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.

The presented work is aimed to deal with the influence of changes in the value of negative (relative) pressure maintained in the die

cavity of pressure die casting machine on the surface quality of pressure castings. The examinations were held by means of the

modified Vertacast pressure die casting machine equipped with a vacuum system. Castings were produced for the parameters selected

on the basis of previous experiments, i.e. for the plunger velocity in the second stage of injection at the level of 4 m/s, the pouring

temperature of the alloy equal to 640°C, and the die temperature of 150°C. The examinations were carried on for three selected values

of negative gauge pressure: - 0.03, - 0.05, and - 0.07 MPa. The quality of casting was evaluated by comparing the results of the surface

roughness measurements performed for randomly selected castings. The surface roughness was measured by means of Hommel Tester

T1000. After a series of measurements it was found that the smoothest surface is exhibited by castings produced at negative gauge

pressure value of - 0.07 MPa.