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Mix and Match for Lightweight Autos

Bruce Morey
By Bruce Morey Senior Technical Editor, SME Media

As automakers expand their menu of materials new concerns arise, from joining techniques to reconsidering body structure architecture

It is common sense—a vehicle that weighs less requires less fuel to move it. A number of studies show that reducing the mass of a vehicle by 10% results in anywhere from 4.5 to 6% better fuel economy—well worth the effort. The dilemma for automakers is that mass reduction needs to happen without compromising safety, NVH, styling, or size. Luxury sedans and half-ton pickup trucks need to remain large, safe, and quiet even if they weigh less. “To accomplish this challenging lightweighting goal, we need to leverage the entire palette of materials that we have and is available to us from the supply base,” said Dr. Hesham Ezzat, Technical Fellow, Body for General Motors (Detroit, MI).

Not that mixed-material light-duty vehicles are new. “Vehicles are in general mixed materials right now, if you talk about the whole vehicle,” said Ezzat. Automakers already use a mix of materials—aluminum engine blocks and suspension parts, steel bodies, plastic trim, and magnesium instrument panels. These parts are typically attached to an all-steel body in final assembly.

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The yellow line indicates typical areas of use for Dow Betamate adhesive, which can bond similar or dissimilar substrates.

Today, body closures are typically made of steel. Automakers are increasingly looking to switch to other materials to make these lighter. “Doors, hoods, and trunk lids, are easier to switch to a material other than steel because it is less disruptive to the manufacturing process,” explained Ezzat. “You assemble them, weld them together in the body shop, hang them on the body” then coat and paint the entire body and send it off to the general assembly area.

Why push it further? Lightweighting attachable parts will have their limit and CAFE requirements continue to increase. The next frontier the industry will explore in mixed materials is in the all-steel body structure itself. “The reason for even thinking about dissimilar materials in the body structure is to use the right material in the right place,” Ezzat said. While lighter than steel, aluminum is in general not as strong. Use stronger material, like steel, in places where strength is needed. Use light materials in places where stiffness is important but strength less so. Use energy absorption materials where crash is important. “In terms of performance, to make a body structure lighter from dissimilar materials makes sense,” he said.

Challenges and Opportunities

However, as Ezzat explains it, integrating dissimilar materials within a body structure (or body-in-white) becomes more complex than attaching lightweight components. For one, joining technologies are different, simple spot welds may no longer be enough—adhesives and mechanical fastening will be needed. It is also imperative to compensate for differing coefficients-of-thermal expansion (CTE). Mismatched CTEs cause ripples and distortions in the manufacturing process as they pass through hot manufacturing processes. Finally, galvanic corrosion between dissimilar metals must be prevented. “You may also need to rethink how you assemble the vehicle if you use more exotic materials, such as composite panels,” he said. If the composite materials cannot tolerate corrosion coating and painting, then the panel needs to skip the body shop operation and be assembled in final.

There can be an upside beyond lighter weight. “For body and chassis, there may be some secondary benefits for using other materials such as aluminum, because you can create parts in forms other than a stamping,” said Ezzat. “For example, you can have aluminum high pressure die-casting or aluminum extrusions, which give you part consolidation and integrating functions as well.”

Using more extruded aluminum parts is a trend that Doug Richman, vice president–Engineering, Kaiser Aluminum Corp (Foothill Ranch, CA) sees as well. “For example, about 35% of the bumpers in North America are now aluminum extrusions. Some seat tracks are extruded aluminum as are spool valves used inside transmissions.” More importantly, in the next generation of aluminum vehicle bodies, he noted, many of the support structures underneath the body will be extruded aluminum sections.

In fact, he noted, about 40% of the hoods and 8–10% of the trunk decks in the vehicle fleet in North America today are made of aluminum. These components do not require strength for crash resistance and so are ideal for a switch to aluminum. “There are no doors yet—but in the next-generation vehicles we’re going to be seeing a lot more of those kinds of components converted to aluminum,” he predicted. Doors are a little trickier for aluminum because they need to be strong enough to withstand side-impacts.

Richman believes that 2014 represents a key inflection point as the light vehicle market will see significantly more use in aluminum. “Today there are no all-aluminum body-in-whites produced in North America. Jaguar, Land Rover, Ferrari and Audi have them but none in North America. By the end of this year Ford Motor Company will be producing a Ford F-150 pickup truck with an almost entirely aluminum body and cargo box. And that will have both sheet product and extruded structural products in the body structure,” he said.

To Richman, it is sensible to examine using alternate materials for the body-in-white structures, given that it contains about 25% of the total mass of the vehicle. He also believes that while a body structure made of dissimilar materials is technically feasible, manufacturing plants may not be in favor of using steel, aluminum, or magnesium all in a single unibody unit. “It is complex. On average there are 250 different components in a car body. They prefer not to mix materials because of the different painting and coating operations required for each,” he said. This argues for a switch to all-aluminum where the manufacturing processes can remain separate and distinct—such as closure panels discussed above.

Structural Adhesives and Glues

Another technology seeing rapid growth is structural adhesive, such as those from Dow Automotive Systems (Auburn Hills, MI). “Growth in automotive adhesives has been very rapid based on having to meet [stringent] CAFE requirements, at the same time maintaining safety,” explained Tonja Sutton, global strategic market manager for Dow Automotive. She reports that adhesives today represent 15–20% of the total automotive joining market. It all started a few decades ago when OEMs began to use adhesives to bond windshields. Today, she believes they are a key enabler in both all-aluminum assemblies as well as mixed-material structures. “There are some automakers that are going to stay with steel, but the majority of those we deal with are moving to add more materials to the mix, in particular aluminum and carbon fiber composites,” she said.

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General Motors industry-first resistance spot welding process for aluminum uses a proprietary multi-ring domed electrode.

She believes that the newer structural adhesives Dow developed help automakers meet many of the challenges of assembling dissimilar materials. Adhesives enable thinner substrates from downgaged high-strength steels (HSS), manage the interfaces between materials with differing CTEs, and provide a galvanic corrosion barrier. Recognizing the practical challenges of high-volume automotive manufacturing, Dow Automotive developed fast-acting two-component polyurethane adhesives for bonding aluminum to steel in the trim shop. The company also developed adhesives that bond aluminum to steel that can withstand the 180°C temperatures in the body shop, managing residual stresses that build up as materials heat and then cool afterwards.

Ensuring compatibility with secondary attachments is also critical. “No OEM is ready to go solely to adhesives, so they combine adhesives with rivets and spot welding. We need to ensure adhesives work well with those,” she said. Combining adhesives with welds (weld bonding) means using fewer welds. It also means improving durability by spreading the load over a larger surface area through the adhesive. Fewer mechanical attachments, either welds or self-piercing rivets, also improves cycle times, another kind of cost entirely.

“There remain applications, such as an aluminum roof panel bonded to a steel body—when done after paint—where two component polyurethane structural adhesives do an excellent job and you do not need secondary attachments,” she said.

She said to look for future adhesives developed specifically for HSS and advanced HSS steels, with their unique property balances compared to more common mild steels. “We are also working on more adhesives that can be applied in the body shop, before the high temperature e-coat bake operation, that will enable adhering aluminum to steel,” she said. “That will take advantage of existing infrastructure and many of our customers have said that would represent cost savings for them.”

Old Becomes New

As aluminum becomes more prevalent, Mike Spodar, welding engineer for the Coldwater Machine Co. (Coldwater, OH), is seeing opportunity with improved methods of joining metals. In particular, Coldwater Machine is introducing a new refill friction-stir spot welding process for nonferrous materials. “Right now, we are joining 5000 and 6000 series aluminum alloys, along with 2000 and 7000 series aluminum alloys,” he said, with magnesium next on the list for development. “We are also interested in future polymer-to-aluminum welding and carbon fiber-to-aluminum welding development,” he added. He notes that 6000 series aluminum tends to be most prevalent in automotive applications, which is ideal for their new process.

The new process uses a rotating sleeve around a rotating pin to create friction, which heats the material. Once the process heats the weld joint to a plastic state, the sleeve is pushed into the material, while the pin is pulled back to create a space for the plasticized material displaced from both plates. “This is an alternative to riveting, resistance spot welding, or adhesive bonding,” he explained. One advantage is that there is no melting of the material, which eliminates issues inherent to fusion welding. The resulting joint is a solid-state bond, with a weld nugget made up from the mixed plasticized materials, rather than melted materials. “It uses less electricity than spot welding, requires no filler wire, no welding gases. It’s robust because no special cleaning of the plates is required, and there is enough stroke on the tool to weld up to 8-mm stack-ups,” he said.

The makers of ferrous steels are also developing advanced new materials. The goal of these steels is to make them stronger and so use less of them—lightweighting through downgaging. A new development includes a third-generation advanced AHSS from NanoSteel (Providence, RI). Advertising high strength combined with excellent formability characteristics, their development goal is to create a new material that “feels, smells, and tastes like regular steel in the manufacturing environment” according to Tad Machrowicz, vice president–Automotive Engineering. “As a third-generation AHSS, NanoSteel sheet provides a significant performance advantage over mild steels but without the complexity and costs associated with second-generation AHSS.”

“The advantage of NanoSteel’s steel from a weldability point of view is that there is not a traditional martensitic formation phase,” explained Machrowicz. “With normal steels and other HSS, cooling rates around welds can create martensite with low ductility. This can cause stress concentrations and discontinuities around welds. Industrial specialists have a way of working around this, but it typically involves increasing weld cycle time and cost,” he explained. “NanoSteel’s goal is to provide steel that welds similar to mild steel with existing manufacturing capital.”

A Peek at the Future—Mixed Materials in High-End Cars

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Audi Space Frame for the TT Coupe uses high-strength and ultra-high-strength steel components for the front end; the structure and the add-on parts are made from aluminum.

High-end luxury cars are often harbingers of technical advances that eventually find their way into everyday use. So it is with mixed material body shells. The 2014 Mercedes-Benz C-Class boasts an aluminum hybrid body shell structure that is 40 kg (88 lbs) lighter than previous models. The design incorporates 24% aluminum by weight, 12% hot-formed steel parts by strength, and 4% ultra-high-strength steels by weight.

Audi uses a hybrid spaceframe with aluminum sheet components in the upper part of the shell and heavier, ultra-high-strength steel components on the bottom. This both saves weight and lowers the center of gravity for a better driving experience.

Richman from Kaiser Aluminum offers observations on aluminum that perhaps pertains to any nonferrous material that costs more than mild steel. “Weight reduction is not free,” he observes. “Perhaps its cost is not dramatically more, but it does increase.” For those car segments where price is critical, do not expect a lot of aluminum (or carbon fiber or magnesium or HSS). “The forecast now is by 2025, 16–18% of the vehicle bodies in North America will be all aluminum—typically larger vehicles,” he said, such as pickup trucks or E-size sedans, luxury cars … and sports cars. ME

This article was first published in the September 2014 edition of Manufacturing Engineering magazine.

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