A feasibility study is currently underway to develop friction stir welding (FSW) technology for titanium- and silicon carbide-based MMCs known as TiSiC, which are potential materials for applications in the aerospace industry, due to their low density and good mechanical properties at elevated temperatures.
The project will develop, characterise and validate the FSW process and tooling system for welding TiSiC materials in flat plate coupons. This will provide the initial process conditions for a future programme to develop the FSW and joint design technology for 3D structures, such as a tubular xenon gas pressure vessel. The work that will be performed under this activity will be carried out in five technical tasks, as follows:
- Literature review, including trade-offs
- Test development plan
- Baseline materials characterisation
- Development of the FSW process
- Characterisation of FSW welds in TiSiC composite
The work, funded by ESA, is a collaboration between TWI and TISICS Ltd, a UK-based company specialising in the development and manufacture of silicon carbide fibre and fibre-reinforced titanium composites. TISICS has previously demonstrated that fusion welding close to the composite fibres is detrimental to the composite performance. A significant monolithic border/margin is therefore required to protect the TiSiC from thermal damage. Fusion welding also degrades the parent metal performance with a relatively wide heat-affected zone (HAZ).
FSW has a number of unique features which address the issues of fusion welding of TiSiC:
- Small HAZ - allowing narrower monolithic margins between the TiSiC composite and the jointline.
- Fine grain structure - which can retain close to parent mechanical strength in optimised scenarios.
- Low weld temperature (below melting point) - this is key to TiSiC welding as the fibre degrades with exposure to temperatures above 1000°C when in titanium matrix. At the 1700°C melting temperature of titanium alloys the fibre and its coatings will react and effectively dissolve into the titanium alloy. The damage will reduce as the temperature decreases with distance from the weld. Friction stir welding can be carried out at two-thirds the melting point and within the safe temperature zone for TiSiC material.
The current feasibility study will conduct welding research and characterisation on flat plate coupons. If FSW can be developed to produce high-performance joints incorporating TiSiC material with monolithic regions, future work is envisaged to develop and demonstrate procedures for thin-walled xenon tanks.
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