CORROSION AND PROTECTION OF FRICTION STIR WELDS IN ... · the onset of localised corrosion in aluminium alloys is known to be able to decrease this parameter (e.g. [25]). Recent work

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29 ottobre 2008 << la metallurgia italiana la metallurgia italiana >> ottobre 2008 29

CORROSION AND PROTECTION OF FRICTION STIR WELDS IN AEROSPACE

ALUMINIUM ALLOYS C. G. Padovani, A. J. Davenport, B. J. Connolly, S. W. Williams,

A. Groso, M. Stampanoni, F. Bellucci

Keywords: aluminium alloys, welding, corrosion

INTRODUCTION

Friction stir welding [1] (FSW) offers the opportunity of obtain-ing high quality welds in the traditionally poorly weldable high strength aluminium alloys of the 2XXX and 7XXX series. Due to the excellent quality of the welded joints, aircraft manufactures are considering the introduction of this technology in aircraft components. Friction stir welding has been used with success in joining primary structures in the Eclipse 500™ jet [2], and will be applied to join external fuel tanks in the NASA Space Shuttle [3]. A review of recent investigations on the properties of FSW has been compiled by Mishra and Ma. [4]. The corrosion performance of the welds has been analysed in a number of studies, which show that the thermal cycle produced by welding leads to significant changes in the microstructure of the metal, leading to enhanced corrosion susceptibility [5-24]. In aerospace alloys of the 2XXX and 7XXX series, this causes con-cerns related to the corrosion-fatigue of FSW components, as the onset of localised corrosion in aluminium alloys is known to be able to decrease this parameter (e.g. [25]). Recent work on AA2024 T351 [16, 17] showed the correlation between welding parameters and precipitation of the age-S phase, while for 7XXX

C. G. Padovani, A. J. Davenport, B. J. ConnollyUniversity of Birmingham, Metallurgy and Materials, Birmingham (UK)

S. W. WilliamsCranfield University, Welding Engineering Research Centre, Cranfield (UK)

A. Groso, M. StampanoniSwiss Light Source, Paul Scherrer Institut, Villigen PSI, (Switzerland)

F. BellucciUniversità degli studi di Napoli Federico II, Dipartimento di Ingegneria

dei Materiali, Napoli (Italia)

alloys changes in electrochemical behaviour have been attributed to the precipitation of η phase.Due to the sensitisation of the weld region, it may be desirable to improve the corrosion performance of friction stir welds by the use of appropriate post treatments. The use of post weld heat treatments to increase and homogenise the corrosion resistance of the weld had limited success [22, 26-30] and tend to be restricted by physical limitations related to the size of the components to be treated.Laser surface melting is able to increase the corrosion resistance of aluminium by dissolving the detrimental constituent particles present in commercial alloys [31] and can be considered for the treatment of FSW due to its ability of forming, in appropriate conditions, corrosion resistant, precipitate free layers. This has been obtained with Excimer lasers [32-39], in which the short du-ration of the thermal cycle induced by laser irradiation leads to limited microsegregation in the molten and resolidified layer. The use of laser surface melting to increase the corrosion resist-ance of friction stir welds has been recently investigated [5, 6, 10, 11, 40, 41]. Apart from increasing the corrosion resistance of the parent material and of the weld region, the use of laser surface melting to increase the corrosion resistance of welds might offer

AA2024AA7449

Si0.500.12

Fe0.500.15

Cu3.8-4.91.4-2.1

Mn0.3-0.90.20

Mg1.2-1.81.8-2.7

Cr0.10

-

Zn0.25

7.5-8.7

Ti + Zr 0.150.25

Al balbal

s

Tab. 1 Nominal chemical composition of AA2024 and AA7449.

Composizione chimica nominale delle leghe AA2024 and AA7449.

the ulterior benefit of reducing galvanic coupling effects between different weld regions that can occur if wetting of the metal with a relatively conductive electrolyte takes place. This paper discuss the application of laser treatment with Excimer laser to increase the corrosion resistance of friction stir welds in AA2024-T351 and AA7449 T7951.

EXPERIMENTAL METHOD

AA2024-T351 and AA7449-T7951 laser surface melted friction stir welds were supplied by BAE SYSTEMS in the form of 4.0 mm and 12.2 mm thick plates respectively; the nominal chemical com-position of the alloys is reported in Tab. 1. Friction stir welding was performed with a Triflute™ carbon steel tool piece at rotation

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speed of 486 rpm and a traverse speed of 195 mm min 1. Me-chanical milling of few mm from the weld surface was performed before laser-treating the welds to remove the characteristic weld “crown”.Laser surface melting was performed in BAE SYSTEMS with a XeCl Excimer laser (wavelength λ = 308 nm) on the surface of FSW plates (L-LT plane). The laser was operated at a fluence of 10 J cm-2 in a 3x3 raster scanning mode in order to obtain 9 pulses per unit area and hence an integrated fluence of 90 J cm2. The laser treatment was performed in air after organic degreasing of the plates with iso propanol. Appropriate degreasing was found to be important as external species (for example from the lubricant used during previous rolling operations) were found to be poten-tially incorporated in the treated layer if not adequately removed. The laser focusing system comprised of micro-array of lenses that produced a spot size of 1.5 mm x 1.5 mm.The morphology of laser-treated FSW was characterised with SEM microscopy of surface and cross section. The cross sections were prepared via cutting with a Struers “Accutom 5” precision machine, cold mounting in a Met Prep “Tri-Hard” epoxy resin and polishing to 1 μm with Struers “DiaDuo” diamond suspen-sion. Before examination, a thin carbon coating was deposited on the sample in order to eliminate charging effects at the metal/resin interface and make it electrically conductive. For this analy-sis a JEOL 7000F FEG-SEM was used in secondary electron and backscattering mode at an accelerating voltage of 15 kV. Chemical analysis of the laser treated material was also carried out using Energy Dispersive X-ray Spectroscopy (EDX), performed at an accelerating voltage of 15 kV with a JEOL 6400 SEM equipped with a Noran Instruments EDX detector.The electrochemical reactivity of laser-treated and untreated welds was tested in a 0.1 M NaCl solution in a micro-capillary cell. In this setup a droplet of solution of controlled dimensions is positioned via a pipette tip on the area to be tested. The pipette is physically connected to a solution reservoir where both reference electrode (Ag/AgCl) and counter electrode (platinum wire) are accommodated. Details on the use of this technique are reported elsewhere [5, 6, 16, 17]. For all the tests performed in this work, a pipette tip with a contact area of 1.2 mm2 was used. Anodic and cathodic polarisation tests were performed on selected areas of la-ser treated and untreated welds; the tests were performed on the weld surface in a scan perpendicular to the welding direction in order to obtain a map of the electrochemical reactivity as a func-tion of position across the weld line. Anodic and cathodic scans were performed separately on different spots after 300 seconds of free corrosion in which the open circuit potential (OCP) was monitored. The scan started at the OCP (± 10 mV) and proceeded to higher (anodic polarisation) and lower (cathodic polarisation) values of the potential. Only the cathodic polarisation measure-ments performed on the laser treated AA2024-T351 started from a value of the potential different from the OCP, which was -800 mV vs. Ag/AgCl. The sweep rate was 1 mV s-1. The surface preparation involved polishing to 1 μm 3 days before testing for untreated welds and 30 seconds immersion in concentrated ni-tric acid immediately before testing for the laser treated welds. Just before testing, untreated and laser treated weld samples were rinsed with deionised water, ethanol and air dried. Nitric acid dipping was adopted as preparation procedure for the laser treated material since the normal grinding/polishing preparation procedure would have removed the laser treated layer. The fact that untreated and laser treated surfaces were pre-treated, before the electrochemical tests, in different manners did not affect the interpretation of the results (not shown). Immersion tests in Cl-containing electrolyte were performed to

verify whether the presence of laser treatment on the alloys would reduce the severity of corrosion and to understand whether the presence of scratches in the treatment would be accompanied by significant corrosion of the substrate. Samples about 1 cm wide and 8 cm long (for an overall area of 8 cm2) were scratched and immersed in corrosive electrolyte. The immersion was carried out in two independent tests for 5 and 20 days in a naturally aerated 0.1 M NaCl solution on untreated and laser treated welds. The samples were covered in several layers of “Stopping off” lacquer to expose only the L-LT surface. The immersion was performed in a beaker immersed in a thermal bath at a temperature of 25°C; deionised water was periodically added to the system in order to keep the solution volume and the electrolyte concentration con-stant with time. The samples were scratched across the weld re-gion perpendicular to the welding direction. Both untreated and laser treated welds were scratched for consistency. The scratch was produced few days before immersion in the electrolyte with a sharp stainless steel tool and was estimated to be ~ 10-15 μm deep after profilometric analysis. As the depth of the LSM layer was only 3 5 μm thick, the scratch was deep enough to expose the substrate. On the corroded samples, 2D analysis was performed via optical and SEM microscopy of surface and cross section and 3D analysis was performed with X ray microtomography (ex situ samples). Before examination, removal of corrosion products was carried out in some cases with immersion for 2 minutes in con-centrated nitric acid.Open circuit potential (OCP) measurements were also performed on untreated, intact laser-treated and scratched laser treated samples to evaluate the effects of galvanic coupling between la-ser treated layer and scratched area (substrate). The dimension of these samples was about 9 cm2, similar to that of scratched weld samples. This ensured similar anode/cathode ratio in both experiments. The tests were performed for 24 hours in naturally aerated 0.1 M NaCl in a beaker containing 500 ml of solution. The data acquisition rate was set to 1 measurement every 100 seconds. The samples were covered with several layers of “Stopping off” lacquer in order to expose only the scratched L-LT surface (laser treated or untreated). For these measurements, the reference elec-trode was a Saturated Calomel Electrode (SCE). The temperature was controlled at values of 25°C with a water bath. X ray microtomography was performed at the Materials Science beamline of the Swiss Light Source at the Paul Scherrer Institut in Switzerland [43]. This technique represents a powerful tool to image the microstructure of relatively small volumes of material in 3 dimensions and was used to understand the mechanism of corrosion propagation in laser treated material. The acquisition apparatus comprised a 28 μm thick Ce-doped YAG scintillator. The beam energy was set to 17.5 keV, the exposure time to 2 sec-onds. 721 radiographs were acquired in a complete 180° rotation around the sample axis at regular angles of ~ 0.25 degrees. The acquisition window of the camera was set to 1024 x 1024 pixels in 2x ‘binning mode’, resulting in a theoretical pixel size of ~ 1.4 μm. The 3D information was reconstructed with traditional filtered (Butterworth) backprojection algorithm. Sample ‘pins’ (parallel-epipeds with base dimension of about 700 μm x 700 μm) were cut with a Struers “Accutom-5” machine and glued with “Araldite” glue in stainless steel holders. Ex situ samples were cut out from nugget, HAZ and parent material of laser treated welds after immersion test, as described in the previous paragraphs. These samples were analysed in order to investigate corrosion propa-gation in damaged LSM layers. Samples for in situ experiments were cut from nugget, HAZ and parent material of laser-treated welds and exposed in situ to a 0.1 M NaCl solution in a radiation transparent silicone rubber tube. In this case, the cut surfaces of

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Fig. 1 Excimer laser treated AA2024-T351 FSW; (a)

and (b) optical micrographs showing surface morpho-logy after the treatment; (c) and (d) SEM micrographs (secondary electron mode) showing absence of precipi-tation on the treated surface.Saldatura FSW in lega 2024-T351 dopo trattamento con Excimer laser; (a) e (b) micrografie che mostrano la morfologia della superficie dopo il trattamento; (c) e (d) micrografie SEM (secondary electron mode) che mostra-no l’assenza di precipitati sulla superficie trattata.

the sample were exposed to the corrosive solution in addition to the laser treated surface. The samples were glued to the stainless steel holders with a continuous layer of glue in order to prevent the simultaneous exposure to the electrolyte of aluminium and stainless steel which would have resulted in undesired galvanic coupling effects. On each in situ sample, analysis before and dur-ing immersion (after 24 hours) was carried out. These samples were analysed to investigate the mechanism of corrosion propa-gation in laser treated layers.

EXPERIMENTAL RESULTS

Laser-treated layer morphologyFig. 1a shows an optical micrograph of a AA2024 T351 laser treated friction stir weld; the characteristic pattern produced

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Fig. 2 Cross section SEM micrographs (backscattered

electron mode) showing melted constituent particles in the LSM layer produced on (a) parent material, (b) FSW HAZ and, (c) FSW nugget on AA2024-T351 laser treated FSW.Micrografie SEM della sezione trasversale (backscattered electron mode) che mostrano la dissoluzione delle parti-celle costituenti nello strato LSM su (a) parent material, (b) FSW HAZ e, (c) FSW nugget su saldature FSW in lega AA2024-T351.

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Fig. 3 EDX elemental analysis of untreated and laser

treated parent material; (a) AA2024 T351; (b) AA-449-T7951. The laser treated material shows slight enrichment in Cu (a) and Cu and Zn (b) relative to the untreated material. The nominal chemical composition of the alloys is also plotted.Analisi EDX su parent material trattato laser e non trat-tato; (a) lega AA2024-T351; (b) lega AA7449 T7951. Il materiale trattato laser mostra arricchimento in Cu (a) e Cu e Zn (b) della superficie rispetto al materiale non trattato. La composizione chimica nominale delle leghe è anche riportata.

a

b

on the metal surface after the LSM treatment is visible from the magnified view displayed in Fig. 1b. Higher magnification SEM micrographs of the treated surface show the absence of the characteristic micron-sized constituent particles found in AA2024-T351 (Figs. 1c and 1d). SEM micrographs of the cross section of the same sample show dissolution of the bright, micron-sized constituent particles and formation of a 3 5 μm thick precipitate-free layer in any weld region (Fig. 2). Similar morphology was found for the AA7449-T7951 (not shown), although less contrast elemental between LSM layer and sub-strate was visible in this case in the SEM backscattered im-ages.Fig. 3 shows the elemental composition of the LSM layer ob-

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tained on AA2024-T351 (a) and AA7449-T7951 (b). For both alloys, the results show that the LSM layer and the matrix ex-hibit similar elemental composition, with some enrichment in Cu (AA2024) and Cu and Zn (AA7449) in the LSM layer.

Electrochemical characterisation of laser-treated weldsThe electrochemical reactivity of the LSM layers was tested with anodic and cathodic polarisation tests performed with mi-cro capillary cell. Anodic polarisation curves and breakdown potentials obtained on laser treated and untreated welds in both alloys are shown in Fig.4. The graphs show typical anodic polarisation curves for parent untreated and LSM material and the breakdown potential across the weld region, evaluated as the potential in which the value of the anodic current density reaches 2 10-5 A cm-2.

Despite the scatter in the values of the breakdown potential, the separation between the curves measured on untreated and laser treated welds (Fig. 4a and 4b) demonstrates that, for AA2024-T351, the laser treatment confers an improvement in corrosion resistance, as the breakdown potential is signifi-cantly increased after the laser treatment. Furthermore the breakdown potential is fairly consistent across the entire weld for the laser treated surfaces, but is lower in the weld region for the untreated weld, showing the greatest susceptibility to anodic dissolution of this region if no post weld treatment is carried out.Different results were found for the AA7449-T7951 (Fig. 4c and 4d), in which anodic polarisation curves and breakdown potentials measured on untreated and laser treated FSW were similar. The values of the breakdown potential on the LSM

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Fig. 4 Anodic reactivity of laser treated and untreated FSWs in 0.1 M NaCl. (a) and (b) AA2024-T351; (c) and (d)

AA7449-T7951. (a) and (c) Are typical anodic polarisation curves in parent material comparing the reactivity of the laser treatment with the reactivity of the untreated metal; (b) and (d) Are nominal breakdown potentials (Eb) at i = 2 x 10-5 A cm2 as a function of position relative to the weld centre for laser treated (dipped in nitric acid) and untrea-ted FSW (polished). A = ‘advancing’ side of the weld; R = ‘retreating’ side of the weld.Caratteristica anodica di saldature FSW dopo trattamento laser in 0.1 M NaCl. (a) e (b) Lega 2024-T351; (c) e (d) lega 7449-T7951. (a) e (c) Sono tipiche curve di polarizzazione anodica nel parent material che confrontano la reattività del trattamento laser con quella del metallo non trattato. (b) e (d) Sono i potenziali di breakdown nominale Eb valutati alla cor-rente i = 2 x 10-5 A cm2 in funzione della posizione rispetto al centro della saldatura. A = parte ‘advancing’ della saldatu-ra; R = parte ‘retreating’ della saldatura.

a b

c d

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weld were scattered and not uniform across the whole sam-ple, while that measured o the untreated weld show lower values in the weld region, indicative of enhanced susceptibil-ity to anodic attack. Cathodic polarisation curves and cathodic currents measured on laser treated and untreated welds for both alloys are shown in Fig. 5. The graphs show typical cathodic polarisation curves in parent material and the values of the cathodic current at a fixed potential of 900 mV vs. Ag/AgCl, which was used to compare the reactivity across the weld region.It is clear that the laser treatment can increase the corrosion resistance of both alloys by reducing the cathodic reactivity. The laser treated material (broken line) shows lower cathodic reactivity in the whole weld region and more uniform reactiv-ity in comparison with the untreated weld (solid line), where,

for both AA2024 and AA7449, a cathodic current density peak is observed in the weld nugget.

Optical and SEM microscopy examination after immer-sion in 0.1 M NaCl solutionTo verify whether the LSM treatment increases the corrosion resistance of FSWs and to understand whether the presence of potential scratches in the treatment would lead to signifi-cant dissolution in the scratched area, immersion for 20 days of scratched laser treated and untreated welds in 0.1 M NaCl solution was performed. Post immersion analysis was per-formed both in the scratched area and in areas ‘away’ from the scratch.Fig.6 shows the appearance of the AA2024-T351 untreated weld after 20 days immersion in 0.1 M NaCl followed by cor-

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Fig. 5 Cathodic reactivity of laser treated and untreated FSWs in 0.1 M NaCl. (a) and (b) AA2024-T351; (c) and

(d) AA7449-T7951. (a) and (c) Are typical cathodic polarisation curves in parent material comparing the reactivity of the laser treatment with the reactivity of the untreated metal. (b) and (d) Are cathodic current densities at 900 mV vs. Ag/AgCl as a function of position relative to the weld centre for laser treated (dipped in nitric acid) and untrea-ted FSW (polished). A = ‘advancing’ side of the weld; R = ‘retreating’ side of the weld.Caratteristica catodica di saldature FSW dopo trattamento laser in 0.1 M NaCl. (a) e (b) Lega 2024-T351; (c) e (d) lega 7449-T7951. (a) e (c) Sono tipiche curve di polarizzazione catodica nel parent material che confrontano la reattività del trattamento laser con quella del metallo non trattato. (b) e (d) Sono le correnti catodiche nominali valutate al potenziale di 900 mV vs. Ag/AgCl in funzione della posizione rispetto al centro della saldatura. A = parte ‘advancing’ della saldatura; R = parte ‘retreating’ della saldatura.

a b

c d

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rosion product removal in concentrated nitric acid. The weld was found susceptible to pitting, with relatively small and sparse pits in nugget (Fig. 6b and 6e) and parent material (Fig. 6d and 6g) and coarser and more numerous pits in the HAZ (Fig. 6c and 6f). Particularly intense attack was found in the scratched area in the HAZ (Fig. 6c).Fig. 7 shows the appearance of the AA2024-T351 laser treated weld after the same test. Residues of not completely removed corrosion products were still visible after immersion in nitric acid in certain areas of the sample (Fig. 7a) so that optical mi-croscopy was necessary to gain a better evaluation of the ex-tent of corrosion damage. Evaluation on the damage in areas ‘away’ from the scratch (Figs. 7b-7g) and comparison with the extent of attack found on untreated welds indicated that local-ised corrosion sites of smaller dimension and fewer in number formed in the HAZ of the laser treated weld, where relatively small pits (comparable in dimension with those found in the parent region) developed in place of the coarse pits present in the HAZ of the untreated weld (Fig. 6c and 6f). The nugget of the laser treated weld was found to be particularly resistant (Fig. 7b).Analysis of the scratched area, however, revealed the presence of few relatively large pits in the exposed substrate after 20 days exposure to 0.1 M NaCl, especially in the HAZ (Fig. 8a). An independent immersion for 5 days confirmed this behav-iour (Fig. 8b) and showed that two relatively large pits devel-

oped in the HAZ in these conditions (note that Fig. 8b shows two large pits surrounded by white corrosion products in the HAZ, while the other black spots visible in the samples were not associated with pitting in an obvious way). From surface examination (Fig. 6c and 8a), the size of the pits developed in the scratched HAZ of the ‘10J 3x3’ laser treated weld were comparable in size with that found in the HAZ of the untreat-ed weld.The results of the immersion tests on untreated and laser-treat-ed FSWs in AA7449 T7951 are shown in Fig. 9, 10 and 11. Fig. 9a shows the appearance of the untreated weld after 20 days immersion. Corrosion products were removed before post im-mersion examination. Only half of the weld is shown in the micrograph: the other half appeared similar. Similarly to the AA2024 T351 FSW, the weld was found to be susceptible to pitting, with relatively small and sparse pits in nugget (Fig. 9b and 9e) and parent material (Fig. 9d and 9g) and coarser pits in the HAZ (Fig. 9c and 9f). Some 100 μm deep pits developed in the HAZ after 20 days immersion.Fig. 10a shows the appearance of the AA7449 T7951 laser treat-ed weld after the same test. Only half of the weld is shown. The reactivity of the laser treated weld was found relatively heterogeneous, with enhanced reactivity found in the HAZ. The weld nugget (Fig. 10b and 10e) and parent material (Fig. 10d and 10g) showed localised corrosion sites of compara-ble number and dimensions with those present in the same

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Fig. 6 Untreated AA2024-T351 FSW after 20 days immersion in 0.1 M NaCl and removal of corrosion products in concentra-

ted nitric acid; (a) weld surface micrograph; (b), (c) and (d) optical micrographs of surface in nugget, HAZ and parent material re-spectively; (e), (f) and (g) optical micrographs of cross section showing typical localised corrosion sites in nugget, HAZ and parent material respectively. Note that micrograph (f) is taken a lower magnification than micrographs (e) and (g).Saldatura FSW in lega AA2024-T351 non trattata dopo immersione per 20 giorni in 0.1 M NaCl e rimozione dei prodotti di corrosione in acido nitrico concentrato; (a) micrografia della superficie; (b), (c) e (d) micrografie ottiche osservate in nugget, HAZ e parent material rispettivamente; (e), (f) e (g) micrografie ottiche della sezione trasversale che mostrano tipici attacchi corrosivi in nugget, HAZ e parent material rispettivamente. Notare che la micrografia (f) è stata acquisita a magnificazione inferiore di quelle mostrate in (e) e (g).

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regions in the untreated weld. However, laser treatment was beneficial in decreasing the reactivity of the HAZ, in which su-perficial attack (Fig. 10c and 10f) was found in place on fairly deep pits (Fig. 9c and 9f).Fig. 11 shows optical micrographs of the scratched area in different regions of the AA7449-T791 laser treated weld after 20 days immersion. Contrarily to what observed for AA2024 T351, the extent of attack in the scratched area was found to be much lower than on the laser treated surface. The number of pits in parent material (Fig. 11a) and nugget (Fig. 11c), for example, was much lower in the scratched area than on the intact LSM surface and much lower that that found on the un-treated weld.Open circuit potential measurements on AA2024-T351 and AA7449-T7951 laser treated and untreated parent material were employed to explain the behaviour of the scratched laser treated material. Measurements performed in 0.1 M NaCl on intact and scratched laser treated parent material and on un-treated parent material are shown in Fig. 12. For AA2024 T351 (Fig.12a), the measurements show higher OCP of the intact laser treated material in comparison with the untreated and scratched laser treated material. For AA7449-T7951 (Fig.12b), in contrast, the OCP of the LSM layer was lower that that

observed on intact parent material and similar to that of the scratched LSM material. Considerations on the OCP measure-ments are presented in the discussion.

X-ray microtomography examination of ex-situ samplesIn order to study corrosion propagation in damaged laser treated layers, X-ray microtomography was used to analyse ex situ samples cut out from a scratched AA7449-T7951 laser treated weld after immersion in 0.1 M NaCl for 5 days. The corrosion products were not removed before examination. Surface observation of the weld (Fig.11) had highlighted attack of the LSM surface in all weld regions but virtually no attack of the underlying substrate in the scratched area. X ray micro-tomography was used to gain a better characterisation of the corrosion damage. The observation that little attack develops in the scratched area of LSM AA7449 when exposed to NaCl is significantly strengthen by the set of micrographs displayed in Fig. 13, which show “slices” parallel to the LSM layer extract-ed form a 3D volume reconstruction of a sample cut out from the HAZ region of a LSM weld. Significant generalised attack, penetrating to a depth of about 30 μm, is visible on the surface of the sample. In contrast, no attack is visible in the scratched area of this sample.

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Fig. 7 Laser treated AA2024-T351 FSW after 20 days immersion in 0.1 M NaCl and removal of corrosion products in concen-

trated nitric acid; (a) weld surface micrograph; (b), (c) and (d) optical micrographs of surface ‘away’ from the scratch in nugget, HAZ and parent material respectively; (e), (f) and (g) optical micrographs of cross section ‘away’ from the scratch showing typical localised corrosion sites in nugget, HAZ and parent material respectively.Saldatura FSW in lega AA2024-T351 trattata laser dopo immersione per 20 giorni in 0.1 M NaCl e rimozione dei prodotti di corrosio-ne in acido nitrico concentrato; (a) micrografia della superficie; (b), (c) e (d) micrografie ottiche della superficie in zone lontane dall’inta-glio in nugget, HAZ e parent material rispettivamente; (e), (f) e (g) micrografie ottiche della sezione trasversale in zone lontane dall’inta-glio che mostrano tipici attacchi corrosivi in nugget, HAZ e parent material rispettivamente.

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s

Fig. 9 Untreated AA7449-T7951 FSW after 20 days immersion in 0.1 M NaCl and removal of corrosion products in concentra-

ted nitric acid; (a) weld surface micrograph; (b), (c) and (d) optical micrographs of surface in nugget, HAZ and parent material re-spectively; (e), (f) and (g) optical micrographs of cross section showing typical localised corrosion sites in nugget, HAZ and parent material respectively. Note that micrograph (f) is taken a lower magnification than micrographs (e) and (g).Saldatura FSW in lega AA7449-T7951 non trattata dopo immersione per 20 giorni in 0.1 M NaCl e rimozione dei prodotti di corrosio-ne in acido nitrico concentrato; (a) micrografia della superficie; (b), (c) e (d) micrografie ottiche osservate in nugget, HAZ e parent ma-terial rispettivamente; (e), (f) e (g) micrografie ottiche della sezione trasversale che mostrano tipici attacchi corrosivi in nugget, HAZ e parent material rispettivamente. Notare che la micrografia (f) è stata acquisita a magnificazione inferiore di quelle mostrate in (e) e (g).

Fig. 8 Optical micrographs showing attack developed in the

scratched area of the AA2024-T351 laser treated welds after immersion in 0.1 M NaCl: (a) high magnification micrograph showing pits developed in laser treated FSW after 20 days immersion (see Fig. 7a); (b) pits developed in the HAZ of laser treated FSW after 5 days immersion. Note that the 5 days and 20 days immersion tests were performed on different samples.Micrografie ottiche che mostrano l’attacco corrosivo sviluppato nell’intaglio prodotto su saldatura AA2024-T351 trattata laser dopo immersione in 0.1 M NaCl: (a) micrografia ad alta magnifica-zione che mostra pit formati nell’intaglio dopo 20 giorni di immer-sione (vedi Fig. 7a); (b) pit formati nella HAZ del trattamento dopo 5 giorni di immersione. Notare che le immersioni di 5 e 20 giorni sono state fatte su campioni diversi.

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X ray microtomography examination of in situ samplesDuring post immersion microscopic characterisation of the corrosion damage (e.g. Fig. 7g) some delamination of the LSM layer on both AA2024-T351 and AA7449-T7951 was observed. In situ X ray microtomography experiments were performed

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on “pins” cut from the nugget, HAZ and parent material of a laser treated weld to investigate this effect. In this case, dif-ferently from the “ex situ” samples, the cut untreated surfaces were exposed together with the laser treated surface.Fig. 14 shows X ray microtomography “slices” perpendicular to the axis of the “pin” sample acquired on LSM AA2024 T351 in situ before (Fig. 14a) and after (Fig. 14b) 24 hours exposure of a parent material sample in 0.1 M NaCl. The distribution of constituent particles clearly identifies the two slices as the same section of the sample. It is evident how delamination of the LSM layer took place during corrosion propagation in the laser treated material. The results obtained on HAZ and nugget samples, however, did not show any sign of delamina-tion after 24 hours exposure, suggesting that this phenomenon might take place only on some areas of a laser treated surface. Similar results were found on AA7449 T7951 (not shown).

DISCUSSION

Electrochemical measurements and immersion tests indicated a higher corrosion susceptibility of the weld region in com-parison with the parent material for untreated FSWs in both AA2024 T351 and AA7449-T7951. These results are in agree-

ment with the findings of other studies that highlighted the decrease in corrosion resistance often obtained in heat treat-able aluminium alloys as a consequence of friction stir welding [5-24].Laser surface melting produced the formation of a homogene-ous, 3-5 μm thick laser treated layer across weld region and parent material. Thermal dissolution of constituent particles and fine precipitates occurred in the LSM weld, leading to the formation of a precipitate free layer. The dissolution of con-stituent particles was enhanced in the nugget region (e.g. Fig. 2b), as in these area the constituent particles are fragmented into smaller pieces by the action of the FSW tool [16, 17]. The morphology of the laser treated layer observed in this study is consistent to that observed by other studies after laser surface melting aluminium alloys with Excimer lasers [32-39].Electrochemical measurements indicated that laser surface melting with an Excimer laser can improve the corrosion resist-ance of AA2024-T351 friction stir welds by decreasing cathod-ic reactivity and increasing the breakdown potential in weld region and parent material. Furthermore the electrochemical measurements showed that laser treating the weld can produce a certain homogenisation of the reactivity, with consequent re-duction of galvanic coupling effects that could occur if wetting

s

Fig. 10 Laser treated AA7449-T7951 FSW after 20 days immersion in 0.1 M NaCl and removal of corrosion products in concen-

trated nitric acid; (a) weld surface micrograph; (b), (c) and (d) optical micrographs of surface ‘away’ from the scratch in nugget, HAZ and parent material respectively; (e), (f) and (g) optical micrographs of cross section ‘away’ from the scratch showing typical localised corrosion sites in nugget, HAZ and parent material respectively.Saldatura FSW in lega AA7449-T7951 trattata laser dopo immersione per 20 giorni in 0.1 M NaCl e rimozione dei prodotti di cor-rosione in acido nitrico concentrato; (a) micrografia della superficie; (b), (c) e (d) micrografie ottiche della superficie in zone lontane dall’intaglio in nugget, HAZ e parent material rispettivamente; (e), (f) e (g) micrografie ottiche della sezione trasversale in zone lontane dall’intaglio che mostrano tipici attacchi corrosivi in nugget, HAZ e parent material rispettivamente.

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achieved through the dissolution of constituent particles and finer precipitates that, being rich in noble elements such as Cu or Fe, act as catalytic sites for oxygen reduction (e.g. [44, 45]). The increase in breakdown potential observed for AA2024 is likely to be related to the dissolution of constituent particles, which can be preferential sites for pitting initiation (e.g. [46]), but also to the formation of a homogeneous Cu-rich laser treat-ed layer: Cu is known to increase the breakdown potential of aluminium alloys when in solid solution [47] and might en-hance the dissolution resistance of the LSM layer. The increase in breakdown potential and the decrease in cathodic reactivity found in this study on AA2024 T351 are consistent with the results found by Chan et al. [33] after LSM of AA6013 with an Excimer laser. In contrast, it has been shown that alloying Zn in Al decreases the breakdown potential [48], and this effect is thought to be responsible of the anodic characteristic of the AA7449 LSM layer. The increase in breakdown potential and decrease in cathodic reactivity achieved on AA2024 T351 after LSM with Excimer laser are superior than that reported by similar studies on AA2024 and AA2014 after LSM with CW Nd:YAG and CO2 lasers [49-54]. This was attributed to the higher homogeneity of the treatments obtained with pulsed lasers such as Excimer lasers in comparison with treatments obtained with CW lasers such as CW Nd:YAG and CO2 lasers, in which, in contrast, higher levels of microsegregation are produced.If LSM can represent an effective way of improving the corro-sion resistance of FSWs, it is important to consider the fact that the laser treated layer is only 3-5 μm thick and that exposure of the substrate (either due to the presence of a pre-existing de-

s

Fig. 11 Optical micrographs showing the appearance of the scratched area of the laser treated AA7449-T7951 FSW shown in

Fig. 10 after 20 days immersion in 0.1 M NaCl. Corrosion products were removed in concentrated nitric acid after immersion. (a) Parent material; (b) HAZ; (c) weld nugget. The amount of corrosion attack present in the scratched area is lower than that found on the LSM surface.Micrografia ottica che mostra l’area intagliata della saldatura FSW in lega AA7449-T7951 trattata laser mostrata in Fig.10 dopo im-mersione per 20 giorni in 0.1 M NaCl. I prodotti di corrosione sono stati rimossi in acido nitrico dopo l’immersione. (a) Parent material; (b) HAZ; (c) weld nugget. L’attacco corrosive presente nell’area intagliata è di entità minore di quello osservato sulla superficie LSM.

a

b

Fig. 12 Open circuit potential of untreated, scratched

laser treated and intact laser treated parent mate-rial specimens during immersion in 0.1 M NaCl; (a) AA2024 T351; (b) AA7449-T7951.Potenziale a circuito aperto di campioni di parent ma-terial non trattato, trattato laser intagliato e trattato laser intatto durante immersione in 0.1 M NaCl; (a) lega AA2024 T351; (b) lega AA7449-T7951.

s

of the metal surface with a continuous, conductive electrolyte takes place. On AA7449-T7951, only decrease in cathodic reac-tivity was found after laser treatment, while the anodic reactiv-ity remained similar to that of untreated material.The reduction in cathodic reactivity produced with LSM is

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fect or as a consequence of corrosion development over time) may occur. In this scenario, considerations related to the ex-posure of damaged (scratched) laser treated samples and to potential galvanic coupling effects between the LSM layer and the substrate become important.The results shown in this paper indicate that, for AA2024 T351, the intact laser treated layer has higher OCP than the untreated parent material. This suggests that, if the substrate is exposed, galvanic coupling effects between laser treated layer and sub-strate tend to drive corrosion preferentially in the substrate. The OCP of the scratched laser treated sample, however, is similar to that of the untreated material indicating that the

s

Fig. 13 X-ray microtomography “slices” of scratched laser treated AA7449-T7951 FSW in HAZ region after ex-situ immersion for

5 days in 0.1 M NaCl. The slices show planes parallel to the laser treatment at different depths below the surface: (a) 7 μm; (b) 15 μm; (c) 31 μm. Although significant corrosion is observed on the sample, little attack developed in the scratched area.‘Sezione’ di microtomografia ai raggi X di un campione di saldatura FSW in lega AA7449-T7951 trattata laser nella HAZ dopo immersio-ne per 5 giorni in 0.1 M NaCl. Le sezioni mostrano piani paralleli al trattamento laser a diverse profondità sotto la superficie: (a) 7 μm; (b) 15 μm; (c) 31 μm. Sebbene l’attacco corrosivo osservato sulla superficie del campione sia notevole, l’entità della corrosione nell’inta-glio è limitata.

Fig. 14 X-ray micro-tomography “slices” of a parent mate-

rial laser treated sample collected in situ before and after immersion for 24 hours in 0.1 M NaCl. The slices show the same plane perpendicular to the pin axis direction (a) before immersion and (b) during immersion (24 hours) and highlight delamination of the laser treated layer during exposure to the electrolyte. The in-situ samples were extracted from a pristine, non scratched, laser treated AA2024-T351 FSW.’Fetta’ di microtomografia ai raggi X di un campione di parent material trattato laser acquisita in situ prima e dopo immersio-ne per 24 ore in 0.1 M NaCl. La fetta mostra lo stesso piano perpendicolare all’asse del campione (a) prima dell’immersione e (b) durante l’immersione (24 ore) ed evidenzia delaminazione dello strato LSM durante esposizione all’elettrolita. I campioni per misure in situ sono stati estratti da saldature FSW trattate laser in lega AA2024-T351 non intagliate

s

galvanic couple formed between the LSM layer and the sub-strate is corroding at the potential that the uncoupled substrate alone would exhibit during free corrosion. This suggests that, at least for the anode/cathode ratio used in this study, the low cathodic reactivity of the LSM layer is unable to significantly polarise the substrate and that galvanic coupling between the substrate (anode) and the LSM layer (cathode) does not re-sult in accelerated corrosion rate of the substrate. For AA7449 T7951, in contrast, the incorporation of Zn into the LSM layer ensured a relatively high anodic reactivity of the laser treated surface. The OCP of the laser treated layer was lower than that of the untreated substrate, ensuring that galvanic coupling of

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the scratched substrate with the intact LSM surface results in sacrificial protection of the former.These observations are confirmed by the results of the immer-sion tests, which indicated a increase in corrosion resistance of the weld after laser treatment in the intact areas away from the scratch. Pitting corrosion developed in the scratched area only for LSM 2024-T351 (especially in the HAZ) and to an ex-tent comparable to that developed in the absence of laser treat-ment. For AA7449-T7951, no localised corrosion was observed in the scratched area as opposed to the rest of the LSM surface, which displayed relatively significant dissolution. Post-exposure cross sectional examination and in situ X-ray to-mography suggested that delamination of the laser treatment from the substrate can take place as a consequence of corrosion propagation. This effect, however, was found only in certain samples after exposure to 0.1 M NaCl and did not result in complete removal of the laser-treated layer after 20 days. The laser treated layer delamination might be related to the forma-tion of fine bands of precipitate in the LSM layer [35, 36].

CONCLUSIONS

The use of laser surface melting with an Excimer laser as corro-sion protection post treatment for friction stir welds in AA2024 T351 and AA7449-T7951 was investigated. The findings of this study can be summarised as follows: - Without laser treatment, friction stir welds showed higher susceptibility to pitting corrosion in the weld region, especial-ly in the HAZ, after exposure to 0.1 M NaCl solution.- The laser surface melting treatment performed with an Exci-mer laser on friction stir welds produced the formation of a 3-5 μm thick layer where constituent particles were dissolved and the alloying elements retained in solid solution. In particular, a Cu-rich solid solution was formed for AA2024 T351, while a Cu and Zn-rich solid solution was formed for AA7449 T7951.- For AA2024-T351, the laser treatment improved the corrosion resistance of friction stir welds by decreasing and homogenis-ing anodic and cathodic reactivity in weld region and parent material. As a consequence, while untreated welds showed increased susceptibility to pitting in the HAZ, laser treated welds showed more uniform and less intense corrosion attack after exposure to 0.1 M NaCl solution.- For AA7449-T7951, the laser treatment improved the corro-sion resistance of friction stir welds by decreasing the cathodic reactivity across weld region and parent material. The anodic reactivity, however, was similar to that observed on untreated material. While untreated welds showed relatively deep pits in the HAZ, laser treated welds showed a more uniform and superficial attack across the whole weld region.- When, before immersion in corrosive electrolyte, a scratch exposing the substrate was produced on a laser treated weld in AA2024-T351, pitting developed in the scratched area, espe-cially in the HAZ. The severity of attack, however, was compa-rable to that found in the HAZ of the untreated material. The behaviour of the scratched laser treated weld was attributed to the formation of a laser treated layer with higher open circuit potential relative to the untreated material but low cathodic reactivity: galvanic coupling between LSM layer and substrate drives corrosion preferentially in the substrate, but the reduced cathodic efficiency of the LSM layer ensures a free corrosion potential of the galvanic couple similar to that of the untreated material, which is incapable of polarising the scratched area and increasing its dissolution rate.- When, before immersion in corrosive electrolyte, a scratch

exposing the substrate was produced on laser treated welds in AA7449 T7951, corrosion did not develop in the scratched area but remained localised on the laser treated layer. This be-haviour was attributed to the formation of an LSM layer with lower OCP than the untreated material, which was able to af-ford sacrificial protection to the substrate if this was exposed by a scratch.- Corrosion propagation beneath the laser-treated layer pro-duced partial delamination of the LSM treatment. This phe-nomenon caused the removal of the protective treatment form part of the surface and might decrease the capability of the LSM treatment to protect the weld.

ACKNOWLEDGMENTS

The authors would like to acknowledge Airbus UK, BAE SYS-TEMS and Airbus D for sponsoring the PhD project which the work shown in this paper is based on. In particular the authors would like to acknowledge Mike Poad (Airbus UK), Stephen Morgan (BAE SYSTEMS), Debbie Price (previously at BAE SYSTEMS), Daniela Lohwasser (Airbus D), Philip Prangnell (University of Manchester), Paul Ryan (formerly at University of Manchester), Manthana Jaryiaboon (formerly at the Uni-versity of Birmingham) and Napachat Tareelap (University of Birmingham) for useful discussion. We would also like to acknowledge the contribution of Nick Stevens (University of Manchester) for his help with the microtomography measure-ments.

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ABSTRACTCORROSIONE E PROTEZIONE DI SALDATURE FRICTION STIR IN LEGHE DI ALLUMINIO PER USO AERONAUTICO

Parole chiave: alluminio e leghe, saldatura, corrosione

L’effetto di un Excimer laser sulla resistenza a corrosione di saldature fric-tion stir in lega di alluminio 2024-T351 e 7449-T7951 è stato studiato con prove elettrochimiche, microscopia ottica e SEM e microtomografia ai raggi X. Il risultato delle prove elettrochimiche mostra una riduzione nella

reattività catodica, ottenuta grazie alla formazione di uno strato di 3-5 μm privo di precipitati. Riduzione nella reattività anodica è stata inoltre osservata per la lega 2024 T351, dovuta alla formazione di uno strato LSM ricco in Cu. Test di immersione in soluzione 0.1 M NaCl confermano che la presenza del trattamento laser riduce l’entità dell’attacco corrosivo ma mostrano anche che, se il trattamento laser è danneggiato da un intaglio, i fenomeni corrosivi tendono o meno a svilupparsi nella zona d’intaglio in relazione al tipo di lega. Delaminazione dello strato trattato laser, inoltre, è stata osservata in seguito ad attacco corrosivo.

and Coatings Technology 179 (2-3): 158-164.40] Connolly, B.J. and Doyle, R. Effect of laser surface melt-ing techniques used for improved corrosion resistance on the fatigue properties of friction stir welded AA2024-T351. Tri-Services Conference. Orlando (USA), 2005.41] Padovani, C., Davenport, A.J., Connolly, B.J., Williams, S.W. and Siggs, E. Corrosion and protection of friction stir welds in 7XXX alloys. Aluminium Surface Science and Technology 2006. Beaune, France, 2006.42] Padovani, C. (2007) Corrosion protection of friction stir welds in aerospace aluminium alloys. PhD Thesis, Metallurgy and Materials, University of Birmingham.43] Stampanoni, M., Borchert, G., Wyss, P., Abela, R., Patter-son, B., Hunt, S., Vermeulen, D. and Rüegsegger, P. (2002) High resolution X-ray detector for synchrotron-based microtomog-raphy. Nucl. Inst. and Meth 491 (1): 291-301.44] Aldykewicz, J., A. J., Isaacs, H.S. and Davenport, A.J. (1995) The Investigation of Cerium as a Cathodic Inhibitor for Alu-minum-Copper Alloys. Journal of the Electrochemical Society 142 (10): 3342-3350.45] Jakab, M.A., Little, D.A. and Scully, J.R. (2005) Experimen-tal and Modelling Studies of the Oxygen Reduction Reaction on AA2024-T3. Journal of The Electrochemical Society 152 (8): B311-B320.46] Suter, T. and Alkire, R.C. (2001) Microelectrochemical stud-ies of pit initiation at single inclusions in Al 2024-T3. Journal of the Electrochemical Society 148 (1): B36-B42.47] Muller, I.L. and Galvele, J.R. (1977) Pitting Potential of High-Purity Binary Aluminum-Alloys .1. Al-Cu Alloys - Pit-

ting and Intergranular Corrosion. Corrosion Science 17 (3): 179-&.48] Muller, I.L. and Galvele, J.R. (1977) Pitting Potential of High-Purity Binary Aluminum-Alloys .2. Al-Mg and Al-Zn Alloys. Corrosion Science 17 (12): 995-1007.49] Chong, P.H., Liu, Z., Skeldon, P. and Thompson, G.E. (2003) Large area laser surface treatment of aluminium alloys for pit-ting corrosion protection. Applied Surface Science 208-209: 399-404.50] Chong, P.H., Liu, Z., Skeldon, P. and Thompson, G.E. (2003) Corrosion behaviour of laser surface melted 2014 aluminium alloy in T6 and T451 tempers. Journal of Corrosion Science and Engineering 6: .51] Li, R., Ferreira, M.G.S., Almeida, A., Vilar, R., Watkins, K.G., McMahon, M.A. and Steen, W.M. (1996) Localized corrosion of laser surface melted 2024-T351 aluminium alloy. Surface and Coatings Technology 81 (2-3): 290-296.52] Liu, Z., Chong, P.H., Butt, A.N., Skeldon, P. and Thomp-son, G.E. (2005) Corrosion mechanisms of laser-melted AA 2014 and AA 2024 alloys. Applied Surface Science 247: 294-299.53] Liu, Z., Chong, P.H., Skeldon, P., Hilton, P.A., Spencer, J.T. and Quayle, B. (2006) Fundamental understanding of the cor-rosion performance of laser-melted metallic alloys. Surface and Coatings Technology 200: 5514-5525.54] Watkins, K.G., Liu, Z., McMahon, M., Vilar, R. and Ferrei-ra, M.G.S. (1998) Influence of the overlapped area on the cor-rosion behaviour of laser treated aluminium alloys. Materials Science and Engineering A 252 (2): 292-300.

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CORROSION AND PROTECTION OF FRICTION STIR WELDS IN ... · the onset of localised corrosion in aluminium alloys is known to be able to decrease this parameter (e.g. [25]). Recent work

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