Thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings and its preparation method

Abstract

The present invention discloses a thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings and a preparation method thereof. The thin-walled graphite mold shell includes an inner mold and an outer mold, a cavity is formed between the inner mold and the outer mold, and the mold shell formed by the inner mold and the outer mold is a thin-walled conformal structure. The preparation method includes the following steps: designing the structure and wall thickness of the thin-walled graphite mold shell; manufacturing the thin-walled graphite mold shell assembly by mechanical processing; performing vacuum degassing treatment on the thin-walled graphite mold shell assembly; assembling the thin-walled graphite mold shell assembly; preheating the whole thin-walled graphite mold shell; placing the whole thin-walled graphite mold shell into a vacuum consumable arc furnace or a vacuum induction furnace, and then performing pouring and filling of titanium and titanium alloy; disassembling the thin-walled graphite mold shell assembly and cleaning it. The present invention makes the casting of titanium alloy easy to implement and convenient to operate, can be applied to the pouring of refractory alloys and alloys with poor fluidity, and can also be applied to the pouring of castings with requirements for refining grains in local tissue properties.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal material casting, and particularly relates to a thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings and a preparation method thereof. Background Art

[0002] As is well known, titanium alloys have become indispensable advanced structural materials in modern industry due to their excellent comprehensive properties. However, titanium alloys themselves have high chemical activity, which makes it difficult to form them under traditional processing methods, especially for some complex components with thin-walled features.

[0003] The casting process is a relatively conventional method for manufacturing titanium alloy castings. Currently, the main casting process is the investment casting technology of near-net-shape forming process, which has also achieved the most remarkable results in the world. Investment casting is a method of obtaining a mold shell by using soluble primary mold material and pouring it into a part. Investment casting has the following advantages: low surface roughness of the casting (Ra = 1.6 - 3.2 μm), accurate dimensions of the casting, high utilization rate of raw materials (70 - 90%), not limited by the type of alloy, and can cast various complex-structured castings, etc. The investment casting technology of titanium alloys has developed into one of the irreplaceable basic processes in the world manufacturing field.

[0004] However, with the continuous improvement of the requirements for aerospace castings, the disadvantages of the investment casting process have become increasingly prominent: (1) The development cycle is long. From mold processing to mold shell preparation, and then to pouring and other processes, small castings require 30 - 40 working days, and large castings require 60 - 90 working days, especially the mold shell preparation process (20 days) is essential; (2) There are many processes, with up to 20 large and small processes, which will lead to unstable quality of the castings, especially large dimensional fluctuations, and the quality of the mold shell will also fluctuate with changes in seasons, climate, temperature and humidity; (3) Usually, the binder will pollute the environment. Therefore, there is an urgent need to develop a mold shell for manufacturing titanium and titanium alloy castings and a preparation method thereof.

[0005] The invention patent with the publication number of CN102974761A discloses a preparation method of a calcium zirconate mold shell for investment precision casting of titanium and titanium aluminide-based alloys, which includes the following steps: pouring calcium zirconate powder into zirconium sol and mixing thoroughly, adding a wetting agent and a sintering aid and mixing evenly to make an inert isolation layer slurry, coating the slurry on a wax pattern, and sprinkling calcium zirconate sand to form an inert isolation layer of the mold shell; mixing bauxite powder or coal gangue powder with silica sol evenly to make a backing layer strengthening layer slurry, coating the slurry, and sprinkling bauxite or coal gangue sand to form a backing layer strengthening layer of the mold shell; removing the wax pattern, and performing high-temperature sintering. This technical solution adopts the traditional investment precision casting process. As is well known, the investment precision casting process has the above-mentioned multiple disadvantages. Moreover, for titanium and titanium aluminide-based alloys, when using the traditional investment precision casting process, the forming effect is poor, and it is even very difficult to form.

[0006] The invention patent with the publication number of CN114535504A discloses a titanium alloy casting, a mold shell and a mold shell preparation method. The mold shell preparation method includes: making a support core and a central core; making a gating system wax pattern with an internal central core; making a structural part wax pattern and a riser wax pattern; connecting the gating system wax pattern and the riser wax pattern to both sides of the structural part wax pattern, connecting the support core to the side of the riser wax pattern away from the structural part wax pattern, and connecting the support core and the central core to obtain an integral wax pattern; performing mold shell preparation on the integral wax pattern, and performing a dewaxing process on the wax pattern after the preparation is completed to obtain a mold shell of the titanium alloy casting. This technical solution also adopts the traditional investment precision casting process. However, the investment precision casting process has many disadvantages, and it is very difficult to form for titanium alloy casting. Summary of the Invention

[0007] To solve the problems existing in the prior art, the present invention provides a thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings, including an inner mold and an outer mold. A casting cavity is formed between the inner mold and the outer mold, and the mold shell formed by the inner mold and the outer mold is a thin-walled conformal structure.

[0008] The present invention adopts a thin-walled conformal structure. The wall thickness of the graphite mold shell varies according to the structure and size of the casting. For castings with small size and complex structure, the wall thickness of the graphite mold shell is thinner, and for castings with large size, the wall thickness of the graphite mold shell is thicker. In addition, the graphite mold shell can have an equal wall thickness or an unequal wall thickness. The specific wall thickness is determined according to the actual pouring situation or through multiple repeated tests. Finally, by adjusting the thickness of the graphite mold shell, the effect that the mold shell does not crack during the pouring of liquid titanium and the mold shell collapses during the solidification of liquid titanium is achieved.

[0009] In the present invention, in order to control the collapsibility of the graphite mold shell, for the parts with relatively large casting stress, the corresponding inner mold and / or outer mold can be interrupted, leaving a gap of 1-2 mm at the fracture, and the interrupted part is fixed with a positioning pin made of a metal material (such as a low-melting-point alloy material like aluminum alloy or copper alloy). When pouring liquid titanium, as the temperature rises, when the temperature reaches the melting point of the metal material, the positioning pin melts, and the modules of the inner mold and / or outer mold at the interrupted part move to provide collapsibility. Since graphite itself has no collapsibility, the collapsibility of the graphite mold shell is controlled by interrupting the inner mold and / or outer mold, enabling better filling of the alloy liquid and reducing casting stress and defects.

[0010] Preferably, an upper fixing plate is provided above the mold shell, a lower fixing plate is provided below the mold shell, and a reinforcing rib plate is provided between the upper fixing plate and the lower fixing plate.

[0011] In any of the above solutions, preferably, the materials of the inner mold and the outer mold are both graphite, and the materials of the upper fixing plate, the lower fixing plate, and the reinforcing rib plate are graphite and / or stainless steel. In the present invention, the inner mold and the outer mold are made of graphite because graphite only reacts weakly with liquid titanium at the pouring temperature of titanium and titanium alloys, and the thickness of its contamination layer is less than that of the contamination layer in the conventional yttrium oxide investment casting process. The graphite mold shell is not easily peeled off, and the internal inclusion defects are fewer than the inclusion defects in the contamination layer of the conventional yttrium oxide investment casting process.

[0012] In any of the above solutions, preferably, heating electrodes are provided on the upper fixing plate and the lower fixing plate.

[0013] In any of the above solutions, preferably, cast iron cooling blocks are provided on the inner mold and / or the outer mold.

[0014] For manufacturing conventional titanium alloy castings: The thin-walled graphite mold shell includes an inner mold and an outer mold. An upper fixing plate is provided above the mold shell, a lower fixing plate is provided below the mold shell, and a reinforcing rib plate is provided between the upper fixing plate and the lower fixing plate.

[0015] For manufacturing titanium-aluminum alloy castings or alloy castings with poor fluidity: The thin-walled graphite mold shell includes an inner mold and an outer mold. An upper fixing plate is provided above the mold shell, a lower fixing plate is provided below the mold shell, and a reinforcing rib plate is provided between the upper fixing plate and the lower fixing plate. At the same time, heating electrodes need to be provided on the upper fixing plate and the lower fixing plate.

[0016] For castings that require regulation of local tissues: The thin-walled graphite shell mold includes an inner mold and an outer mold. An upper fixing plate is provided above the shell mold, a lower fixing plate is provided below the shell mold, and a reinforcing rib plate is provided between the upper fixing plate and the lower fixing plate. At the same time, cast iron condensation blocks need to be provided on the inner mold and / or the outer mold. In actual operation, the shell mold on the inner mold and / or the outer mold corresponding to the part where the local tissue needs to be regulated is removed, and then cast iron condensation blocks of corresponding shapes are supplemented at the removed positions, and finally, sealing treatment is done.

[0017] The present invention also provides a preparation method for a thin-walled graphite shell mold for manufacturing titanium and titanium alloy castings, which is used to prepare the thin-walled graphite shell mold for manufacturing titanium and titanium alloy castings described in any one of the above, and successively includes the following steps:

[0018] Step 1: Design the structure and wall thickness of the thin-walled graphite shell mold according to the structure and size of the titanium and titanium alloy casting to be manufactured.

[0019] Step 2: Use mechanical processing methods to manufacture the thin-walled graphite shell mold components.

[0020] Step 3: Perform vacuum degassing treatment on the thin-walled graphite shell mold components.

[0021] Step 4: After the vacuum degassing treatment is completed, assemble the thin-walled graphite shell mold components according to the design drawings.

[0022] Step 5: After the assembly is completed, perform preheating treatment on the whole thin-walled graphite shell mold.

[0023] Step 6: After the preheating treatment is completed, place the whole thin-walled graphite shell mold into a vacuum consumable arc furnace or a vacuum induction furnace, and then perform pouring and filling of titanium and titanium alloy.

[0024] Step 7: After the pouring and filling are completed, disassemble the thin-walled graphite shell mold components and clean the thin-walled graphite shell mold.

[0025] Preferably, in Step 3, the vacuum degassing treatment process for the thin-walled graphite shell mold components is: heating temperature 920 - 980 °C, heat preservation time 2 - 5 h, vacuum degree not higher than 0.02 MPa, and cooling with the furnace after the heat preservation ends.

[0026] Preferably, in any of the above solutions, in Step 5, the preheating treatment process for the whole thin-walled graphite shell mold is: heating temperature 280 - 320 °C, heat preservation time 1 - 4 h, and cooling with the furnace after the heat preservation ends.

[0027] Preferably, in any of the above solutions, in step six, for titanium-aluminum alloy castings or alloy castings with poor fluidity, the entire thin-walled graphite shell needs to be heated, heated at a heating rate of 4-8 °C / min to 600-800 °C, and held for 20-30 min; after the holding is completed, the power is cut off, and then the alloy liquid is poured into the thin-walled graphite shell; after pouring and filling are completed, it is cooled at a cooling rate of 1-3 °C / min to 280-300 °C, and taken out of the furnace for air cooling.

[0028] In step six, for conventional titanium alloy castings, after the entire thin-walled graphite shell is preheated, it is placed in a vacuum consumable arc furnace or a vacuum induction furnace, and then the pouring and filling of titanium and titanium alloys are carried out. There are no special requirements for the heating process of the thin-walled graphite shell; the pouring process of the alloy can follow the traditional process system and there are no special requirements either.

[0029] In step six, for castings whose local structure needs to be regulated, after the entire thin-walled graphite shell is preheated, it is placed in a vacuum consumable arc furnace or a vacuum induction furnace, and then the pouring and filling of titanium and titanium alloys are carried out. There are no special requirements for the heating process of the thin-walled graphite shell; the pouring process of the alloy can follow the traditional process system and there are no special requirements either.

[0030] The thin-walled graphite shell for manufacturing titanium and titanium alloy castings and its preparation method of the present invention break through the technical barrier of preparing ceramic shells in the investment casting of titanium and titanium alloys, simplify the process flow, reduce the personnel and site costs, make the casting of titanium alloys easy to achieve and convenient to operate. The present invention can be applied to the pouring of refractory alloys and alloys with poor fluidity, and can also be applied to the pouring of castings with the need to refine the grain of local tissue properties, as well as the conventional casting of other metal alloys.

[0031] The thin-walled graphite shell for manufacturing titanium and titanium alloy castings and its preparation method of the present invention have the following beneficial effects:

[0032] (1) By adjusting the thickness of the graphite shell, the effect that the shell does not crack during the pouring of liquid titanium and cracks during the solidification of liquid titanium is achieved.

[0033] (2) For the parts with large casting stress, the corresponding inner mold and / or outer mold can be interrupted, and positioning pins are used to fix the interrupted parts to control the yieldability of the thin-walled graphite shell.

[0034] (3) For titanium-aluminum alloy castings or alloy castings with poor fluidity, the heating electrode is used to conduct electricity to heat the thin-walled graphite shell and improve the fluidity of the metal.

[0035] (4) For castings whose local structure needs to be regulated, the local structure regulation of the casting is realized by embedding cast iron cooling blocks on the inner mold and / or outer mold.

[0036] (5) Locally thin the parts with large shrinkage stress to achieve directional release of internal stress in the casting.

[0037] (6) The thin-walled graphite shell mold adopts a form assembled by multiple components, which is convenient for assembly and disassembly, and also convenient for cleaning the shell mold.

[0038] (7) The graphite shell mold is thin-walled instead of solid and integral, with high machining accuracy and short batch machining cycle, suitable for mass production. Description of the Drawings

[0039] Figure 1 Schematic diagram of the overall structure of the thin-walled graphite shell mold in a preferred embodiment of the thin-walled graphite shell mold and its preparation method for manufacturing titanium and titanium alloy castings according to the present invention;

[0040] Figure 2 is Figure 1 Schematic diagram of the internal structure of the thin-walled graphite shell mold in the illustrated embodiment;

[0041] Figure 3 is Figure 1 Schematic diagram of the structure of the positioning pin in the illustrated embodiment.

[0042] Reference numerals in the figures: 1 - inner mold, 2 - outer mold, 3 - casting cavity, 4 - upper fixing plate, 5 - lower fixing plate, 6 - reinforcing rib plate, 7 - positioning pin. Detailed Description of the Invention

[0043] In order to further understand the content of the present invention, the present invention will be elaborated in detail below in conjunction with specific embodiments.

[0044] Embodiment 1:

[0045] This embodiment is applicable to manufacturing conventional titanium alloy castings. As Figure 1-2 shown, in a preferred embodiment of the thin-walled graphite shell mold for manufacturing titanium and titanium alloy castings according to the present invention, it includes an inner mold 1 and an outer mold 2, a casting cavity 3 is formed between the inner mold 1 and the outer mold 2, and the shell mold formed by the inner mold 1 and the outer mold 2 is a thin-walled conformal structure. An upper fixing plate 4 is arranged above the shell mold, a lower fixing plate 5 is arranged below the shell mold, and a reinforcing rib plate 6 is arranged between the upper fixing plate 4 and the lower fixing plate 5.

[0046] This embodiment adopts a thin-walled conformal structure. The wall thickness of the graphite mold shell varies according to the structure and size of the casting. For small-sized and complex-structured castings, the wall thickness of the graphite mold shell is thinner, and for large-sized castings, the wall thickness of the graphite mold shell is thicker. The specific wall thickness is determined according to the actual pouring situation or through multiple repeated tests. Finally, by adjusting the thickness of the graphite mold shell, the effect that the mold shell does not crack during the pouring of liquid titanium and the mold shell collapses during the solidification of liquid titanium is achieved.

[0047] In this embodiment, in order to control the collapsibility of the graphite mold shell, for the parts with large casting stresses, the corresponding inner mold and / or outer mold can be interrupted, leaving a 1-2 mm gap at the fracture. The positioning pins 7 made of metal materials (such as low-melting-point alloy materials like aluminum alloy and copper alloy) are used to fix the interrupted parts. The specific structure of the positioning pins is as Figure 3 shown. When pouring liquid titanium, as the temperature rises, when the temperature reaches the melting point of the metal material, the positioning pins melt, and the modules of the inner mold and / or outer mold at the interrupted parts move to make a concession. Since graphite itself has no collapsibility, the collapsibility of the graphite mold shell is controlled by interrupting the inner mold and / or outer mold, enabling better filling of the alloy liquid and reducing casting stresses and defects.

[0048] In this embodiment, the materials of the inner mold and the outer mold are both graphite, and the materials of the upper fixing plate, lower fixing plate, and reinforcing rib plate are graphite and / or stainless steel. Using graphite to make the inner mold and outer mold, because graphite only reacts weakly with liquid titanium at the pouring temperature of titanium and its alloys, the thickness of its contamination layer is less than that of the contamination layer in the conventional yttrium oxide investment casting process. The graphite mold shell is not easily peeled off, and the internal inclusion defects are fewer than the inclusion defects in the contamination layer of the conventional yttrium oxide investment casting process.

[0049] This embodiment also provides a preparation method for a thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings, which includes the following steps in sequence:

[0050] Step 1: Design the structure and wall thickness of the thin-walled graphite mold shell according to the structure and size of the titanium and titanium alloy castings to be manufactured;

[0051] Step 2: Use a machining method to make the thin-walled graphite mold shell components;

[0052] Step 3: Perform vacuum degassing treatment on the thin-walled graphite mold shell components;

[0053] Step 4: After the vacuum degassing treatment is completed, assemble the thin-walled graphite mold shell components according to the design drawings;

[0054] Step 5: After the assembly is completed, perform preheating treatment on the entire thin-walled graphite mold shell;

[0055] Step 6: After the preheating treatment is completed, the thin-walled graphite shell is placed as a whole into a vacuum consumable arc furnace or a vacuum induction furnace, and then titanium and titanium alloys are poured and filled.

[0056] Step 7: After pouring and filling are completed, the thin-walled graphite shell assembly is disassembled, and the thin-walled graphite shell is cleaned.

[0057] In Step 3, the vacuum degassing treatment process of the thin-walled graphite shell assembly is as follows: heating temperature is 950 °C, holding time is 3.5 h, vacuum degree is not higher than 0.02 MPa, and it is cooled in the furnace after the holding ends.

[0058] In Step 5, the preheating treatment process of the whole thin-walled graphite shell is as follows: heating temperature is 300 °C, holding time is 2.5 h, and it is cooled in the furnace after the holding ends.

[0059] In Step 6, for conventional titanium alloy castings, after the whole thin-walled graphite shell is preheated, it is placed into a vacuum consumable arc furnace or a vacuum induction furnace, and then titanium and titanium alloys are poured and filled. There are no special requirements for the heating process of the thin-walled graphite shell; the pouring process of the alloy can follow the traditional process system and there are no special requirements either.

[0060] The thin-walled graphite shell for manufacturing titanium and titanium alloy castings and its preparation method in this embodiment break through the technical barrier of preparing ceramic shells in titanium and titanium alloy investment casting, simplify the process flow, reduce personnel and site costs, make the casting of titanium alloy easy to achieve and convenient to operate. It has the following beneficial effects: (1) By adjusting the thickness of the graphite shell, the effect that the shell does not crack during liquid titanium pouring and cracks during liquid titanium solidification is achieved; (2) For parts with relatively large casting stress, interruption treatment can be carried out on the corresponding inner mold and / or outer mold, and positioning pins are used to fix the interrupted parts to control the yieldability of the thin-walled graphite shell; (3) The thin-walled graphite shell adopts the form of multiple components assembled together, which is convenient for assembly and disassembly and also for cleaning the shell; (4) The graphite shell is thin-walled instead of a solid whole, with high machining accuracy and short batch processing cycle, suitable for batch production.

[0061] Example 2:

[0062] This embodiment is applicable to the manufacture of castings where local tissue needs to be regulated. The specific structure, principle, materials used for making each component, beneficial effects, etc. of the thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings are basically the same as those in Embodiment 1: The thin-walled graphite mold shell includes an inner mold and an outer mold. An upper fixing plate is arranged above the mold shell, a lower fixing plate is arranged below the mold shell, and a reinforcing rib plate is arranged between the upper fixing plate and the lower fixing plate. At the same time, cast iron condensation blocks need to be arranged on the inner mold and / or the outer mold. In actual operation, the mold shell on the inner mold and / or the outer mold corresponding to the part where local tissue needs to be regulated is removed, and then cast iron condensation blocks of corresponding shapes are supplemented or inlaid at the removed positions, and finally, sealing treatment is done. The specific wall thickness of the graphite mold shell is determined according to the actual pouring situation or through multiple repeated tests. Finally, by adjusting the thickness of the graphite mold shell, the effect that the mold shell does not break during the pouring of liquid titanium and the mold shell collapses during the solidification of liquid titanium is achieved.

[0063] To control the collapsibility of the graphite mold shell, for the parts of the casting with relatively large stress, the corresponding inner mold and / or outer mold can be interrupted, leaving a 1 - 2 mm gap at the fracture, and the interrupted parts are fixed with positioning pins made of metal materials (such as low melting point alloy materials like aluminum alloy and copper alloy). When pouring liquid titanium, as the temperature rises, when the temperature reaches the melting point of the metal material, the positioning pins melt, and the modules of the inner mold and / or outer mold at the interrupted parts move to make a concession. Because graphite itself has no collapsibility, the collapsibility of the graphite mold shell is controlled by interrupting the inner mold and / or the outer mold, so that the alloy liquid fills the mold better, reducing casting stress and defects.

[0064] This embodiment also provides a preparation method for a thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings, which includes the following steps in sequence:

[0065] Step 1: Design the structure and wall thickness of the thin-walled graphite mold shell according to the structure and size of the titanium and titanium alloy castings to be manufactured.

[0066] Step 2: Use mechanical processing methods to make the components of the thin-walled graphite mold shell.

[0067] Step 3: Conduct vacuum degassing treatment on the components of the thin-walled graphite mold shell.

[0068] Step 4: After the vacuum degassing treatment is completed, assemble the components of the thin-walled graphite mold shell according to the design drawings.

[0069] Step 5: After the assembly is completed, conduct preheating treatment on the whole thin-walled graphite mold shell.

[0070] Step 6: After the preheating treatment is completed, put the whole thin-walled graphite mold shell into a vacuum consumable arc furnace or a vacuum induction furnace, and then conduct the pouring and filling of titanium and titanium alloy.

[0071] Step 7: After pouring and filling are completed, disassemble the thin-walled graphite shell assembly and clean the thin-walled graphite shell.

[0072] In Step 3, the vacuum degassing treatment process for the thin-walled graphite shell assembly is as follows: heating temperature is 920 °C, holding time is 5 h, vacuum degree is not higher than 0.02 MPa, and after holding, it is cooled in the furnace.

[0073] In Step 5, the preheating treatment process for the entire thin-walled graphite shell is as follows: heating temperature is 280 °C, holding time is 4 h, and after holding, it is cooled in the furnace.

[0074] In Step 6, for castings whose local structure needs to be regulated, after the entire thin-walled graphite shell is preheated, place it in a vacuum consumable arc furnace or a vacuum induction furnace, and then carry out pouring and filling of titanium and titanium alloys. There are no special requirements for the heating process of the thin-walled graphite shell; the pouring process of the alloy can follow the traditional process system and there are no special requirements either.

[0075] The thin-walled graphite shell for manufacturing titanium and titanium alloy castings and its preparation method in this embodiment break through the technical barrier of preparing ceramic shells in titanium and titanium alloy investment casting, simplify the process flow, reduce personnel and site costs, make the casting of titanium alloy easy to achieve and convenient to operate. It has the following beneficial effects: (1) By adjusting the thickness of the graphite shell, the effect that the shell does not break during liquid titanium pouring and breaks during liquid titanium solidification is achieved; (2) For parts with large casting stress, the corresponding inner mold and / or outer mold can be interrupted, and positioning pins are used to fix the interrupted parts to control the yieldability of the thin-walled graphite shell; (3) For castings whose local structure needs to be regulated, the local structure of the casting is regulated by inlaying cast iron cooling blocks on the inner mold and / or outer mold; (4) The thin-walled graphite shell is assembled in the form of multiple components, which is convenient for assembly and disassembly and also convenient for cleaning the shell; (5) The graphite shell is thin-walled instead of a solid whole, with high machining accuracy and short batch processing cycle, suitable for mass production.

[0076] Example 3:

[0077] This embodiment is applicable to the manufacture of titanium aluminide castings or alloy castings with poor fluidity. The specific structure, principle, materials used for making each component, beneficial effects, etc. of the thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings are basically the same as those in Embodiment 1: The thin-walled graphite mold shell includes an inner mold and an outer mold. An upper fixing plate is arranged above the mold shell, a lower fixing plate is arranged below the mold shell, and a reinforcing rib plate is arranged between the upper fixing plate and the lower fixing plate. At the same time, heating electrodes need to be arranged on the upper fixing plate and the lower fixing plate. The specific wall thickness of the graphite mold shell is determined according to the actual pouring situation or through multiple repeated tests. Finally, by adjusting the thickness of the graphite mold shell, the effect that the mold shell does not crack during the pouring of liquid titanium and the mold shell collapses during the solidification of liquid titanium is achieved.

[0078] In order to control the collapsibility of the graphite mold shell, for the parts with large casting stress, the corresponding inner mold and / or outer mold can be interrupted, leaving a gap of 1-2 mm at the fracture. A positioning pin made of a metal material (such as a low melting point alloy material like aluminum alloy or copper alloy) is used to fix the interrupted part. When pouring liquid titanium, as the temperature rises, when the temperature reaches the melting point of the metal material, the positioning pin melts, and the modules of the inner mold and / or outer mold at the interrupted part move to make a concession. Since graphite itself has no collapsibility, the collapsibility of the graphite mold shell is controlled by interrupting the inner mold and / or outer mold, so that the alloy liquid fills the mold better, reducing casting stress and defects.

[0079] This embodiment also provides a preparation method for a thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings, which includes the following steps in sequence:

[0080] Step 1: Design the structure and wall thickness of the thin-walled graphite mold shell according to the structure and size of the titanium and titanium alloy castings to be manufactured.

[0081] Step 2: Use a machining method to make the thin-walled graphite mold shell components.

[0082] Step 3: Perform vacuum degassing treatment on the thin-walled graphite mold shell components.

[0083] Step 4: After the vacuum degassing treatment is completed, assemble the thin-walled graphite mold shell components according to the design drawings.

[0084] Step 5: After the assembly is completed, preheat the whole thin-walled graphite mold shell.

[0085] Step 6: After the preheating treatment is completed, put the whole thin-walled graphite mold shell into a vacuum consumable arc furnace or a vacuum induction furnace, and then carry out the pouring and filling of titanium and titanium alloy.

[0086] Step 7: After the pouring and filling are completed, disassemble the thin-walled graphite mold shell components and clean the thin-walled graphite mold shell.

[0087] In Step 3, the vacuum degassing treatment process for the thin-walled graphite shell assembly is as follows: the heating temperature is 980°C, the holding time is 2 h, the vacuum degree is not higher than 0.02 MPa, and after the holding is completed, it is cooled in the furnace.

[0088] In Step 5, the preheating treatment process for the entire thin-walled graphite shell is as follows: the heating temperature is 320°C, the holding time is 1 h, and after the holding is completed, it is cooled in the furnace.

[0089] In Step 6, for titanium-aluminum alloy castings or alloy castings with poor fluidity, the entire thin-walled graphite shell needs to be heated, heated to 700°C at a heating rate of 6°C / min, and held for 25 min; after the holding is completed, the power is cut off, and then alloy liquid is poured into the thin-walled graphite shell; after pouring and filling are completed, it is cooled to 290°C at a cooling rate of 2°C / min and taken out of the furnace for air cooling. The pouring process of the alloy can follow the traditional process system without special requirements.

[0090] The thin-walled graphite shell for manufacturing titanium and titanium alloy castings and its preparation method in this embodiment break through the technical barrier of preparing ceramic shells in titanium and titanium alloy investment casting, simplify the process flow, reduce personnel and site costs, make the casting of titanium alloy easy to achieve and convenient to operate. It has the following beneficial effects: (1) By adjusting the thickness of the graphite shell, the effect that the shell does not crack during liquid titanium pouring and cracks during liquid titanium solidification is achieved; (2) For parts with large casting stress, the corresponding inner mold and / or outer mold can be interrupted, and positioning pins are used to fix the interrupted parts to control the yieldability of the thin-walled graphite shell; (3) For titanium-aluminum alloy castings or alloy castings with poor fluidity, heating electrodes are used to energize to heat the thin-walled graphite shell and improve the fluidity of the metal; (4) The thin-walled graphite shell adopts the form of multiple components assembled together, which is convenient for assembly and disassembly and also convenient for cleaning the shell; (5) The graphite shell is thin-walled instead of a solid whole, with high machining accuracy and short batch processing cycle, suitable for batch production.

[0091] Example 4:

[0092] This embodiment is applicable to manufacturing titanium-aluminum alloy castings or alloy castings with poor fluidity. The specific structure, preparation method, principle, materials used for making each component, beneficial effects, etc. of the thin-walled graphite shell for manufacturing titanium and titanium alloy castings are basically the same as those in Example 3, except for the process parameters in the preparation method of the thin-walled graphite shell, which are specifically as follows:

[0093] In Step 3, the vacuum degassing treatment process for the thin-walled graphite shell assembly is as follows: the heating temperature is 970°C, the holding time is 4 h, the vacuum degree is not higher than 0.02 MPa, and after the holding is completed, it is cooled in the furnace.

[0094] In Step 5, the preheating treatment process for the overall thin-walled graphite shell is as follows: heating temperature is 310°C, holding time is 2 h, and after the holding ends, it is cooled in the furnace.

[0095] In Step 6, for titanium-aluminum alloy castings or alloy castings with poor fluidity, the overall thin-walled graphite shell needs to be heated, heated to 600°C at a heating rate of 4°C / min, and held for 30 min; after the holding ends, power is cut off, and then alloy liquid is poured into the thin-walled graphite shell; after pouring and filling are completed, it is cooled to 280°C at a cooling rate of 1°C / min and taken out of the furnace for air cooling.

[0096] Example 5:

[0097] This example is applicable to manufacturing titanium-aluminum alloy castings or alloy castings with poor fluidity. The specific structure, preparation method, principle, materials used for making each component, beneficial effects, etc. of the thin-walled graphite shell required for manufacturing titanium and titanium alloy castings are basically the same as those in Example 3. The difference lies in the process parameters in the preparation method of the thin-walled graphite shell, which are as follows:

[0098] In Step 3, the vacuum degassing treatment process for the thin-walled graphite shell components is as follows: heating temperature is 940°C, holding time is 3 h, vacuum degree is not higher than 0.02 MPa, and after the holding ends, it is cooled in the furnace.

[0099] In Step 5, the preheating treatment process for the overall thin-walled graphite shell is as follows: heating temperature is 290°C, holding time is 3 h, and after the holding ends, it is cooled in the furnace.

[0100] In Step 6, for titanium-aluminum alloy castings or alloy castings with poor fluidity, the overall thin-walled graphite shell needs to be heated, heated to 800°C at a heating rate of 8°C / min, and held for 20 min; after the holding ends, power is cut off, and then alloy liquid is poured into the thin-walled graphite shell; after pouring and filling are completed, it is cooled to 300°C at a cooling rate of 3°C / min and taken out of the furnace for air cooling.

[0101] Special note: Many parameters are involved in the technical solution of the present invention. The synergistic effects among various parameters need to be comprehensively considered to obtain the beneficial effects and remarkable progress of the present invention. Moreover, the value ranges of various parameters in the technical solution are obtained through a large number of experiments. For each parameter and the combination of various parameters, the inventor has recorded a large amount of experimental data. Due to space limitations, the specific experimental data are not disclosed here.

[0102] Those skilled in the art can easily understand that the thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings and its preparation method according to the present invention include any combination of the above-mentioned invention content, specific implementation manner parts of the specification of the present invention and each part shown in the drawings. Due to space limitations and to make the specification concise, the various solutions formed by these combinations are not described one by one. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings, comprising an inner mold and an outer mold, with a casting cavity formed between the inner mold and the outer mold. Characterized in that: The mold shell formed by the inner mold and the outer mold is a thin-walled conforming structure; An upper fixing plate is arranged above the mold shell, a lower fixing plate is arranged below the mold shell, and a reinforcing rib plate is arranged between the upper fixing plate and the lower fixing plate; the materials of both the inner mold and the outer mold are graphite, and the materials of the upper fixing plate, the lower fixing plate, and the reinforcing rib plate are graphite or stainless steel; heating electrodes are arranged on the upper fixing plate and the lower fixing plate, and cast iron condensation blocks are arranged on the inner mold and / or the outer mold; For the parts of the casting with large stress, the corresponding inner mold and / or outer mold are interrupted, a gap of 1-2 mm is left at the fracture, and positioning pins made of low-melting alloy materials are used to fix the interrupted parts; When pouring liquid titanium, the temperature rises. When the temperature reaches the melting point of the low-melting alloy material, the positioning pins melt, and the modules of the inner mold and / or the outer mold at the interrupted parts move and yield; the thickness of the graphite mold shell achieves the effect that the mold shell does not crack during liquid titanium pouring and cracks during liquid titanium solidification.

2. A preparation method of a thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings, used to prepare the thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings as described in claim 1, in sequence including the following steps: Step 1: Design the structure and wall thickness of the thin-walled graphite mold shell according to the structure and size of the titanium and titanium alloy castings to be manufactured; Step 2: Use a machining method to manufacture the thin-walled graphite mold shell assembly; Step 3: Perform vacuum degassing treatment on the thin-walled graphite mold shell assembly; Step 4: After the vacuum degassing treatment is completed, assemble the thin-walled graphite mold shell assembly according to the design drawings; Step 5: After the assembly is completed, perform preheating treatment on the entire thin-walled graphite mold shell; Step 6: After the preheating treatment is completed, place the entire thin-walled graphite mold shell into a vacuum consumable arc furnace or a vacuum induction furnace, and then perform pouring and filling of titanium and titanium alloy; Step 7: After the pouring and filling are completed, disassemble the thin-walled graphite mold shell assembly and clean the thin-walled graphite mold shell.

3. According to the preparation method of the thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings as described in claim 2, Characterized in that: In step 3, the vacuum degassing treatment process of the thin-walled graphite mold shell assembly is that the heating temperature is 920-980 °C, the heat preservation time is 2-5 h, the vacuum degree is not higher than 0.02 MPa, and it is cooled with the furnace after the heat preservation ends.

4. According to the preparation method of the thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings as described in claim 3, Characterized in that: In step 5, the preheating treatment process of the entire thin-walled graphite mold shell is that the heating temperature is 280-320 °C, the heat preservation time is 1-4 h, and it is cooled with the furnace after the heat preservation ends.

5. According to the preparation method of the thin-walled graphite mold shell for manufacturing titanium and titanium alloy castings as described in claim 4, Characterized in that: In Step 6, for titanium-aluminum alloy castings or alloy castings with poor fluidity, the entire thin-walled graphite mold needs to be heated, with a heating rate of 4 - 8 °C / min to 600 - 800 °C, and held for 20 - 30 min; after the holding is completed, power off, and then pour the alloy liquid into the thin-walled graphite mold; after pouring and filling are completed, cool down at a cooling rate of 1 - 3 °C / min to 280 - 300 °C, and take out of the furnace for air cooling.

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