Ultrasonic Surface Finishing of AISI 1045 Steel Hardened by Laser Heat Treatment with Fibre Laser and Scanning Optics: Layered-Structure-Induced Hardening and Enhanced Surface Morphology

Nowadays, emerging of new technologies causes implicitly the increased requirements for conventional methods and materials. Literature survey shows that combined thermomechanical processes of surface hardening and finishing using highly-concentrated energy sources are promising to enhance the surface integrity and operational properties of structural steels. Some surface-related and microstructural factors can be considered crucial for properties’ enhancement, viz., surface roughness and waviness, grain size and phase composition, residual macrostresses, and microhardness. In this work, a laser heat treatment (LHT) followed by an ultrasonic impact treatment (UIT) also known as high-frequency mechanical impact (HFMI) treatment is sequentially applied to the AISI 1045 steel to demonstrate the efficiency of their combined influence. The near-surface microstructure formed after the combined LHT+UIT process is observed by optical microscopy, transmission electron microscopy, and x-ray diffraction analysis. The surface residual macrostresses, microhardness, roughness, and waviness are also evaluated. The results show that the combined LHT+UIT-treatment induces phase transformation and severe plastic deformation, forming layered hardening and grain structure refinement in the near-surface layers of medium-carbon steel. The subsurface microhardness at a depth up to ≈50 µm after the combined treatment is significantly increased (>10 GPa) due to the severe plastic deformation of the LHT-formed martensitic lamellas, providing the nanoequiaxed grain microstructure in the subsurface layer. The hardening depth (of 140–440 µm) in the LHT+UIT-treated samples depends on the LHT speed (of 40–140 mm/min) and the heating temperature (1200–1300 °C) by scanning laser beam. Additionally, taking into account the surface residual compressive macrostresses (>400 MPa), smoother microrelief on the surface, and reduced roughness parameters (Ra < 0.5 µm) formed by UIT, the studied steel functionality is expected to be improved. The combined laser–ultrasonic surface hardening and finishing process can be used for large-size steel product treatment.