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 1 9 affect strip quality. To gain a better un- derstanding of the impact of nozzle de- sign and operating parameters, a 3D model of the TRC was constructed and multiphase computations were carried out [2] . Key insights reveal that there is significant recirculation of flow in the nozzle, which could lead to insuffi- cient melt supply to the gap between the rolls—therefore contributing to the magnesium strip defects. This finding should help optimize nozzle design. SHEET PRODUCTION THROUGH SOLID STATE ROLLING The sheet metal rolling process consists of passing metal stock through one or more pairs of rollers to generate a flat product with a specific and uni- form thickness. In addition to modify- ing hardware to change deformation characteristics during rolling, there are also efforts to assess the role of all subtle processing parame- ters. The rolling facilities at Can- metMaterials enable research on rolling conditions for a vari- ety of advanced materials includ- ing magnesium alloys. The pilot scale rolling mill is a single stand reverse mill that can be config- ured as 2-high for hot rolling and 4-high for cold rolling under ten- sion (Fig. 5). It is driven by two 300-hp motors applying a max- imum load of 500 tons. For low- scale experiments, the laboratory is equipped with a 50-ton Stanat re- verse rolling mill with preheated rolls. During research on the com- mercial magnesium alloy ZEK100 (Mg-1.2Zn-0.35Zr-0.17Nd, wt%), the im- pact of rolling temperature and post-rolling heat treatment on sheet properties were examined [3] . As-cast plates with 25-mm thickness were rolled to a final sheet thickness of 1.5-1.7 mm at temperatures to 450°C with a thickness reduction of 10-15% per pass. As expected, the tensile and compressive properties of the hot rolled sheet substantially exceeded those for the as-cast state, with values strongly affected by the rolling temperature. An increase in rolling tempera- ture from 350° to 450°C caused a re- duction in tensile strength from 257 to 228 MPa accompanied by a reduction in tensile yield stress from 237 to 185 MPa Fig. 4 — Left, pilot scale TRC facility at CanmetMaterials features a 400-kg capacity furnace for molten metal to produce 355-mmdiameter rolls. Right, close-up of magnesium sheet rolling. (Fig. 6). At the same time, alloy elon- gation increased from 17% to 21%. For comparison, after casting tensile strength was in the order of 175 MPa, yield stress below 65 MPa, and elonga- tion reached 13%. The research shows that alloy grain size was influenced by the rolling temperature with increas- ing temperature in the range from 350-450°C causing grain coarsening with the highest growth seen for the temperature range of 400-450°C. The correlation between alloy grain size af- ter rolling and sheet properties was es- tablished where a reduction in grain size was accompanied by an increase of both the tensile/compressive strength and yield stress. At the same time, a re- duction in alloy grain size was accompa- nied by a reduction in sheet elongation. Based on the Hall-Petch relationship and average grain size, the correlation may be used as the prediction mod- el for properties of the hot rolled magnesium sheet. ALLOY DEVELOPMENT FOR IMPROVED FORMABILITY In addition to devel- oping a modern rolling process, there is a quest for novel magnesium alloys with improved formabil- ity. In this search, experi- mental work and model- ing based on thermody- namic calculations are Fig. 5 — Pilot scale rolling mill at CanmetMaterials.