Cold Spray Induced Strengthening Mechanism Of The Joint Prepared By Friction Stir Welding On AA2024 Alloy

Friction stir welding（FSW）,as a high quality,efficient and environment-friendly solid-state joining technology,has prominent advantages in the welding of high strength aluminum alloys in aerospace,shipbuilding and other industrial fields.However,a large number of studies have shown that FSWed joints of high strength aluminum alloys.Hence,the FSWed joints must be protected for corrosion attack to improve the service life.At present,the methods of corrosion protection for FSWed joints are not perfect.The Al and Al composite coatings deposited by cold spraying（CS）have good corrosion resistance for FSWed joints,but there are few studies about CSed coatings for corrosion protection of FSWed joints.In this research,pure Al,Al-20 vol.%Al₂O₃ composite coatings,puer Cu and pure Ni coatings were CSed on the surface of the FSWed joints of 3.2 mm thick 2024-T3 Al alloy.The effect of the coating on the microstructure,mechanical properties and corrosion performance of FSWed joints was investigated.Furthermore,the mechanisms for improving mechanical properties of FSWed joints were revealed by experiments.The main conclusions are as follows:The effect of rotation speed on microstructure and mechanical properties of joints is analyzed.Results show that the hardness is improved with the rotation speeds;in case of lower rotation speed the hardness profile shows“U”shape,in case of higher rotation speed it shows“W”shape;the reasons are that lower rotation speed can avoid the dissolution of GPB（Guinier-Preston-Bagaryatsk）zones and the precipitation of S phase,but higher rotation speed can promote the dissolution of GPB zones.When in the proper welding heat input the best tensile properties can be achieved.The optimal welding parameters are:600 rpm and 200 mm/min,and the joint has the best ultimate tensile strength（420 MPa）and elongation rate（8.7%）.High-quality Al composite coating can be deposited on the optimal FSWed joint by CS using the proper process parameters.CS has a significant influence on the surface integrity of the FSWed joint.Results show that CS increases the GPB zone of the joint near the surface,makes the grain of the（Stir Zone）SZ refine and the high-angle grain boundaries（HAGBs）increase.Furthermore,the mechanical properties have been improved after CS.The hardness value of CSed joint is higher than the as-welded joint in the whole joint.The ultimate tensile strength and elongation are increased by 7%and 9%,respectively.The fatigue life of the CSed joint is four times as the as-welded joint in 200 MPa.In order to reveal the strengthening mechanism of the FSWed jont induced by CS,the project focuses on the heat flow effect（HFE）caused by the high speed and high temperature gas flow and the shot peening effect（SPE）resulting from the high velocity impact of spray particles during CS.The two effects make the surface integrity and mechanical properties have the similar effect with the CS.The two effects make the GPB zone increase,the grain size refine and the HAGBs increase.However,the“HFE”mainly contributes to the elongation and the“SPE”mainly contributes to the elongation tensile strength.The“HFE”during CS heats up the near-surface layer of the coated joint at a temperature between 200-300^oC,which improves dramatically the dissolution of the GPB zones both in SZ and HAZ.Moreover,the“SPE”during CS may release the residual stresses of the joints,which may also contribute to the hardness improvement.The higher hardness can significantly reduce the formation and growth rate of cracks,and thus improves the joint strength.The coated joint successfully produces superior strength-ductility synergy not due to the simple adding effect of the two effects but synergy work of the two effects.The pure Cu and Ni coatings are produced on the FSWed joints using the same CS process parameters as the Al-20 vol%Al₂O₃ composite coating.The Cu coating has the same thickness as the Al-20 vol%Al₂O₃ coating and the Ni coating has hardly be deposited on the surface of the joint.The influence of the coating with different powder density on the tensile properties of the FSWed joints is compared.The results show that the Cu and Ni coatings have the same effect on the ultimate tensile strength like Al-20 vol%Al₂O₃ coating and the elongation is significatively increased.The elongation of the FSWed joint deposited on Cu and Ni coatings are increased by 44%and 39%,respectively.The reason is that during the cold spraying the kinetic energy of Cu and Ni particle is greater than that of Al-20 vol%Al₂O₃ particle.Thus,the effect of"shot peening effect"on the substrate can be improved significantly,which will make the grain finer and the tensile properties better.The corrosion behavior of the FSWed joint under different solution and different rotation speed is investigated and the mechanism of lower corrosion resistance for the joint is revealed.The reason is due to the different microstructure of the different zones especially the second phase particle distribution of the different zones.Hence,the CSed Al coating can be deposited on the surface of the FSWed joint to supply corrosion protection.The corrosion and wear resistance of the pure Al coating and Al-20 vol%Al₂O₃ coating are tested by electrochemical and wear tests.Results show that the two coatings can both supply corrosion protection to FSWed joint.The corrosion resistance of the Al composite coating is lower than that of the Al coating.It is attributed to the presence of more active sites induced by the high severe plastic deformation in comparison with the unreinforced pure Al coating.Moreover,the interfaces between reinforcement and matrix particles are also active sites easily attacked by the electrolyte.However,The Al-Al₂O₃ coating has better wear resistance than pure Al coating due to the increasement of Al₂O₃ particles.The addition of the hard Al₂O₃ phase into Al coating has improved to a limited extent the wear performance at the expense of corrosion resistance.Based on this finding,what is required next is the combination of the improved corrosion resistance of pure Al with the better wear resistance of the Al-20 vol.%Al₂O₃ coating.The achievements of this project will help realize the integrated control of corrosion resistance and strengthening of FSW joints,to promote the applications of FSW in many important industries.