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Study of the (sub)surface on rolled commercially pure aluminium alloys

Thursday, 13 October, 2005 - 17:00
Campus: Brussels Humanities, Sciences & Engineering campus
Guy Buytaert
phd defence

Rolling is known to alter the surface properties of aluminium alloys and to
introduce disturbed subsurface layers. These layers are defined by a refined grain
structure, incorporated oxide particles, a fine distribution of intermetallic particles
and cracks and voids. In this study the subsurface of rolled commercially pure
aluminium alloys (AA1050 and AA1050 + 0.5 wt% Mg) is investigated. Specimens
from different steps of the production process are characterised: after hot and cold
rolling, with and without annealing. The influence of these subsurface layers on
surface treatments, like etching and AC electrograining, is investigated.

The (sub)surface is characterised by a combined use of surface analytical
techniques. From progressive alkaline etching and measuring the total reflectance
(TR) a thickness of the subsurface layers is estimated. Cold rolling reduces these
layers from 0.50 - 3.0 μm (after hot rolling) to a thickness of 0.03 - 1.1 μm.
Annealing preserves the subsurface layers. Quantitative radiofrequency glow
discharge optical emission spectrometry (rf-GDOES) reveals characteristic
distributions of alloying, trace and impurity elements within the first micrometers
in depth. Moreover, during annealing of a Mg-containing alloy Mg strongly
enriches to the subsurface, while other elemental distributions are not significantly
affected; XPS indicates both MgO and Al2O3 in the surface oxide film. SEM/EDX +
BEI analysis of the bottom of GDOES craters reveals the presence of oxide-rich
islands in the subsurface. High-resolution BEI of ultramicrotomy prepared crosssections
shows rolled-in oxide-rich regions in the subsurface, intermetallics, and
aluminium grains. Cross-sectional TEM analysis of specimens of oxide-rich regions
prepared by focused ion beam thinning (FIB) shows that hot rolled and annealed
aluminium alloys have a complex subsurface structure, lying above large,
recrystallised aluminium bulk grains. The subsurface exhibits a mixture of small
metallic aluminium grains, amorphous oxide particles, cracks and small voids.
TEM confirms the thickness of the subsurface layers found with the method of TR.

Alkaline etching preferentially attacks the aluminium matrix and preserves the
oxide-rich subsurface regions. Treatment with strongly buffered alkaline cleaning
agents such as Nabuclean does not affect the aluminium matrix and the subsurface layers. Treatment with CrO3/H3PO4 preferentially dissolves the oxide-rich regions,
leaving the aluminium matrix un-attacked. AC electrograining in HCl and HNO3
does not (completely) remove the oxide-rich subsurface regions. These regions give
rise to less or not pitted areas upon graining, creating not-desirable plateau-like

Investigating the potential variations during AC electrograining indicates that as
received rolled substrates appear less susceptible for pitting than substrates, which
are pre-treated by alkaline etching. Moreover, alkaline etching gives rise to a more
uniform morphology with more and smaller pits and less un-attacked areas.

Subsurface layers in hot rolled aluminium are investigated with the microcapillary
cell. Areas containing these rolled-in layers exhibit lower breakdown
potentials (in NaCl solution) than areas excluding these regions, indicating them
to be more susceptible to localised attack. In the former areas crystallographic pits
are initiated, whereas in the latter areas a different kind of localised corrosion
occurs, which may be correlated with the local differences in morphology upon AC