Polymer-based composites are used in many industries including automotive, aerospace and home appliances. Application of the polymer-based composites is significantly increasing due to their ease of processing, low weight and cost-effectiveness. Based on the matrix, composites are categorized into two major sections: Thermoplastic and thermoset composites. Thermoplastics become soften when they are heated. The thermosets are solidified after a liquid-solid transition which is called curing. After curing completion the thermosets cannot be remelted or resoften [1]. This leads to introduction of different production methods for thermosets and thermoplastics. However, in products with various shapes and structures the common production methods cannot be applied. In these cases, joining processes can be consider as a solution. Joining processes of polymers and polymeric based composites are divided in three major categories including welding, adhesive joining, and mechanical fastening [2], [3]. Joining by means of screws, rivets, spring clips and metal inserts are the examples of mechanical fastening. Stress concentration, variation of thermal expansion coefficient, damage generation on the two parts, initiation of delamination, galvanic corrosion and increasing the weight of structure are disadvantages of mechanical fastening methods [4], [5] Moreover, in the aerospace structures, application of the mechanical fasteners imposes the high costs [6]. Adhesively joined parts does not have sufficient strength under high loads. Furthermore in some cases curing process in adhesives needs significant time [6]. In these cases, welding of polymeric-based composites could be selected as a proper joining a method. Recently various techniques have been proposed for polymer and polymeric-based composite welding. Vibrational welding, Infrared heating, spin welding, resistance welding, induction welding, Friction stir welding (FSW), and ultrasonic welding are some of the common welding methods of polymeric based composites [2], [4]. Among the above-mentioned welding methods ultrasonic welding is well-known for high speed and low costs [6], [7]. Hence, ultrasonic welding is a proper welding method for mass production and automation. It also provides a clean welding process and produces no fume. Moreover, damages of the welding parts is less than similar methods such as FSW [7]. Considering the above-mentioned advantages, this method is used in automotive, aerospace, medical and electronic applications [7], [8]. Ultrasonic technique utilizes mechanical vibrations at high frequency (almost 20 to 40 KHz) and low amplitude (20- 60 ) to generate enough heat for melting of the polymer at the welding zone. In order to achieve an appropriate quality, the parts of two side shall be imposed under high pressure. In fact, the intermolecular friction generates heat and this provide enough molten polymer to wet the welding surfaces [4], [9]. The mechanical vibrations are provided by a piezoelectric or magnetostrictive transducers and transferred by means of horn. The horn is connected to a sonotrode which imposes the vibrations to the welding parts [2], [10]. The welding process is sensitive to tolerances and clamping accuracy [10]. The weldability is depended on the material’s properties. In thermoplastics, amorphous polymers have higher weld quality in comparison with semi-crystalline and crystalline thermoplastics. The transmission of ultrasonic waves in amorphous materials is better than semi-crystalline. This is due the fact that semi-crystalline structures act as a spring and dissipate the wave’s energy [4], [11]. Melt temperature, flowability, stiffness and chemical structure are the other parameters of materials that affect on the weldability [6]. As previously mentioned thermoset polymers cannot be melted after curing (because of existence of cross-links between polymeric chains), hence the ultrasonic welding cannot be performed between two thermoset parts [12]. In order to solve this problem an intermediary (or coupling) thermoplastic layer is used [12], [13]. Moreover application of surface preparation process such as mechanical abrasion and laser treatment of surfaces could improve the weld quality and elevate the weld strength. [12]–[14]. Laser treatment increases the physical interaction between the welding parts and in other words, increases mechanical interlocking [12]. In this regard, a few researches have been performed previously. Lionetto et al. incorporated PVB films between to epoxy/carbon-fiber composite and investigate the ultrasonic and Induction welding. The ultrasonic welding parameters including welding force, welding amplitude and solidification time were set at 1500 N, 86.2 and 4s. They specify that the thicker coupling layer results superior strength [15]. Fernandez and Van Moorleghem, investigated ultrasonic welding of Carbon/epoxy to Carbon/PEEK composites. They incorporated PEI film as a thermoplastic intermediary layer. They performed SEM analyses to investigate the cross-section of welding [16]. Degen et al. investigated the impact strength of glass fiber reinforced thermoplastics joined by ultrasonic welding method [17]. Balle et al. studied the ultrasonic welding of aluminum 2024 to PA66/CF. They investigate the effects of solution annealing, Natural aging and artificial aging on the weldability [18]. Bhudolia et al. investigated ultrasonic welding of Carbon/Elium® composites [19]. Fernandez and Palardy , investigated ultrasonic welding of CF/PPs composites [20]. On the other hand, Ultrasonic welded parts suffer from the existence of various defects. This indicates that application of Nondestructive Testing (NDT) methods could be useful. In this regard, Rashli et al.[21] placed a transducer on the horn in order to record the welding vibrations. They inspected the welding process by analyzing of the recorded vibrations. In another research Lee et al. located a Linear Variable Differential Transformer (LVDT) sensor to obtain the displacement signals of the horn [22].
In this research Glass Fiber Reinforced Plastic (GFRP) samples were ultrasonically welded. In order to investigate the quality of welding process, Lap shear test and NDT by active thermography were carried out. The SEM analyses were performed to study the microstructure of the welding zone.