| With the development trend of energy saving,environmental protection and automobile lightweight,age-hardening 6xxx series Al-Mg-Si-Cu alloy have low density,medium strength,high specific strength,high corrosion resistance,good formability,good weldability,and good surface quality and other characteristics,are widely used in automobile body panels,engine parts.The ultra-fine grain(UFG)aluminum alloy obtained by equal channel angular extrusion(ECAP)technology has high strength,sufficient ductility,high strain rate and good impact toughness.In addition,after ECAP,an appropriate aging treatment process is introduced to form dispersed nano-metastable precipitates in the ultrafine crystal matrix,which can effectively improve the strength of the alloy while ensuring sufficient toughness.Therefore,it is of great significance to study the ageing precipitation characteristics and strengthening and toughening mechanisms of ultrafine grained Al-Mg-Si-Cu alloys under different aging treatments.Different characterization methods and mechanical properties test were used in this paper to study the precipitation characteristics and mechanical properties of four different compositions of 6000 series aluminum alloy static aging,ECAP state and post-aging state,like differential scanning calorimetry(DSC),X-ray diffraction(XRD),transmission electron microscopy(TEM),high resolution transmission electron microscopy(HRTEM),atomic resolution high-angle annular dark-field scanning transmission electron microscopy(HAADF-STEM)and three-dimensional atom probe(APT).The focus is on the detailed microstructure study of the ECAP state of the No.3 high Si and high Cu alloy with different passes and the aging state after two passes ECAP.The main conclusions include:(1)DSC analysis shows that:compared with solid solution alloys,the exothermic peaks of precipitates of alloys with different composition move forward to different degrees after ECAP,and tend to the low temperature region.The exothermic peak area of the precipitates is also greatly reduced.It has better precipitation characteristics after ECAP.The activation energy calculation shows that the high Cu alloy has faster precipitation kinetics and higher peak temperature.In the low Cu alloy,the?''precipitate precipitation peak is smaller,and the?''precipitate activation energy is also lower.In the low Cu alloy,only a small amount of?''precipitate is precipitated during the thermal analysis.(2)The Mg-Si-Cu atomic clusters and the GP zone are continuously precipitated when the No.3 high Si and high Cu alloy is aged at 180?for 10 min-1 h after two passes ECAP.After aging for 1 h,the GP zone and the Mg-Si-Cu atomic clusters dissolve occurred and the?"precipitate began to precipitation.After aging for 2 h,a large amount of?"precipitate was precipitated in the alloy.After aging for 6 h,the?"precipitate inside the alloy begins to dissolve and transform into a small amount of??and Q'precipitates.The precipitation of??and Q'precipitates is mainly completed between 6-24 h.After aging for 24 h,The?'and Q'precipitates are no longer precipitated inside the alloy,and the?'and Q'precipitates begin to transform into stable precipitates.(3)The XRD pattern analysis showed that the main precipitates of the alloy after three passes of ECAP were L and Q'precipitates.After four passes of ECAP,the types of precipitates in the alloy remained unchanged but the density increased.The precipitates of the alloy aged for 10 min(under-aging)after 2 passes are GP zones and clusters,and the precipitates of the alloy at 2 h(peak-aging)are??and L precipitates.After 24 h(over-aging),The precipitates are Q'and L precipitates.As the number of ECAP deformation passes increases,the grain size and lattice constant of the alloy continue to decrease,and the dislocation density and microstrain increase greatly.After four passes of ECAP,the grain size of the alloy reaches 164 nm,the dislocation density is 1.96×1013m-2,the microstrain is 0.0368%,and the lattice constant is 4.0472.After post-aging treatment,the grain size of the alloy began to increase rapidly,the dislocation density and microstrain began to gradually decrease,and the lattice constant gradually returned to normal.(4)TEM-HRTEM analysis shows that the main precipitates of the alloy are the GP zone due to the lack of storage energy after the two passes ECAP.After two passes of ECAP and 250°C×0.5 h artificial aging,all the GP zone inside the alloy is transformed into L and Q'precipitates.After another passes of ECAP,the density of the precipitates inside the alloy increases greatly,and the size of the precipitates changes.The small part analyzes that the precipitate is spheroidized.At this time,the precipitates in the alloy are a large amount of L precipitate and a small amount of Q'precipitate.After four passes of ECAP,all precipitated phases in the alloy are spheroidized,and the alloy microstructure is ultrafine grain+high-density dislocations+spheroidized precipitates.The two passes ECAP and post-aging TEM showed that the alloy remained ultra-fine grains after 2 hours of aging,with a grain size of 517 nm.At this time,three kinds of nano-precipitates are precipitated in the alloy,which are needle-shaped??precipitate,rod-shaped L precipitate and lath-shaped Q'precipitate.(5)The composition analysis of the three-dimensional atom probe shows that after three passes of ECAP,the measured composition of the L precipitate is 12.1±2.3Al-42.3±3.2Mg-38.5±2.5Si-7.1±1.3Cu(at.%),and The measured composition of the Q'precipitate is 22.3±2.3Al-41.3±2.2Mg-31.5±2.6Si-4.9±1.8Cu(at.%).Both precipitates have high Al atoms and low Cu atoms with the standard chemical composition.The current situation may be that a large amount of Al atoms are solid-dissolved in the Mg,Si and Cu atom columns of the precipitate during the deformation process,and Cu atoms may be segregated on dislocations or grain boundaries in the presence of solute atoms.(6)Hardness experiments show that after two passes of ECAP,the hardness of No.4 high Mg and high Cu alloy reaches 145 HV.After four passes,the hardness of No.3high Si and high Cu alloy is 148 HV.The hardness of the ECAP alloy showed a downward trend at the initial stage of post-aging,and then rose rapidly,reaching the peak hardness at about 80-100 min,and then rapidly decreasing.The higher passes ECAP deformation can significantly accelerate the precipitation kinetics and promote the alloy to reach a higher hardness.The No.4 high Mg and high Cu alloy reaches the peak hardness of 148HV after aging at 180?for 80 min.However,the higher the deformation,the more severe the influence of temperature,and the long-term heat treatment will cause the performance of the alloy to drop significantly.(7)The tensile test shows that the tensile strength of the alloy increases significantly after the ECAP is deformed.No.3 alloy has the best mechanical properties after two passes ECAP.The tensile strength is 410 Mpa,which is 208 MPa higher than the solid solution state,while maintaining a uniform elongation of 11.3%.After the post-aging treatment,the mechanical properties of the alloy were further improved.After the post-aging treatment at 180?for 2 h,the tensile strength and uniform elongation of the alloy were 452 MPa and 7.3%,respectively.(8)Through the characterization of three-dimensional atom probe technology and related literature,the interaction process and evolution law of aging precipitates and dislocations in the ECAP process of ultra-fine grain 6000 series aluminum alloys are proposed:the interaction of high-density dislocations and strong strains.It has undergone rod-shaped precipitates?dislocation cutting?spherical precipitates+rod-shaped precipitates+atomic clusters?atomic cluster nucleation and growth?spherical precipitates,the final alloy microstructure is ultrafine grains+high-density dislocation+dispersedly distributed spherical precipitates. |
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