Microstructure Control And Mechanical Properties Of High-Alloyed Wrought Mg-Al-Zn-Sn Series Alloys

Mg-Al-Zn(AZ)series alloys are among the most widely used commercial Mg alloys.Compared with low-alloyed AZ series alloys,the high-alloyed AZ80 and AZ91 alloys have higher strength.However,the high-alloyed AZ series alloys have low ductility and poor formability,and it is easy to form coarse reticular eutectic phases in their as-cast microstructures.Besides,the AZ series alloys with high alloying content tend to form casting defects such as segregation,porosity,and hot cracking due to their wide solidification intervals.These adverse effects cannot be eliminated by adding alloying elements or modifiers.Therefore,to avoid the adverse effects of the above disadvantages on the properties and manufacturing process of the high-alloyed AZ series alloys,it is urgent to develop new Mg alloys,which have fine as-cast microstructure and can obtain high strength and ductility through deformation.Based on the designing strategy of regulating the Al content to medium level(5-7 wt.%)and moderately increasing the Zn content(> 1 wt.%)in the AZ series alloys,an Mg-5Al-2Zn(wt.%,AZ52)alloy with medium Al content and higher Zn content was selected as the base alloy in this thesis.The effects of small amount of Sn addition on the microstructure and mechanical properties of the wrought AZ52 alloy were studied,and a novel alloy with excellent strength and ductility was optimized.Then,the alloy was compared with the commercial AZ91 alloy.At the same time,a small amount of Bi was added to this new alloy to refine the microstructure and improve the mechanical properties.The effects of sub-rapid solidification on the solidification microstructure and rolling behaviors of multi-component Mg alloys were studied to provide references for the short-process preparation of non-rare earth Mg alloys with high strength and ductility.The main conclusions are as follows:(1)The effect of small amount of Sn on the microstructure and tensile properties of wrought AZ52 alloys was elucidated.Addition of 0.54 wt.% Sn had little effect on the microstructures and mechanical properties of as-extruded and as-annealed AZ52 alloys.When the Sn content reached 0.91 wt.%,the grains of the as-extruded and as-annealed alloys were gradually refined,the precipitate content was gradually increased and the tensile strength of the alloys was also gradually increased with the increase of Sn content.Adding small amount of Sn can weaken the basal textures of as-extruded alloys.The as-extruded and as-annealed AZT522 alloy had good strength and ductility.(2)It is found that the novel AZT522 alloy had finer as-cast microstructure,better rolling formability,and higher annealing tensile properties when compared with AZ91 alloy.Due to its higher Zn content and addition of Sn,the as-cast AZT522 alloy had much finer microstructure than that of the as-cast AZ91 alloy.And the hot rolling formability of AZT522 alloy was superior to that of the AZ91 alloy.The AZT522 alloy annealed at 225 ?/1.5 h had the tensile YS,UTS,and ?_f of ~271 MPa,~343 MPa,and ~16%,respectively,showing better strength and ductility than the annealed AZ91 alloy.(3)The effects of low-temperature solution and ~2% pre-cold rolling deformation on the aging behavior and tensile properties of the AZT522 alloy were clarified.The AZT522 alloy solution-treated at 350 ?/1 h reached its peak hardness at 175 ?/75 h,but there was much discontinuous precipitation in its microstructure,causing poor aging strengthening.The combination of low-temperature solution and pre-cold rolling deformation could promote the aging precipitation,significantly shorten the peak aging time to 15 h and effectively inhibit the discontinuous precipitation,which made the peak-aged AZT522 alloy have YS and UTS increment of ~77 MPa and ~31 MPa,respectively,while its plasticity remained unchanged.(4)The aging behavior of rolled AZT522 alloy with ~70% rolling reduction was studied.The microstructure of the rolled alloy consisted of fine grain region containing a limited amount of precipitates as well as low dislocation density and coarse grain region having a high solid solution as well as high dislocation density.Both coarse and fine regions reached the peak hardness at 175 ?/10 h.Under peak aging state,the precipitates in these two regions were Mg17Al12 and Mg2 Sn phases.The peak-aged AZT522 alloy had good strength and ductility,showing the YS,UTS,and ?_f of ~281 MPa,~368 MPa,and ~13.7%,respectively.(5)The effects of sub-rapid solidification(SRS)on the as-cast microstructure,micro-segregation,and tensile properties of AZTB5220 alloy were elucidated.Compared with the conventional solidification(CS),the SRS with a high cooling rate significantly refined the average dendritic size of the AZTB5220 alloy to ~83 mm and reduced the size of the eutectic phase.The SRS significantly improved the micro-segregation of Al and Sn,but only slightly improved the micro-segregation of Zn and Bi.The mechanical properties of as-cast SRS alloy were greatly improved due to the refinement of microstructure and the increase of solid solution content of alloying elements.(6)The microstructure evolution and properties of AZTB5220 alloy based on SRS during subsequent hot rolling and annealing were investigated.Benefiting from the finer initial grains,the solution-treated SRS alloy had a faster refining rate during the subsequent hot rolling process,and could fastly obtain uniform fine grains and higher strength and ductility after annealing treatment.When hot rolled to ~70% reduction,the annealed SRS sample had an average size of ~3 mm,a large number of nearly spherical precipitates and high dislocation density,leading to the tensile YS,UTS,and ?_f of ~267 MPa,~356 MPa,and ~12.8%,respectively.