Study on hydrogen-induced stress corrosion of 7N01-T4 aluminum alloy for railway vehicles

2.1 Materials

The 7N01-T4 aluminum alloy sheet with a thickness of 7 mm was selected and its aging state was natural aging as the base material. The joint form is butt joint, and 7N01-T4 welding joint is prepared by laser-arc composite welding process. The filling metal is ER5356 aluminum alloy welding wire containing 5% Mg. Table 1 lists the chemical composition of aluminum alloy and welding wire.

The equipment used in the laser-arc composite welding process is TruDisk 10002 continuous laser and Transpuls Synergic 4000 welding machine. The welding process is realized by ABB robot arm. The process parameters of laser-arc welding are shown in Table 2, and the laser output wavelength is 1.06 μm, spot minimum diameter is 0.4 mm, and focal length is 300 mm. The size of the welding test plate is 320 mm × 160 mm × 7 mm (length × width × thickness), and the welding groove is “v” shaped. The heat source configuration with laser in front and arc in back is used for welding.

2.2 Electrochemistry

Electrochemical test can quickly evaluate the influence of environmental factors on the corrosion performance. In order to study the corrosion performance of 7N01-T4 aluminum alloy, Potentiodynamic Polarization Measurement was carried out at the scanning rate of 1 mV/s. Each sample was tested three times, and the average value of electrochemical parameters obtained three times was taken as the final result. The test environment was room temperature and the medium was 3.5 wt% NaCl solution. Polarization test was carried out on CS2000 electrochemical workstation, in which three-electrode system was used for polarization test and the working electrode was connected to the test sample by wire. The sample of base metal and weld area was cut by electric spark wire cutting to a size of 5 × 5 mm². All surfaces except test surfaces shall be sealed with denture powder after welding wires. After polishing the test surface according to the metallographic sample preparation procedure, it was cleaned with anhydrous ethanol to remove oil and water stains on the surface, and dried for later use. The scanning potential range was relative open circuit potential ±0.5 V, and the scanning rate was 1 mV/s.

2.3 Slow strain rate tensile test (SSRT)

The sensitivity of 7N01-T4 laser-arc composite welded joints to hydrogen stress corrosion was evaluated by SSRT test. The reference standard is GB/T15970.7-2000 Corrosion of Metals and Alloys – Stress Corrosion – Part 7: Slow Strain Rate Test. SSRT samples of base metal and joint are taken perpendicular to the welding direction. The length of SSRT sample is 25 mm, and the thickness is 3 mm. MTS-300 tensile testing machine was used for SSRT test.

SSRT tests were carried out in 3.5 wt % NaCl solution at 25°C and strain rate set at 10⁻⁶ s⁻¹. The cathode potential was applied through the CS2000 electrochemical workstation. The sample was used as the working electrode, saturated calomel electrode as the reference electrode, and platinum electrode as the auxiliary electrode, as shown in Figure 1(a). When the cathode potential is applied during the SSRT test, “hydrogen evolution reaction” will occur on the sample surface, and hydrogen atoms can be adsorbed on the sample surface and gradually diffuse into the sample, as shown in Figure 1(b). After the SSRT test, the fracture surface morphology of the sample was observed and analyzed under scanning electron microscope.

Figure 1

Schematic diagram of test design: (a) synchronously applied cathode potential of SSRT test and (b) hydrogen atoms adsorbed on the surface of the material and diffusing into the material under the action of cathode potential.

3.1 Polarization curve

Figure 2 shows the polarization curve of 7N01-T4 aluminum alloy base metal and joint weld in 3.5% NaCl solution (scanning rate is 1 mV/s). In electrochemical corrosion, the corrosion resistance of metal materials can be evaluated by self-corrosion potential. Generally speaking, the greater the self-corrosion potential is, the better its corrosion resistance is, and vice versa. Therefore, it can be obviously found from Figure 2 that the self-corrosion potential of weld is higher and the self-corrosion potential of base metal is lower, indicating that the weld has better corrosion resistance. The reason for the good corrosion resistance of the weld is that the filler material is ER5356 welding wire, and the alloy is Al–Mg, which has good corrosion resistance [16].

Figure 2

Polarization curves of 7N01-T4 aluminum alloy base metal and joint weld zone.

According to the test results, it can be found that the corrosion potential of base metal and weld is basically in the range of −0.8 to −1.4 V. Therefore, the potentials of −0.8, −1.0, −1.2, and −1.4 V were selected as the cathodic potentials applied in the subsequent SSRT hydrogen stress corrosion tests.

Polarization curve analysis shows that the weld self-corrosion potential is higher and the base metal self-corrosion potential is lower, indicating that the weld has better corrosion resistance. The reason for the good corrosion resistance of the weld is that the filler material is ER5356 welding wire, and the alloy is Al–Mg, which has good corrosion resistance.

3.2 SSRT

Figure 3 shows the SSRT stress–strain curves of base metal and joint under the condition of potentiostatic polarization in 3.5 wt% NaCl solution at room temperature. It can be seen from Figure 3 that the slope of stress–strain curves of base material and joint samples at elastic deformation stage is almost the same under different potentiostatic polarization conditions, indicating that potentiostatic polarization has no obvious influence on the elastic deformation behavior of base material and joint. In addition, the overall tensile strength and elongation of the base metal are better than that of the welded joint, mainly because of the performance of the weld. The performance of the weld is related to the chemical composition, fusion ratio, and crystallization process of the filler wire. The weld area is mainly the as-cast structure formed by the solidification of Al–Mg welding wire after melting, resulting in low mechanical properties of the weld. Generally, the as-cast grain size is coarse. According to the Hall–Petch formula, the smaller the grain size is, the greater the yield strength is ref. [17]. Therefore, the yield strength of the weld is lower than that of the base metal, which is also the reason for the low weld strength. On the other hand, the aluminum alloy laser-arc composite welding process will inevitably produce small pores and other defects, and these defects will become the stress concentration position under the action of external tensile force, leading to the joint fracture in advance, resulting in low welding joint fracture strength.

Figure 3

SSRT tensile test results: (a) engineering stress–strain curve of the base metal and (b) engineering stress–strain curve of 7N01-T4 welded joint.

In addition, the same rule can be found in the engineering stress–strain curves of the base metal and the joint, that is, the base metal and the joint have the best tensile properties (breaking strength and elongation are the best) in the air environment. With the increase in the applied cathode potential, the tensile properties of both the base metal and the joint decrease, and the more negative the cathode potential shifts, the worse the fracture resistance. The reason is that when cathode potential is applied to aluminum alloy material, the material indicates that hydrogen evolution reaction will occur, and the reaction intensity increases gradually with the negative shift of the applied potential [18,19]. At this point, hydrogen, driven by external stress and internal stress, will migrate and enrich toward hydrogen trap areas such as grain boundaries, phase boundaries, dislocation, and pores, resulting in hydrogen-induced stress cracking of the material [20,21].

According to the above SSRT tensile test results, stress corrosion sensitivity index I _SSRT was used for quantitative comparative analysis of stress corrosion sensitivity of the base metal and joint. I _SSRT calculation formula (1) is as follows:

where σ _fw is the breaking strength in corrosive environment, δ _fw is the elongation after break in corrosive environment, σ _fA is the breaking strength in air, and δ _fA is the elongation after break in air.

According to formula (1), the engineering stress–strain curves of the base metal and joint were extracted, sorted, and analyzed to obtain the values of various parameters required for the calculation of stress sensitivity index as shown in Table 3. Finally, the stress sensitivity index value I _SSRT of the base metal and joint under different cathode potentials was obtained through comprehensive calculation.

Based on the data calculated in Table 3, the corresponding relation curve between the cathode potential applied by the base material and the joint and the stress sensitivity index were drawn, as shown in Figure 4. As can be seen from Figure 4, the overall curve of the base metal is higher than that of the joint, indicating that the stress sensitivity index of the base metal is higher than that of the joint, and the base metal has a higher stress corrosion sensitivity tendency. Generally speaking, the higher the stress sensitivity index is, the worse the stress corrosion resistance will be. Therefore, compared with the base metal, the laser-arc composite welded joint of 7N01-T4 aluminum alloy has better stress corrosion resistance [22].

Figure 4

Polarization curves of 7N01-T4 aluminum alloy base metal and joint weld zone.

In addition, with the negative shift of the applied cathode potential, the base metal and the joint have the same change trend, and the stress sensitivity index I _SSRT of the two gradually rises to the maximum value. This also confirms the above analysis that negative cathode potential shift results in intensified hydrogen evolution reaction [23]. As the applied cathode potential increases, the I _SSRT index is lowest at the cathode potential of –0.8 V, 0.114 for the base material and 0.058 for the joint. The joint has such a low stress sensitivity index that there is almost no stress corrosion tendency at this potential. Then, with the negative potential shift, the stress index increases gradually. When it reaches –1.4 V, it has the maximum I _SSRT index value, which is 0.430 for the base metal and 0.337 for the joint.

It can be drawn that the overall curve of the base metal is higher than that of the joint, indicating that the base metal has a higher stress corrosion sensitivity index than that of the joint, and the base metal has a higher stress corrosion sensitivity tendency. Therefore, compared with the base metal, 7N01-T4 aluminum alloy laser-arc composite welded joint has better stress corrosion resistance.

3.3 Fracture morphology

Typical tensile fracture at slow tensile rate was selected to study the stress corrosion fracture characteristics of the base metal and welded joint by observing and analyzing the fracture morphology of base metal and welded joint in air environment and 3.5 wt% NaCl corrosion environment. Figure 5 shows the fracture morphology characteristics of base metal in air environment and at various cathode potentials. Figure 6 shows the microstructure characteristics of 7N01-T4 welded joint in air environment and at applied cathode potentials.

Figure 5

Microstructure of base metal fracture: (a) in air condition, (b) in the condition of cathode potential of –0.8 V, (c) in the condition of cathode potential of –1 V, (d) in the condition of cathode potential of –1.2 V, and (e) in the condition of cathode potential of –1.4 V.

Figure 6

Microstructures of K4648 alloy after solid solution treatment: (a) in air condition, (b) in the condition of cathode potential of –0.8 V, (c) in the condition of cathode potential of –1 V, (d) in the condition of cathode potential of –1.2 V, and (e) in the condition of cathode potential of –1.4 V.

It can be seen that the fracture morphology of base metal in non-corrosive media is mainly strip-shaped dimple (Figure 5(a)), indicating that it is ductile fracture. Under the action of corrosion solution and cathode potential, the surface of the fracture showed corrosion characteristics (Figure 5(b–e)). Clusters of corrosion products and mud-like patterns can be found on the surface of the fracture side (Figure 5(b)). Local intergranular fracture morphology can also be found on the surface of the fracture side, with local intergranular cracks (Figure 5(d)) and obvious layered intergranular cracks (Figure 5(e)).

In addition, the fracture positions of welded joints are all located in the weld zone. The fracture morphology of the joint in air is typical of ductile fracture, and the dimple distribution is fine and uniform (Figure 6(a)). Similar to the fracture morphology of the base metal, the fracture surface of the joint also shows corrosion characteristics due to the action of corrosive medium and applied potential (Figure 6(b and c)). At the same time, obvious intergranular fracture characteristics can be found on the surface of the fracture side, and rocky cracking morphology appears in local areas (Figure 6(d and e)).

Many experimental studies show that SCC of 7XXX series high-strength aluminum alloy mainly belongs to the hydrogen-induced cracking mechanism [24,25,26]. According to the hydrogen embrittlement theory, free atomic hydrogen generated in the corrosion process or self-absorbed hydrogen diffuses into the crack tip region along the grain boundary under the action of tensile stress, weakening the grain boundary and causing hydrogen embrittlement, thus accelerating the crack propagation and fracture [27,28,29]. Under the action of hydrogen-induced corrosion cracking, the formation of cracks starts from spot corrosion [20]. Pitting corrosion occurs because in chloride solution, the passivation film is broken by chloride ions, which induces pitting corrosion in local areas and leads to cracks [1,29,30,31]. We know that the reaction of aluminum alloy with water is Al + 3H₂O = Al₃O₂ + 6H⁺ + 6e⁻. As the aluminum alloy is in 3.5 wt% NaCl solution, there will be corresponding chemical reactions at the crack tip: Al + Cl⁻ = AlCl₃ + 3e⁻ and Al³⁺ + H₂O = Al(OH)²⁺ + H⁺.

In addition, under the action of applied stress, there is a great stress concentration at the crack tip. The formula (2) of the stress field of crystal crack tip is as follows:

where σ _yy is the stress value of the crystal crack tip, K _i is the applied load, ρ ₀ is the notch curvature of the crystal crack tip, which is generally considered to be equal to 2–3, and b is the crystal lattice constant. It can be found that the larger the loading load is, the more easily the stress concentration is formed in the front edge of the crack tip, and the larger the load is, the more easily the stress cracking occurs.

Under the joint action of stress field and hydrogen ion, hydrogen will gradually open the crack. The applied stress facilitates the exchange of hydrogen between aluminum alloy and corrosion environment and promotes hydrogen diffusion along grain boundaries. In addition, hydrogen can greatly reduce the bonding strength at grain boundaries, leading to the weakening of grain boundaries, resulting in grain embrittlement and intergranular cracks, resulting in grain boundary cracking and finally collapse.

In summary, observation and analysis of the fracture surface microstructure of the base metal and welded joint of 7N01-T4 aluminum alloy show that the fracture surface morphology of the base metal and welded joint is mainly a large area of dimple distribution in the non-corrosive medium environment, which belongs to the ductile fracture. Under the action of 3.5 wt% NaCl solution and applied cathode potential, the fracture characteristics of the base metal and joint changed, and corrosion traces appeared. Clusters of corrosion products and mud-like patterns were found on the surface of the fracture, and the characteristics of rock intergranular cracking appeared in some areas.

3.4 Stress corrosion crack propagation

Figure 7 shows the stress corrosion crack propagation path of the base metal 7N01-T4 aluminum alloy tested by EBSD. It can be seen from Figure 7(a) that the stress corrosion crack propagation path is discontinuous, and the crack at grain boundary has not been connected with the main crack, presenting a mixed fracture mechanism of transgranular and intergranular. The prefabricated crack of the sample just stays in the grain of the aluminum alloy, inside which the stress corrosion crack starts to propagate. The aluminum alloy matrix at the crack tip is constantly dissolved, and accompanying with the crack propagates through the grain with the diffusion and aggregation of hydrogen.

Figure 7

Stress corrosion crack propagation path of 7N01-T4 aluminum alloy: (a) stress corrosion crack propagation path of base metal 7N01-T4 aluminum alloy and (b) cracks crack path along grain boundary.

Generally, the large angle grain boundary is the position of strong trap for hydrogen, whose formation reduces the mobility of hydrogen atoms, which continue to gather and compound into hydrogen molecules, resulting in hydrogen pressure and cracks fracturing along the grain boundary, as shown in Figure 7(b). Meanwhile, it shows rock sugar pattern and intergranular secondary crack on the fracture surface, and the existence of micropores and hairlines on grain boundaries proves that SCC is related to hydrogen embrittlement. At the same time, small and shallow dimples without inclusions appeared on the crystal plane of the sample fracture, reflecting the deterioration of the toughness of the material. That is because the dimples were formed by hydrogen aggregation rather than ductile fracture. According to the theory of fracture mechanics, the cracks in the middle of the sample are in plane strain state, which is conducive to hydrogen enrichment, which is one of the reasons for the long cracks in the middle of sample. In addition, the stress corrosion crack propagation zone of heat affected zone and base material sample is relatively clean and no corrosion products are attached, also presenting the feature of hydrogen-induced crack fracture.