With die-casting, a hydraulic power source pushes the spindle into a metal mold that acts as a heat sink, absorbs tensions and facilitates the final removal of the piece. Die-casting is used for casting in which the piece requires improved mechanical properties. However, not all copper alloys are castable by die-casting, usually aluminum bronzes and some types of brass are used. This type of process cannot be used for complex shapes or cases where there are variations in section.
Centrifugal casting: in this type of casting the centrifugal force is exploited to obtain thickening of the metal with better filling of the form. Almost all copper alloys can be poured with this technique except for bronzes with a high lead content since this element tends to accumulate in the peripheral area of he pieces, leading to significant changes in the distribution of inclusions.
Shell casting: this type of casting has been in use for about 50 years for copper based alloys; currently it is used for bronzes of various kinds such as those with high content of Pb, phosphorus and gunmetal (alloy with 88% Cu, Sn 10%, Zn 2%). In this process, the metal solidification rate is increased leading to an increase in the delta crystalline phase (which is finely dispersed in the matrix and gives the material great hardness) and a better lead distribution.
The material obtained with this process has better overall mechanical characteristics, in particular hardness increased up to even 50%. The molds used, with great versatility of use, are metallic and conceptually similar to those for die-casting used by laser cutting companies.
Pure copper is an extremely difficult to cast metal because it has a strong tendency, during cooling, to create cracks in the surface and internal cavities giving rise to a very porous final product. The relative casting process, however, can be improved with the addition of small quantities of elements such as beryllium, silicon, nickel, tin, zinc, silver and phosphorus.
As far as copper alloys are concerned, it is necessary to make a classification with regard to their cooling behavior since, unlike pure metal, they solidify in a more or less wide temperature range. In general, the solidification interval is defined as the temperature range between the intersection of the composition between solidus and liquidus of the equivalent state diagram.
The alloys of group 1 and 2 are directionally cast thanks to auxiliary supply channels designed to improve the filling of the mold in critical areas such as variations in section, angles or particular shapes. It is necessary to pay close attention to the possible passage of gaseous hydrogen in solution during the high temperature stay of the alloy used by laser cutting companies.
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