Correlations Between Microstructures And Performances Of Friction Stir Welding/processing Alloys With High Melted Point

Friction stir welding(FSW), an advanced solid-state joining technology, has been widely applied to weld those alloys with low melted point, such as aluminum and magnesium alloys. Recently, the requirements for the joining of various advanced alloys with high melted point aroused the researches on FSW of these alloys. In this thesis, the same and dissimilar joining of nitrogen strengthening high manganese austenitic steel,high nitrogen high manganese steel and Zr-Ti alloys were successfully achieved by using of FSW technology. Furthermore, the microstructures and the mechanical performances at room and cryogenic temperature were systemically investigated for nitrogen strengthening high manganese austenitic steel. Based on the theory of strength match, the effects of microstructural distribution on the tensile property of the same and dissimilar FSW joints were discussed, and the technique methods to improve the mechanical performance of the joints were proposed. In addition, the correlations between microstructure and mechanical performance in the friction stir processed(FSP) zones were also discussed for the unique deformed feature in term of high strain rate and large strain during FSP.The investigations on FSW of nitrogen strengthening high manganese steel showed that the grains of the nugget zone(NZ) in the as-welded joints were significantly refined.Consequently, the inevitable differences in the microstructural distribution resulted in the feature of high strength match along the transversal direction of the as-welded joints. The characteristics of mechanical performances exhibited higher hardness in the NZ, higher strength and lower elongation of the joint. After water quenching treatment, the above differences in microstructure and hardness of as-welded joints were basically removed.Moreover, the mechanical performances at room temperature were improved to the level of base metals(BM). However, the problems of grain abnormal growth and cryogenic intergranular fracture in the joints with water quenching treatment were found. The preliminary analysis reveals that these problems may relate to the non-equilibrium segregation of the solute atoms at grain boundaries. Therefore, the post-welded annealing treatment at lower temperature was employed. As a result, the cryogenic intergranular fracture was effectively restricted, and the mechanical performances were recovered to the level of the BM.The investigations on the friction stir processed microstructures of nitrogen strengthening high manganese and high nitrogen high manganese austenitic steel revealed that the high proportion of low angle grain boundaries resulted in the derivations from linear relationship between strength / hardness and average grain size. Thus, the statistic method of grain orientation spreading was introduced to stress on the contribution of the low angle grain boundaries for the improvement of the mechanical performances.Meanwhile, the effective grain size was defined and used to modify the Holl-Petch equation. The modified Holl-Petch equation can be used to describe the relationship between mechanical performances and the microstructures with high proportion of low angle grain boundaries resulted from server plastic deformation.The investigations on the dissimilar FSW joints of high nitrogen high manganese and304 stainless austenitic steels showed that the satisfied dissimilar joints were achieved by using FSW. The microstructure in all zones of the joints was composed of mono austenite.The order of hardness distribution from low to high value is, BM of 304 steel < BM of high nitrogen high manganese steel < weld metal near to 304 steel < weld metal near to high nitrogen high manganese steel. The yield and ultimate strength of the dissimilar joint were higher than that of 304 austenitic steel, and the elongation was relatively lower.The investigations on the FSW joints of Zr-Ti alloy showed that FSW had been used for the effective joining of Zr-Ti alloy. Moreover, the microstructure of the NZ was significantly refined by the continuous dynamic recrystallization. The microstructure of BM still keeps(α+β) dual phase after welding, but the microstructures of the NZ,thermal-mechanical affected zone and heat affected zone were transformed into mono βphase. The hardness of the as-welded joints was controlled only by the phase composition not the grain size. The hardness of the zones with mono β phase was lower than that of the zone with(α+β) dual phase. By employing the post-welded annealing treatment at temperature in the(α+β) phase range, the hardness distribution and tensile performance of the as-welded joints were all recovered to the level of the BM with(α+β) dual phase.