Effect of coating submicron-sized La2O3 particles on regulating grain structure and mechanical properties of 6061 aluminum alloy CMT welded joints

Abstract

In order to further optimize the microstructure and obtain desirable mechanical properties of cold metal transfer (CMT) welded aluminum alloys joints, this study investigated the effect of coating submicron-sized La2O3 particles on the CMT welded joints of 6061 aluminum alloy. The results showed that the introduction of La2O3 particles improved the welded wettability. However, it did not form any strengthening phase or had a grain refining effect. The introduction of La2O3 at the grain boundaries exhibited a significant segregation phenomenon, accompanied by a tendency for consistent grain orientation. To quantify this effect, the misorientation crack factor (M) was introduced, and the analysis was conducted using a combination of electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The deposition of La2O3 particles at the grain boundaries resulted in increased energy storage due to the grain size effect. As a consequence, the high energy storage region decreased the primary slip plane normal (s→) and the unit vector of the primary slip direction (n→). The reduction in misorientation crack factor was achieved by the addition of La2O3 particles, reducing the dislocation density. The combination of these two factors effectively prevented cracking behavior. Coating the welded joints with the submicron-sized La2O3 particles resulted in a 35.7% increase in their average tensile strength, and the softened area of the welded joint exhibited significant improvements in hardness comparing with that of joints before coating La2O3 particles.