Effects Of Micro-alloying And Novel Extrusion Processing On Microstructures And Properties Of Magnesium Alloy Sheets

Magnesium(Mg)alloy is a new type of structural material in the 21st century,which has the advantages of light weight,high specific strength,high specific stiffness,excellent thermal conductivity and electromagnetic shielding,good compatibility with the environment,excellent damping,shock absorption and noise reduction,etc.At present,casting Mg alloys are widely used because of high precision,but they are lack of more exellent mechanical properties than wrought Mg alloys.Therefore,the development of high performance wrought Mg alloys has become a key technology to expand their application ranges.However,their close-packed hexagonal structures results in a strong basal texture during hot deformation,which induces the sharp deterioration of ductility and formability at room temperature(RT)and greatly impedes their industrial applications.Therefore,controlling the basal texture of Mg sheets favors improving their ductility and formability at RTIn this paper,we used multiple micro-alloying and porthole die extrusion methods to regulate microstructures and textures of Mg alloy sheets and achieve high ductility and formability at room temperature.A dilute Y containing Mg-Sn-Y-(Zn)alloy and a rare earth-free micro-alloying Mg-Al-Zn-Mn-Ca alloy were designed.By optimizing the extrusion mold,we developed an asymmetric porthole die extrusion(APE)and applied this method to the preparation of Mg-2.8Al-0.9Zn(AZ31,wt.%)alloy sheet.We also tried to fabricate an Mg-2.8Al-0.9Zn/Mg-0.3Y(AZ31/W0,wt.%)laminated composite sheet via porthole die extrusion(PE).The main conclusions were summaried as follows:(1)The extruded/rolled-annealed Mg-0.4Sn-0.7Y(TW00,wt.%)sheet displayed a weak texture.During stretch deformation,prismatic slip activity and texture weakening favored decreasing r value and causing the high elongations-to-failure(?33 and?32%,respectively)and Erichsen index(I.E.:?6.2 and?5.4 mm,respectively).An elliptical annular texture extended along the transverse direction(TD)was shown in the extruded Mg-0.4Sn-0.7Y-0.6Zn(TWZ000,wt.%)sheet,accompanied by grain size and second phase refinement.The elliptical annular texture was related to dynamic recrystallization mechanisms including twins,second phases and non-base slip activities.The TWZ000sheet had a better balance in strength(yield strength:?150.0 MPa,ultimate tensile strength:?235.1 MPa),elongation-to-failure(?39.8%)and IE(?7.0 mm).(2)The Mg-1.6Al-0.8Zn-0.4Mn-0.5Ca(AZMX1100,wt.%)alloy also exhibited a TD-extending elliptical annular texture after hot rolling and annealing,which caused a high elongation-to-failure of ?23.9% and an I.E.value of?6.1 mm.At 200 to 300°C,static recrystallization nucleation dominated recrystallization behavior.Al₂Ca phases,contraction twins,double twins,grain boundaries and orientation gradient induced by non-basal slip activities jointly promoted the formation of elliptical annular texture.At300?450°C,the grains with 45-70°TD orientation preferential grew because of the co-segregation of Al,Zn and Ca atoms.(3)The nucleation kinetics of static recrystallization of hot-rolled AZMX1100 sheet was well described by JMAK model in 200?450°C.The Avrami exponent n value was ranged from 0.68 to 1.02,which led to a lower activation energy Q_R of nucleation of ?74.24 KJ/mol.The activation energy Q_g for grain growth was determined by annealing at 300 to 500°C.The Q_g value,fitted to an Arrhenius relationship,was estimated to be?115.48 KJ/mol.It was attributed to a strong pinning of co-segregated Al,Zn and Ca atoms in grain boundaries.(4)Based on the 3D-DEFORM analysis,two symmetric points on the AZ31piecework to show an asymmetric flow during APE,which resulted in an asymmetric shear effect.This effect promoted more dynamic recrystallized grains with random orientations inducing grain refinement and texture weakening in the AZ31 alloy sheet.Compared with CE and PE AZ31 sheets,the APE AZ31 sheets demonstrated high elongations-to-failure and stretch formability,especially for the APE-90 alloy sheet,which increased by?75 and?94%in regard to I.E.value,respectively.(5)Mixed texture was shown in the AZ31/W0 sheet processed by PE processing.The interface of AZ31/W0 sheet contained a diffusion zone of?0.35?m.The sound crystallographic matching between matrix and diffusion zone was a guarantee of high interface shear strength.The I.E.value of the AZ31/W0 sheet increased by?71 and?21%,compared to AZ31 and W0 sheets.The main reasons were concluded as follows:1)massive basal<a>slip and a few prismatic<a>slip systems were activated in the W0layer;2)the formation of mixed texture decreased the in-plane anisotropy of AZ31/W0sheet;3)the activation of{10-12}extension twins in the AZ31 layer contributed to the thickness strains and enhanced the Schmid factor for basal slip during the middle/later stage of stretch forming.