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1 Bin Wang Chuanyong Hao Jinsong Zhang Institute of Metal Research, Chinese Academy of Sciences, Shenyang , P. R. China Hongyan Zhang Department of MIME, University of Toledo, Toledo, OH A New Self-Piercing Riveting Process and Strength Evaluation Self-piercing riveting (SPR) has become an important alternative joining technique for the automotive applications of aluminum sheets. Most existing SPR machines use electrical motors to drive a rivet into the sheets. A significant amount of research has been conducted to improve an SPR joint s strength by increasing the mechanical interlock. In this paper, a new process is presented using gunpowder to drive the riveting process. A joint formed using the new process has different geometric characteristics from one created 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 those of 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: / Introduction Driven by the ever-increasing demands for weight reduction of automobiles to reduce emission and improve fuel economy, new advanced materials, such as advanced high-strength steels and aluminum alloys, have been constantly introduced in automobile body-in-white construction. The use of aluminum alloys for automotive manufacturing poses significant challenges to both forming and joining processes because previously existing equipment and knowledge cannot be directly applied to dealing with the new materials. Because of the metallurgical differences between steels and aluminum alloys, welding especially resistance spot welding of aluminum is not as robust as welding steels. To overcome the difficulties involved in welding aluminum alloys, a selfpiercing riveting SPR technique has been developed, largely to replace resistance spot welding RSW in joining aluminum sheet materials. Aluminum-intensive vehicles often have more SPR joints than resistance spot welds. For instance, Audi A8 has about 500 spot welds and 1100 self-piercing rivets, and about 1500 selfpiercing rivets have totally replaced spot welds on the latest aluminum Audi AL2 1. The existing SPR process is essentially a cold-forming operation in which a semi-tubular rivet is slowly pressed by a punch into two sheets that are supported on a die. The rivet pierces the upper sheet and flares into the bottom sheet, thus, forming a mechanical interlock between the sheets. The piercing and flaring processes during an SPR are usually driven by either a dual-action hydraulic cylinder or an electrical motor. In addition to being expensive and complicated, such equipment is difficult to maintain and the strength of the joints created is not as high as preferred. A large portion of efforts has been devoted to optimizing the geometry of the die cavity to increase the interlock between a rivet and the sheets. As the rivets are usually hardened, in order to pierce through the first sheet, they may endure small amount of plastic deformation during bending before fracture. Therefore, such improvements are significantly limited. In this work, a new type of driving system has been developed for SPR to overcome some of the shortcomings of existing SPR systems. Instead of using slow moving cylinders or electrical motors, the new system as shown in Fig. 1 uses gunpowder to push the rivet into the sheets and form an interlock. The gunpowder is stored in a shell, and when ignited, it directly impacts on a punch, which hits the rivet head. The piercing and flaring processes are similar to those Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received November 12, 2004; final revision received April 27, Review conducted by S. J. Hu. of a conventional SPR system, yet the entire riveting process is usually completed in less than a millisecond. An anvil is placed between the punch and the upper sheet to restrain the stroke of the punch under impact, so the excessive force will not be applied to the rivet or sheet. This is necessary in order to avoid excessive deformation of the rivet head or the sheet, yet still provide sufficient impact energy for the SPR process. Using this system, an aluminum sheet alloy was riveted and tested, both statically and dynamically, in order to quantify the strength level of such riveted joints. Similar specimens with resistance spot welds were also prepared and tested for comparison. For the sake of clarity, the newly developed self-piercing riveting process is called impact SPR, and the conventional process employing an electric motor or hydraulic cylinder is referred to as quasi-static SPR. Experiment A comparison was made between impact and quasi-static SPR joints on their deformation mechanisms and geometric characteristics. The joints were prepared using the newly developed impact SPR system, presented in Fig. 1, and a conventional hydraulic press-based system using same rivets, die, and sheets. The joints were then sectioned for microstructure examination using an optical microscope. Tensile-shear, cross-tension, fatigue, and impact tests were conducted on both SPR joined and resistance spot welded specimens. The specimens were prepared in accordance with the standards of GB tensile-shear, 2, JIS Z3138 cross-tension, 3 GB/ T fatigue, 4, and AWS D8.7M impact, 5. Their configurations are summarized in Figs The cross-tension specimens have flanges, with a set of four holes on each beam bolted to rigid fixtures, to restrain sheet distortion during testing. Impact and quasi-static SPR joints were also compared in their tensile-shear performance. In the experiment, 2 mm thick 5A02 aluminum alloy sheets were used. The composition is listed in Table 1. The surfaces of the sheets for RSW joints were pretreated by a chemical process 6 to ensure welding quality. After removing excessive greases using a metal degreaser, the sheets were dipped in 5% NaOH at C for 4 min, then rinsed in water. A 3 min dip in 30% HNO 3 at room temperature was then followed before a final water rinse. A 300 kva three-phase rectifier-type welding machine was used for welding. The welding parameters were: welding current, 28.5 ka; welding time, 100 ms five 50 Hz cycles ; and welding force, 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 for SPR experiment were not pretreated as it was not needed. The Al Journal of Manufacturing Science and Engineering MAY 2006, Vol. 128 / 1 Copyright 2006 by ASME

2 Fig. 3 Drawing of cross-tension testing specimens unit: millimeters frequency of 20 Hz. The impact tests were done using a new type of 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 consumed by a joint, either an SPR joint or a spot weld, during impact was recorded in the test. At least five replicates were used in tensileshear, cross-tension, and impact tests, and single replicates were done for fatigue tests because such tests are generally time consuming yet fairly consistent for SPR and spot welded joints. Fig. 1 The new SPR system and its working mechanism Fig. 4 Drawing of fatigue testing specimens unit: millimeters 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 various sizes containing different amount of gunpowders were tested for riveting the aluminum sheets, and one of them was chosen for all the joints in this study. The dimensions of the die used are shown 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 performed on a MTS servohydraulic testing machine using a sinusoidal wave form in a tension-tension mode, with R=0.1 and a Fig. 5 Table 1 Drawing of impact testing specimens unit: millimeters Chemical composition wt% of 5A02 aluminum alloy Mg Fe Cu Mn Cr Ti Si Others Al Fig. 2 Drawing of tensile-shear testing specimens unit: millimeters Balance 2 / Vol. 128, MAY 2006 Transactions of the ASME

3 Fig. 6 Drawing of the die used in riveting 2 mm aluminum sheets in this study unit: millimeters Results and Discussion Using the newly developed SPR system, steel rivets can be easily pushed into the sheets and form a mechanical interlock, as shown in Fig. 7. A typical joint formed by such a system is very similar in appearance to one created using traditional, quasi-static, servomotor, or hydraulic-driven systems. However, a closer look at the cross sections Figs. 8 a and 8 b reveals that the amount of metal deformation or distortion in the top sheet in a joint created by the impact SPR system is significantly smaller than in that of a quasi-static SPR joint. The edge of the hole pierced on the top sheet, or edge of the cup formed on the top sheet if it is not penetrated, has been proven by many researchers as the prime site for fatigue crack initiation 8. Large deformation and a tensile stress created during riveting may promote the formation of microcracks in the sheets and, ultimately, weaken the strength, especially the fatigue strength of a riveted joint. A quasi-static SPR joint Fig. 8 b has a uniform deformation on the bottom sheet and a significant stretching along the rivet trunk. This could contribute to a low fatigue strength in a quasi-static SPR joint. The undercut, spread, and bottom-thickness shown in Fig. 8 a are comparable to those in Fig. 8 b. The rivet trunk appears to be thickened 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 quasistatic SPR are revealed by the microstructures of various locations in an SPR joint shown in Figs. 9 and 10. Examining the geometry and material deformation of the sheets and rivet results in the following observations: 1. The impact SPR cuts the top sheet in the early stage of riveting, while the quasi-static SPR stretches the top sheet to a large extent, without fracture on the outside of the rivet s trunk, as shown in Figs. 9 a, 9 c, 10 a, and 10 c. Not only does Fig. 10 d show a larger deformation of the faying interface between the sheets than Fig. 9 c, the deformed grains in the sheets also clearly show a significant material flow in the sheets, dragged by the downward-moving rivet during a quasi-static SPR. 2. The large stretching experienced in the sheets by a quasistatic SPR is also evidenced by comparing sheet deformation inside the rivet trunk. The portion of the top sheet inside the rivet was cut off, then squeezed downward along the rivet inner wall, and the faying interface does not touch the rivet tip in Figs. 9 a and 9 b. Figure 10 a, on the other hand, reveals that the top sheet is dragged down by the rivet tip in the slow riveting process. 3. As the top sheet was cut through by the rivet tip before it was significantly stretched in an impact SPR process, the circumference of the opening was compressed when the tapered rivet head portion, which has a larger diameter than the trunk, entered the cut opening. From Fig. 9 d, it can be seen that the Al grains are slightly compressed, unlike in Fig. 10 d a clear downwards material flow in the top sheet flow results from the dragging by the rivet in a quasi-static SPR. The different amount of material flow seen in Figs. 9 f and 10 f is the result of different deformation mechanisms in these two processes. Fig. 7 An SPR riveted joint: a top view and b back view Fig. 8 Cross sections of a an impact SPR joint and b a quasi-static SPR joint Journal of Manufacturing Science and Engineering MAY 2006, Vol. 128 / 3

4 Fig. 11 Impact load during the impact SPR process Fig. 9 Therefore, the major difference between the new process and the conventional process is the deformation during the riveting process. Because of the large tensile strain induced in the quasi-static SPR, the sheets, both top and bottom ones, were significantly weakened to resist further loading, be it tensile, shear, fatigue, or impact. The less stretching, and even compression, at some places in the sheets in an impact SPR joint should be beneficial in strength. Fig. 10 Cross sections of an impact SPR joint Cross sections of a quasi-static SPR joint As the only difference between the new, impact and conventional SPR processes is the time needed for the rivet to be pushed into the sheets forming a joint, the difference in the joints formed can be attributed to the significantly different deformation rates. Although it is impossible to directly measure the deformation speed, the impact force monitored for the impact SPR process may provide a clue on the magnitude of strain rate in such a process. The force profile in Fig. 11 indicates that the riveting process is completed in about 100 ms, which is a small fraction of conventional quasi-static process in the order of hundreds of milliseconds. The deformation rate of the riveting process can be estimated by considering the process as an impact impulse with the 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 impact impulse is consumed by the piercing and deformation actions, as expressed by the impact speed change. Therefore, an equation can be established for the impulse M v 1 v 2 = impact force dwell time In the above equation, M is the combined mass of the punch and rivet; v 1 and v 2 are the average rivet speed during impact and after riveting, which is zero, respectively. The impact speed is then calculated as impact force dwell time v 1 = M M was measured as 0.84 g rivet 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, / m/s. As the rivet length is 6 mm, therefore, the deformation rate is on the order of / mm/mm s. This is significantly higher than that in a quasistatic SPR. Although the material properties of aluminum sheets at such a high strain rate are unknown, it can be expected that they behave considerably different from those in a quasi-static or slow riveting process. The observation that the top sheet is cut through in Fig. 9, but significantly stretched in Fig. 10 may be attributed to the effects of the different deformation rates in the two processes. The force profile shown in Fig. 11 can be characterized by four stages, corresponding to different processes during an impact riveting, as follows. Stage I. In this stage, the rivet starts to bend the both top and bottom sheets, and an increasing force is needed to continue the deformation. When the force reaches a certain level, the rivet tip starts to penetrate the top sheet. Stage II. The rivet advances in the stack up by wedging into the top sheet. Most of the riveting force is used to cut through the sheet, and the force level constantly decreases as the cutting progresses in the top sheet. Stage III. After the top sheet is pierced through, the rivet encounters the bottom sheet. The bottom side of the sheet touches the tip of the die after being bent a little. Considerable amount of force is needed to push the bottom sheet into the die cavity by deforming the sheet. At the same time, the rivet-head portion, with 4 / Vol. 128, MAY 2006 Transactions of the ASME

5 Fig. 13 Tested cross-tension specimens: a an SPR joint and b a RSW joint Fig. 12 Tensile-shear tested specimens: a impact SPR joints and b RSW joints a larger diameter than its trunk portion, is squeezed into the cut opening on the top sheet created during stage II. Stage IV. Most of the downward extruding on the lower sheet has been completed in stage III. The rivet tip/trunk portion continues to bend, due to the die tip, and the sheet metal is pushed in the lateral direction to fill the die cavity. At this stage, the momentum created by the impulse from the gunpowder explosion is mostly consumed and the riveting force ceases. This is different from a conventional riveting process, which is able to continuously provide riveting force through an electrical or hydraulic driving mechanism in which the riveting force monotonically increases. In order to quantify the quality of the impact SPR joints, they were compared to RSW joints on tensile-shear, cross-tension, fatigue, and impact performances. Tensile-shear-tested SPR joints generally failed through separating the bottom sheet from the rivet and the top sheet, with the rivet loosely hanging on the top sheet with a significantly deformed and enlarged pierced hole Fig. 12 a. These specimens also show a large-sheet distortion around the joint, in contrast to the spot-welded specimens, which have very little base metal deformation Fig. 12 b. From this figure, it can be seen that the spot weld was sheared off through the faying interface. In cross-tension tests, the rivets were pulled out from the bottom sheet Fig. 13 a, similar to that observed in tensile-shear tests. However, the distortion of the pierced hole on the top sheet and the sheet itself is small, mainly due to the restraining of the flanges on the specimens. Spot-welded specimens behaved differently in cross-tension tests from in tensile-shear tests in which interfacial fracture usually occurs. A weld pullout failure was usually observed in cross-tension tests, as shown in Fig. 13 b, with a weld 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 rivet head in the top sheet, and propagated in the transverse direction perpendicular to the loading, as seen in Fig. 14 a. This type of failure is also typical in fatigue testing of spot-welded specimens Fig. 14 b. The deformation in the fatigue tested specimens appears different for the two types of joints Fig. 14. There is a visible distortion around the joint in the failed SPR joint, and the spot-welded specimen has very small rotation in the base metal. Therefore, the loading mode is different when testing the two types of specimens. There is a larger tensile-loading component in the sheet direction when the sheet does not bend much, as in the case of a RSW joint, than when the sheets that bend significantly, as in the case of a SPR. The fatigue strength of SPR joined specimens is several 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. The significantly larger fatigue strength of SPR joints than RSW joints can be attributed to their geometric and metallurgical differences. A spot weld has a sharp corner at the edge of the weld at the faying interface. This corner serves as a stress riser, which reduces the joint s fatigue resistance. On the other hand, in a SPR joint the sheets have either a round cup or a hole at the joint, created by the piercing action, and the stress concentration factor is smaller than that for a spot weld. In a SPR process, no metallurgical process is Journal of Manufacturing Science and Engineering MAY 2006, Vol. 128 / 5

6 Fig. 15 joints Results of fatigue testing of impact SPR and RSW Fig. 14 Tested fatigue specimens: a an impact SPR joint and b a RSW joint involved and the sheets, which are usually the weaker part of the joint than the rivet, experience no metallurgical changes except some mechanical straining. As welding is basically a metallurgical process, the induced structure and properties are usually not as preferable as in the base metal. Such a property difference also works as a stress riser around the weld nugget. Therefore, SPR joints generally have higher fatigue strength than RSW joints, as reported by many other researchers. A comparison of impact strength between SPR and RSW joints has not been reported. However, SPR joints have been impacttested in a work for optimizing riveting dies 9. In this study, the two types of specimens were tested in the same manner using a double pendulum impact tester 7. As shown in Fig. 16 a, the impact action usually separated an SPR specimen at the joint by pulling the rivet out of the bottom sheet, which is similar to the failure modes observed in other tests Figs. 12 and 13. Some of the specimens broke into three pieces under an impact loading: a broken rivet containing the rivet head and upper part of the rivet trunk, the top sheet with a torn hole, and the bottom sheet with the lower part of the broken rivet trunk remaining in it Fig. 16 b. On the other hand, all the spot-welded specimens failed interfacially, with little deformation in the base metal Fig. 17. The amount of distortion in the base metal is directly linked to the strength of the joint and its impact energy absorption capability. The difference in the observed impact strength measurements, as shown in Fig. 18, is reflected by the different amount of base metal 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 the spot-welded joints in tensile-shear tests; SPR joints perform slightly lower in cross-tension tests, and the fatigue strength of Fig. 16 Impact-tested specimens: a the top sheet and the rivet pulled off from the bottom sheet and b the torn-off rivet and a hole left on the top sheet 6 / Vol. 128, MAY 2006 Transactions of the ASME

7 Fig. 17 Impact-tested RSW specimens Fig. 19 Tensile-shear strength comparison between quasistatic and impact SPR, and spot-welded joints SPR joints is three times as great as that of spot-welded joints. Comparison of the fatigue strength uses the load of 3000 N for SPR specimens under which the specimen did not fail after 10 7 cycles, and the strength of RSW specimens is extrapolated from the observed data, as shown in Fig. 15, at the same number of cycles. The SPR joints have a clear advantage over RSW joints as shown in the figure on average impact energy absorption capability. For tensile-shear, cross-tension, and impact strength measurements, the minimum, mean, and maximum values are indicated in the figure to show the variability of the testing data. The fatigue performance was compared using one load level tested on one specimen for each type of joint. The comparisons made in this study between SPR and RSW joints are similar to those by other researchers. It has been reported that self-piercing riveting may provide a static strength similar to that of resistance spot welds, and superior fatigue strength than spot welds The performances of impact SPR and quasi-static SPR joints are compared on their tensileshear strength in Fig. 19. The strength of quasi-static SPR joints is 18% lower than that of impact SPR joints. In addition, impact SPR joints appear to have a smaller variability, which is desirable in industrial joining processes. The figure also shows that quasistatic SPR joints have slightly lower tensile-shear strength than RSW joints, which is consistent with other published results. The results of this study can also be compared to published data on quasi-static SPR joints. Using similar materials as in this investigation Bollhoff reported an average tensile-shear strength of 4.94 kn and a peeling tension strength of 2.87 kn 13, respectively, compared to 6.50 kn tensile-shear and 3.47 kn crosstension 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 10 7 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 generated by gunpowder, possesses notable advantages: equipment is relatively simple, energy consumption is low, operation is simple, and it is especially suitable for repairing. The investigation of the performance of the 5A02 alloy sheets joined by SPR and RSW has led the following main conclusions: 1. Although the impact and quasi-static SPR processes produce SPR joints similar in appearance, they are quite different in structure, as revealed in the cross sections of the joints. The amount of deformation and fracture behavior are different mainly due to the drastically different deformation rates in these two processes. Such differences generate different amount of internal damages/weakening and residual stresses in the sheets, which determine the different behaviors of these joints when they are loaded. 2. When tensile-shear loaded, an impact SPR process yields a higher strength than a quasi-static SPR process. Quasi-static SPR joints have similar tensile-shear peak loads as resistance spot welds, which is consistent with other published results. 3. The impact SPR joints are stronger than or similar to spotwelded joints in most of the tests. In cross-tension tests the RSW joints are slightly higher than SPR joints, and SPR is clearly stronger than RSW in tensile-shear and impact tests. It is noteworthy that the fatigue strength of SPR joints is three times as great as that of spot-welded joints. The impact SPR process shows a clear improvement to conventional SPR processes, and its demonstrated advantages should make SPR a more suitable alternate to RSW in joining aluminum alloys. Acknowledgment The authors would like to express their sincere gratitude for the financial support of the Ministry of Science and Technology of China the 10th Five-Year Plan. Fig. 18 Performance comparison between SPR and spotwelded joints 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 the Automotive Industry, Mater. Sci. Eng., A, A280, pp People s Republic of China Standards GB/T , Test Methods of Tensile Shear of Spot-Welded Joints. 3 Japanese Standards Association JIS Z 3138:1989, Method of Fatigue Testing for Spot-Welded Joints. 4 People s Republic of China Standards GB/T , Test Method for Shear Tensile Fatigue of Spot-Welded Joints. 5 AWS D8.7M:2004, Recommended Practices for Automotive Weld Quality Resistance Spot Welding, American Welding Society, working draft. 6 Li, Z., Hao, C., Zhang, J., and Zhang, H., 2004, Effects of Sheet Surface Journal of Manufacturing Science and Engineering MAY 2006, Vol. 128 / 7

8 Conditions on Electrode Life in Aluminum Welding, Weld. J. Miami, FL, U. S., submitted. 7 Zhang, H., Zhou, M., and Hu, S. J., 2001, Impact Strength Measurement and a New Impact Tester, J. Mech. Manuf., B, 215, pp 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 English Translation of Schweissen and Schneiden, 51 7, pp.. 12 Westgate, S. A., 1998, How do Mechanical Fasteners Measure Up to Spot Welding? Mechanical Fastening Seminar, Jan. 27, Troy, MI Fu, M., and Mallick, P. K., 2003, Fatigue of Self-Piercing Riveted Joints in Aluminum Alloy 6111, Int. J. Fatigue, 25, / Vol. 128, MAY 2006 Transactions of the ASME