In this work, a structural response of 6061-T6 aluminum alloy to friction-stir welding (FSW) was studied in wide range of processing conditions in order to establish a better foundation for the microstructure-strength relationship in produced welds. In contrast to the widely-accepted conception of the FSW-induced precipitation behavior, the welding temperature was found to be often below the dissolution threshold and thus no particle dissolution took place. Moreover, the peak temperature never exceeded 500 � C, and therefore the particles never dissolved completely. Considering a relatively low cooling rate measured in the stir zone, the dissolved precipitates were suggested to partially re-precipitate as solute clusters during weld cooling cycle thus imparting a substantial hardening effect. Accordingly, the precipitation coarsening was deduced to be the major softening mechanism. Due to the extreme sensitivity of this process to a duration of the weld thermal cycle, the welding speed was surmised to be the key factor controlling weld strength.

No abstract

No abstract

No abstract

The influence of cold biaxial forging and annealing on the microstructure evolution and mechanical properties of Fe-18Mn-0.6C-0.1N TWIP steel was investigated. The microstructure after thermomechanical treatment was examined by means of scanning electron microscopy (SEM) with electron back-scatter diffraction (EBSD) analyzer and transmission electron microscopy (TEM). The microstructure containing a high density of deformation twins was evolved by cold forging. Then, the deformation microstructure was recovered or partially recrystallized during subsequent annealing for 1 h at 673 or 873 K, respectively. The cold biaxial forging followed by partial recrystallization resulted in a very attractive combination of mechanical properties. The yield strength and the ultimate tensile strength of the steel after cold biaxial forging were 1320 and 1540 MPa, respectively, with elongation of 6 %. On the other hand, the yield and ultimate tensile strengths of 770 and 1110 MPa, respectively, and elongation about 40 % were obtained after partial recrystallization. The as-forged and recovery annealed samples showed fatigue limit of 600 and 550 MPa, respectively, and the sample after partial recrystallization showed slightly lower fatigue limit of 500 MPa.K e y w o r d s : TWIP-steel, deformation twinning, microstructure, mechanical properties, fatigue