Solidification Behavior And Microstructure Control Of High Performance Twin-roll Casting Al-Mg-Si Alloys

Aluminum alloy is one of the most widely used non-ferrous materials in industry,which is of great significance to the development of lightweight in aerospace,transportation,and other fields.The strip continuous casting,which is represented by twin-roll casting(TRC),is a kind of preparation technology with high performance potential,high efficiency,and energy saving.However,the solidification behavior and the inhibition of macrosegregation are still unclear,which seriously restricts the rapid development of the TRC process.Therefore,based on the AA6005 alloy,the finite element simulation software Pro CAST was used to study the variation law of temperature field,fraction solid,and microstructure under the condition of the inclined TRC process.According to the simulation results,the appropriate parameters were selected to guide the inclined TRC experiment.Then,the selection of the second phase of TRC and direct chill casting(DC)alloys was studied,and the reason for the high performance of the TRC alloy was revealed.Next,the mechanism of Mn element improving macrosegregation of the AA6005 alloy was revealed.Finally,the effect of thermal exposure at 150 °C for 1000 h on the precipitation evolution and mechanical properties of the AA6005 alloy was studied under different aging conditions,and the high thermal stability TRC alloy sheets were successfully prepared by adding the Mn and Cu elements.The main research contents and conclusions are as follows:(1)Based on the finite element simulation software Pro CAST,the finite element simulation analysis of thermo-fluid-microstructure was carried out under the condition of the inclined TRC process of AA6005 alloy,and the TRC model coupled with temperature filed,fraction solid,and microstructure was obtained.Various effects of the pouring temperature,casting speed,and roll gap on the outlet temperature of the cast rolling zone were considered,and the appropriate range of process parameters was finally determined.Moreover,the distribution of the temperature field and the position of the solidification end point(Kiss point)were effectively predicted.Due to the different contact areas between the melt and two rolls,the temperature field and the shape of the liquid hole were asymmetric,and the isotherm and Kiss point shifted to the direction of the upper roll at the steady TRC state.The solidification behavior of the grains in the inclined TRC process was revealed by a cellular automaton-finite element(CAFE)model,and the experimental results were in good agreement with the simulation results.(2)The formation and evolution mechanisms of Fe-bearing phases in the TRC and DC AA6005 alloys were studied.The π-Al Fe Mg Si and β-Al Fe Si phases were formed in the as-cast TRC and DC processes,respectively.The differences in the formation of Fe-bearing phases are attributed to the promotion of the peritectic reaction to form the π-Al Fe Mg Si phase via the TRC process.After homogenization treatment at 550 °C for 2 h,the π-Al Fe Mg Si phase in the as-cast TRC alloy completely broke up into the fine β-Al Fe Si phase,while the morphology of β-Al Fe Si phase in the DC alloy remained unchanged.In addition,the decomposition of the π-Al Fe Mg Si phase increased the solid solubility of Mg and Si atoms in the Al matrix,which promoted the precipitation of the β″ phase.Finally,the yield strength of the TRC alloy(～274 MPa)was increased by ～16MPa compared with that of the DC alloy(～258 MPa).(3)The effects of Mn content on the formation of Fe-bearing phases,macro-segregation behavior,and mechanical properties of the TRC AA6005 alloys were investigated.Severe central and inverse segregations were observed in the as-cast AA6005 alloy,and the macro-segregation region was found to contain the coarse secondary phases containing Al,Fe,Mg,and Si.By the addition of 0.1and 0.3 wt.% Mn,the peritectic reaction to form the π-Al Fe Mg Si phase was inhibited,and the π-Al Fe Mg Si phase was transformed to the α-Al(Fe Mn)Si phase,which weakened the macro-segregation phenomenon.The 0.1Mn alloy exhibited a better combination of yield strength(～287 MPa)and elongation(～18.9%)than that of the AA6005 alloy(～274 MPa,～15.9%)under the peak aging condition.This was because the formation of α-Al(Fe Mn)Si phase had a strong contribution to the dispersion strengthening.(4)The precipitation evolution and mechanical properties of AA6005 aluminum alloy after different aging treatments(175 °C for 2 h,175 °C for 8 h,and 200 °C for 3 h)and thermal exposure at 150 °C for 1000 h were studied.The results showed that the thermal stability of AA6005 alloy after long-term thermal exposure could not be improved by changing the aging process.The precipitated phases of the experimental alloys with different initial aging states grew up completely after thermal exposure at 150 °C for 1000 h.At this time,the precipitation strengthening effect was significantly weakened,and the yield strength of the experimental alloys decreased to ～233 MPa.(5)The effects of Mn and Cu on the microstructure and mechanical properties of the TRC AA6005 alloy after peak aging and long-term thermal exposure were studied.The results showed that the addition of Mn introduces α-Al(Fe Mn)Si dispersoids with good thermal stability.However,after thermal exposure at 150 °C for 1000 h,the mechanical properties of the Mn-containing alloys did not improve significantly due to the coarsening of β″ phase.The addition of Cu changed the aging precipitation sequence,and the needle-like pre-Q′ phase was formed during artificial aging.Due to the low diffusion rate of Cu in the Al matrix,Cu atoms could better stabilize the structure and then improved the coarsening resistance of the precipitates.Therefore,the Cu-containing precipitates exhibited better thermal stability than the β″ phase.In summary,the finite element simulation software Pro CAST was used to simulate and analyze the thermo-fluid-microstructure of the inclined TRC process of AA6005 alloy,and the suitable process parameters were optimized.Then,the evolution of microstructure and mechanical properties during the homogenization,rolling,and heat treatment of AA6005 alloy by TRC and DC processes were systematically studied.Next,the macro-segregation of the TRC AA6005 alloy was weakened by adding Mn element.Finally,the thermal stability of AA6005 alloy after long-term thermal exposure was significantly improved by adding Mn and Cu elements.The results of this work were helpful to further understand the solidification behavior of the TRC aluminum alloy and provide a theoretical basis for the development of weak segregation and high performance Al-Mg-Si alloys based on the TRC technology.