Hybrid welding of thermoset and thermoplastic composites: adhesion improvement

Airplanes are the man-­‐made birds of the sky. Their efficiency for travelling, humanitarian aid, war and transport are self-­‐evident. They have demonstrated to be safe and fast, with a constant increase in the number of users and passengers throughout the years. Intense research is carried out in this field, and the aerodynamics, electronics and materials of the aircrafts have been continuously evolving. The aerospace and automotive industries have been replacing heavy materials, such as steel or aluminium, by lighter and tougher alternatives such as composite materials, which play nowadays a dramatic role in aircraft’s composition. For example, aluminium is being replaced by GLARE (“Glass Laminate Aluminium Reinforced Epoxy"), a structure that interlays aluminium films with glass fibre prepreg laminates, with better damage tolerance, corrosion and fire resistance, and lower specific weight than conventional aluminium. Boeing 787 is made by a 50% of composite materials, including them in its fuselage, wings, tail, doors and interior. Airbus 380, the world’s largest passenger airliner, is made by a 25% of composite materials. Composites used in the aerospace engineering are made of a resin, generally a polymer, and a reinforcement, such as carbon or glass fibres. In particular, according to F. Fischer et al.: “The use of carbon-­‐fibre reinforced plastics (CFRP) is increasing in all areas where weight reduction enables an improvement, either in an economic or ecological way or as regards performance”. Polymers can be, essentially, of three kinds: thermoplastics, thermosets and elastomers. Polymers count with the benefits of excellent bulk and surface properties, low cost and good mechanical applications nowadays. Thermoplastics present high ductility, fracture toughness and impact resistance; moreover, thermoplastic composites (TPCs) can be cost-­‐effectively joined and manufactured. Thermosets, on the other hand, do not count with these properties, but they have good compression, fatigue resistance and creep resistance. Regarding their characteristics, for some applications, thermoplastics may be used, and for others, thermosets are the answer: Thermoplastics are preferably used in small to medium-­‐sized parts, and thermosets are more economical to be used in large parts. Their combination, in order to take advantage of the benefits of both materials for each application, is a fact in current aircraft structures. Here is where the main challenge of using these new materials arises: the joining of thermoplastics and thermosets. Mechanical fastening is the conventional method to join these materials. For example, in the fuselage of A350, the structural frame is joined to the thermoset composite skin through thermoplastic clips and cleats. Apart from the extra weight and stress concentrations they imply – lightweight potential of the composite material is lost due to the need of reinforcement in the areas where the rivets and screws are inserted – when insufficient economical investment is devoted to their insertion, in order to assure the correct performance of the joining procedure, several problems may occur, such as delamination, water intrusion, galvanic corrosion and intensive labour they induce, among others. Then, in order to avoid the damage of the composite materials, and to save weight and economical resources, other joining alternatives are being taken into account...Ingeniería Aeroespacia