The Influence Of Soft Constraint On The Welding Plastic Metal Flow Of Friction Stir Welding

In this paper, the8mm thick plates of2024（O） and2024（T4） aluminum alloywere welded by friction stir welding with a cylindrical tool with left-hand thread. Thesoft constraint state was changed by altering the process parameters, materialproperties and external heat source, and the influence of soft constraint on flowingbehavior of plastic weld metal was analyzed, which to provide theoretical basis of theprocess parameters optimization, and the defect control in friction stir welding.The result shows that in the horizontal marker insert materials experiment, thenugget squeezed on the marking materials in advancing side was weaker thanretreating side. However, when a high-strength constraint body used on retreating side,the amount of material deformation in retreating side is less than advancing side in theperpendicular marker insert materials experiment.With the welding speed reducing or the rotational speed increasing, the migrationamount of the plasticity metal in the axial direction of the pin was increased, so thatthe plastic metal migration distance was showing farther in the transverse direction.More aggregation amount of plastic metal on the end of the pin, the plastic metalextrusion pressure was became strong, thus, the area and width of nugget was greater,and the bending angle of rolling stream line adjacent to nugget was greater.Soft constraints body are mainly constrained transverse migration of plasticmetal, forcing it to migrated upward toward the axial direction, the higher intensity ofsoft constraints body, the stronger the trend of plastic metal axial movement. Underthe same heat-treated state material, due to the smaller constraint of plastic metallateral migration by the soft constraint body, the area and width of the weld nugget of2024（O） aluminum alloy were greater than2024（T4） aluminum alloy. While thedissimilar heat-treated state material was welded and the higher strength of the2024（T4） aluminum alloy was used as the retreated side, the constraint of the plasticmetal migration in the lateral direction was stronger than the advance side, and theupward motion tendency of the plastic metal in the axial direction on the retreatedside was enhanced, and the height of nugget in the retreated side was larger than theadvanced side.Under the preheating conditions of2024（O） aluminum alloy, the plastic metal can not effectively migrate to the end portion of pin, so that the ratio of volume of theplastic metal and the cavity was decreased. Plastic metal squeezing effect wasweakened at the bottom of the weld, the area and width of nugget was reduced, andthe weld nugget metal on the axial upward flow trend weakened, the height of weldnugget reduced. However, under the appropriate preheating conditions of2024（O）aluminum alloy, the ratio of volume of the plastic metal and the cavity was increasedand the strength of the soft constraint body was declined. Thus, the width and area ofthe nugget was increased result from the smaller constraint to the transverse migrationof plastic metal and the migration height of nugget was reduced result from thedecline of the upward motion tendency of the plastic metal in the axial direction.