Wear onset in hot stamping of aluminium alloys sheets

The paper presents experimental investigations on the tribological performances of solid lubricants for the AA6016 aluminium alloy, by reproducing the different thermal conditions at the blank-die interface typical of the hot stamping process. A novel spraying apparatus was developed to apply the lubricants on the metal blank and capable of accurate control of the deposition parameters (i.e. temperature, pressure, film thickness) and quantity of the depositions. The performances of lubricants based on graphite, molybdenum-disulphide and boron-nitride were investigated, in terms of frictional and wear behaviour, by means of a novel strip drawing apparatus in the temperature range 300-400 °C and normal contact pressures up to 15 MPa. The surface characteristics of the metal sheets at the wear onset were analysed by means of optical profilometry, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results showed that the wear onset was primarily due to a loss of adhesion of the lubricants, and it was delayed when using graphite-based lubricants. It was found that at increasing the process temperature both the friction coefficient and the material adhesion increased, while the higher the sliding speed and pressure, the lower the friction coefficient. In particular, the influence of the pressure was found to be larger than the one of the sliding speed.