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The mechanical characteristics, phase composition, residual macroscopic stresses, and surface topography of Co-Cr-Mo-W alloy fabricated using the additive (selective laser melting of powder--SLM) and casting (CT) technologies and modified by ultrasonic impact treatment (UIT) are investigated. A single-contact normal impact-loading mode of UIT is employed. As demonstrated, the macrodefects, including defects of incomplete melting, pores, significant surface roughness, and high levels of residual tensile stresses, which are inherent consequences of 3D printing, are effectively eliminated by short-term UIT in an inert environment. A correlation between the mechanical properties of modified surface layers of the SLM and CT samples and their structural--phase state after UIT exposure for 50 s is established. The strengthening effect of the surface layer of the additively manufactured Co-Cr-Mo-W alloy (by 2 times) is due to the formation of compressive stresses of the first kind (-600 MPa) and martensitic transformation. The Co-Cr-Mo-W alloy obtained by casting technology has lower hardness both in the initial state and after UIT. The strengthening effect does not exceed 1.7 times and is achieved due to a higher level of compressive stresses (-900 MPa) and carbide component refinement. [ABSTRACT FROM AUTHOR]

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The mechanical characteristics, phase composition, residual macroscopic stresses, and surface topography of the Co--Cr--Mo--W alloy manufactured by 3D printing using laser bed-powder fusion and modified by ultrasonic impact treatment (UIT) in an inert environment are studied. The interrelation between the mechanical properties of the modified surface layers and their structural and phase state after UIT of different durations (20-60 s) is established. The maximum hardening of the surface layer of the Co--Cr--Mo--W alloy (by 1.5 times) is achieved after UIT for 30 s and is caused by the formation of compressive stresses of the first kind (-510 MPa, which is ≅ 5.5 times higher as compared to the initial state), a decrease in the crystallite sizes of the ε- (80 nm) and γ- (≅ 140 nm) phases, as well as an increase in the content of the ε-phase from 5% to 95% due to the martensitic transformation. The roughness of the modified surface after the optimal UIT regime (30 s) has parameters that practically do not differ from the initial polished state (Ra = 2.66 µm and Rz = 0.52 µm). The increase in microhardness and wear resistance of the surface layers proves the possibility of using UIT as an effective method for local (surface) post-treatment of medical products made of 3D-printed Co--Cr--Mo--W alloy. [ABSTRACT FROM AUTHOR]

The influence of ultrasonic impact treatment (UIT) on the microhardness, roughness and structural-phase state of the stainless steel AISI 420 surface is studied. Ultrasonic impact treatment is performed in an inert environment under the same conditions—the amplitude of the hub end was 25 µm, the duration of treatment—50 s. Samples of AISI 420 steel are used after softening heat treatment at annealing temperature of 800°C (initial state), after additional hardening from 980°C with oil cooling and after additional hardening from 980°C with subsequent annealing at 250°C for one hour. As established, that after UIT there is a significant decrease in the roughness of the modified surface and increase in microhardness for all cases compared to the initial state. Taking into account the data of x-ray diffraction analysis the main factors of stainless steel AISI 420 surface layers deformation hardening under different combinations of thermal and ultrasonic impact are discussed. [ABSTRACT FROM AUTHOR]