Elektron, Manchester, UK) and WE43B (Alloys. International, Islandia, NY, USA) materials were obtained. These were cold drawn and subsequently annealed ...
European Cells and Materials Vol. NN. Suppl. N, 20xx (page htu)
ISSN 1473-2262
Cold-drawn ZM21 and WE43 wires exhibit exceptional strength and ductility A.J. Griebel1, J.E. Schaffer1 1
Fort Wayne Metals Research Products Corp., Fort Wayne, IN, USA.
INTRODUCTION: Cold drawn wire is extensively used in the production of medical devices. In addition to being a very efficient processing method, cold drawing allows exceptional grain refinement and work-hardening, excellent dimensional control and surface finish, and facilitates many existing subsequent device fabrication steps. However, cold drawn magnesium alloys have not been produced on a commercial scale. Applying cold drawing techniques to magnesium alloys presents significant challenges due to limited deformation mechanisms available at room temperature. However, recent reports would indicate the feasibility of HCP magnesium alloys with moderate levels of cold work [1-3].
Fig. 1: Tensile curves for ZM21 wires.
The aim of this work was to investigate the possible reduction and work hardening in two commercially available HCP alloys, ZM21 and WE43. METHODS: Extruded ZM21 (Magnesium Elektron, Manchester, UK) and WE43B (Alloys International, Islandia, NY, USA) materials were obtained. These were cold drawn and subsequently annealed by conventional methods to reduce the wire diameter. This draw-anneal cycle was repeated as necessary until a final anneal at a diameter of 510 µm. The annealed wires were then cold drawn until failure to determine maximum reduction, with samples taken for tensile testing. Cold-worked samples were then subjected to various thermal treatments to determine aging response. RESULTS: ZM21 was drawn to a diameter of 100 µm (true strain 3.2). UTS increased from 200 MPa annealed to 412 MPa at the highest level of cold work. This was accompanied by a loss in uniform elongation from 10.4% to 3.4% (Figure 1). WE43 was cold drawn to a diameter of 71 µm (true strain 3.94). UTS increased from 267 MPa annealed to 565 MPa at the highest level of cold work. Elongation decreased from 15% to 1.8%. Aging drawn WE43 at 250C increased UTS to 640 MPa (Figure 2).
Fig. 2: WE43 wire drawn to various levels of true strain and aged. Image is 127µm wire with TS=2.8 and knotted closed, indicating good formability. DISCUSSION & CONCLUSIONS: The levels of cold reduction achieved are greater than 3 times those previously reported [1], and the strength increases give the highest reported alloy-specific strengths. The strengths achieved by the aged WE43 are the highest tensile strengths reported for any crystalline Mg alloy. The drop in uniform tensile elongation with increasing cold work is significant, and may limit the utility of the wire in some applications such as balloon-expandable stents, while enabling self-expandable Mg stents. The successes here with two very different alloys would indicate the potential of cold-drawing a wide range of Mg alloys, including those specifically designed for biomedical applications. REFERENCES: 1H.Y. Chao, H.F. Sun, W.Z. Chen, et al (2011) Materials Characterization 62:312-320. 2H.Y. Chao, Y. Yu, and E. Wang (2009) Int. J Modern Physics B 23:927-933. 3G. Fang, W.J. Ai, S. Leeflang, et al (2013) Mat Sci & Eng C 33:3481-3488 http://www.ecmjournal.org
