**4. Discussion**

The results of the design of experiment show the high significance of the factors of layer thickness and laser power on the modulus of elasticity. The recoating speed has A low significance. Interactions between the three factors can only be identified for LT and LP. This interaction effect is the result of the dependence of these factors on the resulting energy density. The higher the energy density, the higher the mechanical properties.

The tensor analysis, for instance, shows that the varied laser power does not influence the fibre orientation, as comparing CT1 (197 J/cm3) and CT4 (87 J/cm3) shows similar X-tensors (0.53 and 0.52). Although the melt viscosity changes with melt temperature, which is directly related to the applied laser power, there is no external force, except gravity, acting on the fibres, which could cause changes in fibre orientation. Lower viscosity can also improve the wetting of the fibres, leading to an improved introduction of load into the fibres and higher part strength, respectively. As such, the effect of higher laser power on part strength results from an improved layer-to-layer interconnection and fibre incorporation, and not necessarily from a change of fibre orientation. This can also be seen in the CT scans, which confirm that the laser energy improves the quality of the layer-to-layer interconnection. Theoretically, perpendicular fibres are able to penetrate several layers (Z direction) more easily if the viscosity is lower. Nevertheless, as mentioned above there is no additional driving force, related to the laser power, which could cause fibre reorientation. Consequently, the CT analysis shows no influence of the energy density on fibre orientation. Thus, fibres mainly follow a 2D orientation inside a layer.

Variation of layer thickness has two effects: First, the laser energy density is proportionately dependent on the layer thickness. As a consequence, the quality of the layer-to-layer interconnection and fibre wetting is affected in the same manner as described above for the laser power. The second effect is directly related to fibre orientation, whereby lower layer thicknesses result in greater orientation in the recoating direction. By reducing the layer thickness, the recoating roller interacts with more fibres, since even smaller fibres can interact and change orientation. In addition, the shear stress between the top and bottom of each layer, which results from the roller's counter-rotation to the recoating direction, is inversely proportional to the layer thickness. Hence, the driving force causing fibre alignment in the

recoating direction increases as the layer thickness decreases. For CT3 with a low layer thickness, the X tensor shows an increase of 7% in comparison to the reference with a medium setting for the layer thickness. Since spherical beads do not have an influence on the tensors, the alignment of the fibres will be higher than the calculated 7% suggests. The shown impact of the layer thickness parameter confirms Jansson's [4] findings, with the effect illustrated in Figure 13. Although the layer thickness has a significant effect on fibre orientation, the layer thickness cannot be set too low, otherwise the powder spreading process will be hindered considering the poorer flowability of the composite powders.

**Figure 13.** Orientation of fibres due to variation of the layer thickness from a high (**left**) to a low level (**right**).

As opposed to the layer thickness, the recoating speed determines the duration of the roller–fibre interaction. Results show that the factor time (interaction between roller and fibre) has little impact on the resulting modulus of elasticity within the chosen settings. The fibre orientation tensors show a slightly higher orientation for the high setting (CT1) in comparison to the low setting (CT5).

#### **5. Conclusions and Outlook**

The current work indicates that for PA6 powder, filled with 30 wt.% glass beads and 10 wt.% glass fibres, the modulus of elasticity can vary strongly (6417–8154 MPa) in dependence on the chosen process parameters. All these variations are related to the applied laser energy density, which was either varied directly by the setting of the applied laser power, or indirectly by layer thickness. The dominant effect can be found in an improved layer-to-layer interconnection and fibre wetting, which is related to the melt viscosity as a result of the varied laser energy density. In addition, reduced layer thickness leads to a slightly higher (7%) fibre alignment in the recoating direction, with smaller fibres now in interaction with the roller and thus aligning in the roller direction. In addition, a change in the melt viscosity due to variation of the energy density does not seem to affect fibre orientation.

No significant impact in terms of fibre orientation related to the parameters of recoating speed and laser power was found. With their apparently low influence on fibre orientation during the processing of fibre-filled PA6 powder, these parameters can be used to focus on process robustness. Conclusively, since the degree of anisotropy in fibre orientation can be influenced by the layer thickness, it would be possible to induce locally adjusted stiffness and anisotropic mechanical behaviour with changes in layer thickness, in order to improve part performance. This study illustrates the need to understand and optimise processing parameters so as to achieve the best mechanical properties of the laser sintered parts, especially with fibre- and bead-reinforced powders.

This study dealt with the effect of selective laser sintering on the fibre orientation in manufactured components. Based on the results shown, it can be assumed that the layer height as well as the laser power have influence on the material structure in the manufactured test specimens. Therefore, the melting process and the herein-induced properties (e.g., porosity, degree of crystallinity, temperature profile) can be investigated in detail in future work.

**Funding:** This research received no external funding.

**Author Contributions:** Conceptualisation, T.H.; methodology, T.H.; validation, T.H. and M.S.; formal analysis, T.H.; investigation, T.H. and M.S.; data curation, T.H. and M.S.; writing—original draft preparation, T.H.; writing—review and editing, M.S.; S.R.R., G.H. and P.M.; visualization, T.H.; supervision, S.R.R., G.H. and P.M. All authors have read and agreed to the published version of the manuscript.

**Acknowledgments:** The authors gratefully acknowledge Farsoon Europe for the technical support and provision of the investigated reinforced PA6 material (FS6140GF). Any opinions, findings, conclusions or recommendations expressed in this study are those of the authors and do not necessarily reflect the views of Farsoon Europe.

**Conflicts of Interest:** The authors declare no conflict of interest.
