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Plastic Joining of Ultra High Strength Steel and Aluminium Alloy Sheets by Self Piercing Rivet
Plastic Joining of Ultra High Strength Steel and Aluminium Alloy Sheets by Self Piercing Rivet 1
K. Mori1 (2), T. Kato2, Y. Abe1 and Y. Ravshanbek1 Department of Production Systems Engineering, Toyohashi University of Technology, Toyohashi, Japan 2 Nippon POP Rivets and Fasteners Ltd, Toyohashi, Japan
Abstract Ultra high strength steel and aluminium alloy sheets were plastically joined by a self piercing rivet driven through the upper sheet and spread in the lower sheet with a die. The self piercing rivet directly pierces into the sheets without drilling the sheets beforehand unlike the conventional rivets. Insufficient driving though the upper sheet and fracture of the lower sheet occur due to the high hardness and low ductility of the ultra sheet, respectively. An ultra high strength steel sheet having a tensile strength of 980MPa and an aluminium alloy sheet were successfully joined by optimising shapes of the die. Keywords: Sheet metal, Plastic joining, Self piercing rivet
1 INTRODUCTION To reduce the weight of automobiles, the use of high strength steel and aluminium alloy sheets tends to increase because of high specific strength. Particularly, the ultra high strength steel sheets having a tensile strength more than 1GPa are hopeful of the reduction [1]. Since both steel and aluminium sheets are generally used for automobiles, the joining of the steel and aluminium sheets is 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 industry to develop new joining processes of aluminium alloy and steel sheets. The mechanical clinching [2], the friction stir welding [3, 4] and the self pierce riveting [5] have been developed as plastic joining processes for aluminium alloy sheets. In the mechanical clinching, sheets are joined by local hemming with a punch and die. Although the mechanical clinching has the advantage of low running costs, the strength of joint is not high. On the other hand, the spot friction stir welding is a new joining process using frictional heat generated by a rotating tool. In this welding, the speed is not enough to join a lot of points in automobile body panels [3]. The self pierce riveting is a cold process for joining two or more sheets by driving a rivet through the upper sheet and upsetting the rivet in the lower sheet without penetration into the lower one. Since this riveting does not require a pre-drilled hole unlike the conventional riveting, the joining speed is the same level with that of the spot welding, and the equipment is also similar. In the self pierce riveting, the difference between melting points of the sheets is not a problem because of the cold process, and thus it is possible to apply this riveting to the joining of dissimilar sheet metals. Cacko et al. [6] have simulated a self pierce riveting process by the finite element method to examine the effect of difference of flow stress between sheets on deformation behaviour. The authors [7] have categorised defects for the self pierce riveting of aluminium alloy and steel sheets to obtain optimum joining conditions. The self
Annals of the CIRP Vol. 55/1/2006
piercing rivet is mainly applied to joining of aluminium sheets, and partly that of aluminium alloy and mild steel sheets. The ultra high strength steel sheets, however, are too hard to be pieced with a rivet shown in Figure 1. The wall thickness of the leg of the rivet gets large due to the compression without driving through the hard upper sheet, and the rivet easily comes off the sheets without joining. SPFC980, 1.4mm A5052 1.5mm
Figure 1: Compression of rivet leg in self pierce riveting of ultra high strength steel sheet SPFC980 and aluminium alloy sheet A5052. In the present paper, self pierce riveting of ultra high strength steel and aluminium alloy sheets is developed. The deforming behaviour in the riveting process is examined in finite element simulation and an experiment to evaluate optimum joining conditions. 2
SELF PIERCE RIVETING
2.1 Joining conditions The high strength steel sheet SPFC980 and aluminium alloy sheet A5052-H34 were joined with a self piercing rivet. SPFC 980 is categorised as an ultra sheet and has a nominal tensile strength of 980MPa. The mechanical properties of the sheets and rivet are shown in Table 1. The rivet is made of boron steel and is plated with zinc to prevent corrosion after the riveting. The flow stresses of the sheets and rivet were measured from the uniaxial tensile and compression tests, respectively. The rivet has not only hardness to piece the upper sheet but also ductility to spread the leg in the lower sheet. The apparatus used for an experiment of self pierce riveting is illustrated in Figure 2. The rivet has a tubular leg for piercing the sheets. The rivet is driven into the sheets with the punch.
High strength steel sheet SPFC980
Thick- Tensile Vickers Fness strength hardness value n-value [mm] [MPa] [HV] [MPa] 1.0 838 339 1337 0.08 1.4 801 348 1415 0.12 2.3
786
342
1275
0.07
Aluminium alloy sheet A5052-H34
1.0 1.5 2.0 2.5
205 211 196 194
81 80 75 73
379 366 353 370
0.12 0.11 0.11 0.15
Rivet, boron steel
-
1955
505
1960
0.013
the joining, it is not easy to attain both driving and spreading for ultra high strength steel sheets 2 or 3 times harder than the high strength steel sheets. 3
JOINING FOR UPPER STEEL SHEET
3.1 Defects in riveting The upper ultra high strength steel and lower aluminium alloy sheets were joined with a self piercing rivet. For the driving though the hard upper sheet, the defects shown in Figure 4 appear. The leg of the rivet is compressed, fractured and bent due to the hard sheet.
Table 1: Mechanical properties of sheets and rivet. φ7.8 Punch
(a) Good joining (b) Leg (c) Leg (d) Leg bending ( ) compression ( ) fracture ( ) (+)
1.5
Upper sheet Lower sheet Die φ9.8
Figure 2: Apparatus used for experiment of self pierce riveting. The self pierce riveting process of the sheets was simulated by the commercial finite element code LSDYNA. Axi-symmetric deformation was assumed by limiting the calculation to the vicinity undergoing plastic deformation. The cross-sections of the sheets and rivet were divided into quadrilateral elements. Not only the sheets but also the rivet undergoes plastic deformation during the riveting, and the die, punch and sheet holder were assumed to be rigid. The coefficient of friction at the interface between the rivet and sheet in the simulation is 0.20. 2.2 Joining of high strength steel and aluminium alloy sheets The deforming shapes of the sheets and rivet obtained from the finite element simulation of self pierce riveting of high strength steel sheet SPFC440 and aluminium alloy sheet are shown in Figure 3, where s is the punch stroke. First, the tubular leg of the rivet is driven though the upper sheet, and next the leg is spread with the die without penetration in the lower sheet. The rivet without the spreading after the joining easily comes off the sheets by upward tension. Although the driving though the upper sheet and the spreading in the lower sheet are required for Punch Rivet
Holder
Figure 4: Defects for joining of upper ultra high strength steel and lower aluminium alloy sheets. 3.2 Driving though upper sheet For the joining of the ultra high strength steel and aluminium alloy sheets, the increase in strength of the rivet may be effective, whereas the increase brings about the decrease in ductility of the rivet necessary for the spreading. In the present study, the shape of the die was optimised to avoid this problem. In the joining for the upper ultra high strength steel sheet, the driving though the hard sheet is required. When the punch load during the driving though the upper sheet exceeds the strength of the rivet, the leg of the rivet undergoes plastic deformation, i.e. the compression, fracture and bending shown in Figure 4. No plastic deformation during the driving though the upper sheet is required, and thus the criterion for the driving though the upper sheet is expressed by P
(1)
where P is the punch load during the driving though the upper sheet, Yr is the yield stress of the rivet and Ar is the tubular cross-sectional area of the rivet leg. The variation of the punch load with the punch stroke for the joining of the upper ultra high strength steel and lower aluminium alloy sheets obtained from the finite element simulation is given in Figure 5. When the punch load P exceeds the rivet strength YrAr, the thickness of the upper
100 80 Load P [kN]
φ5.3
5
Rivet 4
Sheet holder
60 40 20
SPFC980
YrAr
SPFC440
Die (a) s=0mm
(b) s=1.7mm
(c) s=3.7mm
Figure 3: Deforming shapes of sheets and rivet obtained from finite element simulation of self pierce riveting of high strength steel and aluminium alloy sheets.
0 1 2 3 4 5 (d) s=5.0mm Stroke [mm] Figure 5: Variation of punch load with punch stroke for upper ultra high strength steel and lower aluminium alloy sheets obtained from the finite element simulation.
2.5
0.5 1.5
1.5
(a) Conventional die (b) Optimised die Figure 6: Optimised die for joining obtained from trial and error using finite element simulation. The deforming shapes of the sheets and rivet obtained from the finite element simulation of self pierce riveting of upper ultra high strength steel and lower aluminium alloy sheets for the optimised die are shown in Figure 7. By increase the diameter of the cavity and the depth of the projection, the punch force is reduced, and thus plastic deformation of the rivet during the driving though the upper sheet is prevented. Both driving though the upper sheet and spreading in the lower sheet are attained.
3
Leg Leg bendingfracture
2
Leg compression
1
Good joining
0 1 2 3 Lower sheet thickness [mm]
Upper sheet thickness [mm]
3.3 Optimisation of shape of die As the diameter of the die cavity increases, the punch load decreases due to the relaxation of constraint of deformation and may become smaller than the rivet strength. On the other hand, the punch load is reduced by increasing the depth of the projection in the centre of the die cavity, because the reaction force is added by the contact between the lower sheet and die projection. The diameter of the cavity and the depth of the projection were increased in trial and error using the finite element simulation, and the optimised die shown in Figure 6 was obtained. φ10 φ13 φ2 φ2
The joinability for the self pierce riveting of upper ultra high strength steel and lower aluminium alloy sheets using the optimised die for combinations of different thicknesses is compared with that using the conventional die in Figure 9. The range of the joining is extended by using the optimised die. Although the joining for the thick ultra high strength steel sheets is difficult, the joining for such sheets is hardly employed for automobile parts. Upper sheet thickness [mm]
sheet beneath the rivet leg for the high strength steel sheet SPFC440 becomes considerably small, whereas the driving though the upper sheet is not attained for the ultra high strength steel sheet SPFC980. The decrease in punch load is desired for the driving though the upper sheet.
3
2
1
(c) s=5.7mm
Figure 7: Deforming shapes of sheets and rivet obtained from finite element simulation of self pierce riveting of upper ultra high strength steel and lower aluminium alloy sheets. The effectiveness of the optimised die was evaluated from the experiment of self pierce riveting of upper ultra high strength steel and lower aluminium alloy sheets as shown in Figure 8. The occurrence of the compression of the rivet leg shown in Figure 1 is prevented by using the optimised die.
SPFC980, 1.4mm A5052 1.5mm
Figure 8: Self pierce riveting of ultra high strength steel and aluminium alloy sheet using optimised die.
Good joining
(a) Conventional die (b) Optimised die Figure 9: Joinability for upper ultra high strength steel and lower aluminium alloy sheets for combinations of different thicknesses. 3.4 Strength of joint The strength of joint for the good joining of the optimised die shown in Figure 9 was measured from cross-tension and tension-shear tests. The fractures observed in the two tests are shown in Figure 10. Because the strength of the aluminium alloy sheet is considerably smaller than that of the ultra high strength steel sheet, the fractures occur at the aluminium sheets.
Upper： SPFC980, 1.4mm
Upper： SPFC980, 1.4mm
Rivet
Rivet Lower： A5052, 1.5mm
(a) Cross-tension test (b) Tension-shear test Figure 10: Fractures of joined sheets observed in crosstension and tension-shear tests.
150 Joint strength [MPa]
(b) s=4.3mm
Leg compression
0 1 2 3 Lower sheet thickness [mm]
Lower： A5052, 1.5mm
(a) s=3.1mm
Leg fracture
Lower fracture
120 90 60 30
Up 0 pe 0.5 0 r s 1.0 0.5 [mm] he 1.5 1.0 s et 1.5 nes thi 2.0 2.0 t thick ck 2.5 e ne 2.5 she ss r [m 3.0 Lowe m]
Figure 11: Measured strength of joint from cross-tension test for upper ultra high strength steel sheet.
4
JOINING FOR LOWER STEEL SHEET
4.1 Optimisation of shape of die Although the driving though the upper sheet is easy in the joining of the upper aluminium alloy and lower ultra high strength steel sheets, the fracture of the lower sheet during the spreading of the rivet leg is a problem due to low ductility. In this combination, the defects are categorised in Figure 12.
4.2 Strength of joint The measured strength of joint for the lower ultra high strength steel sheet is given in Figure 15. The measure strength is smaller than that for the upper ultra high strength steel sheets shown in Figure 11. 120
Joint strength [MPa]
The measured strength of joint from the cross-tension test for the upper ultra high strength steel sheet is given in Figure 11. The strength of joint is defined as the maximum load divided by the tubular cross-sectional area of the rivet leg. In all combinations, the fracture occurs at the lower aluminium sheet.
90
Upper fracture Lower fracture
60 30
Up pe 0.0 r s 0.5 0.0 m] he 1.0 0.5 m et 1.5 1.0 ess [ thi 1.5 ickn ck 2.0 h t ne 2.0 et ss 2.5 2.5 r she [m m] 3.0 Lowe
Figure 15: Measured strength of joint from cross-tension test for lower ultra high strength steel sheet. (a) Good joining (b) Sheet outer (c) Sheet (d) Separation ( ) fracture (x) inner fracture ( ) ( ) Figure 12: Defects for joining of upper aluminium alloy and lower ultra high strength steel sheets. To prevent the fracture of the lower sheet, the shape of the die is optimised from the finite element simulation as shown in Figure 13. Since the driving though the upper aluminium alloy sheet is easier and the fracture of the lower sheet is prevented, the diameter of the cavity and the depth of the projection in the die are smaller than those for the upper steel sheet shown in Figure 6.
2
0.9
φ11.5 φ2.3
Figure 13: Optimised die for lower ultra high strength steel sheet.
3 Separation 2
1
Good joining
0 3 1 2 Lower sheet thickness [mm]
Upper sheet thickness [mm]
Upper sheet thickness [mm]
The joinability for the self pierce riveting of upper aluminium alloy and lower ultra high strength steel sheets using the optimised die for combinations of different thicknesses is compared with that using the conventional die in Figure 14. The range of the joining is extended by using the optimised die.
3 Separation 2
1
Sheet inner fracture Good joining
Sheet outer fracture
0 1 2 3 Lower sheet thickness [mm]
(a) Conventional die (b) Optimised die Figure 14: Joinability for lower ultra high strength steel sheet for combinations of different thicknesses.
5 CONCLUSIONS A plastic joining process of ultra high strength steel and aluminium alloy sheets using a self piercing rivet has been developed. Although the problem of different melting temperatures for the aluminium and steel sheets is solved due to the cold joining, the high strength of the ultra sheet makes the plastic joining difficult. To attain the joining for the hard ultra sheet, the shape of the die is optimised by means of the finite element simulation without changing mechanical properties of the rivet. The self pierce riveting is effective in combining high strength steel and aluminium alloy sheets in automobile parts. 6 REFERENCES [1] Mori, K., Maki, S., Tanaka, Y., 2005, Warm and hot stamping of ultra high tensile strength steel sheets using resistance heating, Annals of the CIRP, 54/1: 209-212. [2] Barnes, T.A., Pashby, I.R., 2000, Joining techniques for aluminium spaceframes used in automobiles: Part II adhesive bonding and mechanical fasteners, Journal of Materials Processing Technology, 99/1-3: 72-79. [3] Barnes, T.A., Pashby, I.R., 2000, Joining techniques for aluminium spaceframes used in automobiles: Part I solid and liquid phase welding, Journal of Materials Processing Technology, 99/1-3: 62-71. [4] Chen, C., Kovacevic, R., 2004, Joining of Al 6061 alloy to AISI 1018 steel by combined effects of fusion and solid state welding, International Journal of Machine Tools & Manufacture, 44/11: 1205–1214. [5] Cai, W., Wang, P.C., Yang, W., 2005, Assembly dimensional prediction for self-piercing riveted aluminum panels, International Journal of Machine Tools & Manufacture, 45/6: 695-704. [6] Cacko, R., Czyžewski, P., Kocañda, A., 2004, Initial optimization of self-piercing riveting process by means of FEM, Steel Grips, 2, 307-310. [7] Abe, Y., Kato, T., Mori, K., Wu, X., 2005, Finite element simulation of joining process of aluminium alloy and steel sheets using self piercing rivet, Advanced Technology of Plasticity 2005 (e.d. P.F. Bariani et al.), CD-ROM.
K. Mori1 (2), T. Kato2, Y. Abe1 and Y. Ravshanbek1 Department of Production Systems Engineering, Toyohashi University of Technology, Toyohashi, Japan 2 Nippon POP Rivets and Fasteners Ltd, Toyohashi, Japan
Abstract Ultra high strength steel and aluminium alloy sheets were plastically joined by a self piercing rivet driven through the upper sheet and spread in the lower sheet with a die. The self piercing rivet directly pierces into the sheets without drilling the sheets beforehand unlike the conventional rivets. Insufficient driving though the upper sheet and fracture of the lower sheet occur due to the high hardness and low ductility of the ultra sheet, respectively. An ultra high strength steel sheet having a tensile strength of 980MPa and an aluminium alloy sheet were successfully joined by optimising shapes of the die. Keywords: Sheet metal, Plastic joining, Self piercing rivet
1 INTRODUCTION To reduce the weight of automobiles, the use of high strength steel and aluminium alloy sheets tends to increase because of high specific strength. Particularly, the ultra high strength steel sheets having a tensile strength more than 1GPa are hopeful of the reduction [1]. Since both steel and aluminium sheets are generally used for automobiles, the joining of the steel and aluminium sheets is 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 industry to develop new joining processes of aluminium alloy and steel sheets. The mechanical clinching [2], the friction stir welding [3, 4] and the self pierce riveting [5] have been developed as plastic joining processes for aluminium alloy sheets. In the mechanical clinching, sheets are joined by local hemming with a punch and die. Although the mechanical clinching has the advantage of low running costs, the strength of joint is not high. On the other hand, the spot friction stir welding is a new joining process using frictional heat generated by a rotating tool. In this welding, the speed is not enough to join a lot of points in automobile body panels [3]. The self pierce riveting is a cold process for joining two or more sheets by driving a rivet through the upper sheet and upsetting the rivet in the lower sheet without penetration into the lower one. Since this riveting does not require a pre-drilled hole unlike the conventional riveting, the joining speed is the same level with that of the spot welding, and the equipment is also similar. In the self pierce riveting, the difference between melting points of the sheets is not a problem because of the cold process, and thus it is possible to apply this riveting to the joining of dissimilar sheet metals. Cacko et al. [6] have simulated a self pierce riveting process by the finite element method to examine the effect of difference of flow stress between sheets on deformation behaviour. The authors [7] have categorised defects for the self pierce riveting of aluminium alloy and steel sheets to obtain optimum joining conditions. The self
Annals of the CIRP Vol. 55/1/2006
piercing rivet is mainly applied to joining of aluminium sheets, and partly that of aluminium alloy and mild steel sheets. The ultra high strength steel sheets, however, are too hard to be pieced with a rivet shown in Figure 1. The wall thickness of the leg of the rivet gets large due to the compression without driving through the hard upper sheet, and the rivet easily comes off the sheets without joining. SPFC980, 1.4mm A5052 1.5mm
Figure 1: Compression of rivet leg in self pierce riveting of ultra high strength steel sheet SPFC980 and aluminium alloy sheet A5052. In the present paper, self pierce riveting of ultra high strength steel and aluminium alloy sheets is developed. The deforming behaviour in the riveting process is examined in finite element simulation and an experiment to evaluate optimum joining conditions. 2
SELF PIERCE RIVETING
2.1 Joining conditions The high strength steel sheet SPFC980 and aluminium alloy sheet A5052-H34 were joined with a self piercing rivet. SPFC 980 is categorised as an ultra sheet and has a nominal tensile strength of 980MPa. The mechanical properties of the sheets and rivet are shown in Table 1. The rivet is made of boron steel and is plated with zinc to prevent corrosion after the riveting. The flow stresses of the sheets and rivet were measured from the uniaxial tensile and compression tests, respectively. The rivet has not only hardness to piece the upper sheet but also ductility to spread the leg in the lower sheet. The apparatus used for an experiment of self pierce riveting is illustrated in Figure 2. The rivet has a tubular leg for piercing the sheets. The rivet is driven into the sheets with the punch.
High strength steel sheet SPFC980
Thick- Tensile Vickers Fness strength hardness value n-value [mm] [MPa] [HV] [MPa] 1.0 838 339 1337 0.08 1.4 801 348 1415 0.12 2.3
786
342
1275
0.07
Aluminium alloy sheet A5052-H34
1.0 1.5 2.0 2.5
205 211 196 194
81 80 75 73
379 366 353 370
0.12 0.11 0.11 0.15
Rivet, boron steel
-
1955
505
1960
0.013
the joining, it is not easy to attain both driving and spreading for ultra high strength steel sheets 2 or 3 times harder than the high strength steel sheets. 3
JOINING FOR UPPER STEEL SHEET
3.1 Defects in riveting The upper ultra high strength steel and lower aluminium alloy sheets were joined with a self piercing rivet. For the driving though the hard upper sheet, the defects shown in Figure 4 appear. The leg of the rivet is compressed, fractured and bent due to the hard sheet.
Table 1: Mechanical properties of sheets and rivet. φ7.8 Punch
(a) Good joining (b) Leg (c) Leg (d) Leg bending ( ) compression ( ) fracture ( ) (+)
1.5
Upper sheet Lower sheet Die φ9.8
Figure 2: Apparatus used for experiment of self pierce riveting. The self pierce riveting process of the sheets was simulated by the commercial finite element code LSDYNA. Axi-symmetric deformation was assumed by limiting the calculation to the vicinity undergoing plastic deformation. The cross-sections of the sheets and rivet were divided into quadrilateral elements. Not only the sheets but also the rivet undergoes plastic deformation during the riveting, and the die, punch and sheet holder were assumed to be rigid. The coefficient of friction at the interface between the rivet and sheet in the simulation is 0.20. 2.2 Joining of high strength steel and aluminium alloy sheets The deforming shapes of the sheets and rivet obtained from the finite element simulation of self pierce riveting of high strength steel sheet SPFC440 and aluminium alloy sheet are shown in Figure 3, where s is the punch stroke. First, the tubular leg of the rivet is driven though the upper sheet, and next the leg is spread with the die without penetration in the lower sheet. The rivet without the spreading after the joining easily comes off the sheets by upward tension. Although the driving though the upper sheet and the spreading in the lower sheet are required for Punch Rivet
Holder
Figure 4: Defects for joining of upper ultra high strength steel and lower aluminium alloy sheets. 3.2 Driving though upper sheet For the joining of the ultra high strength steel and aluminium alloy sheets, the increase in strength of the rivet may be effective, whereas the increase brings about the decrease in ductility of the rivet necessary for the spreading. In the present study, the shape of the die was optimised to avoid this problem. In the joining for the upper ultra high strength steel sheet, the driving though the hard sheet is required. When the punch load during the driving though the upper sheet exceeds the strength of the rivet, the leg of the rivet undergoes plastic deformation, i.e. the compression, fracture and bending shown in Figure 4. No plastic deformation during the driving though the upper sheet is required, and thus the criterion for the driving though the upper sheet is expressed by P
(1)
where P is the punch load during the driving though the upper sheet, Yr is the yield stress of the rivet and Ar is the tubular cross-sectional area of the rivet leg. The variation of the punch load with the punch stroke for the joining of the upper ultra high strength steel and lower aluminium alloy sheets obtained from the finite element simulation is given in Figure 5. When the punch load P exceeds the rivet strength YrAr, the thickness of the upper
100 80 Load P [kN]
φ5.3
5
Rivet 4
Sheet holder
60 40 20
SPFC980
YrAr
SPFC440
Die (a) s=0mm
(b) s=1.7mm
(c) s=3.7mm
Figure 3: Deforming shapes of sheets and rivet obtained from finite element simulation of self pierce riveting of high strength steel and aluminium alloy sheets.
0 1 2 3 4 5 (d) s=5.0mm Stroke [mm] Figure 5: Variation of punch load with punch stroke for upper ultra high strength steel and lower aluminium alloy sheets obtained from the finite element simulation.
2.5
0.5 1.5
1.5
(a) Conventional die (b) Optimised die Figure 6: Optimised die for joining obtained from trial and error using finite element simulation. The deforming shapes of the sheets and rivet obtained from the finite element simulation of self pierce riveting of upper ultra high strength steel and lower aluminium alloy sheets for the optimised die are shown in Figure 7. By increase the diameter of the cavity and the depth of the projection, the punch force is reduced, and thus plastic deformation of the rivet during the driving though the upper sheet is prevented. Both driving though the upper sheet and spreading in the lower sheet are attained.
3
Leg Leg bendingfracture
2
Leg compression
1
Good joining
0 1 2 3 Lower sheet thickness [mm]
Upper sheet thickness [mm]
3.3 Optimisation of shape of die As the diameter of the die cavity increases, the punch load decreases due to the relaxation of constraint of deformation and may become smaller than the rivet strength. On the other hand, the punch load is reduced by increasing the depth of the projection in the centre of the die cavity, because the reaction force is added by the contact between the lower sheet and die projection. The diameter of the cavity and the depth of the projection were increased in trial and error using the finite element simulation, and the optimised die shown in Figure 6 was obtained. φ10 φ13 φ2 φ2
The joinability for the self pierce riveting of upper ultra high strength steel and lower aluminium alloy sheets using the optimised die for combinations of different thicknesses is compared with that using the conventional die in Figure 9. The range of the joining is extended by using the optimised die. Although the joining for the thick ultra high strength steel sheets is difficult, the joining for such sheets is hardly employed for automobile parts. Upper sheet thickness [mm]
sheet beneath the rivet leg for the high strength steel sheet SPFC440 becomes considerably small, whereas the driving though the upper sheet is not attained for the ultra high strength steel sheet SPFC980. The decrease in punch load is desired for the driving though the upper sheet.
3
2
1
(c) s=5.7mm
Figure 7: Deforming shapes of sheets and rivet obtained from finite element simulation of self pierce riveting of upper ultra high strength steel and lower aluminium alloy sheets. The effectiveness of the optimised die was evaluated from the experiment of self pierce riveting of upper ultra high strength steel and lower aluminium alloy sheets as shown in Figure 8. The occurrence of the compression of the rivet leg shown in Figure 1 is prevented by using the optimised die.
SPFC980, 1.4mm A5052 1.5mm
Figure 8: Self pierce riveting of ultra high strength steel and aluminium alloy sheet using optimised die.
Good joining
(a) Conventional die (b) Optimised die Figure 9: Joinability for upper ultra high strength steel and lower aluminium alloy sheets for combinations of different thicknesses. 3.4 Strength of joint The strength of joint for the good joining of the optimised die shown in Figure 9 was measured from cross-tension and tension-shear tests. The fractures observed in the two tests are shown in Figure 10. Because the strength of the aluminium alloy sheet is considerably smaller than that of the ultra high strength steel sheet, the fractures occur at the aluminium sheets.
Upper： SPFC980, 1.4mm
Upper： SPFC980, 1.4mm
Rivet
Rivet Lower： A5052, 1.5mm
(a) Cross-tension test (b) Tension-shear test Figure 10: Fractures of joined sheets observed in crosstension and tension-shear tests.
150 Joint strength [MPa]
(b) s=4.3mm
Leg compression
0 1 2 3 Lower sheet thickness [mm]
Lower： A5052, 1.5mm
(a) s=3.1mm
Leg fracture
Lower fracture
120 90 60 30
Up 0 pe 0.5 0 r s 1.0 0.5 [mm] he 1.5 1.0 s et 1.5 nes thi 2.0 2.0 t thick ck 2.5 e ne 2.5 she ss r [m 3.0 Lowe m]
Figure 11: Measured strength of joint from cross-tension test for upper ultra high strength steel sheet.
4
JOINING FOR LOWER STEEL SHEET
4.1 Optimisation of shape of die Although the driving though the upper sheet is easy in the joining of the upper aluminium alloy and lower ultra high strength steel sheets, the fracture of the lower sheet during the spreading of the rivet leg is a problem due to low ductility. In this combination, the defects are categorised in Figure 12.
4.2 Strength of joint The measured strength of joint for the lower ultra high strength steel sheet is given in Figure 15. The measure strength is smaller than that for the upper ultra high strength steel sheets shown in Figure 11. 120
Joint strength [MPa]
The measured strength of joint from the cross-tension test for the upper ultra high strength steel sheet is given in Figure 11. The strength of joint is defined as the maximum load divided by the tubular cross-sectional area of the rivet leg. In all combinations, the fracture occurs at the lower aluminium sheet.
90
Upper fracture Lower fracture
60 30
Up pe 0.0 r s 0.5 0.0 m] he 1.0 0.5 m et 1.5 1.0 ess [ thi 1.5 ickn ck 2.0 h t ne 2.0 et ss 2.5 2.5 r she [m m] 3.0 Lowe
Figure 15: Measured strength of joint from cross-tension test for lower ultra high strength steel sheet. (a) Good joining (b) Sheet outer (c) Sheet (d) Separation ( ) fracture (x) inner fracture ( ) ( ) Figure 12: Defects for joining of upper aluminium alloy and lower ultra high strength steel sheets. To prevent the fracture of the lower sheet, the shape of the die is optimised from the finite element simulation as shown in Figure 13. Since the driving though the upper aluminium alloy sheet is easier and the fracture of the lower sheet is prevented, the diameter of the cavity and the depth of the projection in the die are smaller than those for the upper steel sheet shown in Figure 6.
2
0.9
φ11.5 φ2.3
Figure 13: Optimised die for lower ultra high strength steel sheet.
3 Separation 2
1
Good joining
0 3 1 2 Lower sheet thickness [mm]
Upper sheet thickness [mm]
Upper sheet thickness [mm]
The joinability for the self pierce riveting of upper aluminium alloy and lower ultra high strength steel sheets using the optimised die for combinations of different thicknesses is compared with that using the conventional die in Figure 14. The range of the joining is extended by using the optimised die.
3 Separation 2
1
Sheet inner fracture Good joining
Sheet outer fracture
0 1 2 3 Lower sheet thickness [mm]
(a) Conventional die (b) Optimised die Figure 14: Joinability for lower ultra high strength steel sheet for combinations of different thicknesses.
5 CONCLUSIONS A plastic joining process of ultra high strength steel and aluminium alloy sheets using a self piercing rivet has been developed. Although the problem of different melting temperatures for the aluminium and steel sheets is solved due to the cold joining, the high strength of the ultra sheet makes the plastic joining difficult. To attain the joining for the hard ultra sheet, the shape of the die is optimised by means of the finite element simulation without changing mechanical properties of the rivet. The self pierce riveting is effective in combining high strength steel and aluminium alloy sheets in automobile parts. 6 REFERENCES [1] Mori, K., Maki, S., Tanaka, Y., 2005, Warm and hot stamping of ultra high tensile strength steel sheets using resistance heating, Annals of the CIRP, 54/1: 209-212. [2] Barnes, T.A., Pashby, I.R., 2000, Joining techniques for aluminium spaceframes used in automobiles: Part II adhesive bonding and mechanical fasteners, Journal of Materials Processing Technology, 99/1-3: 72-79. [3] Barnes, T.A., Pashby, I.R., 2000, Joining techniques for aluminium spaceframes used in automobiles: Part I solid and liquid phase welding, Journal of Materials Processing Technology, 99/1-3: 62-71. [4] Chen, C., Kovacevic, R., 2004, Joining of Al 6061 alloy to AISI 1018 steel by combined effects of fusion and solid state welding, International Journal of Machine Tools & Manufacture, 44/11: 1205–1214. [5] Cai, W., Wang, P.C., Yang, W., 2005, Assembly dimensional prediction for self-piercing riveted aluminum panels, International Journal of Machine Tools & Manufacture, 45/6: 695-704. [6] Cacko, R., Czyžewski, P., Kocañda, A., 2004, Initial optimization of self-piercing riveting process by means of FEM, Steel Grips, 2, 307-310. [7] Abe, Y., Kato, T., Mori, K., Wu, X., 2005, Finite element simulation of joining process of aluminium alloy and steel sheets using self piercing rivet, Advanced Technology of Plasticity 2005 (e.d. P.F. Bariani et al.), CD-ROM.