Resistance welding of thermoplastic composites-an overview
Introduction
As a result of their growing potential for high performance applications, continuous fibre reinforced thermoplastic composites (CFRTPCs) are becoming of greater interest for the industry. Recently developed matrix materials used for manufacturing thermoplastic composites (TPCs) yield materials with basic mechanical properties (strength, stiffness) much the same, if not better than the thermosets (TS) [1]. Apart from that, thermoplastics have some additional advantages compared to the thermosets, among which improved toughness, better environmental resistance (high temperature, moisture, aggressive fluids), shorter processing times, non-flammability and infinite shelf life [2], [3]. Probably their most important advantage lies in the potential for a low-cost, rapid production [4], [5]. However, due to the limited deformation of the fibre reinforcement, currently produced thermoplastic components have rather simple geometry, which implies that for achieving complex structures, joining of the components is an inevitable process [6].
Joining has proved to be a critical step in the process of manufacturing thermoplastic composite products [7], [8], because it can initiate a number of irregularities in the structure, which can result in weakening of the properties. Traditional joining methods for metals and thermosets (mechanical fastening and adhesive bonding) are feasible, but not ideal for TPCs. Mechanical fastening has a number of disadvantages: introducing stress concentrations in the material, delamination during drilling, different thermal expansion of the fasteners relative to the composite, water intrusion into the joint, possible galvanic corrosion, weight increase and extensive labour and time requirements. Adhesive bonding, although more favourable than mechanical fastening because of avoiding stress concentrations, still presents some difficulties when applied on TPCs. It requires extensive surface preparation, generally difficult to control in industrial environment, and adhesives used (usually epoxies) have long curing cycles. It can also be difficult for the chemically inert thermoplastic matrix to bond.
Fusion bonding is a well established joining method that uses the property of thermoplastic matrices to flow when heated above their glass transition temperature Tg (for amorphous polymers) or the crystalline melting point Tm (for semi-crystalline polymers). Known also as welding, it can be generally described as joining of two parts by fusing their contact interfaces, followed by cooling (consolidating) under pressure, which enables the bond to be made. It appears to overcome all the problems connected to the traditional techniques mentioned above. In spite of the existence of several drawbacks, welding is widely considered to be the ideal bonding technique for TPCs [9].
The fusion bonding techniques are usually classified by the used type of heating. Vast number of means can provide heat to the interface. Hot plates, hot gas, spinning, ultrasonic and radio signal, microwaves, joule effect in a resistor, laser and induction are some of them [10], [11]. From the variety of means, three have emerged as the most promising: induction, ultrasonic and resistance welding [12]. Overviews of these three welding techniques have been presented in several publications [12], [13], [14], [15], [16] that offered full descriptions of the processes and their advantages, as well as some very useful comparative data [12].
This paper presents the first part of an extensive overview dedicated exclusively to resistance welding of TPCs. The aim is to provide a deeper insight into the nature of the resistance welding process and the experimental investigative effort that was put into it by a large group of researchers. After a general description of the resistance welding process, an overview of the experimental procedure is presented, with an emphasis on the experimental set-up and the evaluation methods. The main focus is set on the parameters that govern the welding process (resistance, pressure, power input) and on the main phenomena that have an impact on the quality of the weld. Finally, several alternative resistance welding methods that involve non-thermoplastic materials are discussed.
Section snippets
Resistance welding process
Resistance welding, known also as resistive implant welding, electrical-resistance fusion or electro-fusion [17] is one of the most attractive welding techniques for thermoplastics today. It is a rather simple method that uses electrically resistive implant sandwiched between the bonding surfaces of the laminates to provide the necessary heat to the joint. The principle of the resistance welding process is schematically shown in Fig. 1. When current flows through the heating element, the heat
Test and evaluation methods
Resistance welding is a rather new technique for bonding thermoplastic composite materials, developed as a result of the need for more suitable bonding technique for TPCs than the existing ones. The test methods, however, did not follow the pace of development of the technique itself. No new test method has been developed especially for welding. Currently, standard test methods for adhesive bonding are used for evaluating the strength and the quality of the welds. Although not ideal for
Resistance of the heating element
The resistance of the heating element is one of the most critical parameters in the resistance welding process [20], [25], [29]. Its importance is a result of the close relation between the heating element resistance and the welding energy it produces. Therefore, it is favourable to know or be able to calculate the actual resistance of the heating element used in a particular investigation.
The resistance of the heating element is proportional to its length and inversely proportional to its
Temperature distribution
Uniform heat transfer in the welding stack and uniform temperature distribution as the result of it are essential to obtain optimum performance and quality in the welded joint [44]. The heating rates govern the welding process in general, while the cooling rates have a substantial impact on crystallinity, stiffness and developing of the residual stresses, especially when semi-crystalline thermoplastics are involved. Due to the heat losses in the bulk material, the tooling, the environment and
Alternative resistance welding methods
As a result of its numerous advantages, resistance welding is broadly regarded as a potential for joining of different materials. Numerous joining methods based on the resistance welding technique have been developed in recent years. Especially the use of resistance welding as a hot-melt adhesive for joining different materials has been of interest.
Conclusion
Resistance welding has proven to deservedly hold its place in the group of the fusion joining methods with greatest potential. Its numerous advantages with respect to the traditional joining techniques and the other fusion bonding methods make it very attractive for joining composite parts. The possibility of scaling-up and automatisation of the process has opened the door for the development of alternative resistance welding techniques and application of the resistance welding on
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