Corrosion Protection Of Cold Sprayed High Strength Aluminum Alloy Friction Stir Welding Joints

Friction stir welding（FSW）has an unique advantage in welding lightweight alloys,which has been became the preferred method for welding aluminum alloys at present.However,FSW joint has poor corrosion resistance,which has become a major obstacle to further popularization and application of FSW technology.In order to improve the corrosion resistance of FSW,cold spraying（CS）technology provides protection for FSW joints in this work.A 500μm thick CS coating was deposited densely on the bottom surface of the FSW joint.By introducing the in-situ shot-peening-assisted technology,the quality of CS coatings is significantly enhanced.The porosity of the CS coatings is only 0.77%.The microhardness of CS coatings reaches 120HV_0.2,while the lowest microhardness is located in the thermos-mechanically affected zone（TMAZ）of FSW joints with90HV_0.2.Cold spraying improves the mechanical properties of FSW joints,the average ultimate tensile strength（UTS）and average elongation increase by 7.7%and 16.6%after spraying.The fracture form of the CS coatings is mainly brittle fracture,and the ductile fracture occurs in the local areas.The forming mechanism of CS coatings was revealed by numerical simulation combined with thermogravimetric analysis and transmission electron microscopy.There are three kinds of grains in the CS coatings:refined grains,elongated grains and equiaxed grains,which is caused by the non-uniform plastic deformation and thermal effect.During cold spraying process,the grain boundaries of the refined grain regions are narrowed or even discontinuous,and a large number of dislocations disappear,which improves the corrosion resistance of the coating.At the same time,the second phases of the refined grain regions are precipitated and aggregated.The grain boundaries of the elongated grains regions are elongated and broadened,resulting in a decrease in corrosion resistance.The equiaxed grains regions are not affected by the cold spray process.Corrosion behavior and mechanism of CS coatings were investigated by exfoliation corrosion test（EXCO）,intergranular corrosion test（IGC）and electrochemical tests.The CS coatings exhibit better corrosion resistance than FSW joints and base material（BM）,and the corrosion depth is only 43.5%of the BM and 33.2%of the heat affected zone（HAZ）in EXCO solution for 24h.After 6 hours of intergranular corrosion,the corrosion depth of the CS coating is only 50.0%of the HAZ.Electrochemical results show that the corrosion potential and corrosion current density of the CS coatings are-0.70V and 6.03μA·cm^-2respectively,which is obviously better than the HAZ of FSW joints(-0.75V and 9.90μA·cm^-2)and the BM(-0.81V and 15.00μA·cm^-2).The corrosion behavior of the CS coatings begin in the form of pitting,which is located at the pores between the particles;the CSed particles are occurred typical intergranular corrosion,and the elongated grains zones are preferentially attacked;finally the CS coatings develop into exfoliation corrosion.Conclusively,the CS coatings effectively reduce the corrosion sensitivity of the FSW joints,and significantly improves the corrosion resistance of the FSW joints.Cold spray technology can provides effective corrosion protection for FSW joints.