Reusable support for additive manufacturing

Additive manufacturing (AM) processes such as material extrusion and vat photopolymerization all require supports to print parts with overhang features. These additional supports using the same or different materials are a waste of materials since they need to be removed after the three-dimensional (3D) printing process and cannot be reused. The printing of supports is also time-consuming for the nozzle-based material extrusion processes. A new type of reusable support has been developed to address the support-related challenges in AM. The main idea is to use a set of dynamically controlled metal pins as a programmable building platform. In the layer fabrication, the metal pins will move up one-layer thickness after the printing of each layer. Also, each metal pin will automatically stop at a specified height that is determined by a combination of metal tubes, magnetic discs, and magnetic rings. Additional supports can be 3D-printed on the top surface of the metal pins, while the amount of the supports is dramatically reduced. After the printing process, the metal rods can be separated from the part and reset for the next printing job. A prototype system has been constructed to demonstrate the reusable support principle. The layout optimization and toolpath generation algorithms for the reusable support are also presented. The experimental results of several test cases show an average of nearly 40% saving on the printing time and material with increased reliability and robustness. The reusable support provides a support generation strategy that could be beneficial to other AM processes such as stereolithography and selective laser melting.

Additive Fabrication of Silicon Pillars on Monocrystalline Silicon by Direct Laser Melting

Marie Camille Laëtitia Le Dantec

Additive Manufacturing (AM) is an emerging part production technology that offers many ad-vantages such as high degree of customization, material savings and design of 3D highly complex structures. However, AM is a complex multiphysics process. Therefore, only a limited number of materials can already be commercially used to produce parts and a handful of others are being studied or developed for such process. Consequently, limited knowledge on this process is available, especially concerning materials that present thermomechanical challenges such as brittle materials. The present research focuses on additive fabrication of silicon pillars on a monocrystalline silicon wafer by Direct Laser Melting (DLM) with a pulsed 1064 nm laser beam. The simple geometry of pillars allowed for the first determining steps into process understanding. Several results were achieved through this PhD work. First, crack-free silicon pillars were successfully built onto monocrystalline silicon wafers. With the help of in-situ process monitoring and sample characterization, wafer substrate temperature and laser repetition rate were found to be the main influential parameters to obtain crack-free samples, as minimum substrate temperature of 730°C and a minimum repetition rate of 100 Hz were necessary to reach this goal (for a feed rate of 15 g/min and a pulse duration of 1 ms). The influence of secondary process parameters such as feed rate and energy per pulse were also discussed. A simple Finite Element Modeling (FEM) model validated by the experiments was used to explain crack propagation in the samples. Then, process monitoring of the DLM process was realized. High-speed camera image analysis re-vealed that vertical stage speed and powder feed rate should match to obtain a constant pillar building rate. As all pillars presented necking at their base, estimations of the thermal characteristics of the pillar during growth were carried out by FEM simulations. They were additionally used to explain the pillar final shape. Finally, the microstructure of the pillars built was characterized by the Electron Back-Scattering Dif-fraction (EBSD) technique. In the conditions presented in this work, the microstructure of the pillar was found to be in the columnar growth mode. The feed rate was identified as the most influential parameter on the microstructure, followed by the stage speed, the impurity content of the powder and the crystallographic orientation of the substrate. Epitaxial growth was achieved on more than 1 mm with a feed rate of 1.0 g/min, a stage speed of 0.1 mm/s, a powder with purity of 4N and a oriented wafer substrate. This work could be further continued by making improvements to the DLM setup, studying the influence of additional process parameters on the thermomechanical behavior and the microstructure control of the pillars, and/or using these results to realize more complicated shapes, either with this setup or by using a powder bed technique.

EPFL2018

Periodic cellular aluminum structures for space applications

Florian Thibaut Gallien

Mass is one of the crucial parameters for hardware that has to be placed in Earth orbit. Due to its harsh environment, a material with highest specific properties is desired to achieve space missions. The rise and development of new technologies, such as additive manufacturing (AM), opened new opportunities in part-design complexity, periodic cellular structures (PCS) being one of them. The present thesis investigates the potential implementation of PCS in space applications, particularly for structures and micro-meteoroids and orbital debris (MMOD) impact shields. This was achieved in three steps: Four different types of AlSi12 PCS manufactured by selective laser melting (SLM) were tested under quasi-static compression to measure the mechanical properties dependency versus topology and to characterize the failure mode. Properties ranging from 3 to 4 GPa for the compressive modulus, 5 to 12 MPa for the yield stress, 12 to 20 MPa for the plateau stress, and 2 to 8 MJ/cm3 for the absorbed energy were obtained. An unexpected failure mode was observed when compared to classical cellular metals, namely a brittle failure occurring by global shearing. A predictive failure criterion was established based on topology considerations and correlated to most of the reported results in the literature. A preliminary test campaign on tensile specimens was performed to compute numerical models that were fed into a finite element analysis. Good agreement with experimental data was shown, and the importance of microplasticity effects in this class of material was highlighted. An alternative process was developed to produce AlSi1 PCS by investment casting. The process is based on replication of a polymer preform used to build a NaCl mold. It was observed that the quality of the final cast part depends mainly on the grain size of the salt, with an optimum identified for distributions between 125 and 180 um. Optimization of the process allowed to reduce the drying time by a factor 6. Main process parameters include a drying temperature of 80C and infiltration at 660C under 300 mbar. From this process, PCSs having an energy absorption capacity of 15 MJ/m3 with an efficiency of 80% were produced. Hypervelocity impact tests were conducted on cast PCS and stochastic structures. The objective being to hit the structures with a 2mm-diameter aluminum sphere at velocities close to 7 km/s. Influence of the sample topology, the orientation, and the bumper material was assessed. Stochastic structures successfully stopped the projectile in all configurations. The beneficial effect of the bumper was measured reducing the crater depth from 20 mm to 14 mm. This type of structure exhibited a comparable areal density (0.8 g/cm2) to simple Whipple shield design. PCS poorly performed in mitigating the impact as the debris passed through all the structures, independently of the test configuration due to the open-channels present. PCS are good candidates to be used in space hardware, but their design and the manufacturing process need to be carefully chosen depending on the specific application. AM PCS are suitable for structural application with a high compressive modulus and yield stress. Cast PCS would perfectly fit in shock absorbers. A more random design would be preferable for MMOD shielding applications.

EPFL2018

A miniaturized selective laser melting device for operando X-ray diffraction studies

Nicola Casati, Samy Hocine, Steven Van Petegem, Helena Van Swygenhoven

We report on the development of a miniaturized device for operando X-ray diffraction during laser 3D printing. The printing chamber has a size of only 130 x 135 x 34 mm(3) and is optimized to be installed at synchrotron beamlines. We describe the design considerations, details on the setup and the implementation at two different beamlines of the Swiss Light Source. Its capabilities are demonstrated by ex situ printing of complex shapes and operando X-ray diffraction experiments using Ti-6Al-4V powder. It is shown that the beamline characteristics have an important influence on the X-ray footprints of the microstructural evolution during 3D printing. From the intensity of the diffraction peaks, the evolution of the different phases can be followed during printing. Furthermore, the diffuse scattering signal provides information on the precise location of the laser beam on the sample and the scanning head settling time.

ELSEVIER2020

Additive Fabrication of Silicon Pillars on Monocrystalline Silicon by Direct Laser Melting

Marie Camille Laëtitia Le Dantec

EPFL2018

Periodic cellular aluminum structures for space applications

Florian Thibaut Gallien

EPFL2018