Engineers looking for metals to put in fatigue applications often have some tough choices on their hands. Forged steel, aluminum, and various ductile irons can all do the trick, but each strikes a less-than-perfect balance among mechanical properties, weight, and manufacturing costs. Intermet Inc. (www.intermet.com), a casting manufacturer with research facilities in Lynchburg, VA, has now come up with a ductile iron that offers an alternative to these traditional materials for automotive suspension components and other fatigue- sensitive parts.
Called MADI (machineable austempered ductile iron), this new ductile iron isn't necessarily the strongest metal on the block (see chart). With yield strength ranging from 419 to 524 MPa and an ultimate tensile strength between 623 and 818 MPa, MADI has tensile properties lower than those of forged steel or austempered ductile iron (ADI). At the same time, though, MADI exceeds the tensile properties of as-cast ductile iron of the same hardness. Its strength also tops that of the cast aluminums commonly used in automotive applications. "Our goal with MADI was to put something right in the middle," explains Alan Druschitz, Intermet's director of materials R&D.
And MADI sits in the middle in other important ways too. In terms of hardness, MADI occupies a space between as-cast ductile irons and harder choices like ADI and forged steel. As for weight, Druschitz estimates that MADI offers about a 3% weight penalty versus aluminum and about a 10% weight savings versus forged steel. Both estimates, he notes, reflect both material density and the design capabilities given the different yield strengths.
Intermet metallurgists achieved MADI's balance of properties, as metallurgists often do, by optimizing the material's microstructure. Regular ADI consists of particles of ferrite in an austenite matrix. MADI is the exact opposite. It consists of austenite particles in a ferrite matrix—a microstructure that Intermet achieved after three years of painstaking optimization of ductile iron chemistry and heat treatments.
Druschitz credits this "novel microstructure" for two attributes—fatigue resistance and easy machineability. He says MADI has an edge in fatigue performance because its microstructure resists the formation of defects that can give cracks a foothold in cast parts. And as its name suggests, MADI's microstructure was created with easy machining in mind.
|Material Property Comparison*
ACF Steel 38MnVS6
|Tensile Strength, MPa
|Yield Strength, MPa
|Impact**, joules (RT)
|*Properties determined from samples cut from components