The Effect of Si and Zr on the Formation of Al3X and V-Phase in a 6005A Alloy with Sc—Part 1: Alloy Design and Heat Treatment Selection

Adding Sc to 6xxx series alloys has led to inconsistent results due to the formation of the high-temperature, thermodynamically stable V-phase (AlSc2Si2). Thermo-Calc single-axis equilibrium and phase diagram calculations were employed to identify V-phase formation with varying Si and Zr concentrations, indicating that increasing Zr and decreasing Si lowered the V-phase equilibrium volume fraction. Increasing Zr also shifted the V-phase equilibrium to higher Si concentrations. To access real-world influences of Zr and Si, four compositions were cast with different Si and Zr concentrations: a high-Si, low-Zr alloy; a medium-Si, medium-Zr alloy; a low-Si, high-Zr alloy; and a baseline alloy without Zr and Sc. The compositions were DC-cast followed by multi-step isochronal and isothermal heat treatments, which revealed that increasing Zr concentration did not influence the formation of V-phase but did result in higher hardness at high temperatures, likely due to Al3Zr precipitation. In contrast, higher Si and lower Zr concentrations produced higher hardness in the peak-aged condition but lower hardness at homogenization temperatures in the 400 °C to 520 °C range. Given these conclusions, a new alloy and a multi-step homogenization process are proposed to further develop Sc- and Zr-containing 6xxx extrusion alloys.