Effects of TiO2 coating on the microstructures and mechanical properties of tungsten inert gas welded AZ31 magnesium alloy joints

► The weld penetration and the D/W ratio could be improved dramatically by increasing of the amount of the TiO2 coating. ► The average grain size of the α-Mg grains increased and the β-Mg17Al12 IMC transformed from granular structure to continuous structure with an increase of the amount of the TiO2 coating. ► With an increase of the amount of the TiO2 coating, the microhardness of the FZ of the AZ31 magnesium alloy welded joints decreased slightly at first and then decreased sharply. ► The UTS value of the welded joints increased with an increase of the amount of the TiO2 coating. ► However, too much TiO2 coating caused a significant decrease of the UTS value of the welded joints.