A method for improving the plasticity of cast titanium alloy by refining grains to excite twin effect

Abstract

The application provides a method for improving plasticity of cast titanium alloy by refining grains and exciting twin effect, and relates to the technical field of cast titanium alloy structure and performance control, and the method comprises the following steps: step 1: mixing according to nominal composition of the titanium alloy, melting in a vacuum consumable arc furnace to obtain a primary ingot; step 2: using the primary ingot obtained in step 1 to perform centrifugal casting by cold crucible suspension melting under the condition that the centrifugal rotating speed is 150-300 r / min and the mold shell preheating temperature is 200-300 DEG C to obtain a rod-shaped sample, and the rod-shaped sample is taken out after cooling; and the pouring system used in the pouring process needs to ensure that the rod-shaped samples are spaced; and step 3: using the rod-shaped sample obtained in step 2 to perform hot isostatic pressing at 920 DEG C / 130 MPa for 3h and cooling in the furnace. By the pouring method, the grains of the cast titanium alloy can be refined, the deformation twin can be induced to generate, and the plasticity of the cast titanium alloy can be significantly improved, and the elongation of ZTC4 reaches more than 15%. The method is simple in operation and low in cost.

Description

Technical Field

[0001] This invention relates to the field of microstructure and property control technology for cast titanium alloys, specifically to a method for improving the plasticity of cast titanium alloys by refining grains to induce a twinning effect. Background Technology

[0002] Titanium alloy castings are widely used in aerospace, marine engineering, shipbuilding, and petrochemical industries due to their low density, high specific strength, excellent corrosion resistance, and relatively low processing costs. Among them, ZTC4 (Ti-6Al-4V) alloy has good comprehensive mechanical properties and excellent casting performance, making it the most widely used cast titanium alloy. However, because cast titanium alloys have a coarse composition and are difficult to control through post-processing, although the strength of titanium castings is not significantly different from that of wrought alloys of the same composition, their plasticity is reduced by about 40% to 50%.

[0003] Some literature mentions that secondary processing, such as additive manufacturing, can improve the coarse microstructure, stress concentration, and poor plasticity of titanium alloy components by applying additive manufacturing to the existing cast matrix, thereby increasing the elongation of the alloy castings. However, this secondary processing method is complex and costly. Other literature suggests that refining the microstructure of titanium alloys can be achieved by controlling the solidification path. Compared to adding grain refiners, this method is less likely to introduce foreign inclusions and avoids the performance degradation caused by them. The drawback is that this method requires holding at a certain temperature during the solidification process of the molten metal, making precise temperature control difficult. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention proposes a method for improving the plasticity of cast titanium alloys by refining grains to induce twinning. This method specifically includes casting techniques, hot isostatic pressing (HIP) processes, and casting mold design. Using this method, the grains of cast titanium alloys can be refined, twinning during deformation can be induced, and tensile strength can be increased while significantly improving the plasticity of the cast titanium alloy, achieving an elongation of over 15% for ZTC4.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] This invention provides a method for improving the plasticity of cast titanium alloys by refining grains to induce a twinning effect, comprising the following steps:

[0007] Step 1: Mix the materials according to the nominal composition of the titanium alloy and melt them in a vacuum arc furnace to obtain a primary ingot;

[0008] Step 2: The ingot obtained in Step 1 is smelted using cold crucible suspension melting, and rod-shaped samples are cast using centrifugal casting. After cooling, the samples are removed from the furnace.

[0009] Step 3: Perform hot isostatic pressing on the rod-shaped sample obtained in Step 2 and cool it in the furnace.

[0010] As a further explanation of the present invention, the casting process in step 2 adopts a centrifugal casting method, with specific parameters as follows: centrifugal speed 150~300 r / min, mold shell preheating temperature 200~300℃.

[0011] As a further explanation of the present invention, the casting system used in step 2 must ensure that the rod-shaped samples are spaced apart.

[0012] As a further explanation of the present invention, the casting system used in step 2 includes a pouring cup, a sprue, a runner, and rod-shaped specimens; wherein the runner is composed of an upper runner and a lower runner, both of which adopt a 60° cross-distribution structure design, the sprue is located at the center of the runner and distributed from top to bottom, the top of the sprue extends above the upper runner and connects with the pouring cup, the bottom extends below the lower runner and has a hemispherical sprue recess, and the rod-shaped specimens are distributed intermittently between the upper and lower runners.

[0013] As a further explanation of the present invention, the cross-sectional dimensions of the horizontal sprue are 25x20mm; the diameter of the vertical sprue is 50mm; the diameter of the rod-shaped specimen is 14mm, the length is 140mm, and the spacing between the specimens is 7mm.

[0014] As a further explanation of the present invention, the specific process conditions for the hot isostatic pressing treatment are hot isostatic pressing at 920℃ / 130MPa for 3 hours.

[0015] Compared with the prior art, the present invention has the following beneficial technical effects:

[0016] The casting method of this invention can refine the grain size of cast titanium alloys, induce deformation twinning, and significantly improve the plasticity of cast titanium alloys, achieving an elongation of over 15% for ZTC4. This method is simple to operate and has low cost.

[0017] Other features and advantages of this technical solution will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the technical solution. The objectives and other advantages of this technical solution can be realized and obtained by means of the structures particularly pointed out in the written description and the accompanying drawings.

[0018] The technical solution of this invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0019] The accompanying drawings are provided to further illustrate the present technical solution and form part of the specification. They are used together with the embodiments of the present technical solution to explain the present technical solution, but do not constitute a limitation thereof. In the accompanying drawings:

[0020] Figure 1 This is a schematic diagram of the casting system structure of the present invention.

[0021] Figure 2 This is a schematic diagram of the tissue sheet size according to an embodiment of the present invention.

[0022] Figure 3 This is a tensile stress-strain curve of an alloy in an embodiment of the present invention.

[0023] Figure 4 The alloy deformation structure is shown in the embodiment of the present invention.

[0024] Figure 5 This is a schematic diagram of the tissue lamellar cluster size in Comparative Example 1.

[0025] Figure 6 The figure shows the stress-strain tensile curve of the alloy in Comparative Example 1.

[0026] Figure 7 This is a schematic diagram of the casting system structure for Comparative Example 2.

[0027] Figure 8 This is a schematic diagram of the tissue lamellar cluster size in Comparative Example 2. Detailed Implementation

[0028] The preferred embodiments of this technical solution are described below with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are only for illustration and explanation of this technical solution and are not intended to limit this technical solution.

[0029] This invention provides a method for improving the plasticity of cast titanium alloys by refining grains to induce a twinning effect, comprising the following steps:

[0030] Step 1: Mix the materials according to the nominal composition of the titanium alloy and melt them in a vacuum arc furnace to obtain a primary ingot.

[0031] Step 2: The ingot obtained in Step 1 is smelted using cold crucible suspension melting, and rod-shaped samples are cast using centrifugal casting. After cooling, the samples are removed from the furnace.

[0032] In step 2, a cold crucible suspension melting furnace is selected as the melting equipment. Suspension melting, being a non-contact melting process, reduces inclusions and improves the superheat and compositional uniformity of the melt. Furthermore, cold crucible suspension melting technology eliminates the need for additional stirring equipment; the melt can be thoroughly stirred under electromagnetic force, which helps improve the compositional uniformity of the molten material and reduces contamination.

[0033] Furthermore, in step 2, the casting process employs a centrifugal casting method with the following specific parameters: centrifugal speed of 150~300 r / min and mold preheating temperature of 200~300℃. Centrifugal force can break up grains during alloy solidification, contributing to grain refinement. The lower preheating temperature of the mold allows for faster cooling of the alloy during casting, further aiding grain refinement and promoting the formation of deformation twins during deformation. For example, the centrifugal speed can be set to 150 r / min, 200 r / min, 250 r / min, 300 r / min, etc.; and the preheating temperature can be set to 300℃, 280℃, 250℃, 230℃, 200℃, etc.

[0034] Furthermore, the casting system used in step 2 must ensure that the rod-shaped specimens are spaced apart. Specifically, such as... Figure 1 As shown, the gating system includes a pouring cup 1, a sprue 2, a runner 3, and rod-shaped specimens 4. The runner 3 consists of an upper runner 301 and a lower runner 302, both designed with a 60° intersecting distribution. The runner's cross-sectional dimensions are 25x20mm. This design allows the molten metal to fill the top runner before the casting during the filling process, enabling simultaneous filling of the casting through both runners. It also provides support and fixation. The sprue 2 is located at the center of the runner 3 and extends downwards. Its top extends above the upper runner 301 and connects to the pouring cup 1, while its bottom extends below the lower runner 302 and features a hemispherical sprue recess to prevent turbulence and splashing, ensuring smooth filling. The rod-shaped specimens 4 are spaced between the upper and lower runners. To meet the requirements of vacuum arc remelting of titanium-based alloys, the diameter of the sprue was set to 50 mm to match the electrode dimensions of the vacuum arc remelting equipment. The diameter of the rod-shaped specimens was set to 14 mm, the length to 140 mm, and the spacing between the specimens to 7 mm. This spacing ensures smooth filling during centrifugal casting and optimizes the temperature gradient during solidification, resulting in faster cooling and a finer grain effect.

[0035] Step 3: Perform hot isostatic pressing on the rod-shaped sample obtained in Step 2 and cool it in the furnace.

[0036] The specific process conditions for the hot isostatic pressing treatment are: hot isostatic pressing at 920℃ / 130MPa for 3 hours.

[0037] This invention employs a large isostatic pressing to induce plasticity in the material, causing the voids to collapse under the pressure difference, effectively eliminating defects and achieving a density close to the theoretical density. At the same time, it employs a long heat preservation time, ensuring that the material withstands high temperature and high pressure during the HIP process for a sufficiently long time, thus guaranteeing the degree of densification of the material.

[0038] The present invention will be further described below with reference to specific embodiments.

[0039] The mechanical property tests in the following examples and comparative examples were conducted in accordance with GB / T228.1-2010 Metallic materials, tensile testing—Part 1: Tests at room temperature. The standard tensile specimen, a cylindrical bar, was designed with a clamping end and a gauge length end. The clamping end had an M10 thread and a length of 15 mm. The gauge length had a diameter of 5 mm and a length of 25 mm. The parallel section was 35 mm long. Each end connection had a chamfer with a radius of 4 mm and a length of 3 mm. The tensile rate was 0.45 mm / min. Example 1

[0040] 1) Batching and electrode pressing: Sponge titanium, Al bean, AlV58 and TiO2 powder are mixed and smelted in a vacuum arc furnace.

[0041] 2) Centrifuge the primary ingot obtained in step 1 in a cold crucible suspension melting furnace at a centrifugal speed of 200 r / min, a mold shell preheating temperature of 300℃, and a casting speed of 3 kg / s. After cooling, remove it from the furnace.

[0042] 3) The sample obtained in step 2 was subjected to hot isostatic pressing at 920℃ / 130MPa for 3 hours.

[0043] The size of the lamellar clusters in the obtained samples was characterized using EBSD technology. Figure 2 Chemical composition analysis was conducted at the National Nonferrous Metals and Electronic Materials Analysis and Testing Center. The main components by mass percentage were: Al: 6.16%, V: 4.06%, Fe: 0.30%, O: 0.12%, with the remainder being titanium. Using an electronic universal testing machine, the room temperature mechanical properties were tested as follows: yield strength 760 MPa, tensile strength 831 MPa, elongation 16%. The engineering stress-strain curve is shown below. Figure 3 Deformed tissues such as Figure 4 As can be seen, deformed twins appeared in the tissue.

[0044] This embodiment achieves refined microstructure, induced deformation twinning, and increased plasticity by over 15% in cast titanium alloys by designing the pouring method and pouring model during the casting process, while maintaining the material's strength. This method is simple to operate and has low cost.

[0045] Comparative Example 1

[0046] 1) Batching and electrode pressing: Sponge titanium, Al bean, AlV58 and TiO2 powder are mixed and smelted in a vacuum arc furnace.

[0047] 2) The primary ingot obtained in step 1 is gravity-cast in a cold crucible suspension melting furnace and removed from the furnace after cooling;

[0048] 3) The sample obtained in step 2 was subjected to hot isostatic pressing at 920℃ / 130MPa for 3 hours.

[0049] Characterizing lamellar cluster size using EBSD technology, such as Figure 5 Chemical composition analysis was conducted at the National Nonferrous Metals and Electronic Materials Analysis and Testing Center. The main components by mass percentage were: Al: 5.95%; V: 4.00%; Fe: 0.30%; O: 0.15%, with the remainder being titanium. Using an electronic universal testing machine, the room temperature mechanical properties were measured as follows: yield strength 759 MPa, tensile strength 801 MPa, elongation 6.5%. The engineering stress-strain curve is shown below. Figure 6 .

[0050] Comparative Example 2

[0051] 1) Batching and electrode pressing: Sponge titanium, Al bean, AlV58 and TiO2 powder are mixed and smelted in a vacuum arc furnace.

[0052] 2) The primary ingot obtained in step 1 is centrifugally cast. The casting system adopts... Figure 7 The plum blossom-shaped casting rods shown (without any spacing between the casting rods) are removed from the furnace after cooling.

[0053] 3) The sample obtained in step 2 was subjected to hot isostatic pressing at 920℃ / 130MPa for 3 hours.

[0054] Characterizing lamellar cluster size using EBSD technology, such as Figure 8 The chemical composition of the sample was analyzed at the National Nonferrous Metals and Electronic Materials Analysis and Testing Center. The main components by mass percentage were: Al: 5.97%; V: 4.12%; Fe: 0.30%; O: 0.12%, with the remainder being titanium.

[0055] As can be seen from the microstructure comparison in the attached figures, compared to the coarse microstructure of the gravity-cast Ti-6Al-4V alloy in Comparative Example 1 and the centrifugally cast Ti-6Al-4V alloy in Comparative Example 2, the microstructure of the centrifugally cast Ti-6Al-4V alloy using the casting model of the present invention in Example 1 is finer. After tensile deformation, the fine-grained microstructure of Example 1 shows the appearance of tensile twins, and the elongation is significantly increased.

[0056] Obviously, those skilled in the art can make various modifications and variations to this technical solution without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this technical solution and their equivalents, this technical solution also intends to include these modifications and variations.

Claims

1. A method for improving the plasticity of cast titanium alloys by refining grains to induce a twinning effect, characterized in that, Includes the following steps: Step 1: Mix the materials according to the nominal composition of the titanium alloy and melt them in a vacuum arc furnace to obtain a primary ingot; Step 2: The ingot obtained in Step 1 is smelted using cold crucible suspension melting, and rod-shaped samples are cast using centrifugal casting. The specific parameters are: centrifugal speed 150~200 r / min, mold preheating temperature 200~300℃, casting speed 3kg / s, and then removed from the furnace after cooling. The casting system used in the casting process includes a pouring cup, a sprue, a runner, and rod-shaped specimens. The runner consists of an upper runner and a lower runner, both of which are designed with a 60° cross distribution. The sprue is located in the center of the runner and is distributed from top to bottom. The top of the sprue extends above the upper runner and connects to the pouring cup, while the bottom extends below the lower runner and has a hemispherical sprue recess. The rod-shaped specimens are distributed between the upper and lower runners. Step 3: Perform hot isostatic pressing on the rod-shaped sample obtained in Step 2 and cool it in the furnace.

2. The method for improving plasticity of cast titanium alloys by refining grains to stimulate the twinning effect, as described in claim 1, is characterized in that... The casting system used in step 2 must ensure that the rod-shaped samples are spaced apart.

3. The method for improving plasticity of cast titanium alloys by refining grains to stimulate the twinning effect, as described in claim 1, is characterized in that... The cross-sectional dimensions of the horizontal sprue are 25x20mm; the diameter of the vertical sprue is 50mm; the diameter of the rod-shaped specimen is 14mm, the length is 140mm, and the spacing between the specimens is 7mm.

4. The method for improving plasticity of cast titanium alloys by refining grains to stimulate the twinning effect, as described in claim 1, is characterized in that... The specific process conditions for the hot isostatic pressing treatment are: hot isostatic pressing at 920℃ / 130MPa for 3 hours.

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