Lightweighting initiatives for automotive and aerospace applications have stimulated interest in magnesium as a forging material. Magnesium’s combination of low density and reasonable strength gives it a specific strength greater than either steel or aluminum. The metal costs more than some alternatives, and it is tricky to forge, but research is under way to find the best way to process magnesium and forge it into critical components.
The automotive industry is in the midst of a significant shift, with the use of lightweight materials at the forefront of automotive vehicle design considerations to improve fuel economy. Today, lightweighting represents one of the nearest short-term solutions to help the automotive industry meet the Corporate Average Fuel Economy (CAFÉ) standards set out by government in the U.S. The CAFÉ standards for 2018 are 45 MPG or 19 km/l for small passenger cars and 34 MPG or 14.5 km/l for larger passenger cars.
Advantages and Limitations of Magnesium
The use of magnesium (Mg) is particularly appealing for automotive applications because it provides an attractive advantage over other structural metals such as aluminum and steel. The density of magnesium (0.06 pounds/inch3 or 1.74 gm/cm3) is two-thirds that of aluminum and a quarter that of steel. Magnesium’s combination of low density and reasonable strength leads to a specific strength that is far superior to either steel or aluminum.
Using more magnesium would significantly decrease the weight of an automobile. However, its use in automotive and other applications lags far behind that of aluminum. Wrought magnesium, particularly in the form of forgings, therefore represents a tremendous growth opportunity. At present, the use of magnesium forgings is limited because of: its high cost; its poor room-temperature ductility; the lack of knowledge of magnesium relative to other metals such as steel and aluminum; and its relatively high propensity to corrosion, which means it usually needs a protective coating.
Today, most magnesium automotive applications are in the form of die castings, and the use of wrought magnesium is unusual. The reason for this is magnesium’s atomic crystal structure, which is hexagonal close packed (HCP). This means it has very limited room-temperature ductility. At low temperatures, the active slip systems in the crystal structure that allow deformation to occur are limited to its basal planes. These poor deformation characteristics have hampered our ability to use magnesium in wrought form.
The formability of magnesium can be significantly improved when additional slip systems are activated at higher temperatures. As such, hot deformation is extensively used to produce wrought magnesium products. Magnesium can develop strong textures during deformation, and this can translate into strong mechanical anisotropy in the final product. The texture that develops in magnesium can have a strong effect on its subsequent deformation. Depending on texture, different combinations of deformation systems may be activated.
Hot deformation of the as-cast structure for magnesium leads to a significant improvement of its mechanical properties (Fig. 1) because the plastic strain imposed on the cast structure leads to a more uniform fine-grained structure. The inherent problems of casting processes – such as coarse and non-uniform microstructure evolution and defect formation (such as porosity) – have negative impacts on the final mechanical properties.
A process known as dynamic recrystallization can occur during hot deformation. This leads to the randomization of the initial texture as well as a fine and more uniform grain size (Fig. 2), and it can be of practical interest in the further deformation of magnesium. Hence, forging is an attractive manufacturing process to consider in manufacturing magnesium components for automotive applications. The fine and uniform grain size produced during hot working via dynamic recrystallization can help improve the final mechanical properties of the components, including its fatigue life, and reduce texture effects.
The Effects of Strain Rate on Mg Deformation
Similar to other metals at high temperatures, magnesium alloys become very sensitive to the strain rate that is applied during deformation. In the case of magnesium, different strain rates can also lead to different modes of deformation. Referring to Figure 3, which shows some measured stress-strain curves for AZ31B magnesium alloy compressed at 300°C, higher strain rates cause significant work hardening to occur initially followed by some softening. Tensile twinning is initially the dominant deformation mode. The material does not exhibit any strain hardening at the lower strain rates.
At the University of Waterloo in Ontario, Canada, we are engaged in a collaborative research program to design and validate an automotive fatigue-critical control arm made of forged magnesium. This is a multidisciplinary research program that involves a number of companies and researchers with different backgrounds. The objective to design, produce and validate this part will be achieved through developing the required base knowledge and enabling technology necessary for the optimum design of forged magnesium (Fig. 4).
The forging task in this research relates to developing knowledge of material behavior during forging, final microstructures produced and how these correlate to the final mechanical properties of the forged component. The project includes microstructure optimization, process development, mechanical-property characterization and modeling for selected magnesium alloys.
The Forging Industry Educational and Research Foundation (FIERF) of the Forging Industry Association (FIA) is supporting research on this forging task. Figure 5 shows examples of some of the magnesium shapes that have been hot-deformed to help develop knowledge of magnesium forging and the microstructure and properties that develop for magnesium alloys AZ31B, AZ80 and ZK60.
Figures 6 and 7 show closer views of magnesium shapes made using the 110- and 500-ton presses at the Natural Resources Canada Laboratory (CANMETMaterials) located in Hamilton, Ontario. Research in the forging task involves both conducting experiments as well as developing models of the magnesium forging process using the commercial software package DEFORM 3D.
Forging of magnesium alloys is more challenging than other metals and alloys because of the need to maintain a high deformation temperature spatially at all points in the forging. This means the dies need to be heated and maintained at a minimum temperature (typically 200-250°C) during the forging operation.
Currently, there are very few forging plants specializing in the production of magnesium-alloy forgings. Part of the reason for this is the lack of forging presses with the ability to heat the dies during forging along with the lack of experienced personnel for the forming of these alloys. To improve the hot-forming conditions for magnesium-alloy forging, various lubricating agents such as colloidal graphite, molybdenum disulfide, mineral oils, waxes and fats are used.
Because of its high specific strength, magnesium alloys have great potential as lightweight materials that can be used to produce parts for both the aerospace and automotive industries. In recent years, one can observe a trend to replace parts made of casts with magnesium wrought products, including forgings, because the latter ensures higher mechanical and functional properties.
Author Mary Wells, PhD, PEng, is Professor of Materials Engineering, Department of Mechanical and Mechatronics Engineering, University of Waterloo (a FIERF Magnet School), Waterloo, Ontario, Canada. She is also chairperson of the Ontario Network of Women in Engineering, as well as Associate Dean of Outreach for UW. She may be reached at 519-888-4567 ext. 38356 or at firstname.lastname@example.org.
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