Experimental study and theoretical interpretation of the frictional mechanisms in steel sheet forming
Résumé
The success of many sheet forming operations depends critically on the friction of the sheet on the blankholder. First we review briefly the various frictional mechanisms involved and discuss mainly the occurrence of hydrodynamic effects at macroscopic and microscopic scales, especially with regard to the experimental work performed by Emmens (Proc. 15th Bienn. Congr. IDDRG, Dearborn, MI, May 1988, ASM International, 1988, pp. 63–70). Then we describe the results of friction measurements performed with a tribometer on four low carbon steel sheets: two high formability sheets with electro-discharge texture and laser-textured roughness, a high strength sheet and a hot dip-galvanized sheet with laser-textured roughness; friction tests were performed between two flat tools at two contact pressures p (about 10–30 MPa), two sliding speeds V (about 2–60 mm s−1) and along the rolling direction and the transverse direction. The friction of the galvanized sheet is rather isotropic and decreases markedly as the ratio V/p increases; in contrast, the friction of the uncoated sheets is anisotropic and its decrease as V/p increases is smaller. Observations by optical microscopy of the surface of the sheets after friction testings provide three major results: a small decrease in the friction coefficient μ corresponds generally to a more marked decrease in the final plateaux area. At very low value of the Vp ratio where hydrodynamic effects cannot occur friction is the sum of two mechanisms: at a very small scale the formation of waves, with moderate friction μ ≈ 0.12, and at the greater scale of the plateaux of the sheet surface shearing of transfer film, with higher friction μ ≈0.22; the extent of each mechanism depends on the sheet roughness texture. As the ratio V/p increases the viscosity of the lubricant promotes the formation of a very thin lubricant film (thickness 10–70 nm) on the plateaux which slightly reduces the friction coefficient and more markedly the extent of the two previous mechanisms and the associated surface damage of the sheet and tool surface. This effect is effective before the generation of pressure by viscous shearing of oil pockets proposed by Emmens and responsible of a very marked decrease in friction at high values of the V/p ratio.