Inverse material modeling and optimization of free-cutting steel with graphite inclusions

Abstract

Graphite inclusions in steel are easing chip breakage and act as in situ lubricants. Therefore, graphitized steels are claimed to perform excellent in machining processes. They are considered as environmentally friendly alternatives to widely used Pb-alloyed steels. However, the effectiveness of graphite inclusions in different machining processes is not well understood. With the support of computer simulation techniques, a deeper understanding about chip formation and the role of internal friction (or lubrication) can be obtained. To enter into numerical analysis of material flow and chip formation, the quality of the applied constitutive law is crucial for close-to-reality simulation. In the present work, a most accurate determination of the Johnson-Cook constitutive law of partially graphitized steel 50SiB8 (hardness 214 HB) is aimed for. Johnson-Cook parameters are identified from dynamical compression tests, performed at strain rates up to 20s−1 and temperatures up to 800 °C. The material parameters are derived from two different numerical methods. The first standard method is based on minimizing the sum of squares of the differences between calculation and measurement. The parameters identified from this method are used as initial values for the second method, an inverse material modeling. The second method is based on a multi-case polynomial metamodel optimization in combination with finite element techniques. By applying inverse material modeling, the maximum discrepancy between measurement and calculation is reduced from 12% (regression with standard method) to 1%. A reliable parameter set for the Johnson-Cook model for graphitic steel 50SiB5 as input for future modeling of the chip formation is now available.