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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 1 8 were successfully formed at industrially acceptable forming rates and tempera- tures. In contrast, an annealed AZ31 alloy could not be formed through con- ventional rolling within the same exper- imental settings. This research is now leading to alloyingmagnesiumwith cer- tain elements to improve formability. STRIP PRODUCTION VIA TWIN ROLL CASTING Twin roll continuous casting (TRC), also known as twin roll strip casting, is seen as the most promising technique to overcome barriers of magnesium formability and enable manufacture of sheet or strip products directly from a molten state (Fig. 3). Developed origi- nally by Henry Bessemer in 1865, it only became an industrial practice after the 1950s. The technique combines casting and hot rolling into a single operation, thus reducing manufacturing cycle time, energy, emissions, and final cost compared to traditional sheet produc- tion using direct chill ingot casting. High solidification rates expe- rienced during TRC have positive ef- fects on the strip microstructure and resulting properties: homogeneous microstructures with fine grains, re- duced chemical segregation, increased solid solubility, and enhanced precipi- tate nucleation within the matrix. TRC has gained widespread acceptance over the past several decades in both ferrous and non-ferrous metal industries for producing cast strips of different thick- nesses and widths. However, its appli- cation to magnesium strip production has proven difficult. This is mainly at- tributed to magnesium’s high affinity to oxygen, low specific heat, and wide freezing range, which can cause forma- tion of casting defects such as segrega- tion, hot cracking, and oxide inclusions. The near-rapid solidification expe- rienced by alloys during TRC requires new alloys that can take advantage of the increased amount of solute in the α -Mg solid solution, thus resulting in enhanced nucleation of precipitates within the matrix and their dense distri- bution. The features of microstructure modifications related to rapid solidifi- cation positively influence strip forma- bility at room temperature. Enhanced formability is an important enabler for the adoption of sheet material in auto- motive body part applications. At CanmetMaterials, a pilot scale TRC facility is being used to manufac- ture high ductility, low cost magnesium sheet for automotive lightweighting (Fig. 4). The process modeling provides a quantitative basis for optimizing im- portant process variables to ensure high-quality sheet products based mainly on the AZ31 alloy precursor. However, unsolved challenges remain with regard to hardware performance in a harsh environment of highly reac- tive liquid magnesium. One example involves the ceramic nozzle and roll- ers, which distribute liquid metal at the beginning of the process and adversely Fig. 2 — Factors that influence the low formability of magnesium: (a) anisotropy of magnesium crystal at room temperature; (b) texture formation during rolling; (c) cracking of magnesium alloys during rolling attempts; and (d) brittle fracture surface of magnesium alloy, which cracked during rolling. Note: Images (a) and (b) are based on Reference 9. Fig. 3 — (a) TRCmagnesium strip production process and (b) distribution of molten magnesium alloy to rolls. (a) (b)