Improvement in joint strength and material joinability in clinched joints by electromagnetically assisted clinching

Journal of Manufacturing Processes 41 (2019) 252–266

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Improvement in joint strength and material joinability in clinched joints by electromagnetically assisted clinching

T

⁎

M. Salamati, M. Soltanpour , A. Zajkani, A. Fazli Advanced Forming Technology and Materials Lab, Department of Mechanical Engineering, Imam Khomeini International University, Qazvin, Iran

A R T I C LE I N FO

A B S T R A C T

Keywords: High speed joining Plastic deformation Hybrid processing Strain rate

An electromagnetically assisted clinching process is proposed to join a combination of similar and dissimilar materials using a high-speed punch. As the name suggests, an electromagnetic discharge energy is used to accelerate the punch to sufficiently high speeds. Joining similar and dissimilar combinations of carbon fiber reinforced plastic (CFRP) and aluminum sheets is conducted as the case studies to investigate the performance of the technique. The mechanical behaviour and failure modes of the joints are studied using single-lap shear tests. Superior undercut, neck thickness, and resultantly superior joint strength is obtained compared to the conventional clinching techniques, thanks to the combined effect of thermal softening and strain rate dependent hardening in the joining zone. Such an improvement in the joint strength could be achieved in all strain rate sensitive materials. The influence of the processing and tooling parameters particularly the discharge energy, and toolset geometry are investigated on the joint strength and failure modes. It is suggested to use an optimum range of the discharge energy with respect to the thickness, ductility, yield strength and other mechanical properties of the joining partners especially punch-sided one.

1. Introduction Today high performance, high strength to weight ratio materials such as advanced high strength steel (AHSS), aluminum, magnesium, and carbon fiber reinforced plastic (CFRP) are widely used in hybrid structures. Hybrid structures play an important role in the automotive and aerospace industries. The primary goals are light-weight design (to minimize the fuel consumption [1] while maximizing the motor efficiency), and tailored design. Mercedes Benz, BMW, Audi, Maserati, Lamborghini, Ferrari, Pagani and other high-end automakers use hybrid structures [2] to manufacture body kits, interior parts and even the chassis [3]. Joining is a challenging issue in hybrid structures, due to the significant differences between the physical and mechanical properties of dissimilar materials. Conventional technologies such as adhesive bonding [4], mechanical fastening [5], laser joining [6], ultrasonic welding [7] and friction spot joining [8] have been already utilized to manufacture joints of dissimilar materials. However, they suffer significant limitations. Property degradation, stress concentration, formation of intermetallic compounds (IMCs), increased component weight, long manufacturing time, environmental resistance, etc. are some limitations such conventional techniques face with [9]. Joining by forming is considered as a reliable tool for joining a wide ⁎

range of dissimilar materials [10]. Clinching is a variant of joining by mechanical forming technologies to manufacture spot joints without the need for any kind of joining elements. A lot of combinations of similar and dissimilar materials have already been joined by this technique. However, the process is hard-to-apply on low-formability materials in its conventional form, due to the severe plastic deformation in the joining zone. Accordingly, special considerations are required to apply the process on low-formability materials such as AHSS, aluminum alloys, and CFRP. Lee et al. [11] introduced the hole-clinching as a promising solution to join a formable material suitably to a low formable one. They stated that precise alignment between the center of the clinching tool and the hole on the die-sided sheet is a key factor to achieve a successful clinching process and to obtain a joint of desired strength. Increasing the hydrostatic stress [12] and preheating the workpieces [13] are some desirable solutions to apply the clinching process on low-formability materials. A clinching process with the workpiece preheating is called heat assisted clinching. Various heat-assisted clinching techniques are developed to join brittle materials; either a simple setup consisted of a heating gun (inductive heating or convective heating) or some more complex techniques such as laser assisted clinching, resistance assisted clinching, ultrasonic assisted clinching and friction assisted clinching [9]. Using these techniques, clinching is successfully applied for joining

Corresponding author. E-mail address: [email protected] (M. Soltanpour).

https://doi.org/10.1016/j.jmapro.2019.04.003 Received 7 January 2019; Received in revised form 19 March 2019; Accepted 3 April 2019 1526-6125/ © 2019 Published by Elsevier Ltd on behalf of The Society of Manufacturing Engineers.

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moves the workpiece oppositely away from the coil toward a die and forms it. EMF is a precise method for forming and joining of the metals and other materials. The forming limit diagrams (FLDs) can be extended due to the high strain rate of the process. This results in significant increase in the formability of the material [28], and a considerable reduction in the technical drawbacks such as spring-back and wrinkling [29]. Zajkani and Salamati [30] combined for the first time the EMF and clinching processes as a hybrid process for high speed joining. A clinching punch attached to a high speed aluminium disk actuator is used as the tool for high strain rate processing of the joining partners. The optimum geometries and sizes for different system parts such as the die, blank-holder, actuator, and the punch are selected based on the FE results, to prevent the trial and error procedure. The feasibility of the process was investigated for joining of the AA1050 and CFRP sheets. Babalo et al. [31] proposed a so-called electro-hydraulic clinching (EHC) process in which the EMF/clinching hybrid system is implemented with a working fluid. The energy stored in the pulse generator discharges between the electrodes submerged in the working medium resulting a shock wave that is eventually utilized to form the joining partners into the clinching die. The process is proved as an efficient technique to join low thickness materials [32]. The primary motivations in the present paper were to extend the EMF/clinching hybrid process (electromagnetic clinching “EMC”) for joining materials of lower formability and to improve the strength of the material in the joining zone. The high strain rates obtained by the EMF process are utilized to improve the formability of the material as well as the strength of the joining zone. The EMC process is used to manufacture clinched joints of metal-CFRP as well as CFRP-CFRP or metal-metal components by high strain rate values. The introduced system is designed to be able to join any ductile material regardless of their electrical conductivity. A low formability aluminium alloy is successfully clinched to a pre-drilled CFRP laminate in the CFRP/aluminium joining case study. Enhanced joint characteristics namely neck thickness and undercut are achieved when utilizing proper processing and tooling parameters. The improved joint characteristics can be related to the strain rate dependant hardening of the material in the processing (joining) zone. On the other hand, the feasibility of the joining metal/metal configuration by the process (without the predrilled hole) is investigated. Desirable joint characteristics are also achieved for this configuration.

low-formability materials such as titanium alloys [14], aluminum alloys [15] and magnesium alloys [16]. Using a heating gun seems a simple setup to preheat the workpieces. One can prevent the development of cracks in the workpieces by improved formability, when the hot air flow is towards the punch-sided sheet. On the other hand, directing the hot airflow towards the diesided sheet, thicker necks can be produced, due to the minor restraining effect of the die-sided sheet during the offsetting phase [17]. However, the process may result in decreased local hardness of the material because of increase in grain and precipitates dimensions [9]. Superimposing a clinching process with ultrasonic excitation induces higher temperatures in the sheets. So it can improve the material formability. The process is successfully implemented to join hard-to-form materials such as aluminum alloys [18]. However, the residual effect caused by ultrasonic vibration is quite material sensitive. For example, residual hardening occurs for aluminum and residual softening for titanium [19]. Laser assisted and friction assisted clinching are more efficient technologies. The former, utilizes a laser beam to heat up the workpieces [20]. While the latter, benefits a stirring effect of a rotating tool. Even very hard to form materials such as 22MnB5 are successfully clinched using the laser assisted clinching [21]. However, the process is capital intensive and faces safety issues for the operator [22]. On the other hand, friction assisted clinching is a strong tool to achieve sound joints without cracks at the necking zone, with higher material flows and consequently larger undercuts. It also requires significantly reduced joining force compared to conventional clinching [9]. Despite the unique advantages, the process seems to have the highest proficiency when joining metal to metal combinations. Otherwise, it may require a pre-drilled hole on the non-metallic workpiece [23]. Using a high-speed punching is another solution to increase the material formability. Jäckel et al. [24] applied high-velocity self-pierce riveting to join steel and aluminum sheet metals. The high velocity of the punch results in higher strain rate which can cause a higher flow stress. They concluded that high strain rates, as well as local heating in the material, are the leading causes of the different material behaviour when joining materials by increased velocities. Neugebauer et al. [25] used a high speed riveting process enabled by drop weight system to manufacture hybrid joints of some kinds of AHSS (as the punch-sided sheet) and aluminium alloys (as the die-sided sheet). By means of a finite element analysis it is shown that the strain rate and temperature can increase by about 104 and 20 times respectively, when the punching speed changes from a conventional velocity (below 1 m/s) to an elevated velocity (above 10 m/s). Goldspiegel et al. [26] proposed a high speed nailing process in which a piston with a nail placed at its tip is accelerated towards the sheets being joined until velocities up to 37 m/s to provide enough impact kinetic energy for joining. The nail starts to contact the uppersheet with a maximized velocity. As it progresses into the sheets, both piston and nail velocity decrease and piston kinetic energy is mainly transformed into plastic work leading the workpieces to be joined. The process is used to join AHSS and aluminum sheets. In this technique it is required to access to the both sides of the joining partners and this limits the applicability of the process. Nagel and Meschut [27] introduced functional joining nails consisted of a functional section and joining section. Using such a functional joining elements makes the process applicable for the configurations with one-sided accessibility. Electromagnetic forming (EMF) is a high-strain rate forming technology that utilizes electromagnetic force as the forming force. In this high-speed forming technology, electrical energy stored in a pulse generator, discharges into a coil and creates a time-varying current with an electromagnetic field in it. This time-varying electromagnetic field induces an eddy current in a conductive workpiece nearby the coil that flows in the opposite direction to that of discharge current and therefore creates an opposite electromagnetic field in the workpiece. The repelling force between these two different electromagnetic fields

2. Materials and methods 2.1. Electromagnetic clinching equipment The electromagnetically assisted clinching system is illustrated schematically in Fig. 1 and is consisted of three main parts: the die, the blank-holder and a set of punch and actuator. The die and the blankholder are made of steel, while the actuator is made of aluminum. The main dimensions are also shown in the figure. Metallic sheet and CFRP laminate lie between the die and blank-holder. The blank-holder has a central through-all hole that plays the role of a guide for the punch motion and has four threaded holes to be clamped to the die. The die has clinching cavities in both sides with different diameters to be capable of joining sheets with two different clinch sizes. The cavity depth for both sides is considered to be 1.0 mm, while the cavity diameter is 8.8 mm on one side and 6.6 mm on the other side. These dimensions are determined by the results of the finite element analysis in an earlier work [30]. The die is clamped to the blank-holder by four M10 bolts to hold the sheets tightly. The set of punch and actuator is the other part of the system. The actuator is drilled and threaded in the center. This part should be rigid enough to resist against force applied by the electromagnetic field and does not become deformed. However, this component should be light enough to reach a high velocity in a very small duration. The punch is a surface finished rod made of steel that is 253

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Fig. 1. Schematics of the EMC system.

clamped and are holding the workpieces. The punch attached to the actuator enters the hole in the blank-holder and the complete setup is placed on a flat coil. The clearance between the punch and the hole on the blank-holder is very low (about 0.1 mm) that constrains the radial motion of the punch. Four 100 mm height, M8 bolts are used as stands, to adjust the height of the blank-holder from the punch. The gap between the blank-holder and the punch can be adjusted by the height of these bolts. The set of punch and actuator can quickly move in a vertical direction by an appropriate adjustment of this height. The initial gap between the coil and actuator is considered to be 6.0 mm. In the second phase (Fig. 2b) the energy stored in the pulse power discharges and the electromagnetic pressure translates to the actuator. As a result, the set of punch and actuator accelerates and impacts the punch-sided sheet (aluminum in this case) with a significant velocity (V). In the third phase (Fig. 2c) the aluminum is indented into the hole on the CFRP laminate until it touches the die cavity. The die cavity helps the material to flow radially and to create an undercut. The third phase takes place with a very high speed and lasts only for a few hundred microseconds.

clamped to the actuator by a nut. The complete set of the EMC system is assembled and placed in the electromagnetic twelve turn flat spiral coil of 105.0 mm diameter connected to a capacitor bank of 250 μF. A hydraulic jack is used to keep the assembled die and the coil in place. In the EMC system, electrical energy stored in a pulse generator discharges into the flat coil and creates a time-varying current with electromagnetic field in it. This time-varying electromagnetic field induces an eddy current in the actuator nearby the loop that flows in the opposite direction to that of discharge current and therefore creates a different electromagnetic field in the actuator. The repelling force between these two opposite electromagnetic fields moves the actuator away from the coil toward the sheets. Thereby, the punch connected to the actuator touches and indents the metallic sheet into the die cavity. A considerable amount of the produced pressure will be wasted in the absence of an actuator. Because the punch has a moderate size in comparison to the coil size. Thus, sufficient force will not be provided to move the punch. As illustrated in Fig. 2, the EMC process is consisted of three phases. First, in the assembly phase (Fig. 2a), the blank-holder and the die are

Fig. 2. Process sequence of EMC: Phase 1) Assembly; Phase 2) Discharging the energy stored in the pulse power; Phase 3) Plastic deformation of the punch sided sheet and creation of the undercut. 254

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remarked, applying local pressure on the CFRP laminate, such as drilling or impact punching processes, results in dragging on the laminate, which leads to a consequential delamination. This effect is shown in Fig. 5. In fact, the pressure causes dragging phenomenon around the perimeter of the CFRP hole when the aluminum sheet is passing through the hole during the indentation (bulging) phase. However, deformation of the CFRP laminate around the hole location caused by pulling was of deficient levels (provided that the hole on the CFRP and the punch to be carefully aligned coaxially), and its influence on the development of the interlock was assumed to be negligible. Therefore, the delamination of the CFRP laminate was neglected to facilitate the analysis. The aluminum sheet is the plastically deformed part. Gou et al. [34] investigated the effect of a broad range of strain rates on the plastic behaviour of AA3003 and presented a constitutive model for prediction of flow stress in this alloy (Eq. 1). In the present work, this constitutive model is used to describe the hardening behaviour of the metallic sheet in the processing zone. It should be mentioned that even the same materials produced by two different manufacturers may have different constitutive equations. However, it is beyond the scope of this study to develop a model to describe the flow stress and the constitutive equation of the used aluminum. Therefore, the same constitutive equation obtained by Gou et al (Eq. (1)) is also used in the present work. However, a relatively good agreement was observed between the experimental results of the present study and the predictions of Eq. (1).

Fig. 3. Curing cycle for thermosetting epoxy resin used to manufacture CFRP laminates.

2.2. Materials The materials used in this work were carbon fiber-reinforced thermosetting laminates and hot rolled AA3003 aluminum alloy sheets. Hand lay-up process was used to manufacture the CFRP laminates because this process has a high accuracy and does not need advanced manufacturing equipment. Moreover, parts made by hand lay-up process have very excellent quality and high strength. This process is suitable for low-volume production and usually is used in the high-performance applications such as aerospace. A twill weave of carbon fibers was cut to the desired length and shape to make a composite part. For cutting, the weave was placed on a cutting board and then, was cut using a utility knife and a steel ruler. Part fabrication was done by laying the 5 ply of resin impregnated carbon fiber weaves on top of an open mold with the [0/90/0]s fiber direction. The release agent was applied to the mold for easy removal of the part. Once all the cut weaves were laid in the desired sequence and fiber orientation, vacuum bagging preparations were made for curing and consolidation of the part. Finally, the vacuum bagged part was ready to go inside the autoclave for the curing process. The curing cycle for the resin is illustrated in Fig. 3. The final CFRP laminates have a 1.2 mm thickness. The mechanical characteristics of the CFRP laminates are summarized in Table 1. AA3003-H14 was selected as the metallic joining partner, due to its relatively low strain-to-failure. It allows to investigate the ability of the EMC process to join materials of low formability. Its mechanical properties and chemical composition are listed in Tables 1 and 2, respectively. Moreover, the stress-strain diagrams for aluminum and CFRP used in this study are illustrated in Fig. 4. The aluminum sheets of 0.6, 1.2 and 1.6 mm thickness were used. The specimens were applied under single-lap shear test after joining process to obtain the joint failure load.

1/ q 1/ p

ε˙ σ = 721 − ⎧1 − ⎡−3.2 × 10−5T ⎛In + In f ⎞ (ε . T ) ⎤ ⎥ ⎢ ⎨ ⎠ ⎝ −2 × 1010 ⎦ ⎣ ⎩ T ) + −64ε 0.4

⎫ ⎬ ⎭

f (ε . (1)

Where T and f (ε,T) are given by Eqs. (2) and (3), respectively. Also q and p, are constants in the range (0,1).

∫0

T = T0 + 0.41

ε

σdε

(2)

T 2⎤ 0.05 ⎞ ε f (ε . T ) = 1 + 6 ⎡1 − ⎛ ⎢ ⎥ ⎝ 916 ⎠ ⎦ ⎣

((3))

2.3.2. Analysis of the electromagnetic discharge energy In order to analyse the process, first the electromagnetic pressure applied to the actuator via the coil, is obtained by defining the electromagnetic field produced by the coil. The current in the coil is defined by Eq. (4) [35]: t

I (t ) = I0 e− τ sinωt

(4)

where I0 is the maximum intensity of the discharge current, τ is the damping coefficient of the circuit, and ω is the angular frequency. Because the actuator is a disk, a cylindrical coordinate system can be used. Thus the magnetic field density B possesses a radial component Br and an axial component Bz which are given as follows [35]:

2.3. Process analysis

− 1 ⎛ ∂2 1 ∂ ∂2 1 ∂Br + + 2 − 2 ⎞ Br + =0 2 μ0 σw ⎝ ∂r r ∂r ∂z r ⎠ ∂t

(5.a)

∂Bz − 1 ⎛ ∂2 1 ∂ ∂2 + + 2 ⎞ Bz + =0 μ0 σw ⎝ ∂r 2 r ∂r ∂z ⎠ ∂t

(5.b)

⎜

2.3.1. Strain rate dependent hardening The die, blank holder, set of punch and actuator, flat coil and two workpieces (here, CFRP laminate and aluminum sheet) are the main parts of the EMC system. While an electromagnetic forming machine provides the required forming force for the system. As Ueda et al. [33]

⎜

⎟

⎟

where σw is the conductivity of the material of actuator (aluminium), μ0 is the void permeability and t defines the time. The boundary conditions

Table 1 Material properties of aluminum and CFRP sheets.

Aluminum AA3003-H14 Epoxy 5052 CFRP

Thickness (mm)

Density (Kg/m3)

Young’s modulus (GPa)

Ultimate tensile strength (MPa)

Strain-to-failure (%)

Poisson ratio

0.6, 1.2, 1.6 1.2

2.7e3 1.4e3

69 E1 = 133.86 E2=E3 = 7.706

160 σ1 = 842.7 σ2=σ3= 106.5

8.5 >5 8

0.33 ν12=ν13 = 0.301 ν23 = 0.396

255

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Table 2 Chemical composition of aluminum alloy AA3003. Element

Aluminum (Al)

Manganese (Mn)

Iron (Fe)

Silicon (Si)

Copper (Cu)

Zinc (Zn)

Residuals

Content wt-%

96.8-99

1.0-1.5

0-0.70

0-0.60

0.050-0.20

0-0.10

0-0.15

to solve Eq. (5) are:

B = B0 + B1 at z = dg + u2 (t )

(6.a)

B = 0 at z = dg + u2 (t ) + h

(6.b)

where u2 (t ) is the vertical relocation of the actuator and dg is the initial gap between the coil and actuator. Lorentz force density has two components, a radial and an axial one, which can be determined using following equations:

fr = Jθ Bz

(7.a)

fz = −Jθ Br

(7.b)

where Jθ is the circumferential component of eddy current, and can be obtained using Eq. (8):

Jθ =

∂Bz 1 ∂Br ( − ) μ0 ∂z ∂r

Fig. 5. Dragging due to the compressive force of the punch on a composite laminate which causes consequential delamination.

(8)

Then corresponding magnetic pressures can be determined as follows: z=h

Pr =

∫z=0

Pz =

∫z=0

z=h

fr dz

(9.a)

fz dz

(9.b)

electromagnetic pressure distribution. 2.4. Experiments, mechanical and morphological characterizations Some preliminary experiments were conducted to obtain the range of variation for the discharge energy. The suitable discharge energy for the process, heavily depends on the material (mechanical properties) and thickness of the workpieces. Given that the same material is used in this study, the range of variation for discharge energy is determined

These equations was computed using the analytical approach used in the work of Correia et al. [35]. Then the magnetic pressure acting on the actuator was defined by a MATLAB code. The output is an

Fig. 4. Stress-strain curves of aluminum alloy AA3003-H14 and CFRP laminates. 256

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Table 3 Experimental plan. thickness 0.6 mm Experiment No. Discharge voltage (kV) Discharge energy (kJ)

1 4.00 2.00

1.2 mm 2 4.20 2.20

3 4.40 2.42

1.6 mm 4 4.60 2.53

5 4.80 2.88

6 5.00 3.12

7 5.20 3.38

8 4.80 2.88

HV =

9 5.00 3.12

10 5.10 3.25

11 5.20 3.38

12 5.50 3.78

1.854 × 106 × m d2

13 5.80 4.20

14 6.00 4.50

15 6.30 4.96

16 6.50 5.28

17 6.80 5.78

(10)

Where d is the average diagonal length in micrometers and m is the load in kgf. The load is considered to be m = 0.3 kgf. 3. Results and discussion Fig. 6. Schematic of the single lap shear test specimen.

3.1. Mechanical characterization with regard to the workpiece thickness. As the thickness increases, a wider range of discharge energies could be used in the process. Therefore, the number of experiments for thicker workpiece is higher than the thinner ones. The experimental plan is reported in Table. 3. Accordingly, single-lap shear test specimens are manufactured using the AA3003-H14 and CFRP sheets, in order to study the strength of the joints made by EMC process. The main dimensions of the single-lap shear test specimens are shown in Fig. 6. The apparatus for the test was Zwick/ Roell Z100, and the test speed was 1.0 mm/min at room temperature. Three samples were manufactured for each experimental condition, and applied under single-lap shear test. The average value of the shear test was reported. Furthermore, one sample was manufactured to obtain the geometrical and morphological characteristics of the joint according to the Fig. 7 which are namely neck thickness (tn), undercut (ts), the bulge diameter (d), and the sheet metal thickness at clinched zone (X). In a mechanically clinched joint, tn and ts are the basic parameters, significantly affecting the joint quality and strength. On the other hand, d and X are the secondary parameters which have minor effect on the joint quality. The bulge diameter is a scale of how much percent of the die cavity is filled with the material of sheet metal. When this parameter approaches to die cavity diameter (8.8 mm in this case), it means that the material is fully filled the die cavity. The sheet metal thickness at clinched zone indicates a decrease in the sheet thickness due to the high-velocity impact and material flow. The joints were cut in the longitudinal direction and the parameters ts, tn, and X, were measured using a micrometer and image processing techniques. Furthermore, the Vickers microhardness tests were conducted on the longitudinal cut samples. The microhardness value was evaluated in eight points of each sample as illustrated in Fig. 8. Then, the gathered data was used to study the effect of high strain rate processing on the microhardness. The microhardness value was obtained using Eq. (10):

3.1.1. Static behavior The spatial and temporal distributions of the electromagnetic pressure are depicted for two typical discharge energies in Figs. 9 and 10 respectively. It is evident from Fig. 10 that the distribution of electromagnetic pressure is not constant. This should be considered in the direct forming processes of the workpiece by electromagnetic pressure. But in the case under consideration in this paper, the electromagnetic pressure is used only to accelerate the actuator and then the forming process is performed by a punch attached to the actuator. Therefore, it is possible to ignore the non-constant distribution of the electromagnetic pressure and to consider it as a constant pressure applied to the actuator. By controlling the discharge energy, it is possible to control the punching velocity and thus to control the material flow. As depicted in Fig. 10, utilizing the discharge energy of 5.5 kJ, the maximum electromagnetic pressure acting on the workpiece is almost 1.4 times the acting pressure of 3.0 kJ discharge energy. Thus, increasing the discharge energy leads to a higher punching impact and as will be discussed later, this will result in more material heating and a more severe impact on the workpiece. This will in turn lead to a higher strain hardening at the joining zone. The discharge voltages between 4.40–6.80 kV were used for experiments that provide 2.42–5.78 kJ energy. In lower discharge voltages, the mechanical interlock was not formed, and at the higher ones, the metal sheet was failed. It should be mentioned that the range of discharge energies used to process the material is strongly depended to the frequency and energy dissipation of the EMF machine. The material can be processed with much lower discharge energies using a higher frequency machine with a low energy dissipation. To determine the joints performance, the geometrical parameters indicated in Fig. 7 were measured and investigated experimentally. The results are summarized in Fig. 11. Both the values of tn and ts should be maximized for an optimum joint. However, it is evident that an increase in ts can be achieved only by a decrease in tn, due to the material flow. Thus an optimum value exists and should be obtained. A red vertical line in Fig. 11 depicts the optimum condition, in which the best static performance is achieved for the joint. In case of 1.6 mm thick metallic sheets which are the basic matter of concentration in this work, the optimum value for tn and ts were observed at discharge energies between 4.20–4.50 kJ. As it can be seen from Figs. 11–13, these two joints have the best mechanical performance. It should be noted that same as to conventional clinching, EMC is not a proper technique to manufacture joints of low thickness sheets. To approve this, mechanical interlock was created in the range of 2.00–2.20 kJ discharge energies for 0.6 mm sheets. In the higher energies, the aluminum sheet failed in the clinched zone. The failure load of these specimens was only in the range of 300 N for the different

Fig. 7. Geometrical features of a clinched joint. 257

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Fig. 8. The points considered for Vickers microhardness testing: 1) base metal; 2, 8) punching adjacent zone; 3, 7) neck zone; 4, 6) undercut; 5) joint bottom.

As mentioned before, an increase in undercut results in the decrease in neck thickness because of material flow from necking zone to the clinched button, so an optimum forming force is required to obtain the best joint. In case of 2.88 kJ discharge energy, the undercut was not created, so the joint strength was almost low. By increasing the discharge energy, the undercut also increases, but the neck thickness decreases simultaneously. Thus, an excessive increase in discharge energy results in very low neck thickness and considerably lower joint strength. Accordingly, the joint strength in 5.78 kJ discharge energy was only 450 N. The best forming conditions and joint characteristics are obtained utilizing 4.50 kJ discharge energy. A diagram for joint shear load versus discharge energy is plotted in Fig. 13 for 1.2 mm and 1.6 mm thick sheets. This diagram indicates that an optimum value of discharge energy should be selected for processing of samples with different thicknesses. Lower or higher discharge energies may have negative influence on the joint strength. Generally, this optimum value should be selected according to the thickness and yield strength of the punchsided sheet. Higher thickness and higher yield strength requires higher discharge energies to manufacture a sound joint.

Fig. 9. Temporal spatial distribution of electromagnetic pressure.

3.1.2. Geometrical features Increasing the velocity of punching process through exposing a high strain rate EMF pressing cause to strike metallic sheet by more plastic flow rate inside the die cavity. So, it provides forming by stronger mechanical interlock between the metal sheet and CFRP laminate. While, using the conventional methods in low punching velocity such as hydraulic press may lead to smaller-sized interlock. Fig. 14 illustrates the cross section of a CFRP/Al1.6 joint processed by 4.96 kJ discharge energy. The superior interlock obtained by EMC can be observed in the figure. The magnified views show significantly improved neck thickness and undercut values. The pressure of punch on the sheet metal leads to the material flow from necking zone to the undercut zone. Thus increasing the punching pressure leads to a decrease in the neck thickness and an increase in the undercut. Generally, it is not possible to simultaneously increase these two geometrical features. While in the EMC, as depicted in Fig. 14, the high speed and sudden impact of the punch on the sheet metal, considerably reduces the thickness in the clinched zone (X value) and instead flows the material from the area to the undercut zone. The material flow toward the undercut zone results in material accumulation in this zone and a higher undercut value, while the neck thickness still remains at a high value. The undercut (ts) and neck thickness (tn) have the major responsibility for load bearing in the clinched joints, while the thickness of clinched zone (X) plays a minor role. Therefore, the combination of high undercut and neck thickness values leads to an increase in the joint failure load. A comparison between the geometrical features of aluminum/CFRP joints manufactured by various hole clinching techniques are provided in Table 4. The Table shows the superiority of the geometrical features obtained by EMC compared to similar techniques.

Fig. 10. Spatial distribution of electromagnetic pressure.

discharge energies (Fig. 12). These joints have no good mechanical characteristics because of the very low thickness of the aluminum at the necking zone. As can be seen from Fig. 11, the neck thickness in these samples is less than 0.1 mm. On the other hand, at low discharge energies, undercut cannot be created and a slight increase in the discharge energy leads to fracture at the necking zone before the creation of undercut. Thus further study on the low thickness sheets was neglected and the main concentration was paid to 1.2 mm and 1.6 mm thickness sheets. As listed in Table 3, in case of 1.2 mm thick sheets, the range of variation for discharge energy was 2.42–3.38 kJ. The best joint was observed in case of 3.12 kJ. In this case, the joint strength was 1220 N, and the failure mode was aluminum neck fracture. The CFRP sheet was entirely safe, and no signs of the fracture or delamination had to be seen. It means that a more quality joint can be produced by increasing the thickness of the deforming part. On the other hand, the weakest joint was the case of 3.38 kJ, the strength of this joint was 750 N, and the failure mode was similar to the former specimen. But in this case, the neck thickness of aluminum was very low because of the high energy of the process, so the failure takes place in some small loadings. In case of 1.6 mm thick sheets, the results were much better than 0.6 and 1.2 mm thick specimens. So it can be concluded that the thickness of deforming material is a high effective parameter on the shear strength of the joint. It is evident somehow, since the material flow causes the metal sheet thinning in the necking zone as depicted in phase 3 of Fig. 2. So a higher neck thickness will be obtained when using a higher thickness metal sheet, and therefore the joint strength will be greater. The best joint was in case of 4.50 kJ discharge energy, and the weakest joint was in case of 5.78 kJ discharge energy.

3.2. Tooling considerations Tooling parameters namely the geometries of the punch and die strongly affect the joint characteristics in the EMC. The punch diameter should be selected regarding the metallic sheet thickness and the hole on the CFRP laminate. The value of dP + 2 t should be sufficiently lower than the dH, where dP is the punch diameter, t is the metallic sheet thickness and dH is the hole diameter. When the value of dP + 2 t approaches dH, hoop stress at necking zone reaches high values due to the shear between the hole and punch sided sheet [38]. The increase on punch diameter corresponds to an increase in the forming volume of 258

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Fig. 11. Variation of the joint geometrical features with the discharge energy.

where σf is the fracture shear stress of the punch-sided material. Considering a constant value for the Fj as a desirable joint failure load, different solutions are possible for dP and tn. It is clear that tn decreases drastically when an excessively large diameter punch is used (Compare the neck thickness values schematically illustrated in Fig. 15a and b). A typical sample processed by a large diameter and non-coaxially aligned punch is depicted in Fig. 16. On the other hand, a large neck thickness can be obtained when using smaller diameter punch. However, the

material [39] and a greater material flow in the radial direction, enlarges the damaged region which involves a further increase in the hoop tensile stress [40]. It is known that an increased hoop stress facilitates the formation of radial cracks. So failure may take place before the creation of the interlock when a large diameter punch is used. On the other hand, Lee et al. [39] reported the relationship between the punch diameter and joint failure load (Fj) as Eq. (11).

Fj = σf ∙π (dP ∙tn + tn2)

(11)

Fig. 12. Typical load-elongation curves for CFRP/Al samples of different thicknesses. 259

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Fig. 13. Joint shear load versus discharge energy for different thickness samples.

undercut will be excessively small due to the debilitated material flow to the die cavity. Both the small values for tn and ts negatively affect the joint strength. Thus an optimum punch diameter should be selected to manufacture a high quality joint. Punch corner radius is an important parameter in material flow and creation of mechanical interlock. A high radius (blunt punch corner) gives an incomplete flow of material. It reduces the damage at neck region among with a small-sized undercut [11]. While, low radii (sharp punch corner) results in high shear stress on the punch sided sheet and failure may take place before the creation of mechanical interlock. Indeed, only the upper sheet is deformed in the hole-clinching process; it is not supported by the lower sheet. Therefore, the hydrostatic pressure which helps with geometrical interlocking at high strain state without fracture is less than that in the conventional clinching process. This means that sharp punch corners may cause neck fracture of the upper sheet during the indentation step [39]. Die cavity diameter and depth also affect the joint characteristics. A deep die results in prolonged offsetting phase which may induce circumferential cracks [40] and/or excessive thinning at the neck region (excessively small neck thickness).

Table 4 Maximum neck thickness and undercut values obtained by various hole clinching techniques. Joint materials (thickness of metallic workpiece)

tn

ts

AA6061/CFRP AA5052/CFRP AA5052/CFRP AA6082/CFRP AA5083/CFRP AA5083/CFRP AA3003/CFRP AA3003/CFRP

0.601 0.540 0.420 0.730 0.770 0.830 0.877 0.723

0.512 0.130 0.210 0.220 0.280 0.210 0.833 0.642

(t = 2.0 mm) (t = 2.0 mm) (t = 2.0 mm) (t = 2.0 mm) (t = 1.4 mm) (t = 1.4 mm) (t = 1.6 mm) (t = 1.6 mm)

[11] [36] [36] [3] [37] [37] (present work) (present work)

The precise coaxial alignment of the punch, die, and predrilled hole is the second point to consider, after selecting the dimensions of the punch and the die. The material flow in a coaxially aligned setup and a non-coaxially aligned setup are depicted in Fig. 15b and c respectively. It can be seen that when the punch, the die and the hole on the CFRP

Fig. 14. Superior undercut and neck thickness of EMC joint. 260

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Fig. 15. The effect of coaxial alignment of punch and die cavity: a) excessively large diameter punch; b) coaxially aligned setup; c) non-coaxially aligned setup.

laminate are coaxially aligned (Fig. 15b), the material has the same space to flow in all directions. As a result, the thickness distribution is perfectly uniform in the necking zone, and the undercut is also of high quality in all directions and a joint with the highest possible quality and strength will be manufactured. Now assume that, these three components of the system are not coaxially aligned as in Fig. 15c. In this case, the material experiences severe shear in some areas because of excessive proximity to the workpiece hole. This results in the neck failure or at least severe thinning of the necking zone in such areas. On the other hand, in some areas the material cannot fill in the die cavity due to the excessively large clearance between the punch and the pre-drilled hole. Therefore, the undercut will not have an acceptable quality in any direction. As a result, the joint will experience neck failure, button separation, or a combination of these two failure modes, even for small loadings. The geometrical characteristics of the toolset used in this study are summarized in Table 5. However, it should be mentioned that the coaxiality problem could be addressed using a more constrained setup. The EMC process is in its very earlier stages of development and requires some modifications. A more controlled punch movement and more constrained workpieces can considerably address the problem. On the other hand, Lambiase and Ko [3] showed the feasibility of clinching aluminum and CFRP without a pre-drilled hole. In this case, the aluminum bulge causes the delamination of the CFRP during the offsetting phase. Increasing the punch stroke, causes the CFRP to upset against the die cavity, under the aluminum sheet. Definitely, this leads to the fracture of the CFRP. Thus, a hole in the CFRP sheet is formed since there is a complete cut of the carbon fibers surrounding the aluminum bulge. Implementing the process in such a way, prevents the pre-drilling step and consequent outcomes such as coaxiality problem. However, the product (the joint) may be unacceptable from aesthetic point of view.

Table 5 Tooling parameters for the design of EMC setup. Tooling parameter

t = 0.6 mm

t = 1.2 mm

t = 1.6 mm

Hole diameter (mm) Punch diameter (mm) Punch corner radius (mm) Die diameter (mm) Die depth (mm)

8 5.6 0.6 8.8, 6.6 1.0

8 6.0 0.6 8.8, 6.6 1.0

8 6.4 0.6 8.8, 6.6 1.0

Fig. 17. Flow stress curves for different strain rates.

3.3. Strain rate dependent hardening The joining area is a part of sheet metal in all variants of the clinching process. Thus, the strength of the clinched area significantly influences the joint performance. In fact, the local flow stress in the clinched zone has a considerable influence on the joint strength. The test results, clearly showed an increase in the joint strength compared to the conventional clinching. It can be interpreted as a result of strainrate dependent hardening. Exposing a high strain rate process results in much more work hardening on the clinched area and thus improved joint strength compared to the conventional clinching methods. Fig. 17 shows the flow stress curves of the AA 3003-H14 for different values of strain rates obtained by the Hopkinson pressure bar test. It can be seen that the flow stress is strongly depended to the strain rate, especially for high values of strain rate (above 1000 s−1). It is also observed that the formability of the AA3003-H14 has increased significantly at the high strain rates, from 8.5% in the strain rate of 0.1 s−1 (not included in the figure) to about 20% at a high strain rate. In fact, EMC is a promising technique to improve the joint strength in the strain rate sensitive

Fig. 18. Signs of material heating on the CFRP laminate.

materials such as AA3003. Some signs of material heating can be observed on the CFRP laminates. A heat affected laminate is depicted in Fig. 18. However, the exact amount of temperature increase is not studied in the present work and has been postponed to the future plans. Certainly, some part of the deformation energy converts to the heat, due to the high strain rate of the process. In case of metallic workpiece, an increase in the

Fig. 16. A typical sample processed by non-coaxially aligned excessively large-diameter punch.

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Fig. 19. The Vickers hardness distribution in clinched samples processed by different discharge energies (the hardness of base metal is 44.0 HV).

the Al/CFRP penetration depth; d, tn and ts are the punch diameter, neck thickness and undercut respectively. On the other hand, the joint efficiency is defined as the ratio of the joint experimental failure load to an analytical one, based on a definition for the maximum possible joint shear strength. Apparently in the conventional clinching processes, the sheet thickness in the neck area is thinner than the initial thickness of the sheet, due to the conservation of the mass. Therefore, the upper bond limit for the strength of the joint in the hole-clinching process is when the neck thickness is considered to be equal to the sheet thickness (which is commonly impossible due to the wall thinning effect). This condition is schematically shown in Fig. 22. Based on this figure, the maximum possible joint shear strength (Ftc) that the joint can withstand can be calculated as shown in Eq. (14). In this equation, τc is the ultimate shear strength of the material drawn to the pre-drilled hole and A is the neck area. Consequently, the joint efficiency can be introduced as Eq. (15), where Ft is the joint shear load achieved in the experiment. It can be seen that considering proper tooling and processing parameters for the EMC, a sound joint with a desirable bearing stress, and joint efficiency can be manufactured. Evidently, much more strength can be obtained for joint samples of the EMC, using a higher strength material (e.g. aluminum series 6xxx or 7xxx and steels).

temperature causes the annealing and thermal softening of the material. On the other hand, the strain rate sensitivity of the material increases with increase in temperature. In some aluminum alloys the strain rate sensitivity even becomes negative at room temperature. As a result, the effect of the strain rate at high temperatures will be much higher compared to the room temperature. Accordingly, a combined effect of thermal softening and strain hardening will occur as a result of simultaneous increase in temperature and strain rate in the EMC. The thermal softening relatively increases the material formability and the process can be used to clinch low-formability materials. On the other hand, the strain rate dependent hardening increases the hardness of the material in the processed zone (punching zone and necking zone). Resultantly a higher strength joint can be manufactured. The hardness distribution for the base material and processed material (clinched zone) are illustrated in Fig. 19 for different values of the discharge energy. The figure shows a significant increase in the hardness of the processed zone compared to the base material. It also can be seen that the increase in the discharge energy results in higher strain hardening. As a comparison, the hardness of the necking zone in the workpiece processed by discharge energy of 4.5 kJ is 1.7 times the hardness of the workpiece processed by discharge energy of 2.88 kJ. In addition, it can be seen that for each joint sample, a high hardness value is obtained in the necking and undercut zones, which results in superior performance of the interlock created by EMC compared to the other clinching techniques. The combination of strain rate dependent hardening and superior neck thickness as well as undercut values results in a stronger and more efficiency clinched joint. As a comparison, the best joint strength obtained in the work of Lambiase and Ko [3] was about 2.6 kN, whereas they utilized aluminum AA6082-T6 (t = 2.0 mm) that has very higher ultimate strength than AA3003-H14 (t = 1.6 mm) used in this study (340 MPa vs. 160 MPa). The maximum bearing stress and joint efficiency of all samples are reported in Fig. 20 and Fig. 21, respectively. The maximum bearing stress of the joint can be calculated using Eq. (12) [3]. In this equation, σb is the maximum bearing stress, Ff is the joint failure load in shear and Ab is given by Eq. (13) [41], where h is

σb = Ff / Ab

(12)

Ab = h (d + 2tn + ts )

(13)

Ftc = τc × A =

π 2 [dh − (dh − 2tn )2] 4

η = Ft / Ftc

(14) (15)

3.4. Failure modes There are basically two failure modes in a mechanically clinched joint: button separation and neck fracture. However, in some cases a combination of these two modes can also be observed. These failure modes are illustrated schematically in Fig. 23. Generally, button separation occurs when the undercut is small, and neck failure takes place 262

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Fig. 20. Maximum joint bearing stress.

fracture were observed in this area. Thus some modifications can be applied to the process to prevent the delamination and enhance the joint strength. As mentioned before, applying local pressure on CFRP laminate results in dragging on the laminate, and dragging leads to delamination (Fig. 5). In the EMC process, the velocity of punch and its impact on the materials terminates the pressure on the CFRP laminate and dragging occurs. Resultantly, fracture takes place at the lower load than the desired one. Small undercut also leads to button separation. In this case, the undercut is very small to bear the shear load and aluminum sheet separates from the CFRP laminate easily.

when the sheet thickness is very low at the necking zone. In other words, generally a small value of ts results in button separation and a low value of tn results in neck fracture. Fig. 24 shows the failure modes of some test specimens. The failure mode of specimens of 0.6 mm and 1.2 mm thickness were generally the neck fracture mode. The low thickness of the metallic sheet at necking zone is the reason for this failure mode. For example; the thickness of 0.6 mm aluminum sheet at necking zone is lower than 0.1 mm and this makes the neck fracture reasonable. On the other hand, for 1.6 mm thick metallic sheet specimens, button separation failure mode was observed in addition to neck fracture mode. Button separation has two main reasons: low undercut value and CFRP delamination. In this failure mode, the clinched area generally remained without damage. During the single-lap shear test, the tensile load acting on the hole of the CFRP laminate caused the button separation and clinched area separates from the laminate, no signs of

3.5. Other advantages Beside the improvement in joint strength, another advantage obtained by this high-speed process is the capability of joining materials of low formability. As can be seen from Fig. 4 and Table 1, the strain-to-

Fig. 21. Joint efficiency. 263

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Fig. 22. Schematic of a single-lap shear joint used in definition of the joint efficiency.

The EMC system is not limited to the certain materials. It can be used to join hybrid metal/composite as well as metal/metal or composite/composite joints. Fig. 25 shows a triple layer joint made by the EMC. Two metallic sheets are clinched to a pre-drilled CFRP laminate. Pre-drilling process is not required for joining metallic parts. The superior undercut and neck thickness are also clear in this case. Two 1.2 mm thick aluminum sheets are clinched and the values ts = 0.357 mm and tn = 0.460 mm are obtained. On the other hand, two composite parts can be joined using a small dummy metallic tape. For this purpose, two pre-drilled CFRP sheets lie on each other, and a metallic dummy tape lies under them. Other principles of the process are as former. It is a significant advantage of the system, whereas the thermosetting CFRP has no welding capability. So, it is desirable to join them by another consumable material. Accordingly, in this method, the consumable material is minimal in size and is an inexpensive and costeffective way to join two CFRP laminates.

Fig. 23. Failure modes in a mechanically clinched joint: a) neck fracture, b) button separation [14].

failure for the aluminum used in this work is only 8.5%. Such weak formability limits the forming processes and the locally severe plastic deformation of the clinching process cannot be applied by conventional clinching machines. However, utilizing the high strain rate of the EMC process makes it possible. Joining other low formability materials such as advanced high strength steels is the matter of future works by the authors.

4. Conclusions An electromagnetically assisted high speed clinching process was proposed and investigated. Aluminum and CFRP sheets were used for

Fig. 24. Failure mode of the joint samples; (a) button separation, (b) partial delamination, (c) button separation, (d) to (i) neck fracture. 264

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Fig. 25. Triple layer joint made by EMC.

experiments. The joint specimens were applied under universal shear test. The joining by forming system was designed to overcome the restriction of the necessity of high electrical conductivity of workpieces in EMF process. So, it can be used to join any ductile materials. It is proved that using electromagnetic force as the driving force of the clinching process, can increase the strength of the joint area compared to the conventional clinching. Such an improvement is a result of strain rate dependent hardening. This is a promising technique to improve the joint strength in the strain rate sensitive materials. Furthermore, the process is capable of joining materials of low formability, which cannot be achieved by conventional clinching machines. Thus one important advantage of the system is its capability to join dissimilar materials of low formability and another advantage is its ability to improve the mechanical characteristics of the processing (joining) zone. According to the experimental results, it has been proven that superior undercut and neck thickness can be achieved, when the processing and tooling parameters are selected correctly. This leads to a superior strength and joint efficiency. Dragging and delamination of the CFRP sheet around the clinching zone can negatively influence the joint strength, thus controlling the dragging can be of particular importance to enhance the joint strength. Applying some modifications and optimizations to the proposed joining by forming system to control and avoid the dragging as well as to control the punch movement and to constrain the workpieces can be the fields of further studies. The investigation of some other processing and tooling parameters such as the effect of heat treatment of the punch on the mechanism of deformation and joint quality will also be a field of future study. Finally, some theoretical frameworks such as study on the exact amount of workpiece heating and investigation of combined effect of temperature and strain rate on the mechanism of deformation and characteristics of processing zone in a theoretical manner, will be on the plan.

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