COLD FORMING PROCESSES - SOME EXAMPLES OF PREDICTIONS AND DESIGN OPTIMIZATION USING NUMERICAL SIMULATIONS

Predicting the behaviour of steel during a deformation process, and th en under service conditions, is one of the main challenges in cold for ming. The design of optimized forging schedules, by means of classical trial+errors procedures, has become increasingly heavy in terms of ti me and cost in a competitive environment. Simultaneously, the improvem ent of steel qualities requires the microstructure, constitutive behav iour and deformability to be known a priori regarding a targeted appli cation. During the last few years, numerical simulations have become a very efficient tool to reach these goals. In this paper, we give exam ples of innovating forging sequences developed by numerical simulation s, including the investigation of damage in tools and forged parts. In case of specific processes with very determined geometry - such as wi re drawing - we show how systematic numerical studies may lead to pred ictive models of force, local strains and residual stress... However, reliable predictions from numerical simulations require reliable input data, including constitutive laws, friction conditions and propensity to ductile damage. These data must be characterized under realistic s ollicitations. Typical cold forging loadings are indeed very severe : local strains up to 600%, strain rates locally greater than 1000 s(-1) , and subsequently, plastic heating over 500 degrees C. To characteriz e the constitutive behaviour, the standard upset test between grooved dies is used along with an original methodology to derive the strain h ardening curve from the experimental force-displacement recording. Too l elastic deformations, specimen strain heterogeneity... are taken int o account. This enables a precise determination of the strain hardenin g curve up to about 100% of strain. The extrapolation of the now stres s to greater deformations is then very easy and reliable. Such a test can be performed under quasi-static and isothermal conditions (0.1 s(- 1)) but also adiabatic and rapid conditions (up to 10 s(-1)). This pro cedure was adapted to a Pellini hammer, which enables very simple char acterization at 800 s(-1). The comparison of all these flow curves lea d to the formulation of an original constitutive model, which accounts for the effects of plastic heating, strain rate, dynamic aging... In order to predict ductile fracture during the forging process, the most classical criteria were tested over a wide range of experimental load ing conditions. None of them were general enough to solve all the cold forming problems. On the other hand, mesoscopic models describing the deformation of the metal matrix around inclusions or second phases ha ve proved to be in good agreement with the various experimental observ ations. An original plasticity criterium, based on the recent works in porous plasticity theory, has been developed and already displays pro mising capabilities. Simple experimental procedures enable a reliable classification of steel qualities, heat treatments... in term of forge ability. Finally, the friction problem is treated using different meth odologies based on the forming process considered. For forging operati ons, a fine analysis of the force-displacement curves in direct extrus ion stages may lead to a precise measurement of the friction coefficie nt under pressures from about 200 MPa up to 1000 MPa and for sliding r ates between 1 and 100 mm/s. For wire drawing process, a model relying on an analytical approach using a significantly improved slice method has been developed : the comparison of the experimental drawing force and the predicted one gives the friction coefficient in industrial pr ocessing conditions (speed up to 5 m/s).