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Abstract

Thermoplastic composites offer a wide range of potential applications, but effective composite–composite joining remains challenging. This research work optimizes glass fiber-reinforced polypropylene (GF/PP) composite joints made using carbon fiber resistance welding (CFRW), focusing on the relationship between lap shear strength and process parameters. Carbon fiber is chosen as the heating element due to its light weight, better strength and high conduction in the composite joining using resistive welding. Welding current, time, and clamping pressure were optimized to minimize the Polypropylene matrix deterioration. Lap shear strength (LSS) test was used as the output parameter for the optimization process. The microscopic analysis of joints was done to confirm the formation of strong joints with sound fiber–matrix bonding. Statistical analysis was performed to get the combination of process parameters with optimum lap shear strength. Optimal performance was achieved with moderate currents (10–15 A), clamping forces of 400–500 N, and contact times of 35–54 s, yielding minimal cracks and no matrix burning. In contrast, higher currents, excessive clamping, or binder plies degraded strength and microstructure. Overall, CFRW demonstrates strong potential for thermoplastic composite assembly when moderate parameters are maintained. The obtained results show that CFRW produces reliable, cost-effective joints suitable for automotive, and aerospace applications.

Keywords

Resistance welding joining thermoplastic reinforced composite optimization

Introduction

The composite materials market is rapidly growing due to their versatility in structural and industrial applications. Composites show high strength-to-weight ratio, vibration damping, corrosion resistance, and customizable properties, which makes them vital in aerospace—exemplified by the Boeing 787 and Airbus A350, which are about 50% composite by weight. Beyond aviation, composites are increasingly used in buildings and bridges, highlighting the need for advanced joining methods with metals, wood, plastics, and other materials.¹

The process of joining structures inherently involves the joining of materials, and the growing use of diverse materials and their combinations, for example, composites, underscores the need for careful consideration before proceeding. Before committing substantial time and financial resources, several aspects require thorough study. Adequate information on various joining approaches not only aids in material selection but also informs the overall design, allowing the joining mechanism to complement structural performance rather than risking potential compromises to the system. In essence, a comprehensive understanding of joining methodologies is essential for informed decision-making and ensuring the integrity of the overall structure.²

Thermoplastic composites have many advantages over thermosetting composites such as they can be reheated, reshaped, and recycled leading to a cost-effective and sustainable solution. Moreover, they can be rewelded and easily repaired. Thermoplastic composite offers improved mechanical properties such as toughness and impact resistance over the thermoset counterparts.³ The versatile applications of polypropylene (PP) across sectors like biomedical, automotive, aerospace, and air/water filtration are highlighted.⁴

Various techniques are employed to join different components of composites, and the choice depends on the specific application. Joining methods are generally categorized into adhesive, mechanical, and fusion bonding, as illustrated in Figure 1. Traditional mechanical joining is not a preferred choice for thermoplastic composites due to their ductility, as well as damage to the fibers, stress concentrations around the holes and reducing load-bearing capability.⁵ The adhesive bonding of thermoplastic composites is an intricate method with low molding efficiency, and results in a non-removable joint, significantly influenced by environmental factors.⁶

Figure 1.

Classification of thermoplastic composites joining techniques.

Fusion bonding is a process that entails joining the two thermoplastic materials by heating the interface beyond the glass transition temperature (Tg) for amorphous polymers or above the melt temperature (Tm) for semi-crystalline polymers. This method has been recognized in various reviews and in research articles as an alternative to adhesive bonding and mechanical fastening. It is acknowledged as a capable technique for joining and repairing Thermoplastic Composites (TPCs). Fusion bonding offers a viable approach, particularly in applications where the advantages of reduced weight, enhanced structural integrity, and efficient repair processes are sought.⁷ Melt welding of thermoplastic composites surpasses conventional joining methods due to its ability to endure substantial stresses, achieve elevated productivity, and maintain low manufacturing costs.¹

Yousefpour et al. reviewed various techniques for the melt joining of thermoplastic composite components.⁸ Costa et al. investigated the welding technology of thermoplastic composites in aircraft, analyzing the merits and drawbacks of several welding techniques and their potential applications.⁹ There are benefits and drawbacks to each joining technique, and the selection is based on the needs of the application and design. The fast processing cycles of fusion bonding techniques, including ultrasonic, induction, and resistance welding, make them ideal for volume-intensive applications like bulkheads, surfboards, cars, aerospace fuselage, and wind turbine blades. Their main benefits include improved integrity and durability, reprocessing ability, recyclability, and little surface preparation.²

Despite these advancements, challenges persist in the joining of thermoplastic composites by resistance welding. These challenges include achieving precise control of temperature at the interface and accomplishing the high consolidation pressures necessary to establish intimate contact between the adherends. For a repair technology to be certified, a robust and systematic methodology is crucial. Furthermore, ensuring reliability in the mechanical performance of the welded joints is paramount. Overcoming these challenges will be essential for the successful development and certification of fusion bonding techniques, contributing to the broader acceptance and utilization of TPCs in aerospace applications.¹⁰ Villegas et al. examined the process factors, shear strength, and fatigue characteristics of three welding techniques: ultrasonic welding, induction welding, and resistance welding.⁹ Ahmed et al. analyzed the mechanics of heat generation and parameters controlling the welding process in induction heating.¹¹ Induction welding can join volumes of material; nevertheless, the manufacture of induction elements presents difficulties, resulting in non-uniform interface temperatures that compromise joint strength and incur high process costs.¹²

Wang et al. found that induction-welded joints had better connection quality when process parameters were proper, but high input current can degrade joint quality.¹³ Defects that initiate failure are found near the interface between the base material and the weld zone, and weld porosity seems to have the biggest impact on strength reduction. Friction stir welding (FSW) causes the fibers in the weld zone to break up, releasing pieces that are as short as a few microns.¹⁴ Similarly, an important factor in resistance welding is the heat that is generated by contact resistance when current is supplied. Since thermoplastic composites are insulating in nature, current is delivered through heating elements, with resistive sources placed between material surfaces to provide the bonding heat. This method eliminates the need for complex geometry molding.¹⁵ A feature of resistance-welded joints is that the heating element cannot be removed after molding, so the incorporation of dissimilar materials may influence the interfacial qualities of the connection. Typically, carbon fiber fabric or metal mesh serves as the resistance welding heating element, with carbon fiber fabric being more costly while metal mesh offers a more economical alternative. Resistance welding quality is highly dependent on process settings. As a result, the two important exploration paths for improving the mechanical characteristics of resistance-welded joints in thermoplastic composites are refining the heating element and optimizing process parameters.¹⁶

The heating element made of carbon fiber has its own limitations. Carbon fibers are electrically conductive, but their conductivity is highly anisotropic. They exhibit high electrical conductivity along the fiber axis and comparatively low electrical conductivity in the transverse direction. This anisotropy causes the applied electrical current to preferentially follow specific fiber-aligned pathways. As a result, localized current concentrations or hotspots develop within the adherend. These hotspots increase the risk of localized overheating, matrix burning, and non-uniform weld formation.¹⁷ In an oxygenated environment, significant oxidation began at temperatures ≥550°C, resulting in a reduction in fiber diameter, which reduced further with increasing temperature and exposure duration. Tensile strength and electrical conductivity of carbon fiber decreased with a reduction in fiber diameter¹⁸ Shi et al. have investigated to explore the fabric orientation influences in the failure modes and joint strength of the composites, which were welded by resistance-welding 8-harness satin GF/PEI.¹⁹ The predominant orientation of fibers at the welding zone was identified as a significant factor impacting joint strength. It is observed that when the main apparent orientation of fibers on the welding area is perpendicular to the direction of load, it leads to a notable decrease in the LSS (lap shear strength) of the weld joint. These reductions usually fall within the range of 13 to 20%. Furthermore, fractography analysis of the welded joints confirms that fiber-matrix de-bonding is the main failure mode for configurations such as warp joints, whereas weft joints exhibit substantial laminate ripping. These results highlight the intricate interactions between joint strength, fabric orientation, and failure modes in GF/PEI composite resistance-welded joints.

A number of ultrasonic and resistance heating experiments were conducted in various TPCs; Figure 2 illustrates that the resistance heating has a higher lap shear strength than the others.⁷

Figure 2.

Experimental mean lap shear strength of adhesive and welded joints using TPC substrate (Reis et al., 2020b).

Zhang et al. designed a resistance welding setup, comprising the essential units like DC power supply, a heating element for interface and joining material melting, an electric electrode, an adherend, an insulation board, and an applied pressure control unit, was devised. The resistance values of the heating elements during the resistance welding process adhered to a normal distribution. Utilizing the Taguchi method and analysis of variance (ANOVA), it was determined that, among the three key welding process factors (current, pressure, and contact time) in the GF/PP resistance welding process, the primary factor influencing the resistance welding quality of GF/PP thermoplastic composites (TPCs) was the current, with a substantial contribution rate of 58.12%. The impact rates of time, pressure, and experimental error were found to be 23.07%, 15.29%, and 3.51%, respectively. Applying the signal-to-noise (S/N) method, a correlation between the larger-the-better S/N ratio and process factors was established for optimizing the resistance welding process. Among various combinations of the level of these factors, the optimal resistance welding process parameters were identified as a current of 12.5 A, pressure of 2.5 MPa, and time of 540 s. The divergence between experimental data and projected S/N values was calculated at 6.76%. The LSS of the GP/PP composite joint is reported to be 186 MPa under the optimized parameters, observed using the stainless-steel mesh heating element.²⁰ Cheng et al. reported that a shear strength of 6.656 ± 0.475 MPa was achieved in a GF/PP composite welded joint with process parameters: welding current of 16.7 A, welding pressure of 20 N, and welding time of 75.6 s, when using steel mesh as a heating element.²¹ Wanling Long et al. described joining the GF/PP thermoplastic composite by resistance welding. The optimized welding parameters were identified as a welding pressure of 15.6 N, a current flow time of 1.2 min, and an input current of 10.6 A. Following the welding operation, a 30 s pressure-cooling step was applied. A 40-mesh stainless steel mesh (SSM) heating element, pre-treated with a silane coupling agent, was used during the process. The influence of oxidation temperature, solvent properties, and solution pH on the LSS of the welded joints was systematically evaluated. LSS of the resistance-welded joints treatment under optimal conditions (500°C, ethanol solvent, and pH = 11) improved LSS by 27.2% compared to untreated joints tensile strength 9.2 ± 0.5 MPa.²² Bing DU et al. reported the GF/PP composite materials were joined with the resistance welding, the hot pressed sandwiched with two GF/PP prepregs and a metal mesh heating element was used. The current was transmitted to the heating element through the brass electrode. The resistance values of the heating elements selected in the following research were between 0.19 and 0.23 Ω. With a consistent pressure of 0.05 MPa, variable welding contact time, and welding current, the LSS was optimized. When the welding time was 1 min, and the input current was 10 A, the bending strength of the single lap joint reached a maximum value of 1.46 ± 0.36 MPa.²³

The tensile and compressive failure behavior of single-lap joints in PLA–bamboo fiber green composites joined by resistance welding was studied. The heating elements, polypyrrole, carbon fiber (CF) fabric, and stainless-steel mesh were tested under varying heating time (30, 60, and 90 s) and overlapping area coverage by the heating element (35, 65, and 100%). CF fabric yielded the highest performance, improving tensile and compressive failure loads by 31% and 63% over stainless-steel mesh, and by 187% and 323% over polypyrrole, respectively.²⁴

In their study of CF/PEI resistance welding under displacement and pressure control, Ageorges et al. in his research article has distinguished six stages during the welding of joints: initial compaction, cooling, solidification, contraction, melt flow, and thermal expansion.²⁵ Glass fiber-reinforced polyphenylene sulfide (GF/PPS) welded connections were investigated by Lorena and Samia, who found that fractographic analyses were a useful tool for assessing the quality of the welding.²⁶ Shi et al. reported that proper surface contact prevents microcracks, insufficient heating reduces penetration and strength, and correct welding pressure ensures interface contact while avoiding joint clearances.²⁷ Resistance welding of glass fiber–reinforced semi-crystalline PET using a stainless-steel mesh heating element was studied. Clamping pressure and mesh selection are identified as key parameters for developing a reliable, cost-effective joining method for infrastructure applications.²⁸

The ratio of the heating element’s fraction of open area and wire diameter is the most important parameter to be considered when selecting an appropriate heating element size.²⁹ A finite element model for the resistance welding of thermoplastic composites, using a nanocomposite heating element, allowed us to establish a processing window to target conditions leading to an improvement of welded joints. Furthermore, with this window model, new experiments increased our knowledge of the phenomena at play when welding using these newly developed heating elements. The model demonstrated the possibility of producing welds at lower power densities, and this will be validated experimentally.³⁰

Starrov and Bersee perceived that the cooling rate strongly affects crystallization, weld strength, and weldability in aviation composites. Low power enables isothermal cooling that reduces residual stress. On the other hand, high power leads to non-isothermal cooling, causing stress and distortion. Moderate power provides near-optimal cooling conditions.³¹

The temperature significantly affects the LSS of welded joints, which show a linear decline between 21°C and 150°C. On the other hand, moisture has little to no influence on LSS, regardless of testing temperature or aging conditions in CF/PPS composite joints.³² ISO 12176-2 documented ambient temperatures of −10°C to +40°C for electrofusion welding. In field conditions, heaters may be desired in cold weather and shade in hot weather where cooling takes a longer time. Quenching should be avoided because it can generate internal stresses and compromise joint quality.³³ The cooling rate controlled the crystallinity and morphology, which had a major impact on the welding properties of the joint. To enhance the quality of the welding, it may be crucial to optimize the cooling-down period.³⁴

O’Shaughnessey et al. reported that weld strengths were rigorously affected by polymer type, implant material dimension, and the shape of implant materials, and also indicated that resistance welding (RW) provides better heat dissipation in the weld area as compared to ultrasonic and induction welding (IW) methods.³⁵ Fiber–matrix de-bonding is the main failure approach; fiber–matrix adhesion has a substantial effect on the LSS of welded joints. LSS is reduced by 56% when GF/PEI laminates with aminosilane glass fiber sizing are used instead of laminates with chromium methacrylate glass fiber sizing. Moisture in the laminates causes process-induced porosity during welding, which has a detrimental impact on the LSS and modifies the failure modes.¹⁹

Using an upgraded Continuum Damage Model, a high-fidelity modeling technique is created to precisely forecast matrix failure and splitting while examining the impact of the intricate failure behavior of the welded connection. This model can precisely anticipate the failure mode and offers fresh insights into the joint’s failure behavior.³⁶

Several techniques are used to connect glass fiber-reinforced PP composites. Resistance-welded joints perform better in static tests, but hybrid methods decrease strength because of PP’s high ductility and low modulus. Resistance-welded joints exhibit the least modulus fluctuation, and also the most cycles sustained in dynamic tests. Resistance welding is an excellent technique because it creates consistent, effective welding with no effect on the mechanical performance of thermoplastic composites joining.¹

For thermoplastic composites, resistance welding process optimization requires an understanding of how welding process factors interact. More investigation is required to explore the principles of action and measure the extent to which each parameter influences the creation of strong and effective welding processes for the joining of thermoplastic materials.³⁷

Welding power and time command the amount of heat generated in the heating element, while welding pressure affects the degree of contact and material fusion at the joint interface. Incongruous heat generation caused by either excessive power and prolonged welding time or insufficient power and short duration can lead to resin ablation or inadequate melting, respectively. Similarly, applying too much or too little welding pressure may either extrude molten resin from the weld zone or result in poor bonding due to inadequate resin flow and surface contact. The optimum working temperature of the PP material for RW was determined as 217°C, and the heating time was determined as 13 s by using the Ni-Cr wire of 0.5 mm diameter heating element. However, the welding range between 210 and 250°C is known to give the best results for PP joining.³⁸

Barazanchy et al. selected the initial range of suitable welding parameters and determined them using direct temperature measurement by considering the edge effect, which can lead to non-uniform heating across the weld zone. A temperature gun is used to monitor the heating region. This helps to identify a constant (a product of voltage and time) required to reach the targeted temperature and hence to determine the heat generated during the resistance welding process.³⁹ The resistance welding parameters, such as welding power, welding time, and welding pressure, play a crucial role in determining the quality of the weld.⁴⁰

Power level and energy input are critical parameters in the resistance welding process, which are considered the most important factors. These parameters directly influence the energy supplied to the welding interface, impacting the quality of the joint and overall controllability of the welding process. Moreover, the size and type of heating element play a significant role in the resistance welding process. The heating element is a central component that provides the essential energy to the joining material for welding. The principle of resistance welding is to apply Joule’s law to transform electrical energy into heat, and Joule’s Law equation is as follows:

E = I^{2} Rt

where E is energy, I is current, R is resistance, and t is time. As current passes through the heating element, it generates heat exceeding losses to the material and environment, raising joint temperature. Once the resin matrix melts and diffuses at the interface, the current is stopped, and the joint cools under pressure to form a bond. Thus, resistance welding quality depends on current, time, and pressure.⁸

Extensive research exists on the joining of thermoplastic materials such as GF/PP using metallic heating elements, which have favorable results due to their high electrical conductivity. However, the primary drawback of metal-based heating elements is their added weight. In composite structures subjected to vibration, the presence of a dissimilar, higher-density material like stainless steel mesh, etc, can promote crack initiation and propagation at the interface. This concern highlights the need to explore lightweight heating elements, such as carbon-based materials, which are the focus of the present study. Although carbon fibers have previously been used as heating elements and their application has mainly been reported in other thermoplastic composites such as PEEK, HDPE, and PPS.

This study aims to propose an optimization method for resistance welding using GF/PP thermoplastic composites (TPCs) and carbon fiber heating elements. Glass fiber-reinforced polypropylene (GF/PP) TPCs are typical and low-cost thermoplastic composite materials, which have broad prospects for civil use. The literature review highlights the importance of analyzing the interaction between welding process parameters and the composite matrix to effectively optimize these parameters. To address this, a design of experiments (DoE) approach was developed, as detailed in the methodology section. A self-designed resistance welding equipment suitable for the resistance welding of TPCs was fabricated. A statistical analysis (Minitab) identified optimized parameters yielding the highest LSS. This study work elaborates process–matrix interactions and provides a reference for resistance welding of TPCs.

Experimental

Materials

A commercially available industrial-grade woven glass fabric (400 GSM) and polypropylene (PP) nonwoven sheet (70 GSM), as shown in Figure 3, were used to fabricate glass fiber-reinforced polypropylene composites (GFRPC).

Figure 3.

Polypropylene fabric and glass fiber mats.

Methodology for composite fabrication

The composites were fabricated using a hot compression molding machine (CARVER, 30 Ton Monarch, Model CMG 30H-12-X, Germany) as shown in Figure 4. The GF/PP laminate had a stacking sequence of (4P/1 G/4P/1 G/5P/1 G/4P/1 G/4P), where P and G represent PP matrix and glass fiber layers, respectively, as in Figure 5.

Figure 4.

Sample preparation using compression molding machine (CARVER, 30 Ton Monarch).

Figure 5.

Stacking sequence of matrix and fiber mats.

For the fabrication of GF/PP laminates by hot compression molding, pressure was gradually increased to 14 tons as the temperature reached 200°C. The laminate was held for 10 min, then cooled to 100°C under constant pressure to ensure consolidation. The final sheet achieved a fiber volume fraction of 52% ± 0.4%, and its mechanical and thermal properties were evaluated using standard methods as mentioned in Table 1.

Table 1.

Mechanical and thermal properties of GFRPC.

Material	Tensile	Impact strength	Flexural stress	Thermal conductivity
Test method	ASTM D3039	ISO 179	ASTM D7264	ASTM E1530
Units	MPa	KJ/m²	MPa	W/mK
GFRPC	240	29.29	62	0.167

Resistance welding setup for joining

A self-built resistance welding machine, as shown in Figure 6, was used. It comprised a DC power supply with adjustable controls, a hand-held clamping mechanism, copper electrodes, and insulating materials to ensure uniform current flow and stable conductivity during welding.

Figure 6.

Resistance welding setup.

To minimize power loss, double-sided copper electrodes with ceramic insulating spacers were used, clamping the ends of the carbon fiber heating element to ensure secure connections. The central lap region was set to 25 × 25 mm, with failures mainly observed as fiber–matrix de-bonding and laminate tearing. Variable solid PP non-woven sheet interlayers were inserted to assess matrix content effects on joint strength. Welding employed a DC power supply (80 V, 40 A max) with load cell–measured clamping force and a computer-based system for current, voltage, and contact time monitoring as detailed described in Figure 6. Neat PP butt joints and GF/PP laminates were cut to 25.4 × 115 mm strips for standardized welding trials. Five 3 K carbon fiber yarns having an electric resistance of 0.6–0.9 Ω are arranged with 7 mm spacing (two at edges and three centrally) as described in Figure 7. It served as heating elements to achieve uniform heating and effective bonding. A spacing of 7 mm between adjacent carbon fibers was maintained to ensure uniform distribution across the weld region, thereby promoting consistent heating. In the pre-trial experiments, eight 3K carbon-fiber tows were placed at equal spacing; however, even at low energy input, matrix burning occurred due to localized overheating and short-circuiting caused by the small inter-fiber spacing. To avoid this issue and achieve stable heating, the number of fibers was reduced from eight to five, which is illustrated in Figure 7.

Figure 7.

Composite strip joints dimension.

Resistance welding process parameters

Resistance welding parameters including power, time, and pressure, directly affect the weld quality by controlling heat generation, resin flow, and interfacial bonding. Excessive or insufficient heat and pressure can cause resin ablation, incomplete melting, or poor bonding. A Design of Experiments (Table 2) was conducted using three levels of welding current, welding contact time, and binder ply. Before the DoE, a pre-trial energy optimization was performed to determine the minimum energy required for effective GF/PP welding. The temperature at the outer surface of the sample near the welding zone was measured using an infrared temperature gun (Model CT 44037, Crown) to verify PP melting at the moment when visible PP fumes appeared, as shown in Figure 8. However, direct temperature measurement inside the welding zone was not feasible due to limitations of the experimental setup. Access to the internal region was restricted, and any contact-based sensor, such as a thermocouple, would have been welded into the joint during the process. The welding process follows the Joule heating equation (E = I²Rt = VIt). This means the required energy level is determined by the product of welding current (I), voltage (V), and welding contact time (t). The trials began with 1.4–1.6 kJ and were gradually increased until matrix burning occurred. The optimum range was found to be 4.5–4.6 kJ, which produced good consolidation without burning. As per Design of Experiments, different levels of pressure, current, and solid PP non-woven sheet binder plies were applied, while voltage and contact time were adjusted and measured to maintain constant heat input. According to DoE, in the welding current (I) was fixed/controlled according to the DoE levels, voltage (V) was measured but not controlled externally, and contact time (t) was measured and controlled manually by switching the welding ON/OFF to ensure the final energy delivered remained within the optimized range (4.5–4.6 kJ). Because the target energy was constant, it was observed that the current level and measured voltage dictated how long the welding process needs to run. Low product value of current and voltage leads to a longer required contact time. At low current settings, heat generation is slower, so it takes more time to reach the required energy for matrix melting. High product value of current and voltage leads to shorter contact time needed.

Table 2.

Levels of control parameters in the design of experiment.

Parameters	Level 1	Level 2	Level 3
Welding pressure P (N)	400	500	600
Welding current (Amp)	10	15	20
Binder plies	0	1	2

Figure 8.

Matrix melting during joining process.

Moreover, short contact time exacerbates matrix burning because excessive current generates heat much faster than the composite material can dissipate it due to the matrix (PP) low thermal conductivity as compared to the fiber (glass), leading to localized, rapid temperature spikes that cause thermal decomposition of the matrix before the heat can be distributed. During welding processes, a high electric current passes through carbon conductive heating elements, generating heat through the Joule effect. Excessive current leads to an extremely high rate of heat input. When the heat input rate is very high and the duration is short, the heat is concentrated in a localized zone (at the resistive heating element). This localized overheating causes the polymer to burn, decompose, or ablate, leading to the formation of volatile gases and char, which compromises the material’s integrity and leads to a larger volume of degraded material.

Another important factor, clamping force directly affects resin flow and matrix dispersion during composite welding. If the force is too low, the layers do not consolidate properly, leaving voids, dry spots, and weak bonding that can initiate cracks. If the force is too high, excess resin is squeezed out, creating resin-starved regions with uneven fiber volume fractions. These non-uniform zones develop high thermal and mechanical stresses during cooling, which promote microcrack initiation and growth. Thus, improper clamping force, either too low or too high, leads to poor matrix distribution and increases the risk of cracking in the welded composite.

Therefore, by adjusting the contact time at each current and pressure level, the total delivered energy was maintained within the optimal range of 4.5–4.6 kJ. This control ensured adequate matrix melting, prevented thermal degradation, and resulted in consistent and high-quality welds.

The third parameter of DoE is the PP non-woven sheet binder plies. During resistance welding, the Solid polypropylene (PP) fabric sheets’ ply binder compensates for matrix loss caused by thermal degradation and fuming. However, an excessive number of PP plies increases the volume of molten polymer in the weld zone, which can lead to polymer expulsion from the confined interface and hinder proper joint consolidation. Therefore, optimizing the binder-ply quantity is essential to maintain sufficient matrix availability while preventing overflow.

During the pre-trial investigations after welding, it was observed that under the selected DoE pressure levels, the molten PP in the weld zone solidified and reached near the ambient temperature (well below its melting point) approximately 7 min after the welding current was switched off. Based on these observations, a cooling period of 7 min was adopted for all subsequent welding trials under the DoE. Standardizing this cooling duration ensured a consistent thermal history across specimens, enabling a reliable comparison of joint strengths and isolating the effects of welding current, contact time, and binder-ply quantity. So, the joints were cooled under pressure levels for 7 min before release, ensuring uniform solidification and minimizing post-weld defects. The detailed full factorial scheme incorporated the controlled process parameters listed in Table 3.

Table 3.

Full factorial table included measured process parameters.

Code	Applied load	Current	Voltage	Contact time	No. of binder plies
Code	N	Amp	V	Second	-
CA-1	400	10	14 ± 0.6	33 ± 0.4	0
CA-2	400	10	12.5 ± 0.5	37 ± 0.5	1
CA-3	400	10	8.5 ± 0.5	55 ± 0.5	2
CB-1	400	15	10.5 ± 0.3	30 ± 0.4	0
CB-2	400	15	13 ± 0.4	24 ± 0.4	1
CB-3	400	15	09 ± 0.3	35 ± 0.3	2
CC-1	400	20	14 ± 0.2	22 ± 0.2	0
CC-2	400	20	10.4 ± 0.2	24 ± 0.4	1
CC-3	400	20	12.0 ± 0.2	19 ± 0.3	2
CD-1	500	10	13.0 ± 0.5	35 ± 0.4	0
CD-2	500	10	10.5 ± 0.3	44 ± 0.5	1
CD-3	500	10	8.5 ± 0.4	54 ± 0.3	2
CE-1	500	15	16 ± 0.5	20 ± 0.3	0
CE-2	500	15	08 ± 0.4	37 ± 0.4	1
CE-3	500	15	16 ± 0.4	20 ± 0.3	2
CF-1	500	20	12 ± 0.2	19 ± 0.2	0
CF-2	500	20	11 ± 0.1	20 ± 0.1	1
CF-3	500	20	10 ± 0.2	23 ± 0.2	2
CG-1	600	10	12 ± 0.4	38 ± 0.3	0
CG-2	600	10	8 ± 0.5	80 ± 0.4	1
CG-3	600	10	9 ± 0.4	51 ± 0.3	2
CH-1	600	15	14 ± 0.6	22 ± 0.2	0
CH-2	600	15	9 ± 0.4	35 ± 0.3	1
CH-3	600	15	10 ± 0.4	31 ± 0.2	2
CI-1	600	20	12 ± 0.1	20 ± 0.1	0
CI-2	600	20	13 ± 0.1	18 ± 0.2	1
CI-3	600	20	12 ± 0.2	19 ± 0.1	2

Characterization and analysis

After welding, the joint specimens were tested for Lap Shear Strength (LSS) under shear loading according to standard testing protocols. Additional evaluations and analyses were performed as described in the following sections.

The lap shear strength of the welded joints and base laminates was measured using a Universal Testing Machine (UTM) Z100 (Zwick Roell, Germany) in accordance with ASTM D3039.

Results and discussion

Shear force vs displacement curves in LSS test

As per the LSS test of the sample according to DoE, the sheer force and joint displacement of the sample before rupture are shown in graphs Figure 9(a–c) at different clamping forces and other process parameters like welding current, No. of binder support matrix plies in the welding area.

Figure 9.

Shear force versus displacement curve at: (a) 400 N clamping load, (b) 500 N clamping load, and (c) 600 N clamping load.

In Figure 9(a), at 400 N Clamping load, the maximum shear force and joint displacement have been observed as 2587 N and 3.68 mm in sample CB-3, while the other process parameters, 15 A welding current, welding contact time of 35 s, and usage of two binder plies were observed.

In Figure 9(b), at constant clamping force 500 N, the maximum shear force has been observed 2446 N with other process parameters like 10 A welding current, welding contact time 35 s while no binder plies were used in sample CD-1, but longest displacement 5.46 mm was observed in sample CD-3 with the same constant applied clamping force 500 N with the process parameters welding current 15 Amp and welding contact time 37 s, while one ply of binder was used during welding.

On the other hand, according to Figure 9(c), at a constant applied clamping force 600 N, the maximum shear force has been observed as 2049 N with process parameters welding current 20 A, welding contact time 20 s, and no binder plies were used in sample CI-1. However maximum joint displacement 5.24 mm was observed in sample CH-1 with the same constant applied clamping force 600 N, with the process parameters welding current 15 Amp and welding contact time 22 s, while no strip of binder was used during welding.

Failure analysis of joints after LSS test

As shown in Figure 10, the microscopic examination of sample CB-3 revealed only minor crack propagation with no evidence of matrix burning, while the joint exhibited strong bonding strength. The process parameters for this sample were a clamping force of 400 N, a welding current of 15 A, welding contact time of 35 s. These results suggest that a relatively low clamping load, combined with a moderate welding current and prolonged welding contact time (sufficient for limited matrix melting), can produce a joint with high strength.

Figure 10.

Microscopic view of different samples after shear test.

In contrast, the microscopic analysis of sample CC-3 indicated partial matrix burning complemented by small-scale crack propagation. The process parameters included a clamping load of 400 N and a higher welding current of 20 A were exercised and a short welding contact time of 12 s for matrix melting (joining indication) was observed. The elevated current was the primary cause of matrix degradation. It is noteworthy that increasing the welding contact time under such conditions would likely intensify crack propagation.

Sample CD-3 exhibited only a fine crack line without matrix burning. Its process parameters were a clamping load of 500 N, welding current of 10 A, and observed contact time of 54 s for matrix melting (joining indication). The relatively low current prevented matrix burning, while the extended welding time allowed gradual matrix diffusion, thereby promoting the development of a stronger joint.

On the other hand, sample CF-3 showed distinct signs of matrix burning. The clamping force applied was 500 N, with a welding current of 20 A. And here, a contact time of 23 s was observed for joining. Again, the high current was the dominant factor leading to matrix burning, despite the relatively short welding duration.

The microscopic view of sample CH-2 demonstrated significant crack propagation without matrix burning. This behavior is attributed to the higher clamping force of 600 N, which dispersed the matrix within the composite during welding. A welding current of 15 A was insufficient to induce burning in the matrix, while the 35 s contact time contributed to the observed cracking.

Finally, sample CI-3 displayed both crack propagation and matrix burning. The applied parameters as 600 N clamping force, 20 A welding current, and a short welding contact time of 19 s, created high thermal and mechanical stresses, leading to severe microstructural damage in the joint.

Table 4 presents the detailed microscopic behavior of the glass fiber–reinforced composite joint as influenced by the welding process parameters.

Table 4.

Effect of welding parameters on microstructural behavior of joints.

Sample ID	Clamping force (N)	Welding current (A)	Contact time (s)	Microstructural observation	Key remarks
CB-3	400	15	35	Minor crack propagation; no matrix burning	Low clamping load + medium current + longer contact time → strong joint strength
CC-3	400	20	12	Small cracks; matrix partially burnt	High current + short time → matrix degradation; longer time may worsen cracks
CD-3	500	10	54	Fine crack line; no matrix burning	Low current + long time → slow diffusion; strong joint developed
CF-3	500	20	23	Matrix burning observed	High current dominant factor causing burning despite moderate time
CH-2	600	15	35	Large crack propagation; no matrix burning	High clamping force dispersed matrix; current too low for burning
CI-3	600	20	19	Crack propagation with matrix burning	High force + high current + short time → severe microstructural damage

The experimental observations clearly demonstrate that welding parameters have a decisive influence on the microstructural integrity and overall joint quality of PP-based composites. A moderate welding current (≈15 A), combined with a low-to-medium clamping force (400–500 N) and a relatively longer welding contact time (35–54 s), produced strong joints with minimal crack propagation and no evidence of matrix burning. In contrast, higher welding currents (20 A), particularly when coupled with short contact times, resulted in matrix burning and microstructural degradation regardless of the clamping force applied. Increasing the clamping force to higher levels (600 N) promoted crack propagation due to matrix dispersion but did not necessarily induce burning at moderate currents.

Overall, the results indicate that matrix burning is primarily governed by excessive current and short contact times, while crack propagation is influenced by both clamping force and matrix flow behavior. Therefore, the optimal joint strength is achieved under balanced conditions: moderate clamping force, controlled current, and sufficient contact time to allow gradual matrix diffusion without overheating.

Comparison of lap shear strength

The Lap Shear Strength (LSS) of the welded joints was determined under shear loading conditions and compared graphically across different welding process parameters. The LSS was calculated using the following equation:

L S S = \frac{F_{\max}}{B . L}

where:

• LSS = Lap Shear Strength (MPa)

• F_max = Maximum load applied (N)

• B = Width of the lap joint (mm)

• L = Overlap length (mm)

The results given in Table 5 and Figure 9 show that the lap shear strength of sample CB-3 has the highest value. The welding process parameters of this sample are welding current 15-amp welding current, applied clamping load 400 N, and welding contact time 35 s.

Table 5.

Measured lap shear strength.

Sample #	LSS (MPa)	Sample #	LSS (MPa)	Sample #	LSS (MPa)	Sample #	LSS (MPa)
CA-1	3.03 ± 0.4	CC-3	1.94 ± 0.4	CF-2	3.11 ± 0.3	CI-1	3.28 ± 0.3
CA-2	2.19 ± 0.3	CD-1	3.91 ± 0.3	CF-3	2.52 ± 0.4	CI-2	2.29 ± 0.4
CA-3	3.07 ± 0.5	CD-2	3.23 ± 0.5	CG-1	2.00 ± 0.5	CI-3	2.46 ± 0.5
CB-1	3.04 ± 0.2	CD-3	3.31 ± 0.4	CG-2	2.16 ± 0.4	CF-1	2.83 ± 0.5
CB-2	2.93 ± 0.6	CE-1	2.62 ± 0.4	CG-3	3.05 ± 0.2	CF-2	3.11 ± 0.4
CB-3	4.14 ± 0.3	CE-2	2.13 ± 0.3	CH-1	3.16 ± 0.1	CF-3	2.52 ± 0.3
CC-1	3.28±0.5	CE-3	2.74 ± 0.3	CH-2	3.16 ± 0.3	CG-1	2.00 ± 0.3
CC-2	2.60 ± 0.3	CF-1	2.83 ± 0.4	CH-3	1.39 ± 0.2

The LSS results obtained in the present study have been compared with those reported in the literature, as summarized in Table 6. Previous studies have reported an LSS of 6.656 ± 0.475 MPa using a stainless-steel mesh heating element under the process parameters of a welding pressure of 0.02 MPa, welding current of 16.7 A, and welding time of 75.6 s. In contrast, the maximum LSS achieved in the current work is 4.14 ± 0.3 MPa, corresponding to an applied pressure of 0.004 MPa, welding current of 15 A, and welding time of 35 ± 0.3 s, as recorded for sample CB-3 and presented in Table 5. It is important to note that the applied pressure in the present study is approximately five times lower than that used in the referenced literature.²¹ Moreover, the current research employs a carbon-fiber yarn as the heating element, which is significantly lighter, more durable, and offers greater flexibility compared to stainless-steel mesh.

Table 6.

Comparison of process parameters and LSS of GF/PP composite joint by resistance welding.

Study/Author	Heating element	Welding current	Welding pressure	Welding time/cooling	Resulting strength
Zhang²⁰	Stainless-steel mesh	12.5 A	2.5 MPa	540 s	LSS = 12.186 MPa
Cheng²¹	Steel mesh	16.7 A	0.02 MPa	75.6 s	LSS = 6.656 ± 0.475 MPa
Wanling Long²²	40-Mesh stainless steel mesh (silane-treated)	10.6 A	0.0156 MPa	1.2 min (72 s) + 30 s pressure-cooling	Treated: LSS ↑ 27.2% → 9.2 ± 0.5 MPa
Bing Du²³	Metal mesh (0.19–0.23 Ω)	10 A	0.05 MPa	1 min	Bending strength = 1.46 ± 0.36 MPa

Effect of welding current

The influence of welding current on lap shear strength followed a distinct pattern. At the lowest level of 10 A, weld strength showed a slight improvement, suggesting that the heat input was sufficient to enable proper bonding without damaging the joint. At 15 A, the effect was almost neutral, which indicates that this current may represent a balanced setting where enough energy is provided to form a sound weld without causing defects. In contrast, at 20 A, the strength decreased noticeably. This decline can be linked to overheating, which often leads to the expulsion of molten material, localized weakening, or the development of defects such as cracks and porosity. These issues reduce the effective bonding area and compromise joint performance. From these results, it can be inferred that maintaining a moderate current in the range of 10–15 A is more beneficial, while higher current levels are detrimental.

Effect of welding clamping pressure

Welding pressure also demonstrated a clear trend in its impact on joint strength. At 400 N, the welds achieved a modest improvement in strength, showing that the clamping force was sufficient to hold the parts together during fusion. Increasing the pressure to 500 N resulted in the best performance, suggesting that this level provides an ideal balance between consolidation of the materials and retention of molten metal in the weld zone. However, at 600 N, the strength dropped significantly. Excessive clamping force can force molten material out of the joint, leaving behind weaker bonding surfaces and possibly promoting brittle failure. Therefore, the results highlight that while pressure is necessary to form a good weld, too much clamping can be counterproductive. The optimum condition appears to be around 500 N.

Effect of number of binder plies

The number of binder plies had a noticeable influence on weld quality. When no binder plies were used, the lap shear strength was the highest, reflecting the benefit of direct contact between the workpieces. Introducing one binder strip led to a substantial reduction in strength, likely because the strip interfered with both heat transfer and current flow, disrupting the welding process. With two binder plies, strength remained lower than the baseline, although slightly better than with a single strip. This indicates that while additional plies still weaken the weld, the effect may be distributed more evenly. The use of binder plies consistently reduced the mechanical strength of the joints, and welds performed best when no plies were included.

The results suggest that lap shear strength is strongly influenced by maintaining a balance in the welding parameters. Moderate current levels (10–15 A) and clamping forces around 500 N provide favorable conditions for joint formation, while excessive current and excessive clamping reduce weld performance. In addition, the inclusion of binder plies negatively affects strength, with the best results obtained when they are not used at all. Although the statistical analysis did not identify highly significant effects, the observed patterns provide practical guidance for improving weld quality. These insights can help optimize process settings and avoid conditions that compromise joint strength.

Statistical analysis

To evaluate the main effect of process parameters for lap shear strength (LSS) and interaction relationship, the statistical tool Minitab 19 was used with the multilevel factorial design to run the analysis and find the graphical representation as shown in Table 7 and Figures 11–13.

Table 7.

Factors and levels involved in the study.

Factor	Levels	Values
Clamping force (N)	3	400, 500, 600
Welding current (Amp)	3	10, 15, 20
No. of binder strips	3	0, 1, 2

Figure 11.

Residual plot for LSS (MPa).

Figure 12.

Main effect plot for LSS (MPa).

Figure 13.

Interaction plots for LSS (MPa).

Residual plots

The residual plots for the regression model predicting LSS (Lap Shear Strength) in MPa, as shown in Figure 11, provide valuable insights into how well the model meets the fundamental assumptions of linear regression. These assumptions include normality, linearity, constant variance, and independence of residuals. Proper validation of these conditions ensures that the model is both statistically sound and reliable for making predictions.

The normal probability plot (top-left) is used to assess whether the residuals follow a normal distribution. In this case, the points generally align along the reference line, which indicates that the residuals are roughly normally distributed. There are slight deviations at the ends, but these are minor and typically acceptable in regression analysis, suggesting that the assumption of normality is reasonably met.

The residuals versus fitted values plot (top-right) checks for any patterns that might indicate issues such as non-linearity or changing variance. Here, the residuals appear to be randomly scattered around the horizontal axis, with no visible structure or trend. This randomness supports the idea that the relationship between the independent and dependent variables is adequately captured by the model and that the variance of residuals remains fairly constant across all fitted values.

The histogram of residuals (bottom-left) provides another view of the residual distribution. It shows that the residuals are fairly symmetric and centered on zero, resembling a bell-shaped curve. While there may be a slight skew, it does not appear significant enough to raise concerns. This again supports the assumption of residuals being approximately normally distributed.

Main effect plot

The Main Effects Plot for LSS (MPa) as shown in Figure 12, provides a visual representation of how different process parameters—Welding Pressure (N), Welding Current (Amp), and Number of Binder Plies—influence the mean Lap Shear Strength (LSS) of the welded joints. Each factor is shown on a separate axis, and the plot helps identify which variables have the most significant impact on LSS and how changes in their levels affect the outcome.

Welding clamping pressure (N)

In the first section of the plot, the relationship between welding pressure and LSS is depicted. As the welding pressure increases from 400 N to 500 N, there is a slight increase in the mean LSS. However, increasing the pressure further to 600 N leads to a noticeable drop in LSS, indicating that too much pressure negatively affects the weld strength. This suggests there is an optimal pressure range (around 400–500 N), and going beyond it could compromise the bond quality.

Welding current (Amp)

The middle section of the plot shows the effect of varying welding current. As the current increases from 10 A to 20 A, there is a gradual decline in the mean LSS. This downward trend suggests that higher currents may lead to overheating or material degradation, which reduces the overall weld strength. Therefore, lower current settings around 10 A appear to be more favorable for achieving higher LSS.

Number of binder plies

The last section presents the impact of the number of binder plies used. Here, the mean LSS is highest when no binder plies are used (0 plies). Introducing one binder strip causes a significant decrease in LSS, while using two plies shows a slight improvement but still falls short compared to using none. This implies that binder plies may interfere with the quality of the weld, possibly by acting as a barrier between the surfaces being joined.

Interaction plots

As Figure 13, the Interaction Plot for LSS (MPa) visually represents how combinations of different process parameters—Welding Pressure, Welding Current, and Number of Binder Plies—affect the Mean Lap Shear Strength (LSS). Interaction plots are useful for identifying whether the effect of one factor depends on the level of another. If the lines in these plots are non-parallel or intersect, it suggests that an interaction between factors exists, meaning their combined effect is different from their individual effects.

Welding pressure × welding current (top-left plot)

This section of the plot shows how changes in welding pressure interact with different levels of welding current. The most notable observation is the blue line representing a welding current of 10 A, which shows a significant increase in LSS at 500 N pressure, followed by a sharp drop at 600 N. In contrast, the red (15 A) and green (20 A) lines follow a more stable or slightly decreasing trend.

• This indicates that at a lower current (10 A), pressure has a strong positive effect on LSS—up to a point.

• At higher currents, increasing pressure doesn’t have as much of a beneficial effect and may even reduce strength.

• The crossing and diverging lines confirm that a clear interaction exists between welding pressure and current—their combined impact is not simply additive.

Welding pressure × number of binder plies (bottom-left plot)

This plot compares the effect of welding pressure at different levels of binder plies. The blue line (0 binder plies) remains relatively high and stable at 400 N and 500 N, slightly dropping at 600 N. The red (1 strip) and green (2 plies) lines show more fluctuation, especially a sharp decline in LSS for two binder plies at 600 N.

• The performance without binder plies remains consistently better across all pressure levels.

• Adding binder plies, especially two plies, significantly weakens the joint, particularly at higher pressure.

• Again, the non-parallel nature of the lines indicates an interaction effect between welding pressure and binder plies. The impact of binder plies changes depending on the applied pressure.

Welding current × number of binder plies (bottom-right plot)

This section evaluates how different combinations of current and binder plies affect LSS. The blue line (0 plies) remains relatively high across all current levels, even showing a slight increase as the current rises. In contrast, the green line (2 plies) declines sharply as current increases, reaching the lowest means LSS at 20 A.

• With no binder plies, LSS is consistently higher and unaffected by current increase.

• Using two plies, especially at higher current (20 A), leads to a considerable drop in strength.

• This suggests that higher welding current worsens the impact of binder plies, showing yet another interaction between these two factors.

Conclusions

The lap shear strength and microstructural integrity of PP-based composite joints are highly sensitive to welding parameters, requiring a careful balance for optimal performance. Moderate welding currents in the range of 10–15 A, combined with low-to-medium clamping forces (400–500 N) and relatively longer contact times (35–54 s), yielded strong joints characterized by minimal crack propagation and the absence of matrix burning. Conversely, higher welding currents (20 A), especially when applied with short contact times, led to matrix burning and severe microstructural degradation, irrespective of clamping force. Excessive clamping forces (≈600 N) promoted crack propagation due to matrix dispersion, even without burning at moderate currents. Furthermore, the inclusion of binder plies was found to reduce joint strength, with superior results achieved in their absence. Although statistical analysis did not reveal highly significant effects, the observed trends provide valuable practical insights, highlighting that welding quality can be optimized by maintaining moderate process settings while avoiding excessive currents, high clamping forces, and binder ply usage.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Muhammad Naheed Javed

Khubab Shaker

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Optimization of resistance welding parameters for joining the glass fiber-reinforced polypropylene composites

Abstract

Keywords

Introduction

Experimental

Materials

Methodology for composite fabrication

Resistance welding setup for joining

Resistance welding process parameters

Characterization and analysis

Results and discussion

Shear force vs displacement curves in LSS test

Failure analysis of joints after LSS test

Comparison of lap shear strength

Effect of welding current

Effect of welding clamping pressure

Effect of number of binder plies

Statistical analysis

Residual plots

Main effect plot

Welding clamping pressure (N)

Welding current (Amp)

Number of binder plies

Interaction plots

Welding pressure × welding current (top-left plot)

Welding pressure × number of binder plies (bottom-left plot)

Welding current × number of binder plies (bottom-right plot)

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iDs

References