Author: [Bruce Zhou]
Affiliation: [Jota Machinery Composites Material Prepreg Solution]
Corresponding Author: [jotamachinery@gmail.com]
Published : December 17 , 2025

Abstract

Laser-Assisted Fiber Placement (LAFP) has become a key enabling technology for the in-situ consolidation of thermoplastic composite structures, offering high deposition rates, elimination of autoclave curing, and improved manufacturing efficiency compared with conventional thermoset prepreg processes. In LAFP, bonding quality is governed by the ability to rapidly and uniformly melt the thermoplastic matrix at the tape–substrate interface while maintaining sufficient compaction pressure and residence time. This paper examines the influence of laser beam shaping, optical parameters, and primary process variables—laser power, placement rate, and compaction force—on interlaminar bonding performance in thermoplastic composites. Particular emphasis is placed on the comparison between top-hat and Gaussian laser intensity profiles, the sensitivity of heating uniformity to optical focus and Rayleigh length, and the resulting effects on interlaminar shear strength (ILSS). Experimental results reported in the literature demonstrate that optimized laser heating enables placement rates up to 50% higher than hot-gas-torch-based AFP while achieving comparable or superior bonding quality with reduced variability. The paper further discusses material-related influences, including surface roughness, resin distribution, and polymer crystallization behavior, and outlines practical guidelines for robust LAFP process design.

laser assisted fiber placement thermoplastic composites

Keywords

Laser-Assisted Fiber Placement; LAFP; Thermoplastic Composites; Beam Shaping; Interlaminar Shear Strength; In-Situ Consolidation

1. Introduction

Automated fiber placement technologies have undergone significant evolution as aerospace manufacturing shifts toward higher production rates, reduced energy consumption, and out-of-autoclave processing routes. Thermoplastic composites, particularly carbon fiber reinforced polyaryletherketones such as PEEK and PEKK, have attracted increasing attention due to their recyclability, weldability, and unlimited shelf life. However, their high melting temperatures and rapid cooling behavior impose strict requirements on heat delivery and process control during placement.

Laser-Assisted Fiber Placement (LAFP) addresses these challenges by using high-power laser radiation to directly heat the incoming tape and substrate immediately prior to consolidation. Compared with hot-gas-torch (HGT) heating, laser systems provide concentrated, rapidly controllable energy input and enable higher placement speeds. At the same time, laser processing introduces new sensitivities related to beam profile, focus stability, and material–optical interaction.

The success of LAFP depends on the controlled interaction of three primary process variables: laser power, placement rate, and compaction force. These parameters jointly determine the thermal and mechanical history at the tape–substrate interface, which in turn governs intimate contact development, void suppression, crystallization behavior, and ultimately interlaminar shear strength. This paper reviews and synthesizes recent experimental findings to clarify how optical design and process parameter selection influence bonding quality in LAFP of thermoplastic composites.

2. Literature Review

Early thermoplastic AFP systems relied primarily on hot-gas-torch heating, where convective heat transfer limited achievable deposition rates and resulted in relatively broad heat-affected zones. Subsequent studies demonstrated that laser heating, particularly using near-infrared fiber or diode lasers, offers superior energy efficiency because carbon fibers strongly absorb laser radiation and conduct heat directly into the polymer matrix.

A critical advancement in laser AFP has been the adoption of beam shaping optics. Gaussian beams, which concentrate energy at the center of the spot, tend to overheat the tape center while underheating the edges, leading to non-uniform melting and inconsistent consolidation. To address this issue, industrial systems increasingly employ top-hat (flat-top) beam profiles that distribute energy more uniformly across the tape width. Experimental and numerical studies have shown that flat-top beams reduce thermal gradients, improve edge bonding, and lower variability in ILSS.

Process optimization studies consistently identify placement rate as the dominant factor affecting interlaminar strength, accounting for the majority of observed variation, while laser power and compaction force play secondary roles. However, excessive power at low speeds can lead to thermal degradation, whereas insufficient power at high speeds results in poor fusion. Compaction force is essential to achieve intimate contact but exhibits diminishing returns beyond a threshold, where further increases do not significantly improve bonding.

Material properties also play a decisive role. Tape surface roughness, resin-rich versus fiber-rich surfaces, and polymer crystallization kinetics all influence consolidation behavior. Thermoplastics such as PEKK, with lower and slower crystallinity development than PEEK, are often more tolerant of rapid cooling during in-situ consolidation.

3. Methodology

This paper synthesizes experimental results from published LAFP studies involving carbon fiber reinforced thermoplastic tapes processed under controlled laser heating conditions. The focus is placed on:

  • Optical configuration, including beam profile, beam width relative to tape width, focal position, and Rayleigh length
  • Primary process parameters: laser power, placement rate, and compaction force
  • Material variables such as tape surface roughness, resin distribution, and polymer type

Bonding quality is primarily assessed using short-beam shear tests in accordance with standardized methods for interlaminar shear strength. Additional observations include thermal imaging, process stability, and variability metrics.

4. Results

4.1 Beam shaping and heating uniformity

Studies consistently show that top-hat laser beams provide superior heating uniformity compared with Gaussian profiles. Uniform irradiation across the tape width prevents edge underheating and centerline overheating, leading to more consistent melting and bonding. However, flat-top beams remain sensitive to focal position, and maintaining the working distance within the Rayleigh length is essential to preserve intensity uniformity.

4.2 Influence of laser power and placement rate

Laser power and placement rate must be balanced to achieve sufficient melting within the short nip-point residence time. Experimental data demonstrate that increasing placement rate requires proportionally higher laser power. At optimized settings, LAFP has achieved ILSS values comparable to autoclave-consolidated laminates at placement rates exceeding 100 mm/s, significantly outperforming HGT-based systems.

4.3 Compaction force effects

Compaction force improves bonding by promoting intimate contact and expelling entrapped air. However, beyond a moderate force level, further increases yield minimal improvement in ILSS. Excessive force can distort fibers or expel molten resin, reducing bond quality.

4.4 Comparison with hot-gas-torch heating

Direct comparisons show that laser heating enables equivalent ILSS at approximately 50% higher placement rates than HGT heating. Laser-processed laminates also exhibit lower variability in mechanical performance, attributed to more stable and localized heat input.

5. Discussion

The results confirm that LAFP performance is governed by the controlled delivery of thermal energy at the interface rather than by absolute laser power alone. Beam shaping emerges as a critical design factor, as non-uniform heating directly translates into bonding variability and defects. The dominance of placement rate in determining ILSS highlights the need for precise synchronization between heat input and deposition speed.

Material-related effects further complicate process optimization. Rougher tape surfaces promote intimate contact and higher ILSS, while correct orientation of resin-rich and fiber-rich surfaces prevents sticking to the compaction roller. Polymer selection also influences robustness, with PEKK systems offering greater tolerance to rapid cooling than highly crystalline PEEK.

Overall, the findings indicate that LAFP can achieve high-quality in-situ consolidation when optical design, process parameters, and material characteristics are addressed as an integrated system.

6. Conclusion

Laser-Assisted Fiber Placement represents a mature and highly capable process for manufacturing thermoplastic composite structures at high deposition rates. This paper demonstrates that:

  1. Top-hat laser beam profiles significantly improve heating uniformity and consolidation quality.
  2. Placement rate is the dominant process variable influencing interlaminar shear strength.
  3. Laser heating enables substantially higher productivity than hot-gas-torch systems at comparable bond quality.
  4. Compaction force exhibits an optimal range rather than a monotonic benefit.
  5. Material surface characteristics and polymer crystallization behavior strongly affect LAFP outcomes.

With continued advancements in beam delivery, process monitoring, and adaptive control, LAFP is well positioned to support next-generation aerospace composite manufacturing.

References

Eimanlou, M., Hojjati, M. Effect of process parameters on mechanical properties of thermoplastic composites produced by laser-assisted fiber placement. CANCOM Proceedings, 2017.

Stokes-Griffin, C. M., Compston, P. Laser-assisted tape placement of thermoplastic composites. Composites Part A, 2015.

Pourahmadi, A., et al. Influence of in-situ consolidation on crystallinity and interlaminar strength of CF/PEEK laminates. Composite Structures, 2023.

Lu, S. Deposition behaviour of carbon fibres processed by automated dry fibre placement. PhD Thesis, University of Nottingham, 2024.

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