|Título/s:||Effect of processing parameters on superplastic and corrosion behavior of aluminum alloy friction stir processed|
|Autor/es:||Burgueño, A.; Dieguez; T.; Svoboda, H.|
|Institución:||INTI-Mecánica. Buenos Aires, AR |
Universidad de Buenos Aires, Facultad de Ingeniería. UBA. Buenos Aires, AR
|Palabras clave:||Fricción; Aleaciones de aluminio; Corrosión; Plasticidad; Resistencia a la corrosión; Microestructuras|
| Ver+/- |
Effect of Processing Parameters on Superplastic and Corrosion
Behavior of Aluminum Alloy Friction Stir Processed
A. Burgueño1, a, T. Dieguez2, c, H. Svoboda2, b
1 National Institute of Industrial Technology, General Paz Av. 4554, San Martín, Prov. of
Buenos Aires, Argentina
2 Faculty of Engineering, University of Buenos Aires, Paseo Colon Av. 850, Buenos Aires City,
a email@example.com, b firstname.lastname@example.org, c email@example.com
Keywords: friction stir processing, 7075-T651 aluminum alloy, superplasticity, localized corrosion.
Abstract. Friction Stir Processing (FSP) is a variant of Friction Stir Welding, and can be used to
modify the materials microstructure to functionalize it. Superplastic forming is a technological
process used to produce components with very complex shapes. In the last two decades it has been a
topic of major development. In Fine Structure Superplasticity (FSSP), the initial grain size exerts a
strong influence on the superplastic strain rate and temperatures. Refining grain size (GS) the
parameters (temperature and strain rate) of superplastic forming could be optimized. Thermal
stability is also an important factor to obtain superplasticity. FSP is used to refine GS, but the
optimum processing parameters are still under study over different materials. Corrosion resistance
can be affected by FSP too, but the information about it is scarce.
In the present study, 7075-T651 aluminium alloy was friction stir processed under different
conditions in order to improve superplastic behavior. Tool profile, rotation rate and traverse speed
were analyzed. Microstructures with <4 µm grain size were obtained. The maximum superplastic
elongations, in a range of 740 to 900%, at 400°C were obtained at 1x10-2s-1 strain rate. The results
were discussed in terms of constitutive equations and microstructure evolution. Localized corrosion
potentials were obtained. Localized corrosion resistance was affected by friction stir processing.
The study of the superplastic behavior of metallic materials has been a field of great interest and
development in last years due to the importance of superplastic forming of components to obtain
products with very complex geometries . Superplasticity is one of several micromechanisms of
deformation at high temperature, which is characterized by extensive plastic deformation prior to
fracture , being in the case of fine-structure superplasticity (FSS), the border slip grain (GBS)
mechanism that controls the superplastic deformation . The activation of this mechanism is
mainly determined by temperature, strain rate and grain size (GS) . Superplasticity has been
reported in materials with a fine and stable microstructure, which are deformed in the range of
strain rates between 10-5 and 10-2 s-1 and temperatures usually above 0.5Tm, being Tm the absolute
melting temperature. The refinement of grain size has a strong influence on the optimum strain rate
for FSS, increasing strain rate and decreasing temperature as GS decreases . The thermal stability
of the microstructure of the material is a critical aspect for the applicability of superplasticity.
Processing of materials by friction stir (FSP) has been recently developed and has great potential
as a grain refinement method, having reported the activation of superplastic behavior in alloys
processed by FSP . Like Friction Stir Welding (FSW) a lot of investigations were developed on
Al-alloys, being the 7075Al an interesting alloy due to their applications in aerospace industry and
capability to superplasticity of FSP-microstructure [5-8].
In contrast with the enhancement that represents a friction stir modified microstructure for the
plastic deformation of this alloy the resistance to corrosion in chloride environments decreases .
For this reason in this paper the localized corrosion susceptibility was considered.
The plate thickness of the 7075-T651 aluminum alloy was 4 mm and the nominal composition
5.6%Zn, 2.4%Mg and 1.9%Cu. The conditions of FSP are listed in Table 1. The Velocity Index, Iv,
is the ratio of tool rotation/travel speed, it was reported that the grain size of the stirred zone
becomes smaller as Iv decreases . The tool used was made of H13 tool steel and had a square
side pin with concave shoulder of 12.5 mm in diameter. For all runs the tool inclination was 1.5°.
Table 1. FSProcessing parameters.
Identification Tool rotation [rpm]
514/98 514 98 5.2
388/51 388 51 7.6
514/51 514 51 10.1
The microstructure was characterized by optical microscopy (OM) and the grain size on the
stirred zone was measured by mean of lineal intercept procedure according to ASTM E 112 .
In order to evalute the superplastic behavior tensile tests at 400°C were performed. This
temperature was adopted considering the thermal stability of FSP aluminum alloys studied by the
authors in a previous work . The tensile test specimen were obtained from the stirred zone with
a gage length of 2.90 mm, 2.70 mm width and 1.70 mm in thickness. The tensile tests were carried
out with three different initial strain rates: 5.0x10-3, 1.0x10-2 and 2.5x10-2 s-1. Elongation to fracture
was measured to compare the superplastic behavior.
Anodic potentiodynamic measurements were carried out in de-aerated 3.5wt% NaCl (pH = 6.2)
solution at 24ºC for evaluating the effect of FSP on the material. Electrochemical measurements
were performed using a three electrode cell. A SCE and a Pt sheet were used as reference and
counter electrode respectively. The work electrodes were made with an epoxi resin and
meticulously prepared. They were wet ground with 600 and 1000-grit SiC paper, then polished with
alumina of 1 and 0.3 µm, ultrasonically cleaned in ethanol and dried. Considering the work of
Frankel and Zhao  after polishing each electrode was etched by submerging it in a 1M NaOH
solution at 60°C for 90 sec followed by an immersion in 70% HNO3 for 30 sec in order to eliminate
the severe plastic deformed zone induced by polishing. Results were recorded with an EG&G
Princeton Applied Research model 273A potentiostat, using flat disc samples with a surface area of
0.8 cm2 approximately. The sweep rate was 0.166 mV.s-1. For these experiments a specimen
processed with a tool rotation of 388 rpm and a travel speed of 406 mm.s-1 was compared to the
Results and discussion
In Figure 1a can be observed a sample as processed. A macrostructure of a FSP sample is shown
in Figure 1b. There is neither defects nor discontinuities and can be seen a strong refinement at the
stirred zone. For the operative conditions used in this work no defects were observed.
Figure 1. a. Aspect of a sample processed; b. macrograph of a single pass FSP.
The grain size (mean volumetric grain diameter = 1.571 x mean lineal intercept length) on the
stirred zone of sample 514/98 was 3.53 µm. GS was a little larger for sample 388/51, 3.62 µm,
while the sample 514/51 has a GS of 4.65 µm. These results are in the range of 1 to 7.5 µm reported
previously for similar materials and processing conditions . The influence of the tool geometry
could be the reason for the fine GS. It has been reported that square tools are associated to finer
grain size of FSW-aluminum alloys . In Figure 2 can be seen optical micrographs that have
been taken from base metal (a), and from the stirred zone of different processing conditions: 514/98
(b), 388/51 (c) and 514/51 (d).
Figure 2. Micrographs showing the initial microstructure of: a. base metal and stirred zones of: b.
sample 514/98; c. sample 388/51 and d. sample 514/51.
In the base metal can be seen the structrure of cold worked grains of the T651 temper. In the
other micrographs it can be seen that grain size increased from Figure 2b to Figure 2d. This increase
is related with the Iv . Figure 3 shows the variation of grain size with Iv.
y = 0,2298x + 2,1789
R2 = 0,819
4 5 6 7 8 9 10 11
Velocity Index [rev.mm-1]
Figure 3. Velocity Index (Iv) vs. Grain size (GS) on the stirred zone.
It has been reported lower GS in the stirred zone as Iv is reduced . This behavior is related
with lower peak temperatures reached with lower tool rotation.
Results from tensile tests at 400°C are shown in Figure 4. In this figure can be observed that the
optimum strain rate was 1x10-2 s-1, being the largest superplastic elongation 900% corresponding to
condition 514/98. Also can be observed that for initial strain rates 5x10-3 and 1x10-2 s-1 the largest
elongation correspond to the samples with lowest ratio Iv, 5.2. These results are promissory
considering the elongations reported in previous works at 400°C  or even at higher temperatures
. Considering superplastic forming, temperature and strain rate are parameters of technological
and economical importance, due to its effects on time of processing and energy consumption. In this
sense, processing conditions that allow lower temperatures and higher strain rates superplastic
forming are desired.
1,0E-03 1,0E-02 1,0E-01
Strain rate [s-1]
388 rpm-51 mm/min
514 rpm-98 mm/min
514 rpm-51 mm/min
Figure 4. Elongation to fracture vs. initial strain rate of tensile tests at 400°C, for different
The results obtained for 514/98 are possible due to the relatively small, uniform and thermally
stable grain size reached through the sample thickness .
In Figure 5 is shown a comparison of superplastic elongations obtained from sample 514/98 for
each intial strain rate.
Figure 5. 514/98 tensile samples tested at 400°C under the following strain rate: a. untested
sample; b. 5x10-3s-1; c. 1x10-2s-1 and d. 2.5x10-2s-1.
a b c d
The variation of flow stress (true stress, σ, at a true strain, ε, of 0.1) with the initial strain rate can
be observed in Figure 6.
1,00E-03 1,00E-02 1,00E-01
Strain Rate [s-1]
Figure 6. Variation of flow stress with initial strain rate, m ranged from 0.34 to 0.39.
The superplastic strain rate sensitivity, m, ranged from 0.34 to 0.39. These values of m were
previously reported in the work of Liu and Ma  for low temperature superplasticity of a FSP
Typical potentiodynamic polarization curves are shown in Figure 7. In the figure can be
observed that the breakdown potential for the base metal sample was -750 mV SCE with a mean
value of ten runs of -748 mV SCE, while for the friction stir processed sample an abrupt increase in
current density can be identified at -850 mV SCE being the mean value -850 mV SCE. In all runs
hydrogen bubbles were observed in isolated sites on the work electrode. This can be presumed due
to cathodic reaction near the active sites [17, 18].
Observation of the electrodes by OM indicated that an intragranular attack was predominant in
base metal samples, while for FSP samples pitting attack was prevalent, especially in the HAZ and
lesser in the TMAZ. This is in agreement to previous results reported in the literature for similar
conditions [9, 18, 19].
1,00E-07 1,00E-06 1,00E-05 1,00E-04 1,00E-03
Figure 7. Characteristic potentiodynamic curves for base metal and FSP 7075-T651.
Under conditions analyzed in this work the following can be concluded:
1. successfully grain size refinement was achieved on 7075-T651 alloy by mean of FSP,
reaching GS of stirred zone ranged from 3.53 to 4.65 µm;
2. there was observed a correlation between the velocity index (Iv) and the GS obtained in the
3. superplastic elongation up to 900% were obtained at relative low temperature 400°C and
high strain rate 1x10-2s-1, related to the fine and thermally stable grain structure obtained
using 514 rpm for tool rotation, 98 mm/min for welding speed and a tool with a square pin;
4. the superplastic strain rate sensitivity was between 3.34 to 3.39;
5. the breakdown potential of the FSP was 100 mV below that the breakdown potential of base
6. the HAZ and the TMAZ were more susceptible to localized corrosion that the stirred zone
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