Multivariate relationships between microstructure evolution and strengthening mechanisms in laser powder bed fusion of Al-Mn-Sc alloy: towards improved fatigue performance

Abstract

The effects of direct aging treatment (at 300 °C for 5 hours) on selective laser melted (SLMed) Al-4.5Mn-1.5Mg-0.9Sc-0.2Zr alloy were investigated in this work, with the microstructure, fatigue behaviors, and fracture characteristics examined to determine the primary cause of fatigue crack source. The results revealed that the microstructure of the investigated alloy comprised fine equiaxed and columnar grains. Upon aging treatment, a significant number of nano-scaled Al3(Sc, Zr) precipitates were dispersed within the grains, leading to a substantial increase in strengths. The yield strength improved from 431 MPa to 568 MPa, representing an increase of more than 32%, while the fatigue strength improved from 180 MPa to 220 MPa after aging treatment. Nevertheless, the fracture toughness decreased significantly from \begin{document}$ 25.1\; {\rm{MPa}}\cdot {\sqrt{\rm m}} $\end{document} to \begin{document}$ 12.3 \;{\rm{MPa}}\cdot \sqrt{\rm m} $\end{document}. The results of the fatigue fracture characteristics indicate that the Mn-rich phase and the formation of defects such as pores and poor powder fusion are the sources of fatigue cracking. Although direct aging treatment can significantly increase the yield strength, decrease the rate of fatigue crack propagation, and thus improve the fatigue performance, it deteriorates the fracture toughness, and thus shortens the fatigue life of the alloy as well.

Keywords

• selective laser melting
• al-mn-sc alloys
• high-cycle fatigue