February 2023 Volume 5

MATERIALS

are even mandated on Formula 1 vehicles. They are also available for high-performance motorcycles. However, magnesium wheels are not commonly used on any street vehicles, particularly not those vehicles that would need to travel the streets during the snow season when road salts are applied. WhyMagnesiumUse is Limited Automotive designers and engineers are well aware of magnesium and its potential value—but the material is still not commonly used. There are a number of reasons for this, including its low stiffness, corrosion, lack of reliable supply, and competitive quoting. Low Stiffness: Regardless of the processing method, the modulus of elasticity (stiffness) of magnesium is 45 gigapascals (GPa). By comparison, aluminum is 69 GPa, and steel varies between 190-210 GPa. Many components are stiffness limited, and this is a significant hurdle for magnesium to overcome. Low stiffness materials require a larger cross-section to achieve the same performance. This often leads to increased cost and a larger sized component, which makes it difficult for the component to fit in the allowable space within the vehicle, referred to as package space. The component success stories are designs where the stiffness comes from the component’s inherent geometry, as in a wheel or a U-shaped interior structure underneath the center console of the car. Corrosion: Shock towers can be made of die cast magnesium, and Ford even has a patent on this concept.4 However, there are currently no magnesium shock towers on the market. One of the major limiting factors is the risk of corrosion, and the need to develop methods to protect these castings from corrosion. According to Matt Hamblin, CEO of Keronite Group Ltd., one possible means of achieving this is Keronite PEO,5 which would offer a base coating for initial corrosion protection, enabling the use of magnesium in components like shock towers. “The coating converts the magnesium surface, where circa 50% of the coating grows into the substrate and 50% is outward growth,” explained Hamblin. “PEO by nature is a porous ceramic layer, we also then have the capability to apply primer and topcoats for a complete corrosion solution capable of surviving 2,000+ salt fog hours.” Reliable Material Supply: Around 80-90% of the world’s magnesium is produced in China, and if they stop production, as has happened during the recent Olympic event periods, the price of magnesium skyrockets. In automotive, the suppliers are committed to multi-year contracts with yearly cost-reduction commitments. A massive increase in pricing can (and has had) a devastating effect on these suppliers, who will eliminate magnesium from their material options. In recent news, Kaiser Aluminum Corporation had declared force majeure due to a critical shortage of magnesium (caused when US Magnesium, LLC declared its own force majeure, creating a year long shortage). Fortunately, as of September 2022, Kaiser has since found stable supply of magnesium from alternative suppliers and were able to return to production.6 In this case, the magnesium is used as a critical strengthening element in the production of alloys for aluminum sheet.

Forging grade magnesium has additional supply chain problems. Forging stock generally requires cleaner alloys that are different from the commodity materials commonly used in casting. Because the current demand for magnesium forgings is low, forging stock is not produced very often. Competitive Quoting: In automotive and other markets, the customer needs multiple suppliers to reduce their risk and ensure they are getting a fair price. In die casting, there are a number of well-known Tier 1 suppliers, including Meridian, Spartan Light Metal Products, Aludyne, GF Casting Solutions, and Linamar. In forging, however, there are far fewer producers and most of them target premium markets, like aerospace. This limited amount of producers presents significant risk for the automotive customer. Magnesium Forging Forging materials command a premium over castings, because they deliver better properties, leading to better performance. This makes forging better suited for components that face higher mechanical loads. In general, forged parts also tend to be thicker and smaller in size than castings. Magnesium forgings are produced from forging stock, which is essentially a bar or rod that is placed into the forging press and then compressed into the desired shape. Since all of the stock will become a part of the final forged piece, every inch of the forging stock needs to have the correct chemical composition, even the surface. There are two types of forging stock—as-cast forging stock or extruded forging stock. As-cast forging stock is less desirable, as the cast stock tends to have porosity, which leads to weak spots. Cast stock also has to be further processed in order to achieve the correct properties. “As-cast magnesium forging stock requires full machining, homogenization, and ultrasonic inspection prior to forging,” explained Rick DeLorme at Magnesium USA.7 “Care must be taken in the form of reduced strain and strain rate, along with care to maintain temperature during initial upset to render the wrought structure.” Therefore, extruded forging stock is typically used. In this case, a cast magnesium billet is extruded into a bar to produce the stock material.8 By mechanical working the cast material, such as pushing it through an extrusion die, the forging stock will have less porosity and better overall properties, thus yielding a better forged component. In North America, there are only three suppliers devoted to producing extruded magnesium forging stock. Magnesium USA in Euclid, OH, produces magnesium extrusions and castings for a variety of industries. One of the forging-grade alloys that the company produces is AZ80-T5, which has an ultimate tensile strength of 345 MPa and a yield strength of 262 MPa, along with a 6% elongation.9 Mag Specialties in Denver, CO, produces four forging stock alloys: AZ31B (good forgeability with medium strength), AZ61A (higher strength), AZ80A (high strength and heat treatable), and ZK60A (highest guaranteed minimum strength and higher ductility). 10

FIA MAGAZINE | FEBRUARY 2023 37