Research On Microstructure,Mechanical Property And Corrosion Behavior Of Friction Stir Processed AZ80 Magnesium Alloy

Friction stir processing(FSP)is a new type of fine-grained material preparation technology,which has an obvious effect on improving the mechanical properties of the cast magnesium(Mg)alloys,but limited effect on improving the mechanical properties of deformed Mg alloy.It has been found that FSP,in combination with aging heat treatment can regulate the microstructure of Mg alloy,especially the second phase,and then affect the mechanical properties of Mg alloy.However,the study of the relationship between microstructure characteristics and mechanical properties is not deep enough.Meanwhile,the engineering application of Mg alloy needs not only excellent mechanical property but also good corrosion resistance at the same time.There are few researches on the corrosion properties of FSP Mg alloys,and they are only limited to the single evaluation of corrosion behavior,lacking comprehensive evaluation of various corrosion behaviors,especially the research on the stress corrosion cracking(SCC)behavior of FSP Mg alloy.Therefor,FSP,FSP and aging heat treatment dual process,micro-arc oxidation(MAO)and micro-arc oxidation electrophoresis composite coating(MAOE)are used to modify the surface of AZ80 Mg alloy.The effects of FSP,FSP and aging heat treatment dual process,MAO,MAOE on mechanical properties,static corrosion and SCC behavior of Mg alloys were analyzed.The main research contents and conclusions of this paper are as follows:The microstructure of Mg alloy was modified by FSP and aging heat treatment,meanwhile,the grain refinement mechanism,texture evolution mechanism and the aging precipitation behavior were analyzed.The results indicated that dynamic recrystallization in FSP process led to grain refinement of Mg alloy.Fine-grained structures with grain sizes of 2.7,5.6 and 12.6 μm were obtained in stir zone,respectively.Under the action of shoulder compression stress and stirring needle shear stress,the c-axis of FSP Mg alloy deflected.FSP samples with different processing parameters and working media had similar texture component,and the c-axis deviated about 55°away from transverse direction(TD)and 25° away from processing direction(PD).The dual process of FSP and aging heat treatment had little effect on the grain size and grain orientation of Mg alloy,but it affected the content and morphology of β phase.The mechanical properties of base metal(BM),FSP Mg alloy and aged FSP Mg alloy were investigated.The microhardness and tensile strength of the FSP Mg alloys were improved by grain refinement.The mechanical properties of Mg alloys had the anisotropy due to the deflection of the c-axis.The texture softening effect of PD caused the tensile strength to be lower than TD.The tensile strength of Mg alloy could be increased by the dual process of FSP and aging heat treatment.Compared with BM,the yield strength,tensile strength and elongation of FSP+180℃-24 h sample were increased by 6%,4% and 15%,respectively.Electrochemical corrosion,immersion weight loss and hydrogen evolution behavior in 3.5 wt% NaCl solution were investigated.Grain refinement caused by FSP improved the corrosion resistance of Mg alloy.The effect of β phase on static corrosion behavior had two sides.When the content of β phase was low(FSP+180℃-5h,8.09 wt%),β phase and α-Mg matrix formed the galvanic cell,which reduced the corrosion resistance of Mg alloy.When the content of β phase was high(FSP+180℃-24 h,21.84 wt%),β phase acted as a corrosion barrier,and facilitated the formation of Al-rich film layer,which improved the corrosion resistance of Mg alloy.The SCC behavior of BM,FSP Mg alloy and aged FSP Mg alloy in 3.5 wt% NaCl solution were investigated.The grain refinement induced by FSP was helpful to delay the initiation of SCC crack and hinder the growth of SCC cracks,thus reduced the SCC sensitivity of Mg alloys.The SCC sensitivity of FSP Mg alloy exhibited obvious anisotropy,and the texture hardening effect in TD delayed the formation of pitting corrosion,which made the SCC sensitivity of TD was lower than that of PD.The β phase with high content(FSP 180 ℃-24 h)delayed the initiation of SCC cracks and decreased the SCC sensitivity.The SCC behavior is controlled by anodic dissolution,and hydrogen accelerates the SCC.The surface treatment of BM and FSP Mg alloys by MAO and MAOE was carried out,and the electrochemical corrosion and SCC behavior of the alloys were studied.The corrosion resistance of Mg alloy was improved by the blocking effect of MAO film on corrosive medium.Due to the special chemical composition and physical structure,MAOE coating had better resistance to corrosion.Because of the thicker and denser MAO coating,and the more corrosion-resistant substrate,the FSP-MAO sample had better corrosion resistance than BM-MAO sample.The barrier effect of MAO and MAOE coatings on the corrosion medium in the initial stage of slow strain rate tension could delay the pitting corrosion and thus reduced the SCC sensitivity.