Preheat effect on titanium plate fabricated by sputter-free selective laser melting in vacuum
The dynamics of titanium (Ti) melted by laser irradiation was investigated in a synchrotron radiation experiment. As an indicator of wettability, the contact angle between a selective laser melting (SLM) baseplate and the molten Ti was measured by synchrotron X-rays at 30 keV during laser irradiation. As the baseplate temperature increased, the contact angle decreased, down to 28° at a baseplate temperature of 500 °C. Based on this result, the influence of wettability of a Ti plate fabricated by SLM in a vacuum was investigated. It was revealed that the improvement of wettability by preheating suppressed sputtering generation, and a surface having a small surface roughness was fabricated by SLM in a vacuum.
Selective laser melting (SLM), an additive manufacturing technology, is becoming increasingly popular due to its ability to form three-dimensional (3D) shapes without a mold or cutting. SLM can fabricate complicated shapes, such as a lattice or porous structure, by building an object layer by layer from a powder. Thus, SLM offers cost savings and weight reduction with respect to the conventional forming methods [1, 2, 3, 4, 5, 6]. Owing to its potential, many previous studies have examined the mechanical properties of SLM.
Titanium (Ti) and titanium alloys are attractive materials for medical, aerospace, and automotive applications, because they have properties of biocompatibility, high corrosion and erosion resistance, and mechanical resistance. However, they are difficult to form into complicated structures, such as bionic or lattice structures, because they are difficult to machine. Therefore, SLM can be a suitable method for shaping Ti and Ti alloys. Merkt et al. reported a Ti alloy lattice structure manufactured by SLM , and Sun et al. developed a porous structure from Ti-6Al-4V (Ti64) alloy using SLM . Zhang et al. formed pure Ti under vacuum conditions to prevent oxidization during SLM and clarified that product density depends on laser scanning speed .
However, some studies reported that pores were generated in a sample fabricated by SLM , likely because of sputtering generated by laser irradiation, with a limited correlation between the sputtering and pores. In our previous study, a sputter-free SLM was developed in a vacuum to control laser scanning speed and scanning strategy. In particular, we found that the surface roughness depended on the amount of sputtering generated during laser irradiation [11, 12, 13]. A sample fabricated by the sputter-free SLM showed that surface roughness improved to 1.0 from 30 µm. Matthews et al. numerically modeled the mechanism of sputtering generation by simulating melt pool dynamics and vapor flow patterns . As a result, it was argued that the sputtering was caused by partial pressure expansion, since the metal was vaporized by laser irradiation under inert gas flow. This is, however, not consistent with our experiments under vacuum.
Our previous study suggested that sputtering generation depends on the wettability of molten metal on the SLM baseplate. Thus, this study investigated the dynamics of a metal particle in a vacuum during laser irradiation on a baseplate. The purpose was to determine the relationship between molten metal wettability and baseplate temperature, and then study the influence of wettability on the surface quality of a Ti64 plate fabricated by SLM.
2 Experimental procedure
2.1 Experimental setup for observation of molten Ti particle dynamics
Figure 1 shows the experimental setup for capturing the dynamics of a molten Ti particle, and experimental conditions are listed in Table 1. The experiment employed a direct diode laser with a wavelength of 915 nm, since the light absorption of Ti is 69%. The grain size of the Ti particles was 200 µm made by Ohashi Steel Ball.Corp, as shown in Fig. 2. A single Ti particle was set on an SUS304 baseplate in a vacuum chamber. Then the chamber was evacuated to a pressure of 1 Pa. The Ti particle was irradiated by the laser for 3 s at a power density of 4 × 104 W/cm2. The low power setting facilitated the observation of the Ti melting and spreading process. The laser beam spot diameter was focused to 200 µm to match the Ti particle diameter. The laser beam spot was precisely aligned on the Ti particle by two CCD video cameras, and a CCD video camera was placed coaxially and another one was placed perpendicular to the laser irradiation direction and X-ray irradiation direction.
Laser power density
4 × 104 W/cm2
Beam spot diameter
Ti particle size
2 × 4 mm2
The Ti melting and solidification process was observed in real time using a synchrotron radiation imaging technique at beamline BL22XU of SPring-8, Japan. The resolution of the X-ray transmission image was about 30 µm, so the 200 µm diameter of the Ti particle allowed us to capture the melting dynamics with precision. An X-ray beam with a cross section in the range of 2 mm × 4 mm passed through the Kapton window of the chamber to irradiate the Ti particle during laser melting. The transmitted X-rays passed through another Kapton window and were observed with the high-speed video camera via a scintillator. By two CCD cameras and high-speed video camera, the laser was confirmed to irradiate to the Ti particle. The baseplate temperature was varied from 25 to 500 °C by an electrical heater. The experiment was performed twice for each temperature and the average value was used as the result.
In this way, the wettability of the molten particle could be determined by measuring the contact angle θE between the baseplate and the molten Ti particle using synchrotron X-ray images. The contact angle θE (shown in Fig. 3) is defined by the classical Young equation.
2.2 Experimental procedure for fabrication of Ti64 plate by SLM in vacuum
A Ti64 powder made by a premixed gas atomization process (TILOP64-45, Osaka Titanium Technologies) was used for Ti plate fabrication. We measured its particle size distribution as 4–50 µm with a particle size distribution analyzer (LA-920, HORIBA Co.) that monitored sample scattering in real time (Fig. 4). The most abundant size and the standard deviation were about 25 and 8.9 µm, respectively.
Figure 5 depicts the experimental procedure for fabricating a Ti64 plate by SLM in a vacuum. The chamber was evacuated to a pressure of 5.0 × 10−3 Pa to prevent Ti64 from oxidizing [11, 12]. SLM was performed using a single-mode fiber laser (Fitel, Furukawa Electric Co.) at a wavelength of 1083 nm transmitted through the fused silica window of the processing chamber. The light absorption of Ti is 67% at the 1083 nm, so this wavelength was equivalent to the 915 nm wavelength used in the contact angle measurement. A one-layer 3D fabricated sample (10 × 10 × 0.1 mm) was formed using a linear raster scanning pattern at a scanning speed of 10 mm/s and laser power of 200 W. The laser beam had a Gaussian diameter of 100 µm at the 1/e2 intensity point. The baseplate temperature was varied between 25 and 400 °C. To measure surface roughness, a stylus-type surface roughness tester was used (DSF600KS, Kosaka Laboratory, Ltd.). Amplitude parameters were used to characterize the surface roughness profile based on vertical deviations from the mean. The surface roughness Ra is an arithmetic average of the absolute values.
3 Results and discussion
3.1 Dynamics of molten Ti particle during laser irradiation
Figure 6 shows the high-speed video camera images obtained using synchrotron radiation. The horizontal size of Ti particle was about 200 µm and the contact angle between the Ti particle and baseplate without laser irradiation was 108°. The Ti particle was irradiated with the diode laser and generally was melting after 1.0 s of irradiation. At a baseplate temperature of 25 °C, the molten Ti showed little spreading, as shown in Fig. 6a. However, when the baseplate was preheated to 500 °C, the molten Ti particle gradually spread out on the baseplate at 2.0 s. The contact angle became as small as 28° after 3.0 s at 500 °C, as shown in Fig. 6b. In Fig. 6b, the line shape around the Ti particle was burr remaining on the edge of the baseplate and had no effect on the wettability evaluation of Ti.
Figure 7 shows the correlation between irradiation elapsed time and contact angle between the molten Ti particle and baseplate for four baseplate temperatures. The contact angle generally decreased with increasing baseplate temperature. It is known that the surface tension of many metals is linearly dependent on temperature [15, 16]. This result revealed that the wettability of molten Ti is enhanced by higher baseplate temperature, even in the presence of local heating by laser.
3.2 Effect of baseplate temperature on Ti64 plate fabricated by SLM in vacuum
In our previous study, we reported that the quality of Ti64 plate fabricated by SLM in a vacuum depended on laser irradiation parameters. Especially, the surface morphology and sputtering generation depended on the laser scanning speed and scanning strategy . Here, we employed the same laser scanning speed and scanning strategy used in the previous work , except that we also varied the baseplate temperature from 25 to 400 °C. Figure 8 shows the effect of baseplate temperature on surface roughness Ra with a laser output power of 200 W and laser scanning speed of 10 mm/s. The surface roughness Ra decreased as the baseplate temperature increased, improving from 2.0 to 0.56 µm.
The effect of baseplate temperature on the dynamics of molten Ti powder and surface roughness of the fabricated Ti64 plate was evaluated. From the results, the surface roughness of the plate fabricated by SLM depended on the baseplate temperature. Thus, the melting and solidification dynamics of Ti powder were captured with the high-speed video camera during the laser irradiation at the first line of the linear raster scan. Figure 9 shows the high-speed camera images for baseplate temperatures from 25 to 350 °C. They show that a large amount of sputtering was generated at 25 and 100 °C, but that no sputtering was observed when the baseplate was heated to 350 °C. According to the report by Mathew et al., pressure expansion is large in a vacuum, so at 25 °C, the surrounding powder is scattered and the denudation zone is widened . At a baseplate temperature of 350 °C, the local pressure expansion of the molten Ti would be expected to become larger than it is at 25 °C, with an increase in sputtering. However, in our experiment, the sputtering became smaller.
It is conceivable that oxygen, nitrogen, and water are adsorbed around the Ti powder surface . When the powder temperature rises rapidly during laser irradiation, the adsorbed materials are desorbed as the molten metal evaporates, and the local pressure rises because of these rapid expansions and causes sputtering. Our results indicate that preheating promotes desorption of adsorbed materials before the onset of rapid heating during laser irradiation around 150 °C. When these volatiles are removed, the local pressure expansion can be suppressed. Furthermore, as the baseplate temperature increases over 200 °C, the wettability improves, as shown in Figs. 6 and 7. Titanium melted by laser then spreads across the substrate and solidifies. When the wettability of melted Ti is improved, the contact area between the baseplate and the molten pool increases, and the melted Ti rapidly cools and solidifies. In other words, the amount of Ti evaporated by the vapor pressure from the molten pool decreases and the local pressure expansion decreases. The suppression of sputtering in turn produces a smoother surface.
We demonstrated the dynamics of a molten Ti particle irradiated with laser experimentally with synchrotron X-ray analysis. The results revealed that the contact angle of a molten Ti particle on the baseplate depended on the baseplate temperature. At 500 °C, the contact angle of a molten Ti particle decreased to 28° (from a pre-melt value of 108°). Then, we investigated the influence of wettability on a Ti plate fabricated by SLM in a vacuum. Baseplate preheating improved the wettability and promoted the desorption of volatiles, which suppressed sputtering generation and produced a plate surface with small surface roughness.
This work was supported by the Strategic Innovative Promotion Program of the Japan New Energy and Industrial Technology Development Organization (NEDO). The synchrotron radiation experiments were performed at the BL22XU in SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (Proposal No. 2016B3721, No. 2017A3721).