Second step : 8 hours holding at 90° C metal temperature
Third step : 16 hours holding at 145°C metal temperature
Fatigue tests were carried out by using TOKYOKOKI PBF-30C plane bending
fatigue test machine. Applied loads were controlled by adjusting the
length of a crank arm connected to the camshaft on the fatigue machine.
The applied bending moment can be monitored on a digital screen of the
test machine.
Corrosion fatigue tests were carried out in the seawater obtained from
the Black Sea coastline. The corrosion environment was provided by the
corrosion cell made of PVC film so that the welded area of the test
specimen was completely exposed to the seawater. The schematic view of
the corrosion fatigue test set is shown in Fig. 2.
The hardness values of welded and TSA treated specimens were measured
with a Shimadzu hardness tester. Two measurements were worked out in two
separate regions near the heat affected zone (HAZ) and away from the
HAZ. Scanning electron microscope (SEM) images of the fracture surfaces
were obtained by a Zeiss Sigma 300.
Results and Discussion
Fatigue Curves
In the first stage of this study, fatigue tests were carried out with
unwelded AA 7020 - T651 specimens at the stress ratios of R=-1 and R=0
in air environment. R=-1 strain ratio was chosen for the control group
experiments. The resulting curve with the parameter of ∆σ is shown in
Fig. 3. As well-known fatigue liner under R=-1 were longer than those
under R=0.
In Fig. 4, S - N curves are given for the unwelded AA7020 – T651 in air
and seawater at stress ratio R=0. It is seen from the figure that the
corrosive environment has a detrimental effect on the fatigue behaviour
by significantly reducing the fatigue life of the specimens. It is known
that the pitting formed by the effect of corrosive environment causes
stress concentration, and then accelerates the formation of fatigue
crack and reduces the fatigue life 14.
The S-N curves obtained from welded and unwelded AA7020 – T651
specimens tested in air at R=0 stress ratio are shown in Fig. 5. In this
study nominal stress approach is used to determine the fatigue behavior
of welded specimens. It is seen from the diagram that the fatigue life
of the welded specimens decreases much more especially at high stresses
but less for low stresses. It is known that this decrease on the fatigue
life is due to the stress concentration formed at the weld toe and the
tensile residual stresses formed as a result of phase transformations
due to high temperature in the HAZ. In addition, the effect of welding
defects is one of the main reasons for the degradation of fatigue
properties of welded structures 15.
Fig. 6 shows the S-N curves of unwelded and welded AA 7020 – T651
specimens in the corrosive environment. According to the graph, it is
seen that the corrosion sensitivity of welded specimens is higher. One
of the factors that reduce the corrosion fatigue performance of welded
specimens can be listed as the increase in the corrosion sensitivity of
the material due to the different microstructure formed in the HAZ
region of the weld seam 16.
Fig. 7 shows the effect of TSA on the fatigue properties of welded AA
7020 in air and seawater environments. According to the graph, the TSA
treatment applied to the specimens increased the fatigue performance in
both environments. While, in corrosion environment as the applied stress
value decreased, the fatigue lives of as-welded and TSA treated
specimens approached each other. It is seen that the aging treated
specimens tested in atmospheric environment at the same stress values
are damaged in longer cycle numbers than tested in seawater environment.
Accordingly, seawater reduced the fatigue life of the heat-treated
specimens by about 50% on average.
Vickers Hardness Measurements
TSA process increased the fatigue properties of the present welded
specimen in both corrosion and air environments. This result is
considered as a result of the MgZn2 phase formed as a
result of heat treatment in the aluminium matrix. It is thought that
this hard intermetallic phase, which precipitated in the weld region
(close to HAZ), is dispersed as small and fine second phases and
enhances residual stresses and then fatigue properties17. The hardness values measured by the Shimadzu HMV-G
Vickers method (Table 3) also confirm that heat treatment increases the
hardness in the HAZ region where crack sensitivity is high.
SEM Views
Fig. 8(a) and 8(b) show the fracture surface of unwelded, and Fig. 8(c)
and 8(d) show that of welded AA 7020 – T651 specimen tested in air.
Many tiny fatigue cracks are formed on the fracture surface in 8(a) and
8(b). In 8(a), the main crack propagates into the specimen perpendicular
to the applied stress axis. Fig. 8(c) shows the fracture view of the
welded specimen. Failure of the welded specimen occurred at the
interface between the weld seam and the HAZ. The fracture surface is
harder and smoother than the structure in 8a. In 8d, weld defects formed
during welding are seen.
Fig. 9(a) illustrates the fracture surface of TSA treated specimen
tested in air. Spherical shaped particles dispersed on the fracture
surface are clearly seen in the figure. These large and small
precipitate particles formed at the end of the TSA process are in the
intermetallic (MgZn2) secondary phase. Fig. 9(b) is the fracture surface
image near specimen surface of the TSA treated specimen tested in
seawater. Precipitate particles were seen in the picture, although they
were not as much as seen in Fig. 9(a). Accordingly, we can say that the
precipitate process takes place at a lower level in the near surface
regions. Moreover, it is clearly evident in the picture that a crack
formed on the surface moves into the specimen with the effect of
fatigue.
The surface images of welded AA 7020 – TSA specimen, which was tested
in seawater, are given in Fig. 10. Pitting corrosion and exfoliation
corrosion are the two main types of corrosion that occur in NaCl
solutions of aluminium alloys. Fig. 10(a) shows pitting corrosion on the
surface, and 10b shows exfoliation corrosion. The amount of these
corrosion formations on the specimen surface increased in proportion to
the test time. Corrosion pitting occurred mostly in the HAZ regions of
the welded specimens. The stress concentrations formed by these pits
accelerated the formation of fatigue cracks and caused a decrease in
fatigue life. TSA improved especially exfoliation corrosion resistance.
This resistance is provided by the second phase precipitates which are
dominant on the matrix and grain boundaries 18.
In Fig. 11, the cracked surface images of the welded AA 7020 – TSA
specimen, after the corrosion fatigue tests are given. Fig. 11(a) gives
the general structure of the fracture and exfoliation corrosion
formation is observed close to the surface. The enlarged picture of
11(b) was taken from the region near the specimen surface. The clustered
MgZn2 precipitates are in spherical form and vary in size.
Conclusion
In this study, the effect of the two-step aging process applied to the
welded AA 7020 – T651 aluminum alloy on the fatigue properties in air
and corrosion environment was investigated and the following results
were obtained:
• Welded specimens exhibited lower fatigue properties compared to the
unwelded specimens in air due to weld defects, stress concentration at
the weld toes, and tensile residual stresses in the HAZ.
• TSA treatment increased the hardness of welded material by causing the
MgZn2 phase in the microstructure of the HAZ to disperse
as small precipitates. The corrosion resistance improved with TSA due to
MgZn2 phase formation. Thus, better fatigue properties
were obtained both in air and seawater environments.
• Exfoliation and pitting corrosion occurred on the surface under
corrosion environment. The pits were mostly occurred in the HAZ of the
welded specimens. Corrosion fatigue strength of the TSA treated specimen
was improved compared to that of AA welded specimen due to
MgZn2 phase, which suppresses pit formation.