Microstructure, mechanical properties and corrosion behavior of AA5083 aluminum alloy modified with rare earth elements

Abstract

The commercial AA5083 alloy is a non-heat-treatable alloy with high strengthto- weight ratio. It has good welding properties, ease of machinability and formability, and satisfactory corrosion property resistance in marine and industrial environments. Despite the advantages it possesses, the AA5083 alloy has some limitations such as its moderate strength, and susceptibility to localised corrosion (LC) in the marine environment that increases under high-speed conditions. It is also susceptible to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) at the temperature range of 50°C to 200°C. Previous studies showed that the addition of an appropriate amount of rare earth elements (REEs) can improve the microstructure, mechanical properties, and corrosion resistance of Al-alloys. This research was aimed at investigating the effects of REEs (Ce, Pr, and Ce + Pr) additions on the microstructure, mechanical properties, and corrosion behavior of the AA5083 alloy. Ce and Pr were added to AA5083 alloy using in-situ casting process in the amount of 0.1 wt% to 1.0 wt% whilst the combination of Ce-Pr was between 0.1wt% and 0.5wt%. Non-destructive testing (NDT) was conducted to assess the quality of the samples produced whilst the homogenisation process was performed at 450°C for 24 hr to achieve homogeneous microstructure. Characterisation of the modified AA5083 alloys was conducted using optical microscope (OM), variable pressure scanning electron microscope (VPSEM) equipped with energy dispersive spectrometer (EDS) and x-ray diffraction (XRD). Corrosion behavior was investigated by visual assessment (ASSET-Test), IGC susceptibility by nitric acid mass loss test (NAMLT), electrochemical behaviour testing in 3.5% NaCl solution by Tafel extrapolation (TE) and potentiodynamic polarization (PDP), and full immersion corrosion tests in 3.5% NaCl solution under static and high-speed conditions at room temperature. Tensile properties and impact tests were also conducted to determine mechanical properties of the alloy. In addition, erosion prediction test was performed by impingement of solid spherical particle at normal incidence. The findings indicated that addition of Ce, Pr, and Ce + Pr to the AA5083 alloy resulted in the reduction of grain size. The average grain size were reduced from 105^m to 72 |j,m and 107 ^m to 74 ^m respectively, as addition of Ce and Pr from 0.1 to 1.0wt%. Addition of Ce-Pr combination in the amount of 0.1% to 0.5% resulted in average grain size reduction from 98 |j,m to 65 |j,m. The presence of stable precipitate (REE-rich) phases played a critical role in the grain refinement, thereby improving the mechanical properties. The stable phases reduced the P-phase (AhMg2) and Fe-rich and Mg-rich phases, which are a major cause of LC, IGC, and IGSCC, thus improving the corrosion resistance. These properties were more prominent in the case of addition of Ce + Pr combined than in individual addition of Ce or Pr. The modified AA5083 alloy with 0.5% Ce + 0.5% Pr addition had the best overall improvement in terms of mechanical properties and corrosion behaviour compared to other modified AA5083 alloys. The average grain size was reduced by 39.8%. Toughness, yield strength, and ultimate strength improved by 21%, 27.5% and, 53.3 % respectively whilst corrosion rate decreased by 31.2%. Thus, it is suggested that, REEs can be promising candidates for the alloying elements of AA5083.