Vertical direct chill (VDC) casting of aluminium alloys is a mature process that has
evolved over many decades through gradual change to both equipment design and casting practice.
Today, air-pressurised, continuous lubrication, hot top mould systems with advanced station
automation are selected as the process of choice for producing extrusion billet. Specific sets of
operating parameters are employed on these stations for each alloy and size combination to produce
optimal billet quality. The designs and parameters are largely derived from past experience and
accumulated know-how. Recent experimental work at the University of Queensland has
concentrated on understanding the way in which the surface properties of liquid aluminium alloys,
e.g., surface tension, wetting angle and oxide skin strength, influence the size and shape of the
naturally-stable meniscus for a given alloy, temperature and atmosphere. The wide range of alloyand
condition-dependent values measured has led to the consideration of how these properties
impact the stability of the enforced molten metal meniscus within the hot top mould cavity. The
actual shape and position of the enforced meniscus is controlled by parameters such as the upstream
conduction distance (UCD) from sub-mould cooling and the molten metal head. The degree of
deviation of this actual meniscus from the predicted stable meniscus is considered to be a key driver
in surface defect formation. This paper reports on liquid alloy property results and proposes how
this knowledge might be used to better design VDC mould systems and casting practices.
Enhancement of chip breakability of aluminium alloys by controlling the solidification during direct chill casting
