The effect of forward slip on the surface finish of cold rolled aluminum

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Virginia Polytechnic Institute and State University


Forward slip (FS), the percent difference between the roll velocity and the strip exit velocity, was achieved by altering four variables: reduction, rolling speed, lubricant viscosity, and roll roughness. A two-high laboratory mill was used to roll strips of unalloyed, H-18 temper aluminum. An SEM and a Leitze Orthoplane microscope were used to examine the strip surface.

A grainy pattern, which varied with the rolling condition, was observed on the sample surface. The grainy pattern resulted from three effects: grooves imprinted by the rolls, black spots identified as hydropitting (HP), and horseshoe shaped marks which were attributed to FS. These effects were studied and the following conclusions drawn. Higher viscosity, high speed, and rough rolls produced more HP. Increasing reduction produced HP with the viscous lubricant, oil, but less HP with kerosene. No HP was observed at 59 percent reduction but FS marks occurred and were more prevalent with oil than kerosene. The smooth rolls produced more FS marks than the rough rolls. Two postulates were presented to explain the pattern of these effects.

First, the FS mark lengths were of the same order of magnitude as one set of theoretically calculated relative slip lengths; indicating that FS was responsible for the marks. Second, the absence of HP and presence of FS marks, in particular cases, indicated that a hydraulic effect was responsible for the FS marks. The FS marks would have been created as localized high pressure lubricant flowed across the surface to equalize the film pressure.