Micro-arc Oxidation Treatment On Friction Stir Welding Joint Of AZ31B Magnesium Alloy

Magnesium alloys have higher specific strength and specific stiffness, and better machinability, and are the21century green engineering materials. However, because of poor corrosion-resistence and weldability, Magnesium alloys have hindered their widespread use in many applications. Magnesium alloys are not readily fusion weldable. Consequently, a variety of problems can arise during fusion welding of magnesium alloys, including: oxidation, excessive spatter, porosity, inclusions, liquation and solidification cracking, and high inner stress. As a result, when a magnesium weldment are exposed to corrosive environment, those defects will accelerate its corrosion. Friction stir welding is a novel solid-phase joining technique. Different microstructures and stress distributions are generated at differdent welded zones by FSW process, which exhibit different corrosion behavior correspondingly. Thus, the research on property and corrosion-resistence of magnesium Friction stir welding has an important significance.Test about friction stir welding(FSW) of AZ31B magnesium sheet with6mm was successfully made in butt welding, and AZ31B magnesium base metal and FSW joint were treated by micro-arc oxidation(MAO) under420voltage and490voltage. Studys on the section hardness, microstructure, phase composition and corrosion-resistance of AZ31B FSW joint show that the weld zone can be obviously devided into3different zones: nugget zone, heat affected zone and thermomechanicaliy affected zone. The phase composition of weld is in accordance with base metal: α-Mg、β-MgnAl12�'�AlgMns. The nugget zone has a higher hardness in the joint, and the hardness of whole joint is lower than base metal. The AZ31B FSW joint has a poor corrosion-resistance compared to base metal, nevertheless,the corrosion-resistance of nugget zone is superior to HAZ. They are all pitting corrosion.The property of MAO coatings of wled and base metal under420voltage and490voltage had been studied and the total energy input during MAO process had be calculated. The rusults show that under the same process, the thickness of weld overlop base metal, and the thickness of weld centerline is superior to two sides. The micro morphology of nugget zone, HAZ and base metal is consistent, however, the MAO coatings’density under490voltage is superior to420voltage. The primary elements in MAO coatings under two different voltage is Si and Mg, and the primary phase is Mg2SiO4and MgO. The corrosion-resistance of weld and base metal can be greatly improved by MAO treatment, and the corrosion-resistance of MAO coatings under490voltage is superior to420voltage’s. The corrosion-resistance of weld’s coatings is superior to base metal’s under the same process. The MAO coatings of weld and base metal are well-knit, and the MAO coatings of the whole weld are well after200times thermal shock tests. When weld and base metal make the same thickness MAO coatings under two different voltage, the total energy input under490voltage is far below the energy under490voltage, the total energy input of weld and base metal during MAO treatment is similar under under the same voltage. The thickness and corrosion-resistance of magnesium with the same base metarial but different organization are different. The thickness of MAO coating above the nugget zone is thicker about1-5um than other zones, and the corrosion-resistance of weld’s coatings is superior to base metal’s, but the other properties are silimar.