New process enhances the corrosion-fatigue performance of high strength aerospace aluminium alloys

Indian Scientists have developed an environment-friendly process for improving the lasting of a material in the face of corrosion (corrosion-fatigue life) of high strength aerospace Aluminium alloys that make aerospace structures sufficiently light.

Aluminium alloys are used in aerospace, automobile, and marine applications due to their low density and high specific strength (strength-to-weight ratio). Among various aluminium alloys, heat treatable wrought alloy compositions possess high mechanical strength and are commonly preferred by the aerospace industry. The aerospace components of these aluminium alloys, including landing gear, wing spar, fuselage, skins, pressure cabins, and so on, are expected to possess high fatigue life and corrosion resistance.

Though Aluminium alloys’ corrosion resistance is significantly enhanced by depositing harder alumina (Al2O3) coating through anodic-oxidation-based processes, these processes employ concentrated acids as their electrolyte, leading to emission of toxic fumes that are non-eco-friendly processes. This challenge calls for a multi-functional environment-friendly coating, which can protect the aluminium alloys from corrosion and fatigue damages.

Scientists at the International Advanced Research Centre for Powder Metallurgy and New Materials (ARCI), an autonomous institute of the Department of Science & Technology (DST), Govt. of India, found that Aluminium alloys wear and corrosion resistance are significantly enhanced by depositing harder alumina (Al2O3) coating through anodic-oxidation-based processes — anodizing, hard anodizing, and micro-arc oxidation (MAO).

The presence of beneficial compressive residual stresses beneath the substrate-coating interface delays the crack propagation and results in superior fatigue life, which can be useful for light-weighting of aerospace structures.

While the MAO process employs a dilute concentration of alkaline electrolyte wherein toxic fume liberation is absent, spent electrolyte disposal does not attract any environmental regulations. Therefore MAO is regarded as an eco-friendly process. The Alumina (Al2O3) coatings deposited through the MAO technique possess high hardness (1000-1900 HV), excellent wear, and corrosion resistance as compared to identically thick hard-anodized (HA) coatings (300-500 HV). The work has already been published in the journals like International Journal of Fatigue recently, and patents were granted in Indian and abroad.

A duplex treatment developed at ARCI involving a shot peening (bombardment of a component surface with spherical balls of few hundreds of micro meter size, travelling at specific kinetic energy) operations before the MAO coating (SP+MAO) depositions has led to the significant enhancement in fatigue life of Al alloys by ten times without affecting the essential wear and corrosion resistance of the coating.