The dynamic recrystallization mechanism of ultrasonic power on non-contact ultrasonic-assisted direct laser deposited alloy steel

Abstract

Ultrasonic assistance has been used as one of the more convenient and effective means of grain refinement during alloys direct laser deposition, and the formation of fine grains is closely related to the dynamic recrystallization behavior during laser non-equilibrium metallurgy. To investigate the dynamic recrystallization behavior of the non-contact ultrasonic-assisted process in direct laser deposited alloy steel, this paper used 24CrNiMoY alloy powder and the non-contact ultrasonic-assisted direct laser deposition method to study the mechanism of ultrasonic power on the dynamic recrystallization behavior of alloy steel. The results show that the ultrasonic power had an obvious regulation on the prepared alloy steel samples’ grain size and tensile properties. The microstructure of the prepared samples was mainly composed of upper bainite, lower bainite, and granular bainite. After the non-contact ultrasonic-assisted process, the grain was obviously refined, and the smallest grain was 0.4 μm. The dynamic recrystallization behavior in direct laser deposited alloy steel at different non-contact ultrasonic powers was mainly determined by dislocation density and solidification rate. The introduction of non-contact ultrasonic caused dislocation movement in the plastic deformation zone of direct laser deposited alloy steel samples, and the low angle grain boundaries migration—subgrain growth—the transformation of low angle grain boundaries to high angle grain boundaries initiated by dislocation movement was the key process to trigger dynamic recrystallization. The optimized ultrasonic power of 360 W had the best strength-plasticity matching relationship for the prepared samples, with a tensile strength of 1108 MPa and elongation of 12%. And the strong-plastic product of sample (360 W) was increased from 6 GPa% to 13 GPa% due to the increase of recrystallization fraction. This study provides a valuable theoretical and technical reference for preparing high-performance alloy steel by ultrasonic-assisted direct laser deposition.