Re-inventing the wheel: Magnesium adds traction to lightweighting

Bradley Diak is researching ways to lightweight the spare wheel.
PHOTO: AUTO21

By 2025, the average vehicle sold in North America must achieve a fuel efficiency of 4.21 L/100 km, compared with the 2010 level of 8.51 L/100 km. Achieving this level in 10 years will require the kind of innovations that would make Henry Ford take notice.

Fuel efficiency and performance improves as the car becomes lighter. A 10% reduction in vehicle mass translates into a 3% fuel savings so lightweighting is not just for the racing enthusiast, it’s key to engineering automobiles that are friendlier to the environment.

Lower density magnesium alloys offer significant structural advantages over steel and aluminum alloys, but there are challenges. Magnesium is known for its poor weldability, low formability, poor crash worthiness, greater susceptibility to corrosion, higher cost, reduced alloy selection and limited recycling stream. Researchers are overcoming these issues by targeting specific components.

AUTO21 researchers partnering with Ford Motor Co. have been applying tailor-welded-blank (TWB) and friction stir welding (FSW) technologies to develop an affordable magnesium alloy spare wheel as a test case for making more complicated components.

A spare wheel is effectively dead weight that rests somewhere in a vehicle until it’s needed. In many vehicles the spare has devolved from a full-on replacement wheel to the lighter, space-saving mini-spare. In others it has been replaced with a tire sealer kit. This saves about 10 kilograms (kg), but the sealer kit can’t repair 15% of flats, and it can be difficult to use.
Researchers estimate replacing a steel mini-spare with a full size, 16-inch magnesium alloy spare is about 5 kg. However, there is more to it than changing materials. The manufacturing process must evolve too.

A basic wheel consists of two components. The hub/spokes support considerable compressive and bending stresses and must be strong enough to support the vehicle. The rim distributes the load through the pressurized tire. An ideal design uses different materials for each component.

Casting is the cheapest manufacturing process, and something that magnesium does very well, but the resulting properties do not meet the structural demands of the whole wheel. In contrast, cast hubs made from a stronger, less formable magnesium alloy could be mated to an extruded lower density, lower-alloy-containing magnesium rim. This two-piece solution, the basis of TWB manufacturing used in making complex parts such as doors, saves weight and material costs but requires an additional joining step. The two-piece wheel strategy is used in many aftermarket designs but tends to be unsuitable for mass production.

Arc welding is the standard joining process for many materials, but magnesium alloys are subject to the formation of cracks, porosity and high thermal distortion in the melting and solidification cycle. FSW, a solid-state joining method, uses a rotating tool to plunge and traverse a seam between two pieces. Frictional heat and deformation forms a stir zone, which bonds the components.

FSW was first used successfully by the Fundo Wheels (formerly a Norwegian company) to produce aluminum wheels. AUTO21 researchers hope to bring this production method to Canadian manufacturing.

Bradley Diak is an associate professor of Mechanical and Materials Engineering at Queen’s University in Kingston, Ont. and leader of AUTO21’s Tailor-Welded Blank Manufacturing of Mg Alloy Parts project.

This article appears in the April 2015 issue of PLANT.