Flat die backfill riveting method based on rivet regulation

Abstract

A flat die backfill riveting method based on rivet regulation. First, the whole rivet is pressed into the interior of the plate by an inner punch that matches the shape of the rivet, and then the bulging plate material is evenly extruded by an outer punch. The rivet guides the plastic deformation of the material during the even extrusion and forms a mechanical locking structure between the plates, achieving reliable connection of the plates. In the pre-stamping stage of the present invention, the whole rivet is embedded into the plate by the inner punch, and in the backfill stage, the bulging plate material is backfilled into the central cavity by the outer punch. The effective mechanical connection of the plates can be achieved without the plastic deformation of the rivet. By using rivets that do not require deformation, the high sensitivity of the joint quality to the manufacturing precision of the rivets is significantly reduced; by using a flat die without grooves, a joint with a completely flat bottom surface is formed, and at the same time, the joint quality problems that may be caused by the non-coaxiality of the punch and the bottom die are eliminated. This method does not require matching the rivet and the bottom die, and can significantly reduce the joint design difficulty and shorten the design cycle.

Description

Technical Field

[0001] The present invention relates to a technology in the field of mechanical connection, specifically a flat die backfilling riveting method based on rivet regulation. Background Art

[0002] In the automotive industry, self-piercing riveting technology is still the most widely used aluminum alloy body connection process, which has the advantages of high production efficiency, reliable connection, and simple operation. During the connection process, the rivet undergoes plastic deformation to form a mechanical locking structure between the nail leg and the bottom plate, realizing the mechanical connection of multiple thin plates. This forming principle makes the joint quality of self-piercing riveting more sensitive to the hardness level of the rivet, the size of the nail tip, etc., so the manufacturing and application cost of the rivet is relatively high. During riveting, a bottom die matching the rivet is required to form a joint meeting the quality standard. The numerous combinations of plates result in a wide variety of rivets and bottom dies, with complex joint design, long cycle, and high cost. The punch and the bottom die in the riveting system need to always maintain good coaxiality, otherwise it will cause uneven joint forming and affect the joint quality. In addition, there is an obvious material bulge on the bottom die side of the joint, affecting the flatness of the connecting piece. Summary of the Invention

[0003] Aiming at the deficiencies and challenges of the existing self-piercing riveting technology in terms of application cost, design process, joint flatness, etc., the present invention proposes a flat die backfilling riveting method based on rivet regulation. In the pre-stamping stage, the rivet is integrally embedded into the plate by an inner punch, and in the backfilling stage, the bulging plate material is evenly extruded by an outer punch. The rivet guides the plastic deformation of the material during the even extrusion to form a mechanical locking structure between the plates, realizing the reliable connection of the plates. By using a rivet that does not need to be deformed, the high sensitivity of the joint quality to the manufacturing precision of the rivet (such as hardness level, geometric dimensions, etc.) is significantly reduced; by using a flat die without a groove, a joint with a completely flat bottom surface is formed, and at the same time, the joint quality problems that may be caused by the non-coaxiality of the punch and the bottom die are eliminated. This method does not require matching of the rivet and the bottom die, and can significantly reduce the joint design difficulty and shorten the design cycle.

[0004] The present invention is realized by the following technical solutions:

[0005] The present invention relates to a flat die backfilling riveting method based on rivet regulation. First, the rivet is integrally pressed into the plate by an inner punch matching the shape of the rivet, and then the bulging plate material is evenly extruded by an outer punch. The rivet guides the plastic deformation of the material during the even extrusion to form a mechanical locking structure between the plates, realizing the reliable connection of the plates.

[0006] The rivet is of a solid structure, with its top diameter smaller than the bottom diameter and its height smaller than the total thickness of the plate.

[0007] For the rivet described above, the outer surface profile is preferably a smoothly transitioning stepped structure, an arc structure, or a combination thereof, so as to precisely guide / regulate the plastic deformation behavior of the plates during the backfilling process and promote an increase in the mechanical interlock size between the plates.

[0008] The smoothly transitioning stepped structure described above means that there are several layers of stepped structures, and the protruding corners and concave corners of the steps are all replaced with arc transitions.

[0009] The arc structure described above means a side profile that is an arc and the distance from any point on the arc to the central axis gradually increases.

[0010] The middle part of the inner punch is provided with a concave structure, and the shape of this concave structure is completely matched with the outer contour of the rivet.

[0011] The method described above specifically includes:

[0012] Step 1: Stack the upper plate and the lower plate to be connected and place them on the flat die after sufficient contact. Press down the blank holder to compress the plates to restrict the relative movement of the plates during the riveting process.

[0013] Step 2: The inner punch pushes the solid rivet downward to press the entire rivet into the upper plate and the lower plate until, after reaching the preset displacement, the inner punch moves in the reverse direction and separates from the rivet, and then remains stationary after retracting.

[0014] The stamping depth of the inner punch needs to be adjusted according to the thickness of the upper plate to ensure that the lower half of the rivet is below the interface between the upper plate and the lower plate.

[0015] For the retraction described above, the distance needs to be adjusted according to the thicknesses of the upper plate and the lower plate to promote the effective formation of the mechanical interlock structure.

[0016] Step 3: The outer punch moves downward to squeeze the protruding plate material to flow into the central cavity, guides and controls the plastic deformation of the plates through the side profile of the rivet, forms a mechanical interlock structure between the upper plate and the lower plate to connect the plates together. Until reaching the predetermined position, it moves back to the initial position in the reverse direction, and the inner punch and the blank holder also retract to the initial position. The plates and the rivet experience springback to form the final joint, and the riveting process is completed.

[0017] Technical effects

[0018] The present invention regulates the plastic deformation behavior of the sheet through a special rivet structure, promotes the further growth of the mechanical locking structure between the sheets during the backfilling stage, and finally forms a reliable mechanical connection. Compared with the prior art, the present invention uses rivets that do not require deformation, significantly reducing the sensitivity of the joint quality to the manufacturing accuracy of the rivets; the rivet structure is simple, and the manufacturing and application costs are low; a flat die without grooves is used, which can form a joint with a completely flat bottom surface, eliminating the joint quality problems caused by the non-coaxiality of the punch and the bottom die; there is no problem of matching between the rivet and the bottom die, and the joint design is simple and the design cycle is short. Description of the Drawings

[0019] Figure 1 It is a flow chart of the method of the present invention;

[0020] Figure 2 It is a schematic structural diagram of the double-punch riveting device proposed in the present invention;

[0021] In the figure: 1 outer punch, 2 blank holder, 3 inner punch, 4 rivet, 5 upper plate, 6 lower plate, 7 flat die;

[0022] Figure 3 It is a schematic structural diagram of four different rivets and the corresponding inner punches;

[0023] Figure 4 It is a schematic diagram of the connection process of the flat die backfilling riveting process based on rivet regulation;

[0024] Figure 5 It is a schematic diagram of the cross-sectional morphology and key quality characteristics of the process joint. Detailed Embodiment

[0025] As Figure 2 shown, this embodiment relates to a double-punch riveting system, including: an outer punch 1, a blank holder 2, an inner punch 3, a rivet 4 and a flat die 7. Among them, the outer punch 1, the blank holder 2 and the inner punch 3 need to ensure good coaxiality and the radial clearance is less than 0.05 mm to prevent the sheet from being squeezed into the gap during the riveting process and affecting the appearance quality of the joint. The internal contour of the inner punch needs to be completely matched with the geometric structure of the rivet, so as to drive the rivet to press into the sheet and avoid obvious plastic deformation of the rivet.

[0026] The upper end of the said rivet 4 has a small diameter and the lower end has a large diameter, and the side profile is as Figure 3 shown, a and b are smooth transition stepped structures, c and d are arc structures. Specifically: the side profile of the first type of rivet 4 is composed of a straight line + two arcs, the side profile of the second type of rivet 4 is composed of a straight line + one arc, the side profile of the third type of rivet 4 is composed of one arc, and the side profile of the fourth type of rivet 4 is composed of one arc + a straight line.

[0027] In this embodiment, Figure 3The rivet structure shown in a. The rivet 4 is made of medium carbon steel processed by annealing. The height, upper diameter, and bottom diameter of the rivet are 2.4 mm, 2.3 mm, and 4.8 mm respectively. The upper straight line length of the side profile is 0.8 mm, the middle arc radius is 0.3 mm, the lower arc radius is 0.6 mm, and the transition chamfer radius is 0.1 mm.

[0028] As Figure 1 shown, an in - plane die backfilling riveting method based on rivet regulation involved in this embodiment includes: the stages of sheet placement, blank - holder pressing down, inner punch pressing down and retracting, outer punch pressing down, and all actuators retracting. Specifically, as Figure 4 shown, it includes:

[0029] Step 1: Stack the upper layer plate 5 and the lower layer plate 6 together so that they are fully close to or in contact with each other.

[0030] In this embodiment, the upper layer plate 5 to be riveted is a 1.2 - mm - thick aluminum alloy AA5754 thin plate, and the lower layer plate 6 is a 1.8 - mm - thick aluminum alloy AA5754 thin plate. The sheets are not surface - treated before riveting.

[0031] Step 2: Place the stacked sheets above the flat die 7 without grooves.

[0032] Step 3: The blank - holder 2 moves downward to press the sheets tightly against the flat die 7, restricting the relative movement of the sheets during the riveting process.

[0033] Step 4: The inner punch 3 pushes the rivet 4 downward, and the rivet 4 is entirely pressed into the interior of the sheets.

[0034] Step 5: After reaching the preset displacement, the inner punch 3 moves in the reverse direction to separate from the rivet 4, retracts a certain distance, and then remains stationary.

[0035] Step 6: The outer punch 1 moves downward, squeezing the protruding sheet material to flow into the central cavity, regulating the plastic deformation of the sheets through the side profile of the rivet 4, and forming a mechanical interlock structure between the upper layer plate 5 and the lower layer plate 6 to connect the sheets together.

[0036] Step 7: After the outer punch 1 reaches the predetermined position, it moves back to the initial position in the reverse direction. The inner punch 3 and the blank - holder 2 also retract to the initial position. The sheets and the rivet 4 experience springback to form the final joint, and the riveting process is completed.

[0037] The riveting process was modeled and analyzed by the numerical simulation software Simufact.Forming 16. The downward stamping speed of the inner punch 3 was set to 100 mm / s, the stamping depth, that is, the downward movement distance of the rivet 4, was 2.4 mm, the upward retraction speed was 50 mm / s, and the retraction distance was 1.7 mm. The downward stamping speed of the outer punch 1 was 100 mm / s, and the stamping ended when the lowest point of the outer punch 1 was at the same height as the lowest point of the blank holder 2, forming a connection joint with a reliable mechanical locking structure. The measured mechanical locking dimensions L1, the minimum residual thickness L2 of the neck, and the minimum residual thickness L3 of the bottom were 0.57 mm, 0.22 mm, and 0.23 mm respectively. The lower bottom surface of the joint was completely flat, and there was only a slight bulge on the upper surface, as Figure 5 shown.

[0038] As shown in Table 1, when using combinations of upper plates 5 and lower plates 6 with other thicknesses, this method can successfully rivet the sheets to be joined tightly together. Among them, for the 1.2 mm + 1.5 mm sheet combination, the downward pressing depth of the inner punch was 2.3 mm, and the retraction distance was 1.6 mm; for the 1.2 mm + 2.5 mm sheet combination, the downward pressing depth of the inner punch was 3.0 mm, and the retraction distance was 2.3 mm; for the 1.5 mm + 1.5 mm sheet combination, the downward pressing depth of the inner punch was 2.5 mm, and the retraction distance was 1.8 mm; for the 2.0 mm + 2.0 mm sheet combination, the downward pressing depth of the inner punch was 3.4 mm, and the retraction distance was 1.5 mm; other riveting parameters remained unchanged. The simulation results fully demonstrate the versatility of this method.

[0039] Table 1 Quality characteristic values of joints under different sheet thickness combinations

[0040]

[0041] Compared with the prior art, the present invention uses a rivet that does not require deformation to control the deformation behavior of the sheet. The forming quality of the joint does not depend on the plastic deformation of the rivet itself, and the influence of the manufacturing precision fluctuation of the rivet on the joint quality is small; the rivet structure is simple, and the manufacturing and application costs are low; a flat die without grooves is used, and a joint with a completely flat bottom surface can be formed, which is suitable for connection scenarios with high requirements for the surface flatness of the joint, eliminating the joint quality problems that may be caused by the non - coaxiality of the punch and the bottom die; there is no problem of matching between the rivet and the bottom die, the joint design difficulty is low, and the design cycle is short.

[0042] The specific implementation described above can be locally adjusted in different ways by those skilled in the art without departing from the principles and purposes of the present invention. The protection scope of the present invention is defined by the claims and is not limited by the above - mentioned specific implementation, and all implementation solutions within its scope are subject to the present invention.

Claims

1. A flat die backfill riveting method based on rivet regulation, characterized in that, First, the rivet is pressed into the interior of the sheet material as a whole by an inner punch that matches the shape of the rivet. Then, the raised sheet material is flattened and extruded by an outer punch. The rivet guides the plastic deformation of the material during the flattening extrusion and forms a mechanical locking structure between the sheets to achieve reliable connection of the sheets. The rivet is of solid structure, with its top diameter smaller than the bottom diameter and its height smaller than the total thickness of the sheet. An inner concave structure is provided in the middle of the inner punch, and the shape of the inner concave structure completely matches the outer contour of the rivet.

2. The flat die backfill riveting method based on rivet regulation according to claim 1, characterized in that, For the rivet, its outer surface contour is a smoothly transitioning stepped structure, an arc structure, or a combination thereof to precisely guide and regulate the plastic deformation behavior of the sheet during the backfilling process and promote the increase of the mechanical interlocking size between the sheets. The smoothly transitioning stepped structure means that there are several layers of stepped structures, and the protruding corners and concave corners of the steps are replaced with arc transitions.

3. The flat die backfill riveting method based on rivet regulation according to claim 2, characterized in that, The arc structure means a side profile that is an arc and the distance from any point on the arc to the central axis gradually increases.

4. The flat die backfill riveting method based on rivet regulation according to any one of claims 1-3, characterized in that specifically It includes: Step 1: Stack the upper and lower sheets to be connected and place them on the flat die after full contact. The blank holder moves downward to press the sheets tightly to restrict the relative movement of the sheets during riveting. Step 2: The inner punch pushes the solid rivet downward to press the rivet into the interior of the upper and lower sheets as a whole. After reaching the preset displacement, the inner punch moves in the reverse direction and separates from the rivet, and remains stationary after retracting. The stamping depth of the inner punch needs to be adjusted according to the thickness of the upper sheet to ensure that the lower half of the rivet is below the interface between the upper and lower sheets. Step 3: The outer punch moves downward to extrude the raised sheet material to flow into the central cavity. The plastic deformation of the sheet is guided and controlled by the side profile of the rivet to form a mechanical interlocking structure between the upper and lower sheets and connect the sheets together. After reaching the predetermined position, it moves in the reverse direction to the initial position, and the inner punch and the blank holder also retract to the initial position. The sheet and the rivet experience springback to form the final joint, and the riveting process is completed.

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