METHOD FOR CASTING Ti-Al BASED ALLOY
20220250141 · 2022-08-11
Assignee
Inventors
Cpc classification
B22D21/02
PERFORMING OPERATIONS; TRANSPORTING
B22D21/005
PERFORMING OPERATIONS; TRANSPORTING
B22D21/06
PERFORMING OPERATIONS; TRANSPORTING
International classification
Abstract
The objective of the present invention is to improve non-defective product yield by reducing shrinkage cavities inside small-diameter ingots, in a method for casting active metals. In this Ti—Al based alloy casting method for casting an ingot of Ti—Al based alloy by tapping molten metal from a tapping hole (5) provided in a bottom portion of a water-cooled copper crucible (2), in an induction melting furnace (3) employing said crucible (2), into a casting mold (4), the degree of vacuum inside the induction melting furnace (3) when the Ti—Al based alloy is being melted or cast is in a range of 80 to 700 Torr, and the Al concentration in the cast ingot is within ±1.0 mass % of a target value.
Claims
1. A method for casting a Ti—Al based alloy comprising: tapping molten metal from a tapping hole provided in a bottom portion of a water-cooled copper crucible in an induction melting furnace; and casting an ingot of Ti—Al based alloy, wherein a degree of vacuum in the induction melting furnace at the time of melting or casting the Ti—Al based alloy is set within a range of 80 Torr to 700 Torr, and a concentration of Al of the cast ingot is set within ±1.0 mass % of a target value.
Description
BRIEF DESCRIPTION OF DRAWINGS
[0018]
[0019]
[0020]
DESCRIPTION OF EMBODIMENTS
[0021] Hereinafter, embodiments of a method for casting a Ti—Al based alloy according to the present invention will be described in detail with reference to the drawings.
[0022] In the method for casting a Ti—Al based alloy according to the present embodiment, a molten metal M in which a titanium-aluminum based alloy (Ti-Al-based alloy) having an active high melting point is melted is poured into a mold 4 to perform casting, thereby producing a small diameter ingot S (ingot). As the Ti—Al based alloy, various alloys can be considered, and alloys such as a Ti-3Al-2.5V alloy, a Ti-6Al-6V-2Sn-0.5Fe-0.5Cu alloy, a Ti-3Al-10V-2Fe alloy, a Ti-5Al-5V-5Mo-3Cr-0.5Fe alloy, a Ti-3Al-8V-6Cr-4Mo-4Zr alloy, a Ti-3Al-15V-3Cr-3Sn alloy, a Ti-6Al-4V alloy, a Ti-3Al-15Mo-2.7Nb-0.2Si alloy, a Ti-5Al-2Sn-2Zr-4Mo-4Cr alloy, a Ti-6Al-2Sn-4Zr-6Mo alloy, a Ti-6Al-2Sn-4Zr-2Mo alloy, a Ti-6Al-5Zr-0.5Mo-0.25Si alloy, a Ti-5.5Al-3.5Sn-3Zr-1Nb-0.25Mo-0.3Si alloy, and a Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si-0.06C alloy are used as alloys used in aircraft parts.
[0023] Hereinafter, a casting apparatus 1 used in the method for casting a Ti—Al based alloy according to the present embodiment will be described.
[0024] As shown in
[0025] According to the present invention, gas defects caused by gas involvement or the like inside the ingot can be reduced by controlling the degree of vacuum (atmospheric pressure) in the melting and casting chamber to a predetermined value, and the yield of the cast product can be improved by keeping a concentration of Al of a cast product within a standard range.
[0026] The induction melting furnace 3 used in the casting apparatus 1 of the present embodiment generates an induced current inside a material to be melted to utilize resistive-heating thereof, and is generally called a cold crucible induction melting apparatus. The induction melting furnace 3 melts a Ti—Al based alloy by using the water-cooled copper crucible 2, and is formed of copper without using a refractory that is often used as a material for the crucible 2 in a general melting furnace. Therefore, the casting using the induction melting furnace 3 is less likely to be affected by contamination from the refractory.
[0027] As shown in
[0028] A wall of the crucible 2 is formed of copper as described above and water-cooled. When the wall of the crucible 2 is formed of such water-cooled copper, the temperature of an inner wall of the crucible 2 does not rise to a predetermined temperature (for example, 250° C.) or higher even when the melted Ti—Al based alloy is accommodated in the crucible 2. Specifically, a solidified shell called a scull is formed between an inner peripheral surface of the wall of the crucible 2 and the molten metal when the melted Ti—Al based alloy described above is put into the water-cooled copper crucible 2. Since the scull serves as a crucible, the molten metal is not contaminated from the crucible 2.
[0029] The crucible 2 of the present embodiment is a bottom tapping type, and a tapping hole 5 that can guide the accommodated Ti—Al based alloy downward is formed in the bottom portion of the crucible 2. An opening diameter of the tapping hole 5 can be adjusted, and the amount of the molten metal M guided downward can be adjusted. The opening diameter of the tapping hole 5 may be adjustable by an electromagnetic method or a mechanical method, or a plurality of valve members having different opening diameters may be prepared in advance, and the opening diameter of the tapping hole 5 may be adjusted by replacing the valve members.
[0030] The mold 4 is formed in a bottomed cylindrical shape opened upward.
[0031] Various inner dimensions of the mold 4 can be considered. Assuming that the diameter of the ingot S is D, the height of the ingot S is H, and the weight of the molten metal M is W, it is preferable that the inner dimension of the mold 4 is set to a size that falls within the following application range.
[0032] Diameter D (mm) of ingot: 10≤D≤150
[0033] Height H (mm) of ingot: 15≤H≤1600
[0034] Molten metal weight W (kg): 0.2≤W≤200
[0035] Next, a procedure for casting an active metal using the induction melting furnace 3 described above, in other words, a method for casting a Ti—Al based alloy will be described.
[0036] In the method for casting a Ti—Al based alloy according to the present embodiment, in the induction melting furnace 3 using the water-cooled copper crucible 2, the molten metal M is tapped from the bottom portion of the crucible 2 to the mold 4 to cast the small diameter ingot S of an active metal. At this time, casting is performed under the casting conditions in which the diameter of the small diameter ingot S to be cast is 10 mm or more, the ratio H/D of the height (H) of the ingot S to the diameter (D) of the ingot S is 1.5 or more, and the weight of the molten metal M tapped by casting is 200 kg or less. In addition, when casting is performed, the tapping hole 5 whose opening diameter can be adjusted is provided in the bottom portion of the crucible 2, the temperature of the molten metal M during casting is made higher than the melting point of the active metal, and the opening diameter of the tapping hole 5 is adjusted, whereby casting is performed while controlling a casting speed V (kg/sec), which is a speed at which casting proceeds in the mold 4, to V≤0.1H in relation to the height H of the ingot S, thereby reducing the number of shrinkage cavities inside the ingot S and improving the casting yield. In order to prevent “molten metal clogging” in which the molten metal tapped during casting is clogged and the molten metal does not flow, the temperature of the molten metal M during casting is preferably 20° C. or more higher than the melting point of the active metal, and more preferably 40° C. or more higher than the melting point of the active metal.
[0037] Incidentally, even when the ingot S of a Ti—Al based alloy is cast at the casting speed V (kg/sec) described above, the concentration of Al is gradually decreased during casting since Al evaporates from the molten metal when the inside of the furnace is in a vacuum state, and finally, there is a possibility that the concentration of Al in a composition of the cast product falls below the standard value of the target concentration of Al. Therefore, in the method for casting a Ti—Al based alloy according to the present embodiment, the degree of vacuum in the induction melting furnace 3 at the time of melting or casting the Ti—Al based alloy is set to be in the range of 80 Torr to 700 Torr and the concentration of Al of the cast product (ingot) is set to be within ±1.0 mass % of a target value when the molten metal M is tapped into the mold 4 from the tapping hole 5 provided in the bottom portion of the water-cooled copper crucible 2 of the induction melting furnace 3 to cast the cast product (ingot) of a Ti—Al based alloy.
[0038] Strictly speaking, the concentration of Al described above differs between a state of being cast into a cast product (solid state) and a state of a molten metal (liquid state). However, in the present embodiment, the concentration of Al is set to be within ±1.0 mass % in both cases of solid and liquid.
[0039] The reason why the degree of vacuum in the induction melting furnace 3 described above is defined within the range of 80 Torr to 700 Torr is as follows.
[0040] That is, in the present embodiment, the weight of the molten metal used for casting is 50 kg. A casting time of 15 minutes is required until all of the molten metal is cast as the ingot S. It is needless to say that the casting time varies depending on a diameter of a nozzle. Even during the casting time of 15 minutes, the concentration of Al of the molten metal is gradually decreased due to evaporation. However, when an amount of decrease in the concentration of Al lost by evaporation is within an allowable variation range of the standard value of the concentration of Al, the cast product (ingot) after casting has an appropriate composition as a Ti—Al based alloy.
[0041] The standard value (target value) of the concentration of Al of the Ti—Al based alloy may be high or low depending on the alloy type of the cast product to be cast. However, the applicant has confirmed that an appropriate composition can be obtained as a Ti—Al based alloy when the concentration of Al is kept within ±1.0 mass % of the standard value of 23.3 mass % to 43.3 mass %. For example, even when the standard value of the concentration of Al of 30 mass % is targeted, the variation range of the concentration of Al allowable for the actual cast product with respect to the standard value is within ±1.0 mass %.
[0042] Here, in the casting method of the present embodiment, first, a difference between an upper limit and a lower limit of the allowable variation range with respect to the standard value, in other words, the variation range of the concentration of Al is obtained. The variation range of 2.0 mass % thus obtained is divided by the casting time of 15 minutes. It is nothing but determining a rate of change in the concentration of Al such that the amount of decrease does not exceed the variation range of the concentration of Al even when the concentration of Al is decreased in the casting time of 15 minutes. The rate of change in the concentration of Al as a result of the calculation described above is calculated to be: 2.0 mass %÷15 min=0.13 mass %/min or less.
[0043] After the 0.13 mass %/min described above is calculated, the degree of vacuum at which the rate of change in the concentration of Al is 0.13 mass %/min or less may be obtained by actually performing an experiment. The degree of vacuum of the casting obtained by the experiment in this manner is 80 Torr to 700 Torr described above. The experiment for determining the range of the degree of vacuum will be described in detail in Examples described later.
[0044] When casting is performed under the condition in which the degree of vacuum (pressure in a vacuum container) described above is less than 80 Torr, a large amount of Al in the molten metal evaporates during casting, and thus the composition of the molten metal greatly fluctuates during casting, and the fluctuation in the composition of the cast product cannot be ignored. As a result, the composition (concentration of Al) of the cast product deviates from the standard value, and a cast product having poor composition is likely to be generated, which leads to a decrease in yield.
[0045] In addition. when the casting is performed under the condition in which the degree of vacuum (pressure in the vacuum container) exceeds 700 Torr, the Ti—Al based alloy is cast in an environment in which a large amount of gas is present. Thus, there is a high possibility that the gas existing in the surroundings is involved when the molten metal is solidified in the mold, the gas defects are increased, and the yield is decreased.
[0046] The gas defects are generated due to the involving of the gas existing in the surrounding when the molten metal is solidified in the mold 4, and are often formed on a surface layer of the mold. Therefore, when the degree of vacuum in the furnace at the time of melting and casting is fine, the amount of gas to be involved is also reduced. That is, the gas defects can be reduced by improving the degree of vacuum in the furnace. When the gas defects are reduced, the amount of cutting the cast product in order to remove the gas defects from the cast product is also reduced, so that the yield of the cast product can be improved.
[0047] Although there is no quantitative evaluation result of the number of gas defects in the case of casting at atmospheric pressure, there is a qualitative discussion that the number of gas defects tends to be decreased as the casting environment for casting is changed from atmospheric pressure to vacuum. Therefore, by setting the degree of vacuum at the time of melting or casting to a higher degree of vacuum than 700 Torr described above (lower pressure than 700 Torr), it can be considered that the number of gas defects can be significantly reduced as compared with the case where the degree of vacuum is 760 Torr (atmospheric pressure) or more.
[0048] As described above, by performing casting with the degree of vacuum (air pressure in the vacuum container) set to 80 Torr to 700 Torr, it is possible to appropriately control the composition (concentration of Al) of the cast product to a standard value while preventing the generation of the gas defects, and it is possible to greatly improve the comprehensive quality in consideration of the defect in the composition.
EXAMPLE
[0049] In Examples, a maximum of 50 kg of a Ti—Al based alloy material (Ti-33.3Al-4.8Nb-2.55Cr (mass %)) was melted, and a molten metal obtained by melting was used for casting.
[0050] Specifically, the materials described above were heated using the water-cooled copper crucible 2 in the cold crucible induction melting furnace 3 shown in
[0051] As shown in
[0052] After the relationship between the degree of vacuum in the furnace and the evaporation rate of Al was obtained in advance by the procedure described above, a cast product (ingot) was cast by actually tapping into the cast mold 4.
[0053] Note that casting was performed by tapping the molten metal from a graphite nozzle (tapping hole 5) installed at a bottom portion of the crucible 2 in which the molten metal was sealed, and solidifying the molten metal with the graphite mold 4 located below the crucible 2. The casting was also performed under an Ar atmosphere with reduced pressure, in other words, under an Ar atmosphere in which the degree of vacuum was controlled in the range of 0.001 Torr to 700 Torr. The cast product (ingot) to be produced can be changed in accordance with the shape of the graphite mold 4. In the present embodiment, an ingot was produced by casting with two types of the graphite mold 4 having a rectangular nozzle opening shape and the graphite mold 4 having a circular nozzle opening shape. Specifically, there are two types of nozzle opening shapes, 65 mm×65 mm and 55 mm×55 mm for a rectangular shape, and φ72 mm and φ50 mm for a circular shape. A cylindrical ingot is obtained when the nozzle opening shape is circular, and a prismatic ingot is obtained when the nozzle opening shape is rectangular.
[0054] A height of the ingot is between 620 mm and 1520 mm, and a casting speed of the ingot is in a range of 0.18%/s to 0.4%/s, although they depend on experiments.
[0055] With respect to the obtained ingot product, the applicant has confirmed that the evaporation rate of Al is suppressed in the range of the degree of vacuum of the present embodiment, and an ingot in which the concentration of Al is controlled to be in the range of ±1.0 mass % of a target value is obtained. When a Ti—Al alloy having a target value of concentration of Al of 33.3 mass % was cast at 80 Torr by the method of the present invention, the concentration of Al was within ±1 mass % of the target value of 33.3 mass % and was 32.79 mass %.
[0056] Among the obtained ingots, the number of gas defects per unit height of the ingot was measured for the cast product cast at a degree of vacuum of 200 Torr, and was 30/mm or less. This shows a result that the number of gas defects is significantly reduced compared with the number of gas defects being on order of several hundred in the case of casting under atmospheric pressure, and that the generation of gas defects is prevented.
[0057] As shown in the above results, the concentration of Al can be controlled with high accuracy by setting the degree of vacuum in the induction melting furnace 3 at the time of melting or casting the Ti—Al based alloy within the range of 80 Torr to 700 Torr when the molten metal is tapped from the tapping hole 5 provided in the bottom portion of the water-cooled copper crucible 2 of the induction melting furnace 3 to the mold 4 to cast the ingot of the Ti—Al based alloy, and by setting the concentration of Al of the cast ingot within ±1.0 mass % of the target value, and the comprehensive quality of the cast product can be greatly improved considering not only the defect of the shape but also the defect of the composition.
[0058] It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in all respects. In particular, items that are not explicitly disclosed in the embodiments disclosed herein, such as operating conditions, operational conditions, various parameters, and dimensions, weights, volumes, and the like of components do not depart from the scope of normal implementation by those skilled in the art, and values that can be easily assumed by those skilled in the art are adopted.
[0059] The present application is based on Japanese Patent Application No. 2019-143776, filed on Aug. 5, 2019, and the contents thereof are incorporated herein by reference.
REFERENCE SIGNS LIST
[0060] 1 Casting apparatus
[0061] 2 Crucible
[0062] 3 Induction melting furnace
[0063] 4 Mold
[0064] 5 Tapping hole
[0065] M Molten metal
[0066] S Cast Product (ingot)