Induction heating of continuous carbon-fibre-reinforced thermoplastics

Transcription

1 Induction heating of continuous carbon-fibre-reinforced thermoplastics Abstract This paper addresses the experimental investigation of induction heating of continuous carbon-fibre reinforced thermoplastics. The influence of the process parameters electromagnetic frequency, generator power, distance between induction coil and laminate, coil geometry and laminate lay-up on the heating rate and the heat distribution have been investigated in stationary experiments. It was found that all investigated parameters have significant influence on the heating behaviour and that a quadratic dependence is dominating. Heat is only generated when closed fibre loops exist, through which current can flow. The quality of the fibre junctions in a laminate, especially the contact length, was found to be of major importance. Thus, for example laminates with unidirectional fibre reinforcement, which do not contain fibre junctions, cannot be heated. Experimental evidence has shown that induction heating of carbon-fibre-reinforced thermoplastics is based on Joule losses Elsevier Science Ltd. All rights reserved. Keywords: E. Joining; Induction heating 1. The continuous induction welding process Today s market for high-performance thermoplastic composites is dominated by small and medium series production and prototyping. Therefore there is a strong need for process technologies with minor capital investment and high flexibility. For that reason a continuous induction welding process (CIW) for carbon-fibre-reinforced thermoplastics (CFRT) has been developed at the Institut fuer Verbundwerkstoffe GmbH, Kaiserslautern (Germany), which is sketched in Fig. 1. The process meets the market needs very well because it requires only minor capital investment and is especially designed for joining complex shaped parts [1]. With induction heating the transferable heat is for example 1500 times that of heat conduction (cf. Fig. 2) [2]. Some work has been performed on induction welding of composites. In most cases metal susceptors have been placed at the welding interface in order to heat up the composite indirectly. Yarlagadda [3] reports on a novel concept for metal mesh susceptors which were especially designed to achieve a uniform in-plane heat distribution. In the EMAWELD process welding is achieved by inductively heating a ferromagnetically filled thermoplastic medium layer placed at the joint interface to the fusion temperature of the abutting composite [4]. Benatar and Gutowski [5] placed a nickel-coated graphite/j-polymer prepreg and two pure J-polymer sheets at the joint interface and heated and joined such composites. Xiao [6] examined the heat affected zones caused by different coil geometries and found in agreement with [7,8] that the heated composite area is a mirror image of the coil. The calculation of the heated zone showed good agreement with the experiments. Most of the earlier work was performed either with laboratory devices and a stationary process was used or there is no exact description of the used devices. Since a stationary process limits the complexity of the parts to be joined the present work focuses on a continuous process. In the developed CIW the parts to be welded are moved with a constant velocity under an induction coil and the magnetic field heats up the laminates. The welding pressure is applied by a cooled roller, positioned at a distance l from the induction coil, which is dependent on the laminate cooling behaviour. The most important quality relevant feature of the CIW is the temperature of the laminate during the four process phases (cf. Fig. 3). 1. Passing the induction coil the laminate surface and interfacial temperature rises to u 1, which represents the maximum temperature to which the laminate is heated. u 1 has

2 1192 R. Rudolf et al. / Composites: Part A 31 (2000) Fig. 1. Schematic representation of the continuous induction welding process. to be slightly higher than the welding temperature (stock temperature), because the laminate temperature decreases until the pressure roller applies the welding pressure. However, u 1 has to be lower than the maximum permissible temperature of the matrix material to avoid thermal degradation. 2. When the laminate reaches the pressure roller it has cooled down to the actual welding temperature u 2 by heat convection to the surrounding air and heat conduction to the workpiece fixture as well as to the adjacent laminate regions. This temperature has to be high enough to enable the interdiffusion of the macromolecules and molecular chains in order to effect the weld. 3. After the laminate has passed the roller the temperature u 3 has to be low enough to prevent delamination and deconsolidation of the laminate. In order to cool down the laminate sufficiently the roller is water-cooled. 4. Behind the roller an increase in the laminate temperature to a peak temperature u 4 can be observed, which is caused by the residual heat stored in the laminate. To prevent defects in the laminate, u 4 has to be lower than the recrystallisation temperature of the matrix material. This can be achieved by an additional cooling by compressed air. Indeed, the laminate temperature should Fig. 3. Typical temperature-time-curve of the continuous induction welding process. be held at the recrystallisation temperature for a certain period of time to complete the crystallisation of the matrix material. Otherwise, if the laminate is used at a temperature above the recrystallisation temperature, recrystallisation and shrinkage of the matrix material might occur, which in turn can lead to warpage or laminate defects like matrix cracks or delamination. This paper focuses on the inductive heat generation in phase I. 2. Induction heating principle When an electrically conductive, non-magnetic material is exposed to an alternating magnetic field, eddy currents are induced and the material is heated due to resistive losses of the eddy currents. In magnetic materials, hysteretic losses occur which lead to an additional heat generation [2]. Carbon-fibre reinforced thermoplastics can be induction heated without any additional material since the carbonfibres are electrically conductive. Glass-fibre reinforced Fig. 2. Transferable heat with different heating mechanisms [2].

3 R. Rudolf et al. / Composites: Part A 31 (2000) Fig. 4. Power output of the induction generator. thermoplastics on the other hand, can only be induction heated by means of an additional electrically conductive susceptor material which is placed between the parts to be joined. The principle of induction heating of carbon-fibre reinforced thermoplastics was investigated by Miller et al. [7] and Fink [8]. However, both found contrary reasons for the heat generation. Miller et al. found that eddy currents are induced in the carbon fibres, and that electrical current transfers between the fibres and fibre layers since they are in contact or close proximity. The heat generation is caused by Joule losses in the fibres. However, the existence of conductive loops is required, so that the unidirectional carbon-fibre laminates cannot be induction heated. In a subsequent work to [7] a two-dimensional model was developed for induction heating and a Macintosh based software programme was developed [9]. With this, the eddy current distribution, the field strength distribution and the temperature distribution of the laminate surface is made possible. The agreement between the measured and the calculated temperature distribution was very good. However, the programme is not commercially available. Fink, on the other hand, claimed that even for laminates with fibre volume fraction of 60% there is not contact or close proximity between the fibres. Therefore, the heat generation is caused by dielectric losses in the polymer between the fibres. The laminate is modelled as a capacitor with several capacitive layers represented by the fibre layers, which are insulated from one another by polymer regions. Fink also calculated the inductive heat generation and temperature distribution in CFRT. Despite the contrary approaches for the heat generation mechanisms both Fink and Miller reported good agreement between the experiment and the theory. However, the heat generation mechanisms are not clear yet. Therefore, the investigations presented in this paper focus on the experimental determination of the parameter influences on the induction heating process. To obtain basic quantitative information for subsequent process modelling a stationary process was used for the experiments. 3. Equipment Fig. 5. Experimental set-up for the induction heating investigations. The experiments were performed with a khz, 5.2 kw induction generator. While the voltage is hold constant at 220 V, the machine self-tunes the frequency and coil current for each particular coil and load used. The power is pulsed as depicted in Fig. 4 and the power setting is proportional to the coil current. The maximum inductance that can be fixed to the generator is 25 mh, resulting in a maximum coil area of 500 mm 2.

4 1194 R. Rudolf et al. / Composites: Part A 31 (2000) Fig. 6. Influence of the frequency on the heating rate (triangular coil, power 20%, distance coil laminate 5 mm). The magnetic field emitted by the coil could not be measured due to the difficulty of measuring fields at high frequencies. The investigations of the process parameter influences were carried out with carbon-fibre fabric reinforced polyphenylensulfide (CF-PPS) (5-harness satin; fibre volume fraction: 46%; thickness: 2 mm). The influences of the laminate structure were investigated with carbon-fibre reinforced polyamide 66 (CF-PA66) (fibre volume fraction: 50%). 4. Experimental The aim of this investigation is to predetermine the process parameters of the CIW with the help of a minimum number of preliminary experiments and a simple experimental set-up. Therefore, stationary experiments were first carried out to investigate the influence of the process parameters electromagnetic frequency, generator power, distance between the induction coil and workpiece, coil geometry and laminate lay-up. Fig. 5 shows the experimental set-up. The laminate was placed between wooden clamps, such that free convection to both sides was possible and heat transfer to the fixture could be neglected. The laminate temperature was measured by means of an infrared camera. 5. Materials 6. Results 6.1. Influence of the electromagnetic frequency The alternating magnetic field B induces a voltage u ind in a conductive fibre loop which is defined as u ind ˆ 4 B A ˆ 2p f m H A where f is the frequency and 4 the angular frequency of the magnetic field, m the permeability of the workpiece material, H the magnetic field intensity and A is the area enclosed by the conductive fibre loop [10]. In an ohmic resistor (carbon fibre in this study) the induced current is dissipated as Joule losses P [11], given by P ˆ u2 ind R f ˆ 4p2 f 2 m 2 H 2 A 2 R f where R f is the electrical resistance of the carbon fibres. Hence, the inductive heat generation Q ind is proportional to the frequency squared Q ind f

5 R. Rudolf et al. / Composites: Part A 31 (2000) Fig. 7. Influence of the generator power on the heating time. Experiments with different induction generators showed, that while a carbon-fibre fabric reinforced thermoplastic cannot be heated beyond 50 C at 20 khz; nevertheless it can be heated to 300 C at 1 MHz in less than 5 s and at 26 MHz in less than 1 s. Fig. 6 shows that the influence of the frequency on the heating rate is very strong even in the small range of khz. These heating experiments were carried out with a triangular coil, which was placed at a distance of 5 mm on top of the laminate, and a generator power of 20%. The time Fig. 8. Influence of the distance between induction coil and laminate on the heating time.

6 1196 R. Rudolf et al. / Composites: Part A 31 (2000) Fig. 9. Dependence of the heating time on the distance between coil and laminate. Fig. 10. Dependence of the heating time on the generator power.

7 R. Rudolf et al. / Composites: Part A 31 (2000) Fig. 11. Selection of the investigated coil geometries and schematic representation of heat-affected areas (left: double-d; middle: circular pancake with square tube; right: clip). was measured when the hottest point on the laminate surface reached 300 C Influence of the generator power and the distance between coil and laminate Heating experiments with different power levels and distances between coil and laminate were performed in order to investigate the significance of these process parameters for he heating phase of the CIW. Differently from induction heating of metals there was no coupling effect measured between coil and workpiece with CFRT so that the coil current is constant independent of the distance between the coil and the laminate. This can be explained with the low magnetic permeability of CFRT that is close to that of air. In Fig. 7 the heating time is depicted as a function of the temperature for different power levels, a constant distance of 4 mm between coil and laminate and a frequency of 1 MHz. The heating time is growing exponentially with the temperature. Fig. 8 shows the heating time versus the temperature for different distances between coil and laminate and a constant power of 40%. Again the heating time is growing exponentially with the temperature. This corresponds with the theory of ohmic heat generation Q ˆ Q max a e t=t where t is the time, Q the temperature, Q max is the equilibrium temperature, (Q max a) represents the temperature at t ˆ 0 and t represents the time at which Q ˆ 0:63 u max : Notice that in Fig. 8 the inverse function is depicted. 4 Fig. 12. Maximum reachable temperatures for different power levels and distances between coil and laminate.

8 1198 R. Rudolf et al. / Composites: Part A 31 (2000) ) Q i 2 ind 6 While the heating time is increasing quadratically with the distance between the coil and the laminate (cf. Fig. 9), it is decreasing quadratically with the power (cf. Fig. 10). In another set of experiments the influence of the power and the distance between the coil and the laminate on the magnetic field intensity was investigated, which in turn influences the induced current and the maximum achievable temperature. A double-d coil (cf. Fig. 11a) was placed on top of a CF- PPS laminate at distance of 10, 15 and 20 mm, respectively. The laminate was heated using power levels of 10 and 20%, respectively, and the laminate surface temperature was measured by means of an infrared camera. The induced current determines the maximum temperature to which a laminate can be heated. Analogous to Eqs. (2) and (3) it can be written as P ˆ u ind i ind ˆ R f i 2 ind Fig. 13. Orientation of the coil to the laminate. 5 where i ind represents the induced eddy currents. Fig. 12 shows that the maximum temperature and, therefore, the current intensity are growing with increasing power. This means that the pulsation of the power (cf. Fig. 4) causes a summation of the heating effects of the single current pulses induced in the laminate. Furthermore, the current intensity is decreasing with the distance between the coil and the laminate. This is due to the decrease of the magnetic field intensity H with the distance from the coil, which is given by H ˆ i Z 1 4p ~r 2 d ~l ~r 7 ~r where i is the coil current, dl a section of the coil length, and r is the distance between the coil and some point P [10]. The degree of the field intensity decrease with the distance from the coil is dependent upon the coil geometry. The field intensity of a circular coil, for example, decreases with 1/ r 3, whereas for a linear coil of infinite length it decreases with 1/r [12] Influences of the induction coil geometry and position For the CIW the heating rate is not solely important but also the temperature distribution in the plane of the workpiece and in through-thickness direction, which is mainly influenced by the induction coil geometry and, therefore, the field intensity. The aim is a uniform temperature Fig. 14. Influence of the coil geometry on the heat generation (N: number of windings).

9 R. Rudolf et al. / Composites: Part A 31 (2000) Fig. 15. Experimental set-up for the resistivity measurements. distribution. Heating experiments with several induction coil geometries were performed and the heating rate and temperature distribution was measured by means of an infrared camera. The coils were produced such that the electromagnetic frequency was 1 MHz for all coil geometries. This distance between coil and laminate was 5 and 10 mm, respectively, and the power was 35%. Since the 5-harness satin weave is an asymmetrical fabric, it was also investigated if the coil orientation has an influence on the heating rate and temperature distribution. For that purpose the experiments were carried out with the coil axis oriented in 0, 45 and 90 direction to the laminate, as depicted in Fig. 13. Fig. 11 shows a selection of the investigated coil geometries and the shape of the heat-affected area which was measured with an infrared camera. It can be seen that the heating pattern is a mirror image of the shape of the induction coil. The double-d coil and the clip coil effect an elliptical heating zone with a uniform temperature distribution in the plane of the workpiece. The heating zones of all other investigated coil geometries show no closed structure but have a cold spot in their middle or at the edge. In the CIW this cold spot will disappear because of the movement of the workpiece but it leads to a reduced feed velocity. Fig. 14 shows the period of time until the hottest point on the laminate surface has reached 300 C. Since the frequency and the power were constant for all experiments the difference of the heating times is caused by the different electromagnetic field intensities. It can be seen that a decrease of the distance between coil and laminate from 10 to 5 mm leads to a decrease of the heating time of %. This is caused by the decrease of the electromagnetic field intensity with the distance from the induction coil (refer to Eq. (7)). Exception represents the circular pancake coils and the little triangular coil. The experiments were stopped after 120 s since the laminate did not reach 300 C. These results show that for some coil geometries the decrease of the field intensity with the distance has bigger influences on the heating behaviour than for others, and that the distance between coil and laminate is a very sensitive parameter of the induction heating process. The biggest heating rate (about 100 C/ s) was achieved with the oval pancake coil. The orientation of the coil has no measurable effect on the heating behaviour. The differences in the heating times can be traced back to measuring inaccuracies Temperature distribution over the laminate thickness A temperature gradient over the laminate thickness can be caused by two phenomena. 1. The skin depth is smaller than the laminate thickness, which causes a skin effect. 2. The field intensity decrease over the thickness is strong enough to cause a temperature gradient. The skin depth d is defined as r r d ˆ in m 8 p f m where r is the electrical resistivity of the heated material (in V m), f the electromagnetic frequency (in Hz) and m is the magnetic permeability (in H/m) [12]. For non-ferromagnetic materials like CFRT the magnetic permeability m is equal to that of air m ˆ m 0 ˆ 1: H=m : The measurement of the resistivity of the investigated CF-PPS gave r ˆ 3 V m and r k ˆ V m where r is the resistivity in through-thickness direction perpendicular to the fibre-axis and r k is the resistivity in the plane of the workpiece parallel to the fibre-axis. The set-up for the resistivity measurements is depicted in Fig. 15. Insertion of m, f and r k in Eq. (8) yields a skin depth of d ˆ 12:6 mm for the investigated CF-PPS. Hence a temperature gradient due to a skin effect is impossible. Stationary heating experiments with different power levels and distances between induction coil and laminate were carried out in order to investigate the influence of the field intensity decrease on the through-thickness heating. The laminate was oriented vertically to achieve equal convective conditions on both surfaces and the surface temperature was measured by means of an infrared camera.

10 1200 R. Rudolf et al. / Composites: Part A 31 (2000) Fig. 16. Influence of the laminate lay-up on the heating rate for a pancake coil (pw, plain weave; s, satin weave; f, fleece; ud, unidirectional; p, polymer interlayer). The investigations showed no temperature gradient in through-thickness direction of a 2-mm thick CF-PPS laminate Influence of the laminate structure Heating experiments were carried out with different laminate lay-ups in order to determine the influence of the textile structure on the heating rate and the temperature distribution. The following carbon-fibre textiles were investigated separately and in combination with each other: plain weave (pw); 5-harness satin weave (s); Fig. 17. Schematic representation of the investigated fabrics. fleece (f); unidirectional tape (ud). The distance between the induction coil and the laminate was 5 mm and the power was 20% for all experiments. The tendency of the heating results was equal for all investigated coil geometries, only the absolute heating rates varied. Therefore, the results are shown in Fig. 16 exemplary for a circular pancake coil (cf. Fig. 11b). The influence of the coil orientation (0, 90 ) on the heating rate can be neglected, since the differences in the heating rates are small. Notice that the laminate with the fleece and the unidirectional fibre orientation could not be heated. This indicates that there are no electrical-conductive paths in such laminates through which current can flow. However, other fleece materials could be heated to temperatures of up to 250 C. Thus, further investigation into the heating mechanisms of fleece materials has to be performed. The plain weave is heated faster than the satin weave, which indicates that the plain weave contains more electrical-conductive loops. This can be explained by the structure of the fabrics (cf. Fig. 17). The total number of fibre junctions is equal for both fabrics, but in the plain weave there are more fibre junctions, at which the fibres have a big curvature (see arrows in Fig. 17). This indicates that the current flow is enhanced at these junctions resulting in a bigger heating rate. Microscopic studies of laminate crosssections showed that there is no closer proximity of the fibres in 0 and 90 direction at these junctions. However

11 R. Rudolf et al. / Composites: Part A 31 (2000) Fig. 18. Fibre contact-length of the fibre junction types and total number of junction types for an area of 7 7 threads (cross sections are composed of fibre bundles). the contact length is larger compared to the fibre junctions with a straight fibre orientation, as schematically outlined in Fig. 18 for a 5-harness satin weave. Indeed, the total contact length in a laminate with a plain weave reinforcement is 15% higher than that of a laminate with a 5-harness satin weave reinforcement. This correlates well with the experiments (cf. Fig. 16). The laminates having polymer interlayers with a different thickness (e.g. pw-p-pw, cf. Fig. 16) show a dependence of the heating rate on the thickness of this polymer layer. The thicker the interlayer, the longer is the heating time. Fink [8] found the same for cross-ply laminates and explained it with the capacitive layer model. The dielectric heat generation decreases when the dielectric layer between the capacitor plates increases. The differing behaviour of the satin weave reinforced laminates is not clear and might be caused by differences in the processing conditions. The study of micrographs of laminate cross-sections revealed that there is contact or at least a close proximity of less than 5 10 mm between the crossing fibre bundles allowing the current to flow. Therefore, in the case of the investigated laminates with fabric reinforcement, the heat generation seems to be dominated by Joule losses in the fibres. The heat generation is increasing with the current density, which in turn is dependent on the fibre fibre contact at the fibre junctions. This parameter is not changed when a polymer layer is added between the fabrics. Therefore, the increasing heating times can only be caused by the additional mass that has to be heated Mechanism of induction heating of carbon-fibre fabricreinforced thermoplastics Induction heating experiments were carried out with a polyamide 12 composite containing one layer of plain weave on one hand and one layer of pure plain weave without matrix polymer on the other hand. Both samples were inductively heated using the same process parameters and the temperature was monitored with an infrared camera. The experiments showed that the maximum achievable temperature was equal for both samples, which means that the matrix polymer does not influence the heating mechanism, and dielectric heating, as proposed by Fink [8], can be excluded. This is supported by the fact that a pure polyamide 12 film could not be heated, too, at the investigated electromagnetic frequency. Hence, induction heating of fabric reinforced thermoplastics is based on current flow at the fibre junctions and Joule losses in the carbon fibres, which supports the theory of Miller et al. [7]. 7. Conclusions The influence of the process parameters electromagnetic frequency, generator power, distance between induction coil and laminate, coil geometry and laminate lay-up on the heating rate and the heat distribution have been investigated in stationary experiments. In accordance with Miller et al. and Fink [3,4] it was found that heat is only generated when closed fibre loops exist. Thus, for example laminates with unidirectional fibre

12 1202 R. Rudolf et al. / Composites: Part A 31 (2000) reinforcement, which do not contain fibre junctions, could not be heated. Moreover, the texture of the reinforcement was found to be of major importance, namely the quality of the fibre junctions. A model is presented for the correlation of the total contact length between the fibres in 0 and 90 direction in a laminate and the achievable heating rate. The induction heating mechanism was found to be based on Joule losses in the carbon fibres since current is flowing in the laminate. The strongest influence on the heating behaviour has the magnetic field intensity distribution, which in turn is influenced by the power, the induction coil geometry and the distance between coil and laminate. It was found that the heating time increases quadratically with the distance between induction coil and laminate and decreases quadratically with the generator power. The maximum temperature to which a laminate can be heated is dependent on the power and the distance between induction coil and laminate. The heat distribution in through-thickness direction was found to be uniform, whereas the heat-affected area in the plane of the workpiece is a mirror image of the magnetic field. The coil orientation had no influence on the heating rate and the maximum achievable temperature even for induction heating of asymmetrical reinforcements like satin weaves. References [1] Rudolf R, Mitschang P, Neitzel M, Rueckert C. Welding of highperformance thermoplastic composites. Polymers and Polymer Composites 1999;7(5):1 6. [2] Benkowsky G. Induktionserwärmung. 5. Auflage. Berlin: Verlag Technik GmbH, [3] Yarlagadda S, Fink BK, Gillespie Jr. JW. Resistive susceptor design for uniform heating during introduction bonding of composites. Journal of Thermoplastic Composite Materials 1998;11(7): [4] Topping MH. Electromagnetic welding of thermoplastics and specific design criteria. Society of Automotive Engineering, Technical Paper Series No , [5] Benatar A, Gutowski TG. Methods for fusion bonding thermoplastic composites. SAMPE Quarterly 1986;10: [6] Xiao XR, Hao SV, Street KN. Development of fusion bonding repair of thermoplastic resin composites. Proceedings of ICCM-8, Honolulu, vol. 1., p. 8.A.1 8.A.10. [7] Miller AK, et al. The nature of induction heating in graphite-fibre, polymer matrix composite materials. SAMPE Journal 1990;26(4): [8] Fink BK, McCullough RL, Gillespie Jr., JW. A local theory of heating in cross-ply carbon fibre thermoplastic composites by magnetic induction. Centre for Composite Materials, University of Delaware, USA. CCM-Report , [9] Lin W, Miller AK, Buneman O. Predictive capabilities of an induction heating model for complex-shape graphite fiber/polymer matrix composites. Proceedings of 24th International SAMPE Electronics Conference, p [10] Frohne H. Elektrische und magnetische felder. Stuttgart: Teubner Verlag, [11] Lindner H. Physik fuer ingenieure. Braunschweig/Wiesbaden: Vieweg Verlag, [12 Miner GF. Lines and electromagnetic fields for engineers. New York: Oxford University Press, 1996.