The article was provided by our partner company DEMIN SRM GmbH (www.demin-srm.de).
The main purpose of using risers is to obtain a good casting without shrinkage cavities and porosity. In general, the formation of these defects can be presented as follows:
– in a very short period, a crust of solidified alloy is formed on the surface of the casting, which defines its outer contour;
– the decrease in the volume of liquid metal inside the formed solid crust is ahead of the volumetric shrinkage of the hard casings itself;
– in the absence of the liquid metal inflow, compensating the decrease in volume, voids, which are called cavities, are formed, their size depends on the alloy properties, the overheating temperature and the initial volume. The supply conditions for the solidifying casting determine the possibility of obtaining its dense walls and minimal mechanical inhomogeneity in different parts of the casting, which is most noticeably manifested in the manufacture of large-sized cast parts.
Investigation of the conditions for cast feeding and the development of methods for using the risers, determining their sizes, as well as methods for improving the action of the risers are based on the study of the solidification processes of alloys.
Gas or air pressure increases the reliability of the risers, helping to reduce metal consumption and porosity in castings. However, additional costs are required for implementation and careful testing of the technological process. The use of such risers is advisable in the manufacture of large castings subjected to hydrotesting, as well as with a large length of the fed units, on which several conventional risers are to be installed.
No riser with an endothermic or exothermic casing will have the same effect as those with overpressure.
Exothermic casings significantly reduce the size of the ingot top and the shrinkage chamber itself, however, in those areas of the casting that are adjacent to the exothermic riser, local overheating of the metal to a temperature of 2000 °C or more is created, which disturbs the structure of the metal entering the casting from the bottom. Such overheating is especially dangerous when casting spheroidal graphite and carbon steel cast iron.
In addition, in the castings, areas with different microstructure are formed at the boundaries of the overheated metal and metal with the calculated pouring temperature. The formation of such areas is fraught with internal stresses and various strength characteristics in these areas.
There are several ways to create excess pressure in the riser – this is the compressed air supply and the gas-emitting cartridges installation; however, all these options require additional costs and, most importantly, are possible only on large-sized castings.
We propose a new trend in casting using riser casings that create excess pressure due to the casing material expansion, or rather, due to expansion in the volume of the coating that is applied to the working part of the casing. The coating material is capable of increasing tens of times without gaseous substances release, just by changing its structure at temperatures above 300 °C.
By adjusting the thickness of the coating along the height and surface of the riser casing, it is possible to control the crystallization processes of the resulting castings.
The operation principle the riser casings developed by us is as follows: when metal enters the riser, the coating material changes its crystal lattice and increases in volume over the entire surface of the applied coating by an amount depending on the coating concentration and the its application thickness. The pressure in the head can be created from atmospheric to several atmospheres. It also depends on the thickness and concentration of the coating.
This effect is possible only on closed risers.
In this case, with excess pressure in the riser, it is possible to obtain castings with thinner walls and castings with different wall thicknesses without reducing their quality.
Moreover, which is also very important, it is possible to reduce the temperature of the metal during pouring, which will entail, in addition to improving the quality of castings, also saving energy for heating the metal to a higher temperature.