La tribologie de l'emboutissage

The tribology of sheet metal forming

Responsable adjoint du Groupe Surfaces et Tribologie Ecole des Mines de Paris - CEMEF - CNRS URA 1374, Sophia-Antipolis

Abstract

We review the various aspects of the friction between a thin sheet and the fanning tools ; we consider most often examples related to the forming of autobody parts manufactured from steel sheets with or without coating. This paper presents the following aspects:

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  the bulk and surface properties of the sheet before and after forming, their control by the skinpass and by addition of coatings,

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  the various modes of plastic deformation of the sheet, the related contact conditions and the effects of friction on a pure streching process, a deep drawing operation and the forming of a part of complex shape with drawbeads,

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  the main tribological problems of the twenty last years: antigalling roughness profile, restraining characteristics of drawbeads, lubricants and tool surface treatments for zinc coated sheets, friction data for numerical simulation of sheet metal forming, occurrence of dry coatings and aluminium alloys sheets,

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  the friction tests : Inland test, tribometry with flat and curved tools, drawbead test, Erichsen and Swift tests,

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  the methods for characterizing the surface properties of the sheet : roughness measurements, microscopy and image analysis, microhardness testing, tape test and differential weighting,

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  an example of a numerical simulation including a lubrication model : the Erichsen test performed with a newtonian lubricant,

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  the microscopic mechanisms of friction below the blankholder : microplastic interactions (ploughing with a mobile wave, buid-up-edge, cutting, plateaus ironing and galling), transfer films, microhydrodynamic effects,

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  the restraining characteristics of drawbeads.

Endly we discuss the problem involved in the development of numerical simulation : the formulation of a reliable friction law for sheet metal forming.