**1. Introduction**

In polymer and composite technologies, reinforcement fillers of various materials are commonly used to increase part strength and stiffness. In injection molding, it is well known that fibres align inside the melt flow and display characteristic fibre orientation patterns inside the center of the flow and outer shell [1]. In addition to fibre orientation, fibre content distribution and length can vary within different locations inside a part [2]. Due to the effect of fibre matrix separation, areas within a part can appear with significant deviations in filler content [3].

In Selective Laser Sintering (SLS), part strength and stiffness can also be increased with the use of fibre-reinforced powders. While the typical layer-by-layer build-up differs greatly from the injection molding process, changes in filler orientation and part anisotropy are also observed. This is mainly caused by fibre alignment during recoating.

Jansson and Pejryd [4] investigated the orientation of carbon short fibres in SLS processing. Their results show greater fibre alignment in the direction of powder recoating, while longer fibres exhibit higher levels of orientation. During recoating, the recoating blade touches longer fibres and aligns them into recoating direction, while short fibres with a length below the layer thickness align more randomly. This investigation is supported by Arai et al. [5], who determined the anisotropy of glass fibres in polybutylene terephthalate (PBT). Arai showed that the tensile strength and modulus of elasticity show their highest values in the recoating direction, and lowest mechanical properties perpendicular to it. Furthermore, Jansson and Pejryd [2] and Schmid [6] state that fibre-reinforced SLS parts can show worse mechanical properties in the perpendicular build-up direction than unreinforced parts with equal polymer materials. This is caused by reduced fibre orientation in the thickness direction, and hence lower layer-to-layer connection [4].

In addition, the influence of powder recoating and the energy density on the mechanical properties of unreinforced SLS material was investigated. Beitz [7] compared di fferent recoating systems, such as roller and blade. He found that the recoating unit applies pressure onto the powder bed, dependent on the recoating speed and type of recoating unit. Drexler et al. [8] showed no significant e ffects on part density with di fferent recoating speeds, but an impact on the mechanical properties can be observed. Tensile strength as well as elongation at break show the highest values for the lowest recoating speed. This e ffect is explained via the longer interaction time of the recoating unit with the powder particles. As a consequence, a more homogenous part surface results.

The influence of laser energy density was previously studied for unreinforced materials [9]. Lanzl et al. [10] conducted the first experiments to investigate the e ffect of the energy density on the mechanical properties of fibre-reinforced materials. In their investigations, fibre content was varied from 10 to 50 vol.%, while particularly fibre contents of up to 30 vol.% showed a high impact on the modulus of elasticity. The variation of energy density from 0.04 to 0.06 J/mm<sup>2</sup> had only a little influence on part performance. At very low energy density levels, the modulus of elasticity decreased significantly due to the low melting behaviour of the polymer particles. These results are supported by Arai's et al. [5] investigation. The authors investigated the orientation of glass fibres in a PBT SLS material. At a layer thickness of 100 μm, the tensor shows an orientation of 50% in the recoating direction and 35% in the cross direction, while the build-up direction Z displays only 15%. This behaviour is also observed in the mechanical properties, whereas the highest tensile strength and modulus of elasticity were found in the recoating direction, while the lowest properties were found in the build-up direction. Furthermore, it was found that the samples in the coating direction showed the highest heat deflection temperature and the lowest shrinkage e ffects.

Besides the process influence, the powder and fibre composition also have an influence on fibre orientation and part behaviour. Zhu et al. [11] developed a method whereby a green body manufactured with SLS is combined with epoxy resin infiltration. After the modification of carbon fibres (CF) with a Polyamide 12 (PA12) surface treatment, the PA12/CF composite powder was used for the SLS process. This method enables a better random three-dimensional distribution of the fibres.

The literature review shows that prior studies investigated the mechanical properties as a consequence of fibre orientation. The characterisation of fibre orientation is only described by di fferent sample orientations in the build job. Therefore, this study describes the fibres' orientation by calculation of the orientation tensors in the X, Y and Z direction. In this context, the impact of the processing conditions on the fibre orientation tensor is described. The parameters of recoating speed, layer thickness and laser power are varied, and the interactions between these three parameters are identified in this study.

#### **2. Materials and Methods**

This research explores the influence of process parameters on fibre orientation. Based on a literature review, an experimental design was created to identify the influence of processing parameters on the fibre orientation. After identifying a suitable sintering window, samples were manufactured according to the experimental design and chosen parameters. The results of mechanical testing are used to identify samples with remarkable mechanical performance, to be investigated by means of X-ray computed tomography (CT).

#### *2.1. Design of Experiment*

A full factorial design of experiment (DoE) with the parameters of recoating speed (RS), layer thickness (LT) and laser power (LP) was created. The parameters were identified based on a literature review, as mentioned earlier. For each parameter, three factor levels were set: low, medium and high. The bottom and top limit values were maximised in order to observe high effects on the fibre orientation. Although going to the process limits, safe processing had to be ensured, characterised by low curling and warpage effects. The values of the DoE are summarised in Table 1. All other processing parameters were kept constant. Three parameters with three levels leads to a 33 DoE.


**Table 1.** Varied parameters for effect identification on fibre orientation.

#### *2.2. Manufacturing of Samples*

For the manufacturing of samples, a HT252P SLS printer made by Farsoon (Hunan, China), was applied. The printer features a maximum chamber temperature of 220 ◦C and is equipped with a 60 W CO2 laser (wavelength 10.6 μm). The build chamber is heated from all side: from the top by infrared radiation (IR) heaters, and from the cylinder walls and piston plate by integrated heating bands. The surface of the powder reservoir is heated by IR heaters. In addition, the chamber is flooded with nitrogen during the complete build job, leading to a residual oxygen content below 0.3% throughout all build stages. Prior to the heat-up stage, the machine was flooded manually with nitrogen and heated up to 150 ◦C for 1h. After this period, the actual warm-up stage began. The warm-up stage started with a ramp from the start temperature of 150 ◦C up to the final build temperature. The optimum build temperature was identified prior to testing by differential scanning calorimetry (DSC) analysis, supplier recommendations and printing pre-trials. The cylinder, piston and feed temperatures had been optimised in pre-tests with the target of minimizing curling and warpage effects. After the core temperature of the part cake had cooled down to below 50 ◦C, samples were removed and cleaned. As the impact by blasting could lead to uneven surfaces, the removal of the remaining powder from the specimens was done manually with a brush, and additionally with a vacuum cleaner with an attached brush.

Based on the DSC analysis (Section 3.1) and knowledge from pre-trials, the parameters for the temperatures were identified as summarised in Table 2. The temperature of the cavity walls (cylinder temperature) and piston plate was set to 180 ◦C. The surface temperature (build temperature) of the powder after recoating was set to 210 ◦C. While RS, LT and LP were changed according to the DoE, the temperatures, the scanning speed and the hatch distance were kept constant.

For tensile testing, tensile bars according to DIN EN ISO 527-2 1A were printed. For CT scans, cubes with a geometry of 3 mm × 3 mm × 3 mm on a base platform were chosen. The base allowed labelling as well as easier positioning on the CT platform. Both samples are illustrated in Figure 1.

#### *2.3. Investigated PA6 Material*

A non-commercial Polyamide 6 (PA6) powder, supplied by Farsoon (FS6140GF), filled with 30 wt.% glass beads and 10 wt.% glass fibres, was used for this study. Due to the effects whereby fibres increase the anisotropy and decrease the flowability of the powder, beads facilitate the powder recoating, the isotropy and the warpage behaviour. The reinforcing fillers were added to the powder and were not integrated into the powder particles. The particle size distribution was D50 = 63 μm. Carbon black was added as colourant as well as additives for oxidation resistance. The melting point was 225 ◦C and the bulk density of the powder was 0.6–0.73 g/cm3, while the density of the printed parts was 1.4 g/cm<sup>3</sup> at sufficient energy input.


**Table 2.** Processing parameters of PA6-GB-GF (Farsoon FS6140GF).

**Figure 1.** Overview of the manufactured samples. (**a**) Tensile bars to identify the mechanical properties and (**b**) computed tomography (CT) cubes to investigate the fibre orientation.

#### *2.4. Mechanical Testing and Fibre Orientation Determination*

Before the determination of the mechanical properties, the samples were vacuum dried at 80 ◦C for 168 h. The modulus of elasticity was measured with an extensometer at 1 mm/min according to DIN EN ISO 527-2/1A/5. After determining the linear-elastic strain area, the testing speed was increased to 5 mm/min. For the determination of the tensile properties, a Z050 by Zwick Roell (Ulm, Germany), with an extensometer was used. The measurements were performed under environmental conditions at room temperature of 23 ◦C and humidity of 50%.

X-ray computer tomography was applied for investigation of the fibre orientation and to collect images for qualitative and quantitative analysis. A v|tome|x m CT machine by GE Inspection Technologies, United States, with a resolution of 2 μm for detailed measuring range and 6 μm for overview images, was used. Figure 2 illustrates the high resolution area (green). The lower resolution is only needed for identification of the labels and positioning of the samples for calculation analysis.

The software VG Studio Max allows the detection of fibre orientation and evaluation of the differences due to different processing parameters. For the calculation of fibre orientation, a cube of 2.8 mm × 2.8 mm × 2.8 mm inside the CT cube (3 mm × 3 mm × 3 mm) was defined and analysed, as shown in Figure 3a. This way, typical SLS effects at the edge zones, such as shrink marks, are neglected, which increases the comparability of the samples. The cube (blue coloured) was further sliced into

21 layers (1 × 1 × 21, X × Y × Z mesh) as illustrated in Figure 3b. Thus, a better understanding of the fibre orientation through the single layers could be obtained.

**Figure 2.** Illustration of the CT resolution within the CT cube. Green represents the high detail area with a resolution of 2 μm.

**Figure 3.** Illustration of the measuring cube within the sample for tensor calculation (**a**). The measuring cube is split into a 1 × 1 × 21 mesh (**b**).
