The Interface Migration Behavior And Mechanical Property Of Friction Stir Welded Lap Joints

In this paper, friction stir lap joint of3mm thick plates of LY12(M)、LF6andLY12(CZ) aluminum alloys were conducted using the cylindrical tool with left-handand right-hand thread that the length of pin is3.3mm. The influence of pin geometry、welding process parameters and material property on interface migration wasinvestigated，and the tensile-shear strength of lap joints was tested. The transferbehavior of plasticized metal in the weld thickness was investigated to observe thedistribution of the horizontal tracing material, and combining the force of stir tool toplasticized metal, the formation mechanism of interface migration was analyzed.The results indicate that the pin geometry、welding process parameters and materialproperty are important factors influencing the migration of lap interface. Themigration-height of interfaces at both sides of weld nugget, moving toward uppersurface of welds welded by the tool with left-hand thread, is greater than that movingdownward welded by the tool with right-hand thread. Using the tool with right-handthread, the effective width of lap welds increases obviously with the increase of thediameter of the pin, while the migration-height of interface increases firstly and thendecreases. With the plunge depth of shoulder and the welding speed increasing, themigration-height of interface increases resulted from the strengthening of thesucking-extruding effect, as the weeding speed exceeding47.5or60mm/min, themigration-height of interface would reduces owing to the weakening of thesucking-extruding effect. As to the lap welds of dissimilar aluminum alloy, the materialproperties such as yield strength and thermal conductivity are important factorsaffecting the migration of interface. If the lower yield strength for the sheet placed onthe bottom, the migration-height of interface will be shorter; if the lower thermalconductivity for the sheet placed on the bottom, the deformation-width of interface willbe shorter.The transfer behavior of plasticized metal in weld is the essential reason whichaffects the migration of interface. The formation of migration interfaces at both sides ofthe weld nugget are determined by the combined action of the three factors such as therotation-friction action of plasticized metal in weld horizontal direction, thesucking-extruding effect of plasticized metal and the extruding effect of plasticizedmetal at the top face of stir pin in weld thickness direction, affected by the stir tool. The sucking-extruding effect is the chief reason which influenced the migration-height ofinterface.The results of tensile-shear test exhibit that the migration interfaces at both sides ofweld nugget are the source of fracture. The joints welded using the tool with left-handthread are broken at the side which has the migration interface exists on top sheet, whilethe joints using right-hand thread are broke generally at the side which had migrationinterface on bottom sheet and the tensile-shear strength is stronger than that of the jointsusing the tool with left-hand thread. The tensile-shear strength of the joints is mainlydepended on the migration-height of interface, that is, it will decrease with increasing ofthe migration-height of interface. As to the joints of dissimilar aluminum alloy, if thesheets with the lower yield strength placed on bottom, the lower the migration-height ofinterface is and the milder the bending-degree of interface is, the stronger thetensile-shear strength is.