A cold heading multi-stage impact forming process for improving the fracture resistance of screw head

Abstract

The present application relates to the technical field of fastener cold forming processing, and discloses a cold upsetting multi-stage impact forming process for improving the anti-fracture performance of a screw head, comprising the following steps: S1, providing a metal wire, cutting the metal wire into a blank with a predetermined length, and the predetermined length being 4-8 times the diameter of the blank; and S2, performing shaping treatment on the blank to make the end face of the blank flat, wherein the blank is formed by three-stage gradient compression ratio, the deformation amount and impact force are stepwise decreased, the tensile stress of the head surface layer is reduced, the arc-shaped punch and the rotation of the blank are matched to improve the metal flow line, the stress concentration of the head and rod is eliminated, and the formation of micro cracks is avoided. The time, speed and temperature of the thread rolling after the cold upsetting are specified to prevent secondary cracks. The anti-fracture load and fatigue life of the finished product head are significantly improved, the mold loss is low, the defective rate is small, the cold upsetting equipment can be directly used for production of screws of multiple materials and multiple specifications, and the practicality is high.

Description

Technical Field

[0001] This invention relates to the field of fastener cold forming technology, specifically to a cold heading multi-stage impact forming process for improving the fracture resistance of screw heads. Background Technology

[0002] As the most commonly used fastener, screw head fracture is one of the most common failure modes during service. The neck region where the screw head connects to the shank is the area of ​​greatest stress concentration. Under large tightening torques or alternating loads, this region is highly susceptible to fracture failure, seriously threatening the safety and reliability of the connection structure.

[0003] Currently, screw heads are primarily manufactured using cold heading. Cold heading offers advantages such as high material utilization, high production efficiency, and high dimensional accuracy, and has been widely applied in the fastener industry. However, existing cold heading processes generally employ one or two impact forming methods to complete the head upsetting, with single-stage deformation often exceeding 60%. This large-deformation, one-time impact forming subjectes the surface material on the head's sides to significant circumferential tensile stress, easily inducing axial microcracks. Furthermore, the R-angle region at the head-shank junction is obstructed, creating a deformation dead zone that disrupts the continuity of metal flow lines, making this area a weak point in the screw's mechanical properties. In addition, the thread rolling process in existing processes is typically performed at any time after cold heading, without considering the impact of the work-hardened material's aging characteristics on thread forming quality, further exacerbating the risk of screw head fracture. Therefore, we propose a multi-stage impact forming process for cold heading to improve the screw head's fracture resistance and address these problems. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a cold heading multi-stage impact forming process to improve the fracture resistance of screw heads. This process solves the problems of large single-stage deformation, easy generation of micro-cracks on the surface of screw heads, disordered metal flow lines and severe stress concentration at the head-shaft joint, and head fracture failure due to lack of control over the thread rolling process in traditional single and double-stamp cold heading processes.

[0005] To achieve the above objectives, the present invention provides the following technical solution: a cold heading multi-stage impact forming process for improving the fracture resistance of screw heads, comprising the following steps:

[0006] S1. Provide metal wire, cut the metal wire into blanks of a predetermined length, the predetermined length being 4-8 times the diameter of the blank;

[0007] S2. The blank is shaped to make the end face of the blank flat and a guide chamfer is formed at at least one end of the blank;

[0008] S3. The head of the blank is pre-upset using a first impact die to form a pre-formed head with a first preset diameter and a first preset height, wherein the compression ratio of the pre-formed head is 38%-45%;

[0009] S4. The preformed head is subjected to progressive impact forming using at least two stages of precision upsetting impact dies, including at least:

[0010] S41. The preformed head is subjected to secondary upsetting using a second impact die, causing the head metal to flow outward and initially forming the head outline. The compression ratio of the second impact process is 22%-28%.

[0011] S42. A third impact die is used to perform final upsetting on the head after the second impact process to form a screw head with the final shape and size. The compression ratio of the third impact process is 12%-16%.

[0012] In the multi-stage precision upsetting process, the compression ratio of adjacent processes decreases sequentially along the forming direction, and the impact force of each impact die decreases sequentially along the forming direction.

[0013] S5. Eject the formed screws from the mold.

[0014] Preferably, the ratio of the diameter D1 of the preformed head to the diameter D0 of the blank is 1.3-1.4:1, and the ratio of the height H1 of the preformed head to the diameter D0 of the blank is 0.9-1.1:1.

[0015] Preferably, in the second impact process, the impact end face of the second impact die is provided with an arc-shaped transition surface, and the radius of curvature of the arc-shaped transition surface is R, which satisfies: R=(0.4-0.5)×D0, where D0 is the diameter of the blank.

[0016] Preferably, in the third impact process, the impact end face of the third impact die is provided with a protrusion for forming a head groove, and the draft angle of the protrusion is 1°-1.5°.

[0017] Preferably, the impact forces of the first impact die, the second impact die, and the third impact die decrease sequentially, and the difference in impact force between adjacent impact dies is 12%-18% of the impact force of the first impact die.

[0018] Preferably, a billet rotation step is provided between adjacent steps of the multi-stage precision upsetting process, so that the billet is rotated by a predetermined angle relative to the impact die of the previous process before entering the next process.

[0019] Preferably, the predetermined angle is 80°-100°.

[0020] Preferably, each impact process in the multi-stage fine upsetting process is carried out at room temperature, and the temperature of the billet in each impact process does not exceed 260°C.

[0021] Preferably, after the S5 ejection process, a thread rolling process is further included: the ejected screw blank is fed into a thread rolling machine, and the shank of the screw blank is rolled into threads at room temperature to form threads; in the thread rolling process, the feed speed of the thread rolling plate is V, which satisfies: V=(0.2-0.5)×V0, where V0 is the impact speed of the third impact die in the multi-stage precision upsetting process; the thread rolling process is carried out within 2-4 hours after the S5 ejection process is completed, and the temperature of the screw blank in the thread rolling process does not exceed 120°C.

[0022] This invention provides a screw, which is manufactured by cold heading multi-stage impact forming process, after being ejected and then formed by thread rolling. The metal flow lines at the connection between the head and the shank of the screw are continuously distributed. When observed under a metallographic microscope at 100x magnification, there are no cracks or defects on the surface of the side of the head, and the shank of the screw is provided with external threads.

[0023] Beneficial effects

[0024] This invention provides a cold heading multi-stage impact forming process to improve the fracture resistance of screw heads. Compared with the prior art, it has the following advantages:

[0025] This invention, a multi-stage cold heading process for improving the fracture resistance of screw heads, employs a three-stage gradient compression ratio for step-by-step cold heading compared to existing single-punch and double-punch cold heading processes. The compression ratio and impact force at each stage decrease progressively along the forming direction, significantly reducing the amount of plastic deformation per pass and effectively alleviating the circumferential tensile stress on the screw head surface, thus suppressing axial micro-cracks from the forming source. The second impact die features an arc-shaped transition surface adapted to the blank diameter, eliminating the deformation dead zone at the head-shank transition position. Combined with an 80°-100° blank rotation process between adjacent processes, it improves the unilateral metal accumulation defects caused by unidirectional impact, ensuring continuous and complete metal flow lines and uniform circumferential distribution at the head-shank joint, significantly reducing stress concentration weak points. The final punch die features a 1°-1.5° draft angle in the groove protrusion, ensuring full cavity filling and preventing micro-defects caused by pulling on the head groove during demolding.

[0026] Strictly limiting the cold-forged semi-finished product to low-temperature, low-speed thread rolling within 2-4 hours avoids the risk of secondary cracks caused by the work-hardening aging of steel. Performance testing shows that the load fracture points of screw wedges prepared using this process all occur in the shank. Under the same material specifications, the head fracture load resistance is increased by 25%-40%, and the alternating fatigue cycle life is increased by more than 50%. Multi-stage step-by-step forming reduces the impact load of single-station dies, extends the service life of the die, and significantly reduces the scrap rate of defects such as head cracking and flow line disorder. The entire process is adaptable to various wire materials such as carbon steel and alloy steel, and is compatible with various screw head structures such as pan head and countersunk head. It can be directly produced using existing three-die, three-punch cold forging equipment, demonstrating outstanding industrial adaptability and economic value. Attached Figure Description

[0027] Figure 1 This is a diagram of a cold heading multi-stage impact forming process for improving the fracture resistance of screw heads according to the present invention. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] like Figure 1 As shown:

[0030] Example 1: M4 carbon steel pan head screw;

[0031] S1. Select SWRCH10K cold heading steel wire, billet diameter D0=4mm, cut billet length 20mm (length is 5 times D0, in the range of 4-8 times).

[0032] S2. Mill the end faces of both ends of the cut blank flat, and machine a C0.5 guide chamfer on the head forming end;

[0033] S3, First impact die pre-upsetting, compression ratio 40%; pre-formed head D1=5.4mm (D1:D0=1.35:1), H1=4mm (H1:D0=1:1); billet temperature controlled ≤230℃ throughout the process;

[0034] S4, Multi-stage Precision Upsetting Impact: S41 Secondary Upsetting: The second impact die has an arc transition R=1.8mm (0.45×D0), with a compression ratio of 25%; the first impact force is 100kN, the second impact force is 85kN, with a difference of 15kN (15% for the first impact, within the 12%-18% range); the billet rotates 90° between stations; S42 Final Upsetting: The third impact die has a cross groove protrusion with a draft angle of 1.2°, and a compression ratio of 14%; the third impact force is 70kN, with a difference of 15kN from the second impact; the billet rotates 90° before entering the final upsetting; the highest billet temperature throughout the process is 242℃;

[0035] S5. The ejection mechanism ejects the semi-finished screws out of the mold cavity;

[0036] S6. After cold heading is completed, the blank is fed into the thread rolling machine 3 hours later. The final impact speed V0 = 0.5 m / s and the thread rolling feed speed V = 0.2 m / s (0.4 × V0). The maximum temperature of the blank during the thread rolling process is 105℃, forming an M4 standard external thread.

[0037] Finished product inspection: 100x metallographic observation showed that the streamline at the screw head-shank joint was continuous and unbroken, and there were no microcracks on the head sidewall; in the wedge load test, the fracture location was always the shank, and the head fracture load was 32% higher than that of the traditional double-punch process.

[0038] Example 2, M6 alloy steel countersunk screw;

[0039] S1, 45# steel wire, D0=6mm, billet length 36mm (6 times D0).

[0040] S2, both ends are flat, and the head end has a guide chamfer C1;

[0041] S3, compression ratio 42%, D1=7.8mm (1.3:1), H1=5.4mm (0.9:1);

[0042] S4 multi-stage precision upsetting;

[0043] S41, Secondary upsetting: R=2.4mm (0.4×D0), compression ratio 26%; first stroke 150kN, second stroke 129kN, difference 21kN (14%), billet rotation 85°;

[0044] S42, one-slot protrusion draft angle 1°, compression ratio 13%; three-stroke 108kN, blank rotation 95°; maximum forming temperature 251℃;

[0045] S5, eject and demold;

[0046] S6. After cold heading, 2.5 hours later, the teeth are rolled, V0=0.4m / s, the rolling speed is 0.12m / s (0.3×V0), and the rolling temperature is ≤112℃.

[0047] Finished product inspection revealed no surface cracks at the head, and the streamlines were symmetrical and continuous; the number of fatigue alternating cycles was increased by 56% compared to traditional processes.

[0048] Example 3, M2 small precision electronic screw;

[0049] S1, D0=2mm, billet length 14mm (7 times D0).

[0050] S2, micro C0.2 chamfer;

[0051] S3, compression ratio 44%, D1=2.8mm (1.4:1), H1=2.2mm (1.1:1).

[0052] S4 multi-level precision upsetting

[0053] S41, second stroke R=1mm (0.5×D0), compression ratio 23%; first stroke 40kN, second stroke 33.6kN, difference 6.4kN (16%), rotation 100°;

[0054] S42, three-stroke draft angle 1.5°, compression ratio 15%, three-stroke 27.2kN; maximum forming temperature 228℃;

[0055] S5, eject;

[0056] S6. Processing 3.5 hours after cold heading, V0=0.6m / s, thread rolling speed 0.12m / s (0.2×V0), temperature ≤98℃.

[0057] Finished product inspection revealed that the miniature head has no microcracks and no risk of tipping over or failing, meeting the requirements for long-term vibration conditions in precision electronics.

[0058] Using the existing two-die two-punch process, the total compression ratio in a single pass is 62%, with no graded deformation, no billet rotation, and no thread rolling time control; after forming, metallographic imaging shows axial microcracks on the head sidewall and streamlined folding of the head rod R angle; in the wedge load test, 35% of the samples experienced head fracture, and the fracture load resistance was only 70% of that in Example 1 of this invention.

[0059] In this implementation plan: the metal wire is fed into the cutting station by the feeding mechanism and precisely cut to a length of 4-8 times the billet diameter; the cut billet enters the shaping station, where the end face is flattened and a guide chamfer is processed to ensure the coaxiality of the subsequent mold positioning; the billet is transferred to the first impact station, where a large impact force completes the pre-polymerization of the head, allocating 38%-45% of the deformation amount to initially form the head base; the transfer robot rotates the pre-formed billet 80°-100° and sends it into the second impact station, where the arc-shaped transition punch gently extrudes it, allocating 22%-28% of the deformation amount to initially unfold the outer contour of the head and eliminate the right-angle shear stress of the head rod; the billet is rotated again into the third final punch station, where a smaller impact force completes the groove shape and final dimension finishing, allocating only 12%-16% of the micro-deformation to avoid surface cracking; after forming, the ejection mechanism ejects the semi-finished product from the mold cavity; within 2-4 hours, it is sent to the thread rolling machine, where low-speed and low-temperature thread rolling is completed, finally obtaining a high-fracture-resistant screw product with no cracks in the head and continuous streamline.

[0060] The forming mechanism involves splitting the total deformation amount in three stages, gradually reducing the single-pass plastic deformation amplitude, and simultaneously reducing the impact load. Combined with circumferential rotation between workstations, arc transition molds, and matching and controlling thread rolling processes, the metal flow behavior is optimized in all aspects, eliminating four major defects: tensile stress concentration, deformation dead zone, flow line disorder, and secondary cracks. This improves the overall mechanical strength of the screw head from the microstructure level of the forming process.

[0061] Compared to existing single-punch and double-punch cold heading processes, this invention employs a three-stage gradient compression ratio for step-by-step upsetting. The compression ratio and impact force at each stage decrease progressively along the forming direction, significantly reducing the amount of plastic deformation per pass and effectively alleviating the circumferential tensile stress on the screw head surface, thus suppressing axial micro-cracks from the forming source. The second impact die features an arc-shaped transition surface adapted to the billet diameter, eliminating the deformation dead zone at the head-and-shank transition position. Combined with the 80°-100° billet rotation process between adjacent processes, it improves the unilateral metal accumulation defects caused by unidirectional impact, ensuring continuous and complete metal flow lines and uniform circumferential distribution at the head-and-shank joint, significantly reducing stress concentration weak points. The final punch die features a 1°-1.5° draft angle in the groove protrusion, ensuring full cavity filling and preventing micro-defects caused by pulling on the head groove during demolding.

[0062] Strictly limiting the cold-forged semi-finished product to low-temperature, low-speed thread rolling within 2-4 hours avoids the risk of secondary cracks caused by the work-hardening aging of steel. Performance testing shows that the load fracture points of screw wedges prepared using this process all occur in the shank. Under the same material specifications, the head fracture load resistance is increased by 25%-40%, and the alternating fatigue cycle life is increased by more than 50%. Multi-stage step-by-step forming reduces the impact load of single-station dies, extends the service life of the die, and significantly reduces the scrap rate of defects such as head cracking and flow line disorder. The entire process is adaptable to various wire materials such as carbon steel and alloy steel, and is compatible with various screw head structures such as pan head and countersunk head. It can be directly produced using existing three-die, three-punch cold forging equipment, demonstrating outstanding industrial adaptability and economic value.

[0063] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A cold heading multi-stage impact forming process for improving the fracture resistance of screw heads, characterized in that: Includes the following steps: S1. Provide metal wire, cut the metal wire into blanks of a predetermined length, the predetermined length being 4-8 times the diameter of the blank; S2. The blank is shaped to make the end face of the blank flat and a guide chamfer is formed at at least one end of the blank; S3. The head of the blank is pre-upset using a first impact die to form a pre-formed head with a first preset diameter and a first preset height, wherein the compression ratio of the pre-formed head is 38%-45%; S4. The preformed head is subjected to progressive impact forming using at least two stages of precision upsetting impact dies, including at least: S41. The preformed head is subjected to secondary upsetting using a second impact die, causing the head metal to flow outward and initially forming the head outline. The compression ratio of the second impact process is 22%-28%. S42. A third impact die is used to perform final upsetting on the head after the second impact process to form a screw head with the final shape and size. The compression ratio of the third impact process is 12%-16%. In the multi-stage precision upsetting process, the compression ratio of adjacent processes decreases sequentially along the forming direction, and the impact force of each impact die decreases sequentially along the forming direction. S5. Eject the formed screws from the mold.

2. The cold heading multi-stage impact forming process for improving the fracture resistance of screw heads according to claim 1, characterized in that: The ratio of the diameter D1 of the preformed head to the diameter D0 of the blank is 1.3-1.4:1, and the ratio of the height H1 of the preformed head to the diameter D0 of the blank is 0.9-1.1:

1.

3. The cold heading multi-stage impact forming process for improving the fracture resistance of screw heads according to claim 1, characterized in that: In the second impact process, the impact end face of the second impact die is provided with an arc-shaped transition surface, and the radius of curvature of the arc-shaped transition surface is R, which satisfies: R=(0.4-0.5)×D0, where D0 is the diameter of the blank.

4. The cold heading multi-stage impact forming process for improving the fracture resistance of screw heads according to claim 1, characterized in that: In the third impact process, the impact end face of the third impact mold is provided with a protrusion for forming the head groove, and the draft angle of the protrusion is 1°-1.5°.

5. The cold heading multi-stage impact forming process for improving the fracture resistance of screw heads according to claim 1, characterized in that: The impact forces of the first impact die, the second impact die, and the third impact die decrease sequentially, and the difference in impact force between adjacent impact dies is 12%-18% of the impact force of the first impact die.

6. The cold heading multi-stage impact forming process for improving the fracture resistance of screw heads according to claim 1, characterized in that: Between adjacent processes of the multi-stage precision upsetting process, there is also a billet rotation step, which rotates the billet relative to the impact die of the previous process by a predetermined angle before entering the next process.

7. The cold heading multi-stage impact forming process for improving the fracture resistance of screw heads according to claim 6, characterized in that: The predetermined angle is 80°-100°.

8. The cold heading multi-stage impact forming process for improving the fracture resistance of screw heads according to claim 1, characterized in that: All impact processes in the multi-stage precision upsetting process are carried out at room temperature, and the temperature of the billet in each impact process does not exceed 260°C.

9. The cold heading multi-stage impact forming process for improving the fracture resistance of screw heads according to claim 1, characterized in that: Following the S5 ejection process, a thread rolling process is also included: the ejected screw blank is fed into a thread rolling machine, and the shank of the screw blank is rolled into threads at room temperature to form threads; in the thread rolling process, the feed speed of the thread rolling plate is V, which satisfies: V=(0.2-0.5)×V0, where V0 is the impact speed of the third impact die in the multi-stage precision upsetting process; the thread rolling process is carried out within 2-4 hours after the completion of the S5 ejection process, and the temperature of the screw blank in the thread rolling process does not exceed 120℃.

10. A screw, characterized in that, The screw is manufactured by cold heading multi-stage impact forming process according to any one of claims 1 to 8, followed by thread rolling. The metal flow lines at the connection between the head and the shank are continuously distributed. When observed under a metallographic microscope at 100x magnification, there are no cracks or defects on the surface of the head side, and the shank of the screw is provided with external threads.

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