In situ observation of a rolling interface and modeling of the surface texturing of rolled sheets

Abstract

To observe the tool–workpiece interface in sheet rolling, a visual rolling system is constructed on a laboratory scale. In situ observation is by a small CCD camera with a macro-lens embedded into the upper roll so that the optical axis coincides with the roll axis. A strip specimen of half-hard pure aluminium 2.0 mm thick was rolled at 167 mm s −1 at a thickness reduction of 5–13%. Paraffinic base oils with differing viscosities were used as lubricants. The original ridges and valleys of the sheet specimen were set to be transverse to the experimental rolling direction. From observation of the rolling interface with image processing together with scanning laser microscopy after rolling, a model of surface texture formation on the sheet is proposed as follows. At a low thickness reduction of 5% the initial ridge-and-valley pattern of the sheet survives, but the lubricant introduced into the rolling interface is hydrodynamically compressed and is emitted across the outlet. At an increased reduction of 10%, after contact of the top of the ridges of the sheet surface and the tool, and pressurization of lubricant, the saddle points of the ridges provide new channels for the pressurized lubricant. As a result, the original valleys and the new “valleys” parallel to rolling direction are connected. This causes local and fragmented emission of lubricant at the outlet of the roll. Under tighter contact at 13% reduction, the lubricant present in the initial valleys play a moderating role in the contact, enabling partial free surface deformation and consequent roughening. Parallel ridges and valleys fragment but asperities are finally flattened to leave minor isolated micropools.