.: J. Mater. Sci. Technol., 2011, 27(1), 8–14
Fig. 1 Microstructure of 6061 alloy of semi-solid billet: (a) as-cast, (b) as pre-deformed
Fig. 2 Microstructures of the remelted 6061 alloys at different temperatures (as cast) with holding time of 15 min:
(a) 610◦ C, (b) 620◦ C, (c) 630◦ C, (d) 640◦ C
and pre-deformed 6061 alloy used for remelting
process. It can be seen from Fig. 1(a) that the grains
of as LSC cast 6061 alloy clearly exhibits the nearspheroide and rosette character. After cold rolling
with the reduction of 60%, some of the rosette grains
were broken up to fine blocks, and the grains of the
alloy were elongated and tended to be oriented along
the rolling direction, as shown in Fig. 1(b).
The series of optical micrographs of 6061 alloys experienced isothermal heat treatment at different temperatures for 5, 10, 15 and 20 min, respectively, are
shown in Figs. 2 to 5.
Figures 2 and 3 show the microstructures of 6061
alloy isothermally treated at different temperatures
for 15 min. It could be seen that the liquid phase increases with the increase of temperature. The shape
of the solid grains is more globular when the temperature is lower than 630◦ C, but anomalous at 640◦ C
during the partial remelting. The shape of the solid
grains has a tendency to become more globular with
the temperature varying from 610◦ C to 640◦ C, as
shown in Fig. 3, during the partial remelting of the
pre-deformed samples. A comparison between Figs. 2
and 4 reveals that, with increasing temperature, the
grain boundary liquid film becomes thicker and the
grains become more spheroidal in appearance in the
course of partial remelting of the pre-deformed samples. The spheroidizing degree of grains is gradually
improved with the increase of temperature under the
driving of the specific surface energy. This can be
explained by the growth mechanism named Ostwald
ripening[25–27] . Larger grains grow continuously and
smaller ones dissolve quickly because of the difference
in curvatures of the grains. Meanwhile, the grains
spheroidize. The possible reasons were elucidated as
follows. On one hand, with increasing the tempera-
N. Wang et al.: J. Mater. Sci. Technol., 2011, 27(1), 8–14
11
Fig. 3 Microstructures of the remelted 6061 alloys at different temperatures (pre-deformed) with holding time of
15 min: (a) 610◦ C, (b) 620◦ C, (c) 630◦ C, (d) 640◦ C
Fig. 4 Microstructures of the remelted 6061 alloys at 630◦ C for different time (as-cast): (a) 5 min, (b) 10 min,
(c) 15 min, (d) 20 min
ture, the diffusion of atoms would be enhanced. According to the Ostwald ripening mechanism, the dissolution and reprecipitation can be regarded as the
diffusion-controlled processes that would be enhanced
and result in the coarsening of solid grains. On the
other hand, the dynamic equilibrium of dissolution
and reprecipitation coexist in the semisolid slurry, and
the energy of the system has a trend to reduce by
altering the surface of solid phase, which ultimately
results in the spheroidizing of solid grains.
Figures 4 and 5 show the microstructures of 6061
alloy at 630◦ C for 5, 10, 15 and 20 min, respectively, of
the two routes. After isothermal treatment for 5 min,
the eutectic phase at the grain boundary remelted and
12
N. Wang et al.: J. Mater. Sci. Technol., 2011, 27(1), 8–14
Fig. 5 Microstructures of the remelted 6061 alloys at 630◦ C for different time (pre-deformed): (a) 5 min,
(b) 10 min, (c) 15 min, (d) 20 min
then the periphery of solid grains remelted partially
through the solute diffusion at the solid/liquid interface (Figs. 4(a) and 5(a)). After isothermal treatment
for 10 min, grains start to amalgamate and grow. The
grains obviously tend to grow and spheroidize. But
in this case, grain boundary is ambiguous (Figs. 4(b)
and 5(b)). The grains become more spheroidic and
the boundaries clearer, as shown in Fig. 5(b). After isothermal treatment for 15 min, the grains keep
growing, spheroidizing and being surrounded by continuous liquid phase (see Figs. 4(c) and 5(c)). After the holding time lasts for 20 min the fraction
of liquid phase increases obviously. Eutectic liquid
phases in the grain interior also gradually amalgamate and interpenetrate with the surrounding liquid
phase. However, some coarsen grains still exist and
the size of which even exceeds 200 µm (Figs. 4(d)
and 5(d)). During the partial remelting at a given
holding temperature and for an appropriate time, the
relative content of liquid and solid phases in semisolid slurry will reach a dynamic equilibrium state,
namely the rate of the melting of solid phases corresponds to that of the melting of liquid phases. It
usually needs a relatively long time to reach the said
dynamic equilibrium, which means that the holding
time is of significance for obtaining globular grains.
As-cast microstructures of 6061 alloy has many
complex compounds which distribute in the grain
boundary. They were broken and distributed on
the α-Al matrix as large granular after cold rolled.
Small needle-like Mg2 Si precipitates during isothermal treatment and the present Mg2 Si particles at
grain boundary inhibit the migration of liquid film
grain boundaries, either through a pinning mechanism or through impeding diffusion through the liquid
film at the boundary. In contrast, the insoluble Fe,
Mn-containing particles will be present in the semisolid state and are of a size which can obstruct the
migration of the liquid boundary, either through a
pinning type of mechanism or through inhibiting diffusion through the liquid from one boundary position
to another[30] . The results show the inhibiting effect
of second phase particles.
Figure 6 demonstrates the effect of holding temperature and time on grain size and shape factor of
semi-solid 6061 alloy. It can be seen that the grain
size increases with the increase of holding temperature
and time. The shape factor is the smallest at 630◦ C
and for 15 min during partial remelting. The main
reason might be that the liquid fraction increases
rapidly when the semi-solid billet firstly remelted below 630◦ C and isothermally held for less than 15 min.
The rapid increasing of liquid phase leads to the separation of grains and the formation of individual polygonal grains which have high interfacial energy due to
high specific surface area. The protruding positions of
polygonal grains remelt firstly under surface tension
due to low balance melting point, in which the grains
spheroidize rapidly to reduce the interfacial energy.
Therefore, the grains are much closer to be round.
With the increase of holding temperature and time,
the small grains dissolute and large grains grow into
larger ones accompanied with the reduction in amount
of globular grains. When the holding tempera-
13
N. Wang et al.: J. Mater. Sci. Technol., 2011, 27(1), 8–14
2.1
Grain size (as-cast)
Grain size (pre-deformed)
1.8
1.7
80
1.6
60
1.5
40
1.4
1.3
20
1.2
0
610
615
620
625
630
Holding temperature /
635
640
o
C
2014
RAP
0
3
3
AA2014 spray cast
2014
CS
201
CS
2700
Al-4.5Cu-1.5Mg
2400
2100
[23]
[29]
[29]
3000
3
Roundness (pre-deformed)
100
-18
1.9
m /s) assuming d -d =kt
2.0
Roundness (as-cast)
120
Coarsening coefficient K ( 10
(a)
Average roundness
Average grain size /
m
140
[28]
[30]
DC-cast with grain refiner
[31]
A356
electromagnetic
6061
RAP without MnCr
6061
RAP
7075
RAP
[23]
[23]
[29]
6061
as-cast[this work]
1800
6061
pre-deformed[this work]
2017
Shear Cooling Roll
1500
Al-4Cu
SIMA with grain refinement
[31]
[32]
1200
900
600
300
0
520
540
560
580
600
620
640
660
200
Grain size (as-cast)
180
o
2.0
Temp. / C
Grain sizer (pre-deformed)
1.9
Roundness (as-cast)
160
Roundess (pre-deformed)
1.8
140
1.7
120
1.6
100
1.5
80
1.4
60
40
1.3
20
1.2
0
Average grain roundness
Average grain size /
m
(b)
1.1
5
10
15
20
Holding time / min
Fig. 6 Grain size and the shape factor of remelted 6061
alloy
ture is 640◦ C for holding time 15 min or 630◦ C for
20 min, the grains might not adequately ripen though
the grains spheroidize rapidly but the holding time is
not long enough. However, the samples are too soft to
clamp. The shape factor decreases with holding temperature and time during the partial remelting of the
pre-deformed samples. The difference in the kinetics
of grain growth should be attributed to the different
eutectic melting activation energy. During the predeformed process, a fraction of external work remains
in alloys and forms strain energy. The energy stored
in the alloy will provide the driving force to promote
recovery and recrystallization during subsequent partial remelting. During remelting, recovery is induced
by the release of the stored energy through the virtue
of dislocation motion, which accelerates the atomic
diffusion at the elevated temperature. The grains are
much finer than that shown in Figs. 2 and 3. The predeformed billet appears to be relatively more resistant
to grain growth than the as-cast billet, in which the
grain is refined in advance via recrystallization.
The coarsening rates are calculated by using
Eq. (1). The pre-deformed 6061 alloy after heat treatment exhibits lower coarsening rate constants than
that of the as-cast one because of a number of recrystallization nuclei. During the isothermal treatment, the solid particle size of the pre-deformed 6061
alloy is smaller, which result in an increase in both
grain nucleation and growth; however, the increment
Fig. 7 Dependence of coarsening coefficient (K) on holding temperature for wrought Al alloys
of nucleation rate is faster than that of the growth
rate. The lower coarsening rate may be attributed to
the presence of Mg2 Si particles at grain boundaries
in pre-deformed 6061 alloy, which hinder the mobility of solute atoms. Inspection of the composition
shows that it includes 0.07 wt% Ti which may inhibit
boundary migration and hence reduce the coarsening
rate constant.
It can be derived from the present work that coarsening rate is relevant to the microstructure of the alloy. As shown in Fig. 7, the value of coarsening rate
constant is lower for RAP2014 than that for CS2014,
and similarly lower for pre-deformed 6061 than that
for as-cast 6061[28] . In addition, Fig. 7 further supports the argument that the alloys giving relatively
low coarsening rates contain stuffs pining the liquid
film grain boundaries (insoluble intermetallics based
around elements such as Fe, Mn, Cr, porosity, grain
refining species such as those based on Ti). It is shown
in Fig. 7 that the pre-deformed alloy in this work
has lower coarsening rates in the process of partial
remelting, which indicates that combination of nearliquidus semi-continuous casting and pre-deformation
treatment is a promising method to fabricate the billet of semisolid alloy.
4. Conclusions
(1) Preparation of semi-solid billet by using the
combination of near-liquidus semi-continuous casting
and pre-deformation treatment helps to further improve the microstructure of the semi-solid alloy during partial remelting process, which leads to a lower
coarsening rate of grains.
(2) The optimal remelting parameters for 6061
alloy cold rolled with 60% reduction in height are
heat treated at 630◦ C for 10-15 min. The obtained microstructure is better than that without predeformation processing.
(3) The coarsening rate for partial remelting of
14
N. Wang et al.: J. Mater. Sci. Technol., 2011, 27(1), 8–14
the pre-deformed 6061 alloy is lower than those for
other wrought alloys from literature, showing that the
method used in this work is promising for fabricating
high quality semi-solid alloys.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China (Grant No.
50674032).
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