Modelling of the dynamic behaviour of hard-to-machine alloys

Abstract

Machining of titanium alloys and nickel based superalloys can be difficult due to their excellent mechanical properties combining high strength, ductility, and excellent overall high temperature performance. Machining of these alloys can, however, be improved by simulating the processes and by optimizing the machining parameters. The simulations, however, need accurate material models that predict the material behaviour in the range of strains and strain rates that occur in the machining processes. In this work, the behaviour of titanium 15-3-3-3 alloy and nickel based superalloy 625 were characterized in compression, and Johnson-Cook material model parameters were obtained from the results. For the titanium alloy, the adiabatic Johnson-Cook model predicts softening of the material adequately, but the high strain hardening rate of Alloy 625 in the model prevents the localization of strain and no shear bands were formed when using this model. For Alloy 625, the Johnson-Cook model was therefore modified to decrease the strain hardening rate at large strains. The models were used in the simulations of orthogonal cutting of the material. For both materials, the models are able to predict the serrated chip formation, frequently observed in the machining of these alloys. The machining forces also match relatively well, but some differences can be seen in the details of the experimentally obtained and simulated chip shapes.