An approach to modeling the cost-strength-weight tradeoff in aluminum and magnesium extrusions for automotive applications

Abstract

In light of volatile fuel prices and tightening emissions regulations, automobile manufacturers have been increasingly considering the use of light-weight magnesium in their efforts to improve fuel economy. While mainly used in minor components now, greater weight savings lie in its replacement of heavier structural components now made of extruded aluminum and stamped steel. However, as a material with generally lower mechanical properties on a volumetric basis and higher unit materials cost, magnesium introduces a strength-weight tradeoff with non-obvious total cost implications. Accordingly, manufacturers could greatly benefit from a method of systematically studying this weight-strength relationship in cost terms for extruded magnesium beams in a variety of loading scenarios. In this paper, we describe the development of an interface within a Process Based Cost Model of the extrusion process for quantifying these relationships on user defined parts. This interface consists of Visual Basic functions which dynamically compute dimensions of hollow Mg or Al extruded tubes necessary to achieve some strength constraint, input them into the cost model, and return the results.

(cont.) This capability was demonstrated on a representative system - a 1 m long, 70 or 75 mm wide, 6 or 8 mm thick Mg or Al tube - for three distinct loading conditions - axial loading as quantified by Euler buckling load, deflection from center load, and deflection from end load. Results show that in non-package constrained scenarios, cost and weight savings can be achieved by switching from Al to a larger diameter Mg extrusion of equivalent strength; however, when diameter is constrained, it is neither cost nor weight-effective unless some geometric, processing, or strength constraint is somewhat relaxed. In general, switching to Mg is favorable when specific strength rather than absolute strength is more important. While intrinsic characteristics of the model limit practical usefulness in some cases, it is nevertheless very helpful in studying relative differences between the strength, weight, and cost of extruded Mg and Al beams.