Formation Mechanism of Thicker Intermetallic Compounds in Friction Stir Weld Joints of Dissimilar AA2024/AZ31B Alloys

Abstract

The hybrid structures of AA2024 aluminum alloy and AZ31B magnesium alloy have the advantages of being lightweight, having high specific strength, etc., which are of great application potentials in the aerospace industry. It is a key problem to realize the high-quality welding of these two dissimilar alloys. In this study, the friction stir welding (FSW) tests of AA2024 aluminum alloy and AZ31B magnesium alloy plates of thickness 3 mm were carried out. The intermetallic compounds (IMCs) at the bonding interface were characterized by scanning electron microscope, electron probe, and transmission electron microscope. It was found that the IMCs at the bonding interface in weld nugget zones of dissimilar AA2024/AZ31B FSW has a double-layer structure and a much larger thickness. During the welding process of AA2024/AZ31B, when the boundary of magnesium grains bulges and nucleates, the aluminum atoms diffuse into the magnesium grains, and the γ phase (Al₁₂Mg₁₇) nucleates at the bonding interface. The β phase (Al₃Mg₂) then precipitates at the grain boundary of the γ phase and preferentially grows into γ phase grains. The continuous grain growth to the aluminum side makes the copper contained in AA2024 aluminum alloy concentrate on the side of β phase, which reduces the nucleation work of recrystallization and phase transformation, and further promotes the nucleation and growth of IMCs grains. This is the main reason for the thicker IMCs in the FSW weld of dissimilar AA2024/AZ31B alloys.

Keywords: Al alloy AA2024; Mg alloy AZ31B; dissimilar alloys; friction stir welding; intermetallic compounds.

Figure 1

Schematic diagrams of Al/Mg FSW experimental set-up.

Figure 2

Schematic of FSW tool.

Figure 3

Schematic illustration of weld cross section and SEM characterization locations: TMAZ-thermo-mechanically affected zone, HAZ- heat affected zone. TOP, MID, and BOT represent the top, middle, and bottom regions of the weld, respectively.

Figure 4

SEM images at three locations in FSW weld: (a) location TOP, (b) location MID, (c) location BOT.

Figure 5

EPMA scanning at location BOT (a), distribution map of Al (b), Mg (c), Cu (d), and Si (e).

Figure 6

STEM image at location BOT (a), line scan results (b), element distribution of Cu (c), Al (d), and Mg (e).

Figure 7

TEM diffraction pattern at location BOT: (a) diffraction location (corresponding to Figure 4a), and (b–f) diffraction pattern at locations b–f in (a).

Figure 8

Schematic of IMCs formation at the bonding interface of AA2024/AZ31B FSW weld: (a) before welding, (b) first stage of welding;(b) second stage of welding, (c) third stage of welding, (d) fourth stage of welding, and (e) completion of welding.