Influence Of PWA On Microstructure And Properties Of Friction Stir Welded 6061-T6 Aluminum Alloy

Lightweight is a critical factor that decides the whole performance of high-speed railway. Although aluminum rolling stock produces effective lightweight, the strength of the welding joint will considerably reduce due to fusion welding. This will seriously influences the safety of high-speed railway. The friction stir welding is characterized with the excellent properties of the welding joint and low welding stress and distortion, and has been applied in the aluminum alloy rolling stock of high-speed railway. Aiming to the fabricating of the aluminum alloy rolling stock, in this paper the microstructure and properties of friction stir welded 6061-T6 aluminum alloy was investigated, and the rule and mechanism of the post welding aging on the welding joint was discussed so as to provide the theory and experiment basement for the application of friction stir welding in the high-speed train.With the different heating temperature and mechanical stir on the welding joint of FSWed 6061-T6 aluminum alloy, the various base microstructure and Mg₂Si were obtained. The welding nugget happened to dynamically recrystallize and the refined crystal grains were obtained and the majority of Mg₂Si particles dissoluted due to the intensive mechanical stir and higher high temperature. In the thermal-mechanical affect zone that was heated under the recrystallized temperature, the crystal grains were elongated due to the mechanical stir and Mg₂Si particles partly dissoluted and the others grew. In the heat affect heat without the mechanical stir, to some extent, both of the crystal grains and the majority of Mg₂Si particles grew.Post welding aging prompted that Mg₂Si precipitated again. However, with the different aging temperature and time, and with the different supersaturation in the welding nugget, TMAZ and HAZ, the aging proceed at the different extent and rate and as a result the different aging effectiveness appeared at the various zone of the welding joint. The aging increased the strength and hardness, but decreased the plastics. Because the base microstructure deformed and Mg₂Si grew irreversibly, the natural and artificial aged joint strength respectively reached to 75% and 88% of parent metal. The tensile specimens all fractured ductile at the retreated side of the heat effect zone with the lowest microhardness.The various zones of the welding joint showed the difference in the corrosive resistance. The ranking of the corrosive resistance was welding nugget, parent metal, and heat affect zone. The corrosion mechanism was pin corrosion and the influencing factor was mainly heterogeneous in the microstructure. The artificial aging prompted Mg₂Si precipitated and grew, consequently the corrosion resistance of the welding joint decreased. On the other hand, the difference of the various zones in the corrosive resistance also reduced as a result of the reduced heterogeneous microstructure.