Preparation method of TC18 titanium alloy free forge piece with large length-thickness ratio

Abstract

The invention relates to the technical field of TC18 titanium alloy processing, and discloses a preparation method of a TC18 titanium alloy free forge piece with a large length-thickness ratio, which comprises (alpha + beta) two-phase region forging with the heating temperature of T beta-(40-60) DEG C and beta-phase region forging with the heating temperature of T beta + (15-20) DEG C. The deformation of each heating number is controlled to be 15-35%, and the deformation of each heating number is controlled to be 15-35%. And the deformation in the width direction of the blank is reduced in the flat square drawing forging in the blank making stage of the blank, and the feeding amount in beta-phase region forging is increased, so that the head and tail structure performance difference is reduced, and the problem that the structure is not uniform when the TC18 titanium alloy free forge piece with the large length-thickness ratio is prepared is solved. The TC18 titanium alloy free forge piece with the large length-thickness ratio, prepared through the method, is uniform in structure and stable in performance, and can meet the standard performance requirements of aircraft forge pieces.

Description

Technical Field

[0001] The present invention belongs to the technical field of titanium alloy processing, and particularly relates to a preparation method for a free forging of TC18 titanium alloy with a large length-to-thickness ratio. Background Art

[0002] The nominal composition of TC18 titanium alloy is Ti-5Al-5Mo-5V-1Cr-1Fe, which is a near-β type titanium alloy and belongs to high-strength and tough titanium alloys. The forgings prepared from it have characteristics such as high specific strength, significant weight reduction effect, and excellent impact resistance, etc., and are widely used in some load-bearing components of landing gears and fuselages such as middle struts, frame beams, etc. in aircraft.

[0003] Currently, the traditional process route for free forgings of TC18 titanium alloy is blank making, near-β forging (forging heating temperature is T β +15 °C) + double annealing. For free forgings of TC18 titanium alloy with a small length-to-thickness ratio, the anisotropy of the structure during the near-β forging process is small. However, when the length-to-thickness ratio of the free forging of TC18 titanium alloy is large, problems such as large anisotropy of the structure, poor flatness of the forging, and obvious differences in mechanical properties in the length direction of the forging are likely to occur. Traditional preparation methods often have difficulty in ensuring the tissue uniformity and performance stability of the material. Summary of the Invention

[0004] In view of this, in order to overcome the above problems in the prior art, the purpose of the present invention is to provide a preparation method for a TC18 titanium alloy forging with a large length-to-thickness ratio. This preparation method can prepare a TC18 titanium alloy forging with a large length-to-thickness ratio that has uniform tissue and stable performance, and the forgings prepared meet the requirements of aviation standards.

[0005] To solve the above problems, the present invention adopts the following technical solutions:

[0006] On the one hand, the present invention provides a preparation method for a free forging of TC18 titanium alloy with a large length-to-thickness ratio, which includes the following steps:

[0007] S1 Forging and blank making of TC18 titanium alloy bars in the (α + β) two-phase region:

[0008] Heat the TC18 titanium alloy bars in the furnace to T β -(40 - 60) °C, keep the temperature for ((0.6 - 1.2) × the thickness of the smallest cross-section of the bar blank) min, take out of the furnace and perform flat-square forging, control the deformation amount per heat treatment at 15% - 30%, and control the pressing speed at 5 - 10 mm / s to obtain a cuboid blank A;

[0009] S2 Perform β-phase region forging on the cuboid blank A obtained in step S1:

[0010] Preheat the cuboid blank A in the furnace to T β -(20 - 60)°C with a preheating and holding time of ((0.5 - 1) × the thickness of the minimum cross-section of the bar) min, then heat it, with the heating temperature being T β +(15 - 20)°C and a holding time of ((0.3 - 0.5) × the thickness of the minimum cross-section of the bar) min. After taking it out of the furnace, wrap it with a heat-insulating sleeve and put it into the furnace again. Heat it at a temperature of T β +(15 - 20)°C. After holding, perform flat-square forging by the method of pressing down with two staggered hammers, with the deformation amount controlled at 30 - 35% to obtain blank B;

[0011] S3 Divide the blank B obtained in step S2 into two equal parts along the width direction to obtain blank C1 and blank C2;

[0012] S4 Perform a first annealing treatment on the blank C1 obtained in step S3. The temperature of the first annealing is (T β -35)°C to (T β -30)°C. Hold the temperature, cool it in the furnace to 750 - 760°C, then hold the temperature again, and take it out of the furnace and air-cool it to room temperature to obtain blank D1;

[0013] S5 Perform a second annealing treatment on the blank D1 obtained in step S4. The temperature of the second annealing is 600 - 630°C. After taking it out of the furnace, air-cool it to room temperature to obtain a free forging of TC18 titanium alloy;

[0014] The T β is the β transformation temperature of TC18 titanium alloy.

[0015] Further, in the flat-square forging in step S1, perform 2 - 3 times of reheating the hot material back to the furnace, and the reheating time of the hot material back to the furnace is ((0.3 - 0.6) × the thickness of the minimum cross-section of the bar) min.

[0016] Further, in step S1, perform two times of reheating the hot material back to the furnace with heat preservation coefficients of 0.43 and 0.47 respectively.

[0017] Further, in step S2, after holding for 30 min, use a 100 MN quick forging machine to perform flat-square forging by the method of pressing down with two staggered hammers.

[0018] Further, in step S2, the heat-insulating sleeve is rock wool.

[0019] Further, in step S2, the final forging temperature ≥ 820°C.

[0020] Further, in step S4, after holding for 3.5 h, cool it in the furnace to 750 - 760°C, then hold for 2.5 h, and take it out of the furnace and air-cool it to room temperature.

[0021] Further, after step S4, it further includes sawing, surface turning and polishing the forging obtained in step S4 to obtain a machined free forging of TC18 titanium alloy.

[0022] On the other hand, the present invention also provides a free forging of TC18 titanium alloy with a large aspect ratio, which is prepared by the above preparation method.

[0023] Further, the single weight of the free forging of TC18 titanium alloy is 90 kg, and the specification is 65 mm thick × 125 mm wide × 2060 - 2500 mm long.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] The preparation method of the free forging of TC18 titanium alloy in the present invention breaks the conventional deformation amount in the width direction in the (α + β) two-phase region forging and β-phase region forging and flattening deformation methods. When flattening the blank in the blank preparation stage, the deformation amount in the width direction of the blank is reduced, and an intermediate blank A with a smaller width direction is obtained to increase the feed amount during the subsequent β-phase region forging, reduce the β forging time, so as to achieve the excellent effect of finally reducing the anisotropy of the forging, making it able to meet the performance requirements of the forging standard.

[0026] In addition, the preparation method of the present invention increases the deformation amount in the thickness direction during β forging, and adopts the method of two-stage cross-hammer pressing. One is to reduce the deformation concentration area of the blank, making the blank structure more uniform; the other is to ensure the deformation amount below the phase transformation point, improve the alloy strength, and at the same time obtain high plasticity and high fracture toughness. Its comprehensive performance is excellent, and the forgings prepared meet the requirements of the aviation standard. Description of the Drawings

[0027] Figure 1 It is the transverse macrostructure diagram of the free forging of TC18 titanium alloy prepared in Example 1 of the present invention.

[0028] Figure 2 It is the transverse microstructure diagram of the free forging of TC18 titanium alloy prepared in Example 1 of the present invention. Detailed Embodiments

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the description of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0030] In the present invention, when it comes to numerical intervals (i.e., numerical ranges), unless otherwise specified, the selectable numerical values are considered continuous within the above-mentioned numerical intervals, and include the two numerical endpoints (i.e., the minimum value and the maximum value) of the numerical range, as well as each numerical value between these two numerical endpoints. Unless otherwise specified, when the numerical interval only refers to the integers within the numerical interval, it includes the two endpoint integers of the numerical range, as well as each integer between the two endpoints. In this article, it is equivalent to directly listing each integer. For example, when t is an integer selected from 1 - 10, it means that t is any integer selected from the integer group consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. In addition, when multiple ranges are provided to describe features or characteristics, these ranges can be combined. In other words, unless otherwise specified, the ranges disclosed in this article should be understood to include any and all sub-ranges subsumed therein.

[0031] For the temperature parameter in the present invention, unless otherwise specified, it allows both isothermal treatment and variation within a certain temperature range. It should be understood that the isothermal treatment allows the temperature to fluctuate within the accuracy range controlled by the instrument. It is allowed to fluctuate within a range such as ±5°C, ±4°C, ±3°C, ±2°C, ±1°C.

[0032] The present invention provides a method for preparing a TC18 titanium alloy free forging with a large length-to-thickness ratio, which is characterized by including the following steps:

[0033] A method for preparing a TC18 titanium alloy free forging with a large length-to-thickness ratio, which is characterized by including the following steps:

[0034] S1 Forging and blanking the TC18 titanium alloy bar in the (α + β) two-phase region:

[0035] Heat the TC18 titanium alloy bar in the furnace to T β -(40 - 60)°C, the holding time is ((0.6 - 1.2) × the thickness of the smallest cross-section of the bar blank) min. Take it out of the furnace and perform flat-square forging. The deformation amount per heating is controlled within 15% - 30%, and the pressing speed is controlled within 5 - 10 mm / s to obtain a cuboid blank A;

[0036] S2 Perform β-phase region forging on the cuboid blank A obtained in step S1:

[0037] Adopt the step-by-step heating method to preheat the cuboid blank A in the furnace to T β -(20 - 60)°C, the preheating holding time is ((0.5 - 1) × the thickness of the smallest cross-section of the bar) min, and then perform heating. The heating temperature is T β +(15 - 20)°C, the holding time is ((0.3 - 0.5) × the thickness of the smallest cross-section of the bar) min. Take it out of the furnace and wrap it with a heat-insulating jacket, then put it into the furnace again, at Tβ Heat at a temperature of (15 - 20)°C, and after heat preservation, perform flat-square forging by the method of pressing with a hammer twice, with the deformation amount controlled at 30 - 35% to obtain blank B;

[0038] S3 Divide the blank B obtained in step S2 into two equal parts along the width direction to obtain blank C1 and blank C2;

[0039] S4 Perform a first annealing treatment on the blank C1 obtained in step S3, and the temperature of the first annealing is (T β - 35)°C to (T β - 30)°C, keep warm, cool in the furnace to 750 - 760°C, then keep warm again, take out of the furnace and air-cool to room temperature to obtain blank D1;

[0040] S5 Perform a second annealing treatment on the blank D1 obtained in step S4, and the temperature of the second annealing is 600 - 630°C. After taking out of the furnace, air-cool to room temperature to obtain a free forging of TC18 titanium alloy;

[0041] The T β is the β transformation temperature of TC18 titanium alloy.

[0042] In some embodiments, the present invention provides a method for preparing a free forging of TC18 titanium alloy with a large aspect ratio, including the following steps:

[0043] S1 Perform forging and blank making on a TC18 titanium alloy bar in the (α + β) two-phase region:

[0044] Heat the TC18 titanium alloy bar in the furnace to T β - (40 - 60)°C, with the heat preservation time being ((0.6 - 1.2) × the thickness of the smallest cross-section of the bar blank) min. Take out of the furnace and perform flat-square forging. The deformation amount per heating is controlled at 15% - 30%. Perform hot material re-heating 2 - 3 times in the middle. The hot material re-heating time is ((0.3 - 0.6) × the thickness of the smallest cross-section of the bar) min, and the pressing speed is controlled at 5 - 10 mm / s to obtain a cuboid blank A;

[0045] S2 Perform β-phase region forging on the cuboid blank A obtained in step S1:

[0046] Adopt the method of stepped heating to preheat the cuboid blank A in the furnace to T β - (20 - 60)°C, with the preheating and heat preservation time being ((0.5 - 1) × the thickness of the smallest cross-section of the bar) min, and then perform heating. The heating temperature is T β + (15 - 20)°C, with the heat preservation time being ((0.3 - 0.5) × the thickness of the smallest cross-section of the bar) min. After taking out of the furnace, wrap it with a heat preservation jacket, put it into the furnace again, and at T βHeat at a temperature of +(15 - 20)°C, hold for 30 min, and then perform flat - square forging on a 100 MN quick - forging machine in a way of pressing down with two offset hammers. Control the deformation amount within 30 - 35%, and the final forging temperature ≥820°C to obtain blank B;

[0047] S3 Divide the blank B obtained in step S2 into two equal parts along the width direction to obtain blank C1 and blank C2;

[0048] S4 Perform a first annealing treatment on the blank C1 obtained in step S3. The temperature of the first annealing is (T β - 35)°C to (T β - 30)°C, hold for 3.5 h, cool in the furnace to 750 - 760°C, then hold for 2.5 h, and take out of the furnace and air - cool to room temperature to obtain blank D1;

[0049] S5 Perform a second annealing treatment on the blank D1 obtained in step S4. The temperature of the second annealing is 600 - 630°C, and after taking out of the furnace, air - cool to room temperature to obtain a TC18 titanium alloy open - die forging.

[0050] The T β is the β - transformation temperature of the TC18 titanium alloy.

[0051] In some other embodiments, the present invention provides a TC18 titanium alloy open - die forging with a large length - to - thickness ratio prepared by the above - mentioned preparation method. Its single weight is 90 kg, and the specification is 65 mm thick × 125 mm wide × 2060 - 2500 mm long.

[0052] The following will further illustrate the present invention with specific embodiments, but the present invention is not limited thereto.

[0053] Example 1

[0054] A method for preparing a TC18 titanium alloy open - die forging with a large length - to - thickness ratio uses a φ250 mm bar for blank making and near - β forging.

[0055] S1 Perform (α + β) two - phase region forging to make a blank from the TC18 titanium alloy bar

[0056] Billets are made on a 25MN quick forging machine. A billet with a single weight of 280 kg and a specification of φ250mm×1235mm is cut from a finished TC18 titanium alloy bar of φ250mm×L, and is forged for billet making in one heat. First, it is heated to 50°C below the phase transformation point, and the holding time is 250 min (the holding coefficient is 1, and the holding time is (the minimum cross-sectional size of the billet × the holding coefficient) min). After taking out of the furnace, it is forged into a flat square, with the deformation amount controlled at 25%, the pressing speed at 10 mm / s, the reduction amount at 40 - 60 mm, and the feed amount at 130 - 200 mm. It is forged to a thickness of 188 mm (billet size: 188×250×1293 mm) and then the hot billet is returned to the furnace. The holding time for returning to the furnace is 80 min (the holding coefficient is 0.43). After taking out of the furnace, it is forged into a flat square, with the deformation amount controlled at 21%, the pressing speed at 10 mm / s, the reduction amount at 30 - 40 mm, and the feed amount at 100 - 160 mm. It is forged to a thickness of 149 mm (billet size: 149×260×1569 mm) and then the hot billet is returned to the furnace. The holding time for returning to the furnace is 70 min (the holding coefficient is 0.47). After taking out of the furnace, it is forged into a flat square, with the deformation amount controlled at 16%, the pressing speed at 10 mm / s, the reduction amount at 25 - 35 mm, and the feed amount at 70 - 120 mm. The size of the forged billet after forging is 125×260×1862 mm. The final forging temperature throughout the forging process is controlled above 700°C.

[0057] S2 Forging the forged billet obtained in step S1 in the β-phase region

[0058] Using a 100MN quick forging machine to forge the forged billet obtained in step S1 in the β-phase region. The forged billet obtained in step S1 is charged into the furnace. First, it is heated to 30°C below the phase transformation point, and the holding time is 90 min (the holding coefficient is 0.72). Subsequently, it is heated to 18°C above the phase transformation point and held for 50 min (the holding coefficient is 0.4). After reaching the temperature, it is taken out of the furnace, wrapped with cotton as a whole and then returned to the furnace, and held for another 30 min after reaching the temperature in the furnace. After taking out of the furnace, it is forged into a flat square, with the deformation amount controlled at 32%, the pressing speed at 10 mm / s, the reduction amount: pressed in two times, the feed amount at 400 - 500 mm, pressed 5 - 6 times for the first time, pressed 5 - 6 times for the second time. After the head and tail of the billet are turned around, it is pressed down in two hammers. First, the whole billet is pressed 1 time, and the second time is pressed 2 - 4 times. Finally, the billet is shaped. First, shape the width direction, 7 hammer times, then shape the thickness direction, 6 hammer times; after turning around, first shape the thickness, press 2 - 3 hammer times, then first shape the width direction, and then shape the thickness direction continuously for 2 times. The final size of the forged billet is 85×300×2332 mm, and the final forging temperature is controlled above 820°C. Subsequently, the billet is leveled and straightened using the remaining temperature of the billet. When straightening, the billet can be placed parallel on the lower anvil, and the upper anvil is pressed down as a whole.

[0059] When forging this batch, select the heating furnace closest to the forging machine for heating and heat preservation, and the number of billets charged ≤ 2; extension of heat preservation is not allowed, and the work should be completed within 10 minutes after the heat preservation time expires.

[0060] In order to reduce the performance difference between the head and tail of the forgings, it is necessary to reduce the forging time in the β-phase region in the last heat. In the flat-square forging, the feed rate needs to be increased. However, when the feed rate is increased, the billet hardly moves in the length direction and moves more in the width direction. Therefore, the billet size after blank preparation should be longer in the length direction and narrower in the width direction.

[0061] S3 Heat Treatment

[0062] Before heat treatment, first divide the billet evenly in the width direction. After cutting, the billet size is 85(0, +5)mm × 145(-5, +0)mm × L, and then perform heat treatment on this billet. Use a heat treatment furnace that can control the cooling rate for heat treatment. The heat treatment process is as follows: primary annealing: 840°C / 3.5h, furnace cooling to 750°C / 2.5h (furnace cooling rate is 0.5°C / min), after taking out of the furnace, level and straighten, and then air cool; secondary annealing: 610°C / 5h, AC.

[0063] S4 Machining

[0064] Machining requirements: thickness 65(0, +5)mm × width 125(0, +5)mm × length 2060(0, +5)mm, the roughness of the 4 large surfaces in the width and height directions is required to be Ra ≤ 1.6μm; surface defects shall not exceed the negative tolerance of the forgings after grinding;

[0065] The specifications of the prepared finished forgings are 65mm × 125mm × 2060mm.

[0066] Perform macrostructure observation on the forgings prepared in Example 1. The microstructure is as Figure 1 shown. It can be seen that the macrostructure of the forgings is uniform, without cracks, inclusions, segregation, shrinkage cavities, pores, laminations, fine-grained bright bands (rings) and other metallurgical defects. The streamline of the forgings conforms to the regulations of the forging drawing, without obvious turbulent flow and severe eddy current. The streamline of the open-die forgings is not significantly cut off, meeting the requirements of the aviation standard.

[0067] Table 1 shows the mechanical property test results of the forgings prepared in Example 1

[0068]

[0069]

[0070] In the table, ST represents the high direction, LT represents the transverse direction, and L represents the longitudinal direction.

[0071] The forgings prepared in Example 1 were subjected to ultrasonic flaw detection, and the results were uniform, all meeting the requirements of above φ1.2 - 12 dB.

[0072] Example 2

[0073] The difference between Example 2 and Example 1 is that: before starting the quasi-β forging after the forging blanking is completed, the blank is ground, and the surface defects of the blank are inspected and ground to obtain forgings with better quality.

[0074] In addition, in order to ensure the overall performance of the forgings, the heat treatment system adopted in Example 2 is a single annealing system: charged into the furnace at the appropriate temperature, heated to 840 °C, held for 3.5 h, controlled the furnace cooling rate at 0.67 °C / min (i.e., the time from 840 °C to 760 °C is 120 min), furnace cooled to 760 °C and held for 2.5 h, taken out of the furnace, leveled and straightened, and then air cooled; double annealing system: charged into the furnace at the appropriate temperature, heated to 608 °C for 5 h, and air cooled.

[0075] Table 2 Mechanical properties of the forgings prepared in Example 2

[0076]

[0077] As can be seen from Table 2, the forgings prepared in Example 2 have good mechanical properties, and their structures are uniform and the properties are stable, meeting the requirements of aviation standards.

[0078] It should be noted that the above-described embodiments are only the preferred embodiments of the present invention. For those of ordinary skill in the art of this technology, without departing from the principle of the present invention, several modifications, improvements and equivalent replacements can be made to the present invention, and these modifications, improvements and equivalent replacements are also regarded as falling within the protection scope of the claims of the present invention.

Claims

1. A method for preparing a TC18 titanium alloy free forging with a large aspect ratio, characterized in that: The steps include: S1 forges TC18 titanium alloy bars in (ɑ+β) two-phase zone: The TC18 titanium alloy rod is heated in a furnace to T β -(40~60)℃, the holding time is ((0.6~1.2)×thickness of the minimum cross section of the bar blank)min, and the billet is flattened and forged after being taken out of the furnace. The deformation amount of each fire is controlled at 15%~30%, and the pressing speed is controlled at 5~10mm / s to obtain a rectangular billet A; S2 performs β phase forging on the rectangular blank A obtained in step S1: The rectangular blank A is preheated in a furnace to T β -(20~60)℃, preheating and holding time is ((0.5~1)×thickness of the minimum cross section of the bar)min, then heating, heating temperature is T β +(15~20)℃, the holding time is ((0.3~0.5)×the thickness of the minimum cross section of the bar)min, after being taken out of the furnace, it is wrapped with a heat-insulating bag and put back into the furnace. β Heating at a temperature of +(15-20)°C, and then maintaining the temperature, performing flattening forging by two times of hammer pressing, with the deformation controlled at 30-35%, to obtain billet B; S3: dividing the blank B obtained in step S2 into two along the width direction to obtain blanks C1 and C2; S4 performs an annealing treatment on the blank C1 obtained in step S3, and the temperature of the annealing is (T β -35)℃~(T β -30)℃, keep warm, furnace cool to 750℃~760℃, keep warm again, take out of the furnace and air cool to room temperature to obtain billet D1; S5: performing secondary annealing on the blank D1 obtained in step S4, wherein the temperature of the secondary annealing is 600° C. to 630° C., and air cooling to room temperature after being taken out of the furnace to obtain a TC18 titanium alloy free forging; The T β It is the lowest temperature at which TC18 titanium alloy is completely transformed into β phase structure during heating, referred to as β transformation temperature.

2. The method for preparing a TC18 titanium alloy free forging with a large aspect ratio according to claim 1, characterized in that: During the flattening forging in step S1, the hot material is returned to the furnace 2 to 3 times, and the heating time of the hot material is ((0.3 to 0.6) × the thickness of the minimum cross section of the bar) min.

3. The method for preparing a TC18 titanium alloy free forging with a large aspect ratio according to claim 2, characterized in that: In step S1, two hot material recycling processes are performed with insulation coefficients of 0.43 and 0.47 respectively.

4. The method for preparing a TC18 titanium alloy free forging with a large aspect ratio according to claim 1, characterized in that: In step S2, after keeping warm for 30 minutes, a 100MN fast forging machine is used to perform flattening forging by two times of hammer pressing.

5. The method for preparing a TC18 titanium alloy free forging with a large aspect ratio according to claim 1, characterized in that: In step S2, the thermal insulation bag is rock wool.

6. The method for preparing a TC18 titanium alloy free forging with a large aspect ratio according to claim 1, characterized in that: In step S2, the final forging temperature is ≥820°C.

7. The method for preparing a TC18 titanium alloy free forging with a large aspect ratio according to claim 1, characterized in that: In step S4, the temperature is kept at 3.5 hours and then cooled to 750°C-760°C in the furnace, and then kept at 2.5 hours, and then taken out of the furnace and air-cooled to room temperature.

8. The method for preparing a TC18 titanium alloy free forging with a large aspect ratio according to claim 1, characterized in that: After step S4, the method further includes sawing, surface turning and polishing the forging obtained in step S4 to obtain a machined TC18 titanium alloy free forging.

9. A TC18 titanium alloy free forging with a large aspect ratio, characterized in that: The method is prepared according to any one of claims 1 to 8.

10. The TC18 titanium alloy free forging with a large aspect ratio according to claim 9, characterized in that: The unit weight of the TC18 titanium alloy free forging is 90 kg, and the specifications are 65 mm thick×125 mm wide×2060-2500 mm long.

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