CN112828060B

CN112828060B - Ultrasonic-assisted blocking type rotary gradient extrusion method and device for titanium alloy fastener

Info

Publication number: CN112828060B
Application number: CN202110014055.8A
Authority: CN
Inventors: 李萍; 薛克敏; 严思梁
Original assignee: Hefei University of Technology
Current assignee: Hefei University of Technology
Priority date: 2021-01-06
Filing date: 2021-01-06
Publication date: 2023-06-16
Anticipated expiration: 2041-01-06
Also published as: CN112828060A

Abstract

The invention provides a titanium alloy fastener ultrasonic auxiliary blocking type rotary gradient extrusion method and a device, wherein a fixed female die and a rotary female die are utilized to form an upper and lower blocking type floating female die structure, a blocked variable-section cavity is formed in the floating female die by a punch and an ejector rod, and a gradient forming area with a diameter tapered ladder structure is formed in the inner cavity of the rotary female die; the fixed concave die can be tightly attached to the rotary concave die all the time in the forming process; mechanical vibration formed by converting an ultrasonic generator through an acoustic guide rod is used as driving force for extrusion blank forming of a punch, and a titanium alloy fastener superfine crystal gradient tissue blank is formed by a gradient extrusion and high-pressure torsion compound process based on an ultrasonic vibration auxiliary forming technology. The invention can enhance the shearing deformation capability and grain refinement effect of the gradient extrusion process, reduce the forming load, improve the forming performance of the material, improve the forming limit of the material and greatly deform the titanium alloy difficult to deform under the condition of relatively low temperature.

Description

Ultrasonic-assisted blocking type rotary gradient extrusion method and device for titanium alloy fastener

Technical Field

The invention relates to the technical field of material processing, in particular to an ultrasonic-assisted blocking type rotary gradient extrusion method and device for a titanium alloy fastener.

Background

The titanium alloy fastener has the excellent comprehensive performances of light weight, high strength, high temperature resistance, fatigue resistance, corrosion resistance and the like, and is an important development direction of high-performance light-weight structural members in the modern aerospace field. Titanium alloy fasteners, represented by titanium alloy bolts, are generally formed by upsetting a bar stock formed by forging and forging; in order to increase the strength of the screw, there are also processes in which the stem is formed by upsetting a head, extruding or other process. Titanium alloys have poor thermal conductivity, large friction coefficient, strong adhesion and difficult forming. Cold forming is adopted, so that defects such as folding, cracking, internal cavity, adiabatic shearing, surface cracking and the like are easy to generate; by adopting warm forming, a larger temperature gradient can be formed in the material due to the skin effect of induction heating, so that the grain structure of the surface layer is thicker than that in the material. In addition, the traditional deformation process has small deformation amount and unsatisfactory material deformation refinement strengthening effect.

The titanium alloy fastener is subjected to shearing, fretting wear, stretching, compression, stress corrosion, bending, fatigue and the like in the working process, and the stress condition is very complex. The titanium alloy has low surface hardness and poor thermal conductivity, and is extremely easy to generate adhesive wear and abrasive particle wear; in addition, under the action of micro-motion and alternating stress, fatigue cracks are usually initiated on the surface, and the service life and reliability of parts are seriously affected. The difficulty in improving the strong plasticity simultaneously weakens the industrial application potential of the titanium alloy fastener to a certain extent, and becomes one of bottleneck problems restricting the development of the titanium alloy fastener. The existing plastic forming process is difficult to meet the requirements of the development of aerospace technology on higher strength plasticity, fatigue resistance, wear resistance and the like of the titanium alloy fastener, and the limitation of the prior art needs to be broken through, so that a new forming process is developed.

Research shows that the multi-stage construction of the material structure can effectively overcome the performance defects of the material and exert the performance advantages of the material, and the gradient structure is one important construction type. In particular, when the surface layer of the material is nanocrystalline or ultrafine grain and the grain size is distributed in a gradient manner from small to large from the surface layer to the core part, the material has excellent plasticity and high strength, and the wear resistance, fatigue resistance and surface alloying performance are obviously improved, so that a new thought is provided for improving the service performance of the aerospace titanium alloy fastener. Currently, gradient tissue is generally prepared by gradient plastic deformation, gradient physical or chemical deposition methods. The newly developed gradient extrusion process is one of the most developed plastic deformation processes for preparing gradient structures. The gradient extrusion is to realize gradual accumulation of large strain on the surface layer of the material through a gradient forming unit, effectively promote crushing refinement of crystal grains from outside to inside, further realize gradual reduction of the size of blank crystal grains from inside to outside, greatly improve the mechanical property of the surface layer, improve the ductility of the core part, and further improve the comprehensive mechanical property of the material, thereby providing a new technical approach for processing high-performance titanium alloy fasteners. However, the gradient extrusion process has large forming load, is difficult to form the difficult-to-deform high-strength titanium alloy, and is easy to generate defects such as surface cracking. Although the formability of the titanium alloy can be improved by increasing the forming temperature, the accumulation of high-energy defects in the material can be reduced, and the grain crushing and refining effects in the plastic deformation process are affected; and the crystal grains are easily coarsened due to long-time heating and high-temperature deformation. In particular, the gradient extrusion process has limited deformation and grain refinement effect, and the preparation of superfine grain gradient structure is difficult to realize.

Disclosure of Invention

Aiming at the defects and limitations of the traditional production process applied to the titanium alloy fastener difficult to deform, the invention provides an ultrasonic auxiliary blocking type rotary gradient extrusion method of the titanium alloy fastener and a device for realizing the method, so that the integrated manufacturing of the forming property of a high-performance titanium alloy fastener blank can be realized, the shearing deformation capacity and the grain refinement effect of the gradient extrusion process are enhanced, the forming load is reduced, the forming property of a material is improved, and the forming limit of the material is improved, thereby greatly deforming the titanium alloy difficult to deform under the condition of relatively low deformation temperature, and obtaining a stable and controllable superfine grain gradient tissue structure from the surface layer to the inside.

In order to achieve the above purpose, the invention adopts the following technical scheme:

an ultrasonic auxiliary blocking type rotary gradient extrusion method for a titanium alloy fastener is characterized in that:

the method comprises the steps that a fixed female die and a rotary female die which can do rotary motion around a central axis relative to the fixed female die form an up-down block blocking type floating female die structure, a punch head and an ejector rod which can do linear motion along the central axis of the floating female die are used for closing two ends of a cavity in a forming process, so that a blocked variable cross section cavity is formed in the floating female die, and the variable cross section cavity is a gradient forming area with a stepped structure with a diameter gradually reduced along the forming direction in an inner cavity of the rotary female die; the elastic mechanism is arranged, so that the fixed concave die can float along the central axis direction in the forming process so as to be tightly attached to the rotary concave die all the time; mechanical vibration formed by converting an ultrasonic generator through an acoustic guide rod is used as driving force for extrusion blank forming of a punch, and a titanium alloy fastener superfine crystal gradient tissue blank is formed by a gradient extrusion and high-pressure torsion compound process based on an ultrasonic vibration auxiliary forming technology.

The method is also characterized by comprising the following steps:

step 1, taking a conventional uniform coarse-grain titanium alloy bar after smelting and forging as an initial blank, putting one end of an initial blank with a round cross section from one end of a fixed female die into a cavity of a floating female die, and coaxially pressing a punch on the end face of one outward side of the initial blank along the forming direction;

step 2, starting an ultrasonic generator, converting ultrasonic frequency electric oscillation into mechanical vibration by a sound guide rod and descending, driving a punch to perform ultrasonic vibration, and enabling the punch to extrude an initial blank in a forming direction to descend in a cavity of a floating female die;

step 3, the initial blank enters into the inner cavity of a rotary female die which performs rotary motion under the extrusion of a punch, and axial-tangential dual-mode severe gradient shear deformation occurs in a gradient forming area in the rotary female die until the extrusion is completed in the rotary female die, so that a titanium alloy fastener superfine crystal gradient tissue blank is formed;

and 4, ejecting the superfine crystal gradient tissue blank of the titanium alloy fastener by using an ejector rod.

The invention also provides a device for realizing the ultrasonic auxiliary blocking type rotary gradient extrusion method of the titanium alloy fastener, which is structurally characterized in that:

a floating die formed by an upper fixed die and a lower rotary die is arranged, a vertical variable-section die cavity is formed in the floating die and is used as a forming channel of a titanium alloy fastener, and a punch and an ejector rod are respectively sealed at a fixed die end and a rotary die end of the variable-section die cavity, so that the variable-section die cavity is blocked and can linearly displace along the central axis of the die cavity; the bottom end of the rotary female die is arranged on the lower die plate and can synchronously rotate along with the lower die plate around the central axis of the die cavity, the inner cavity forms a gradient forming area with a stepped structure with a gradually-reduced diameter along the forming direction, the fixed female die is elastically connected with the lower end of the upper die plate above through an elastic mechanism, and the fixed female die can float up and down along the central axis of the die cavity under the elastic action of the elastic mechanism in the forming process so as to be always tightly attached to the rotary female die;

the ultrasonic vibration auxiliary forming component comprises an ultrasonic generator and a sound guide rod, the sound guide rod converts ultrasonic frequency electric oscillation of the ultrasonic generator into mechanical vibration, the lower rod end of the sound guide rod is connected with the upper end of the upper template, and the punch is driven to perform ultrasonic vibration through the upper template during descending, so that the punch forms extrusion of initial blanks in a variable-section cavity of the floating female die.

The device is also characterized in that the structure of the elastic mechanism is as follows:

the lower end of the upper die plate is connected with the upper end of the fixed die through a plurality of springs which are circumferentially equidistantly arranged.

The variable cross-section cavity of the floating female die is used for receiving an initial blank at a fixed female die end, and the inner diameter of the inner cavity of the fixed female die is adapted to the outer diameter of the initial blank;

the lower die plate is provided with an ejection hole by taking the central axis of the floating die cavity as the center, and the ejection rod is penetrated in the ejection hole in an adaptive manner;

the top end of the inner cavity of the floating die is connected with the inner cavity of the fixed die through an upper transition section which is adapted to the external dimension of the inner cavity of the fixed die, and the bottom end of the floating die is connected with the ejection hole through a lower transition section which is adapted to the external dimension of the ejection hole; the gradient forming area is connected between the upper transition section and the lower transition section, and is a short column section which is of a multi-section ladder structure and is of a diameter decreasing type along the forming direction, the cross section of the last short column section is round and is of a shape and size which is adapted to the superfine crystal gradient tissue blank of the titanium alloy fastener to be formed, the cross sections of other short column sections are oblate, and the adjacent two short column sections, the uppermost short column section and the upper transition section and the last short column section and the lower transition section are connected through inclined planes for smooth transition;

the inner diameter of the ejection hole is larger than that of the short column section of the last section.

The ultrasonic generator is supported by the bracket and is positioned above the upper template.

Compared with the prior art, the invention has the beneficial effects that:

1. the invention adopts the ultrasonic auxiliary blocking type rotary gradient extrusion method to manufacture the titanium alloy fastener blank, can obtain an ultra-fine grain gradient tissue structure, obviously improves comprehensive mechanical properties such as strong plasticity, fatigue resistance, wear resistance and the like, and avoids the defects of limited grain refinement effect, difficult satisfaction of strong plasticity and the like of the existing manufacturing process;

2. the invention adopts the ultrasonic auxiliary blocking type rotary gradient extrusion method, can obviously enhance the hydrostatic pressure and the process plasticity of the process, improve the forming property of the titanium alloy at relatively low temperature, improve the plastic deformation capacity of the material, and overcome the defects of small deformation and coarsening of crystal grains of the traditional titanium alloy fastener cold and hot manufacturing process;

3. the invention adopts the ultrasonic auxiliary blocking type rotary gradient extrusion method, can obviously enhance the shearing deformation capacity and the grain refining effect of the gradient extrusion process, improve the gradient distribution of grain size, improve the comprehensive mechanical property of the material and overcome the defect of limited shearing deformation capacity and grain refining effect of the traditional gradient extrusion process;

4. the invention adopts the ultrasonic auxiliary blocking type rotary gradient extrusion method, can greatly reduce the forming load, improve the formability of the titanium alloy difficult to deform, improve the accumulation limit of severe plastic deformation, and avoid the defects of large forming load, high extrusion temperature, large friction and abrasion, easy cracking and the like in the extrusion process of the difficult-to-deform material.

Drawings

FIG. 1 is a schematic diagram of the structure of the present invention;

FIG. 2 is a schematic view of the structure of the initial blank;

FIG. 3 is a schematic illustration of a forming process of a titanium alloy fastener superfine grain gradient texture blank;

FIG. 4 is a schematic structural view of a titanium alloy fastener superfine crystal gradient structure blank obtained by forming;

FIG. 5 is a schematic diagram of the structure of an upper and lower block-blocked floating die;

fig. 6 is a schematic top view of the upper and lower block-type floating die.

In the figure, 1 initial blank; 2, a sound guide rod; 3, punching heads; 4, upper template; 5, lower template; 6, fixing a female die; 7, rotating the female die; 8, a spring; 9, an ejector rod; 10 titanium alloy fastener superfine crystal gradient tissue blank; an ultrasonic generator; 12 brackets.

Detailed Description

For the purpose of making the objects, technical solutions and advantages of the embodiments of the present invention more apparent, the technical solutions in the embodiments of the present invention will be clearly and completely described in the following in conjunction with the embodiments of the present invention, and it is apparent that the described embodiments are some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.

The aerospace fastener has high strength and good plasticity and toughness, and the commonly adopted structural material is high-temperature titanium alloy. However, titanium alloys have poor thermal conductivity and formability, and the existing plastic forming process is difficult to meet the requirements of higher strength plasticity, fatigue resistance, wear resistance and the like. The invention aims to enhance the shearing deformation capacity and the grain refining effect of the gradient extrusion process, reduce the forming load, improve the forming performance of the material and improve the forming limit, so that the titanium alloy difficult to deform can be greatly deformed at a relatively low temperature, an ultrafine grain gradient tissue structure from the surface layer to the inside can be obtained, the performance potential of the material is fully excavated, and a novel method with great potential is provided for manufacturing the high-performance lightweight titanium alloy fastener.

The titanium alloy bolt for aerospace according to the prior design is provided with a head part, a rod part and threads, and the high-performance manufacturing method is provided for the rod part only in the embodiment, and the high-performance manufacturing method is expressed in the form of unprocessed head parts and threads in the embodiment.

The ultrasonic auxiliary blocking type rotary gradient extrusion method of the titanium alloy fastener provided by the embodiment of the invention comprises the following steps:

the fixed die 6 and a rotary die 7 which can do rotary motion around a central axis relative to the fixed die 6 form an up-down block blocking type floating die structure, and the punch 3 and the ejector rod 9 which can do linear motion along the central axis of the floating die are used for blocking two ends of a die cavity in the forming process, so that a blocked variable-section die cavity is formed in the floating die, and the variable-section die cavity is a gradient forming area with a stepped structure with a diameter gradually reduced along the forming direction in the inner cavity of the rotary die 7; an elastic mechanism is arranged, so that the fixed die 6 can float along the central axis direction in the forming process to be tightly attached to the rotary die 7 all the time; the mechanical vibration formed by the ultrasonic generator 11 through the transformation of the sound guide rod 2 is used as the driving force for the extrusion blank forming of the punch head 3, so that the ultra-fine grain gradient tissue blank 10 of the titanium alloy fastener is formed based on the gradient extrusion and high-pressure torsion compound technology of the ultrasonic vibration auxiliary forming technology.

The method comprises the following steps:

step 1, taking a conventional uniform coarse-grain titanium alloy bar after smelting and forging as an initial blank 1, placing one end of an initial blank 1 with a round cross section from a fixed female die 6 into a cavity of a floating female die, and coaxially pressing a punch 3 against the end face of the outward side of the initial blank 1 along the forming direction;

step 2, starting an ultrasonic generator 11, converting ultrasonic frequency electric oscillation into mechanical vibration by a sound guide rod 2 and descending, driving a punch 3 to perform ultrasonic vibration, and enabling the punch 3 to extrude an initial blank 1 in a forming direction to descend in a cavity of a floating female die;

step 3, the initial blank 1 enters an inner cavity of a rotary female die 7 which performs rotary motion under the extrusion of a punch 3, and axial-tangential dual-mode severe gradient shear deformation occurs in a gradient forming area in the rotary female die 7 until the extrusion is completed in the rotary female die 7, so that a titanium alloy fastener superfine crystal gradient tissue blank 10 is formed;

and 4, ejecting the superfine crystal gradient tissue blank 10 of the titanium alloy fastener by using the ejection rod 9.

Specifically, in step 3, the gradient forming area in the rotary die 7 adopts a stepped structure with an oblate-circular section, so that gradient distribution of deformation from edge to center and effective transmission of tangential torque are realized, and the last section of the gradient forming area is a circular section.

Referring to fig. 1 to 6, the present embodiment also provides a device for implementing the ultrasonic-assisted blocking type rotary gradient extrusion method of the titanium alloy fastener, which has the following structure:

a floating die formed by an upper fixed die 6 and a lower rotary die 7 is arranged, a vertical variable cross-section die cavity is formed in the floating die and is used as a forming channel of a titanium alloy fastener, and a punch 3 and an ejector rod 9 are respectively sealed at the fixed die 6 end and the rotary die 7 end of the variable cross-section die cavity so as to block the variable cross-section die cavity and can linearly displace along the central axis of the die cavity; the bottom end of the rotary female die 7 is arranged on the lower die plate 5 and can synchronously rotate along with the lower die plate 5 around the central axis of the die cavity, a gradient forming area with a diameter-tapered stepped structure is formed in the inner cavity along the forming direction, the fixed female die 6 is elastically connected with the lower end of the upper die plate 4 above through an elastic mechanism, and the fixed female die can float up and down along the central axis of the die cavity in the forming process under the elastic force of the elastic mechanism so as to be always tightly attached to the rotary female die 7;

the ultrasonic vibration auxiliary forming component comprises an ultrasonic generator 11 and a sound guide rod 2, the sound guide rod 2 converts ultrasonic frequency electric oscillation of the ultrasonic generator 11 into mechanical vibration, the lower rod end of the sound guide rod 2 is connected with the upper end of an upper template 4, and the upper template 4 drives a punch 3 to perform ultrasonic vibration during descending so that the punch 3 forms extrusion of an initial blank 1 in a variable-section cavity of a floating female die.

The ejector rod 9 is arranged, on one hand, the ejector rod 9 is matched with the punch 3 to block the variable-section cavity, the ejector rod 9 does not act in the forming process, the process plasticity is enhanced, the forming performance of the titanium alloy is improved, the plastic deformation accumulation limit is improved, on the other hand, the ejector rod 9 can eject the superfine crystal gradient tissue blank 10 of the titanium alloy fastener after forming, and the blank is taken out conveniently. The driving device for the linear displacement of the ejector rod 9 and the driving device for the rotary motion of the lower die plate 5 can be of a driving type commonly used in the art, and will not be described in detail.

In a specific implementation, the corresponding structure arrangement also includes:

the structure of the elastic mechanism is as follows: the lower end of the upper die plate 4 is connected with the upper end of the fixed die 6 through a plurality of springs 8 which are equidistantly arranged along the circumferential direction.

The variable cross section cavity of the floating female die is used for receiving the initial blank 1 at the fixed female die 6 end, and the inner diameter of the inner cavity of the fixed female die 6 is adapted to the outer diameter of the initial blank 1;

the lower die plate 5 is provided with an ejection hole by taking the central axis of the floating die cavity as the center, and an ejection rod 9 is penetrated in the ejection hole in a matched manner;

the top end of the inner cavity of the floating female die is connected with the inner cavity of the fixed female die 6 through an upper transition section which is adapted to the external dimension of the inner cavity of the fixed female die 6, and the bottom end of the floating female die is connected with the ejection hole through a lower transition section which is adapted to the external dimension of the ejection hole; the gradient forming area is connected between the upper transition section and the lower transition section, and is a short column section which is of a multi-section ladder structure and is of a diameter decreasing type along the forming direction, the cross section of the last short column section is round and is of a shape and size which is adapted to the superfine crystal gradient tissue blank 10 of the titanium alloy fastener to be formed, the cross sections of other short column sections are oblate, and smooth transition is realized between two adjacent short column sections, between the uppermost short column section and the upper transition section and between the last short column section and the lower transition section through inclined plane connection;

the inner diameter of the ejection hole is larger than that of the last short column section.

The ultrasonic generator 11 is supported by a bracket 12 and is positioned above the upper die plate 4.

In the ultrasonic-assisted occlusion type rotary gradient extrusion method for the titanium alloy fastener of the embodiment, the following steps are adopted:

based on an up-and-down block blocking type floating female die structure, a gradient extrusion and high-pressure torsion composite process is adopted to enhance the gradient shear deformation capability and the grain refinement effect, so that the gradient distribution of a microstructure can be improved, and a high-performance titanium alloy fastener blank with an ultrafine grain gradient structure is formed;

moreover, as the technology of the upper and lower block blocking type floating female dies is adopted, the process plasticity is increased, the forming performance of the titanium alloy is improved, the plastic deformation accumulation limit is improved, and the formed blank is convenient to take out;

in the upper and lower block blocking type floating female die, the upper part is a fixed female die 6, the inner cavity is simple in structure and floats up and down only under the action of an elastic mechanism so as to prevent the instability of bars; the lower part is a rotary female die 7 which rotates along with the lower die plate 5 and is provided with a gradient forming area with a flat circular-circular section ladder structure, so that tangential torque of a material in the deformation area can be effectively transmitted, severe gradient shearing deformation of metal in the deformation area in an axial-tangential dual mode is realized, the cumulative shearing deformation capacity of the material and the effect of refining grains are greatly improved, the cumulative deformation of bars from edge to center from large to small is realized, and gradient distribution of a microstructure is optimized;

meanwhile, an ultrasonic vibration auxiliary forming technology is adopted, ultrasonic vibration transmission quality and structural torsional strength are guaranteed through the structure of the sound guide rod 2, so that forming load is reduced, forming limit is further improved, an ultrafine grain gradient tissue structure from the surface layer to the inside is obtained, and the performance of the titanium alloy fastener is remarkably improved.

In summary, the invention combines the ultrasonic vibration assisted plastic forming technology, the high-pressure torsion and the gradient extrusion technology:

the ultrasonic vibration assisted plastic forming technology can obviously reduce forming force and flow stress of the material, reduce friction between a die and a workpiece, improve forming performance of the material, improve forming limit of the material, and provide a new way for plastic forming of high-strength titanium alloy difficult to deform;

the gradient extrusion combines the traditional extrusion with large plastic deformation, realizes gradual accumulation of large strain on the surface layer of the material through a gradient forming area, effectively promotes the breaking and refinement of grains from outside to inside, further realizes gradual reduction of the size of blank grains from inside to outside, greatly improves the mechanical property of the surface layer, improves the ductility of the core part, and further improves the comprehensive mechanical property of the material;

the high-pressure torsion process is a large plastic deformation process which applies torque on the contact surface of a workpiece and a die through active friction while applying high hydrostatic pressure on the axial direction of the deformed body, so that the deformed body generates axial compression and tangential shear deformation, has obvious grain refinement effect, and the refinement capability is distributed in a descending gradient from edge to center;

therefore, by adopting the ultrasonic-assisted blocking type rotary gradient extrusion method, the high-performance ultra-fine grain gradient structure blank is formed by uniformly coarse grain bar stock of the titanium alloy, the forming load is low, the process plasticity is good, the friction and abrasion are small, the shearing deformation capacity is strong, the grain crushing and refining and gradient distribution effects are good, the comprehensive mechanical properties of the blank are excellent, and the defects of high loading, small accumulated shearing deformation capacity, poor grain crushing and refining effect, unstable process and the like of the traditional gradient extrusion forming are avoided, so that the method has good application value and development prospect.

While embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that: many changes, modifications, substitutions and variations may be made to the embodiments without departing from the spirit and principles of the invention, the scope of which is defined by the claims and their equivalents.

Claims

1. An ultrasonic-assisted blocking type rotary gradient extrusion method for a titanium alloy fastener is characterized by comprising the following steps of:

the method comprises the steps that a fixed concave die (6) and a rotary concave die (7) which can rotate around a central axis relative to the fixed concave die (6) form an up-down block blocking type floating concave die structure, two ends of a die cavity are closed in a forming process by a punch (3) and an ejector rod (9) which can do linear motion along the central axis of the floating concave die, a blocked variable-section die cavity is formed in the floating concave die, and the variable-section die cavity is a gradient forming area with a diameter tapered step structure formed in the inner cavity of the rotary concave die (7) along the forming direction; an elastic mechanism is arranged, so that the fixed concave die (6) can float along the central axis direction in the forming process so as to be tightly attached to the rotary concave die (7) all the time; mechanical vibration formed by converting an ultrasonic generator (11) through an acoustic guide rod (2) is used as driving force for forming an extrusion blank of a punch (3) so as to form a titanium alloy fastener superfine crystal gradient tissue blank (10) by using a gradient extrusion and high-pressure torsion composite process based on an ultrasonic vibration auxiliary forming technology, and the ultrasonic auxiliary blocking type rotary gradient extrusion method of the titanium alloy fastener is carried out according to the following steps:

step 1, taking a conventional uniform coarse-grain titanium alloy bar after smelting and forging as an initial blank (1), putting one end of a self-fixing female die (6) with a round cross section of the initial blank (1) into a cavity of a floating female die, and coaxially pressing a punch (3) against the end face of the outward side of the initial blank (1) along the forming direction;

step 2, starting an ultrasonic generator (11), converting ultrasonic frequency electric oscillation into mechanical vibration by the sound guide rod (2) and descending, driving the punch (3) to perform ultrasonic vibration, and enabling the punch (3) to extrude the initial blank (1) in the forming direction to descend in the cavity of the floating female die;

step 3, the initial blank (1) enters an inner cavity of a rotary female die (7) which performs rotary motion under the extrusion of a punch (3), and axial-tangential dual-mode severe gradient shear deformation occurs in a gradient forming area in the rotary female die (7) until the extrusion is completed in the rotary female die (7), so that a titanium alloy fastener superfine crystal gradient tissue blank (10) is formed;

and 4, ejecting the superfine crystal gradient tissue blank (10) of the titanium alloy fastener by using an ejector rod (9).

2. An apparatus for carrying out the ultrasonic assisted occlusion type rotary gradient extrusion method of the titanium alloy fastener of claim 1, which is characterized in that:

a floating die formed by an upper fixed die (6) and a lower rotary die (7) is arranged, a vertical variable-section die cavity is formed in the floating die, and a punch (3) and an ejector rod (9) are respectively sealed at the fixed die (6) end and the rotary die (7) end of the variable-section die cavity as forming channels of a titanium alloy fastener, so that the variable-section die cavity is blocked and can linearly displace along the central axis of the die cavity; the bottom end of the rotary female die (7) is arranged on the lower die plate (5) and can synchronously rotate around the central axis of the die cavity along with the lower die plate (5), the inner cavity forms a gradient forming area with a stepped structure with a gradually-reduced diameter along the forming direction, the fixed female die (6) is elastically connected with the lower end of the upper die plate (4) above through an elastic mechanism, and the fixed female die can float up and down along the central axis direction of the die cavity under the elastic action of the elastic mechanism so as to be tightly attached to the rotary female die (7) all the time in the forming process;

the ultrasonic vibration auxiliary forming component comprises an ultrasonic generator (11) and a sound guide rod (2), wherein the sound guide rod (2) converts ultrasonic frequency electric oscillation of the ultrasonic generator (11) into mechanical vibration, the lower rod end of the sound guide rod (2) is connected with the upper end of the upper template (4), and the punch (3) is driven to perform ultrasonic vibration through the upper template (4) during descending, so that the punch (3) forms extrusion of an initial blank (1) in a variable-section cavity of a floating female die.

3. The ultrasonic-assisted occlusion type rotary gradient extrusion device of a titanium alloy fastener according to claim 2, wherein the elastic mechanism is structured as follows:

the lower end of the upper die plate (4) is connected with the upper end of the fixed die (6) through a plurality of springs (8) which are equidistantly arranged along the circumferential direction.

4. The ultrasonic-assisted occlusion type rotary gradient extrusion device of a titanium alloy fastener according to claim 2, wherein:

the variable cross-section cavity of the floating female die is used for receiving an initial blank (1) by the end of a fixed female die (6), and the inner diameter of the inner cavity of the fixed female die (6) is adapted to the outer diameter of the initial blank (1);

the lower die plate (5) is provided with an ejection hole by taking the central axis of the floating die cavity as the center, and the ejection rod (9) is penetrated in the ejection hole in an adaptive manner;

the top end of the inner cavity of the floating female die is connected with the inner cavity of the fixed female die (6) through an upper transition section which is adapted to the external dimension of the inner cavity of the fixed female die (6), and the bottom end of the floating female die is connected with the ejection hole through a lower transition section which is adapted to the external dimension of the ejection hole; the gradient forming area is connected between the upper transition section and the lower transition section, and is a short column section which is of a multi-section ladder structure and is of a diameter decreasing type along the forming direction, the cross section of the last short column section is round and is of a shape and size which is adapted to a titanium alloy fastener superfine crystal gradient tissue blank (10) to be formed, the cross sections of other short column sections are oblate, and the cross sections of two adjacent short column sections, the uppermost short column section and the upper transition section and the last short column section and the lower transition section are connected through inclined planes for smooth transition;

5. The ultrasonic-assisted occlusion type rotary gradient extrusion device of a titanium alloy fastener according to claim 2, wherein: the ultrasonic generator (11) is supported by a bracket (12) and is positioned above the upper template (4).