2006-01-0775
Effect of sheet material coatings on quality and strength of
self-piercing riveted joints
L. Han*, K.W. Young, R. Hewitt, M. R. Alkahari
Warwick Manufacturing Group, University of Warwick, Coventry, CV4 7AL, UK
A. Chrysanthou
School of Aerospace, Automotive and Design Engineering, University of Hertfordshire, Herts, AL10 9AB, UK
Copyright © 2005 SAE International
ABSTRACT
A study examining the effect of coatings on the
quality and behaviour of self-piercing riveted joints
joining aluminium NG5754 to HSLA350 sheets was
conducted. Two types of coatings: E-coated and Zinc
plated were applied to the HSLA350 sheet for this
study. The results showed that coating affected the
joint quality in terms of head height, interlock and
remaining material thickness. Examination of joint
sections indicated that the self-piercing riveting
process needed different setting parameters for coated
and uncoated sheets in order to achieve optimum
setting conditions. The joint strength was also
affected by sheet coatings. However, whether the
joints with coated sheet had higher strength was
dependent on joint setting conditions. Different
coatings had different effects on joint quality and
behaviour and different setting conditions led to
different behaviour of the joints with or without
coatings.
INTRODUCTION
Self-piercing riveting (SPR) is a relatively new
joining method in the automotive field where it can be
used to join thin sheet material [1, 2]. It differs from
conventional riveting in that it does not require a predrilled hole [3]. The technique involves clamping the
sheets to be joined between a blank-holder and an
upset die and forcing a rivet to pierce the upper sheet
and flare into the bottom sheet under the influence of
an upset die. A button is left on the underside of the
bottom sheet due to the effect of the upset die. The
joint can be set flush in one side if a countersunk rivet
is used. Oval head rivets can also be used if non-flush
on both sides is acceptable. Ideally, the rivet tail
should not break through the bottom sheet. The
technique has many advantages. Apart from no predrilled hole requirement, a wide range of materials
can be joined, including combinations of similar or
dissimilar materials, such as aluminium to steel. In
comparison to spot-welding, the process is
environmentally friendly due to the low energy
requirement, no fume and low-noise emissions and it
involves no heating. Research in this area has shown
that self-piercing riveting of aluminium alloys gives
joints of comparable static strength and superior
fatigue behaviour to spot-welding [4, 5]. Therefore
the technique offers a solution to the automotive
industry as the increasing use of lightweight
aluminium alloys for automobile body-in-white
applications needs to be balanced against the wellknown problems of spot-welding of aluminium alloys
[3].
The use of steel as a reinforcement in aluminium
body assembly has raised corrosion concerns due to
galvanic effects. Coating is therefore desirable in
order to prevent corrosion. At the same time, coating
may also improve resistance to wear and provide a
decorative finish. One of the widely used coatings for
automotive parts to enhance corrosion resistance is
electrophoretic-coating (E-coat). An organic polymer
is used and applied to conductive parts under a
medium-range voltage and relatively high electrical
current [6]. There are four common types of Ecoatings; anodic epoxy, anodic acrylic, cathodic
epoxy and cathodic acrylic. For automotive parts,
cathodic epoxy is the most common application.
Another means of corrosion protection for steel parts
is by zinc plating (Zn-plate) [7]. Zinc-tin alloy is
commonly used. It is applied to the steel surface by
electroplating and protects it from direct contact with
air and oxygen and therefore from corrosion. If
scratched, zinc being more electronegative than mild
steel, will oxidize repairing itself in the process. Both
E-coat and zinc plate can be successfully applied on
HSLA parts. SPR is capable of joining dissimilar
materials and combinations as well as coated material
[8, 9]. However, how the coatings would affect the
quality and behaviour of the SPR joints has not
previously been reported in public domain. The aim
of the study that is reported here was therefore to
examine the effects of coatings on the quality and
mechanical behaviour of self-piercing riveted joints in
aluminium alloy sheets joined with HSLA which had
been coated with E-coat or Zn-plate.
EXPERIMENTAL PROCEDURE
Aluminium alloy NG5754 in wrought form of
1.8mm and 2.0mm gauge thickness and high
strength low alloy steel (HSLA350) of 1.2mm and
2.5mm gauge thickness were used to produce
(1.8mm+1.2mm) and (2.0mm+2.5mm) joints for
this investigation. The joint material combination
involved the aluminium alloy as the top or pierced
sheet and the HSLA350 as the bottom or locked
sheet. The aluminium alloy NG5754 had previously
been pre-treated with a thin chromate-free film and
a wax-based solid lubricant. The 1.2mm HSLA350
had an E-coat whilst the 2.5mm HSLA350 had a
Zn-plate and non-passivated coating. The
compositions and mechanical properties of the
aluminium and the HSLA350 sheet materials are
listed in Table 1 and 2 respectively.
Table 1: Composition and mechanical properties
of 5754 alloy
MECHANICAL PROPERTIES
Yield
Tensile
Elongation
Hardness
strength
strength
(HV)
(MPa)
(MPa)
110
240
22%
63.5
NOMINAL COMPOSITION (BALANCE Al) (%)
Si
Fe
Cu
Mn
Mg
0-0.40
0-0.40
0-0.10
0-0.50
2.60-3.60
Table 2: Composition and mechanical properties
of HSLA350
MECHANICAL PROPERTIES
Yield strength
Tensile strength
Elongation
(MPa)
(MPa)
390
430
22%
NOMINAL CHEMICAL COMPOSITION (%)
C
Si
Mn
P
S
Al
Nb
Ti
0.1
0.5
1.2 0.025 0.025 0.015 0.09 0.15
Steel rivets with a countersunk head and mechanical
zinc/tin surface coating were used throughout to join
the sheet materials. The steel rivets were supplied by
Henrob Ltd. Two groups of samples were prepared
for (1.8mm+1.2mm) and (2.0mm+2.5mm) sheet
combinations respectively. For each sheet
combination, the top aluminium sheet was joined with
coated or uncoated HSLA350 sheet respectively. The
setting parameters used for coated and uncoated
samples were the same for (1.8mm+1.2mm) and
(2.0mm+2.5mm) correspondingly. Two set-up
conditions, conditions 1 and 2, were used for
(1.8mm+1.2mm) groups with E-coated or un-coated
HSLA350, whilst only one set up, condition 3, was
applied for (2.0mm+2.5mm) samples with zinc plated
or un-plated HSLA350. The setting parameters for
condition 1 to 3 are listed in Table 3. The rivets used
for condition 1 and 2 had a diameter of 5.0mm and
length of 6.0mm. The same die profiles but with
different die diameter were adopted for conditions 1
and 2. For condition 1, the die diameter was 9mm
whilst 10mm was used for condition 2. For condition
3, 5.0mm diameter and 7.0mm length of rivets were
used together with a die diameter of 9.0mm like
condition 1. Henrob Servo-riveting equipment, which
is force driven, was used to produce all the samples.
In order to achieve around zero head height, different
setting velocities were chosen for different setting
conditions. The setting velocity for the condition1
was 240mm/min, whilst 220mm/min for the condition
2 and 320mm/min for the condition 3.
Table 3: Setting parameters for condition 1 to 3
Condition
Combination
Rivet
diameter
Rivet length
Die diameter
Setting
velocity
Condition 1
1.8mm+1.2m
m
5.3mm
Condition 2
1.8mm+1.2m
m
5.3mm
Condition 3
2.0mm+2.5m
m
5.3mm
6.0mm
9.0mm
240mm/min
6.0mm
10.0mm
220mm/min
7.0mm
9.0mm
320mm/min
The samples were sectioned and macro-inspection
was carried out in order to examine the joint quality.
Ai4 image analysis software was used to measure the
head height, interlock and the remaining material
thickness in order to assess whether the joint quality
had been optimized. Static lap-shear and T-peel tests
were performed using an Instron tensile test machine
with a load capacity of 10kN at a 10mm/min crosshead speed. The dimensions of the specimens used in
the shear test are shown in Fig.1. Fig. 2 shows the
specimen geometry for the T-peel test. At least 6
samples were tested at each condition.
Fig. 1: Specimen geometry for shear and fatigue
tests
Fig. 2: Specimen geometry for T-peel tests
RESULTS AND DISCUSSION
OBSERVATION OF CROSS SECTIONS
Fig. 3 shows typical cross-sections of samples from
condition 1. Ai4 image analysis software revealed the
head height, interlock and remaining material
thickness for both E-coated and un-coated samples.
The results that were averaged from at least 4 samples
are tabulated in table 4. The head height for the Ecoated samples was 0.03mm, whilst a value of
0.08mm was measured for the un-coated samples.
The E-coated samples had a lower head height than
the un-coated samples. The interlock for the E-coated
samples was 0.47mm, which was greater than the
0.34mm interlock distance for the un-coated samples.
However, the section of the E-coated samples is
unsymmetrical. The remaining material thickness for
the E-coated samples was only 0.05mm from
breaking through the bottom on right hand side of the
section, while it was enough on left side. In contrast,
the un-coated samples are symmetrical and had a
much thicker remaining material thickness of
0.29mm. These results indicated the effect of the Ecoat on the joint quality. Under condition 1, the
quality of the uncoated samples was better than that of
the E-coated samples in terms of remaining material
thickness.
(a) U-coated sample
(b) E-coated sample
Figure 3: Section of the samples from condition 1
Fig. 4 shows sections of the samples from condition 2.
Visual inspection indicated that small gaps appeared
at the interface between the two riveted panels for
both E-coated and un-coated samples. These gaps
may lead to corrosion concerns and easy loosening of
the joints under dynamic loading. Ai4 software
revealed the head height, interlock and remaining
material thickness, as listed in table 4. Under this
condition, the E-coated samples are still
unsymmetrical, but the least remaining material
thickness was 0.24mm, which was enough to prevent
the joints from breakthrough. The uncoated samples
had a thicker remaining material thickness of
0.28mm. The head height for the E-coated samples
was 0.05mm which was lower than the value of
0.08mm for the head height measured for the un-
coated samples. The interlock distance for the Ecoated samples was 0.30mm and was marginally
higher than the 0.25mm value that was measured for
the un-coated samples. Under condition 2 and based
on the values of head height, interlock distance and
remaining material thickness, both the E-coated and
un-coated samples had an acceptable joint quality. In
terms of the interlock value, the joint quality for the
E-coated samples was judged to be slightly better than
that for the un-coated samples. These observations
suggested that the optimum setting conditions for the
coated and uncoated HSLA was different. In addition,
under condition 2 both the E-coated and un-coated
samples had smaller interlock values compared with
condition 1 indicating the dependency of the SPR
joint quality on the setting parameters.
for the Zn-plated samples was 0.22mm which was
thinner than the value of 0.59mm measured for the
un-plated samples. The joint quality for both groups
of samples was acceptable. The difference in the head
height, interlock and remaining material thickness
indicated the effect of zinc plating on the joint quality.
(a) Un-plated
(a) Un-coated sample
(b) Zn-plated
Figure 5: Section of the samples from condition 3
Table 4: Measurement results for coated and
Un-coated samples
(b) E-coated sample
Figure 4: Section of the samples from condition 2
Fig. 5 shows typical cross-sections of the Zn-plated
and un-plated samples. The average Ai4 measurement
results of 4 samples are listed in table 4. The head
height for the un-plated samples was -0.08mm which
was very close to the value of -0.10mm measured for
the Zn-plated sample. The slightly more negative
value for the head height for the Zn-plated samples,
resulted in a greater interlock distance for the Znplated samples. A value of 1.07mm was measured for
the Zn-plated samples compared to 0.91mm for the
un-plated samples. The remaining material thickness
Condition
Samples
1
E-coated
/Un-coated
E-coated
/Un-coated
Zn-plated
/Un-plated
2
3
Head
height
(mm)
0.03
/0.08
0.05
/0.08
-0.10
/-0.08
Interlock
(mm)
0.47
/0.34
0.30
/0.25
1.07
/0.91
Remaining
material
(mm)
0.05
/0.29
0.24
/0.28
0.22
/0.59
Self-piercing riveting is a cold forming process.
During this process, the rivet pierces the top sheet
and flares into the bottom sheet. In order to
facilitate the process, the rivet is specially designed
in terms of tip geometry and coating. As a result,
A similar principle may also be applied to the zinc
plated samples. Sadykov and Barykin [10] have
shown that zinc-based coatings can have a lubricating
effect during cold forming of steels. The lubricating
effect of the zinc coating was therefore to reduce the
head height and to increase the interlock distance in
comparison to the joints for the uncoated samples.
The surface roughness test results for Zn-plated and
un-plated 2.5mm HSLA 350 are shown in Fig.7. A
1.409µm Ra value for the zinc coated HSLA350
provided smoother surface compared with the
uncoated 2.5mm HSLA350 which had a 1.431µm Ra
value. The smoother surface of the Zn-plated samples
may also contribute to lower head height, greater
interlock and consequently thinner remaining material
thickness compared with the un-plated samples.
It should be mentioned here, in figures 3(b) and 4(b),
the sections for E-coated samples are unsymmetrical
leading to process stability concern. Although only 4
samples were measured using Ai4 software, more
than 10 samples were sectioned and the appearance of
the sections were similar. However, in condition 2
where the die profile was changed from condition 1,
the unsymmetrical feature is not as bad as in condition
1. This might indicate that the die profile might also
contribute to the asymmetry apart from the coating
effect. For detailed process stability analysis, further
examination is necessary.
1.435
Surface roughness Ra (um)
the resistance encountered by the rivet in piercing
and flaring the sheet material mainly depends on the
material properties. For instance, the harder the
sheet material the more difficult the process is. For
both E-coated and Z-plated samples, the substrate
sheet material properties are the same except for the
surface conditions. During the E-coating process the
organic polymer film was introduced to the surface
of the HSLA 350 sheet. The effect of this was to
alter the surface roughness of the 1.2mm HSLA350
sheet. Figure.6 shows the results of surface
roughness measurements. The mean surface
roughness, Ra, for the E-coated HSLA 350 sheet
was 0.485μm, while a value of 1.424μm was
recorded for the uncoated HSLA 350 sheet. These
results suggested that after E-coating, the surface of
the1.2mm HSLA350 sheet became smoother and
had a lower coefficient of friction. As the rivet
pierced and flared into the HSLA 350, the soft
polymer coating may have acted as a solid lubricant
at the rivet-sheet interface enabling the rivet to
encounter a lower amount of friction. As the
resistance to the rivet during piercing and flaring
was reduced, the rivet was able to lock deeper into
the E-coated HSLA 350 in comparison to the
uncoated samples. The lower head height that was
observed for the E-coated samples was indicative of
the rivet reaching deeper into the locked sheet, thus
achieving
a
greater
interlock
distance.
Consequently, the remaining material thickness for
the coated samples was smaller compared with the
uncoated samples.
1.43
1.425
Coated
Uncoated
1.42
1.415
1.41
1.405
1.4
1.395
(a) E-coated, Ra: 0.485µm
Figure 7: Surface roughness results for zinc
coated and uncoated 2.5mm HSLA350
STATIC BEHAVIOUR OF THE JOINTS
(b) Uncoated, Ra: 1.424
Figure 6: Surface roughness of 1.2 mm E-coated
and uncoated HSLA 350
Fig. 8 shows the shear and peel test results for the Ecoated and un-coated samples that were prepared
using condition 1 and 2. Under condition 1, the Ecoated samples exhibited both higher shear and peel
strength in average than the uncoated samples; the
the riveted sheet for the E-coated samples. The
frictional force which mainly governs the interlock
strength for the E-coated samples was higher than for
the un-coated samples. This was because under the
same setting pressure, the frictional force for both Ecoated and un-coated samples was proportional to the
contact area between the rivet and the sheet. The
greater interlock provided the joints with bigger
contact area between the rivet and the sheet leading to
greater frictional force at this interface for the Ecoated samples. This contributed to the higher
strength for the E-coated samples. However, under
condition 2 the relatively small difference in the
interlock values could not compromise the reduction
in the coefficient of friction and therefore the joint
strength was lower for the coated samples compared
with the uncoated one. These observations indicated
that the overall joint strength was also dependent on
factors other than the interlock value. The very thin
remaining material for the coated samples at
condition 1 led to a crack on the bottom sheet during
shear testing, as shown in Fig.9(a). By contrast, there
was no crack formation at the bottom of the buttonhole, following fracture of the un-coated samples as
presented in Fig.9(b). Under dynamic loading, this
may lead to crack propagation as well as to corrosion
concerns as water or moisture may penetrate into the
joint. This is why the joint quality for the coated
samples at condition 1 was not as good as for the
uncoated samples.
Mean Load (kN)
shear strength was marginally higher by 4%, and the
peel strength by 14%, whilst under condition 2, the Ecoated samples had 17% lower shear strength and 8%
lower peel strength compared with the uncoated
samples. Overall, the samples prepared under
condition 1 exhibited higher strength than the samples
prepared under condition 2. These results indicated
the effect of coating and setting condition on the joint
strength. During shear and peel tests, all samples
failed by means of rivet pull-out of the bottom sheet.
This indicates that the joint interlock strength
dominated the joint strength. In general, the larger
interlock provides the joint with greater strength. The
quality assessment of joints using A4i software had
indicated that the interlock was significantly superior
for the samples prepared under condition 1 in
comparison to condition 2, as listed in Table 3.
Therefore the joints prepared under condition 1 were
expected to have higher strength compared to the
joints prepared under condition 2. The results shown
in Fig.8 support this view. Previous work has shown
that the surface roughness and coefficient of friction
at the interfaces between the rivet-sheet and the two
riveted sheets were important factors in determining
the behaviour of SPR joints [11]. During loading, part
of the load will be taken up in the form of frictional
force at the interfaces between the rivet-sheet and the
two riveted sheets. If the frictional force at the rivetsheet interface and at the interface between the two
sheets is minimized then the joint strength is likely to
be lower. Although the low surface roughness and the
lubrication effects of the E-coating led to a low
coefficient of friction at these interfaces for the Ecoated samples, this was not necessarily accompanied
by a low frictional force. For the samples prepared
under condition 2, a low amount of frictional force at
these interfaces for the E-coated samples
consequently led to the lower strength of these joints
under both shear and peel conditions. This was in
spite of the fact that the joints for the E-coated
samples had exhibited a higher interlock dimension.
For these samples the interlock observed for the Ecoated samples was greater by only 0.05mm
compared to the uncoated samples. In the case of the
samples produced under condition 1, the interlock for
the E-coated samples was greater by about 0.13mm. It
appears that for the samples produced under condition
1, the big difference in the interlock between the
coated and uncoated samples might have
compromised the reduction in the coefficient of
friction at the interface between the rivet shank and
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
0
4%
-17%
coated
uncoated
14%
-8%
Shear
Peel
condition 1
Shear
Peel
condition 2
Figure 8: Shear and peel test results for
condition 1 and 2
peeling. This was because during the riveting process,
the top sheet for the coated samples was pierced by
the rivet head more than that for the uncoated samples
in terms of head height. This contributed to the high
amount of tearing of the top sheet for the coated
samples.
Crack
8000
(b) E-coated condition 2
7000
Mean Load (N)
(a) E-coated condition 1
coated
uncoated
2.7%
6000
5000
4000
3000
-6.5%
2000
1000
0
(c) Uncoated condition 1&2
Figure 9: Failure modes occurred during shear test
Fig. 10 shows the shear and peel test results for the
zinc plated and un-plated samples under condition 3.
The Zn-plated samples had 6.5% lower peel strength
but 2.7% higher shear strength in average compared
with the un-plated samples. In both cases, the
variation in the strength values between the coated
and uncoated samples was insignificant. Fig. 11
shows the failure modes that occurred during the
shear and peel tests for these samples. The interlock
distance for these samples was very high giving very
strong joints. The failure mechanism for these
samples did not involve rivet pull-out. Instead,
peeling of the top sheet was the main failure mode for
both the coated and uncoated samples. This indicated
that the sheet material strength dominated the sample
strength. The effect of the zinc plated coating on the
joint strength was therefore shielded by the failure of
the NG5754 riveted sheet. For this specific sheet
combination, the loading that would be expected to be
taken up by the 2.0mm NG5754 top sheet would be
much lower than that taken by the 2.5mm HSLA 350
bottom sheet. As a result, the 2.0mm NG5754
aluminium sheet underwent plastic deformation
followed by peeling and tearing during the shear and
peel tests. Therefore the strength values for these
samples were actually dependent on the NG5754 top
sheet, rather than the strong interlock of the joints.
However, comparison of Figures 11(a) and 11(b),
show that the amount of tearing of the top sheet for
the zinc coated sample was higher than that for the
uncoated samples, whose top sheet mainly suffered
Peel
Shear
Figure 10: Shear and peel test results for
condition 3
(a) Uncoated in shear
(b) Zinc coated in shear
(c) Zinc coated and uncoated in peel
Figure 11: Failure modes occurred during shear
and peel tests under condition 3
ACKNOWLEDGEMENTS
2. King. R.P, “Analysis and quality monitoring selfpierce riveting process”, PhD dissertation, 1997,
University of Hertfordshire.
3. Barnes. T. A and Pashby. I. R, “Joining
techniques for aluminium spaceframes used in
automobiles, Part Ι- solid and liquid phase
welding adhesive bonding and mechanical
fasteners”, Journal of Materials Processing
Technology, 99 2000 62-71.
4. Krause. A. R and Chernenkoff. A.R.A, “A
comparative study of the fatigue behaviour of spot
welded and mechanically fastened aluminium
joints”, SAE Technical paper, 1995, 950710.
5. Booth. G.S, Olivier. C.A and Westgate. S.A,
Liebrecht. F and Braunling. S, 2000, “Selfpiercing riveted joints and resistance spot welded
joints in steel and aluminium”, SAE Technical
Paper, 2000/01, 2681.
6. Oravitz. J. J, “Electrocoating rectifier PRODUCT
REVIEW”, Metal finishing, 101 [1] 2003.
7. Azzerri. N, Bruno. R, Ferrari. V, Memmi. M and
Splendorini. L, “Electrochemical test methods for
the development of coated steels with improved
corrosion resistance practical application to food
packaging and automotive industry products”,
Materials Chemistry 7 1982 221-240.
8. Sibley. M, “Quick-fix solutions”, Professional
Engineering, 1997.
9. Bonde. N and Grange-Jansson. S, “Self-piercing
riveting in high strength steel – A way to increase
fatigue life”, Advanced Technologies & Process,
IBEC Conference, 25, 1996, 16-20.
10. Sadykov. F. A, and Barykin. N. P, “Effect of state
of surface layer of electodeposited zinc coating on
its anti-friction properties in metal pressureworking”, Trenie I Iznos, 2 1991 1089-1093.
11. Han. L, Chrysanthou. A, O’Sullivan. J. M,
“Fretting behaviour of self-piercing riveted
aluminum alloy joints under different interfacial
conditions”, Journal of Materials & Design, 27
[3] 2006 200-208.
The authors wish to thank West Midlands for funding
the project.
CONTACT
It should be mentioned here, although different
coatings are applied to the surface of sheet material to
prevent the joints from corrosion, after the riveting
process, the coating is damaged as the riveted sheet is
deformed. This can be seen in Figs. 9 (a) & (b), which
show damage to the coating on the bottom sheet. The
loss of coating, even in a very small area will
accelerate corrosion. This is a current project, the
results of which will be reported later.
CONCLUSIONS
This study has examined the effect of two types of
coatings on the joint quality and behaviour. It was
observed that the choice of coating had an important
effect on the joint quality and behaviour. Previous
research reported the effect of the interfacial
conditions on the joint behaviour under dynamic
conditions [11]. The results from the present study
were in good agreement with previous research.
Based on the results obtained from this study, the
following conclusions can be drawn:
ƒ
ƒ
ƒ
ƒ
The type of coatings on the HSLA steel
affected the quality and strength of a SPR
joint.
The extent of the effects on the quality and
behaviour of a SPR joint differed significantly
according to the type of coatings on the
HSLA steel.
The use of different setting conditions and
sheet combinations altered the effect of
coating on the joint strength.
The use of coatings might necessitate the use
of different optimum setting conditions in
comparison to uncoated material.
REFERENCES
1. Han. L, Chen. Y. K, Chrysanthou. A and
O’Sullivan. J. M, “Self-pierce riveting – a new
way for joining structures”, ASME, 2002; PVPVol. 446(2): 123-127.
* Corresponding author
Warwick Manufacturing Group, University of
Warwick, Coventry, CV4 7AL, UK
Tel: +44(0)2476575385
Fax: +44(0)2476575366
E-mail: li.han@warwick.ac.uk