# Enhancing weld in ultrasonic spot welding of carbon fibre-reinforced polyetheretherketone with polyetherimide energy director

Adrian Korycki, Christian Garnier, France Chabert, Fabrice Carassus, Toufik Djilali

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## ► To cite this version:

Adrian Korycki, Christian Garnier, France Chabert, Fabrice Carassus, Toufik Djilali. Enhancing weld in ultrasonic spot welding of carbon fibre-reinforced polyetheretherketone with polyetherimide energy director. Material Forming (ESAFORM 2025), May 2025, Paestum, Italy. pp.2273 - 2280, 10.21741/9781644903599-245. hal-05125552

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Submitted on 27 Jun 2025

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# Enhancing weld in ultrasonic spot welding of carbon fibre-reinforced polyetheretherketone with polyetherimide energy director

Adrian KORYCKI<sup>1,2,a*</sup>, Christian GARNIER<sup>1,b</sup>, France CHABERT<sup>1,c</sup>, Fabrice CARASSUS<sup>1,2,3,d</sup> and Toufik DJILALI<sup>2,3,e</sup>

<sup>1</sup>LGP-ENIT-UTTOP, 47 Avenue d'Azereix, 65016 Tarbes, France

<sup>2</sup>LAUAK France, Innovation Service, 8 Rue Louis Caddau, 65000 Tarbes, France

<sup>3</sup>POTEZ, Research and Development Service, 89 Avenue de Lourdes, 65310 Odos, France

<sup>a</sup>adrian.korycki@uttop.fr, <sup>b</sup>christian.garnier@uttop.fr, <sup>c</sup>france.chabert@uttop.fr, <sup>d</sup>fabrice.carassus@potez.com, <sup>f</sup>toufik.djilali@potez.com

**Keywords:** Thermoplastic Composites, Ultrasonic Spot Welding, Mechanical Properties, Weld Failure

**Abstract.** Static ultrasonic welding (USW) is a widely used technique for assembling thermoplastic composites. However, joining them remains challenging due to the lack of knowledge of the impact of process parameters (time, pressure, temperature) on welded joints. Continuous USW of large parts is a challenge for industrial applications. Spot welding is a more credible way of assembling large industrial parts. Improving the manufacturing efficiency and weld attributes such as welded area and strength are essential. To achieve this goal, the research was carried out in two steps: i) single-lap welding (SLW) was optimized and assessed in displacement single mode, ii) single-spot welding (SSW) with integrated and loose energy director (ED) was studied. The effect on the welding processes and weld attributes of the manufactured welded joints was investigated. The displacement single mode on integrated ED and a new method using multi-mode ultrasonic welding on loose ED were tested. The results of both approaches were compared to achieve robust sequential ultrasonic welding of thermoplastic composites with integrated or loose energy director. The ultrasonic welding parameters such as time, displacement and pressure with a specific amplitude of vibrations were explored to define the optimum conditions for the chosen composite material. The parts were assembled in a single lap shear (SLS) configuration to evaluate mechanical resistance and to observe the interfaces after failure. Mechanical results indicate that the SSW with a smaller sonotrode size can reach similar values as SLW. The loose ED significantly improved the welding efficiency. The weld resistance was effectively enhanced by adopting multi-mode ultrasonic welding.

**Introduction**

Ultrasonic spot welding has emerged as a highly effective technique for joining thermoplastic composites, offering significant advantages such as rapid processing, minimal energy consumption, and the ability to join complex geometries without requiring additional adhesives or fasteners. The ultimate challenge to achieve is continuous USW for assembling large parts. Despite some attempts, continuous USW is still a challenge for industrial applications. Mastering continuous welding will bring a very significant gain to companies due to the speed of execution (around 1 second) and energy savings. Gaining more knowledge about spot welding is a step towards continuous welding.

Among thermoplastic composites, carbon fibre-reinforced polyetheretherketone (CF/PEEK) stands out for its exceptional mechanical, thermal, and chemical resistance, making it a preferred material in aerospace and other applications [1]. However, achieving high-quality welds with

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CF/PEEK can be challenging due to its high melting temperature and poor thermal conductivity, which leads to anisotropic heat distribution during welding.

The introduction of polyetherimide (PEI) as energy director has been shown to address these challenges effectively. The energy director is a polymeric film located between the parts to be assembled with the aim to localize and enhance heat generation at the weld interface, promoting efficient melting and bonding. Their tailored morphology and the compatibility of PEI [2] with CF/PEEK ensure a robust and reliable joint while mitigating defects such as voids or insufficient fusion. A critical factor in concentrating heat in the weld area is the energy director, with previous research works showing that the ED's thickness and surface finish significantly influence the final assembly quality [3, 4].

A recent review by Arumugam and Pramanik [5] examined spot welding for forming composite joints, highlighting the effectiveness of ultrasonic welding in producing high-quality composite assemblies. While SLW demonstrates superior capability in creating robust joints, it is primarily suited for small components and may increase manufacturing costs due to the need for specialized clamping systems. The potential of spot welding as an alternative to mechanically fastened joints for thermoplastic composite assemblies was investigated by Zhao et al. [6-8] with CF/PEEK composites. Their findings revealed that multi-spot welded joints exhibited comparable load-carrying capabilities, with nearly double the stiffness of mechanically fastened joints under pure shear loading. Moreover, the failure of welded joints caused less damage to the composite laminate compared to mechanically fastened joints.

This study delves into optimizing the ultrasonic spot-welding process for CF/PEEK using PEI energy directors, exploring their influence on weld quality, mechanical performance, and process efficiency. By leveraging the synergy between advanced thermoplastic materials and innovative welding techniques, this research aims to expand the potential of ultrasonic spot welding in high-performance composite applications, setting new benchmarks for joint strength, durability, and manufacturing reliability.

## Materials and Methods

Materials. The thermoplastic composite material utilized in this study was a fabric carbon fibre-reinforced polyetheretherketone supplied by Porcher. A 0.25 mm thick polyetherimide film, provided by Goodfellow, was used as the energy director.

Methods. The composite laminates, comprising seven semi-impregnated fabric layers arranged in a [0/90] stacking sequence, were consolidated using a Pinette Emidecau Industries Lab800 thermo-compression device. The manufacturing process was conducted in two steps. In the first step, the CF/PEEK plies were consolidated within a steel frame to reach 2 mm thick plates. In the second step, a PEI film was integrated onto the surface of the composite plates. Specimens were prepared in dimensions of 100 × 25 mm², Fig. 1. The edges were deburred after cutting, and all samples were cleaned with ethanol to remove impurities and residual release agents.

Omega 4X ultrasonic welder from Mecasonic was used for the experiments. Its specifications are a working frequency of 20 kHz and a maximum load of 2600 N. The displacement amplitude of the waves generated by the generator is increased by a factor of 1.7 through the booster and further amplified by a factor of 1.7. The standard welds and spot welds in this study are created by a 40 mm and 10 mm diameter circular titanium sonotrode (bleu circles in Fig. 1), respectively. The welder's sensors and actuators are connected to an interface card located inside the module. The welder can work in three welding modes: controlled sonotrode displacement, power and time.
<figure><img src="image_4.png" /><figcaption>Figure 1 – Scheme showing configurations for SLW with a 40 mm diameter sonotrode (a) and SSW with a 10 mm diameter sonotrode (b) of studied assemblies</figcaption></figure>

The quality of the assemblies was assessed by lap shear strength (LSS). The assemblies tested were made following ASTM D1002, with an overlapping area of 12.5 x 25 mm² (SLW, Fig. 1.a) and 25 x 25 mm² (SSW, Fig. 1.b). The single lap shear tests were performed on the Instron Universal 33R4204 tensile machine equipped with a 100 kN load cell with a crosshead speed of 2800 N.min⁻¹. The value of LSS was calculated by Eq. 1.

$$LSS = \frac{F_{max}}{W_{area}} \qquad (1)$$

In SLS, the strength of a weld depends on the maximum load (Fmax) at the breaking point divided by the real welded area (Warea) after failure.

## Results and Discussion

Standard single-lap ultrasonic welding (SLW) with integrated ED. The process parameters were initially evaluated using a 40 mm diameter circular titanium sonotrode, based on findings from a previous study on a long series of trial-error tests. The welding parameters that generate heat at the welding interface such as vertical pressure, vibration amplitude and displacement of the sonotrode, were adapted to the material assemblies. Controlled sonotrode displacement mode was employed for process regulation. The curves for the displacement of the sonotrode and power as a function of time are presented in the graph of Fig. 2.a. An optimal operating pressure of 0.30 MPa corresponding to a welding force of 1400 N, and an amplitude of 68 µm were identified. A cooling time of 2.5 s at the same pressure was applied. Specimens with a displacement of 0.15 mm exhibited the highest mechanical resistance, achieving a peak lap shear strength of 44 MPa, with an average LSS of 31 ± 11 MPa.

The maximum displacement represents the thickness of two PEI layers which is 0.5 mm. The power curves are stopped after a defined displacement of the sonotrode of 30 % of ED thickness. The power-displacement curves reveal a noticeable increase in sonotrode displacement, which correlates with the heating and subsequent flow of the polyetherimide. The power peaks observed during the process are indicative of the physical phenomena occurring within the two polymers. The initial power rise is attributed to the heating of the PEI to its glass transition temperature in the first peak. This is followed by a power increase leading to a second peak, corresponding to the flow of PEI as the sonotrode moves and the polyetheretherketone begins to melt in the outer layers of the specimen.
<figure><img src="image_5.png" /><figcaption>Figure 2 – Power and displacement curves (a) and fracture (b) of CF/PEEK specimen with integrated PEI during ultrasonic welding: amplitude of 68 µm, operating pressure of 0.30 MPa, displacement of sonotrode of 0.15 mm</figcaption></figure>

Post-fracture analysis of the welded area, Fig. 2.b, performed using ImageJ software, revealed an average bonded area of 250 ± 15 mm². The vibration phase of the welding process consistently lasted approximately 0.8 s, a duration also determined to be optimal for time-controlled welding in standard ultrasonic welding. We observe the mix of two different failure types, adhesive failure between PEI and CF/PEEK layers or cohesive failure within the PEI layers, whereas within the specimens themselves, mostly cohesive failure between CF/PEEK plies occurs.

Single-spot ultrasonic welding (SSW) with integrated ED. Single-spot configuration effectively managed heat generation at the interface, using an optimized combination of parameters in controlled sonotrode displacement mode and a 10 mm diameter circular titanium sonotrode. The optimal welding parameters included a displacement of 0.15 mm, an operating pressure of 0.30 MPa, an amplitude of 68 µm, and a cooling time of 2.5 s at the same pressure.

Sonotrode displacement and power curves with time are illustrated in Fig. 3.a. The maximum displacement scale corresponds to the combined thickness of the two PEI layers which is 0.5 mm. During the vibration phase, most of the power output ranged between 1500 W and 2000 W, ensuring efficient heat transfer from the energy director to the composite material.

The vibration phase lasted 1.31 ± 0.11 s. Unlike standard ultrasonic welding, the obtained power curves exhibit distinct characteristics. In standard USW, physical phenomena at the interface can be readily identified. However, in SSW, the complete coverage of the composite by the PEI energy director, with a surface area larger than the sonotrode diameter, changes the curve's profile. The additional material from the ED prevents the sonotrode from lowering smoothly, leading to a "broken curve" effect. Several stages in the power-displacement curve can be correlated to interface phenomena. The initial power increase corresponds to the heating of the ED at the interface due to hammering effect [9], which is the heating when there is a detachment of the sonotrode on the surface of the composite. When the energy director softens (viscoelastic solid state), there is an accompaniment of the movement with the SLW, which reduces and then ends up making this "hammering effect" disappear (and which makes the slope decrease before the first power peak). Power increases faster in SSW due to the peripheral regions of the PEI overlap area remaining solid around the thermally affected zone. During the plateau of the broken curves (displacement and power), localized melting of the ED occurs. This phase is complex and challenging to characterize. Subsequently, a decrease in power coupled with an increase in displacement indicates that the ED is sufficiently melted to allow it to be flattened, but there is no indication of localized melting of the PEEK matrix in the composite. Stabilization of the curves
indicates an extension of the heat-affected zone, allowing the ED material to accompany the vibratory movement. Its measurement in Fig. 3.b. shows that it extends beyond the 10 mm diameter of the sonotrode.

<figure><img src="image_6.png" /><figcaption>Figure 3 – Power and displacement curves (a) and fracture (b) of single-spot weld CF/PEEK specimen with integrated PEI during ultrasonic welding: amplitude of 68 µm, operating pressure of 0.30 MPa, displacement of sonotrode of 0.15 mm</figcaption></figure>

The mean LSS achieved during sonotrode displacement-controlled welding was 33 ± 3 MPa. It is noteworthy that this LSS value was calculated based on a real welded area of approximately 170 ± 23 mm², including the secondary welded zone in the corner, shown in Fig. 3.b. This area significantly exceeds the 78.5 mm² surface area of the sonotrode, confirming that ultrasonic welding thermally affects a much larger region. As a reminder, the sonotrode surface is smaller than the overlap area of PEI energy director in Fig. 4. Indeed, we see that the thermally affected zone can spread out and it is around 91.5 mm².

<figure><img src="image_7.png" /><figcaption>Figure 4 – Scheme showing SSW with a 10 mm diameter sonotrode</figcaption></figure>

The fractured surfaces of the welded samples reveal the large region of unmelted PEI surrounding the spot welds. During ultrasonic welding, molten material tends to flow outward from the overlap area. As the melted PEI progresses towards the edges, it creates secondary welded zones. These secondary welds, formed as by-products, may compromise the weld quality at the overlap edges. This is attributed to the reduced distance required for the excess melted energy director material to evacuate. In the SSW configuration, cohesive failures are evident within the PEI layers.

Single-spot ultrasonic welding (SSW) with loose ED. The innovative approach of this work involves studying the effects of the multi-mode ultrasonic welding method on the mechanical resistance of the interfaces. It is worth noticing that most of works on ultrasonic welding are carried out using a single controlling mode. The division of the vibration phase is intended to enhance the reliability of results by automating and ensuring the consistency of critical stages within the
welding curves, which are essential for achieving a high-quality assembly. The parameters controlled in multimode welding are presented in Table 1.

<table><caption>Table 1 – Multimode welding-controlled parameters</caption><thead><tr><th colspan="4">Welding parameters</th></tr><tr><th></th><th>Time [s]</th><th>Displacement [mm]</th><th>Amplitude [%]</th><th>Pressure [MPa]</th></tr></thead><tbody><tr><td>Stage 1</td><td>0.9</td><td></td><td>60</td><td>0.3</td></tr><tr><td>Stage 2</td><td></td><td>Detection mode</td><td>100</td><td>0.3</td></tr><tr><td>Stage 3</td><td>Detection mode</td><td></td><td>80</td><td>0.3</td></tr></tbody></table>

In these tests, the vibration phase was divided into three distinct stages with different sonotrode amplitudes:

* The first stage (time-controlled mode) is designed to ensure that the initial power peak occurs at a relatively stable position. A lower amplitude (60 % of the maximum capacity of the welder) is chosen to facilitate effective heat transfer from the energy director to the composite material

* The second stage is tailored to detect a rapid downward displacement of the sonotrode, utilizing a specialized motion detection mode for this purpose

* The third stage is designed to halt the ultrasonic vibrations at the optimal time, achieved through an automated position stabilization mode provided by the welder

For single-spot welding with a circular 10 mm diameter and 0.5 mm thick loose ED, the specimen length was 100 mm, with an overlap joint area of 25 × 25 mm².

<figure><img src="image_8.png" /><figcaption>Figure 5 – Power and displacement curves (a) and fracture (b) of single-spot weld CF/PEEK specimen with loose PEI during multi-mode ultrasonic welding</figcaption></figure>

The assemblies were kept under an operating pressure of 0.30 MPa during the welding process. A cooling time of 4 s at the operating pressure of 0.40 MPa was then applied. Curves of displacement and power with time are presented in Fig. 5.a. The maximum displacement scale corresponds to the total thickness of the PEI energy director. In this multi-mode method, it can be observed that almost the whole PEI thickness was flattened during ultrasonic welding which is shown by the displacement curve: it reaches 0.47 mm (the initial thickness is 0.50 mm). Automation of the welding stages according to the state of the welding interphase thus led to greater flattening of the ED. Moreover, the power curve is very similar to the one for SLW, with
a longer time dedicated to the welding phase. Modulating the welding amplitude influences the power dissipated during welding, and the overall welding time.

Specimens with multi-mode welding exhibited the highest mechanical resistance, achieving a peak lap shear strength of 53 MPa, with an average LSS of 50 ± 1 MPa. It was found that to obtain such exceptional resistance, the energy director should be flattened or evacuated during ultrasonic welding to possibly allow for a connection between PEEK matrices. Cohesive failures between PEI and CF/PEEK layers are observed, Fig. 5.b, which also improves the quality of the welds.

## Summary

To ensure a meaningful comparison, the optimization of ultrasonic welding in the sonotrode displacement-controlled single mode, which is recognized as the most reliable single-mode approach, was conducted first. This initial study identified several challenges associated with this welding mode.

In conclusion, the study highlights the optimization of ultrasonic welding parameters for thermoplastic composites using sonotrode displacement control and multi-mode ultrasonic welding. Key findings include the identification of optimal operating conditions, such as a pressure of 0.30 MPa, vibration amplitude of 68 µm, and a cooling time of 2.5 seconds, which resulted in superior mechanical performance. Specimens welded with a displacement of 0.15 mm achieved the highest lap shear strength of 44 MPa. Notably, multi-mode welding with loose energy director exhibits exceptional mechanical resistance, achieving an average LSS of 50 ± 1 MPa, surpassing single-mode approaches. The expanded welding area and cohesive failure patterns indicate improved interface quality.

The study also reveals that multi-mode ultrasonic welding with loose energy directors significantly enhances the welding efficiency and mechanical resistance of thermoplastic composites. These findings underscore the potential of tailored ultrasonic welding strategies to achieve robust and high-quality welds, with implications for advancing the fabrication of composite materials in structural applications.

## References

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[2] A. Korycki, F. Carassus, O. Tramis, C. Garnier, T. Djilali, F. Chabert, Polyaryletherketone based blends: a review, Polymers, 15/19 (2023) 3943, https://doi.org/10.3390/polym15193943

[3] F. Carassus, A. Korycki, C. Garnier, F. Chabert, M. Yahiaoui, T. Djilali, Influence of Roughness in Ultrasonic Welding of Carbon Fiber/PEEK Composites, in: Materials Research Proceedings, Material Forming - ESAFORM 2023, Krakow, Poland (2023) 1841-1850, https://doi.org/10.21741/9781644902479-199

[4] A. Korycki, F. Carassus, C. Garnier, F. Chabert, T. Djilali, Effect of Energy Director Thickness on Thermal Diffusion and Joint Quality during Ultrasonic Welding of CF/PEEK Composites, in: Materials Research Proceedings, Material Forming - ESAFORM 2023, Krakow, Poland (2023) 1819-1828, https://doi.org/10.21741/9781644902479-197

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