Performance of graphite and boron-nitride-silicone based lubricants and associated lubrication mechanisms in warm forging of aluminum

Although water/oil-graphite emulsions are widely used in warm forging processes, they carry environmental concerns. In an attempt to replace graphite-based lubricants in warm forging of aluminum alloys, two variants of boron-nitride-silicone lubricants were formulated. The two variants were made by dispersing boron nitride powder in polydimethyl siloxane oil at concentrations of 1% and 8%. The formulated lubricants were initially tested for their thermal degradation characteristics using a thermogravimetric analyzer and compared to the thermal degradation behavior of graphite and silicone oil lubricants. Ring compression tests were then carried out at 260 degrees C and 370 degrees C. Boron-nitride-silicone lubricant variants did not show significant difference in performance as die temperature was increased from 260 degrees C to 370 degrees C. This is in contrast to graphite, which performed much better at 260 degrees C than at 370 degrees C, due to thermal oxidation. On the other hand, silicone oil exhibited the worst performance at 260 degrees C and the best performance at 370 degrees C. In both boron nitride lubricant variants, the polydimethyl siloxane facilitated hydrostatic/hydrodynamic lubrication at 260 degrees C, with boron nitride acting as a barrier film that reduced friction. However, the lubrication mechanisms changed at 370 degrees C, where the depolymerization of polydimethyl siloxane led to formation of silica due to thermal oxidation. Silica, together with boron nitride, acted as a film barrier with low shear strength. The dual lubrication mechanisms make boron-nitride-silicone lubricants suitable for a wide range of aluminum forging temperatures.