2.1 Materials for welding
2.1.1 Thermoplastic composite
In this work, carbon fiber reinforced PEEK consolidated laminate supplied by Toray Advanced Composites has been selected. The 1.86mm thick laminate is made of 6 plies of 5H satin weave from 3K T300 carbon fiber roving with a quasi- isotropic lay-up.
2.1.2 Interlayer surface preparation
Two reference of 25 µm PEEK film have been chosen at an amorphous (PEEK A) and semi-cristalline (PEEK SC) state (Aptiv®2000 from Victrex®) with a matte/gloss finishing. A 50 µm PEI (ULTEMT M from Sabic) film is also included in this study as reference DSC analysis presented on Figure 1 shows that PEEK A presents a low level of cristallinity, 8% as received, compared to the 34,7 % of cristallinity of PEEK SC as received. PEEK A start to recrystallize by heating up to 160◦C. Per convention, the matte surface of the films is in contact with the TSC, and the gloss one is welded on the TPC.
Cleaning up and activation steps were performed using an atmospheric pressure Dielectric Barrier Discharge (DBD) reactor. Briefly, plasma was ignited in a DBD (3 mm gap between the electrodes) in which each of the two high voltage aluminum plate electrodes is protected by a 3.25 mm thick glass plate. Plasma discharge was generated by an AC power supply set at 450W and 6 kHz, with a flow of 80% N2 and 20% O2 . The top electrode moved over the bottom one at a constant 4 m.min−1 speed in a to and fro movement. The deposition time was set to 1 minute.
Through the proposed surface treatment by N2/O2(80/20) atmospheric plasma Kuzminova et al. (2014) , the wettability of the three interlayer films is improved through a cleaning and chemical activation. In order to better understand the time contribution of the treated surface, three post-treatment time are selected for this study: 1 minute, 3 days and 7 days after treatment. An interlayer film without treatment of each interlayer materials is also included in the present work.
It is well-known in the literature Fricke et al. (2012) that plasma treatment can affect significantly the surface roughness of the polymer substrate. In or- der to quantify the expected surface roughness modification, 3D profilometry measurements have been performed on both non-treated and plasma-treated interlayer films. Due to the initial roughness of the selected layer, profilometry have been preferred over Atomic Force microscopy (AFM). The used pro- filometer is a KAR TENCOR P17 and a scanning area of 50 µm per 50 µm have been selected.
Plasma treatment is also industrially used to activate the polymer surface and consequently increase their wettability. Wettability measurements have been done by a contact angle system OCA 15 from Dataphysics. A series of 5 sessile water droplets of 2 µL were deposited on the surface of each sample by means of a syringe pump. The contact angle value has been extracted from the droplet shape by using a numerical fit based on the Laplace-Young model. Plasma treatment is also responsible for major surface chemical changes. X- ray photoelectron spectroscopy (XPS) was performed to track chemical surface modifications with a VG SCIENTA SES-2002 spectrometer equipped with a concentric hemispherical analyzer. The incident radiation used was generated by a monochromatic Al Kα x-ray source (1486.6 eV) operating at 420W (14 kV; 30 mA). Photo-emitted electrons were collected at a take-off angle of 90◦ from the surface substrate, with electron detection in the constant analyzer energy mode (FAT). Wide scan spectrum (survey) signal was recorded with a pass energy of 500 eV and for high resolution spectra (C1s and O1s) pass energy was set to 100 eV. The analyzed surface area was approximately 24mm2 and the base pressure in the analysis chamber during the experiment was about 10−9 mbar. The spectrometer energy scale was calibrated using the Ag 3d5/2, Au 4f7/2 and Cu 2p3/2 core level peaks, set respectively at binding energies of 368.2, 84.0, and 932.6 eV. Spectra were subjected to a Shirley background and peak fitting was made with mixed Gaussian-Lorentzian components with equal full-width-at-half maximum (FWHM) using CASAXPS version 2.3.18 software. The surface composition, expressed in atom%, was determined using integrated peak areas of each components and took into account transmission factor of the spectrometer, mean free path and Scofield sensitivity factors of each atom. All the binding energies (BE) were referenced to the C1s peak at 285.0 eV and given with a precision of 0.1 eV. The O/C and N/C ratio have been obtained by using the value from the surface composition analysis.
2.1.3 Functionalized thermoset composite by Liquid Resin Infusion (LRI)
Liquid Resin Infusion (LRI) process are used to co-cure the thermoplastic interlayer onto the TSC surface. The interlayer is first placed on the molding tool surface. Dry carbon fibers reinforcement, made of 6 plies of carbon fabric layup at 0◦, supplied by Hexcel with following type of yarns: warp, HexTow AS4C GP 3K; weft : HexTow AS4C GP 3K . The fabric’s weave style is twill 2/2 for a nominal weight 200 g/m2. One 600x 600 mm composite plate is injected per interlayer treatment conditions. All three interlayer material , 200 mm x 600mm will be integrated on each plate.
Onto the dry carbon fiber reinforcement a peel ply, resin distribution media and a vacuum bag complete the set-up. The latter is place in a 2 m3 oven from SAT Thermique, +/-5 ◦C accurate at 410◦C. An ISOJET injection machine composed of a pressured heated tank of 25L, a resin flow rate control system and a heated transfer pipe, fully controlled automatically through a Schneider Citect/SQL monitoring automate.
The selected thermoset resin is the RTM6-2, bi-component supplied by Hexcel. The cure cycle has been adapted in order to launch the resin poly- merisation above the PEEK Tg and below the recrysatlisation temperature of the PEEK A. This cycle should promote the Interlayer/TSC interface by molecule diffusion Shi et al. (2017) . The thermal cycle of 2h at 150◦C + 2h @ 180◦C with 5◦C heating ramp rate have been selected. The applied receipt is described in Table 1 .
On Figure 2 the functionalized TSC surface is shown after demoulding. It should be noted, that the non-treated PEEK SC configuration present no adhesion after demoulding and consequently, which is why no welding tests are possible. Plates are water jet cut to a size 100 mm x 170 mm for be then welded in the next stage.
2.2 Infrared welding process for dissimilar join
2.2.1 IR Welding principle
IR welding experiment were carried out using FRIMO’s Infrared-Welding Machine ECO800 equipped with a welding tool enabling the assembly of two 100 mm length composite plates with an overlap of 12.7 mm. Welding line is 170 mm width. Single lap shear samples are cut in a later stage through water jet cutting at width of 25mm.
The welding tool is composed of an upper and lower moulds (Cf. Figure 3) where the plates are positioned through vacuum gripper. Both moulds are fixed on mobile table enabling the definition at each process step of they position in vertical direction. A mobile heating frame able to move along the horizontal direction. On both side of the heating frame are fixed a set of infrared lamps. Each lamp is control individually enabling a control of the power as function of the time. By dissimilar material welding, the cycle time of the upper and the lower set of IR lamp are adapted to the thermal profile required by each material melting behavior.
2.2.2 IR welding process for dissimilar material welding
For dissimilar welding, the principle (Cf. Figure 4 ) consists to heat up each surface to assemble though an appropriate thermal cycle by controlling individually short wave infrared emitters, on the thermoset composite side and the thermoplastic composite side. Our approach consist to heat up the thermoplastic composite up to the decompaction of around half of the material initial thickness, and heat up just before the end of the cycle the thermoset composite side as quick as possible, enabling the fusion of the thermoplastic film without permanent major degradation of the thermoset composite substrate. In this work, two different heating cycle time have been proposed: 27 seconds at 90% of the maximum infrared lamp power for the TPC, and 17 seconds for the TSC. The TSC heating cycle starts after 10 s delay compared to the TPC heating cycle start.
Figure 5 shows the thermal profile of both the welded surface and back surface of the TPC and TSC. Type K Thermocouples, with a 0.25mm diameters, have been positioned on the surface to weld of each welding partners, as well as between the welding partner and the tool. A SEFRAM DAS 240 data logger have been used for temperature acquisition with a sampling rate of 20 ms.
2.2.3 Thermal degradation of the TS matrix
Prior to the welding of TSC with TPC a more in depth study of the effect IR light towards the resin alone need to be investigated as such material is sensitive to high energy light or temperature which cause severe degradation and damage. The modification during the IR treatment was tracked by FTIR in reflection mode to assess the chemical variation. FTIR spectrum was performed on the surface of the as manufactured resin at different IR radiation time from 1 to 15s. From the spectrum we can noticed an increase of the peaks at 1660 cm−1 and the apparition of a shoulder at 1735 cm−1 which is related to the formation of carbonyls groups linked to the thermal degradation when the sample are subjected to the IR radiation in presence of oxygen Doblies et al. (2019) Villegas and Rubio (2015) . Moreover, a decrease of the bond at 1050 cm−1 with the time of irradiation increase which can be attributed to the epoxy backbone ether bridge. Such decrease can be assigned to chain scission and argue for the degradation/decomposition of the resin under the IR light. By using high energy IR irradiation modify the surface of the resin by a thermal degradation mechanism where oxidation and chain scission occur. Such observation can explain the failure observed in the next section where the failure occurs in the thermoset composite. For a future application the control of the power of IR lamp will be optimized to avoid the degradation but just applied enough energy to weld the material.