Effect of Transient Thermal Tensioning (TTT) on Fatigue Crack Growth Rate of Friction Stir Welded T-Joints in Aluminium Alloy 6061-T6 Welded Structures
Abstract
T-weld joints are extensively used in the fabrication of structural members in railway car body, automotive, ship building and other industries. Under dynamic loads, T-welded joints are usually suffered from fatigue failure which initiates at weld toes due to high stress concentration and tensile residual stress. This study describes an application of transient thermal tensioning (TTT) to enhance fatigue performance of T-weld joints fabricated using friction stir welding (FSW). The FSW process was carried out using tool rotation and tool travelling speed of 1450 rpm and 0.2 mm/s respectively. Subsequently, the T-weld joints were subjected to TTT process with various heater positions, i. e., ahead of and behind the tool. The fatigue crack growth rate (FCGR) test was carried out using a constant amplitude fatigue experiment with stress ratio (R) and frequency of 0.1 and 11 Hz respectively whereas specimens used were centre-cracked tension (CCT) type with the initial cracks located at the weld toes. Results of this investigation show that at lower AK, typically below 9 MPa.m05, the TTT process with heaters ahead of the tool significantly reduces FCGR of the T-weld joints. This beneficial effect could be associated with thermal stress generated by locally moving heat sources which give stretching effect and oppose weld tensile residual stress.
Keywords: friction stir welding , residual stress, transient thermal tensioning, fatigue crack growth rate
