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Abstract

This study introduces detailed characterisations of a novel highly-filled PLA-Al6061 polymer-metal composite filament (69.0 wt.% Al6061, remainder PLA) for metal additive manufacturing (AM) via material extrusion, specifically fused filament fabrication (FFF). The filament, synthesised through controlled blending, compounding, and extrusion processes, was evaluated for its microstructural, thermal, and physical properties. Scanning electron microscopy (SEM) revealed a homogeneous dispersion of Al6061 particles (average size 60 ± 10 μm) within the PLA matrix, with energy-dispersive X-ray spectroscopy (EDX) confirming Al dominance alongside C and O from the polymer. X-ray fluorescence (XRF) indicated 91.08% Al in the metal phase, consistent with Al6061 alloy composition, while X-ray diffraction (XRD) identified a face-centered cubic (FCC) Al phase. Thermal analysis via differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) showed PLA melting at 172.61°C (ΔH = 5.01 J/g) and decomposition at 310.53°C (23.60% weight loss), potentially supporting subsequent debinding and sintering processes for fully dense metallic parts later. Physical properties, including average surface roughness (R_a = 10.36 ± 1.22 μm) and Vickers microhardness (60.4 HV) suggest green part robustness, which was successfully printed by using a typical desktop FFF 3D printer. These results demonstrate the potential of the filament for producing robust, cost-effective metal parts via MEX-based FFF technology, advancing metal AM applications.

Keywords

Highly-filled PLA-Al6061 composite metal additive manufacturing (AM)material extrusion (MEX)fused filament fabrication (FFF)microstructure thermal stability surface roughness hardness

Introduction

Additive manufacturing (AM), or more commonly known as 3D printing has transformed modern manufacturing by enabling the layer-by-layer construction of complex geometries unattainable through traditional subtractive methods. Technologies such as powder bed fusion (PBF), directed energy deposition (DED), and material extrusion (MEX) have expanded its applications across aerospace, automotive, and biomedical industries.¹ Among these, MEX-based AM technology, particularly fused filament fabrication (FFF), stands out for its simplicity, cost-effectiveness, and accessibility, utilising thermoplastic filaments to build parts via nozzle-based deposition.² The versatility of FFF AM technique has spurred innovations in material development, transitioning from purely polymeric filaments to advanced composites that incorporate ceramics, fibers, and metal/alloys, broadening its scope beyond prototyping to functional component production.³ In fact, recent trends have emerged in utilising FFF AM using widely available desktop FFF 3D printers to produce fully metallic parts for various applications.^2,4 Metal AM via FFF represents a hybrid approach, enabling the production of fully metallic components through a three-stage process: printing, debinding, and sintering. In the printing stage, a polymer-metal composite filament is extruded into a green part using standard FFF equipment.³ Debinding follows, where the polymer binder is thermally and/or chemically removed, leaving a porous metal skeleton termed ‘brown part’.⁵ Sintering then densifies this structure into a solid metallic component, e.g. through solid-state sintering (SSS) or liquid phase sintering (LPS) mechanism, typically achieving >90% density depending on process parameters.⁶

In this context, polylactic acid (PLA), a biodegradable thermoplastic derived from renewable sources, is a cornerstone of FFF AM technology due to its low melting point (∼150°C–180°C), ease of printing, and environmental benefits.⁷ In MEX-based metal FFF AM, PLA serves as a temporary binder to encapsulate highly-filled metal powders (>50 wt.%) to synthesise printable filaments for producing green parts, which are subsequently debound and sintered to yield fully metallic parts.^1,2 Al6061, a precipitation-hardened aluminum alloy composed primarily of aluminum with magnesium, silicon, and copper, is widely used in established metal AM technologies, e.g., PBF and DED for its high strength-to-weight ratio, good corrosion resistance, and weldability.^8–10 In PBF and DED, Al6061 is processed directly into metallic parts using high-energy heat sources like laser/electron beams or plasma arcs. In contrast, metal FFF AM necessitates an indirect approach; A composite PLA-Al6061 filament, in which PLA binds Al6061 particles (>50 wt.%) is required to enable printing on standard FFF printers, producing green parts. Here, it is important to clarify that the printing stage of the Al60616-PLA filament leverages the thermoplastic properties of PLA, with extrusion temperatures governed by the aforementioned melting range of PLA. This stage resembles conventional polymer FFF AM processes, as the PLA matrix facilitates filament bonding during printing. However, the filament is specifically engineered for metal AM, where the PLA serves as a temporary binder that is removed via a subsequent thermal debinding stage, followed by sintering to yield a fully metallic Al6061 part with high densification levels (>90%).^6,11,12 This hybrid approach aligns with established metal FFF techniques, ensuring compatibility with standard FFF printers while achieving metallic components post-processing.^6,12

Thus, for metal FF AM, Filament production is critical to this workflow, requiring precise control over metal particle size (∼10–60 μm), polymer-metal ratio (typical metal content of >50 wt.%), and mixing homogeneity to ensure printability and structural integrity.^13,14 Despite the rapid advancements of metal FFF AM technology thus far, especially on filament production and process-microstructure-property relationships, studies on aluminum-based filaments, particularly Al6061, remain scarce,^15,16 with most efforts focusing on higher-density metals like stainless steel, titanium alloys, and nickel superalloys.^17–20 The limited exploration of Al-based alloys in FFF-based metal AM, e.g., stems from challenges in achieving uniform particle dispersion, adequate mechanical properties in the green state, and efficient debinding and sintering without compromising the aluminum structure, particularly its high affinity to oxygen, i.e. oxidation and its relatively low melting point (<600°C).²¹ This paucity of research on Al6061-based filaments for FFF AM represents a significant opportunity, given its lightweight advantages and industrial relevance. Thus, the problem lies in the lack of open-source, systematically characterised Al6061 filaments, hindering their adoption in cost-effective metal FFF AM.

Therefore, this study addresses this gap by characterising a novel, highly-filled PLA-Al6061 composite filament tailored for FFF-based metal AM. Comprising 69.0 wt.% Al6061 powder and 31 wt.% PLA binder, the filament was synthesised to balance printability with suitability for the ensuing debinding and sintering stages. Subsequently, the microstructures, chemical composition, crystal structure, as well as thermal, and physical (surface and microhardness) properties are comprehensively analysed through extensive microscopy and analytical tools. The results provide a robust framework for evaluating its suitability for the printing-debinding-sintering cycle, offering insights into the processing of high-metal-loading filaments for metal AM applications via MEX-based FFF AM process. Overall, this study uniquely distinguish itself from present studies that primarily focus on polymer composites with metallic fillers for structural enhancements.^22,23 Additionally, these contributions position this study as a novel advancement in the development of thermoplastic composite filaments for metal AM, presenting a scalable, eco-friendly solution for producing lightweight metallic parts via FFF AM technology.

Materials and methods

Filament synthesis and printability test

The PLA-Al6061 composite filament was synthesised by The Virtual Foundry company, (TVF, Wisconsin, USA), composed of 69.0 wt.% Al6061 alloy powder (average particle size ∼60 μm) and 31 wt.% polylactic acid (PLA) binder. Based on the densities of Al6061 (∼2.7 g/cm³) and PLA (∼1.24 g/cm³), the volume fraction of Al6061 in the filament is approximately 51 vol.%, which supports high metal loading for subsequent debinding and sintering in metal AM.¹ The composition was selected based on their in—house preliminary trials that ensure filament extrudability (continuous filament formation) and compatibility with the printing-debinding-sintering cycle described in a later section (Implications for subsequent printing-debinding-sintering stages). The synthesis process involved dry blending Al6061 powder and PLA pellets in a high-shear mixer, followed by compounding in a twin-screw extruder, and forced through a 1.75 mm die into spools (each spool holds 500 g filaments), suitable for standard FFF printers. Further details on parameter values and conditions were not disclosed due to the proprietary nature of the process.

Additionally, while rheological characterisation was not conducted, literature on similar highly-filled metal-polymer composite filaments indicate suitable viscosity for FFF. For instance, Spoerk et al.²⁴ investigated metal-PLA composites and reported that high filler loadings (e.g., 40–50 vol.%) increase viscosity but maintain extrudability within typical FFF temperature ranges (180°C–200°C) due to the shear-thinning behavior of PLA. Similarly, Kukla et al.²⁵ found that metal-PLA composites with comparable loadings exhibit stable flow properties at 190°C, suitable for FFF 3D printing without nozzle clogging. Thus, it could be reasonably inferred that the current PLA-Al6061 composition selection is also expected to successfully yield green parts during the printing process.

Subsequently, to evaluate the printability of the PLA-Al6061 composite filament, cube samples (20 20 mm × 20 mm) were fabricated by using an entry-level desktop 3D printer Artillery Sidewinder X1, with a nozzle diameter of 0.6 mm. The nozzle temperature, bed temperature, layer height, and infill printing parameters were 220°C, 60°C, 0.20 mm, and 100%, respectively.

Sample preparation

Filament samples were prepared for characterisation by cutting the 1.75 mm diameter strand into segments of appropriate lengths using a precision blade. For microstructural and compositional analyses (SEM, EDX, XRD, XRF), 10 mm segments were used as-extruded. For thermal analyses (DSC, TGA), samples were finely chopped into ∼10 mg pieces to ensure uniform heating. For Surface roughness analysis, the filament strands were analysed in its present form. On the other hand, samples for Vickers microhardness tests, 20 mm filament segments were mounted in epoxy resin, ground with SiC papers (grit 320–1200), and polished with 3 μm diamond paste to expose a flat surface. All samples were stored in a desiccator at 25°C and 20% relative humidity to prevent moisture absorption prior to testing.

Scanning electron microscopy

Microstructural analysis was conducted using a JEOL JSM-6010PLUS/LV scanning electron microscope (JEOL, Tokyo, Japan) operated at 5 kV in high-vacuum mode. Samples were sputter-coated with a 10 nm gold layer (Quorum Q150R) to enhance conductivity. Images were captured at magnifications of 50x to 500x for surface morphology and 1000x to 2000x for cross-sectional particle distribution and qualitative assessment of bonding between the Al6061 powder particles and the PLA binder. Energy dispersive X-ray spectroscopy (EDX) analysis was performed concurrently to map elemental distribution (Al, C, O), with spectra analysed using INCA software equipped with the SEM facility. The average particle size was quantified from SEM images using ImageJ from 20 particles across three fields.

X-ray-based characterisations

The elemental composition of the filament was determined via x-ray fluorescence (XRF) using a PANanalytical Epsilon 1 analyser through quantification of elemental percentages, calibrated against Al6061 reference standards, focusing on major elements (Al, Mg, Cu, Si) and trace impurities.

X-ray diffraction (XRD) was performed to identify the crystalline phases within the filament using a Bruker D8 Advance Diffractometer equipped with a Cu Kα radiation source (λ = 1.5406 Å). Samples were scanned over a 2θ range of 10–80° at a step size of 0.02° and a scan rate of 2°/min. Diffraction patterns were analysed using the International Centre for Diffraction Data (ICDD) database (reference 98-024-0129) to confirm the presence of the Al6061 face-centred cubic (FCC) phase and/or any other crystal structures present.

Characterisation of thermal properties

Thermal transitions, melting behaviour, and heat absorption characteristics were assessed through differential scanning calorimetry (DSC) by using a DSC-60 Plus (Shimadzu). Samples (∼9.6 mg) were weighed into sealed copper pans, with an empty pan as reference, and heated from 25°C to 600°C at 10°C/min under a nitrogen atmosphere (100 mL/min flow rate) to prevent oxidation. Melting temperature (T_m), onset and endset temperatures (T_m _Onset and T_m _Endset, respectively), and enthalpy (ΔH) were determined from endothermic and/or exothermic peaks using TA Instruments software.

Thermal stability and decomposition behaviour were evaluated through thermogravimetric analysis (TGA) by using a PerkinElmer STA 8000. Samples (∼9.0 mg) were placed in platinum crucibles and heated from 25°C to 600°C at 10°C/min under nitrogen (100 mL/min). Weight loss (%) and decomposition temperatures were recorded continuously, analysed with Pyris software to identify degradation stages.

Surface roughness

For surface roughness analysis, the filament strand was measured using a ZYGO ZeGage™ 3D optical profilometer. Three 5 mm × 5 mm areas along the filament length were scanned, and average roughness (R_a), root mean square roughness (R_q), and peak-to-valley height (R_z) were calculated using Mx software, averaged for statistical reliability.

Vickers microhardness

Mechanical properties were assessed via Vickers microhardness tests by using a Shimadzu HMV-G series tester. A 100 gf load (0.98 N) was applied for 15 s with a diamond indenter on polished filament surfaces. Fifty indentations were made at random locations, and hardness values (HV) were averaged to ensure statistical precision.

Results and discussion

Microstructural analysis

The representative scanning electron microscopy (SEM) image shown in Figure 1 reveal homogeneous dispersion of Al6061 alloy particles within the PLA matrix across the surface and cross-section of the filament (Figure 1(a) and (b), respectively). The average particle size, determined from multiple SEM images was approximately 60 ± 10 μm (see Figure 1(c) for example particle size measurements via SEM in), consistent with the ∼60 μm Al6061 powder used during synthesis. Cross-sectional images indicated a well-bonded interface between the metal particles and PLA binder, with minimal voids or agglomeration. Higher magnification showed that the Al6061 particles were predominantly spherical, with some irregularity likely due to powder processing, embedded uniformly throughout the filament volume.

Figure 1.

SEM images showing dispersions of Al6061 powder particles within the PLA matrix of the filament viewed from its (a) surface and (b) cross-section. (c) Example of powder particle size measurements through SEM.

This uniformity suggests that the high-shear mixing and twin-screw extrusion processes effectively distributed the 69.0 wt.% Al6061 content, preventing sedimentation or clustering that could disrupt FFF printing. The strong metal powder particle-polymer binder bonding observed is critical for maintaining structural integrity during extrusion and subsequent debinding, as poor adhesion could lead to delamination or cracking.¹³ Compared to PLA-stainless steel composites, where particle sizes of 10–45 μm are common,¹⁴ the larger ∼60 μm Al6061 particles may enhance metal content retention post-sintering. This is because larger particles have lower surface area that reduces the total binder surface area in contact with the metal particles, thereby potentially^13,26: (i) reducing binder entrapment that potentially reduces oxidation and improves packing density, and (ii) minimising metal loss during debinding that promotes better particle-particle contact during sintering. Nevertheless, melt viscosity could be increased and risk nozzle clogging if the relatively larger particles are not uniformly dispersed, thereby necessitating careful processing to ensure smooth extrusion during FFF process.^24,25 On the other hand, the absence of significant porosity contrasts with reports of gas entrapment in metal-polymer filaments,²⁷ indicating optimised extrusion conditions (e.g., temperature <200°C) that avoided PLA degradation. Overall, this microstructure supports the suitability of the filament for producing green parts with consistent metal distribution, a prerequisite for high-density sintered components.

Chemical composition

X-ray fluorescence (XRF) analysis (Table 1) quantified the elemental composition of the filament’s metal phase, revealing 91.08% Al, 4.03% Mg, 1.82% Cu, 1.65% Si, and trace elements (e.g., 0.32% Cl, 0.21% Fe), aligning with the typical composition of Al6061 alloy.⁸ Energy-dispersive X-ray spectroscopy (EDX) spectrum (Figure 2) from SEM analysis corroborated these findings, detecting Al as the dominant element alongside C and O from the PLA binder, with minor O peaks suggesting slight surface oxidation. Being organic, the PLA component was not detected by XRF, consistent with its focus on inorganic elements.

Table 1.

Elemental composition of the PLA-Al6061 composite filament obtained from XRF.

Element	Composition (wt.%)
Al	91.08
Mg	4.03
Cu	1.82
Si	1.65
Cl	0.32
Ca	0.28
S	0.25
Fe	0.21
Cr	0.16
Others (K, Ti, Mo, W, V, Mn)	0.2

Figure 2.

(a) Location of EDX analysis. (b) EDX spectrum showing internal elemental composition at analysis location in (a).

The high Al content (91.08%) confirms that the filament retains the chemical identity of Al6061 alloy, crucial for achieving metallic properties post-sintering. The presence of Mg, Cu, and Si matches the precipitation-hardening characteristics of Al6061 alloy, potentially enhancing strength in the fully metallic final part.²⁸ The consistency between XRF (surface) and EDX (internal) results indicates uniform elemental distribution, suggesting efficacy of the filament synthesis process. However, the minor O detected by EDX suggests some oxidation of Al6061 particles, possibly during powder handling or extrusion, though not significant enough to alter the alloy’s phase (see XRD results in Figure 3). Nevertheless, such compositional stability of PLA-Al6061 filament is advantageous for predictable outcomes in the fully metallic parts post-sintering.⁶ This chemical fidelity supports its potential for aerospace-grade applications, though trace impurities (e.g., Cl, Fe) warrant monitoring to avoid embrittlement during sintering.

Figure 3.

XRD spectrum of the PLA-Al6061 composite filament.

Crystal structure

X-ray diffraction (XRD) patterns exhibited prominent peaks at 38.41° (111), 44.65° (200), 64.98° (220), and 78.08° (311), corresponding to the face-centered cubic (FCC) α-Al phase of Al6061 (ICDD 98-024-0129). No distinct PLA peaks were observed, reflecting its amorphous nature, while the sharp, intense Al peaks indicated that the crystallinity of this alloy was preserved within the composite. No secondary phases (e.g., oxides or intermetallics) were detected, suggesting minimal thermal or chemical alteration during filament synthesis.

The retention of the FCC Al phase is critical as it ensures that the metallic skeleton post-debinding retains the inherent properties of Al6061 alloy, such as ductility and strength.⁹ The absence of PLA crystallinity aligns with its typical amorphous state below 200°C processing temperatures, avoiding complications like shrinkage during printing.²⁹ Compared to steel-PLA composites, where feedstock characteristics can lead to secondary phase formation (e.g., carbides) during subsequent debinding and sintering stages post-processing,³⁰ the absence of secondary phases in the PLA-Al6061 filament (this study) indicates a stable Al6061 structure, likely due to the low extrusion temperature (<200°C) preventing alloy decomposition. This crystalline purity enhances the suitability of this filament for subsequent printing, debinding, and sintering stages, in which phase stability influences densification and mechanical performance. However, the potential for minor oxide formation (suggested by EDX), but below XRD detection limits should be considered in future high-temperature studies.

Thermal Properties

Differential scanning calorimetry (DSC) results (Figure 4(a)) identified two endothermic peaks. The first, at 172.61°C (T_{m Onset}: 164.39°C, T_{m Endset}: 177.86°C, ΔH = 5.01 J/g), corresponds to PLA melting, consistent with its reported range of 150°C–180°C.^31,32 The second, at 346.08°C (T_m _Onset: 324.44°C, T_m _Endset: 360.24°C, ΔH = 51.11 J/g), reflects a major thermal event, likely PLA decomposition or interaction with Al6061. Thermogravimetric analysis (TGA) (Figure 4(b)) showed two weight-loss stages: (i) 23.60% at 310.53°C, attributed to the primary degradation of PLA, and (ii) 6.91% at 363.84°C, possibly residual organic breakdown or minor oxidation.

Figure 4.

(a) DSC analysis results. (b) TGA analysis results.

The low enthalpy of the first DSC peak (5.01 J/g) suggests a small crystalline fraction in PLA, typical of its semi-crystalline behavior when processed below 200°C, facilitating easy extrusion in FFF.³³ The second peak’s high enthalpy (51.11 J/g) and broad range align with TGA’s 23.60% loss at 310.53°C, confirming the thermal scission of PLA into volatile compounds (e.g., lactide, CO₂).³⁴ The additional 6.91% loss at 363.84°C may indicate carbon residue decomposition, a common secondary stage in metal-polymer composites.³⁵ These thermal profiles define a debinding window of ∼300°C–400°C, well below Al6061’s melting point (582°C–652°C), ensuring metal stability during subsequent debinding and sintering stages. Nevertheless, it is important to note that the existence of different types and grades of PLA could exhibit varying thermal degradation behaviour depending on molecular weight, crystallinity, and additives, which may influence the decomposition temperature and thus the binder removal window.^36–38

Notably, this decomposition range (300°C–400°C) is lower than that reported for PLA-stainless steel filaments (∼400°C–500°C),^14,39,40 despite both using PLA as the binder. This difference likely stems from the higher thermal conductivity of Al6061 (∼167 W/m·K) compared to stainless steel (∼15–30 W/m·K),⁴¹ enabling faster heat transfer to PLA and accelerating its degradation. Additionally, Al6061 or its surface oxides may catalyse PLA chain scission, lowering the onset temperature, whereas stainless steel exhibits less catalytic influence.^34,42,43 The 69.0 wt.% Al6061 content and larger average particle size (∼60 μm) versus typical PLA-stainless steel (50–80 wt.%, 10–45 μm) may further enhance local heating effects, reducing the thermal stability of PLA.^6,12,14 In contrast, the higher decomposition range of PLA-stainless steel often includes complete binder and residue burnout, requiring elevated temperatures due to slower heat distribution.^5,44 This earlier decomposition in PLA-Al6061 offers energy efficiency for debinding, and the observed 30.51% total weight loss closely matches the expected 31.0% for 69.0 wt.% Al6061, with the minor deviation likely within experimental error and thus negligible for practical purposes.

Surface properties and microhardness

The surface roughness measurements yielded an average roughness, R_a of 10.36 ± 1.43 μm, root mean square roughness, R_q of 13.46 ± 1.22 μm, and peak-to-valley height (R_z) of 108.91 ± 7.22 μm, as revealed in Table 2 and Figure 5. On the other hand, Vickers microhardness tests averaged 60.4 ± 4.5 HV across 50 measurements under a 100 gf load (15 s dwell time), indicating consistent mechanical performance.

Table 2.

Surface properties of PLA-Al6061 composite filament in this study.

Surface properties
Parameter	Value (μm)
Average roughness (R_a)	10.36 ± 1.43
Root mean square, RMS roughness (R_q)	13.46 ± 1.22
Peak valley height (R_z)	108.91 ± 7.72

Figure 5.

3D plot visualisation of the surface roughness of the filament obtained from ZYGO ZeGage™ 3D optical profilometer.

The R_a of 10.36 μm reflects a moderately rough surface, likely due to Al6061 particles protruding from the PLA matrix, a common feature in metal-filled filaments.⁴⁵ This roughness exceeds that of pure PLA filament (∼2–5 μm), but is comparable to steel-PLA composite counterparts (∼8–12 μm), suggesting a trade-off between metal content and surface finish.^45,46 For metal FFF AM process, this relatively higher R_a value may affect layer adhesion during the printing process, but is acceptable for green parts destined for sintering, in which surface quality is secondary to metal retention.⁴⁷ The average Vickers microhardness of 60.4 ± 4.5 HV significantly surpasses that of pure PLA filament (∼15–20 HV), attributable to the reinforcing Al6061 particles and their uniform dispersion (Figure 1).⁴⁵ Furthermore, the hardness of this composite filament is comparable to that of as-cast Al6061 (45–56 HV), reflecting the impact of high metal content (69.0 wt.% in this study) on the mechanical properties even in the green state.^48,49 The consistency across HV measurements, indicated by the low error bar value underscores effective metal particle-polymer binder bonding, critical for load transfer during printing and handling.

Printing trial results

To demonstrate the printability of the highly-filled PLA-Al6061 composite filament, printing trials were conducted using a standard entry-level desktop FFF 3D printer (Artillery Sidewinder X1) with a 0.6 mm nozzle, 220°C nozzle temperature, 60°C bed temperature, 0.20 mm layer height, and 50 mm/s print speed. Cube samples (20 mm × 20 mm × 20 mm) were successfully printed without nozzle clogging or filament breakage. The printed green parts exhibited consistent layer adhesion and dimensional accuracy within ±0.2 mm, as verified by digital caliper measurements. Figure 6 shows representative images of the printed samples, validating the processability of the composite filament in standard FFF printers, supporting its application in the printing-debinding-sintering cycle for producing fully metallic Al6061 parts as previously explained.

Figure 6.

Representative images of successfully 3D printed green parts of the highly-filled PLA-Al6061 composite filament by using a standard entry-level desktop FFF 3D printer.

The filament exhibited smooth and continuous extrusion throughout the printing process, suggesting adequate thermal and rheological behavior at typical PLA printing temperatures (190°C–220°C).⁵⁰ This is supported by the thermal properties of the filament (Figure 4), where the PLA binder demonstrated a melting temperature of 172.61°C as determined by DSC, ensuring sufficient flow during nozzle deposition without premature degradation. Furthermore, the average Al6061 particle size of 60 ± 10 μm is significantly smaller than the nozzle diameter (0.6 mm), reducing the risk of clogging and supporting uninterrupted material flow. The homogeneous particle distribution observed in SEM analyses (Figure 1) further indicates that no large agglomerates or uneven dispersion occurred, which could otherwise hinder printability. The moderate surface roughness (Ra = 10.36 ± 1.43 μm) and sufficient Vickers microhardness (60.4 HV) contribute to stable feeding and handling during printing, ensuring that the filament can be guided smoothly through the extruder without breakage or deformation. Overall, these observations confirm that the synthesis filament is feasibly printed by using typical desktop FFF printers to produce green parts with good dimensional integrity.

Implications for subsequent printing-debinding-sintering stages

The characterisation results collectively underscore the potential and suitability of the PLA-Al6061 filament for the printing-debinding-sintering cycle in metal FFF, with specific implications for each stage. In the printing stage, the homogeneous microstructure (average Al6061 particle size: 60 ± 10 μm) and moderate surface roughness (R_a = 10.36 ± 1.43 μm) suggest reliable extrudability through standard nozzles used in typical FFF AM machines, also known as 3D printers (e.g., 0.4–0.6 mm) as the particle size is well below typical nozzle diameters, minimising clogging risks.^45,51 The low PLA melting point (172.61°C) aligns with common FFF 3D printer extruder capabilities (180°C–220°C), thereby ensuring smooth deposition, while the Vickers microhardness (60.4 ± 4.5 HV) indicates sufficient green part durability to withstand handling and layer stacking without deformation.¹³ Furthermore, the relatively higher surface roughness of the PLA-Al6061 composite filament compared to its conventional PLA counterpart do not bring a huge impact to overall part quality as the green parts will be subjected to debinding and sintering for producing fully metallic parts.⁴⁷

Nevertheless, the adhesion between Al6061 particles and the PLA matrix is critical for ensuring filament integrity during FFF and effective load transfer in the green parts. While specific experiments on adhesion mechanisms were not conducted in this study, insights from literature provide a basis for understanding the interfacial behavior. Spoerk et al.²⁴ suggest that in highly-filled PLA-metal composites, adhesion is primarily driven by mechanical interlocking between the polymer matrix and metallic particles, as the non-polar nature of PLA limits covalent bonding. The Al6061 powder used in this study exhibits a native oxide layer, as evidenced by minor oxygen peaks in EDX analysis (Figure 2(a)), which can reduce chemical bonding with PLA, as noted by Kukla et al.²⁵ No surface treatments, such as silane coupling or acid etching, were applied to the Al6061 powder, relying instead on mechanical interlocking to achieve sufficient adhesion for FFF AM processing. This is supported by the consistent Vickers microhardness (60.4 ± 5.5 HV) and successful printing trials, which indicate adequate interfacial bonding for filament extrudability and green part formation. Future studies could explore surface treatments to enhance adhesion, particularly for improving the mechanical properties of sintered parts.

On the other hand, the thermal analysis (DSC/TGA) results provide a clear roadmap for the debinding process. The primary decomposition of PLA at 310.53°C (23.60% weight loss) and secondary loss at 363.84°C (6.91%) define an effective debinding window of ∼300°C–400°C, well below the melting range of Al6061 (582°C–652°C). This temperature range allows complete binder removal without compromising the Al6061 skeleton, critical for avoiding residual carbon that could form carbides during sintering.³⁰ The observed 30.51% total weight loss (vs 31 wt.% PLA content) suggests complete binder removal, potentially resulting in a clean, fully metallic structure after sintering.⁶ The thermal stability of the filament up to 310°C also supports its integrity during prolonged printing, reducing the risk of premature degradation.

The PLA-Al6061 filament is designed for a printing-debinding-sintering cycle to produce metallic Al6061 parts. The printing stage leverages the thermoplastic properties of PLA at low extrusion temperatures (150°C–180°C), followed by thermal debinding to remove the PLA binder and sintering to densify the Al6061 particles into fully metallic structures. The use of a single-component PLA binder necessitates purely thermal debinding, typically conducted at 350°C–450°C to decompose and volatilise the PLA. Based on literature for similar PLA-based composites with high metal loadings (45–50 vol.%),^24,25 thermal debinding requires approximately 6–12 h, depending on part thickness and heating rates, to ensure complete binder removal without causing defects like cracking or bloating. This duration, while longer than multi-component binder systems with solvent or catalytic debinding, is standard and comparable with metal FFF AM process incorporating metal powder-polymer binder filaments for producing high-density, fully metallic parts post-sintering.^6,12,13,52

In the sintering stage, the chemical composition (91.08% Al, XRF) and FCC Al phase (XRD) confirm that the filament retains the metallurgical properties of Al6061, essential for densification into a fully metallic part. The uniform particle dispersion (SEM) and absence of secondary phases (XRD) enhance sintering potential, as irregular particle packing or impurities can lead to porosity or cracking.⁵³ Literature suggests Al6061 achieves >95% density when sintered at 600°C–650°C,⁵⁴ and the high metal loading (69.0 wt.%) here possibly supports this outcome, potentially yielding lightweight yet high-strength components. However, the slight oxidation indicated by EDX (O peaks) could form Al₂O₃ during sintering that could negatively impact ductility unless controlled via vacuum or inert atmospheres, e.g., argon).^6,11 The hardness increases from 60.4 HV (green state) to ∼90–110 HV (sintered Al6061) is anticipated, reflecting the transition from a polymer-metal composite to a fully metallic structure.

Overall, these results position the PLA-Al6061 filament as a viable candidate for metal FFF AM process. Its printability, thermal processability, and metallurgical fidelity address key requirements for the printing-debinding-sintering workflow, potentially expanding the available range of filament materials for metal FFF alongside stainless steel, Inconel, Ti6Al4V, and many more.^1,2 Challenges such as surface roughness and minor oxidation are manageable with process optimisation, while the high Al6061 content promises lightweight parts ideal for aerospace or automotive applications. Future studies should validate these implications through actual printing trials, debinding efficiency tests, and sintered part mechanical testing (e.g., tensile strength, density) to confirm scalability and performance.

Conclusions

In this study, a highly-filled PLA-Al6061 composite filament (69.0 wt.% Al6061), optimised for metal FFF AM process have been successfully developed and characterised. SEM observations confirmed uniform Al6061 particle dispersion (average particle size ∼60 μm), while XRF and EDX validated the alloy composition (91.08% Al) and XRD identified a stable FCC Al phase. DSC and TGA revealed PLA melting at 172.61°C and decomposition at 310.53°C (23.60% weight loss), defining a 300°C–400°C debinding window. Average surface roughness (R_a = 10.36 μm) and Vickers microhardness (60.4 HV) values, although surpassing those of pure PLA filament, most likely ensure green part robustness after printing. These properties—low melting point, high metal loading, and thermal stability—demonstrate the suitability of the PLA-Al6061 composite filament for metal FFF AM, potentially enabling reliable printing, efficient debinding, and sintering into fully metallic parts. Additionally, cube samples measuring 20 × 20 × 20 mm were successfully printed by using an entry-level desktop 3D printer (Artillery Sidewinder X1), further verifying the processability and feasibility of the PLA-Al6061 composite filament to produce green parts with good dimensional accuracy.

These properties and the subsequent successful printing trial collectively affirm the suitability of this composite filament for metal FFF. The low melting point and homogeneous microstructure facilitate extrusion through standard FFF equipment, while the high metal loading and good thermal stability are expected to ensure effective debinding and sintering into dense, lightweight Al6061 parts. The enhanced hardness and moderate roughness suggest a balance between green part strength and print quality, making it a cost-effective, eco-friendly, and sustainable option for producing scalable complex metallic components. These characteristics are highly promising, particularly for applications requiring the high strength-to-weight ratio of Al6061 alloy, such as aerospace or automotive sectors.

Future work should include debinding and sintering experiments to assess final part density and mechanical properties (e.g., density and tensile strength). Optimisation of Al6061 content or particle size, as well as debinding and sintering parameters could further enhance performance, solidifying the role of this filament as a viable solution for metal FFF AM technology.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The research is supported by Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2025R4), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

ORCID iD

Shahir Mohd Yusuf

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Microstructural,thermal,and physical characterisations of highly-filled PLA-Al6061 polymer-metal composite filament for metal additive manufacturing via material extrusion

Abstract

Keywords

Introduction

Materials and methods

Filament synthesis and printability test

Sample preparation

Scanning electron microscopy

X-ray-based characterisations

Characterisation of thermal properties

Surface roughness

Vickers microhardness

Results and discussion

Microstructural analysis

Chemical composition

Crystal structure

Thermal Properties

Surface properties and microhardness

Printing trial results

Implications for subsequent printing-debinding-sintering stages

Conclusions

Footnotes

Declaration of conflicting interests

Funding

ORCID iD

References