Effects Of Thermal-mechanical Treatments On Microstructures And Local Corrosion Behavior Of Al-Mg-Si-Cu Alloy

Due to their excellent properties such as light weight,aging strengthening,good ductility,exellent formability and good weld-ability performance,Al-Mg-Si-(Cu)alloys are widely used in transportation.They can be manufactured to be complex parts by rolling,extrusion,drawing,forging,casting and other processes.Al-Mg-Si alloy generally has a relatively low strength.Al-Mg-Si-Cu alloys have higher strength.However,with higher Cu content,Al-Mg-Si-Cu alloys are susceptible to local corrosion because copper-rich precipitates and copper-enrich films formed at the grain boundaries,causing micro-couples between precipitate free zones and grain boundary precipitates(and/or copper-enrich films).Effects of pre-aging and post-aging treatment on local corrosion behavior of samples with various deformation strain ratios(DSRs)is still unclear.In the present study,effects of microstructure,including grain microstructure,as well as size,type and distribution of precipitates in the matrix and grain boundary precipitates,on properties,including strength,ductility and local corrosion behavior by modifying processing parameters to tune microstructures,was investigated.Microhardness,tensile and accelerated corrosion tests were used to analyze their performance.Scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),and elemental energy spectrum analysis(EDS)were used to investigate the relationship between local corrosion behavior and microstructure.Main conclusions can be drawed as follows:Compared with traditional T6 artificial aging,a combinatorial process consisting of pre-aging(aging at 180 ~oC for 10 min),cold rolling(80%)and post-aging(peak aging at 120 ~oC)can obtain enhancing combinatorial properties,including excellent local corrosion resistance,high strength,and good ductility.Microstructures analyses reveal that high strength is mainly due to finely dispersed lath-like phases within the matrix,dislocations,dislocation cells and sub-grain boundaries.Meanwhile,a more uniform distribution of solute atoms on grain boundaries reduces the mirco-electro-chemistry between grain boundary precipitates and precipitates free zones.Additionally,a modification of grain boundary structure by cold rolling makes it difficult for intergranular corrosion to expand deeply along the grain boundaries.By controlling cold rolling reductions,the structure of grains and grain boundaries of Al alloy can be modified,and then after the subsequent aging treatment,the segregation state of solute atoms can be further changed,thereby affecting the local corrosion behavior of the alloy.Peak-aged samples with small DSRs(5–10%)are sensitive to intergranular corrosion(IGC)along the transverse-sections,cross-sections and rolling planes.When DSRs increase to medium strength(20–40%),IGC sensitivity decreases in these planes,and corrosion propagates anisotropically along cross-sections and rolling planes.When DSRs are higher(60–80%),IGC sensitivity along the transverse-sections,cross-sections decrease.However,it increases along the rolling planes,showing pronounced corrosion anisotropy.Microstructural characterization indicates that the decreased IGC sensitivity mainly attributed to a reduction in element segregation at high-angle grain boundaries and a kinked and hard grain boundary corrosion path.Whereas the increased IGC sensitivity observed in the rolling planes results from the flattened grains boundary corrosion paths.By introducing different dislocation densities,the segregation state of solute atoms in the alloy in grains and on grain boundaries during post-aging process can be changed,thereby changing the properties of the alloy.IGC resistance of Al-Mg-Si-Cu alloy was improved by over aging treatment for samples with 20%DSR.For the samples with 40%DSR,IGC sensitivity of the peak aged sample is low,and that of the over-aged sample increase.TEM observation shows that Q phases precipitate at grain boundaries of the sample with 20%DSR after peak aging treatment.Grain boundary precipitates during overaging treatment get coaser.Such that,continuous corrosion paths are interrupted by increasing the spacing of precipitates.Therefore,overaging treatment can improve corrosion resistance.For the sample with 40%DSR,amount of solute atoms precipitates on high angle grain boundaries decreases,thus increasing corrosion resistance of high angle grain boundaries.During post over-aging treatment,defects can recovery and annihilation at high angle grain boundarie,which can promote more solute atoms diffusion to grain boundary,leading to increases of solute atoms segregating at grain boundaries precipitates.High resolution TEM observation results show that some Q phases precipitate on grain boundaries.Q phases precipitate at grain boundaries and precipitate free zones at the adjacent of grain boundaries could form corrosion microcouples at grain boundaries,so IGC sensitivity of grain boundaries is improved.By adjusting properly cold rolling and peak aging treatment,a combination of high strength and good corrosion resistance can be obtained in Al-Mg-Si-Cu alloy.Re-precipitation of solute atoms during post-aging is related to the segregation state of solute atoms during pre-aging.It is an effective way to modifiy the microstructure and improve local corrosion resistance of Al-Mg-Si-Cu alloy by controlling solute atoms segregation during pre-aging treatment.Samples with natural aging pre-treatment had excellent IGC resistance.After post peak-aging treatment corrosion morphology was no longer typical IGC,but pitting corrosion,and the maximum corrosion depth was significantly reduced.The main reason is that amount of precipitates on grain boundaries are less.Samples with pre-artificial aging for 12 h(AA12h)had high IGC susceptibility.It is because that there are a large number of Q phases on the grain boundaries.Such that grain boundaries are very sensitive to IGC.Two kinds of precipitates are formed in the natural aging pre-treated samples,one is the long and curved continuous distribution near the dislocation cell,the other is the dispersion distribution.For samples with AA12h pre-aging,mainly lath-like phase precipitate in the grains.After subsequent cold rolling and post-aging treatment,the precipitate still exists in the grains.After subsequent annealing treatment,there are still a large number of dislocation in the grains.Matrix is still in the state of high strain.