130 High Performance Thermoplastic Resins and Their Composites 6.4.6 Ultrasonic Welding Ultrasonic welding is one of the most widely used techniques for joining thermoplastics [246J.In using this technique,parts to be assembled are held together under pressure and are subjected to ultrasonic vibrations [104,236,237,238,2461.These high frequency mechanical vibrations,usually greater than 18 kHz [238,246].cause surface and intermolecular frictions which generate heat.Heat is generated at the desired location at the weld interface,by the addition of wedge shaped thermoplastic protrustons called energy directors(Figure 58).When above the melting temperature,the thermoplastic matrix flows and solidifies after the vibrations stop. 6.4.7 Friction/Vibration Welding This technique consists in the generation of heat by rubbing together the two parts to be joined [236.238.239].The two parts are held together under pressure and the relative vibrational motion generates heat.Friction welding refers to rotational motion while vibration welding consists of linear oscillations.Vibration frequencies are typically between 100-240 Hz [2381.Once the matrix is molten the motion is stopped,then the parts are aligned and the interfaces solidify during cooling.Sometimes,the motion may result in some fibre misalignment but this can be reduced by adding a resin rich layer at the bond surface. 6.4.8 Thermoplastic Amorphous Bonding Thermoplastic amorphous bonding is a novel technique that has been applied to APC-2 composites [104,206.244].In this process.a polyetherimide (PEI)film,an amorphous thermoplastic resin,is placed on the surface during lay-up and co-consolidated with the APC-2 parts to be welded.The two parts are subsequently bonded at a temperature above the Tg of PEI (210C)but well below the melting temperature of PEEK(340C).Hence the APC-2 parts do not melt and they retain superior dimensional tolerances and fibre orientation in the vicinity of the weld interface [206,2441.This technique produces good joint strength [104,206,244]and requires minimal equipment.In the experimental study of Don et al.[244].APC-2/PEI resistance welded specimens produced slightly lower lap shear strength (35.9t 1.0 MPa)than resistance welded APC-2 with PEEK film(44.8 t 7.6 MPa),but the results were more reproducible and no fibre movement was detected as was the case with APC-2/PEEK. Thermoplastic amorphous bonding has the major disadvantage of reducing the solvent resistance of the joined APC-2 composite parts compared to the semi-crystalline joint obtained with PEEK film [2441.No other amorphous thermoplastics other than PEI used to bond APC-2 composites was found in this literature search.However,perhaps amorphous thermoplastics having a Tg closer to,but still less than the melting temperature of the parts to be bonded may give better performance.130 High Performance Thermoplastic Resins and Their Composites 6.4.6 Ultrasonic Welding Ultrasonic welding is one of the most widely used techniques for joining thermoplastics [246]. In using this technique, parts to be assembled are held together under pressure and are subjected to ultrasonic vibrations (104, 236,237, 238.2463. These high frequency mechanical vibrations, usually greater than 18 kHz [238, 2461, cause surface and intermolecular frictions which generate heat. Heat is generated at the desired location at the weld interface, by the addition of wedge shaped thermoplastic protrusions called energy directors (Figure 58). When above the melting temperature, the thermoplastic matrix flows and solidifies after the vibrations stop. 6.4.7 Friction/Vibration Welding This technique consists in the generation of heat by rubbing together the two parts to be joined [236,238,239]. The two parts are held together under pressure and the relative vibrational motion generates heat. Friction welding refers to rotational motion while vibration welding consists of linear oscillations. Vibration frequencies are typically between 100 - 240 Hz [238]. Once the matrix is molten the motion is stopped, then the parts are aligned and the interfaces solidify during cooling. Sometimes, the motion may result in some fibre misalignment but this can be reduced by adding a resin rich layer at the bond surface. 6.4.8 Thermoplastic Amorphous Bonding Thermoplastic amorphous bonding is a novel technique that has been applied to APC-2 composites [104,206.244]. In this process, a polyetherimide (PEI) film, an amorphous thermoplastic resin, is placed on the surface during lay-up and co-consolidated with the APC-2 parts to be welded. The two parts are subsequently bonded at a temperature above the Tg of PEI (2 10’ C) but well below the melting temperature of PEEK (340” C). Hence the APC-2 parts do not melt and they retain superior dimensional tolerances and fibre orientation in the vicinity of the weld interface [ZOS, 2443. This technique produces good joint strength [104,206, 2441 and requires minimal equipment. In the experimental study of Don et al. (2441. APC-P/PEI resistance welded specimens produced slightly lower lap shear strength (35.9 _+ 1.0 MPa) than resistance welded APC-2 with PEEK film (44.8 k 7.6 MPa), but the results were more reproducible and no fibre movement was detected as was the case with APC-2/PEEK. Thermoplastic amorphous bonding has the major disadvantage of reducing the solvent resistance of the joined APC-2 composite parts compared to the semi-crystalline joint obtained with PEEK film (2441. No other amorphous thermoplastics other than PEI used to bond APC-2 composites was found in this literature search. However, perhaps amorphous thermoplastics having a Tg closer to, but still less than the melting temperature of the parts to be bonded may give better performance