**3. Results**

## *3.1. Powder Analysis*

The SEM images in Figure 4 show that the fillers are added to the powder and are not integrated into the polymer particles. The length of fibres varies up to a maximum of 500 μm, while the fibre diameters have a constant value of about 15 μm. Furthermore, the diameter of the beads varies up to 80 μm. The potato-shaped geometry of the powder particles is a result of the fabrication process of the powder. Farsoon reported that the PA6 powder is fabricated by precipitation of a dissolved PA6 [12]. Additionally, small spherical additives are commonly added to the powder, which act as separating agents to decrease the particle-particle attractive forces for a better powder flowability [13]. In these SEM images, dust on the particles is identifiable, which could also support the flowability.

The dynamic DSC analysis of the powder is illustrated in Figure 5. The melting and crystallisation peaks are clearly defined, which allows one to derive the optimum processing parameters. The chosen build temperature of 210 ◦C is 6 ◦C lower than the onset melt temperature (216 ◦C). The fillers do not significantly influence the melting curve. The temperature of the cavity walls and piston plate was set to 180 ◦C to allow the melted material to cool down as slowly as possible, so as to avoid warpage effects. Furthermore, it can be observed that the crystallisation is initiated at higher temperatures (195 ◦C) in comparison to non-filled materials (BASF Ultrasint® PA6: 188 ◦C). The glass fibres, glass beads, carbon black and other additives act as crystallisation nuclei. Depending on the size and geometry of the crystallisation nucleus, the crystalline structure is influenced.

**Figure 4.** Powder composition of the investigated reinforced PA6 powder.

**Figure 5.** Characterisation of the melting and crystallisation behaviour of PA6-GB-GF powder.

#### *3.2. Process Influence on the Mechanical Properties*

For the evaluation of influences on the mechanical properties, the modulus of elasticity is chosen as a key factor. The analysis of the DoE shows that the values of the modulus of elasticity follow a normal distribution (*p*-value < 0.005). All factors show significant effects (significance level: 1.98). LT and LP show similar effects, with effect values above 7.2. RS shows low significance with 2.5. Furthermore, the correlation of LT and LP has an effect of 5.5. This high significance is due to the impact of both parameters on to the resulting volumetric energy density. The other correlations show no significance.

Figure 6 illustrates the results of mechanical testing. The result shows averaged values for the modulus of elasticity. As observed at effect significance, RS has only a little influence on the sample's modulus of elasticity. There is a maximum at medium setting, while the low and high setting leads to lower values. In total, the impact of RS is hardly measurable. LT and LP, on the other hand, show higher impacts. The higher LT, the lower is the modulus of elasticity. The opposite trend is observed for LP. A high increase from low to medium setting can be observed. At the medium setting, the maximum is achieved and remains on this level with a small decrease in the high setting. Both effects can be explained by the resulting volumetric energy density. The lower LT and/or the higher LP, the higher is the resulting energy. The higher modulus of elasticity can be found with the better layer-to-layer interconnection [9]. For the influence of fibre orientation on the mechanical properties, the orientation tensors in the following chapters are necessary.

**Figure 6.** Effect of processing parameters on the modulus of elasticity of the produced samples.

#### *3.3. Sample Selection for CT Investigation*

The analysis of the modulus of elasticity itself does not directly allow a conclusion as regards the fibre orientation. Therefore, samples for the CT are selected, which display significant differences in their mechanical values. The selected samples are summarised in Table 3. The CT1 cube reflects the parameter combination with the highest modulus of elasticity of 8154 MPa, and is taken as the reference. The selection for the other CT cubes is reasoned to be below. In addition, the resulting volumetric energy density for each sample is calculated.

The selected samples also vary in their tensile strength and elongation at break. The values are summarised in Figure 7. The results show a correlation with the resulting laser energy density, whereas the three settings with sufficiently high laser energy density (CT1 (197 J/cm3), CT3 (236 J/cm3) and CT5 (197 J/cm3)) lead to high strengths as well as elongations. Lower mechanical properties are observed for CT2 (169 J/cm3) and CT4 (87 J/cm3) if the laser energy density is much lower. For high layer-to-layer interconnection, the threshold value can be found between 169 J/cm<sup>3</sup> and 197 J/cm3. In addition, the recoating speed seems to influence the reproducibility. The setting with the lowest recoating speed (CT5) shows the lowest standard deviation of 1.72 MPA (2.1%) for the tensile strength.

**Figure 7.** Effects of the varied parameters on tensile strength and elongation at break.



#### *3.4. Fibre Orientation Dependent on Processing Parameters*

The evaluation of CT1 shows that fibres tend to align in the powder recoating direction (Figure 8a). The SLS's typical rough surface can be detected all around the edge areas. At the right bottom edge, curling e ffects can be observed, which on a macroscopic view lead to a smooth transition between right side and bottom side. Next to the aligned glass fibres, glass beads of di fferent sizes, as well as shrinkage-related voids, occur inside the part. Furthermore, the top layers show less filler particles than the bottom layers. Another di fference in the comparison of the top/bottom layer is seen in the melting behaviour of the particles. The first layer of the sample displays partially melted powder particles, while particles in the top layer melted almost completely. This e ffect can be explained via the layer's temperature distribution before a new layer of powder is added. While in the first layer, the temperature of the applied powder corresponds to the build temperature, the existing temperature is much lower in comparison to the top layer, where a higher temperature results from the heat of previously melted layers. Beyond that, Figure 8b shows the fibre orientation in the X and Y direction. Fibres are mainly orientated in the X direction. In addition, the orientation in the Y direction is more highly detectable than orientation in the Z direction.

**Figure 8.** 2D cross-sectional slice from the CT scan of CT1. Fibre orientation in (**a**) XZ plane and (**b**) XY plane. Parameters: RS = 288 mm/s, LT = 120 μm and LP = 36 W (CT1).

The calculation of the tensors for CT1 is illustrated in Figure 9. As already seen in the qualitative results above, the X alignment is the dominating tensor, followed by Y and Z. The value of the X tensor varies from 0.49 to 0.58 along the grid elements. The Y tensors shows a value of 0.28, and Z of 0.18.

Mean values of the 21 tensor measuring ranges are calculated for each orientation tensor and CT scan sample. The mean values of all investigated tensors are summarised in Figure 10. Except CT3, the tensors show similar characteristics for the X, Y and Z directions. With a higher value of approximately 0.04, the X tensor of CT3 shows a greater orientation of fibre in the recoating direction.

The CT3 image in Figure 11 shows similar results as that of CT1. Despite the 39 J/cm<sup>3</sup> higher energy input, voids are still visible. The slightly increased fibre orientation is not recognisable from this single image, but could be measured by tensor evaluation across all layers. Due to the lower layer thickness, more fibres interact with the roller and an increased orientation is the result. Despite the increased laser energy and higher orientation, the modulus of elasticity decreases compared to CT1. At this point two e ffects overlap: the modulus of elasticity increases due to the fibre orientation in the recoating and tensile test direction, and the high energy density leads to a degradation in the polymer material.

**Figure 9.** Fibre orientation in CT1 with the parameters RS = 288 mm/s, LT = 120 μm and LP = 36 W.

**Figure 10.** Comparison of all tensor values for all investigated CT scans.

**Figure 11.** 2D cross-sectional slice from the CT scan of CT1. Parameters: RS = 288 mm/s, LT = 100 μm and LP = 36 W (CT3).

Figure 12 illustrates the result of insufficient laser energy in sample CT4. Many voids within the part occur, which reduces the layer-to-layer interconnection. This leads to a low tensile strength of 60 MPa and low elongation at break of 1.2%. In addition, the reduced laser energy leads to a reduction in the curling. Because of the lower melt temperature, the temperature difference between the melt and the recoated powder is comparably low. Therefore, internal stresses are reduced and less curling occurs.

**Figure 12.** Insufficient laser power leads to defects in the part's material and consequently to a low layer-to-layer interconnection. Parameters: RS = 288 mm/s, LT = 120 μm and LP = 16 W (CT4).
