Precipitation behavior of intermetallic compounds and their effect on mechanical properties of thick plate friction stir welded Al/Mg joint

Abstract

Brittle intermetallic compounds are prone to form in dissimilar joints, which significantly affected the reliability of the joints. To avoid or control the production of brittle intermetallic compounds, an inclined butt joint of 5A06 Al and AZ31B Mg alloys with 20 mm thickness using friction stir welding was designed. Microstructures and chemical composition of the joint were investigated by optical microscopy, scanning electron microscopy, micro X-ray diffraction, transmission electron microscopy. The intermetallic compounds in the weld nugget zone presented different precipitation behavior along the thickness direction. In the upper of the weld nugget zone, intermetallic compounds Al3Mg2, Al12Mg17, and non-equilibrium phase AlMg precipitated during solidification due to excessively high temperature of the upper of the weld nugget zone. The Al3Mg2 phase merely precipitated and grown along the grain boundaries in the middle and lower of the weld nugget zone. The maximum Vickers hardness value was approximately 289 HV, which was obtained at the upper of the Mg side interface, existing a thick eutectic zone consisted of Mg and Al12Mg17. The nanohardness of the Al12Mg17 layer and the Al3Mg2 layer were 5.39 GPa and 4.18 GPa, respectively. The precipitation of the Al12Mg17 and the Al3Mg2 phases plays a precipitation strengthening effect.