|Over the last three decades, additive manufacturing (AM), which is based on fashioning 3D objects through a layer-by-layer construction, has greatly expanded. The progress in this field is due to the capability of fabricating parts with more complexity in the topology and diversity in the array of applicable materials. One such an example is structuring of cellular solids and composite materials appropriate for the vast range of applications.
Among different AM methods, powder-bed AM of heterogeneous structures has been utilized to make complex parts. However, the adoption of this process is hindered due to the inability to create closed internal channels that is an essential factor, defining the properties of solids. This inability is mainly related to the de-powdering issue associated with trapped powders inside closed voids. A novel combined approach has been used to attain the design criteria, by selectively encapsulating sacrificial materials within the designated layers during manufacturing and decomposing the substances in post-processing steps.
The main objective of this dissertation is to investigate two hypothesized methods for the functional printing of titanium cellular structures with graded pores arranged in a controlled manner. The products in this study were characterized through employing several methods including shrinkage measurements, porosity measurement, mechanical compression test, hardness test, scanning electron microscopy, micro and nano computerized tomography, and X-ray diffraction.
The employed methodologies have been classified into two approaches according to the shape of the secondary material embedded into the main substance: 1) crystalline solid particles and 2) liquid resin droplets. For the first approach, a micro-punching system was proposed as a means of positioning the sacrificial particles at designated locations in the AM process. The prototype of the unit was developed and its performance and quality of the products were evaluated. The performance analysis revealed the potential of deploying the method for selective encapsulation of particles from multiple materials and different sizes: (0.3±0.0) µm-(0.5±0.0) µm.
In the second approach, an application of a hybrid AM system integrating binder jetting and material extrusion is investigated to form the graded cellular solids. A proper polymeric sacrificial resin combined bisphenol-A ethoxylated diacrylate and cellulose acetate butyrate was developed to attain the green specimens’ quality and the dimensional accuracy in the morphology of the encapsulated pores. The statistical significance of employing this method was explored on the mechanical and porosity of the titanium structures. Compare to the standard samples (no polymer included), the porosity was increased between 6% to 16% depending on the polymer layout in the structures. The stiffness and yield stress measurements suggested a range of 2.48±0.37 GPa to 3.55±0.49 GPa and 107.65±18.14 MPa to 145.75±13.85 MPa, respectively. Moreover, the influence of layer thickness and binder saturation level while selectively varied throughout the structure was evaluated to accompany the proposed ideas.
Additionally, a novel hybrid AM approach was introduced for developing titanium matrix composites. A highly loaded titanium di-boride ceramic resin was developed and pressure-less sintering protocols were adapted. The quality of the composite with graded periodic reinforcement was determined. Evaluating the data suggested a higher possibility for the formation and growth of titanium boride whisker as the temperature elevated in the heat treatment step (1400°C). The stiffness of the samples was enhanced significantly by increasing the temperature and volume fraction of reinforcement elements. In particular, those samples sintered up to 1400°C displayed 6.4% to 15.2% improvement in the stiffness. The similar improvement trend in the density of the porous matrix was observed (4.5%-19%).
The pore morphology and properties of the Ti structures developed in this thesis are comparable with the size of trabecular separation and properties of the cancellous bones. However, the proposed studies can be deployed in other applications which required the production of lightweight structures.