Synergetic effect of surface-active metallic additions on structure modification in aluminium alloys

Abstract

Control of Fe-containing intermetallic size and morphology is often achieved using external physical fields, but chemical modification via metallic additions offers a faster, more cost-effective alternative without requiring production-line changes. This work developed a screening method for such additions by adapting Maltsev’s (1964) adsorption-based theory, using a generalised moment formula to rank elements and guide targeted phase modification. However, this theory did not explain the observed changes. Instead, other mechanisms such as Ca-Sr-containing intermetallics behaving as nucleation sites, the reduction in the concentration of Si in the liquid during solidification, and the build-up of metallic additions in the liquid, are the mechanisms suggested to achieve modification resulting in an improved homogenous microstructure. Model alloy experiments demonstrated the critical role of the Fe:Si ratio in phase formation and showed that Ca/Sr additions help retain α(AlFeSi) in the microstructure. In AA6050, optimisation of Ca–Sr-containing intermetallic morphology highlighted the importance of addition ratio and concentration in controlling their size and shape. A mechanism for Bi removal via Ca/Sr additions was proposed, involving oxide formation. The morphology of Al/Mg oxides, combined with the added weight of Bi-, Ca-, and Sr-containing phases, determines whether oxides sink or float in the melt. Machinability of AA6050 improved with combined Ca and Sr additions, attributed to the lower hardness of Alx(Ca,Sr)Siy intermetallics compared to Fe-containing phases. Their relative softness, favourable morphology, and increased intermetallic area fraction likely enhance machinability. Finally, a novel method for assessing alloy machinability was introduced, offering a faster and more process-relevant alternative to current industry practices.