2019 Volume 59 Issue 1 Pages 104-112
The nitriding behavior of titanium sponges with nitrogen gas and the dissolution behavior of titanium nitride titanium sponges in titanium alloy melt were examined.
A titanium nitride sponge was produced using nitrogen gas. A high nitriding temperature corresponded to a longer nitriding time, and thus to a higher nitrogen concentration in the sponge. Both the titanium sponge and titanium nitride sponge featured a porous structure. Porous structures at both the surface layer and inside were formed at intervals of about 5.0 × 10−5 m. When the titanium nitride sponge was immersed into a titanium alloy melt, the melt permeated into the pores. The nominal dissolution rate of the titanium nitride sponge in the titanium alloy melt depends on the temperature of the melt. Higher melt temperatures corresponded to higher nominal dissolution rates. However, the concentration of nitrogen in the titanium nitride sponge had no influence on the nominal dissolution rate. Nitriding models of the titanium sponge with nitrogen gas, and the dissolution model of the titanium nitride sponge into the titanium alloy melt were proposed. These models considered the structure of the sponge; thus, the behavior of both the nitriding and dissolution sponges was predicted and confirmed.
The demand for high-quality titanium alloy ingots for the aerospace industry is growing. Therefore, it is important to remove the inclusions from the melt before casting.
There are two kinds of inclusions in titanium alloys: high-density and low-density inclusions. High-density inclusions are bits of tools and machine components, which contaminate the raw material before the ingots are cast; therefore, such inclusions can be avoided in an environment with controlled conditions for handling raw materials. However, contamination due to low-density inclusions is more difficult because they are caused by a reactions, which originates in the raw materials. To prevent contamination due to with low-density in the ingot, these inclusions need to be dissolved in the titanium alloy melt before casting. Therefore, many studies1,2,3,4,5,6,7,8,9) on the dissolution rate of low-density inclusions in titanium alloy melt have been conducted. It is necessary to understand the nitriding and dissolution behaviors of the titanium sponge. However, systematic experimental studies for these phenomena have not yet been conducted.
Titanium nitride sponges with different nitrogen concentrations were prepared from titanium sponges as the raw material, and the nitriding behavior was studied. Moreover, the dissolution behavior of the titanium nitride sponge in the titanium alloy melt was examined using a small electron beam furnace.
In this study, the nitride formation in a titanium sponge was examined as a simulated low-density inclusion formation. For this purpose, it was necessary to prepare a titanium nitride sponge with a known concentration of nitrogen. There are many studies10,11,12,13,14,15,16,17) on the nitriding behavior for titanium alloy plates. However, only a single study16) focused on the nitriding behavior of titanium sponges subjected to heat treatment under an Ar-5 vol% N2 mixture gas atmosphere. To ensure uniformity in the concentration of nitrogen in the titanium nitride sponge, a lower nitriding temperature to decrease the temperature gradient and longer nitriding time were used, as compared to the previous study.
Figure 1 shows a schematic diagram of the experimental apparatus for the heat treatment used in this study. The sponges were passed through sieves of sizes 5.6 × 10−3 m and 6.7 × 10−3 m, and thus separated according to their size. These titanium sponges were then placed in a molybdenum container, which was set in the heat-treating furnace. The atmosphere in the vessel was replaced by an Ar-5 vol% N2 mixed gas after evacuation with a diffusion pump to 1.0 × 10−1 Pa before the experiment. Then, two valves of the vessel were opened, and the mixed gas was fed from one valve side at a flow rate of 9.5 L·min−1. The concentration of nitrogen for the titanium nitride sponge was varied with the heat treatment temperature over 3 h. The concentration of nitrogen in the sponge was controlled by the thermal conductivity technique.
Schematic diagram of heat treatment furnace for titanium sponges with nitrogen gas.
Figure 2 shows a schematic diagram of the experimental apparatus with the small electron beam furnace. The titanium alloy ingot, which was cast previously, was set in a water-cooled copper crucible in a vacuum vessel. This ingot had an upper diameter of 1.8 × 10−1 m, lower diameter of 1.2 × 10−1 m, and height of 9.0 × 10−2 m, and its composition was Ti-6.4 mass% Al-4.2 mass% V. The power of the electron beam was set to 30 kW and the current was varied from 0.8 A to 1.0 A. A molten pool was formed in the upper part of the ingot by irradiating its top surface with an electron beam. To avoid direct irradiation of the center region of the top surface of the ingot, the electron beam shape was set to a circular ring. After about 1 h, the temperature of the melt was adjusted to the prescribed temperature. The titanium nitride sponge, which was located above the melt, was immersed into the melt about 1.5 × 10−2 m below the surface melt. At this position, the titanium nitride sponge was completely immersed. After 20 to 120 s, the titanium nitride sponge was removed out of the melt. In this study, the titanium nitride sponge, with sizes in the range of 1.1 × 10−2 m to 1.3 × 10−2 m, as determined by sieving, was used. The concentration of the titanium nitride sponge was chosen in the range of 5 to 15 mass% by heat treatment with the mixed gas. A hole of 3.5 × 10−3 m in diameter was made at the center of this titanium nitride sponge, and then a tungsten rod of 3.0 × 10−3 m in diameter, with an L-type-shaped front edge, was inserted in this hole. The titanium nitride sponge was then moved to the central area of the circular ring of the electron beam. This was done so that that area would not be irradiated directly; thus, the direct dissolution of the titanium nitride sponge was prevented by electron beam irradiation before immersion into the melt.
Schematic diagram of electron beam furnace for dissolution of titanium nitride sponge in titanium alloy melt.
The temperature of the alloy melt was measured with a two-color pyrometer from outside the vacuum vessel through the window. To calibrate the values of a two-color optical pyrometer, the temperature of the alloy melt was measured using a thermocouple of W-5 mass% Re/W-26 mass% Re with 5.0 × 10−3 m in diameter.
There are many studies on the nitriding behavior for a titanium alloy plate. In this study, a titanium sponge, which featured a porous structure, was used as feed material of nitride, the validity of this experimental procedure was examined.
Figure 3 shows the relationship between the concentration of nitrogen in the titanium nitride sponge and the time for the 1323 K heat treatment. The concentration of nitrogen increased with time until 12 h, after which it exhibited a constant value.
Relationship between concentration of nitrogen in titanium nitride sponge and time.
Figure 4 shows the relationship between the concentration of nitrogen in the titanium nitride sponge and the temperature at a time of 3 h after the experiment was started. The concentration of nitrogen increased with the temperature until 1273 K, following which the rate of increase declined. By changing the heat treatment temperature, the desired concentration of nitrogen for the titanium nitride sponge can be achieved.
Change in concentration of nitrogen titanium nitride sponge with temperature for a duration of 3 h.
It is thought that the nitriding behavior of the titanium sponge depends on its structure. Therefore, the structure of the titanium sponge was examined.
Figure 5 shows the structure of the titanium sponge. Figure 5(b) is a secondary electron image of the surface of sponge. The image shows that surface structure features porosity with an interval of about 5.0 × 10−5 m. Figure 5(c) shows the microstructure in the center region of the titanium sponge observed by using an optical microscope. A porous structure was also formed in the center region of the sponge with an interval of about 5.0 × 10−5 m, in addition to on the surface. It was found that the titanium sponge formed a completely porous structure. Therefore, the nitrogen gas permeated into the sponge easily, and the nitriding reaction progressed from both sides of the solid to a distance of about 5.0 × 10−5 m in the thickness direction.
(a) External view, (b) surface, and (c) center of titanium sponge before nitriding, respectively. (Online version in color.)
Figure 6 shows the microstructure of the titanium nitride sponge. Figure 6(b) is the secondary electron image of the surface of the sponge and Fig. 6(c) is the optical microscope image in the center region of the sponge. It was found that the titanium nitride sponge formed as a porous structure with an interval of about 5.0 × 10−5 m, in addition to that observed for the titanium sponge.
(a) External view, (b) surface and (c) center of titanium nitride sponge, respectively. (Online version in color.)
Figure 7(a) shows the optical microscopy image of the microstructure of the cross-section of the center of the titanium nitride sponge when the concentration of nitrogen was 8 mass%. The outer white region of the titanium nitride sponge was perfused with in an embedded resin. Figure 7(b) shows the concentration distribution for nitrogen and titanium in the titanium nitride sponge obtained via energy-dispersive X-ray spectrometry (EDS). From these results, it was confirmed that both the surface and the center of the sponge were uniformly nitrided. It is thought that the nitriding reaction in both the surface and center of the titanium sponge progressed because the nitrogen-mixed gas permeated into the center region. These results necessitated an examination of the reaction in the sponge to examine the nitriding behavior of the titanium sponge.
EDS analysis of N and Ti at surface and center of titanium nitride sponge. (Online version in color.)
The type of the reaction product formed by the reaction between titanium and nitrogen gas was hypothesized to change depending on the concentration of nitrogen. The phases in the titanium nitride sponge varied with the concentration of nitrogen, and were identified and characterized by X-ray diffraction.
Figure 8 shows the XRD patterns of the titanium sponge and titanium nitride sponge. Both were composed of TiN and Ti2N.
Phases of titanium nitride in titanium nitride sponge determined by X-ray diffraction analysis. Concentration of nitrogen: (a) 0 mass%, (b) 3, (c) 8 and (d) 15.
Figure 9 shows the relationship between the fraction of nitriding and the concentration of nitrogen in the titanium sponge and titanium nitride sponge. TiN and Ti2N were formed, and the fraction of these nitrides depended on the concentration of nitrogen.
Relationship between fraction of titanium nitride phase and concentration of nitrogen determine by X-ray diffraction analysis.
However, these nitrides are reaction products, which are obtained from the reactions with 1 mol of nitrogen; moreover, these reactions are limited by the diffusion of nitrogen. Therefore, the difference between the generation of TiN and Ti2N is thought to be small. Therefore, in the following analysis, it was assumed that the only titanium nitride is TiN, in the following analysis.
Figure 10 shows the analytical model of the nitriding reaction for the titanium sponge. A two-dimensional simple analytical model was constructed, because the entire titanium sponge consisted of porous structures, and elemental titanium that was part of the structure was surrounded by the nitrogen-mixed gas.
Schematic diagram of model for nitriding titanium sponge using nitrogen gas.
According to the kinetic theory of gases,18) the molecule flux of nitrogen JN2, at the surface of titanium, can be predicted using the following equation.
| (1) |
The diffusion of nitrogen in titanium is predicted by the following equation. In this study, a generalized software20) was used for this calculation.
| (2) |
Mass balance between the nitrogen on the titanium side and nitrogen on the gas side at the interface was assumed.
| (3) |
Figure 11 shows the concentration change of nitrogen in titanium within the analytical element at a distance away from the interface when the time is 0.25 h, 0.5 h, and 3 h, respectively. The calculation temperature was 1073, 1173, and 1323 K. A longer heat treatment time corresponded to a higher concentration of nitrogen. However, the concentration gradient of nitrogen was small, because the diffusion length was small as well, i.e., 2.5 × 10−5 m.
Relationship between concentration of nitrogen in titanium sponge and distance. (a) 1073 K, (b) 1173 K and (c) 1323 K.
The change in the calculated concentration of nitrogen with the temperature is also shown in Fig. 4 for a heat treatment time of 3 h. The calculated data agree with the experimental data, thus confirming the validity of this analysis.
3.2. Dissolution of Titanium Nitride SpongeTo calibrate the measured value of the two-color pyrometer, this value was compared with the measurement result of the titanium alloy melt by using a thermocouple. An example of the measurement result is shown in Fig. 12. After holding and preheating the thermocouple above the alloy melt, it was immersed. The end position of the thermocouple corresponded to the center of the titanium nitride sponge. Thus, it was confirmed that the temperature of the alloy melt could be maintained for about 200 s at 1173 K. The temperature of the alloy melt varied according to the power of the current of the electron beam. The temperature of the alloy melt was measured with a thermocouple while changing the power current of the electron beam repeatedly, in order to calibrate the output of the two-color pyrometer.
Measurement result of melt temperature by thermocouple made with W-5%Re/W-26%Re.
Figure 13 shows the shape of the pool for the alloy melt where the titanium nitride sponge was immersed. After the upper part of the ingot had been remelted, that melt was solidified again. The longitudinal cross-section of the solidification microstructure at the center of the ingot was revealed by nitric-5 vol% hydrofluoric acid, and the boundary in the molten pool was evaluated. As the depth of the molten pool was about 4.0×10−2 m, a titanium nitride sponge could be immersed completely in the molten pool.
Molten pool depth near top surface of titanium alloy ingot. Dashed line shows interface between melt and solid. (Online version in color.)
Figure 14 shows photographs of the titanium nitride sponge before and after immersion in the molten pool. The concentration of nitrogen of the titanium nitride sponge was 8 mass% in this sample. The diameter of the titanium nitride sponge after immersion reduced as result of immersion. It was confirmed that it dissolved almost evenly without any localized dissolution. It is thought that the titanium nitride sponge was held in the uniform temperature region of the molten pool. The nominal dissolution rate was calculated from the diameter difference of the titanium nitride sponge before and after immersion, and immersion time.
Titanium nitride sponge (a) before and (b) after immersion into titanium alloy melt. (Online version in color.)
Figure 15 shows the relationship between the nominal dissolution rate obtained by the experiment and the reciprocal temperature when the concentration of nitrogen was 5, 8, 10, and 15 mass%. When the temperature of the alloy melt was high, the nominal dissolution rate was high in all cases. However, the experimental values deviated slightly; thus, the influence of the concentration of nitrogen on the nominal dissolution rate was not observed. It is thought that the influence of the concentration of nitrogen was not evident because the nominal dissolution rate of the titanium nitride sponge, which was formed with a porous structure of about 5.0 × 10−5 m in thickness of porous part, was high as a result of the permeation of the alloy melt into the porous structure. In this figure, the relationship between the nominal dissolution rate and the reciprocal of the temperature, which were obtained from previous studies, are also shown. These values are close to the lower limit obtained for this study. For this reason, it is thought that a simulated inclusion, which was added to the top surface of the melt, went to the bottom of the molten pool, where the temperature was lower, and cooled.
Change in nominal dissolution rate of titanium nitride sponges with temperature of titanium alloy melt.
To predict the nominal dissolution rate of the titanium nitride sponge, a simple dissolution model was prepared, and the validity of the predicted values was examined by comparing them with the experimental results. In a previous study,4) a dissolution model assumed that the α phase and β phase, which were part of simulated inclusions, were arranged in a concentric pattern and that the outer of these phases was surrounded by the alloy melt. Then, the diffusion of nitrogen was calculated between the phases. By using the relationship between the concentration of nitrogen and temperature in the phase diagram, it was examined whether a simulated inclusion would melt or not at a given temperature.
Figure 16 shows the photograph of the cross section of the titanium nitride sponge that was immersed into the melt, held for 60 s, and pulled out to be inspected with an optical microscope. It was found that the alloy melt filled the pores of the titanium nitride sponge at the center in addition to the surface. Therefore, it is necessary to prepare a dissolution model of the titanium nitride sponge, which has a porous structure. Subsequently, a structural model was assumed in which the solid phase of the titanium nitride was staggered in the alloy melt as liquid at 5.0 × 10−5 m intervals, as shown in Fig. 17(a). By considering structural symmetry, an analyzed element was cut, as shown in Fig. 17(b). The nominal dissolution rate was evaluated using that element.
Titanium nitride sponge after immersion into titanium alloy melt.
Analytical model for dissolution of titanium nitride sponge immersed in titanium alloy melt. (a) Entire titanium nitride sponge, (b) ordered configuration of sponge and melt and (c) interface movement by diffusion in the volume element.
The nitrogen diffused from the titanium nitride sponge as a solid to the alloy melt as a liquid. Conversely, aluminum and vanadium both diffused from the melt to the sponge. As local equilibrium is gradually established at the interface between the titanium nitride sponge and alloy melt, the interface shifts with time. As the thermal conductivity of the titanium nitride sponge is low, there is a temperature gradient between the surface and the core when the sponge is immersed in the alloy melt. Therefore, it is thought that the outermost nth layer, in which the temperature is the highest in the sponge, begins to dissolve in series. If the dissolution time of the nth layer is evaluated, the nominal dissolution rate can be calculated from the length of the analyzed element.
Figure 18 shows the change in the concentration of nitrogen with time when the initial concentration of nitrogen in the titanium nitride sponge is 8 mass% and the initial concentration of aluminum and vanadium in the alloy melt are 6 mass% and 4 mass%, respectively. The nitrogen diffused into the alloy melt and its concentration decreased. Further, both aluminum and vanadium diffused into the titanium nitride sponge and their concentrations decreased.
Calculated relationship between concentration of (a) nitrogen, (b) aluminum and (c) vanadium and distance.
Figure 19 shows the relationship between a fraction of the liquid in the analyzed element and the time when the temperature was 2023, 2073, and 2123 K. When the fraction of liquid reaches 1.0, the titanium nitride sponge in an analyzed element will dissolve completely. At any temperature, the fraction of liquid increased with to time and reached 1.0 finally. The shorter the time taken by the fraction of the liquid to reach 1.0, the higher the temperature of the element.
Calculated relationship between fraction of liquid and time at (a) 2023 K, (b) 2073 K and (c) 2123 K.
The relationship between the calculated nominal dissolution rate of the titanium nitride sponge and temperature is also shown in Fig. 15. It was confirmed that the nominal dissolution rate of the titanium nitride sponge could be predicted by using this analytical model because the calculated value was in the range of experimental results.
The demand for high-quality titanium alloy ingots for the aerospace industry is growing. Therefore, it is important to remove the inclusions from the melt before casting, even when the inclusions caused by the raw materials are mixed. However, it is difficult to prevent the contamination by nitrides, because it involves the generation of a reaction product in the raw material. Therefore, it is necessary to dissolve the nitride in the alloy melt before casting.
In this study, the nitriding and dissolution behavior of a titanium sponge as the raw material was examined systematically. From these results, the following conclusions were obtained.
(1) The titanium sponge and titanium nitride sponge were formed with a porous structures. The porosity interval was about 5.0 × 10−5 m at both the surface and center of the sponge. When the nitride sponge was immersed in the titanium alloy melt, this melt permeated into the pores of the sponge.
(2) The titanium nitride sponge could be produced with a titanium sponge as the raw material by heat treatment with nitrogen gas. A high concentration of the titanium nitride sponge corresponds to a higher nitriding temperature, and thus a longer nitriding time.
(3) The nominal dissolution rate of the titanium nitride sponge in the titanium alloy melt changed with the temperature of the melt. The nominal dissolution rate of the sponge increased with increasing temperature. The influence of the concentration of the titanium nitride sponge on the nominal dissolution rate was small.
(4) An analytical model was constructed to nitride titanium sponge with nitrogen gas. In addition, an analytical model for the dissolution of the titanium nitride sponge in the titanium alloy melt was constructed. By using these models, both, the nitriding and dissolution behaviors of the sponge could be predicted.
