This study intends to contribute to the state of the art of Fused-Filament Fabrication (FFF) of short-fiber-reinforced polyamides by optimizing process parameters to improve the performance of printed parts under uniaxial tensile loading. This was performed using two different approaches: a more traditional 2k full factorial design of experiments (DoE) and multiple polynomial regression using an algorithm implementing machine learning (ML) principles such as train-test split and cross-validation. Evaluated parameters included extrusion and printing bed temperatures, layer height and printing speed. It was concluded that when exposed to new observations, the ML-based model predicted the response with higher accuracy. However, the DoE fared slightly better at predicting observations where higher response values were expected, including the optimal solution, which reached an UTS of 117.1 ± 5.7 MPa. Moreover, there was an important correlation between process parameters and the response. Layer height and printing bed temperatures were considered the most influential parameters, while extrusion temperature and printing speed had a lower influence on the outcome. The general influence of parameters on the response was correlated with the degree of interlayer cohesion, which in turn affected the mechanical performance of the 3D-printed specimens.

The scope of this work is to provide an overview of the influences of process parameters, print orientation, and post-process treatments of Ti6AlV4 processed by laser powder bed fusion on its microstructure and physical and mechanical properties and their anisotropic behavior. To avoid the influence of changes in powder quality and ensure comparability, experiments were carried out using a single batch of virgin powder. First, characterization of the density and surface roughness was performed to optimize the process parameters utilizing design of experiment. Tensile, notched bar impact and compression test specimens were built in three different orientations: vertically, horizontally, and inclined at 45° to the build plate. Later, the influence of the staircase effect and the possible course of anisotropy from vertical to horizontal were investigated. Subsequently, heat treatments for stress relief, furnace annealing, and hot isostatic pressing were performed. In addition to as-built samples, mechanical machining and a two-step electrochemical polishing surface treatment were applied to investigate the influence of the surface roughness. With parameter optimization, a relative density of 99.8% was achieved, and surface roughness was improved over default parameters, reducing Ra by up to 7 µm. Electrochemical polishing is a viable way to decrease the surface roughness. An Ra value of 1 µm and an Rz value of 4 µm can be achieved for 45° downskin surfaces with as-built surface roughness values of Ra 24 µm and Rz 117 µm. As-built and stress-relieved conditions show little anisotropy in their yield and tensile strength (max 2.7%), but there is a strong influence of the build orientation on necking, and brittle fracture behavior is shown due to the martensitic microstructure (up to 70%). Heat treatment can increase the ductility and further decrease the strength anisotropy with both furnace annealing and hot isostatic pressing delivering similar results for tensile properties, while angled samples exhibit behavior that is closer to vertical than horizontal, indicating a non-linear change in break behavior. Electrochemical polishing increases fracture necking, and its isotropy drastically increases from 4% to over 30% compared with as-built parts, which is close to the level of the machined specimen.

As-printed Laser-Powder Bed Fusion (LPBF) surfaces can provide anchoring spots for the infiltration of polymer printed by Fused Filament Fabrication (FFF), enhancing metal-polymer joint strength. This work evaluates the influence of the as-printed LPBF surface roughness and FFF parameters on the strength of Ti-6Al-4V/PA-CF joints produced by this process combination. A three-point bending testing method based on ISO 14679:1997 was deployed, whereby the energy dissipated by the joint interface was measured. Roughness was varied by 3D-printing the substrate with different inclination angles; Height and printing speed of the coating layer (the polymer layer in direct contact with the metal) were also varied. Data was interpreted using a combination between Decision Tree and Gradient Boosting Regression, ultimately suggesting that printing speed is the prominent parameter followed by inclination angle for joint strength. Additionally, the combined effect of low printing speed and inclination angle resulted in the highest energy absorption at the interface (>200 J).

This study evaluated the manufacturing of metal–polymer hybrid parts using a 3-axis desktop Fused Filament Fabrication (FFF) printer. Two printing strategies were employed: a more trivial one, consisting of 3D-printing the polymer directly onto the metal surface, and an alternative one, consisting of encasing the metal with printed polymer. Materials used were Ti-6Al-4V (both rolled/sandblasted and 3D-printed by laser powder bed fusion) and polyamide-based polymers. Demonstrators were designed to resemble omega-shaped skin stringers commonly used in vehicular applications. Several challenges were addressed, including harvesting the heat emanating from the deposited polymer to locally increase the substrate temperature, as well as positioning the metallic parts to avoid undesired collisions during the print job. Furthermore, to better understand the behavior of the encased metal under load, pullout tests were conducted on commercially available M6 and M8 steel nuts that were enclosed in a 3D-printed composite block. Results revealed that the length of the edge shared by the enclosure and metal significantly impacted the pullout strength.

Fused-filament fabrication (FFF) is an extremely popular additive manufacturing process due to its affordability, relative ease to operate, and wide range of possible materials. It is also notorious for the hundreds of different process variables, which often are overlooked in favor of parameters considered to be more relevant for mechanical performance, such as printing and bed temperatures, printing speed, and layer height. Thus, this study is aimed at evaluating some of the less frequently studied process variables, namely raster orientation angles (and their stacking sequence) and feeding rate. Based on this, the influence of these variables on the tensile and flexural properties of short carbon fiber-reinforced polyamide printed by FFF was assessed. The study concluded that stacking layers with raster angles of 0°/90° and +30°/−30° resulted in the best trade-off between tensile and bending properties, with the former reaching ultimate tensile and flexural strengths of 111 ± 1 and 137 ± 5 MPa, respectively. The study also found that there was no increase in part density or mechanical properties when the volumetric flow was increased up to 120% of the intended road volume. Therefore, the hypothesis that an increase in flow rate would result in less inter-road gaps could not be confirmed with the current setup.