Investigation on microstructure evolution of Mg-Gd-Zn-Zr alloys during friction stir processing by liquid CO2 cooling assisted stop action

Abstract

The microstructure evolution of Mg-14Gd-2.5Zn-0.5Zr (wt.%) alloys during friction stir processing (FSP) were investigated through liquid CO2 cooling assisting stop action technology. In particular, solutionizing at 480 °C (LT4) and 520 °C (HT4) was carried out to clarify the evolution of different second phases. The results showed that grain size decreased from ∼20 μm to ∼1–3 μm in the stir zone, and a weak texture intensity could be found after FSP. Although the type of the second phases in as-cast, LT4 and HT4 samples did not change during FSP, different evolution sequences about the second phase and grain refinement could be found. Bulk X phase was broken into finer one later than recrystallization and refinement of α-Mg grain in LT4 alloy, while β-phase fragmentation and formation of recrystallized grains occurred simultaneously in HT4 alloy. In addition, the intragranular 14H-LPSO structure retained in three samples after FSP. Due to the different types and volume fractions of the second phases, the as-cast and LT4 alloy were mainly refined by continuous dynamic recrystallization (CDRX), while both CDRX and geometric dynamic recrystallization (GDRX) occurred in HT4 alloy. After FSP, the yield strength increased by 74%, 110% and 90% for as-cast, LT4, and HT4 alloy, respectively.