Joining aluminium alloys with reduced

ductility by mechanical clinching

F. Lambiase1, A. Di Ilio and A. Paoletti,

Dept. of Industrial and Information Engineering and Economics, University of L’Aquila, v. G.

Gronchi 18, Nucleo Industriale di Pile, 67100 (AQ), Italy

Abstract

The joinability of aluminium alloy sheets with reduced ductility produced by mechanical clinching

is analysed. A modified tool set geometry was developed to reduce the localization and magnitude of

plastic strain. Sheets pre-heating was adopted to increase the material formability. Optical microscopy

and scanning electron microscopy were utilized to observe possible presence of cracks in clinched

connections and to perform dimensional analysis. Mechanical characterization tests comprising

micro-hardness test and single lap shear tests were conducted to evaluate the influence of the

processing conditions on joints strength.

Keywords: Joining, Clinching, Sheet metal, Formability, Warm deformation, Forming,

Aluminium

1 Corresponding author:

F. Lambiase

francesco.lambiase@univaq.it

Monteluco di Roio, 67040 (AQ), Italy

Tel. N.: (+39) 0862 434343

Fax N.: (+39) 0862 434303

1. Introduction

The employment of materials with high strength to weight

ratio represents a key feature to improve the efficiency in the

transportations industries. In addition, in order to gain the best

performances from sub-assemblies, the joining technologies have

to be improved too. Aimed by such needs, several joining

processes based onto mechanical joining [1-3] have been recently

developed to produce high performing joints within a limited time.

Among these, self-pierce riveting [3-5] and mechanical clinching

are gaining considerable attention because they allow producing

joints with high static strength and fatigue life [6], and require

small processing time [7]. In addition, the employment of such

processes allow overcoming the main problems encountered

during welding process of dissimilar materials as well as materials

with poor weldability, such as aluminium alloys. Referring to

hybrid joints or materials difficult to weld, clinching process

offers additional advantages over conventional mechanical or

adhesive bonding processes, e.g. process cleanness, simplicity of

machines and absence of surface pre-treatments, low cost per joint

and easiness and robustness of the process. Because of these

advantages, clinching process has been extended to thick plates

[8] either to materials other than metals [9-12].

Static and dynamic strength of clinched connections have been

extensively investigated [13-18] as well as free [19] and forced

[20] vibration. The mechanical strength of clinched joints depends

on the interlock dimension between the sheets and the neck

thickness of punch-sided sheet [21].

The dimensions of the clinching tools represents a key aspect

to improve the mechanical strength of clinched connections. To

this end, Lee et al. [22,10] have proposed a method to design the

clinching tools based on the punch-die cavity volume. Since the

complex material flow during the process, such a model represents

a valid design tool especially at the early design phase, while non-

linear FE models should be preferred as a subsequent design step

[23,7,13,15]. The influence of process parameters on quality

criteria [24-26] is essential for the optimization of clinching tools

[27-29]. Once the quality parameters (mainly the interlock and the

neck thickness) are known, the mechanical strength of clinched

connections can be predicted by using analytical models in pull-

out tests [30]. FE models have been also extensively adopted for

this purpose and in the recent past, more complex models

involving material damage have been developed [31-33,29].

One of the main limitations in the employability of clinching

process is represented by the high localization of stress that may

result in occurrence of cracks in both punch-sided [34] and die-

sided [35] sheets. Therefore, materials with low ductility present

additional problems while joined by mechanical clinching with

round tools.

In the present study, the mechanical clinching of a material

with reduced formability is investigated using different

approaches including the adoption of a special designed punch,

the employment of dies with reduced die anvil depth and pre-

heating. The former approach is intended to reduce the high

localization of shear and tear stress in the zone surrounding the

punch corner, the adoption of shallow dies should reduce the

amount of plastic strain while the material pre-heating is aimed at

increasing the material formability.

Optical microscopy and Scanning Electron Microscopy (SEM)

were employed to verify possible occurrence of cracks as well as

perform dimensional characterization of clinched connections.

Mechanical characterization tests on single lap shear joints and

micro-hardness tests were used to compare the joints produced

under different processing conditions.

The main results show that the adoption of punches with

smoother fillet radius and preheating can notably improve the

joinability and the mechanical performances of the clinched joints.

On the other hand, although the adoption of shallow dies can

prevent crack development during the joining process, the

resulting joints exhibit poor mechanical behaviours due to a

reduced interlock.

2. Clinching Process

Clinching consists in producing a mechanical interlock

between two or more metal sheets by using a punch and a die.

During the process (especially in the offsetting phase), the punch

sided sheet in subjected to a severe thinning. In addition, a highly

localized strain is produced around the punch corner that may

result in local crack and even complete breakage of the sheet [34].

The mechanical strength of clinched connections depends on

both neck thickness, tn and the interlock ts: joints having small

values of neck thickness often fail by necking while joints with

small interlock are likely to fail by unbuttoning, as shown in Fig.

1.

Fig. 1 (a) Main geometrical parameters of clinched connections; (b) failure

modes of clinched connections.

Clinched connections on materials with reduced formability

may also produce cracks at the die-sided sheet [35] if the punch-

die cavity volume is not designed properly. Nevertheless, different

solutions can be adopted to join materials with reduced

formability by heating up the sheets before joining [36]. Warm

and hot forming processes resulting from the employment of a

heating source can both reduce the forming forces and increase the

formability of the sheets.

2. Materials and Methods

2.1. Methods for enhancing joinability

Mechanical clinching process involves a localized

deformation near the punch round corners, the magnitude of the

local strain often being larger than unity. Improper design of

clinching tools or reduced ductility of the sheets can lead to the

production of weak joints or even formation of cracks on the upper

sheet during the joining process.

In order to improve the joinability of such materials, the

influence of the following solutions on mechanical strength and

joints profile have been investigated:

reduction in the die anvil depth;

increase of material formability by pre-heating;

reduction of shear by changing the punch geometry;

2.2. Instrumented Clinching

AA6082 T6-alloy sheets of 1 mm in thickness have been

employed in this investigation. Tensile tests according to ASTM

E08 standards were conducted to measure the main mechanical

behaviours of the analysed material, summarized in Table 1, using

a universal testing machine by MTS model 322.31.

Table 1. Mechanical properties of AA6082T6 sheets

E

[GPa]

Rp0.2

[MPa]

Rm

[MPa]

Etot

[%]

Flow

Stress [MPa]

67.2 171 340 10.8 =

467.5 0.098

Clinched connections were produced using an experimental

apparatus mounted on the above-mentioned universal testing

machine MTS 322.31 with 25 kN full scale to measure the load

variation during the joining operation. The clinching process was

performed at a constant punch speed of 1 mm/s.

In the tests involving sheets pre-heating, convection heating

was used to heat up the sheets before the clinching operation. A

common industrial heater gun model Makita HG651CK was

employed. The schematic of the process involving the material

pre-heating is reported in Fig. 2.

Fig. 2 Schematic representation of clinching phases with preheating.

The process begins by pre-heating the upper sheet surface for

a prescribed time; thus, the heater gun is switched off and the

punch rapidly approaches (in 0.5 s) the upper sheet surface. Then

the punch further moves to produce the clinched joint and finally

the punch is retracted. A schematic representation of the

equipment used for preheating and clinching is depicted in Fig. 3.

Fig. 3 Schematic representation of the clinching equipment with preheating

adapter.

The temperature variation produced with different heating

times was measured by means of a thermo-camera model

ThermaCAM S65 HS by Flir Systems; herein, the exposed surface

was preventively covered by graphite powder to reduce the

reflectivity of the aluminium sheets.

To study the effect of the die anvil depth h, three different

extensible dies [37,38] were used (h = 0.6 mm, h = 0.8 mm and h

= 1.1 mm). In addition, two punches were used, as shown in Fig.

4: the former, designated as P1, is a common punch with a

cylindrical tool tip and a relatively sharp fillet radius of 0.1 mm;

the latter, P2, has a truncated cone shape with a smoother fillet

radius of 0.2 mm.

Fig. 4 Main dimensions of clinching tools.

Single lap shear tests were carried out to evaluate the

mechanical properties of the clinched joints on sheets samples 60

× 20 mm2 as illustrated in Fig. 5. The samples were cut from a

unique sheet by CO2 laser. The mechanical tests of clinched joints

were performed on the previous mentioned MTS machine at a

constant velocity of 2 mm/min. For each testing conditions, three

repetitions were performed.

Fig. 5 Schematic representation of specimen used in single lap shear tests.

3. Results and discussion

Fig. 6 depicts the temperature variation of the punch-sided

sheet (referred as joining temperature Tj) achieved with different

heating times th as measured by the infrared camera. The

temperature variation was measured on the developed system. As

can be observed in Fig. 6a, a uniform spatial distribution of

temperature is ensured over the joining area. Fig. 6b depicts the

mean temperature variation with time. As can be inferred, the

temperature rapidly rises in the early 25 s of the heating; therefore,

it saturates since conductive heat exchanges almost compensates

the convective heating. Finally, as the heat gun is switched off, a

rapid reduction in temperature is noted owing to the heat exchange

mainly towards the underlying die.

Fig. 6 (a) Temperature field distribution as measured by infrared camera and

(b) variation of average temperature with time according to the adopted preheating

schemes.

Following clinching tests were performed at three different

temperatures (Tj): room temperature, Tj1 = 120°C (achieved with

a preheating th1 = 12 s) and Tj2 = 180°C (achieved with a

preheating th2 = 50 s). The choice of the higher level for the joining

temperature was mainly driven by the limitations of the adopted

setup that limited the maximum reachable temperature to 180 -

185 °C. On the other hand, the joining temperature Tj1 was chosen

as the intermediate value between room and temperature Tj2.

3.1. Clinching with conventional punch (P1)

Fig. 7 depicts the variation of the force during the clinching

joining using three dies having different depth and the punch P1.

As can be inferred, the load-displacement curves using the three

dies are qualitatively similar. Indeed, the load slightly increases as

long as the lower sheet reaches the die anvil; from this point, a

steep increase in the force is shown due to the high hydrostatic

stress, which is developing (offsetting phase). The first path of the

three curves is similar, this is because the die-sided sheet is not yet

in contact with the die anvil and the arising deformation is almost

identical. Differently from the curves produced with the shallow

anvils (h = 0.8 mm and h = 0.6 mm), the force pertaining to h =

1.1 mm exhibits a gentle drop before the lower sheet contacts with

the die anvil.

Fig. 7 Variation of joining force with punch stroke using the standard punch

P1 and different dies.

By observing the macrographs of the joints cross sections

reported in Fig. 8, it could be concluded that the force reduction

can be ascribed to the damage initiation and subsequent

development in the punch-sided sheet, which finally results into

crack formation. Using the die with h = 1.1 mm, the upper sheet

completely separates before the onset of the upsetting phase. In

addition, the crack development initiates before the formation of

the interlock between the sheets (which is produced by the sheets

upsetting), therefore, under the above-mentioned conditions, the

clinching die with h = 1.1 mm cannot be used to join the AA6082

alloy at room temperature. Although in a minor manner, also the

joint produced with the die having h = 0.8 mm shows a relatively

deep crack near the punch-sided sheet neck which leads to the

production of a relatively weak joints under static or fatigue loads.

As a matter of facts, the only specimen that is exempt from cracks

is that produced with the shallowest die (h = 0.6 mm). However,

as it can be observed from Fig. 8, the adoption of such a die leads

to the formation of a very small interlock (ts = 0.03 mm), which

affects the joints strength.

Fig. 8 Cross sections of clinched connections produced with the punch P1 and different dies at room temperature.

Scanning electron microscopy was used to analyse the fracture

zone on the upper sheets. As can be inferred from Fig. 9a and 9b,

the upper sheet of the joint produced with the deeper die (h = 1.1

mm) exhibits a crack which originates from concentration of

deformation around the punch corner during the offsetting phase,

as reported by Abe et al. [34]. This crack stems from the relatively

high depth of the die anvil and the sharp corner radius of the

punch. As can be noted in Fig. 9d, the employment of dies with

deep die anvils (h = 1.1 mm) may result in crack development in

the die-sided sheet due to high positive hydrostatic stress. Indeed,

during the upsetting phase, the die sided sheet undergoes to a

severe radial spread leading to the development of high tensile

circumferential stresses which finally may cause radial cracks.

Similar conditions were observed in some experiments conducted

by Mucha in [2] and Abe et al. [35] while joining high strength

steel sheets. The adoption of shallower dies relieves the strain

around the punch corner; thus, it may prevent cracks formation in

the punch-sided sheet, as shown in Fig. 9c.

Fig. 9 SEM macrographs of clinched connections: punch-sided surface (punch

P1 die with h = 1.1 mm) (a) low magnifications, (b) high magnifications on the

crack zone, (c) punch sided surface (punch P1 with h = 0.6 mm) high magnification

on neck and (d) die sided surface (punch P1 with h = 1.1 mm) with low magnification.

3.2. Clinching with customized punch (P2)

In order to overcome the limitations encountered when using

the customized punch P1, a new punch, called P2, was used for

further tests. The new punch differed from the previous one by

with conical shape and a larger fillet radius R = 0.2 mm, which is

intended to spread the contact force arising at the punch-sheet

interface over a larger area.

Fig. 10 depicts the cross sections of the clinched joints realized

with the three types of die. Using the punch P2, the joints produced

with the die with h = 0.8 mm were free from cracks; however, the

joints produced with the deeper die (h = 1.1 mm) still showed a

thought crack, as shown in the macrograph in Fig. 10a both the

sheets exhibit a crack.

Fig. 10 (a) Macrograph of clinched connection produced with the punch P2

and the die with h = 1.1 mm (the lower sheet was metallized to better distinguish

between the sheets) (b) Protrusion of the die-sided sheet. (c-e) Cross sections of

clinched connections produced with different die depths h.

Fig. 11 Variation of joining force with punch stroke using the customized

punch P2 and different dies.

The variation of the force during clinch joining using the punch

P2 is reported in Fig. 11. The lack of the load drop before the

upsetting phase, as occurred with the punch P1, in the force-

displacement indicates a delay of crack initiation around the punch

corner after the offsetting phase owing to the smoother corner

radius of the punch P2.

The die with h = 0.6 mm is characterized by a minor punch-

die cavity-volume between the punch and the die which causes a

faster cavity volume fulfilment during the joining operation. In

addition, when such a die is used, a high thickness reduction at the

bottom is required to achieve the mechanical interlock between

the sheets. As a result, the adoption of the shallowest die (h = 0.6

mm) implies a major forming force as compared with deeper ones.

The main quality criteria (interlock and neck thickness) as well

as the shear strength of clinched connections produced using

different dies and pre-heating schemes (resulting in different

joining temperatures, Tj) are reported in Fig. 12.

The increase in the die depth results in an increase of interlock

ts and a slight reduction in the neck thickness tn, as shown in Fig.

12b and Fig. 12c, respectively. Actually, deeper dies involve a

major thinning effect on the sheets (especially during the

offsetting phase) which reduce the wall thickness of the punch-

sided sheet. On the other hand, the increase in the die depth

produces a higher material flow, which contributes to the

formation of a larger interlock. Indeed, when deeper dies are

employed, the wall thickness of the die-sided sheet is reduced

(during the offsetting phase) and the die-sided sheet exerts a minor

resistance to the spreading of the punch-sided sheet during the

upsetting phase which results in a larger interlock, as reported in

[25].

The employment of the most shallow die (with h = 0.6 mm)

produces relatively high values of neck thickness tn = 0.36 mm

and low values of interlock, ts = 0.045 (these values are the

average of tn and ts of joints produced at different temperatures,

Tj) leading to small values of the shear strength Fs = 0.32 kN.

Increasing the die depth h = 0.8 mm results in an increase in the

interlock ts = 0.13 mm and a slight decrease in the neck thickness

tn = 0.34 mm resulting in a significant improvement of the shear

strength of the joints Fs = 0.8 kN. Further increase in the die depth

h = 1.1 mm allows producing joints with higher strength Fs = 1.23

kN since the increase in the interlock ts = 0.22 mm; however, the

employment of such a die required the adoption of sheet pre-

heating since crack development at room temperature.

Fig. 12 Influence of joining temperature and die depth on (a) Shear strength

Fs, (b) interlock ts and (c) neck thickness tn of clinched connections (X-parameter

= 0.6 mm).

The depth of the die anvil also influences the joint failure mode,

namely pull-out, shear or a combination of them [30,39,13].

Indeed, the joints produced by means of the die with h = 1.1 mm

mainly failed by shearing mode, see fig 13a and b, due to the

higher interlock. Such failure mode is characterized by the

formation of a crack under the punch-sided sheet cap, which

progressively develops as the load increases up to the complete

separation of the cap from the sheet (fig. 13a). On the other hand,

joints produced with shallow dies are characterized by the pull out

failure mode. Herein, the interlock was much smaller than the

neck thickness and the punch-sided sheet is pulled away from the

joint causing a sudden drop in the load carrying ability.

Fig. 13 Main failure modes: (a-b) shearing and (c-d) unbuttoning.

The adoption of preheating allowed increasing the material

formability and avoiding cracks initiation during the joining

phase. As a result, sheets pre-heating allowed to produce sound

joints even with the deepest die (h = 1.1 mm).

As can be inferred from Fig. 12, in the examined range of

temperatures, the employment of preheating had marginal effect

on the quality criteria resulting in a slight increase in the interlock

and negligible difference in neck thickness that led to marginal

difference of shear strength of the joints. Actually, the highest

analyzed temperature is relatively small as compared to the

melting temperature of the aluminum alloy. Thus, only reduced

variation in the yield stress of the material as well as variation in

the friction coefficient are expected, resulting in negligible

variation of metal flow during clinching.

In order to evaluate the effect of the joining temperature on the

sheet deformation, micro-hardness test were performed on the

joint neck and the bottom protrusion. The micro-hardness of the

base material was 208 7 HV0.1. At each location, three

repetitions were performed. Fig. 14 depicts the distribution of the

average micro-hardness measurements of three joints produced at

different temperatures with h = 0.8 mm. As can be inferred, the

high deformation results in a drastic hardness increase stem from

material strain-hardening. However, because of the reduced work

hardening characterizing the AA6082T6 alloy, such an increase is

much smaller than that exhibited by joints made of other materials

[40].

The highest increase in micro-hardness was shown at the

bottom protrusion, regardless the joining temperature, due to the

severe plastic flow. Comparing the micro-hardness distribution of

specimens produced at different temperatures, it emerges that the

micro-hardness of the punch-sided sheet zones reduces with the

joining temperature Tj. This could be ascribed to a higher strain-

hardening of the material when joined at room temperature and

partial recovery annealing arising at higher temperatures.

On the other hand, the die-sided sheet seems to be only

marginally influenced by the joining temperature. Indeed, because

of the heating flux is directed towards the punch-sided sheet, the

die sided sheet experiences a lower temperature increase which

could explain the lower sensitivity to the joining temperature.

Fig. 14 Micro-hardness distributions on clinched connections performed at

different temperatures (Tj) using a die with h = 0.8 mm.

4. Conclusions

The joinability of an aluminium alloy sheets with low ductility

by means of mechanical clinching was investigated

experimentally. A prototypal apparatus for pre-heating the sheets

was developed to evaluate the effect of joining temperature on

joinability window. The occurrence of cracks was assessed using

optical and SEM microscopy. Mechanical characterization of

joints was performed to evaluate the effect of the adopted

processing conditions. The main results are summarized as

follows:

the employment of shallow dies can precede the initiation

and development of cracks on the joint during the joining process;

however, reduced interlocks are produced resulting in weak joints

with low values of maximum strength;

the employment of a punch with a smoother corner radius

and a conical shape allowed to delay the initiation of cracks during

the joining process and extend the joinability to much deeper dies

(h = 0.8 mm);

the adoption of the developed system for sheets

preheating has allowed to further extend the joinability by

clinching to much deeper dies (h = 1.1 mm);

the joints produced with deeper dies were characterized

by higher mechanical behaviours as compared to shallower ones

due to an increase in the interlock;

the employment of deeper dies allowed a drastic

reduction in the forming force because of lower hydrostatic

stresses were produced;

Acknowledgements

The authors would like to thank Jurado (Rivotorto di Assisi,

Italy) and CAMS (Chieti, Italy) for providing the extensible dies

and the customized punches used in this research. The authors

would also like to thank Mr. G. Organtini and Mr. G. Pasqualoni

(DIIIE - University of L’Aquila) for contribute during the setup

and conduction of the experimental tests.

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List of Figure Captions

Fig. 1 (a) Main geometrical parameters of clinched

connections; (b) failure modes of clinched connections Fig. 2 Schematic representation of clinching phases with

preheating. Fig. 3 Schematic representation of the clinching equipment

with preheating adapter Fig. 4 Main dimensions of clinching tools Fig. 5 Schematic representation of specimen used in single lap

shear tests. Fig. 6 (a) Temperature field distribution as measured by

infrared camera and (b) variation of average temperature with

time according to the adopted preheating schemes. Fig. 7 Variation of joining force with punch stroke using the

standard punch P1 and different dies. Fig. 8 Cross sections of clinched connections produced with

the punch P1 and different dies. Fig. 9 SEM macrographs of clinched connections: punch-

sided surface (punch P1 die with h = 1.1 mm) (a) low

magnifications, (b) high magnifications on the crack zone, (c)

punch sided surface (punch P1 with h = 0.6 mm) high

magnification on neck and (d) die sided surface (punch P1 with

h = 1.1 mm) with low magnification. Fig. 10 (a) Macrograph of clinched connection produced with

the punch P2 and the die with h = 1.1 mm. (the lower sheet was

metallized to better distinguish between the sheets) (b)

Protrusion of the die-sided sheet. (c-e) Cross sections of clinched

connections produced with different die depths h. Fig. 11 Variation of joining force with punch stroke using the

customized punch P2 and different dies. Fig. 12 Influence of joining temperature and die depth on (a)

Shear strength Fs, (b) interlock ts and (c) neck thickness tn of

clinched connections (X-parameter = 0.6 mm). Fig. 13 Main failure modes: (a-b) shearing and (c-d)

unbuttoning. Fig. 14 Micro-hardness distributions on clinched connections

performed at different temperatures (Tj) using a die with h = 0.8

mm.

List of Table Captions

Table 1. Mechanical properties of AA6082T6 sheets