Corrosion study of the friction stir lap joint of AA7050-T76511 on AA2024-T3 using the scanning vibrating electrode technique
Introduction
High strength aluminium alloys, as the ones belonging to the 2xxx and 7xxx series are the most used for aircraft structural elements. However, these alloys show some defects when joined by conventional welding techniques [1], [2]. Because of this, the riveting technique is largely used to join aircraft structural elements [2]. The Friction Stir Welding (FSW) process developed by The Welding Institute in 1991 [3], [4] can be used to eliminate conventional welding defects. In this process, a non-consumable rotating tool moves along the surface of the base material in a face-to-face contact to produce the weld. The FSW is a solid-state process, i.e., the temperature generated by the rotation of the tool does not exceed the melting temperature of the workpiece.
The typical microstructure formed by the FSW joining of aluminium alloys is characterized by the nugget zone (NZ), the thermo-mechanically affected zone (TMAZ), the heat-affected zone (HAZ) and the base material (BM), whose microstructure is not affected by the welding process [1], [2], [5], [6], [7], [8]. NZ is subjected to the highest temperatures and strains of the process and the resulting dynamic recrystallization produces a microstructure with small sized equiaxed grains. The TMAZ material also suffers strain, due to the tool action. However, the heat-strain input is not enough to induce recrystallization. Grain sizes and shapes of HAZ are usually similar to that of the base material, however showing a different response to chemical attack and hardness [5], [6]. It has been reported that the weld region is more susceptible to localized corrosion compared to the base material [5], [7], [8], [9], [10]. Jariyaboon et al. [7] studied the effect of FSW parameters on the corrosion of the AA2024-T3 aluminium alloy and showed that the weld region behaves always anodic, when in contact with the unaffected base material. Furthermore, they observed that for low welding rotations intergranular corrosion takes place in NZ, while for high rotation values the attack occurs in HAZ. Kang et al. [8] reported that during exfoliation corrosion tests of welded AA2024-T3 according to ASTM G34-01 norm [11] in 4.0 M NaCl + 0.5 M KNO3 + 0.1 M HNO3 solution (EXCO) of pH 0.4 pitting corrosion nucleates by the dissolution of the S phase (Al2CuMg), followed by the dissolution of the surrounding matrix. Thus, the verified higher density of S phase particles in NZ would increase the susceptible to pitting corrosion. The authors also showed that the Fe-containing particles Al–Cu–Fe–Mn–(Si) also contribute to pitting corrosion [8]. Wadeson et al. [12] studied the corrosion behaviour of the AA7108 friction stir welded using a modified EXCO solution. They verified the sensitization of weld, being the edges of TMAZ the region most vulnerable to corrosion. According to the authors, the heterogeneous distribution of MgZn2 precipitates in the TMAZ occurred during the FSW leads to intergranular attack.
Usually, the corrosion of FSW welded AA7050 occurs on the NZ-TMAZ interface. For this alloy, with higher Zn and lower Cu contents than AA2024, sensitization occurs along the grain boundaries on the nugget neighbouring TMAZ as a consequence of the presence of Cu–Zn-rich precipitates, leading to intergranular corrosion [10]. Paglia et al. [10] studied the environmental corrosion susceptibility of the FSW AA7050 alloy with scandium additions. For this material, fracture takes place on the region lying between NZ and TMAZ, both in air and in 3.5 wt.% NaCl solution. Proton et al. [13] verified the influence of post-weld heat treatments on the electrochemical behaviour of the nugget AA2050 alloy. The gel visualisation test showed that after the heat treatment the nugget change its behaviour from anodic to cathodic. The test also showed the heterogeneous corrosion between the bottom and the top of the nugget, being the bottom more anodic. Laser surface melting (LSM) is an alternative method for improving the corrosion resistance of FSWs [14]. Padovani et al. [14] tested the electrochemical reactivity of LSM treated friction stir welds of AA7449-T7951 using the micro-capillary cell. The laser treatment enhanced the corrosion resistance by reducing the cathodic reactivity of the weld and base material. The authors also reported that the post-treatment was more effective on the HAZ.
Compared to butt welds, lap welds need more complex tool designs that increase the volume of disturbed metal at the interface, and thus the energy input [15]. This kind of weld is used for the union of internal structural elements (stringers) of Al alloys of higher strength to the aircraft external skin of softer Al alloys of higher corrosion resistance. Since the weld faces are in the internal side of the aircraft, eventual corrosion occurrences are hidden and difficult to inspect, thus of practical relevance. Moreover, exposed cross sections of the weld may be subject to an even more aggressive situation of cut-edge corrosion.
Although FSW is a very promising welding technique presently under regular industrial use, there are only a few electrochemical studies using local techniques concerning its influence on the local corrosion behaviour of welded Al alloys as atomic force microscopy (AFM) and scanning Kelvin probe force microscopy (SKPFM) for Al/Cu lap joint [16] and scanning electrochemical microscopy (SECM) and local electrochemical impedance spectroscopy (LEIS) for AA2050-AA7449 butt weld [17]. However, to our knowledge, there is no report in the literature of using SVET to elucidate the involved corrosion mechanisms of FSW joints. In the present work, the corrosion behaviour of friction stir lap weld of AA7050-T76511 on AA2024-T3 was examined using the scanning vibrating electrode technique (SVET), allowing the identification of galvanic elements and associated cathodic and anodic sites on the different weld zones. Moreover, conventional cyclic voltammetry (CV) and open circuit potential (OCP) measurements were carried out.
Section snippets
Experimental
AA2024-T3 sheet and AA7050-T76511 extruded profile were lap-joined by FSW using typical parameters for Al Alloys (welding speed of 960 mm/min and rotation speed of 1700 RPM) specially prepared for this work. The sheet and the extruded profile have 1.3 mm of thickness and the set is schematically presented in Fig. 1. The chemical analyses of these alloys were determined by optical emission spectroscopy and are listed on Table 1.
For the experiments, base metal and cross sections of the FSW samples
Microstructure analysis
Fig. 2 shows OM view of the microstructure of the base materials and a macrograph of the cross section of the welded sample. The AA2024-T3 alloy presents an almost equiaxed grain structure with grain sizes ranging from 30 to 50 μm (Fig. 2a), while the AA7050-T76511 has an elongated grain structure aligned with the extrusion direction (Fig. 2b). The microstructures of the weld are shown in Fig. 2c. The narrow TMAZ with flow lines and HAZ close to the base material (BM) of the two alloys are seen
Conclusions
The corrosion behaviour of a dissimilar friction stir lap weld of the AA7050 and AA2024 alloys has been investigated for the first time using localized microelectrodes (SVET) and conventional technique, leading to the following findings:
- 1.
FSW lowers Erep of the more active alloy AA7050, while Epit remains unaffected.
- 2.
For OCP conditions more intense pitting occurs preferentially on the AA7050 nugget zone. This is probably due to the intense local galvanic coupling, as well as to the higher
Acknowledgments
The authors greatly acknowledge the Empresa Brasileira de Aeronáutica S.A. (EMBRAER) for providing samples specially prepared for this study, the Microscopy Centre of UFRGS for the use of facilities, CNPq for financial support (MSc. fellowships) and also G. Knörnschild for helpful discussions.
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2022, Corrosion ScienceCitation Excerpt :The reverse curves evidenced a sharp slope change at a potential intermediate between the vertex potential and the pit repassivation potential, Erep, at which negligible anodic currents are obtained. This potential, generally named “pit transition potential” (Eptp) [36–40], is instead characterized by high anodic currents (above 10−4 A cm−2 in Fig. 9). According to [37,39], Eptp represents the potential below which full repassivation of surface pits and partial repassivation of deep pits may occur, while Erep actually corresponds to the potential of complete repassivation of deeper pits.