N. AKAR et al.: INVESTIGATION OF THE EFFECT OF HOLDING TIME AND MELT STIRRING ...
433–437
INVESTIGATION OF THE EFFECT OF HOLDING TIME AND
MELT STIRRING ON THE GRAIN REFINEMENT OF AN A206
ALLOY
PREISKAVA VPLIVA ^ASA ZADR@EVANJA IN ME[ANJA TALINE
NA ZMANJ[ANJE VELIKOSTI ZRN ZLITINE A206
Neºet Akar1, Ziya Tanyel3, Kadir Kocatepe1, Ramazan Kayikci2
1Gazi University, Department of Metallurgical and Materials Engineering, Ankara, Turkey
2Sakarya University, Department of Metallurgical and Materials Engineering, Sakarya, Turkey
3Gazi University, Graduate School of Natural and Applied Sciences, Ankara, Turkey
rkayikci@sakarya.edu.tr
Prejem rokopisa – received: 2014-12-12; sprejem za objavo – accepted for publication: 2015-06-17
doi:10.17222/mit.2014.302
Effects of melt stirring and holding time were studied with an Al-4.5 % of mass fractions of Cu (A206) alloy. The optimum
level of a grain refiner was determined in conjunction with both continuously stirred and non-stirred melts during the holding
time. Results showed that a mass fraction of a Ti addition of as low as 0.03 % was sufficient to obtain the 82 μm average grain
size, while a Ti addition exceeding the mass fraction of 0.03 % showed no significant effect on the grain size of castings. The
grain refinement tends to fade with a long holding time in a non-stirred liquid. The results also showed that an effective grain
refinement of the A206 alloy can be achieved with a long holding time as long as the liquid alloy is continually stirred.
Keywords: grain refinement, melt stirring, holding time, Al-4.5Cu alloy, A206 alloy
Raziskan je bil vpliv me{anja in ~asa zadr`evanja taline zlitine Al-4,5 % masnega dele`a Cu (A206). Dolo~ena je bila optimalna
koli~ina udrobnjevalca zrn, v povezavi s stalnim me{anjem ali brez me{anja taline med zadr`evanjem. Rezultati so pokazali, da
je bilo `e 0,03 % masnega dele`a dodatka Ti, dovolj za doseganje povpre~ne velikosti zrn 82 μm, medtem ko dodatek Ti ve~ji od
0,03 % masnega dele`a, ni pokazal vpliva na velikost zrn ulitkov. U~inkovitost drobnjenja zrn se zmanj{a pri dolgih ~asih
zadr`anja in brez me{anja taline. Rezultati so pokazali {e, da je mogo~e dose~i u~inkovito drobnjenje zrn tudi pri dolgih ~asih
zadr`evanja, dokler se talina stalno me{a.
Klju~ne besede: drobnjenje zrn, me{anje taline, ~as zadr`evanja, zlitina Al-4.5Cu, zlitina A206
1 INTRODUCTION
Al-Cu alloys are one of the most important Al-based
alloys because they provide good castability and
excellent mechanical properties.1–6 Due to their superior
mechanical properties, Al–Cu alloys can be used in
many areas such as aircraft construction, military field
and automobile manufacturing.4 Grain refinement of
Al–Cu alloys significantly improves the microstructure
and mechanical properties.7–9 Along with many advant-
ages, hot tearing, which frequently occurs during solidi-
fication due to a long freezing range, is a severe problem
in producing cast components with these alloys. Previous
works on casting and solidification of Al–Cu alloys
consistently indicated that hot tearing can be eliminated
with a good grain refinement.1–4,7–9 Grain refinement was
also found to be effective for reducing the amount of the
porosity and size of the pores, and improving the feeding
of cast Al–Cu alloys.9 Titanium and boron are added in
the form of Al–Ti–B master-alloy rods to cast aluminium
alloys for the grain refinement. Rod-type additions were
found to be more effective for providing, controlling and
optimizing TiB2 particles than the salt form.10,11 It was
reported in recent years that Al–Ti–C master alloys are
also effective grain refiners.4,12 Due to its high refining
potential, an Al5Ti1B alloy in the rod form is one of the
most commonly used grain refiner and has been widely
accepted in controlling the grain size and microstructures
of aluminium alloys in industrial applications.13
The A206 alloy is a well-known Al-Cu casting alloy
and a research on grain refinement with this alloy indi-
cated that a proper grain refinement can be achieved with
a 0.15–0.30 % titanium mass fraction of the final cast
part.14 On the contrary, in recent years other resear-
chers3,5,7–9 showed that modern grain refiners containing
Al–Ti and B are more suitable for an acceptable grain
refinement of A206 alloys if the Ti content is lower than
0.15 %.
Despite a number of studies conducted in the past to
investigate the grain refinement of Al–Cu
alloys,3,5–9,12,15–16 studies on the effects of the holding time
and melt stirring on the grain-refinement process have
not been published. Therefore, in this work, the optimum
amount of the grain refiner for the A206 alloy, using an
Al5Ti1B rod-shape grain refiner, in combination with the
holding time and melt stirring, was studied.
Materiali in tehnologije / Materials and technology 50 (2016) 3, 433–437 433
UDK 620.181:622.795:669.3:669.715 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 50(3)433(2016)
2 EXPERIMENTAL PROCEDURES
A206 alloys with two different chemical compo-
sitions were prepared using commercial-purity Al ingots
and commercially pure Cu wires in a new SiC crucible.
The final chemical compositions of these alloys before
the grain-refiner additions are given in Table 1. A Spec-
tro-type optical spectrometer was employed to perform a
chemical analysis of the alloys throughout this study. An
industrial electrical-resistance furnace with a 600 kg
capacity and a SiC crucible were used for the melting.
An Al5Ti1B master alloy was introduced into the
liquid A206 alloy at 730 °C followed by rotary degassing
for 10 min with dry argon. The temperature of the liquid
metal in the melting furnace was continuously controlled
with a K-type thermocouple connected to the control unit
of the furnace to ensure the holding of the liquid at
730 °C.
Grain-refining experiments were carried out using the
Alcan standard grain-refining test10 with two sets of
samples. In the first set of experiments, samples were
taken from the crucible at 730 °C, before the grain-refi-
ner addition. Samples were also taken after each addition
of the Al5Ti1B rod-type master alloy introducing (0.01,
0.02, 0.03, 0.05, 0.1, 0.2, 0.3) % Ti contents into the
liquid alloy. After each addition, the liquid alloy was re-
degassed for 10 min at 730 °C. This resulted in 10-min
intervals between the sample-taking processes.
The second set of experiments was carried out to
determine the effects of the holding time coupled with
melt stirring on the grain refinement. To implement this,
a new heat of the A206 alloy was melted in a new cru-
cible and heated up to 730 °C. The Al5Ti1B master alloy
was introduced into the melt to obtain a 0.05 % Ti con-
tent within the alloy. After a 10-min degassing treatment,
the purging argon was turned off and the molten alloy
was continuously stirred using the graphite lance of the
rotary degassing unit with a rotating speed of 150 min–1
for 90 min. Samples for the Alcan grain-refining tests
were taken during the whole stirring period, in 30-min
intervals from the beginning to the end.
Finally, the melt was re-degassed for 10 min and held
for another 90 min without any stirring actions. During
this second 90-min period, Alcan test samples were also
taken in 30-min intervals, in the same manner as ex-
plained above. At the end of this period, the melt was
re-stirred for only 1 min and the final sample was taken.
The specimens for the metallographic examinations
were cut as shown in Figure 1. The surface of each
specimen was electro-polished using 5 mL of HClO4,
15 mL of 2-Butoxyethanol, 60 mL of ethanol and 20 mL
of distilled water. The average grain size was determined
with the linear intercept method according to the ASTM
E112 standard, at different regions of each sample.
3 RESULTS AND DISCUSSION
3.1 Grain-size measurements
The mean values of the measured grain sizes with va-
rious Ti contents are shown in Table 2, which indicates
that the titanium contents in the alloy were determined
within a narrow variance. Table 2 also shows that the
grain size of the samples decreased dramatically with the
addition of the Al5Ti1B master alloy regardless of the
titanium content. The average grain size versus the Ti
content is also shown in Figure 2. Figure 2 indicates
that the addition of the grain refiner, even with a Ti con-
tent as low as 0.01 %, resulted in a remarkable reduction
in the grain size of the alloy. Figure 2 also indicates that
the lowest grain size of the alloy is about 80 μm, which
was obtained with a 0.03 % Ti content. A higher Ti con-
N. AKAR et al.: INVESTIGATION OF THE EFFECT OF HOLDING TIME AND MELT STIRRING ...
434 Materiali in tehnologije / Materials and technology 50 (2016) 3, 433–437
Table 1: Chemical compositions of A206 alloys before the grain refinement, in mass fractions (w/%)
Tabela 1: Kemijska sestava zlitin A206, pred udrobnjenjem zrn, v masnih dele`ih (w/%)
Experiment
number
in mass fractions (w/%)
Si Fe Cu Mn Mg Zn B Ti Al
1 0.041 0.105 4.510 0.281 0.214 0.098 0.001 0.000 bal.
2 0.039 0.107 4.610 0.353 0.199 0.084 0.002 0.007 bal.
Table 2: Average grain size of the samples with different Ti contents
Tabela 2: Povpre~na velikost zrn pri vzorcih z razli~no vsebnostjo Ti
Ti (w/%)
Target 0 0.01 0.02 0.03 0.05 0.1 0.2 0.3
Realized 0 0.013 0.019 0.032 0.044 0.098 0.194 0.292
Average grain size (μm) 970 112 92 82 81 79 81 79
Figure 1: Alcan test dimensions and grain-size measuring surface
Slika 1: Dimenzije Alcan preizku{anca in povr{ina za merjenje
velikosti zrn
tent within the A206 alloy did not further reduce the
grain size of the samples. These results are in good
agreement with the previous work carried out by Sig-
worth and co-workers.3,7–8
The relationship between the titanium recovery and
the microstructure is given in Figure 3. Figure 3a shows
the microstructure of the sample obtained with no grain-
refiner addition. The microstructure consists of coarse
dendrites, heterogeneously distributed in equiaxed
grains. Figure 3b shows a small addition of Ti, as low as
a 0.01 % mass fraction, which caused the grain refine-
ment of the A206 alloy. However, a Ti content of up to
0.05 % was found to be more effective for further
reducing the average grain size of the alloy, as seen in
Figure 3c. The microstructure obtained with a 0.3 %
mass fraction of Ti in the melt is shown in Figure 3d. It
indicates that the increased amount of Ti no longer
affected the reduction of the grain size of the A206 alloy
used in this study.
The Al5Ti1B master alloy is an effective grain refiner
of Al-Cu alloys as it increases the number of hetero-
geneous nucleation sites for achieving a finer equiaxed
grain structure.10 Figure 3 clearly shows that a Ti content
exceeding the mass fraction of 0.01 % is adequate to turn
the solidification morphology of the alloy from a fully
dendritic to a globular or near-dendritic structure. Thus,
the grain refinement of the A206 alloy by adding the
Al5Ti1B master alloy can be expected to increase the
mechanical properties of A206 castings via reducing the
amount of solidification defects such as hot tearing,
micro-shrinkage and micro-segregation. These results
are in good agreement with the report from H. Kamali et
al.5 who reported that Ti additions of 0.05–0.3 % showed
no significant effect on the grain size, although the mini-
mum Ti mass fractions of 0.05 % was necessary to
eliminate the hot-tearing defects.
3.2 Effects of the holding time and melt stirring on the
grain size
In the present study, possible effects of melt stirring
during the holding period after the master-alloy addition
were also investigated. Experiments were carried out
with the castings from two different heats of stirred and
non-stirred melts. Samples were cast in 30-min intervals
throughout the 90-min holding time, during which the
amount of Ti was fixed at around 0.05 % of mass
fractions.
N. AKAR et al.: INVESTIGATION OF THE EFFECT OF HOLDING TIME AND MELT STIRRING ...
Materiali in tehnologije / Materials and technology 50 (2016) 3, 433–437 435
Figure 3: Microstructure of the A206 alloy with different Ti
additions: a) 0 % of mass fractions, b) 0.01 % of mass fractions, c)
0.05 % of mass fractions and d) 0.3 % of mass fractions
Slika 3: Mikrostruktura zlitine A206 z razli~nimi dodatki Ti: a) 0 %
masnega dele`a, b) 0,01 % masnega dele`a, c) 0,05 % masnega dele`a
in d) 0,3 % masnega dele`a
Table 3: Effect of the holding time on the average grain size
Tabela 3: Vpliv ~asa zadr`evanja na povpre~no velikost zrn
Liquid-metal
condition
Holding time
(min)
Ti recovery
(w/%)
B recovery
(w/%) Ti:B ratio
Average grain size
(μm)
Heat 1
Stirred melt
0 0.049 0.009 5.4 84
30 0.046 0.008 5.8 84
60 0.047 0.008 5.9 85
90 0.048 0.009 5.3 84
Heat 2
Non-stirred melt
0 0.046 0.009 5.1 84
30 0.036 0.005 7.2 88
60 0.035 0.004 8.8 102
90 0.031 0.003 10.3 111
90* 0.048 0.009 5.3 85
(90*) Stirring after 90 min.
Figure 2: Effect of the Ti content on the grain size of the A206 alloy
Slika 2: Vpliv vsebnosti Ti na velikost zrn zlitine A206
A summary of the quantitative results obtained from
the test samples throughout these experiments are shown
in Table 3. Titanium and boron measured in the samples,
cast with the stirred melt, show that both elements
remained at almost their initial values during the 90-min
holding time. Accordingly, the initial Ti:B ratio also
remained constant, which was around 5:5. The average
grain size of the stirred melt was about 85 μm regardless
of the holding time. These results emphasise the fact that
the refining efficiency of the Al5Ti1B type master alloy
is consisted, at least during the 90-min holding time,
even with the Ti recovery as small as 0.05 % of mass
fractions of Ti in the A206 alloy. On the other hand, the
results obtained for the non-stirred melt reveal that both
titanium and boron recovery decreased with the
increasing holding time. Interestingly, the diminution in
boron was faster compared to titanium, especially in the
first 30 min of the holding time, for the non-stirred melt.
Therefore, the measured Ti:B ratio shows a continuous
increase as the period of the holding time increases.
In Table 3, the average measured grain size for the
samples of the non-stirred melt also shows a consisted
increase with the holding time. This is associated with
the diminishing of the Ti and B recovery and the ever
increasing Ti:B ratio with the increasing holding time for
the non-stirred heat. It is also obvious from Table 3 that
an immediate stirring action followed by a 90-min hold-
ing time facilitated the Ti and B levels as well as the Ti:B
ratio to remain almost at their initial values. This also
caused a similar consequence in the measured grain size.
These results are in good agreement with a number of
previous studies on Al-Cu alloys. Grain-refining studies
on non-stirred melts were concluded so that the grain
size increased continuously with the increasing holding
time;11–12,16 however, it began to decrease when the
stirring action was resumed.16
The phenomenon of decreasing Ti and B with the in-
creasing holding time in the non-stirred A206 alloy can
be attributed to the settling of TiB2. This compound has
been widely accepted as one of the potential nucleation
sites during the solidification of aluminium.19–22 Since
the density of a solid TiB2 compound is higher (4.48
g/cm3)17 than that of the liquid A206 alloy (2.78 g/cm3),
it is quite probable that some potential TiB2 nuclei are
disqualified because they sink to the bottom of the
crucible as the holding time increases.
The microstructures representing the Alcan test sam-
ples obtained from the heats after the 90-min holding
time are shown in Figures 4a and 4b. A comparison of
the two microstructures provides an explanation of the
difference between the stirred and the non-stirred melts.
The larger grain size for the non-stirred melt in Figure 4b
can be associated with a weaker grain-refining action of
the master alloy during the 90-min holding. This can also
be related to the fading of the potential nuclei probably
due to the gravity action of the TiB2 compound.18
4 CONCLUSIONS
The grain-refining effects of the Al5Ti1B rod-type
master alloy on a commercial A206 alloy with different
addition levels were studied. The effects of the holding
time under stirring and no-stirring conditions were also
studied. From the experimental results, the following
conclusions can be drawn:
1) Different amounts of titanium recovery ranging from
0.01 to 0.3 were formed in the A206 alloys. Measure-
ments showed that the grain size of the Alcan test
samples decreased dramatically with an addition of
the Al5Ti1B master alloy regardless of the titanium
recovery.
2) The smallest average grain size of the A206 alloy
was 82 μm, achieved with a 0.03 % of mass fraction
of Ti recovery in the melt. Increasing the Ti recovery
up to a 0.3 % of mass fractions did not result in a
further decrease in the grain size of the A206 alloy.
3) Grain-size measurements for two different heats
showed that the average initial grain size of the
samples increased from 84 μm to 111 μm at the end
of the 90-min holding time for the non-stirred liquid.
However, a constant grain size was achieved for the
stirred melt throughout the holding period.
4) The results obtained for the non-stirred melt revealed
that both titanium and boron recovery decreased with
N. AKAR et al.: INVESTIGATION OF THE EFFECT OF HOLDING TIME AND MELT STIRRING ...
436 Materiali in tehnologije / Materials and technology 50 (2016) 3, 433–437
Figure 4: Microstructures of Alcan test samples after a 90 min
holding time and an Al5Ti1B master-alloy addition. Samples were
cast from: a) continuously stirred melt and b) non-stirred melt.
Slika 4: Mikrostruktura Alcan preizku{ancev po 90 min zadr`evanju
po dodatku predzlitine Al5Ti1B. Vzorci so bili uliti iz: a) kontinuirno
me{ane taline in b) taline brez me{anja.
the increasing holding time. During the holding pe-
riod, the diminution in boron was larger compared to
titanium, as measured on the cast samples. This was
attributed to the formation of a TiB2 compound in the
melt, which is widely accepted as a heterogeneous
nucleation site for aluminium. Since the density of
TiB2 is higher than that of the liquid A206 alloy, the
compound tends to settle in the non-stirred melt,
which may be the reason for the fading of the grain-
refining efficiency as the holding time increases.
5) The refining efficiency of the Al5Ti1B-type master
alloy can be stimulated via re-stirring the melt. The
results showed that an immediate stirring action
followed by a 90-min holding time reverted Ti and B
and also the Ti:B ratio almost to their initial con-
ditions. The Alcan test samples of the re-stirred melt
also showed a well-refined grain structure.
Acknowledgements
Authors thank Mr. Ahmet Cevdet Altun at Altun
Döküm A.ª., Konya, Turkey, for the casting experiments
carried out at the premises of the company.
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