The Influence of Microstructural Heterogeneity on Mechanical Properties of Friction Stir Welded Joints of T6-Treated Al-Zn-Mg Alloy 7A52

Abstract

A high strength Al-Zn-Mg alloy 7A52 with T6 treatment was successfully friction stir welded. The grain structure, dislocations and precipitates in typical regions of the weld joint, including the weld nugget zone (WNZ), thermos-mechanically affected zones (TMAZ) and heat affected zones (HAZ) were investigated to understand the mechanical properties of each zone and the weld joint. In WNZ, a relatively higher density of dislocations is observed on the advancing side, caused by vacancy collapse induced by severe plastic deformation during stirring. However, in the center and on the retreating side, the dislocation density is very low. The strength of the WNZ is influenced by grain refinement, solution strengthening, or natural ageing hardening. In TMAZs, different mechanical properties on each side are due to different grain structures and precipitates introduced by the asymmetrical thermo-mechanical cycle. In HAZs, the mechanical properties are a strong function of the ratio of η to η phase. Compared to the micro-tensile results, premature failure of the weld joint occurs in HAZs on the advancing side, resulting from stress concentration near the area with the lowest hardness.welding joints [4,[9][10][11][12][13]. These improvements are a result of reduced heat input during FSW compared with conventional fusion weld. The solid phase joining of FSW is achieved by introducing frictional heat, interface deformation and solid-state diffusion, which results in gradual local microstructure changes in the aluminum alloy [14,15]. Understanding the microstructural evolution during the thermomechanical process imposed by FSW is a very important step in understanding the weld's mechanical properties.A number of research papers have been published on the development of microstructures and mechanical properties [4,9,11,[15][16][17]. Concerning Al-Zn-Mg alloys, such studies in alloys 7075 [18][19][20][21], 7050 [10,12,22], 7449 [11], 7039 [4,23-27], 7010 [28] are presented. Results from these studies suggested six different microstructure zones: the nugget in the center of the weld (WNZ), thermo-mechanical affected zones (TMAZ) on each side of the nugget and under the shoulder contact zone, heat affected zones (HAZ) in between the TMAZ and the unaffected parent material. An asymmetrical microstructure produced by FSW has been revealed by hardness testing. This is due to differences in strain levels and thermal cycles on the advancing side and on the retreating side. In addition to the grain structure evolution, different precipitate distributions are induced within each zone by the severe thermo-mechanical condition, which results in higher deformation and temperature gradients to the passing tool.Although microstructural evolution has been reported, the direct link between microstructures and tensile properties has not been established. Previous works have noted more interesting observations on microstructures but not macroscopic tensile properties. The present paper focuses on the direct relationship between microstructures a...

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