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Laser cleaning of aluminium alloys
[Thesis]. Manchester, UK: The University of Manchester; 2019.
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Abstract
Laser cleaning has been employed in a wide range of industries as a potential substitution to conventional surface cleaning methods using chemical or mechanical techniques. The interest of using it to remove oxide layers formed in manufacturing processes on metallic substrates, which is essential for bonding and jointing applications, has been increased in recent years, especially in aerospace and automotive industries. However, the understanding of surface modification by laser cleaning is still limited. The present work studies the effect of laser cleaning on three aluminium alloys, including AA7024-T4, AA5083-O and AA2024-T3 alloys, in terms of surface characteristics, corrosion behaviour and welding performance. The surface analysis has revealed that laser cleaning effectively removes the oxide layers originated from manufacturing processes, comprising of MgO and MgAl2O4 for AA7024-T4 and AA5083-O alloys which results in brighter surface with an increase of surface reflectance. A formation of thin oxide layer induced by laser cleaning, which comprises of magnesium oxide and aluminium oxide, has been observed on the cleaned surface. The composition and thickness of this newly formed oxide layer are varied by several factors including the applied laser fluence, laser absorptivity of alloy surface and alloy compositions. The corrosion evaluation by immersion in 3.5 wt.% NaCl solution has shown that the surface modification by laser cleaning increases the corrosion resistance, suggesting that the formation of MgO and Al2O3 oxide layer is more protective than the porous oxide layer presented on the as-received surface. The study of laser cleaning as a surface preparation prior to laser welding of AA2024-T3 and AA5083-O alloys has achieved a significant reduction of porosity in the weld to an average ratio of porosity area of 1% from an average ratio of 3% in AA2024 alloy and 5% in AA5083 alloy without laser cleaning. Based on the surface characterisation, the decrease of pores is attributed to the removal of hydroxide/oxide layer which is related to the hydrogen source. The mechanical tests have provided further confirmation, showing an increase of weld hardness and the ultimate tensile strength in laser cleaned samples compared with uncleaned alloys.