A slotted mandrel extrusion strengthening process for improving the uniformity of residual stress around the hole

Abstract

The present invention proposes a slotted mandrel extrusion strengthening process for improving the degree of uniformity of residual stress around the hole, which comprises the following steps: (1) placing a sample with a hole on a slotted mandrel guide section; (2) installing a support rod; (3) performing a single hole extrusion strengthening on the sample with a hole, and forming a convex ridge on the hole wall of the sample after the hole extrusion strengthening of the slotted mandrel hole; (4) flipping the sample 180°, so that the intrusion end of the first hole extrusion strengthening sample of the slotted mandrel is used as the extrusion end of the second hole extrusion strengthening sample, and the extrusion end is used as the intrusion end of the second hole extrusion strengthening sample, and rotating the slotted mandrel by 10° to 30° to perform the second extrusion strengthening of the slotted mandrel; (6) reaming. The present invention adopts a slotted mandrel two-time hole extrusion strengthening process to perform extrusion strengthening on the sample with a hole, and after the hole extrusion strengthening, the plastic deformation degree of the intrusion end and the extrusion end of the hole wall of the sample is similar, and the residual compressive stress formed on the hole wall is uniform, thereby improving the fatigue performance of the sample with a hole.

Description

Technical Field

[0001] The invention relates to a hole extrusion strengthening method for homogenizing residual stress around a hole by double extrusion strengthening with a slotted mandrel, belongs to the technical field of anti-fatigue manufacturing of hole extrusion strengthening, and particularly relates to using a slotted mandrel to perform double extrusion strengthening in opposite directions on a sample with a hole. Background Art

[0002] Connection holes are common connection structures on aircraft. Fasteners are installed in the connection holes to connect the aircraft structures to each other. The existence of connection holes causes material discontinuity. Fatigue cracks are easily generated in the specimen with holes under the action of external alternating loads, which affects the fatigue life of the specimen with holes. Hole extrusion strengthening is a simple and effective process in anti-fatigue manufacturing technology and is widely used in the aviation field. However, when the hole specimen is extruded and strengthened with a slotted bushing and a solid mandrel, uneven residual compressive stress is formed on the hole wall of the specimen after extrusion strengthening, which affects the fatigue performance of the specimen with holes.

[0003] The Chinese patent document with publication number CN111118270B (hereinafter referred to as Document 1) proposes a cold extrusion process and device for homogenizing residual stress around a hole. The cold extrusion process for homogenizing residual stress around a hole is suitable for straight-through holes. The cold extrusion process includes strengthening the straight-through hole for the first time along the first axial direction by a first mandrel; strengthening the straight-through hole for the second time along the second axial direction by a second mandrel; wherein the first axial direction is opposite to the second axial direction, and the maximum diameter of the strengthened portion of the first mandrel is less than or equal to the maximum diameter of the strengthened portion of the second mandrel. After two-step bidirectional cold extrusion strengthening, the circumferential residual stress of the straight-through hole is more evenly distributed along the thickness direction, and no residual tensile stress is generated at the hole edges of the two end faces, which is beneficial to improving the fatigue resistance of the strengthened hole. The main differences between this patent document and the present application document are: (1) The hole extrusion strengthening tool is different. The hole extrusion strengthening tool used in Document 1 is a bushing, and the bushing thickness used for the two hole extrusion strengthenings is different, while the hole extrusion strengthening tool used in the present application is a slotted mandrel, and the slotted mandrel used for the two hole extrusion strengthenings is the same, which reduces the process cost. (2) Hole extrusion strengthening process. The slotted mandrel mentioned in this application is extruded twice. During the second extrusion strengthening, the slotted mandrel needs to be rotated at a certain angle to remove the ridge formed on the hole wall of the hole sample during the first extrusion strengthening of the slotted mandrel. The patent document of the cold extrusion process and device for homogenizing the residual stress around the hole does not limit the angle of the extrusion mandrel. (3) Reaming. This application uses a slotted mandrel for two extrusion strengthenings. Reaming is required to trim the hole wall of the sample after extrusion strengthening. The purpose is to make the hole diameter of the sample reach the final hole diameter after hole extrusion strengthening. Document 1 does not mention the need for reaming after two extrusion strengthenings.

[0004] The Chinese patent document with publication number: CN114085968A (hereinafter referred to as Document 2) proposes a method for strengthening the cold extrusion assembly hole of a slotted bushing and a matching tooling device, the method comprising: steps S1-S5. The matching tooling comprises an extrusion gun, an extrusion core rod and a bushing top block; a piston cavity is arranged in the outer shell, a telescopic movable mechanism is arranged in the piston cavity, an extension end is arranged at the front end of the outer shell, a telescopic channel connected to the piston cavity is arranged in the extension end, a telescopic rod is arranged in the telescopic channel, and one end of the telescopic rod is connected to the telescopic movable mechanism; an annular first extrusion part is arranged at the end of the extrusion core rod, and a transition section for placing the slotted bushing is arranged between the first extrusion part and the second extrusion part; the bushing top block is a conical tubular structure with a large upper part and a small lower part, and the bushing top block is sleeved on the extrusion core rod. It proposes a method for secondary extrusion strengthening of the assembly hole using the slotted bushing to achieve uniformity of the stress value of the entire assembly hole, better ensure the strengthening quality and effect, and further significantly improve the fatigue resistance. The main differences between this patent document 2 and the present application document are: (1) Extrusion strengthening tool. The hole extrusion strengthening tool used in Document 2 is a slotted bushing. Each time an extrusion strengthening is performed, a slotted bushing needs to be replaced. However, in this application document, the same mandrel is used in both extrusion strengthening processes of the holed sample. (2) The slot position of the extrusion strengthening tool during hole extrusion strengthening. The slot positions of the slotted bushings used in the two extrusion strengthening processes in Document 2 are aligned, so that the ridges on the hole wall of the sample still exist after two extrusion strengthening processes. However, in this application document, the slotted mandrel is rotated by 10-30° when the holed sample is subjected to the second slotted mandrel hole extrusion strengthening process. This can eliminate the ridges formed on the hole wall of the holed sample during the first extrusion strengthening process of the slotted mandrel. (3) The extrusion mandrel structure is different. The mandrel used in the second extrusion strengthening in Document 2 is a solid mandrel, and the solid mandrel includes two extrusion parts, the first extrusion part and the second extrusion part, and the diameter of the first extrusion part is larger than the diameter of the second extrusion part. The extrusion mandrel used in this application document is a slotted mandrel, and the extrusion strengthening part of the extrusion mandrel is provided with a slot. The perforated specimen can achieve single-side extrusion strengthening. The structure of the slotted mandrel consists of a connecting section, a guide section, a front cone section, a working ring, a rear cone section, etc., among which the front cone section, the working ring, and the rear cone section are the extrusion strengthening parts of the slotted mandrel. The mandrel used for the two extrusion strengthening of the slotted mandrel is the same slotted mandrel. (4) The two extrusion strengthening forms of the perforated specimen are different.In Document 2, the slit bushing is inserted into the assembly hole from one end of the assembly hole, the extrusion tooling device drives the extrusion mandrel to contract, and the first extrusion part on the extrusion mandrel contracts backward to extrude the slit bushing. After the first extrusion part is completely pulled out of the slit bushing, the first extrusion is completed, the slit bushing is taken out of the assembly hole, and a new slit bushing is replaced and assembled on the extrusion mandrel, the new slit bushing is inserted into the assembly hole from the other end of the assembly hole, and the slit position of the new slit bushing is aligned with the slit position of the slit bushing used for the first extrusion, the extrusion tooling device drives the extrusion mandrel to contract, and the first extrusion part on the extrusion mandrel contracts backward to extrude the slit bushing. After the first extrusion part is completely pulled out of the slit bushing, the second extrusion is completed. In the present application document, the slit mandrel passes through the auxiliary plate and the perforated sample from the front, and the slit mandrel inserted into the support rod is pulled out at a uniform speed to complete the first hole extrusion strengthening of the perforated sample, and the perforated sample is flipped 180°, and the extrusion end of the hole wall of the slit mandrel sample for the first hole extrusion strengthening is used as the extrusion end of the hole wall of the slit mandrel sample for the second hole extrusion strengthening, and the extrusion end of the hole wall of the slit mandrel sample for the first hole extrusion strengthening is used as the extrusion end of the hole wall of the slit mandrel sample for the second hole extrusion strengthening, and the slit mandrel is rotated 10° to 30°, and force is applied to the extrusion strengthening part of the slit mandrel to shrink the slit at the extrusion strengthening part of the slit mandrel, so that the slit mandrel can pass through the auxiliary plate and the first hole extrusion strengthening sample of the slit mandrel from the front, and the support rod is inserted into the slit mandrel, and the slit mandrel inserted into the support rod is pulled out at a uniform speed, and the perforated sample completes the second hole extrusion strengthening of the slit mandrel.

[0005] In summary, slotted bushings and solid mandrels are widely used in hole extrusion strengthening technology. However, the slotted bushing has a thin wall and is prone to large plastic deformation during hole extrusion strengthening, which makes the slotted bushing unable to be reused, seriously affecting the cost of the slotted mandrel hole extrusion strengthening process. The diameter of the extrusion strengthening part of the solid mandrel is larger than the initial hole diameter of the hole specimen, which leads to limitations in the application scope of the solid mandrel hole extrusion strengthening process. Combining slotted bushings with solid mandrels, the slotted mandrel hole extrusion strengthening process proposed can solve the problems of slotted bushings and solid mandrels. However, the slotted mandrel is used to perform one-time extrusion strengthening on the hole specimen. After the hole extrusion strengthening, the extrusion pressures at the extrusion end, middle layer, extrusion end and other positions of the hole wall of the specimen are different, and the degree of plastic deformation of the hole wall of the specimen is different, resulting in uneven residual compressive stress on the hole wall of the hole specimen after hole extrusion strengthening. To this end, the present invention proposes a slotted mandrel extrusion strengthening process for improving the uniformity of residual stress around the hole, aiming to achieve uniform residual compressive stress on the hole wall of the hole specimen after two extrusion strengthenings of the slotted mandrel, thereby improving the fatigue performance of the hole specimen. Summary of the invention

[0006] In the process of hole extrusion strengthening of the perforated sample using the slit bushing, the extrusion mandrel squeezes the slit bushing, and the slit bushing undergoes elastic deformation to extrude the hole wall of the perforated sample. Since the wall of the slit bushing is thin, the slit bushing is prone to large plastic deformation during the hole extrusion strengthening process, resulting in the inability of the slit bushing to undergo secondary extrusion strengthening, which increases the process cost. If a solid mandrel is used to extrude and strengthen the hole of the perforated sample, the working ring diameter of the solid mandrel is larger than the initial hole diameter of the perforated sample, and the solid mandrel passes through the perforated sample from the mandrel extrusion strengthening position, that is, the unilateral extrusion strengthening of the perforated sample cannot be achieved, resulting in the inability of the solid mandrel to perform extrusion strengthening of large perforated samples, which has process limitations.

[0007] Based on slotted bushing and solid mandrel, a slotted mandrel hole extrusion strengthening process is proposed. There is a slot in the slotted mandrel extrusion strengthening part. Before the hole extrusion strengthening, force is applied to the slotted mandrel extrusion strengthening part, and the slot in the slotted mandrel extrusion strengthening part shrinks. The working ring diameter of the slotted mandrel is smaller than the initial hole diameter of the sample. The slotted mandrel can pass through the holed sample from the extrusion strengthening part to achieve unilateral extrusion strengthening of the holed sample. During the slotted mandrel hole extrusion strengthening process, the intrusion end and the extrusion end of the sample hole wall are not constrained as free ends, and the middle layer of the hole wall is constrained by the intrusion end and the extrusion end. The extrusion force borne by the middle layer of the hole wall is greater than the intrusion end and the extrusion end. The slotted mandrel moves in the opposite axial direction along the hole wall of the sample with a hole. The hole wall material is subjected to the extrusion force and the metal flows along the axial direction of the sample hole wall, resulting in a small extrusion amount at the intrusion end of the sample hole wall and a large extrusion amount at the extrusion end of the sample hole wall. Therefore, the degree of plastic deformation of the middle layer of the hole wall is greater than the degree of plastic deformation of the hole wall extrusion end and greater than the degree of plastic deformation of the hole wall entry end, which causes the residual compressive stress of the middle layer of the hole wall to be greater than the residual compressive stress of the hole wall extrusion end and greater than the residual compressive stress of the hole wall entry end. In view of the uneven residual compressive stress formed on the hole wall of the holed specimen after slotted mandrel hole extrusion strengthening, the present invention proposes a slotted mandrel extrusion strengthening process for improving the degree of uniformity of residual stress around the hole. The present invention is used to perform two hole extrusion strengthenings in opposite directions on the holed specimen, so that the degree of plastic deformation of the sample hole entry end and extrusion end is more uniform, and the residual compressive stress formed on the hole wall is more uniform, thereby improving the fatigue performance of the holed specimen.

[0008] Technical solution: In order to achieve the above-mentioned invention object, the present invention adopts the following technical solution:

[0009] The slotted mandrel extrusion strengthening process for improving the uniformity of residual stress around the hole includes the following steps:

[0010] (1) Installation of the perforated specimen. There is a slot in the extrusion strengthening section of the slotted mandrel. Before the hole is extruded and strengthened, force is applied to the extrusion strengthening section of the slotted mandrel. The slot of the extrusion strengthening section of the slotted mandrel shrinks. The working ring diameter of the slotted mandrel is smaller than the initial hole diameter of the perforated specimen. The slotted mandrel can pass through the auxiliary plate and the perforated specimen from the extrusion strengthening section in sequence. The auxiliary plate and the perforated specimen are placed in the guide section of the slotted mandrel. During the installation of the perforated specimen, the slotted mandrel does not perform extrusion strengthening on the specimen hole wall.

[0011] (2) Install the support rod. On the same side where the slit mandrel passes through the auxiliary plate and the perforated specimen in sequence, insert the support rod into the slit mandrel. Due to the supporting effect of the support rod, the extrusion strengthening section of the slit mandrel does not shrink, and the working ring diameter of the slit mandrel is larger than the initial hole diameter of the perforated specimen;

[0012] (3) Primary hole extrusion strengthening of the hole specimen. The slotted mandrel inserted into the support rod is pulled out from the hole specimen at a uniform speed. The slotted mandrel is in direct contact with the hole wall of the hole specimen. The slotted mandrel extrudes and strengthens the hole wall of the hole specimen. The hole wall material undergoes elastic-plastic deformation, and beneficial residual compressive stress is formed on the hole wall, completing the primary extrusion strengthening of the hole specimen.

[0013] (4) Double extrusion strengthening of slotted mandrel. Due to the slot in the extrusion strengthening section of the slotted mandrel, a ridge is formed on the hole wall of the sample after extrusion strengthening of the slotted mandrel hole. The ridge affects the quality of the hole wall of the sample, and fatigue cracks are easily generated at the root of the ridge. Therefore, reaming is required after extrusion strengthening of the slotted mandrel hole. However, during the extrusion strengthening of the slotted mandrel hole, the intrusion end and the extrusion end of the sample with a hole are not constrained as free ends, and the middle layer of the sample hole wall is constrained by the intrusion end and the extrusion end. The extrusion force on the middle layer of the sample is greater than the intrusion end and the extrusion end of the hole wall. During the movement of the slotted mandrel along the axial direction of the sample with a hole, the hole wall material undergoes metal flow along the axial direction, causing the actual extrusion amount of the hole wall intrusion end to be small and the actual extrusion amount of the hole wall extrusion end to be large. Therefore, after extrusion strengthening of the slotted mandrel hole, the residual compressive stress formed at the intrusion end of the sample hole wall is less than the residual compressive stress formed at the extrusion end of the sample hole wall, which is less than the residual compressive stress formed in the middle layer of the sample hole wall. Therefore, the slotted mandrel is used to perform double extrusion strengthening on the hole specimen to increase the plastic deformation degree of the hole wall material of the hole specimen and increase the residual compressive stress of the hole wall of the hole specimen.

[0014] The "double extrusion strengthening of the slotted mandrel" is one extrusion strengthening of the slotted mandrel + two extrusion strengthenings of the slotted mandrel by rotating the slotted mandrel by a certain angle and in different directions. The "rotation by a certain angle" is to rotate the slotted mandrel by 10° to 30°.

[0015] The above-mentioned "slit mandrel double hole extrusion strengthening process" is preferably:

[0016] (1) Plan:

[0017] Solution 1: The slotted mandrel is extruded twice in the same direction, where the direction of the second hole extrusion strengthening is the same as that of the first hole extrusion strengthening.

[0018] Solution 2: Two-way extrusion strengthening with slotted mandrel, where the direction of the second hole extrusion strengthening is opposite to that of the first hole extrusion strengthening.

[0019] Solution 3: One extrusion strengthening of the slotted mandrel + two extrusion strengthenings in the same direction with the slotted mandrel rotated at a certain angle. It is based on Solution 1, and the slotted mandrel is rotated at a certain angle when the slotted mandrel is subjected to the second hole extrusion strengthening.

[0020] Solution 4: One extrusion strengthening of the slotted mandrel + two extrusion strengthenings of the slotted mandrel by rotating the slotted mandrel by a certain angle in different directions. It is based on Solution 3, and the slotted mandrel is rotated by a certain angle when the slotted mandrel is subjected to the second hole extrusion strengthening.

[0021] The "rotation of a certain angle" in Scheme 3 and Scheme 4 refers to "rotation of the slit mandrel by 10° to 30°".

[0022] (2) Analysis of results and conclusions:

[0023] Among them, Scheme 1 (two extrusion strengthenings in the same direction with a slotted mandrel) cannot eliminate the ridges formed on the hole wall after one extrusion strengthening, and will increase the height of the ridges formed on the hole wall. Since the hole wall has been extruded twice, the degree of plastic deformation of the hole wall material after extrusion strengthening is greater than that after one extrusion strengthening with a slotted mandrel, and the residual compressive stress formed on the hole wall is greater than that after one extrusion strengthening with a slotted mandrel.

[0024] Scheme 2 (two anisotropic extrusion strengthening with a slotted mandrel rod), the extrusion end (extrusion end) of the hole wall of the first hole extrusion strengthening specimen is the extrusion end (extrusion end) of the second hole extrusion strengthening specimen. This scheme cannot eliminate the ridge formed on the hole wall by one extrusion strengthening with a slotted mandrel rod, and increases the height of the ridge formed on the hole wall. Since the hole wall is extruded and strengthened twice, the degree of plastic deformation of the hole wall material after extrusion strengthening is greater than that of one extrusion strengthening with a slotted mandrel rod, and the residual compressive stress formed on the hole wall is greater than that of one extrusion strengthening with a slotted mandrel rod. After two anisotropic hole extrusion strengthenings with a slotted mandrel rod, the residual compressive stress formed on the hole wall of the specimen is more uniform.

[0025] Scheme 3 (one extrusion strengthening with a slotted mandrel + two extrusion strengthenings in the same direction with the slotted mandrel rotated at a certain angle). The second hole extrusion strengthening can eliminate the ridge formed on the hole wall during the one-time extrusion strengthening with the slotted mandrel, and increase the degree of plastic deformation of the hole wall material of the specimen. The degree of plastic deformation of the hole wall material after hole extrusion strengthening is greater than that after the one-time extrusion strengthening with the slotted mandrel, and the residual compressive stress formed on the hole wall is greater than that after the one-time extrusion strengthening with the slotted mandrel.

[0026] Scheme 4 (one extrusion strengthening with a slotted mandrel + two extrusion strengthenings in different directions with the slotted mandrel rotated at a certain angle), the second hole extrusion strengthening can eliminate the ridge formed on the hole wall during the one-time extrusion strengthening with the slotted mandrel, and increase the degree of plastic deformation of the hole wall material of the specimen. The degree of plastic deformation of the hole wall material after hole extrusion strengthening is greater than that after the one-time extrusion strengthening with the slotted mandrel, and the residual compressive stress formed on the hole wall is greater than that after the one-time extrusion strengthening with the slotted mandrel. The directions of the two extrusion strengthenings of the hole specimen are opposite, and the residual stress formed on the hole wall of the specimen after hole extrusion strengthening is more uniform.

[0027] In general, the hole extrusion strengthening effects of Scheme 3 "one extrusion strengthening with a slotted mandrel + two extrusion strengthenings in the same direction with a certain angle rotation of the slotted mandrel" and Scheme 4 "one extrusion strengthening with a slotted mandrel + two extrusion strengthenings in different directions with a certain angle rotation of the slotted mandrel" are more significant than those of Scheme 1 "two extrusion strengthenings with a slotted mandrel in the same direction" and Scheme 2 "two extrusion strengthenings with a slotted mandrel in different directions". Among them, the residual stresses formed on the hole wall at the extrusion end and the extrusion end of the sample hole after extrusion strengthening in Scheme 4 are more uniform, and the hole extrusion strengthening effect is better than that of Scheme 3. Due to the existence of slots in the extrusion strengthening part of the slotted mandrel, a ridge is formed on the hole wall of the sample after the slotted mandrel hole extrusion strengthening. When the sample with a hole is subjected to the second extrusion strengthening, the slotted mandrel is rotated 10° to 30°. The second extrusion strengthening of the slotted mandrel can eliminate the ridge formed on the hole wall of the sample after the first extrusion strengthening of the slotted mandrel.

[0028] (5) Reaming. After the slotted mandrel is extruded and strengthened twice, the specimen needs to be reamed to make the specimen hole diameter reach the final hole diameter.

[0029] Beneficial Effects The slotted mandrel extrusion strengthening process for improving the uniformity of residual stress around the hole of the present invention has the following advantages:

[0030] 1. The residual stress of the hole wall is uniform. After the slit mandrel is used for one extrusion strengthening, the sample is turned 180°. The extrusion end (extrusion end) of the first hole extrusion strengthening sample with the slit mandrel and the extrusion end (extrusion end) of the second hole extrusion strengthening sample with the open mandrel are interchanged. The two hole extrusion strengthening directions are opposite, and the residual compressive stress formed on the hole wall is more uniform.

[0031] 2. Before the second extrusion strengthening of the slotted mandrel, rotate the slotted mandrel (clockwise or counterclockwise) 10° to 30° to achieve the best hole extrusion strengthening effect. After hole extrusion strengthening, the plastic deformation degree of the hole wall of the hole specimen at the extrusion end and the extrusion end is more uniform.

[0032] 3. The same slit mandrel is used for the two hole extrusion strengthening with the slit mandrel, and there is no need to replace the extrusion strengthening tool. The slit mandrel has a slit, and a convex ridge is formed on the hole wall of the hole specimen after the hole extrusion strengthening; during the hole extrusion strengthening process of the slit mandrel, the extrusion pressures at the extrusion end, middle layer, and extrusion end of the hole wall of the hole specimen are different, and the hole wall of the specimen is uneven after the hole extrusion strengthening; there are processing errors during the processing of the slit mandrel. These three reasons cause the hole diameter of the specimen after the hole extrusion strengthening with the slit mandrel to be smaller than the final hole diameter of the specimen. Use the same slit mandrel to perform the second extrusion strengthening of the hole specimen, completing the two extrusion strengthening of the hole specimen. The two hole extrusion strengthening with the slit mandrel does not require the replacement or addition of hole extrusion strengthening tools, thereby saving the process cost of the slit mandrel hole extrusion strengthening. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the process of one-hole extrusion strengthening of the slotted mandrel. (a) Extrude the extrusion strengthening section of the slotted mandrel to make the diameter of the extrusion strengthening section of the slotted mandrel smaller than the initial hole diameter of the holed specimen; (b) The front of the slotted mandrel passes through the auxiliary plate and the holed specimen in sequence; (c) Insert the support rod into the slotted mandrel; (d) Pull out the slotted mandrel inserted with the support rod at a uniform speed.

[0034] Figure 2 It is a schematic diagram of the double extrusion strengthening in the same direction with a slotted mandrel (Scheme 1);

[0035] Figure 3 It is a schematic diagram of the two-way extrusion strengthening with slotted mandrel (Scheme 2);

[0036] Figure 4 This is a schematic diagram of one extrusion strengthening with a slotted mandrel + two extrusion strengthenings in the same direction with the slotted mandrel rotated at a certain angle (Scheme 3). (a) One extrusion strengthening with a slotted mandrel; (b) Second extrusion strengthening in the same direction with the slotted mandrel rotated at a certain angle.

[0037] Figure 5 This is a schematic diagram of the single extrusion strengthening with a slotted mandrel and the double extrusion strengthening with the slotted mandrel rotated at a certain angle (Scheme 4). (a) to (d) are the single hole extrusion strengthening process with a slotted mandrel; (e) the sample is flipped 180° after the first extrusion strengthening with the slotted mandrel; (f) the second anisotropic extrusion strengthening with the slotted mandrel rotated at a certain angle.

[0038] Figure 6 It is the residual stress distribution law of the hole wall at the extrusion end of the slotted mandrel hole extrusion strengthening specimen.

[0039] Curve 1 is the one-time extrusion strengthening of the slotted mandrel;

[0040] Curve 2 is one extrusion strengthening with a slotted mandrel and two extrusion strengthenings with the slotted mandrel rotated at a certain angle in the same direction (Scheme 3);

[0041] Curve 3 represents one extrusion strengthening with a slotted core rod + two extrusion strengthenings with the core rod rotated at a certain angle in different directions (Scheme 4).

[0042] Figure 7 It is the residual stress distribution law of the middle layer of the hole wall of the slotted mandrel hole extrusion strengthening specimen.

[0043] Curve 1 is the one-time extrusion strengthening of the slotted mandrel;

[0044] Curve 2 is one extrusion strengthening with a slotted mandrel and two extrusion strengthenings with the slotted mandrel rotated at a certain angle in the same direction (Scheme 3);

[0045] Curve 3 represents one extrusion strengthening with a slotted core rod + two extrusion strengthenings with the core rod rotated at a certain angle in different directions (Scheme 4).

[0046] Figure 8 It is the residual stress distribution law at the extrusion end of the hole wall of the slotted mandrel hole extrusion strengthening specimen.

[0047] Curve 1 is the one-time extrusion strengthening of the slotted mandrel;

[0048] Curve 2 is one extrusion strengthening with a slotted mandrel and two extrusion strengthenings with the slotted mandrel rotated at a certain angle in the same direction (Scheme 3);

[0049] Curve 3 represents one extrusion strengthening with a slotted core rod + two extrusion strengthenings with the core rod rotated at a certain angle in different directions (Scheme 4).

[0050] Among them, 1-connecting section; 2-guide section; 3-front cone section; 4-working ring; 5-rear cone section; 6-auxiliary plate; 7-hole sample; 8-support rod; 9-slotted mandrel one-time extrusion strengthening sample; 10-slotted mandrel second extrusion strengthening sample in the same direction; 11-slotted mandrel sample flipped 180° after the first extrusion strengthening; 12-slotted mandrel after the first extrusion strengthening of the sample hole wall extrusion end; 13-slotted mandrel after the first extrusion strengthening of the sample hole wall middle layer; 14-slotted mandrel after the first extrusion strengthening of the sample hole wall extrusion end. DETAILED DESCRIPTION

[0051] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0052] Example 1 Slotted mandrel hole extrusion strengthening process

[0053] Figure 1 It is a one-hole extrusion strengthening process of the slotted mandrel. It includes the following steps:

[0054] (1) Installation of the perforated specimen. There is a slot in the slotted mandrel extrusion strengthening section (including: front cone section 3, working ring 4, rear cone section 5). When force is applied to the slotted mandrel extrusion strengthening section before the hole extrusion strengthening, the slot shrinks. The diameter of the working ring 4 is smaller than the initial hole diameter of the perforated specimen 7. The slotted mandrel passes through the auxiliary plate 6 and the perforated specimen 7 from the extrusion strengthening section in sequence. The auxiliary plate 6 and the perforated specimen 7 are placed in the slotted mandrel guide section 2, and during the installation of the perforated specimen 7, the slotted mandrel does not perform extrusion strengthening on the specimen hole wall.

[0055] (2) Install the support rod. Insert the support rod 8 into the slotted mandrel from the end away from the sample. Under the support of the support rod 8, the extrusion strengthening section of the slotted mandrel does not shrink, and the diameter of the working ring 4 is larger than the initial hole diameter of the perforated sample 7.

[0056] (3) Primary hole extrusion strengthening of the perforated sample. The slotted mandrel inserted into the support rod 8 is pulled out from the perforated sample 7 at a uniform speed. The slotted mandrel is in direct contact with the hole wall of the perforated sample 7. The slotted mandrel extrudes and strengthens the hole wall of the sample. The hole wall material undergoes elastic-plastic deformation, and beneficial residual compressive stress is formed on the hole wall, completing the primary extrusion strengthening of the perforated sample.

[0057] Example 2 Slotted mandrel double hole extrusion strengthening process

[0058] 1. Program settings:

[0059] Solution 1: The slotted mandrel is extruded twice in the same direction, where the direction of the second hole extrusion strengthening is the same as that of the first hole extrusion strengthening.

[0060] Solution 2: Two-way extrusion strengthening with slotted mandrel, where the direction of the second hole extrusion strengthening is opposite to that of the first hole extrusion strengthening.

[0061] Solution 3: One extrusion strengthening of the slotted mandrel + two extrusion strengthenings in the same direction with the slotted mandrel rotated at a certain angle. It is based on Solution 1, and the slotted mandrel is rotated at a certain angle when the slotted mandrel is subjected to the second hole extrusion strengthening.

[0062] Solution 4: One extrusion strengthening of the slotted mandrel + two extrusion strengthenings of the slotted mandrel by rotating the slotted mandrel by a certain angle in different directions. It is based on Solution 3, and the slotted mandrel is rotated by a certain angle when the slotted mandrel is subjected to the second hole extrusion strengthening.

[0063] The "rotation of a certain angle" in Scheme 3 and Scheme 4 refers to "rotation of the slit mandrel by 10° to 30°".

[0064] 2. Implementation process:

[0065] Figure 2After completing the extrusion strengthening of the holed sample 7 once according to Example 1, the same process is repeated once to achieve the extrusion strengthening of the holed sample 7 twice in the same direction.

[0066] Figure 3 Schematic diagram of the slotted mandrel two-way extrusion strengthening. After the perforated sample 7 is extruded once according to Example 1, the perforated sample is flipped 180 degrees, and the extrusion end of the hole wall of the slotted mandrel for the first extrusion strengthening sample is used as the extrusion end, and the extrusion end is used as the extrusion end, and the hole extrusion strengthening process of Example 1 is repeated to achieve the slotted mandrel two-way extrusion strengthening of the perforated sample 7.

[0067] Figure 4 After completing the one-time extrusion strengthening of the perforated sample 7 according to Example 1, the slit mandrel is rotated by a certain angle, and the process of Example 1 is repeated to achieve one-time extrusion strengthening of the slit mandrel of the perforated sample 7 + two-time extrusion strengthening of the slit mandrel with a certain angle rotated in the same direction.

[0068] Figure 5 After completing the one-time extrusion strengthening of the perforated sample 7 according to Example 1, the obtained perforated sample is flipped 180°, the extrusion end of the sample is used as the extrusion end, and the extrusion end is used as the extrusion end. The slit mandrel is rotated at a certain angle, and the process of Example 1 is repeated, so that the one-time extrusion strengthening of the slit mandrel of the perforated sample 7 + the two-time extrusion strengthening of the perforated sample 7 by rotating the mandrel at a certain angle can be achieved.

[0069] 3. Results and Analysis

[0070] Among them, in Scheme 1 and Scheme 2, the second hole extrusion strengthening of the slotted mandrel cannot eliminate the ridge formed on the hole wall during the first hole extrusion strengthening process of the slotted mandrel, and the second hole extrusion strengthening of the slotted mandrel increases the height of the ridge formed on the hole wall after the first hole extrusion strengthening of the slotted mandrel. Therefore, the hole extrusion strengthening effects of Scheme 3 and Scheme 4 are compared by using the finite element simulation analysis method, and the slotted mandrel two-time hole extrusion strengthening process method with the best hole extrusion strengthening effect is selected.

[0071] 1. By establishing a three-dimensional finite element simulation analysis model for double hole extrusion strengthening with a slotted mandrel, the tangential residual stresses at the extrusion end, middle layer, and extrusion end of the hole wall of the hole specimen are extracted. The variation law of the residual stress of the hole wall of the hole specimen strengthened with a slotted mandrel double hole extrusion is as follows: Figure 6 shown.

[0072] At the extrusion end of the sample hole wall, the residual compressive stress formed on the hole wall of Scheme 4 is greater than that of Scheme 3;

[0073] In the middle layer of the hole wall of the specimen, the residual compressive stresses formed by schemes 3 and 4 are similar;

[0074] At the extrusion end of the sample hole wall, the residual compressive stress formed on the hole wall of Scheme 3 is greater than that of Scheme 4.

[0075] The residual compressive stresses formed at the injection end and the extrusion end of the hole wall in Scheme 4 are relatively close.

[0076] The residual compressive stress formed on the hole wall of Scheme 3 is greater than that of the single extrusion strengthening with a slotted mandrel.

[0077] 2. Scheme 1 "two extrusion strengthenings in the same direction with a slit mandrel". The extrusion strengthening direction of the holed specimen is the same. During the extrusion strengthening process of the slit mandrel hole, metal flow occurs in the hole wall material along the axial direction, which reduces the actual extrusion amount at the entry end of the specimen hole wall and increases the actual extrusion amount at the extrusion end of the specimen hole wall. As a result, after two extrusion strengthenings in the same direction, the residual compressive stress at the extrusion end of the specimen hole wall is greater than that at the entry end.

[0078] Scheme 2 "two extrusion strengthenings with slit mandrel in different directions" is as follows: the extrusion strengthening directions of the hole specimen are opposite, and the hole wall material undergoes metal flow along the axial direction during the extrusion strengthening process of the hole of the slit mandrel. The metal flow of the hole wall material of the first hole extrusion strengthening of the slit mandrel flows from the hole wall extrusion end of the specimen to the hole wall extrusion end. After the first extrusion strengthening of the slit mandrel, the hole specimen is flipped 180°, and the hole wall extrusion end of the first extrusion strengthening of the slit mandrel is the extrusion end of the second extrusion strengthening of the slit mandrel. The extrusion end of the hole wall of the first extrusion strengthening of the slit mandrel is the hole wall extrusion end of the second extrusion strengthening of the slit mandrel. The metal flow of the hole wall material of the second hole extrusion strengthening of the slit mandrel flows from the hole wall extrusion end of the specimen to the extrusion end. After two unidirectional hole extrusion strengthenings with the slit mandrel, the plastic deformation degree of the hole wall extrusion end and extrusion end of the specimen is relatively uniform, and the residual compressive stress formed at the hole wall extrusion end and extrusion end is relatively uniform.

[0079] In scheme 3, "two extrusion strengthening with slotted mandrel in the same direction", and scheme 4, "two extrusion strengthening with slotted mandrel in different directions", the slotted mandrel extrudes and strengthens the hole sample twice, and the slotted mandrel is rotated by a certain angle when the slotted mandrel is extruded and strengthened for the second time, eliminating the ridge formed on the hole wall during the first hole extrusion strengthening of the slotted mandrel. The plastic deformation degree of the hole wall material of the hole sample after the two extrusion strengthening is greater than that of the one extrusion strengthening with the slotted mandrel, causing the residual compressive stress formed on the hole wall of the hole sample after the two extrusion strengthening is greater than that of the one extrusion strengthening with the slotted mandrel. When the slotted mandrel is extruded and strengthened for the second time, the slotted mandrel needs to be rotated by 10° to 30°. Due to the slotted mandrel, a ridge is formed on the hole wall of the sample after the slotted mandrel hole is extruded and strengthened. If the position and direction of the slotted mandrel are the same when the slotted mandrel is extruded and strengthened for the second time as when the slotted mandrel is extruded and strengthened for the first time, the ridge formed on the hole wall of the slotted mandrel after the second extrusion strengthening does not disappear, and the height of the ridge is greater than that of the ridge formed on the hole wall of the sample after the first hole extrusion strengthening. The slotted mandrel is rotated 10° to 30° during the second hole extrusion strengthening of the slotted mandrel. The second hole extrusion strengthening of the slotted mandrel can eliminate the ridge formed on the hole wall after the first hole extrusion strengthening of the slotted mandrel. If the angle of rotation of the slotted mandrel is too small or too large during the second hole extrusion strengthening of the slotted mandrel, the ridge formed on the hole wall after the first hole extrusion strengthening of the slotted mandrel cannot be eliminated.

[0080] In summary, scheme 4 has the best hole extrusion strengthening effect, that is, after the first hole extrusion strengthening of the slotted mandrel hole of the hole specimen, due to the slotted mandrel, a convex ridge is formed on the hole wall of the specimen after the slotted mandrel hole extrusion strengthening. Then the specimen is flipped 180°, so that the extrusion end of the first hole extrusion strengthening specimen of the slotted mandrel is used as the extrusion end of the second hole extrusion strengthening specimen of the slotted mandrel, and the extrusion end of the first hole extrusion strengthening specimen of the slotted mandrel is used as the extrusion end of the second hole extrusion strengthening specimen of the slotted mandrel, and the slotted mandrel is rotated 10° to 30° for the second extrusion strengthening of the slotted mandrel to improve the uniformity of the residual stress around the hole.

Claims

1. A slotted mandrel extrusion strengthening process for improving the homogenization of residual stress around a hole, characterized in that: The following steps are involved: (1) Installation of the perforated specimen: There is a slot in the extrusion strengthening section of the slotted mandrel. Before the hole is extruded and strengthened, force is applied to the extrusion strengthening section of the slotted mandrel, causing the slot of the extrusion strengthening section of the slotted mandrel to shrink. The working ring diameter of the slotted mandrel is smaller than the initial hole diameter of the perforated specimen. After the slotted mandrel passes through the auxiliary plate and the perforated specimen from the extrusion strengthening section in sequence, the auxiliary plate and the perforated specimen are placed in the guide section of the slotted mandrel. During the installation of the perforated specimen, the slotted mandrel does not perform extrusion strengthening on the specimen hole wall. (2) Install the support rod: insert the support rod into the slotted mandrel from the end away from the specimen. Under the support of the support rod, the extrusion strengthening section of the slotted mandrel does not shrink, and the working ring diameter of the slotted mandrel is larger than the initial hole diameter of the perforated specimen. (3) Primary hole extrusion strengthening of the hole specimen: The slotted mandrel inserted into the support rod is pulled out from the hole specimen at a uniform speed. The slotted mandrel is in direct contact with the hole wall of the hole specimen. The slotted mandrel extrudes and strengthens the hole wall of the hole specimen. The hole wall material undergoes elastic-plastic deformation, and beneficial residual compressive stress is formed on the hole wall, completing the primary extrusion strengthening of the hole specimen. (4) Two extrusion strengthening with slotted mandrel: The two extrusion strengthenings use the same slotted mandrel with the same working ring diameter. After the slotted mandrel is extruded once, the slotted mandrel is rotated 10° to 30° counterclockwise or clockwise for two different extrusion strengthenings. Since the slotted mandrel has a slot, a convex ridge is formed on the hole wall of the sample after the slotted mandrel hole is extruded and strengthened. During the second hole extrusion strengthening, the slotted mandrel is rotated 10° to 30°, which can eliminate the convex ridge formed on the hole wall after the slotted mandrel hole is extruded and strengthened for the first time. (6) Reaming: The specimen after extrusion strengthening is reamed to make the specimen hole diameter reach the final hole diameter.

2. The slotted mandrel extrusion strengthening process for improving the uniformity of residual stress around the hole according to claim 1 is characterized in that: In step (4), the two extrusion strengthenings are one extrusion strengthening with a slit mandrel, and the slit mandrel moves in an axial direction along the hole wall of the sample. When the sample with a hole is subjected to a second hole extrusion strengthening, the sample is flipped 180° so that the extrusion end of the sample with the slit mandrel for the first hole extrusion strengthening is used as the extrusion end of the sample with the slit mandrel for the second hole extrusion strengthening, and the extrusion end of the sample with the slit mandrel for the first hole extrusion strengthening is used as the extrusion end of the sample with the slit mandrel for the second hole extrusion strengthening, and the slit mandrel moves in an axial direction along the hole wall of the sample; when the sample with a hole is extruded twice, the slit mandrel moves in the same axial direction along the sample with a hole.

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