**6. Magnesium solution in the automotive sector: the present**

One significant value that manufacturers usually give to magnesium is its excellent die-castability resources, compared with aluminum. It is mainly due to very low viscosity in the molten state and reduced (or absent) die-soldering phenomena with steel mold-die that broadly extend mold-dies lifespan. The high castability is one metallurgy factor that allows die-casters to realize large, thin-walled, and complex casting shapes. It is due to a less costly manufacturing process that would replace steel-made components by assembling numerous steel stamped pieces or heavily reinforced plastic members [37]. As magnesium alloys can be cast with thinner walls than aluminum, the lower elastic modulus of magnesium alloys can be compensated using located ribs of thin wall thickness that allow restoring stiffness at required values. Secondly, the lower latent heat for solidification of magnesium compared to aluminum leads to considerably shorter casting cycle times, compensating for the lower heat conductivity of magnesium. For a comprehensive overview of die-casting processes and recent advancements, you may refer to [38], here in the following, shortly resumed. Two main casting processes are available for magnesium, the pressure-assisted cold, and hot-chamber injection, with an alternative represented by low-pressure die casting. In pressurized injection casting processes, high pressure is exerted after the liquid metal injection to compensate for metal shrinkage and remove as much possible air entrapped during the shot sleeve movement that accelerated to pressurize liquid metal into the die. The metal solidifies at high cooling rates (higher for the cold chamber than the hot chamber), leaving a fine-grained material (more satisfactory for the cold-chamber process) with secondary dendrite arm spacing in the range of 5–10 μm. As it is usual for any metal, particularly for magnesium alloys, grain refinement is one primary strengthening mechanism capable of saving good ductility and though properties, generally lower for the common magnesium-aluminum alloys containing more than 3–4% of aluminum. If, on one hand, aluminum promotes a strengthening mechanism based on a solid solution, on the other hand, an excess of aluminum (it is limited up to 9%) produces an almost continuous secondary phase of aluminum enriched—the magnesium aluminide, Mg17Al12. The magnesium aluminide decreases local plastic resources at the alpha-solution grain boundaries, where magnesium aluminide precipitates.

The long tradition of magnesium automotive part die-casting is proper for magnesium manufacturers, as shown in **Figure 8**, where an example of Meridien's timeline for automobile products is summarized. Magnesium die-casting is evolving in Mercedes-Benz automatic transmission cases, from the first seven-speed automatic transmission case developed in 2003 (**Figure 9**) to the current eight-speed transmission case, still manufactured by magnesium alloy.

**Figure 10a** shows the recent magnesium die-cast liftgate inner for the 2017 Chrysler Pacifica Mini-Van realized by Meridian Company with Fiat-Chrysler Automobiles. The liftgate assembly consists of:


### **Figure 8.**

*Meridian product development timeline (courtesy of IMA).*

### **Figure 9.**

*The 2003 case of the 7G-TRONIC, the world's first seven-speed automatic transmission.*

### **Figure 10.**

*(a) Diecast liftgate inner by Meridien (courtesy of IMA), 1450 mm wide, 1210 mm in height, the mass of 6.9 kg, (b) Strut bar Audi A8 (courtesy of IMA).*

An AM60B alloy has been used due to elongation, strength, castability, and energy-absorbing properties. The magnesium casting allows replacing seven steel stampings, including reinforcements in hinge & latch areas, two plastic pieces, joining technologies. In the final assembly, a powder coat was applied to all structures to prevent galvanic corrosion problems. **Figure 10b** shows the new die-cast strut bar of *Magnesium Alloys for Sustainable Weight-Saving Approach: A Brief Market Overview, New… DOI: http://dx.doi.org/10.5772/intechopen.102777*

### **Figure 11.**

*Recent die cast parts from magnesium industry [courtesy of GF Casting Solutions]: (a) Porsche Control Box Cover, made of MgAl4RE4 alloy, 2.6 kg weight, realized by multistage process (casting, stamping, machining), then assembled; (b) E-vehicle upper door frame alloy made of AM 50 alloy, 2.9 kg weight, realized by multistage process (casting, punching, machining, e-coating); (c) Ranger Rover Front End & Cross Car beam made of AM60 alloy, 6.0 kg weight realized by the multistage process then assembled (casting, machining, stamping); (d) Daimler SLK 2 seat back frame, AM 50 alloy by die-casting, 2.6 kg weight (courtesy of GF Casting Solution AG).*

Strut bar for Audi A8 realized by Stihl Magnesium. In **Figure 11**, several magnesiummade parts have been recently put onboard vehicles.

Magnesium part manufacturers deploy a long-tradition cumulated in warm and hot deformation processes (**Figure 12**). Opposite to the common thought about magnesium deformation resources, wrought magnesium alloys are suitable for sheet forming contributing to weight-saving projects in the automotive sector. Indeed, significant drawbacks in magnesium alloys' sheet forming and deformation processes exist, especially compared to aluminum alloys. Due to its hexagonal structure, to activate enough slip systems for assuring sufficient plasticity, magnesium alloys must be formed above 200°C. Furthermore, the different heat transfer capability compared to aluminum is an issue to consider for optimizing pre and re-heating temperatures in the hot-deformation multistage processes. Extrusion of magnesium alloys is usually carried out in multiple steps, starting with a pre-extrusion of large billets into smaller diameter billets. After the preliminary stage, the billet can be re-heated and subsequently extruded into the final shape. Generally, the pressure per unit volume material

**Figure 12.** *The Chevrolet Corvette SS Race Car, 1957, made of magnesium-formed panels.*

extruded is higher than in aluminum alloys. Thus, extrusion speed shall be carefully controlled and optimized for specific magnesium alloy to avoid local melting and local oxidation phenomena, particularly critical for hollow sections extrusion process by porthole dies, as typically employed for aluminum alloys. These aspects are firm limits for semi-finishing and net-shape forming processes, prominently for affordability.

But on the other hand, warm deep drawing is also possible for magnesium alloys, as for aluminum alloys; in the range of 100–180°C thin sheet of 0.5 mm approximately can be drawn, with precise temperature control and at a lower speed [39]. The recent application of magnesium sheets we can find in the literature illustrates the successful use of a novel Mg-Zn-Ca-Zr alloy in sheet form produced by twin-roll casting. This alloy has been used to manufacture a Volkswagen Passat decklid magnesium-made that saved half 6 kg of the original 12 kg steel weight [40]. Large magnesium components can also be produced by die casting (see **Figure 13**).

Net-shape semi-solid forming has attracted automakers with alternate attention since the middle 1990s. The net-shape semi-solid forming is possible for magnesium alloys thanks to its thixotropic state realized when vigorously sheared in a semi-solid state. Shearing reduces the viscosity of the slurry mass to a similar value of the liquid metal, providing similar (sometimes better) castability of the liquid form. Still, the semi-solid state allows shaping with lower latent heat in the mass; this creates advantages for shorter casting cycles than die-casting (depending on chosen semisolid process) lower metal shrinkage to compensate, and consequently near-net shaping. The industrial application of semi-solid net-shaping in the magnesium industry commenced in the early 1980s with the Dow Chemical Company. Dow Chemical patented the Thixomolding technology based on the architecture of plastic injection molding machinery for injecting magnesium alloys in the semi-solid state into a mold die.

Further advantages of injection molding of magnesium alloys are that this technology's highly complex shaping capability allows for more innovative design concepts and a multi-body-material concept design. Direct assembly of different

*Magnesium Alloys for Sustainable Weight-Saving Approach: A Brief Market Overview, New… DOI: http://dx.doi.org/10.5772/intechopen.102777*

**Figure 13.**

*(a) The Porsche window frame realized by AM50 magnesium alloy with multi process stages (casting, laser cutting, machining), finally coated and joined; (b) The Aston Martin cover door made AM50 with multi-stage process (Casting, Stamping, Flattening) [courtesy of GF Casting Solutions AG].*

parts during injection molding in a molded-in technique, thanks to inserting aluminum parts directly in the tool. As a semi-solid process, less energy is consumed by the Thixomolding apparatus; the power energy is estimated to be on the order of 12–24% lower than the total energy required by a conventional casting process. An additional benefit is that the Thixomolding product cycle employs inert gas, usually argon, to protect magnesium feedstock from oxidation once introduced in the hopper in particulate and solid forms (pellets or chips) [41]. However, it is worth noticing that current die-casting processes align with the environmental sustainability of the Thixomolding process thanks to much more environmentally friendly cover gases mixtures today used instead of the banded SF6. With relevant advantages of the Thixomolding process in net-shaping part of high complexity in a single shot, two are the most drawbacks acknowledged by part manufacturers. The high price and the limited number of suppliers of chip or pellet forms of magnesium alloys, namely the material feedstock of Thixomolding machinery, and the maximum clamping force exerted during the metal injection into mold dies. Clamping forces of 2700–6500 kN generally allow the production of thin flat surfaces (0.8 mm, not possible by high pressure die-casting) such as that for tablet terminals, notebook computers, electronics, sports goods. Instead, the interest of the automotive sector is to even thicker and wider structural components with a weight of over 2 kg. This would require more giant machines with increased clamping forces over 8000 kN [41].
