Effect of Spinning Deformation Processing on the Wear and
Corrosion Properties of Al-7Si-0.3Mg Alloys
˴
Y.C. Cheng ˴*, A.H. Tan c, S.L. Lee a,b, C.K. Lin a,b, J.C. Lina,b
Department of Mechanical Engineering National Central University
b
Institute of Materials Science and Engineering National Central University
c
Department of Mechanical Engineering Ching-Yun University
ʳ
ʽCorresponding author:
Tel:+886-931-249207 Fax: +886-3-4254501
Email: [email protected]
a
Abstract
This study dedicated to modify the casting structure of Al-7Si-0.3Mg (A356) alloys by spinning
deformation processing which was analyzed by OM and SEM, correlated with the wear and
corrosion durability. The spinning deformation processing elongated typical casting structure of
A356 alloys, broke and distributed the eutectic silicon phase over the Al-matrix, eliminated the
casting defect. The contribution of spinning deformation processing to microstructure diminished
the crack nucleation at sliding wear environment. The improved wear rate of A356 alloys related
to refinement of eutectic silicon phase and elimination of casting defect. The resistance to
corrosion was increased by elimination of casting defect. Eliminating casting defect by spinning
deformation processing reduced the pitting susceptibility of A356 alloys and effect of Al/Si interface
to nucleate galvanic corrosion.
Keyword: spinning, aluminum alloys, wear, corrosion
Introduction
Spinning deformation processing was a kind
of method to form hollow cylinder part for
application of military, aerospace and automotive
industry.
The process involved applying
compression to outside diameter of a cylindrical
preform, attached to a rotating mandrel.
Compression was applied by a combination of
axial and radical force using set of roller(s) that
were simultaneously moved along the length of
the rotating preform, flowed the material
plastically in both radical and axial directions.
Several
researches
had
conducted
experimental and simulated analysis in spinning
deformation of tubes to evaluate the power and
requirements as well as the effects of process
variables such as feed rate [1], approach angle [2]
and percentage reduction [3] on the purpose of
light weight-high strength [4]. Presently, the
innovative applications in spinning deformation
processing were to design different profile of
mandrel for producing the cylindrical component
with inner gear or ribs [5, 6]. However, those
parts that were fabricated by spinning deformation
processing generally served in a fatigue, wear and
oxidation environment. As a result, there is a
lack of understanding for resistance to hostile
environment of those spinning deformed parts.
Al-7Si-0.3Mg (A356) alloys offered a
combination of good mechanical properties and
castability, which accounted for their use in
automotive as well as aerospace applications.
Hot spinning deformation processing of Al-Si-Mg
alloys was developed to eliminate casting defect
with a desired distribution of wall thickness [7].
Considering the development of spinning
deformation for aluminum parts resisting hostile
environment, the effect of spinning deformation
on the wear and corrosion properties needed to be
fully understood. Thus the object of this study
was to examine the wear and corrosion properties
of the different microstructure in Al-7Si-0.3Mg
alloys developed by spinning deformation
processing further compared with those
Al-7Si-0.3Mg alloys from permanent mold
casting.
Experimental procedures
The casting sample and spinning preform
were prepared by melting commercial Sr-modified
A356 alloys which was poured into permanent
mold of 500oC following by air cooled. The
composition for A356 alloys in this study was
Al-7.1Si-0.3Mg-0.12Fe-0.11Ti-0.01Srwt%
measured by Glow Discharge Spectrometer. The
preform was designed to a symmetric casting of
solid line pattern in figure 1. The destination in
rim thickness was 8.2 mm as the dash line in
figure 1. The reduction of initial, middle and
final portions started from 0 to 62% depending on
reduction in the original rim thickness of preform.
The spinning sample of this study was sawed from
middle and final portion to focus the effort of
spinning deformation. The preform was attached
1
with mandrel, was rotated with a speed of 200 rpm
and heated to temperature of 350oC.
Three-rollers spinning deformation processing was
performed over the taper mandrel axially and
radially. Optical microscopy and IMAQ 6.0
software were employed to define the average area
and aspect ratio of eutectic silicon phase. To
explore detailed microstructure by SEM
(JEOL-6360), the etchant consisted of 15ml HCl,
10ml HF and 90ml of distilled water was utilized
to remove the Al-matrix. Both casting and
spinning deformed specimens for hardness,
density, wear and corrosion examination were heat
treated with T6 treatment (solid solution at 540oC
for 6 hours and artificially aged at 155oC for 3
hours ). The hardness of A356 alloys was
measured using a Rockwell tester with F scale
indentor in 60Kg load configuration. The bulk
density of A356 alloys was determined using
Archimedes’ principle.
Figure 1 Schematic illustration of wokpiece for
spinning deformation processin˺; preform with
solid line pattern, spinning deformed area with
dash line.
The cylindrical A356 pins, with 5mm
diameter and 15mm length, for a pin-on-disc dry
sliding wear test at a load of 40N, with velocity of
1m/s and distance of 3600m was conducted for all
samples. The counterpart of SK5 steel plate had
a hardness of 60HRC. Before testing, samples
were ground with SiC paper then polished with
0.3µm Al2O3. The wear rate in the present paper
was defined as the weight loss divided by the
sliding distance. Potential dynamic polarization
tests were performed to analyze the corrosion
resistance of A356 alloys in 3.5wt.% NaCl
aqueous solution at room temperature.
The
potential of the samples was swept using an
EG&G Model 273A Potentiostat at a scan rate of
1mVs−1 from the initial potential of −250mV
versus OCP to the final potential of 250mV versus
OCP.
The samples were immersed in the
solution for 30 minutes prior to each corrosion test
to stabilize the system. All potentials were
recorded with respect to a SCE, using platinum
gauze as the counter electrode.
Result and discussion
Figure 2a clearly exhibited a typical Al-Si
hypoeutectic casting structure consisting of
primary α-aluminum with inter-dendritic region of
fibrous eutectic silicon phase. Shrinkage and gas
porosities were also found in the castings. The
fibrous eutectic silicon phase displayed the
average area 3.8±0.6µm2 and aspect ratio
1.58±0.29 that was well modified structure with
strontium (Sr).
The effect of spinning
deformation processing on the microstructure was
illustrated in figure 2b.
The A356 casting
experienced principal stress from radical and axial
direction that elongated the casting structure,
distributed the eutectic silicon phase over the
Al-matrix and eliminated the casting defect. The
average area and aspect ratio of eutectic silicon
phase was 3.2±0.4µm2 and 1.46 ± 0.34,
respectively.
It was 16% refinement
(3.8→3.2µm2) in average area of eutectic silicon
phase and 8% modification (1.58→1.46) in aspect
ratio of eutectic silicon phase.
After T6
treatment, the eutectic silicon phase was sphered
and significantly coarsened in comparison with
as-received A356 alloys. Eutectic silicon phase
of spinning-T6 A356 alloys with an average area
of 7.5±2.2µm2 (figure. 2c) was smaller than the
casting-T6 A356 alloys with an average area of
8.1±3.1µm2 (figure 2d). The average aspect ratio
of eutectic silicon phase for casting-T6 alloys was
1.5±0.5 that approached to 1.41±0.3 of
spinning-T6 A356 alloys. After deep etching
Al-matrix, the eutectic silicon phase with a coral
shape appeared on casting-T6 A356 alloys as
shown in figure 2e. The spinning-T6 A356
alloys displayed fine particle shape for eutectic
silicon phase in figure 2f. It was an interesting
tendency for the average area and aspect ratio of
eutectic silicon phase. The difference in eutectic
silicon phase measurement between casting-T6
and spinning-T6 A356 alloys stayed in the range
of error bar that probably was just a local
characterization for coral-like eutectic silicon
phase in two dimensional observations.
Nevertheless, the break up of eutectic silicon
phase attested to the severe plastic deformation
caused by spinning deformation processing. The
effect of reducing the aspect ratio of eutectic
silicon phase was exceeding to that of using Sr to
modify melts prior to casting.
2
thought to diminish crack nucleation at sliding
wear environment.
Figure 2 Microstructures of A356 alloys: OM
micrographs (a) as-cast, (b) as-spinning deformed,
(c) casting-T6, (d) spinning-T6; deep etching SEM
images, (e) casting-T6 and (f) spinning-T6.
The wear property of A356 alloys was
strongly dependent on porosity level, scale of
eutectic silicon phase and heat treatment [8-10].
The independent effect of a single variable may be
difficult to isolate.
Figure 3 revealed the
hardness, density and wear rate of A356 alloys
with the casting-T6 and the spinning-T6
processing. The hardness of A356 alloys was
identical between the casting-T6 and spinning-T6
processing. But the lower density and much
scattering in hardness of casting-T6 A356 alloys
was observed. The variation found in hardness
due to porosities and concentration of eutectic
silicon phase was reduced by spinning
deformation processing. Thus the elevating in
bulk density and minor variation in the hardness of
spinning-T6 A356 alloys was due to its uniform
microstructure.
However, casting-T6 A356
alloys exhibited a higher wear rate than the
spinning-T6 A356 alloys although that was
identical hardness for both of casting-T6 and
spinning-T6 A356 alloys. At sliding wear test,
large subsurface strains penetrated under the
contact surface which caused micro-crack
nucleation and growth [11]. Kori et al. [12]
added Sr in Al-7Si alloys that changed the
microstructure from a plate-like shape to a round
shape of eutectic silicon phase, which reduced
stress concentration at the interface of
α-aluminum and eutectic silicon phase to improve
the wear failure tendencies. In addition to being
Sr-modified, the spinning deformation processing
transformed the eutectic silicon phase into fine
particles and eliminated the casting defect.
Above contribution of spinning deformation
processing to microstructure of A356 alloys was
Figure 3 Hardness, density and wear rate of A356
T6-treated alloys with different processing.
Figure 4 presented the curve of
potential-dynamic polarization for both casting
and spinning-T6 A356 alloys. The corrosion
potential of A356 alloys was shifted toward the
noble direction from -685 to -661.2mV by
spinning deformation processing.
The lower
corrosion current density in 5.8µA/cm2 of
spinning-T6 A356 alloys was compared with
12.0µA/cm2 of casting-T6 A356 alloys, meaning
better corrosion resistance with spinning-T6 A356
alloys. Al-7Si alloys consisted of α-aluminum
and eutectic silicon that were usually accompanied
with casting defect of shrinkages and gas
porosities. These pores typically occurred in the
inter-dendritic region which was the last parts of
the structure to freeze, intensified the pitting
susceptibility for Al-Si alloys [13]. Apart from
that, the different potential between Al and Si
caused the galvanic corrosion in the aggressive
environment. The corrosion property of Al-Si
alloys was also strongly dependent on size and
content of eutectic silicon phase [14]. The
relatively small scale eutectic silicon phase in the
same Si content possessed more Al/Si interfaces to
nucleate galvanic corrosion.
Relative to
casting-T6 A356 alloys, spinning-T6 A356 alloys
exhibited 7% (8.1→7.5µm2) refinement in average
area of eutectic silicon phase and elimination of
casting defect. The effect of Al/Si interface for
degradation to corrosion was diminished by
elimination of casting defect that improved the
corrosion resistance. Apparently, the casting
defect could be the critical factor for
anti-corrosion in this study. In terms of plastic
deformation for corrosion resistance, identical
improvement was found as the other ECAPed
aluminum alloys [15].
3
4.
5.
6.
Figure 4 Potential dynamic polarization curves of
A356 T6-treated alloys with different processing.
Conclusions
The conclusions for the effect of spinning
deformation processing on the wear and corrosion
properties of A356 alloys revealed as follows:
1.
Applying
the
spinning
deformation
processing elongated casting structure of
A356 alloys that distributed the eutectic
silicon phase over Al-matrix, eliminated the
casting defect and transformed eutectic
silicon phase into a fine particle shape.
2.
The wear rate of casting-T6 A356 alloys was
decreased with contribution of spinning
deformation processing to microstructure.
3.
The resistance to corrosion was improved
with eliminating casting defect by spinning
deformation
processing.
Corrosion
potential of A356 alloys was reasonably
shifted toward the noble direction and
diminished corrosion current density.
7.
8.
9.
10.
11.
Acknowledgements
12.
The authors would like to thank the National
Science Council of the Republic of China, Taiwan
and Chung-Hsin Electric & Machinery Mfg. Corp.,
Taiwan for financially supporting this research
13.
under Contract No. NSCxx-xxxx-x-xxx-xxx-xxx.
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