EVOLUTION OF THE FRICTION FACTOR OF A MOLYBDENUM WORKPIECE DURING UPSETTING TESTS AT DIFFERENT TEMPERATURES

A constitutive law has been determined by means of compression tests a nd then used in an F.E. code in order to estimate the coefficient of t he Tresca or Coulomb law that fits with the experimental data. For the se experiments, cylinders of molybdenum of 12 mm diameter of 20 mm hei ght were used with circles machined in the top and the bottom faces to enable the sliding to be followed precisely. The experiments were con ducted at temperatures ranging from 600 to 1000 degrees C. The numeric al simulations have been performed with the code POLLUX, developed in collaboration with INSA Lyon and especially with B. Michel. It is a tw o-dimensional code, handling both the mechanical and the thermal prope rties of the material. It uses a thermo-elasto-viscoplastic formulatio n which can be used for numerous applications. Comparison between expe rimental and numerical data is made for: (i) the forging load; (ii) th e sliding of the top and the bottom faces, which provides the value of the best coefficient for the friction laws, and (iii) explaining how the metallurgical observations made on the upset molybdenum parts can be linked with the strains predicted by the code. How this friction co efficient evolves versus temperature is explored. (C) 1998 Elsevier Sc ience S.A. All rights reserved.