| The hot forging process is one of the manufacturing processes where metal is pressed, pounded or squeezed under great pressure into high strength parts. Before the billet is forged, lubricant is applied to heated dies to promote the flow of metal, to reduce friction and wear, and to aid in the release of the finished part. The most commonly used lubricant in the hot forging process is water-based graphite and it is applied to dies by spraying large numbers of atomized lubricant droplets through nozzles. An understanding of film formation and heat transfer of a single droplet colliding on a heated wall will result in more precise control of spray to obtain required film patterns and heat transfer. The precision of spray control will, in turn, reduce energy consumption and pollution in the hot forging process through reduction of billet pre-heat temperature and enhancement of lubricant wetting with the die.; Interactions between the die surface and droplets of water based graphite lubricant are the subjects of this study. First, the spreading of droplets which impact on the die surface is modeled based on Navier-Stokes equations and this model is solved using a commercial CFD code, Flow-3D. Special attention is paid to the influence of lubricant dilution ratio, lubricant properties, droplet parameters, and die surface temperature on the formation of the lubricant film in this model. A non-dimension analysis of the simulation results provides an insight into the physical phenomena and the model is validated through controlled experiments using a high speed camera.; The lubricant film is formed after all the residue liquid on the heated die surface is dried off. The evaporation time of the droplets is not only a productivity index in hot metal processing but also an important factor for product quality and life of the die. This study presents an analytical model to predict the evaporation time of a droplet impacted on a die surface, heated to temperatures beyond Leidenfrost point, beyond which are the working temperatures for hot forging dies. Calculated evaporation times from the model are compared with experimental measurements and agree well in a right magnitude.; A methodology for calculating the heat transfer coefficients in lubricant sprays is then presented. The heat transfer characteristics of the lubricants at various thermo-physical regimes on the hot surface are determined using single droplet experiments. The results of these experiments are used along with equations based on a non-dimensional and averaging approach to predict heat transfer coefficients for a complicated spray. Finally, the outcomes of dynamic interactions between lubricant droplets and heated die surface are categorized as bounce off, wetting and breakup, based on the coupling of Weber number (We) and die surface temperature. These characteristic outcomes of droplets with different dilution ratio are mapped through the experimental results.; Predictions by both the fluid dynamic model and the dryoff model agree well with the results obtained in experiments. Results from single droplet studies show that both Weber number (We) and die surface temperature play an important role on the lubricant wetting, dryoff and heat transfer on the die. Selection of lubricant dilution ratio has also been shown to be important on interactions between sprayed lubricants and the die surface. |
