Back fixation plate - Al Alloy - Modern Times

Marianne. Senn (EMPA, Dübendorf, Zurich, Switzerland) & Christian. Degrigny (HE-Arc CR, Neuchâtel, Neuchâtel, Switzerland)

Stratigraphic representation: none

Fig. 4: Stratigraphic representation of the object in cross-section using the MiCorr application. This representation can be compared to Fig. 9.

Analyses performed:
Metallography (nital etched), Vickers hardness testing, SEM/EDX.

The metal is a relatively pure aluminium alloy with numerous inclusions (Table 1). From the chemical composition of the inclusions they can be interpreted as alpha-AlFeSi intermetallic compounds. In bright field we observe elongated inclusions indicating that the metal was rolled (Fig. 4). The alloy composition is similar to an unalloyed primary aluminium (Al content between 99 and 99.8 mass%). The O content reflects the immediate oxidation of the metal and is not part of the alloy. After etching the organisation of inclusions in rows is more easily seen (Fig. 5). The SEM image shows large grains formed after annealing (Fig. 6). The average hardness of the metal is HV1 40.

 

Elements

Al Si Fe O Total
Metal (average) 95 0.8 < 0.7 97
Inclusion (average) 60 8.6 31 1.5 100

Table 1: Chemical composition (mass %) of the metal and inclusions (from Fig. 4). Method of analysis: SEM/EDX, Laboratory of Analytical Chemistry, Empa.

The metal is covered by a very thin corrosion layer. Locally thicker “corrosion products” (or adhering material) can be observed (dark-grey in Fig. 7). Under polarized light, they appear blue-brown (Fig. 8). Analysis indicates that the metal is, as expected, covered by a very thin Al and O-rich layer whereas the particles and adhering material contain C,O,Si,Ca,Fe,Zn,S and even Ti (Figs. 9 and 10). The location of the adhering material studied and the presence of both Zn and Ti suggest the presence of a paint coating on the metal surface.

Corrected stratigraphic representation: none

This aluminium alloy has a composition similar to a primary aluminium with an Al content between 99 and 99.8 mass%. The main impurities are Si and Fe. Because of their insolubility in the aluminium they form intermetallic (alpha-AlFeSi) inclusions. The metal was hot rolled and annealed. The metal is covered by a very thin corrosion layer (probably aluminium oxide) and in areas adhering materials are present, most likely the remains of a paint system (Zn and Ti-rich) mixed with environmental pollutants.

References object

1. Rumo, L. (2008) Analyse et caractérisation des alliages constitutifs de l'avion Dufaux IV. Mémoire Filière conservation-restauration, Haute Ecole art appliqués, La Chaux-de-Fonds, 101-105.

 

References sample

2. Rumo, L. (2008) Analyse et caractérisation des alliages constitutifs de l'avion Dufaux IV. Mémoire Filière conservation-restauration, Haute école art appliqués, La Chaux-de-Fonds, 101-105.

References on analytic methods and interpretation