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, Volume 10, Issue 4, pp 34–39 | Cite as

Automated welding of thermoplastic joining element on CFRP

  • Thomas Meer
  • Matthias Geistbeck
Production Joining Techniques
  • 242 Downloads

Adhesive bonded joining elements need a high manufacturing effort. Robot based welding onto aerospace CFRP with epoxy based matrix can facilitate a reliable and efficient joining. One prerequisite here is the modification of the CFRP surface with a thermoplastic functional layer.

In aircrafts today a multitude of joining elements are mounted. The program of semifinished products includes cage nuts, stud bolts and cable supports. They are used both for applications with low requirements, like the attachment of heat insulation, and for the fixation of complex component assembly to the aircraft structure. The manual adhesive bonding process of the supports is connected with a high manufacturing effort. Several specific work steps are required,shown in Figure 1 and the curing time of 8 to 12 h constitutes a significant productivity limiting factor. With the aim to come to a more efficient fastener installation that also shows potential for structural applications the possibility of a welding application of fasteners was investigated.
Figure 1

Diagram of adhesive bonding process for brackets. Highlighted in green are the steps that can be omitted using a welding process (© Airbus)

Carbon fibre reinforced polymers (CFRPs) since many years are an important material in aerospace. The predominant CFRP material is built of carbon fibres with a thermoset epoxy matrix. Two relevant manufacturing processes can be distinguished, the dominant prepreg technology with autoclave curing and the vacuum assisted resin infusion (VAP) technology. For future applications thermosets will predominate, although in the meantime also parts with thermoplastic matrix are used.

The investigations described aimed at the modification of the thermoplastic CFRP with a thermoplastic surface layer to allow for the welding of standard joining elements. In a first step a thermoplastic film had to be applied to the thermoset CFRP surface. It had to be ensured that the interface between the thermoplastic film and the base epoxy CFRP does not constitute the weak point in the later assemblage. The thermoplastic counterpart could then be welded on the functionalised surface [1]. For this an appropriate welding process was required that allows the processing of aerospace high performance thermoplastics without a thermal degradation of the epoxy matrix within the CFRP.

Thermoplastic Functional Layers

The thermoplastic materials that are today certified for structural aerospace applications belong to the category of high performance thermoplastics (e.g. PPS, PPSU, PEI, PEEK). Especially polyetherimide (PEI) is already well investigated concerning its toughening effect on epoxy based matrix systems and the compatibility with epoxy compounds has been proven [2, 3, 4, 5] .

Hence the investigated approach dealt with the equipment of the thermoset CFRP material with a thin PEI foil. Application of the foil took place before the curing of the epoxy, Figure 2. Curing of the epoxy resin is typically performed at 180 °C. This temperature is well below the glass transition temperature of PEI at 217 °C, so a melting of thermoplastic resin is not yet taking place. However the PEI is partly chemically dissolved by the liquid epoxy resin and a fusion zone with both epoxy and PEI is existent. During the curing process the thermoplastic precipitates and a mixing zone respectively an interphase region is built. The sequence of the dissolution and curing process could be followed at a PEI/epoxy resin probe on a heated sample table under a microscope. It was possible to observe the formation of the interphase during a curing cycle, Figure 3.
Figure 2

Diagram of welding process for thermoplastic brackets; to apply the welding process the orange additional steps are necessary compared to an adhesive bonding process (left); schematic illustration of the welded joint between thermoplastic (TP) bracket and thermoset (TS) CFRP part by formation an interphase (right) (© Airbus)

Figure 3

Formation of mixing zone between Polyetherimide (PEI) and epoxy resin in uncured state — microscopic picture on heated sample (160 °C) (© Airbus)

The dissolution process started at about 150 °C. Until gelation the thickness of the fusion zone growth up to 60 to 80 μm and is nearly independent from the applied heating rate or possible holding stages.

The structure of the interphase was investigated with photomicrographs and scanning electron microscopy (SEM) of CFRP sample with thermoplastic functional top layer that had been cured according to the material specific curing cycle. To uncover the differing epoxy and PEI domains the polished cross-sections had been etched. The etching dissolves some micrometres of the PEI so that a contrast to the surrounding epoxy can be observed.

The investigations have been performed with epoxy prepreg skins and honeycomb core.

As can be seen in Figure 4 within the epoxy close to the interphase region there is an increased amount of PEI spheres of sizes up to 1 μm. This increases the ductility of the epoxy resin in this region, what counteracts the stiffness jump between the epoxy and the thermoplastic resin. The epoxy itself within the PEI also forms sphere like structures, that are connected and built-up a network filled with thermoplastic.
Figure 4

Scanning electron microscopic picture of mixing zone (interphase) between PEI top layer and epoxy CFRP after etching with Dichloromethane (© Airbus)

With increasing depth in the PEI the epoxy spheres decrease in size. This results in a continuous transition zone from the epoxy to the PEI. The connection therefor does not only result from adhesion forces but also is based on mechanical interlocking. This structure can be detected particularly well in the resin rich areas of the CFRP. The penetration of the interphase region in the first fibre layers further increases the mechanical interlocking [1].

For evaluation of the quality of a joint between a PEI film and an epoxy CFRP the interlaminar energy release rate (GIc) has been tested according to DIN EN 6033. Based on a continuous peeling along the connection line the test allows to estimate the joint strength. For the trials different CFRPs typical for aerospace have been tested that during manufacturing were equipped with a PEI intermediate layer, Figure 5. Additionally half of the samples before testing were aged under hot-wet conditions (1000 h; 70 °C; 85 %r.h.) to estimate potential negative influence of humidity on the joint.
Figure 5

Influence of thermoplastic PEI interlayer on the interlaminar energy release rate (GIc) for different CFRP base materials used in aerospace, tested both under dry and wet conditions (hot-wet aged for 1000 h at 70 °C and 85 % r.h.) (© Airbus)

The GIc values of the samples modified with a 125 μm thick PEI film were always higher compared to the reference without thermoplastic intermediate layer. Hot-wet ageing slightly decreases the G1c values, what can be explained by the reduced sample stiffness after the ageing. The sometimes high mean variations result from the fact that for some test samples the crack runs from the ductile thermoplastic interlayer into the brittle base laminate, where only its reference GIc value can be reached. In no case the interface between PEI and epoxy was the weak point in the lay-up, what could have been detected by adhesive failure between those materials.

Welding Operation

The described material combination of high performance thermoplastic for the fastener and carbon fibre reinforced epoxy resin for the structure has an effect on the selection of the appropriate welding technique. The high performance thermoplastic materials require welding temperatures in the range of 300 °C to 400 °C. On the other hand the epoxy resin of the CFRP base laminate may not thermally degrade during the welding operation. The extent of thermal damage depends on temperature, time and the presence of oxygen. As a benchmark for the maximum allowable temperature, the resin curing temperature during the CFRP manufacturing of 180° can be used. Importance for staying below the maximum allowable temperature loading is the use of a very fast welding technique that only affects the surface of the parts to be welded. This requires very high heating rates combined with a homogenous heating over the joint area with only a very low depth effect.

These criteria are met with the circular friction welding technique that is used in machines of the company Firma Fischer Kunststoffschweißtechnik GmbH. The applied eccentric movement of the part to be welded allows a homogenous energy introduction in the welding area also for not rotation-symmetric parts.

A forced-guided friction element allows the use of a wider welding parameter range (amplitude, frequency, welding time) compared to spring-mass system excited oscillation systems. The movement during the circular welding process is designed in a way that the position and the orientation of the bracket are identical at the beginning and at the end of the welding. The welding module used within the investigation has been a prototype machine that has been designed as a mobile version for robot integration, Figure 6.
Figure 6

Mobile friction circular welding module used for investigations [5] (© Airbus)

Since the mobile welding head was guided by a robot some adaptions compared to a stationary welding machine had to be made. The occurring contact and friction forces have to be transferred with the robot arm. Because of the reduced stiffness of the robot arm the contact forces can cause in a tilting of the join partners. Additional due to the reduced stiffness of the robot arm the introduced welding power had to be increased to compensate power losses in the robot arm.

Welding Trials

For the welding trials commercial available brackets from the company Clickbond were used. They consist of a metallic load introduction element and of a fibre reinforced thermoplastic foot with different fibre contents, Figure 7. The bracket foot, which was originally designed for application with adhesive bonding, has a diameter of 32 mm. The welding investigations have been performed both on monolithic epoxy CFRP and on sandwich samples with thin epoxy prepreg skins and honeycomb core. Both featured the thermoplastic PEI functional surface layer as described above.
Figure 7

Welded bracket on thermoset CFRP with thermoplastic Polyetherimide (PEI) functional layer (© Airbus)

The sandwich because of its lower compressive strength is limiting the allowable contact force during the welding process. Mechanical and ultrasonic testing proved that the contact forces used in this investigation did not cause any damages to the sandwich structures.

The welding process itself includes the gripping of the brackets, the application of the joining force, the circular friction movement, the consolidation under retained joining force and as a last step the removal of the welding head from the part.

Depending from the used material of the bracket foot and the degree of its fibre reinforcement the described welding process takes around 10 s [6]. The joint after welding can be immediately loaded and no significant rework has to be performed. Only the resulting weld squeeze out, Figure 8, has to be removed.
Figure 8

Welded brackets on sandwich structure with thermoplastic Polyetherimide (PEI) functional layer, before removal of weld squeeze out (© Airbus)

This weld squeeze out can be used for process control, since it only appears in case of a successful welding operation. Additionally he transports potential surface contaminations out of the joint area.

Pull-off testing of circular welded brackets proofed the capability of the process to weld on contaminated surfaces. For these tests sandwich samples with residues of silicone containing mould release agents in surface concentrations significantly higher than what can be expected in real part manufacturing were used.

The produced weld joints were tested after hot-wet ageing again and under increased testing temperature (135 °C) and were compared to adhesive bonded brackets with fibre reinforced epoxy feet. Those are state of the art in current shop floor pro duction.

The joint strength of the welded brackets in initial stage was slightly below the strength of adhesive bonded reference, what can be explained by the different bracket foot resin materials.

Under wet conditions the adhesive bonded joint reaches only 20 % of initial strength, the additional influence of a mould release agent on the adherent surface nearly no strength can be obtained. For both bracket types a failure appears in the adhesive.

For the welded joints in contrast also on the contaminated surfaces a strength level of nearly the uncontaminated value can be achieved, Figure 9. The failure here is occurring in the bracket foot.
Figure 9

Tensile strengths of two bracket variants welded on sandwich structures (initial state and release agent contaminated); tested at room temperature, at 135°C after hot-wet ageing (1000 h at 70 °C and 85 %r.h.) (© Airbus)

Besides the mechanical testing the joint quality was characterised by microsections. The microscopic images show the very good quality of the weld. Using a 125 μm thick PEI functional layer allows completely closed joint zones without cavities or tilting. Within the interface a uniform PEI layer of about 100 μm remains after welding, Figure 10.
Figure 10

Fully closed joint zone with constant thickness of PEI layer in micrographs of circular welded thermoplastic brackets on epoxy base CFRP with PEI surfacing layer (TP foil) (© Airbus)

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Conclusion

Thermoplastic PEI functional layers on epoxy based thermoset carbon fibre reinforced poly mers allow producing fastener joints by circular welding. The strength of the joints especially at higher temperatures or when welded on contaminated surfaces is higher compared to the adhesive bonded joints. This can be attributed to the excellent connection between the thermoplastic surface film and the matrix epoxy resin due to the formation of a distinct TP/epoxy interphase.

The investigations aimed at the modification of the thermoplastic CFRP to allow for the welding of standard joining elements.

It was demonstrated that an automated and robot based placement of PEI brackets is possible on these PEI surface films without introducing thermal damage to the underlying CFRP epoxy resin. This was confirmed also for sandwich substrates and for the application of PEEK brackets.

While adhesive bonded brackets can be loaded only after 8 to 12 h and need a refinishing operation the circular welded joints can be joint in less than 10 s and immediately achieve full strength. The opportunity of a process control by the weld flash can have additional benefits compared to the adhesive technology. |

Notes

Thanks

The research and development project “MAIplast” has been funded by the German Federal Ministry for Education and Research (BMBF) (funding code 03MAI01F) and was overseen by project executing agency Jülich (PTJ).

References

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    Bastien, L. J.; Gillespie, J. W. Jr.: A non-isothermal healing model for strength and toughness of fusion bonded joints of amorphous thermoplastics. In: Polymer Engineering and Science, Volume 31, Issue 24, December 1991, pp. 1720–1730CrossRefGoogle Scholar
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    Hodgkin, J. H.; Simon, G. P.; Varley, R. J.: Thermoplastic toughening of epoxy resins — a critical review. In: Polymers for Advanced Technologies. Volume 9, Issue 1, January 1998, pp. 3–10CrossRefGoogle Scholar
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    www.fischer-st.de/unternehmen/mediathek/

Copyright information

© Springer Fachmedien Wiesbaden 2017

Authors and Affiliations

  • Thomas Meer
    • 1
  • Matthias Geistbeck
    • 1
  1. 1.Airbus Group InnovationsTaufkirchen close to MunichGermany

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