The paper presents the results of investigations concerning the influence of negative (relative) pressure in the die cavity of high pressure
die casting machine on the porosity of castings made of AlSi9Cu3 alloy. Examinations were carried out for the VertaCast cold chamber
vertical pressure die casting machine equipped with a vacuum system. Experiments were performed for three values of the applied gauge
pressure: -0.3 bar, -0.5 bar, and -0.7 bar, at constant values of other technological parameters, selected during the formerly carried initial
experiments. Porosity of castings was assessed on the basis of microstructure observation and the density measurements performed by the
method of hydrostatic weighing. The performed investigation allowed to find out that – for the examined pressure range – the porosity of
castings decreases linearly with an increase in the absolute value of negative pressure applied to the die cavity. The negative pressure value
of -0.7 bar allows to produce castings exhibiting porosity value less than 1%. Large blowholes arisen probably by occlusion of gaseous
phase during the injection of metal into the die cavity, were found in castings produced at the negative pressure value of -0.3 bar. These
blowholes are placed mostly in regions of local thermal centres and often accompanied by the discontinuities in the form of interdendritic
shrinkage micro-porosity. It was concluded that the high quality AlSi9Cu3 alloy castings able to work in elevated temperatures can be
achieved for the absolute value of the negative pressure applied to the die cavity greater than 0.5 bar at the applied set of other parameters
of pressure die casting machine work.