Heat treatment process of Ti2AlNb alloy casting

Abstract

The invention discloses a heat treatment process of a Ti2AlNb alloy casting. The process sequentially comprises the steps of surface pretreatment, penetrant flaw detection, secondary cleaning, primary heat treatment, gradient cooling solution treatment, cleaning after oil cooling and aging treatment. By adopting a method of combining triple surface cleaning and triple heat treatment, particularly in a solid solution treatment stage, a stepped precise cooling and heat preservation process is creatively implemented, namely furnace cooling is performed in stages at the speed of not higher than 0.3 DEG C / min from the temperature higher than a phase transformation point by 15-30 DEG C, the adjacent temperature interval is smaller than or equal to 3 DEG C, and the heat preservation temperature is not higher than 30 DEG C; and finally, oil cooling is performed after long-time heat preservation at the temperature slightly higher than the phase transformation point. By means of the method, the as-cast coarse Widmannstatten structure can be converted into an ideal microstructure with the discontinuous equiaxial alpha phase at the grain boundary and the uniform double-size lath structure in the grain. The room-temperature tensile strength of the treated casting is larger than or equal to 1050 MPa, the ductility is larger than or equal to 7%, matching of high strength and good plasticity is achieved, the process is stable, the period is short, and the method is suitable for industrial production.

Description

Technical Field

[0001] This invention belongs to the field of titanium-based intermetallic compound processing, specifically relating to a heat treatment process for Ti2AlNb alloy castings. Background Technology

[0002] Ti2AlNb-based alloys, dominated by an ordered orthorhombic O phase, have attracted widespread attention due to their low density, excellent plasticity, high specific strength, and superior high-temperature performance, showing broad application prospects in the aerospace field. Ti2AlNb-based alloys are multiphase alloys, containing α2 phase, O phase, and β / B2 phase. The B2 (ordered) / β (disordered) phase has a body-centered cubic (bcc) structure, the O phase has an ordered orthorhombic structure, and the α2 phase has an ordered hexagonal close-packed structure. Alloy composition, thermomechanical processing, and heat treatment processes directly affect the composition, distribution, and content of phases in Ti2AlNb-based alloys, thus directly impacting the alloy's performance. Therefore, it is necessary to rationally control the phase content and composition during heat treatment to achieve performance requirements suitable for service applications. Using casting processes can significantly save on metal raw materials, machine tool processing time, and reduce costs; therefore, developing cast Ti2AlNb alloys is an effective way to realize complex structural components. However, Ti2AlNb alloy castings have a coarse Widmanstätten structure, poor room temperature tensile properties, low elongation, and the mechanical properties of the castings generally cannot be improved through deformation processes, which limits the large-scale engineering application of Ti2AlNb alloy castings. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention discloses a heat treatment process for improving the mechanical properties of Ti2AlNb alloy castings. The microstructure of the Ti2AlNb alloy castings prepared by this method consists of flat lamellar α2 phase separation at grain boundaries, which is then spheroidized into discontinuous equiaxed α2 phases. The interior of the grains exhibits a uniform double-size lath structure with good microstructure uniformity, resulting in high strength and excellent plasticity. This method has a simple production process, a short production cycle, and is suitable for large-scale industrial applications.

[0004] The specific technical solution is as follows: A heat treatment process for Ti2AlNb alloy castings includes the following steps: Step 1): First, use a grinding wheel to rough grind the surface of the cast billet, then use silicon carbide sandpaper of 120# to 800# for progressive grinding, and then use acetone, anhydrous ethanol and deionized water for ultrasonic cleaning in sequence, and then dry it. Step 2): Spray penetrant onto the surface of the casting, covering the entire area of ​​the casting; penetration time is 10~20 minutes. After low-temperature drying, spray developer onto the surface of the casting and check for cracks on the surface of the casting. Step 3): After the surface inspection is qualified, use acetone and anhydrous ethanol to ultrasonically clean the surface of the casting in sequence, and then dry it; Step 4): When the furnace temperature drops below 200℃, the casting is placed into the furnace. The heating rate is ≤5℃ / min. When the temperature reaches 10℃ away from the holding temperature, the heating rate is adjusted to 1℃ / min. The casting is heated to 10~40℃ above the α2+B2 / α2+B2+O phase transformation temperature and held for 2~4 hours. After the heating is completed, the casting is immediately removed and air-cooled. Then, it is air-cooled to room temperature. Step 5): Select a box-type resistance furnace. Place the casting into the furnace when the furnace temperature drops below 200℃. The heating rate should be ≤5℃ / min. When the temperature reaches 10℃ below the holding temperature, adjust the heating rate to 1℃ / min. Heat the casting to 15~30℃ above the α2+B2 / α2+B2+O phase transformation temperature (T1) and hold for t1. Then, furnace cool to T2 and hold for t2. Continue furnace cooling to T3 and hold for t3, and so on, until T1 is reached. n Keep warm for 120-240 minutes. Among them, t i (i=1, 2, 3...) represents the heat preservation time, 30min≤t i ≤60min, T i (i=1, 2, 3...) represents the insulation temperature, satisfying T1>T2>T3...>T n Furthermore, the temperature interval between adjacent temperatures should not exceed 3℃, and the furnace cooling rate should not exceed 0.3℃ / min. n The final holding temperature for the casting, (T) n After holding the oil at 0~10℃ above the α2+B2+O / B2+O phase transition temperature, immediately remove it from the oil cooler and then cool it to room temperature. Step 6): Remove the quenching oil from the surface of the Ti2AlNb alloy casting, then immerse the casting in acetone, anhydrous ethanol and deionized water for ultrasonic cleaning in sequence and dry it. Step 7): Select a box-type resistance furnace. When the furnace temperature drops below 200℃, put the casting into the furnace. The heating rate is ≤5℃ / min. When the temperature reaches 10℃ away from the holding temperature, adjust the heating rate to 1℃ / min. Heat the billet to 780~830℃ and hold it for 18~24h. After the process, take it out and air cool it immediately.

[0005] The preferred embodiment of the heat treatment process for Ti2AlNb alloy castings is that, after the heat treatment process, the continuous and straight grain boundary α2 phase in the microstructure of the Ti2AlNb alloy castings is spheroidized into discontinuous equiaxed α2 phase, and a uniformly distributed double-size lath structure is formed inside the grains.

[0006] The preferred embodiment of the heat treatment process for Ti2AlNb alloy castings is that, after the heat treatment process, the room temperature tensile strength of the Ti2AlNb alloy castings is greater than 1050 MPa, and the room temperature elongation is greater than 7%.

[0007] The preferred embodiment of the heat treatment process for Ti2AlNb alloy castings is that, in steps 1) and 3), acetone, anhydrous ethanol, and deionized water are used sequentially for ultrasonic cleaning, with each ultrasonic cleaning session lasting no less than 10 minutes.

[0008] In the preferred embodiment of the heat treatment process for Ti2AlNb alloy castings, in step 2), due to the high density of the Ti2AlNb alloy castings, the penetration time shall not be less than 15 minutes.

[0009] The preferred embodiment of the heat treatment process for Ti2AlNb alloy castings is that, in step 5), the gradient cooling process includes at least three successively decreasing holding stages, and the holding temperature interval between each stage does not exceed 3°C.

[0010] In the preferred embodiment of the heat treatment process for Ti2AlNb alloy castings, in step 6), the transfer time of the oil-cooled castings after the heat treatment is completed shall not exceed 30 seconds.

[0011] The beneficial effects of this invention are: A triple surface pretreatment and triple heat treatment system is adopted. The first surface pretreatment of the casting prevents carburization and the formation of brittle carbides, while also preventing surface contaminants from hindering atomic diffusion, ensuring uniform microstructure between the surface and core of the casting after heat treatment. The second surface pretreatment prevents residual chemicals after carburization from corroding the surface. The first heat treatment improves the uniformity of the casting microstructure, and the flat lamellar α2 phase at the grain boundaries is spheroidized into discontinuous equiaxed α2. The second solution treatment introduces a gradient cooling and holding process to eliminate internal stress to the maximum extent, while obtaining a highly uniform and stable microstructure, avoiding casting deformation or cracking. After the second solution treatment, the microstructure of the casting is a uniform coarse lath α2 / O phase within the grains, while the grain boundaries are still spheroidized discontinuous equiaxed α2 phase. Then, a third surface cleaning is performed to prevent the surface oil film from hindering uniform heat transfer and causing uneven aging in subsequent stages. The third aging treatment results in a double-size lath microstructure within the grains and a discontinuous equiaxed α2 composition at the grain boundaries, achieving a good match between alloy strength and plasticity. Attached Figure Description

[0012] Figure 1 A schematic diagram of a heat treatment process for a Ti2AlNb alloy casting. Figure 2 Photograph of the original microstructure of the Ti2AlNb alloy casting before heat treatment; Figure 3 This is a high-magnification microstructure photograph of the Ti2AlNb alloy casting in Example 1; Figure 4 This is a high-magnification microstructure photograph of the Ti2AlNb alloy casting in Example 2. Detailed Implementation

[0013] The present invention will now be described in full and in detail with reference to the embodiments and corresponding drawings.

[0014] Example 1

[0015] The material selected in this embodiment is a Ti2AlNb alloy cast rod with a diameter of 20 mm and a length of 100 mm. Its composition is: Ti-21.91Al-24.26Nb-0.53Mo. β The phase transition point is 1055℃. Step 1): First, use a grinding wheel to rough grind the surface of the casting rod, then use 120#→800# silicon carbide sandpaper to grind it step by step, and then use acetone, anhydrous ethanol and deionized water in sequence to ultrasonically clean for 10 minutes in sequence, and then dry at 80℃. Step 2): Spray fluorescent penetrant over the entire area of ​​the casting; penetration time is 18 minutes. After that, remove excess penetrant from the surface of the casting. After drying at a low temperature of 50°C, spray developer on the surface of the casting and check for cracks on the surface of the casting. Step 3): After passing the inspection, the surface of the casting is ultrasonically cleaned with acetone and anhydrous ethanol for 10 minutes in sequence, dried at 80°C and then heat-treated. Step 4): The casting is placed in the furnace at a furnace temperature of 150℃. The heating rate is 5℃ / min. The temperature is raised to 1005℃. The heating rate is then adjusted to 1℃ / min. The casting is heated to 1015℃ and held for 120 minutes. After the heat treatment process is completed, the casting is immediately removed and air-cooled. Step 5): After the casting has cooled to room temperature in Step 4), the casting is placed into the furnace at a furnace temperature of 150℃. The billet is heated to 965℃ using a box-type resistance furnace at a heating rate of 5℃ / min. The heating rate is then adjusted to 1℃ / min to 975℃ and held for 30 minutes. The casting is then furnace cooled to 972℃ and held for 30 minutes. The casting is then furnace cooled to 969℃ and held for 30 minutes. The casting is then furnace cooled to 966℃ and held for 30 minutes. The casting is then furnace cooled to 963℃ and held for 30 minutes. The casting is then furnace cooled to 960℃ and held for 120 minutes. The program ends when the furnace cooling rate is 0.2℃ / min. The casting is then immediately removed and oil-quenched. The oil-quenched casting is transferred in 20 seconds and cooled to room temperature.

[0016] Step 6): Wipe the oil stains off the surface of the casting obtained in Step 5), then immerse the casting in acetone and anhydrous ethanol, and ultrasonically clean it for 10 minutes in sequence, and dry it at 80°C. Step 7): Select a box-type resistance furnace and put the casting into the furnace at a furnace temperature of 150℃. Heat the casting to 790℃ at a heating rate of 5℃ / min, then adjust the heating rate to 1℃ / min and heat to 800℃. Hold for 24 hours and then immediately remove and air cool.

[0017] Conclusion: The microstructure of the casting prepared in Example 1 consists of discontinuous equiaxed α2 phases at grain boundaries, with double-sized laths and a B2 matrix within the grains, and α2 phase precipitated in the core of the primary O phase laths. Figure 3 As shown, the Ti2AlNb casting prepared in this embodiment has a tensile strength of 1063 MPa, a yield strength of 998 MPa, and a room temperature tensile elongation of 8.9%. Its mechanical properties are stable and superior to those of the comparative casting 1 and comparative casting 2 in Table 2. Table 1 shows the room temperature tensile properties of Example 1 and comparative castings 1 and 2.

[0018] Table 1. Room temperature tensile properties of Ti2AlNb-based alloy castings

[0019] Example 2

[0020] The material selected in this embodiment is a Ti2AlNb alloy cast rod with a specification of Ф40mm×200mm, and its composition is Ti-22.19Al-24.01Nb-0.49Mo. The alloy's T β The phase transition point is 1060℃. Step 1): First, use a grinding wheel to rough grind the surface of the material, then use 120# to 800# silicon carbide sandpaper to grind it step by step. Use acetone, anhydrous ethanol and deionized water to ultrasonically clean for 15 minutes in sequence, and then dry at 80°C. Step 2): Spray penetrant over the entire area of ​​the casting; penetration time is 20 minutes, then remove excess penetrant from the surface, dry at 50℃, then spray developer on the casting surface and check for cracks on the casting surface. Step 3): After passing the inspection, the surface of the casting is ultrasonically cleaned with acetone and anhydrous ethanol for 15 minutes in sequence, dried at 80°C and then heat-treated. Step 4): The casting is placed in the furnace at a furnace temperature of 180℃. The heating rate is 5℃ / min. After the temperature reaches 1010℃, the heating rate is adjusted to 1℃ / min. Then the casting is heated to 1025℃ and held for 120 minutes. After the program ends, the casting is immediately taken out and air-cooled. Step 5): After the casting has cooled to room temperature in Step 4), the casting is placed in the furnace at a furnace temperature of 180℃. The casting is heated using a box-type resistance furnace at a heating rate of 5℃ / min to 950℃, then the heating rate is adjusted to 1℃ / min to 970℃, held for 40 min, furnace cooled to 968℃ and held for 40 min, furnace cooled to 966℃ and held for 40 min, furnace cooled to 964℃ and held for 40 min, furnace cooled to 962℃ and held for 40 min, furnace cooled to 960℃ and held for 150 min. The program ends, with the furnace cooling rate being 0.2℃ / min. The casting is then immediately removed and oil quenched. The transfer time for the oil-quenched casting is 25 s, and it is cooled to room temperature.

[0021] Step 6): Wipe the oil stains off the surface of the casting obtained in Step 5), then immerse the casting in acetone and anhydrous ethanol, and ultrasonically clean it for 15 minutes in sequence, and dry it at 80°C. Step 7): The casting is placed in the furnace at a furnace temperature of 180℃. A box-type resistance furnace is used to heat the casting to 790℃ at a heating rate of 5℃ / min. Then the heating rate is adjusted to 1℃ / min and the casting is heated to 810℃. The casting is held at this temperature for 24 hours. After the heating is completed, the casting is immediately removed and air-cooled.

[0022] In conclusion, the microstructure of the casting prepared in Example 2 is basically consistent with that in Example 1, such as... Figure 4 As shown, the Ti2AlNb casting prepared in this embodiment has a tensile strength of 1080 MPa, a yield strength of 1018 MPa, and a room temperature tensile elongation of 9.1%. Its performance is significantly better than that of comparative castings 1 and 2 in Table 2, which shows the room temperature tensile properties of Example 2 and comparative castings 1 and 2.

[0023] Table 2 Room temperature tensile properties of Ti2AlNb-based alloy castings

[0024] Matters not covered in this invention are common knowledge.

[0025] The above descriptions are merely some embodiments of the present invention. It should be emphasized that, within the scope of the inventive concept, various improvements and modifications can be made to the technical solutions of the present invention. These all fall within the protection scope of the present invention.

Claims

1. A heat treatment process for Ti2AlNb alloy castings, characterized in that, Includes the following steps: Step 1: Surface pretreatment; First, the cast billet is subjected to rough grinding and step-by-step grinding, and then ultrasonically cleaned with acetone, anhydrous ethanol and deionized water in sequence before drying. Step 2: Surface penetrant testing; Spray a penetrant onto the surface of the casting, covering the entire area; after penetrating for 10–20 minutes, dry at low temperature, then spray a developer onto the surface of the casting to check for surface cracks. Step 3): Secondary surface cleaning; ultrasonically clean the casting surface with acetone and anhydrous ethanol in sequence, then dry. After passing inspection, the surface of the casting is ultrasonically cleaned with acetone and anhydrous ethanol in sequence, and then dried. Step 4): First heat treatment; When castings are placed into the furnace at a temperature below 200℃, the heating rate should be ≤5℃ / min. When the temperature reaches 10℃ away from the holding temperature, the heating rate should be adjusted to 1℃ / min. Heat the casting to 10-40°C above the α2+B2 / α2+B2+O phase transformation temperature, hold for 2-4 hours, and immediately remove and air cool to room temperature. Step 5): Gradient cooling solution treatment; Castings are introduced into the furnace when the furnace temperature drops below 200℃. The heating rate is ≤5℃ / min. When the temperature reaches 10℃ away from the holding temperature, the heating rate is adjusted to 1℃ / min. The casting is heated to 15~30℃ above the α2+B2+O / B2+O phase transformation temperature as the first holding temperature T1, with a holding time t1. Then, the casting is furnace cooled to T2 at a rate not exceeding 0.3℃ / min, with a holding time t2. The furnace cooling continues to T3, with a holding time t3. This stepwise cooling and holding process is repeated until the final holding temperature T is reached. n T n Located 0~10℃ above the α2+B2+O / B2+O phase transition temperature, with adjacent temperature intervals not exceeding 3℃, satisfying T1>T2>T3...>T n The heat preservation time for each section is 30-60 min, and the final heat preservation time is 120-240 min; after the heat preservation is completed, oil cooling is performed immediately to cool to room temperature; Step 6): Clean after the oil cools; After removing the quenching oil from the surface of the Ti2AlNb alloy casting, the casting was sequentially immersed in acetone, anhydrous ethanol and deionized water for ultrasonic cleaning and then dried. Step 7): Time-sensitive processing; When the furnace temperature drops below 200℃, the casting is put into the furnace. The heating rate is ≤5℃ / min. When the temperature reaches 10℃ away from the holding temperature, the heating rate is adjusted to 1℃ / min. The billet is heated to 780~830℃ and held for 18~24h. After the holding time is completed, it is immediately taken out and air-cooled.

2. The heat treatment process for Ti2AlNb alloy castings according to claim 1, characterized in that: After the heat treatment process described above, in the microstructure of the Ti2AlNb alloy casting, the continuous and straight grain boundary α2 phase spheroidizes into discontinuous equiaxed α2 phase, and a uniformly distributed double-size lath structure is formed inside the grains.

3. The heat treatment process for a Ti2AlNb alloy casting according to claim 1, characterized in that: After the heat treatment process described above, the room temperature tensile strength of the Ti2AlNb alloy casting is greater than 1050 MPa, and the room temperature elongation is greater than 7%.

4. A heat treatment process for Ti2AlNb alloy castings according to claim 1, characterized in that: In steps 1) and 3), acetone, anhydrous ethanol, and deionized water are used to perform ultrasonic cleaning in sequence, with each ultrasonic cleaning session lasting no less than 10 minutes.

5. A heat treatment process for Ti2AlNb alloy castings according to claim 1, characterized in that: In step 2), due to the high density of the Ti2AlNb alloy casting, the penetration time must not be less than 15 minutes.

6. A heat treatment process for Ti2AlNb alloy castings according to claim 1, characterized in that: In step 5), the gradient cooling process includes at least three successively decreasing heat preservation stages, and the temperature interval between each heat preservation stage does not exceed 3°C.

7. A heat treatment process for Ti2AlNb alloy castings according to claim 1, characterized in that: In step 6), after the heat treatment is completed, the transfer time of the oil-cooled casting shall not exceed 30 seconds.

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