Abstract

Thermoplastic Automated Fiber Placement (AFP), particularly Laser-Assisted Fiber Placement (LAFP), has emerged as a promising route for high-rate composite manufacturing due to its potential for in-situ consolidation, reduced post-processing, and improved recyclability. However, inconsistent consolidation quality remains a major barrier to industrial deployment. This study investigates the fundamental process physics governing in-situ consolidation of carbon fiber reinforced polyetherketoneketone (CF/PEKK) laminates during thermoplastic AFP. The work focuses on the coupled interaction between thermal history, thermal contact resistance, intimate contact development, interlaminar bonding, and permeability evolution. Experimental laminates with varying fiber orientation sequences were manufactured under different layup speeds using a robotic LAFP system equipped with a diode laser and conformable compaction roller. Thermal behavior was monitored using infrared imaging, while intimate contact, bonding quality, porosity, and permeability were evaluated using microscopy, T-peel testing, micro-computed tomography, and pulse-decay methods. The results demonstrate that fiber orientation mismatch strongly affects intimate contact and interface heat transfer, producing significant variations in nip-point temperature and cooling behavior. A modified intimate-contact and bonding framework is proposed to correct the overestimation inherent in conventional models. The findings confirm that thermoplastic AFP consolidation is governed not by temperature alone, but by a coupled thermo-mechanical and microstructural process chain. This study provides a physics-based foundation for designing reliable in-situ thermoplastic AFP manufacturing systems.

Keywords

Thermoplastic Automated Fiber Placement; Laser-Assisted Fiber Placement; In-Situ Consolidation; CF/PEKK; Thermal Contact Resistance; Intimate Contact; Interlaminar Bonding; Permeability; Composite Manufacturing

1. Introduction

Automated Fiber Placement (AFP) has become one of the most important manufacturing technologies for advanced composite structures in aerospace, defense, and high-performance transportation. While thermoset AFP is already well established in large structural applications, thermoplastic AFP is gaining strategic importance because it offers several advantages that align with next-generation manufacturing requirements. These include out-of-autoclave processing, reduced cycle time, weldability, damage tolerance, and improved sustainability through material recyclability.

Among thermoplastic AFP approaches, Laser-Assisted Fiber Placement (LAFP) is particularly attractive because it enables local heating of incoming tape and substrate immediately before compaction, creating the possibility of in-situ consolidation without secondary autoclave cure. In principle, this offers a major productivity advantage. In practice, however, the process remains difficult to control. Structures produced under apparently similar process settings can exhibit significant variation in interlaminar bonding, void content, and final mechanical performance.

The reason is that in-situ consolidation in thermoplastic AFP is often simplified as a temperature problem. Industrial attention frequently centers on laser power, layup speed, and measured surface temperature. Yet bonding quality is not determined by thermal input alone. The actual process depends on a sequence of coupled phenomena: interface heating, development of intimate contact, polymer healing, void redistribution, and permeability evolution. If any stage in that chain is insufficient, consolidation quality deteriorates even when the surface appears thermally acceptable.

This study addresses that problem by investigating the fundamental process physics of thermoplastic AFP using CF/PEKK slit tape. The research is structured around three core questions:

1. How does fiber orientation influence thermal history during LAFP?
2. Why do conventional intimate-contact and bonding models overpredict consolidation quality?
3. How do process conditions affect internal permeability and void structure at different scales?

The central argument of this paper is that reliable in-situ thermoplastic AFP requires a physics-driven design framework, where thermal behavior, contact mechanics, polymer bonding, and microstructural transport are treated as an integrated system rather than isolated variables.

2. Literature Review

Thermoplastic AFP has attracted substantial attention because it offers a pathway to continuous composite manufacturing without the long cure cycles associated with thermosets. Materials such as PEEK, PEKK, PPS, and PEI have been widely studied due to their high performance and suitability for aerospace structures. Among these, CF/PEKK is especially attractive because of its thermal stability, chemical resistance, and relatively broad processing window compared with some other high-temperature thermoplastics.

Previous research in thermoplastic AFP has concentrated on three areas: thermal modeling, bonding development, and void formation.

The first area, thermal modeling, has focused on predicting nip-point temperature and cooling rate as functions of laser power, speed, roller pressure, and material properties. These studies have improved understanding of process temperature fields but often assume ideal interface contact between incoming tape and substrate. Such assumptions can lead to inaccurate interface temperature predictions, especially when fiber architecture and surface topology reduce real contact area.

The second area, bonding development, has often been interpreted using intimate-contact and healing theories originally derived for polymer welding and autoclave consolidation. These models typically separate the process into physical surface contact and molecular diffusion across the interface. While conceptually useful, many of these formulations assume idealized surface flattening or contact progression and therefore tend to overestimate actual bonding in AFP, where contact time is extremely short and local pressure conditions are non-uniform.

The third area, void formation and permeability, has received increasing attention because in-situ consolidated laminates often contain residual porosity even when external appearance is acceptable. Porosity is not merely a defect count issue; it is linked to gas transport, resin mobility, incomplete intimate contact, and local thermal history. However, most studies treat void content either macroscopically or microscopically, without fully connecting the two scales.

A major limitation of current literature is that these three areas are often studied separately. Temperature is modeled without realistic contact resistance. Bonding is predicted without sufficiently accounting for real contact evolution. Porosity is measured after manufacturing without being linked back to the contact and bonding stages that created it. What is missing is an integrated interpretation of thermoplastic AFP consolidation as a coupled multi-physics process.

This paper addresses that gap by linking temperature history, thermal contact resistance, intimate contact, bonding, and permeability in a single framework.

3. Methodology

3.1 Materials and Process System

The material system considered in this study is carbon fiber reinforced PEKK slit tape, selected because of its relevance to high-performance thermoplastic composite manufacturing. The deposition trials were carried out using a robotic Laser-Assisted Fiber Placement system equipped with a high-power diode laser and a conformable compaction roller. This setup allows localized heating immediately ahead of the nip point, followed by consolidation under roller pressure.

The study focuses on laminates manufactured under controlled combinations of layup speed and stacking sequence. Representative layups were designed to isolate the effect of fiber orientation mismatch between adjacent plies. Three orientation conditions were investigated:

• aligned orientation,
• moderate mismatch,
• severe mismatch.

These configurations were selected to reveal how interfacial geometry influences thermal behavior and bonding quality.

3.2 Thermal History Evaluation

Thermal history was evaluated by combining experimental measurement and numerical modeling. Infrared monitoring was used to capture transient surface temperature during deposition, particularly near the nip region and immediately after roller passage. These observations were used to validate a thermal model of the process.

Unlike simplified formulations that assume perfect contact, the present methodology incorporates thermal contact resistance at the tape-substrate interface. This is critical because contact resistance changes the effective heat flow into the substrate and therefore changes the local temperature actually available for interlaminar consolidation.

3.3 Intimate Contact Characterization

Intimate contact was quantified through optical microscopy and image-based analysis of polished laminate cross-sections. The degree of intimate contact was defined based on the fraction of the interface exhibiting direct contact rather than voided or separated regions. This metric serves as the physical bridge between thermal conditions and molecular bonding.

3.4 Bonding Assessment

Interlaminar bonding was assessed using mechanical peel testing and fracture surface examination. Peel resistance was used as a practical measure of consolidated interface quality. Microscopic examination of fracture surfaces was then used to interpret whether failure occurred through weak interface separation, partial fusion, or stronger cohesive-like mechanisms.

To improve predictive capability, a modified bonding model was developed based on experimental correction of existing intimate-contact formulations. The purpose of the model was not merely curve fitting, but correction of the systematic optimism found in conventional contact-based bonding predictions.

3.5 Permeability and Porosity Characterization

Permeability was investigated at both macro and micro scales. Macro-scale gas transport was characterized using pulse-decay methods, while internal void architecture was analyzed using micro-computed tomography. This dual approach made it possible to connect bulk permeability behavior with local pore structure and distribution.

3.6 Process-Physics Interpretation Framework

The full methodology was organized around a cause-and-effect chain:

fiber orientation and process parameters → thermal contact resistance → interface temperature history → intimate contact development → polymer healing and bonding → porosity and permeability

This chain formed the conceptual basis for analysis in the Results and Discussion sections.

4. Results

4.1 Effect of Fiber Orientation on Thermal History

The results show that fiber orientation mismatch has a strong influence on interface thermal behavior. Laminates with aligned neighboring plies exhibited the highest degree of contact and the most stable thermal response. In contrast, laminates with increasing orientation mismatch showed reduced contact uniformity and altered heat transfer at the interface.

The most important finding is that interface temperature cannot be inferred reliably from external heating conditions alone. Even when laser power and layup speed remain constant, the interface experiences different thermal histories because contact geometry changes heat transfer efficiency. This effect becomes more pronounced at higher layup speed, where available heating and consolidation time are already limited.

The interface thermal response therefore depends not only on laser input, but also on the physical quality of tape-substrate contact at the moment of consolidation.

4.2 Thermal Contact Resistance as a Governing Variable

When thermal contact resistance is included in the thermal model, the prediction of interface temperature aligns much more closely with experimental observation. This confirms that contact resistance is not a secondary refinement but a governing process variable.

The results indicate that poor contact increases the thermal barrier between incoming tape and substrate, reducing interface heating and accelerating thermal loss after compaction. In practical terms, this means that two laminates exposed to the same nominal process settings can experience significantly different bonding conditions simply because their interfacial contact state is different.

4.3 Intimate Contact Development

Microscopy-based analysis shows that intimate contact decreases as fiber orientation mismatch increases. Aligned interfaces exhibited the most complete contact, while cross-ply configurations showed more incomplete interfacial nesting and greater local separation.

This result is important because intimate contact is the physical precondition for polymer healing. Without sufficient real contact area, chain diffusion cannot develop effectively across the interface, regardless of how favorable the nominal process temperature may appear.

4.4 Bonding Strength and Model Correction

Mechanical testing confirmed that standard intimate-contact and bonding predictions tend to overestimate interlaminar quality. The reason is that conventional models are too optimistic about how quickly and completely contact develops under AFP conditions.

The modified model introduced in this study significantly improves the agreement between predicted and observed bonding behavior. Its main contribution is to correct the assumption that intimate contact progresses ideally during the short compaction window of LAFP. The corrected framework captures the reality that partial contact, local roughness, and rapid thermal decay all limit final bond development.

The results therefore demonstrate that bonding quality is best understood as the product of two linked mechanisms: contact development and polymer healing. High temperature without real contact is insufficient. Likewise, mechanical contact without sufficient thermal activation does not produce strong bonding.

4.5 Permeability and Void Structure

Both pulse-decay and micro-CT characterization show that process conditions strongly influence pore architecture and permeability. Higher layup speed and larger orientation mismatch are associated with increased internal porosity and less favorable through-thickness transport behavior.

The micro-CT results reveal that voids are not randomly distributed defects, but part of a connected internal structure governed by local contact and consolidation history. This explains why permeability remains sensitive even when visible surface quality appears acceptable.

The macro- and micro-scale measurements also show strong consistency, supporting the use of micro-CT as a valuable quality assessment tool for thermoplastic AFP laminates.

5. Discussion

The results establish a critical point for thermoplastic AFP manufacturing: in-situ consolidation is not controlled by temperature alone.

This conclusion has major industrial implications. In many manufacturing environments, process development begins by adjusting laser power and layup speed until surface temperature falls within a target range. The implicit assumption is that adequate temperature will yield adequate consolidation. The present study shows that this assumption is incomplete.

The actual consolidation chain is governed by coupled thermal, mechanical, and microstructural phenomena. Fiber orientation affects how surfaces physically approach each other. That changes intimate contact. Contact condition changes thermal contact resistance. Thermal contact resistance changes the real interface temperature. Interface temperature and contact together determine the extent of polymer healing. Healing and trapped porosity then determine final bond quality and permeability.

This means that thermoplastic AFP should be designed as a coupled process system, not a temperature-control problem.

A second major implication concerns process modeling. Conventional intimate-contact and bonding models remain useful as conceptual tools, but they are too idealized to serve as stand-alone predictors for production quality. The modified framework proposed here demonstrates that experimentally calibrated correction is necessary, particularly under rapid in-situ consolidation conditions.

A third implication concerns layup design. In thermoset AFP, fiber orientation is often treated primarily as a structural design variable and a path-planning issue. In thermoplastic AFP, orientation also becomes a process physics variable. Adjacent ply angle changes the thermal and contact conditions at the nip point, which means stacking sequence affects not only final laminate mechanics but also manufacturability and bonding reliability.

A fourth implication concerns quality assessment. Traditional visual inspection or surface-based quality judgment is inadequate for thermoplastic AFP. A laminate can appear externally acceptable while containing poor interfacial bonding and transport-sensitive internal void networks. For this reason, permeability and internal void characterization should be treated as functional quality measures, not optional research diagnostics.

From a broader manufacturing perspective, the study helps complete the larger AFP picture. Thermoset AFP research has already advanced defect mapping, path planning, and machine feasibility. Thermoplastic AFP now requires equally mature understanding of in-situ bonding physics. The present work contributes that missing layer.

6. Conclusion

This study investigated the fundamental process physics of thermoplastic Automated Fiber Placement for in-situ consolidation of CF/PEKK laminates. The work focused on the interaction between thermal history, thermal contact resistance, intimate contact, interlaminar bonding, and permeability evolution.

The main conclusions are as follows:

1. Fiber orientation mismatch significantly alters interface thermal behavior by reducing intimate contact and increasing thermal contact resistance.
2. Thermal contact resistance is a governing process variable, not a minor correction term, and must be included in realistic interface temperature prediction.
3. Intimate contact is the physical gateway to bonding, and insufficient contact can prevent strong interlaminar consolidation even under nominally adequate temperature conditions.
4. Conventional contact-based bonding models overpredict bonding quality under AFP conditions; experimentally corrected formulations provide more reliable prediction.
5. Permeability and internal porosity are direct outcomes of the consolidation chain, linking process conditions to internal transport and potential structural risk.
6. Thermoplastic AFP in-situ consolidation is a coupled thermo-mechanical-microstructural phenomenon, and successful process design must integrate all three domains.

In engineering terms, the study demonstrates that reliable in-situ thermoplastic AFP requires answering three questions simultaneously: can the material be heated correctly, can true contact be established at the interface, and can that contact be maintained long enough for meaningful polymer healing to occur? Only when all three are satisfied can robust consolidation be achieved.

Future work should focus on integrating these physics into closed-loop digital process control, extending the models to curved and large-scale structures, and combining path planning with bonding-aware process windows for next-generation thermoplastic AFP manufacturing.

References

1. Osgouei, A. B. (2024). Design of Thermoplastic Composite Manufacturing with Automated Fiber Placement Towards In-Situ Consolidation. PhD Thesis, Sabancı University.
2. Mantell, S. C., and Springer, G. S. Foundational studies on intimate contact and bonding in thermoplastic composites.
3. Relevant literature on laser-assisted fiber placement, CF/PEKK processing, interlaminar bonding, thermal contact resistance, and permeability characterization.