Liquid-Solid Contact at Evaporation of Water Liquid Droplet Emulsified With Lubricant and Its Adhesion Situation

Abstract

In aluminum die casting process, water-soluble lubricant which is a kind of emulsion and consists of oil, surfactant and water are sprayed onto metal die surfaces, where the surface temperature is relatively high, over 300°C. When a lot of small liquid droplets of the water-soluble lubricant impact on a superheated surface, it is important to assure that liquid-solid contacts occur and oil component adhere onto the surface. However, many parameters such as liquid temperature, concentration, diameter and impact speed, affect this phenomena, so the phenomena themselves has not been totally revealed and it has not been fully established how to assure the liquid-solid contact and adhesion of oil component. Therefore, as a fundamental and experimental research, this paper aims to obtain essential information and data of liquid-solid contact of water-soluble lubricant and its adhesion situation in order for improving aluminum die-casting process. Firstly, evaporation curves, especially Leidenfrost temperature, of water droplets including water-soluble lubricant on metal surface were measured. Experimental parameters were as follows: temperature of droplet, 20 ∼ 80°C, concentration, 0 ∼ 50% corresponding to surface tension 20 ∼ 70mN/m, falling height of droplet, 1 ∼ 9cm, and two kinds of lubricant. The metal surface was made of SKD61, usually used as materials of die casting. Experimental results show that Leidenfrost temperature is not strongly affected by droplet velocity, but significantly affected by concentration and liquid temperature. It is worthy to note that there was found to be optimum temperature and concentration of droplet from view points of aluminum die casting. These results can be explained by effects of surface tension and interfacial temperature between solid and liquid. Secondly, visual observations of these phenomena by high-speed camera and measurements of electric resistance between droplet and metal surface were conducted. Surface chemical analysis was also performed by SEM/EDX after evaporation experiments for detecting carbon and silicone components on the surface to check adhesion situations of lubricant. The obtained results show that liquid-solid contact becomes gradually localized and limited as initial surface temperature increases. However, liquid-solid contact surely occurred at higher surface temperature than Leidenfrost temperature, 210 ∼ 270°C, and lubricant adhesion was clearly confirmed even if liquid-solid contact duration was limited to very small time scale. This result implies that oil component can adhere on die surface of higher temperature than Leidenfrost temperature.