Research on Corrosion Fatigue Crack Propagation Behavior of Welded Joints of A 7 N 01 PT 4 Aluminum Alloys

Corrosion fatigue crack propagation behavior of welded joints a of A7N01P-T4 aluminum alloys were investigated. The corrosion fatigue crack propagation rate of base metal is higher than that of the heat-affected zone and the welding seam has a better corrosion fatigue resistance. Introduction Corrosion fatigue failure is one of failure forms of structures which under the service of cyclic load with corrosive environment[1].This failure form often occurs in 2xxx,7xxx series aluminum alloys of aircraft, vehicles and other important transportation structures and more than half of failures were in correlation with corrosion fatigue [3~6].The fatigue life of structures in corrosion environment is much shorter than that of in air, especially when the applied stress is much closer to the fatigue limit of the structures. This paper reports the results from our investigation on the corrosion fatigue properties especially on the crack propagation behavior. Material and experimental techniques The experiment materials are A7N01P-T4aluminum alloy plates (P stands for plate and T4 stands for a steady state after solution treatment and natural cooling according to ISO XXXX[35]).Welding was performed by the Metal Inert-Gas (MIG) technique with a PHOENIX 421 EXPERT welding machine. The welding wires are ER5356 of1.6 mm diameter. The chemical composition of the A7N01P Al alloy base metal, A7N01P, and the welding wire are listed in Table 1. The welding parameters are listed in Table 2. To remove the oxides and reduce the porosity of the joints, the surface of the alloy was chemically cleaned before welding. Table 1 Chemical composition of base metal and welding wire Note: 1. The chemical composition of A7N01P-T4 refers to GB/T 16475-2008: Temper designation system for wrought aluminum and aluminum alloy. 2. The chemical composition of ER5356, which is similar to filler metal, was listed for future analysis. Material Zn Mg Cu Mn Ti Si Fe Cr Al A7N01P-T4 4.0~5. 0 1.0~1 .8 0.10 0.20~0. 70 0.01~0 .06 0.35 0.40 0.06~0 .20 Bal. ER5356 0.10 4.5~5 .5 0.1 0.05~0. 20 0.2 0.25 0.10 -Bal. Residual Stresses 2016: ICRS-10 Materials Research Forum LLC Materials Research Proceedings 2 (2016) 599-602 doi: http://dx.doi.org/10.21741/9781945291173-101 600 Corrosion fatigue crack propagation experiment was performed according to ISO 12108: Metallic materials—Fatigue experimenting—Fatigue crack propagation method. The specimen in this paper was improved from single edge notched tension specimen(SENT) in ISO 12108.The specimens of base metal and welding joints was machined along Y-X (X stands for the direction paralleling to the welding line and Y stands for the direction perpendicular to the welding line). BM, HAZ and WM respectively stand for the specimen in base metal, the heated-affected zone and the welded joint. Sampling method and dimension of corrosion fatigue specimen are shown in fig.1.


Introduction
Corrosion fatigue failure is one of failure forms of structures which under the service of cyclic load with corrosive environment [1].This failure form often occurs in 2xxx，7xxx series aluminum alloys of aircraft, vehicles and other important transportation structures and more than half of failures were in correlation with corrosion fatigue [3~6].The fatigue life of structures in corrosion environment is much shorter than that of in air, especially when the applied stress is much closer to the fatigue limit of the structures.
This paper reports the results from our investigation on the corrosion fatigue properties especially on the crack propagation behavior.

Material and experimental techniques
The experiment materials are A7N01P-T4aluminum alloy plates (P stands for plate and T4 stands for a steady state after solution treatment and natural cooling according to ISO XXXX [35]).Welding was performed by the Metal Inert-Gas (MIG) technique with a PHOENIX 421 EXPERT welding machine.The welding wires are ER5356 of1.6 mm diameter.The chemical composition of the A7N01P Al alloy base metal, A7N01P, and the welding wire are listed in Table 1.The welding parameters are listed in Table 2. To remove the oxides and reduce the porosity of the joints, the surface of the alloy was chemically cleaned before welding.

Table 1 Chemical composition of base metal and welding wire
Note: 1.The chemical composition of A7N01P-T4 refers to GB/T 16475-2008: Temper designation system for wrought aluminum and aluminum alloy.2. The chemical composition of ER5356, which is similar to filler metal, was listed for future analysis.Corrosion fatigue crack propagation experiment was performed according to ISO 12108: Metallic materials-Fatigue experimenting-Fatigue crack propagation method.The specimen in this paper was improved from single edge notched tension specimen(SENT) in ISO 12108.The specimens of base metal and welding joints was machined along Y-X (X stands for the direction paralleling to the welding line and Y stands for the direction perpendicular to the welding line).BM, HAZ and WM respectively stand for the specimen in base metal, the heated-affected zone and the welded joint.Sampling method and dimension of corrosion fatigue specimen are shown in fig. 1.

Sampling method and dimension of corrosion fatigue specimen (a)Sampling method (b)dim ension of corrosion fatigue specimen
Specimens in the direction of Y-X (see Fig. 1) for corrosion fatigue cracking experimenting were cut and machined from the BM, HAZ and WM of the welded joints according to ISO 7539-6:2003 and ISO 7539-8:2000.A notch was cut at the edge of each specimens with the same way.The presetting notch was1~ 2 mm long and the pre-crack length was 1~2 mm long.
To assure the accuracy of the experimenting data, only the region of interest of each sample was immersed into the solution during experimenting, and the rest of the sample surface was masked with the BONLE type cyanoacrylate adhesive glue.The sharp incision and the extensometers were also isolated using glue and then covered with epoxy.A paper cup was used as the corrosion solution container.The entire set up for the corrosion fatigue cracking experimenting is shown in Fig. 2. The specimens were clamped through two fixtures.From the results, corrosion fatigue crack propagation rate of BM, HAZ and WM increased with the increase of stress ratio R. The propagation rate of BM was the highest, the propagation rate of HAZ was the medium and the WM was the lowest.

Conclusion
The behavior of corrosion fatigue crack propagation of base metal and welded joint of A7N01P-T4 aluminum alloy in 3.5%wt.NaCl under the different stress ratio was studied in this paper.On the basis of the experiment results, the conclusions are drawn as follows: (1)The welding seam of A7N01P-T4 aluminum alloy has a better corrosion fatigue resistance than that of the heat-affected zone in 3.5%wt.NaCl and base metal has the worst corrosion fatigueresistant performance.
Fig.1 Sampling method and dimension of corrosion fatigue specimen (a)Sampling method (b)dim ension of corrosion fatigue specimen

Fig. 2
Fig.2 Clamping sketch of the specimen Table2 Fitting parameters of Paris formula of base metal and welded joint