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.