Thermophysical Properties Measurement of Liquid Al and Al-Cu by the Discharge Crucible Method

Abstract

The demand to develop Materials Genomics and Integrated Materials Computation requires the availability of high temperature property data of liquid metals. High temperature metallurgical processes, such as refining, casting, welding and additive manufacturing, can be optimized and operate more efficiently with the accurate knowledge of thermophysical properties of metals and alloys, such as viscosity, surface tension and density. As computing power and algorithms are constantly being improved, the accuracy of thermophysical property data has emerged as one of the limiting factors. Knowledge of these properties for materials such as Al and Al-alloys is a critical factor in numerical simulations and modelling, which are essential for not only the development but also the optimization of production processes. This work reports on the simultaneous measurements of viscosity, surface tension and density of liquid Al and Al-Cu alloys using the discharge crucible method. The method is based on a mathematical formulation that describes the fluid dynamics of a liquid draining through an orifice under the influence of gravity. It is based on the Bernoulli formulation, but accounts for surface tension effects induced by Laplace pressure at the exit of the orifice. It is used to describe the liquid flow rate in terms of orifice exit radius, discharge coefficient, experimental head, and the three thermophysical properties. In gathering data of the experimental flow rate, viscosity, surface tension and density can be solved iteratively using a multiple non-linear regression. The results for Al and Al-22.5wt.%Cu as a function of temperature will be presented and compared to literature values obtained using classical methods and theoretical and empirical numerical models. The results have achieved, with a varying degree of success, the goal of measuring the thermophysical properties of Al and Al-Cu. The results for viscosity and surface tension have provided an important validation to the measurement technique and are in good agreement with published literature data and certain numerical models. Conversely, results for density were found to be much lower than reported in literature. Through analysis, it was determined that wetting at the orifice affected the flow rate, since modelling of the flow did not account for accelerative losses created by the meniscus. Dimensionless number analysis identified that wetting had an immediate effect on the flow rate, becoming more dominant with drain time. This work recommends that efforts be made to further modify the discharge crucible formulation to account for wetting and validate the technique by measuring different liquid metals and alloys at various temperatures.