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Joining of high strength steel and aluminium alloy sheets by mechanical clinching with dies for control of metal flow
Journal of Materials Processing Technology 212 (2012) 884–889
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Joining of high strength steel and aluminium alloy sheets by mechanical clinching with dies for control of metal flow Y. Abe a,∗ , K. Mori a , T. Kato b a b
Department of Mechanical Engineering, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan Department of Mechanical Engineering, Ishikawa National College of Technology, Kahoku-gun, Ishikawa 929-0392, Japan
a r t i c l e
i n f o
Article history: Received 12 July 2011 Received in revised form 22 November 2011 Accepted 25 November 2011 Available online 6 December 2011 Keywords: Joining Mechanical clinching High strength steel sheet Aluminium alloy sheet Small ductility Die shape
a b s t r a c t High strength steel and aluminium alloy sheets were joined by mechanical clinching with dies for control of metal flow. Since the sheets undergo plastic deformation for the joining during the mechanical clinching, the high strength steel sheets tend to fracture due to the small ductility. For the upper high strength steel sheet, fracture was caused by the concentration of deformation around the corner of the punch, and cracks were caused by the tensile stress generated in the bulged bottom into the groove of the die for the lower high strength steel sheet. To prevent these defects, metal flow of the sheets was controlled by optimising a shape of the die. For the upper high strength steel sheets, the depth of the die was decreased to prevent the concentration of deformation around the corner of the punch. On the other hand, the groove of the die was eliminated to reduce the tensile stress for the lower high strength steel sheets. The sheets below SPFC780 and SPFC980 were successively joined with the aluminium alloy sheet for the upper and lower high strength steel sheets, respectively. © 2011 Elsevier B.V. All rights reserved.
1. Introduction To reduce the weight of automobiles, Kleiner et al. (2003) show the use of high strength steel and aluminium alloy sheets tends to increase because of high specific strength. Since both steel and aluminium sheets are generally used for automobiles, joining processes of the steel and aluminium sheets are required. Although the resistance spot welding is usually used to join steel sheets for automobile body panels, the welding of aluminium alloy sheets is not easy because of the high thermal conductivity, low melting point and natural surface oxide layer. Moreover, it is difficult to weld aluminium alloy and steel sheets together, because the two melting points are very different. It is desirable in the automobile industry to develop new joining processes of aluminium alloy and steel sheets. Plastic joining processes without melting are attractive for joining dissimilar sheet metals. Although the friction stir spot welding, the mechanical clinching and the self-piercing riveting were developed as plastic joining processes of aluminium ally sheets, these processes have been extended to joining of dissimilar sheets. The friction stir welding is a joining process using frictional heat generated by a rotating tool. Although this welding has been extended
∗ Corresponding author. Tel.: +81 532 44 6705; fax: +81 532 44 6690. E-mail address: [email protected] (Y. Abe). 0924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2011.11.015
to joining of steel and aluminium ally sheets, Briskham et al. (2006) show the joining speed is not enough to join a lot of points in automobile body panels, and an intermetallic compound generated around the interface of the two metals is risky for the fatigue strength. The self pierce riveting is a cold process for joining two or more sheets by directly piercing the sheets with a rivet without a predrilled hole. Voelkner (2000) reported that sheets are joined by driving the rivet through the upper sheet and spreading the leg of the rivet in the lower sheet in this riveting. Since the self-pierce riveting does not require a pre-drilled hole unlike the conventional riveting, the joining speed is the same level with that of the spot resistance welding. The authors (Mori et al., 2006) have developed to join the ultra high strength steel and aluminium alloy sheets by the self pierce riveting. The authors (Abe et al., 2009) have investigated the joinability of aluminium alloy and high strength steel sheets in the conventional self pierce riveting, and then have improved the joinability In the mechanical clinching, sheets are joined by local hemming with a punch and die. In the mechanical clinching and self pierce riveting, the intermetallic compound is not generated due to mechanical joining without bonding unlike the friction stir welding. Although the strength of joint by the mechanical clinching is lower than that by self pierce riveting, Varis (2006) indicated that the mechanical clinching has the advantage of low running costs without a rivet. The mechanical clinching is widely used for
Y. Abe et al. / Journal of Materials Processing Technology 212 (2012) 884–889
Interlock Δ x
Sheet holder
φ5.2
Upper sheet
φ8
Lower sheet Die
Upper sheet Material flow
Lower sheet
1.8
Punch
Sheet holder
2.6
Punch
885
Die groove Fig. 2. Conventional die for joining aluminium alloy sheets.
Die Die groove (a) Initial
(b) Interlocking
(c) Final
Fig. 1. Joining of sheets by mechanical clinching: (a) initial; (b) interlock; (c) final.
electrical appliances, automobiles, etc. Pak and Kwon (1995) investigated the tension force of the joined steel and aluminium alloy sheets by the mechanical clinching. The authors (Abe et al., 2007a) have joined mild steel and aluminium alloy sheets by the mechanical clinching, and Varis (2003) has joined high strength steel sheets. In the mechanical clinching of high strength steel and aluminium alloy sheets, the authors (Abe et al., 2007b) showed the high strength steel sheet tends to fracture because of small ductility, and thus the combination of sheets is limited still. In the present paper, high strength steel and aluminium alloy sheets were joined by the mechanical clinching. Metal flow of the sheets in the mechanical clinching was controlled by optimising a shape of the die. 2. Mechanical clinching of high strength steel and aluminium alloy sheets 2.1. Mechanism of joining In the mechanical clinching, the sheets are penetrated with the punch and die to generate the interlock x between the lower and the upper sheets as shown in Fig. 1. The upper and lower sheets are joined by being hooked on the interlock generated by the material flow between the corners of the punch and die. The groove of the die has the function of accelerating the material flow, i.e., the increase in interlock. The requisites for the joining using the mechanical clinching are given by • Generation of interlock. • No facture of upper and lower sheets. Appropriate strength of the joined sheets is generated from the interlock. The fracture of the sheets brings about the corrosion of the sheets. The mechanical properties of the high strength steel and aluminium alloy sheets used for the mechanical clinching measured from the tensile test are given in Table 1. The steel sheets are from the ultra-high strength steel sheet SPFC980 having a nominal tensile strength of 980 MPa to the mild steel sheet SPCC, and the aluminium alloy sheet is fixed to A5052. As the tensile strength increases, the critical reduction in area representing ductility decreases, i.e., the joining becomes more difficult.
sheets shown in Fig. 2. When the high strength steel sheet was set to be upper and lower, the deformation behaviour was observed. For the joining of the upper high strength steel sheet, the upper sheet fractures as shown in Fig. 3. Since deformation of the upper sheet during the mechanical clinching concentrates around the corner of the punch, the facture occurs due to the small ductility of the high strength steel sheet, even for SPFC440. For the joining of the lower high strength steel sheet, the lower sheet fractures due to the tensile hydrostatic stress generated in the bulged bottom into the groove of the die (see Fig. 4). Although cracks were not observed for SPFC590, the radial cracks occur for SPFC780. For the mechanical clinching of the high strength steel and aluminium alloy sheets, the conventional die is inappropriate. The small ductility of the high strength steel sheets brings about the occurrence of fracture. In addition, the difference of flow stress between the high strength steel and aluminium alloy sheets makes the joining difficult. Therefore, the control of metal flow of the sheets is required for the joining. 3. Joining for upper high strength steel sheets 3.1. Prevention of concentration of deformation around corner of punch In the joining for the upper high strength steel sheets, the concentration of deformation around the corner of the punch is prevented by optimising the shape of the die. The effects of the depth and diameter of the die on metal flow of the sheets are illustrated in Fig. 5. As the depth of the die decreases, the concentration of deformation prevents, whereas the interlock between the upper and lower sheets decreases due to the reduction in material flow between corners of the punch and die. On the other hand, appropriate interlock is not generated for excessive small and large diameter of the die due to reduction in material flow. The condition of the mechanical clinching of the high strength steel and aluminium alloy sheets is illustrated in Fig. 6. The shape of the die was optimised from the commercial finite element code LS-DYNA. Hamel et al. (2000), Varis and Lepistö (2003) and de Paula et al. (2007) have calculated the deformed sheets in mechanical clinching by the commercial finite element code ABAQUS, MARC and DEFORM, respectively. In the simulation, axi-symmetric
2.2. Mechanical clinching with conventional die for joining of aluminium alloy sheets High strength steel and aluminium alloy sheet were mechanically clinched with the conventional die for joining aluminium alloy
Fig. 3. Occurrence of fracture of upper high strength sheet SPFC440 in mechanical joining with conventional die for joining aluminium alloy sheets.
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Table 1 Mechanical properties of high strength steel and aluminium alloy sheets used for mechanical clinching measured from tensile test. Sheet SPFC980 SPFC780 SPFC590 SPFC440 SPCC A5052-H34
Thickness (mm)
1.4 1.6 1.5
Tensile strength (MPa) 976 764 623 473 334 244
Critical reduction in area (%) 33 38 53 60 63 58
Flow stress (MPa) = 1415ε0.12 = 1178ε0.15 = 865ε0.16 = 754ε0.18 = 517ε0.17 = 366ε0.11
Fig. 4. Occurrence of cracks of lower high strength steel sheet SPFC780 in mechanical joining with conventional die for joining aluminium alloy sheets: (a) experimental; (b) calculated hydrostatic stress.
deformation was assumed by limiting the calculation to the vicinity undergoing plastic deformation. The cross-sections of the sheets were divided into quadrilateral solid elements. The die, punch and sheet holder were assumed to be rigid. The coefficient of friction at the interfaces equivalent to an unlubricated condition in the simulation was 0.20. 3.2. Determination of shape of die The effect of the depth of the die on the reduction in wall thickness of the upper sheet at the corner of the punch obtained from the calculation for d = 8 mm is shown in Fig. 7, where the critical reduction in wall thickness with fracture obtained from the Fig. 6. Conditions of mechanical clinching.
experiment is added. As the depth of the die decreases, the concentration of deformation around the corner of the punch prevents. The effect of the diameter of the die on the reduction in wall thickness of the upper sheet at the corner of the punch and the interlock obtained from the calculation for h = 1.0 mm and SPFC590
Fig. 5. Effects of die shapes on deforming behaviour of sheets: (a) diameter and depth of die; (b) small depth; (c) small diameter; (d) large diameter.
Fig. 7. Effect of depth of die on reduction in wall thickness of upper sheet at corner of punch obtained from calculation for d = 8 mm.
Y. Abe et al. / Journal of Materials Processing Technology 212 (2012) 884–889
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Fig. 8. Effect of diameter of die on reduction in wall thickness of upper sheet at corner of punch and interlock obtained from calculation for h = 1.0 mm and SPFC590.
Table 2 Depth and diameter of die determined from finite element simulation for upper high strength steel sheets. Upper sheet
Depth of die h (mm)
Diameter of die d (mm)
SPFC780 SPFC590 SPFC440 SPCC
0.7 1.0 1.3 1.8
10.0 9.5 9.0 8.0 Fig. 10. Relationship between interlock and tensile strength of upper high strength steel obtained from experiment.
is given in Fig. 8. As the diameter of the die increases, the reduction in wall thickness of the upper sheet at the corner of the punch decreases, whereas the interlock has a peak for d = 9.5 mm. The depth and diameter of the die determined from the finite element simulation for the upper high strength steel sheets are given in Table 2. For the upper high strength steel sheet SPFC980, the occurrence of fracture cannot be prevented for the combination of the depth and diameter of the die. 3.3. Joinability for present die The joining range for various thicknesses obtained from the experiment for the upper SPFC590, d = 9.5 mm and h = 1.0 mm is illustrated in Fig. 9. When the thickness of the lower sheet is excessive small, the interlock is not generated by insufficient material flow between the corners of the punch and die because of the small flow stress of the aluminium alloy sheet. The relationship between the interlock and the tensile strength of the upper high strength steel obtained from the experiment is shown in Fig. 10. As the tensile strength increases, the interlock decreases. Although the high strength steel sheet and aluminium alloy sheet were not joined with the conventional die for joining of aluminium alloy sheets, the sheets were successfully joined with the present die for control of metal flow except for SPFC980.
Fig. 9. Joining range for various thicknesses obtained from experiment for upper SPFC590, d = 9.5 mm and h = 1.0 mm.
4. Joining for lower high strength steel sheets 4.1. Reduction in tensile stress in bottom In the joining for the lower high strength steel sheets SPFC780 and SPFC980, the lower sheet fractures due to the tensile stress generated in the bulged bottom into the groove of the die. To reduce the tensile stress, the groove of the die was eliminated, i.e., the flat bottom. Since the contact area between the lower sheet and die becomes large, the tensile stress is reduced. Since the material flow between the corners of the punch and die is decreased by eliminating the groove of the die, the amount of interlock is reduced. The amount of interlock is compensated by increasing the depth of the die. The groove of the die shown in Fig. 6 was eliminated, and the depth and diameter of the die were optimised. 4.2. Determination of shape of die The occurrence of cracks for the lower high strength steel sheets obtained from experiment for d = 8 mm is shown in Fig. 11. As the depth of the die increases, the tensile stress around the corner of the sheet bottom becomes large, and thus the fracture occurs. As the tensile strength increases, the critical depth of the die decreases due to the reduction in ductility.
Fig. 11. Occurrence of cracks for lower high strength steel sheets obtained from experiment for d = 8 mm.
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Fig. 12. The interlock has a peak for d = 8.25 mm. Effects of depth and diameter of die on interlock obtained from calculation for lower SPFC780. Table 3 Depth and diameter of die determined from finite element simulation for lower high strength steel sheets. Lowe sheet
Depth of die h (mm)
Diameter of die d (mm)
Bottom of die
SPFC980 SPFC780 SPFC590 SPFC440 SPCC
1.6 1.8 1.8 1.8 1.8
8.25 8.25 8.0 8.0 8.0
Flat Flat Groove Groove Groove
Fig. 14. Relationship between interlock and tensile strength of lower high strength steel obtained from experiment.
The effects of the depth and diameter of the die on the interlock obtained from calculation for lower SPFC780 are illustrated in Fig. 12. The interlock has a peak for d = 8.25 mm. The depth and diameter of the die determined from the finite element simulation for the lower high strength steel sheets are given in Table 3. The depth for the lower high strength steel is larger than that for the upper high strength steel sheets due to the elimination of groove. Below SPFC590, the bottom of the die has a groove, because the fracture does not occur. 4.3. Joinability for present die The joining range for various thicknesses obtained from experiment for the lower SPFC780, d = 8.25 mm and h = 1.8 mm is illustrated in Fig. 13. Although the radial cracks were observed for the conventional die, the occurrence of cracks was prevented by the present die. When the thickness of the lower sheet is excessive large, the interlock is not generated by insufficient material flow between the corners of the punch and die because of the small flow stress of the aluminium alloy sheet. The difference of flow stress between the high strength steel and aluminium alloy sheets is also a problem for the joining.
Fig. 15. Maximum load measured from cross-tension test of joined high strength steel and aluminium alloy sheets.
The relationship between the interlock and the tensile strength of the lower high strength steel obtained from the experiment is shown in Fig. 14. As a comparison, the relationship for the conventional die is added. As the tensile strength increases, the interlock decreases. The joining range is extended by the present die for control of metal flow, and the joining becomes possible even for the lower SPFC980. The maximum load measured from the cross-tension test of joined high strength steel and aluminium alloy sheets is shown in Fig. 15, where Steel-A5052 and A5052-steel denote the upper and lower high strength steel sheets, respectively. For all combinations, the aluminium alloy sheet fractured. As the tensile strength increases, the cross-tension load decreases due to the reduction in interlock. 5. Conclusions
Fig. 13. Joining range for various thicknesses obtained from experiment for lower SPFC780, d = 8.25 mm and h = 1.8 mm.
To join high strength steel and aluminium alloy sheets, metal flow of the sheets was controlled by optimising a shape of the die. The mechanical clinching has the advantage of simple joining by
Y. Abe et al. / Journal of Materials Processing Technology 212 (2012) 884–889
local hemming with a punch and die. The following results are obtained: (1) In the joining for the upper high strength steel sheets, the occurrence of fracture induced by the concentration of deformation around the corner of the punch was prevented by decreasing the depth of the die. (2) In the joining for the lower high strength steel sheet, the occurrence of cracks induced by the tensile stress generated in the bulged bottom into the groove of the die was prevented by eliminating the groove of the die. (3) The depth and diameter of the die were optimised to increase the amount of interlock between the upper and lower sheets. (4) The sheets below SPFC780 and SPFC980 were successively joined with the aluminium alloy sheet for the upper and lower high strength steel sheets, respectively. References Abe, Y., Kato, T., Mori, K., 2007a. Joining of aluminium alloy and mild steel sheets using mechanical clinching. Mater. Sci. Forum 561–565, 1043–1046. Abe, Y., Kato, T., Mori, K., 2007. Finite element simulation of plastic joining processes of steel and aluminum alloy sheets. In: Cesar de Sa, J.M.A. et al. (Eds.), Materials
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Processing and Design: Modeling, Simulation and Application. AIP Conf. Proc., vol. 1, pp. 197–202. Abe, Y., Kato, T., Mori, K., 2009. Self-piercing riveting of high tensile strength steel and aluminium alloy sheets using conventional rivet and die. J. Mater. Process. Technol. 209 (8), 3914–3922. Briskham, P., Blundell, N., Han, L., Hewitt, R., Young, K., Boomer, D., 2006. Comparison of self-pierce riveting, resistance spotwelding and spot friction joining for aluminium automotive sheet. SAE paper 2006-01-0774. de Paula, A.A., Aguilar, M.T.P., Pertence, A.E.M., Cetlin, P.R., 2007. Finite element simulations of the clinch joining of metallic sheets. J. Mater. Process. Technol. 182 (1–3), 352–357. Hamel, V., Roelandt, J.M., Gacel, J.N., Schmit, F., 2000. Finite element modeling of clinch forming with automatic remeshing. Comput. Struct. 77 (2), 185–200. Kleiner, M., Geiger, M., Klaus, A., 2003. Manufacturing of lightweight components by metal forming. Ann. CIRP 52 (2), 521–542. Mori, K., Kato, T., Abe, Y., Ravshanbek, Y., 2006. Plastic joining of ultra high strength steel and aluminium alloy sheets by self piercing rivet. Ann. CIRP 55 (1), 283–286. Pak, S.W., Kwon, S.Y., 1995. Application of Mechanical Clinching Method to Aluminum Hood, SAE Tech. Pap. Ser. (Soc. Automot. Eng.) 950714. Varis, J., 2003. The suitability of clinching as a joining method for high-strength structural steel. J. Mater. Process. Technol 132 (1–3), 242–249. Varis, J., 2006. Economics of clinched joint compared to riveted joint and example of applying calculations to a volume product. J. Mater. Process. Technol. 172 (1), 130–138. Varis, J.P., Lepistö, J., 2003. A simple testing-based procedure and simulation of the clinching process using finite element analysis for establishing clinching parameters. Thin-Wall. Struct. 41 (8), 691–709. Voelkner, W., 2000. Present and future developments of metal forming: selected examples. J. Mater. Process. Technol. 106 (1–3), 236–242.
Contents lists available at SciVerse ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Joining of high strength steel and aluminium alloy sheets by mechanical clinching with dies for control of metal flow Y. Abe a,∗ , K. Mori a , T. Kato b a b
Department of Mechanical Engineering, Toyohashi University of Technology, Toyohashi, Aichi 441-8580, Japan Department of Mechanical Engineering, Ishikawa National College of Technology, Kahoku-gun, Ishikawa 929-0392, Japan
a r t i c l e
i n f o
Article history: Received 12 July 2011 Received in revised form 22 November 2011 Accepted 25 November 2011 Available online 6 December 2011 Keywords: Joining Mechanical clinching High strength steel sheet Aluminium alloy sheet Small ductility Die shape
a b s t r a c t High strength steel and aluminium alloy sheets were joined by mechanical clinching with dies for control of metal flow. Since the sheets undergo plastic deformation for the joining during the mechanical clinching, the high strength steel sheets tend to fracture due to the small ductility. For the upper high strength steel sheet, fracture was caused by the concentration of deformation around the corner of the punch, and cracks were caused by the tensile stress generated in the bulged bottom into the groove of the die for the lower high strength steel sheet. To prevent these defects, metal flow of the sheets was controlled by optimising a shape of the die. For the upper high strength steel sheets, the depth of the die was decreased to prevent the concentration of deformation around the corner of the punch. On the other hand, the groove of the die was eliminated to reduce the tensile stress for the lower high strength steel sheets. The sheets below SPFC780 and SPFC980 were successively joined with the aluminium alloy sheet for the upper and lower high strength steel sheets, respectively. © 2011 Elsevier B.V. All rights reserved.
1. Introduction To reduce the weight of automobiles, Kleiner et al. (2003) show the use of high strength steel and aluminium alloy sheets tends to increase because of high specific strength. Since both steel and aluminium sheets are generally used for automobiles, joining processes of the steel and aluminium sheets are required. Although the resistance spot welding is usually used to join steel sheets for automobile body panels, the welding of aluminium alloy sheets is not easy because of the high thermal conductivity, low melting point and natural surface oxide layer. Moreover, it is difficult to weld aluminium alloy and steel sheets together, because the two melting points are very different. It is desirable in the automobile industry to develop new joining processes of aluminium alloy and steel sheets. Plastic joining processes without melting are attractive for joining dissimilar sheet metals. Although the friction stir spot welding, the mechanical clinching and the self-piercing riveting were developed as plastic joining processes of aluminium ally sheets, these processes have been extended to joining of dissimilar sheets. The friction stir welding is a joining process using frictional heat generated by a rotating tool. Although this welding has been extended
∗ Corresponding author. Tel.: +81 532 44 6705; fax: +81 532 44 6690. E-mail address: [email protected] (Y. Abe). 0924-0136/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2011.11.015
to joining of steel and aluminium ally sheets, Briskham et al. (2006) show the joining speed is not enough to join a lot of points in automobile body panels, and an intermetallic compound generated around the interface of the two metals is risky for the fatigue strength. The self pierce riveting is a cold process for joining two or more sheets by directly piercing the sheets with a rivet without a predrilled hole. Voelkner (2000) reported that sheets are joined by driving the rivet through the upper sheet and spreading the leg of the rivet in the lower sheet in this riveting. Since the self-pierce riveting does not require a pre-drilled hole unlike the conventional riveting, the joining speed is the same level with that of the spot resistance welding. The authors (Mori et al., 2006) have developed to join the ultra high strength steel and aluminium alloy sheets by the self pierce riveting. The authors (Abe et al., 2009) have investigated the joinability of aluminium alloy and high strength steel sheets in the conventional self pierce riveting, and then have improved the joinability In the mechanical clinching, sheets are joined by local hemming with a punch and die. In the mechanical clinching and self pierce riveting, the intermetallic compound is not generated due to mechanical joining without bonding unlike the friction stir welding. Although the strength of joint by the mechanical clinching is lower than that by self pierce riveting, Varis (2006) indicated that the mechanical clinching has the advantage of low running costs without a rivet. The mechanical clinching is widely used for
Y. Abe et al. / Journal of Materials Processing Technology 212 (2012) 884–889
Interlock Δ x
Sheet holder
φ5.2
Upper sheet
φ8
Lower sheet Die
Upper sheet Material flow
Lower sheet
1.8
Punch
Sheet holder
2.6
Punch
885
Die groove Fig. 2. Conventional die for joining aluminium alloy sheets.
Die Die groove (a) Initial
(b) Interlocking
(c) Final
Fig. 1. Joining of sheets by mechanical clinching: (a) initial; (b) interlock; (c) final.
electrical appliances, automobiles, etc. Pak and Kwon (1995) investigated the tension force of the joined steel and aluminium alloy sheets by the mechanical clinching. The authors (Abe et al., 2007a) have joined mild steel and aluminium alloy sheets by the mechanical clinching, and Varis (2003) has joined high strength steel sheets. In the mechanical clinching of high strength steel and aluminium alloy sheets, the authors (Abe et al., 2007b) showed the high strength steel sheet tends to fracture because of small ductility, and thus the combination of sheets is limited still. In the present paper, high strength steel and aluminium alloy sheets were joined by the mechanical clinching. Metal flow of the sheets in the mechanical clinching was controlled by optimising a shape of the die. 2. Mechanical clinching of high strength steel and aluminium alloy sheets 2.1. Mechanism of joining In the mechanical clinching, the sheets are penetrated with the punch and die to generate the interlock x between the lower and the upper sheets as shown in Fig. 1. The upper and lower sheets are joined by being hooked on the interlock generated by the material flow between the corners of the punch and die. The groove of the die has the function of accelerating the material flow, i.e., the increase in interlock. The requisites for the joining using the mechanical clinching are given by • Generation of interlock. • No facture of upper and lower sheets. Appropriate strength of the joined sheets is generated from the interlock. The fracture of the sheets brings about the corrosion of the sheets. The mechanical properties of the high strength steel and aluminium alloy sheets used for the mechanical clinching measured from the tensile test are given in Table 1. The steel sheets are from the ultra-high strength steel sheet SPFC980 having a nominal tensile strength of 980 MPa to the mild steel sheet SPCC, and the aluminium alloy sheet is fixed to A5052. As the tensile strength increases, the critical reduction in area representing ductility decreases, i.e., the joining becomes more difficult.
sheets shown in Fig. 2. When the high strength steel sheet was set to be upper and lower, the deformation behaviour was observed. For the joining of the upper high strength steel sheet, the upper sheet fractures as shown in Fig. 3. Since deformation of the upper sheet during the mechanical clinching concentrates around the corner of the punch, the facture occurs due to the small ductility of the high strength steel sheet, even for SPFC440. For the joining of the lower high strength steel sheet, the lower sheet fractures due to the tensile hydrostatic stress generated in the bulged bottom into the groove of the die (see Fig. 4). Although cracks were not observed for SPFC590, the radial cracks occur for SPFC780. For the mechanical clinching of the high strength steel and aluminium alloy sheets, the conventional die is inappropriate. The small ductility of the high strength steel sheets brings about the occurrence of fracture. In addition, the difference of flow stress between the high strength steel and aluminium alloy sheets makes the joining difficult. Therefore, the control of metal flow of the sheets is required for the joining. 3. Joining for upper high strength steel sheets 3.1. Prevention of concentration of deformation around corner of punch In the joining for the upper high strength steel sheets, the concentration of deformation around the corner of the punch is prevented by optimising the shape of the die. The effects of the depth and diameter of the die on metal flow of the sheets are illustrated in Fig. 5. As the depth of the die decreases, the concentration of deformation prevents, whereas the interlock between the upper and lower sheets decreases due to the reduction in material flow between corners of the punch and die. On the other hand, appropriate interlock is not generated for excessive small and large diameter of the die due to reduction in material flow. The condition of the mechanical clinching of the high strength steel and aluminium alloy sheets is illustrated in Fig. 6. The shape of the die was optimised from the commercial finite element code LS-DYNA. Hamel et al. (2000), Varis and Lepistö (2003) and de Paula et al. (2007) have calculated the deformed sheets in mechanical clinching by the commercial finite element code ABAQUS, MARC and DEFORM, respectively. In the simulation, axi-symmetric
2.2. Mechanical clinching with conventional die for joining of aluminium alloy sheets High strength steel and aluminium alloy sheet were mechanically clinched with the conventional die for joining aluminium alloy
Fig. 3. Occurrence of fracture of upper high strength sheet SPFC440 in mechanical joining with conventional die for joining aluminium alloy sheets.
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Table 1 Mechanical properties of high strength steel and aluminium alloy sheets used for mechanical clinching measured from tensile test. Sheet SPFC980 SPFC780 SPFC590 SPFC440 SPCC A5052-H34
Thickness (mm)
1.4 1.6 1.5
Tensile strength (MPa) 976 764 623 473 334 244
Critical reduction in area (%) 33 38 53 60 63 58
Flow stress (MPa) = 1415ε0.12 = 1178ε0.15 = 865ε0.16 = 754ε0.18 = 517ε0.17 = 366ε0.11
Fig. 4. Occurrence of cracks of lower high strength steel sheet SPFC780 in mechanical joining with conventional die for joining aluminium alloy sheets: (a) experimental; (b) calculated hydrostatic stress.
deformation was assumed by limiting the calculation to the vicinity undergoing plastic deformation. The cross-sections of the sheets were divided into quadrilateral solid elements. The die, punch and sheet holder were assumed to be rigid. The coefficient of friction at the interfaces equivalent to an unlubricated condition in the simulation was 0.20. 3.2. Determination of shape of die The effect of the depth of the die on the reduction in wall thickness of the upper sheet at the corner of the punch obtained from the calculation for d = 8 mm is shown in Fig. 7, where the critical reduction in wall thickness with fracture obtained from the Fig. 6. Conditions of mechanical clinching.
experiment is added. As the depth of the die decreases, the concentration of deformation around the corner of the punch prevents. The effect of the diameter of the die on the reduction in wall thickness of the upper sheet at the corner of the punch and the interlock obtained from the calculation for h = 1.0 mm and SPFC590
Fig. 5. Effects of die shapes on deforming behaviour of sheets: (a) diameter and depth of die; (b) small depth; (c) small diameter; (d) large diameter.
Fig. 7. Effect of depth of die on reduction in wall thickness of upper sheet at corner of punch obtained from calculation for d = 8 mm.
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Fig. 8. Effect of diameter of die on reduction in wall thickness of upper sheet at corner of punch and interlock obtained from calculation for h = 1.0 mm and SPFC590.
Table 2 Depth and diameter of die determined from finite element simulation for upper high strength steel sheets. Upper sheet
Depth of die h (mm)
Diameter of die d (mm)
SPFC780 SPFC590 SPFC440 SPCC
0.7 1.0 1.3 1.8
10.0 9.5 9.0 8.0 Fig. 10. Relationship between interlock and tensile strength of upper high strength steel obtained from experiment.
is given in Fig. 8. As the diameter of the die increases, the reduction in wall thickness of the upper sheet at the corner of the punch decreases, whereas the interlock has a peak for d = 9.5 mm. The depth and diameter of the die determined from the finite element simulation for the upper high strength steel sheets are given in Table 2. For the upper high strength steel sheet SPFC980, the occurrence of fracture cannot be prevented for the combination of the depth and diameter of the die. 3.3. Joinability for present die The joining range for various thicknesses obtained from the experiment for the upper SPFC590, d = 9.5 mm and h = 1.0 mm is illustrated in Fig. 9. When the thickness of the lower sheet is excessive small, the interlock is not generated by insufficient material flow between the corners of the punch and die because of the small flow stress of the aluminium alloy sheet. The relationship between the interlock and the tensile strength of the upper high strength steel obtained from the experiment is shown in Fig. 10. As the tensile strength increases, the interlock decreases. Although the high strength steel sheet and aluminium alloy sheet were not joined with the conventional die for joining of aluminium alloy sheets, the sheets were successfully joined with the present die for control of metal flow except for SPFC980.
Fig. 9. Joining range for various thicknesses obtained from experiment for upper SPFC590, d = 9.5 mm and h = 1.0 mm.
4. Joining for lower high strength steel sheets 4.1. Reduction in tensile stress in bottom In the joining for the lower high strength steel sheets SPFC780 and SPFC980, the lower sheet fractures due to the tensile stress generated in the bulged bottom into the groove of the die. To reduce the tensile stress, the groove of the die was eliminated, i.e., the flat bottom. Since the contact area between the lower sheet and die becomes large, the tensile stress is reduced. Since the material flow between the corners of the punch and die is decreased by eliminating the groove of the die, the amount of interlock is reduced. The amount of interlock is compensated by increasing the depth of the die. The groove of the die shown in Fig. 6 was eliminated, and the depth and diameter of the die were optimised. 4.2. Determination of shape of die The occurrence of cracks for the lower high strength steel sheets obtained from experiment for d = 8 mm is shown in Fig. 11. As the depth of the die increases, the tensile stress around the corner of the sheet bottom becomes large, and thus the fracture occurs. As the tensile strength increases, the critical depth of the die decreases due to the reduction in ductility.
Fig. 11. Occurrence of cracks for lower high strength steel sheets obtained from experiment for d = 8 mm.
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Fig. 12. The interlock has a peak for d = 8.25 mm. Effects of depth and diameter of die on interlock obtained from calculation for lower SPFC780. Table 3 Depth and diameter of die determined from finite element simulation for lower high strength steel sheets. Lowe sheet
Depth of die h (mm)
Diameter of die d (mm)
Bottom of die
SPFC980 SPFC780 SPFC590 SPFC440 SPCC
1.6 1.8 1.8 1.8 1.8
8.25 8.25 8.0 8.0 8.0
Flat Flat Groove Groove Groove
Fig. 14. Relationship between interlock and tensile strength of lower high strength steel obtained from experiment.
The effects of the depth and diameter of the die on the interlock obtained from calculation for lower SPFC780 are illustrated in Fig. 12. The interlock has a peak for d = 8.25 mm. The depth and diameter of the die determined from the finite element simulation for the lower high strength steel sheets are given in Table 3. The depth for the lower high strength steel is larger than that for the upper high strength steel sheets due to the elimination of groove. Below SPFC590, the bottom of the die has a groove, because the fracture does not occur. 4.3. Joinability for present die The joining range for various thicknesses obtained from experiment for the lower SPFC780, d = 8.25 mm and h = 1.8 mm is illustrated in Fig. 13. Although the radial cracks were observed for the conventional die, the occurrence of cracks was prevented by the present die. When the thickness of the lower sheet is excessive large, the interlock is not generated by insufficient material flow between the corners of the punch and die because of the small flow stress of the aluminium alloy sheet. The difference of flow stress between the high strength steel and aluminium alloy sheets is also a problem for the joining.
Fig. 15. Maximum load measured from cross-tension test of joined high strength steel and aluminium alloy sheets.
The relationship between the interlock and the tensile strength of the lower high strength steel obtained from the experiment is shown in Fig. 14. As a comparison, the relationship for the conventional die is added. As the tensile strength increases, the interlock decreases. The joining range is extended by the present die for control of metal flow, and the joining becomes possible even for the lower SPFC980. The maximum load measured from the cross-tension test of joined high strength steel and aluminium alloy sheets is shown in Fig. 15, where Steel-A5052 and A5052-steel denote the upper and lower high strength steel sheets, respectively. For all combinations, the aluminium alloy sheet fractured. As the tensile strength increases, the cross-tension load decreases due to the reduction in interlock. 5. Conclusions
Fig. 13. Joining range for various thicknesses obtained from experiment for lower SPFC780, d = 8.25 mm and h = 1.8 mm.
To join high strength steel and aluminium alloy sheets, metal flow of the sheets was controlled by optimising a shape of the die. The mechanical clinching has the advantage of simple joining by
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local hemming with a punch and die. The following results are obtained: (1) In the joining for the upper high strength steel sheets, the occurrence of fracture induced by the concentration of deformation around the corner of the punch was prevented by decreasing the depth of the die. (2) In the joining for the lower high strength steel sheet, the occurrence of cracks induced by the tensile stress generated in the bulged bottom into the groove of the die was prevented by eliminating the groove of the die. (3) The depth and diameter of the die were optimised to increase the amount of interlock between the upper and lower sheets. (4) The sheets below SPFC780 and SPFC980 were successively joined with the aluminium alloy sheet for the upper and lower high strength steel sheets, respectively. References Abe, Y., Kato, T., Mori, K., 2007a. Joining of aluminium alloy and mild steel sheets using mechanical clinching. Mater. Sci. Forum 561–565, 1043–1046. Abe, Y., Kato, T., Mori, K., 2007. Finite element simulation of plastic joining processes of steel and aluminum alloy sheets. In: Cesar de Sa, J.M.A. et al. (Eds.), Materials
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Processing and Design: Modeling, Simulation and Application. AIP Conf. Proc., vol. 1, pp. 197–202. Abe, Y., Kato, T., Mori, K., 2009. Self-piercing riveting of high tensile strength steel and aluminium alloy sheets using conventional rivet and die. J. Mater. Process. Technol. 209 (8), 3914–3922. Briskham, P., Blundell, N., Han, L., Hewitt, R., Young, K., Boomer, D., 2006. Comparison of self-pierce riveting, resistance spotwelding and spot friction joining for aluminium automotive sheet. SAE paper 2006-01-0774. de Paula, A.A., Aguilar, M.T.P., Pertence, A.E.M., Cetlin, P.R., 2007. Finite element simulations of the clinch joining of metallic sheets. J. Mater. Process. Technol. 182 (1–3), 352–357. Hamel, V., Roelandt, J.M., Gacel, J.N., Schmit, F., 2000. Finite element modeling of clinch forming with automatic remeshing. Comput. Struct. 77 (2), 185–200. Kleiner, M., Geiger, M., Klaus, A., 2003. Manufacturing of lightweight components by metal forming. Ann. CIRP 52 (2), 521–542. Mori, K., Kato, T., Abe, Y., Ravshanbek, Y., 2006. Plastic joining of ultra high strength steel and aluminium alloy sheets by self piercing rivet. Ann. CIRP 55 (1), 283–286. Pak, S.W., Kwon, S.Y., 1995. Application of Mechanical Clinching Method to Aluminum Hood, SAE Tech. Pap. Ser. (Soc. Automot. Eng.) 950714. Varis, J., 2003. The suitability of clinching as a joining method for high-strength structural steel. J. Mater. Process. Technol 132 (1–3), 242–249. Varis, J., 2006. Economics of clinched joint compared to riveted joint and example of applying calculations to a volume product. J. Mater. Process. Technol. 172 (1), 130–138. Varis, J.P., Lepistö, J., 2003. A simple testing-based procedure and simulation of the clinching process using finite element analysis for establishing clinching parameters. Thin-Wall. Struct. 41 (8), 691–709. Voelkner, W., 2000. Present and future developments of metal forming: selected examples. J. Mater. Process. Technol. 106 (1–3), 236–242.