Enhanced Tensile Properties of Mg Sheets by a Unique Thermomechanical Processing Method
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Lightweight magnesium (Mg) alloys have attracted considerable attention in the past 15 years for potential application in the automotive industry.[1–3] However, the vast majority of their applications are confined to casting alloys, and the applications of wrought alloys, particularly sheet products, are very limited due to their high processing cost and inadequate mechanical properties.[4–6] Since conventional Mg alloys have poor formability at near room temperature,[7–9] they have to be produced by multi-pass hot or warm rolling. A small thickness reduction per pass (10 to 20 pct), together with an annealing between individual passes, is necessary to prevent edge-crack formation.[10,11] Moreover, rolling speeds (roller surface velocity) adopted by the Mg industry are lower than the speeds commonly used for
MINGZHE BIAN and ZHUORAN ZENG, Research Fellows, and NICK BIRBILIS and JIAN-FENG NIE, Professors, are with the Department of Materials Science and Engineering, Monash University, Melbourne, VIC, 3800, Australia Contact e-mail: [email protected]. SHIWEI XU, Chief Scientist, and WEINENG TANG, Scientist, are with the Research Institute (R & D Centre), Baosteel Group Corporation, Shanghai, 201900, P.R. China, CHRISTOPHER H.J. DAVIES, Professor, is with the Department of Mechanical and Aerospace Engineering, Monash University, Melbourne, VIC, 3800, Australia. Manuscript submitted July 9, 2016. Article published online September 26, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
Al sheet alloys, which makes Mg sheet more expensive. One way to solve this problem is to develop a low-cost thermomechanical process to produce Mg sheet with improved mechanical properties. A recent study[12] indicates that ultrahigh speed rolling (450 to 2000 m/minute) can remarkably improve the rollability of commercial Mg alloy AZ31 (Mg-3Al-1Zn-0.3Mn, wt pct). With this ultrahigh speed rolling, the maximum thickness reduction per pass can be increased up to 60 pct without any surface cracks, even when rolling is done at relatively low temperatures of 473 K to 623 K (200 C to 350 C)[13] However, these rolling speeds are extremely fast, making this process challenging an industrial setting. A more recent study[14,15] shows that a larger thickness reduction per pass (80 pct) is achievable in Mg alloy ZK60 (Mg-6Zn-0.5Zr, wt pct) under a fast rolling (FR) speed of 26 m/minute and at temperatures of 523 K to 673 K (250 C to 400 C). However, the resultant sheets fabricated by this process have an incomplete dynamic recrystallization (DRX) microstructure or relatively coarse recrystallized grains, and consequently their mechanical properties are not fully optimized. Attempts were made by the present authors to develop a thermomechanical process that could produce Mg sheets at lower processing cost while enhancing their tensile properties. It was found that an increase in rolling speed from 2 to 15 m/minute allowed the thickness reduction in a single pass of commercial alloys of AZ31 and ZEK100 (Mg-1Zn-0.2Nd-0.2Zr, wt pct) to be consid
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