induction spot welding of metal/cfrpc hybrid joints

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DOI: 10.1002/adem.201200273

Induction Spot Welding of Metal/CFRPCHybrid Joints**
By Peter Mitschang,* Rudi Velthuis and Mirja Didi
Today’s areas of application for lightweight materials range from consumer goods and sports to hightechnology applications in transportation like aerospace or automotive. Thermoplastic compositesmanufactured by compression molding, thermoforming, tape laying, or injection molding will play amajor role due to their weldability, their suitability for automated production (robot), and theirrecyclability. To reach a further step of weight reduction, the use of carbon fiber reinforced polymercomposites (CFRPC) is unavoidable. A full substitution of metal is unlikely and new developments willconsist of a combination of metal and CFRPC. This paper shows new developments in joining hybridsof metal (steel DC01 and aluminum AlMg3) and CFRPC (organic sheets CF-PA66 and CF-PEEK).Induction heating is chosen as appropriate joining technology for the bonding of metal/CFRPC as it ischaracterized by a rapid heating. An explanation of the process, the equipment, the influence of surfacetreatments, the characterization of the bonding mechanisms, as well as a first step to automation arepresented. Basic experiments on the influence of pretreatments and process parameters show greatinfluence of corundum blasting, acidic pickling and temperature control on the shear tensile strength.Joints shear tensile strength of 14.5 MPa for AlMg3/CF-PA66 and of 20 MPa for DC01/CF-PEEK,respectively is measured. The documentation of the process parameters shows a high reproducibility

onstrator parts are successfully manufactured.
and reliability of the developed equipment and dem
1. Introduction

Today’s global efforts to reduce the overall energy

consumption often correlate with lightweight design or

lightweight construction allowances. Areas of application

for lightweight materials range from consumer goods and

sports to high technology applications in transportation like

aerospace or automotive. In a first step of lightweight design,

there was a material replacement from steel to aluminum,

magnesium, or titanium. To increase the weight reduction

[*] Prof. P. Mitschang, Dr. R. Velthuis, M. DidiUniversity of Kaiserslautern,Institut fur Verbundwerkstoffe GmbH,Erwin-Schrodinger Str. Geb. 58, 67663, GermanyE-mail: [email protected]

[**] The authors acknowledge the financial support providedby ‘‘Deutsche Forschungsgemeinschaft’’ for the funding inthe frame of the DFG-Research-Group 524 ‘‘Herstellung,Eigenschaftsanalyse und Simulation geschweißter Leichtbaus-trukturen aus Metall/Faser-Kunststoff-Verbunden’’ (http://mechanik.mv.uni-kl.de/forschergruppe/index.html)

804 wileyonlinelibrary.com � 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ADVANCED ENGINEERING MATERIALS 2013, 15, No. 9

aerospace industry began to use carbon fiber reinforced

thermoset polymers. These materials fit to aerospace require-

ments in quality and production rates very well, and new

airplanes will have a material share with of 50% polymeric

composites by weight. In the last two decades, the automotive

industry also began to use polymeric composites for semi

structural parts like front ends, under body covers or

dashboards. Glass fiber reinforced thermosets (sheet molding

compounds, SMC) as well as glass fiber reinforced thermo-

plastics (glass mat reinforced thermoplastics, GMT, and long

glass fire reinforced thermoplastics, LFT) were chosen due to

technical advantages in combination with cost-saving and

mass production abilities. In addition, thermoplastic compo-

sites manufactured by compression molding, thermoforming,

tape laying, or injection molding will play a major role due to

their weldability their suitability for automated production

(robot), and their recyclability. The latter, for example,

meets the demands of state regulations like the end-of-life

vehicle law.[1] Current developments are focusing on the

use of continuous glass fiber reinforced thermoplastics, so

called organic sheets, for structural applications like

bumper systems. The adequate manufacturing technology

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P. Mitschang et al./Induction Spot Welding of Metal/CFRPC Hybrid Joints

is thermoforming in match metal tools similar to metal sheet

stamp forming or the in process combination of thermoform-

ing and injection molding.[2,3] To reach a further step of weight

reduction in automotive industry, new developments have to

compete with aluminum used for space frame architectures or

body side panels. Therefore, the use of carbon fiber reinforced

polymer composites (CFRPC) is unavoidable. Major dis-

advantages of CFRPC are the high costs compared to metal

solutions and the already not adequate developed manufactur-

ing technologies especially for high mass production. Conse-

quently, a full substitution of metal is unlikely and new

developments will consist of a combination of metal and

CFRPC. This paper shows new developments in joining hybrids

of metal (steel DC01 and aluminum AlMg3) and CFRPC

(organic sheets CF-PA66 and CF-PEEK). Induction heating is

chosen as appropriate joining technology for the bonding of

metal/CFRPC as it is characterized by a rapid heating. An

explanation of the process, the equipment, the influence of

surface treatments, the characterization of the bonding

mechanisms by single-lap joint experiments and by microscopic

analysis, as well as a first step to automation are presented.

2. State of the Art Metal/CFRPC Joints

There are some results presented in literature dealing

with joining of metal and CFRPC. Only a few are directly

comparable to this presented study joining steel (DC01; 1.0330 to

DIN 17163) or aluminum (AlMg3, 3.3535 to DIN 1725-1)

and carbon fiber reinforced thermoplastics (CF-PA66 and

CF-PEEK). The experiments in literature study were performed

in accordance with DIN EN 1465 (single-lap joints) or in the style

of the standard. A comprehensive presentation is given in

Table 1. Depending on material combinations, pretreatment,

Table 1. State of the art of metal-polymer joints.[4]

Manufacturing process Joiningpartner 1

Joiningpartner 2

P

Hot pressing Al (6061-T6) PP Metal: anodizing i

Resistance welding Al (7075-T6) CF/PEEK Metal: Cleaning w

anodizing in H3PO

Heat bonding with

PEKEKK

Ti-6Al-4V PEKEKK Grit blasting with

anodizing in NaOH

Heat bonding Ti-6Al-4V Ti-6Al-4V PPQ, GF/PEI, PEI,

Heat bonding with

polyimide 422

Ti-6Al-4V Ti-6Al-4V Polyimide 422

Heat bonding Ti-6Al-4V Ti-6Al-4V Ti: Nitric-hydroflu

chromic acid anod

Al: FPL as well as

chromic acid, with

Induction welding Al (2024-T3) CF/PEEK Metal: pickling in

Heat bonding with PEEK DC01 DC01 Metal: Acetone tre

Adhesive bonding with 1K,

2K epoxide; 2K polyurethane

AlMg4,5

Mn0,4

AlMg4,5

Mn0,4

Metal: acetone – co

Adhesive bonding with

epoxy resin

AlMg3 AlMg3 Sulfuric acid pickl

NaOH-pickling, gr

Adhesive bonding with

1k-epoxy resin

AlMg3 CF/PA66 Sulfuric acid pickl

ADVANCED ENGINEERING MATERIALS 2013, 15, No. 9 � 2013 WILEY-VCH Verl

and testing method shear tensile strength values from 4 up to

25 MPa are reported within this compilation.

To obtain comparable testing results in this study and in

relation to the literature study (Table 1) all experiments are

performed according to DIN EN 1465. For the single-lap joint

the specimen length of both welding parts is set to 100 mm, the

specimen width to 25 mm, and the overlap to 12.5þ/� 0.25 mm.

The thickness of the aluminum specimen is 1 mm and the

thickness of the CFRPC specimen 2 mm. To ensure a parallel

load introduction two plates with a length of 37.5 mm, a width

of 25 mm, and a thickness of 1 or 2 mm, respectively are attached

to both ends of the sample. The tests are performed on a

standard universal testing machine (1485, Zwick) with a testing

speed of 1 mm �min�1. The inhomogeneous stress distribution

due to differences in thickness is determined by Schmeer.[16]

The organic sheets are manufactured of a 5H satin, 3k carbon

fiber fabric, and a plain weave. Both fabrics have an area weight

of 285 g �m�2. Six layers of fabric are fully impregnated with

PA66 or PEEK in a film stacking process and consolidated in the

autoclave to a height of 2 mm. The fiber volume fraction is

calculated to 0.48. The used aluminum sheets is AlMg3 and

the steel sheet is DC01 with a thickness of 1.0 mm.

To improve the surface area, the joining partners are

treated before joining. In this study, degreasing, sandblasting,

and chemical treatment are used. The effects are explained in

Table 2. A detailed study is given in literature.[4,21]

3. Technology of Induction Heating andExperimental Set-Up

Inductive heating is well known for induction hardening in

metal technology. The same effect can also be used to heat up

carbon fiber reinforced polymers by inducing eddy current into

retreatment Testingmethod

Joiningstrength[MPa]

Reference

n phosphoric acid; PP: plasma ASTM 3163 19 [5]

ith trichloroethane,

4, Coating with PEI

ASTM 3163 Up to 20 [6]

60 grit Al2O3 and Bar with ; 19 mm 95–130 [7]

and PES ASTM D-1002 17–34 [8]

ASTM D-1002 30–41 [9]

oric acid and

ized

ASTM D-1002 44–55 [10]

grit blasting,

and without primer

chromic acid ASTM D-1002 31–33 [11]

atment, grit blasting, polishing DIN EN 1465 20–45 [12]

rundum blasting – acetone DIN EN 1465 15–18 [13]

ing, phosphoric acid pickling,

it blasting, polishing

DIN EN 1465 4–19 [14]

ing, NaOH-pickling DIN EN 1465 17 [15]

ag GmbH & Co. KGaA, Weinheim http://www.aem-journal.com 805

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Table 2. Effect of different surface treatment methods.

Method Treatment Effect Reference

Cleaning, degreasing Washing with a solvent, like acetone Cleaning of the joining partners

Mechanical treatment Sandblasting Geometrical changes of the surface! roughness [17]

Removal of contamination layers

Chemical treatment Alkaline pickling; acidic pickling Change in the chemical structure of the surface (e.g. oxidation) [18–20]

the carbon fibers. Some restrictions in relation to fiber content

and fiber orientation are given and reported in literature.[22]

Welding of thermoplastic fiber reinforced polymer composites

means joining by melting the thermoplastic polymer, diffusion

or adhesion of molecule chains at the bonding line and

reconsolidation under pressure.[23,24] Specially for fiber rein-

forced polymer composites (FRPC) welding seems to be an ideal

joining technology and compared to other heating mechanisms,

induction heating is characterized by contact free, fast, and

locally concentrated energy input directly into the laminate.[25,26]

3.1. Principle of Inductive Heating

If carbon fibers are used as reinforcing material, the fibers can

be heated directly giving special opportunities for automa-

tion.[27] Different heating mechanisms are operating depending

on the fiber architecture, manufacturing conditions, and

consolidation quality. When the carbon fibers are in good

contact, joule losses will lead to fiber heating. If a low fiber

content or bad consolidation is given, junction heating

by dielectric hysteresis and fiber contact is the main heating

mechanism (Figure 1).[23] Especially fiber heating by joule losses

is very effective. Woven textiles are best to guarantee a good fiber

contact and to realize close conductor loops and inductive fiber

heating. To calculate inductive heating electromagnetic and heat

transfer have to be taken into account.[28] Analytical models will

help to get a basic idea of the influence of different parameters,

but they are not sufficient to calculate real complex material

systems like CFRPC-CFRPC joints or metal-CFRPC joint.[29]

The heat generation P inside the fiber in accordance to

Joules law can be described as shown in Equation (1).[22,29]

P ¼ 4p2f2m2H2A2

R(1)

P is the generated power, f is the field frequency, m is the

permeability of the material to be heated, H is the magnetic

Fig. 1. Heating mechanism in carbon fiber reinforced laminates.[23]

806 http://www.aem-journal.com � 2013 WILEY-VCH Verlag GmbH & C

field intensity, A is the area enclosed by a conductive fiber

loop, and R is the resistance.

Another very important effect is the so called skin-effect

which leads to a non-homogeneous heat distribution during

inductive heating, especially for materials with a high

magnetic permeability like fero alloys. Alternating current

flowing through a resistive conductor generates an uneven

current distribution on and perpendicular to the surface

where the maximum is located directly on the surface.[28] The

characteristic value is the depth of penetration d which can

be calculated by Equation (2).

d ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

1

pfm0mrs

s(2)

The depth of penetration d depends on the frequency of the

magnetic field, the magnetic permeability m0 of vacuum, the

electrical conductivity s and the relative permeability mr of

the material. The described depth is the distance perpendicular

to the surface where the flux density is reduced to 1/e of the

value acting on the surface. Based on the depth of penetration,

the current density along a circular conductor in radial direction

can be calculated according to Equation (3) where J is the current

density at the distance y to the surface, d the depth of penetration

and J0 the current density at the surface.

J ¼ J0ey=d (3)

Due to the high complexity of induction heating effects,

developments are carried out mainly empirically. To reduce

the number of experiments and to improve the process

understanding new modeling techniques are necessary.

Three-dimensional models will help to receive a spatial

impression of the effectiveness of changing the inductor

geometry or the influence of other process parameters. As an

example, the use of Finite-Element codes allows the combined

transient simulation of induction heating and thermal

analyses and to calculate the temperature distribution in a

given work-piece (Figure 2).

Starting point is the creation of the geometry and the

parameter set of material characteristics, like temperature,

dependent heat capacity, or heat conductivity, and the

calculation requirements. The next step is the calculation of

the magnetic vector potential and the distribution of the

turbulent flow. In a transient thermal simulation, the heat

generation and temperature distribution is determined. The

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Fig. 2. Principle scheme of the simulation of inductive heating.

temperature dependent material parameters are checked each

time step (Figure 2; ti) and corrected, if necessary, for the next

time step. The calculation ends when the set heating time

(Figure 2, thj) is reached.

3.2. Experimental Set-Ups

Three different equipment configurations are used. In a

first series of basic experiments, the influence of different

pretreatments and process parameters are investigated by use

of configuration C1, a three phase discontinuous induction

welding process (Figure 3). In phase I, the joining partner

nearest to the inductor (e.g. the metal part) is heated up. The

polymer matrix of the organic sheet (e.g. CF-PA66) is melted

due to heat conduction. The metal sheet protects the

composite part from being heated by induction caused by

the magnetic field. In phase II, the joining partners are

transported by a linear motion to the consolidation station.

Fig. 3. Principle of concept one C1, a 3 phase discontinuous induction welding.


Joining pressure is applied in phase III and the joining

partners are cooled down and re-consolidated under pressure.

The important process parameters are the temperature in

the joining area, the consolidation pressure, holding time,

and cooling rate. These process parameters are recorded

(Figure 4). The welding process is controlled by a computer

and runs automatically. Figure 4 shows the pressure and

temperature development during the welding process. It can

be seen that a significant temperature drop of 50 K occurs

during the transport phase (II). To assure the melting state

of the polymer at the beginning of Phase III a maximum

temperature near the degradation temperature of the polymer

is necessary in Phase I.

In a second step configuration C2 is developed. The major

difference to C1 is a modification to apply temperature and

pressure at the same place and at the same time (Figure 5). Due

to this the transport phase (II) was eliminated and heating

takes place during pressure application, which leads to less

deconsolidation of the composite joining partner. A detailed

description of the developed temperature-pressure-stamp is

given in Figure 6. The inductor is placed above an inlay. If the

inlay is nonmetallic, the magnetic field directly affects the

upper welding part. If the inlay is metallic, it will be heated up

and the welding parts are affected by conduction of heat.

Simultaneously to the temperature the pressure is applied by a

Fig. 4. Pressure and temperature record of configuration one.

Fig. 5. Configuration two C2 with simultaneous temperature and pressure application.


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Fig. 6. Detailed description of the integrated temperature-pressure-stamp, configur-ation two C2.

ring stamp, which is water-cooled from the inside. This

configuration leads to a spot heating while the surrounding

material, especially the high conductive metal, remains cool

and problems related to different thermal expansion coeffi-

cients are dramatically reduced. Thermal calculations show

the heat flow inside the stamp (Figure 7). The heating zone is

set to a radius of 5 mm (point 2 in Figure 7) related to the center

point and the cooling zone in a distance of 10 to 20 mm from

the center point (point 3 in Figure 7). As a requirement the

temperature at the borderline of the heating zone (point 2 in

Figure 7) has to be higher than the melting temperature of

the polymer (e.g. PA66 260 8C). If the cooling is set to zero,

the temperature (upper curve in Figure 7) decreases very

slow and even outside the pressure zone (distances higher

than 25 mm from the center point) the temperature is in a

range that deconsolidation of the CFRPC part will take

place. An optimized cooling shows two major advantages.

The temperature peak (point 1 in Figure 7) in the center can

be increased by 70 K, which indicates faster heating and the

material temperature stays below crystallization temperature

outside the pressure zone preventing uncontrolled decon-

solidation. The temperature in the contact zone between

stamp-inlay and welding part is controlled by the power of the

generator and the temperature of the cooling water. Thus, a

defined heat distribution is realized and reproducibility as

well as reliability are significantly improved.

Fig. 7. Calculation of feat flow in stamp configuration two, C2.


Finally, a third configuration C3 is investigated where the

temperature-pressure integrated stamp was modified to be

integrated in a robot cell (Figure 8) to demonstrate an

automatic spot welding process. Figure 8 also shows the final

demonstration part which consists of a hat shaped CF-PA66

part manufactured in a thermoforming process and an

aluminum (AlMg3) sheet to close the open profile. The parts

are joined by a minimum of 5 and a maximum of 10 welding

points on both flanges.

4. Basic Results on Hybrid Joining byInduction Heating

To set the allowed maximum and minimum temperature

for the polymer materials and to assure a good bonding, the

minimum temperature to enable welding is the melting point.

Depending on the heating rate and the environment, the

maximum allowed temperature is reached, when a weight

loss of 5% occurs. Thermogravimetric analysis (TGA) is

used to define the process window for PA66 or CF/PA66,

respectively (Figure 9) and CF/PEEK. The use of an inert gas

atmosphere (nitrogen) increases the degradation temperature.

The melting temperature for CF-PA66 is 260 8C and the

maximum temperature is set to 350 8C. Figure 9 shows

that it is not necessary to use nitrogen to reach a sufficient

temperature level. For CF-PEEK the melting temperature is

345 8C and the maximum temperature is set to 500 8C. In both

cases, a large process window is given.

Figure 10 gives an example for a load displacement curve

of a single-lap joint tensile test performed on an AlMg3/

CF-PA66 specimen. The load at brake is above 4 kN and all

three specimens show a high reproducibility.

To investigate the influence of different process parameters

on the tensile strength of the metal/CFRPC joints, all

previously described facility configurations are used (Table 3).

4.1. Effect of Pretreatments

Table 4 gives an overview of the used pretreatments and

their application to the metal joining partner and the CFRPC,

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Fig. 8. Configuration three C3, a robot cell for automated inductive spot welding and demonstration part.

respectively. The experiments are performed with configura-

tion C1. Major influence can be found in using corundum

blasting and acidic pickling which increase the shear tensile

strength by 60% in relation to a simple degreasing with

acetone (Figure 11). A combination of these pretreatments

Fig. 9. Weight loss of PA66 and CF-PA66 by increasing temperature.

Fig. 10. Single-lap joint tensile test.


does not lead to higher strength. Also other physical

treatments like plasma do not show any further improvement.

Similar results can be seen for AlMg3/CF-PA66 and

DC01/CF-PEEK. In practical use, corundum blasting is much

easier than acidic pickling. For this reason corundum blasting

for the metal partner and acetone cleaning for the CFRPC

partner are set as standard pretreatments.

Due to the high fiber volume content (48%) of the organic

sheets, the amount of polymer remaining directly at the

surface near the joining zone is very low. Adding additional

polymer to the joining zone by supplying a polymer

compatible film of 100 mm PA66 or 300 mm PEEK leads to

an additional increase in shear tensile strength of 15% for

AlMg3/CFPA66 and 55% for DC01/CF-PEEK, respectively

(Figure 12). The effect of adding polymer to the joining zone is

also shown in Figure 12. It can be seen that the additional

polymer creates an interlayer. This also helps to prevent

galvanic corrosion between the aluminum and the carbon

fibers.[21] In relation to the starting point, the shear tensile

Table 3. Use of the three different configurations to investigate metal/CFRPC joints.

Configuration Abbreviation Investigations

3 phase process C1 Pretreatment

Additional polymer in the

joining zone

Fast cooling

Comparison to adhesive

bonding

Simultaneous heating

and pressure

application

C2 Optimization of time

dependent process

parameters

Geometrical limitation of

joining zone

Integration in a

robot cell

C3 Integrated control system

Manufacturing of

demonstration parts


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Table 4. Surface treatments used in this study.

Treatment Description

Cleaning with acetone Metal: cleaning for 3 min in an acetone

Polymer: wiped with acetone

Blasting with aluminum oxide

(white corundum)

Metal: cleaning for 15 min in an acetone and lasting with aluminum oxide at a pressure of 6 bar


Alkaline pickling Metal: alkalic in 1 M NaOH for (15 min); rinsing; pickling in 20% nitric acid (3 min); rinsing with water


Acidic pickling Metal: submersion pickling in 65% nitric acid (15 min); rinsing in water


strength of a single-lap joint can almost be doubled by using

adequate pretreatment techniques.

To improve the weld quality, configuration C1 was slightly

modified to reduce the transportation time to vary the cooling

rate. To investigate the influence of a minimized transporta-

tion time and variable cooling a standard pretreatment by

corundum blasting and acetone cleaning is used. Additional

polymer is added to the joining zone. Switching on the

generator the temperature increases up to melting tempera-

Fig. 11. Principle influence of pretreatment on tensile shear strength for AlMg3/CF-PA66.

Fig. 12. Influence of pretreatment on tensile shear strength for AlMg3/CF-PA66 and DC


ture. By controlling the power of the generator the maximum

temperature is kept constant before a fast transport to the

consolidation station takes place. Then a fast cooling takes

place by using an actively cooled consolidation stamp. The

maximum temperature is set to 290 8C for CF-PA66 and 370 8Cfor CF-PEEK, respectively. Table 5 summarizes the parameter

settings. The welding pressure is removed after sufficient

cooling. A measured temperature distribution during welding

of AlMg3/CF-PA66 is shown in Figure 13. The two curves

show the difference in cooling in relation to a preset cooling

temperature of the pressure tool. In both cases, the cooling

starts very fast and results in a similar crystallinity. A slight

increase of 5–10% of the shear tensile strength can be

observed for the induction welding due to the minimization

of the transportation phase. Another positive effect is an

improvement of the overall reproducibility (reduced standard

deviation).

4.2. Comparison to Adhesive Bonding

To get a quantified criterion for the quality the spot-welded

metal/CFRPC joints are compared with adhesive bonded

joints. For adhesive bonding the AlMg3 specimen is

pretreated using alkaline pickling. For induction welding,

corundum blasting cleaned by acetone and additional

polymer are chosen as standard pretreatment. The DC01

specimen is pretreated with corundum blasting and cleaned

by acetone for both joining technologies. Spot welding

experiments are performed with the modified configuration

01/CF-PEEK.

o. KGaA, Weinheim

C1. The used adhesive bonding system is a

high performance 1k epoxy system with a

curing temperature of 180 8C. Induction spot

welding reaches about 85% of the compar-

able adhesive bonding shear tensile strength

(Figure 14). Taking into account that the spot

welding process takes <2 min these are

excellent results that demonstrate compe-

tiveness to standard joining processes.

4.3. Combination of Temperature and

Pressure Application

The experimental set-up configuration C2

combines temperature and pressure applica-

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Table 5. Optimized parameter setting for spot welding configuration one.

AlMg3/CF-PA66 DC01/CF-PEEK

Pretreatment metal Corundum blasting Corundum blasting

Pretreatment

polymer

Acetone

cleaningþ 100 mm

PA66

Acetone

cleaningþ 300 mm

PEEK

A: Max.

temperature [8C]

290 370

C: Cooling rate 25 K � s�1 6 K �min�1

Fig. 13. Temperature development during spot welding of AlMg3/CF-PA66.

Fig. 14. Comparison between spot welding and adhesive bonding.

Table 6. Parameter setting for spot welding configuration two.

AlMg3/CF-PA66 Processwindow

Pretreatment metal Corundum blasting

Pretreatment polymer Acetone cleaningþ 100 mm PA66

A: Max. temperature [8C] 290 �15

B: Holding time [s] 53 �5

C: Cooling rate 22 K � s�1 �2

tion at the same place at the same time. In Figure 5 the inductor

can be seen in the middle of the cooled stamp. This allows

heating and cooling under pressure, which dramatically

reduces deconsolidation of the CFRPC part. The further

investigations concentrate on AlMg3/CF-PA66 and are

performed with combination C2. A typical parameter set

up is given in Table 6. To improve the polymer adhesion to the


metal the temperature is kept constant for a specific holding

time. In this case, the holding time is set to 53 s. Corundum

blasting and acetone cleaning are used as standard pretreat-

ments. A typical temperature and pressure development over

time is shown in Figure 15. While pressure is applied the

temperature is increased over melting temperature and kept

constant for a specific time. After cooling down the pressure is

removed. The melted area, noticeable at the specimen

(Figure 15), shows clearly a copy of the circular heated inlay

inside the spot welding stamp. The pressure is controlled in a

range of 10% to the ultimate value. The outside temperature of

the stamp is constant at room temperature. With these

experiments the concept of integrated heat transfer under

pressure is verified.

5. First Steps to Automation

The first step towards automation is the integration of

configuration C2 into a numerical controlled robot cell. This

configuration C3 is shown in Figure 8. Different modifications

are done to use the robot control as a master system and to run

the spot welding head in a slave configuration. The robot

system is used for positioning the spot welding head at the

right position relative to the part geometry. The welding

pressure is applied by a pressurizing system integrated in the

spot welding head, measured and documented by the master

control system. Also the surface temperature of the inlay, as

well as the temperature of the cooling water and the outside of

the stamp is documented. To test the integration and the

performance of the new configuration C3, single lap joints are

welded and measured by tensile testing. The tests are

performed in accordance to DIN EN 1465 but an increased

specimen width (70 mm) was used. The increase of the

specimen width should represent infinite part geometry to

simulate boundary conditions for the spot welding process

nearer to application requirements. A first set of experiments

are used to control whether the results are comparable to the

configuration C1 results and to control whether the improve-

ment of a polymer rich joining zone (added polymer film) is

still present. Experiment with and without a 100 mm PA66 film

as additional polymer in the welding zone are performed.

Specimens with an additional polymer film in the joining

zone, welded by configuration C1, in comparison to config-

uration C3 show a slight decrease in the mean value by 9% and

a comparable standard deviation. Comparing the configura-


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Fig. 15. Temperature and pressure over time for AlMg3/CF-PA66 spot welding.

tion C3 results for specimens welded without additional

polymer to the configuration C1 results, a slight increase of

12% is noticeable (Figure 16). One possible reason for this is an

inhomogeneous pressure distribution under the pressure

Fig. 16. Shear tensile strength of AlMg3/CF-PA66 joints manufactured by differentfacility configurations.

Fig. 17. Spot welding of demonstrator part AlMg3/CF-PA66.

812 http://www.aem-journal.com � 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

stamp. The polymer film melts in the joining

zone (area equivalent to the inlay dimension,

e.g. Figure 15) while the surrounding poly-

mer film stays as a solid. By using welding

configuration C1, the specimens are smaller,

the whole polymer film is melted and the

pressure application is homogeneous and

independent from the temperature area.

These results indicate that a further optimi-

zation of the process parameter is possible

and necessary. To demonstrate the ability of

the new welding technology (C3) a demon-

strator part consisting of a thermoformed

CFRPC hat profile and a flat metal sheet was

defined (Figure 8). A set of five spot welds is

applied to both flanges. Figure 17 shows a

measurement documentation of a series of five spot welds

characterized by the inlay surface temperature (indicating the

welding temperature), the temperature of the cooling water,

the outside temperature of the stamp and the pressing force

indicating a constant welding pressure. It is clearly demon-

strated that all parameters stay inside the defined process

windows. Due to the constant process parameters and the

high reproducibility, each spot weld has the same boundary

conditions resulting in a comparable welding strength. The

high quality can also be seen by observing the fracture surface

after testing (Figure 18). Individual carbon fibers still stick on

the metallic welding partner which clearly indicates a partly

cohesive break inside the CFPRC laminate.

6. Conclusions

The present study deals with a new technology to perform

metal/CFRPC hybrid joints. The used materials are aluminum

AlMg3 in combination with CF-PA66 and steel DC01 in

combination with CF-PEEK. Based on induction heating three

configurations are developed. In a first configuration C1,

induction heating and reconsolidation are realized in two

individual process steps. Basic experiments on the influence of

pretreatments show great influence of corundum blasting and

acidic pickling, which lead to the highest shear tensile

strength. The increase of the polymer amount in the welding

zone leads to a further increase of the shear tensile strength.

The integration of heating device and pressure application in

one spot welding stamp (configuration C2) shows higher

process reliability and also a slight increase in welding

strength. The induction welded samples mainly show

physical, but also mechanical adhesion. The temperature

control has a large influence on the shear tensile strength.

Joints shear tensile strength of 14.5 MPa for AlMg3/CF-PA66

and of 20 MPa for DC01/CF-PEEK, respectively is measured.

Comparisons with standard adhesive bonding demonstrate a

good competitiveness of the new technology. The high

potential of the inductive spot welding technology for process

automation is verified by a third configuration C3 where the

spot welding stamp is an integrated part of a computer

ADVANCED ENGINEERING MATERIALS 2013, 15, No. 9

REVIE

W


Fig. 18. Spot welded AlMg3/CF-PA66 demonstrator part and fracture surface.

controlled robot cell. The increase of part dimensions show an

effect of the boundary conditions on the pressure application

which leads to higher shear strength without the use of

an additional polymer film. This result indicates the need of

further investigations and process parameter optimization.

Nevertheless, the documentation of the process parameters

shows a high reproducibility and reliability of the developed

equipment. Demonstrator parts made of thermoformed

CF-PA66 hat profiles and flat AlMg3 sheets are successfully

manufactured.

Received: September 7, 2012

Final Version: October 31, 2012

Published online: January 29, 2013

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