Joining of Composite Materials

Turkchem 02 Oct 2019 39 4 dk okuma

TURKCHEM

Composite materials can be used in applications such as aerospace, defense industry, biomedical applications, automotive industry, electronic communication antennas, and aircraft exterior surfaces due to their low density and multifunctional properties. However, the joining ability (weldability) of plastic composite materials is a current research topic. Traditional mechanical joining methods, such as bolting, for thermoplastic and thermoset composite materials cause delamination problems due to stresses generated during mechanical drilling, corrosion between the material and fasteners, and unwanted deformation in the composite material. Alternative joining techniques to mechanical joining methods have become an important topic in the aerospace industry. Alternative joining techniques to mechanical joining are co-curing, co-bonding, and secondary-bonding. Figure 1. Drawings of manufacturing processes used in joining composites Co-curing is the simultaneous curing of two parts using adhesive film. Co-bonding is achieved by bonding a previously cured part to an uncured or partially cured part. In secondary-bonding, two previously cured parts are joined using adhesive. Co-curing is carried out in a vacuum bag or autoclave together with the adhesive. Secondary-bonding and co-bonding are the most commonly used joining processes. Co-curing and co-bonding are preferred more than secondary-bonding because the number of parts or curing cycles produced is reduced. However, for large and complex parts, secondary-bonding provides stronger bonds [1].

Joining Methods for Thermoplastic Composite Materials Resistance Welding

Resistance welding is a welding method performed by applying heat generated from the resistance displayed by workpieces against electric current passing through them, along with the application of pressure. No additional heat is applied other than the heat generated by the electric current passing through the material. Heat is generated in the areas to be welded and pressure is applied through electrodes or jaws on the welding machine. A transformer converts grid current from high voltage/low amperage to low voltage/high current. Welding is performed by compressing the materials to be welded with a certain pressure and using electrodes that allow current to flow through the material. Current flows from one electrode to the other through the material. These electrodes are made of special copper alloys that allow high current to pass through and have sufficient physical strength under pressure. There are two approaches for joining thermoplastic composites by heating with resistance welding: using carbon fabric and metal. The advantages of using carbon fabric include the absence of any foreign material remaining at the joint area after joining and the better adhesion of thermoplastic resin to carbon fibers compared to metallic materials. To provide electrical insulation from carbon fibers and fill gaps with additional matrix material, thin polymer film layers are typically added on both sides of the resistance heater. When welding all thermoplastic composites, adequate pressure must be maintained in all areas heated above the melting temperature. Joining by resistance welding is performed by applying pressure between 7-14 bar and takes an average of 30 seconds to 5 minutes.

Ultrasonic Welding

This method, commercially widely used for unreinforced thermoplastics with low melting temperatures, has recently also been used for advanced thermoplastic composite materials. In ultrasonic welding, electrical energy is converted to mechanical energy. Ultrasonic welding of thermoplastics is based on joining plastics without adding extra material by heating them through mechanical vibrations transmitted from a horn that vibrates in the form of ultrasonic waves at high frequency and low amplitude (10 micrometers to 250 micrometers) between 10 kHz and 70 kHz, compressed between the horn and an anvil. In addition to the heat applied to the weld area, pressure must also be applied. This welding process works well if there is any roughness on either of the composite surfaces. This process takes on average less than 10 seconds and applies pressure between 5-14 bar. Welding large parts with ultrasonic welding is difficult [2].

Some Research Conducted

Hıdıroğlu et al. 2016 examined an example ultrasonic welding application in the automotive industry [3]. Goto et al., 2019, examined the shear and tensile strengths of carbon fiber reinforced thermoplastic (CFRTP) laminate joints welded with ultrasonic welding using lap-shear tests (LSTs) and cross-tension tests (CTTs) [4]. Tao et al., 2019, performed ultrasonic assisted welding on Carbon Fiber (CF)/Poly (ether ether ketone) PEEK composites. The shear strength of the joint reached 28 MPa [5]. Villegas et al., 2018, investigated ultrasonic welding of carbon/epoxy and carbon/PEEK composites using a PEI thermoplastic coupling layer [6].

Induction Welding

In the induction welding process, heat is generated using an induction coil used to create an electromagnetic field. Heat conductive materials include iron, nickel, carbon fiber, and copper meshes. As in heating by resistance welding, placing a polymer film layer on both sides of the metallic heat conductive material is a normal practice. The induction welding process takes an average of 5-30 minutes and applies pressure between 4-14 bar. Assoc. Prof. Özgür Demircan / Ondokuz Mayıs University - Faculty of Engineering / Metallurgy and Materials Engineering