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Influence of heat treatment on the microstructure and wearbehavior of end-chill cast Zn–27Al alloys with different copper
content
R. Arabi Jeshvaghani1 • H. Ghahvechian1 • H. Pirnajmeddin1 • H. R. Shahverdi1
Received: 19 September 2015 / Accepted: 29 October 2015 / Published online: 15 March 2016
Springer-Verlag Berlin Heidelberg 2016
Abstract The aim of this paper was to study the effect of
heat treatment on the microstructure and wear behavior of Zn–27Al alloys with different copper content. In order to
study the relationship between microstructure features and
wear behavior, the alloys prepared by an end-chill cast
apparatus and then heat treated. Heat treatment procedure
involved solutionizing at temperature of 350 C for 72 h
followed by cooling within the furnace to room tempera-
ture. Microstructural characteristics of as-cast and heat-
treated alloys at different distances from the chill were
investigated by scanning electron microscopy (SEM),
energy dispersive spectroscopy (EDS) and X-ray diffrac-
tion. Wear tests were performed using a pin-on-disk test
machine. To determine the wear mechanisms, the worn
surfaces of the samples were also examined by SEM and
EDS. Results showed that heat treatment led to the com-
plete dissolution of as-cast dendritic microstructure and
formation of a fine lamellar structure with well-distributed
microconstituents. Moreover, addition of copper up to
1 wt% had no significant change in the microstructure,
while addition of 2 and 4 wt% copper resulted in formation
of e (CuZn4) particle in the interdendritic regions. The
influence of copper content on the wear behavior of the
alloys was explained in terms of microstructural charac-
teristics. Delamination was proposed as the dominant wear
mechanism.
1 Introduction
During the last three decades, zinc–aluminum alloys (ZA)
have occupied attention of both researchers and industries
for their unique combination of properties. Some of the
attractive properties of these alloys are lower density,
excellent castability, good machinability, excellent corro-
sion resistance in a variety of environments and superior
wear properties [1–5]. Desirable characteristics and low
manufacturing cost of these alloys enable them to be
considered as competing materials for cast metals such as
high strength aluminum alloys, cast iron, copper alloys and
brass. Typical uses of these alloys include machine tools,
internal combustion engines, bearing bushings and flanges,
fuel-handling components, pulleys, electrical fittings and
hardware components [6, 7].
It is known that mechanical and tribological properties
of ZA alloys strongly depend on their microstructures and
can be further improved by controlling the as-cast
microstructure. The final microstructure of casting is a
complex function of the composition and cooling rate [8].
In this regard, several studies have been done on the
relationships between cooling rate, composition and prop-
erties of ZA alloys.
Savaskan and Turhal [9] have studied the effect of
cooling rate and copper content on the structure and
mechanical properties of monotectoid Zn–40Al–Cu alloys,
which were cast in a permanent mold. They reported that
with increasing cooling rate the hardness, tensile strength,
elongation and impact energy increased in a linear manner.
Furthermore, copper content was found to be more effec-
tive in optimizing the mechanical properties than cooling
rate. Savaskan et al. [10] have also investigated the effect
of copper content on the sliding wear behavior of mono-
tectoid-based Zn–Al–Cu alloys. They found that the
H. Ghahvechian and H. Pirnajmeddin have contributed equally to this
work.
& H. R. Shahverdi
1Department of Materials Engineering, Tarbiat Modares
University, P.O. Box 14115-143, Tehran, Iran
1 3
Appl. Phys. A (2016) 122:397
DOI 10.1007/s00339-016-9820-5
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highest wear resistance obtained with the monotectoid-
based alloy containing 2 wt% copper and the wear resis-
tance deteriorated at higher copper contents. Mojaver and
Shahverdi [11] observed that decrease in cooling rate of
Zn–27Al during end-chill casting resulted in increase in
dendrite arm spacing, percentage porosity and the inter-
dendritic region.
It is well known that addition of copper improvesmechanical and tribological properties of these alloys, but
the existence of copper more than 1 wt% results in the
formation of metastable e (CuZn4) phase during solidifi-
cation [12]. The tendency of the e phase to transform to the
stable T 0 (Al4Cu3Zn) phase leads to the dimensional
instability, which limits the applications of ZA alloys at
low operating temperatures [13]. One of the possible
measures for overcoming this problem is applying a suit-
able heat treatment. Accordingly, several heat treatment
procedures have been suggested by authors, which mainly
involve solutionizing with subsequent water quenching and
artificial aging [2, 14].Babic et al. [1, 2] studied the effects of heat treatment,
involving solution treatment at temperature of 370 C for 3
and 5 h followed by quenching in water, on tribological
behavior of ZA27 alloys. They reported that higher wear
resistance of the heat-treated alloys over as-cast ones is
attributed to the finer and more uniform distributed
microconstituents. Savaskan and Bican [14] investigated
the wear behavior of Al–25Zn–3Cu–3Si alloy subjected to
T7 heat treatment, which includes solutionizing at a tem-
perature of 375 C for 36 h followed by rapid water
quenching and aging at 180 C for 8 h. They found that
heat treatment reduced the wear resistance of the Al–25Zn–
3Cu–3Si alloy, but greatly increased its ductility.
Another interesting procedure is the furnace cooling
process, which includes solutionizing at a high temperature
for a period of time to obtain one or more phases at that
temperature following by slow cooling to room tempera-
ture within the furnace. Slow cooling is common in cast
ingot production. It is interesting to note that phase trans-
formation and microstructural changing in the slow cooling
process are much more complicated than in solution-trea-
ted quenched alloy. In this process the phase or phases
obtained by solution treatment decompose to various pha-
ses during cooling, which is similar to what happens in
some of the advanced metallurgical processes, e.g. con-
tinuous casting. Thus, investigation of the furnace-cooled
eutectoid Zn–Al based alloy is of practical importance for
the advanced metallurgical processes [15, 16].
To the best of our knowledge, most of published papers
have focused on the aging characteristics of conventional
zinc–aluminum alloys [1, 2]. However, little effort has
been made to investigate the microstructural changes and
phase transformation during slow cooling process.
Furthermore, up to now, very limited information exists on
the influence of heat treatment on the response of copper
modified zinc–aluminum alloys. In view of the above, the
aim of this paper is to study the effect of slow cooling
process on the microstructure and wear behavior of Zn–
27Al alloys containing different copper content. In this
regard, an end-chill cast was conducted on Zn–27Al alloys
with different copper content to produce a variety of microstructural features. Then heat treatment, involving
solutionizing at temperature of 350 C for 72 h followed
by cooling within the furnace, was carried out.
2 Experimental procedures
2.1 Alloy preparation
In the present work, Zn–27Al alloys containing different
copper content were produced from pure zinc (99.99 wt%),
pure commercial aluminum (99.9 wt%) and an Al–Cumaster alloy. The alloys were melted in a resistance fur-
nace and degassed with zinc chloride and poured into the
end-chill sand mold in the form of cylindrical castings
(size: 40 mm in diameter, 160 mm height). The details of
the end-chill apparatus, used to promote downward heat
extraction, have been described in our previous works [11,
17]. The chemical compositions of the produced alloys
which determined by atomic absorption spectroscopy are
listed in Table 1.
2.2 Heat treatment procedure and microstructural
examination
In order to study the effect of heat treatment, the as-cast
alloys were solutionized at 350 C for 72 h and then cooled
to room temperature in the furnace. For complete disso-
lution of as-cast dendritic microstructure and interdendritic
phases, solutionizing time was chosen so long. Due to the
shrinkage pipe at the top of the casting and the limited
effective amplitude of the chill, only the first 110 mm of
the castings was considered for study. After cutting the
cylinder longitudinally, one of the halves was cut at 10-mm
intervals for microstructural examinations and the other
Table 1 Chemical composition of prepared alloys
Alloys Elements (wt%)
Zn Fe Al Cu Mg
Zn–27Al Bal. 0.1 25.5 – 0.01
Zn–27Al–1Cu Bal. 0.1 26.2 1.1 0.01
Zn–27Al–2Cu Bal. 0.1 26.9 2.2 0.01
Zn–27Al–4Cu Bal. 0.1 27.6 4.2 0.01
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half was machined into two pins for wear tests. Figure 1 is
a schematic of the preparation of samples. To study the
microstructural features, selected surfaces were polished
and etched using Palmerton’s reagent. After etching, the
samples were rinsed with a solution of CrO3 (20 mg) in
H2O (100 ml) [18]. Finally, the microstructures were
examined by scanning electron microscope equipped with
energy dispersive spectroscopy. The phase identificationwas also conducted by means of X-ray diffraction analysis.
2.3 Wear testing
Dry sliding wear tests were performed using a pin-on-disk
machine. The test materials in the form of pins of 9 mm
diameter were made to slide against a rotating steel disk
with hardness of 58 HRC. Wear tests were carried out in an
applied normal pressure of 1 MPa, a sliding speed of
0.5 m/s and total sliding distance of 3000 m. Three sec-
tions of each alloy, taken at distances of 20, 50 and
110 mm from the chill, as shown in Fig. 1, were tested.These sections were selected based on the data obtained
from prior tests to show enough difference in microstruc-
tural features. Each sample was ultrasonically cleaned and
then accurately weighed before the wear test using a bal-
ance with an accuracy of ±1 mg. After 100, 300, 600,
1000, 1500, 2000 and 3000 m of sliding, the test sample
was removed, cleaned with solvents and weighed to
determine the weight loss. The weight loss of each section
reported as average of two observations. To determine the
wear mechanisms, the worn surfaces of the samples were
examined by SEM and EDS.
3 Results and discussion
3.1 Microstructures of as-cast alloys
Figure 2a shows SEM micrographs of Zn–27Al alloy. As
can be seen in this figure, the as-cast microstructure indi-
cated dendritic structure comprising primary a-dendrites
surrounded by eutectoid a ? g and residual g phase in the
interdendritic regions. This obviously inhomogeneous
structure is as a consequence of solidification under non-equilibrium conditions. A magnified view of the
microstructure and results of EDS analysis are shown in
Fig. 2b. The EDS results indicated that aluminum had the
Fig. 1 Positions of
metallography and wear
samples in the end-chill casting
Fig. 2 a SEM micrographs of
as-cast Zn–27Al alloy and b
higher magnification image of
the microstructure and the EDS
results of the corresponding
compounds
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highest concentration in the dendrite core (EDS 1), then
decreased on the periphery of the dendrite (EDS 2) and was
lowest in the interdendritic region (EDS 3). Furthermore,
the highest concentration of zinc in the interdendritic
regions revealed that zinc-rich g phase precipitated in these
areas.
According to the aluminum–zinc phase diagram [19] in
Fig. 3, solidification of Zn–27Al alloy begins with pre-cipitation of the Al-rich a phase with a dendritic structure
at 493 C. With decreasing the melt temperature to 443 C,
the peritectic reaction ( L ? a ? b) occurs and due to the
rapid solidification condition only a thin layer of the b
phase forms at the edges of the a phase. The residual liquid
becomes enriched with zinc, and the solidification is fre-
quently completed by a divorced eutectic reaction,
L ? b ? g. The eutectic b phase attaches to the peritectic
b phase, and g phase distributes in the form of interden-
dritic layers. The b phase is unstable and decomposes intoa and g at eutectoid temperature [20].
The X-ray diffraction pattern of this alloy is shown in
Fig. 4. It is clear that a and g were predominant phases in
the microstructure and there was no evidence of the pres-
ence of the b phase. This confirms that the b phase has
entirely decomposed into a and g phases.
Figure 5 shows the microstructures of Zn–27Al alloys
with different copper content. The composition of labeled
regions in Fig. 5 determined by EDS is given in Table 2.
The microstructure of the ternary Zn–27Al–1Cu alloy is
shown in Fig. 5a. As can be seen in this figure, addition of
copper up to 1 wt% made no obvious change in thestructure. The results of EDS of areas marked by A and B in
Fig. 5a verified that the area A with a lamellar structure
was eutectoid a ? g and the area marked by B was inter-
dendritic g phase. According to the aluminum–zinc phase
diagram and the lever rule, the majority of dendritic
structure of Zn–27Al alloy transforms into eutectoid a ? g
at 275 C. This was confirmed by the presented
microstructures in Fig. 5, which showed extensive lamellar
eutectoid a ? g. The EDS results also revealed the
Fig. 3 Binary Al–Zn equilibrium phase diagram [19]
Fig. 4 X-ray diffraction pattern of as-cast Zn–27Al alloy
Fig. 5 SEM micrographs of as-cast Zn–27Al alloys with different copper (wt%): a 1 %, b 2 % and c 4 %
Table 2 Chemical composi-
tions of selected areas shown inFig. 5
Area Elements (wt%)Zn Al Cu
A 57.7 38.7 3.6
A0 49.0 42.4 8.6
B 65.5 31.6 2.9
C 85.1 0.1 14.8
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existence of copper in the matrix, which led to solid
solution strengthening effect. As reported by Costa et al.
[21], added copper in low concentration (\1 wt%) dis-
tributes almost uniformly in a macro- and microscopic
scale, presenting complete solubility in the matrix. Since
the solubility of copper in a-Al is generally higher than that
in g- Zn, the copper content in primary dendrites was higher
than that of interdendritic regions.Figure 5b and c shows the microstructures of Zn–27Al
alloys containing 2 and 4 wt% copper, respectively. It is
obvious that the addition of copper more than 1 wt% led to
the formation of a new phase in the interdendritic areas.
The composition of area marked with C in Fig. 5b con-
firmed formation of copper-rich phase in the interdendritic
regions. The X-ray diffraction patterns of these alloys in
Fig. 6 revealed the existence of a, g and copper-rich e
phase in the microstructure.
As mentioned earlier, copper has low solubility in zinc.
Therefore, when the copper content exceeds 1 wt%, the
surplus copper becomes concentrated in the eutectic liquidduring the final stage of solidification and as a result a
metastable copper-rich e phase rejects from the liquid in theFig. 6 X-ray diffraction patterns of as-cast Zn–27Al alloys withdifferent copper (wt%): a 2 % and b 4 %
Fig. 7 SEM micrographs of as-cast Zn–27Al alloys with different copper (wt%) at two different positions from the chill: a, b 1 %, c, d 2 % and
e, f 4 %
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form of discrete irregular particles [21, 22]. Another fea-
ture in Fig. 5b, c is size dependency and distribution of e
phase on the copper content. It is obvious that increasing
copper content increased the size and density of the e
phase.
Figure 7 shows the microstructures of Zn–27Al alloys
with different copper content at two different positions (i.e.
20 and 110 mm) from the chill. It can be seen that withincreasing distance from the chill, the interdendritic
regions became more extensive and size as well as density
of the e phase increased. It is clear that with increasing
distance from the chill, cooling rate decreases. Thus, there
is enough time for melting of small dendrite arms, which
have high surface area to volume ratio. Dissolution of
small dendrite arms increases dendrite arm spacing (DAS),
which leads to significant change in size and density of
interdendritic phases [4].
3.2 Microstructures of heat-treated alloys
Figure 8 shows SEM micrographs of heat-treated Zn–27Al
alloy. Comparing with the as-cast microstructure, it can
clearly be seen that applied heat treatment refined the
microstructure and reduced the segregation effects.
According to the aluminum–zinc phase diagram, solu-
tionizing of this alloy at 350 C results in dissolution of
interdendritic phases into the matrix and formation of b
phase. This phase is unstable below the eutectoid temper-
ature. Thus, during furnace cooling decomposes into the
equilibrium a and g phases through a eutectoid reaction and
forms a lamellar structure [12, 23].
It can be seen in Fig. 8 that during prolonged solution-
izing, the dendrite cores and interdendritic phases com-
pletely dissolved. Therefore, the final microstructure
consisted of a fine lamellar structure of eutectoid a ? g
with a distinct contrast; the dark a and light g phases are
also visible along the grain boundaries which is one of the
characteristics of the b phase decomposition through the
cellular reaction: b ? a ? g. The EDS results of the areas
labeled with D and E in Fig. 8b reveal the presence of a
and eutectoid a ? g phases, respectively. The results of
EDS are listed in Table 3.
Figure 9 illustrates SEM micrographs of heat-treated
Zn–27Al alloys with different copper content in the dis-
tance of 20 mm from the chill. According to the Fig. 9a,
the microstructure of alloy containing 1 wt% copper was
quite similar to the microstructure of copper-free alloy. As
mentioned before, copper dissolves in the matrix and
results in solid solution strengthening [4].
Figure 9b, c shows the microstructures of Zn–27Al con-
taining 2 and 4 wt% copper, respectively. The microstruc-
tures comprised the lamellar a ? g and e phases. By
comparing Fig. 9b, c, it is possible to observe an increase in
the relative quantity of the e phase in the alloy with higher
copper content. Liu et al. [23] also reported that the addition
of copper more than 1 wt% resulted in formation of the e
phase as well as with increasing the copper content the size,
density and distribution of this phase increased.
Figure 10 shows SEM micrographs of heat-treated Zn–
27Al–4Cu alloy at three different positions from the chill. It
is obvious that with increasing distance from the chill, heat-
Fig. 8 a SEM micrographs of heat-treated Zn–27Al alloy in the
distance of 20 mm from the chill and b higher magnification image
Table 3 Chemical composi-
tions of selected areas shown in
Figs. 8 and 10
Area Elements (wt%)
Zn Al Cu
D 18.1 81.9 –
E 44.7 55.3 –
F 10.5 38.8 50.7
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treated structures became coarser. These microstructural
features are due to the use of chill that establishes different
cooling rates in the various distances from the chill and
creates different microstructures along the cast. For this
reason, heat-treated sections nearer the chill showed a finer
structure. Meanwhile, with increasing the size and density of
the e phase at higher position from the chill, the gray pre-
cipitates (area marked by F in Fig. 10c) observed inside the e
phase, which was recognized as T 0 (Al4Cu3Zn). It is reported
that this phase forms during decomposition of the e phase
through the four-phase transformation, a ? e?
T 0
? g[24]. The composition of this phase was determined using
EDS and is listed in Table 3.
3.3 Wear test results
The weight losses obtained for three different sections for
each alloy are plotted versus distance from the chill in
Fig. 11. As can be seen, addition of copper reduced the
weight loss of Zn–27Al alloy. However, addition of more
than 1 wt% copper increased the weight loss. As cited
before, Savaskan et al. [10] also reported that addition of
copper more than 2 wt% deteriorated the wear resistance
of the monotectoid-based Zn–Al alloy. This observation
can be explained in terms of microstructural changes that
occur during solidification and heat treatment.
As mentioned earlier, copper up to 1 wt% completely
dissolves in the matrix and leads to the solid solution
strengthening effect, which improves wear resistance [21].
When the copper content exceeds 1 wt%, the e phase forms
in the interdendritic regions. Seemingly, microstructureswith two phases, one hard and the other soft, are ideal for
bearing materials [25], but formation of the e phase results
in a reduction in the copper content of the a phase which is
the matrix of the alloys and hence reduces the effect of
solid solution strengthening. In addition, formation of the e
phase can increase the cracking susceptibility of the alloys
[10]. Therefore, decrease in wear resistance of the alloy
with 2 wt% copper can be related to the above-mentioned
reasons. Moreover, the lower wear resistance of the alloy
Fig. 9 SEM micrographs of heat-treated Zn–27Al alloys with different copper (wt%) in the distance of 20 mm from the chill: a 1 %, b 2 % and
c 4 %
Fig. 10 SEM micrographs of heat-treated Zn–27Al–4Cu alloy at three different positions from the chill: a 1 mm, b 20 mm and c 110 mm. The
high-magnification images of each position also are shown in top right-hand corner of each micrograph
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with 4 wt% copper may be attributed to the formation of
the T 0 phase, which reduces hardness [13].
According to Fig. 11, the weight losses of alloys without
and with 1 wt% copper increased with increasing distance
from the chill, while for alloys containing 2 and 4 wt%
copper the trend reversed. In the case of Zn–27Al and Zn–
27Al–1Cu alloys, deterioration of wear resistance with
increasing distance from the chill is attributed to thecoarsening of structure. Applying heat treatment breaks the
dendrite structure, decreases the fraction of interdendritic
regions and forms a fine lamellar structure. However, the
as-cast structure affects the characteristics of the heat-
treated structure. With increasing distance from the chill
the grain size increased greatly. It is reported that with
decreasing grain size the strain hardening occurs because
of the dislocation pileup. Moreover, a reduction in grain
size increases the shear strength. Therefore, the structureswith smaller grain size show better wear resistance. This
can be ascribed to the increase in work hardening and shear
strength of the alloy [26, 27]. Concerning alloys with 2 and
4 wt% copper, the inverse trend was due to the increase in
the size and density of the e phase. As reported by Mojaver
and Shahverdi [17], the morphology of the e phase is also
effective on the wear resistance of Zn–27Al alloys.
Comparing the wear results of heat-treated and as-cast
alloys, reported in our previous papers [11, 17], showed
that heat treatment had a positive effect on the wear
resistance of Zn–27Al alloys with different copper content,
because heat treatment dissolved the dendriticmicrostructure of as-cast alloys and created a fine lamellar
structure with well-distributed microconstituents. Further-
more, Prasad [13] reported that the presence of various
phases with different thermal characteristics and mechan-
ical properties in the structure of non-homogeneous as-cast
alloys results in creation of residual stresses on a micro-
scale. Based on this fact, Miroslav et al. [2] concluded thatFig. 11 Weight losses obtained from all the tested sections versusdistance from the chill
Fig. 12 Worn surfaces of heat-
treated Zn–27Al alloys withdifferent copper (wt%) in the
distances of 20 and 110 mm
from the chill: a, b 1 % and c, d
2 %
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obtained uniformity after heat treatment relieves the
residual stresses and enhances the wear characteristics.
3.4 Morphology of the worn surfaces
Since the worn surfaces of all the sections were similar to
each other, typical micrographs of the worn surfaces are
shown in Fig. 12. It is clear that the worn surfaces arerough and have patches with a fractured appearance. From
these patches, it can be deduced that a layer of a material
has been removed as debris. These features propose the
delamination wear as the main wear mechanism.
The delamination is based on the shear plastic defor-
mation, crack initiation and crack propagation [28]. During
the sliding process because of the applied stresses, micro-
cracks nucleate at the phase/matrix interfacial regions.
When these cracks grow and become interconnected,
delamination occurs. It is worthwhile to be mentioned that
the presence of second-phase microconstituents can pro-
mote crack initiation at the phase/matrix interfacial regionsbecause of the inferior coherency between the phase and
the surrounding matrix, which makes the interfacial regions
weak and more susceptible to the cracking [29].
Except the patches, shallow grooves are also visible on
the worn surfaces. These grooves are the consequence of
plowing action of hard debris particles emanated from the
sample surface and the steel disk, which activates three-
body abrasion [13].
4 Conclusions
The influence of heat treatment on the microstructure and
wear behavior of Zn–27Al alloys with different copper
content has been studied. Based on the results and dis-
cussion, the following conclusions can be drawn:
• End-chill casting established different cooling rates in
the various distances from the chill and created
different microstructures along the cast.
• Addition of copper more than 1 wt% resulted in the
formation of copper-rich e phase. The number, size and
distribution of this phase increased with the increase in
copper content.• Solutionizing followed by furnace cooling completely
dissolved the as-cast dendritic structure. The
microstructure of heat-treated alloys is refined with
uniform distribution of microconstituents.
• Heat-treated microstructure of the sections solidified at
higher cooling rates showed a fine lamellar structure
compared with those solidified at lower cooling rates.
The lamellar structure mainly consisted of a and g
phases.
• Copper addition improved the wear resistance of Zn–
27Al alloy. The best wear resistance obtained with
adding copper up to 1 wt%. Wear surfaces of the alloys
were characterized by delamination and shallow
grooves.
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