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Microstructural characterization and corrosion behavior of multipass friction stir processed AA2219 aluminium alloy


Abstract and Figures

Multipass friction stir processing of AA 2219-T87 aluminium alloy to a depth of 2 mm in a 5 mm plate resulted in fine α-Al grains, reduction and dissolution of both eutectic phase (CuAl2) and the strengthening precipitates (CuAl2). Anodic polarization and electrochemical impedance tests in 3.5% NaCl showed an improved corrosion resistance of the processed alloy, which increased with the number of passes. Salt spray and immersion tests also showed improved resistance to corrosion. The increased resistance to corrosion is attributed to the dissolution of CuAl2 particles, which was established by XRD and DSC studies.
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Microstructural characterization and corrosion behavior of multipass friction
stir processed AA2219 aluminium alloy
K. Surekha , B.S. Murty , K. Prasad Rao
Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai, 600 036, India
Received 20 August 2007; accepted in revised form 1 February 2008
Available online 11 February 2008
Multipass friction stir processing of AA 2219-T87 aluminium alloy to a depth of 2 mm in a 5 mm plate resulted in fine α-Al grains, reduction
and dissolution of both eutectic phase (CuAl
) and the strengthening precipitates (CuAl
). Anodic polarization and electrochemical impedance
tests in 3.5% NaCl showed an improved corrosion resistance of the processed alloy, which increased with the number of passes. Salt spray and
immersion tests also showed improved resistance to corrosion. The increased resistance to corrosion is attributed to the dissolution of CuAl
particles, which was established by XRD and DSC studies.
© 2008 Elsevier B.V. All rights reserved.
Keywords: Friction stir processing; AA2219 Aluminium alloy; Corrosion
1. Introduction
AA2219 (Al6.5%Cu) alloy is a widely used age hardenable
alloy in aerospace and defence applications. It is a high strength
alloy with excellent weldability. However, its only disadvantage
is its poor corrosion resistance owing to the galvanic coupling
between the noble CuAl
precipitate and the matrix. If there
could be some means by which CuAl
could be taken into
solution or removed, the corrosion resistance of the alloy can be
improved, though this might lower the mechanical properties to
some extent. It is generally accepted that laser surface melting
(LSM) can be used for improving the corrosion resistance of
metallic alloys as a result of homogenization/refinement of
microstructure, dissolution/redistribution of precipitates or inclu-
sions and phase transformation. However, LSM needs costly
equipment and the operator should be well protected with all
accessories to avoid the hazardous effects of the laser. Similar
changes can be induced in an alloy by friction stir processing
(FSP), which is a cost-effective, environment and user friendly
technique. The present study is undertaken to study the effect of
FSP on corrosion behaviour of AA2219 alloy.
FSP is an emerging surface-engineering technology, which
uses the principles of friction stir welding [1,2] to process materials
in a variety of ways besides joining them. Mishra et al. [2]
developed FSP as a generic tool for microstructural modification
based on the basic principles of friction stir welding (FSW). FSP
can locally eliminate casting defects and refine microstructures,
thereby improving strength, ductility, resistance to corrosion,
formability and other properties. FSP is carried out by rotating and
plunging a specially designed cylindrical, shouldered tool with a
small diameter pin into the plate that is clamped firmly to the bed.
Frictional heat causes the metal to soften and allows the tool to
traverse along the plate. The FSP generates three distinct micro-
structural zones: the nugget, the thermo mechanically affected
zone (TMAZ) and the heat-affected zone (HAZ). The nugget is the
region through which the tool piece pin passes, and thus ex-
periences large deformation and high temperatures. It generally
consists of fine equiaxed grains due to recrystallisation. The
TMAZ adjacent to the nugget is the region where the metal is
plastically deformed as well as heated to a temperature, which is
not sufficient to cause recrystallisation. The HAZ experiences only
heating effect, with no mechanical deformation.
vailable online at
Surface & Coatings Technology 202 (2008) 4057 4068
Corresponding author. Tel.: +91 44 2257 4754; fax: +91 44 2257 4752.
E-mail addresses: (K. Surekha), (B.S. Murty).
0257-8972/$ - see front matter © 2008 Elsevier B.V. All rights reserved.
The corrosion properties of various zones of FSW have been
examined by a number of investigators. Corrosion attack in the
nugget has been found for AA2024-T351, AA5456-H116 and
AA7010-T7651 alloys [3]. Studies have shown attack to be pre-
dominantly in heat affected zone (HAZ), for example in AA2024-
T351 [4], AA7075-T651 [5], AA7075-T6 [6] alloys and for the
AA7050-T7651 alloy, corrosion occurred in the interface between
nugget and thermomechanically affected zone (TMAZ) [7].
Frankel and Xia [8] investigated pitting and stress corrosion
cracking behavior of FSWAA5454 alloy and compared them with
those of basealloy and gas tungsten arc welded (GTAW)samples.
In FSW, the pits formed in HAZ and in GTAW in the fusion zone.
FSW samples also showed pitting resistance greater than the
basemetal (BM) and GTAW welds. Corral et al. [9] investigated
the effect of FSW on the corrosion behavior of AA2024-T4 and
AlLi alloys (AA2195) and showed that the diffusion-limiting
current densities and corrosion potentials of both AA2024 and
AA2195 FSW welds were nearly identical to those of the base
alloys for a 0.6 M NaCl solution. Zucchi et al. [10] reported that
the friction stir welded AA5083 alloy exhibited a higher corrosion
resistance in EXCO solution (4 M NaCl0.5 M KNO
0.1 M
) and a lower pitting tendency than the base alloy.
Meletis et al. [11] investigated stress corrosion cracking be-
havior of FSW AA7075-T6, AA2219-T87 and AA2195-T87
alloys and reported that SCC resistance of the welds is better than
the base metal. Pao et al. [12] studied corrosion fatigue crack
growth of friction stir welded AA7050 alloy and reported that in
both air and 3.5% NaCl solution, fatigue crack growth rates in FSW
HAZ are significantly lower than those in the base metal and the
weld. Lumsden et al. [5,13] and Paglia et al. [14] demonstrated that
the HAZ regions of friction stirred welds of AA7075, AA7010,
AA2024 and AA7050 alloys were more susceptible to intergra-
nular attack than the base alloy, TMAZ and nugget regions.
Jariyaboon et al. [15] reported the effect of welding parameters
during FSW, especially rotation speed and traverse speed, on
corrosion behaviour. All the corrosion studies reported till date
have been on FSW alloys. Though the principles of FSW and FSP
are same, the effect of FSP, a novel surface engineering technique,
on corrosion behaviour has not been studied yet. There have been a
few studies on the microstructure evolution, improvement in fa-
tigue resistance and superplasticity by multipass FSP [1618].
However, no work has been reported on the effect of multipassing
on corrosion behaviour. The present study is focused on this aspect.
2. Experimental procedure
The material used in this work is AA2219-T87 alloy with the
nominal composition in wt.% of Cu6.1, Mn0.25, Zr0.16,
V0.09, Ti0.05 and rest Al. The AA2219-T87 plates (250 ×
150 × 5 mm in size) were friction stir processed, with an
indigenously developed machine (3000 rpm, 15 HP and 25 kN)
at a constant axial force of 12 kN with a non-consumable threaded
tool made up of high-speed tool steel. The basic principle of FSP
is described in the previous section. Multipassing was done such
that one bead was laid exactly over the other. Up to three passes
were given at three rotation speeds of 800 (slow S), 1200
(medium M), 1600 (fast F) rpm and two welding speeds (0.37
(slow S) and 0.76 (fast F) mm/s). The depth of the processed
region was 2 mm in a 5 mm thick plate. Single pass FSP was
carried out at a rotation speed of 800 rpm and traverse speed of
0.37 mm/s are named SS1, two passes as SS2 and three passes are
termed as SS3. Similarly, all other joints were named according to
the parameters used for the process and their nomenclature is
shown in Table 1. FF and FS3 samples are not shown in the table
as it was impossible to get a crack free bead with these parameters.
The parameter optimization and the selection of tool profile in the
present work was implemented taking cue from the work of
Elangovan et al. [19] on friction stir welding. The present work
being FSP, the effective volume of the material which acts as a
heat sink differs from that of welding. The microstructural ana-
lysis of the friction stir processed samples was carried out by
Table 1
Parameters used for friction stir processing
Samples Rotation speed (rpm) Travel speed (mm/s) Number of passes
SS 800 0.37 SS1, SS2, SS3
SF 800 0.76 SF1, SF2, SF3
MS 1200 0.37 MS1, MS2, MS3
MF 1200 0.76 MF1, MF2, MF3
FS 1600 0.37 FS1, FS2
Fig. 1. Optical micrographs of (a) BM and (b) nugget region in MS3 sample.
4058 K. Surekha et al. / Surface & Coatings Technology 202 (2008) 40574068
Leica optical microscope and FEI scanning electron microscope
(SEM). The samples were etched with Keller's reagent to reveal
the grain boundaries. Microhardness measurements were carried
out using Matsuzawa Vicker's micro hardness tester at 25 g load
for 30 s.
Corrosion studies have been carried out using potentiody-
namic polarization tests (ASTM G3) and Electrochemical Im-
pedance tests (EIS). Software based PAR basic electrochemical
system is used for the tests. A flat cell was used for all the
experiments and the 5 mm wide nugget is used as working
electrode, carbon is used as auxiliary electrode and saturated
calomel electrode as reference electrode. 3.5% NaCl was used as
the electrolyte. Potentiodynamic polarization tests were carried
out a scan rate of 0.166 mV/s with an initial potential of
0.25 mV. The E
value signifies the breakage of the passive
film and hence the corrosion current increases drastically with the
applied voltage after E
. For the electrochemical impedance
tests, the samples were immersed in the electrolyte for 30 min
before the test. The samples were exposed (0.16 cm
only the nugget is subjected to the corrosion tests and the rest of
the areas were masked. EIS measurements were carried out in the
frequency range of 10 mHz to 100 kHz.
Immersion corrosion tests were performed on etched sam-
ples according to ASTM standard G110. The samples were
immersed in a solution of 57 g/l (0.98 M) NaCl and 10 ml/l
(0.09 M) for 6 h and the extent of corrosion attack was
observed in SEM. Salt Spray corrosion (ASTM B117) tests
Fig. 2. (a) and (b) SEM micrographs of base metal at different magnifications.
Fig. 3. SEM micrographs of nugget regions in (a) MS1, (b) MS2 and (c) MS3
4059K. Surekha et al. / Surface & Coatings Technology 202 (2008) 40574068
were carried out and weight loss was measured. SEM was used to
know the extent of corrosion. The top surface of the sample alone
was exposed to the fog and the rest of the sides were masked.
Grain size and particle size measurements were carried out
using an Image analyzer attached to an optical microscope. X-
ray diffraction (XRD) and differential scanning calorimetry
(DSC) measurements were carried out by Philips x-ray diffrac-
tometer and Netzsch differential scanning calorimeter, respec-
tively, to find the amount of CuAl
dissolved during FSP.
3. Results and discussion
The corrosion behaviour of the nugget region of the FSP
samples obtained at various rotational speeds and traverse speeds
(SS, MS andFS, SF and MF) followed a similar trend. The traverse
speed did not have any significant influence on the corrosion
behaviour, similar to the earlier observations by Jariyaboon et al.
[15]. In the present paper, the results of multipassing with one set of
rotational speed and traverse speed (MS) are discussed. Inter-
metallic phases formed during casting (eutectic solidification) and
those formed during aging are two types of second phase particles
in precipitation hardenable aluminium alloys. Both these second
phase particles influence corrosion behavior of the alloy and hence
the effect of multipassing on these particles is discussed below.
3.1. Microstructural characterization
Fig. 1 shows the optical micrographs of the nugget region of
the MS3 sample along with the base metal. Significant grain
refinement can be noticed in the alloy on FSP in comparison to
the base metal. Since the second phase particles were not dis-
cernible by optical microscopy, SEM studies were carried out to
know the dissolution of intermetallic particles formed during
solidification. Fig. 2(a)(b) shows SEM micrographs of the
base metal at different magnification, which indicate that the as
cast grain size is very large in this alloy. Fig. 3(a)(c) shows the
nugget region of MS samples on multipassing, which clearly
demonstrates that the grain size of the α-Al has significantly
decreased after FSP. In high strength Al alloys the nugget zone
contains both soluble and insoluble second phase particles
[20,21]. In AA2219 alloy, CuAl
precipitates are soluble and
Zr and dispersoids formed with Ti and V are insoluble. The
second phase particles in Figs. 2 and 3 are identified as CuAl
particles by energy dispersive x-ray (EDX) microanalysis.
The histograms of grain sizes of different MS samples are
shown along with that of BM in Fig. 4.Table 2 gives the
average grain size of α-Al and second phase particle size values.
The average grain and particle sizes were calculated considering
Fig. 4. Average grain size of (a) BM, (b) MS1, (c) MS2 and (d) MS3 samples.
Table 2
Average grain and particle size in the nugget region of different friction stir
processed samples along with the base metal
Alloy condition Average grain size (μm) Average particle size (μm)
BM 67.4 20.9
MS1 6.2 5.2
MS2 6.7 4.5
MS3 7.0 3.5
4060 K. Surekha et al. / Surface & Coatings Technology 202 (2008) 40574068
at least hundred grains and particles. The BM shows a large
average grain size of 67.4 μm while the friction stir processed
sample showed an average grain size of 6.2 μm in the first pass
itself. With subsequent passes the average grain size showed a
marginal increase with MS2 and MS3 samples showing 6.7 μm
and 7 μm, respectively. The formation of these fine grains during
FSP can be attributed to dynamic recrystallization [22,23].Hassan
et al. [24] have reported that a low heat input during FSW results
in an exceptionally fine grain structure along with dissolution of
the precipitates. When the FSP is carried out with higher heat
inputs, the grains in the nugget are coarser. On multipassing, the
heat input is slightly increased and hence the small increase in
grain size on multipassing can be attributed to this factor. Fig. 5
shows a significant reduction in the average particle size of CuAl
on multipassing as observed earlier by Hassan et al. [24].The
average particle size of CuAl
of BM reduced from 20.9 to 4.9 μm
on the first pass and to 4.5 μm and 3.5 μm on second and third
passes. Thus, the present results confirmed that FSP reduces the
size of intermetallic particles, formed during solidification of the
alloy, by fracture of the particles.
The decrease in the number of Cu rich particles in the pro-
cessed alloys is further confirmed by EDX line scan studies. Fig. 6
shows the SEM images of MS samples along with the base metal
and the variation of copper concentration at various passes. It can
be observed that the copper concentration of the particles is very
high (~55 wt.%) in the base metal and it has been lowered
significantly by single pass (~15 wt.%). Further decrease in
concentration of copper is noticed in MS2 (~10 wt.%) and MS3
samples (~5 wt.%). This is due to refinement of the particle size,
which makes the matrix contribute significantly to the line scan
analysis as the number of passes increase.
TEM analysis was carried out to show the dissolution of
strengthening precipitates (CuAl
) since they cannot be resolved
by SEM studies. Fig. 7 shows the TEM images of MS samples
along with the base metal. TEM studies also showed that the size
and volume fraction of the precipitates decreases with increase in
the number of passes. Both eutectic CuAl
and the strengthening
precipitates influence corrosion and hence it has been established
by SEM and TEM studies that size and volume fraction of both
the CuAl
of the eutectic mixture and the strengthening pre-
cipitates decreased with multipass FSP.
The amount of second phase (both eutectic and strengthening
precipitates) dissolved during FSP has been calculated from the
XRD and DSC studies. Fig. 8 shows the XRD patterns of MS
samples along with the base metal. The XRD pattern is magnified
to clearly show the CuAl
peaks, which have much lower
intensity than Al peaks. The figure clearly shows that the intensity
of the CuAl
(JCPDS file number 25-0012) peaks decrease with
multipassing, which indicates dissolution of the precipitate par-
ticles. Similar dissolution of particles on heavy deformation has
been reported earlier [25,26]. The ratio of the area under the most
intense CuAl
XRD peak and the sum of the areas under most
intense CuAl
and Al peaks (JCPDS file number 04-0787) is
found for all FSP parameters and the base metal. Let the value
obtained be X. One minus Xof the processed metal divided by
the Xof the base metal gives the fraction of CuAl
dissolved at a
particular FSP parameter. It was found that the dissolution of
is more at higher rotation speeds and with multipassing.
Fig. 5. Average CuAl
particle size in (a) BM, (b) MS1, (c) MS2 and (d) MS3 samples.
4061K. Surekha et al. / Surface & Coatings Technology 202 (2008) 40574068
Tab le 3 shows the amount of CuAl
dissolved on multipassing
based on the XRD calculations.
The amount of CuAl
dissolved was estimated from DSC
traces by considering the area of the peak corresponding to
dissolution. For a particular FSP parameter, the
difference between the area under the CuAl
peak of the base
metal and the processed metal divided by the area under CuAl
peak of the base metal gives the fraction of CuAl
dissolved at a
particular FSP condition. Fig. 9 shows the DSC traces of the MS
samples. The dissolution is higher with higher rotation speeds
and multipassing, which confirmed the XRD results. However,
the DSC calculations showed a lower amount of CuAl
dissolution than XRD calculations for any given condition.
For example, the amount of CuAl
dissolved in MS1, MS2 and
MS3 samples were found as 42.8, 57.1, and 71.4%, respectively
by XRD measurements and as 23.1, 29.2, 36.9% respectively
Fig. 6. SEM-EDX line scan analysis of MS samples along with the BM.
4062 K. Surekha et al. / Surface & Coatings Technology 202 (2008) 40574068
by DSC measurements. The amount of CuAl
dissolved at other
parameters is shown in Table 2. The DSC results are more
reliable than the XRD results as the intensity of the XRD peak
of the precipitate not only depends on its weight fraction but
also its structure factor and a number of corrections to it.
3.2. Hardness
Dissolution of strengthening precipitates impairs the mecha-
nical properties. To have an insight into the mechanical pro-
perties, hardness measurements were carried out. The average
hardness values of MS samples on multipassing are shown in
Fig. 10. In all parameters, the nugget showed a lower hardness
compared to the base metal. This suggests dissolution of
Fig. 7. TEM images of (a) BM and nugget regions in (b) MS1, (c) MS2 and (d) MS3 samples.
Fig. 8. XRD patterns of MS samples on multipassing along with that of base metal.
Table 3
XRD and DSC results showing the amount of CuAl
dissolved during FSP
Intensity of
of Al
% of CuAl
(XRD result)
% of CuAl
(DSC result)
BM 124.37 1519.0 0.07 ––
MS1 82.02 1604.0 0.04 42.8 23.1
MS2 74.54 1851.8 0.03 57.1 29.2
MS3 70.75 2821.4 0.02 71.4 36.9
FS1 209.3 8243.9 0.02 71.4 27.5
FS2 24.6 1713.7 0.01 85.7 34.8
SS1 42.6 807.2 0.05 28.6 17.4
SS2 209.1 4825.6 0.04 42.8 24.7
SS3 95.6 2746.5 0.03 57.1 29.6
4063K. Surekha et al. / Surface & Coatings Technology 202 (2008) 40574068
precipitate particles, which was confirmed by XRD and DSC
results. Sato et al. [27] have examined the hardness profiles
associated with the microstructure in an FSW precipitation
hardenend AA6063-T5 alloy and observed similar results as in
the present case.
It was found that the area of the softened zone (nugget region)
and the hardness in the nugget region increased with increase in
attributed to the increase in the heat input and the increase in
hardness with multipassing may be attributed to the decrease in
particle size of insoluble dispersoids. FSP creates a softened
region around the weld center in a number of precipitation-
hardened aluminium alloys. It was suggested that such a softening
is caused by coarsening and dissolution of strengthening pre-
cipitates during the FSW [28,29].
3.3. Corrosion behaviour
Potentiodynamic polarization tests were carried out to find the
pitting corrosion resistance. Anodic polarization curves were
obtained by exposing the nugget area alone to 3.5% NaCl solution,
whose pH is maintained at 10 by adding KOH. Fig. 11 shows the
potentiodynamic polarization curves of MS samples on multi-
passing. It was found that with three passes the pitting potential
was nobler (466 mV) compared to single pass (498 mV), two
passes (484 mV) and the base metal (587 mV). I
gives the
direct measure of corrosion rate. The I
value of the base metal is
869 µA and it is 77, 31 and 6 µA for MS1, MS2 and MS3 samples,
respectively. Table 4 lists the E
and I
values of MS samples
along with the base metal. From these results it can be seen that the
corrosion rate decreased drastically with FSP and further im-
provement in corrosion resistance is observed with increase in
number of passes.
Electrochemical Impedance Spectroscopy (EIS) results for
base material and the nugget exposed to 3.5% NaCl solution for
30 min are plotted in Fig. 12. The low frequency impedance
indicates the corrosion resistance of the surface. It was found that
the nugget of MS3 exhibits higher electrochemical corrosion
resistance (18.3 kΩcm
) than the base material (484.6 Ωcm
It was also found that the electrochemical resistance increased
Fig. 9. DSC traces of MS samples on multipassing.
Fig. 10. Average hardness value of MS samples on multipassing along with that
of the base metal.
Fig. 11. Potentiodynamic polarization curves of MS samples on multipassing.
Table 4
Corrosion values of FSP samples after pitting, impedance and salt spray tests
Corrosion rate
in mpy
(mV) I
(μA) Z(kΩcm
BM 15.7 587 869.6 0.484
MS1 4.5 498 77.8 9.6
MS2 4.0 484 31.5 14.2
MS3 3.8 466 6.3 18.3
Fig. 12. EIS curves for MS samples on multipassing.
4064 K. Surekha et al. / Surface & Coatings Technology 202 (2008) 40574068
with increase in the number of passes. The impedance values of
MS1 and MS2 samples were 9.6 and 14.3 kΩcm
, respectively.
Table 4 lists the impedance values of the MS samples along with
the base metal.
The galvanic coupling between the Al matrix and the CuAl
precipitate is the main reason for corrosion of AA2219 alloy.
Dissolution of the CuAl
precipitates decreases the sites for
galvanic coupling and hence increases the corrosion resistance.
Dissolution of the precipitates was confirmed by both XRD and
DSC techniques. From Fig. 13 it is evident that the corrosion
resistance increases with increase in number of passes. Fig. 13
shows the influence of CuAl
dissolved on the impedance
values. The amount of dissolution increased with the increase in
number of passes and hence the corrosion resistance also fol-
lowed the same trend.
Salt spray tests were carried out in 5% NaCl solution for
100 h to assess the uniform corrosion resistance. Weight loss
measurements were made to find the corrosion rate. Pits of very
small diameter were observed on the surface after corrosion test
and a continuous decrease in thickness over the entire surface
area of the metal was observed throughout the corrosion test.
Fig. 14 shows the SEM images of the MS samples after salt
spray test along with the base metal. Tab le 4 shows the cor-
rosion rate of the processed alloys after salt spray test. It is seen
that the base metal has corroded very severely and corrosion
products are seen throughout the surface. But in MS1 only a few
pits are seen. It can further be seen that the density and size of
pits in MS2 and MS3 are lower in comparison to MS1. This
Fig. 13. Influence of amount of CuAl
dissolved on impedance values.
Fig. 14. SEM images after salt spray test of MS samples on multipassing.
4065K. Surekha et al. / Surface & Coatings Technology 202 (2008) 40574068
increased corrosion rate of the base metal is due to the large
number of CuAl
sites available for the galvanic coupling
between the Al matrix and the CuAl
particles, which is clearly
revealed by the particle size. The corrosion rate in the base
metal is 15.7 mpy where as it is 4.5, 4.0 and 3.8 mpy in MS1,
MS2 and MS3 samples, respectively. From these results, it
can be inferred that with the increase in number of passes the
corrosion resistance increases. The corrosion product formed is
known to be Al(OH)
. EDX microanalysis of the corroded
products was carried out. However, as this technique can not
reveal the presence of hydrogen, the presence of Al(OH)
not be proved at the moment. Attempts are on to carry our FTIR
studies to prove the presence of Al(OH)
as a part of future
To investigate the susceptibility to intergranular attack, 6 h
immersion test was carried out in a solution containing 57 g/l
NaCl and 10 ml/1 H
(30 vol.%). Fig. 15 shows the SEM
images of the samples after immersion test. The results are
consistent with the impedance, potentiodynamic and salt spray
tests. The number of passes is the primary factor in determining
the rate of attack. The corrosion attack is less in MS3 sample
compared to MS1, MS2 samples and the BM. It is seen the base
metal has corroded intergranularly whereas only a few pits were
seen in the processed alloys. Intergranular corrosion will occur
only when the following three conditions are simultaneously
met [30]:
1. Presence of a corrosive medium,
2. Difference in potential in the order of 100 mV between the
intermetallics and the matrix,
3. Continuous network of the intermetallics at the grain
boundaries such that intergranular cracks can propagate.
All the above three conditions are met in the case of base
metal. Hence, the base metal corroded intergranularly. The
corrosion products are identified as Cu
O in this case based on
the EDX microanalysis results as shown in Fig. 16. Copper
enrichment (5458 wt.%) in the corrosion deposit was
observed, which was due to the selective dissolution of copper
rich intermetallic followed by redeposition of copper on the
surface in the form of oxide. In the case of FSP alloys, con-
tinuous network of CuAl
at grain boundaries is not observed
due to the breakage and dissolution of the intermetallic par-
ticles. Hence, there was no copper enrichment in the corrosion
Fig. 15. SEM images after G110 corrosion of MS samples on multipassing.
4066 K. Surekha et al. / Surface & Coatings Technology 202 (2008) 40574068
products and only a few pits were observed. The propagation of
intergranular corrosion starts at pits. This indicates that it needs
some more time for the intergranular cracks to propagate.
Hence, the processed alloys have better corrosion resistance
compared to the base metal. Fig. 17 shows the EDX analysis of
the corrosion products of MS1, which clearly indicates that the
corrosion product is Al
in this case.
4. Conclusions
FSP is a novel surface modification technique and in the
present work multipass FSP is carried out to improve the cor-
rosion resistance of AA 2219 aluminium alloy. The following
conclusions can be arrived at based on the present work.
1. The effectiveness of FSP in improving the corrosion
resistance has been demonstrated on the AA2219 alloy. All
the processed alloys, irrespective of the processing para-
meter studied showed superior corrosion resistance com-
2. Number of passes during FSP has a significant effect on
the corrosion resistance. Three pass and two pass FSP alloy
showed better corrosion resistance in comparison to the
single pass processed alloy.
3. Dissolution of the CuAl
particles during FSP reduces the
number of sites available for galvanic coupling and hence a
reduction in the corrosion rate is observed in the processed
The authors gratefully acknowledge the help and support ren-
dered in FSP by Prof. V. Balasubramanian of the Department of
Manufacturing Engineering, Annamalai University, Chidambaram.
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... This might be explained by the rise in heat input from faster spinning speeds. 81 The FSP zones had a more significant temperature and a slower cooling rate due to the high tool rotational speed. ...
... Hardness may decline as a result of this. 81 Therefore, the tool's rotational speed must be balanced to avoid grain growth while providing adequate heat to soften the material, enabling significant plastic deformation, proper material flow, elimination of porosity and refinement of grains through recrystallization. ...
As a noble surface modification technique, friction stir processing (FSP) has been established based on the principle of friction stir welding that refines the microstructure and localized plastic deformation of the material. Nevertheless, despite notable technological progress, challenges persist in utilizing FSP to improve material characteristics. This review paper offers a concise overview, summarizing the influence of process parameters on the microstructure mechanical and tribological properties of aluminium alloy subjected to FSP. In particular, the performance of aluminium alloy is discussed about FSP factors such as tool travel speed, rotating speed, reinforcement particles, multiple passes and tool tilt angle. This paper also reviewed the effect of different process parameters on the wear properties, corrosion and tribological characteristics of the processed alloy, which will facilitate researchers and business organizations in creating the needed upgraded composite.
... For further investigation of the structure of the commercially available Al foils coated with GRMs, a comparison is made using bare Al foil in tandem with X-ray diffraction (XRD). As can be seen in Figure S7, the typical graphite peak at 26° is observed in all coated samples, while their corresponding peak intensities of crystalline aluminum planes are variable among the specimens examined, attributed to the crystalline structure of the foils [42]. ...
Full-text available
Commercial aluminum foils were coated by graphene oxide, and its functionalized derivatives and the corrosion performance of the coated specimens were examined in acidic conditions (lithium perchlorate and sulfuric acid). Electrochemical experiments have shown that all graphene oxide-coated specimens provided up to 96% corrosion inhibition efficiency with a corresponding lower corrosion rate compared to the bare aluminum foil. Our results clearly show that graphene-related materials offer viable alternatives for the protection of aluminum, and this opens up a number of possibilities for its use in a number of commercial applications.
... The (Al2Cu) precipitates were less thermally stable as compared to the (Al2CuLi) precipitates [4]. α-Al grains, reduction, and dissolution of both the eutectic phase (CuAl2) and the strengthening precipitates (CuAl2) improved the corrosion resistance of friction stir processed (FSPed) Al-Cu-Li alloys [5][6][7]. Aging treatment also improves the tensile properties and intergranular corrosion resistance of Al-Cu-Li alloys [8]. The microhardness test reveals that magnesium has a vital role in influencing the microhardness of the specimens, as they diffuse more into aluminum and copper [9]. ...
- In this work, AA2050 alloy was friction stir processed at various tool rotation speeds and feed rates. The material was subjected to artificial aging to investigate its influence on macrostructure, microstructure, microhardness, and tensile strength of friction stir processed AA2050. Besides, a hot salt corrosion test was done on the test specimens at 130 °C for 168 hours. The results indicate that grain refinement and dispersion of secondary phase particles improved the microhardness and strength of friction stir processed specimens. The artificial aging of the friction stir processed specimens improved the ductility by 81.5%. However, the tensile strength of the specimens decreased by 2.8%. The corrosion (mass loss per unit area) of the specimen processed with a single pass at a speed of 600 rpm and feed rate of 60 mm/min decreased by 90% compared to the base material.
... 2219 aluminum alloy is one of the most commonly used alloys for advanced applications, dues to its excellent weldability, good mechanical properties, high specific strength, and good corrosion resistance [1][2][3]. The thin sheets made of this alloy are used to form thin-walled components, which are widely applied in the aerospace, aviation, and automotive industries [4]. ...
Full-text available
To facilitate the manufacturing of the thin-walled components of 2219 aluminum alloy, the quenching–forming–aging (Q–F–A) process has been increasingly utilized. However, natural aging (NA) after quenching significantly affects the cold forming performance of this alloy. In this study, experiments are conducted to investigate the effect of NA time on the cold forming performance of 2219 aluminum alloy. The results indicate that NA can weaken the Portevin–Le Chatelier (PLC) effect, thereby reducing its influence on the cold forming performance of the alloy. The PLC effect becomes indistinct when the aging time reaches 2 years. The yield strength of 2219 aluminum alloy increases monotonically with aging time, while the elongation first increases rapidly and then decreases. After an aging time of 2 years, the yield strength increases by 28.6% from that of newly quenched alloys. The strain hardening index and hardening coefficient indicate that short-term NA (less than 4 days) increases the work hardening rate, while long-term NA reduces it. Microstructural analysis shows that the strengthening effect of NA on 2219 aluminum alloy is mainly due to the growth of G.P. zones and the precipitation of θ″ phases. The NA precipitation behavior can also cause the aggregation of solute atoms and weaken the PLC effect.
... Homogenization would also mean the reduction of porosity in cast materials [3]. Through such microstructural modifications, FSP has been shown to improve ductility [4]- [6], fatigue resistance [7], yield stress [8], wearability [9], and corrosion resistance [10]. In some cases, the property enhancement is greater with more passes of FSP [11]. ...
Full-text available
This paper investigates the thermal-pseudo mechanical (TPM) model's residual stress prediction capability for its utility in developing friction stir processing (FSP). Specifically, two FSP tests under different processing conditions were conducted, and the corresponding simulations were carried out to verify if the TPM model can predict residual stresses for various tool radii and workpiece materials. The model successfully predicted residual stresses with an error less than 4% for one of the tests but failed to work for the other test. Further simulations under different FSP conditions proved that the TPM model works for cast aluminum alloys and wrought aluminum alloys. In addition, the large FSP tool used was found to be the reason for the model's failure on one of the tests. This indicates that there is a range of tool radii for which the TPM model is applicable. As a solution, this paper suggests modifications to the TPM model based on calibration to the FSP test temperatures. The resulting residual stress prediction is accurate and differs from the experimentally characterized stress values by only 6.5 MPa. The calibrated TPM model requires FSP to be carried out when using a tool with a different radius. Following that, the effect on residual stresses due to changes in the other process parameters, such as the tool traverse & rotation speeds and the clamping conditions, can be predicted.
... M Jariyaboon et al [26] reported that rotational rate is primary in finding the locations of corrosion activity for Al2024 joints, resulting in higher rotational speed giving better corrosion resistance. K Surekha et al [27] also confirmed that the corrosion performance of the Al2219 depends on the rotational speed and dissolution of precipitates. They found that the corrosion rate of the stir zone decreased with the increment in the rotational rate. ...
Full-text available
In the present investigation, the influence of tool rotational speed (900, 1100, 1300, and 1500 rpm) on microstructure, mechanical characteristics, and corrosion behavior of friction stir welding (FSW) of dissimilar Al5083-6061 alloys was studied. Optical and scanning electron microscopes were used to study the microstructural features. The grain size measured at the stir zone (SZ) decreased considerably in contrast with the base materials (BM) but increased with tool rotational speed. The mixing degrees of two materials at the SZ are enhanced by increasing rotational speeds. The mechanical properties, such as hardness and tensile properties, were studied using Vicker's hardness tester and universal testing machine. The hardness of the weld joints is inferior to that of the base metal, and uniform hardness distribution and the highest average hardness in SZ were obtained at 1100 rpm. The higher strength and elongation of 202 MPa and 5.2 %, respectively, were achieved at 1100 rpm, with joint efficiency of 65 %, due to optimum heat input, and the lowest tensile strength and elongation of 156 MPa and 3.2%, respectively, were obtained at 1300 rpm with a joint efficiency of 50 % due to excess heat input. It is curious to note that an increase in tensile elongation accompanies the enhancement of weld strength. The fracture occurred at the retreating side of the heat-affected zone as it is the weakest zone in the FSWed joint. Different corrosion tests, such as immersion, open circuit potential, and Tafel polarization tests, were carried out using an electrochemical workstation. The corrosion findings showed that the corrosion resistance of the samples increased after the welding, and the highest corrosion resistance was obtained at a rotational speed of 1100 rpm.
... This can result in the lack of the necessary plastic martial flow for particle dispersal. Multi pass FSP technique may therefore be adopted to improve reinforcement particle distribution [35][36][37]. By and large it is observed that increasing tool rotation speed and decreasing too feed rate will improve reinforcement particle dispersion [38]. ...
In this study, the Repeated Upsetting (RU) process as a severe plastic deformation method was performed on the aluminum alloy 5083 (AA5083) to refine the microstructure. The microstructure, mechanical properties, and corrosion behavior of AA5083 before and after applying 1, 3, and 5-pass through the RU process were investigated. Based on the EBSD analysis, the average grain size of AA5083 decreased from 21.9 μm for the base metal (BM) to less than 5 μm for RUed samples. The potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) results have shown that the corrosion resistance of the AA5083 increased after applying the RU process due to the grain refinement and homogenous distribution of the second phase. By increasing the RU passes from 1 to 5, the corrosion rate of AA5083 was enhanced because of rising localized plastic deformation. Scanning electron microscopy and optical microscopy images confirmed the electrochemical results, in which the BM showed a severe corrosion surface compared with the RUed samples. On the other hand, tensile tests confirmed that the maximum improvement in mechanical properties occurred after one process pass. Accordingly, the 1-pass RU product can be an ideal candidate for application under severe corrosive environments and high-loading conditions.
In this study, the microstructure of aerospace-grade aluminium 7075 alloy was modified using multi-pass friction stirring, and its mechanical and corrosion properties were investigated. Macroscopic observations indicated the formation of defect-free stir zones with better material consolidation. Electron backscatter diffraction investigation revealed significant grain refinement on the specimen subjected to 4 friction stir processing passes with grain size averaging 2.6 μm. Transmission electron microscopy observations confirmed the phenomenon of grain boundary pinning by second-phase MgZn2 precipitates, which favoured the formation of a fine-grained structure due to dynamic recrystallization. X-ray diffraction analysis confirmed the occurrence of lattice distortion in the matrix due to variations in the coefficient of thermal expansion between the matrix and the precipitate. Bulk texture results exhibited a maximum texture index of 2.0800 in the 4-pass friction stir processed specimen with copper and S being major components. The average microhardness and tensile strength increased to 122.97 HV and 470 ± 15 MPa after 4 friction stir processing passes due to finer grains, dense dislocations and finely dispersed precipitates in the microstructure. Potentiodynamic polarization analysis displayed enhanced corrosion resistance in the friction stir processed specimens compared to the base alloy. Maximum resistance to corrosion with a corrosion potential of –1.0852 V was exhibited by the 4-pass specimen due to grain refinement and the formation of a stable passive film.
In this work, the materials from different positions of 2219 aluminum alloy/304 stainless steel inertia friction welded tank with long‐term storage of N 2 O 4 were studied. The results showed that the corrosion product layer composed of Fe 3 O 4 and FeCr 2 O 4 was covered on the surface of stainless steel. The corrosion product layer on the surface of stainless steel can effectively improve the protection effect. Severe corrosion occurred in both the sidewall and bottom aluminum alloy, with intergranular corrosion depths of about 30 to 50μm observed in the cross‐section, accompanied by typical exfoliation corrosion. A Fe‐Al dominated intermetallic compound layer was formed at the weld. A dynamic recrystallization zone and thermo‐mechnical affected zone were included on the aluminum alloy side with intergranular corrosion depths of 4.3 and 13.3μm, respectively. The granular Al matrix can be seen on the peeling surface of the aluminum alloy. Combining scanning Kelvin probe and potentiodynamic polarization curve results, the high potential of the stainless steel could further promote the corrosion of the aluminum alloy. This article is protected by copyright. All rights reserved.
Full-text available
The localized corrosion and environmentally assisted cracking of high strength aluminum alloys 7075-T651 and 7050-T7451 friction stir welds have been investigated in the weld transverse and longitudinal orientation by using conventional electrochemical measurements, corrosion immersion tests as well as constant extension rate testing. The weld micro-zones of both 7075-T651 and 7050-T7451 friction stir welds were found to be more susceptible to corrosion as compared to the unaffected parent metals. The heat affected zones of the weld were found to be the most susceptible to corrosion and environmental assisted cracking for the 7075-T651 FSW. An asymmetrical distribution of the' localized corrosion susceptibility (intergranular corrosion) correlated with the slightly different Cu depletion along the grain boundaries on both sides of the weld. In this alloy, fracture occurred constantly within the "soft" heat affected zones for samples tested in air and in 3.5 wt. % NaCl solution. In 7050-T7451, the TMAZ-nugget boundary exhibited the greatest corrosion susceptibility. In this alloy, the fracture position changed from the "soft" heat affected zones for samples tested in air to the TMAZ-nugget interface for samples tested in 3.5 wt. % NaCl solution. In this case a high environmentally susceptibility was found within the nugget and the TMAZ which exhibited grain boundaries heavily populated with Cu-enriched corrosion susceptible Mg (Zn, Cu)2. In constant extension rate tests, 7075-T651 FSW generally exhibited better environmental cracking resistance as compared to 7050-T7451 FSW, while 7075-T651 parent metal indicated lower resistance then the parent metal of 7050-T7451.
Full-text available
The microstructure and hardness of a friction stir welded (FSW) 7075-T6 alloy has been correlated with mechanical properties, corrosion and environmental cracking behavior for transverse and longitudinal orientations. The strength and ductility of the weld micro-zones are controlled by grain size, the presence and size of the coherent intragranular precipitates, along with the degree of sensitization achieved during the weld process. The corrosion behavior is influenced by the intermetallic phases as well as the grain boundary phases and/or precipitate-free zones, although a discrimination on the intergranular susceptibility between the micro-zones could not be clearly achieved with conventional polarization techniques. The heat affected zone in the trailing side of the weld is the highest susceptible micro-zone to corrosion as well as environmental assisted cracking which correlate with the sensitization level i. e. with the extent of Cu depletion at grain boundary.
The corrosion properties of high strength aluminum alloys are strongly dependent on grain boundary composition as well as the composition and morphology of the intermetallics. The recrystallized microstructure of the weld nugget usually has corrosion properties different from those of the parent material. In addition, the local temperature occurring during FSW is sufficiently high to cause dissolution, nucleation, and/or coarsening of the strengthening intermetallics in the weld heat affected zone (HAZ). Our investigations have shown that these transformations produce a sensitized microstructure increasing susceptibility to intergranular corrosion, pitting, and stress corrosion cracking (SCC). This work investigates post weld heat treatments of FSW and active cooling of AA7050-T7651 as corrective actions for restoring SCC resistance. Results using the slow strain rate (SSR) technique showed that an artificial aging treatment of 100°C for 1 week restored a significant amount of the SCC resistance. Other artificial aging treatments investigated, although they restored the SCC resistance, caused an unacceptable loss in mechanical properties under ambient conditions.
Friction stir welding (FSW) was utilized to join three high-strength aluminum alloys, namely Al 7075, 2219 and 2195. In the present investigation, the stress corrosion cracking (SCC) behavior of these FSW alloys was studied by conducting two types of experiments: (i) four-point bending at three loading levels (50%, 65% and 85% of the ultimate tensile strength) under alternate immersion (AI) conditions in 3.5% NaCl solution for 90 days and (ii) slow extension rate testing (SERT) at 3.3×10-5 s-1 of specimens that were pre-exposed (PE) under AI in 3.5% NaCl solution. The latter set of experiments also involved testing of all specimens that survived the 90-day exposure under four-point bending. The results showed that all FSW alloys exhibited significantly lower strength compared to their parent alloy counterparts. Also, with respect to the corresponding parent alloys, FSW Al 7075 and 2195 exhibited lower ductility (∼50% reduction), whereas FSW Al 2219 exhibited comparable ductility. The four-point bending results revealed no SCC susceptibility for any of the FSW alloys for the given exposure period and loading levels. Significant ductility reductions were observed only for Al 7075 in the more severe SERT experiments. In this case susceptibility increased with increasing PE time. The present evidence suggests that the observed environmental susceptibility in FSW Al 7075 is due to hydrogen embrittlement.
Five-pass friction stir processing (FSP), with 50% overlap, was conducted on cast A356. Overlapping FSP did not exert a significant effect on the size and distribution of the Si particles. In the as-FSP condition, the strength and ductility of the transitional zones between two FSP passes were slightly lower than those of the nugget zones. Further, in the multiple-pass material the strength of the previously processed zones was lower than that of the subsequent processed zones due to overaging from the FSP thermal cycles.
The corrosion properties of friction-stir-welded (FSW) AA7050-T765 I have been investigated. Immersion in a modified exfoliation corrosion (EXCO) solution showed that the grain boundaries in the nugget, partially recrystallized zone (PRZ), and heat-affected zone (HAZ) were sensitized. The most heavily sensitized region was the nugget/PRZ interface. Pitting potentials were determined potentiodynamically in 0.6 M sodium chloride (NaCl). The nugget had the lowest pitting potential, which was 75 mV less than the parent material. The pitting potential of the HAZ was 50 mV less than the parent material. In results from the slow strain rate test, the percent elongation in 0.6 M NaCl for the slowest strain rate used was ∼20% that in air, indicating susceptibility to stress corrosion cracking. Metallography showed that the fracture was intergranular and that the crack path was on the nugget side of the nugget/PRZ interface. Analytical electron transmission microscopy (ATEM) does not conclusively identify a grain boundary, precipitate-free zone, or precipitate chemistry, which correlates with sensitization. These chemistries are complex and vary with distance from the center line of the weld.
The susceptibility of welded and unwelded samples of Al 5454 (UNS A95454) in the -O and -H34 tempers to pitting corrosion and stress corrosion cracking (SCC) in chloride solutions was studied. Welded samples were fabricated using the relatively new friction stir welding (FSW) process as well as a standard gas-tungsten arc welding process for comparison. Pitting corrosion was assessed through potentiodynamic polarization experiments. U-bend and slow strain rate tests were used to determine SCC resistance. The FSW samples exhibited superior resistance to pitting corrosion compared to the base metal and arc-welded samples. U-bend tests indicated adequate SCC resistance for the FSW samples. However, the FSW samples exhibited discontinuities that probably were associated with remnant boundaries of the original plates. These defects resulted in intermittent increased susceptibility to pitting and, particularly for Al 5454-H34 samples, poor mechanical properties in general.
Friction stir welding (FSW), a relatively new solid-state joining process, is used to join Al alloys of all compositions, including alloys essentially considered unweldable. This study focused on microstructures in FSW Al alloy 7075-T651 (AA 7075-T651 [UNS 97075-T651]), an alloy not commonly fusion welded, and the resultant corrosion susceptibility. Although the heat input associated with FSW was relatively low and the time at temperature was short compared to fusion welding, localized microstructures, chemical segregation, and precipitate distributions were created that generally are not present in parent metal AA 7075-T651. Typically, in the weld and heat affected zone (HAZ), the times at peak temperature were short, cooling was relatively rapid, and peak temperatures were > â500 C. Accordingly, a corresponding microstructural gradient developed from the weld nugget into the unaffected parent metal with the precipitate distribution in and around grain boundaries reflecting this temperature excursion. Some of these microstructures, when exposed to a corrosive environment, showed selective grain boundary attack and a decrease in the pitting potential relative to the parent metal. A characterization of the microstructure and localized chemistry differences within the weld zones suggested that the decrease in corrosion resistance correlated with a depletion of Cu within the grain boundaries and precipitate-free zones. These results provided evidence that the lowered resistance to intergranular corrosion following FSW of AA 7075-T651 was caused by a difference in pitting potentials.
This paper investigates the potential of Laser Surface Melting (LSM), carried out using an excimer laser, to suppress the localised intergranular corrosion of friction stir welds (FSW) from the alloys 2024-T351 and 7010-T7651. The laser-melted surfaces have been characterised using optical microscopy, Scanning Electron Microscopy (SEM), immersion testing and atmospheric corrosion panels. A micro-electrochemical technique has also been used to obtain profiles of the anodic and cathodic reactivity across the width of each weld and laser treated surface. LSM treatment has been shown to produce a chemically homogeneous melt layer (∼10μm thick), which is more corrosion resistant than the untreated alloy. By laser-treating the surface of FS welds, it has been possible to suppress intergranular corrosion of the HAZ and nugget regions, displacing it instead into the surrounding untreated material where a more general pitting attack occurs.
A staggered pass sample of friction stir processed (FSP) 7075 aluminum was created to make samples with one through four passes of FSP under identical conditions. The tensile testing temperatures ranged from 673 to 763K with initial strain rates ranging from 1×10−3 to 1×10−1s−1. Materials processed by single as well as multiple passes exhibited superplasticity across various testing temperatures and strain rates while the as received materials exhibited elongations below 200%. This study demonstrated the effectiveness of four consecutive FSP passes in creating large areas of superplastic material. However, the largest elongations were observed for the single pass material.