Laser In Situ Joining as a Novel Approach for Joining Large‐Scale Thermoplastic Carbon Fiber‐Reinforced Polymer Aircraft Structures

Thermoplastic matrix composites are a viable option to reduce the carbon footprint during the life of an aircraft due to their ability to be molten and resolidified again. Tape‐based layup processes, such as automated tape placement, are well‐examined but have not seen extensive use in large‐scale joining applications, since they have to be processed layer‐by‐layer. In contrast, the advanced laser in situ joining method (CONTIjoin) utilizes fully consolidated and cut‐to‐size multilayered laminates, enabling the continuous layup of tailored laminates aligned with the mechanical application requirements. Herein, sample joints are manufactured using CONTIjoin technology, describing the general process principle, and are compared with samples produced using a standard heat press process. The base material, carbon fiber‐reinforced LMPAEK (low‐melt polyaryletherketone) is characterized using infrared spectroscopy. Using a 3.5 kW carbon dioxide (CO2) laser (10.6 μm wavelength) coupled with highly dynamic beam deflection, multidirectional reinforced laminates with up to six plies are processed to produce 24‐ply plates. The influence of joining temperatures up to 400 °C on the joint quality is investigated. Cross‐cuts are examined and interlaminar shear strength tests are conducted. With CONTIjoin, maximum strengths of 48.5 MPa are observed, reaching over 90% of the heat press reference.

intimate contact are the main factors in achieving sufficient bond strength during joining. [15,18][21] These are well-examined for the manufacturing of standalone parts such as stringers, frames, and fuselage segments but have not been used in joining applications.As these layup techniques rely on the use of single-layer unidirectional tapes, they are limited regarding the constraints of joining applications.While cutting angles of the tapes can be modified to some degree with modern blades, overrun or cuts short of the part edge will still occur in multidirectional laminates. [20]hen placed in scarfed joint structures (see Figure 1a), layup in 45°angles leads to the formation of a sawtooth pattern with areas missing fiber reinforcement, as overrun is not possible.Furthermore, as tapes are prone to ripping, an extensive clamping concept has to be present, as small movements or deviations can lead to delamination of the just laid up tapes.Particularly when dealing with large-scale structures such as aircraft fuselages (see Figure 1b), this can be a huge cost driver.
In contrast, fully consolidated laminates with multidirectional reinforcement have not been used as base materials for layup processes in global research efforts.Laminates can be manufactured perfectly tailored to the joint geometry and stress situation, as imperfections such as overrun can be simply removed during cutting to the final size.Especially, continuous processes using equipment like double-belt presses open up manufacturing possibilities for high-performance, large-scale laminates. [19,21]A laminate layup similar to the one proposed in Figure 1 would adopt the corrosive and structural advantages of monomaterial products created by AFP and ATL and could overcome the disadvantages of their process management.
Advanced laser in situ joining (CONTIjoin) is a process based on the principles of AFP and ATL: a thermoplastic CFRP prepreg is guided onto and over the surface of a substrate material while heat and pressure are applied constantly.Due to the reversible meltability of the polymer matrix, a continuous joint is formed.The major characteristic, that distinguishes CONTIjoin from the layup processes named above, is the used material.In contrast to unidirectionally reinforced single-ply tapes, fully consolidated and multidirectionally reinforced laminates can be processed. [22]ue to the relatively high absorption of the matrix polymers at larger wavelengths, a carbon dioxide (CO 2 ) laser with a wavelength of 10.6 μm can be used as the energy source to heat up the incoming laminate and substrate material. [23]umerous investigations have been conducted to determine the most effective processing techniques and parameters for laser-assisted layup of carbon fiber-reinforced materials.However, these studies have been limited to unidirectional single-layer tapes. [24,25]Thus, parameter screenings must be conducted to obtain a deeper understanding of the processes during joining when working with laminates instead of tapes.Especially, thermal process management is of major interest, as laminates show higher thicknesses and possibly inhomogeneous heat transfer due to multidirectional reinforcement.The process of joining laminates directly, known as co-consolidation, must be evaluated with respect to the achievable mechanical performance of the joints, when produced continuously.
This study assesses the feasibility of using multidirectionally reinforced laminates for joining applications through the use of monolithic laminate stacks.After conducting a spectroscopic analysis of the base material, the mechanical performance of the joints created using CONTIjoin is evaluated.The influence of various process parameters on mechanical performance is analyzed through parameter screenings, including an in-depth processing temperature screening.ILSS tests are carried out to assess mechanical characterization and the laminate quality is evaluated for defects and compaction behavior using crosscuts.The results are compared to a reference panel prepared using static heat press co-consolidation.

Thermoplastic Laminates
As the base material, fully consolidated, endless carbon fiberreinforced laminate straps made of low-melt PAEK (LMPAEK, Cetex TC1225, Toray Advanced Composites) with a stacking sequence of þ45°/À45°/90°/90°/À45°/þ45°were used (from hereon described as CF-LMPAEK).On both sides of the laminates, an additional neat resin layer of 60 μm thickness (APTIV AE 250 LMPAEK, Victrex) was placed, resulting in a total laminate thickness of approximately 1.25 mm (from hereon described as LMPAEK).The straps had a width of 70 mm and a length of 1000 mm and were cut from a larger sheet (750 mm Â 1700 mm) with a guillotine shear.The sheet itself was distributed by Fraunhofer IGCV, laid up via AFP and later consolidated using a double-belt press.

Spectroscopic Analysis
The utilized material was characterized by Fourier-transform infrared spectroscopy.The absorptivities of both the reinforced laminate as well as the neat thermoplastic resin were examined.The spectral range was chosen to include the frequently used near-infrared (NIR) laser systems with wavelengths of around 1 μm and the less frequently used CO 2 laser radiation at 10.6 μm for comparison. [14,16,22,25]In this frame, the radiation wavelength was varied from 250 nm to 12 μm using two different spectrometers (250 nm to 1.67 μm: Varian Cary 5000, 1.67-12 μm: Perkin Elmer Frontier).

Continuous Co-Consolidation by Advanced Laser In Situ Joining
The laser in situ joining process was carried out using a continuous wave CO 2 laser with a wavelength of 10.63 μm and 3.5 kW maximum power (Rofin, Germany).An optical setup was used in which the emitted laser beam was guided into a laser scanner (Raylase, Germany) via beam-bending mirrors, where it is aligned with the mating point (nip point) of the incoming laminate strap and the substrate.The schematic setup is shown in Figure 2a and the respective realized setup is displayed in Figure 2c.The laser beam oscillates over the material perpendicular to the feed direction (in width direction of the incoming laminate) at a speed of approximately 8.8 m s À1 .The focal length after passing the laser scanning system was 600 mm and the distance to the nip point was 1000 mm, relative to the position of the laser scanner.The chosen defocus results in a laser spot diameter at the nip point of approximately 10 mm.As the laser scanner has fixed X-and Y-positions in the used setup and can only be moved vertically (Z-direction), a mechanical setup was developed to simulate a continuous process along an aircraft fuselage.A linear axis system was placed on top of a machine table that is able to move in X and Y.During the process, the machine table moved away from the laser scanner (positive Y-direction) while the linear axis system moved inversely (negative Y-direction) with the same velocity, as shown in Figure 2b.The linear axis system carries a tool head that contains a guidance mechanism for centering the incoming laminate strap to the substrate laminate as well as a consolidation mechanism to apply pressure to the joining partners when meeting at the nip point.Due to the synchronous inverse movement, the strap is laid up onto the substrate while maintaining the designated working distance between the laser scanner and the nip point at all times.
The consolidation tool is based on roller segments that are mounted on needle bearings to ensure smooth rotation during the process.Onto these bearings, silicon sleeves with a thickness of 7 mm and a hardness of 40 Shore A are placed to distribute the consolidation force evenly to the surface and to maximize the interaction area between the joining partners.The consolidation force was applied using two pneumatic cylinders.During the layup process, a pyrometer (Sensortherm, Germany) constantly measured the temperature at the center of the nip point.The measurement position of the pyrometer was set using a second scanner system.The data was fed into a proportionalintegral-derivate (PID) control setup every 40 ms to adjust the laser output power accordingly, holding a given set temperature as constant as possible.In addition, the nip point was flooded with nitrogen gas through three active nozzles at a pressure of 4 bar to mitigate thermal degradation and oxidation effects.The interaction with the laser radiation in the nip point area results in plastification and melting of the laminate surfaces.For manufacturing a joint, the substrate is attached to the substrate tooling.The incoming laminate strap is then placed inside the guidance mechanism to be pulled underneath the consolidation roller beyond the nip point, where it is also attached to the tooling.After the consolidation force is applied, the described inverse movement of all axes is triggered by a computerized numerical control system, which also sets the starting point for the laser emission one second later.Once the desired joint length is achieved, laser emission and movement are terminated.
The procedure was carried out in a way that for the first layup, both materials were laminate straps (6 layers) and the outcoming joined material was used as the substrate afterward.This was repeated twice, resulting in a final composite structure of four 6-ply laminates (see Figure 3a).In each joint plane, one parameter set was used for a joint of approximately 150 mm length.After that, the parameters were changed for the next 150 mm.Therefore, several parameter sets could be tested with one setup, resulting in the individual parameter set area, as shown in Figure 3b.
In addition to a standard parameter set "0", three different parameters (feed rate, set temperature, and consolidation force) were varied to evaluate their influence on joint quality.In a second set of experiments (denoted as Parameter set D), the temperature was systematically varied from 360 to 400 °C, since this parameter has a high impact on the quality of the produced parts.All parameter sets are described in Table 1.

Static Co-Consolidation by Heat Press
To compare the quality of the continuously formed joints, reference samples were created via static co-consolidation in a heat press (COLLIN Lab & Pilot Solutions, Germany) according to the manufacturer's processing guidelines.Comparable to the description of the continuous process, a stack (275 Â 275 mm 2 ) of the same four 6-ply laminates was used but co-consolidated in a single process step.Starting at room temperature, the stack was heated at a rate of 5 K min À1 to 365 °C.After holding this temperature for 10 min, cooling of the parts was performed at the same rate of 5 K min À1 until room temperature.During this cycle, a pressure of 2 bar was maintained constantly.All heat press procedures were performed using UPILEX-25S (UBE, Japan) polyimide films as separators between the material and the mold to avoid direct contact with release agents.

Mechanical Testing
For evaluating the mechanical performance of the created joints, ILSS testing was conducted (Figure 4a).The respective samples were extracted by water jet cutting.During cutting, the nozzle angle was adjusted to ensure a cut orthogonal to the laminate plane, avoiding trapezoidal edges.The sample and testing equipment geometries were selected according to DIN EN ISO 14 130, as shown in Figure 4b. [26]The orientation was chosen such that the 90°direction follows the longitudinal dimension of the ILSS samples, as there are two layers with parallel fiber orientation in the initial 6-ply laminate, resulting in eight 90°layers in the 24-ply monolithic plate after joining.The loading speed during testing was set to 1 mm s À1 .As DIN EN ISO 14 130 does not specify any termination criteria, they were chosen according to ASTM D2344. [27]Testing was aborted if either the crossbeam travel exceeded the sample thickness, two-piece specimen failure occurred or a load decrease above 30% of the maximum load was observed.

Microscopic Analysis
To evaluate the bonding quality, cross-cuts of the joining areas adjacent to the sample extraction spots were prepared by mechanical cutting and a subsequent polishing procedure.Cross-section images were taken with an inverted optical microscope (OLYMPUS GX51, Olympus, Germany).Sample thickness of these cross-cuts was measured at ten points spread equidistantly over the sample width using ImageJ in form of the Fiji distribution.To examine the failure planes in the joined laminate stacks, high-resolution digital microscopic images were made using a digital microscope (KEYENCE VHX-5000, KEYENCE, Japan).

Spectroscopic Analysis
First, the materials used were examined using Fourier-transform infrared spectroscopy.Transmittance and reflectance of the samples were measured and subtracted from 100% to determine the resulting absorption.30] While indications of these peaks are also detectable in the carbon-fiber reinforced materials, their absorption difference in comparison with the respective baseline is low in comparison to the neat resin, probably as a result of the high absorption of the   reinforcement fibers over the whole measurement spectrum.[30][31] The C = O stretching vibration of ketone groups can be detected at wavenumbers around 1645 cm À1 . [29][30][31] In contrast to the other spectra in literature, multiple peaks were detected between 3000 and 1700 cm À1 .These are not characteristic for the PAEK polymer family itself and could be caused by additives in the matrix compound.
The corresponding absorption, reflection, and transmission values of LMPAEK and CF-LMPAEK at the typical wavelengths of fiber (λ = 1.064 μm-9399 cm À1 ) and CO 2 laser systems (λ = 10.63 μm-941 cm À1 ) are displayed in Table 2.The fiberreinforced LMPAEK samples showed higher absorption levels at both the evaluated wavelengths.In comparison, the neat resin was only absorbing a lower amount of the incoming radiation at λ = 1.064 μm, meaning that the higher absorption levels at this wavelength result from fiber reinforcement.At a wavelength, λ, of 10.6 μm, the absorption values were more similar, ranging from 77.5% to 90.5%.CF-LMPAEK and neat resin material nearly show the same absorption at this wavelength, resulting in an even energy intake during processing which enables homogeneous heating off all composite phases.In the case of CF-LMPAEK with an additional resin film applied to the surface, the highest absorption at the CO 2 laser wavelength was observed, despite the lower absorption of both its constituents.Baseline CF-LMPAEK showed significantly higher reflectivity (21.1%) than the resin-coated material (9.5%).Therefore, it can be confirmed that the additional neat resin layer can be used to increase the absorption of the laminate structure.In addition, the transmission for the CF-LMPAEK materials was zero, due to their thickness of approximately 1.2 mm.Transmission was observed only for the 60 μm thick neat resin film.
Regarding the results of the infrared spectroscopy investigations, it can be concluded that the CO 2 laser radiation is absorbed by both major components of the composite material, whereas energy input with a fiber laser would lead to heating mainly occurring in the carbon fiber phase.Therefore, a CO 2 laser will be used for the joining experiments to observe the influence of a more homogeneous heating of all CF-LMPAEK phases on the mechanical performance of such joints.The material for those trials will be equipped with an additional neat resin layer on the surface as the absorption of the composite material can be increased as a result of decreasing reflectivity, promising processing with higher energy efficiency.

Cross-Sections
After performing the optical measurements described in the previous section, CO 2 laser heating was used for assembling the 24-ply stack using four 6-ply stacks, as described in Section 2.3 (Figure 2b).Cross-sections of all resulting stacks are displayed in Figure 6.Regions with representative defects (e.g., voids) are indicated by yellow arrows.The initial 6-ply (Figure 6a) laminate used for assembly of the 24-ply stack presented voids located in all layers, especially in the À45°layer (second layer from top and bottom).This was probably caused by the manufacturing method used for the laminate sheets which were the origin of the laminate straps for sample manufacturing.As the conducted experiments for evaluating the CONTIjoin process aim to be as near as possible to a real aircraft manufacturing environment, the base material production is part of that as well.Laminate straps suitable for welding of a longitudinal fuselage joint would have to be much bigger in size than the conventional static tooling equipment for post-layup treatment of AFP-made sheets.Therefore, a continuous consolidation process had to be considered.However, double belt press consolidation is not able to reach the low void contents of static press consolidations as of yet, especially at higher feed rates. [19,21]These initial defects were also visible in the reference sample manufactured via heat press co-consolidation (Figure 6b).In addition, a significant number of pores inside the joint plane were found, since the applied heat press co-consolidation cycle was not capable of achieving the complete bonding of the laminate interfaces.This is contrary to the findings in literature regarding the porosity of automated tape placement made laminates in comparison with autoclave reference samples. [32]egarding the samples joined via CONTIjoin treatment, the cross-sectional images of samples joined with parameter sets 0, A and B (Figure 6c-e) presented similar characteristics.All of them inhibited a minor degree of pores in the joint plane in comparison with the reference, presenting voids with similar sizes (150-1000 μm).In the case of the samples processed with parameter set C (Figure 6f ), additional damage inside individual layers was observed.This could be a result of the increased consolidation force (4200 N compared to 2800 N), resulting in slippages of the fibers inside the layers.
Based on these results, parameter set B was found to be the most adequate for producing the 24-ply segments (feed rate = 250 mm min À1 , consolidation force = 2800 N, temperature = 360 °C).34] Thus, similar to parameter set B, the temperature was varied while maintaining the same feed rate and consolidation force (see Parameter set D in Table 2), resulting in equidistant temperature intervals of 10 K from 350 to 400 °C.For these samples, the acceleration dynamic of the laser scanner was adjusted from 2.5 to 32 m s À2 to minimize the dwell time of the laser spot at the turning points, as thermal degradation was observed at these positions.However, due to the samples being extracted from the center of the laminate stacking, this should not directly influence the laminate quality of the samples.
The cross-section of the co-consolidated coupons produced at 360 °C (Figure 6g) shows no major differences compared to the parameter set B sample (Figure 6e), with typical minor defects in the joint plane and between some laminate layers.In contrast, the samples joined at 380, 390, and 400 °C (Figure 6h-j) presented a significant deformation in the same area (pink arrows), with an increase of the part thickness.This was caused by a gap between the elastomeric roller sleeves of the consolidation tool due to slight slipping.Therefore, less force was applied in this area, resulting in increased void concentration and layer distortion as a result of missing compaction force.In comparison with samples produced at 370 °C (Figure 6j), the joint planes appear more uneven with an increase in the amount of darker spots, indicating possible defect locations.
The influence of the processing temperature on the thickness of the joints is displayed in Figure 7.The thicknesses of laser in situ joined samples ranged from 5.09 to 4.91 mm.Overall, higher set temperatures led to lower thicknesses, an effect that is presented in literature as well. [35]The samples joined at 350 and 360 °C exceeded the reference thickness of 5.06 mm, whereas higher temperatures led to lower thicknesses in comparison to the reference.In all cases, standard deviations of CONTIjoin samples were higher than those of the reference samples which is comprehensible given the respective manufacturing mechanisms.The influence of set temperature on mean sample thickness was fairly linear in the ranges from 350 to 360 °C and 380 to 400 °C.In the linear sections, the increased compaction could be a result of higher melt flow of the thermoplastic matrix material at elevated temperatures, as this promotes faster spreading of the molten material across the surfaces of the joining partners, transporting material away from the interface. [35,36]The nonlinear correlation of throughthickness compaction and processing temperature, as seen at 370 °C set temperature was also observed for other semicrystalline thermoplastics. [37]

Mechanical Testing
The results of the ILSS testing are shown in Table 3.In all cases, the sets with parameter variation exceeded the strength of the standard parameter set 0 and scored below the reference strength of 53.8 MPa.Regardless of the significant number of voids inside the joining planes of the reference samples, it still exhibited the highest mechanical performance.This leads to the assumption that the remaining interfaces showed sufficient bond quality.
The increase in strength of parameter sets A, B, and C in comparison with parameter set 0 complied with the expectations, as all the parameters were varied in a way that they should be beneficial for the bonding process according to the findings regarding their influence on the autohesion presented earlier in this study.Elevated temperature and increased interaction time (at lower feed rates) are beneficial for the formation of a strong bond, as both amplify diffusive processes.In other studies, increases in nip-point temperature, compaction force, and feed rate resulted in elevated interlaminar bond strength. [38]Between parameter sets A, B, and C, the temperature increase in parameter set B led to the highest strength increase to over 87% of the reference value, with an effective improvement of 17% compared to parameter set 0, which correlates with results observed previously. [35,39]n increase in compaction force (parameter set C) led to an increased sample strength as well, an effect also observed in other studies. [40,41]Similarly, a lower feed rate resulted in elevated mechanical performance as reported in literature. [32,39,42,43]In most cases, the homogeneity of the laser in situ joints in the form of standard deviation was comparable to that of the reference.The higher standard deviation of parameter set C was caused by one sample not being evaluable due to signs of pressure-related failure, therefore, lowering the total sample size to two.
The influence of the chosen set temperature on the mechanical performance (ILSS) of the laser in situ joined samples is shown in Figure 8.The results reveal that the normalized ILSS increased for temperatures over 350 °C, with a maximum at 360 °C, where the ILSS reached over 90% of the reference strength, corresponding to a 21% increase compared with parameter set 0. One explanation for this observation could be the decrease in viscosity at increasing temperatures, as this is beneficial for the bonding process at the interface. [36,44]With further increase in the process temperature, lower ILSSs were measured, eventually settling at approximately 80% of the reference strength between 380 and 400 °C.This decrease could be caused by the deformations shown in the cross-sections in Figure 6h-j or by an increasing amount of thermal degradation at higher temperatures, even when the viscosity decreases further.For instance, the thermoplastic polyetheretherketone (PEEK), which is very similar to LMPAEK, starts to decompose at 450 °C.The subsequent significant weight loss occurs in air at approximately 500 °C. [26]As the pyrometric measurement system is only capable of collecting mean temperature values over the 40 ms cycle time, these temperatures could have been reached or exceeded locally, resulting in local thermal damage.Additionally, the local concentration of nitrogen could have had an influence as well, as thermal degradation in air starts at a similar temperature level but is much more severe than in a pure nitrogen atmosphere. [45]hen increasing the laser energy deposited on the surface of PEEK, this can also lead to surface carbonization, where a shielding char layer is left on the surface, which could prevent sufficient bonding at higher temperatures. [46]n Figure 9, logged and averaged graphs of the heat press reference and the CONTIjoin samples at 360 °C set temperature generated through ILSS testing are displayed.For obtaining the average curves, only the common range of displacement of the respective curves was evaluated to ensure homogeneous requirements for the calculation.In this range, 1000 equidistant observation points were placed.For each of these individual points, the average force was evaluated through interpolation of the source curves.Behavior under load was found to be similar with the maximum average load observed at approximately 1 mm.Failure occurred similarly as well, as after withstanding the maximum load capacity, a steady decrease in load over displacement was observed, before several abrupt failures led to termination of the test according to the termination criteria mentioned before.

Failure Behavior
Following the ILSS measurements, the specimens were subjected to cross-sectional investigations to obtain further insights.For the statically joined reference samples, failure mainly occurred between the þ45°and À45°layer as indicated by the arrows in Figure 10a.Of the four reference samples that were tested, only one exhibited failure within the joining plane (Figure 10b).In some cases, delaminations were also found between the two adjacent 90°layers (Figure 10c) and in several planes simultaneously (Figure 10d).
Regarding the samples that have been co-consolidated via laser in situ joining, the most common failure locations were observed between 90°and À45°layers as well as between þ45°and À45°.In contrast, failure within the joining plane was a less common occurrence in the examined samples.In contrast, the samples of parameter set B showed an increased amount of failure in the joining plane (Figure 11a) in addition to the other failure planes   described previously (Figure 11b).This could be the result of an observed pressure loss in the pneumatic actuator system, leading to lower compression of the laminates during the laser joining process.

Conclusion
In this work, using advanced laser in situ joining, a process for continuous co-consolidation of carbon fiber-reinforced thermoplastic laminates was developed.For the first time, carbon fiber-reinforced thermoplastic laminates with six multidirectional plies were laid up and joined onto a substrate in a continuous process.Spectroscopic analysis has shown that matrix and carbon fibers are highly absorbent for the used CO 2 laser radiation.An additional neat resin film on the surface of the CF-LMPAEK reduces the reflectivity of the material at 10.6 μm resulting in an increase in absorption to over 90%.Due to the absorption of the matrix polymer itself in contrast to NIR laser radiation, the matrix at the joint interface can be heated directly.
With selection of a sufficient parameter set (360 °C set temperature, 2800 N consolidation force, and 200 mm min À1 feed rate), ILSSs of up to 48.5 MPa were reached, corresponding to approximately 90% of the strength of the heat press reference.Higher temperatures led to a decrease in the joint's performance.With this parameter set, the mechanical behavior under load of CONTIjoin joints can be assumed to be similar to heat press joints as the average force maximum of both sample pools occurs at similar value of crossbeam travel during testing.][49][50][51] Furthermore, it has to be mentioned that these values were achieved with in situ layup of unidirectional and single-layer tapes instead of using laminates directly.It has been shown that an increase in set temperature correlates with a higher compaction of the joined structure,  with the 360 °C sample being the closest to the reference thickness of 5.06 mm for a 24-layer stack.Further investigations regarding the thermomechanical properties (such as Melt Flow Index measurements) of the materials should be conducted to underline the process window findings.In addition, the gradient of temperature around the nip point area during the process should be investigated further to validate the pyrometer data and set temperature.A lower feed rate and higher consolidation forces also resulted in increased ILSS, with the optimum set point still to be determined, as well as the performance of the joints under cyclical load as they occur during the service time of an aircraft.While the standard deviation of both the heat press reference and samples manufactured via CONTIjoin was comparable, evaluations involving a higher number of samples have to be conducted in future studies to ensure homogeneous bonding strength and reliable processing for aircraft application validation.Further internal studies show a promising tendency toward sufficient reproducibility.Failure occurred mainly in areas outside the joining plane.Additionally, the used base material originated from an early generation and is still in development, with newer generations showing less initial porosity, according to the manufacturer, which could increase overall performance.The results obtained here are promising for future applications in industrial joining scenarios, e.g., in aircraft manufacturing.The potential avoidance of autoclave postprocessing could be a huge benefit for large-scale thermoplastic composite structures.

Figure 1 .
Figure 1.Joining concept for large-scale thermoplastic structures using laser in situ joining: a) scarfed structure with laminates laid up within with saw-tooth pattern on the edges due to manufacturing via layup and b) rendered image of the concept being applied to a longitudinal fuselage joint.

Figure 2 .
Figure 2. Schematic drawing of a) the optical and mechanical setup used for the laser in situ joining process, b) the movement that the components describe over time, and c) the components shown in the real-world lab setup.

Figure 3 .
Figure 3. Illustration of a) the dimensions of the laminates and the stacking scheme during continuous co-consolidation and b) the resulting 24-ply laminate with different parameter set areas.

Figure 4 .
Figure 4. a) Setup for ILSS testing and b) dimensions of ILSS samples, 90 °orientation across the longitudinal side, dimensions in mm.

Figure 5 .
Figure 5. Absorption spectra of LMPAEK and CF-LMPAEK, fiber and CO 2 laser radiation wavelengths shown with dotted lines.

Figure 7 .
Figure 7. Influence of set temperature on the mean thickness of laser in situ joined samples.

Figure 8 .
Figure 8. Influence of set temperature on normalized ILSS of laser in situ joined samples.

Figure 9 .
Figure 9. Curves from ILSS testing of a) heat press reference samples and b) CONTIjoin samples manufactured with 360 °C set temperature and the respective averaged curves.

Figure 10 .
Figure 10.Microscopic images of observed failure planes of the static heat press reference samples: a) between þ45°and À45°layers, b) failure in joint plane, c) between adjacent 90°layers, and d) multiple failure planes occur in the same sample; arrows indicate areas of delamination.

Figure 11 .
Figure 11.Representative microscopic images of observed failure planes of ILSS samples joined with set B parameters: a) increased amount of joint plane failure and b) joint plane failure occurring together with other failure planes.

Table 1 .
Parameter sets for samples made by laser in situ joining (varied parameters shown in italics).