Research On Microstructure And Properties Of AZ91 Magnesium Alloy By Friction Stir Weldin

As the lightest material in metal components,magnesium alloys are widely used in aerospace,transportation,automotive manufacturing,and other fields.For components with complex structures,it is often necessary to fabricate them by splicing,and welding is a common and efficient method for component splicing.In the process of welding magnesium alloys using traditional fusion welding,it is easy to generate holes,thermal cracks,shrinkage holes,and burning loss of alloy elements.It is difficult to obtain high-quality welded joints of magnesium alloy,which affects the performance of magnesium alloys.As a green solid-state welding method,friction stir welding can effectively avoid the adverse effects of traditional fusion welding methods on magnesium alloys,and is an excellent welding method for magnesium alloys.However,during friction stir welding,the flow of materials is an extremely complex and unstable process,and the microstructure of welded joints is very complex.In order to obtain welded joints with good surface quality and high joint coefficient,and increase the application efficiency of magnesium alloy in components,the microstructure and mechanical properties of welded joints under different processes are analyzed by “direct friction stir welding”,“pre solution before welding+friction stir welding”,“direct friction stir welding+homogenization after welding”.The main results are as follows:(1)After direct friction stir welding of as cast AZ91 magnesium alloy,there were obvious differences in the microstructure of different areas of the weldment.When the welding speed was maintained at 100 mm/min and the rotation speed were 600 r/min,800 r/min,900 r/min and 1000 r/min respectively,the tensile strength of the welded joint were 135 MPa,136 MPa,138 MPa,and 136 MPa,the joint coefficients were above 90 %,.When the rotation speed was maintained at 1000 r/min and the welding speeds were 50 mm/min and 150 mm/min respectively,the tensile strength of the welded joint were 126 MPa and 124 MPa,and the joint coefficient were 84 % and 83 %,respectively.(2)The interface between the β phase which was easily segregation at retreated side and matrix became the source of crack initiation,which made this area become the“weak zone” of the joint.The pre-solution treatment before welding for 12 h made the weldment has the best mechanical properties,and its tensile strength was 184 MPa and the elongation was 6.7 % respectively,which benefited from the elimination of the partial β phase on the retreated side,homogenization of weld structure and the reduction of the strength difference of each part of the weldment.(3)The direct friction stir welded joints were homogenized for 8h and 12 h respectively,although the aggregated β phase was eliminated and it improved the uniformity of the microstructure of the welded joint,the mechanical properties of the welded joints have not been effectively improved.