Study On Technology And Mechanical Properties Of Joint Of Dissimilar 5754Al/AZ31Mg Alloys By Friction Stir Welding

Both of Al alloy and Mg alloy have very good properties, and it is significant to lower weight of industrial structures by joining Al alloy and Mg alloy. It is found that the parameters window is narrow after welding dissimilar Al alloy and Mg alloy by friction stir welding in air, and there are three kinds of defects formed, namely, groove, surface peeling, and void(tunnel defect).First of all, when the rotational speed is too low or welding speed is too high, the weld metal is not able to flow fully due to short of heat input, leading to the groove defect formation. When the rotational speed is too high or welding speed is too low, too much heat input to the weld metal lead to the enclosed tool by weld metal and further lose the function of pin thread. Thus the tool cannot stir the weld sufficiently, leading to the formation of groove. Due to inappropriate rotational speed and welding speed, too much Al/Mg alloy concentrate in the concave shoulder and overflow the edge of the shoulder, which form the surface peeling.Too much surface peeling or heat input to the weld metal lead to the void defect. With the Mg alloy placed on the advancing side, groove form at all welding parameters, at which all sound joints has been formed with Al alloy on the advancing side.On the basis of obtaining a sound weld, there are still two weak zones in the joint: 1) the bonding interface of weld center;2) localized band structure and severely deformed zone of Al side.Effect of water cooling on the microstructure and properties of the joint was studied. It is noted that when the welding speed is relatively low(30mm/min), a large amount of Mg element, as a plastic layer, is transformed to the advancing side to mix with Al alloy layer with the traveling and spinning spin, onion ring formed. Moreover, effect of the tool placement on the weld surface, microstructure property and fracture location at 1500rpm-30mm/min was also studied. It is shown that the good weld surfaces were obtained with the tool placed at weld center and 1mm away from weld center to Al side. When the tool was placed at 1mm away from weld center to Mg side, weld surface crack and voids formed, Al and Mg alloy cannot be welded together, this is because Mg alloy has less mobility than Al alloy, when the tool was placed at 1mm away from weld center to Mg side, much Mg alloy was influenced by spin and less heat input was created, which become harder to form a sound weld.When the weld is short of heat input, ultrasonic energy could eliminate surface peeling, improve the mobility of bottom metal and eliminate voids. At the same time, ultrasonic energy strengthen the Al/Mg atom reaction to some degree and increase IMC formation. The joint failed at bonding interface of weld center by ductile and brittle mixed fracture modes.