Titanium is an outstanding material, due to its unique set of properties, which is sought after in almost every industry. However, with great properties comes a great price, which for titanium is the ironic statement as it is the 9th most abundant element in the Earth’s crust. The price, instead, comes from the extraction and fabrication conventionally adopted by the industry.
The extraction of titanium is currently done via the Kroll Process, which was established in 1940 and has almost reached saturation in its development and optimisation. Due to this, many novel extraction techniques have emerged, such as the FFC-Cambridge Process; an electrolytic reduction process conducted in molten salt. This process has been shown to reduce the cost associated with the production of titanium and has been intensively investigated since its development.
As previously stated, the fabrication of titanium is another problematic area due to the low heat dissipation, low Young's modulus and high reactivity of titanium. Therefore, titanium is required to be treated in an inert atmosphere and when shaped the additional coolant needs to be supplied, which still does not remove the increased wear of milling tools that is common to titanium forming.
These obstacles were reported to be overcome by utilising the unique solid-state transformation seen in the FFC-Cambridge Process, which results in the near-net-shaped reduction of the precursor into a metallic product. Previous studies covered the use of slip casting to shape the precursor, which reduces the versatility of the process due to the low variety of possible products. In this project for the first time, the combination of the FFC-Cambridge Process with the ceramic 3D Printing was decided to be investigated and optimised, creating a new additive manufacture technique named the Near-net-shape Electrochemical Metallisation (NEM) Process.
The first step of the NEM Process was Direct Ink Writing (DIW), which allowed the viscous ink to be extruded in layers that built up the 3D structure. Optimisation of DIW for titanium dioxide printing was conducted, along with some of the modification to the mechanics of the process. The initial study on the formulation of the titanium dioxide ink was facilitated by the extrudability and rheological tests. These tests indicated that among the wide variety of inks, the one containing Polyethylene Glycol (PEG) was performing the best on a small scale. When the size of the products was increased, the inconsistency of flow became more distinct. This change was found to be caused by a complex interaction within the components of the ink at higher shear rates, which was overcome by the addition of surface lubricant. The high-quality products were achieved from DIW with the titanium dioxide ink that contained the following components in the weight ratio: 1 of TiO2, 0.9 of 10% aqueous solution of PEG and 0.1 of mixed oil.
The next step of the NEM process focuses on the electrochemical metallisation of these titanium dioxide components via the constant voltage electrolysis in 900 oC CaCl2, which resulted in a noticeable deformation of the titanium product. It is important to highlight that this phenomenon has not previously been reported before and, thus, required an in-depth investigation. Following results showed the dependencies between salt temperature, pre-sintering and design with the deformation. It was noted that the temperature of the salt during the electrolysis affected the sintering of the produced titanium parts, causing products to shrink by up to 40% and dramatically deform. This, however, was found to be mitigated to some degree by the sintering of the precursors for 1 hour at 1100 oC. The resulted product had a suitable density and porosity to be successfully reduced to metallic titanium with compressive strength of 111.4 MPa and Young’s modulus of 1.39 GPa that was reported to be similar to the conventionally produced titanium foams with similar porosity (around 50%). In addition to that, the oxygen content was recorded to be as low as 1000 PPM, another sign of the successful reduction to metallic titanium, but it was found to be depended on the geometry of the product. Moreover, some attention was given to deformations that were caused by the design flaws, which unfortunately were not possible to predict and could be eliminated only by the iteration in the design.
Finally, a lot of work was done to evaluate the feasibility of the NEM Process in both cost and environmental aspects. Conducted cost evaluation demonstrated an incredible cost reduction compared to conventional additive manufacture technique, measuring a cost reduction of up to 4 times. In addition to that, the cost breakdown was found to be heavily dependent on the labour cost, which reached 56% of the cost of one tooth implant.
The environmental impact of the NEM Process was assessed on the gate-to-gate principle using the Life Cycle Assessment (LCA) following the established international standard. This evaluation was done in two parts, first was the investigation of the main contributors towards the environmental impact, pointing out two major contributors that were argon consumption and electrode materials. Due to this a few possible optimisations to the process were proposed such as ``smart'' argon consumption and the need for reusable electrode materials. Implementing both showed a possible reduction of the overall environmental impact of 10-15%.
The second part of the LCA was comparative analysis where the NEM Process was compared against the commercially used Kroll - Electron Beam Melting (EBM) process. This showed that the overall impact of the NEM Process was lower than the Kroll - EBM Process by around 67%. Additionally, acquired data were also applied to the legislations that mainly monitor NOx, SO2, PM2.5 and CO2, the emission of which was found to be more than 70% lower for the NEM Process.
This project resulted in the development of a feasible and green manufacturing route of titanium components in a safe and relatively fast way. Along with that numerous new ideas and discoveries were made that deserve further research and have the potential to improve our quality of life.
|Date of Award||8 Nov 2020|
- Univerisity of Nottingham
|Supervisor||Di Hu (Supervisor) & George Chen (Supervisor)|
- 3D Printing
- Direct Ink Writing
- Molten salts
- Titanium Dioxide
- Life Cycle Assessment
- the FFC-Cambridge Process
- Additive Manufacturing