Marcel Tiffe

Fitting of Constitutive Material Parameters for FE-Based Machining Simulations for Functionally Graded Steel Components

The composition of different materials and their specific properties like tensile strength and toughness is one way to achieve workpiece characteristics which are tailored to the later application. Another approach is the subsequent local heat treatment of workpieces made of homogeneous materials. However, both ways are costly and go along with several subsequent process steps. Therefore, mono-material workpieces which were manufactured by thermo-mechanical forming processes may provide such tailored properties in the form of functional gradations. Furthermore, the process chain is shortened by the combination of forming and heat treatment, but nevertheless machining processes are still needed for proper workpiece finish. This puts the challenge of varying process conditions due to hardness alterations within a single process step, e.g. turning. In addition to experimental investigations simulative analysis techniques are desired to evaluate mechanical as well as thermal loads on tool and workpiece. In the case of FE-based microscopic chip formation simulations proper material behaviour needs to be determined with respect to material hardness. This paper describes the approach of fitting Johnson-Cook material parameters as a function of workpiece material hardness. In order to achieve realistic stress states within the process zone, this approach considers the yield strength as a linear function of the hardness. It is shown how the hardness influences the cutting conditions and how the Johnson-Cook parameters are identified. Then these parameters are validated in three-dimensional simulations of exterior dry turning by comparison of simulated process forces and chip formation to experimentally achieved ones.

Turning Residual Stresses in Functionally Graded Steel Components

Introduction Metallic components with graded materials properties are of particular importance for the realization of lightweight design. The scientific basis of such concepts was provided by the Collaborative Research Centre TRR 30, funded by the German Research Foundation DFG between 2006 and 2015. As part of the project, thermo-mechanically graded components were manufactured by hot metal forming processes [1, 2]. Their characteristic properties are to a large extent determined by final machining operations. In this context, residual stresses play an important role for strength and lifetime of the produced parts. A survey about near surface residual stress states after hard turning of differently heat treated quenched and tempered steel AISI 6150 (51CrV4) is given in [3]. In the present paper, results of similar investigations carried out of Jominy end quench samples are reported and compared with results of samples with homogeneous microstructures.