Effects of preheating-ultrasonic synergistic on the microstructure and strength-ductility of 24CrNiMoY alloy steel by laser directed energy deposition

Laser additive manufacturing alloy steel with good strength-ductility has important application prospects in the manufacturing of critical equipment parts. However, the poor matching of strength-ductility caused by high tensile strength and low elongation is a bottleneck problem that restricts the laser directed energy deposition (LDED) technology application. In this paper, based on the previous research on single physical field treatment, a new method to prepare alloy steel samples with better strength-ductility match by using compound physical field treatment (preheating/ultrasonic dual-field) to assist LDED alloy steel is proposed, thus solving the technical bottleneck problem of poor strength-ductility. The results show that the dual-field synergy has a more significant modulating effect on the densities, phase composition ratio, grain size, and strength-ductility matching properties of 24CrNiMoY alloy steel than the single field assistance by LDED. The density of the sample prepared with dual-field assistance was as high as 99.9%, and the phase compositions were ultra-fine crystalline lower bainite, granular bainite and residual austenite, with the average grain size refined from 0.46 mu m to 0.26 mu m. The ultimate tensile strength, elongation and strength-plastic product at break of the sample prepared by 360 W and 100 degrees C were (1058 +/- 4.2) MPa, (13.8 +/- 0.9)% and (14.6 +/- 3.7) GPa%, respectively. A remarkable improvement of 12.3% in the strength-plastic product at break was achieved compared with the single ultrasonicated sample. The dual-field synergistic mechanism is the sufficient heterogeneous nucleation and cooling rate, which promoted the transformation of bainite morphology from lath to granular. The combined effect of the two aspects resulted in a satisfactory strength-ductility matching of the prepared alloy steel. This study will provide a promising theoretical and practical reference for the preparation of better strength-ductility matching alloy steel by dual-field-assisted LDED.