| 6xxx series alloys are the key materials of automobile lightweight,and replacing steel parts with these alloys is the development goal in the automobile field.Therefore,it is of great value to develop low-cost and high-performance 6xxx series alloys.Twin-roll casting(TRC)has broad development prospects due to its advantages including micro structure refinement and process shortening.However,to promote the application of TRC 6xxx series alloy strips,there are three key issues to be studied and solved.First,the influencing mechanism of sub-rapid solidification during TRC on the strength-ductility of alloys is still unclear so far,which results in the lack of scientific application guidance.Second,TRC strips often suffer from serious center macro-segregation,which can damage the processability.Third,the thermal stability of 6xxx series alloys is poor,which limits the application environment of these alloys.Accordingly,sub-rapid solidified TRC Al-Mg-Si-Cu alloys were chosen as the research object in this paper.Based on the study of microstructure and mechanical properties of different solidification modes,the mechanism of the high strengthductility synergy of TRC alloys was revealed;based on the control of the thermalmechanical coupling process during TRC,the center segregation was alleviated,and the strips with uniform microstructure and high processability were obtained;based on the control of Mg/Si ratio,the thermal stability of alloys was improved and the influencing mechanism of precipitate type on thermal stability was elucidated.The purpose of this paper is to provide a reference for the development and application of aluminum alloy sheets with high strength-ductility synergy and high thermal stability for vehicles.The main conclusions are as follows:(1)The solidified microstructure and aging behavior of TRC and ingot casting(IC)alloys were studied.It was found that the TRC refined the microstructure and promoted the formation of Si-riched quaternary AlFeMgSi phases.During the solution treatment process,the Si-riched AlFeMgSi phase could decompose into the Fe-riched a-AlFeSi phase,which expanded the solid solubility of Si and Mg.Therefore,an enhanced aging hardening response and high volume fraction of β"phases were obtained,and excellent tensile properties with yield strength(～340 MPa),tensile strength(～370 MPa),and elongation(～19%)were achieved in the peak-aged TRC sample,which were higher than those of the IC counterpart(～320 MPa,～350 MPa,and 14%).(2)The thermal-mechanical coupling process of TRC strips with different thicknesses(3 mm and 4 mm)was studied.It was found that the π-AlFeMgSi phase was formed in the 3 mm strip,while the α-AlFeSi phase was formed in the 4 mm strip.Based on the crystallographic calculations and experimental verification,two pairs of matching planes between the α-AlFeSi and Mg2Si phases as well as one pair of matching planes between the α-AlFeSi and Q-AlCuMgSi phases were identified:(0001)α-AlFeSi//(110)Mg2Si,(11(?)0)α-AlFeSi//(1(?)0)Mg2Si,and(11(?)0)α-AlFeSi//(0001)Q.(3)The mechanism of center segregation’s mitigation based on the thermalmechanical coupling control was elucidated,which was mainly attributed to the changes in the solidification behavior and mechanical stress.From the perspective of solidification behavior,the α-AlFeSi phase could serve as nucleation substrates for the Mg2Si and Q-AlCuMgSi phases,thus promoting the uniform distribution of elements from the surface to the center;from the perspective of mechanical stress,the RSF of the 4 mm strip was smaller,thus the liquid flow behavior induced by the external force was weakened,resulting in the alleviation of the center segregation.Thus,excellent ductility was obtained in the homogenized 4 mm strip(～29%),which was higher than that of the 3 mm counterpart(～20%).(4)The mechanical properties and thermal stability of alloys with various Mg/Si ratios(0.5,1,2,and 4)under the T6 condition were studied.It was found that the thermal stability of Mg-riched alloys was significantly better than that of Si-riched alloys.After thermal exposure at 150 °/1000 h,for Mg/Si～0.5 and 1 samples,the yield strength reduction rate reached～10%and the elongation was less than 10%.While the yield strength of Mg/Si～2 and 4 samples did not decrease significantly and the elongation was more than 10%.The best performance after thermal exposure was obtained in Mg/Si～2 sample,showing the yield strength,tensile strength,and elongation of～330 MPa,～350 MPa,and～11%,respectively.(5)The micro structural evolution during thermal exposure of alloys with various Mg/Si ratios was studied.It was found that the formation of Cu-containing precipitates was promoted in Mg-riched alloys.First,the aging kinetics of Mgriched alloys was relatively slow,thus leaving a relatively long diffusion time for Cu atoms;meanwhile,Cu atoms entering Mg-riched clusters could provide a beneficial reduction in the strain energy;therefore,β" phase was the main strengthening phase in the peak-aged Si-riched alloy(Mg/Si～1),while L phase was the main strengthening phase in the peak-aged Mg-riched alloy(Mg/Si～2).(6)The mechanism of thermal stability difference of alloys with various Mg/Si ratios was analyzed,and the reason for the excellent thermal stability of Mg-riched alloys was that the L phase had a better coherent with the matrix and had lower interfacial energy,thus resulting a lager resistance towards coarsening.Therefore,during the thermal exposure process,the pre-Q’ and β" phases in Si-riched alloy were transformed into Q’ phase,and then the Q’ phase was significantly coarsened.While the L phase in Mg-riched alloy was not obviously coarsened or transformed,showing better thermal stability. |
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