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Friction Stir Welded Low-Cost Titanium Propellant Tank, Pt. 1

An update to this project was published in August 2016 –  view Friction stir welded low-cost titanium propellant tank, pt. 2

In 2014 the European Space Agency (ESA) initiated a programme to investigate the capability of friction stir welding (FSW) as a manufacturing method for making titanium propellant tanks for its future space programmes.

TWI, in partnership with Airbus Defence and Space in Stevenage, developed a proposal to conduct a feasibility study of FSW for titanium alloys suitable for launch vehicle propellant tanks.

The two-year project started in December 2014 and comprises two phases of work, with the main aim to raise the technology readiness level of FSW of titanium alloys from TRL3 to TRL6 in conjunction with a potential reduction of costs against the current conventional approach for manufacturing propellant tanks.

Figure 1 Schematic of FSW process
Figure 1: Schematic of FSW process
  1. Develop, characterise and validate the FSW process and system required for welding titanium alloy(s)
  2. Manufacture two titanium demonstrator tanks using the FSW process. The manufacturing process will be compared with current conventional technical approaches in terms of cost, reliability, efficiency and environmental impact.

The current production method for propellant tank components comprises a Ti-6Al-4 forging of 15-30mm thickness which is machined down to 1–2mm thickness ready for EB or TIG welding. The long procurement time for the forging (up to one year) coupled with the machining and welding makes propellant tanks one of the most expensive items on the spacecraft.

Figure 2 Propellant tank example
Figure 2: Propellant tank example

Current fabrication methods for propellant tanks have several drawbacks:

  • High-cost forging
  • Long delivery time for forging
  • High-cost machining process
  • High buy-to-fly ratio
  • Fusion-based welding process limitations

TWI and Airbus have worked together to undertake an extensive review of materials and manufacturing routes, including the following:

  • Ti-6Al-4V – forged, machined and friction stir welded
  • Ti-6Al-4V – spin formed, machined and friction stir welded
  • Ti-6Al-4V – cast, machined and friction stir welded
  • Ti-15V-3Cr-3Al-3Sn – spin formed, machined and friction stir welded

Following the conclusion of this review the team has subsequently procured forged and cast material for preliminary trials.

A representative demonstrator tank design has been developed and agreed with ESA and Airbus.

Figure 3 Exploded view of the titanium alloy tank
Figure 3: Exploded view of the titanium alloy tank

Welding trials using TWI’s recent developments in stationary shoulder friction stir welding (SSFSW) tools have demonstrated excellent results so far, producing high-quality welds and, importantly, demonstrating a meaningful extension in the lifetime of the tool.

Figure 4a The surface of a Ti-6Al-4V weld (left) and macrostructure of a weld (right) made with SSFSW tool
Figures 4a and 4b: The surface of a Ti-6Al-4V weld (top) and macrostructure of a weld (bottom) made with SSFSW tool
Figures 4a and 4b: The surface of a Ti-6Al-4V weld (top) and macrostructure of a weld (bottom) made with SSFSW tool

TWI plans to continue development of the welding parameters supported by a comprehensive testing regime. The project will use non-destructive and destructive testing techniques to allow verification of the weld and material quality.

Design and manufacture of the jigging and fixturing is ongoing. TWI expects to undertake production of demonstrator propellant tanks in the latter part of 2016.

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