Extrusion thread machining method and application thereof

Abstract

The invention discloses an extrusion thread machining method and application thereof. The method comprises the steps that a prefabricated hole is formed in a workpiece; performing extrusion processing on the prefabricated hole by adopting an extrusion tap, so that the peripheral wall of the prefabricated hole is plastically deformed to form an internal thread; and the crest of the internal thread is passivated, the crest is truncated to a preset truncation amount, and a passivated contour is formed. According to the scheme, the sharp crest is accurately shaped immediately after the extrusion forming procedure, stress concentration and crest crack initiation sources in the closing-in stage are eliminated, the residual compressive stress of an extrusion layer and metal flow line integrity are kept, meanwhile, the machining process is simplified, the axial tensile strength of the thread is remarkably improved, and the cyclic fatigue life of the thread is remarkably prolonged. And the requirements of aviation self-locking nuts and other high-specification connecting pieces on strength and reliability are met.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal internal thread precision forming processing, and particularly to an extrusion thread processing method and application thereof. BACKGROUND

[0002] Extrusion thread processing is an internal thread forming method based on plastic deformation rather than cutting to remove metal. During processing, the ridge top of the extrusion tap generates a high strain zone on the preformed hole wall, driving the hole wall material to flow along the flank and arch to the crest to form a continuous metal fiber streamline. Compared with traditional cutting taps, the extrusion process eliminates the structural fragmentation caused by chips, significantly improving the static tensile limit and cyclic fatigue life of the thread.

[0003] The existing manufacturing process usually includes the steps of making a preliminary hole, extruding a thread, closing a thread (for a self-locking nut), surface treatment, and final performance testing. In order to maintain the integrity of the fiber streamline and shorten the beat, most manufacturers directly enter the closing or surface treatment after extrusion without shortening or rounding the thread crest. The sharp triangular crest left in this way is first pressed during subsequent radial closing plastic deformation, and is prone to peak folding deformation and the initiation of microcracks; at the same time, the interference between the sharp top and the outer thread arc root during screwing forms a bite burr and a local high stress area. For materials such as titanium alloy and stainless steel with low elongation, these cracks can expand into through cracks in the early stage of service, directly weakening the strengthening effect brought by extrusion, and making the fatigue life and assembly reliability of the self-locking nut difficult to meet the high-specification standards of aviation and other industries. SUMMARY

[0004] The main purpose of the present application is to solve the technical problem that the existing extruded thread is prone to peak folding cracks and has insufficient fatigue life due to the sharp thread crest before closing.

[0005] The first aspect of the present application provides an extruded thread processing method, which comprises: preparing a preformed hole on a workpiece; extruding the preformed hole using an extrusion tap to make the peripheral wall of the preformed hole plastically deform to form an internal thread; performing a passivation treatment on the crest of the internal thread to shorten the crest to a preset shortening amount and form a passivation profile.

[0006] Preferably, the passivation treatment on the crest of the internal thread comprises: after completing the extrusion processing and while the extrusion tap is still in the internal thread without being withdrawn, continuously collecting the high-frequency contact impedance spectrum between the tap root of the extrusion tap and the crest of the internal thread along the thread axial direction; determining the residual sharp height of the internal thread crest in real time according to the comparison result of the impedance peak value in the high-frequency contact impedance spectrum and a preset threshold value; automatically adjusting the passivation energy applied to the local area of the internal thread crest corresponding to the impedance peak value and the axial micro-feeding amount of the extrusion tap according to the determination result, so that the truncated amount of the passivated internal thread crest is kept within a target window range.

[0007] Preferably, the scanning frequency range of the high-frequency contact impedance spectrum is 50 kHz-1 MHz. When the impedance peak value Z p The increment ΔZ=Z p of the impedance peak value Z start relative to the same scanning segment impedance average value Z0 start satisfies ΔZ / Z0>pre-set proportion threshold η, it is determined that the residual sharp height of the internal thread crest is out of limit. The control system determines the passivation energy applied to the local area of the internal thread crest corresponding to the impedance peak value and the axial micro-feeding amount of the extrusion tap in real time according to the pre-set corresponding relationship between the increment ΔZ and the target truncated amount window through table lookup or interpolation algorithm.

[0008] Preferably, the passivation energy is provided by the high-frequency induction heating energy output by the micro-induction coil arranged in the belt-shaped area on the tap root opposite to the internal thread crest, and the belt-shaped area keeps the metal in a conductive state and forms an electrical contact loop with the internal thread crest. The frequency of the high-frequency induction heating is 200 kHz-350 kHz, the single heating power is not greater than 10 W, the heating duration is not greater than 150 ms, the local metal of the internal thread crest corresponding to the impedance peak value is softened to 450°C-550°C, and the thickness of the softened layer is not more than 30 μm.

[0009] Preferably, within 1 ms after the high-frequency induction heating is completed, the extrusion tap is driven by the machine tool servo system to feed 1 μm-5 μm along the thread axial direction, the softened internal thread crest is flattened and shaped by the micro-convex rib on the tap root, the metal flows laterally and the burr is rolled into the thread side groove, and a passivation profile with a circular arc radius of 0.1P-0.2P is formed, where P is the pitch.

[0010] Preferably, during the implementation of the high-frequency induction heating on the local area of the internal thread crest corresponding to the impedance peak value, the real-time change value Z(t) of the contact impedance of the local area is synchronously monitored at a sampling period of 0.2 ms-1 ms, and the induction heating is immediately terminated and the axial micro-feeding action of the extrusion tap is triggered when Z(t) satisfies one of the following conditions: Condition one: Z(t) is decreased by an amount ΔZ=Z start relative to the impedance Z start at the start of the heating, and Z(t) is not less than 15% of Z start . Condition two: the rate of impedance change dZ / dt is less than -0.5Z start s -1 ; Thus, the local softening temperature is controlled below 550℃, the softening layer thickness is controlled below 30μm, and the shortening amount of the internal thread crest after passivation is kept within the target window.

[0011] Preferably, when the local area of the internal thread crest corresponding to the impedance peak value is subjected to high-frequency induction heating, the induction power supply outputs the heating power in the form of pulse trains, and the duty cycle of the next pulse is updated in real time according to the following formula: D new =D old ×(1–k·ΔZ rel ); wherein 0<k≤0.4, D old is the duty cycle of the previous pulse, ΔZ rel =(Z start –Z(t))⁄Z start is the real-time impedance drop relative value, Z start is the impedance at the start of heating, and Z(t) is the current-time impedance; When ΔZ rel ≥0.15 or D new ≤0.20, the high-frequency induction heating is immediately terminated, and the micro-feeding shaping action of the extrusion tap along the thread axial direction is triggered, so as to control the local temperature rise rate in sections, prevent one-time overheating from causing surface layer annealing, and ensure that the shortening amount of the internal thread crest after passivation is stable within the target window.

[0012] Preferably, during the extrusion of the extrusion tap into the preformed hole and the subsequent extrusion processing and crest passivation, ester-based micro-atomized lubricating oil is sprayed to the contact area between the tap and the hole wall at a flow rate of 0.5mL·h -1 ~3mL·h -1 , the conductivity of the lubricating oil is not higher than 10μS·cm -1 , and the water content is not higher than 0.05%, so as to provide lubrication and heat dissipation while inhibiting arc discharge and maintaining the stability of high-frequency contact impedance spectrum measurement.

[0013] Preferably, when any of the following conditions is detected, the control system immediately terminates the passivation process, withdraws the extrusion tap along the original feeding path from the internal thread, and automatically replaces the cutting tap with adjustable shortening amount to continue the shortening process of the internal thread crest: the signal-to-noise ratio of the high-frequency contact impedance spectrum is lower than the preset lower limit, the induction heating power feedback exceeds the safe upper limit, or the axial micro-feeding torque fluctuation exceeds the allowable range.

[0014] The second aspect of the present application provides an application of the extruded thread, which adopts the extruded thread processing method of any of the above embodiments.

[0015] The technical scheme provided by the embodiment of the application is that the workpiece is first drilled to leave uniform and controllable plastic allowance for subsequent metal flow; the extrusion tap then enters the hole, the ridge top generates a high strain area on the hole wall, guides the material to continuously flow along the tooth side and arch to generate a triangular tooth top; the tooth top is immediately truncated and passivated after plastic forming, the sharp top corner is flattened or rounded according to a predetermined truncation amount, and finally a smooth and continuous circular arc or platform profile is reserved.

[0016] Such a process combination forms a continuous closed chain in the stress mechanism. The continuous fiber flow line and the residual compressive stress layer provide body reinforcement for the internal thread, and the tooth top truncation passivation converts the potential high stress peak into a circular arc area with a larger pressure area, and the compression and shear load is dispersed to a wider surface to avoid peak tearing during closing shaping; the truncation height is pre-set to ensure that the internal tooth top and the external tooth root maintain a specified gap, and the whole stress path gradually connects compression and shear, and the crack initiation position is transferred from the sharp top to the circular arc area with higher strength and greater residual compressive stress. Through the connection of this additive flow forming and local geometric passivation, the material reinforcement brought by extrusion is preserved, and the peak tearing crack initiation condition is eliminated from the geometric source, thereby simultaneously improving the tensile limit, fatigue life and assembly reliability. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 An embodiment of the extrusion thread machining method in the embodiment of the application is shown. DETAILED DESCRIPTION

[0018] The technical scheme in the embodiments of the application will be described clearly and completely below with reference to the drawings in the embodiments of the application. Obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments of the application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0019] It should be noted that if the embodiments of the application involve directional indications (such as up, down, left, right, front, back, etc.), the directional indications are only used to explain the relative positional relationship, movement condition, etc. between components in a certain posture, and if the certain posture changes, the directional indications also change accordingly.

[0020] In addition, the description related to "first", "second" and the like in the present application is only for the purpose of description, and cannot be understood as indicating or implying the relative importance of the indicated technical features or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first", "second" can explicitly or implicitly include at least one of the features. In addition, "and / or" throughout the text includes three schemes, for example, A and / or B includes A technical scheme, B technical scheme, and A and B simultaneously meet the technical scheme; in addition, the technical schemes of each embodiment can be combined with each other, and it must be based on the realization of the technical personnel in the art, when the combination of technical schemes appears contradictory or cannot be realized, it should be considered that the combination of such technical schemes does not exist, nor in the protection scope required by the present application.

[0021] An embodiment of the present application provides a thread extrusion processing method. Figure 1 An embodiment of the present application provides a thread extrusion processing method. In the embodiment, the method comprises: Please refer to Figure 1 , a preformed hole is prepared on the workpiece; Specifically, the preformed hole refers to a cylindrical hole prepared in advance for subsequent thread extrusion processing, and the diameter thereof is determined according to the target thread specification. For example, for M10x1.25 thread, the preformed hole diameter is 8.8mm. A numerical control drilling machine is used to drill a preformed hole at the axial position of the nut workpiece by using a high-speed steel drill bit. The drilling depth is matched with the target thread depth, the hole wall surface roughness is controlled within Ra3.2μm, and the roundness and straightness of the hole meet the subsequent extrusion processing requirements.

[0022] Please continue to refer to Figure 1 , the preformed hole is extruded by using an extrusion tap to make the peripheral wall of the preformed hole plastically deform to form an internal thread; Specifically, the extrusion tap is clamped on the spindle by using a numerical control tapping machine. The extrusion tap is a thread forming tool without cutting edge, and the surface thereof has a spiral protrusion matched with the target thread (since the extrusion tap is a widely used tool, it is not expanded here). The extrusion tap is rotated into the preformed hole at a constant rotational speed (for example, 100rpm is used for M10 thread) and an axial feed speed. The spiral protrusion of the extrusion tap applies radial extrusion force to the peripheral wall of the preformed hole, so that the metal of the hole wall flows plastically. The metal material is arched upward along the thread surface direction under the action of the extrusion force, forming the thread crest, and flows to both sides to fill the thread root, and finally forms a complete internal thread profile on the inner wall of the preformed hole. No metal chips are generated in the whole process, and the metal fibers remain in a continuous state.

[0023] Extrusion processing forms thread by plastic deformation, avoids cutting processing to cut metal fiber streamline, forms thread with continuous fiber organization and surface residual compressive stress, and significantly improves tensile strength and fatigue life of thread.

[0024] Please continue to refer to Figure 1 The crest of the internal thread is passivated, truncated to a preset amount, and a passivation profile is formed.

[0025] In this scheme, there are at least two embodiments that can implement the passivation process of the above steps. Specifically, in embodiment one: after the extrusion tap completes thread forming, it exits the preformed hole, and a cutting tap is replaced for crest passivation. The cutting tap is a thread processing tool (the cutting tap is a widely used tool, and this application will not be described in detail), which is similar in structure to a common tap but has opposite functions: the root of the cutting tap is designed with sharp cutting edges, while the crest is relatively flat. When the cutting tap is screwed into the formed internal thread, the cutting tap's root cutting edge contacts the internal thread's crest, and the cutting force generated by relative movement removes the sharp triangular apex of the internal thread's crest. In specific operation, the cutting tap is screwed into the internal thread at a low speed (e.g., 50 rpm), and the cutting tap's root edge gradually scrapes the internal thread's crest metal, cutting the original sharp triangular crest into a flat top or a passivated shape with a small circular arc. The preset amount of truncation refers to the height dimension of the crest that is removed, for example, for M10x1.25 thread, the truncation amount is controlled at 0.15P (P is the pitch 1.25mm), i.e., about 0.19mm. The passivation profile refers to the non-sharp geometric shape of the crest after processing, which eliminates the original sharp angle feature.

[0026] Embodiment two is described in detail below.

[0027] In one embodiment of the present application, the passivation of the crest of the internal thread includes: After the extrusion processing is completed and the extrusion tap is still in the internal thread without exiting, the high-frequency contact impedance spectrum between the extrusion tap root and the internal thread crest is continuously collected along the thread axial direction; According to the comparison result of the impedance peak value in the high-frequency contact impedance spectrum and the preset threshold, the residual sharp height of the internal thread crest is determined in real time; According to the determination result, the passivation energy applied to the local area of the internal thread crest corresponding to the impedance peak value and the axial micro-feeding amount of the extrusion tap are automatically adjusted, so that the truncated amount of the passivated internal thread crest is kept within the target window range.

[0028] The following describes the steps involved in the above embodiments: The application adds a calibration section after the forming section of the extrusion tap. The calibration section is a functional area specially used for passivation treatment, and the root area is processed with a conductive band with a width of about 0.25P, and a copper alloy micro-induction coil with a diameter of 0.25mm is embedded in the conductive band. The root and flank surfaces on both sides of the conductive band are covered with a 5μm thick alumina-silicon nitride gradient insulating layer, and the insulating layer is further covered with a 0.5μm thick diamond-like coating to ensure wear resistance. The tap shank leads out two-way wires through a rotating slip ring: the first way is connected with the conductive band, which is used for high-frequency detection and induction heating; the second way is connected with the thread gauge ground, forming a complete electrical circuit. This calibration section structure makes the extrusion tap increase the electrical detection and local induction heating capability on the basis of maintaining the original thread forming function, providing a hardware basis for realizing in-situ passivation treatment.

[0029] After the extrusion thread forming is completed, the extrusion tap stays in the internal thread without withdrawing, and the machine spindle speed is reduced to 40rpm for pressure maintaining rotation. At this time, the impedance analyzer outputs a step frequency square wave signal in the frequency range of 50kHz to 1MHz to the conductive band of the calibration section through the rotating slip ring. The high-frequency contact impedance spectrum refers to the characteristic curve of the electrical impedance of the contact interface between the extrusion tap conductive band and the internal thread tooth top varying with frequency. The contact interface is equivalent to a circuit network composed of contact resistance and contact capacitance, and the size of the contact capacitance directly depends on the contact area. The sharp tooth top has a very small contact width (usually less than 20μm), and the equivalent capacitance formed is significantly smaller than that of the passivated arc tooth top, so it shows significantly higher impedance value in the high-frequency band. The impedance analyzer records the voltage amplitude and phase angle of each frequency point in real time with a sampling interval of 0.5ms, and the control system synchronously collects the angle signal of the spindle encoder, arranges the impedance data according to the axial position of the thread, and generates a two-dimensional data curve of impedance-axial position. On this curve, each sharp tooth top position will appear as a clear impedance peak point, and the peak value is increased by more than 20% relative to the average value of the same section. For example, for M10×1.25 thread, the system can accurately identify the sharpness of each tooth top and its axial distribution within each 1.25mm pitch range. This detection method utilizes the physical coupling relationship between contact geometry and electrical characteristics, and converts the tooth top geometry characteristics that are difficult to directly observe in a closed internal hole into accurately measurable electrical parameters.

[0030] The control system performs data analysis and processing on the collected high-frequency contact impedance spectrum. The software algorithm calculates the ratio of the impedance peak increment ΔZ and the impedance average value Z0 of the same section at each detection position point by point. When the ratio of a certain point exceeds the preset threshold value 0.15, the system determines that the residual sharp height of the tooth top at this position exceeds the target truncation window 0.12P-0.18P of the allowable range, and needs to perform blunting processing. The preset threshold value 0.15 is a parameter obtained by experiment calibration: when the tooth top truncation amount is less than 0.10P, the contact area change is not enough to produce obvious capacitance difference, and the impedance change is weak and difficult to accurately identify; when the truncation amount exceeds 0.25P, although the geometric shape meets the blunting requirements, excessive removal will weaken the engagement strength and carrying capacity of the thread. The residual sharp height refers to the geometric gap between the current tooth top state and the ideal blunting profile, which is indirectly quantified and represented by the relative increment of the impedance peak value. This determination method establishes a quantitative correspondence between the electrical signal characteristics and the geometric size, realizes the automatic and accurate identification of the blunting requirement of each tooth top, and at the same time provides accurate spatial positioning coordinates for subsequent local heating power control.

[0031] When the control system confirms that a tooth top position needs to be blunted, the micro inductive coil at the corresponding position is immediately started, and an inductive heating pulse with a frequency of 250 kHz is output, and the pulse duty cycle is initially set to 0.6. The blunting energy is generated by electromagnetic induction principle: the high-frequency alternating current generates an alternating magnetic field in the inductive coil, the magnetic field induces eddy current in the tooth top metal, and the Joule heating effect of the eddy current makes the local metal quickly heat up and soften to a temperature range of 450-550℃. During the inductive heating process, the monitoring module continuously records the real-time impedance value Z(t) of the position with a high-frequency sampling period of 0.2ms, and calculates the impedance drop amplitude ΔZ rel =(Z start -Z(t)) / Z start , where Z start is the initial impedance value at the beginning of heating. When ΔZ relWhen the impedance peak value Z

[0032] In an embodiment of the present application, the scanning frequency range of the high-frequency contact impedance spectrum is 50 kHz-1 MHz. When the impedance peak value Z p The increment ΔZ=Z p When ΔZ / Z0> preset proportion threshold η, it is determined that the residual sharp height of the internal thread crest exceeds the limit. The control system determines the blunting energy applied to the local area of the crest corresponding to the impedance peak value and the axial micro-feeding amount of the extrusion tap in real time according to the preset corresponding relationship between the increment ΔZ and the target truncation amount window through table lookup or interpolation algorithm.

[0033] The following describes the steps involved in the above embodiment: The impedance analyzer outputs a sweep signal in the frequency range of 50 kHz to 1 MHz for contact impedance detection. The selection of the sweep frequency range is based on the physical relationship between contact capacitance and frequency: at low frequencies (below 50 kHz), contact resistance dominates, and geometric differences have little effect on impedance; at high frequencies (above 1 MHz), parasitic parameters of the circuit and electromagnetic interference can mask the true contact characteristic signal. The frequency range of 50 kHz to 1 MHz is exactly in the window range where the contact capacitance effect is significant and the interference is relatively small. In specific implementation, the impedance analyzer sets 128 test frequencies in this frequency range according to the logarithmic scale, such as 50 kHz, 63 kHz, 79 kHz, etc. The measurement time of each frequency point is 4 ms, and the complete scanning of a tooth top position requires about 0.5 seconds. In this frequency range, the micro-capacitance formed by the sharp tooth top due to the extremely small contact area produces a large capacitive reactance at high frequencies, making the total impedance significantly higher than that of the arc tooth top after passivation treatment. The setting of this frequency range fully utilizes the coupling effect of contact geometry and electrical characteristics, ensuring that there is a large enough impedance difference between the sharp tooth top and the passivated tooth top to accurately identify.

[0034] The control system analyzes the impedance data obtained by scanning in real time and calculates the impedance characteristic parameters of each detection point using a sliding window algorithm. The scanning section refers to the data interval containing the current detection point and the previous and next three tooth top positions, i.e., the impedance data of a total of seven teeth form an analysis window. The system first calculates the impedance mean Z0 of the seven data points in the scanning section, then extracts the impedance peak Z p of the current detection point, and calculates the increment ΔZ = Z p -Z0. The preset proportion threshold η is determined by a large number of experiments and has different values for different materials and thread specifications: for titanium alloy M10 threads, η is set to 0.15; for stainless steel M8 threads, η is set to 0.18. When the calculated ratio ΔZ / Z0 is greater than the preset proportion threshold η, the system determines that the residual sharp height of the thread tooth top at this position exceeds the allowed range and needs to be passivated. For example, the 8th tooth top position of an M10 titanium alloy thread, Z p is measured to be 850Ω, the impedance mean Z0 of the scanning section where it is located is 720Ω, the calculation ΔZ = 130Ω, and the ratio ΔZ / Z0 = 0.18, which exceeds the threshold value 0.15, so it is determined that the tooth top needs to be passivated. This relative ratio determination method effectively eliminates the influence of environmental factors such as material resistivity difference and temperature change on the absolute impedance value, improving the accuracy and stability of the determination result.

[0035] The control system determines the corresponding passivation energy and axial micro-feeding amount parameters according to the value of impedance increment ΔZ through a preset lookup table algorithm. The lookup table algorithm is based on a ΔZ-passivation parameter corresponding relationship table calibrated in advance, which is established through a large number of test tests on different materials and different thread specifications. The corresponding relationship table divides the ΔZ value into several intervals, and each interval corresponds to a specific passivation parameter combination. For example, for M10 titanium alloy threads: when ΔZ is in the range of 100-150Ω, the passivation energy is set to 6W for 80ms, and the axial micro-feeding amount is set to 2μm; when ΔZ is in the range of 150-200Ω, the passivation energy is increased to 8W for 120ms, and the axial micro-feeding amount is increased to 3μm. For ΔZ values between the calibration points, the system uses a linear interpolation algorithm to calculate the accurate parameter value. The specific calculation process of the interpolation algorithm is as follows: assuming that ΔZ=135Ω is located in the interval [100Ω, 150Ω], the corresponding passivation energy is calculated by linear interpolation as 6W+(135-100) / (150-100)×(8W-6W)=7.4W. The target truncation window refers to the qualified range of the truncation amount of the tooth crest after passivation, which is usually set to 0.12P to 0.18P, where P is the pitch. Through the combination of lookup table and interpolation algorithm, the system can accurately match the optimal passivation processing parameters according to the actual sharpness of each tooth crest, realize individualized adaptive control, and ensure the consistency and repeatability of the passivation effect.

[0036] In an embodiment of the present application, the passivation energy is provided by high-frequency induction heating energy output by a micro-induction coil arranged in a strip-shaped area at the bottom of the extrusion tap, which maintains a metal conductive state and forms an electrical contact loop with the tooth crest of the internal thread; The frequency of the high-frequency induction heating is 200kHz-350kHz, the single heating power is not greater than 10W, and the heating duration is not greater than 150ms, so that the local metal of the internal thread tooth crest corresponding to the impedance peak is softened to 450℃-550℃, and the softening layer thickness is not more than 30μm.

[0037] The following specifically describes the steps involved in the above embodiment: The passivation energy is generated by a micro-induction coil arranged in a band region of the shank of the extruded tap. The band region refers to the conductive band of the shank described above, which has a width of about 0.25P and is directly opposite and in physical contact with the crest of the internal thread. The surface of the conductive band remains bare metal without an insulating coating, forming a direct metal-to-metal electrical contact with the crest of the internal thread. The micro-induction coil is embedded in the conductive band, has a diameter of 0.25 mm, is made of copper alloy wire with 10-12 turns, and is integrally formed with the body of the extruded tap by a powder metallurgy process. The electrical contact loop refers to the complete current path formed by the conductive band of the shank of the extruded tap, the metal of the crest of the internal thread, the body of the workpiece, and the machine tool grounding system. In specific implementation, the high-frequency power supply supplies power to the micro-induction coil through the rotating slip ring described above, the alternating magnetic field generated by the coil induces eddy currents in the conductive band and the metal of the crest of the internal thread contacted thereby, and the eddy currents generate Joule heat in the metal resistance to achieve local heating. For example, for M10x1.25 threads, the conductive band has a width of about 0.31 mm and a contact area of about 0.8 square millimeters with the crest of the internal thread, forming a stable electrical contact interface. This induction heating method achieves precise positioning of the energy delivery, ensuring that the heating area is strictly limited to the local crest that needs to be passivated, avoiding thermal effects on the entire thread structure.

[0038] High-frequency induction heating uses a frequency range of 200 kHz to 350 kHz for power output. The selection of the induction heating frequency is based on the skin effect principle: in a metal conductor, high-frequency current mainly flows in the surface layer, and the current density decreases exponentially with depth. At a frequency of 200 kHz, the skin depth in titanium alloy is about 120 μm, and in stainless steel it is about 180 μm; at a frequency of 350 kHz, the skin depth is further reduced to about 80 μm and 120 μm. This frequency range ensures that the eddy currents are mainly concentrated in the 30 μm range of the crest surface, achieving efficient local heating without affecting the deep metal structure. The single heating power is limited to 10 W, and the heating duration is controlled to be less than 150 ms. The power and time parameters are set based on heat conduction calculations: a power density of about 12.5 W / square millimeter is generated on a contact area of 0.8 square millimeters at a power of 10 W, which can heat the 30 μm thick surface layer metal to a temperature range of 450-550°C within 150 ms. In specific implementation, the high-frequency power supply uses a pulse output mode, the pulse width can be adjusted in the range of 10-150 ms, and the pulse power can be adjusted in the range of 3-10 W. For example, for crests with a large residual sharp height, a heating parameter of 8 W for 120 ms is used; for slightly over-limit crests, a parameter of 5 W for 60 ms is used. This parameterized control method avoids material performance degradation caused by excessive heating, while ensuring sufficient softening for subsequent mechanical shaping.

[0039] The internal thread crest corresponding to the impedance peak locally softens under the action of high-frequency induction heating. The softening temperature range of 450-550°C is determined according to the recrystallization temperature and phase transition temperature of commonly used thread materials: the α-β phase transition temperature of titanium alloy is about 882°C, and the softening temperature of 450-550°C is far below the phase transition point, ensuring that the material structure does not change; the recrystallization temperature of stainless steel is about 650°C, and the material maintains the original austenitic structure within this temperature range; the softening temperature of aluminum alloy is relatively low, and 450°C is sufficient to achieve full softening. The softening layer thickness is strictly controlled within 30μm, and this thickness limit is based on the geometric size and strength requirements of the thread crest: for standard threads, the crest width is about 0.125P, and the softening layer thickness of 30μm only accounts for a very small proportion of the total crest thickness, ensuring that the mechanical strength of the thread body structure is not affected. The softening layer thickness is achieved by controlling the three parameters of power, frequency and time of induction heating: the power determines the heat input per unit time, the frequency determines the distribution depth of heat in the metal, and the time determines the degree of heat diffusion. For example, using a combination of 250kHz frequency, 7W power, and 100ms time, a softening layer with a thickness of about 25μm and a temperature of about 500°C can be formed on the surface of the titanium alloy crest. Precise control of temperature and thickness ensures that the metal has good plasticity in the softened state, facilitating subsequent flattening and shaping operations, while maintaining the continuity of the fiber flow lines formed during extrusion and the integrity of the surface layer residual compressive stress, achieving the removal of sharp features while maintaining the high strength and fatigue resistance of the thread.

[0040] In one embodiment of the present application, within 1ms after the high-frequency induction heating is completed, the extrusion tap is driven by the machine tool servo system to feed 1-5μm along the thread axial direction, and the softened internal thread crest is flattened and shaped by the micro-ridge of the extrusion tap root, causing the metal to flow laterally and the burr to be rolled into the thread side groove, forming a blunt profile with a circular arc radius of 0.1P-0.2P, where P is the pitch.

[0041] The following describes the steps involved in the above embodiment: After high-frequency induction heating ends, the control system immediately triggers the machine tool servo system to perform precise axial feed. This 1ms time window is determined by the thermal conductivity of metals: softened metal cools rapidly after heating ceases, remaining in a highly plastic state for 1ms. Beyond this time window, the metal begins to reharden. Upon receiving the control command, the machine tool servo system drives the spindle to advance the extrusion tap a minute distance of 1μm to 5μm in the axial direction of the thread. The axial feed rate is determined based on the impedance analysis results previously described: a 1-2μm feed rate is used for the tip of the thread with a small residual tip height, while a 4-5μm feed rate is used for the tip of the thread with a large residual tip height. For example, for an M10×1.25 thread, when the peak impedance of the tip exceeds a threshold of 15%, the system sets an axial feed rate of 3μm. The feed process is driven by a servo motor, achieving a positional accuracy of ±0.1μm. This micro-feed ensures that the extrusion tap maintains synchronous engagement with the internal thread, avoiding additional cutting paths or thread damage.

[0042] The micro-ridges at the bottom of the calibration section of the extrusion tap flatten and shape the softened internal thread crest. Micro-ridges refer to the original tiny protrusions on the bottom surface of the calibration section of the extrusion tap, with a height of approximately 0.02P and a width of approximately 0.05P, distributed along the thread profile. Under the action of axial feed, the micro-ridges contact the softened internal thread crest and apply pressure, causing the crest metal to plastically deform. Under the action of pressure, the softened metal flows laterally toward the tooth side, flattening the originally sharp triangular crest into an arc-shaped profile. At the same time, the tiny burrs generated during the extrusion process are drawn into the tooth side grooves by the metal flow, achieving synchronous deburring. During the flattening process, the metal flows along the path of least resistance and is evenly distributed on both sides of the tooth surface, forming a symmetrical blunt profile. For example, for a crest with a residual sharp height of 0.08mm, after flattening with an axial feed of 3μm, the crest height is reduced to 0.19mm, forming a smooth arc transition.

[0043] After flattening and shaping, the top of the internal thread forms a blunt profile with an arc radius of 0.1P to 0.2P. The blunt profile refers to the non-sharp geometric shape of the tooth top after processing, eliminating the original sharp angle features. The setting of the arc radius range is based on the thread standard requirements and fatigue performance optimization: the arc radius of 0.1P meets the minimum blunting requirements of aviation thread standards such as MJ and UNJ, and the arc radius of 0.2P provides better stress dispersion effect but does not excessively weaken the thread engagement strength. For example, for M10×1.25 threads, the arc radius of the blunt profile is controlled in the range of 0.125mm to 0.25mm. This arc profile effectively eliminates the stress concentration source during the closing process, significantly reduces the probability of folding peak cracks, and at the same time maintains the continuous fiber streamlines formed by extrusion and the residual compressive stress on the surface, achieving the dual effects of crack prevention and strengthening.

[0044] In one embodiment of the application, during the high-frequency induction heating of the local area of the internal thread crest corresponding to the impedance peak, the real-time change value Z(t) of the contact impedance of the local area is monitored synchronously with a sampling period of 0.2ms to 1ms, and the induction heating is terminated immediately and the axial micro-feeding action of the extruding tap is triggered when Z(t) meets one of the following conditions: Condition one: the decrease amount ΔZ = Z start -Z(t) of Z start (t) relative to the impedance Z start at the start of heating is not less than 15% of Z Condition two: the impedance change rate dZ / dt is less than -0.5Z start s -1 ; Thereby, the local softening temperature is controlled below 550°C, the softening layer thickness is controlled below 30μm, and the truncation amount of the internal thread crest after passivation is kept within the target window range.

[0045] The following specifically describes the steps involved in the above embodiment: While the high-frequency induction heating of the local area of the internal thread crest corresponding to the impedance peak is being performed, the impedance measurement system continuously monitors the real-time change of the contact impedance of the local area with a sampling period of 0.2ms to 1ms. The sampling period refers to the time interval between adjacent two impedance measurements, and the high-frequency sampling of 0.2ms can capture the rapid change of the impedance during the heating process, and the sampling period of 1ms is suitable for the heating stage with relatively slow change. The real-time change value Z(t) of the contact impedance of the local area refers to the impedance value of the specific thread crest position measured at time t, which continuously changes with the increase of the metal temperature and the change of the contact state. In specific implementation, the impedance analyzer measures through the electrical contact loop of the calibration section conductive strip and the internal thread crest as described above, and uses the lock-in amplifier technology to extract the effective signal and filter out the noise interference. The impedance Z start at the start of heating is recorded and stored as a reference value, and the system calculates the difference between the current impedance Z(t) and the reference value in real time. For example, for a certain M10 titanium alloy thread crest, Z start is measured as 820Ω at the start of heating, and Z(t) is measured as 710Ω at 50ms of heating. The monitoring system stores all the sampling data in the buffer area and performs real-time data analysis and processing. This synchronous monitoring mode realizes the accurate association of the heating process and the impedance change, and provides real-time data support for the accurate determination of the heating termination time.

[0046] The control system analyzes the impedance data obtained by monitoring in real time to determine whether the preset heating termination condition is met. The determination process of condition one is as follows: the system calculates the decrease amount ΔZ of Z(t) relative to the impedance Z start at the start of heating, and the decrease amount ΔZ is equal to Zstart Subtract Z(t), when the drop is not less than 15% of Z start , the termination condition is triggered. Take the previous example: Z start is 820Ω, Z(t) is 710Ω, the drop ΔZ is 110Ω, the drop ratio is 110÷820=13.4%, which does not reach the threshold condition of 15%. The condition two requires the calculation of the impedance change rate dZ / dt, i.e. the rate of change of impedance over time. The system uses a sliding window difference algorithm to calculate the change rate: take the impedance data of the last 5 sampling points, calculate the impedance difference between adjacent points divided by the sampling time interval, and get the instantaneous change rate. When the calculated dZ / dt is less than -0.5 times Z start per second, condition two is triggered. For example, the impedance drops from 750Ω to 730Ω in 1ms, the change rate dZ / dt is -20000Ω / s, and if Z start is 820Ω, the threshold is -410Ω / s, then -20000<-410, which satisfies condition two. The 15% drop threshold is determined based on the metal softening characteristics: when the contact impedance drops by 15%, the corresponding metal temperature has reached a fully softened state; the change rate threshold -0.5Z start s -1 corresponds to the characteristic change speed of the rapid softening stage of the metal, and exceeding this rate indicates that the heating process has entered an out-of-control state and needs to be terminated immediately.

[0047] When the impedance monitoring data meets any of the termination conditions, the control system immediately cuts off the high-frequency induction heating power supply and synchronously triggers the axial micro-feeding action of the extrusion tap. The response time of heating termination is controlled within 0.1 ms, ensuring that the metal softening degree is accurately controlled within the target range. The system quickly cuts off the power supply of the induction coil through the solid-state relay, while sending a feeding instruction to the machine tool servo system. The axial micro-feeding action is performed in the range of 1 μm to 5 μm as described above, and the specific feeding amount is determined according to the impedance drop amplitude: when the drop amount reaches 15%, the standard feeding amount is used, and when the change rate triggers, the feeding amount is appropriately adjusted according to the rate size. For example, when the impedance drop amount just reaches 15%, the system performs 3 μm of axial feeding; when the change rate significantly exceeds the threshold, the system reduces the feeding amount to 2 μm to avoid excessive shaping. The local softening temperature is indirectly controlled through the correspondence between impedance change and temperature: a 15% impedance drop corresponds to a metal temperature of about 500°C, ensuring that the softening temperature is controlled below 550°C. The softening layer thickness is controlled below 30 μm by limiting the heating time and power, and the softening metal in this thickness range has good plastic deformation ability, facilitating subsequent flattening and shaping operations. The truncation amount of the passivated internal thread crest is kept within the target window range by controlling the axial feeding amount, realizing accurate closed-loop management from softening control to geometric shaping. This real-time monitoring and adaptive termination control method avoids over-heating-induced material performance degradation, while ensuring consistent passivation effect for each crest, significantly improving the stability and repeatability of processing quality.

[0048] In an embodiment of the present application, when high-frequency induction heating is performed on the local area of the internal thread crest corresponding to the impedance peak, the induction power supply outputs the heating power in the form of pulse train, and the duty cycle of the next pulse is updated in real time as follows: D new =D old ×(1–k·ΔZ rel ); where 0<k≤0.4, D old is the duty cycle of the previous pulse, ΔZ rel =(Z start –Z(t))⁄Z start represents the real-time impedance drop relative value, Z start is the initial impedance at the start of heating, and Z(t) is the impedance at the current time; When ΔZ rel ≥0.15 or D new ≤0.20, the high-frequency induction heating is immediately terminated, and the micro-feeding shaping action of the extrusion tap along the thread axis is triggered, to control the local temperature rise rate in segments, prevent annealing caused by one-time overheating, and ensure that the truncation amount of the passivated internal thread crest is stable within the target window range.

[0049] The following describes the steps involved in the above embodiments in detail: The inductive power source outputs heating power to the micro inductive coil of the calibration section in the form of pulse trains. The pulse train form means that the power source does not continuously output constant power, but outputs a series of pulse signals at certain time intervals, each pulse lasting for 5-20 ms, and the pulse interval being 2-10 ms. The duty cycle is the ratio of the duration of a single pulse to the pulse period (the duration of a pulse plus the pulse interval), for example, if a pulse lasts for 10 ms and the interval is 5 ms, the duty cycle is 10 ÷ (10+5) = 0.67. The system first sets the initial duty cycle D old to be 0.6, and the corresponding actual heating power is 60% of the nominal power. In a specific implementation, the high-frequency power source adjusts the duty cycle of the output pulse through a pulse width modulation controller, and the pulse frequency is kept within the range of 250 kHz as described above. For example, for a certain M10 titanium alloy thread crest, the first pulse is output with a duty cycle of 0.6, and the duration is 15 ms, and the corresponding average power is 6 W x 0.6 = 3.6 W. The control system collects impedance data during the interval after each pulse ends and calculates the duty cycle parameter of the next pulse. This pulse output method can provide more accurate energy control than continuous heating, and can avoid local overheating while ensuring sufficient softening effect.

[0050] The control system updates the duty cycle of the next pulse according to a specific algorithm according to the real-time impedance change. The duty cycle update formula is: D new new = D old old x (1-k x ΔZ rel ), where D new new represents the duty cycle of the new pulse to be output, D old old represents the duty cycle of the previous pulse, k is an adjustment coefficient and has a value in the range of 0 to 0.4, and ΔZ rel represents the real-time impedance drop relative value. The calculation method of the real-time impedance drop relative value ΔZ rel is to subtract the current impedance Z(t) from the initial heating instantaneous impedance Z start , and then divide by Z start to obtain the relative change ratio. Z start is the initial impedance value recorded when the inductive heating is started for the thread crest position, and Z(t) is the impedance value measured at the current time for the position. The specific calculation process is as follows: assuming that the thread crest position Z start is 800 Ω, and the current Z(t) is 720 Ω, then ΔZ rel = (800-720) ÷ 800 = 0.1, if k is 0.3, and the previous pulse duty cycle D old old is 0.6, then the new duty cycle D new= 0.6 x (1 - 0.3 x 0.1) = 0.582. The range of k is 0 to 0.4 based on a large number of experiments: when k is too small, the duty cycle adjustment range is insufficient to effectively respond to impedance changes; when k is too large, the adjustment is too aggressive, which can lead to instability in the heating process. The system uses a digital signal processor to execute the algorithm in real time, and after the calculation is completed, the parameter settings of the pulse generator are immediately updated.

[0051] The control system continuously monitors ΔZ rel and D new , and when the termination condition is met, it immediately stops heating and triggers the mechanical shaping action. The termination condition includes two criteria: the first criterion is that ΔZ rel is greater than or equal to 0.15, i.e., the relative impedance drop reaches 15%; the second criterion is that D new is less than or equal to 0.20, i.e., the new duty cycle calculated is reduced to below 20%. The 0.15 impedance drop threshold corresponds to a softening state of about 500℃ for the metal, and when this state is reached, the metal has good plastic deformation ability; the 0.20 minimum duty cycle limit prevents the heating power from being too low, resulting in insufficient softening. For example, when ΔZ rel of a certain tooth tip gradually increases from 0.12 to 0.15, the system immediately cuts off the induction power and triggers an axial micro-feeding action of 2-3 μm. The segmented control of the local temperature rise rate refers to allowing heat diffusion through the pulse interval period to avoid rapid temperature rise; preventing over-heating of the surface layer refers to controlling the maximum temperature to be no higher than 550℃ to avoid irreversible changes in the metal organization. This adaptive pulse control method achieves precise regulation of the heating process, ensuring that each tooth tip achieves the most suitable softening degree, and the passivated truncation amount is stably controlled within the target window range, significantly improving the consistency of the machining and the repeatability of the thread quality.

[0052] In one embodiment of the present application, during the extrusion of the tap into the pre-made hole and the subsequent completion of the extrusion and tooth tip passivation, ester-based micro-atomized lubricating oil is sprayed at a flow rate of 0.5 mL·h -1 ~ 3 mL·h -1 to the contact area between the tap and the hole wall, and the conductivity of the lubricating oil is not higher than 10 μS·cm -1 , and the water content is not higher than 0.05%, so as to provide lubrication and heat dissipation while suppressing arc discharge and maintaining the stability of high-frequency contact impedance spectrum measurement.

[0053] The following describes the steps involved in the above embodiment: The micro-lubrication system continuously supplies ester-based micro-atomized lubricating oil to the contact area between the tapping screw and the internal thread during the entire process of the tapping screw entering the preformed hole to start the machining. The contact area between the tapping screw and the hole wall refers to the direct contact interface between the surface of the tapping screw and the inner wall of the preformed hole and the tooth surface of the internal thread. The micro-lubrication system uses a compressed air atomizing nozzle to atomize the ester-based lubricating oil into fine oil droplets with a diameter of 1-5 μm, which is delivered to the machining area through the cooling channel inside the machine tool spindle. The flow rate is controlled within the range of 0.5 mL·h -1 to 3 mL·h -1 , and a precision metering pump is used to adjust the oil supply. For example, for M10 thread machining, the flow rate is set to 2 mL·h -1 , and the total consumption of lubricating oil is about 0.5 mL during the entire 15-minute machining cycle. The ester-based micro-atomized lubricating oil refers to a special lubricant based on synthetic ester oil and treated by atomization, which has good lubricating performance and volatility. This flow rate range ensures sufficient lubrication effect while avoiding excessive lubricating oil affecting the accuracy of electrical measurement.

[0054] The lubricating oil must meet strict electrical performance requirements to ensure the accuracy of high-frequency contact impedance spectrum measurement. Electrical conductivity refers to the electrical conductivity of lubricating oil per unit volume, and the lower the value, the better the insulation performance. The electrical conductivity of the lubricating oil is strictly controlled to be not higher than 10 μS·cm -1 , and the water content is controlled to be not higher than 0.05%. The electrical conductivity is measured by a conductivity meter, and the water content is detected by Karl Fischer titration. For example, a synthetic ester-based lubricating oil with an electrical conductivity of 8 μS·cm -1 and a water content of 0.03% is selected. Water is the main factor that causes the electrical conductivity of the lubricating oil to rise, and the 0.05% water content limit ensures that the lubricating oil maintains excellent insulation performance. The lubricating oil needs to be dehydrated before use, and molecular sieve adsorption or vacuum dehydration equipment is used to reduce the water content to the required range. These electrical performance requirements ensure that the lubricating oil does not form a low impedance path between the calibration section conductive strip and the internal thread tooth top, avoiding interference with the high-frequency contact impedance spectrum measurement described in the foregoing.

[0055] The ester-based micro-atomized lubricating oil plays multiple technical roles in the machining process. The lubrication function reduces the friction coefficient by forming a very thin lubricating film on the contact interface, reducing the wear of the tapping tap and the torque requirement; the heat dissipation function takes away the excess heat generated by induction heating through the heat absorption of oil mist vaporization, assisting in maintaining local temperature control. Suppressing arc discharge refers to the insulation performance of the lubricating oil preventing the arc breakdown phenomenon between the calibration section conductive strip and the surrounding metal parts when a transient high voltage occurs during high-frequency induction heating. Maintaining the stability of high-frequency contact impedance spectrum measurement refers to the low conductivity of the lubricating oil ensuring that the impedance measurement loop is not affected by external conductive media, ensuring the reproducibility and accuracy of the measurement data. The lubrication system realizes the dual functions of mechanical lubrication and electrical insulation, providing a stable and reliable process environment for the entire electro-inductive-mechanical collaborative passivation process.

[0056] In one embodiment of the present application, when any of the following conditions is detected: the signal-to-noise ratio of the high-frequency contact impedance spectrum is lower than the preset lower limit, the induction heating power feedback exceeds the safety upper limit, or the axial micro-feeding torque fluctuation exceeds the allowed range, the control system immediately terminates the passivation process, withdraws the tapping tap along the original feeding path, and automatically replaces the adjustable truncated cutting tap to continue the truncation process on the internal thread crest.

[0057] The following describes the steps involved in the above embodiment: The control system monitors the abnormal state of three key parameters in real time to determine whether to start the safety protection mechanism. The signal-to-noise ratio of the high-frequency contact impedance spectrum refers to the ratio of the effective impedance signal to the background noise. When the signal-to-noise ratio is lower than the preset lower limit, it indicates that the impedance measurement data is unreliable. The signal-to-noise ratio is calculated by a digital signal processor, and the preset lower limit is set to 20 dB. When the value is lower than this, the accuracy of impedance peak identification significantly decreases. The induction heating power feedback refers to the ratio of the actual power consumed by the micro-induction coil to the set power. When the ratio exceeds the safety upper limit of 1.2, it indicates that the coil or circuit is abnormal. The power feedback is monitored in real time by a power meter. For example, if the set power is 8W but the actual power feedback is 10W, the ratio is 1.25, which exceeds the safety upper limit. The axial micro-feeding torque fluctuation refers to the change amplitude of the torque value when the machine tool servo system performs micro-feeding action. The allowed range is set to ±15%. The torque fluctuation is monitored by the spindle torque sensor. When the fluctuation exceeds the allowed range, it indicates that the tap or thread is abnormally stuck. Real-time monitoring of these three parameters ensures that the system responds promptly in abnormal conditions, avoiding equipment damage or workpiece rejection.

[0058] When any abnormal situation is detected, the control system immediately executes a safety exit procedure. The system first cuts off all heating power, stops the pulse output, and then controls the machine tool spindle to slowly exit the extruded tap along the original feed path into the internal thread. The original feed path refers to the helical trajectory of the extruded tap when it enters the preformed hole. When exiting, it must strictly follow the same helical path in the opposite direction to avoid damaging the formed internal thread. The exit speed is set to 50% of the feed speed to ensure smooth exit. For example, if the feed speed is 100 rpm, the exit speed is set to 50 rpm. During the exit process, the system continuously monitors the torque change, and if abnormal resistance occurs, the exit speed is further reduced. After complete exit, the machine tool automatically switches to the spare tool magazine and selects a cutting tap with adjustable truncation. The cutting tap with adjustable truncation refers to a special tool whose tooth base cutting edge height can be adjusted. By adjusting the cutting depth through the adjusting mechanism, the tooth crest truncation is controlled. The system determines the required truncation based on the previous impedance detection results, automatically adjusts the cutting tap parameters, and continues to complete the tooth crest blunting process. This safety protection mechanism ensures that even if the electro-inductive-mechanical cooperative system fails, the thread blunting can still be completed through traditional cutting, ensuring production continuity and product quality.

[0059] The application also provides an application of the extruded thread, which is processed by the extruded thread processing method of any one of the above embodiments. The internal thread processed by the extruded thread processing method is applied to the demand for high-strength thread connecting pieces in the fields of aerospace and automobile manufacturing.

[0060] In the field of aerospace, the extruded thread is mainly applied to the manufacturing of key components such as self-locking nuts, connecting rings, and fasteners. Aviation thread connecting pieces need to meet strict international standard requirements such as MJ and UNJ, and have very high requirements for the fatigue life, tensile strength, and reliability of the thread. The traditional cutting blunting method will cut off the continuous metal fiber flow line formed by extrusion, weaken the surface residual compressive stress, and cause the thread to easily fatigue fracture under high-frequency vibration and alternating load. The method of the application eliminates the sharp tooth crest while completely maintaining the continuity of the fiber flow line and the distribution of residual compressive stress through local induction softening and slight flattening, which significantly improves the fatigue life and tensile strength of the thread.

[0061] In the field of automobile manufacturing, the extruded thread is suitable for threaded connection of high stress parts such as engine connecting rod, gearbox shell, chassis connecting piece, etc. The automobile industry has increasingly strict requirements for lightweight and high strength of parts, and the threaded connection needs to realize weight reduction under the premise of ensuring strength. The present application can use smaller size threads or thinner wall thickness parts under the same strength requirement by optimizing the internal strength characteristics of the thread, realizing effective lightweight design. At the same time, automobile parts bear complex load spectrum and environmental conditions in long-term use, the thread processed by the method has better fatigue resistance and dimensional stability, which can effectively prolong the service life of the automobile and reduce the maintenance cost.

[0062] The above only describes the preferred embodiments of the present application, and does not limit the patent scope of the present application, and any equivalent structural transformation made under the inventive concept of the present application, or direct / indirect application in other related technical fields, is included in the patent protection scope of the present application.

Claims

1. A method for processing an extruded thread, characterized in that: include: preparing prefabricated holes in the workpiece; The prefabricated hole is extruded by using an extrusion tap to plastically deform the peripheral wall of the prefabricated hole to form an internal thread; The crest of the internal thread is passivated, the crest is shortened to a preset shortening amount and a passivation profile is formed.

2. The extruded thread processing method according to claim 1, characterized in that: The passivation treatment of the crest of the internal thread comprises: After the extrusion process is completed and the extrusion tap remains in the internal thread and has not been withdrawn, continuously collecting high-frequency contact impedance spectra between the thread bottom of the extrusion tap and the thread top of the internal thread along the axial direction of the thread; Determining the residual sharp height of the internal thread crest in real time based on a comparison result of an impedance peak in the high-frequency contact impedance spectrum and a preset threshold; The passivation energy applied to the local area of ​​the internal thread crest corresponding to the impedance peak and the axial micro-feed of the extrusion tap are automatically adjusted according to the judgment result, so that the shortening amount of the internal thread crest after passivation is kept within the target window range.

3. The extruded thread processing method according to claim 2, characterized in that: The scanning frequency range of the high-frequency contact impedance spectrum is 50kHz to 1MHz; When the impedance peak Z p The increment ΔZ=Z relative to the impedance mean Z0 of the same scanning section p - When Z0 satisfies ΔZ / Z0>preset ratio threshold η, it is determined that the residual sharp height of the internal thread crest exceeds the limit; The control system determines the passivation energy applied to the local area of ​​the tooth top corresponding to the impedance peak and the axial micro-feed of the extrusion tap in real time through a table lookup or interpolation algorithm based on the preset corresponding relationship between the increment ΔZ and the target truncation amount window.

4. The extruded thread processing method according to claim 2, characterized in that: The passivation energy is provided by high-frequency induction heating energy output by a micro-induction coil arranged in a strip-shaped area at the bottom of the extrusion tap and opposite to the internal thread crest. The strip-shaped area remains in a metallic conductive state and forms an electrical contact loop with the internal thread crest. The frequency of the high-frequency induction heating is 200kHz~350kHz, the single heating power is not more than 10W, and the heating duration is not more than 150ms, so that the local metal of the internal thread top corresponding to the impedance peak is softened to 450℃~550℃, and the thickness of the softened layer does not exceed 30μm.

5. The extruded thread processing method according to claim 4, characterized in that: Within 1 ms after the high-frequency induction heating is completed, the machine tool servo system drives the extrusion tap to feed 1 μm to 5 μm along the thread axis, and uses the micro-ridges on the bottom of the extrusion tap to flatten and shape the softened internal thread top, so that the metal flows laterally and the burrs are rolled into the tooth side grooves, forming a blunt profile with an arc radius of 0.1P to 0.2P, where P is the pitch.

6. The extruded thread processing method according to claim 5, characterized in that: During high-frequency induction heating of a local area of ​​the internal thread crest corresponding to the impedance peak, the real-time change value Z(t) of the contact impedance of the local area is synchronously monitored with a sampling period of 0.2ms to 1ms. When Z(t) meets one of the following conditions, the induction heating is immediately terminated and the axial micro-feeding action of the extrusion tap is triggered: Condition 1: Z(t) relative to the impedance Z at the start of heating start The decrease of ΔZ=Z start -Z(t) is not less than Z start 15%; Condition 2: Impedance change rate dZ / dt is less than -0.5Z start s -1 ; In this way, the local softening temperature is controlled below 550°C, the softening layer thickness is controlled below 30 μm, and the shortening amount of the internal thread crest after passivation is kept within the target window range.

7. The extruded thread processing method according to claim 6, characterized in that: When high-frequency induction heating is applied to the local area of ​​the internal thread crest corresponding to the impedance peak, the induction power supply outputs heating power in the form of a pulse train and updates the duty cycle of the next pulse in real time according to the following formula: D new =D old ×(1–k·ΔZ rel ); where 0 <k≤0.4,D old is the duty cycle of the previous pulse, ΔZ rel =(Z start –Z(t))⁄Z start Indicates the relative value of real-time impedance drop, Z start is the instantaneous impedance at the start of heating, Z(t) is the impedance at the current moment; When ΔZ rel ≥0.15 or D new When the value is ≤0.20, the high-frequency induction heating is terminated immediately and the micro-feed shaping action of the extrusion tap along the thread axis is triggered to control the local temperature rise rate in sections, prevent surface annealing caused by overheating, and ensure that the shortening amount of the internal thread crest after passivation is stable within the target window range.

8. The extruded thread processing method according to claim 2, characterized in that: During the period when the extrusion tap enters the prefabricated hole and completes the extrusion process and the tooth top passivation, the -1 ~3mL·h -1 The flow rate is to spray a small amount of ester-based atomized lubricating oil to the contact area between the tap and the hole wall, and the conductivity of the lubricating oil is not higher than 10μS·cm -1 , the water content is not higher than 0.05%, so as to suppress arc discharge and maintain the stability of high-frequency contact impedance spectroscopy measurement while providing lubrication and heat dissipation.

9. The extruded thread processing method according to claim 2, characterized in that: When it is detected that the signal-to-noise ratio of the high-frequency contact impedance spectrum is lower than a preset lower limit, the induction heating power feedback exceeds the safety upper limit, or the axial micro-feed torque fluctuation exceeds the allowable range, the control system immediately terminates the passivation process, withdraws the extrusion tap from the internal thread along the original feed path, and automatically replaces the cutting tap with an adjustable shortening amount to continue shortening the internal thread crest.

10. An application of extruded thread, characterized in that, The extruded thread processing method according to any one of claims 1 to 9 is adopted.

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