Die coatings influence evaluation and friction model selection in aluminium extrusion by means of visioplasticity technique

Visioplasticity represent a powerful technique for investigating material flow during bulk metal forming processing and particularly in extrusion where complex metal flows occur. In particular, gridded billets or rod insertion techniques are used for characterize metal flow in direct and indirect hot aluminium extrusion. In the paper reduced scale experiments were innovatively proposed in order to analyze the process tribology by means of visioplasticity of coated tools (TiN layer by means of Physical Vapor Deposition (PVD) Technique, TiC-TiN-Al2O3 layer through Chemical Vapor Deposition (CVD) Technique) comparing the results to classical uncoated tools. Reduced scale experiments provides friction conditions comparable to real scale extrusion as revealed by the comparison with a real scale extrusion of a round profile. The deformed grids were measured in order to estimate the friction effect throughout the billet length in coated and uncoated tools conditions. Numerical investigations for the qualitative and quantitative determination of friction models were carried out by means of the Deform code with different friction formulations and coefficients. It was found that uncoated tools realize a fully sticking behaviour, while die coatings allowed reducing process friction; CVD coating better fits extrusion requirements because PVD coating lost its effect after a limited number of billets.