Homogenization of EN AW 6005A Alloy for Improved Extrudability

The homogenization practice for EN AW 6005A was investigated. It requires an 8-hour soaking at 853 K (580 °C) to even out the Mg and Si distribution in the aluminum matrix and to fully transform the monoclinic β-Al₅FeSi plates into relatively small round α_c-Al₁₂(Fe,Mn)₃Si particles with a “necklace” type configuration. It is predominantly the equilibrium β-Mg₂Si phase that precipitates during subsequent cooling at 1000 K h⁻¹ while orthogonal β′-Mg₂Si platelets are also noted starting at 500 K h⁻¹. The hardness decreases with decreasing cooling rate from 41.1 ± 1 HV to 35.8 ± 0.8 HV, by 15 pct, over the range investigated. The precipitation capacity retained in homogenized samples cooled to room temperature at less than 250 K h⁻¹ is reduced substantially. The solvus temperatures of these samples are below 773 K (500 °C) ensuring full solutionizing of the Mg₂Si precipitates below the press exit temperatures typically employed by extruders. An 8-hour soaking at 853 K (580 °C) followed by cooling at 250 K h⁻¹ is identified to be the optimum homogenization treatment as it gives a homogeneous structure with predominantly spherical-shaped α_c-Al₁₂(Fe,Mn)₃Si particles and sufficiently low hardness for improved extrudability and a solvus temperature below 773 K (500 °C) to ensure adequate age-hardening capacity.