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Title: Microstructure and crystallographic texture evolution in stationary shoulder friction stir welded Ti-6Al-4V
Author: Jiang, Xiaoqing
ISNI:       0000 0004 2723 7876
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2012
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Stationary shoulder friction stir welding with Ti-6Al-4V of 7mm thickness was undertaken by TWI Ltd and six welds with varying traverse speeds and rotation speeds were studied in terms of microhardness measurement, optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). Data were processed by Sigma Plot 10.0, HKL channel 5 and quantitative metallographic analysis including linear intercept method and point counting method. The weld zones were divided into four zones in this work, and they are the base material (BM) with equiaxed morphology, the heat affected zone (HAZ) with bimodal structure, thermo-mechanically affected zone (TMAZ) and the stir zone (SZ). Both the TMAZ and the SZ exhibit lamellar structure. Microhardness tests have been carried out on the cross sections, the normal section and the side sections for all the three welds with a constant rotation speed and on the cross sections for all the three welds with a constant traverse speed. Results showed that weld zones have slightly higher hardness than the BM; hardness at the advancing side (where tool rotation is as the same direction as the forward motion) is slightly harder than that at the retreating side (where tool rotation opposes the forward motion); an increased hardness was observed at the top surface regions of all the six welds and at the weld bottom of the two welds with 100 and 150mm/min traverse speed and microhardness distribution is most uniform in the four welds with a traverse speed of 150mm/min. Optical microscopy and scanning electron microscopy were used to investigate the cross sections on both horizontal and vertical lines revealing the microstructural evolution of the weld zones and the effect of traverse speed and rotation speed on the grain morphology of the bimodal structure and the lamellar structure, respectively. EBSD runs were collected from the cross sections of all the six welds for texture analysis with step sizes of 2μm and 3μm, and the texture analysis consists of the BM texture characterisation, the definition of TMAZ and HAZ, the representation of shear texture in the SZs, misorientation analysis of the welding edges of all the cross sections and two normal sections on the horizontal line. The BM was characterized by a classic rolling texture with equiaxed αp grains showing macrozones; the HAZ and the TMAZ both have developed a texture type containing three components: TD component, 45° component and WD component, using Ari-Gur and Semiatin's notation (1998); the SZ exhibited shear texture dominated by D1 (112)[111] simple shear texture component.
Supervisor: Wynne, Bradley Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available