Page 311 - 00. Complete Version - Progress Report IPEN 2014-2016

Materials and Nanotechnology | Progress Report 311

atively to the two aluminium base metals tively to the stir zone (nugget) or to the zones
tested individually. Corrosion of the welded unaffected by FSW.
system resulted in the establishment of a gal-
vanic coupling, clearly shown by local electro- Corrosion resistance investigation
chemical impedance measurements, at which of Al-Cu-Li alloys
the AA7475 alloy behaves anodically with re-
spect to the AA2024 alloy. A Zn deposit was Aluminum-copper-lithium alloys are widely
observed on the intermetallic particles of the used in the aircraft industry due to their ad-
AA2024 alloy after 24 h of immersion in the vantageous properties of low weight and high
electrolyte resulting from the galvanic cou- mechanical resistance. Precipitation hardening
pling. Such a behaviour in combination with is one of the mechanisms to increase their re-
LEIS results allowed to describe the develop- sistance caused by second phase particles pre-
ment of the galvanic coupling between two cipitates in the aluminum alloy matrix. These
different aluminium alloys (AA2024-T3 and particles are formed during solidification or
AA7475-T761) butt-welded by FSW as a func- thermomechanical treatments. Addition of Cu
tion of time from the early stage of immersion. and Mg leads to increased mechanical resis-
tance of Al-Li alloys in solid solution condition
Friction stir welding effects on and to homogeneous precipitation of coherent
microstructure and localized phases, such as ο’(Al3Li) in the matrix with
corrosion resistance of S82441 spherical morphology, besides other phases,
lean duplex stainless steel such as ο ‘(Al2Cu), S (Al2LiMg) or T1 (Al2CuLi).

It is important to note that hardening by pre-
Friction stir welding (FSW) has been consid- cipitation of particles with Li involves the for-
ered an alternative to replace fusion welding mation of metastable precipitates, which are
processes of stainless steels. In this study, the phase precursors, such as T1. The aim of this
UNS S82441 lean duplex stainless steel (LDSS), study is to investigate the localized corrosion
a recently developed alloy, was FSWed and the resistance of aluminum-copper-lithium alloys.
effect of this welding process on the micro- This study has been carried out by surface ob-
structure and localized corrosion resistance servation as a function of exposure to various
of the steel was investigated by assessment corrosion environments and by electrochem-
of phase volumetric fraction in each zone re- ical techniques. The microstructure of Al-Cu-
sulting from FSW, either affected or unaffect- Li alloys is also under evaluation.
ed zones of the steel by X-ray diffraction (XRD),
scanning electron microscopy, transmission The effect of cerium in the sealing
electron microscopy and electrochemical tests. of anodized aluminum alloys
The pitting resistance of each zone was inves-
tigated by polarization tests and by determi- Aluminum alloys for application in the aircraft
nation of the pitting critical temperature. The industry are highly susceptible to localized cor-
results indicated that microstructural chang- rosion. Consequently, it is usually protected by
es promoted by FSW did not affect the pitting anodizing and sealing treatments. In this study,
resistance of the LDSS studied at room tem- Al-Cu-Li alloys were anodized in tartaric sul-
perature. However, the critical pitting tem- furic acid solution, simulating the industrial
perature at the thermomechanically affected process adopted by the aircraft industry, and
zones of the LDSS slightly decreased compara- subsequently sealed, either in boiling water