Influence Of Ageing On The Microstructure And Corrosion Resistance Of Al-Zn-Mg-Cu Alloys Refined With Nano TiC/Ti Refiner

As the technology of aerospace and national defense war industry developed rapidly,the requirements of formability,mechanical properties and corrosion resistance for Al-Zn-Mg-Cu high strength alloys are increasing as well.Adding nano TiC/Ti refiner can effectively refine the as-cast microstructure and reduce alloying elements segregation of alloys,thus,improve their plastic deformation properties and mechanical properties.The mechanical properties of Al-Zn-Mg-Cu alloys can be further improved by ageing strengthening,but,there is no report about the influence of ageing on the corrosion resistance of Al-Zn-Mg-Cu alloys refined with ultrafine ceramic particles.Therefore,after using the TiC/Ti refiner to modify the as-cast microstructure,and then research the microstructure and corrosion behavior of aged Al-Zn-Mg-Cu alloy refined with ultrafine ceramic particles,explore the corrosion mechanism will have profound theoretical and practical significance.A novel grain refiner consisting of TiC nanoparticulates suspended in a metal Ti powder?abbr.nano TiC/Ti refiner?was prepared by high-energy ball milling.The TiC/Ti refiner was used to refine the as-cast microstructure of Al-Zn-Mg-Cu alloys,then the refined alloys were proceeded with homogenization and hot extrusion deformation.After that,the specimens were treated with solution treatment,and followed with peak ageing and over ageing treatment.The effect of ageing on the microstructure,microhardness and corrosion resistance were studied.Moreover,the electrochemical corrosion method combined with microstructure analysis were used to explore the corrosion mechanism.The conclusions can be summarized as follows:TiC/Ti refiner shows remarkable performance in refining the as-cast microstructure of Al-Zn-Mg-Cu alloy,its average grain size decreased to 119?m from 525?m when Al-Zn-Mg-Cu alloy is unrefined.The average grain size of peak-aged and over-aged Al-Zn-Mg-Cu alloy refined with TiC/Ti are 57?m and 65?m,and vickers hardness are203HV and 189HV,respectively.The soaking corrosion rate and intergranular corrosion depth are decreased by over ageing treatment.When soaked in exfoliation corrosion solution for 48h,the peak-aged Al-Zn-Mg-Cu alloy refined with TiC/Ti refiner is blasting and layering severely,while the corrosion only occurs in minorly region and no layering at over-aged alloy.Agminated TiC particles and coarse Al₇Cu₂Fe phase would accelerate the corrosion process.The stress corrosion resistance of over-aged alloy is higher than that of the peak-aged alloy,and they all show as intergranular corrosion fracture morphology of the stress corrosion fracture.The stress corrosion mechanism of Al-Zn-Mg-Cu alloy refined with TiC/Ti refiner including two aspects,anodic dissotion and hydrogen embrittlement.Anodic dissolution is the main factor for stress corrosion cracking in 3.5wt.%NaCl solution,while in 3.5wt.%NaCl+2.0wt.%H₂O₂ solution,its corrosion carcking is dominated by hydrogen embrittlement,and the corrosion resistance decreased with respect to immerse in 3.5wt.%NaCl solution.When the alloys are refined with TiC/Ti refiner,the open circuit potential,polarization voltage and polarization resistance of over-aged Al-Zn-Mg-Cu alloy are larger than that of peak-aged alloy.In 3.5wt.%NaCl aqeous solution,the impedance spectrum is shown as high frequency capacitive arc,it is starting pitting at the passive film of Al-Zn-Mg-Cu alloy refined with TiC/Ti refiner.However,in 3.5wt.%NaCl+2.0wt.%H₂O₂solution,the impedance spectrum of peak-aged alloy are high frequency capacitive arc and low frequency reactance arc,it is pitting seriously of the passive film and the alumina matrix starts denude,while the impedance spectrum of over-aged alloy is only high frequency capacitive arc,there is only pitting at the passive film,the corrosion resistance is improved.The main influencing factors of corrosion resistance are grain boundary precipitates and PFZ,and the latter plays a leading role.TiC partcles can improve the precipitation power,promote solute atoms diffuse to grain boundary.When the alloys are refined with TiC/Ti refiner,the discontinuous grain boundary precipitates of peak-aged alloy can cut off the corrosion channel which is favorable to the corrosion resistance,while the wide PFZ will reduce the corrosion resistance significantly.However,with respect to over-aged alloy,the discontinuous grain boundary precipitates,narrow PFZ and their increasing content of Cu atoms will improve the corrosion resistance evidently.