This is a look at the structure of the 2016 Cadillac CT6. The skin of the car is all aluminum, but as is evident here, the structure is a combination of steel and aluminum. According to Travis Hester, Cadillac CT6 executive chief engineer, âThe structure of the CT6 is one of the most advanced body systems weâve ever produced. The innovation surrounding our joining techniques have enabled us to create a vehicle structure with the highest torsional rigidity of any Cadillac while achieving one of the most mass-efficient vehicles in the segment.â Note that steel is used for the B-pillar, which is thin enough to provide ingress, egress and visibility, while being strong enough to provide cabin safety. The material is also used in other areas around the cabin to handle crash energy and to resist intrusion. Although 64% of the CT6âs body structure is aluminum, it should be noted that it is the most premium of the Cadillac sedan lineup. Said Cadillac President Johan de Nysschen of the approach to materials use, âWith the CT6, we used high-strength aluminum and high-strength steels; lightweight chassis components; we integrate aluminum and steel where it makes sense; we eliminate every gram of mass possible, while achieving world-class performance.â
This is what is sometimes referred to as a âbanana chart,â given the general shape of arc that starts on the left side at âConventional Steelsâ and goes down toward the right. As Jody Hall of the Steel Market Development Institute explains, the materials along the bottom in orange are high in strength, but difficult to form. These are the first-generation advanced high-strength steels. Consequently, work is being done to improve the formability while not giving up on strength, which is the grey bubble, â3rd Gen AHSS.â The second generation? Theyâre the austenitic stainless (annealed) and TWIP.
Jody N. Hall has a PhD from the department of Materials Science and Engineering at the University of Michigan. She had a GM Fellowship while pursuing her PhD. Her Mastersâ is from the same department. Her BSE comes from the Metallurgical Engineering Dept. at Michigan.
Whatâs interesting to note is the subject of Hallâs PhD thesis: âFatigue Behavior of SiC Particulate Reinforced Aluminum.â Interesting because of where Hall, who had been with General Motors for some 30 years, having joined the company in 1984, now works.
She is the vice president, Automotive Market, Steel Market Development Institute (SMDI), a business unit of the American Iron and Steel Institute (AISI).
Thatâs right, Hall is heading up an organization that is committed to advancing the interests of steel manufacturers in the automotive space.
She knows her stuff about nonferrous materials as well as the ferrous that she promotes.
âI started out my career at General Motors doing powertrain advanced manufacturing technology development. At that time we were looking at silicon-carbide reinforced aluminum for our connecting rods and crankshafts,â Hall says. âThe first 12 years of my career was powertrain development. The last 18 were on body materials,â she says.
In the mid-90s, when she transitioned from powertrain to body, she worked on a program for developing a tailgate that had an aluminum outer and a polymer composite inner. âItâs easy to stamp an aluminum outer but it is difficult to stamp the inner because it is a deeper draw and tighter radii and things.â
âLater I went on to manage a group that did advanced stamping materials and we focused on both aluminum and steel.â
All of which is to say that Hall has moreÂ than passing familiarity with aluminum, the material that is providing the greatest competition for the material of which her current organization is a proponent.
One of the things that Hall learned early on in the auto industry was that costs are a huge consideration. âYou have to be very careful in your selection of materials. You have to look at the cost of materials. Cost of manufacturing, including any infrastructure changes you have to make.â
As in: âWhen we introduced aluminum into our stamping plants at General Motors, it required multi-million scrap-sorting systems for each plant.â
Thatâs just for segregating the materials. Hall also points out that there can be significant changes in the body shop necessitated by making a move from steel to aluminum, as in using mechanical fasteners in place of spot welds, which can mean a whole new automation setup.
But there are some details that canât be overlooked. Details that may measure just tenths of a millimeter.
Hall explains that when stamping aluminum, trimming can present problems, particularly if there is die wear, and she points out that wear is a common problem with aluminum stamping due to the abrasiveness of the material. âAluminum tends to create slivers if you donât trim it properly, so the clearance between the working tools is critical.â
The issue with these small slivers, she says, is that because the sheets tend to be covered with lubricant, they get carried along to the next station.Â âIf it is an exposed outer panel, youâll get little imprints of the slivers.â Which is not good for a Class A surface.
Steel? âIt doesnât sliver.â If the clearances arenât right, there can be a jagged trim. While this generally isnât a problem, she acknowledges, âIf you get a ragged trim and stretch it later on in a flanging operation, you could get a split.â
Mild steels are one thing. But they are giving way to higher-strength steels. These steels, like aluminum, have less formability. So might a company not decide that if they have to start dealing with different material requirements when it comes to handling and processing them, they might as well make a switch from the traditional to something different, from steel to aluminum?
Hall answers that overall, the required changes to handle advanced high-strength steels (AHSS) from mild steels are âmore incrementalâ than going from mild steel to aluminum. âYou can use the existing structure, but maybe make some modifications.â For example, there might be increased tooling costs for handling AHSS, but the front-of-the-line systemsâlike the magnetic loading devices for the stamping pressesâdonât have to be replaced.
If cost is the key consideration, then thatâs something that needs to be considered.
But there are some instances where the AHSS materials require a significant change in the way parts for body-in-white applications are performed. For example, there are the first-generation AHSS materials, martensitic grades, that have to be hot stamped or roll formed in order for them to achieve their properties. The materials may have the high strength that OEMs are looking for (>800 MPa) for applications like B-pillars and roof rails (the material is very good at repelling intrusion into the passenger compartment), but the processing is costly. âAutomakers,â HallÂ says, âwould like to replace that technology with something they can stamp inÂ their current plants at room temperature.â
Which leads to the development ofÂ third-generation advanced high strength steels (3GAHSS).
(âWait a minute,â you might be thinking.Â âThis has gone from first-generation to third-generation. What about the second?â According to Hall, there are second-generation materials that offer âreally good elongation and pretty good strength.â These materials are austenitic stainless and TWIP (twinning-induced plasticity) steels. Because of their stain-less chemistries, which means alloying elements like chromium and nickel, they tend to be more expensive. Whatâs more, they weld differently than other steels. So, Hall says, these materials donât have a great deal of application in auto.)
And 3GAHSS leads to another acronym,Â ICME. That stands for âIntegrated Computational Materials Engineering.â Which is essentially a method by whichÂ there is the use of computer-aidedÂ design of multiphase steel microstructures with the goal of developing materials that have high strength and good ductility.*
A four-year ICME project with funding from the U.S. Department of Energy ($6-milion with 30% in-kind matching) and the participation of FCA, Ford and General Motors, SMDI, universities, engineering companies, and a national lab was initiated in 2013. The objective is to develop two steels that are highly ductile but strong:
â¢Â Â Â 12,000 MPa tensile strength with 30% elongation
â¢Â Â Â 15,000 MPa tensile strength with 25% elongation
So far, two years in, theyâve developed limited quantities of the first material, a duplex TRIP (transformation-induced plasticity) steel. They are getting closerÂ to the second, Hall says, but the elongation metric hasnât been met.
These steels, she explains, would allow vehicles to be built that are both light and strong (light because the materials are strong, thereby resulting in the opportunity to use thinner gauges), and without huge changes to the stamping or body shops.
The elephant in the office is the 2015 Ford F-150. The Ford Dearborn Truck Plant is approximately 15 miles to the southeast of Hallâs office. If anyone talks about the F-150, the words aluminum truck are certainly part of the conversation. âBut with a steel frame,â Hall interjects.
Clearly, the auto industry is undergoing a transition from what has, with few exceptions, been a steel-centric process and product to one that is more of a multi-material mix, with the primary alternative material being aluminum. (Hall cites an article from 1953 in an automotive magazine that claimed that by 1960, steel would be replaced in cars by aluminum, magnesium and plastics. Itâs not that some things never change; itâs that some things donât change a whole lot.)
But what is happening is that there isÂ the replacement of steel for things likeÂ closure panels, while the bodies-in-whiteâor, in the case of pickups, framesâtend to remain primarily steel because of its capability to absorb crash energy or to provide occupant protection.
So it seems that whatâs happening is that the âskinâ of some vehicles is being replaced by aluminum. Whatâs seemingly lost in the discussion of things like the âaluminum F-150â is that closure and hang-on panels account for about 8% of the mass of a vehicle, while the body-in-white (or frame) accounts for as much as 28%, according to a report from World Steel Dynamics (October 2014).
So it is almost as though when it comes to materials used in cars, rhetoric is trumping reality.
One more comment from Hall in this regard: âAdvanced high-strength steel is the fastest growing material on a vehicle.â