Metal additive manufacturing helps automakers meet fuel standards

As one example of lightweighting vehicles to improve fuel economy, BMW used a combination of magnesium and aluminium for its N52 six-cylinder crankcases from 2004 to 2015. (©BMW AG)

Smoggy cities, roller coaster gas prices, environmental concerns over fracking and offshore drilling – there are many reasons to increase fuel efficiency in both passenger and commercial vehicles. One of the best ways is by reducing vehicle weight. Lighter cars mean consumers use less fuel, and less fuel means cleaner air and reduced dependence on fossil fuels.

The U.S. government thinks so as well. Since 1975, the U.S. Department of Transportation has imposed strict Corporate Average Fuel Economy (CAFE) standards on automakers, telling them to reduce fuel consumption or face stiff penalties. The next round of efficiency bar-raising is just around the corner; by 2025, cars and light trucks are expected to boast an “average” fuel economy of 54.5 mpg.

The calculations behind those standards are complex, making interpretation somewhat subjective, yet the fact remains that automotive manufacturers and their suppliers will be fighting a continuous uphill battle to design and produce lighter, more efficient automotive components for decades to come.

All these regulatory and market-driven redesign initiatives are creating a perfect storm of product development activity for automotive suppliers. Even in the commercial sector, countless opportunities exist to simplify designs, reduce weight and use less material, all of which benefits consumers and the planet alike. A move to lightweight products and components often begins with prototyping, where material and manufacturing process selection is paramount.

A Move to Magnesium

One thing that should be remembered before embarking on any lightweighting project is to take small bites. Unless you’re creating a fuel-efficient redesign of the transmission housing for an 18-wheeler, you’re not going to remove 50 pounds of weight from any single part. Instead, shaving ounces and even fractions of ounces out of each component that makes up a passenger vehicle is the clear path to CAFE compliance.

The rear view mirror used in passenger cars, for example, was once heavy enough to pound a nail. Today, most rear view mirrors are made of a magnesium frame and a plastic shell, yet retain the same strength and functionality as their corpulent predecessors. The trick is to develop products that fulfill cost and duty requirements but use alternate materials and clever designs to reduce weight.

Fortunately for designers and engineers, today’s array of prototyping materials and advanced manufacturing technologies mean never before possible opportunities for iterative, even parallel-path design testing. For example, suppose you’ve been tasked with lightweighting the headlight bezel for a 2019 model year reintroduction of the four-door Studebaker Lark. When you begin to explore material selection, magnesium might be a good place to start.

With a density of 106 lb. per cubic foot, magnesium is the lightest of all structural metals, and has the highest strength-to-weight ratio as well. It carries a proven track record in the automotive, aerospace, medical and electronics industries, and is used in everything from fuel tanks to gearboxes. And because magnesium is one of the most abundant minerals in the human body, it’s not only biocompatible but also biodegradable, so it is a logical choice for self-dissolving screws, pins and other implants requiring greater strength than those made of biodegradable polymers.

Magnesium is routinely milled into a variety of prototype parts. Compared to aluminum, the lightweighting runner-up, it is more expensive per pound, but that cost delta is offset somewhat by magnesium’s 33-percent lighter weight and comparable strength. It’s also easily machined although some care must be taken to control fine chips and metal particles, as these can be flammable in oxygen-rich environments.

A metal additive manufacturing process, like DMLS, can be used to build complex aluminum parts that are difficult to machine as in this motor mount.

For those concerned over fires with magnesium components in the field, don’t be – magnesium is everywhere. The Volkswagen Beetle sported a magnesium alloy engine block for decades, and BMW started using magnesium for its N52 six-cylinder crankcases and cylinder head covers in 2005. The AZ31 and AZ91 grades of magnesium alloy are even weldable, and have melting points of roughly 900° F (482°C). Unless you’re designing a lightweight furnace liner, magnesium is an excellent choice for many different components.

The Big Three are also big on magnesium. In 2006, General Motors began using die cast magnesium for the engine cradle in its Z06 Corvette, shaving 12 pounds off the old design. Ford Motor Company’s started using magnesium in the liftgate of the 2010 Lincoln MKT, and the third row passenger seats for the 2011 Ford Explorer. In that same year, Chrysler Group introduced a magnesium instrument panel for its Jeep Grand Cherokee, helping the vehicle achieve highway fuel economy of 23 mpg. In summary, domestic and foreign automakers alike are turning to magnesium for its strength, light weight and – because it can be extracted from seawater – its abundant supply.

A growing method to rapidly manufacture magnesium parts is through injection molding (also known as thixomolding). Here, chips of magnesium feedstock are loaded into the hopper of a molding press. Heat and agitation are then applied, thus bringing the magnesium payload to a semisolid state, whereupon it is “shot” under pressure into a mold cavity via a feeder screw. The result is that fully functional magnesium components can be produced in low volumes at a fraction the cost of “production-tooled” parts.

Many manufacturing engineers associate magnesium with die casting, as this has long been the traditional, high-volume method of forming this ubiquitous metal. Yet magnesium injection molding offers a number of distinct advantages over its more mature counterpart. Thixomolding is essentially a “cold” process, operating just short of magnesium’s melting point. Because of this, there is less shrinkage and warp compared to die-cast parts, and the mechanical properties of thixomolded parts are generally better as well.

The cooler process also requires less sophisticated tooling, as there is little need for cooling channels. And since the magnesium slurry is fed into the mold at very high pressure – in some cases twice that of die casting – very fine part details are produced. All things considered, thixomolding is the clear choice for many prototyped or low-volume magnesium components.

Another increasingly used additive manufacturing process offered is DMLS, or direct metal laser sintering. DMLS melts layers of metal powder, as thin as 0.0008 in. (20 microns), to create complex, 98-percent dense part shapes that are often impossible to manufacture otherwise. It is highly accurate with tolerances of +/- 0.003 in. plus an additional 0.001 in./in. that are typically achieved on well-designed parts.

DMLS works with aluminum and titanium, so is an obvious contender for manufacturing lightweight parts, but is also used with 316L and 17-4PH stainless steel, cobalt chrome alloy and Inconel, super strong metals known for their extreme heat resistance and durability rather than weight reduction. You might be wondering how an additive process that uses metals such as these has made its way into a lightweighting article.

But here’s the thing: DMLS can fabricate metal parts that, until now, were nothing more than a designer’s fantasy. Parts more hollow than a chocolate Easter egg, Escher-like curves and spheres, ultra-thin walls and spider web-like lattices, consolidation of multipart assemblies into a single sintered component – these are a few of the lightweighting possibilities DMLS can offer.

DMLS is slower than other additive processes, and more expensive – if your part design can be efficiently machined or molded, DMLS may not be the right manufacturing method. But for complex assemblies, improbable shapes or parts where small amounts of superalloy go a long way, DMLS might be just the ticket to reduce part weight and cut manufacturing costs. Lastly, DMLS isn’t limited to prototype quantities – given a small, complex workpiece too difficult or expensive to manufacture via conventional methods, DMLS is often a viable alternative for low-volume production volumes in the thousands.

There are many good reasons for lightweighting. Designing lighter, stronger and more cost-effective manufactured products is good for everyone, and provides a competitive edge to the companies that produce them. The drive to greater fuel efficiency in cars and trucks will continue to be the Holy Grail of these industries, a goal that rests on a daunting three-legged stool of limited fossil fuels, increasing greenhouse gases, and growing government regulations.
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Jeff Schipper is the global industry manager at Proto Labs.
This article is an excerpt from his white paper “Reducing Component Weight for Automotive Applications”.