Hardness depth-profiles of laser-treated cast titanium

Objectives: To investigate the effect of laser treatment on the hardness depth-profiles of cast titanium surfaces. Methods: Cast titanium rods (3mm-diam, 15mm-length) were prepared using Ticast casting system (Selec Co.). After the cast surfaces were air-abraded with 50µm alumina, entire surfaces were laser-treated with a dental Nd:YAG laser (Neolaser L, Girrbach) under argon-gas shielding. The laser conditions used were: 1.4mm (spot-diameter), 10ms (pulse duration), and 240V or 300V (voltage). Control specimens were also reserved without laser treatment. Cross sections of titanium rods were subjected to hardness measurement. Hardness depth profiles were made by measuring the Vickers' hardness (VHN) at 25µm from the cast surface to 1,500µm in depth (50µm increment for 50-500µm; 250µm increment for 500-1,500µm). For each depth, four measurements were averaged for statistical analysis (ANOVA/Duncun, p=0.05). Results: Mean hardness values ± SD for the control group were 485±128 (25µm), 199±36 (averaged for 150-300µm) and 199±22 (averaged for 1,000-1,500µm). Laser-treatment with 240V was 352±23 (25µm), 358±27 (150-300µm) and 216±10 (1,000-1,500µm). Laser-treatment with 300V was 338±27 (25µm), 325±30 (150-300µm) and 220±21 (1,000-1,500µm). At 25µm, both laser treatment groups showed significantly lower hardness than the control group. On the contrary, the laser treatment groups exhibited greater VHN values than the control group for the 150-300µm in depth. The 240V group exhibited significantly higher average hardness for 25-300µm than the 300Vgroup .There were no statistical differences in average hardness among the groups for 1,000-1,500µm in depth. Conclusion: The results of this hardness depth-profiling indicated that laser treatment significantly improved surface integrity by modifying the cast surface contamination.