Abstract
Objective
To investigate the effect of laser surface treatment of cast titanium alloy on microstructure and mechanical properties.
Methods
Dumbbell- and plate-shaped cast titanium specimens were prepared for mechanical testing and microstructure analysis. After the cast surfaces of each specimen were laser-treated using a dental Nd:YAG laser machine at 240 V and 300 V with and without argon gas shielding, tensile testing and microstructure analysis were conducted. Hardness depth profiles were also made from the cross-section of laser-treated cast specimens. Microstructural and chemical analysis were performed by means of the SEM, XRD, AES and WDS.
Results
The results of tensile testing and Vickers hardness depth profiling showed that laser treatment improved the mechanical properties. Bulk microstructure of as-cast titanium was mainly composed of α-grains with acicular and widmanstatten patterns. The laser melted zone was characterized by columnar beta grains. When the emission voltage of laser increased to 300 V, a larger grain size was promoted. The XRD analysis indicated that the beta phase formation was clearly noticeable after laser surface treatment. Supplementary marked peaks of the TiO, TiO 2 and Ti 2 N were detected without argon gas shielding. When argon shielding gas was used, the presence of titanium oxide was significantly reduced and the peaks of titanium nitride disappeared.
Significance
Laser treatment on cast titanium surfaces showed significant enhancement of mechanical properties and modification of microstructures, and therefore could produce reliable titanium metal frameworks for dental prostheses.
1
Introduction
Titanium and titanium alloys have been widely used for aerospace technology and surgical implants because of their high corrosion resistance, excellent biocompatibility, good oxidation resistance, strength–weight ratio and low mass-to-volume ratio. However, the commercially pure titanium (CP-Ti) possesses low mechanical strength, low surface hardness and poor wear resistance . Consequently, an increased interest has been shown to modify the surface microstructure and composition with the addition of alloying elements to improve these properties.
Laser beams are widely used for surface modification because specific thermal characteristics induced by laser irradiation can generate specific microstructures including metastable phases and nano-crystalline grains. The convective fluxes and hydrodynamic instabilities inside the irradiated surface layers contribute to the heat transport to mix the ambient gas and molten metal. Melting of surface layers for direct laser surface treatment of metallic targets should be conducted in controlled reactive atmosphere since it enhances the chemical reactions and avoids significant vaporization and particulates removal. The inclusion of light elements in titanium target occurs depending on the interactions among laser beam, substrate composition, and gaseous environment. Microstructural feature of titanium alloys, which is one of the important factors to control mechanical properties, is sensitively affected by heating temperature, holding time and cooling rate of the substrates. Laser parameters have to be optimized in order to reduce defects and to optimize microstructure . Morphology of the layers created and nature of the phases formed mostly depend on the laser beam overlapping , the nature of surrounding gas , and the intensity of laser beam .
This study focused on a cast commercially pure titanium (CP-Ti) used for dental restorations. Laser treatment on cast titanium surfaces could produce reliable titanium metal frameworks for dental prostheses since a significant enhancement of their mechanical properties can be obtained by laser treatment . The aim of this study was to investigate the effect of the Nd:YAG laser modification of the cast titanium surface on the changes of microstructure and mechanical properties.
2
Materials and methods
2.1
Specimen preparation
Plates (10 mm × 10 mm × 3 mm) and dumbbell-shaped patterns (ISO6871, 18 mm for gauge length, 3 mm in diameter) were prepared for casting of a commercially pure titanium (CP-Ti) (grade 2, T-Alloy M. GC Corp., Tokyo, Japan). The CP-Ti was cast with a magnesia-based investment material (Selevest CB, Selec Co., Osaka, Japan) using an argon-arc melting/centrifugal titanium casting machine (Ticast Super R, Selec Co.). After casting, the molds were bench-cooled to room temperature and the cast specimens were retrieved. The surfaces of the cast specimens were air-abraded with 50 μm Al 2 O 3 particles to uniform surface conditions. The specimens were then ultrasonically cleaned with acetone for 10 min.
2.2
Laser surface treatment
Laser surface treatment was performed using a dental Nd:YAG laser welding machine (Neolaser L, Girrbach Dental Systems, Pforzheim, Germany) using the following parameters: spot diameter of 1.4 mm, pulse duration of 10 ms and emission voltages of 240 V or 300 V . During the laser treatment, argon gas shielding was applied from two nozzles set at a 45° angle on both sides above the treatment area of the specimen. Laser single pulses were applied perpendicular to the long axis of each specimen in order to avoid misshaping of the straight gauge due to the induction of compressive stresses by the laser . The laser-treated specimens without argon shielding were also prepared for both plate-shaped and dumbbell-shaped tensile (240 V treatment only) specimens. A schematic surface aspect of the overlapped laser treatment is presented in Fig. 1 .
2.3
Mechanical tests
Tensile testing was conducted using a universal testing machine (Model 5567, Instron Corp., Canton, MA) at a crosshead speed of 1.0 mm/min. Tensile strength, modulus of elasticity and elongation were recorded. Tensile test specimens prepared were non-treated control specimens, specimens laser-treated at 240 V with and without argon gas shielding and specimens treated at 300 V with argon gas shielding. The data were statistically analyzed by ANOVA and Tukey’s post hoc test at a significant level set at α = 0.05.
2.4
Hardness measurements
Hardness depth-profiles were made on the cross sections of plate specimens. Vickers microhardness measurements (100 g load) were performed using a microhardness tester (VMHTAUTO, Leica, Wetzlar, Germany). Measurements started at 25 μm from the cast surface to 750 μm in depth with 50 μm increments. Five measurements were averaged for each depth.
2.5
Material characterization
To determine the compounds formed during surface treatments, X-ray diffraction (XRD) was carried out on a /2 reflection geometry using CuK α radiation (PW1820, Philips, Eindhoven, Netherlands). The Cu X-ray tube voltage was set to 40 kV, tube current to 40 mA, a two-theta scanning rate of 0.02°/min and an analysis region size of 0.5 cm 2 . SEM morphology observations (JMS 6360A, JEOL, Tokyo, Japan) were performed on the cross-sections of treated plate specimens. Wavelength-energy dispersive X-ray spectrometric (WDS) analysis was used for quantitative element analysis (EPMA SX100, Cameca, Mahwah, NJ). Auger Electron Spectroscopy (AES) profiling was carried out under the argon ion beam etching (4 kV and 0.5 mA) using a scanning auger system (MICROLAB 310F, VG Scientific, East Grinstead, UK). The spot surface area was 3 μm 2 and the theoretical etching rate was 0.2 nm s −1 .
2
Materials and methods
2.1
Specimen preparation
Plates (10 mm × 10 mm × 3 mm) and dumbbell-shaped patterns (ISO6871, 18 mm for gauge length, 3 mm in diameter) were prepared for casting of a commercially pure titanium (CP-Ti) (grade 2, T-Alloy M. GC Corp., Tokyo, Japan). The CP-Ti was cast with a magnesia-based investment material (Selevest CB, Selec Co., Osaka, Japan) using an argon-arc melting/centrifugal titanium casting machine (Ticast Super R, Selec Co.). After casting, the molds were bench-cooled to room temperature and the cast specimens were retrieved. The surfaces of the cast specimens were air-abraded with 50 μm Al 2 O 3 particles to uniform surface conditions. The specimens were then ultrasonically cleaned with acetone for 10 min.
2.2
Laser surface treatment
Laser surface treatment was performed using a dental Nd:YAG laser welding machine (Neolaser L, Girrbach Dental Systems, Pforzheim, Germany) using the following parameters: spot diameter of 1.4 mm, pulse duration of 10 ms and emission voltages of 240 V or 300 V . During the laser treatment, argon gas shielding was applied from two nozzles set at a 45° angle on both sides above the treatment area of the specimen. Laser single pulses were applied perpendicular to the long axis of each specimen in order to avoid misshaping of the straight gauge due to the induction of compressive stresses by the laser . The laser-treated specimens without argon shielding were also prepared for both plate-shaped and dumbbell-shaped tensile (240 V treatment only) specimens. A schematic surface aspect of the overlapped laser treatment is presented in Fig. 1 .
2.3
Mechanical tests
Tensile testing was conducted using a universal testing machine (Model 5567, Instron Corp., Canton, MA) at a crosshead speed of 1.0 mm/min. Tensile strength, modulus of elasticity and elongation were recorded. Tensile test specimens prepared were non-treated control specimens, specimens laser-treated at 240 V with and without argon gas shielding and specimens treated at 300 V with argon gas shielding. The data were statistically analyzed by ANOVA and Tukey’s post hoc test at a significant level set at α = 0.05.
2.4
Hardness measurements
Hardness depth-profiles were made on the cross sections of plate specimens. Vickers microhardness measurements (100 g load) were performed using a microhardness tester (VMHTAUTO, Leica, Wetzlar, Germany). Measurements started at 25 μm from the cast surface to 750 μm in depth with 50 μm increments. Five measurements were averaged for each depth.
2.5
Material characterization
To determine the compounds formed during surface treatments, X-ray diffraction (XRD) was carried out on a /2 reflection geometry using CuK α radiation (PW1820, Philips, Eindhoven, Netherlands). The Cu X-ray tube voltage was set to 40 kV, tube current to 40 mA, a two-theta scanning rate of 0.02°/min and an analysis region size of 0.5 cm 2 . SEM morphology observations (JMS 6360A, JEOL, Tokyo, Japan) were performed on the cross-sections of treated plate specimens. Wavelength-energy dispersive X-ray spectrometric (WDS) analysis was used for quantitative element analysis (EPMA SX100, Cameca, Mahwah, NJ). Auger Electron Spectroscopy (AES) profiling was carried out under the argon ion beam etching (4 kV and 0.5 mA) using a scanning auger system (MICROLAB 310F, VG Scientific, East Grinstead, UK). The spot surface area was 3 μm 2 and the theoretical etching rate was 0.2 nm s −1 .
3
Results
3.1
Tensile properties
The results of tensile testing are summarized in Table 1 . The highest tensile strength was obtained for the specimens laser-treated at 300 V with argon shielding (significant, p < 0.05), followed by 240 V with argon shielding, 240 V without argon shielding, and as-cast CP-Ti. The specimens laser-treated at 300 V with argon shielding also showed the significantly highest elongation value among the groups. There was no statistical difference in tensile strength, elongation and elastic modulus between as-cast CP-Ti specimens and specimens laser-treated at 240 V without argon. The laser treated specimens with argon shielding showed the higher values of elastic modulus than the as-cast specimen.
| Tensile strength (MPa) | Elongation (%) | Elastic modulus (GPa) | |
|---|---|---|---|
| CP-Ti | 437 (94) | 9 (5) | 109 (19) |
| 240 V with Ar | 561 (51) | 8 (3) | 160 (35) |
| 300 V with Ar | 789 (17) | 19 (5) | 139 (38) |
| 240 V without Ar | 457 (43) | 5 (3) | 114 (6) |
3.2
Hardness depth profiles
Microhardness depth profiles obtained from the plate specimens are shown in Fig. 1 . The hardness of the laser treated groups gradually decreased from the top surface down to 200 μm and reached to bulk hardness. Regardless of voltage, the hardness values of specimens treated with argon shielding were lower than those without shielding. Bulk hardness values of all treated groups were similar to that of the as-cast control specimens (deeper than 250 μm).
3.3
Microstructure analysis of the surfaces
3.3.1
XRD analysis
Fig. 2 shows XRD patterns from as-cast CP-Ti surface, laser-treated surfaces at 300 V with and without argon shielding. Regardless of the voltage, spectra were similar in the fixed gas condition. The as-cast CP-Ti was mainly composed of alpha-phase even though the peak (38.3°) widening revealed the presence of beta phase. After laser surface treatment, beta phase formation was clearly noticeable. The supplementary marked peaks of titanium oxide (TiO JCPDS-12-0754 and TiO 2 JCPDS-21-1272) and titanium nitride (Ti 2 N JCPDS-23-1455) were noticed when argon shielding gas was not used. Under the argon gas shielding, the presence of titanium oxide was significantly reduced, and the peaks of titanium nitride disappeared.
