Research Of Organization And Lectrochemical Properties Of Friction Stir Weld Of AZ31Magnesium Alloy

Magnesium alloy is widely used in industry such as automotive, aerospace,3C electronic products, because of the characteristics of light weight, fine heat radiating performance, high strength-to-weight ratio, high stiffness-to-weight ratio etc. Extensive application of magnesium alloy can not only play the role of energy saving and emission reduction, but also enhance the car-body and body maneuvering performance with the protection of the environment. Without welding, magnesium alloy will be narrowly applied. The traditional melting welding of magnesium alloy tends to cause overheating, porosity and hot brittleness defect. Friction stir welding(FSW) solves this problem successfully with the nearly intact retention of basic properties of the parent materials.Firstly, the microstructure of FSW weld joint of AZ31magnesium alloy was observed and analysed by optical microscope. Analysis results show that:the microstructure of FSW weld joint of AZ31magnesium alloy is divided into two distinct areas,namely,weld nugget zone(NZ) and thermomechanically affected zone(TMAZ).Close to weld the thermomechanically affected zone lateral grain coarse as heat affected zone(HAZ). The grains of weld nugget zone are fine and uniform caused by the severely plastic deformation from the FSW; The grains of thermomechanically affected zone distribute unevenly with elongated trend because of the interaction of shoulder and stirring pin; The grains of heat affected zone are relatively coarse because of the effect of the heat.Then, by static weight loss method, potentiodynamic polarization curves and electrochemical impedance spectroscopy in neutral5%(wt.%) NaCl aqueous solution, the AZ31magnesium alloy was tested and compared with the weld at room temperature. The results show that:(1) The average corrosion rate of FSW weld of AZ31magnesium alloy and the parent material were0.135g/m2·h and0.154g/m2·h respectively, the corrosion rate of the welding seam is less than the parent material;(2) By the potentiodynamic polarization curves, the corrosion potential of FSW weld joint of AZ31magnesium alloy is similar to that of parent metal, the corrosion current densities are0.00045A/cm-2and0.00163A/cm-2respectively, the corrosion current density of weld was lower than that of the parent metal;(3) By the electrochemical impedance spectra(EIS), The AC impedance spectrum radius of FSW weld of AZ31magnesium alloy is bigger than that of parent metal. The anti-corrosion ability is stronger after the FSW welding.Finally,FSW weld of AZ31magnesium alloy is tested with the same method by changing the pH value of the solution,the results indicate: (1) With the increasing of pH value, the corrosion rate of the welding seam is lower by static weight loss method;(2) The corrosion current density of the measured weld decreased with increasing pH value, by the potentiodynamic polarization curves;(3) By electrochemical impedance spectroscopy,with pH value increasing, electrochemical impedance spectrum radius increases.