An investigation of new tribological variables in friction modeling of bulk metal forming

The present 'realistic' friction model is examined by a series of upsetting tests. Specimens made of different aluminum alloys and prepared by various processes display diverse results. Severe surface roughening takes place in some cases using A6061T4 bar manufactured by hot extrusion. It results in a lubrication condition which is very different from the theoretical prediction. From hardness tests and microstructure observations, it is found that there is no obvious correlation between the hardness and the roughening process for A6061T4 alloy. The severe roughening is attributed to the distortion of the coarse grains recrystallized during the preprocess, that is, the hot extrusion. This is rare in sheet metal forming where most workpieces are prepared from cold rolled sheet metals. The orange peel appearance not only changes the lubrication mechanics, but also makes the surface finish of the product unacceptable. Since the existing friction model fails to predict such phenomena, surface roughening and all its related factors must be considered as new, important tribological variables in the friction modeling of bulk metal forming.