Page 42 - MetalForming-May-2018-issue

The Science of Forming By Daniel J. Schaeffler, Ph.D.
The Aluminum-Steel Battle
in Automotive Applications
The aluminum and steel indus- tries continuously battle for supremacy in becoming the metal of choice in automotive body construction. This makes sense—with about 2 tons of sheetmetal purchased for each of the 17-million vehicles sold annually in the United States, there isn’t another industry that offers such high-volume sales opportunities. Each metal grade provides critical charac- teristics that help optimize the more than 500 stampings found in a typical vehicle.
A material’s strength is a function of its chemical composition, its ther- mo-mechanical processing route (e.g., converting from a thick slab to a thin sheet), and subsequent heattreatments. These variables make it impossible to state that one material always is stronger than another. For example, many aluminum grades are stronger than steel grades, but rarely are they intended for the same application. Fur- thermore, strength is not the only con- sideration. Weight, ability to be formed into the desired shape, ability to be
Danny Schaeffler, with
30 years of materials and applications experi- ence, is co-founder of 4M Partners, LLC and founder and president of Engineering Quality Solu- tions (EQS). EQS provides product-applications assistance to materials and manufacturing com-
panies; 4M teaches fundamentals and practical details of material properties, forming technolo- gies, processes and troubleshooting needed to form high-quality components. Schaeffler, who also spent 10 years at LTV Steel Co., received his Bach- elor of Science degree in Materials Science and Engineering from the Johns Hopkins University in Baltimore, MD, and Master of Science and Doctor of Philosophy degrees in Materials Engineering from Drexel University in Philadelphia, PA.
Danny Schaeffler
248/66-STEEL • www.EQSgroup.com
E-mail ds@eqsgroup.com or Danny@learning4m.com
joined to the neighboring parts, stiff- ness and cost represent some of the many other facets to be considered.
Stiffness of a component is a func- tion of material type and geometry. On the material side, stiffness depends on a property known as the modulus of elasticity, Young’s modulus. Unlike strength, which can vary from grade to grade (or even coil to coil), Young’s modulus essentially is constant for a given alloy composition, independent of heattreatment, processing or cold work. Young’s modulus for steel is about three times that of aluminum. For a fixed geometry, a part made of sheet steel will be three times as stiff as if the part were made from aluminum. Put another way, an aluminum part under load will deflect three times as much as a similarly loaded steel part.
The thickness and shape of a formed part also contributes to its stiffness. Stiffness is proportional to the cube of the thickness. To neutralize the effect of aluminum being one-third the stiff- ness of steel, an aluminum part must be made 44-percent thicker than a steel part of the same shape. Even this increased thickness offers the potential for weight savings, as aluminum has
one-third the density of steel.
For skin panels such as hoods, doors and liftgates, automakers prefer to use sheetmetal with strength as high as possible so that the panel will offer sufficient dent resistance. Thickness affects dent resistance as well, limiting the degree to which panel thickness can be reduced in the name of weight savings. Stiffness constraints further limit how thin skin panels can be. If too thin, the panels may start to wave and flutter as the vehicle travels down the road—unless character lines are
added for “styling benefits.” Automakers also must specify sheet-
metal that can be produced to Class A surface standards. Current technology limits this to using sheet steel or alu- minum with strength below about 300 Mpa, although grades under develop- ment of both materials exceed this amount.
Joining does not present a significant hurdle, since several cost-effective tech- niques exist to mate the outer panel to the inner panel. With a similar upper strength available in either material, and several grades of either material that can be produced sufficiently thin, the one offering the best combination
40 MetalForming/May 2018
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