Electron beam melting toward defects-free titanium alloys

Titanium alloys used for rotating parts of aeroplanes must meet the strict standards of quality, so that it is required to demonstrate that electron beam melting is capable of producing defects-free ingots. However, complete elimination by dissolution of low density inclusions (LDI) having high nitrogen content would be difficult, so that a new way of eliminating LDI was needed. Therefore, control of a state of the molten titanium flow with the aide of Marangoni flow induced by electron beam has been studied, and the effects on eliminating LDI have been verified.


Introduction
Since it was introduced in 1980s, electron beam cold hearth melting [1] has been used to produce not only commercially pure titanium products but also aerospace-quality products. Among of them, titanium alloys used for rotating parts of aeroplanes must meet strict standards of quality, so that it is required to demonstrate that electron beam melting is capable of producing defectsfree ingots. There are two kinds of inclusions (see table 1) to be eliminated. One is high density inclusions (HDI) such as WC tool bits and other refractory metals. The other is low density inclusions (LDI) such as nitrided sponge particles and burned sponge particles. Since density of HDI is greater than that of molten titanium (e.g., Ti-6Al-4V≒3920 [kg/m 3 ]), they can sink in the molten titanium (pool) and can be trapped in the skull (solidified titanium formed on a crucible). On the other hand, LDI can have density lower than molten titanium, so that they do not sink but can move inside or on the surface of the pool. In order to eliminate those kinds of LDI, removal by dissolution is often used, but complete removal of LDI having high nitrogen content might be impossible [2,3]. Therefore, a new way of eliminating LDI has been studied.

Methodology
It is well known that there is a temperature gradient on the surface of a fluid, and thus Marangoni flow is induced. Therefore, some area of a molten titanium in a hearth is intensively heated by electron beam (hereafter, we call Intensive Heating (IH)), and thus Marangoni flow originating the heated area is generated. Based on the fact, we considered the control of a state of the molten titanium flow with the aide of the Marangoni flow, and inclusions coming to the area got stuck in front of the area (see figure 1).

Verification of effectiveness of IH by numerical simulation
In order to verify that Intensive Heating can really get LDI stuck in front of the heated area, a computational model describing a molten titanium flow and behaviour of inclusions in a molten titanium has been developed [4][5][6].

Computational model
Equations describing molten metal flow are shown:

Numerical Conditions
Flow of a molten titanium and behaviour of LDI moving on the surface of the pool was investigated under the following numerical conditions:

Numerical Results
Behaviour of LDI without Intensive Heating is shown in figure 2. In the figure, molten titanium flows from the left to the right, so that the solid lines in figure 2(b) representing LDI path on the surface of the pool are reaching to the lip where refined molten titanium is poured to a mold. This result indicates that LDI cannot be eliminated, and three is a risk that LDI contaminate ingots.
On the other hand, behavior of LDI with IH is shown in figure 3. In the calculation, IH was carried out at the center of the hearth, so that the surface temperature becomes greater than other. Because of this temperature gradient, Marangoni flow is induced and LDI coming from the left get stuck in front of the hotter area and cannot move to the lip. This result does not change even if the position of beam spot changes as shown in state 1 to 3 in figure 3.

Verification of effectiveness of IH by experiments
Experiments to verify the effects in actual facilities have been carried out. In the experiments, carbon tracer particles were used instead of true LDI because the density is much less than that of molten titanium. And then we mixed them with raw materials. Time transition of carbon tracer particles in molten titanium is shown in figure 4. In the figure, the molten titanium in the cold hearth flowed from the upstream side to the downstream side, so that carbon tracer particles behaved in the same manner. In the case of a heat pattern without IH (see figure 4(a)), the particle was not eliminated and flowed to a mold. On the other hand, in the case of a heat pattern with IH (see figure 4(b)), the particle got stuck in front of the IH area and was not able to flow to the mold. Therefore, the effects of Intensive Heating on eliminating LDI have been confirmed.

Conclusion
Titanium alloys used for rotating parts of aeroplanes must meet strict standards of quality, so that it is required to demonstrate that electron beam melting is capable of producing defects-free ingots. However, complete elimination by dissolution of LDI having high nitrogen content would be difficult, so that a new way of eliminating LDI was needed. Therefore, control of a state of the molten titanium flow with the aide of Marangoni flow induced by Intensive Heating has been studied and effects on eliminating LDI has been verified by the numerical simulations and the experiments using actual electron beam facility and carbon tracer particles.