Development of the Conform™ process for the recycling of waste titanium into wire

• Main Author: Smythe, Sarah Anne
• Format: Dissertation
• Language: English
• Published: University of Sheffield 2021

The Conform™ process was first established in the early 1970's for the recycling of copper and aluminium rod. Since then, the process has been developed further with the expansion into powder feedstocks and other materials such as titanium, magnesium and zinc. However, there are still significant gaps in the understanding of Conform™ - particularly for high strength feedstocks such as titanium alloys. This work sets out to develop the Conform™ process for titanium particulate feedstocks including machining swarf, in order to provide scientific insight into the challenges and process parameter requirements for the next stage of industrialisation. Through the use of discrete element modelling and experimental trials, bespoke toolsets were designed for use in the Conform™ machine and their effectiveness discussed. In this work it was shown that different powder feedstocks require different toolset geometries in order to increase the chance of extruding a consolidated product. This was further influenced by a smallscale testing rig, which assessed the densification and consolidation of powder feedstocks used in the Conform™ process and helped to determine the 'Conform-ability'. This testing rig, known as the Arbitrary Strain Path (ASP) test machine, used bespoke tooling to replicate similar conditions to those found in the Conform™ machine, in that the feedstocks undergo both load and shear during processing. Large scale trials methods were completed using a Conform™ 350i at BWE Ltd., Ashford, UK. The trials were conducted using bespoke tooling designed for this project and resulted in, in a number of cases, the successful extrusion of a titanium rod product. The experimental data from each trial was recorded and analysed to attempt to show further insight into the Conform™ process and demonstrate how this can influence the parameters used during a trial. The results showed that the spherical particles require more pressure to consolidate, and when coupled with the results from the simulations and experimental trials, this has shown that the smaller Conform™ toolset is more appropriate. By using the ASP and DEM simulations, there is a significant reduction in the costs of experimental testing, as the large Conform™ toolsets can cost upwards of £10000, whereas the small-scale testing and computational modelling fall in the £100s. Large scale extrusion trials were conducted on Ti-6Al-4V swarf and Ti-6Al-4V GA. The microstructure of the products often demonstrated an equiaxed fine-grained structure with grain sizes < 20 µm and limited porosity. This was reflected in the mechanical properties, which, in the case of the extruded CP Ti HDH Gr 2 feedstock, were closer to CP Ti HDH Gr 4. These results would meet requirements for additive manufacturing feedstocks and therefore the wire holds the potential of being used for downstream welding and wire-based additive manufacturing processes. This in turn highlights a potential recycling stream for aerospace waste. Overall, the thesis provides a novel, small-scale testing method for determining whether a feedstock would be suitable for Conform™ and the conditions under which it should be processed, resulting in significant cost savings. The work also develops understanding of the processing occurring during Conform™ with an insight into the trial data and how this can be analysed to provide details about the optimum extrusion parameters. Finally, the project, for the first time, reports the extrusion of waste titanium feedstocks, including swarf, into a potential product for wire-based additive manufacturing.