Forming magnesium alloys with improved ductility

Abstract

A magnesium alloy comprising up to about one weight percent of cerium may be hot worked to produce an intermediate or final alloy workpiece that exhibits enhanced ductility at room temperature. The addition of a small amount of cerium may affect the magnesium alloy by reducing yield strength, refining grain size, and improving the work hardening behavior. Recrystallization during hot deformation of the rare earth containing magnesium material alters the texture of the alloy and orients the grains in a manner that favors basal slip activity. The alloy thus deforms at room temperature by a combination of twinning and slip mechanisms.

Description

[0001]This application claims priority based on provisional application 60 / 952,018, titled “Forming Magnesium Alloys with Improved Ductility,” filed Jul. 26, 2007 and which is incorporated herein by reference.TECHNICAL FIELD[0002]This invention generally relates to processed magnesium alloy compositions exhibiting improved ductility at room temperature. More specifically, magnesium alloyed with cerium is subjected to high temperature deformation to improve the alloy's formability at room temperature.BACKGROUND OF THE INVENTION[0003]Magnesium is the lightest structural metal. In engineering applications it is alloyed with one or more elements, for example, aluminum, manganese, rare earth metals, lithium, zinc, and silver. Magnesium usually constitutes eighty-five percent by weight or more of these alloys.[0004]The cost of magnesium has decreased dramatically in recent years and magnesium and its alloys have become attractive structural materials for a wide range of applications due i...

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Benefits of technology

[0009]It was observed that the grain size of the extruded cerium-containing magnesium billets was smaller than the grain size of the extruded pure magnesium billet. The room temperature tensile strength of the cerium-containing billet material was decreased and the percent elongation was significantly increased. Elongation at ultimate load in tensile testing is a measure of ductility. As will be seen in data presented below in this specification, the ductility of the hot deformed cerium-containing magnesium alloys was surprisingly increased compared with magnesium and other known magnesium alloys.

[0010]The addition of cerium in amounts up to about one percent by weight is found to enhance the room temperature ductility and workability of magnesium alloys following suitable hot deformation processing. In a specific embodiment, the hot deformation is accomplished by extrusion at billet temperatures of about 350° C. to about 475° C. with extrusion ratios in the range of about 10:1 to about 60:1 at suitable extrusion speeds. During the hot deformation the billets were suitably lubricated with graphite based lubricants or boron nitride, although this is not necessarily required.

[0011]The presence of the small amount of cerium favors the formation of recrystallized grains with their basal planes oriented at 40-50 degrees to the extrusion axis. The magnesium-cerium alloy provides easier basal slippage during subsequent straining along the extrusion axis which is effectively precluded in the extruded rods of pure magnesium. But whatever the straining mechanism, the presence of about 0.2 to about 0.5 weight percent cerium in the hot worked magnesium matrix markedly increased the ability to further shape the extruded bar material at room temperature. And the improvement was realized whether the hot worked material was solid in cross-section or hollow. Thus, hot extruded magnesium alloy bars or tubes, for example, may then be subjected to bending or hydroforming steps, for example, at an ambient temperature to more easily form more complex shapes for automotive vehicle structures or parts, or the like.

[0012]Significant increases in room temperature ductility have been demonstrated in binary magnesium-cerium alloys containing up to about 0.5 percent by weight cerium and the process may be practiced using cerium in amounts up to about one percent of the magnesium alloy. The cost of the cerium addition may be reduced by using cerium-containing mischmetal which typically comprises fifty percent by weight or more cerium with smaller amounts of lanthanum and other lanthanum-group elements. In this embodiment, the magnesium-based alloy may contain around one half to one percent cerium together with other rare earth elements in the mischmetal.

[0013]Relatively small amounts of other alloying elements are sometimes added to magnesium for physical properties other than room-temperature ductility. For example, aluminum in amounts up to about nine percent by weight, and / or zinc in amounts up to about three percent by weight, and / or manganese in amounts up to about one percent by weight have been used in commercial magnesium based alloys. And small amounts of titanium have been added for grain-refinement of magnesium alloys. Cerium additions in an amount up to about one weight percent may be used to improve the room-temperature ductility of these many different magnesium alloys but the room temperature ductility may not be as high as in the magnesium-0.2-1 cerium binary alloys. In most embodiments, magnesium will constitute at least eighty-five percent by weight of the magnesium-cerium alloy when these other alloying constituents are used for other properties of the resulting alloy.

Problems solved by technology

However, the use of magnesium in wrought products like sheet and extrusions has been limited due to the poor workability of magnesium castings and the lower formability and ductility of magnesium in the primary fabricated stage.

At room temperature, pure magnesium is generally characterized by limited ductility as a result of its hexagonal close-packed crystal structure and resulting limited number of active slip systems.

This inherent limitation often discourages widespread use of magnesium in wrought products made from sheets and extrusions because it is difficult and expensive to process the poorly workable metal into useable finished shapes.

However, the high operating expenses associated with ECAE often render the process economically unattractive and cause those in industry to utilize other alloys more susceptible to cheaper processing techniques, such as steel and aluminum alloys.

Unfortunately, knowledge in this area is scarce because only a limited amount of attention has been devoted to identifying alloy additions capable of favorably manipulating formability of wrought magnesium materials at room temperature.

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