| The 2XXX series high-strength aluminum alloy characterizes low density,high stiffness,high strength,good impact property and high recovery rate,etc.,and is one of the leading materials for manufacturing aircraft structural parts of aviation and aerospace.However,the safety and reliability of aluminum alloy welded structures are significantly reduced due to the characteristics of microstructure heterogeneity,weld softening,stress concentration and residual tensile stress,resulting in the limitation of wide application of aluminum alloy welded structures.In recent years,relevant scholars have applied the gradient self-nanocrystallization technology to the surface modification of metal materials to improve the wear resistance,corrosion resistance and fatigue resistance of metal materials.After welding,the welded joint could be treated by the gradient self-nanocrystallization technology to realize the surface modification and improve the quality.Based on the advantages of self-nanocrystallization technology on metal materials,combined with the deficiency of aluminum alloy welded joints,it can be seen that self-nanocrystallization technology is expected to significantly improve the performance of aluminum alloy welded joints.Meanwhile,there is a lack of systematic research on the relationship between grain refinement mechanism and properties of welded joint by gradient self-nanocrystallization technology.Therefore,it is of great significance to carry out this research of welded joint surface modification.In this paper,gradient nanocrystalline was produced on the surface of 2A12 aluminum alloy welded joint by using high frequency impact and rolling(HFIR)technology.Grain self-nanocrystallization mechanisms of the base metal and weld seam were explored in detailed,respectively.The effect of HFIR parameters(static pressure)on the friction and wear behavior,corrosion performance and fatigue performance was systematically discussed.HFIR process could be affected by static pressure,amplitude,frequency,etc.Static pressure has the most significant influence on the processing process.When the static pressure is small,the grain refinement is not obvious;when the static pressure is too lager,the surface quality of the material will be damaged.After comprehensive consideration,the surface of the 2A12 aluminum alloy welded joint after removing weld reinforcement was treated by HFIR treatment with different static pressures(200N,300N,400N and 500N).The grain self-nanocrystallization mechanisms of base metal and weld seam was discussed.The results show that the coarse grains of the base metal and weld seam were refined to about 400nm and 13 nm after 500N static pressure treatment,respectively.Along the direction of deformation depth,the grain sizes of the base metal and weld seam shows a gradient change.The realization of surface self-nanocrystallization of the base metal and weld seam is mainly controlled by the dislocation slip movement.However,T precipitated phase of the base metal and S precipitated phase of the weld seam show completely distinct characteristics in the grain self-nanocrystallization process.During the grain self-nanocrystallization process of the base metal,the size of the T precipitated phase did not change.With the depth from the treated surface of the base metal decreasing,the motion of dislocation was blocked by T precipitated phase and dislocation lines gradually intersected.The T precipitated phases were the intersection of sub-grain boundaries.While,in the WC,the shape and size of S precipitated phase changed significantly,and the size of S precipitated phase changed gradiently along the deformation depth.With the depth from the treated surface of the weld seam decreasing,S precipitated phases were gradually rotated,sheared and broke by dislocation motion,and broke down to the nanometer scale.In addition,the nanoscale S precipitated phases can effectively pin grain boundaries,inhibit grain growth,and increase phase difference on both sides of grain boundaries,which is conducive to obtaining large angular grain boundaries and promoting grain nanocrystallization.HFIR treatment can have a significant impact on the surface properties of 2A12aluminum alloy welded joints.Hence,the surface roughness,surface nanomicrohardness,elastic modulus,cross section microhardness and friction and wear behavior of the base metal and weld seam treated by HFIR treatment with different static pressures were studied.The results show that the surface roughness of welded joint decreased and the surface quality improved after HFIR treatment.After HFIR treatment with 500N static pressure,the improve degree of the surface nanomicrohardness and elastic modulus of the base mater and weld seam was greatest.The cross section microhardness of the HFIR-treated base metal and HFIR-treated weld seam showed a gradient change trend along the deformation depth direction,which was consistent with the depth range of the cross section deformation layer.After HFIR treatment with 500N static pressure,the friction and wear resistance of the base metal and weld seam was improved,which was attributed to the combined effect of high microhardness surface nanocrystalline and stable mechanical mixed layer formed during the friction and wear process.The poor corrosion resistance of aluminum alloy welded joint is one of the key problems affecting its performance.Corrosion sensitivity and corrosion micro-process of the base metal and weld seam of 2A12 aluminum alloy welded joint before and after HFIR treatment were studied,and the corrosion kinetics law of the base metal and weld seam with and without500N static pressure were obtained.The results show that the corrosion resistance of the 2A12aluminum alloy welded joints treated by 500N static pressure was improved.The improvement of corrosion resistance is attributed to the fast formation of the dense passivation film and the reduction of the precipitated phase size.After different corrosion time,the first-order exponential function model of (?) could better predict the dynamic change law of corrosion pit depth and diameter distribution of the HFIR-untreated and HFIR-treated base metal;the corrosion pit diameter distribution parameters and depth distribution parameters of HFIR-untreated weld seam obey they (?) first-order exponential function model and they(28)at~bfunction model,respectively.The distribution parameters of corrosion pit diameter and depth of the 500N-HFIR weld seam both follow the first-order exponential function model of (?).Fatigue crack growth tests were carried out on HFIR-untreated and 500N-HFIR treated welded joints,and the influence of HFIR treatment on fatigue performance of 2A12aluminum alloy welded joints was studied.By fatigue crack growth tests,the fatigue crack growth rate(P-da/d N-(35)K)models of the base metal and weld seam before and after HFIR treatment were obtained under different survival rates.The results show that the crack growth rate of the 2A12 aluminum alloy welded joint after 500N static pressure treatment was lower than that of the HFIR-untreated welded joint,the fatigue crack growth life of base metal and weld seam was increased by about 1.594～1.744 times and 2.902～3.231 times after HFIR treatment,respectively.The fatigue fracture mechanism of the base metal and weld seam was explained by analyzing the fracture morphology of different fracture stages(pre-crack region;fatigue crack growth region;transient fracture region).Compared with HFIR-untreated BM,the fatigue fracture of HFIR-treated BM shows smaller fatigue striations spacing and bigger and deeper dimples,implying that FCG rates were decreased and the ductility was improved by HFIR treatment.The fatigue fracture of weld seam CT samples before and after HFIR treatment was mainly cleavage fracture.The dense fatigue striations and more secondary cracks of HFIR-treated WS effectively reduced the FCG rate.In addition,HFIR region of HFIR-treated WS existed the dimple fracture characteristics,implying that the ductility was improved by HFIR treatment. |
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