EBSD Study of Microstructural and Textural Changes of Hot-Rolled Ti–6Al–4V Sheet After Annealing at 800 ℃

In this paper, electron backscatter diffraction and various other characterization and analysis techniques mclucilng X-raydiffraction, electron channeling contrast imaging and energy-dispersive spectrometry were jointly employed to investigatemicrostructural and textural changes of a hot-rolled Ti-6A1-4V （TC4） sheet after annealing at 800 ℃ for 5 h. In addition,the hardness variation induced by the annealing treatment is rationalized based on revealed microstructural and texturalcharacteristics. Results show that the TC4 sheet presents a typical dual-phase （α＋β） microstructure, with α-Ti as themajor phase and short-rod-shaped β-Ti （minority） uniformly distributed throughout the α matrix. Most of α grains cor-respond to the un-recrystallized structures with a typical rolling texture （c//TD and 〈11-20〉//ND） and dense low angleboundaries （LABs）. After the annealing, the stored energy in the as-received specimen is significantly reduced, along withgreatly decreased LABs density. Also, the annealing allows recrystallization and grain growth to occur, leading toweakening of the initial texture. Furthermore, the water quenching immediately after the annealing triggers martensitictransformation, which makes the high-temperature β phases be transformed into submicron α plates. The hardness of theannealed specimen is 320.5 HV, lower than that （367.0 HV） of the as-received specimen, which could be attributed toreduced LABs, grain growth and weakened texture. Nevertheless, the hardening effect from the fine martensitic platescould help to suppress a drastic hardness drop.

Variant selection in stationary shoulder friction stir welded Ti-6Al-4V alloy

Stationary shoulder friction stir welding of Ti-6Al-4V of 7 mm thickness was conducted with varying welding speeds and rotation speeds. Variant selection analysis was carried out based on the inherited α phase texture and the reconstructed 13 phase texture. The weld surfaces became significantly smoother with increasing welding speed and decreasing rotation speed. Heat input decreased greatly with increased welding speed and it decreased slightly with decreased rotation speed. The orientation relationship between the prior 13 grains was measured based on the reconstructed electron backscattered diffraction （EBSD） data. Weak Variant selection has occurred in all the welds because most of the prior 13 grains did not share { 110} poles. Strong links between crystal orientation of the prior 13 grains and hardness have been found.