Materials Transactions,Vol.46.No.7(2005)pp.1646 to 1650 C2005 The Japan Institute of Light Metals Ultra-Fine Grain Development in an AZ31 Magnesium Alloy during Multi-Directional Forging under Decreasing Temperature Conditions* Jie Xing*2,Hiroshi Soda*2,Xuyue Yang,Hiromi Miura and Taku Sakai Department of Mechanical Engineering and Intelligent Systems,UEC Tokyo (The University of Electro-Communications) Grain refinement of a magnesium alloy,AZ31,was studied in multi-directional forging (MDF)with decreasing temperature from 623 to 423 K.The MDF was carried out up to cumulative strains of around 5 with changing the loading direction during decreasing temperature from pass to pass.The structural changes are characterized by the development of many mutually crossing kink bands accompanied by MDF at low strains,followed by full development of very fine grains at high strains.The dynamic changes in grain size evolved can be expressed by two different power law functions of flow stress for the regions of flow stress above or below around 100 MPa.The MDF under decreasing temperature condition can accelerate the uniform development of much finer grains and the improvement in plastic workability,leading to the minimal grain size of 0.36um at a final processing temperature of 423 K.The mechanism of grain refinement is discussed in detail. (Received January 26.2005;Accepted May 6,2005;Published July 15,2005) Keywords:magnesium alloy AZ31.multi-directional forging (MDF).continuous dynamic recrystallization,fine grain.hall-petch relation 1.Introduction at high strain.As a result,fine grains cannot be fully generated throughout the material even in high strain.4.5)In Magnesium (Mg)and its alloys are the lightest metallic the present study,optimum processes for fine grain develop- structural materials and have the following several features, ment and improvement of the mechanical properties are i.e.high specific strength,good electromagnetic interference studied in multi-directional forging (MDF)of Mg alloy with shielding,good recyclability,and etc.Mg alloys are very decreasing temperature from pass to pass.Fine-grained attractive in application to automotive parts and casing of microstructure developed is investigated by optical and portable electrical devices in recent years,however,are transmission electron microscopy (OM and TEM). categorized as hard plastic materials because of a limited ductility and formability due to the hexagonal closed-packed 2.Experiment Procedure (HCP)crystal structure at room temperature.Then,structural Mg alloys have been manufactured less frequently by plastic A commercial AZ31 magnesium alloy was provided as a working such as rolling and other forming processes hot-extruded rod with the chemical composition as follow- compared with casting route.2)It is expected that several ing:Al2.86.Zn0.75,Mn0.68.Cu0.001,Si0.003.Fe0.003 slip systems can be operated in addition to the basal slip and balance Mg (all in mass%).The rectangular samples with system during warm and hot deformation,leading to increase a dimension of 31,21 and 14 mm in each side (i.e.the axial in the plastic workability.It is also known23)that fine grains ratio =2.22:1.49:1)were machined from the rod parallel to are developed in Mg alloys at relatively low strains during the extrusion direction (see Fig.1(a)).The samples were warm and hot working and result in much improvement of annealed for 7.2 Ks at 733 K and then furnace cooled,leading the plastic workability.Yang et al.4.5)investigated the grain to the evolution of equiaxed grains with an average size of refinement mechanism occurring in Mg alloy AZ31 during about 22 um (see Fig.3(a)). high temperature deformation and found that the formation Compression tests were carried out at a constant true strain mechanism of new grains is clearly different from conven- rate of 3x 10-3s-by using a testing machine equipped tional discontinuous dynamic recrystallization (dDRX).The with a quenching apparatus,which made it possible to number and the misorientation angle of kink bands evolved at quench a sample in water within 1.5s after hot deformation low strains rapidly increase with further deformation,finally was ceased.)The sample was deformed by multi-directional resulting in development of new fine-grains in high strain. forging(MDF)with changing the loading axis at an angle of They concluded that dynamic grain evolution in Mg alloy can 90 from pass to pass and with decreasing temperature from be based on grain fragmentation taking place in original 623 to 423K(Fig.1).When a pass strain As is fixed at a coarse grains and so controlled by deformation-induced constant,i.e.As =0.8,the dimension ratio of the sample continuous reactions assisted by dynamic recovery,i.e. does not change in each pass and so the compression continuous dynamic recrystallization (cDRX). processing can be carried out to infinity.7 Deformed samples Kink bands are formed roughly perpendicular to the basal were cut along a plane parallel to the last compression axis. plane.5)In single pass compression,the basal plane of Microstructural observation was carried out by using OM and extruded Mg rod,which is parallel to the compression axis,is TEM under an accelerating voltage of 200kV.The Vickers rotated and approaches perpendicular to the compression axis hardness was also measured at room temperature. 3.Results and Discussion *IThis Paper was Originally Published in J.Jpn.Inst.Light Met.54(2004) 527-531. 2Graduate Student.UEC Tokyo (The University of Electro-Communica- 3.1 Deformation behavior tions) Typical true stress-cumulative strain (o-Ag)curves ofUltra-Fine Grain Development in an AZ31 Magnesium Alloy during Multi-Directional Forging under Decreasing Temperature Conditions*1 Jie Xing*2, Hiroshi Soda*2, Xuyue Yang, Hiromi Miura and Taku Sakai Department of Mechanical Engineering and Intelligent Systems, UEC Tokyo (The University of Electro-Communications) Grain refinement of a magnesium alloy, AZ31, was studied in multi-directional forging (MDF) with decreasing temperature from 623 to 423 K. The MDF was carried out up to cumulative strains of around 5 with changing the loading direction during decreasing temperature from pass to pass. The structural changes are characterized by the development of many mutually crossing kink bands accompanied by MDF at low strains, followed by full development of very fine grains at high strains. The dynamic changes in grain size evolved can be expressed by two different power law functions of flow stress for the regions of flow stress above or below around 100 MPa. The MDF under decreasing temperature condition can accelerate the uniform development of much finer grains and the improvement in plastic workability, leading to the minimal grain size of 0.36 mm at a final processing temperature of 423 K. The mechanism of grain refinement is discussed in detail. (Received January 26, 2005; Accepted May 6, 2005; Published July 15, 2005) Keywords: magnesium alloy AZ31, multi-directional forging (MDF), continuous dynamic recrystallization, fine grain, hall-petch relation 1. Introduction Magnesium (Mg) and its alloys are the lightest metallic structural materials and have the following several features, i.e. high specific strength, good electromagnetic interference shielding, good recyclability, and etc. 1) Mg alloys are very attractive in application to automotive parts and casing of portable electrical devices in recent years, however, are categorized as hard plastic materials because of a limited ductility and formability due to the hexagonal closed-packed (HCP) crystal structure at room temperature. Then, structural Mg alloys have been manufactured less frequently by plastic working such as rolling and other forming processes compared with casting route.1,2) It is expected that several slip systems can be operated in addition to the basal slip system during warm and hot deformation, leading to increase in the plastic workability. It is also known2,3) that fine grains are developed in Mg alloys at relatively low strains during warm and hot working and result in much improvement of the plastic workability. Yang et al.4,5) investigated the grain refinement mechanism occurring in Mg alloy AZ31 during high temperature deformation and found that the formation mechanism of new grains is clearly different from conven￾tional discontinuous dynamic recrystallization (dDRX). The number and the misorientation angle of kink bands evolved at low strains rapidly increase with further deformation, finally resulting in development of new fine-grains in high strain. They concluded that dynamic grain evolution in Mg alloy can be based on grain fragmentation taking place in original coarse grains and so controlled by deformation-induced continuous reactions assisted by dynamic recovery, i.e. continuous dynamic recrystallization (cDRX). Kink bands are formed roughly perpendicular to the basal plane.5) In single pass compression, the basal plane of extruded Mg rod, which is parallel to the compression axis, is rotated and approaches perpendicular to the compression axis at high strain. As a result, fine grains cannot be fully generated throughout the material even in high strain.4,5) In the present study, optimum processes for fine grain develop￾ment and improvement of the mechanical properties are studied in multi-directional forging (MDF) of Mg alloy with decreasing temperature from pass to pass. Fine-grained microstructure developed is investigated by optical and transmission electron microscopy (OM and TEM). 2. Experiment Procedure A commercial AZ31 magnesium alloy was provided as a hot-extruded rod with the chemical composition as follow￾ing: Al 2.86, Zn 0.75, Mn 0.68, Cu 0.001, Si 0.003, Fe 0.003 and balance Mg (all in mass%). The rectangular samples with a dimension of 31, 21 and 14 mm in each side (i.e. the axial ratio = 2.22:1.49:1) were machined from the rod parallel to the extrusion direction (see Fig. 1(a)). The samples were annealed for 7.2 Ks at 733 K and then furnace cooled, leading to the evolution of equiaxed grains with an average size of about 22 mm (see Fig. 3(a)). Compression tests were carried out at a constant true strain rate of 3
103 s1 by using a testing machine equipped with a quenching apparatus, which made it possible to quench a sample in water within 1.5 s after hot deformation was ceased.6) The sample was deformed by multi-directional forging (MDF) with changing the loading axis at an angle of 90 from pass to pass and with decreasing temperature from 623 to 423 K (Fig. 1). When a pass strain
" is fixed at a constant, i.e.
" ¼ 0:8, the dimension ratio of the sample does not change in each pass and so the compression processing can be carried out to infinity.7) Deformed samples were cut along a plane parallel to the last compression axis. Microstructural observation was carried out by using OM and TEM under an accelerating voltage of 200 kV. The Vickers hardness was also measured at room temperature. 3. Results and Discussion 3.1 Deformation behavior Typical true stress-cumulative strain (
–
") curves of *1 This Paper was Originally Published in J. Jpn. Inst. Light Met. 54 (2004) 527–531. *2 Graduate Student, UEC Tokyo (The University of Electro-Communica￾tions) Materials Transactions, Vol. 46, No. 7 (2005) pp. 1646 to 1650 #2005 The Japan Institute of Light Metals