The Controlled Rolling Process And Superplastic Deformation Behaviors Of Mg-Al-Sn-Zn System Alloys With High Al Content

Recently,with the energy and environmental issues becoming prominent,energy-conservation and environment-protection have become the necessary choices for the healthy development of human civilization.The concept of green,scientific,and sustainable development has been deeply rooted among the people.Therefore,as the lightest metallic structural material,meanwhile showing an excellent specific strength,specific stiffness,and recyclability,magnesium?Mg?alloys have been widely recognized and the development and application of Mg alloys have become a hotspot of worldwide research.Due to the poor room-temperature formability of Mg alloys,high-temperature superplastic deformation is considered as a good choice for forming Mg-based products for automobile components with complex shapes.However,in the existing commercial Mg alloy brands,there are few low-cost components with excellent mechanical properties at room and high temperature,so it is urgent to develop new Mg alloy systems.Meanwhile,the twin-roll cast?TRC?Mg alloys with high alloy content face the difficulty of relieving segregated microstructure and the problem of hard machining deformation,and there are few effective methods as so far,which seriously hinders the application of TRC technology in Mg alloys.In this study,based on the most widely used Mg-Al-Zn alloy system,the Mg-Al-Sn-Zn alloy system with high Al?⁸%?and low Sn content was designed.This alloy system has excellent mechanical properties at room and high temperature because of the high volume fraction of low-melting second phase(Mg₁₇Al₁₂)and fine particles?Mg₂Sn?with good thermal stability.The twin roll cast alloy?TRC-ATZ821?,ingot cast alloy?IC-ATZ821?,and extruded alloys?AT82 and ATZ811?were chosen as the experimental objects.The microstructure of each alloy was observed and the mechanical property was tested.The rolling process and superplastic deformation behavior were analyzed in detail,and the microstructural evolution was emphasized,expecting to provide some guidance in the development and application of high-performance Mg sheets.The main conclusions are shown as follows:?1?The difference of initial microstructure and properties between the TRC-ATZ821 and IC-ATZ821 was studied.It was found that compared with the IC-ATZ821 alloy,the TRC-ATZ821 alloy has obvious advantages in microstructural refinement,which leads to the higher UTS of TRC alloy than that of IC alloy ?TRC:²70 MPa,IC:²00 MPa?.However,the TRC alloy suffers from the occurrence of segregated Mg₁₇Al₁₂2 phase in chain-shaped and can be up to a millimetre in size,which seriously affects the plasticity of the material.Therefore, the elongation of TRC alloy at room temperature is significantly lower than that of IC alloy?TRC:¹%,IC:⁷%?.?2?The controlled rolling process was proposed to relieving the severe segregation in TRC-ATZ821 alloy.It was found that under the same total deformation,the microstructural homogeneity improves greatly by increasing the number of rolling pass.Correspondingly,a homogeneous microstructure with fine grains?⁴?m? and near-spherical precipitates?⁰.4?m?was obtained after 12-pass rolling process,and remarkable properties was produced with the YS,UTS and elongation reaching²45 MPa,³47 MPa and¹7%,respectively.?3?The microstructural evolution during 12-pass controlled rolling was analyzed in detail.The evolution of the second phases can be classified into three stages:the net dissolution,balancing and net precipitation stages.During the first stage,the quantity of the precipitates decreases gradually with the increase in the number of rolling pass,which exhibits a general dissolution trend.During the second stage, there is no remarkable change in the morphology of the precipitates,achiving dynamic equilibrium.And during the third stage,a great quantity of second phases continuously precipitates.?4?The superplastic deformation behavior between AT82 and ATZ811 alloys was compared and analyzed.Excellent superplasticity was found in the ATZ811 alloy, which exhibits a tensile elongation of⁵10%under a strain rate of 10^-3 s^-1 at 300 ?,in contrast to the inferior elongation of³80%for the AT82 alloy.This is because that the incompatible deformation between Mg₂Sn particles and matrix can cause the formation of cavities during high-temperature tensile deformation. Compared with AT82 alloy,the substitution of 1 Zn for 1 Sn?wt.%?results in a decreasing amount and a better uniformity of Mg₂Sn particles in ATZ811 alloy, thus inhibiting the formation of cavities and leading an excellent superplasticity.?5?The microstructural evolution during tensile deformation was observed,and the superplastic mechanism was analyzed.It was found that during tensile deformation,there was no obvious grain growth in both alloys,due to the high-volume fraction of second phases located at grain boundaries.Meanwhile, the texture weakening was observed,suggesting that grain boundary sliding?GBS? is the dominant superplastic deformation mechanism at 300?,which agrees well with the strain rate sensitivity?m?and the activation energy?Q?calculations.