Fabrication and Performance of Continuous 316 Stainless Steel Fibre-Reinforced 3D-Printed PLA Composites

Abstract

This study investigates the feasibility of 3D printing continuous stainless steel fibre-reinforced polymer composites. The printing study was carried out using 316L stainless steel fibre (SSF) bundles with an approximate diameter of 0.15 mm. This bundle was composed of 90 fibres with a 14 μm diameter. This fibre bundle was first coated with polylactic acid (PLA) in order to produce a polymer-coated continuous stainless steel filament, with diameters tailored in the range from 0.5 to 0.9 mm. These filaments were then used to print composite parts using the material extrusion (MEX) technique. The SSF's volume fraction (Vf) was controlled in the printed composite structures in the range from 4 to 30 Vf%. This was facilitated by incorporating a novel polymer pressure vent into the printer nozzle, which allowed the removal of excess polymer. This thus enabled the control of the metal fibre content within the printed composites as the print layer height was varied in the range from 0.22 to 0.48 mm. It was demonstrated that a lower layer height yielded a more homogeneous distribution of steel fibres within the PLA polymer matrix. The PLA-SSF composites were assessed to evaluate their mechanical performance, volume fraction, morphology and porosity. Composite porosities in the range of 2-21% were obtained. Mechanical testing demonstrated that the stainless steel composites exhibited a twofold increase in interlaminar shear strength (ILSS) and a fourfold increase in its tensile strength compared with the PLA-only polymer prints. When comparing the 4 and 30 Vf% composites, the latter exhibited a significant increase in both the tensile strength and modulus. The ILSS values obtained for the steel composites were up to 28.5 MPa, which is significantly higher than the approximately 13.8 MPa reported for glass fibre-reinforced PLA composites.

Keywords: 3D printing; mechanical properties; thermoplastic polymers.

Figure 1

Stainless steel fibre bundle: (a) spool and (b) 90 fibres per bundle.

Figure 2

Schematic of 3devo filament making. (a) Schematic with the fibre introduction region indicated by the blue dashed ovals. (b) Photograph of the PLA-SSF filament co-extrusion. (c) Velocity and shearing profile of a non-Newtonian polymer-fibre co-extrusion die.

Figure 3

Photograph of the printing head with an insert showing a schematic of the 1 mm in diameter polymer pressure vent (arrow), which facilitated the removal of excess polymer during printing. Note also the 4.5 mm flat area on the print head, which was found to facilitate ‘ironing’ of the part during printing.

Figure 4

Three-dimensional printing continuous PLA-SSF composite. (a) Geometry shape trial prints. (b) Close-up of PLA-SSF continuous 3D printing path. (c) Tensile sample printed at a layer height of 0.22 mm, (10 mm scale bar).

Figure 5

Cross-section SEM images of PLA-SSF filaments mounted in resin: (a) 0.7 mm PLA-SSF filament and (b) 0.5 mm PLA-SSF filament. Note the greater dispersion of fibres in the bundle for the filament with the smaller diameter.

Figure 6

The $\mu$CT scans of PLA-SSF 3D-printed part cross-sections (scale bar = 550 μm). (a) Printing head travel path, with uneven perimeters of the 0.35 mm layer height samples. (b) Printing head travel path, with even perimeters of the 0.22 mm layer height samples. (c) Composite (12 $V_f$%) printed with layer height of 0.35 mm. (d) Composite (30 $V_f$%) printed with layer height of 0.22 mm.

Figure 7

SEM cross-section image of the PLA-SSF composite matrix printed using a 0.22 mm layer height, demonstrating good diffusion of the PLA between the fibres in the SSF bundle.

Figure 8

Interlaminar shear strength results for the PLA 0.22, 0.35 and 0.48 mm layer height prints, along with the results reported in the literature for PLA-glass fibres (cGF) and a PLA-Basalt composite investigated in a previous study at UCD (not reported) [32].

Figure 9

Interlaminar shear strength results for the 0.22, 0.35 and 0.48 mm layer heights investigated with statistical analysis.

Figure 10

Examination of PLA-SSF composites after ILSS testing. (a) Fractured composite printed with a layer height of 0.35 mm, indicating interlaminar shearing and tensile fracture. The yellow dashed square indicates the region at higher magnification in (b). (b) The fibre pull-out and necking for this sample given in a higher magnification. (c,d) Corresponding images for a composite printed with a layer height of 0.22 mm, demonstrating similar fracturing along with fibre pull-out and SSF necking. The dashed blue square in (c) indicates the higher magnification image in (d).

Figure 11

Tensile properties of PLA composite samples printed at the four print head heights in this study (purple squares), with examples of metal- and carbon fibre-reinforced composites reported in the literature [17,19,20,30,33,36,41,44,64].

Figure 12

SEM images of PLA-SSF composite (35 mm layer height) after tensile testing. The image on the right is a higher magnification of the region in the purple box in the left image.