| Currently,the government came up with the concept of Ecological Civilization,including Green Manufacturing,energy-saving,emission-reduction and so on.Al-Mg-Si-Cu alloys have aroused great interest among industry,owing to their lightweight,recyclable,good formability and excellent corrosion resistance.Because Al-Mg-Si-Cu alloys have medium-high strength,so the study on strength improvement could bring great social benefits.There is still some dispute about the precipitation strengthening mechanisms,and the influence of natural aging on the material's properties during subsequent artificial aging of the Al-Mg-Si-Cu alloys still need to be identified.Moreover,grain refinement could significant strengthening the alloy,but lack of research focus on the grain refinement of Al-Mg-Si-Cu alloy.Therefore,exploring the process of grain refinement and the mechanism of fine-grained strengthening on the Al-Mg-Si-Cu alloy are worth to be deeply inquired.AA6061 was selected as main research object in this study,which is widely used in industry.The same composition of Al-Mg-Si-Cu alloy was melted,and then the panels and bars were prepared by rolling and hot extrusion,respectively.Vickers hardness measurement,tensile test,differential scanning calorimetry(DSC),scanning electron microscopy(SEM),transmission electron microscopy(TEM),high resolution transmission electron microscopy(HRTEM),electron backscatter diffraction(EBSD)were adopted to systematically study:(1)the precipitation strengthening mechanisms during natural ageing and subsequent artificial aging in the Al-Mg-Si-Cu alloy;(2)the influence of pre-treatment on the precipitation behavior during natural aging and subsequent artificial aging;(3)the process of grain refinement and the fine-grain strengthening mechanisms in the Al-Mg-Si-Cu alloy.The key conclusions include:1.For the panel of Al-Mg-Si-Cu alloy,the yield strength after peak artificial aging of the as-quenching sample is 287.5 MPa(T6 condition).But when the as-quenching sample has 7 days natural aging,the yield strength after peak artificial aging is only 229.6 MPa.The formation of Mg-Si clusters induced by natural aging is beneficial for the hardness and yield strength,but the clusters still occupied a mount of Mg,Si atoms and quenched-in vacancies during artificial aging,which inhibited the precipitation and growth of ?" phase.Microstructures of the peak-aged samples show that the ?" precipitates in the naturally aged sample are smaller in size and much lower in volume fraction than the sample without natural aging,resulting in a decline of peak-aging hardness and yield strength.Moreover,the precipitation strengthening effect of the ?" phase was quantitatively evaluated by Orowan mechanism,which can well explain the negative effect of natural aging on subsequent artificial aging.2.It was found that the pre-deformation treatment could inhibit the negative influence of natural aging to some degree.Although the 5 % pre-deformation treatment cannot increase the peak artificial aging hardness,but it could reduce 50% of the artificial aging time to reach the peak aging condition compared with T6 treatment.Then adopt 5 % pre-deformation+210 ?Cpre-aging(10 min)as the pretreatment,the method completely suppressed the negative effect of natural aging.In addition,it significantly increased the peak-aging hardness(higher than T6 peak-aging condition)and shortened the time(by 50%)to reach peak hardness of the alloy during subsequent artificial aging.3.The Al-Mg-Si-Cu alloy foils with ultra-fine grains were first obtained using melt spinning,followed by hot extrusion in which the grains almost did not grow in the alloy rods.The yield strength of the extrusion rod was 114 % higher than that of the extruded rods of the alloy by a conventional direct chill casting.The study of microstructure reveals that the higher extrusion temperature(400 ?C,450 ?C,500 ?C),resulting in the larger grain size and less <111> fiber texture in the rods.Since few debondings defects exist in the 400 hot extruded rod,so 450 ?C is the best hot extrusion temperature.Moreover,the contribution of fine grains and texture to the extended yield strength were also been discussed,respectively.4.Under the process of artificial aging(180 ?C),the precipitation rate of the extruded melt spun sample is more faster than other Al-Mg-Si-Cu samples.The hardness of the fine grain rod is increased to 127.34 HV when aging 30 min,which is higher than T6 condition in the Al-Mg-Si-Cu panel.Microstructure results show that the strengthening phase in fine grain rod is also ? " phase,higer volume fraction of the ?" phase result in better mechanical properties. |
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