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 7 in the automotive market these parts could include hoods, trunks, inner door panels, and seat components, among other applications (Fig. 1). The key obstacle to industrial scale magnesium rolling is the element’s in- herently poor formability at room tem- perature. To address this challenge, the search is on for both new mag- nesium alloys that feature improved formability and novel rolling techni- ques. This article describes activities at CanmetMaterials aimed at expanding manufacturing capabilities of magne- sium-based sheet materials for future applications in transport vehicles. MAGNESIUM FORMABILITY The poor plastic flow properties of magnesium at room temperature are inherently related to its crystal anisot- ropy and hexagonal close packed (hcp) structure, which limit the number of ac- tive slip systems and primarily involve only the basal planes [1] . The critical re- solved shear stress for basal plane slip in magnesium single crystal is about two orders of magnitude lower than that for non-basal plane slip involving prismatic or pyramidal planes. There- fore, the distribution of the former plays an important role in determin- ing Mg formability. Deformation resis- tance along the direction parallel to the basal plane is small, but deformation resistance in direction parallel to the prismatic plane is very large. As shown in Fig. 2, due to the strong preferred crystallographic orientation of grains (texture), a metal cannot deform along the thickness direction during rolling. Because very strong basal texture de- velops in magnesium sheet during roll- ing deformation, changing the texture type—or at least weakening the basal component—is an effective route to en- hancing formability. In order to overcome the funda- mental issues with magnesium form- ing, research activities are focusing on two fronts—developing novel alloys with improved formability and inves- tigating new rolling methods that con- sider both processing parameters and machine hardware. An example with regard to deformation temperature shows that as temperature increas- es, the critical resolved shear stress of the non-basal slip systems decreases. At about 220-250°C or above, a signifi- cant increase in metal formability oc- curs. Formability modification may also be achieved through changing defor- mation conditions as is the case with superplastic forming, equal channel angular rolling, cross-roll rolling, and asymmetric or shear rolling. Efforts focused on understanding texture development in wrought mag- nesium alloys are also taking place. Research by the NSERC Magnesium Network (MagNET) reveals a very dif- ferent texture in the commercial mag- nesium alloy ZEK100, which contains a small amount of neodymium compared to the strong basal texture of conven- tional alloys such as the AZ31 grade. Us- ing the ZEK100 alloy, automotive parts D ue to their lightweighting charac- teristics, magnesium alloys have proven to be attractive structural materials for a number of transport vehi- cle applications. They offer opportunities for significant weight reduction as well as improvement in fuel efficiency, handling, and overall performance from a materi- al that is 75% lighter than high-strength steel and 33% lighter than aluminum. To- day, casting is the dominant manufactur- ing process for magnesium components, representing about 98% of all structural applications for magnesium [1] . This is due to the excellent castability of magnesium alloys, which exceeds other metals such as aluminum and copper. Magnesium’s unique solidification features, including high fluidity and low susceptibility to hy- drogen porosity, make it a good candi- date for successful casting operations. However, it is generally accepted that to expand the existing magnesium applications and manufacture wrought components in commercially viable sizes with tight dimensional toleranc- es, adequate surface quality, and opti- mal mechanical properties, sheet metal forming is required. Semi-finished prod- ucts such as strip and sheet can further be transformed into net-shape final parts through a variety of manufactur- ing processes including laser or water jet cutting, stamping, bending, perfo- rating, punching, incremental or press brake forming, curling, roll forming, and spinning. Development of low-cost magnesium sheet is thus an enabler to downstream processing. For example, (a) (b) (c) Fig. 1 — Examples of automotive magnesium applications developed using sheet metal technology: (a) trunk lid inner panel by General Motors, used on production Cadillac SLS sedans; weight of the AZ31B alloy panel is 2.4 kg, about 30% less than the Al version; (b) back seat/trunk separa- tion in the new SM7 Nova fromRenault Samsung Motors using POSCO sheet, reducing weight from 3.6 kg to 1.4 kg [7] ; (c) roof of Porsche 911 GT3 RS (991 Series) produced from AZ31 magnesium sheet (POSCO) with a thickness of 1.1 mm. Total weight is 2.2 kg, 700 g lighter than carbon fiber. Reducing weight at the top lowers the center of gravity [8] .