Directional solidification device and method suitable for titanium alloy casting

Abstract

The invention discloses a directional solidification device and method suitable for a titanium alloy casting. The water-cooling base plate is communicated with a cooling system of the water-cooling copper crucible of the skull furnace; the lower mold is installed on the top of the water-cooling base plate, the upper mold is installed on the side, away from the water-cooling base plate, of the lower mold, and multiple through holes are formed in the side wall and the top of the upper mold. The risers communicate with the corresponding through holes, and exhaust holes are formed in the sides, away from the upper mold, of the risers. The casting pipe is fixedly arranged on the side, away from the lower mold, of the upper mold and close to the side wall of the upper mold. The core is of a hollow structure; the wall thickness of the upper mold and the wall thickness of the core are smaller than the wall thickness of the lower mold, and the wall thickness of the riser is smaller than the wall thickness of the upper mold and the wall thickness of the core. The fastening assembly is arranged on the outer side of the mold assembly in the circumferential direction. Through the differential design of the thick wall of the lower mold and the thin wall of the upper mold / riser, the high thermal conductivity of the graphite material is combined, the stable vertical temperature gradient is constructed, directional solidification of the titanium alloy melt from bottom to top is achieved, internal shrinkage cavities are reduced, and the internal quality of the titanium alloy casting is improved.

Description

Technical Field

[0001] This application relates to the technical field of titanium alloy casting, and particularly to an apparatus and method for directional solidification of titanium alloy castings. Background Art

[0002] Due to its excellent specific strength, specific modulus, high-temperature creep resistance and oxidation resistance, titanium alloy has become the core material in high-end equipment fields such as aviation and aerospace. Traditional titanium alloy castings mostly adopt the bottom-pouring casting process, and its solidification process progresses layer by layer from the outer surface of the casting to the inside, forming an inverse temperature gradient with a low temperature at the top and a high temperature at the bottom. This solidification mode easily leads to premature closure of the liquid metal feeding channels, making it difficult to discharge defects such as pores and shrinkage cavities inside the casting, seriously affecting its density and mechanical properties. Summary of the Invention

[0003] Embodiments of this application solve the problems raised in the background art by providing an apparatus and method for directional solidification of titanium alloy castings.

[0004] In a first aspect, embodiments of this application provide an apparatus for directional solidification of titanium alloy castings, including a mold assembly, a fastening assembly and a water-cooled backing plate; the water-cooled backing plate is connected to the cooling system of the water-cooled copper crucible of the ingot shell furnace, and a coiled cooling channel is integrated inside it; the mold assembly includes an upper mold, a lower mold, a core, a casting tube and a plurality of risers; the lower mold is installed on the top of the water-cooled backing plate, the upper mold is installed on the side of the lower mold away from the water-cooled backing plate, and a plurality of through holes are provided on its side wall and top; a plurality of the risers are respectively communicated with the corresponding through holes, and exhaust holes are provided on the side away from the upper mold; the casting tube is fixedly arranged on the side of the upper mold away from the lower mold and is close to the side wall of the upper mold; the core is a hollow structure and is arranged in the cavity structure formed by enclosing the upper mold, the lower mold and the risers on the top of the upper mold; the wall thickness of the upper mold and the wall thickness of the core are less than the wall thickness of the lower mold, and the wall thickness of the riser is less than the wall thickness of the upper mold and the wall thickness of the core; the upper mold, the lower mold, the casting tube, the core and the risers are all made of graphite material; the fastening assembly is arranged on the circumferential outer side of the mold assembly for pressing the mold assembly.

[0005] In combination with the first aspect, in a possible implementation manner, the apparatus for directional solidification of titanium alloy castings further includes a plurality of exhaust pipes; a plurality of the exhaust pipes are respectively arranged on the top of the risers on the side wall of the upper mold.

[0006] In combination with the first aspect, in a possible implementation, the upper mold is a split structure, and the upper mold includes a housing and a cover; a plurality of grooves and support platforms in an annular array are provided on the top wall of the housing; the cover is disposed on the top of the support platform and is flush with the top wall of the housing, and a central hole communicating with the riser at the top of the upper mold is provided in the middle of the cover; the cover and the grooves form a hollow sandwich structure.

[0007] In combination with the first aspect, in a possible implementation, the directional solidification device for titanium alloy castings further includes a first refractory insulation layer, a plurality of second refractory insulation layers, and a plurality of third refractory insulation layers; the first refractory insulation layer is coated on the outer wall of the upper mold; a plurality of the second refractory insulation layers are respectively filled in the hollow structure of the core and the hollow sandwich structure of the upper mold; a plurality of the third refractory insulation layers are coated on the outer walls of the corresponding risers.

[0008] In combination with the first aspect, in a possible implementation, the wall thickness of the first refractory insulation layer is 50 mm, and the wall thickness of the third refractory insulation layer is 100 mm.

[0009] In combination with the first aspect, in a possible implementation, the directional solidification device for titanium alloy castings further includes a mold assembly plate and a vibration meter; the mold assembly plate is installed at the bottom of the water-cooled backing plate and is provided with slots corresponding to the fastening components; the vibration meter is installed at the bottom of the mold assembly plate.

[0010] In a second aspect, an embodiment of the present application provides a method for directional solidification of titanium alloy castings, including the directional solidification device for titanium alloy castings in the first aspect or any possible implementation of the first aspect. The method includes:

[0011] S1: Put the components of the prepared mold assembly into a vacuum degassing furnace, first heat it to 400 - 420 °C, keep it warm for 4 - 5 h, then raise the temperature to 900 - 950 °C, keep it warm for 4 - 5 h, the vacuum degree ≤ 100 Pa, and cool it to below 300 °C before taking it out of the furnace;

[0012] S2: After cleaning the residues on the surfaces of the components of the mold assembly, install the lower mold on the top of the water-cooled backing plate, install the core, the upper mold, and the riser in sequence, press the mold assembly through the fastening components, and connect the cooling systems of the water-cooled backing plate and the water-cooled copper crucible of the ingot shell furnace to obtain a first casting mold;

[0013] S3: Put the first casting mold into a preheating furnace, preheat it at 300 - 400 °C, and keep it warm for 2 - 4 hours to obtain a second casting mold;

[0014] S4: Inject titanium alloy melt into the pouring tube of the second casting mold, and demold it after cooling to room temperature with the preheating furnace to obtain a titanium alloy casting.

[0015] Combined with the second aspect, in a possible implementation, before S1, it further includes preparing a coating with a thickness of 0.4 - 0.5 mm on the inner wall of the riser and the inner wall of the upper mold by spraying process, and curing it at room temperature for 2 - 3 h.

[0016] Combined with the second aspect, in a possible implementation, when the upper mold is a split structure, before S3, it further includes wrapping the first refractory heat-insulating layer around the outer wall of the upper mold;

[0017] Filling the second refractory heat-insulating layer into the hollow structure of the core and the hollow sandwich structure respectively;

[0018] Wrapping a plurality of third refractory heat-insulating layers around the outer walls of the corresponding risers.

[0019] Combined with the second aspect, in a possible implementation, during the casting process of S4, start the vibrator at the bottom of the mold assembly plate to make the second casting mold generate micro-vibrations, and the vibrations continue until the titanium alloy melt is completely solidified, for refining the grains and releasing the casting stress.

[0020] One or more technical solutions provided in the embodiments of the present application have at least the following technical effects:

[0021] The directional solidification device for titanium alloy castings provided in the embodiments of the present application includes a mold assembly, a fastening assembly, and a water-cooled backing plate. The directional solidification device of the present application realizes the directional solidification of the titanium alloy melt from bottom to top through the collaborative control of the structured heat conduction design and material properties. The specific process is as follows:

[0022] First, (1) The water-cooled backing plate of the present application is connected to the cooling system of the water-cooled copper crucible of the skull furnace. The internal coiled cooling channels quickly remove heat through forced circulation of the coolant (such as water or ethylene glycol solution), making the temperature at the bottom of the lower mold significantly lower than that at the top. (2) The upper mold, lower mold, core, and riser are all made of graphite. The lower mold forms an efficient heat dissipation layer due to its wall thickness ≥ 50 mm. Utilizing the high thermal conductivity of graphite (thermal conductivity ≥ 100 W / (m·K)), a rapid quenching zone is formed. The wall thickness of the upper mold (10 - 15 mm) and the wall thickness of the riser (≤ 10 mm) are relatively thin, forming a high-temperature zone at the top. The temperature gradient in the vertical direction is 200 - 300 °C / cm (the temperature at the top is high and the temperature at the bottom is low), driving the directional solidification of the titanium alloy melt.

[0023] Second, (1) After the titanium alloy melt is injected into the cavity structure through the casting tube, it contacts the surface of the lower mold graphite at the bottom, and immediate contact chill occurs, forming a fine equiaxed crystal zone; (2) Before solidification, it advances upward at a certain rate, and heat is continuously conducted through the lower mold graphite to the water-cooled backing plate. Columnar crystals preferentially grow along the heat flow direction (vertically upward), inhibiting lateral grain competition, and the proportion of the columnar crystal zone reaches more than 80%; (3) Feeding and exhaust functions of the riser: The riser delays cooling through a thin-wall structure (≤10 mm) and refractory insulation cotton, provides liquid metal compensation for solidification shrinkage, and reduces shrinkage cavities. The top exhaust hole discharges the gas in the melt to prevent gas shrinkage cavities from entering the casting.

[0024] Therefore, through the differential design of the thick-walled lower mold and the thin-walled upper mold / riser in this application, combined with the high thermal conductivity of the graphite material, a stable vertical temperature gradient is constructed to achieve the directional solidification of the titanium alloy melt from bottom to top, reduce internal gas shrinkage cavities, and improve the internal quality of titanium alloy castings. Brief Description of the Drawings

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments of the present application. Obviously, the drawings in the following description are some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0026] Figure 1 It is a schematic structural diagram of a directional solidification device applicable to titanium alloy castings provided by an embodiment of the present application;

[0027] Figure 2 It is a schematic structural diagram of the upper mold provided by an embodiment of the present application;

[0028] Figure 3 It is a schematic structural diagram of the cover body provided by an embodiment of the present application;

[0029] Figure 4 It is a schematic structural diagram of the cooling system connection between the water-cooled backing plate and the water-cooled copper crucible of the skull furnace provided by an embodiment of the present application;

[0030] Figure 5 、 Figure 6 and Figure 7 They are simulation comparison diagrams of Example 1, Example 2, and Example 3.

[0031] Icon: 1 - Mold assembly; 11 - Upper mold; 111 - Shell; 112 - Cover body; 113 - Groove; 114 - Support platform; 12 - Lower mold; 13 - Risers; 14 - Cores; 15 - Casting tube; 2 - Fastening assembly; 3 - Exhaust pipe; 4 - Molding plate; 5 - Cooling system of the water-cooled copper crucible of the skull furnace; 6 - Water-cooled backing plate. Detailed Description of the Embodiments

[0032] The following will clearly and completely describe the technical solutions in the embodiments of the present application with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without making creative efforts fall within the scope of protection of the present application.

[0033] In the description of the embodiments of the present application, it should be noted that the orientation or positional relationship indicated by the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer", etc. is based on the orientation or positional relationship shown in the accompanying drawings. It is only for the convenience of describing the embodiments of the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be construed as a limitation to the present application. The terms "first", "second", "third" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance. In addition, the terms "installation", "connection", "connection" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to specific circumstances.

[0034] The embodiments of the present application provide a directional solidification device applicable to titanium alloy castings, as Figures 1 to 4 shown. The directional solidification device applicable to titanium alloy castings includes a mold assembly 1, a fastening assembly 2, and a water-cooled backing plate 6. The water-cooled backing plate 6 is connected to the cooling system 5 of the water-cooled copper crucible of the skull furnace, and a coiled cooling channel is integrated inside it. The mold assembly 1 includes an upper mold 11, a lower mold 12, a core 14, a casting tube 15, and a plurality of risers 13. The lower mold 12 is installed on the top of the water-cooled backing plate 6, and the upper mold 11 is installed on the side of the lower mold 12 away from the water-cooled backing plate 6, and a plurality of through holes are provided on its side wall and top. A plurality of risers 13 are all connected to the corresponding through holes, and exhaust holes are provided on the side of each riser 13 away from the upper mold 11. The casting tube 15 is fixedly arranged on the side of the upper mold 11 away from the lower mold 12 and is close to the side wall of the upper mold 11. The core 14 is a hollow structure and is arranged in the cavity structure formed by enclosing the upper mold 11, the lower mold 12, and the risers 13 on the top of the upper mold 11. The wall thickness of the upper mold 11 and the wall thickness of the core 14 are smaller than the wall thickness of the lower mold 12, and the wall thickness of the riser 13 is smaller than the wall thickness of the upper mold 11 and the wall thickness of the core 14. The upper mold 11, the lower mold 12, the casting tube 15, the core 14, and the risers 13 are all made of graphite. The fastening assembly 2 is arranged on the circumferential outer side of the mold assembly 1 and is used to compress the mold assembly 1.

[0035] It should be noted that the directional solidification device of the present application achieves the upward directional solidification of the titanium alloy melt through the collaborative control of the structured heat conduction design and material properties. The specific process is as follows:

[0036] First, (1) The water-cooled bottom plate 6 of the present application is connected to the cooling system 5 of the water-cooled copper crucible of the chill casting furnace. The internal coiled cooling channels quickly remove heat through forced circulation of a coolant (such as water or ethylene glycol solution), making the temperature at the bottom of the lower mold 12 significantly lower than that at the top. (2) The upper mold 11, lower mold 12, core 14, and riser 13 are all made of graphite. The lower mold 12 forms an efficient heat dissipation layer due to its wall thickness ≥ 50 mm, and a rapid quenching zone is formed by utilizing the high thermal conductivity of graphite (thermal conductivity ≥ 100 W / (m·K)). The wall thickness of the upper mold 11 (10 - 15 mm) and the wall thickness of the riser 13 (≤ 10 mm) are relatively thin, forming a high-temperature zone at the top. The temperature gradient in the vertical direction is high at the top and low at the bottom, driving the directional solidification of the titanium alloy melt.

[0037] Second, (1) After the titanium alloy melt is injected into the cavity structure through the casting tube 15, it contacts the graphite surface of the lower mold 12 at the bottom and immediately undergoes contact quenching, forming a fine equiaxed crystal zone; (2) Before solidification, it advances upward at a certain rate, and heat is continuously conducted from the graphite of the lower mold 12 to the water-cooled bottom plate 6. Columnar crystals preferentially grow along the heat flow direction (vertically upward), suppressing lateral grain competition, and the proportion of the columnar crystal zone reaches more than 80%; (3) The feeding and exhaust functions of the riser 13: The riser 13 delays cooling through a thin-wall structure (≤ 10 mm), provides liquid metal supplement for solidification shrinkage, and reduces shrinkage cavities. The exhaust hole at the top discharges the gas in the melt, preventing gas shrinkage cavities from entering the casting.

[0038] Therefore, the present application constructs a stable vertical temperature gradient through the differential design of the thick wall of the lower mold 12 and the thin walls of the upper mold 11 / riser 13, combined with the high thermal conductivity of the graphite material, to achieve the upward directional solidification of the titanium alloy melt, reduce internal gas shrinkage cavities, and improve the internal quality of the titanium alloy casting.

[0039] In the embodiment of the present application, the directional solidification device applicable to titanium alloy castings further includes a plurality of exhaust pipes 3. The plurality of exhaust pipes 3 are respectively arranged at the top of the riser 13 on the side wall of the upper mold 11.

[0040] It should be noted that in the initial stage of solidification, the residual gas in the titanium alloy melt, due to density difference and surface tension, preferentially accumulates in the high-temperature zone (the top of the riser 13). The exhaust pipe 3 enables the gas to quickly escape before the solid-liquid interface of the titanium alloy melt advances through negative pressure suction or natural convection (utilizing the temperature difference between the riser 13 and the cavity structure to drive gas flow), preventing it from being captured by the solid-liquid interface to form gas shrinkage cavities.

[0041] In the embodiment of the present application, the upper mold 11 is of a split structure, and the upper mold 11 includes a housing 111 and a cover 112. The top wall of the housing 111 is provided with a plurality of grooves 113 and support platforms 114 arranged in an annular array. The cover 112 is arranged on the top of the support platform 114 and is flush with the top wall of the housing 111. A central hole communicating with the riser 13 at the top of the upper mold 11 is provided in the middle of the cover 112. The cover 112 and the groove 113 form a hollow sandwich structure.

[0042] It should be noted that in the present application, by decoupling the top wall of the traditional integral upper mold 11 into a modular structure of a housing 111 (integrating an annular groove 113 and a support platform 114) and an independent cover 112, a differential thinning design is implemented for the non-load-bearing core area, achieving a significant reduction in the overall heat capacity. The thinned top wall of the upper mold 11 weakens the sensitive response to ambient temperature changes in the initial stage of cooling through a thermal inertia regulation mechanism, effectively suppressing the rapid dissipation of heat to the external environment and avoiding solidification instability caused by sudden changes in local thermal stress. By reducing the system heat capacity, the cooling rate of the high-temperature feeding area (the core area of liquid metal filling and densification) at the top of the upper mold 11 is dynamically regulated, extending the effective filling time window of the titanium alloy melt in the semi-solid state area, providing sufficient thermodynamic conditions for the dynamic feeding during the advancement of the liquid metal at the solid-liquid interface, and finally achieving a reduction in the porosity and an increase in the density of the titanium alloy casting.

[0043] In the embodiment of the present application, the directional solidification device for titanium alloy castings further includes a first refractory insulation layer, a plurality of second refractory insulation layers, and a plurality of third refractory insulation layers. The first refractory insulation layer is coated on the outer wall of the upper mold 11. A plurality of second refractory insulation layers are respectively filled in the hollow structure of the core 14 and the hollow sandwich structure of the upper mold 11. A plurality of third refractory insulation layers are coated on the outer walls of the corresponding risers 13.

[0044] It should be noted that the first refractory insulation layer is coated on the outer wall of the upper mold 11 to form a continuous thermal resistance barrier, effectively blocking the convective heat transfer and radiative heat dissipation between the surface of the device and the external environment. By reducing the radial temperature gradient of the outer wall, the lateral diffusion of heat in the horizontal direction is inhibited, and the directional transfer of heat in the axial path of the riser 13 - casting is strengthened, providing a heat flow driving condition for directional solidification. The second refractory insulation layer filled in the hollow structure of the core 14 delays the cooling rate of the central hot spot area of the casting through local thermal resistance strengthening, making both the center and the outer layer slow down in solidification; the second refractory insulation layer simultaneously filled in the hollow sandwich structure of the upper mold 11 maintains the high-temperature state in the riser 13 area, extends the effective feeding time window of the titanium alloy melt at the end of solidification, ensures sufficient densification of the thick-walled part, and significantly reduces shrinkage cavity / shrinkage porosity defects. The third refractory insulation layer is coated on the outer wall of the riser 13 to form a hot melt layer with high heat storage capacity, delaying the temperature drop rate in the riser 13 area, enabling the titanium alloy melt to achieve directional solidification from the bottom to the top of the casting under thermodynamic drive, and ensuring the orderly growth of columnar crystals along the axis.

[0045] In an embodiment of the present application, the wall thickness of the first refractory insulation layer is 50 mm, and the wall thickness of the third refractory insulation layer is 100 mm. The first refractory insulation layer, the second refractory insulation layer, and the third refractory insulation layer are all zirconium refractory insulation cotton.

[0046] In the embodiment of the present application, the apparatus for directional solidification of titanium alloy castings further includes a mold plate 4 and a vibration meter. The mold plate 4 is installed at the bottom of the water-cooled backing plate 6 and is provided with slots corresponding to the fastening assembly 2. The vibration meter is installed at the bottom of the mold plate 4.

[0047] It should be noted that the mold plate 4 of the present application is connected to the fastening assembly 2 through the bottom slots, accurately fixing the water-cooled backing plate 6 and the mold assembly 1, and eliminating the assembly offset caused by thermal expansion.

[0048] Specifically, the fastening assembly 2 includes a plurality of pressing plates and fastening screws. The plurality of pressing plates are divided into two categories: one category is arranged on the tops of the plurality of risers 13 to apply a downward pressure to the risers 13 to prevent the risers 13 from shifting due to the impact or thermal expansion of the titanium alloy melt during the pouring process; the other category of pressing plates is evenly arranged around the circumference of the lower mold 12 to position and fix the mold assembly 1 from all around, ensuring the overall structural stability of the mold during the pouring and solidification processes.

[0049] Bolt holes are provided on both sides of each pressing plate. The positions and sizes of these bolt holes are precisely designed to ensure matching with the fastening screws. One end of the plurality of fastening screws extends into the corresponding bolt holes of the pressing plates and is further screwed into the corresponding slots pre-set on the mold plate 4. The inside of the slots is provided with a thread structure adapted to the fastening screws. When the fastening screws are screwed into the slots, through the engagement of the threads, the pressing plates are tightly pressed against the mold assembly 1, thereby achieving reliable clamping of the mold assembly 1. This fastening method can not only effectively prevent the mold from loosening or shifting due to the impact force of the titanium alloy melt during the pouring process, but also ensure that the components of the mold are closely fitted together, reducing the risk of titanium alloy melt leakage caused by gaps, ensuring the smooth progress of the directional solidification process of titanium alloy castings, and improving the forming quality and dimensional accuracy of titanium alloy castings.

[0050] The embodiment of the present application provides a method for directional solidification of titanium alloy castings, including the apparatus for directional solidification of titanium alloy castings described above. The method includes:

[0051] S1: Put the components of the prepared mold assembly 1 into a vacuum degassing furnace, first heat it to 400 - 420 °C, keep it warm for 4 - 5 h, then raise the temperature to 900 - 950 °C, keep it warm for 4 - 5 h, the vacuum degree ≤ 100 Pa, and cool it to below 300 °C before taking it out of the furnace.

[0052] It should be noted that, on the one hand, in a vacuum environment (high vacuum degree), through two-stage gradient heating from low temperature to high temperature, the present application realizes the rapid desorption of adsorbed gas on the mold surface and the deep desorption of residual gas in the material pores, avoiding the reaction of gas with the titanium alloy melt to generate inclusions, thereby ensuring the purity of the casting; on the other hand, through stepped heating + long-time heat preservation, the internal thermal stress of the materials of each component of the mold assembly 1 is evenly released, reducing the risk of casting deformation caused by uneven thermal expansion of each component of the mold assembly 1 during the subsequent directional solidification process.

[0053] S2: After cleaning the residues on the surfaces of each component of the mold assembly 1, install the lower mold 12 on the top of the water-cooled backing plate 6, and sequentially install the core 14, the upper mold 11 and the riser 13. Compress the mold assembly 1 through the fastening assembly 2, and connect the cooling system 5 of the water-cooled backing plate 6 and the water-cooled copper crucible of the skull furnace to obtain the first casting mold.

[0054] It should be noted that, in the present application, an air gun is used to blow the floating dust on the surfaces of each component of the mold assembly 1, eliminating the surface defects of the casting caused by impurities, and at the same time avoiding the pollution of the vacuum environment and the purity of the casting by the gas generated by the decomposition of the residues at high temperature.

[0055] S3: Put the first casting mold into a preheating furnace, preheat it at 300 - 400 °C, and keep it warm for 2 - 4 hours to obtain the second casting mold.

[0056] It should be noted that during the low-temperature preheating process of the first casting mold, through creep deformation and microscopic dislocation recombination, the internal stress gradient caused by processing residual stress and material phase change difference is released, avoiding the crack propagation caused by thermal shock during subsequent high-temperature pouring.

[0057] S4: Inject titanium alloy melt into the pouring tube 15 of the second casting mold, and demold after cooling to room temperature with the preheating furnace to obtain a titanium alloy casting.

[0058] In the embodiment of the present application, before S1, it also includes preparing a coating with a thickness of 0.4 - 0.5 mm on the inner wall of the riser 13 and the inner wall of the upper mold 11 by using a spraying process, and curing it for 2 - 3 h at room temperature.

[0059] The coating in the embodiment of the present application is prepared by mixing yttrium oxide powder and zirconium diacetate colloid, and improves the quality of the titanium alloy casting through the following synergistic effects: the incompletely melted yttrium oxide particles in the coating form a micro-protrusion structure, enhancing the spreadability during melt filling and reducing the subsequent machining allowance; the dense coating structure blocks the diffusion path of oxygen molecules, inhibits surface oxidation pollution, reduces the thickness of the oxidation brittle layer, and improves the stability of the surface mechanical properties of the casting; the low thermal conductivity characteristic and high radiation heat dissipation ability of the coating act synergistically to reduce the surface heat load of the casting, inhibit the concentration of thermal stress and the risk of thermal cracking. Since the lower mold 12 needs to accelerate heat dissipation, it does not need to be sprayed with a coating.

[0060] In the embodiment of the present application, when the upper mold 11 is of a split structure, before S3, it further includes wrapping the first refractory insulation layer around the outer wall of the upper mold 11.

[0061] Fill the second refractory insulation layer into the hollow structure of the core 14 and the hollow sandwich structure respectively.

[0062] Wrap a plurality of third refractory insulation layers around the outer walls of the corresponding risers 13.

[0063] In the embodiment of the present application, during the casting process of S4, start the vibrator at the bottom of the mold assembly plate 4 to make the second casting mold vibrate slightly, and the vibration continues until the titanium alloy melt is completely solidified, which is used to refine the grains and release the casting stress.

[0064] The present application designs three different embodiments, and compares the defect conditions of titanium alloy castings under each embodiment through simulation analysis:

[0065] Embodiment 1:

[0066] Adopt conventional bottom gating. The mold assembly 1 includes an upper mold 11, a lower mold 12, a core 14, a casting tube 15 and a plurality of risers 13; the wall thicknesses of the upper mold 11, the lower mold 12 and the risers 13 are all 60 mm, and the core 14 is of a solid structure. Put each component of the prepared mold assembly 1 into a vacuum degassing furnace, first heat it to 400 °C and keep it warm for 4 h, then raise the temperature to 900 °C and keep it warm for 4 h, the vacuum degree ≤ 100 Pa, and cool it to below 300 °C before taking it out of the furnace; after cleaning the residues on the surfaces of each component of the mold assembly 1, press the mold assembly 1 through the fastening assembly 2 to obtain the first casting mold; put the first casting mold into a preheating furnace, preheat it at 400 °C and keep it warm for 3 hours to obtain the second casting mold; inject titanium alloy melt into the casting tube 15 of the second casting mold, and demold it after cooling to room temperature with the preheating furnace to obtain a titanium alloy casting.

[0067] Embodiment 2:

[0068] The water-cooled backing plate 6 is connected to the cooling system 5 of the skull furnace water-cooled copper crucible, and its interior is integrated with a coiled cooling channel;

[0069] The mold assembly 1 includes an upper mold 11, a lower mold 12, a core 14, a casting tube 15, and a plurality of risers 13; the upper mold 11 is installed on the side of the lower mold 12 away from the water-cooled backing plate 6, and a plurality of through holes are provided on its side wall and top; a plurality of risers 13 are all communicated with the corresponding through holes, and exhaust holes are provided on the side away from the upper mold 11; the casting tube 15 is fixedly arranged on the side of the upper mold 11 away from the lower mold 12 and close to the side wall of the upper mold 11; the core 14 is a hollow structure and is arranged in the cavity structure formed by the enclosure of the upper mold 11, the lower mold 12, and the risers 13 on the top of the upper mold 11; wherein, the wall thickness of the upper mold 11 is 12 mm, the wall thickness of the lower mold 12 is 60 mm, and the wall thickness of the riser 13 is 8 mm; the upper mold 11, the lower mold 12, the casting tube 15, the core 14, and the risers 13 are all made of graphite material.

[0070] Put the components of the prepared mold assembly 1 into a vacuum degassing furnace, first heat to 400 °C, keep warm for 4 h, then raise the temperature to 900 °C, keep warm for 4 h, the vacuum degree ≤ 100 Pa, and cool to below 300 °C before taking out of the furnace;

[0071] After cleaning the surface residues of the components of the mold assembly 1, install the lower mold 12 on the top of the water-cooled backing plate 6, install the core 14, the upper mold 11, and the risers 13 in sequence, press the mold assembly 1 through the fastening assembly 2, and connect the cooling system 5 of the water-cooled backing plate 6 and the water-cooled copper crucible of the skull furnace to obtain the first casting mold;

[0072] Put the first casting mold into a preheating furnace, preheat at 400 °C, and keep warm for 3 hours to obtain the second casting mold;

[0073] Inject the titanium alloy melt into the casting tube 15 of the second casting mold, cool in the furnace to room temperature and then demold to obtain the titanium alloy casting.

[0074] Example 3:

[0075] Compared with Example 2, the structure of the upper mold 11 in Example 3 is different. It adopts a split structure. The upper mold 11 includes a shell 111 and a cover 112. The top wall of the shell 111 is provided with a plurality of grooves 113 and support platforms 114 arranged in a circular array; the cover 112 is arranged on the top of the support platform 114 and is flush with the top wall of the shell 111. A central hole communicating with the riser 13 at the top of the upper mold 11 is provided in the middle of the cover 112; the cover 112 and the grooves 113 form a hollow sandwich structure. This split structure is convenient for installing the refractory insulation layer, and at the same time, the hollow sandwich structure also helps to adjust the heat dissipation speed of the upper mold 11. When the upper mold 11 is of a split structure, before putting the first casting mold into the preheating furnace, wrap the first refractory insulation layer around the outer wall of the upper mold 11;

[0076] Fill the second refractory insulation layer into the hollow structure of the core 14 and the hollow sandwich structure respectively;

[0077] Wrap multiple third refractory insulation layers around the outer walls of the corresponding risers 13.

[0078] Prepare titanium alloy castings according to the processes of Example 1, Example 2, and Example 3 respectively, and conduct simulation analysis. The defect situations of each example are as follows:

[0079] After simulation, the defect situation of the process in Example 1 is as Figure 5 shown. The internal gas shrinkage hole defects in the titanium alloy casting are widespread. This is because in the conventional bottom gating and furnace cooling process, the high-temperature melt fills the mold cavity from bottom to top. When the melt fills the cavity, the temperature gradient in the cavity shows a state where the temperature at the top is low and the temperature at the bottom is high. As heat continues to spread, the riser solidifies first, and the feeding channel closes in advance, resulting in a large amount of gas remaining in the casting to form gas shrinkage holes, seriously affecting the quality and performance of the titanium alloy casting.

[0080] After simulation, the defect situation of Example 2 is as Figure 6 shown. After thinning the upper mold 11 and chilling the bottom of the lower mold 12, the defect situation is significantly improved, and the gas shrinkage holes at the riser 13 at the top move upward. The wall thickness of the upper mold 11 is thinned to 12 mm, reducing the chilling effect and the heat dissipation rate of the upper mold 11, forming a reverse temperature gradient with the chilling at the bottom of the lower mold 12, which is beneficial to the sequential solidification of the titanium alloy casting from bottom to top. The setting of the riser 13 and the design of the exhaust holes provide channels for gas discharge and melt feeding, reducing the number of gas shrinkage hole defects inside the titanium alloy casting, and some gas shrinkage holes move towards the riser 13, facilitating the improvement of the quality of the titanium alloy casting by cutting off the riser 13 later.

[0081] After simulation, the defect situation of Example 3 is as Figure 7 shown. Further delay the heat dissipation of the upper mold 11, form a strong temperature gradient with the chilling of the lower mold 12, and achieve sequential solidification from bottom to top, with a further obvious improvement effect. By adopting a split upper mold 11 structure and adding a refractory insulation layer, the heat dissipation rate of the upper mold 11 is effectively delayed, making the chilling effect at the bottom of the lower mold 12 more significant, and a large temperature gradient is formed between the upper mold 11 and the lower mold 12. Under the action of this strong temperature gradient, the titanium alloy casting can achieve sequential solidification from bottom to top. The melt starts to solidify from the bottom and gradually advances upward, and the riser 13 always remains liquid, providing sufficient feeding for the titanium alloy casting, eliminating the gas shrinkage hole defects inside the casting, and significantly improving the quality of the casting.

[0082] The various embodiments in this specification are described in a progressive manner. For the same or similar parts between the various embodiments, reference can be made to each other. The key point of each embodiment is to illustrate the differences from other embodiments.

[0083] The above embodiments are only used to illustrate the technical solutions of the present application, rather than limiting the present application; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the present application.

Claims

1. A directional solidification device applicable to titanium alloy castings, characterized in that, It includes a die assembly (1), a fastening assembly (2) and a water-cooled backing plate (6); The water-cooled backing plate (6) is connected to the cooling system (5) of the water-cooled copper crucible of the skull furnace, and a coiled cooling channel is integrated inside it; The die assembly (1) includes an upper mold (11), a lower mold (12), a core (14), a casting tube (15) and a plurality of risers (13); The lower mold (12) is installed on the top of the water-cooled backing plate (6), and the upper mold (11) is installed on the side of the lower mold (12) away from the water-cooled backing plate (6), and a plurality of through holes are provided on its side wall and top; A plurality of the risers (13) are all connected to the corresponding through holes, and exhaust holes are provided on the side away from the upper mold (11); The casting tube (15) is fixedly arranged on the side of the upper mold (11) away from the lower mold (12) and is close to the side wall of the upper mold (11); The core (14) is of a hollow structure and is arranged in the cavity structure formed by enclosing the upper mold (11), the lower mold (12) and the risers (13) on the top of the upper mold (11); The wall thickness of the upper mold (11) and the wall thickness of the core (14) are smaller than the wall thickness of the lower mold (12), and the wall thickness of the riser (13) is smaller than the wall thickness of the upper mold (11) and the wall thickness of the core (14); The upper mold (11), the lower mold (12), the casting tube (15), the core (14) and the riser (13) are all made of graphite; The fastening assembly (2) is arranged on the circumferential outer side of the die assembly (1) for pressing the die assembly (1).

2. The directional solidification device for titanium alloy castings according to claim 1, wherein It also includes a plurality of exhaust pipes (3); A plurality of the exhaust pipes (3) are respectively arranged on the top of the risers (13) on the side wall of the upper mold (11).

3. The directional solidification device for titanium alloy castings according to claim 1, characterized in that, The upper mold (11) is of a split structure, and the upper mold (11) includes a shell (111) and a cover (112); An annular array of a plurality of grooves (113) and support platforms (114) are provided on the top wall of the shell (111); The cover (112) is arranged on the top of the support platform (114) and is flush with the top wall of the shell (111), and a central hole communicating with the riser (13) on the top of the upper mold (11) is provided in the middle of the cover (112); The cover (112) and the groove (113) form a hollow sandwich structure.

4. The directional solidification device for titanium alloy castings according to claim 3, characterized in that, It also includes a first refractory insulation layer, a plurality of second refractory insulation layers and a plurality of third refractory insulation layers; The first refractory insulation layer is coated on the outer wall of the upper mold (11); A plurality of the second refractory insulation layers are respectively filled in the hollow structure of the core (14) and the hollow sandwich structure of the upper mold (11); A plurality of the third refractory insulation layers are coated on the outer walls of the corresponding risers (13).

5. The directionally solidifying device for titanium alloy castings according to claim 4, characterized in that, The wall thickness of the first refractory insulation layer is 50 mm, and the wall thickness of the third refractory insulation layer is 100 mm.

6. The directional solidification device for titanium alloy castings according to claim 1, characterized in that, It also includes a mold assembly plate (4) and a vibrator; The mold assembly plate (4) is installed on the bottom of the water-cooled backing plate (6) and is provided with slots corresponding to the fastening assembly (2); The vibration meter is installed at the bottom of the modular plate (4).

7. A directional solidification method applicable to titanium alloy castings, characterized in that, Including the directional solidification device for titanium alloy castings according to any one of claims 1-6, the method comprising: S1: Put the components of the prepared mold assembly (1) into a vacuum degassing furnace, first heat to 400-420 °C, keep warm for 4-5 h, then raise the temperature to 900-950 °C, keep warm for 4-5 h, the vacuum degree ≤ 100 Pa, cool to below 300 °C and then take out of the furnace; S2: After cleaning the residues on the surfaces of the components of the mold assembly (1), install the lower mold (12) on the top of the water-cooled backing plate (6), sequentially install the core (14), the upper mold (11) and the riser (13), press the mold assembly (1) through the fastening assembly (2), and connect the cooling system (5) of the water-cooled backing plate (6) and the water-cooled copper crucible of the skull furnace to obtain the first casting mold; S3: Put the first casting mold into a preheating furnace, preheat at 300-400 °C, and keep warm for 2-4 hours to obtain the second casting mold; S4: Inject the titanium alloy melt into the pouring tube (15) of the second casting mold, demold after cooling to room temperature with the preheating furnace to obtain the titanium alloy casting.

8. The directional solidification method for titanium alloy castings according to claim 7, characterized in that, Before S1, it also includes preparing a coating with a thickness of 0.4-0.5 mm on the inner walls of the riser (13) and the upper mold (11) by a spraying process, and curing at room temperature for 2-3 h.

9. The directionally solidified method for titanium alloy castings according to claim 8, characterized in that, When the upper mold (11) is of a split structure, before S3, it also includes wrapping the first refractory insulation layer around the outer wall of the upper mold (11); Filling the second refractory insulation layer into the hollow structure and the hollow sandwich structure of the core (14) respectively; Wrapping a plurality of third refractory insulation layers around the outer walls of the corresponding risers (13).

10. The directionally solidifying method for titanium alloy castings according to claim 8, characterized in that, During the casting process in S4, start the vibration meter at the bottom of the modular plate (4) to make the second casting mold generate micro-vibrations, and the vibration continues until the titanium alloy melt is completely solidified, which is used to refine the grains and release the casting stress.

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