Ultrasonic spot welding of aluminum sheet/ carbon fiber reinforced polymer joints

Transkript

1 DOI: /mawe Mat.-wiss. u. Werkstofftech. 2007, 38, No. 11 Ultrasonic spot welding of aluminum sheet/ carbon fiber reinforced polymer joints Ultraschallpunktschweißen von Aluminiumblech/C-Faser-Kunststoff Verbunden Dedicated to Professor Dr. E. Roeder on the occasion of his 80 th birthday F. Balle, G. Wagner, D. Eifler Current demands on light weight constructions lead to an increasing use of light weight metals such as aluminum or magnesium alloys as well as fiber reinforced polymers (FRP). Suitable welding methods are necessary to join these dissimilar material groups and to integrate them in engineering structures. At the Institute of Materials Science and Engineering the ultrasonic metal welding technique was successfully applied to realize aluminum sheet/carbonfiber-reinforced polymer (CFRP) - joints. The welding during this process occurs in two steps: First the ultrasonic shear waves lead to a softening and displacement of the polymer matrix out of the welding zone. In the second step a direct weld between the load bearing carbon fibers of the CFRP and the aluminum alloy sheet is generated. A welding time of less than 5 s and no damage of the carbon fibers are important advantages of the process. Actually tensile shear strengths of about 30 MPa were achieved for the joints. By means of light microscopic and SEM investigations the bonding mechanisms can be described in detail. Possible application fields of such welds can be seen in the automotive or in the aircraft industry. Keywords: Light weight metals, CFRP, ultrasonic metal welding, joining technology Aus ökonomischen und ökologischen Gründen gewinnen Leichtbaustrukturen in der industriellen Fertigung ständig an Bedeutung. Dies führt zu einem verstärkten Einsatz von Leichtmetallen wie Aluminum- und Magnesiumlegierungen sowie von Faserverbundwerkstoffen. Zum stoffschlüssigen Fügen derart unterschiedlicher Werkstoffgruppen müssen geeignete Schweißverfahren zur Verfügung stehen. Am Lehrstuhl für Werkstoffkunde der TU Kaiserslautern wurde erstmals die Metall-Ultraschallschweißtechnik erfolgreich zum Fügen von Aluminiumblech/Kohlenstofffaserverstärkter Kunststoff (CFK) - Verbunden eingesetzt. Die Schweißung erfolgt in zwei Schritten: Im ersten Schritt wird durch die einwirkenden Ultraschallscherwellen die Matrix des CFK im Fügebereich plastifiziert und aus der Schweißzone verdrängt. Die lasttragenden C-Fasern des CFK werden freigelegt und können somit im zweiten Schritt direkt mit dem metallischen Fügepartner verschweißen. Während des Fügevorgangs tritt keine Schädigung des in der Matrix eingebetteten C-Faser-Textils auf. Innerhalb von maximal 5 s ist der Ultraschall-Punktschweißvorgang abgeschlossen. Durch den Einsatz des Metall-Ultraschallschweißverfahrens können bisher Metall/CFK-Verbunde mit Zugscherfestigkeiten von bis zu 30 MPa realisiert werden. Mittels licht- und rasterelektronenmikroskopischen Untersuchungen der Fügezonen können zudem die Bindungsmechanismen im Detail beschrieben werden. Anwendungsfelder für die Leichtmetall/CFK-Verbunde sind beispielsweise in der Fahrzeug- oder in der Luftfahrttechnik zu sehen. Schlüsselworte: Leichtmetalle, CFK, Metall-Ultraschallschweißen, Fügetechnik 1 Introduction The ultrasonic welding of similar materials e.g. wires for cable harnesses or plastics in the packaging industry is already established in industrial manufacturing [1, 2, 3]. One field of research at the Institute of Materials Science and Engineering (WKK) at the University of Kaiserslautern is to expand the application fields of the ultrasonic metal welding technique by joining dissimilar materials like glass or ceramics with metals [4, 5, 6]. In comparison to other joining techniques such as adhesive bonding, brazing or soldering ultrasonic welding is characterized by a low energy input and hence low temperatures in the welding zone as well as short welding times. So far the ultrasonic plastic welding method is typically used for joining CFRP. However this welding method only realizes a joining between the matrix of the CFRP. In this case the high-strength textile of the CFRP can not be used to transmit mechanical loads directly between the metallic joining partner and the CFRP [1, 2]. Recent investigations at the WKK show that ultrasonic metal welding is also suitable to join CFRP with metal sheets like aluminum-alloys or aluminum plated steel. In the following the mechanical properties of the realized joints, possible bonding mechanisms and the future applicability fields will be presented. 2 Experimental setup The main components of an ultrasonic spot welding system are the ultrasonic generator (1), the converter (2), the booster (3) and the welding tool called sonotrode (4), Fig. 1. The ultrasonic generator converts the 50 Hz main voltage into a high frequency alternating voltage output of 20 khz. The converter uses the reversed piezoelectrical effect to transform this high frequency voltage into a mechanical oscillation of the same frequency. The necessary amplitude of oscillation (u) in the welding zone, 5 to 50 lm, is achieved by an appropriate design of the booster and the sonotrode. Simultaneously to the input of the welding energy (W US ) which is transferred by ul- 934 F 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

2 Fig. 1. Principle of the ultrasonic metal spot welding technique Abb. 1. Prinzip des Metall-Ultraschallpunktschweißverfahrens trasonic shear waves a static pressure (F US ) is applied pneumatically (7) on the welding partners (5), which are positioned on an anvil (6), Fig. 1. Beside a high reproducible clamping of the specimens on the anvil it is necessary to control and regulate the welding force during the joining process by an integrated force measuring device. Therefore a special anvil was developed at the WKK, Fig. 2. An optimal adjustment of the three process-related parameters welding force, welding energy and amplitude of the sonotrode is a necessary requirement for the realization of high strength joints. 3 Experimental design The statistical model named central composite design circumscribed (CCC) was used to investigate the weldability of Al99.5/CF-PA66-joints. In comparison to a stepwise variation of each welding parameter this model for non-linear relationships allows to find the optimal parameters with considerably less weldings. An important advantage of the CCC-model is the description of the mutual dependence of the three central welding parameters oscillation amplitude, welding force and energy in relation to the achievable tensile shear strength of the joints. As an appropriate arrangement of the CCC-model for the performed investigations an orthogonal equally spaced cube (8 cube corners) supplemented by 6 star corners (star distance coefficient a = 1.5) and a center were defined [8], Fig. 3. The statistical table of the CCC-model is based on the three significant process parameters with five different settings for each parameter. The suitable ranges of the process parameters were approximately determined in pre-investigations. For joints of Al99.5 and CF-PA66 composite appropriate welding parameters concerning for the force (F US ) lie between 55 and 85 N, for the amplitude (u) in the range of 29 up to 35 lm and for the energy (W us ) between 1550 and 1850 J. The described structure of the CCC-model leads to 18 different parameter triples. The use of the statistic experimental design reduced the number of experiments by approximately a factor of 7 in comparison to a stepwise investigation of the influence of the three welding parameters. Fig. 2. Anvil for ultrasonic metal spot welding of metal/cfrpjoints, developed at WKK Abb. 2. Amboss zum Metall-Ultraschallpunktschweißen von Metall/CFK-Verbunden, entwickelt am WKK Fig. 3. Central composite design circumscribedmodel (CCC-model) [8] Abb. 3. Zentral zusammengesetzter Versuchsplan [8] Mat.-wiss. u. Werkstofftech. 2007, 38, No. 11 Ultrasonic spot welding of aluminum sheet/carbon fiber 935

3 4 Materials and specimen geometry In the following selected results for weldings between Al99.5 with a thickness of 1 mm and the thermoplastic composite material CF-PA66 with a thickness of 2 mm will be presented. The fiber reinforcement of CF-PA66 is a carbon fiber textile Atlas 1/4-fabric with a weight per unit area of 285 g/m 2. The fiber volume in the CFRP is about 48 %. It was manufactured in an autoclave process. The specimen geometries are shown in Fig. 4. The welding area of the sonotrode is mm 2. Since it is not possible to determine the geometry of the real joining area the shear strength is calculated by the ratio of the achieved tensile shear force related to the sonotrode contact area. 5 Welding results Fig. 5 shows the results of the investigations based on the CCC-model for a constant welding force of 70 N. A maximum tensile shear strength of MPa can be determined slightly left to the center of the diagram. However two-dimensional cuttings of the diagram are necessary to ascertain the welding parameters exactly. In Fig. 6 the progression of the tensile shear strength for a constant welding amplitude of 32 lm and a welding force of 70 N is presented. The welding energy is varied between 1550 and 1850 J. Beside the course of the average tensile shear strength the lower and upper confidence interval for 95% is specified. An increasing welding energy leads up to 1750 J to an increased displacement of the matrix of the CFRP and a better contact between the metal sheet and the fibers. As a result the tensile shear strength of the joints increases. After the peak value a damage of the textile and thus a decrease of the tensile shear strength follows due to increased welding energies. An optimum range appears between 1700 and 1800 J. The higher standard deviation for the boundary values can be traced back to the chosen CCC-model. The variation of the welding amplitude and the welding force show the same effects. The highest average tensile shear strength for the Al99.5/ CF-PA66 joints with 24.6 MPa can therefore be expected for a welding force of 65 N, an oscillation amplitude of 32 lm and Fig. 5. Tensile shear strengths of Al99.5/CF-PA66 joints (F US =70 N) Abb. 5. Zugscherfestigkeiten für Al99.5/CF-PA66 - Verbunde (F US =70N) a welding energy of 1725 J. In further welding tests the identified optimal welding parameters were validated. Fig. 7 shows four characteristic load-extension-curves welded with the listed parameters. In spite of the absolutely different material groups it is possible to weld Al99.5/CF-PA66 joints with a low spread of the deformation curves, Fig. 7. Furthermore the maximum tensile strength lies in the expected range with MPa. That the achieved maximum is determined by the tensile strength of aluminum sheet is clearly documented in Fig. 8. The failure occurs in the edge area of the sonotrode welding zone. There is no adhesive failure of the welding zone. 6 Microscopic investigations Microscopic investigations help to understand the bonding mechanisms. A cross section of a spot welded Al99.5/CF- PA66 - joint shows the characteristic development of the welding zone, Fig. 9. During the ultrasonic welding of metal/metal-joints the oxide surface layers on the metal are torn off. As a result an intensive adaptation of the joining partners and intermolecular reactions can occur in the welding zone. A mechanical Fig. 4. Specimen geometry Abb. 4. Probengeometrie Fig. 6. Tensile shear strength of Al99.5/CF-PA66 joints versus welding energy (u = 32 lm, F US = 70 N, confidence level 95 %) Abb. 6. Zugscherfestigkeit der Al99,5/CF-PA66 - Verbunde in Abhängigkeit der Schweißenergie (u = 32 lm, F US = 70 N, Vertrauensbereich 95 %) 936 F. Balle, G. Wagner, D. Eifler Mat.-wiss. u. Werkstofftech. 2007, 38, No. 11

4 Fig. 7. Load-extension-curves of ultrasonic welded Al99.5/CF- PA66 - joints Abb. 7. Kraft-Verlängerung-Kurven ultraschallgeschweißter Al99,5/CF-PA66 - Verbunde Fig. 9. Light microscopic and scanning electron microscope (SEM) micrographs of the bonding zone of an Al99.5/CF-PA66 - joint (cross section) Abb. 9. Licht- und rasterelektronenmikroskopische Aufnahmen der Fügezone eines Al99,5/CF-PA66-Verbundes (Querschliff) Fig. 8. Characteristic failure of Al99.5/CF-PA66 - joints Abb. 8. Charakteristisches Versagen der Al99,5/CF-PA66 - Verbunde interlocking of the joining partners can enhance the strength of the weld [1]. Regarding this the purpose of the present work is to realize a direct contact between the load bearing fibers and the metal sheet. Fig. 9 shows a characteristic welding area. In a higher resolution (Fig. 9) it can be seen that the polymer matrix is displaced out of the welding zone and the ductile aluminum nearly encased the carbon fibers. For these joints it could be proved that both an intermolecular contact and a mechanical interlocking have been developed by using ultrasonic metal welding. Furthermore no damage of the fibers caused by the ultrasonic welding process was observed in the cross section. For a precisely description of the welding zone, fracture surfaces of joints welded with suitable but non-optimized process parameters were also investigated, Fig. 10. The tensile shear strengths of these joints lie in the range of MPa. In this case cohesive failure occurs. These areas show carbon fibers pulled out of the CFRP welded on the aluminum sheet. The investigations carried out clearly showed that it is possible to realize high strength joints between metal sheets and CFRP by ultrasonic metal welding. Fig. 10. SEM micrograph of the fracture surface of an Al99.5/CF- PA66-joint Abb. 10. REM-Aufnahme der Bruchfläche eines Al99,5/CF-PA66 - Verbundes 6 Conclusions and outlook With the presented work for the first time the ultrasonic metal welding technique was applied successfully to join metal sheets with CFRP. The welding times were always less than 5 s. By using the CCC-Model it was possible to find the optimal welding parameters with only 15 % of the necessary tests in comparison to a stepwise investigation of the relevant welding parameters. The identified applicable parameters lead to tensile shear forces up to 2460 N corresponding to a tensile shear strength of about 25 MPa. In this case the fracture of the metal sheet limits the achievable strength. The cross sections and the micrographs of the fracture surface show that an intensive joint between the metallic material and the load bearing carbon fibers of the CFRP has developed during the ultrasonic welding process. The polymer matrix was displaced out to the edge of the welding zone. Finally no damage of the carbon fiber reinforcement was detected. The option to join different metal sheets to CFRP with high strength extends the application fields for the ultrasonic metal welding technique. Regarding the efficiency, automation, ecological compatibility and the achievable mechanical and technological properties ultrasonic metal welding is an attractive alternative to existing plastic joining techniques. Mat.-wiss. u. Werkstofftech. 2007, 38, No. 11 Ultrasonic spot welding of aluminum sheet/carbon fiber 937

5 Beside the represented joints it is possible to weld aluminum alloys and unalloyed steel (Al-plated) with CF-PA66. Furthermore the weldability of CF-PEEK with metal sheets was investigated successfully. For these combinations shear strengths of more than 30 MPa were realized so far. Application fields for the presented ultrasonic welding technique can be seen in automotive industry, e.g. to join different lightweight crash structures by using fiber reinforced materials, or in the aircraft engineering for construction elements of the fuselage, the aerofoil or the tail fin. Acknowledgement The authors would like to thank the German research foundation (DFG) for the financial support (FOR 524). References 1. J. Wodara, Ultraschallfügen, DVS-Verlag, Düsseldorf H. Potente, Fügen von Kunststoffen, Carl Hanser Verlag, München, Wien G. W. Ehrenstein, Handbuch Kunststoffverbindungstechnik, Carl Hanser Verlag, München, Wien C. Born, G. Wagner, D. Eifler, Adv. Eng. Mat., 2006, 8, No. 9, H. Kuckert, Ch. Born, G. Wagner, D. Eifler, Mat.-Wiss. u. Werkstofftech., 2003, 34, No. 1, S. Krüger, G. Wagner, D. Eifler, MP Mat.-Prüf., 2004, 46, No. 3, S. Krüger, G. Wagner, D. Eifler, Adv. Eng. Mat., 2004, 6, No. 3, W. Kleppmann, Taschenbuch Versuchsplanung, Carl Hanser Verlag, München, Wien Korrespondenzautor: D. Eifler, University of Kaiserslautern, Institute of Materials Science and Engineering, Kaiserslautern, Germany, Received in final form: July 26, 2007 [T 212] 938 F. Balle, G. Wagner, D. Eifler Mat.-wiss. u. Werkstofftech. 2007, 38, No. 11