# Developing Tungsten-Filled Metal Matrix Composite Materials Using Laser Powder Bed Fusion
## 3 Results
### Process Parameter Development
This leads to the necessity of adapting the process parameters in order to counteract the descending densities with increasing tungsten content. Consequently, adapted DoE matrices were applied to shift and expand the process parameter variations. Additionally, the powder bed, which was not heated for the starting parameters, was heated to ca. 90 \({}^{\circ}\)C. These measures led to an improvement in the overall density level in the case of 60 vol% tungsten. However, a further descending density from 60 to 70 vol% tungsten was monitored (red data points). Concentrating on the 60 vol% tungsten volume content, another optimization iteration of the process parameters resulted in densities close to the theoretical density (green data points). The measured density is influenced by porosities as well as the actual tungsten volume content. Thus, low porosity leads to high densities, whereas high porosity results in lower densities. Figure 2a,b highlights these two cases. The tungsten volume content can differ from the nominal value. Illustrative images of corresponding microstructures are presented in Figure 2c,d, respectively. This phenomenon can arise because of inhomogeneity originating in the mixing process or because of the mechanical interaction of the recoater blade of the L-PBF machine with the powder bed. Another reason for this phenomenon can be the application of an excessive energy density. This can lead to temporary, high local temperatures above the melting point of tungsten, allowing single tungsten particles to fuse or deform during a short period. The resulting change in morphology is shown in Figure 2c. The change in tungsten content explains why positive deviations of the expected relative densities can be observed. Figure 1: Archimedean density in comparison to the theoretical density for a variation of tungsten volume content and manufacturing parameters. The large markings present the mean of the central points. The observed characteristics \"porosity\", \"tungsten content\" and \"particle morphology\" proved to be challenging for an analysis of the experimental plans in Table 1. Density measurements by the Archimedean principle, as they are common for the development of L-PBF process parameters, are insufficient because all characteristics mentioned above impact the result. The use of \(\upmu\)CT measurements has been tested within this work. However, because of the high X-ray absorption of tungsten, the quality of the results was insufficient for an evaluation (not presented within this publication). While the tungsten content and the porosity can be quantified based on microscopical images, there is no approach to quantify the particle morphology. DoE approaches, as they are used for the development of process parameters for nanomaterials, should therefore be adapted for MMCs. This could be done by moving the process window of the experimental plans into an area where no or low fusion of filling particles can be observed. Single laser track experiments, as shown by [PERSON] [11], could be used to define a suitable process window. Because of the time-consuming sample preparation and analysis by microscopy, it is advisable to keep the experimental design as lean as possible (e.g., fractional designs). Because of the described challenges, a thorough analysis of the experimental designs in Table 1 was not possible. However, because the process parameter variation should mainly serve to fabricate a material of sufficient quality for a proof-of-concept of the material combination, the parameter combination resulting in the lowest porosity was picked out of all samples showing no excessive deformation or fusing of the tungsten particles. All following investigations were based on the main process parameters of 90 W, 500 mm/s, 0.07 mm and W 60% vol%, resulting in the following characteristics: \(\sim\)1.1% porosity, \(\sim\)58% tungsten content and \"spherical\" morphology. Energy-dispersive X-ray spectroscopy (EDS) measurements confirmed the presence of tungsten particles surrounded by an iron-based matrix.