August 2016 update: Weld development trials – TWI completes first full circumferential friction stir weld of titanium alloy propellant tank.
This is an update on an ongoing project – view Friction stir welded low-cost titanium propellant tank, pt. 1
As part of the friction stir welding (FSW) of titanium propellant tanks project, TWI Ltd has been developing weld parameters for the welding of titanium alloy propellant tank demonstrators.
The work commenced with weld trials on rolled titanium 6Al-4V flat plate to define the optimum process parameters. These welds underwent preliminary testing, allowing identification of the optimised process window for this application.
Following completion of initial trials TWI progressed to verify the welding parameters on cast flat-plate coupons. Welds were performed to confirm the process conditions were comparable from wrought plate to cast material. A comprehensive testing regime assessed the performance of FSW of cast titanium.
During the development of the flat plate welding trials, TWI established the required tools, fixtures and jigging to enable the welding of curved coupons representative of the final demonstrator.
TWI undertook the first curved weld trial, making a full circumferential weld to join two 420mm-diameter cylinders to form a barrel. Following successful completion of the weld, it was subjected to non-destructive testing (NDT) and mechanical testing to review its quality and properties.
A cylinder-to-hemisphere circumferential weld was made to complete the weld development work package. The weld was completed using the same welding parameters and was a full circumferential weld on the 420mm-diameter cylinder configuration.
As part of this General Support Technology Programme (GSTP) project, the residual stresses within the friction stir welded Ti-6Al-4V hemisphere-cylinder will be measured. An outline proposal was submitted to utilise the STFC ISIS pulsed spallation neutron source to assess the FSW.
The hemisphere-cylinder weld will undergo neutron radiography analysis utilising the IMAT for characterisation. The weld will then travel to ESTEC (Netherlands) for X-ray diffraction (XRD) measurements, enabling near-surface residual stresses to be determined.
The welded hemisphere-cylinder will then return to ESA Harwell (UK) for residual stress experiments on the ENGIN-X facility, to determine the residual stress across the friction stir weld using neutron diffraction.
Finally the welded hemisphere-cylinder will return to TWI Cambridge (UK) for the application of a hole-drilling residual stress measurement technique.
The results of the residual stress analysis undertaken by XRD, neutron diffraction and hole drilling will enable the team to compare the analytical techniques. It is also hoped that the results will form a useful comparison between friction stir welded propellant tanks and the current state-of-the-art techniques for propellant tank welding.
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