May/June_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 1 8 2 0 used to provide a deeper understand- ing of the complex chemistry of mag- nesium alloys that will assist in the design of new alloys for sheet produc- tion. Although the universal objective is improved formability, alloys tailored specifically for TRC should also take advantage of the unique solidification characteristics of this technology. Recent studies [4,5] show that mag- nesium, when alloyed with rare earth elements, leads to weakening of the basal texture and a corresponding im- provement in room temperature form- ability. In addition to these benefits, advantages of rare earths include high strength, high creep resistance, accept- able corrosion behavior, and excellent flammability resistance. In this respect, rare earth elements are considered ef- fective. Research results suggest that a strong interaction of solutes with dis- locations and grain boundaries is re- sponsible for the significant impact rare earth additions have on the extruded grain size and texture at very low alloy- ing levels [4] . To overcome the high cost and supply issues of rare earths, other effective modifiers such as Ca, Sr, and Ba are being investigated, with Ca of particular interest. For example, there is evidence that the synergistic effect of Ca combined with Ce enhances precipi- tation hardening, refines the grain size, and improves texture and ductility to a greater extent than separate additions of individual elements [5] . In this research, a combination of calculations using FactSage thermo- chemical software and examinations using a number of experimental tech- niques were explored to determine so- lidification characteristics of wrought magnesium alloys containing neo- dymium [6] . An example of the experi- mental compositions tested is shown in Table 1. Under equilibrium solidification conditions, FactSage software pre- dicted the same phases of α -Mg, Mg12Zn13, and Nd5Mg41 for the ZEK100, Mg-1Zn-0.5Nd, and Mg-1Zn-1Nd alloys. During non-equilib- rium solidification, FactSage software predicted that an additional phase of Mg51Zn10 would form, mainly at the expense of Mg12Zn13. Thermal analy- sis performed during solidification con- firmed two major reactions: formation of α -Mg dendrites followed by eutectic transformation. There was a slight dif- ference in both the liquidus and solidus temperature between the alloys tested, aligned with growing Nd content. The TABLE 1 – SELECTED ALLOYS AND LIQUIDUS TEMPERATURES Alloys Liquidus, o C Mg-1Zn-0.5Nd 647 Mg-1Zn-1Nd 646 Mg-2Zn-1Nd 640 Mg-2Zn-2Nd 637 Mg-4Zn-1Nd 635 Mg-4Zn-2Nd 634 ZEK100 648 Alloys containing neodymium and their liquidus temperatures determined by thermal analysis. cooling rate during solidification affect- ed the microstructure refinement and a volume fraction of intermetallic precip- itates. Increasing the cooling rate from 30°C/s to 110°C/s resulted in a reduc- tion of dendrite arm spacing in Zr-free alloys from roughly 45 to 25 μ m. For all alloys, an increase in the solidification rate was accompanied by a noticeable reduction in the number of intermetal- lic precipitates. SUMMARY Metal rolling is one of the most important manufacturing processes in modern industry and therefore its ex- pansion to magnesium alloys is of par- amount interest. Although magnesium has poor room temperature formabili- ty due to its crystallographic structure, efforts are underway to overcome this barrier in order to produce magnesium sheet at the industrial scale. Canmet- Materials with its state-of-the-art pi- lot scale casting, twin roll casting, and rolling facilities is well suited to contrib- ute to the progress in development of low cost magnesium strip and sheet. To achieve this, researchers are focused on further development of twin roll cast- ing/rolling and creating new magne- sium alloys with improved formability at room temperature. ~AM&P For more information: Amjad Javaid, research scientist, CanmetMaterials, 183 Longwood Rd. South, Room 259C, Hamilton, Ontario L8P 0A5 Canada, 905.645.0781 ,amjad.javaid@canada.ca, www.canmetmaterials.nrcan.gc.ca. Fig. 6 — Tensile properties of hot rolled ZEK100 sheet [2] .