A method for preparing titanium alloy bar for aerospace fastener

Abstract

The application belongs to the technical field of titanium alloy processing, and discloses a preparation method of TC4 titanium alloy bar for aerospace fasteners. The preparation method adopts the full-process temperature control and rolling production line to roll the large single heavy hot-rolled coil with uniform rolling organization and consistent performance on the basis of electron beam cold hearth furnace and vacuum consumable arc furnace smelting, including ingot smelting, forging, polishing, first rolling, centerless peeling, pickling and polishing, second rolling and the like. The finished bar with a diameter of 6-21 mm is prepared through drawing, peeling, sawing, heat treatment and other machining processes. Compared with the preparation of finished bar by transverse rolling mill hot-rolled bar, the TC4 titanium alloy bar for fasteners prepared by the method has more uniform organization performance and higher microstructure grade, and is more in line with the service requirements of the fastener factory upsetting and the finished fastener.

Description

Technical Field

[0001] This invention belongs to the field of titanium alloy processing technology, and particularly relates to a method for preparing titanium alloy bars for aerospace fasteners. Background Technology

[0002] Titanium alloys possess advantages such as high specific strength, good corrosion resistance, and non-magnetic properties. The extensive use of titanium structural components in aircraft can significantly reduce weight and improve aircraft maneuverability and combat performance. Besides large structural components, aircraft use tens of thousands of fasteners. With the increasing demand for composite materials and titanium alloys in future advanced aircraft, the demand for titanium alloy fasteners will also increase year by year. Given the extensive use and complex operating conditions in aerospace, titanium alloy fasteners must possess high consistency and reliability to ensure no failures during use. This places higher demands on the purity, performance consistency, and microstructure grade of the bars used for titanium alloy fasteners.

[0003] Currently, most TC4 titanium alloy bars in China are smelted into ingots using a three-stage vacuum arc remelting (VAR) process. Due to the relatively low melting temperature of this technology, inclusions with high melting points and varying densities are difficult to remove, posing a risk of poor ingot purity. This significantly impacts subsequent fastener processing and operations. Furthermore, most domestic manufacturers use small-weight forging billets for straight bars, employing multi-stand horizontal rolling mills. The main grades are 430, 350, 280, and 250. This production process involves numerous uncontrollable factors, leading to potential performance differences within the same batch of billets. Moreover, limitations imposed by tooling, operator intervention, and billet temperature drop necessitate continuous sawing and grinding during billet transfer. The closer to the finished product specifications, the smaller the individual billet weight, ultimately resulting in batch-to-batch variations in microstructure and properties. As the size of the finished bar stock increases, the microstructure becomes coarser and the microstructure grade becomes lower and lower. After being upset into bolts / nuts and then heat-treated, the number of needle-like α structures increases, which seriously affects the service life of the tooling. Summary of the Invention

[0004] To address the aforementioned shortcomings of existing technologies, the present invention aims to provide a method for preparing titanium alloy bars for aerospace fasteners. This method, based on electron beam cold hearth (EB) and vacuum consumable arc furnace melting, employs a fully controlled temperature and rolling production line to roll large single-weight (200kg) hot-rolled coils with uniform microstructure and consistent properties. Finished bars (Φ6~21mm) are then produced through machining processes such as drawing, peeling, sawing, and heat treatment. Compared to producing finished bars using a transverse rolling mill, this method reduces processing costs and losses, and improves the consistency of the bar's microstructure and properties.

[0005] To achieve the above objectives, the present invention provides the following technical solution.

[0006] On one hand, the present invention provides a method for preparing TC4 titanium alloy bars for aerospace fasteners, the method comprising the following steps:

[0007] S1 ingot smelting:

[0008] The ingot electrode alloy feedstock is prepared according to a predetermined ratio. The feedstock includes sponge titanium, aluminum vanadium, aluminum granules, iron granules, TiO2, and titanium carbide. The feedstock is then smelted in an electron beam cold hearth furnace and a vacuum consumable arc furnace to obtain a TC4 titanium alloy ingot billet. The TC4 titanium alloy ingot billet is then peeled and the riser and ingot bottom are removed to prepare a TC4 titanium alloy ingot with a diameter of 720-820 mm.

[0009] S2 ingot forging:

[0010] The TC4 titanium alloy ingot obtained in step S1 is heated to 1100℃~1170℃ and held for a period of time. It is then forged to □490~520×L, ground and sawn. The forging billet is then heated to 920℃~980℃ and held for a period of time. It is then forged again to □210~240×L. The final forging temperature is ≥800℃.

[0011] S3 forging blank grinding:

[0012] The oxide scale and cracks on the surface of the forging billet obtained in step S2 are removed by grinding to obtain a polished □210~240×L forging billet.

[0013] S4 first rolling:

[0014] The □210~240×L forging billet obtained in step S3 is heated to 40~100℃ above the phase transformation point and held at that temperature. It is then rolled in multiple passes using a two-roll reversible rolling mill to obtain □140×170×L~□160×190×L rolled square billet. The rolled square billet is then heated to 40~100℃ below the phase transformation point and held at that temperature. It is then rolled in multiple passes using a two-roll reversible rolling mill to obtain Φ99~102×L rolled bar.

[0015] S5 centerless machining, peeling, acid washing and polishing:

[0016] The rolled bar obtained in step S4 is peeled off by a centerless turning process with a thickness of not less than 0.5 mm on one side, and then pickled and polished until there are no cracks on the surface.

[0017] S6 Second Rolling:

[0018] The rolled bar obtained in step S5 is heated to 40-100°C below the phase transformation point and held at that temperature. It is then subjected to low-temperature rolling in one pass using a BD600 two-roll reversible mill, an intermediate mill, and a finishing / Kocks tandem mill to obtain a hot-rolled coil with a diameter ≤22mm.

[0019] S7 machining:

[0020] The hot-rolled coil obtained in step S6 is machined by hot drawing, peeling, straightening, grinding, and chamfering to obtain TC4 titanium alloy bars for aerospace fasteners.

[0021] Furthermore, step S7 specifically includes the following steps:

[0022] S7.1 The hot-rolled coil obtained in step S6 is hot-drawn at 720-760°C using a hot drawing machine, so that the peeling and grinding amount is 70-80 filaments;

[0023] S7.2 Peel the drawn material at least 0.3mm using a centerless peeling machine until the surface is free of black skin and cracks, and then cut it to length of 2.5-3m.

[0024] S7.3 The sawn billet is placed in a box annealing furnace for heat treatment at 740-800℃ and held for 60-120 minutes. The billet is then straightened using a straightening machine to ensure that the straightness is ≤0.5mm / m.

[0025] S7.4 uses a centerless grinding mill to rough and fine grind the annealed and straightened billet, and performs multiple grinding passes depending on the specifications.

[0026] S7.5 flat-head chamfering removes burrs from the end face and chamfers 2×C1mm;

[0027] S7.6 uses a polishing machine to remove the few scratches and pulls on the surface of the bar that have not been ground off, and ensures that the dimensions of the finished bar are within the tolerance range, so as to obtain the finished TC4 titanium alloy bar.

[0028] Further, in step S1, the weight percentage of each component in the ingot electrode alloy batch includes: Al content of 6.1-6.5%, V content of 3.9-4.3%, Fe content of 0.10-0.25%, C content of 0.01-0.03%, N content of 0.001-0.010%, O content of 0.10-0.18%, and the balance being Ti.

[0029] Furthermore, in step S2, the deformation amount from blanking to □490~520×L is 47~53%; the deformation amount from secondary forging to □210~240×L is 77~84%.

[0030] Further, in step S4, the □210~240×L forging billet obtained in step S3 is heated and held at a temperature for 160~200min, and then rolled in multiple passes using a two-roll reversible rolling mill with a deformation of 32~58% to obtain a □140×170×L~□160×190×L rolled square billet; the rolled square billet is heated and held at a temperature for 120~150min, and then rolled in multiple passes using a two-roll reversible rolling mill with a deformation of 66~75% to obtain a Φ99~102×L rolled bar.

[0031] Furthermore, in step S6, the heat preservation time is 60–90 min.

[0032] Furthermore, in step S6, the deformation amount of the low-temperature rolling is 95-99%.

[0033] Furthermore, in step S7.1, the drawing speed is 1 to 1.2 m / min.

[0034] On the other hand, the present invention also provides a TC4 titanium alloy bar for aerospace fasteners, which is prepared according to the above-described preparation method.

[0035] Furthermore, the diameter of the TC4 titanium alloy rod is 6mm to 21mm.

[0036] In this invention, □ represents the side length of the cross-sectional square of the object being processed, and L represents the length of the object being processed.

[0037] Compared with the prior art, the present invention has the following beneficial effects:

[0038] 1. This invention adjusts the chemical composition and smelting method of TC4 titanium alloy ingots, and controls the forging, rolling, machining, and heat treatment processes of the bars, so that the microstructure and properties of TC4 titanium alloy bars reach the best and most stable state, making them suitable for fasteners.

[0039] 2. This invention changes the TC4 ingot melting method from 3×VAR to EB+VAR, benchmarking against top international technologies. During the melting process, the EB furnace can remove high and low density inclusions in the ingot, and the VAR melting process can make the chemical composition of the ingot more uniform.

[0040] 3. The present invention uses a 100MN fast forging mill to fully break the as-cast structure, providing a favorable environment for subsequent low-temperature forging and rolling; the use of a full-process temperature-controlled rolling production line to roll bars / hot-rolled coils reduces the temperature rise during the process, resulting in good material consistency and further ensuring the uniformity of the structure, thus maximizing the satisfaction of the standard requirements for titanium alloy bars used in aerospace fasteners.

[0041] 4. This invention regulates the microstructure of billets of different specifications by adjusting the rolling temperature and the number of heat treatments. It adopts a full-process temperature and rolling production line to produce large single-weight (200kg) hot-rolled coils with Φ8~22mm specifications and good consistency in rolling microstructure properties.

[0042] 5. Compared to producing finished bars by rolling hot-rolled bars on a transverse mill, finished bars with a diameter of Φ6 to 21 mm produced by machining processes such as drawing, peeling, sawing, and heat treatment from large single-weight (200 kg) hot-rolled coils have a higher microstructure grade and more uniform microstructure properties, which better meet the requirements of fastener manufacturing and the service requirements of finished fasteners.

[0043] 6. This invention limits the types of heating furnaces used in the production process to ensure heating uniformity in each stage of heat treatment, and specifies the transfer time of the billet after heat treatment to ensure the effect of heat treatment. Attached Figure Description

[0044] Figure 1 (a)-(b) are transverse and longitudinal micrographs of the Φ8.7mm TC4 titanium alloy rod prepared in Example 1 of the present invention.

[0045] Figure 2 (a)-(b) are transverse and longitudinal micrographs of the Φ14.5mm TC4 titanium alloy rod prepared in Example 2 of the present invention.

[0046] Figure 3 (a)-(b) are transverse and longitudinal micrographs of the Φ16.5mm TC4 titanium alloy rod prepared in Example 3 of the present invention. Detailed Implementation

[0047] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the invention.

[0048] In this invention, numerical intervals (i.e., numerical ranges) are involved. Unless otherwise specified, the selected numerical distributions within the aforementioned numerical intervals are considered continuous and include the two endpoints (i.e., the minimum and maximum values) of the numerical range, as well as every value between these two endpoints. Unless otherwise specified, when a numerical interval refers only to integers within that interval, it includes the two endpoint integers of the numerical range, as well as every integer between the two endpoints. In this document, this is equivalent to directly listing every integer. For example, if t is an integer selected from 1 to 10, it means that t is any integer selected from the group of integers consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. Furthermore, when multiple ranges are provided to describe features or characteristics, these ranges can be merged. In other words, unless otherwise specified, the ranges disclosed herein should be understood to include any and all subranges to which they are included.

[0049] Unless otherwise specified, the temperature parameters in this invention can be either constant temperature treatment or variations within a certain temperature range. It should be understood that the constant temperature treatment allows temperature fluctuations within the precision range controlled by the instrument. Fluctuations are permitted within ranges such as ±5℃, ±4℃, ±3℃, ±2℃, and ±1℃.

[0050] This invention provides a method for preparing TC4 titanium alloy bars for aerospace fasteners, characterized by comprising the following steps:

[0051] A method for preparing TC4 titanium alloy bars for aerospace fasteners, characterized by comprising the following steps:

[0052] S1 ingot smelting:

[0053] The ingot electrode alloy feedstock is prepared according to a predetermined ratio. The feedstock includes sponge titanium, aluminum vanadium, aluminum granules, iron granules, TiO2, and titanium carbide. The feedstock is then smelted in an electron beam cold hearth furnace and a vacuum consumable arc furnace to obtain a TC4 titanium alloy ingot billet. The TC4 titanium alloy ingot billet is then peeled and the riser and ingot bottom are removed to prepare a TC4 titanium alloy ingot with a diameter of 720-820 mm.

[0054] S2 ingot forging:

[0055] The TC4 titanium alloy ingot obtained in step S1 is heated to 1100℃~1170℃ and held for a period of time. It is then forged to □490~520×L, ground and sawn. The forging billet is then heated to 920℃~980℃ and held for a period of time. It is then forged again to □210~240×L. The final forging temperature is ≥800℃.

[0056] S3 forging blank grinding:

[0057] The oxide scale and cracks on the surface of the forging billet obtained in step S2 are removed by grinding to obtain a polished □210~240×L forging billet.

[0058] S4 first rolling:

[0059] The □210~240×L forging billet obtained in step S3 is heated to 40~100℃ above the phase transformation point and held at that temperature. It is then rolled in multiple passes using a two-roll reversible rolling mill to obtain □140×170×L~□160×190×L rolled square billet. The rolled square billet is then heated to 40~100℃ below the phase transformation point and held at that temperature. It is then rolled in multiple passes using a two-roll reversible rolling mill to obtain Φ99~102×L rolled bar.

[0060] S5 centerless machining, peeling, acid washing and polishing:

[0061] The rolled bar obtained in step S4 is peeled off by a centerless turning process with a thickness of not less than 0.5 mm on one side, and then pickled and polished until there are no cracks on the surface.

[0062] S6 Second Rolling:

[0063] The rolled bar obtained in step S5 is heated to 40-100°C below the phase transformation point and held at that temperature. It is then subjected to low-temperature rolling in one pass using a BD600 two-roll reversible mill, an intermediate mill, and a finishing / Kocks tandem mill to obtain a hot-rolled coil with a diameter ≤22mm.

[0064] S7 machining:

[0065] The hot-rolled coil obtained in step S6 is machined by hot drawing, peeling, straightening, grinding, and chamfering to obtain TC4 titanium alloy bars for aerospace fasteners.

[0066] In some embodiments, the present invention provides a method for preparing titanium alloy bars for aerospace fasteners, comprising the following steps:

[0067] S1 ingot smelting:

[0068] The ingot electrode alloy feedstock is prepared according to a predetermined ratio. The feedstock includes sponge titanium, aluminum vanadium, aluminum granules, iron granules, TiO2, and titanium carbide. The feedstock is then subjected to electron beam cold hearth furnace melting and vacuum consumable arc furnace melting to obtain a TC4 titanium alloy ingot billet. The TC4 titanium alloy ingot billet is then peeled and the riser and ingot bottom are removed to prepare a TC4 titanium alloy ingot with a diameter of 720-820 mm. Preferably, the feedstock electrode alloy feedstock is subjected to one electron beam cold hearth furnace melting and one vacuum consumable arc furnace melting. The weight percentages of each component in the ingot electrode alloy batch include: Al content of 6.1-6.5%, V content of 3.9-4.3%, Fe content of 0.10-0.25%, C content of 0.01-0.03%, N content of 0.001-0.010%, O content of 0.10-0.18%, and the balance being Ti.

[0069] S2 ingot forging:

[0070] The TC4 titanium alloy ingot obtained in step S1 is placed in a box-type resistance heating furnace and heated to 1100℃~1170℃ and held. It is then forged using a 100MN high-speed forging mill to □490~520×L with a deformation of 47~53%. The ingot is then polished until the surface is free of cracks and sawn off. The forging billet is then placed in a box-type resistance heating furnace and heated to 920℃~980℃ and held. It is then forged again using a 2500T high-speed forging mill to □210~240×L with a deformation of 77~84%. The final forging temperature is ≥800℃.

[0071] S3 forging blank grinding:

[0072] The oxide scale and cracks on the surface of the forging billet obtained in step S2 are removed by grinding to obtain a polished □210~240×L forging billet.

[0073] S4 first rolling:

[0074] The □210~240×L forging billet obtained in step S3 is placed in a box-type resistance heating furnace and heated to 40~100℃ above the phase transformation point, and held for 160~200min. It is then rolled in multiple passes using a two-roll reversible rolling mill with a deformation of 32~58% to obtain □140×170×L~□160×190×L rolled square billet. The □150×180×L square billet is placed in a box-type resistance heating furnace and heated to 40~100℃ below the phase transformation point, and held for 120~150min. It is then rolled in multiple passes using a two-roll reversible rolling mill with a deformation of 66~75% to obtain Φ99~102×L rolled bar.

[0075] S5 centerless machining, peeling, acid washing and polishing:

[0076] The rolled bar obtained in step S4 is peeled off by a centerless turning process with a thickness of not less than 0.5 mm on one side, and then pickled and polished until there are no cracks on the surface.

[0077] S6 Second Rolling:

[0078] The rolled bar obtained in step S5 is placed in a box-type resistance heating furnace and heated to 40-100°C below the phase transformation point, and held for 60-90 minutes. Then, the bar is subjected to one-fire low-temperature rolling in sequence using a BD600 two-roll reversible rolling mill, an intermediate rolling mill, and a finishing / Kocks tandem rolling mill. The cumulative deformation is 95-99%, resulting in a hot-rolled coil with a diameter ≤22mm.

[0079] S7 machining:

[0080] The hot-rolled coil obtained in step S6 is machined by hot drawing, peeling, straightening, grinding, and chamfering to obtain TC titanium alloy finished bars.

[0081] Preferably, step S7 specifically includes the following steps:

[0082] S7.1 The hot-rolled coil obtained in step S6 is hot-drawn at 720-760°C using a hot drawing machine to ensure that the subsequent peeling and grinding amount is 70-80 filaments. Preferably, the drawing speed is 1-1.2 m / min.

[0083] S7.2 Peel at least 0.3mm of the drawn material using a centerless peeling machine until the surface is free of black skin and cracks, and then saw it to length.

[0084] S7.3 The sawn billet is placed in a box annealing furnace for heat treatment at 740-800℃ and held for 60-120 minutes. The billet is straightened using a straightening machine so that the straightness is ≤0.5mm / m. Preferably, the time interval between straightening the first billet and the last billet does not exceed 45 minutes.

[0085] S7.4 uses a centerless grinding mill to rough and fine grind the annealed and straightened billet, and performs multiple grinding passes depending on the specifications.

[0086] S7.5 flat-head chamfering removes burrs from the end face and chamfers 2×C1mm.

[0087] S7.6 A polishing machine is used to remove the few scratches and scratches on the surface of the bar that were not ground off, ensuring that the dimensions of the finished bar are within the tolerance range, thus obtaining the finished TC titanium alloy bar. The specific machining process in this step is as follows: Finished bar machining process: billet → hot drawing → centerless turning for peeling → sawing → annealing and straightening → centerless grinding → flattening and chamfering → polishing → 100% ultrasonic testing → quality inspection (Φ6mm~21mm).

[0088] To clarify the purpose, technical solution, and advantages of this invention, the invention will be described in detail below with reference to specific embodiments and accompanying drawings. The specific embodiments described herein are only used to explain the invention, and the invention is not limited thereto.

[0089] Example 1

[0090] This embodiment provides a method for preparing titanium alloy bars for aerospace fasteners, including the following steps:

[0091] S1 ingot smelting:

[0092] The ingot electrode alloy feedstock is prepared according to a predetermined ratio. The feedstock includes sponge titanium, aluminum vanadium, aluminum granules, iron granules, TiO2, and titanium carbide. The feedstock is then subjected to one electron beam cold hearth furnace melting and one vacuum consumable arc furnace melting to obtain a TC4 titanium alloy ingot billet. The TC4 titanium alloy ingot billet is then peeled and the riser and ingot bottom are removed to prepare a TC4 titanium alloy ingot with a diameter of 800 mm. The percentage of each component in the TC4 titanium alloy ingot billet in this embodiment is shown in Table 1 below.

[0093] S2 ingot forging:

[0094] The TC4 titanium alloy ingot obtained in step S1 is placed in a box-type resistance heating furnace and heated to 1100℃ and held. It is then forged to □500×L using a 100MN high-speed forging machine, with a deformation of 50.2%. The ingot is then polished until there are no cracks on the surface and sawn off. The forging billet is then placed in a box-type resistance heating furnace and heated to 950℃ and held. It is then forged again to □220×L using a 2500T high-speed forging machine, with a deformation of 80.6%. The final forging temperature is ≥800℃.

[0095] S3 forging blank grinding:

[0096] The oxide scale and cracks on the surface of the forging billet obtained in step S2 are removed by grinding to obtain a polished □220×L forging billet.

[0097] S4 first rolling:

[0098] The □220×L forging billet obtained in step S3 is placed in a box-type heating resistance furnace and heated to 1040℃, and held for 180 min. It is then rolled in multiple passes using a two-roll reversible rolling mill, with a deformation of 55.8%, to obtain a □150×180×L rolled square billet. The □150×180×L square billet is then placed in a box-type heating resistance furnace and heated to 940℃, and held for 120 min. It is then rolled in multiple passes using a two-roll reversible rolling mill, with a deformation of 69.7%, to obtain a Φ102×L rolled bar.

[0099] S5 centerless machining, peeling, acid washing and polishing:

[0100] The rolled bar obtained in step S4 is peeled off by a centerless turning process with a thickness of not less than 0.5 mm on one side, and then pickled and polished until there are no cracks on the surface.

[0101] S6 Second Rolling:

[0102] The Φ100×L rolled bar obtained in step S5 was placed in a box-type resistance heating furnace and heated to 920℃ and held for 60 minutes. It was then subjected to one-fire low-temperature rolling in sequence using a BD600 two-roll reversible rolling mill, an intermediate rolling mill, and a finishing / Kocks tandem rolling mill. The cumulative deformation was 98.7%, resulting in a hot-rolled coil with a diameter of 11.2 mm.

[0103] S7 machining:

[0104] S7.1 The hot-rolled coil obtained in step S6 is hot-drawn at 740°C using a hot drawing machine at a drawing speed of 1 to 1.2 m / min, and drawn to Φ9.4 mm.

[0105] S7.2 Use a centerless peeling machine to peel the drawn material to 9.1mm, with no black skin or cracks on the surface, and then saw it at 3000mm intervals;

[0106] S7.3 The sawn billet is placed in a box annealing furnace for heat treatment at 750℃ and held for 60 minutes. The billet is straightened using a straightening machine so that the straightness is ≤0.5mm / m. Preferably, the time interval between straightening the first billet and the last billet does not exceed 45 minutes.

[0107] S7.4 uses a centerless grinding mill to rough and fine grind the annealed and straightened billet, and grinds it to Φ8.7mm in multiple passes depending on the specifications.

[0108] S7.5 flat-head chamfering removes burrs from the end face and chamfers 2×C1mm;

[0109] S7.6 uses a polishing machine to remove the few scratches and scratches on the surface of the bar that have not been ground off, ensuring the dimensional tolerance of the finished bar, and obtaining a TC4 titanium alloy finished bar with a diameter of 8.7mm.

[0110] In this embodiment, the box-type resistance heating furnace in steps S2, S4, and S6 meets the Class III furnace requirements in HB5425 "Method for Determining the Effective Heating Zone of Heat Treatment Furnaces for Aerospace Parts", and the box-type annealing furnace used for heat treatment in step S7 meets the Class 2 furnace requirements in AMS2750E "High Temperature Determination Method".

[0111] Example 2

[0112] This embodiment provides a method for preparing titanium alloy bars for aerospace fasteners, including the following steps:

[0113] S1 ingot smelting:

[0114] The ingot electrode alloy feedstock is prepared according to a predetermined ratio. The feedstock includes sponge titanium, aluminum vanadium, aluminum granules, iron granules, TiO2, and titanium carbide. The feedstock is then subjected to one electron beam cold hearth furnace melting and one vacuum arc furnace melting to obtain a TC4 titanium alloy ingot billet. The TC4 titanium alloy ingot billet is then peeled, and the riser and ingot bottom are removed to prepare a TC4 titanium alloy ingot with a diameter of 800 mm. The percentages of each component in the TC4 titanium alloy ingot billet in this embodiment are shown in Table 1 below.

[0115] S2 ingot forging:

[0116] The TC4 titanium alloy ingot obtained in step S1 is placed in a box-type resistance heating furnace and heated to 1100℃ and held. It is then forged to □500×L using a 100MN high-speed forging machine, with a deformation of 50.2%. The ingot is then polished until there are no cracks on the surface and sawn off. The forging billet is then placed in a box-type resistance heating furnace and heated to 950℃ and held. It is then forged again to □220×L using a 2500T high-speed forging machine, with a deformation of 80.6%. The final forging temperature is ≥800℃.

[0117] S3 forging blank grinding:

[0118] The oxide scale and cracks on the surface of the forging billet obtained in step S2 are removed by grinding to obtain a polished □220×L forging billet.

[0119] S4 first rolling:

[0120] The □220×L forging billet obtained in step S3 is placed in a box-type heating resistance furnace and heated to 1040℃, and held for 180 min. It is then rolled in multiple passes using a two-roll reversible rolling mill, with a deformation of 55.8%, to obtain a □150×180×L rolled square billet. The □150×180×L square billet is then placed in a box-type heating resistance furnace and heated to 940℃, and held for 120 min. It is then rolled in multiple passes using a two-roll reversible rolling mill, with a deformation of 69.7%, to obtain a Φ102×L rolled bar.

[0121] S5 centerless machining, peeling, acid washing and polishing:

[0122] The rolled bar obtained in step S4 is peeled off by a centerless turning process with a thickness of not less than 0.5 mm on one side, and then pickled and polished until there are no cracks on the surface.

[0123] S6 Second Rolling:

[0124] The Φ100×L rolled bar obtained in step S5 was placed in a box-type resistance heating furnace and heated to 920℃ and held for 60 minutes. It was then subjected to one-fire low-temperature rolling in sequence using a BD600 two-roll reversible rolling mill, an intermediate rolling mill, and a finishing / Kocks tandem rolling mill. The cumulative deformation was 97.7%, resulting in a hot-rolled coil with a diameter of 15.2 mm.

[0125] S7 machining:

[0126] S7.1 Use a centerless peeling machine to peel the drawn material to 14.9mm, with no black skin or cracks on the surface, and then saw it at 3000mm intervals;

[0127] S7.2 The sawn billet is placed in a box annealing furnace for heat treatment at 750℃ and held for 60 minutes. The billet is straightened using a straightening machine so that the straightness is ≤0.5mm / m. Preferably, the time interval between straightening the first billet and the last billet does not exceed 45 minutes.

[0128] S7.3 uses a centerless grinding mill to rough and fine grind the annealed and straightened billet, and grinds it to Φ14.5mm in multiple passes according to different specifications;

[0129] S7.4 Flat-head chamfering removes burrs from the end face and chamfers 2×C1mm;

[0130] S7.5 uses a polishing machine to remove the few scratches and scratches on the surface of the bar that have not been ground off, ensuring the dimensional tolerance of the finished bar, and obtaining a 14.5mm TC4 titanium alloy finished bar.

[0131] In this embodiment, the box-type resistance heating furnace in steps S2, S4, and S6 meets the Class III furnace requirements in HB5425 "Method for Determining the Effective Heating Zone of Heat Treatment Furnaces for Aerospace Parts", and the box-type annealing furnace used for heat treatment in step S7 meets the Class 2 furnace requirements in AMS2750E "High Temperature Determination Method".

[0132] Example 3

[0133] This embodiment provides a method for preparing titanium alloy bars for aerospace fasteners, including the following steps:

[0134] S1 ingot smelting:

[0135] The ingot electrode alloy feedstock is prepared according to a predetermined ratio. The feedstock includes sponge titanium, aluminum vanadium, aluminum granules, iron granules, TiO2, and titanium carbide. The feedstock is then subjected to one electron beam cold hearth furnace melting and one vacuum arc furnace melting to obtain a TC4 titanium alloy ingot billet. The TC4 titanium alloy ingot billet is then peeled, and the riser and ingot bottom are removed to prepare a TC4 titanium alloy ingot with a diameter of 800 mm. The percentage content of each component in this embodiment is shown in Table 1 below.

[0136] S2 ingot forging:

[0137] The TC4 titanium alloy ingot obtained in step S1 is placed in a box-type resistance heating furnace and heated to 1100℃ and held. It is then forged to □500×L using a 100MN high-speed forging machine, with a deformation of 50.2%. The ingot is then polished until there are no cracks on the surface and sawn off. The forging billet is then placed in a box-type resistance heating furnace and heated to 950℃ and held. It is then forged again to □220×L using a 2500T high-speed forging machine, with a deformation of 80.6%. The final forging temperature is ≥800℃.

[0138] S3 forging blank grinding:

[0139] The oxide scale and cracks on the surface of the forging billet obtained in step S2 are removed by grinding to obtain a polished □220×L forging billet.

[0140] S4 first rolling:

[0141] The □220×L forging billet obtained in step S3 is placed in a box-type heating resistance furnace and heated to 1040℃, and held for 180 min. It is then rolled in multiple passes using a two-roll reversible rolling mill to obtain a □150×180×L rolled square billet with a deformation of 55.8%. The □150×180×L square billet is then placed in a box-type heating resistance furnace and heated to 940℃, and held for 120 min. It is then rolled in multiple passes using a two-roll reversible rolling mill to obtain a Φ102×L rolled bar with a deformation of 69.7%.

[0142] S5 centerless machining, peeling, acid washing and polishing:

[0143] The rolled bar obtained in step S4 is peeled off by a centerless turning process with a thickness of not less than 0.5 mm on one side, and then pickled and polished until there are no cracks on the surface.

[0144] S6 Second Rolling:

[0145] The Φ100×L rolled bar obtained in step S5 was placed in a box-type resistance heating furnace and heated to 920℃ and held for 60 minutes. It was then subjected to one-fire low-temperature rolling in sequence using a BD600 two-roll reversible rolling mill, an intermediate rolling mill, and a finishing / Kocks tandem rolling mill. The cumulative deformation was 97.0%, resulting in a hot-rolled coil with a diameter of 17.2 mm.

[0146] S7 machining:

[0147] S7.1 Use a centerless peeling machine to peel the drawn material to 16.9mm, with no black skin or cracks on the surface, and then saw it at 3000mm intervals;

[0148] S7.2 The sawn billet is placed in a box annealing furnace for heat treatment at 750℃ and held for 60 minutes. The billet is straightened using a straightening machine so that the straightness is ≤0.5mm / m. Preferably, the time interval between straightening the first billet and the last billet does not exceed 45 minutes.

[0149] S7.3 uses a centerless grinding mill to rough and fine grind the annealed and straightened billet, and grinds it to Φ16.5mm in multiple passes according to different specifications;

[0150] S7.4 Flat-head chamfering removes burrs from the end face and chamfers 2×C1mm;

[0151] S7.5 uses a polishing machine to remove the few scratches and scratches on the surface of the bar that were not ground off, ensuring the dimensional tolerance of the finished bar, and obtaining a 16.5mm TC4 titanium alloy finished bar.

[0152] In this embodiment, the box-type resistance heating furnace in steps S2, S4, and S6 meets the Class III furnace requirements in HB5425 "Method for Determining the Effective Heating Zone of Heat Treatment Furnaces for Aerospace Parts", and the box-type annealing furnace used for heat treatment in step S7 meets the Class 2 furnace requirements in AMS2750E "High Temperature Determination Method".

[0153] Table 1 shows the component content of TC4 titanium alloy ingots in Examples 1 to 3.

[0154] Table 1. Ingot component content (wt%)

[0155] Location Al V Fe C N O superior 6.17 4.08 0.10 0.019 0.004 0.144 middle 6.16 4.09 0.13 0.02 0.004 0.145 Down 6.15 4.01 0.12 0.02 0.004 0.141

[0156] Figures 1 to 3 The figures show schematic diagrams of the transverse and longitudinal microstructures of 8.7mm, 14.5mm, and 16.5mm TC4 titanium alloy bars prepared in Examples 1 to 3 of this invention. As can be seen from the figures, the microstructure of the finished TC4 titanium alloy bars consists of equiaxed α+ intergranular β. Compared with other straight bar preparation processes, the microstructure of this invention has smaller grains, which better meets the requirements of A1~A3 and AA1~AA3 in the ETTC2 standard for TC4 titanium alloy bars used in aerospace fasteners. The higher microstructure grade is beneficial for improving the service life of tooling after upsetting into bolts / nuts and subsequent heat treatment.

[0157] Table 2 below shows the properties of the TC4 titanium alloy bars prepared in Examples 1 to 3 of the present invention.

[0158]

[0159] In Table 2, M represents the annealed state. As can be seen from Table 1, the mechanical properties of the TC4 titanium alloy bars prepared in Examples 1 to 3 of this invention all meet customer requirements. The preparation of straight bars from coiled wire results in better batch-to-batch performance stability and is beneficial for the upsetting of TC4 titanium alloy bars.

[0160] It should be noted that the embodiments described above are merely preferred embodiments of the present invention. For those skilled in the art, various modifications, improvements, and equivalent substitutions can be made to the present invention without departing from its principles, and such modifications, improvements, and equivalent substitutions are also considered to fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing TC4 titanium alloy bars for aerospace fasteners, characterized in that, Includes the following steps: S1 ingot smelting: The ingot electrode alloy feedstock is prepared according to a predetermined ratio. The feedstock includes sponge titanium, aluminum vanadium, aluminum granules, iron granules, TiO2, and titanium carbide. The feedstock is then smelted in an electron beam cold hearth furnace and a vacuum consumable arc furnace to obtain a TC4 titanium alloy ingot billet. The TC4 titanium alloy ingot billet is then peeled and the riser and ingot bottom are removed to prepare a TC4 titanium alloy ingot with a diameter of 720-820 mm. In step S1, the weight percentages of each component in the ingot electrode alloy batch are as follows: Al content 6.1–6.5%, V content 3.9–4.3%, Fe content 0.10–0.25%, C content 0.01–0.03%, N content 0.001–0.010%, O content 0.10–0.18%, with the balance being Ti. S2 Ingot Forging: The TC4 titanium alloy ingot obtained in step S1 is heated to 1100℃~1170℃ and held for a period of time. It is then forged to □490~520×L, ground and sawn. The forging billet is then heated to 920℃~980℃ and held for a period of time. It is then forged again to □210~240×L. The final forging temperature is ≥800℃. S3 Forging Billet Grinding: The oxide scale and cracks on the surface of the forging billet obtained in step S2 are removed by grinding to obtain a polished □210~240×L forging billet. S4 First rolling: The □210~240×L forging billet obtained in step S3 is heated to 40~100℃ above the phase transformation point and held at that temperature. It is then rolled in multiple passes using a two-roll reversible rolling mill to obtain □140×170×L~□160×190×L rolled square billet. The rolled square billet is then heated to 40~100℃ below the phase transformation point and held at that temperature. It is then rolled in multiple passes using a two-roll reversible rolling mill to obtain Φ99~102×L rolled bar. In step S4, the □210~240×L forging billet obtained in step S3 is heated and held at a temperature for 160~200min, and then rolled in multiple passes using a two-roll reversible rolling mill with a deformation amount of 32~58% to obtain a □140×170×L~□160×190×L rolled square billet; the rolled square billet is then heated and held at a temperature for 120~150min, and then rolled in multiple passes using a two-roll reversible rolling mill with a deformation amount of 66~75% to obtain a Φ99~102×L rolled bar. S5 centerless machining, peeling, acid washing and polishing: The rolled bar obtained in step S4 is peeled off by a centerless turning process with a thickness of not less than 0.5 mm on one side, and then pickled and polished until there are no cracks on the surface. S6 Second Rolling: The rolled bar obtained in step S5 is heated to 40-100°C below the phase transformation point and held at that temperature. It is then subjected to low-temperature rolling in one pass using a BD600 two-roll reversible mill, an intermediate mill, and a finishing / Kocks tandem mill to obtain a hot-rolled coil with a diameter ≤22mm. S7 machining: The hot-rolled coil obtained in step S6 is machined by hot drawing, peeling, straightening, grinding, and chamfering to obtain TC4 titanium alloy bars for aerospace fasteners. Step S7 specifically includes the following steps: S7.1 The hot-rolled coil obtained in step S6 is hot-drawn at 720-760°C using a hot drawing machine, so that the peeling and grinding amount is 70-80 filaments; S7.2 Peel the drawn material at least 0.3 mm with a centerless peeling machine until the surface is free of black skin and cracks, and then cut it to length of 2.5-3m. S7.3 The sawn billet is placed in a box annealing furnace for heat treatment at 740-800℃ and held for 60-120 minutes. The billet is then straightened using a straightening machine to ensure that the straightness is ≤0.5mm / m. S7.4 uses a centerless grinding mill to rough and fine grind the annealed and straightened billet, and performs multiple grinding passes depending on the specifications. S7.5 flat-head chamfering removes burrs from the end face and chamfers 2×C1mm; S7.6 uses a polishing machine to remove the few scratches and pulls on the surface of the bar that have not been ground off, and ensures that the dimensions of the finished bar are within the tolerance range, so as to obtain the finished TC4 titanium alloy bar.

2. The method for preparing TC4 titanium alloy bars for aerospace fasteners according to claim 1, characterized in that, In step S2, the deformation amount from blanking to □490~520×L is 47~53%; the deformation amount from secondary forging to □210~240×L is 77~84%.

3. The method for preparing TC4 titanium alloy bars for aerospace fasteners according to claim 1, characterized in that, In step S6, the heat preservation time is 60-90 minutes.

4. The method for preparing TC4 titanium alloy bars for aerospace fasteners according to claim 1, characterized in that, In step S6, the deformation amount of the first-fire low-temperature rolling is 95-99%.

5. The method for preparing TC4 titanium alloy bars for aerospace fasteners according to claim 1, characterized in that, In step S7.1, the drawing speed is 1 to 1.2 m / min.

6. A TC4 titanium alloy bar for aerospace fasteners, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 5.

7. The TC4 titanium alloy bar according to claim 6, characterized in that, The diameter of the TC4 titanium alloy rod is 6mm to 21mm.

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