The effect of friction on pressure in upsetting at low diameter-to-height ratios

Abstract

The upsetting of relatively slender cylinders, with a diameter-to-height ratio of 0.5–1.0, is often encountered in hot forging and cold upsetting and is also one of the preferred methods of flow stress determination. Because of friction, barrelling and even folding-over of the side surfaces occurs but, for reasons of convenience, pressure-multiplying factors derived from theories based on homogeneous deformation are generally used not only for the predictive calculation of upsetting pressures, but also for correcting pressures for friction effects when determining the flow stress of materials. Upsetting experiments on 6061-T6 and hot-rolled mild steel cylinders with both good lubrication and unlubricated grooved anvils showed that pressures are much lower than predicted by theory even when sticking friction is attained, as long as the final diameter-to-height ratio of the workpiece is less than 2. From the experiments, a realistic pressure-multiplying factor curve is derived for predictive calculations of average upsetting pressure. It is also found that friction increases interface pressures by less than 5% and thus can be ignored in flow stress determination to a strain of ϵ = 0.9 (or 60% reduction in height) by one of the two suggested techniques: Firstly, and preferably, a slender cylinder of d 0/h 0 = 0.5 is upset between unlubricated grooved anvils. Secondly, upsetting may be carried out with good lubrication, but then a stubbier cylinder of d 0/h 0 = 0.67 must be used to prevent instability (skewing) during upsetting.