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Djelali Attaf
Abstract
Mechanisms of wear in metal tools for stamping
Djelali Attaf - 30 january 2003
In all the operations of metal forming, friction plays an important part on the result. The tribological problems encountered in the sheet deep drawing are particularly complex. The complexity is due to surface interactions between the sheet and the tool with in addition, in the contact, the influence of wear debris or lubricant and furthermore to the synergy that exists between the sheet bulk deformation and the problems of interface (contact, friction...). From these interactions, friction, sheet surface quality evolution and tool wear are resulted.
The thesis objectives are to study degradations of the deep drawing die radius. These degradations have a direct impact on the sheet results. The study on a press of this problem is very complicated also we developed our own test facility. This pilot, called deep drawing wear pilot, allows friction between a sheet and a die radius. Moreover, it associates the sheet plastic deformation induced by bending and unbending on the die radius with the effects of retained forces caused in particular by the blank holder. In addition, it can dissociate the effects on the die radius because, it allows to study elementary conditions found in deep drawing. Interpretation and the relationships between degradation causes and effects are then easier. The experimental study based on a DC04 sheet and a X160CrMoV12 steel die radius, without surface coating, revealed that the surface die radius are degraded by abrasion and transfer mechanisms. The distribution of these degradations on the die radius is heterogeneous and localised in two sites, depending on the exit angle between the sheet and die radius. The die radius friction coefficient modelling analyses these particular positions according to the cycle number. We thus established a correlation between the friction coefficient evolution and the kinetics of the two mechanisms of degradation.
The study of the mechanical answers of the die radius, carried out by numerical simulation, attests heterogeneity of the contact pressure distribution with the presence of two zones of strong pressure. These results, in agreement with measurements by photoelasticity device, confirm the localisation of the die radius degradations observed on the experimental results. To explain the mechanism of degradation by transfer of the sheet on the die radiu, a model was developed. With this model, we can found an energy criterion which expresses the localisation of the transfer degradation on the die radius versus the roughness and the mechanical properties of the sheet and of the die. Its application, in the present, case ables us to understand the zones of experimental degradation.
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