A Review of Welding Technologies for Thermoplastic Composites in Aerospace Applications

Q Heat generation I Electrical current R Ohmic resistance t Time IR Infrared ITA Instituto Tecnológico de Aeronáutica LBW Laser Beam Welding P Pressure PEEK Poly-etheretherketone PES Poly-ethersulphone PPS Poly-phenylenesulphide PEI Poly-etherimide T Temperature Tg Glass transition temperature Tm Melting temperature UNESP Universidade Estadual Paulista Júlio de Mesquita Filho USP Universidade de São Paulo


INTRODUCTION
laminates have shown great promise as materials for current and reinforced composite laminates, thermoplastic laminates are tion nor lengthy curing process, they are easily recycled and do moisture uptake, which means their mechanical properties are less degraded under hot/wet conditions, not to mention their higher an important role in manufacturing of composite structures in marine, automotive and aerospace industry.Mechanical metals or composite components.However, there are disadvantages associated with these methods such as stress plate, electromagnetic induction, dielectric/microwave and IR et al., 2008;Botelho and Rezende, 2007;Wang and Hahn, 2007;Jones, 1994;Loos et al., 1981).
Researches involving welding in composites in Brazil were USP.The work started in these universities with cooperation in Portuguese) focused on the mechanical and thermal PPS thermoplastic polymer) reinforced with continuous resistance welding technology whereas USP is advancing in IR lamp welding methodology.
niques for thermoplastic composite laminates devised to structural aerospace applications, focusing on recent developments in this area.
More recently, thermoplastic composites have found utiliza-et al., 2011;Rezende et al., 2011;Chevali et al., 2010;Bates et al., 2009;Arici, 2007;Espert et al., 2004;Lee et al., 1993).A wide range of high performance thermoplastic matrices studied and reported.Both of them are semi-crystalline polymers, and crystallinity in high-performance polymers is quite mechanical properties.Crystallinity tends to increase stiffness and tensile strength while amorphous areas are more effective et al., 2009;Mazur et al. 2008).Today, joining thermoplastic composite structures composite usage is rapidly replacing metallic and thermoset fatigue loads applied to aerospace vehicles.Many welding own advantages and pitfalls.

WELDING PROCESSES
In choosing a structural engineered material for aerospace application, an important consideration is how easy is to manufacture, join, inspect, repair and replace it in service.One of the many advantages of thermoplastic over thermoset composites reformed.In theory, using adequate temperature and P levels, a developed and evaluated for thermoplastic applications in the ing the order they appear in the schematic provided in Fig. 1.

Ultrasonic welding
Ultrasonic welding is a process in which high frequenin order to induce molecular motion, thus creating friction, components using ultrasonic welding depends on material physical properties, frequency and amplitude of ultrasonic T g and T m et al., 2008;Siddiq and Ghassemieh, et al. 2004;Ageorges and Ye, 2002).et al., 2004;Yousefpour et al., 2004;Siddiq and Ghassemieh, 2008;Levy et al., 2008;Ageorges and Ye, 2002).automotive, aerospace, energy, medical and packaging.In particular, aerospace industry employs this methodology to parameters and process windows to produce high quality et al., 2004).
laminates.A capacitor is then created and dielectric heating occurs due to the movement of charge and rotation of A special type of induction welding is the so-called the joining parts.The Emaweld-process is fast compared to conventional induction welding.However, due to the metal in the welding zone thus reducing the mechanical properties et al. 1999).
ing resistance and induction welding showed that, under equivalent heating conditions, induction welded specimens --
The particular advantage of microwave welding over other and Froment, 2001).

et al., 2004).
The technique is still in the development stages and, as such, there are currently no reported industrial applications.However, it is anticipated that microwave welding may prove and aerospace parts.
and the power circuit is critical to the resistance welding process.In order to introduce adequate current into the among others.Instead of using continuous power during the pulses.This process is called impulsive resistance welding et al., 2009).
constant load or displacement control.The method can produce lap shear strength values greater than 33 MPa, and et al., 2006).

Hot plate welding
interface causing a decrease of viscosity thus allowing polymer chains to interdifuse as the surfaces are held tightly together, with et al., 2004; Ageorges et al., 2001;Ageorges et al., 2000).
Inside of this concept, hot plate or hot-tool welding is one of Often mechanical stops are employed to prevent squeezing the weld'.In case of too little pressure, full contact and thus good hot element is removed and melted surfaces are pressed against each other until consolidation occurs.Small parts demand at least 30 minutes to guarantee a good connection.Risks of al., 2001;Ageorges et al., 2000).
strong joints in automated systems.For instance, retention Besides high consistent, low scattered mechanical properties of IR welded joints, which is translated in high structural nent devised to aeronautical industry, huge potential of this et al., 1993;Chen, 2000;Miller, 2006).In the other hand, a main shortcoming of IR welding is the detrimental effect of colorants and pigments the quality of the end product.Darker polymers are especially prone to display overheating surface degradation due to low IR energy transfer rates down through melting zone, stage, as well as thermal residual stresses development along et al., 1993;Chen, 2000;Ageorges et al., 2001;Bray, 2003;Grewell et al. and Benatar, 1995).Figures 7 to 9 feature some important aspects related to IR radiation welding, particularly in regard to the welding system operation, applied thermomechanical

Laser welding
In order to produce integral composite structures and thus or thermoplastic composite components.Thermoplastic poly-Through the activation of this physicochemical mechanism, et al., 2010;hakaran et al., 2006;Grewell et al., 2004).joining thin, as well as medium to thick thermoplastic components used in the aeronautic sector.Moreover, size, -et al., 2010).LBW allows joining thermoplastics, provided joining separate parts ensuring full continuity of the resin system without the addition of an adhesive system and avoid-   applications in which short processing cycles are necessary.reduced surface preparation requirements, reprocessing, recy-edly the ultrasonic, induction and the resistance welding.
Process integration is a critical aspect of joining technology.Particular requirements of each joining techniques should ments at the very early stage of the design process.
involved in development and/or application of welding technolothe development of the resistance welding technology associated cal properties of thermoplastic laminates, whereas USP is in the forefront of IR lamp welding methodology and advanced destructive and nondestructive characterization of the joints.literature, since this manuscript is a state-of-the-art review.Therefore, such data do not provide information for deals of an initial mapping of potential techniques over a wide range Melting temperature UNESP Universidade Estadual Paulista Júlio de Mesquita Filho USP Universidade de São Paulo Figure 2 presents a schematic of an ultrasonic welding frequency.All ultrasonic welding systems are composed of P -ultrasonic generator to provide and control the ultrasonic et al.
and the square of the induced I - thermoplastic composites: et al., 2008); A Review of Welding Technologies for Thermoplastic Composites in Aerospace Applications Costa, A.P. et al.   dielectric loss factors using focused microwave energy up is shown in Fig.4.Joint elements with low-to-medium increases and reaches the polymer T m underneath the focused microwave energy.Meanwhile, localized fusion occurs in the presence of P, resulting in a weld.Joint elements with high dielectric loss factor elements, and then evaporate, leaving a localized melting P, producing a weld.Materials process, some of these materials evaporate and some remain Figure 7. Schematic of infrared lamp welding system conceived to join thermoplastic laminates highlighting the infrared radiation source, polymer parts deployment and pressing et al., 2004; Ageorges et al., 2001; Potente et al., 1993).