Improvement of Surface Mechanical and Tribological Characteristics of L-PBF Processed Commercially Pure Titanium through Ultrasonic Impact Treatment

Multi-pass ultrasonic impact treatment (UIT) was applied to modify the microstructure and improve the mechanical and tribological characteristics at the near-surface region of commercially pure Ti (CP-Ti) specimens produced by the laser powder bed fusion (L-PBF) method. UIT considerably refined the L-PBF process-related acicular martensites (α′-M) and produced a well-homogenized and dense surface microstructure, where the porosity content of 1-, 3-, and 5-pass UITed samples was reduced by 43, 60, and 67%, respectively. The UITed samples showed an enhancement in their near-surface mechanical properties up to a depth of about 300 μm. The nanoindentation results for the 3-pass UITed sample revealed an increase of about 53, 45, and 220% in its nanohardness, H/E_r, and H³/E_r² indices, respectively. The stylus profilometry results showed that performing the UIT removed the L-PBF-related features/defects and offered a smooth surface. The roughness average (R_a) and the skewness (R_sk) of the 3-pass UITed sample were found to be lower than those of the L-PBFed sample by 95 and 223%, respectively. Applying the UIT also enhanced the material ratio, where the maximum load-bearing capacity (~ 100%) in as-L-PBFed (as-built) and 3-pass UITed samples was obtained at 60- and 10-µm depths, respectively. The tribological investigations showed that applying the UIT resulted in a significant reduction of wear rate and average coefficient of friction (COF) of CP-Ti. For instance, under the normal pressures of 0.05 and 0.2 MPa, the wear rate and COF of the 3-pass UITed sample were lower than those of the L-PBFed sample by 65 and 58%, and 20 and 17%, respectively.