The effect of multi-scale second-phases on the microstructure evolution of a Mg–Al–Sn–Ca alloy during friction-stir processing

In the present work, the type, morphology and size of secondary phase particles in a Mg–4Al–2Sn-0.5Ca (ATX420) alloy are tailored through sub-rapid solidification (SRS) and aging treatment before friction stir processing (FSP) to systematically investigate their impact on microstructures and mechanical properties of the FSP ATX420 Mg alloy. The smaller eutectic CaMgSn phase with micro-defects in the as-cast SRS alloy is more likely to be broken during FSP, compared to the as-cast conventional solidification (CS) alloy. Additionally, nano-sized CaMgSn and Mg17Al12 particles precipitate in as-aged SRS alloy due to a high supersaturation solubility of solutes in the as-cast SRS alloy. These densely-distributed nano-sized precipitates in the as-aged SRS alloy increase the processing temperature by 60 °C during FSP, which results in less nano-sized precipitates in the stirring zone and grain growth of the α-Mg matrix. This leads to a decrease in hardness in the FSP-aged-SRS alloy (∼10 HV). Besides, the grain structure evolutions in the stirring zone and transition zone are systematically investigated for understanding the microstructure evolution during FSP in Mg alloys with eutectic phase.