The Microstructure Evolution And Aging Behavior Of The Rolled Al-Mg-Si Alloys Prepared By Sub-rapid Solidification

Al-Mg-Si alloy,as the typical heat-treated aging aluminum alloy,is considered to be the most promising automotive material due to the characteristics of great formability during processing and high yield strength after aging hardening.How to produce high-performance Al-Mg-Si alloy with high efficiency and low cost is an urgent issue in the aluminum industry.The cooling rate of sub-rapid solidification could reach 10²-10³K/s,which provides a feasible method for the efficient production of aluminum alloys.However,the recent research on sub-rapidly solidified aluminum alloys,including microstructure evolution under sub-rapidly solidified conditions,aging behavior and alloying element effects,is not in-depth.The development of production of high-performance sub-rapidly solidified Al-Mg-Si alloy is obviously restricted.In this study,the Al-Mg-Si alloy prepared by sub-rapid solidification(SRS)was investigated by optimizing the heat treatment process.The microstructure evolution mechanism of SRS Al-Mg-Si alloy were systematically studied.The aging behavior and mechanical property strengthening mechanism were revealed,and the influence law of the subrapid solidification process on the subsequent mechanical properties was clarified.Meanwhile,the effect of Zr and Ti on the thermal stability of Al-Mg-Si alloy during solution quenching was discussed.Furthermore,the effect of Zr element on the aging behavior and mechanical properties of SRS Al-Mg-Si alloy were studied in depth.The high-performance Al-0.7Mg-1.2Si-0.15Fe-0.2Zr(wt.%)alloy had been optimized,which provides a reference for the study of the strong-plasticity optimization of Al-Mg-Si alloy.The main research contents and conclusions are as follows:(1)The microstructure evolution of Al-Mg-Si alloy at different cooling rates was studied,and the influence of microstructure on the aging behavior and mechanical properties was revealed.There are more Mg₂Si phase and coarse Al Fe Si phase in the alloy prepared by CS.Due to the low solid solubility of Si element in Al alloys,a considerable number of coarse Si particles is observed,which would decrease the content of Si atoms which could participate in aging precipitation and also reduce the mechanical properties.The increase of cooling rate leads to a more uniform and finer Al Fe Si phase,and a significant decrease of Mg₂Si phase content,which result in improving the potential of ductility and aging potential.The grain size is obviously refined and the coarse Si particles are hardly observed.Based on SRS,Si atoms would be supersaturated solid soluble in the matrix in SRS Al-Mg-Si alloy,which provides the dynamic conditions for the subsequent artificial aging.(2)The aging behavior and strengthening mechanism of mechanical properties of sub-rapid solidification Al-Mg-Si alloy were studied.SRS Al-Mg-Si alloy with short-time homogenizing after T4 treatment has the lower yield strength(?133 Mpa)and better ductility(elongation,?30%),which is good for manufacturing processing.The solidification microstructure of the CS counterpart has the coarse Si particles and Al Fe Si phases,which would reduce the plastic and aging potential.The yield strength(?305 MPa)and elongation(?19%)of the SRS alloy after T6 treatment are significantly better than the CS alloy(yield strength?254 MPa,elongation?13%).In sub-rapid solidified Al-Mg-Si alloy Si and Mg atoms that supersaturated solid soluted provides the faster aging dynamics,resulting in higher yield strength aging response(?170 MPa),much higher than that of conventional solidification alloy(?100 MPa).The main strengthen precipitate??of SRS alloy after T6treatment has finer size and more number density than those of CS samples,which contribute to the better precipitation strengthen effect.In SRS alloy,there is hardly none coarse Si particles,while Si phases of the nanometer size(?40 nm)are observed after T6treatment,which may be because of the precipitation of supersaturated solid soluted Si atoms and improve the aging harden response and mechanical properties.(3)It is found that Ti and Zr elements can improve the thermal stability after the high temperature solidification treatment of the SRS Al-Mg-Si alloy.The distribution of the second phase and grain size is also optimized.The addition of Zr element was found to refine the grain size and improve the distribution of microstructure and mechanical properties.Al₃Zr phase has high thermal stability,and the average grain size of SRS alloy can be maintained as 13-15?m after the long-time homogenization treatment.The mechanical properties of SRS alloy have been improved and the excellent performance(yield strength?355 MPa,elongation?16%)is obtained before reaching the peak of the hardness.The addition of Zr elements improves the aging hardening response of Al-Mg-Si alloy,which is due to the synergistic effect of Zr atoms in supersaturated solution with Si and Mg atoms in matrix during artificial aging,which accelerates the precipitation of solute elements and forms the nanoscale Zr-containing second phase.The results show that the sub-rapid solidification process can improve the content range of Zr in Al-Mg-Si alloy.