PSI - Issue 60

Chinnam Sivateja et al. / Procedia Structural Integrity 60 (2024) 245–255 Sivateja et al. / Structural Integrity Procedia 00 (2023) 000 – 000

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1. Introduction Aluminium 2024-T3 alloy in the cladded form has been used extensively in transport aircraft wing and fuselage skin structures. Corrosion protection is paramount in preserving such alloy materials' structural integrity and longevity, especially in aerospace applications. The aircraft operates in varied climatic conditions like seaport to the desert.

Nomenclature CAA

Cladded Aluminium Alloy

CDF

Cumulative distribution function

CMAS-AA

Chemically milled, anodized and sealed aluminium alloy

COV

Coefficient of variation Plane stress fracture toughness Scanning electron microscope

K c

SEM

SIF

Stress intensity factor X-ray diffraction

XRD

It is subjected to variable amplitude loading during its operation, leading to fatigue of materials under various environmental factors such as moisture, salinity, and temperature. It has been shown that the fatigue life of corroded aluminium 2024-T3 alloy has been reduced by ten times that of uncorroded samples, see CCFAM 2000. Among the numerous methods available, two prominent techniques are often employed for effective corrosion prevention: Cladding and anodization. Cladding involves the application of a protective layer of corrosion-resistant material over the surface of a substrate, creating a barrier that shields the underlying material from corrosive agents. This process enhances the material's resistance to degradation caused by environmental factors and exposure to harsh elements. On the other hand, anodization is a surface treatment primarily applied to metals, such as aluminium, to form an oxide layer. This oxide layer is a robust protective shield, significantly reducing the material's susceptibility to corrosion, oxidation, and wear. Both Cladding and anodization are standard and reliable methods for safeguarding against corrosion, ensuring the extended service life and structural reliability of essential components and structures, see ASM Vol. 13B (2007). The commercially available aluminium sheets are of the standard thickness, and it is essential to reduce the thickness of the material to the optimum size to reduce weight at some required places. Such thickness reduction in thin sheet materials is achieved by chemical milling. However, chemical milling removes the Cladding, and subsequent anodizing followed by sealing creates a corrosion protection layer. It has been shown in the literature that anodization reduces fatigue life significantly (Schneider and Fürbeth, 2022). Limited data has been available on the effect of chemical milling followed by the anodization process on the fatigue life of 2024-T3 aluminium alloys. Chemical milling reduces thickness through a chemical reaction using either acid or basic etchant, generally an alkaline solution. The advantage of this process is stress-free milling and adaptability to complex shapes and thin sections (Schneider and Fürbeth, 2022; Sesana et al., 2019; Çakır , 2008). The manufactured parts by chemical milling need no subsequent sanding, polishing, or machining of the milled surfaces. However, during the process, the chemical attack on sheets is selective. The reaction rate for grain, grain boundary, and the precipitates inside or at the grain boundaries differs metallurgically. This may lead to residual pits of various sizes and depths and consequential surface roughness on the chemically milled sheet (Sesana et al., 2019). The pits' sizes and distribution depend on the chemical used and milling time, and they influence the mechanical properties, including fatigue behaviour. Accordingly, the process for chemical milling should be chosen as per the composition of the alloy. A study on the effect of chemical milling on Al-Mg-Si alloy showed that the average roughness of samples increased 3-4 times that of electropolished samples (Spear and Ingraffea, 2013). Available literature indicates