| Al-Mg-Si(6xxx series)alloy has a wide application prospect in the lightweight development of automobiles,rail transit,and other fields because of its low density,high formability,good weldability,and corrosion resistance.To address the lack of mechanical properties of traditional Al alloy sheets,high cost of preparation and complicated process,the sub-rapid solidification twin-roll casting(TRC)is proposed in this paper.It facilitates the refinement of solidification structure,improvement of solid solubility and shortening of process flow,providing a feasible solution for the production of high-performance aluminum alloys.However,there are still three key issues facing the TRC 6xxx series aluminum alloy sheets,which are great limitations to its application promotion.First,the TRC 6xxx series Al alloy is prone to central segregation,which seriously impairs the formability and mechanical properties of the sheets.Second,the mechanism of formation and evolution of sub-rapid solidification structure in TRC is still unclear,the aging precipitation behavior of sub-rapid solidification aluminum alloys is not realized,and the effective control of high-strength and ductility of the structure is still not clarified.Third,the strength and thermal stability of 6xxx series Al alloys are mismatched,which limits the application environment of these alloys.Therefore,sub-rapid solidification Al-Mg-Si alloys were taken as the research object in this paper.Based on Zr microalloying to refine the dendrites and the second phase,promoting solute trapping at the solid-liquid interface and mitigating central segregation,highly solid-solution and highly formable cast and rolled plates were obtained;based on the study of the structure and properties of aluminum alloys with different Zr content additions,the formation mechanism of the fine grain organization and the aging strengthening mechanism of high solid solution aluminum alloys were revealed;based on the modification of precipitation phase characteristics,the strength and thermal stability of the alloys were improved,and the influence law and action mechanism of Zr and Ag composite addition on the thermal stability of the alloys were clarified;based on multiple alloying and heat treatment regime modification,a new kind of Al-Mg-Si-Cu-Zn(-Ag)alloy with both high forming in T4 state,high strength and high thermal stability in T6 state has been successfully developed.The main conclusions are as follows:(1)The evolution of solidification structure of Al-Mg-Si alloy with the addition of Zr element was investigated.It was found that the(Al,Si)3Zr phase,introduced by the addition of Zr elements,can serve as a nucleation substrate for α-Al and πAlFeMgSi phase morphology to achieve synergistic refinement of the dendrites and the second phase.The refinement of dendrites can shorten the solute diffusion distance,resulting in more solute atoms being solidified in the aluminum matrix,which reduces the solute content in the residual liquid phase,thus mitigating central segregation.Benefiting from the reduction of central segregation and the modification of uniform grain distribution,the fracture elongation of the homogenized alloy increased from～17.0%to～31.5%.(2)The formation of the fine grain structure and aging strengthening mechanism of high solid solution Al-Mg-Si alloy were revealed.With the addition of Zr element,the formation of(Al,Si)3Zr dispersoids achieved highly homogeneous finegrained structure control by pinning grain boundaries and inhibiting recrystallization.The high density of(Al,Si)3Zr dispersoids improved the effect of dispersion strengthening(～18 MPa),moreover,the Zr element contributed to accelerating the diffusion of solute atoms by allowing vacancies to be retained after quenching and released during aging,which promoted the precipitation of the β" phase,resulting in the alloys containing 0.13 wt.%Zr with higher volume fractions than those without the β" phase and much greater precipitation strengthening effect(～260 MPa vs.～244 MPa)than Zr-free alloys.It showed that the T6 state 0.13Zr alloy has a high strength-ductility match(yield strength～342 MPa,tensile strength～379 MPa and elongation of fracture～17.3%).(3)The effect of Zr and Ag elements on the thermal stability of T6 Al-Mg-Si alloy and its mechanism were clarified.Although the addition of Zr element introduced the high heat-resistant(Al,Si)3Zr dispersoids,the thermal stability of the alloys was not improved,which was related to the coarsening of the Mg-Si phase and phase transformation resulting from the accelerated diffusion of solutes.In contrast,the addition of Ag significantly improved the stability of the Mg-Si phase.At the early stage of thermal exposure,Ag element could be doped into the Mg-Si phase to stabilize the structure of the Mg-Si phase,and at the late stage of thermal exposure,Ag element tended to be enriched at the interface of the Mg-Si phase to increase the aspect ratio of the precipitated phase.It was noticed that the alloy with 0.07 wt.%Zr and 0.26 wt.%Ag additions exhibited an optimal match of strength and thermal stability,with the yield strength decreased by only～22 MPa with respect to the peak-aged state after 150℃/1000 h thermal exposure(～325 MPa vs.～347 MPa).(4)The effect of aging precipitation behavior of Al-Mg-Si-Cu-Zn(-Ag)alloys and its effect on the mechanical properties were investigated.It was observed that the addition of Zn and Ag elements promoted the aging precipitation kinetics but without changing the peak aging precipitation phase species,where Zn and Ag could be polarized at the phase interface of QP2 and thus stabilize the phase structure.It was developed that Al-Mg-Si-Cu-Zn(-Ag)alloys not only exhibited high formability(fracture elongation～34%,comparable to that of 6xxx series Al alloys)in the T4 state,but also high yield(390～409 MPa,close to that of highstrength 7xxx series Al alloys),and high thermal stability(yield retention of the alloys is higher than 98%after thermal exposure of 150℃/1000 h)in the T6 state.(5)The multi-phase synergistic precipitation mechanism and strengthening mechanism of high-performance Al-Mg-Si-Cu-Zn(-Ag)alloys were revealed.Pre-aging treatment at 120℃/12 h and addition of Ag elements resulted in the formation of number-dense GP zones,which could serve as nucleation sites to promote the precipitation of precipitated phases(β" and QP phases)upon aging at 180℃.The high thermal stability of the alloys was primarily attributed to the fact that the Mgrich Mg-Si-Cu clusters were basically transformed into the highly thermally stable QP2 phase upon aging at 180℃,without significant coarsening or transformation of the phase after long-term thermal exposure.It was calculated that the strengthening mechanism of the alloy in the T6 state,and the high yield strength were mainly obtained from the multi-phase synergistic precipitation strengthening(82～84%of the total yield strength). |
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