Resistance Welding Of Thermoplastic Composites Research Articles

Resistance Welding of Thermoplastic Composites, Including Welding to Thermosets and Metals: A Review

This review paper presents the current progress in the development of resistance welding techniques for thermoplastic composites, with a particular emphasis on their application in hybrid joints, such as those involving thermosetting composites and metals. Resistance welding, a fusion bonding method, offers significant advantages over adhesive bonding and mechanical joining by eliminating the need for additional adhesive materials and enabling integration into automated manufacturing processes. The study highlights the unique benefits of resistance welding, including lower energy consumption compared to other methods and its compatibility with automated manufacturing, which can reduce production costs by up to 40%. Key findings from the literature indicate that resistance welding is particularly effective in achieving strong, durable joints for complex and large structures, such as those used in the aerospace industry. The review also identifies the main challenges associated with resistance welding, including temperature control, current leakage in carbon-fiber-reinforced polymers, and potential corrosion when using metal meshes. To address these challenges, various strategies are discussed, including surface treatments, the use of nanocomposites, and the integration of carbon nanotubes. The review concludes by emphasizing the need for further research to optimize welding parameters and to develop non-destructive testing methods for industrial applications, ensuring the reliability and long-term performance of welded joints.

Experimental characterisation and statistical modelling of woven carbon fibre weld conductors for electrical resistance welding of thermoplastic composites

Resistance welding of high-performance thermoplastic composites is a promising joining technique for the structural assembly of aerospace components. Knowledge of the weld conductor resistance as a function of its dimensions, as well as external boundary conditions such as contact preparation and pressure, can be used to scale welding parameters. This paper presents a comprehensive review on the weld conductor contact preparation, length, width and contact pressure dependent resistance with respect to the Toray 5HS T300JB carbon woven prepreg 281 gsm fibre architecture. The study is based on a 4-wire resistance measurement of the carbon fibre fabric weld conductors and was conducted to provide a solid basis for defining the weld conductor’s resistance, considering the dependence on length, width and contact pressure. The experimentally determined resistance values were approximated using linear and non-linear regression functions and combined into a general formulation with respect to the investigated carbon fibre fabric architecture. The least squares fit of experimental versus model data confirmed a very high model confidence. Thus, the model allows a simplified transfer of electrical properties for process pre-design and scaling considering weld conductors with the same fibre architecture.

Synergy of carbon fiber fabric-based heating element and h-BN modified insulated film on resistance welding of thermoplastic composites

A sandwich-structure heating element (HE) of hexagonal boron nitride (h-BN) decorated polyetherimide (PEI) and plain weave carbon fiber fabric (CFF) of different fabric weight was fabricated to bond carbon fiber reinforced polyetherimide (CF/PEI) laminates by resistance welding. The effects of fabric weight of CFF and thermal conductivity of insulating films on the lap shear strength (LSS) of resistance welding joints and failure mode of CF/PEI composites were investigated. The LSS of the composite joints increased by 31.4% due to the improvement the thermal conductivity of PEI composite film, as well as the reduction of the current leakage during welding process. Intralaminar failure of joints and breakage of carbon fibers of CFF in HE was observed.

Resistance welding of thermoplastic composites with nanofiber films prepared by electrospinning technique

In resistance welding of thermoplastic composites, a stainless-steel (SS) heating element (HE) is usually applied with current to provide heat to melt the thermoplastic polymer and remains at the joint. In this study, electrospinning technique was proposed to produce nanofiber films on the SS HE to achieve the hole pre-filling in the mesh and to provide a resin-rich zone in the bondline to enhance resin impregnation and void reduction, which improved the mechanical interlocking of the joints. The lap shear strength (LSS) of welded joints with nanofiber films was increased by 2.3 times to 36.98 MPa in comparison to the film-free joints and the indefinite fatigue life of the joint was reported at 20% of the static LSS. Taguchi method and Analysis of Variance approach were used to investigate the influence of the process parameters on the LSS. A comparative analysis of the welding curves, defects distributions, and fractographic evolutions was carried out for welded joints with and without the nanofiber films. With nanofiber films, the main failure modes of the joints changed from HE debonding to interlayer debonding.

Resistance welding of thermoplastic composites via a novel carbon nanofilm implant

A novel composite heating element constructed by carbon nanotube film and stainless steel meshes was used to resistance welding of carbon fiber reinforced polyetherimide (CF/PEI) laminates. The microstructure and mechanical property of the resistance welding joints under different t w were respectively investigated by scanning electron microscopy (SEM) and tensile test. The failure interface and failure model matched the mechanical properties were analyzed in detail, and the characteristic area shown a strong correlation with morphological changes of CNT film.

Modelling resistance welding of thermoplastic composites with a nanocomposite heating element

Electrically conductive nanocomposite heating elements are being developed as a complement to traditional carbon fibre or stainless steel heating elements in resistance welding of thermoplastic composites. Here we present the development of a finite element model of the resistance welding process with these new heating elements, from which we establish a process window for high quality welded joints. The finite element model results were validated experimentally and a lap shear strength improvement of 28% is reported relative to previously published results. Fractography analysis of the broken joints revealed a thin-layer cohesive failure mode due to the brittleness of the nanocomposite heating elements.

Improved adhesion between stainless steel heating element and PPS polymer in resistance welding of thermoplastic composites

Resistance welding of thermoplastic composites involves applying an electric current to a heating element, usually a stainless-steel (SS) mesh, which heats up by Joule effect, melts the surrounding polymer matrix and remains at the joint interface following the welding operation. In this study, the adhesion between a thermoplastic matrix, i.e., polyphenylene sulfide (PPS), and the SS mesh heating element is evaluated and improved through the application of surface coatings on the SS. The coatings are based on an organosilane, a bivalent molecule often used to create an organic coating on metals. The efficiency of the surface coatings is evaluated by measurement of the water contact angle on treated SS plates, characterization of the coating by ATR-FTIR, observations by SEM and TEM and mechanical testing of resistance-welded quasi-isotropic carbon fibre (CF)/PPS joints. A lap shear strength (LSS) improvement of 32% is reported for joints welded using the treated heating elements relative to joints welded using untreated heating elements. Double cantilever beam tests between a SS plate and the CF/PPS composite also show a much better adhesion between the PPS and SS following surface coating, with a higher load at failure and fracture surfaces showing polymer and fibre remaining attached to the SS plate.

Resistance welding of thermoplastic composites with a nanocomposite heating element

In this study, we propose a new heating element (HE) for the resistance welding of thermoplastic composites. This HE is made of polyetherimide (PEI), rendered electrically conductive by the addition of 10% wt. multi-wall carbon nanotubes (MWCNT) (conductivity of 0.79 S cm-1). The new HE was successfully used to weld carbon fibre/poly (ether ketone) (CF/PEEK) laminates in a single lap shear configuration, leading to a lap shear strength of up to 19.6 MPa. Observations of the fracture surfaces revealed a cohesive failure mode within the nanocomposite HE and non-uniform heating over the weld area. It is believed that PEI/MWCNT HE present an interesting alternative to current HE, although more work is needed to improve the temperature homogeneity over the weld area.

A displacement-detection based approach for process monitoring and processing window definition of resistance welding of thermoplastic composites

The evolution of weld displacement, or the thickness of welding stack, with welding time during resistance welding of thermoplastic composites was characterised, and based on this the possibility of using displacement data for process monitoring and processing window definition was investigated. Resistance welding of glass fabric reinforced polyetherimide using a metal mesh as the heating element was studied, and weld displacement was detected using a laser sensor. The effect of welding parameters on the displacement curve was studied. Welding defects, such as voids and squeeze flow, could be detected by monitoring the weld displacement. Fast definition of the welding processing window was found to be possible using displacement curves, and the predicted processing window showed good agreement with the processing window determined from mechanical tests.

Continuous resistance welding of thermoplastic composites: Modelling of heat generation and heat transfer

A process model composed of electrical and heat transfer models was developed to simulate continuous resistance welding of thermoplastic composites. Glass fabric reinforced polyphenylenesulfide welded in a lap-shear configuration with a stainless steel mesh as the heating element was considered for modelling and experimental validation of the numerical results. The welding temperatures predicted by the model showed good agreement with the experimental results. Welding input power and welding speed were found to be the two most important parameters influencing the welding temperature. The contact quality between the electrical connectors and the heating element was found to influence the distribution of the welding temperature transverse to the welding direction. Moreover, the size of the electrical connectors was found to influence the achievable welding speed and required power input for a certain welding temperature.

Characterisation of a metal mesh heating element for closed-loop resistance welding of thermoplastic composites

Resistance welding is one of the most suitable and mature welding techniques for thermoplastic composites. It uses the heat generated at the welding interface when electric current flows through a resistive element, mostly a metal mesh. Closed-loop resistance welding relies on indirect temperature feedback from the weld line for process control. Its implementation is more complex than the most common open-loop welding, but on the contrary, it does not, in principle, require the definition of processing windows for each welding configuration and it allows for constant-temperature welding. The temperature at the welding interface can be indirectly monitored through the resistance of the heating element. The relationship between resistance and temperature, expected to be approximately linear for a metal mesh heating element, can then be used to translate the welding temperature into a target resistance value for the process-control routine. Despite the apparent straightforwardness of this procedure, the research results presented in this article prove that different types of characterization tests yield different resistance versus temperature relations for a metal mesh heating element, which can lead to significant temperature deviations when used in closed-loop processes.

Study on resistance welding of glass fiber reinforced thermoplastic composites

Thermoplastic polymer industry has expanded a new impact with the introduction of Eco-friendly thermoplastic matrix composites (TPCs), which finds applications in medical, aerospace, electronic and automotive areas. Joints of non-conductive thermoplastic composite materials are processed by Resistance Welding (RW) by incorporating a conductive corrosion resistive conductive material, as an interlayer. Glass fiber reinforced polypropylene thermoplastic composite sheets were used for this study. The experimental set-up for resistance welding was fabricated. Welding current, clamping pressure and welding duration (Time) are the control parameters. The joining trials on composites were carried out using Taguchi method to reduce time and cost effective experimental studies. The effect of parameters which govern the quality of resistance welding of thermoplastic composites is also emphasized in this work. The purpose of this study is to determine the optimum process parameters for RW. The joints obtained were analyzed microscopically; it revealed the good integration of thermoplastic composite with the interlayer material. The mechanical strength of the joints is tested through lap shear strength testing.

Optimization of thermoplastic composites resistance welding parameters based on transient heat transfer finite element modeling

The use of resistance welding technology to join thermoplastic composite aerospace structures is still contingent upon a better understanding of the heat transfer mechanisms occurring during welding, which govern the joint quality and mechanical performance. In this study, two-dimensional (2D) and three-dimensional (3D) transient heat transfer finite element models were developed to simulate resistance welding of thermoplastic composites. The 2D model was used to investigate the effect of the length of the exposed areas of the heating element to air (clamping distance) on the local overheating at the edges and the effects of the input power level on the thermal behavior of the welds. It is shown that controlling the clamping distance improves the thermal uniformity of the weld. The 3D model shows that heat conduction along the length of the laminates influences the thermal uniformity of the weld interface. An optimization chart is developed in order to minimize the undesirable edge effect and to define the conditions required to obtain a complete weld. The results of the 3D model are compared with experimental data.

Metal mesh heating element size effect in resistance welding of thermoplastic composites

The objective of this work is to determine the effects of metal mesh heating element size on resistance welding of thermoplastic composites. The materials to be resistance-welded consisted of carbon fiber/poly-ether-ketone-ketone (CF/PEKK), carbon fiber/poly-ether-imide (CF/PEI) and glass fiber/PEI (GF/PEI). Four different metal mesh sizes were used as heating elements. The samples were welded in a lap shear joint configuration and mechanically tested. Maximum Lap Shear Strengths of 52, 47 and 33 MPa were obtained for the CF/PEKK, CF/PEI and GF/PEI specimens, respectively. The ratio of the heating element’s fraction of open area and wire diameter was shown to be the most important parameter to be considered when selecting an appropriate heating element size.

Resistance welding of thermoplastic composites skin/stringer joints

An experimental investigation of resistance welding of APC-2/AS4 PEEK/carbon fibre composite using a stainless steel mesh heating element is presented. A special specimen geometry, the skin/stringer configuration, was used to represent a typical reinforced aerospace structural joint. The specimens consisted of a flange, representing a stringer or frame, welded onto a skin laminate. The effects of the welding parameters such as the input power level and clamping distance on the weld quality and performance were investigated. The welding parameters were optimised using short beam shear tests, ultrasonic C-scan inspection and optical microscopy. The mechanical performance of the resistance-welded skin/stringer configuration was investigated using three- and four-point bending tests and the failure mechanisms were characterised by optical and scanning electron microscopy. Two methods were used to reduce the stress concentration at the flange tip. The first method was to machine a 20° taper angle at the edge of the flange and the second one was to create a resin fillet at the flange tip. No mechanical performance improvement was obtained with the resin fillet method but the taper angle method showed 25% mechanical performance improvement when the taper angles were machined after the welding operation.

Resistance welding of thermoplastic composites-an overview

This paper presents an extensive overview of resistance welding of thermoplastic composites. The objective is to provide a deeper insight into the nature of the resistance welding process and a summary of the vast experimental investigative effort put into it over the years. The main focus is set on the parameters that govern the welding process and the principal phenomena that affect the quality of the joint. The standard experimental procedure, the experimental set-up and the main evaluation methods are also looked at in detail. Finally, several alternative resistance welding methods that involve non-thermoplastic materials and offer possibilities for future applications are briefly reviewed.

Two‐Dimensional thermal analysis of resistance welded thermoplastic composites

AbstractA transient two‐dimensional thermal model for resistance welding of thermoplastic composites is presented. A parametric study is conducted that yields insight into the welding process enabling some critical process and material parameters to be identified. Time to melt is predicted by the model and is successfully compared to experimental observations. Local heating and meltthrough can also be explained by the transient thermal model in agreement with experimental observations. Mode I fracture toughness of unidirectional graphite reinforced poly(etheretherketone) resistance welded double cantilever beam specimens are conducted under various process conditions. Experimental results indicate that under optimum process conditions, the interlaminar fracture toughness of the bulk compression‐molded thermoplastic composite material can be achieved using resistance heating as a joining technique.