Casting of the Year: Magnesium Liftgate Cuts Weight, Adds Value
Click here to see this article as it appears in the May issue of Modern Casting.
The liftgate closure inner casting for the 2017 Chrysler Pacifica is a collaboration between Fiat Chrysler Automobiles, Meridian Lightweight Technologies and Thai Summit America Corp., and represents the first high-volume magnesium application of its kind in the automotive industry. The casting forms part of a four-piece assembly with aluminum sheet upper and lower outers and aluminum stamping wiper reinforcement to reduce the weight of the entire assembly by more than 22 lbs. (10 kg)—nearly a 50% reduction over the previous generation design.
The Pacifica’s liftgate is one of the most recent examples of the growing use of lightweight material in automotive design to help improve fuel economy. The 2017 Pacifica is a new minivan creation from Chrysler, replacing its traditional Town & Country. Rather than a variation of the existing minivan, the Pacifica was reengineered as a new product within the minivan segment and features a new platform and body structure. It weighs 250 lbs. (113 kg) less than the Town & Country; the total liftgate assembly accounts for 22 of those lost pounds.
“We started from the ground up to design the most technologically advanced minivan, which offers bold styling, class-leading ride and handling, and unmatched fuel economy, with the Pacifica Hybrid delivering up to 80 miles per gallon equivalent (MPGe) in city driving,” Timothy Kuniskis, head of passenger car brands, FCA-North America, said in a press release.
Chrysler touts the magnesium liftgate as one of its innovative “firsts” for a minivan, along with 37 other innovations tallied with the 2017 Pacifica. For Meridian Lightweight Technologies, this first was a natural progression from what the magnesium casting company had been doing over the last several years.
“We have been casting parts like this in magnesium at lower productions, so a lot of the things we learned then we could apply to the higher volume required for the Pacifica,” said Tom Faupel, engineering manager, Magnesium Products of America, Eaton Rapids, Michigan.
Magnesium was a lightweight material choice for the liftgate because of its mechanical properties, particularly its ductility, which helps in meeting the crash testing requirements. Originally a weldment of seven steel stampings, the redesigned liftgate was combined into a single magnesium part. The joining technologies required, such as spot welds and rivets, were reduced from 84 to 10.
“When we search for potential applications, we look at what magnesium does well,” Faupel said. “It doesn’t handle high temperature, so that rules out engine applications, but its mechanical properties make it a very good structural material.”
The magnesium liftgate surpassed all its crash requirements—the same ones required by the original steel version. Two of the main challenges of the liftgate were the casting size and volumes required.
“You want to have large, thin-walled parts like this cool as evenly as possible to maintain dimensional stability,” Faupel said. “Magnesium wants to cool rapidly, and filling the mold is made more challenging with large, thin-walled parts.”
Meridian relied on multiple advanced thermal management technologies it had developed over the years to help the metal stay molten as it fills the mold. This thermal management allowed Meridian to incorporate extra features into the casting that otherwise would have been added as extra parts or secondary processes, such as pockets for the tail lamps and rear speakers. It also incorporates an aluminum stamping wiper bracket reinforcement in part to provide for the gating area to feed the extremities of the casting.
Because of the high volume required, Meridian runs two die casting machines, with a third die in rotation at its Eaton Rapids, Michigan, location. As the two dies are used in production, the third will undergo preventive maintenance to address any wear on the die and assure dimensional stability and repeatability. The two casting lines merge into a single process flow for secondary finishing, which includes surface preparation, dimensional measurements, piercing and machining operations, and powder coating.
The finished castings are shipped to the Tier 1 supplier for assembly with the rest of the liftgate substructure before it is delivered to Fiat Chrysler’s plant in Windsor, Ontario, Canada, to be placed on the Pacificas.
According to Jon Weiler, metallurgist at Meridian’s Global Technology Centre, cutting weight in one area of the vehicle, like the liftgate, provides secondary benefits to the automotive design. “Because of the liftgate being lighter to start with, the designers can use smaller motors, struts, hinges, etc. All of that allows them to save additional weight, and it’s significant.”
Beyond the weight reduction, the magnesium diecast liftgate allowed for an improved rearward visibility by close to an inch, achieved better noise, harshness and vibration performance by more than 16 Hz in bending, and prevents cavitation from idle boom and road boom through local ribbing, integrated gussets and added thickness in select areas of the component.
The Pacifica’s liftgate structure opens up the possibility for other mass market automobiles to incorporate magnesium in its rear hatches or side doors.
The 2017 Pacifica was unveiled in January and will be available in five models in dealerships this spring. Two hybrid models will be available later this year. The magnesium liftgate frame will be in all of them.