Khademzadeh, Saeed (2019) Assessment and Development of Laser-Based Additive Manufacturing Technologies For Metal Microfabrication. [Ph.D. thesis]
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Abstract (italian or english)
Nowadays many devices are produced in very small sizes or containing small features for particular application such as biomedical and microfluidic devices. Based on this demand, manufacturing processes should be developed for implementation of micro features in different ranges of sizes. A broad range of microfabrication technologies have been developed which have different applications and capabilities such as laser ablation, plating, photolithography, lithography and electroplating. However, such techniques are restricted when utilized to new microproducts which need the employment of a diversity of materials and have complicated three-dimensional geometries. Additive manufacturing (AM) needs each layer to be fabricated according to an exact geometry defined by a 3D model. This concept seems suitable for production of complicated parts with micro features. Development of robust metal additive manufacturing for microfabrication opens a new window toward miniaturization of metallic parts such as design and production of porous implants containing micro features and micro pores (50-500 µm). This work covers the development of micro additive manufacturing through two laser based AM processes with two different concepts: Micro direct metal deposition (µDMD) and selective laser melting (SLM). Nowadays, NiTi shape memory alloys are among the most interesting materials in the field of bioengineering and medical applications. Assessment of both techniques for production of NiTi porous scaffolds for biomedical application was carried out in this thesis. Long-term fixation of biomedical implants is achievable by using porous materials. These kinds of materials can develop a stable bone-implant interface. A critical aspect in production of porous implants is the design of macro and micro pores.
At the first step of this thesis, the process parameters of both technologies were optimized to obtain full density samples. Secondly, porous scaffold structures with geometry controlled porosity were designed and manufactured using both technologies. Investigations using X-ray diffraction and scanning electron microscopy equipped with energy dispersive spectroscopy showed that B2-NiTi phase with small quantity of unwanted intermetallics can be obtained by micro direct metal deposition of mechanically alloyed Ni50.8Ti49.2 powder. Micro direct metal deposition was optimized through a set of process parameters and designed experiments to improve the geometrical accuracy and repeatability of micro fabrication. Micro X-ray computed tomography were used to analyze the surface topography, micro porosity, and deviations of products with respect to nominal geometrical models. Below 10% deviation to nominal geometrical models was achieved in hollow NiTi samples through a set of micro direct metal deposition process parameters and designed experiments.
A comprehensive study was conducted on Ni50.8 Ti49.2 (at%) alloy to discover the influence of SLM process parameters on different aspects of physical and mechanical properties of NiTi parts. The provided knowledge allowed choosing different optimized parameters for production of complicated geometry with micro features maintaining the phase composition through the sample. For the first time and in this thesis, without going through any solid solution and heat treatments, single phase austenite was obtained in SLM NiTi parts with the selection of three different regimes of process parameters. This knowledge led to manufacture of NiTi bony structure applying different process parameters for the border and internal parts. The experimental results showed that SLM process with specific process parameters is a feasible micro additive manufacturing method to implement the complicated internal architecture of bone. It is an important issue in production of customized prostheses.
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