Effect of Processing Steps on the Mechanical Properties and Surface Appearance of 6063 Aluminium Extruded Products

Abstract

6063 aluminum anodized extrusions may exhibit a common surface defect known as streaking, characterized by the formation of narrow bands with a surface gloss different from the surrounding material. The origin of this banding lies in the differential surface topography produced after etching during the anodizing stage, shown to be connected to certain microstructural characteristics. The present study has attempted to determine the origin of these defects and measure the mechanical properties in these zones, properties which were either barely acceptable or did not meet the specification’s requirements. Quantitative metallography and mechanical testing, both tensile and microhardness, were used for materials assessment at the different steps of the process of manufacturing 6063 anodized extrusions. The results of this research show that nonequilibrium solidification rates during billet casting could lead to the formation of coarse eutectic Mg2Si particles which have a deleterious effect on both mechanical properties and surface appearance in the anodized condition. However, differences in the size and density of the coarse Mg2Si particles have been found to exist in the streak profile compared to the surrounding zones. The study revealed the importance of these particles in explaining the origin of the marginal or sub-marginal properties and anodizing surface defects found.

Thermodynamic analysis of 6xxx series Al alloys: Phase fraction diagrams

“…The as-cast billet of 6xxx series Al alloys cannot always satisfy the subsequent extrusion process due to the harmful precipitates formed during casting. For instance, the size and distribution of Mg 2 Si critically determine the extrusion speed, and the grain boundary particles determine the surface finish [5]. Fe impurity forms Fe-bearing intermetallic phases, such as Al 13 Fe 4 , α_AlFeSi, β_AlFeSi, Al 8 FeMg 3 Si 6 , etc., since the solubility of Fe in Al solid solution is very low.…”

Evolution of Fe Bearing Intermetallics During DC Casting and Homogenization of an Al-Mg-Si Al Alloy

“…[6] Therefore, a post-cast homogenization heat treatment is used commercially to encourage transformation of b to a to allow: (a) more reliable downstream deformation, typically by extrusion, (b) improved mechanical properties, especially toughness and elongation to failure, and (c) improved surface finish. [5,[10][11][12][13][14][15][16] Despite the apparent maturity of AA6xxx alloys, there is a significant on-going effort to optimize homogenization conditions in terms of properties, while also minimizing the homogenization time. Among the factors that govern the homogenization response, the initial cast microstructure and the alloy chemical composition play key roles.…”

Effect of the shear layer on the etching behavior of 6060 aluminum extrusion alloys

“…Before extrusion, the billets were heat treated to homogenize the microstructure. The heat treatment involves the formation of α‐AlFeSi and the dissolution of Mg 2 Si particles into the matrix. This heat treatment also assures the uniform distribution of alloying elements in the matrix.…”