Research On The Zigzag-curve In Friction Stir Welding Joints For LY12 Aluminum Alloay

LY12 is one high strength aluminum alloy which has good mechanical properties. It has been widely applied in aviation, astronautics, automobiles, ships and other industrial fields. It is easy to generate crack, porosity and distortion with fusion welding. In this paper, LY12 aluminum alloy was joined by friction stir welding with different types of pin, different welding parameters and pin offsets. The formation patterns of zigzag-curve and its influences on mechanical properties of joint were analyzed. It can provide the control methods for the FSW joints of this kind high strength aluminum alloy. It is improvement and application of FSW actually.The zigzag-curve is the accumulation of the oxide layer on the butt surface of the welded plates. It is continuous along the length direction. From SEM, zigzag-curve is composed of holes. When the holes are dense, the zigzag-curve is similar to fracture. EDS analysis shows that the main components are O and Al. The shape of pin influences the formation of zigzag-curve greatly. The zigzag-curve is scarcely formed with columnar-pin and screw threads but easily formed with non-whorl pin. When welding speed is fixed, tool rotating speed is high or the latter is fixed, the former is slow, the zigzag-curve can't exist no longer.When welding speed is fixed, the zigzag-curve gets across centerline, its extended length toward the advancing side decreases and its angle increases with the tool rotating speed increasing used the columnar-pin. When the pin offsets the advancing side, the zigzag-curves in the weld have similar shape. The extended length toward the advancing side decreases and the zigzag-curve moves to the retreating side with the offset decreasing. The zigzag-curve discontinuous runs through the cross section in the end. When the pin offsets the retreating side, the zigzag-curve gradually moves to the advancing side with the offset increasing and it covers more area in the weld thickness direction.When welding speed is fixed, the length of zigzag-curve in the advancing side decreases and the distance from disappearance point to weld bottom increases when zigzag-curve extends from up to down with the tool rotating speed increasing used cone-pin. When tool rotating speed is fixed, the zigzag-curve mostly locates in advancing side and it covers litter area in the weld cross section. When the cone-pin offsets advancing side, the variation of zigzag-curve is in agreement with the results used columnar-pin.The zigzag-curve can affect the fracture characteristics of the joint. The tension specimens with zigzag-curve all fracture along the zigzag-curve in the weld nugget and fracture surfaces show"s"shape. While the ones without zigzag-curve fracture along the aluminum clad in the advancing side. The tensile strength of the former isn't lower than the one of the latter. Bend specimens all crack. The initiation angles of the positive curving specimens are larger than the ones in back bend. Along zigzag-curve the specimens crack. While the ones without zigzag-curve all crack in the weld nugget with a positive curving and crack along the aluminum clad in advancing side.