Friction stir welding (FSW) is a maturing welding technique using a rotating tool for simultaneous heating and stir deforming th~ material interface to form a solid bond. Significant tensile residual stresses (RS) and component distortion may be produced even with optimized FSW parameters. Recent stress engineering techniques such as global mechanical or roller tensioning may reduce tensile RS and distortion. This dissertation reports on the first systematic investigation into the efficiency of insitu global mechanical tensioning (IS GMT) as well as roller tensioning applied in-situ (ISRT) and post welding (PWRT) for mitigation of tensile RS and plate distortion in high strength aluminium alloy plates joined by FSW. The techniques were evaluated by measuring the distribution of RS across the weld-line by means of neutron and synchrotron X-ray diffraction as well as' the levels of plate distortion. In each case the weld microstructure and hardness distribution were characterised. The data were rationalised against the ISGMT load and roller tensioning down force respectively. The results have shown that ISGMT and PWRT significantly mitigate longitudinal tensile RS and component distortion. ISGMT was found to decrease the tensile RS by an amount approximately equal to that of the load applied. Consequently, a stress free weld is produced with an ISGMT load equal to the magnitude of the weld-line RS in the as-welded condition~ PWRT decreases the tensile RS as the rolling down force increases and significant compression may be introduced once a certain magnitude is exceeded. ISRT, at least for the range tested, was found to be less effective. The component distortion reduced along with RSÇ'ú mitigation. No effects on the microstructure or hardness distribution due to mechanical stress engineering were observed. Furthermore, it was demonstrated that in order to make accurate stress measurements by diffraction, the effect of precipitation on the stress free lattice spacing must be taken into account for age hardening alloys. In order to complete this study an automated robotic sample manipulation system was developed. Finally, the degree of stress relaxation occurring on cutting down large welds was evaluated by progressively shortening test welds and determining the RS for each length. The amount of stress relaxation for each weld follows the same behaviour and appears to depend on the width of the tensile weld zone only.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:505389 |
| Date | January 2009 |
| Creators | Altenkirch, Jens |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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