Study On The Relationship Between Welding Technology,Weld Structure And Joint Property During Al/Mg FRICTION STIR WELDING

Friction stir welding,a new solid state welding technology,has been widely used to produce similar/dissimilar joints between metals or alloys and is promising to achieve effective welds between Al alloy and Mg alloy.Through systematic change of welding tool design and welding parameter,the thesis completed three series of welding experiments and conducted comprehensive characterization of macro-and micro-structure and mechanical properties.According to the technique-structure-property analysis,findings including the effect of welding parameter on nugget structure,the effect of nugget structure on joint property,materials flow and intermixing pattern and formation mechanism of intermetallic compounds were obtained.Research results suggest that the nugget zone is mainly composed of Al/Mg intermixing zone,Al/Mg interface and Al in the retreating side,which play important roles in determining the mechanical property and fracture mode of the welding joints.For Al/Mg intermixing zone,large amounts of intermetallic compounds of Al₁₂Mg₁₇are developed,majority in the form of interconnected porous skeleton inside Mg matrix,minority in the form of lamella.The quantity and distribution of intermetallic compounds play decisive role in the strength of Al/Mg intermixing zone.For Al/Mg interface,it is composed of continuous or non-continuous intermetallic compound layer.The morphology and thickness of the intermetallic compound layer play decisive role in the strength of Al/Mg interface.For Al in the retreating side,its strength is mainly determined by the extent of reduction of strength owning to the dissolution and coarsening of precipitate in the Al alloy.Under different welding conditions,the relative strength of three weak zone would lead to the variation of fracture mode and resultant mechanical property of the welding joints.Formation of intermetallic compounds during the welding is intimately related to the materials flow and intermixing pattern.During FSW process,Mg alloy in the advancing side enters into the rotation layer and the extrusion layer from the retreating side is adjacent to the rotation layer.Al pieces in the retreating side are sheared off the extrusion layer by the friction between rotation layer and extrusion layer.Then the Al pieces are transported from the retreating side to the advancing side with the rotation layer and finally deposit as the rotation layer detaches from the pin at its rear side.The intermetallic compounds’structure at the banded structure zone is possibly because of the occurrence of constitutional liquidation during welding under strong shearing effect during solidification.