| Magnesium alloys are the lightest metals in the current manufacturing industry as structural materials.It has the advantages of high strength,low density,high specific modulus,easy cutting and recycling,and has been popularized in marine,rail transit,aerospace and other fields.As a high-performance new structural material,the connection between structural members is often encountered in practical applications.Friction stir joining is a new type of solid phase joining method.It is mainly used in the solid connection of low melting point metals and their alloys,breaking the constraints of traditional magnesium welding and avoiding defects such as loose joints and hot cracks in the joint zone.The complete performance analysis of the 10 mm thick AZ31 B magnesium alloy stirring joint zone is carried out in the paper.Through a certain orthogonal optimization test of the joint process parameters,a well-formed joint zone is obtained.Analyze the influence of process parameters on the microstructure,mechanical properties and corrosion resistance of the joint zone.The following conclusions were obtained:(1)The surface of the shoulder of the stirring head is concave with concentric circular groove ring.The stirring needle with spiral threads is circular and conical.The lower pressure of the shoulder is 0.2mm,and the inclination angle of the spindle is 2.6°.Under the parameters of 600-1000r/min rotating speed and 100-120 mm/min forward speed of the mixer,the connection of AZ31 B magnesium alloy with 10 mm thickness was realized,and the joining zone with high surface forming quality and internal flawless was obtained.(2)The grain refinement in the joint zone under different parameters is remarkable.The grain size increases with the increase of the rotating speed of the stirring head.Improved grain size uniformity as the forward speed increases.Long deformation,and obvious grain dislocation phenomenon;(3)The impact resistance is the best when the rotating speed and the forward speed are 800,1000 r/min and 120 mm/min,respectively,which is about 94.2% of the base metal.When the rotating speed is 1000 r/min and the advancing speed is 150 mm/min,the tensile strength reaches 85.8% of the base metal,and the elongation after breaking is 1.9 times of that of the base metal.The overall dimples on the fracture surface are uniformly distributed in size and direction,and the fracture mechanism of the joint after stirring connection is mainly ductile fracture.(4)The overall variation curve of the microhardness of the joint zone is "W" shape,which is roughly symmetrical around the weld nugget.When the current feed speed is 100mm/min,the joint hardness is 69.5HV when the rotation speed is 600r/min.Slightly higher than the base metal,the average hardness is lower than the base metal.As the distance from the coronal layer increases,the microhardness of the joint increases first and then decreases.The hardness of the coronal layer in the joint zone is lower,and it decreases about 8 mm from the surface of the joint zone.As the rotation speed increases,the microhardness of the joint decreases.(5)The corrosion resistance of the joint zone is inferior to that of the base metal.The surface of the electrochemical corrosion was found by electron microscopic scanning.It was found that the surface of the joint surface was corroded with severe corrosion,and obvious cracks were observed.A large number of small corrosion pits appeared on the flat surface,and there were dense and fine particles in the corrosion pit.After 12 h and 24 h salt spray corrosion,it was found that the corrosion rate of the joint zone increased significantly with the increase of corrosion time. |
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