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Powder plasma arc additive manufacturing (PPA-AM) is a promising method for fabricating high-entropy alloys (HEAs). However, the characteristics of thermal cycling during the PPA-AM process and the mechanism of subsequent heat treatment on the microstructure and mechanical properties are unclear. In this study, an Al 1.2 CoCrFeNi 2.1 HEA with dual-phase (FCC + B2) was fabricated via PPA-AM. The microstructure and mechanical properties of the as-built and heat-treated samples were explored. The crystallographic orientations of the FCC and B2 phases were also determined. Because the alloy in the bottom region was affected by thermal cycling, B2 particles precipitated in the FCC phase, and FCC particles precipitated in the B2 phase. During the tensile test, the phase boundary between B2 matrix and FCC particles produced cracks preferentially. These cracks reduced the strength and elongation of the alloy. After heat treatment at 1200 °C, the FCC and B2 precipitates formed globular shapes inside the B2 and FCC grain matrix, respectively. The alloy showed a uniform structure and the ductility of the alloy was improved significantly. This work elucidates the mechanisms of thermal cycling and heat treatment on the microstructure and mechanical properties of the alloy. [ABSTRACT FROM AUTHOR]