| dc.description.abstract | Binder jetting (BJ) is a branch of additive manufacturing (AM) technology that uses progressive addition of powders and adhesive binders to build parts with complex geometries. Compared to the other AM methods, the BJ provides a cost-effective process with a faster printing speed, high design of freedom, and negligible residual stresses generated during the printing process. This suggests the BJ as one of the most applicable technology for many industries including automotive and aerospace in fabricating functional parts with geometries such as lattice structures or internal cooling channels. The printed parts are treated with a series of thermal processes that are required to remove the excess binders and consolidate the powders. The most important factor for the BJ is the binder selection and post-process parameter determination. A failure to understand these process parameters may result in generating a porous part. Despite the possibility of high porosity, the BJ remains attractive to industries due to the many benefits it can provide with a heat-free printing process. Hence, the BJ technology was selected as a fabrication method for this study.
Aluminum is a ductile, corrosion-resistive, light-weight material with a high electrical conductivity which are being used in many industries including the automotive and aerospace. However, the surface oxide layers surrounding the aluminum powder provided a challenge for powder metallurgy industry in promoting the sintering. These oxide layers have very high thermal stability which cannot be disturbed below the melting temperature of the aluminum powder. Therefore, the sintering mechanisms are needed to disrupt the passivating layers to allow the particle diffusions. In this thesis, the conventional powder metallurgical approaches for sintering aluminum powder were addressed. These methods often involve the modification of the alloy compositions that are typically avoided by many industries due to the required resources and time. Therefore, the focus of this work was to introduce an alternative sintering agent that can interrupt the oxide films and facilitate the consolidation of aluminum powder without a need for alloy modification. In this study, the AlSi10Mg was used as a base material and the melting properties were obtained by performing the differential scanning calorimetry (DSC) technique. The first part of the research studied the polymeric binders to identify their compatibility with the aluminum powder. The selected polymers in this research were polyvinylpyrrolidone (PVP), poly (vinyl) alcohol (PVA), polyacrylic acid (PAA), polyacrylonitrile (PAN), and polyvinylidene fluoride (PVDF). Thermogravimetric analysis (TGA) was conducted to ensure that the polymer degradation temperature is below the melting point of the AlSi10Mg. Moreover, the elemental analyses were completed for the samples fabricated with the polymeric binders to investigate possible residues left after the de-binding cycle. Based on these characterization results, the most compatible polymer for the AlSi10Mg was determined. The introduction of different sintering agents was then explored to investigate the possibility of the surface oxide layer disruption. Metallic and oxide nanoparticles, dispersed in the selected polymeric binder and metal-organic decomposition (MOD) inks, were suggested as candidates for this research. The effect of these sintering agents was studied by investigating the sintered microstructure and compositions of the test specimens using optical and scanning electron microscopy (SEM).
The elemental analyses of the de-bound test specimens were considered to determine the polymeric binder with the lowest residue contents. Polyacrylic acid (PAA) displayed the best result, where no contribution to powder composition was observed. Hence, the PAA was selected and used as a base polymeric solution to integrate the nanoparticle into the process. Fortunately, the metal-organic decomposition inks were able to fabricate the part without the help of the binding agent.
In the porosity analysis, the polymeric binders did not outperform in achieving a high-density part. The maximum density attained with the binding agent was 95.1% with the PVP solution. Conversely, all sintering agents except for the silver MOD ink achieved a density as high as 99.9%. The sintered microstructure was studied to better understand the sintering mechanisms. The SEM equipped with the energy-dispersive X-ray spectroscopy (EDX) was used to obtain high magnification micrographs and elemental compositions at the grain boundaries. The specimen constructed with the PAA was selected as a reference material, and other sintering agents having low porosities were examined in this analysis.
Only partially disrupted oxide films were observed in the test specimens fabricated with PAA and copper oxide nanoparticles, resulting in incomplete sintering. The specimen with the aluminum oxide exhibited unusual behavior, where it promoted the sintering of the oxide films together with the neighboring oxide layer. The bonded oxide films obstructed the aluminum particles from forming sinter bonds with each other. This resulted in the formation of a brittle structure susceptible to mechanical failure. In contrast, the specimens built with the copper nanoparticle and copper MOD ink demonstrated promising results. The intergranular oxide films were reconstructed with particle agglomerates composed of copper, magnesium, titanium, aluminum, and oxygen. A completely disrupted oxide film allowed the aluminum particles to diffuse through the neighboring particles and facilitated sintering. It was concluded that the copper nanoparticle and copper MOD ink were the most effective sintering agents for the aluminum alloys considered in this study.
In future work, mechanical characterization is recommended to further investigate the influence of the sintering agents. In addition, more intensive study to develop the binder suitable for the binder jetting machine is also suggested. This study may involve performing a viscosity test and adding other elements to customize the flowability of the liquid binder. Furthermore, an in-depth study can be performed to integrate the newly introduced sintering agent into the binder jetting machine. | en |
