Microstructure evolution and properties of direct laser deposited 24CrNiMoY alloy steel assisted by non-contact ultrasonic treatment

Abstract

To solve the problems of high porosity and uneven microstructure of high-performance alloy steel parts, 24CrNiMoY samples were prepared by non-contact ultrasonic cavitation-assisted (UC-A) direct laser deposited (DLD) process. The samples in the UC-A DLD process were treated with ultrasonic application at three angles of 30°, 45°, and 60° to the horizontal plane and without ultrasonic treatment. The ultrasonic frequency and amplitude were 40 kHz and 30 μm, respectively. The laser energy density of the deposited samples was 83.3 J/mm2. Characterization tests such as OM, XRD, SEM, EBSD, TEM, hardness, wear and tensile tester were performed to study the structural evolution and mechanical properties of the samples under different ultrasonic treatments. The results showed that the microstructure of the prepared samples were a large amount of lath bainite (LB) and a small amount of granular bainite (GB), and the introduction of ultrasound could significantly reduce the porosity in the sample. When the angle of ultrasonic application was 60°, the dense sample with fine-grained and better mechanical properties was obtained. With the ultrasonic application angle increased to 60°, the average strip width of LB was reduced from 34.9 μm to 14 μm, and the grain size of alloy sample was decreased from 0.87 μm to 0.47 μm. The average microhardness, wear loss, tensile strength, and elongation of the samples treated with ultrasonic application angle of 60° were 381.97 HV0.2, 1.0×10-3 g, 1125.1 MPa, and 11.5%, respectively, which increased by 10.6%, 33.3%, 31.7%, 88.5% compared with the sample untreated. Finally, the evolution mechanism between the angle of ultrasonic application and the pores in 24CrNiMoY alloy steel sample was established, and the effect of ultrasonic cavitation on the microstructure and performance of the samples were analyzed through the experimental procedure. Therefore, it is an effective method to reduce porosity, refine microstructure, and improve the mechanical properties of DLD 24CrNiMoY alloy steel by applying non-contact UC-A with a proper application angle (60°).