Corrosion behaviour study of the forged AA6082 aluminum alloy from different feedstock

Preliminary forging and caustic cleaning tests were carried out on cast-homogenized materials and raised some further questions on the corrosion resistance of this material compared to the conventional extruded feedstock. This corrosion study serves as a milestone key to ensure the quality and the possibility of using this material as feedstock in the forging industry. The corrosion behavior of the forged-T6 AA6082 aluminum alloy was studied by comparing cast-forged and extruded-forged materials in both forged and aged tempers. The main conclusion of this comparative study is that no significant difference that could have a possible negative impact on the final product has been measured in corrosion susceptibilities between these two forging feedstocks.;Corrosion tests were conducted on the AA6082 aluminum alloy to focus on the susceptibility in corrosion of both forging materials. Three tests show slight variations on the corrosion results but not enough to distinguish a better feedstock material resistance. The OCP curves have shown that the times necessary for a passive state equilibrium of cast-forged materials were longer than those extruded-forged after 24 hours immersion in a 3.5 w/v% NaCl solution. Similar anodic polarizations of impressed voltage permitted to identify Epit between -0.63 to -0.54 V/SCE for all conditions. Cast feedstock conditions showed a continuous pitting phenomenon after Epit. The cast feedstock T6 condition seemed to have a slightly better pitting resistance when the voltage was applied with the nobler Epit result at -0.54 V/SCE. Optical microscopy analyses revealed uniform round corroded pit patterns for the metallurgically cast-forged conditions and an asymmetrical aspect with large corrosion pits for the extruded-forged material.;Image analyses by the CLEMEX(c) software on salt sprayed corroded specimens identified a better resistance of the extruded-forged material without subsequent solutionizing and aging heat treatments when compared with the same cast-forged condition (13.46% cast-forged-T6 vs 6.77% extruded-forged-T6). The T6 thermal treatment on both forged materials appears to have obtained the same percentage of area affected by pitting corrosion (3.62% cast-forged-T6 vs 3.68% extruded-forged-T6) in the cabinet exposure test. No significant variation in corrosion-fatigue resistance was identified in the two types of materials. In the air environment, fatigue S-N curves established two cases for both forged materials: the cast-forged material shows a better fatigue resistance at low stresses and the extruded-forged material revealed an improved fatigue resistance during the high stresses applied. In a devastating corrosive environment addition, the corrosion-fatigue test results into a decrease in the number of cycles by a factor of 1x103 on both types of forged-T6 materials.;The macroscopic fatigue fractures analyses revealed flat and grainy fades of cast-forged samples opposed to important cleavage zones for the extruded-forged surfaces. Many fatigue specimens seem to rupture with a striation propagation mechanism with persistent shear bands (PSB). The corrosion factors during fatigue-corrosion tests have an impact on the softening of microscopic fatigue striations for both materials.;A metallographical investigation before and after forging allowed to highlight microstructural patterns and chemical differences between forging feedstocks. The microstructural analysis puts into perspective two materials that could have different affinities to forging and corrosion assessment, especially due to the presence of special layers at the surface. The cast material has an equiaxed grain structure with an enriched surface layer in segregated alloying elements and the extruded material has a fibrous structure having a recrystallized surface layer with a fine grain structure. The elongated texture generated for the cast-forged condition changed to a recrystallized fine microstructure after the T6 heat treatment. This thermal treatment produced an opposite result for the extruded-forged material by revealing a static recrystallization that promotes a coarse grain structure.