Effect Of Aging And Extrusion On The Microstructure And Mechanical Properties Of ATZ331 Alloy

Magnesium and its alloys are characterized by some distinct properties such as superior specific strength and rigidness,high electrical and thermal conductivities,and good electromagnetic shielding.Furthermore,Magnesium alloys are easy to machine and recycle,and thus green engineering materials of 21st century.In this thesis,ATZ331(Mg-3Al-3Sn-1Zn)alloys were fabricated by permanent mould casting under a protective flux.Cylindrical ingots were subjected to pre-deformation ranging from 0%to 9%compression in 3%increments in solution-treated ATZ331 alloys at room temperature,and then ATZ331 alloys with four kinds of pre-deformation strain,were aged at 160?,180? and 200?,respectively.The aging processing has been optimized by observations of microstructure and microhardness evolutions of ATZ331 alloys during aging.Both the as-solutioned and peak-aged ATZ331 alloys were extruded into rods(10 mm and 20 mm in diameter,respectively)at three different temperatures(such as 250?,300? and 350?)and two different extrusion ratio(such as 72 and 18)corresponding to the above bars of diameter 10 mm and 20 mm,respectively.Microstructures and mechanical properties of the ATZ331 alloys extruded under various conditions of temperature and extrusion ratio were investigated by OM,SEM,TEM,XRD and uniaxial tensile tests,etc.The results show that a large number of twins,which were induced by room temperature pre-deformation in the ATZ331 alloys,are favorable positions for the formation of the nuclei of the second phases.The higher peak-aged hardness of ATZ331 alloys achieved at 160? is progressively reduced due to coarsening of the fine precipitates as the aging temperature rises up to 180? or 200?.The microhardness value of the 9%pre-deformed ATZ331 alloy peak-aged at 160? is 84.0HV.Both the as-solutioned ATZ331 alloys without pre-deformation and peak-aged ATZ331 alloys without and with 9%pre-deformation,which were aged to the first and to the second peak hardness at 160?,respectively,were extruded with an extrusion ratio of 72 at 250?,300? and 350?,respectively.The results show that the combination of mechanical properties of the second peak-aged ATZ331 alloys without and with 9%pre-deformation is enhanced in comparison to that of the first peak-aged ones.The 9%pre-deformed ATZ331 alloys peak-aged at 160? were extruded with an extrusion ratio of 72 at 250?.The average grain size of the extruded ATZ331 alloys decreases,and thus mechanical properties of the extruded ATZ331 alloys are extremely improved by refining the grain size in comparison to the ATZ331 alloys without aging prior to extrusion.The tensile strengths of the second peak-aged ATZ331 alloys without and with 9%pre-deformation were increased by 33 MPa and 37 MPa after extrusion,respectively.Meanwhile,the elongation of the ATZ331 alloys is slightly increased.After the above ATZ331 alloys were extruded with a constant extrusion ratio of 72 at 300?,the tensile strengths of the ATZ3 31 alloys were increased by 9MPa and 15 MPa,respectively.Likewise,the elongation of the ATZ331 alloys is slightly improved.However,after the ATZ331 alloys mentioned above were extruded with a constant extrusion ratio of 72 at 350?,there is no change in the mechanical properties of the extruded ATZ331 alloys.In addition,extrusion ratios have little effect on the microstructure and mechanical properties of the extruded ATZ331 alloys.The 9%pre-deformed ATZ331 alloys aged to the second peak hardness at 160?,which were extruded with an extrusion ratio of 72 at 300?,exhibit the best combination of mechanical properties due to a large number of extremely small and uniformly dispersed Mg2Sn particulates,as well as the average grain size of approximately 3.44 m,with an ultimate tensile strengths of 305 MPa and an elongation of 28.1%.