a new self-piercing riveting process and strength evaluation

8/22/2019 A New Self-Piercing Riveting Process and Strength Evaluation

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Bin Wang

Chuanyong Hao

Jinsong Zhang

Institute of Metal Research,

Chinese Academy of Sciences,

Shenyang 110016, P. R. China

Hongyan ZhangDepartment of MIME,

University of Toledo,

Toledo, OH 43606

A New Self-Piercing RivetingProcess and Strength EvaluationSelf-piercing riveting (SPR) has become an important alternative joining technique forthe automotive applications of aluminum sheets. Most existing SPR machines use elec-trical motors to drive a rivet into the sheets. A significant amount of research has beenconducted to improve an SPR joints strength by increasing the mechanical interlock. In

this paper, a new process is presented using gunpowder to drive the riveting process. Ajoint formed using the new process has different geometric characteristics from onecreated using a conventional system. The tensile-shear, cross-tension, fatigue, and impact

performances of self-piercing riveted joints using the new device are compared to thoseof spot-welded joints on aluminum sheets. The experiment has proven that the new SPR

joints have provided a similar or higher strength than resistance spot welds. DOI: 10.1115/1.2137746

Introduction

Driven by the ever-increasing demands for weight reduction ofautomobiles to reduce emission and improve fuel economy, newadvanced materials, such as advanced high-strength steels and

aluminum alloys, have been constantly introduced in automobilebody-in-white construction. The use of aluminum alloys for auto-motive manufacturing poses significant challenges to both form-

ing and joining processes because previously existing equipmentand knowledge cannot be directly applied to dealing with the newmaterials. Because of the metallurgical differences between steelsand aluminum alloys, weldingespecially resistance spot weld-

ing of aluminumis not as robust as welding steels. To overcomethe difficulties involved in welding aluminum alloys, a self-piercing riveting SPR technique has been developed, largely toreplace resistance spot welding RSW in joining aluminum sheet

materials. Aluminum-intensive vehicles often have more SPRjoints than resistance spot welds. For instance, Audi A8 has about500 spot welds and 1100 self-piercing rivets, and about 1500 self-piercing rivets have totally replaced spot welds on the latest alu-

minum Audi AL2 1 .The existing SPR process is essentially a cold-forming opera-

tion in which a semi-tubular rivet is slowly pressed by a punchinto two sheets that are supported on a die. The rivet pierces theupper sheet and flares into the bottom sheet, thus, forming a me-chanical interlock between the sheets. The piercing and flaringprocesses during an SPR are usually driven by either a dual-actionhydraulic cylinder or an electrical motor. In addition to beingexpensive and complicated, such equipment is difficult to main-tain and the strength of the joints created is not as high as pre-ferred. A large portion of efforts has been devoted to optimizingthe geometry of the die cavity to increase the interlock between arivet and the sheets. As the rivets are usually hardened, in order topierce through the first sheet, they may endure small amount ofplastic deformation during bending before fracture. Therefore,such improvements are significantly limited. In this work, a newtype of driving system has been developed for SPR to overcomesome of the shortcomings of existing SPR systems. Instead ofusing slow moving cylinders or electrical motors, the new system as shown in Fig. 1 uses gunpowder to push the rivet into thesheets and form an interlock. The gunpowder is stored in a shell,and when ignited, it directly impacts on a punch, which hits therivet head. The piercing and flaring processes are similar to those

of a conventional SPR system, yet the entire riveting process isusually completed in less than a millisecond. An anvil is placedbetween the punch and the upper sheet to restrain the stroke of thepunch under impact, so the excessive force will not be applied tothe rivet or sheet. This is necessary in order to avoid excessivedeformation of the rivet head or the sheet, yet still provide suffi-cient impact energy for the SPR process.

Using this system, an aluminum sheet alloy was riveted andtested, both statically and dynamically, in order to quantify thestrength level of such riveted joints. Similar specimens with resis-tance spot welds were also prepared and tested for comparison.For the sake of clarity, the newly developed self-piercing rivetingprocess is called impact SPR, and the conventional process em-ploying an electric motor or hydraulic cylinder is referred to asquasi-static SPR.

Experiment

A comparison was made between impact and quasi-static SPRjoints on their deformation mechanisms and geometric character-

istics. The joints were prepared using the newly developed impactSPR system, presented in Fig. 1, and a conventional hydraulicpress-based system using same rivets, die, and sheets. The jointswere then sectioned for microstructure examination using an op-tical microscope.

Tensile-shear, cross-tension, fatigue, and impact tests were con-ducted on both SPR joined and resistance spot welded specimens.The specimens were prepared in accordance with the standards ofGB2651-81 tensile-shear, 2 , JIS Z3138 cross-tension, 3 GB/T15111-94 fatigue, 4 , and AWS D8.7M impact, 5 . Theirconfigurations are summarized in Figs. 25. The cross-tensionspecimens have flanges, with a set of four holes on each beambolted to rigid fixtures, to restrain sheet distortion during testing.Impact and quasi-static SPR joints were also compared in theirtensile-shear performance.

In the experiment, 2 mm thick 5A02 aluminum alloy sheetswere used. The composition is listed in Table 1. The surfaces ofthe sheets for RSW joints were pretreated by a chemical process 6 to ensure welding quality. After removing excessive greasesusing a metal degreaser, the sheets were dipped in 5% NaOH at

6065C for 4 min, then rinsed in water. A 3 min dip in 30%

HNO3 at room temperature was then followed before a final water

rinse. A 300 kVA three-phase rectifier-type welding machine wasused for welding. The welding parameters were: welding current,

28.5 kA; welding time, 100 ms five 50 Hz cycles ; and weldingforce, 7.5 kN Dome-shaped Cu-Cr-Zr electrodes of face radius of

100 mm and 20 mm dia were used for welding. The sheets forSPR experiment were not pretreated as it was not needed. The Al

Contributed by the Manufacturing Engineering Division of ASME for publication

in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript receivedNovember 12, 2004; final revision received April 27, 2005. Review conducted by S.

J. Hu.

Journal of Manufacturing Science and Engineering MAY 2006, Vol. 128 / 1

Copyright 2006 by ASME

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sheets have a yield strength of 176 MPa and an elongation of 7%at fracture. The rivets made of carbon steel have a barrel diameter

of 5.4 mm and a total length of 6 mm. Gunpowder shells of vari-ous sizes containing different amount of gunpowders were testedfor riveting the aluminum sheets, and one of them was chosen forall the joints in this study. The dimensions of the die used areshown in Fig. 6. The die was made of tool steel.

The quasi-static tests were conducted at a constant speed of

0.02 mm/s on a MTS testing machine. Fatigue tests were per-formed on a MTS servohydraulic testing machine using a sinu-

soidal wave form in a tension-tension mode, with R =0.1 and a

frequency of 20 Hz. The impact tests were done using a new typeof impact tester developed by Li et al. 6 and Zhang et al. 7 .The testing mechanism is as explained in Ref. 7 , and the impact

speed was set at about 5.78 m/s 13 mph . The energy consumedby a joint, either an SPR joint or a spot weld, during impact was

recorded in the test. At least five replicates were used in tensile-shear, cross-tension, and impact tests, and single replicates weredone for fatigue tests because such tests are generally time con-suming yet fairly consistent for SPR and spot welded joints.

Fig. 1 The new SPR system and its working mechanism

Fig. 2 Drawing of tensile-shear testing specimens unit:millimeters

Fig. 3 Drawing of cross-tension testing specimens unit:millimeters

Fig. 4 Drawing of fatigue testing specimens unit: millimeters

Fig. 5 Drawing of impact testing specimens unit: millimeters

Table 1 Chemical composition wt% of 5A02 aluminum alloy

Mg Fe Cu Mn Cr Ti Si Others Al

2.02.8 0.4 0.1 0.150.4 0.15 0.40 0.7 Balance

2 / Vol. 128, MAY 2006 Transactions of the ASME



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Results and Discussion

Using the newly developed SPR system, steel rivets can beeasily pushed into the sheets and form a mechanical interlock, asshown in Fig. 7. A typical joint formed by such a system is verysimilar in appearance to one created using traditional, quasi-static,servomotor, or hydraulic-driven systems. However, a closer lookat the cross sections Figs. 8 a and 8 b reveals that the amountof metal deformation or distortion in the top sheet in a joint cre-

ated by the impact SPR system is significantly smaller than in thatof a quasi-static SPR joint. The edge of the hole pierced on the topsheet, or edge of the cup formed on the top sheet if it is notpenetrated, has been proven by many researchers as the prime sitefor fatigue crack initiation 8 . Large deformation and a tensilestress created during riveting may promote the formation of mi-crocracks in the sheets and, ultimately, weaken the strength, espe-cially the fatigue strength of a riveted joint. A quasi-static SPR

joint Fig. 8 b has a uniform deformation on the bottom sheetand a significant stretching along the rivet trunk. This could con-tribute to a low fatigue strength in a quasi-static SPR joint. Theundercut, spread, and bottom-thickness shown in Fig. 8 a arecomparable to those in Fig. 8 b . The rivet trunk appears to bethickened in a quasi-static SPR joint. The definitions of SPR

joints geometric attributes can be found in 9 .

The differences in deformation between impact SPR and quasi-static SPR are revealed by the microstructures of various locationsin an SPR joint shown in Figs. 9 and 10. Examining the geometryand material deformation of the sheets and rivet results in thefollowing observations:

1. The impact SPR cuts the top sheet in the early stage ofriveting, while the quasi-static SPR stretches the top sheet toa large extent, without fracture on the outside of the rivetstrunk, as shown in Figs. 9 a , 9 c , 10 a , and 10 c . Notonly does Fig. 10 d show a larger deformation of the fayinginterface between the sheets than Fig. 9 c , the deformed

grains in the sheets also clearly show a significant materialflow in the sheets, dragged by the downward-moving rivetduring a quasi-static SPR.

2. The large stretching experienced in the sheets by a quasi-static SPR is also evidenced by comparing sheet deforma-tion inside the rivet trunk. The portion of the top sheet insidethe rivet was cut off, then squeezed downward along therivet inner wall, and the faying interface does not touch therivet tip in Figs. 9

a

and 9

b

. Figure 10

a

, on the otherhand, reveals that the top sheet is dragged down by the rivettip in the slow riveting process.

3. As the top sheet was cut through by the rivet tip before itwas significantly stretched in an impact SPR process, thecircumference of the opening was compressed when the ta-pered rivet head portion, which has a larger diameter thanthe trunk, entered the cut opening. From Fig. 9 d , it can beseen that the Al grains are slightly compressed, unlike inFig. 10 d a clear downwards material flow in the top sheetflow results from the dragging by the rivet in a quasi-staticSPR. The different amount of material flow seen in Figs. 9 fand 10 f is the result of different deformation mechanismsin these two processes.

Fig. 6 Drawing of the die used in riveting 2 mm aluminum sheets in this studyunit: millimeters

Fig. 7 An SPR riveted joint: a top view and b back viewFig. 8 Cross sections of a an impact SPR joint and b aquasi-static SPR joint




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Therefore, the major difference between the new process and theconventional process is the deformation during the riveting pro-cess. Because of the large tensile strain induced in the quasi-staticSPR, the sheets, both top and bottom ones, were significantlyweakened to resist further loading, be it tensile, shear, fatigue, orimpact. The less stretching, and even compression, at some placesin the sheets in an impact SPR joint should be beneficial instrength.

As the only difference between the new, impact and conven-tional SPR processes is the time needed for the rivet to be pushedinto the sheets forming a joint, the difference in the joints formedcan be attributed to the significantly different deformation rates.Although it is impossible to directly measure the deformationspeed, the impact force monitored for the impact SPR processmay provide a clue on the magnitude of strain rate in such aprocess. The force profile in Fig. 11 indicates that the riveting

process is completed in about 100 ms, which is a small fraction ofconventional quasi-static process in the order of hundreds of mil-liseconds . The deformation rate of the riveting process can beestimated by considering the process as an impact impulse withthe help of the riveting force profile shown in Fig. 11.

When the gunpowder is ignited, the punch is pushed down,together with the rivet, to impact the sheet stack up. The impactimpulse is consumed by the piercing and deformation actions, asexpressed by the impact speed change. Therefore, an equation canbe established for the impulse

M v1 v2 = impact force dwell time

In the above equation, M is the combined mass of the punch and

rivet; v1 and v2 are the average rivet speed during impact and afterriveting, which is zero, respectively. The impact speed is thencalculated as

v1 = impact force dwell time

M

M was measured as 0.84 g

rivet

+39.44 g

punch

=40.28 g. if

impact force is taken as 20 kN, and dwell time as 100 s, the

impact speed is estimated as 20,000 100106 / 40.28

103 50 m/s. As the rivet length is 6 mm, therefore, the de-

formation rate is on the order of 50103 / 68

103 mm/mm s. This is significantly higher than that in a quasi-static SPR. Although the material properties of aluminum sheets atsuch a high strain rate are unknown, it can be expected that theybehave considerably different from those in a quasi-static or slowriveting process. The observation that the top sheet is cut throughin Fig. 9, but significantly stretched in Fig. 10 may be attributed tothe effects of the different deformation rates in the two processes.

The force profile shown in Fig. 11 can be characterized by fourstages, corresponding to different processes during an impact riv-eting, as follows.

Stage I. In this stage, the rivet starts to bend the both top andbottom sheets, and an increasing force is needed to continue thedeformation. When the force reaches a certain level, the rivet tipstarts to penetrate the top sheet.

Stage II. The rivet advances in the stack up by wedging into thetop sheet. Most of the riveting force is used to cut through thesheet, and the force level constantly decreases as the cuttingprogresses in the top sheet.

Stage III. After the top sheet is pierced through, the rivet en-counters the bottom sheet. The bottom side of the sheet touchesthe tip of the die after being bent a little. Considerable amount offorce is needed to push the bottom sheet into the die cavity bydeforming the sheet. At the same time, the rivet-head portion, with

Fig. 9 Cross sections of an impact SPR joint

Fig. 10 Cross sections of a quasi-static SPR joint

Fig. 11 Impact load during the impact SPR process




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a larger diameter than its trunk portion, is squeezed into the cutopening on the top sheet created during stage II.

Stage IV. Most of the downward extruding on the lower sheethas been completed in stage III. The rivet tip/trunk portion con-tinues to bend, due to the die tip, and the sheet metal is pushed inthe lateral direction to fill the die cavity. At this stage, the momen-tum created by the impulse from the gunpowder explosion is

mostly consumed and the riveting force ceases. This is differentfrom a conventional riveting process, which is able to continu-ously provide riveting force through an electrical or hydraulicdriving mechanism in which the riveting force monotonically in-creases.

In order to quantify the quality of the impact SPR joints, theywere compared to RSW joints on tensile-shear, cross-tension, fa-tigue, and impact performances. Tensile-shear-tested SPR jointsgenerally failed through separating the bottom sheet from the rivetand the top sheet, with the rivet loosely hanging on the top sheetwith a significantly deformed and enlarged pierced hole Fig.12 a . These specimens also show a large-sheet distortion aroundthe joint, in contrast to the spot-welded specimens, which have

very little base metal deformation Fig. 12 b . From this figure, itcan be seen that the spot weld was sheared off through the fayinginterface.

In cross-tension tests, the rivets were pulled out from the bot-tom sheet Fig. 13 a , similar to that observed in tensile-sheartests. However, the distortion of the pierced hole on the top sheetand the sheet itself is small, mainly due to the restraining of theflanges on the specimens. Spot-welded specimens behaved differ-ently in cross-tension tests from in tensile-shear tests in which

interfacial fracture usually occurs. A weld pullout failure was usu-ally observed in cross-tension tests, as shown in Fig. 13 b , with aweld button left on one sheet and a hole created on the other.

When an SPR joint was under a repetitive or fatigue loading,cracking initiated from the edge of the pierced hole near the rivethead in the top sheet, and propagated in the transverse directionperpendicular to the loading, as seen in Fig. 14 a . This type offailure is also typical in fatigue testing of spot-welded specimens Fig. 14 b .

The deformation in the fatigue tested specimens appears differ-ent for the two types of joints Fig. 14 . There is a visible distor-tion around the joint in the failed SPR joint, and the spot-weldedspecimen has very small rotation in the base metal. Therefore, theloading mode is different when testing the two types of speci-mens. There is a larger tensile-loading component in the sheet

direction when the sheet does not bend much, as in the case of aRSW joint, than when the sheets that bend significantly, as in thecase of a SPR. The fatigue strength of SPR joined specimens isseveral orders higher than that of RSW joints as shown in Fig. 15.In addition, the SPR joints have a run out at 10 million cycles. Thesignificantly larger fatigue strength of SPR joints than RSW jointscan be attributed to their geometric and metallurgical differences.A spot weld has a sharp corner at the edge of the weld at thefaying interface. This corner serves as a stress riser, which reducesthe joints fatigue resistance. On the other hand, in a SPR joint thesheets have either a round cup or a hole at the joint, created by thepiercing action, and the stress concentration factor is smaller thanthat for a spot weld. In a SPR process, no metallurgical process is

Fig. 12 Tensile-shear tested specimens: a impact SPR jointsand b RSW joints

Fig. 13 Tested cross-tension specimens: a an SPR joint andb a RSW joint




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involved and the sheets, which are usually the weaker part of thejoint than the rivet, experience no metallurgical changes except

some mechanical straining. As welding is basically a metallurgicalprocess, the induced structure and properties are usually not aspreferable as in the base metal. Such a property difference also

works as a stress riser around the weld nugget. Therefore, SPRjoints generally have higher fatigue strength than RSW joints, asreported by many other researchers.

A comparison of impact strength between SPR and RSW jointshas not been reported. However, SPR joints have been impact-tested in a work for optimizing riveting dies 9 . In this study, thetwo types of specimens were tested in the same manner using adouble pendulum impact tester 7 . As shown in Fig. 16 a , theimpact action usually separated an SPR specimen at the joint bypulling the rivet out of the bottom sheet, which is similar to thefailure modes observed in other tests Figs. 12 and 13 . Some ofthe specimens broke into three pieces under an impact loading: abroken rivet containing the rivet head and upper part of the rivettrunk, the top sheet with a torn hole, and the bottom sheet with thelower part of the broken rivet trunk remaining in it Fig. 16 b .On the other hand, all the spot-welded specimens failed interfa-cially, with little deformation in the base metal Fig. 17 . Theamount of distortion in the base metal is directly linked to thestrength of the joint and its impact energy absorption capability.The difference in the observed impact strength measurements, asshown in Fig. 18, is reflected by the different amount of basemetal distortion shown in Figs. 16 and 17.

Figure 18 summarizes the differences in strength between SPR

and RSW joints. It shows that the SPR joints are stronger than thespot-welded joints in tensile-shear tests; SPR joints performslightly lower in cross-tension tests, and the fatigue strength of

Fig. 14 Tested fatigue specimens: a an impact SPR joint andb a RSW joint

Fig. 15 Results of fatigue testing of impact SPR and RSWjoints

Fig. 16 Impact-tested specimens: a the top sheet and the rivet pulled off from the bottom sheetand b the torn-off rivet and a hole left on the top sheet




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SPR joints is three times as great as that of spot-welded joints.

Comparison of the fatigue strength uses the load of 3000 N forSPR specimens under which the specimen did not fail after

107 cycles, and the strength of RSW specimens is extrapolatedfrom the observed data, as shown in Fig. 15, at the same numberof cycles. The SPR joints have a clear advantage over RSW joints as shown in the figure on average impact energy absorptioncapability. For tensile-shear, cross-tension, and impact strengthmeasurements, the minimum, mean, and maximum values are in-dicated in the figure to show the variability of the testing data. Thefatigue performance was compared using one load level tested onone specimen for each type of joint.

The comparisons made in this study between SPR and RSWjoints are similar to those by other researchers. It has been re-ported that self-piercing riveting may provide a static strengthsimilar to that of resistance spot welds, and superior fatiguestrength than spot welds 1012 . The performances of impactSPR and quasi-static SPR joints are compared on their tensile-shear strength in Fig. 19. The strength of quasi-static SPR joints is18% lower than that of impact SPR joints. In addition, impactSPR joints appear to have a smaller variability, which is desirablein industrial joining processes. The figure also shows that quasi-

static SPR joints have slightly lower tensile-shear strength thanRSW joints, which is consistent with other published results. Theresults of this study can also be compared to published data onquasi-static SPR joints. Using similar materials as in this investi-gation Bollhoff reported an average tensile-shear strength of

4.94 kN and a peeling tension strength of 2.87 kN 13 , respec-tively, compared to 6.50 kN tensile-shear and 3.47 kN cross-tension produced by the impact SPR joints. Using AA6111-T4,

an average of 4.31 kN tensile-shear strength was obtained 14 .The same paper also reported a fatigue strength of 1.29 kN at

107 cycles for quasi-static SPR joints, which is significantly lower

than that obtained on impact SPR joints 3.0 kN .

Summary

The new impact SPR process, using an impact impulse gener-ated by gunpowder, possesses notable advantages: equipment isrelatively simple, energy consumption is low, operation is simple,and it is especially suitable for repairing. The investigation of theperformance of the 5A02 alloy sheets joined by SPR and RSWhas led the following main conclusions:

1. Although the impact and quasi-static SPR processes produceSPR joints similar in appearance, they are quite different instructure, as revealed in the cross sections of the joints. Theamount of deformation and fracture behavior are differentmainly due to the drastically different deformation rates inthese two processes. Such differences generate differentamount of internal damages/weakening and residual stressesin the sheets, which determine the different behaviors ofthese joints when they are loaded.

2. When tensile-shear loaded, an impact SPR process yields ahigher strength than a quasi-static SPR process. Quasi-staticSPR joints have similar tensile-shear peak loads as resis-tance spot welds, which is consistent with other publishedresults.

3. The impact SPR joints are stronger than or similar to spot-

welded joints in most of the tests. In cross-tension tests theRSW joints are slightly higher than SPR joints, and SPR isclearly stronger than RSW in tensile-shear and impact tests.It is noteworthy that the fatigue strength of SPR joints isthree times as great as that of spot-welded joints.

The impact SPR process shows a clear improvement to conven-tional SPR processes, and its demonstrated advantages shouldmake SPR a more suitable alternate to RSW in joining aluminumalloys.

Acknowledgment

The authors would like to express their sincere gratitude for thefinancial support of the Ministry of Science and Technology ofChina the 10th Five-Year Plan .

References 1 Miller, W. S., Zhuang, L., Bottema, J., Wittebrood, A. J., De Smet, P., Haszler,

A., and Vieregge, A., 2000, Recent Development in Aluminum Alloys for theAutomotive Industry, Mater. Sci. Eng., A, A280, pp. 3749.

2 Peoples Republic of China Standards GB/T2651-81, Test Methods of TensileShear of Spot-Welded Joints.

3 Japanese Standards Association JIS Z 3138:1989, Method of Fatigue Testingfor Spot-Welded Joints.

4 Peoples Republic of China Standards GB/T15111-94, Test Method for ShearTensile Fatigue of Spot-Welded Joints.

5 AWS D8.7M:2004, Recommended Practices for Automotive Weld QualityResistance Spot Welding, American Welding Society, working draft.

6 Li, Z., Hao, C., Zhang, J., and Zhang, H., 2004, Effects of Sheet Surface

Fig. 17 Impact-tested RSW specimens

Fig. 18 Performance comparison between SPR and spot-welded joints

Fig. 19 Tensile-shear strength comparison between quasi-static and impact SPR, and spot-welded joints




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Conditions on Electrode Life in Aluminum Welding, Weld. J. Miami, FL, U.

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8 Li, B., 2003, Deformation and Fatigue Behavior of Riveted Joints, Ph.D.

dissertation, The University of Toledo, Toledo, OH.

9 Zheng, H., 2003, Study of Impact Performance of Self-Piercing Riveting

Joints, M.S. thesis, The University of Toledo, May.

10 Hahn, O., and Schulte, A., 1998, Performance and Reliability of Self-Piercing

Riveted Joints in Steel and Aluminum Alloys, Mechanical Fastening Seminar,Jan. 27, Troy, MI.

11 Hahn, O., Meschut, G., and Peetz, A., 1999, Mechanical Properties of Punch-Riveted and Adhesive-Bonded Aluminum Sheets, Welding and Cutting En-glish Translation of Schweissen and Schneiden , 51 7 , pp. .

12 Westgate, S. A., 1998, How do Mechanical Fasteners Measure Up to SpotWelding? Mechanical Fastening Seminar, Jan. 27, Troy, MI.

13 http://www.bollhoff-rivnut.com/ 14 Fu, M., and Mallick, P. K., 2003, Fatigue of Self-Piercing Riveted Joints in

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a new self-piercing riveting process and strength evaluation

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