Mechanical Properties And Thermal Stability Of High Solid Solution Bimodal Structured Al–Mg Alloys Processed By Hard-plate Rolling Route

Due to a good combination of properties such as formability,mechanical strength,weldability and corrosion resistance,Al-Mg alloys are widely used for commercial application.However,the strength of conventionally deformed Al-Mg alloys is hardly comparable to steel Magnesium has a high solid solubility in Al matrix,leading to a strong work hardening ability.The strength of Al-Mg alloys can increased by severe plastic deformation technology,but their ductility would reduce significantly and limiting the industrial applications.Traditional large deformation techniques include ECAP and HPT and so on.However,the ECAP and HPT techniques can only prepare small-volume bulk materials.Therefore,industrial production of aluminum alloy sheets generally adopt the conventional rolling method.But due to the conventional rolling process has a large shear stress along rolling direction,making the hard-to-deformed Al-Mg alloys easy to crack.Furthmore,for the preparation of difficult-to-deformed aluminum alloy sheet,a multi-pass and small-reduction rolling method is generally adopted,leading to a long production process.In response to the above bottleneck problems,this paper developed a new type of short flow liner hard-plate rolling(HPR)route for high so lid-solution Al-Mg alloys.The microstructure evolution of high so lid-solution Al-Mg alloys processed by HPR has been investigated via electron backscattering diffraction(EBSD)and transmission electron microscope(TEM)and so on.By evaluating the mechanical properties of Al-Mg alloys with different Mg contents,finding that high so lid-solution Mg content can promote the formation of bimodal structure,and thus enhance the strength and ductility simultaneously.The conclusions are as follows:(1)The new HPR technology was demonstrated to be an efficient and short flow route in preparing high solid solution Mg(>5 wt.%)content Al-Mg alloys.For the hard-to-deformed high so lid-solution Al-Mg alloys,the single-pass rolling reduction reached to 80%and much higher than that of conventional rolling.The optimal Mg content for HPR route can be up to 9 wt.%.The HPRed sheets remain smooth and unbroken and have a good industrial application prospect.(2)Based on EBSD and TEM analysis,The HPR is inhomogeneous deformation process and the high Mg content can restrain dynamic recovery effectively.Hence,a bimodal microstructure consisting of coarse micron grains and ultrafine grains(?200-400 nm)were formed in HPRed Al-5Mg and Al-9Mg alloys.The contained coarse grains dominated by(101)orientation,and four slip systems could be activated simultaneously during deformation process and the distribution of dislocations is relatively uniform in these grains,resulting in a higher strain.These(101)oriented grains are relatively stable and hard to be refined,and still remained coarse in HPRed high so lid-solution Al-5Mg and Al-9Mg alloys.(3)The HPRed Al-Mg alloys possess promising mechanical properties.Especially,the HPRed Al-9Mg alloy possesses a superior combination of high ductility(?140-%)and strength(UTS of?525 MPa).The high strength is mainly attributed to a high dislocation density,a high level of Mg solid solution and the ultrafine grains.The reason for the good ductility is the strong work hardening ability resulting from the bimodal grain structure and high Mg solute content.(4)Compared with the ultrafine-grained Al-Mg alloys processed by severe plastic deformation,the HPRed bimodal structured Al-9Mg alloy possesses high thermal stability.When annealed at 275 ?,the recrystallized fraction is still below 10%and considerable grains are mainly(101)oriented although the annealing time reached 24 h.While the annealing temperature is higher than 300 ?,the microstructure would recrystallize gradually and the optimum recrystallization temperature is?300-350 ?.Furthermore.we establish the transformation relationship of annealing temperature and time for severely deformed high Mg-containing Al-Mg alloys.At relatively lower annealing temperatures(<275 ?),dislocation recovery and rearrangement occur in the coarse grains firstly and the stored energy is consumed gradually,thus the recrystallization process is hard to occur.when annealeded at moderate to high temperatures(300-350 ?),recovery and recrystallization occur firstly at boundaries of shear bands and coarse grains,and the recrystallized fraction increases with the annealing time.When the annealing temperature is relatively high(>400 ?),the recrystallization could occur rapidly due to the high driving force and activation energy.