Effect of surface treatment on the ultrasonic weldability of AZ31B magnesium alloy plate

摘要

Magnesium and its alloys have been widely used in various fields because they are the lightest metal in the structural materials and they possess many excellent properties such as high specific strength, low density, good and economic processability by using cast technology and high recycling potential. The recent progress in rolling technology enables the production of magnesium alloy plates. In this study, we tried to ultrasonically weld the magnesium plate of AZ31B, and examined the effect of the surface treatment on the ultrasonic weldability. Four kinds of surface treatments were adopted: (I) Wet polishing, (II) Alkaline cleaning, (III) Pickling with CrO_3 solution and (IV) Pickling with CrO_3 plus H_2SO_4 solution were adopted. The following results were obtained in this study. The outer surface of the magnesium cleaned in alkaline solution is covered with Mg (OH)_2 thick film, resulting in the deterioration of the ultrasonic weldability. The magnesium alloy of AZ31B contains inevitably some contents of Mn_6Al_7 intermetallic compounds, and these compounds appearing on the faying surface deteriorate the ultrasonic weldability. The surface treatment by pickling in aqueous solution consisting of CrO_3 and H_2SO_4 is effective to remove the Mg (OH)_2 film and the Mn_6Al_7 intermetallic compound and leads to increase the tensile strength of the ultrasonic weld. Heating the specimen to 100-200 deg C during ultrasonic welding can increase the tensile strength of the weld approximately 2.5 times as high as that of the weld performed at a room temperature.