Friction measurement by the tip test for cold forging

Tip test is a friction measurement technique for forging process based on upsetting and backward extrusion with a specimen of which the diameter is larger than the punch and smaller than container. Due to the smaller diameter of the specimen, a sharp tip was formed during deformation and a unique linear relationship between the tip distance and maximum load measured was found. In the present paper, the effect of flow stress of the material of AL2024-O and 6061-O on sensitivity of the downsized tip test was investigated. Different friction conditions on the punch, counter punch, and container were introduced to determine their effects on calibration of the shear friction factor. A non-dimensional equation to determine the shear friction factor from the non-dimensional tip distance was derived based on tip test results under the various experimental conditions with different surface topographies of the counter punch, different order of deformation speeds, and different types of lubricant such as grease, VG100, VG32, and corn oil using the experimental data available in the literature. The linear relationships among the shear friction factor, measured tip distance, and maximum load were reconfirmed for all the experimental conditions in this study. It was found that the tip test was more sensitive and versatile in determining a representative shear friction factor in cold forging than other testing methods available so far.