Atomic clusters induced rapid hardening behavior in an early stage of isothermal aging for a high-strength Al alloy produced by laser powder bed fusion additive manufacturing

Abstract

Due to the transient interaction between laser and powder and layer-by-layer rapid melting and solidification, laser additive manufacturing-fabricated metal components can exhibit unique microstructure evolution behaviors and strengthening mechanisms that are normally not available in traditional processes. In this work, a previously unreported rapid hardening behavior at the very early stage of isothermal aging for laser powder bed fusion-processed high-strength Al-5024 alloy was revealed. The microstructures and mechanical properties of specimens aged from 10 min to 120h were systematically analyzed. It showed that the specimens underwent two peak hardening processes during an isothermal aging at 325 °C. The mechanical properties of the specimens including microhardness, yield strength, and elastic modulus were significantly enhanced after an extremely short aging time of 10 min and then reached a secondary peak hardening at an aging time of 4h, where the yield strength of 450 ± 10.3 and 463.2 ± 13.2 MPa were obtained, respectively. The unusual aging responses were attributed to the formation and decomposition of Sc-rich clusters with a high number density of 2.7 × 1023 m−3 and nano-size of 2.71 nm. These clusters were characterized by transmission electron microscopy analyses and further supported by differential scanning calorimetry measurements, where a significantly higher activation energy of 147.6 ± 21.1 kJ/mol corresponding to the precipitation/coarsening process of Al3(Sc,Zr) was measured for rapid hardening specimens. In addition, the relationship between the aging process, the evolution of nano-precipitates, and the mechanical properties was systematically demonstrated.