Direct Metal Laser Sintering of Titanium implant with Tailored structure and Mechanical Properties
Abd Aziz, I. (2014). Direct Metal Laser Sintering of Titanium implant with Tailored structure and Mechanical Properties (Thesis, Doctor of Philosophy (PhD)). University of Waikato, Hamilton, New Zealand. Retrieved from http://hdl.handle.net/10289/8855
Permanent Research Commons link: http://hdl.handle.net/10289/8855
Direct Metal Laser Sintering has attracted much attention over the last decade for producing complex parts additively based on digital models. The capability and reliability of this process has stimulated new design concepts and has widened the manufacturing perspective of product customisation. This research work is designed to gain a deep understanding of laser sintering processing parameters, the corresponding microstructures and the mechanical properties. The main purpose is to have a body of fundamental knowledge about the laser and titanium powder material interactions, thus establishing the factors that influence the process-structure-properties relationships of the Direct Metal Laser Sintering process. Finally, a route for manufacturing customised craniofacial implants was described. This is to evaluate the DMLS processing capabilities in medical areas, particularly those parts having porous and lattice design structures. The interaction between a laser beam and the powder bed creates a distinctive structure; a ball shaped (blob) consists of solid and porous regions. All the blobs have the same shape and morphology which may well suggest that there is a tendency for the powder particles to form a spherical droplet prior to a movingless laser beam. Surrounding the melted core is a sintered region of partially melted powder particles where the powder particles were fused together to form inter-particle necks. There is a linear relation between size, weight and density of a blob and the laser power. The surface temperature obtained exceeds the melting and vaporization temperature of the titanium and this creates a hole on the top part of a blob as a result of a massive temperature rise. Laser power of 140W gives a consistent structure and hardness in a blob. Metallographic analyses of a blob’s cross-section show an α+β structure with prior-beta grains. The morphology of the lamellar structure consisted of acicular needles with a basket-weave pattern. The pores were characterised as having flat and spherical features with the size ranging from 2µm to 6µm. The micro-porosity observed may be associated with shrinkage which occurs during solidification or with the presence of entrapped gases from the atmosphere or argon gas from the shrouding environment Laser power and scan speed are the two most crucial factor controlling the laser-powder interactions. Result shows that laser power is capable of widening the processing parameters particularly the scan speed. Increased laser power causes more powder to melt thus creating a bigger melt pool. Contrary to this, increasing the scan speed reduces the interaction time thus a smaller amount of powder melts. The right combination of these two parameters results in inducing an appropriate exposure time where continuously scanned tracks can be formed. Most of the parts were successfully built using a specific volume energy density of 50Jmm⁻³, which was considered to be the optimum processing parameter for this research work. The ideal laser-material interaction time was calculated at 0.0008secs. The microstructural analysis revealed a fully lamellar structure with acicular morphology. XRD analysis confirmed the presence of α’ martensite, which explains the thermal history of a high isothermal condition and rapid cooling. The cross section of a solid part exhibited an acicular, needle-like structure with a herring bone pattern, parallel to building direction, due to directional solidification. The microstructure had a high tensile strength but with low ductility. It is also worth mentioning that a slight change in scan speed, with the intention of providing more energy density to the powder, may cause instability in the melt pool and cause deterioration in the mechanical properties. It is therefore confirmed that there is an upper limit and allowable processing window where a good balance of tensile strength and hardness in a DMLS part is achievable. A framework prior to an implant’s fabrication was established and the associated design and manufacturing software are reported. The processing route required software like MIMICS and MAGICS to manipulate the medical images and design data and equitable skills must be acquired to handle the machine in order to successfully fabricate the desired parts. Employing MAGICS new lattice function proved to be more efficient, saving time compared to a manual procedure, especially when dealing with large medical data manipulation. In conclusion, the proposed method from this study is capable of producing a customised part with the highest degree of design complexity compared with other conventional manufacturing methods. This has proved to be very suitable for manufacturing titanium medical implants, particularly craniofacial implants which require a customised and lightweight structure and at the same time still provide good mechanical properties
University of Waikato
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