Formability of Ultrafine-Grain Mg Alloy AZ31B at Warm Temperatures
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TO improve fuel economy and minimize environmental impact, Mg alloys have attracted much attention for lightweight structural applications in the transportation industry due to their high specific strength and stiffness.[1] However, Mg alloys suffer from poorer formability compared to widely used structural materials, such as steels and Al alloys. The limited number of independent slip systems in hexagonal-close-packed (hcp) Mg cause low ductility,[2] but increasing deformation temperature is known to enhance its formability. At higher temperatures, additional slip systems are activated to enhance flow.[3,4] Superplasticity has also been reported at high temperatures and low strain rates for coarse-grained Mg alloys,[5,6] but surface oxidation can be severe at elevated temperatures, thus increasing the time and cost associated with cleaning operation, and complicating part manufacturing process. Therefore, warm forming of Mg at low levels of elevated temperature (£250 C) is a desirable goal. Grain refinement can be an effective way to improve the formability of wrought Mg alloys in the warm forming temperature range. In recent years, grain refinement and superplasticity have been documented for many magnesium alloys.[7–9] For example, Mabuchi et al.[9] reported that a conventionally extruded AZ91 with a grain size of 4.1 lm can exhibit an elongation-tofailure of 425 pct at 3 9 10-4 s-1 and 523 K, and an Q. YANG, formerly with the Department of Materials Science and Engineering, The University of Michigan, is Research Engineer, Hitachi America Ltd., Farmington Hills, MI 48335. A.K. GHOSH is with the Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109-2136. Contact e-mail: [email protected] Manuscript submitted September 1, 2005. Article published online August 8, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
extruded ZK60 with a grain size of 2.4 lm can exhibit an elongation of 730 pct at 4 9 10-4 s-1 and 573 K. However, ZK60 is more expensive than AZ31 alloy, the latter being closer to production readiness in sheet form and currently of interest to industry. The ultrafine-grain (UFG) structure derived from severe plastic deformation has enabled superplastic forming capability at low temperatures and high strain rates in various alloys such as Al,[10,11] Ti,[12] Ni,[13] and Mg[14] alloys. Therefore, this approach is worth investigating for warm forming AZ31 alloy. Tensile elongation at elevated temperatures is a function of strain rate sensitivity of flow stress (m) and strain hardening exponent (n). For superplasticity, m is generally near or >0.5, but n is low. On the other hand, a higher n leads to a more uniform deformation. During deformation of UFG Al[10] and Ti[12] alloys at relatively low temperatures, a moderate combination of m and n values gives rise to high elongations (>350 pct). Recently, a new process for severe deformation has been developed for grain refinement. Deformation is imparted to a billet by repeated sine-wave die pressing as described in References 15 and
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