An active die with spring washers for self-piercing riveting of low ductility materials

Abstract

An active die comprising a tubular housing, a piston disposed in an interior of the housing, a piston stop coupled with the housing and a spring device proximal a rearward end of the housing. The spring device comprises a spring washer stack with a plurality of disc spring washers. The piston comprises a cylindrical piston body with a piston crown, and the piston stop comprises a piston stop die surface adjacent and about the piston crown. The piston crown and the piston stop die surface together define a die cavity. The piston is responsive to force applied at the piston crown to compress the spring device to increase a variable depth of the die cavity and to increase a variable counterforce at the piston crown. The active die and method are useful to prevent cracking of a mechanical joint button caused by joining low ductility materials during self-piercing riveting.

Description

AN ACTIVE DIE WITH SPRING WASHERS FOR SELF-PIERCING RIVETING OF LOW DUCTILITY MATERIALSFIELD

[0001] The present disclosure relates generally to techniques for assembling low ductility materials, including particularly self-piercing riveting with an active die.BACKGROUND

[0002] Self-piercing riveting is a technique widely used in the automotive industry for joining multiple layered sheets of materials including, but not limited to, aluminium, aluminum alloys, steel, iron, copper, and magnesium. The technique generally uses a rivet that pierces through a first or uppermost layer of material, and a receiving die that allows the rivet to flare within a last or bottommost layer of material. The material of the bottommost layer plasticises and flows into, fills, and takes the shape die, thereby forming a button. The flared rivet thereby forms a mechanical interlock that joins the multi-layer sheets together.

[0003] Often, during the self-piercing riveting process, gaps between the sheet materials can form as a result of differences in the deformation properties of the different materials. These gaps, however, are closed or reduced by a force applied through the rivet head during the riveting process. A relatively high force is often required to set a rivet into multiple layered sheets of materials to form a joint. In the automotive industry, higher rivet inserting forces may be required with the increasing use of high-strength sheet materials. However, an issue arises when employing stronger metallic materials in the self-riveting process, as the reduced formability of these materials may lead to cracks at the joint button, especially when less ductile materials are used as the bottom piece, as shown in FIG. 1.

[0004] One technique for preventing the formation of cracks includes preheating low ductility materials prone to cracking during the riveting process using an AC current or a laser heat source to improve ductility and reduce deformation.

[0005] Such known techniques, while addressing some of the issues involved in cracking, nevertheless suffer from certain shortcomings, includingincreased costs associated with heat treatments necessary to improve ductility, loss of mechanical strength, higher risk of material distortion, and longer processing time with respect to the mass production of self-piercing riveting joints.

[0006] There remains, therefore, a need for improved techniques to form crack-free joint buttons with less ductile materials, which addresses at least some of these shortcomings, provides yet further advantages, and thus provides a material value over prior techniques.BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Embodiments will now be described, by way of example only, with reference to the attached Figures.

[0008] FIG. 1 shows images a cracked button formed from self-piercing riveting in Aural2-T5 cast aluminum.

[0009] FIG. 2 shows an image of a button without cracks formed from selfpiercing riveting of 7075-T6 aluminum sheet, as bottom material, according to disclosed embodiments.

[0010] FIG. 3 shows an image of a button without cracks and with a central indentation formed from self-piercing riveting of Aural2-T5 cast aluminum, as bottom material, according to disclosed embodiments.

[0011] FIG’s 4 & 5 show perspective views of active dies according to disclosed embodiments.

[0012] FIG. 6 shows an image of an active die according to a disclosed embodiment.

[0013] FIG. 7 shows a cross-sectional elevation view of an active die according to a disclosed embodiment.

[0014] FIG. 8 shows a cross-sectional elevation view of the active die of FIG.7 configured with a preloaded piston crown counterforce.

[0015] FIG. 9 shows a cross-sectional elevation view of the active die of FIG.8 with the piston translated rearwardly to a maximum extent limited by maximum compression of the spring device.

[0016] FIG. 10 shows a cross-sectional elevation view of an active die according to a disclosed embodiment having a die post with shoulder limiting rearward translation of the piston.

[0017] FIG. 11 shows a cross-sectional elevation view of the active die of FIG. 10 configured with a preloaded piston crown counterforce.

[0018] FIG. 12 shows a cross-sectional elevation view of the active die of FIG. 11 with the piston translated rearwardly to a maximum extent limited by the die post shoulder.

[0019] FIG. 13 shows a cross-sectional elevation view of an active die according to a disclosed embodiment having a die post without shoulder limiting rearward translation of the piston.

[0020] FIG. 14 shows a flowchart of a method according to disclosed embodiments.

[0021] FIG. 15 shows a figurative diagram of an arrangement of a selfpiercing rivet, workpieces, and active die figuring in the method of FIG. 14.

[0022] FIG. 16 shows an assembly of workpieces interlocked by a flared selfpiercing rivet formed by a method and active die according to disclosed embodiments.

[0023] Throughout the drawings, sometimes only one or fewer than all of the instances of an element visible in the view are designated by a lead line and reference character, for the sake only of simplicity and to avoid clutter. It will be understood, however, that in such cases, in accordance with the corresponding description, that all other instances are likewise designated and encompassed by the corresponding description.DESCRIPTION

[0024] An active die as disclosed herein is useful in a self-piercing riveting method, including when a bottom workpiece, which is in a stack of workpieces to be joined by a rivet, is formed of low ductility materials. A surface of the active die applies a counterforce against an opposing surface of the bottom workpiece throughout the formation of a button, or beginning at a preconfigured point duringthe formation of the button. The counterforce may change, which may be increase, as the formation of the button proceeds.

[0025] Without wishing to be bound by any particular theory, it is believed that in so doing, stresses operative in the workpiece material during the formation of the button are changed, which may include obtaining or increasing a shear / compression stress states as opposed to or relative to a tensile / plane strain stress states. Doing so may increase the ductility of the material, with the result that the button may form without cracking even in the absence of heat treatment, for example at ambient or room temperature. Examples of buttons formed in accordance with the present disclosure are shown in FIG’s 2 & 3, and it is observable that they are free of cracks.

[0026] One such active die 100 is shown in FIG’s 4 & 7-9. The active die 100 has a generally tubular rigid housing 110 having a longitudinal axis L (shown in FIG. 7) extending between a rearward end 112 and a forward end 114 of the housing 110. The housing 110 has an opening 116 at the forward end 114 and a floor 118 at the rearward end 112. The housing 110 may also have a mounting shaft 115 extending rearwardly from the floor 118, which may be sized and shaped for rigid grasping by, for example, an end effector of a robotic arm or other robotic apparatus, or a collet coupled with a frame or other structure.

[0027] The active die 100 has a piston 120 disposed at least partly in an interior 117 of the housing 110. The piston 120 has a generally cylindrical piston body 122 having a piston crown 124 at a forward end 126 of the piston body 122. The piston crown 124 is disposed proximal the opening 116 of the housing 110. The piston 120, and thus the piston crown 124, is translatable along the longitudinal axis L relative to the housing 110. The active die 100 further has a piston stop 130 which may have a generally cylindrical shape and which is operable to limit forward translation of the piston 120 to a forwardmost piston position relative to the piston stop 130 (shown in FIG’s 7 & 8), and thereby to limit forward translation of the piston crown 124 to a forwardmost piston crown position relative to the piston stop 130 (shown in FIG’s 7 & 8). The piston stop 130 may define a piston stop channel 137 extending through the piston stop 130 along the longitudinal axis L. The piston stopchannel 137 may be generally cylindrical so as to fittingly and slidingly receive the piston body 122 fortranslation of the piston body 122 within the piston stop channel 137 along the longitudinal axis L. The piston stop 130 has a piston stop die surface 132 adjacent and about the piston crown 124. The piston crown 124 and the piston stop die surface 132 together define a die cavity 150. The piston stop die surface 132 may be tapered to form a generally frustoconical shape, and with the piston crown 124 may form a generally frustoconical shape of the die cavity 150 when the piston 120 is at the forwardmost piston position. Any suitable configuration is contemplated, and an angle formed between the piston stop die surface 132 and the forward piston stop surface 135 may be between 0° and about 15°.

[0028] The active die 100 further has a spring device 140 proximal the rearward end 112 of the housing 110. The spring device 140 is arranged to urge forward translation of the piston 120, and thus the piston crown 124, relative to the housing 110. For example, the spring device 140 may include a spring washer stack 142 comprising a plurality of disc spring washers 144 aligned along the longitudinal axis L. The spring washer stack 142 may be sandwiched between a rearward end 123 of the piston 120 and the floor 118 of the housing 110, and have an uncompressed height huextending along the longitudinal axis L. The piston 120, and thus the piston crown 124, is translatable rearwardly along the longitudinal axis L relative to the housing 110 responsive to a variable force Fxapplied at the piston crown 124 to compress the spring device 140 along the longitudinal axis L. Doing so increases a variable depth d of the die cavity 150 extending along the longitudinal axis L, and further increases a variable counterforce CFXat the piston crown 124, which is equal in magnitude to and opposite in direction to the applied force Fx.

[0029] The piston 120 further comprises a piston flange 128 disposed rearwardly from the piston body 122. The piston flange 128 has a piston flange width wPf extending along a transverse axis T of the housing 110 perpendicular to the longitudinal axis L. The piston flange width wPf is greater than a piston body width wPb of the piston body 122 extending along the transverse axis. Any suitable dimensions are contemplated. For example, the piston body width wPb of the piston body 122 may be between about 5 mm and about 12 mm. A rearward end 138 ofthe piston stop 130 abuts a forward end 121 of the piston flange 128 when the piston 120 is in the forwardmost piston position to limit forward translation of the piston 120 to the forwardmost piston position. The piston stop 130 has a forward piston stop surface 135 adjacent and about the piston stop die surface 132, and the piston crown 124 is spaced rearwardly from the forward piston stop surface 135 by a preconfigured minimum die cavity depth dminwhen the piston 120 is in the forwardmost piston position relative to the piston stop 130, as shown in FIG’s 7 & 8. The active die 100 may be configured to provide any desired preconfigured minimum die cavity depth dmin. For example, the preconfigured minimum die cavity depth dmin may be about 0.0 mm to about 1.0 mm. At 0.0 mm, the piston crown 124 is substantially flush with the forward piston stop surface 135 when the piston 120 is in the forwardmost piston position.

[0030] The piston stop 130 may be adjustable such that a position of the piston stop 130 relative to the housing 110 along the longitudinal axis L is adjustable. For example, the piston stop 130 may be coupled with the housing 110 proximal the forward end 114 of the housing 110. In particular, the piston stop 130 may have a piston stop threading 134 at a radially outer surface 136 of the piston stop 130. In such case, the housing 110 has a housing threading 111 at a radially inner surface 113 of the housing 110. The piston stop 130 is coupled with the housing 110 by coupling of the piston stop threading 134 and the housing threading 111 , and the piston stop 130 is rotatable about the longitudinal axis L relative to the housing 110 to selectively translate the piston stop 130 forwardly or rearwardly along the longitudinal axis L to adjust a piston stop position of the piston stop 130 along the longitudinal axis L. Inasmuch as, as mentioned above, the piston stop 130 is operable to limit forward translation of the piston 120 to the forwardmost piston position, the piston stop 130 may be selectively rotated as described to selectively adjust that forwardmost piston position. For this purpose, the piston stop 130 may have at a forward end 131 thereof at least one, which may be two (shown particularly in FIG’s 4 & 5), or four (shown particularly in FIG. 6), or any other number, piston stop keyseats 133, which may be recesses or openings sized, shaped, and positioned at the forward end 131 of the piston stop 130 to receive atleast a part of a tool (not shown), such as a turning key, operable selectively to rotate the piston stop 130 clockwise or counterclockwise thereby to adjust the piston stop position along the longitudinal axis L, and therefore the forwardmost piston position.

[0031] In addition, and with reference to FIG. 8 in view of FIG. 7, inasmuch as the spring device 140 is arranged to urge forward translation of the piston 120, and downward translation of the piston 120 further compresses the spring device 140 and therefore the variable counterforce CFXat the piston crown 124, the piston stop 130 is further operable as described above to selectively adjust the piston stop position along the longitudinal axis Z to selectively compress the spring device 140 to a preconfigured starting spring device height hsextending along the longitudinal axis L which results in a preconfigured non-zero starting counterforce CFSat the piston crown 124, shown in FIG. 8, when the piston 120 is at the forwardmost piston position relative to the piston stop 130. As variable force Fxis applied at the piston crown 124, the variable corresponding counterforce CFXrises accordingly while the piston 120 and therefore piston crown 124 does not translate along the longitudinal axis L. Once the variable force Fxreaches and surpasses the starting counterforce CFs however, the piston 120 is translated rearwardly from the forwardmost piston position relative to the piston stop 130 by the application of the variable force Fxapplied at the piston crown 124, and the spring device 140 is further compressed thereby generating a corresponding counterforce CFX. The piston 120 may be further translated rearwardly by the application of variable force Fxapplied at the piston crown 124 to a position where the spring device 140 is fully compressed and has a minimum spring device height hmin, the piston crown 124 is at a rearwardmost crown position, and the die cavity 150 has a maximum depth dmax. Any suitable maximum depth dmaxis contemplated, which may be from about 0.5 mm to about 10 mm, or about 1 mm to about 5 mm, or about 1.5 mm, or about 1.45 mm. In this configuration, the variable counterforce CFXgenerated at the piston crown 124 equals whatever variable force Fxis applied at the piston crown 124 without further rearward translation of the piston 120 and therefore the piston crown 124, such that the die cavity 150 maintains the maximum depth dmax. The progression between thestarting position and this final, fixed position described is illustrated in FIG’s 8 & 9 by means of arrows 900.

[0032] The components of the active die 100 may be formed of any suitable materials, and without limitation may be formed of high-strength tool steel.

[0033] As noted above, and with reference to FIG’s 14-16 the active die 100 is useful in a self-piercing riveting method 200, including when a bottom workpiece 300, which is in a stack of workpieces to be joined by a self-piercing rivet 320, is formed of low ductility materials. Without limitation, the low ductility material may include magnesium, or high strength and cast aluminum alloys, which may include Aural2-T5 cast aluminum. An active die 100 as disclosed herein is provided at an opposing surface 310 of workpieces 300,302 (step 210), which may be any number of workpieces, and which may be layered workpieces. The workpieces may include an opposing workpiece 300 having the opposing surface 310.

[0034] In the method 200, the self-piercing rivet 320 is driven along the longitudinal axis into the workpieces 300,302 by application of a driving force 330 at a head 321 of the rivet 320 through a near surface 312 of the workpieces against the active die 100 to flare the legs 326 of the rivet 320 to form a flared end 322 (shown in FIG. 16) to form an interlock between the workpieces 300,302, and to form a button 350 at the opposing surface 310 of the workpieces 300,302 (step 220). A variable counterforce CFXis applied by a die cavity surface 340, which may be the piston crown 124, along the longitudinal axis against the opposing surface 310 of the workpieces 300,302 during the formation of the button 350, while the die cavity surface 340 translates along the longitudinal axis during the formation of the button (step 230).

[0035] The die cavity surface 340 may be in continuous contact with the opposing surface 310 of the workpieces 300,302 during the formation of the button 350, such as when the minimum die cavity depth dmin, as described above, is zero. Alternatively, the die cavity surface 340 may come into contact with the opposing surface 310 after partial formation of the button 350 in the die cavity 150. In either case, the starting counterforce CFS, being the variable counterforce CFXwhen the opposing surface 310 comes into first contact with die cavity surface 340, may beabout 0.5 to about 1.5, or about 1 to about 1.3 times the product of an ultimate tensile strength of a material of the opposing workpiece 300 and an area of the die cavity surface 340, although other values are possible and contemplated. For example, the starting counterforce CFSmay be at least 5 kN, or about 5 kN to about 30 kN, or about 10 kN, although other values are possible and contemplated. As the formation of the button 350 proceeds, the die cavity surface 340 may apply a progressively increasing variable counterforce CFXagainst the opposing surface 310 of the workpieces 300,302 during a remainder of the formation of the button 350. At the end of the piston stroke, the counterforce may be any suitable value, which may be up to about 40 kN, or up to 25 kN.

[0036] Thus, as shown in FIG. 16, the method 200 produces an assembly including the layered plurality of workpieces 300,302 interlocked by the self-piercing rivet 320 having a head 321 and a flared end 322, with the lowermost one of the workpieces 300 proximal the flared end 322 forming the button 350. Importantly, as shown in the examples of FIG’s 2 & 3, the button is free or substantially free from cracks, even when the lowermost one of the workpieces 300 is formed of a material having low ductility.

[0037] Although the principles disclosed herein are described and shown in the embodiment of the active die 100 as described and shown, it is intended to include in the scope of the present disclosure any functionally equivalent arrangement operative to apply a variable counterforce to the opposing surface of the lowermost workpiece as the button is formed, with all of the alternatives and variations described herein. It is contemplated that numerous different structural selections and arrangements will be operative to produce this result, and they are intended to be included in the present disclosure.

[0038] For example, an alternative active die 400 is shown in FIG. 13. It is substantially identical to the active die 100 in every respect, except as now described. (The reference characters and lead lines shown in FIG. 7 apply equally in FIG. 13 to the active die 400 in like place and relation to the parts of the respective active die 100, and so the reference characters and corresponding lead lines not now mentioned are not replicated in FIG. 13 in order to avoid clutter.) In the activedie 400, piston 405, which is otherwise identical to piston 120, but in this case has a piston body 401 , which is otherwise identical to piston body 122, but in this case defines a generally cylindrical piston die post channel 410 extending through the piston 405 along the longitudinal axis Z. The active die 400 further comprises a die post 420 fittingly and slidingly received in the piston die post channel 410 for translation of the piston 405 along the longitudinal axis L while the die post 420 remains stationary relative to the housing 110. For example, a rearward end 422 of the die post 420 may abut the floor 118 of the housing 110. The die post 420 has a die post crown 424 at a forward end 426 of the die post. The piston crown 407, which is otherwise identical to piston crown 124, but in this case may have a generally annular shape (shown particularly in FIG’s 5 & 6), and with the piston stop die surface 132 and the die post crown 424 may together define the die cavity 450, which is otherwise identical to die cavity 150. With this arrangement, the active die 400 may function substantially identically to the active die 100 in the method 200 described above, except only in that the die post 420, and thus the die post crown 424, remains stationary during the method and during the formation of the button 350, with the result that the button 350 is formed with a generally circular indentation as shown in FIG. 3. The die post 420 so positioned may operate to assist, improve, or facilitate the flaring of the legs 326 of the rivet 320 to form the flared end 322 of the rivet 320.

[0039] As noted above, the active die 400 with die post 420 may operate substantially identically to the active die 100, including with respect to the limits of translation of the piston 120. In particular, and as shown in FIG. 13, the die post 420 of active die 400 may have a substantially constant die post width wdPalong the transverse axis T matching a piston die post channel width wdpcof the piston die post channel 410, such that the die post 420 itself permits translation of the piston 405 along its entire length, or in other words does not impede and presents no limit to translation of the piston 405 along the longitudinal axisZ. Thus, as with active die 100 and as described above with reference to FIG. 9 and piston 120, the translation of the piston 405 may be limited to a rearwardmost position when the spring device 140 is fully compressed, and the die cavity 450 has a maximum depth dmax.

[0040] An alternative active die 500 may be identical in all respects to active die 400, including in its function in the method 200, except as now described. (The reference characters and lead lines shown in FIG’s 7 & 13 apply equally in FIG’s 5, 6 & 10-12 to the active die 500 in like place and relation to the parts of the respective active die 100 and active die 400, and so the reference characters and corresponding lead lines not now mentioned are not replicated in FIG’s 5, 6 & I Q- 12 in order to avoid clutter.) As shown in FIG’s 10-12, active die 500 also has a die post 510, which is identical in all respects to die post 420, except in it does not have a substantially constant width along the transverse axis T. Instead, the die post 510 has a forward section 432 fittingly and slidingly received in the piston die post channel 410 for translation of the piston 405 along the longitudinal axis Z while the die post 510 remains stationary relative to the housing 110. The forward section 432 has a forward section width wdp along the transverse axis T matching the piston die post channel width wdpcthe piston die post channel 410, such that the forward section 432 itself permits translation of the piston 405 along its entire length, or in other words presents no limit to translation of the piston 405 along the longitudinal axis Z. Any suitable dimensions are contemplated, and without limitation the piston die post channel width wdpcmay range from about 3 mm to about 5 mm, or be about 3 mm. The die post 510 also has a rearward section 434 having a rearward section width wdpralong the transverse axis T which is greater than the forward section width wdPf, and thus the forward section 432 and the rearward section 434 form a die post shoulder 436 sized, shaped, and positioned to abut a rearward end 409 of the piston 405 when the piston 405 is translated rearwardly a preconfigured distance. In particular, the rearward section width wdPr may be selected based on a corresponding width of openings in the spring device 520, such that the die post 510 is operable to maintain or urge longitudinal alignment of the spring device 520 during compression as described.

[0041] Thus, as shown in FIG. 11 , and in the same manner as described above with active die 100 in view of FIG’s 7 & 8 and the corresponding description, the active die 500 may be operable to selectively adjust the piston stop position of the piston stop 130 along the longitudinal axis Z to selectively compress the springdevice 520, which is otherwise identical to spring device 140 but shown as having a different size, to a preconfigured starting spring device height extending along the longitudinal axis L which results in a preconfigured non-zero starting counterforce at the piston crown 407. As the piston 405 is translated rearwardly by the application of a variable force Fxapplied at the piston crown 407, as described, in this case the spring device 520 does not become fully compressed, but instead, as shown in FIG. 12, before this state is reached, the rearward end 409 of the piston 405 abuts the die post shoulder 436, thereby preventing further rearward translation of the piston 405 while the spring device 520 is in a less than fully compressed state, which may be any suitable degree, which in some embodiments is 75%. Thus, active die 500 is one example of a configuration where the die cavity 450 maximum depth is determined not by the spring device 520 minimum height, but instead by the placement of the die post shoulder 436 relative to a height of the piston 405.

[0042] As with active die 400, active die 500 is useful in the method 200 in a substantially identical manner as described, and as with active die 400 the die post 510 may operate to assist, improve, or facilitate the flaring of the legs 326 of the rivet 320 to form the flared end 322 of the rivet 320, and to produce a button 340 that is formed with a generally circular indentation as shown in FIG. 3.

[0043] In any of the embodiments disclosed herein, the housing 110 may be filled with lubricant, such as oil, to enhance the fatigue life of the spring device 140, which may be by minimizing wear at contact surfaces and reducing additional load caused by friction, such as between the spring washers 144.

[0044] Non-limiting embodiments of the present disclosure are presented below.

[0045] Embodiment 1. An active die comprising: a generally tubular rigid housing having a longitudinal axis extending between a rearward end and a forward end, the housing having an opening at the forward end and a floor at the rearward end; a piston disposed at least partly in an interior of the housing, the piston having a generally cylindrical piston body having a piston crown at a forward end of the piston body, wherein the piston crown is disposed proximal the opening of the housing and the piston is translatable along the longitudinal axis relative to thehousing; a piston stop coupled with the housing proximal the forward end of the housing to limit forward translation of the piston to a forwardmost piston position, the piston stop having a piston stop die surface adjacent and about the piston crown; and a spring device proximal the rearward end of the housing and arranged to urge forward translation of the piston relative to the housing, wherein: the piston crown and the piston stop die surface together define a die cavity; and the piston is translatable rearwardly along the longitudinal axis relative to the housing responsive to force applied at the piston crown to compress the spring device along the longitudinal axis to increase a variable depth of the die cavity extending along the longitudinal axis and to increase a variable counterforce at the piston crown.

[0046] Embodiment 2. The active die of Embodiment 1 , wherein: the piston stop is cylindrical and has a piston stop threading at a radially outer surface of the piston stop; the housing has a housing threading at a radially inner surface of the housing; the piston stop is coupled with the housing by coupling of the piston stop threading and the housing threading; and the piston stop is rotatable about the longitudinal axis relative to the housing to selectively adjust a piston stop position of the piston stop along the longitudinal axis thereby to selectively adjust the forwardmost piston position.

[0047] Embodiment 3. The active die of Embodiment 2, wherein the piston stop position is selectively adjustable along the longitudinal axis to selectively adjust a non-zero starting counterforce at the piston crown when the piston is at the forwardmost piston position.

[0048] Embodiment 4. The active die of Embodiment 2 or 3, wherein: the piston stop has a forwardmost piston stop surface adjacent and about the piston stop die surface; and the piston crown is spaced rearwardly from the forward piston stop surface by a preconfigured minimum die cavity depth when the piston is in the forwardmost piston position.

[0049] Embodiment 5. The active die of Embodiment 4, wherein: the preconfigured minimum die cavity depth is about 0.0 mm to about 1.0 mm.

[0050] Embodiment 6. The active die of Embodiment 4, wherein: the piston crown is substantially flush with the forwardmost piston stop surface when the piston is in the forwardmost piston position.

[0051] Embodiment 7. The active die of any one of Embodiments 1 to 6, wherein: the piston further comprises a piston flange disposed rearwardly from the piston body; the piston flange has a piston flange width extending along a transverse axis of the housing perpendicular to the longitudinal axis; the piston flange width is greater than a piston body width of the piston body extending along the transverse axis; and a rearward end of the piston stop abuts a forward end of the piston flange when the piston is in the forwardmost piston position to limit forward translation of the piston to the forwardmost piston position.

[0052] Embodiment 8. The active die of any one of Embodiments 1 to 7, wherein: the piston stop defines a piston stop channel extending through the piston stop along the longitudinal axis; the piston stop channel is generally cylindrical so as to fittingly and slidingly receive the piston body for translation of the piston body within the piston stop channel along the longitudinal axis.

[0053] Embodiment 9. The active die of any one of Embodiments 1 to 8, wherein: the spring device comprises a spring washer stack comprising a plurality of disc spring washers aligned along the longitudinal axis; the spring washer stack is sandwiched between a rearward end of the piston and the floor of the housing.

[0054] Embodiment 10. The active die of any one of Embodiments 1 to 9, wherein: the piston stop die surface is tapered to form with the piston crown a generally frustoconical shape of the die cavity when the piston is at the forwardmost piston position.

[0055] Embodiment 11. The active die of any one of Embodiments 1 to 10, wherein: the piston defines a piston die post channel extending through the piston along the longitudinal axis; the piston die post channel is generally cylindrical; the active die further comprises a die post fittingly and slidingly received in the piston die post channel for translation of the piston along the longitudinal axis while the die post remains stationary relative to the housing; the die post has a die post crown ata forward end of the die post; and the piston crown, the piston stop die surface, and the die post crown together define the die cavity.

[0056] Embodiment 12. The active die of Embodiment 11 , wherein: the piston crown and the die post crown are flush when the piston is at the forwardmost piston position.

[0057] Embodiment 13. A self-piercing riveting method comprising: driving a self-piercing rivet along a longitudinal axis into workpieces through a near surface of the workpieces against an active die provided at an opposing surface of the workpieces to flare the rivet, to form an interlock between the workpieces, and to form a button at the opposing surface of the workpieces; and applying a variable counterforce along the longitudinal axis against the opposing surface of the workpieces during the formation of the button, wherein a die cavity surface of the active die applies the variable counterforce and translates along the longitudinal axis during the formation of the button.

[0058] Embodiment 14. The method of Embodiment 13, wherein the die cavity surface is in continuous contact with the opposing surface of the workpieces during the formation of the button.

[0059] Embodiment 15. The method of Embodiment 13 or 14, wherein: the workpieces comprise an opposing workpiece having the opposing surface of the workpieces; the opposing workpiece is formed of a material having an ultimate tensile strength; and the initial variable counterforce is between 1 to 1 .3 times a product of the ultimate tensile strength of the material of the opposing workpiece and an area of the of the die cavity surface.

[0060] Embodiment 16. The method of any one of Embodiments 13 to 15, wherein: the die cavity surface applies: a starting counterforce against the opposing surface of the workpieces at first contact between the die cavity surface and the opposing surface of the workpieces during the formation of the button; and an increasing counterforce against the opposing surface of the workpieces during a remainder of the formation of the button.

[0061] Embodiment 17. The method of Embodiment 16, wherein the starting counterforce is at least 5 kN.

[0062] Embodiment 18. The method of any one of Embodiments 13 to 17, wherein: the active die is the active die as defined in any one of Embodiments 1 to 12; the longitudinal axis is the longitudinal axis of the housing; and the die cavity surface is the piston crown.

[0063] Embodiment 19. An assembly comprising a plurality of workpieces interlocked by a self-piercing rivet, the self-piercing rivet having a head and a flared end, a lowermost one of the workpieces proximal the flared end forming a button, wherein the button is free or substantially free from cracks.

[0064] Embodiment 20. The assembly of Embodiment 19, wherein the lowermost one of the workpieces is formed of a material having low ductility.

[0065] So that the present disclosure may be more readily understood, certain terms are defined. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which embodiments of the invention pertain. While many methods and materials similar, modified, or equivalent to those described herein can be used in the practice of the embodiments of the present invention without undue experimentation, the preferred materials and methods are described herein.

[0066] All terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting in any manner or scope. For example, as used in this specification and the appended claims, the singular forms "a," "an" and "the" can include plural referents unless the content clearly indicates otherwise. Numeric ranges recited within the specification are inclusive of the numbers defining the range and include each integer within the defined range. Throughout this disclosure, various aspects of this invention are presented in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the invention. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges, fractions, and individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6,etc., as well as individual numbers within that range, for example, 1 , 2, 3, 4, 5, and 6, and decimals and fractions, for example, 1.2, 3.8, 11 , and 4%. This applies regardless of the breadth of the range.

[0067] The terms “about” or “approximately” as used herein refer to variation in the numerical quantity that can occur, for example, through typical measuring techniques and equipment, with respect to any quantifiable variable, including, but not limited to, mass, volume, time, distance, voltage, and current. Further, given solid and liquid handling procedures used in the real world, there is certain inadvertent error and variation that is likely through differences in the manufacture, source, or purity of the ingredients used to make the compositions or carry out the methods and the like. The terms “about” and “approximately” also encompass these variations. Expressions which combine the terms “about” or “approximately” with one or more bounds of a range refer to a union of the bound modified by the term “about” or “approximately” as described above, and the range having the unmodified bound. Thus, for example, the expression “at least about X” means the union of “at least X” and “about X”. Similarly, “at most about Y” means the union of “at most Y” and “about Y”.

[0068] Embodiments of the disclosed subject-matter are described herein using the auxiliary verb “may”. When used herein, unless required otherwise by the context of usage, the auxiliary verb “may” designates an embodiment of the disclosed subject-matter which possesses the addressed object without requiring necessarily that any other embodiment of the disclosed subject-matter possesses the addressed object. Thus, a statement such as “X may include Y” indicates that the disclosed subject-matter includes embodiments where X includes Y, without requiring that all disclosed embodiments include Y, and without excluding any other embodiments which do not include Y.

[0069] In the preceding description, for purposes of explanation, numerous details are set forth in order to provide a thorough understanding of the embodiments. However, it will be apparent to one skilled in the art that these specific details are not required. In particular, it will be appreciated that the various additional features shown in the drawings are generally optional unless specificallyidentified herein as required. The above-described embodiments are intended to be examples only. Alterations, modifications and variations can be effected to the particular embodiments by those of skill in the art. The scope of the claims should not be limited by the particular embodiments set forth herein, but should be construed in a manner consistent with the specification as a whole.

Claims

WHAT IS CLAIMED IS:1 . An active die comprising: a generally tubular rigid housing having a longitudinal axis extending between a rearward end and a forward end, the housing having an opening at the forward end and a floor at the rearward end; a piston disposed at least partly in an interior of the housing, the piston having a generally cylindrical piston body having a piston crown at a forward end of the piston body, wherein the piston crown is disposed proximal the opening of the housing and the piston is translatable along the longitudinal axis relative to the housing; a piston stop coupled with the housing proximal the forward end of the housing to limit forward translation of the piston to a forwardmost piston position, the piston stop having a piston stop die surface adjacent and about the piston crown; and a spring device proximal the rearward end of the housing and arranged to urge forward translation of the piston relative to the housing, wherein: the piston crown and the piston stop die surface together define a die cavity; and the piston is translatable rearwardly along the longitudinal axis relative to the housing responsive to force applied at the piston crown to compress the spring device along the longitudinal axis to increase a variable depth of the die cavity extending along the longitudinal axis and to increase a variable counterforce at the piston crown.

2. The active die of claim 1 , wherein: the piston stop is cylindrical and has a piston stop threading at a radially outer surface of the piston stop; the housing has a housing threading at a radially inner surface of the housing;the piston stop is coupled with the housing by coupling of the piston stop threading and the housing threading; and the piston stop is rotatable about the longitudinal axis relative to the housing to selectively adjust a piston stop position of the piston stop along the longitudinal axis thereby to selectively adjust the forwardmost piston position.

3. The active die of claim 2, wherein the piston stop position is selectively adjustable along the longitudinal axis to selectively adjust a non-zero starting counterforce at the piston crown when the piston is at the forwardmost piston position.

4. The active die of claim 2 or 3, wherein: the piston stop has a forwardmost piston stop surface adjacent and about the piston stop die surface; and the piston crown is spaced rearwardly from the forward piston stop surface by a preconfigured minimum die cavity depth when the piston is in the forwardmost piston position.

5. The active die of claim 4, wherein the preconfigured minimum die cavity depth is about 0.0 mm to about 1 .0 mm.

6. The active die of claim 4, wherein the piston crown is substantially flush with the forwardmost piston stop surface when the piston is in the forwardmost piston position.

7. The active die of any one of claims 1 to 6, wherein: the piston further comprises a piston flange disposed rearwardly from the piston body; the piston flange has a piston flange width extending along a transverse axis of the housing perpendicular to the longitudinal axis;the piston flange width is greater than a piston body width of the piston body extending along the transverse axis; and a rearward end of the piston stop abuts a forward end of the piston flange when the piston is in the forwardmost piston position to limit forward translation of the piston to the forwardmost piston position.

8. The active die of any one of claims 1 to 7, wherein: the piston stop defines a piston stop channel extending through the piston stop along the longitudinal axis; and the piston stop channel is generally cylindrical so as to fittingly and slidingly receive the piston body for translation of the piston body within the piston stop channel along the longitudinal axis.

9. The active die of any one of claims 1 to 8, wherein: the spring device comprises a spring washer stack comprising a plurality of disc spring washers aligned along the longitudinal axis; and the spring washer stack is sandwiched between a rearward end of the piston and the floor of the housing.

10. The active die of any one of claims 1 to 9, wherein the piston stop die surface is tapered to form with the piston crown a generally frustoconical shape of the die cavity when the piston is at the forwardmost piston position.11 . The active die of any one of claims 1 to 10, wherein: the piston defines a piston die post channel extending through the piston along the longitudinal axis; the piston die post channel is generally cylindrical; the active die further comprises a die post fittingly and slidingly received in the piston die post channel for translation of the piston along the longitudinal axis while the die post remains stationary relative to the housing; the die post has a die post crown at a forward end of the die post; andthe piston crown, the piston stop die surface, and the die post crown together define the die cavity.

12. The active die of claim 11 , wherein the piston crown and the die post crown are flush when the piston is at the forwardmost piston position.

13. A self-piercing riveting method comprising: driving a self-piercing rivet along a longitudinal axis into workpieces through a near surface of the workpieces against an active die provided at an opposing surface of the workpieces to flare the rivet, to form an interlock between the workpieces, and to form a button at the opposing surface of the workpieces; and applying a variable counterforce along the longitudinal axis against the opposing surface of the workpieces during the formation of the button, wherein a die cavity surface of the active die applies the variable counterforce and translates along the longitudinal axis during the formation of the button.

14. The method of claim 13, wherein the die cavity surface is in continuous contact with the opposing surface of the workpieces during the formation of the button.

15. The method of claim 13 or 14, wherein: the workpieces comprise an opposing workpiece having the opposing surface of the workpieces; the opposing workpiece is formed of a material having an ultimate tensile strength; and the initial variable counterforce is between 1 to 1 .3 times a product of the ultimate tensile strength of the material of the opposing workpiece and an area of the of the die cavity surface.

16. The method of any one of claims 13 to 15, wherein the die cavity surface applies:a starting counterforce against the opposing surface of the workpieces at first contact between the die cavity surface and the opposing surface of the workpieces during the formation of the button; and an increasing counterforce against the opposing surface of the workpieces during a remainder of the formation of the button.

17. The method of claim 16, wherein the starting counterforce is at least 5 kN.

18. The method of any one of claims 13 to 17, wherein: the active die is the active die as defined in any one of claims 1 to 12; the longitudinal axis is the longitudinal axis of the housing; and the die cavity surface is the piston crown.

19. An assembly comprising a plurality of workpieces interlocked by a selfpiercing rivet, the self-piercing rivet having a head and a flared end, a lowermost one of the workpieces proximal the flared end forming a button, wherein the button is free or substantially free from cracks.

20. The assembly of claim 19, wherein the lowermost one of the workpieces is formed of a material having low ductility.

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