The Mechanism Research Of Strength,Toughness And Ageing Stability Of AA7021 Aluminum Alloy Thin Plates

With the gradual enhancement of people’s safe travel consciousness,vehicle crash safety has drawn much more attention under the background of energy-conservation and emission-reduction.The aluminum alloys are widely used in the field of transportation and aerospace,especially in the field of automobile industry which is of great significance for automobile lightweight,due to their higher strength-to-weight ratio,good plasticity,excellent weldability and processability.etc.Therein,6000 series aluminum alloys(Al-Mg-Si)have played a crucial role in lightweight of automobile for the applications of outer panels,but it is difficult to realize the lightweight of bearing structure with the low strength.For service safety concerns,the high strength of 7000 series aluminum alloys(Al-Zn-Mg-Cu)can satisfy the strength demands of bearing structure,while application thickness range(>4.5 mm),weak thin-plate preparation technology and inferior formability still confine further uses in automotive field.Therefore,the moderate-strength 7000 series aluminum alloys(Al-Zn-Mg),which can well make a compromise between strength and formability,increasingly turn into the ascendant candidate materials for lightweight automotive structural.Up to now,the exploitation of moderate-strength 7000 series cold rolled thin plate is still in the blank stage and the knowledge about strength,toughness and room temperature formability of cold rolled thin plate is also scarce.In addition,both the poor corrosion resistance and weak natural ageing stability are the disadvantages for the applications of moderate-strength 7000 series aluminum alloy thin plates.All the above-mentioned reasons restrict their application ranges in automobile Field.So,in this,the AA7021 aluminum alloy was selected to investigate the thermos-mechanical treatment for the purpose of grain refinement and systematically analyse the recrystallization mechanism based on deformation induced precipitation(DIP)effect and particle stimulated nucleation(PSN)effect.In terms of heat treatment and service safety of thin plates,this work explored enhanced baking hardening response,aging stability and corrosion behavior.Moreover,this work also examined the crack propagation behavior around different crystallographic orientation grains.The following conclusions were drawn:1)The hot working process window of AA7021 aluminum alloy is accurately obtained at the temperature ranges of 380～450 ℃ and strain rate ranges of 0.01～0.5 s-1 on the basis of constitutive equation and processing maps.At whole hot deformation processes,the cross slip and three dimensional dislocation network debonding dominate the dynamic recovery process.When the deformation temperatures exceeds 350 ℃,the discontinuous dynamic recrystallization nucleation mechanism is grain-boundary bulging(BLG),and the continuous dynamic recrystallization nucleation mechanism is sub-grain rotation(SRG)around and inside the grains.2)400 ℃+3 h pre-aging treatment contributes to the coarsening of MgZn2 particles,the coarsened particle size ranges and quantity density are 0.5～1μm and 7.2 ×1010 per square meter respectively,which are similar to those in classical RI-ITMT that surpasses 8 h at 400 ℃.Large size MgZn2 particles(>0.5μm)can act as nucleation sites to facilitate grain refinement in high stacking fault energy AA7021 aluminum alloys by PSN effect.STMT samples exhibits similar grain refinement effect with the RI-ITMT samples,the grain size is 12.53 μm in STMT samples and 10.48 in RI-ITMT samples.3)Grain orientation analysis reveals that Goss and random grains with high twist angle boundary components present a significant crack deflection effect.Besides,there is no evidence that tensile crack deflections are relevant to tilt angle boundaries in uniaxial tension tests.In the condition of transverse rolling,it is indicated that Brass grains with 8 movable slip systems can effectively facilitate intracrystalline extension for secondary cracks,however,in Copper grains the secondary cracks only propagate along the boundaries because of the less movable 6 slip systems.4)The pre-aging treatment(PA’,120 ℃+20 min)can produce plenty of stabilized GP zones acting as precursors(GP-2 zones)to form the metastable η’phase and then improve the bake-hardening response of AA7021 aluminum alloy thin plates.The ageing stability of AA7021 aluminum alloy thin plates is developed with PA’and Sn sddition,and the ageing stability reaches the peak with the coupling effect of PA’and Sn sddition,increasing from 2 h(natural ageing,NA)to 11 d(PA’+Sn sddition).On the contrast,the η’phase precipitation activation energy declines dramatically from 95.38 kJ/mol(NA)to 38.2 kJ/mol(PA’+Sn addition).In terms of corrosion,the corrosion resistance of AA7021 aluminum alloy thin plates after bake hardening treatment(BH)is higher than PA’.Furthermore,we attribute the unstable passivation sites indwelled in potentiodynamic polarization curves before and after BH to the damage effect of ageing precipitation phases on passive films on the sample surfaces.In addition,some Sn containing second phase particles before and after BH exist in trace Sn containing AA7021 aluminum alloy matrix,it is deduced that Sn addition weakens the corrosion resistance of AA7021 aluminum alloy thin plates.