Development of empirical equation to predict shear friction factor in cold forging

Abstract

Friction is generated between two bodies in contact under a normal load and is defined as the resistance to relative motion between the two. Conventionally, friction was described by Coulomb friction (τ = μ??p, μ = coefficient of friction and p = normal pressure) or the constant shear model (τ = mf ??ks, mf = shear friction factor and ks = shear yield stress of the material), known as the Tresca friction model. For the high contact pressure arising in metal forming, the latter works better under the condition that mf is characterized correctly at the given condition. It is well known that frictional behavior is highly nonlinear and it also depends on a number of variables such as contact area, lubricants, sliding velocity, temperature, surface condition and the environment in general cases. Thus, it is not easy to characterize mf correctly for practical purposes. In the present investigation, an empirical equation composed of process variables in cold forging to predict shear friction factor is suggested. To decide deterministic relationships between those process variables and shear friction factors, the tip test based on a linear relationship among the dimensionless load, tip distance and shear friction factors (mfp at the punch and mfd at the counter punch interfaces) was utilized. The tip test was conducted under various combination of different surface roughness of the specimens and forming tools, viscosity of the lubricants, deformation speeds, and hardness of the deforming materials on friction. For the test, cylindrical specimens made of aluminum alloys of 2024-O, 6061-O and 7075-O, a single punch moving downwards at speeds of 0.01, 0.1, 0.5, 1.0 and 5.0 mm/s, and three die sets with different surface topographies of 0.08, 0.097 and 0.63 μm were used with three different kinds of commercial lubricants, namely the VG series (viscosity grade 22, 32, 46, 68 and 100), corn oil and grease. The load levels and tip distances were measured and compared to de...