Extrusion Limits of Magnesium Alloys
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INTRODUCTION
IT is reported that magnesium alloys are generally 1/3 to 2/3 slower to extrude than aluminum alloys.[1] This makes a significant contribution to the cost of production. Both hydrostatic[2–6] and indirect[4,6,7] extrusion methods have been employed to permit magnesium alloys to be extruded at higher rates. Nevertheless, direct extrusion is very common, and improved knowledge of the performance of magnesium in this process is expected to lead to alloys with enhanced extrudability.[8] An effective means of assessing relative extrusion rates is the extrusion limit diagram. These have been used to describe the extrusion performance of a number of aluminum alloys,[9–14] but the approach appears to have been applied to magnesium alloys only in a few cases.[15–18] Despite the infrequent use of the full limit diagram, the degree to which the maximum extrusion speed of the Mg-Al-Zn alloy series is raised by lowering the aluminum or zinc level has been quantified in a number of cases.[16,18–20] As expected, lowering these alloying additions also comes at a cost to the mechanical performance of the product.[4] In contrast to this, it has been shown that manganese additions improve mechanical properties without significantly lowering the extrusion speed.[18,20] Furthermore, in the Mg-Zn-Zr alloy series, the addition of zirconium actually increases the extrudability by raising the solidus temperature.[21] There are also a limited number of cases where the extrusion performance has been compared for isolated alloys.[2,3,5,22–25] Systematic comparison over a range of alloys does not appear to have been carried out. The present work aims to quantify the extrusion speed limits for key wrought magnesium alloys using a DALE L. ATWELL, Research Engineer, and MATTHEW R. BARNETT, QEII Research Fellow, are with the Cooperative Research Centre for Cast Metals Manufacturing (CAST), Geelong Technology Precinct, Deakin University, Geelong, VIC 3217, Australia. Contact e-mail: [email protected] Manuscript submitted December 18, 2006. Article published online October 12, 2007 3032—VOLUME 38A, DECEMBER 2007
laboratory scale extrusion press in a manner that allows the effects of the different alloying additions to be assessed. A common fast extruding 6XXX series aluminum alloy is also examined for comparison.
II.
EXPERIMENTAL METHOD
The billets used in this investigation measured Ø30 mm · 20 mm and were of the commercial magnesium alloy grades M1, ZM21, AZ31, AZ61, and ZK60, and the aluminum alloy AA6063. The chemical composition of the magnesium billets is shown in Table I. The M1, ZM21, AZ31, and AZ61 alloy billets were acquired in the extruded (wrought) condition. These should be thought of as being in a homogenized state. This condition can also be considered to replicate the first stage of a two-stage extrusion process, which is not uncommon for many magnesium alloys.[7] The AA6063, ZK60 alloys and an additional sample of the AZ31 alloy were acquired as billets in the cast condition. The cast AA6063 billets were given a stand
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