Fatigue Properties of Ti-6Al-4V Titanium Alloy Friction Stir Welding Joint
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JMEPEG https://doi.org/10.1007/s11665-018-3490-8
Fatigue Properties of Ti-6Al-4V Titanium Alloy Friction Stir Welding Joint Zhenlei Liu, Yue Wang, Shude Ji, and Dejun Yan (Submitted October 8, 2017; in revised form June 6, 2018) Friction stir welding was used to consolidate 2-mm-thick Ti-6Al-4V titanium alloy sheets, and the static and fatigue properties of the joint were investigated. The results reveal that uniform microhardness distribution of the nugget zone (NZ) after post-weld heat treatment was associated with the uniform equiaxed microstructure of the NZ. The joint fatigue limit predicted by the S–N curve reached 640.84 MPa, up to 64.76% of joint tensile strength. Fatigue specimens fractured at the transition zone between the base material and NZ, and the positions of fatigue crack initiation sites were relatively random. The fracture surface of the fatigue crack propagation region was composed of radial strips and striations, and numerous dimples were observed on the final fast fracture surface. Keywords
equiaxed microstructure, fatigue limit, fracture surface morphology, friction stir welding, Ti-6Al-4V titanium alloy
1. Introduction Titanium and titanium alloys are widely applied to many industries of aviation, aerospace, automobile and shipbuilding due to their great specific strength, excellent specific stiffness and good corrosion resistance (Ref 1, 2). Ti-6Al-4V alloy, in particular, is a general-purpose a-b alloy and accounts for about 60% of all productions of titanium alloys (Ref 3). Joining methods of Ti-6Al-4V alloys have become a research hotspot at home and abroad. Friction stir welding (FSW), a solid-state welding technology invented by The Welding Institute in 1991, can avoid hot crack, big deformation and coarse microstructure produced by fusion welding technologies (Ref 4, 5). The FSW joint of Ti-6Al-4V alloy has attracted considerable attention of researchers. Related investigations reveal that the fatigue fracture of welding structures accounts for about 80% of all failure modes. Hence, it is of vital importance to investigate the fatigue property of the Ti-6Al-4V-alloy FSW joint. The fatigue properties of titanium alloys vary with different microstructures of the materials (Ref 6-10). The inhomogeneity of microstructures and incompatibility of deformation between a and b phases of Ti-6Al-4V alloy can lead to the initiation of internal fatigue cracks (Ref 8). The preferred fatigue crack initiation site is always within the equiaxed a grains for the titanium alloys with bimodal or fully equiaxed microstructures (Ref 9), and the finer and more uniform grains are beneficial to improving the very high-cycle fatigue properties of Ti-6Al-4V alloy (Ref 10). Zhenlei Liu, Yue Wang, and Shude Ji, Faculty of Aerospace Engineering, Shenyang Aerospace University, Shenyang 110136, PeopleÕs Republic of China; and Dejun Yan, Guangdong Key Laboratory of Enterprise Advanced Welding Technology for Ships, CSSC Huangpu Wenchong Shipbuilding Company Limited, Guangzhou 510715, PeopleÕs Republic of China. Contact
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