Page 27 - PR 2014 2016 10 Materials and Nanotechnology

Materials and Nanotechnology | Progress Report 303

comparatively to the two aluminium base fected zones of the LDSS slightly decreased
metals tested individually. Corrosion of the comparatively to the stir zone (nugget) or to
welded system resulted in the establishment the zones unaffected by FSW.
of a galvanic coupling, clearly shown by local
electrochemical impedance measurements, at Corrosion resistance investigation of Al-
which the AA7475 alloy behaves anodically Cu-Li alloys
with respect to the AA2024 alloy. A Zn deposit
was observed on the intermetallic particles Aluminum-copper-lithium alloys are widely
of the AA2024 alloy after 24 h of immersion used in the aircraft industry due to their ad-
in the electrolyte resulting from the galvanic vantageous properties of low weight and high
coupling. Such a behaviour in combination mechanical resistance. Precipitation harden-
with LEIS results allowed to describe the de- ing is one of the mechanisms to increase their
velopment of the galvanic coupling between resistance caused by second phase particles
two different aluminium alloys (AA2024-T3 precipitates in the aluminum alloy matrix.
and AA7475-T761) butt-welded by FSW as These particles are formed during solidifica-
a function of time from the early stage of tion or thermomechanical treatments. Addi-
immersion. tion of Cu and Mg leads to increased mechan-
ical resistance of Al-Li alloys in solid solution
Friction stir welding effects on condition and to homogeneous precipitation
microstructure and localized corrosion of coherent phases, such as δ’(Al Li) in the
3
resistance of S82441 lean duplex matrix with spherical morphology, besides
stainless steel other phases, such as Θ ‘(Al Cu), S (Al LiMg)
2 2
or T1 (Al CuLi). It is important to note that
2
Friction stir welding (FSW) has been consid- hardening by precipitation of particles with
ered an alternative to replace fusion welding Li involves the formation of metastable pre-
processes of stainless steels. In this study, the cipitates, which are phase precursors, such
UNS S82441 lean duplex stainless steel (LDSS), as T1. The aim of this study is to investigate
a recently developed alloy, was FSWed and the the localized corrosion resistance of alumi-
effect of this welding process on the micro- num-copper-lithium alloys. This study has
structure and localized corrosion resistance been carried out by surface observation as
of the steel was investigated by assessment a function of exposure to various corrosion
of phase volumetric fraction in each zone re- environments and by electrochemical tech-
sulting from FSW, either affected or unaffected niques. The microstructure of Al-Cu-Li alloys
zones of the steel by X-ray diffraction (XRD), is also under evaluation.
scanning electron microscopy, transmission
electron microscopy and electrochemical tests. The effect of cerium in the sealing of

The pitting resistance of each zone was in- anodized aluminum alloys
vestigated by polarization tests and by deter-
mination of the pitting critical temperature. Aluminum alloys for application in the aircraft
The results indicated that microstructural industry are highly susceptible to localized
changes promoted by FSW did not affect the corrosion. Consequently, it is usually pro-
pitting resistance of the LDSS studied at room tected by anodizing and sealing treatments.
temperature. However, the critical pitting In this study, Al-Cu-Li alloys were anodized
temperature at the thermomechanically af- in tartaric sulfuric acid solution, simulating