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Journal of Manufacturing Processes 35 (2018) 254–260
Contents lists available at ScienceDirect
Journal of Manufacturing Processes
journal homepage: www.elsevier.com/locate/manpro
Durability of orbital riveted steel/aluminium joints in salt spray
environment
T
⁎
G. Di Bellaa, , C. Borsellinob, L. Calabreseb, E. Proverbiob
a
b
NAVTEC, c/o CNR ITAE, Via Comunale S. Lucia 40, Messina, 98125, Italy
University of Messina, Contrada di Dio, Messina, 98166, Italy
A R T I C LE I N FO
A B S T R A C T
Keywords:
Joining
Durability
Orbital forming
Corrosion
In this work, aluminium alloy/steel joints were realised with an orbital riveting process. Two series of joints were
made; i.e. in the first, the head of rivet touches with the aluminium sheet; in the second, it touches with the steel
one. After their manufacturing, during tests were carried out by exposing the joints to a salt spray fog up to
10 weeks into a climatic chamber. Finally, mechanical tests were performed with the aim to know the effect of
corrosion on performance and failure.
1. Introduction
Nowadays, industries develop innovative leading technologies to
respond to market requirements [1]. In joining technology, riveting is
significant in various industries (i.e. especially in aeronautical industry
and, particularly, in the connection among the sheets in aircraft).
Therefore, many progresses have been done for its improvement in
terms of performances (i.e. resistance and durability of the joint). The
process of riveting presents different procedures; i.e. pop riveting, press
riveting, explosion and radial frictional riveting [2].
The main critical issue is the resistance and the sensitivity to failure.
In particular, this depends directly on tension status of the joint.
Various parameters (i.e. riveting load, friction ratio and tolerance) affect directly on the stress and the fatigue life of the riveted joint.
To increase the joint properties, in pressing operations, Collette et al
applied non-uniform-impact force [3]. In such methods, study on rivet
microstructure [4], evidenced the starting of numerous cracks and the
dissolution of rivets’ structure. Moreover, after applying desired forces,
it much needed to consider tension [5], torsion tolerance [6] and residual stress in work pieces. According to experiments conducted on
sheets connected by changing the type of riveting, sheets riveted by
pressing operations [7] respect to modern methods, are characterised
by a less dissolution of rivets’ particle structure.
A new vision in manufacturing, including just-in-time (JIT) manufacturing and measurable process control and the requirement of a joint
with less residual stress, increased the use of a new technology: the
orbital forming.
This is a cold forming process, alternative to conventional fastening
⁎
operations (i.e. staking, peening, crimping, pressing, swaging, spinning,
rolling, riveting, welding, upsetting) [8].
It is comparable to impact and compression forming, where is applied with a tool a compressive axial load to deform the piece. The
difference with the previous processes is that, in orbital forming, the
tool rotates at a fixed angle (i.e. typically 3° to 6°) and applies axial and
radial forces to progressively deform material until the specific shape is
reached (see Fig. 1 [9]). Moreover, the process requires more tool revolutions and typically takes 1.5–3.0 s to complete.
During the process, the deformation work interests only the tool/
rivet line of contact, not the whole tool surface.
This fact reduces axial loads of about 80% by inducing several advantages; i.e.:
• lower level of stress on the parts that have to be fastened or mated;
• smooth surface finish;
• elimination of cracks caused by impact riveting;
• cold-head forming by avoiding bending or swelling of the fastener
shank;
• use of smaller presses in terms of sizes and costs;
• less rigid fixturing and longer lasting tools.
The process is employed with different materials; i.e. metals (ferrous
and nonferrous) and plastics [10].
However, the industrial applicability of this joining technology is
strongly limited by highly aggressive environmental conditions that can
induce localised corrosion mechanisms [11]: i.e. the junction between
dissimilar substrates (i.e. steel/aluminium) can induce corrosion
Corresponding author.
E-mail address: guido.dibella@itae.cnr.it (G. Di Bella).
https://doi.org/10.1016/j.jmapro.2018.08.009
Received 17 April 2018; Received in revised form 11 July 2018; Accepted 8 August 2018
1526-6125/ © 2018 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.
Journal of Manufacturing Processes 35 (2018) 254–260
G. Di Bella et al.
Fig. 2. Geometry of substrate [mm].
Fig. 3. Geometry of rivet [mm].
The rivet was realised with a S11aluminium alloy.
Fig. 1. Orbital forming.
2.2. Geometry
phenomena for galvanic effects. In fact, these metals have a quite different electrochemical behaviour [12]. This phenomenon can be intensified by crevices (i.e. crevice corrosion attack) or irregularities. For
this fact, in joining design, the metal sheets have to be chosen by
evaluating also electrochemical and corrosion properties [13]. Moreover, internal stresses can influence the durability. This can induce
starting and diffusion of local cracks (i.e. effect of stress corrosion
cracking - SCC), by reducing the joint resistance by increasing the
ageing times [14].
Porcaro et al. [15] shown that the mechanical strength of aluminium riveted joints is stable after 3 days of natural ageing. Moroni et al.
[16] investigated the behaviour of hybrid adhesive-mechanical joints
after thermal cyclic ageing.
Calabrese el al. [14] showed that, after long durability time (i.e. 60
days), the mechanical performance of aluminium self-riveting joints
decreases significantly, by evidencing that the corrosion phenomena
influence performance and failure.
Although the durability of dissimilar joints in a corrosion fog is well
known, few works operate on the ratio between joint durability and
electrochemical characteristics of the metal [13,14].
In particular, the goal of this research is to study the performance of
a hybrid joint between an aluminium sheet and a steel one, realised
with an orbital forming process by focusing the attention not only on
the mechanical resistance but mainly on the durability in salt spray fog.
This work follows other studies performed by the Authors that in
recent years have investigated several joining techniques between dissimilar materials: i.e. self-piercing riveting [17,14], clinching and [18],
clinch-bonding [19].
In Figs. 2 and 3 are reported, respectively, the geometry of the
substrate and the rivet.
The thickness of the substrate is 1 mm for steel alloy and 2 mm for
aluminium one.
Two configurations of joints have been made: in the first, the rivet
part subjected to orbital forming touches with the aluminium sheet (in
the next, series A); in the second, it touches with the steel (in the next,
series F).
2.3. Joining process
Orbital forming was performed using a BK-TAUMEL “BK80” machine (Fig. 4). Its characteristics are reported in Table 1.
Table 2 reports the setup parameters; i.e. F is the punch force that
deforms the rivet, t is the working time and Δx is the displacement of
the punch.
2. Experimental setup
2.1. Materials
The substrates of the joint were realised using, respectively, a 6082
aluminium alloy, subjected to a heat treatment process (i.e. T6), and a
steel alloy A570.
6082 aluminium alloy is characterised by a good strength and a
really good corrosion strength. It is the better of the 6000 series alloys
and, for this fact, is used mainly in structural applications.
In plate form, the alloy is used for machining.
Carbon steel A570 is widely used in production. It has good corrosion strength, high harness, toughness and strength.
Fig. 4. BK-TAUMEL “BK80” machine.
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Journal of Manufacturing Processes 35 (2018) 254–260
G. Di Bella et al.
Table 1
Characteristics of the BK-TAUMEL “BK80” machine.
Technical features
BK80
Max rivet shaft diameter
(with material strength 400 N/mm2)
Stroke min - max
Max push strength
Motor output
Air pressure
Air consumption
8 mm
0–40 mm
13.5 kN
0.37 KW
5.2 bar
10.4 Nlt
Table 2
Setup parameters.
F [kN]
t [s]
Δx [mm]
8
4
0.85
Fig. 5. Typical load/displacement curves for un-aged samples.
2.4. Corrosion treatment
After the orbital riveting, the samples were placed into a climatic
chamber and corrosion tests are performed in according with ASTM B
117 standard. In particular, the main conditions are: i) a 5% NaCl solution with a pH of 6.5–7.2 as salt spray fog and ii) a temperature of
35 °C. This creates a controlled corrosion environment that was used to
obtain important information about the corrosion behaviour of the
exposed joints. After each week, 5 specimens for each series were removed and tested. Such samples were cleaned out, dried and stored in
plastic bag with desiccant silica gel.
70 samples has been realized (five for each series and for each time
interval).
The aim of corrosion testing is to evaluate how the fracture mechanisms of hybrid joints are affected by the environmental conditions
where they work. Therefore, the corrosion mechanisms within the joint
were investigated to study the ratio between the corrosion degradation
mechanisms and the failure ones.
•
A similar discrimination in three main stage can be defined for the
load displacement trend observed in series F. In particular:
• Concerning the first stage, after the mechanical set-up adjustments,
2.5. Single lap shear test
•
Mechanical tests were performed in according to ISO/CD 12996, by
an UTM (Zwick-Roell Z600) with a 50 kN load cell. The cross-head rate
is equal to 1 mm/min.
2.6. Analisys of joints
•
To analyse the corrosion effect at the materials interfaces, the joints
were cut obtaining the Cross-section profiles. The cross section was
investigated by a Zeiss Stemi 2000-C stereomicroscope.
3. Results and discussion
In Fig. 5, typical load/displacement curves related to the un-aged
samples for both series are reported.
For the joints of series A, it is possible to identify several regions:
• Initial region. In this stage two sub-steps can be identified. Firstly,
•
contact for effect of a bending deflection at the ends of the sheets
caused by the joint asymmetry. In this phase, the performance of the
joint depends only on the rivet and the resistance is bear by the
joining point. Secondly, the load reaches the maximum value, and
by increasing the deformation of the joint, a plateau is shown evidencing a progressive joint failure. This region is significant because
of a bearing phenomenon around the button on steel side produced
by a sliding action, where the steel sheet progressively and plastically deforms, the steel hole loses its circularity (Fig. 6).
Residual resistance stage. The ΔP/ΔL ratio reduces over 50% respect
its maximum value by evidencing that the joint results critically
damaged. The residual strength progressively decreases, a drastic
reduction of the load is not evident, but it is possible to notice a
gradual evolution of the damage. Finally, for higher deformation, a
complete joint fracture occurs for net tension with a drastic load
reduction (Fig. 6).
we observe a linear relationship between load and displacement,
related to joint stiffness. The curve slope is quite similar to that
noticed for the series A.
Maximum load stage. In this stage, the non-linear trend is not evident and, after the stage I, the load reaches maximum value that is
lower than the one observed in the series A. This phenomenon takes
place for the start of cracks induced by the orbital forming on the
steel sheet. The higher workability of aluminium avoids the creation
of these cracks on the samples of the series A.
Residual resistance stage. The ΔP/ΔL ratio reduces over 50% compared with its maximum value by evidencing that the joint results
critically damaged. Load progressively decreases for the diffusion of
the cracks but the failure does not drastically occurs due to the
bearing around the button on steel side - caused by a sliding action –
(Fig. 6). This failure mechanism is in competition with the cracks’
propagation. The load slightly increases and, then, it decreases again
with a gradual evolution of the damage. This causes the complete
joint fracture that takes place exclusively for bearing.
Further information concerning the mechanical properties of hybrid
joint configuration can be obtained by analysing in detail Fig. 6. In
particular, although both the series shown a fracture that interests the
steel sheet, the mechanical collapse is affected by the joint configuration. In the joint A, where the steel sheet is between the rivet head and
the aluminium one, the fracture occurs for mixed bearing and net
tension mode. This failure mechanism could be caused by the interlocking that avoids the unbuttoning of steel sheet. Thus, a progressive
bearing plastic deformation of steel sheet is evident in the hole area
until the collapse for net tension. Whereas, in the joint F, the steel sheet
the trend is related to mechanical adjustments (i.e. presence of
clearance and off-axis). This sub-step is not representative and it is
not considered for performance evaluation of orbital riveted joints.
In the second sub-step a load linear increase versus displacement is
observed; i.e. the joint strength is due to the shear resistance of the
rivet.
Maximum load stage. Also in this case, two sub-steps can be defined.
Firstly, the load–displacement curve become not-linear with a progressive reduction of the slope ΔP/ΔL. The sheets progressively loses
256
Journal of Manufacturing Processes 35 (2018) 254–260
G. Di Bella et al.
Fig. 6. Typical failure mechanisms of un-aged samples.
unbuttoning is possible by considering the geometry configuration. In
fact, a local plastic deformation in the hole area, due to an extensive
bearing collapse of the steel sheet, is evident. The unsymmetrical configuration of the single lap joint configuration induces a joint twisting
that potentially, at longer displacement, can induce an unbuttoning
along the rivet shaft. These considerations can be confirmed by considering that the joints of the two series are characterised also by different values both of maximum load and displacement. In particular,
the curve of series A is characterised by a wider area under the curve.
Then, these joints requires a higher energy before reaching the failure.
This behaviour is a consequence of the different failures mechanisms,
observed in Fig. 6.
In Fig. 7 are reported the typical load/displacement curves related
to the samples by varying the ageing time for both the series.
In particular:
• For series A (Fig. 7a): the curves present different trends by chan•
ging the time of corrosion. It is possible to evidence three groups: (i)
the curve of the un-aged sample; (ii) the curves of the treated
samples after 1, 3, 4 and 6 weeks; (iii) the curves of the treated
samples after 8 and 10 weeks.
For series F (Fig. 7b): the curves present similar trends where the
Fig. 8. Analysis of failure mechanisms: (a) series A; (b) series F.
one related to the un-aged sample gradually modify in the curve
related to the treated sample after 10 weeks
This can be explained by analysing the failure mechanisms by
changing the time of corrosion, as evidenced in Fig. 8 for both the
series.
As for the series A, when the sample is not corroded (i.e. week 0),
the failure occurs for bearing and net tension (Fig. 6). The cracks develop from the hole along an orthogonal direction than the load axis.
Moreover, it is evident also the bearing along the load axis on the steel
sheet. At this stage the steel become the critical substrate of the joint.
Between 1 and 6 weeks, the failure takes place mainly for unbuttoning and, in few cases, for bearing (Fig. 9). The unbuttoning is
characterised by three steps: (i) the sample bends because of the deformation of the sheet; (ii) this last causes the load transfer to the rivet
head until its failure; (iii) the sheets pull out of the rivet shank. The
bearing interests the steel sheet.
After 8 weeks the failure mechanism is different. Failure occurs
mainly for shear out of the aluminium sheet (Fig. 9). Occasionally,
bearing was also observed. This is due to the degradation mechanism
occurring during ageing time. Aluminium sheet acts as anode and steel
sheet as cathode for effect of the galvanic coupling. Thus, a rapid
Fig. 7. Typical load-displacement curves by varying the corrosion time: (a)
series A; (b) series F.
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Journal of Manufacturing Processes 35 (2018) 254–260
G. Di Bella et al.
Fig. 9. Typical failure mechanisms – Series A.
Fig. 10. Typical failure mechanisms – Series F.
maximum load with its standard deviation is reported by increasing
ageing time.
In such Figure it is evident that the mean maximum load is higher,
after 1, 3 and 4 weeks, than the untreated sample. This fact is caused by
the formation of corrosion products that in an initial phase promote the
joining (i.e. interlocking effect [18,19]). Then, by increasing the corrosion time, the maximum load decreases due to the damage of substrates and rivets. In addition is it evident that the joint of series F
present lower mechanical properties, due to the different deformation
of the sheet steel.
To know how the corrosion affects the samples’ behaviour, the
section of the joints were investigated. In particular, Fig. 12 reports the
sections of all the joints by varying ageing time and series.
In Figure are evident the corrosion products. Their volume increases
by increasing the ageing time. The steel corrosion products are brown
and red, whereas the aluminium corrosion products are white.
In the steel/aluminium contact region is evident the galvanic effect.
In fact, this induces the dissolution of aluminium and its thinning, as
evident at the week 10. The steel shows only the creation of a slight
layer of oxide.
Specifically, for series A:
dissolution of anodic metal near the contact region takes place by inducing a premature failure in the hybrid joint.
By analysing joints’ failure mechanisms of series F, the different
configuration (i.e. the rivet part subjected to orbital forming touches
with the steel sheet) induces another trend in the failure modes. In this
case, bearing and unbuttoning are in competition (Fig. 10). For untreated samples and for low corrosion times, failure occurs mainly for
bearing. Whereas, for high corrosion times, failure occurs mainly for
unbuttoning by following the same scheme described for the samples of
series A.
Despite series F, where a progressive evolution from bearing to
unbuttoning of the steel sheet occurs, the series A shows a very high
sensitivity of the failure modes to the ageing time. In fact, firstly, at low
ageing time, a transition from bearing/net tension of steel sheet to
unbuttoning of aluminium sheet takes place. At long ageing time, the
shear out of aluminium sheet became the dominant failure mechanism.
This behaviour is strictly related to the corrosion phenomena. At low
ageing time the creation of corrosion products at the steel/aluminium
interface progressively reduces the joint interlocking forces by stimulating the joint unbuttoning. At long ageing time the galvanic coupling
is the cause of the aluminium degradation and, as a consequence, the
aluminium thin and it breaks for shear-out.
To better evidence the difference, both between the two series and
within each one, regarding the mechanical joint resistance, in Fig. 11
• After 1 week: It is evident the creation of the corrosion products
•
Fig. 11. Maximum load by varying corrosion time.
258
between the sheets where there is maximum contact between the
different materials (i.e. galvanic corrosion). Moreover, the interstice
is really low by promoting a corrosion for crevice. The corrosion
products present high volume and the sheets deform. It is possible to
notice also the pitting localised corrosion caused by the damage of
the oxide layer on aluminium (box A). The chlorides into the salt
spray fog damage the weakest points of this layer. The corrosion
products fill up the gap between sheets and rivet by promoting the
interlocking phenomenon (box B).
After 4 weeks: It is evident the high increase of volume of the products. The corrosion phenomenon interests mainly the aluminium
(box C). The galvanic corrosion, coupled with pitting and crevice,
induces the disappearance of an entire surface layer of aluminium. It
is possible to notice also the creation of corrosion products between
the formed rivet and the aluminium due to the different alloys that
constitute the two parts (box D).
Journal of Manufacturing Processes 35 (2018) 254–260
G. Di Bella et al.
Fig. 12. Sections of joints.
• After 6 weeks: Corrosion products further increase and the rivet
•
•
failure occurs for bearing (low corrosion time) and unbuttoning (high
corrosion time).
For both series, at low corrosion time, the strength of the joints
slightly increases for effect of the interlocking phenomenon.
The corrosion test evidenced that joints of series A are characterised
by better performance because the rivet part subjected to orbital
forming is highly preserved.
head presents localised corrosion for pitting caused by the chlorides
that attack the oxide layer (box E). Moreover, the aluminium tins
due do the metal dissolution caused by the galvanic coupling.
After 8 weeks: In the photo is evident the thinning of the aluminium
steel and the anodic behaviour of this last preserves the alloy of the
rivet to resists corrosion.
After 10 weeks: It is possible to notice the galvanic corrosion of the
rivet head that touches with steel substrate.
References
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4. Conclusions
From the analysis of the data from experimental tests conducted on
hybrid steel/aluminium joints it is possible to observe that: the mechanical behaviour of the joints, realised by orbital forming, are influenced by both the corrosion time and the configuration (i.e. position
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The joint configuration affect both the failure modes for the untreated samples (bearing/net tension of A versus bearing/potentially
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