Microstructural Evolution and Mechanical Behavior of AZ61 Alloy Plate Prepared by Multi-Pass Friction Stir Processing

Author:Luo Xi Cai

Supervisor:zhang da tong


Degree Year:2019





Magnesium alloys,as the lightest structural metallic material,have received considerable attention for their potential applications in the aerospace and automotive industries,in which magnesium alloys are used for weight reduction solutions as their low density,high specific strength and abundant resources.Magnesium alloys with HCP crystal structure have a limited number of slip systems,resulting in low formability and ductility at room temperature,which seriously restrains their applications.It is an effective method to improve the ductility of magnesium alloy via grain refinement.Traditional severe plastic deformation(SPD)techniques,such as equal channel angular pressing(ECAP)and high pressure torsion(HPT),can achieve significant grain refinement,while the problems including complex process,high energy consumption and the size limitation restrain their applications.Friction stir processing(FSP),as a novel SPD technique,is an effective grain refinement technique with many advantages such as one-step processing and good repeatability.Multi-pass FSP(M-FSP)with a certain overlapping ratio can produce large-scale fine-grained magnesium platesIn this study,the cast AZ61 magnesium alloy was selected as the base material(BM)and multi-pass FSP was conducted in air and underwater cooling conditions,respectively.The work mainly focused on the effect of processing parameters,number of the pass and heat treatment on the microstructural evolution,texture development and mechanical behavior of AZ61 alloy.The relationship between the microstructures and mechanical behavior at room and elevated temperature were investigated,with an aim of providing theoretical base and key technique for large-scale fine-grained magnesium plate prepared by FSP.Firstly,FSP was conducted on the cast AZ61 alloy in air by using different processing parameters.The effect of rotational rate(ω)and processing speed(ν)on microstructural evolution and mechanical behavior of FSP sample was investigated.The microstructures in stir zone(SZ)of FSP alloy become finer than BM due to dynamic recrystallization during FSP.The center width of SZ and average grain size of FSP samples increase with the increasing ratio ofω/νdue to an increase of heat input.The relationship between the grain size and Zener-Hollomon parameter is constructed by analyzing the effect of processing parameters on recrystallized grain size.The mechanical properties of FSP specimens are significantly improved compared with BM.The yield strength and micro-hardness increase with the decrease of grain size.The sample prepared by FSP at the rotational speed of 1000 rpm and processing speed of 60 mm/min(noted as 1000-60)presents a combination of high tensile strength and good ductility,which is attributed to the favorable factors such as weak texture,low dislocation density and high fraction of high angle grain boundaries.Secondly,M-FSP was carried out on the cast AZ61 alloy with the processing parameter of1000-60.The effect of M-FSP on microstructural characteristics was systematically investigated,and the tensile properties were measured at room and elevated temperature.Stretch formability was also tested at room temperature.M-FSP can produce large-scale fine-grained Mg alloy plate,while non-uniform microsturctues exist in the transitional zone(TZ).Moreover,there is a great difference in texture distribution between TZ and SZ.Due to the effect of non-uniform microstructures,M-FSP specimens present mechanical anisotropy with the best tensile strength and ductility in the processing direction(PD).Erichsen value(IE)of3.7 mm was achieved via Erichsen tests at room temperature and a punch speed of 0.1 mm/min.It is very difficult for the M-FSP plate to achieve high-temperature superplasticiy due to the non-uniform microstructures resulting in inhomogeneous deformation and strain localization during high-temperature tensile test.M-FSP Mg plate also presents mechanical anisotropy at elevated temperature,and only the PD specimen exhibits superplasticity with an elongation of211%at 623 K and 3.3×10-4 s-1.Thirdly,M-SFSP was conducted on the cast AZ61 magnesium alloy underwater adopting the processing parameter of 800-240 due to low heat input and high grain-refinement ability.The research is to investigate the effect of multi-pass SFSP with overlapping of 50%and 100%under enhanced cooling condition on microstructural evolution and tensile behavior at room and elevated temperature.And the high-temperature tensile behaviors of M-SFSP alloy plate were investigated along different orientations.Two-pass SFSP refines and homogenizes the microstructures,resulting in a great improvement of mechanical properties,especially the ductility of SFSP specimen.M-SFSP successfully produces large-size AZ61 alloy plate with an average grain size of approximately 4μm attributed to high cooling rate of water.Compared with BM,M-SFSP Mg alloy plate exhibits a significant improvement in the mechanical properties,which benefits from several strengthening mechanisms,including grain refinement strengthening,dislocation strengthening and solid solution strengthening.Fracture is easily to originate in TZ because of inhomogeneous micro-strain distribution during deformation resulted from significant inhomogeneity of texture distribution,which leads to mechanical anisotropy of M-SFSP specimens tested at room temperature.Due to grain refinement and weak texture,M-SFSP specimens present excellent superplasticity with elongations of 332%467%in different orientations tested at high temperature of 623 K and strain rate of 1×10-33 s-1,and the anisotropic tensile behaviors are significantly weakened at high temperature.Finally,M-SFSP specimens are subjected to annealing treatment at different conditions with an aim to investigate the effect of annealing treatment on grain size,texture development,distribution of second-phase and mechanical behavior.At the annealing conditions of low temperature and short time,the grain size of annealing sample increases slightly with some small second phase precipitated fromα-Mg matrix,and the texture is not changed just with a bit decrease in texture intensity.On the other hand,at the annealing conditions of high temperature and long time,the grains of the annealing specimen are greatly coarsened with some large second-phase precipitations distributed along grain boundaries.Furthermore,the texture is significantly changed and weakened,and the plate tends to exhibit isotropic behavior under room temperature tensile test.