**5. Metal injection casting**

The production of castings by high pressure die casting (HPDC) are generally limited to the low melting point metals Al, Mg, Zn and Pb. Some brasses are cast by this technique but attempts to cast stainless steels seem to have been abandoned. This brief description will concentrate only on the casting of Al alloys.

Although the term 'high pressure' seems to offer reassurance of a wellconsolidated pore-free product, as most readers will be aware, this can be far from the truth and should never be forgotten by potential users. In general, the HPDC process can never guarantee freedom from porosity and leakage. Nevertheless, the process has valuable features and capabilities which distinguish it widely from other casting methods.

The process is often described as high productivity. It is true it benefits enormously from its ability to cast thin sections which can freeze quickly. But in common with all metal mold casting processes, the metal mold cannot be opened until the casting has frozen, or nearly frozen. This waiting time for the casting to solidify is a major contribution to the production cycle. High production sand casting systems can be much faster for any thickness of casting section, because after pouring, the mold can be moved away, allowing the immediate pouring of

**9**

blades.

*Perspective Chapter: A Personal Overview of Casting Processes*

impressions, giving multiple castings per filling.

a second mold, and so on. Both sand and die systems can benefit from multiple

For most HPDC machines, metal is spooned from an open holding furnace, and poured into a shot sleeve, from where it is rammed into a steel die by a piston. The steel die is sunk into a massive steel bolster, which is kept closed during the shock of the filling process by hydraulic rams developing hundreds or thousands of tons of force. This brutal description is not too far from reality, although the injection stroke and filling pattern is now often optimized by computer simulation to reduce air entrainment, which has resulted in significant improvements to the reduction in

The turbulence during the injection process, in which the metal velocity usually exceeds 50 to 100 m/s, is so great that defects are necessarily created but are accepted as a feature of the process. Interestingly, the high density of bifilms is not necessarily the disadvantage that might be imagined; the long oxide flow tubes (the oxide tubes which surrounded the jets of metal entering the mold cavity) and other bifilms are aligned along the flow direction, giving a fibrous microstructure whose properties somewhat resemble the directional features of wood. The rapidity of the filling process, being completed within milliseconds, probably also suppresses the degradation of the casting by bifilms, whose constituent films have so little time to grow and are necessarily extremely thin. Their limited thickness may permit some bonding between the two films as a result of atomic rearrangements during their transformation from pure alumina to spinel as Mg in the alloy diffuses into the bifilm. The high pressure, keeping the two sides of the bifilm closely in contact is a further aid to bonding and, in any case, provides strength by the bifilm being enabled to resist shear force, because of jogs and wrinkles, if not direct tensile force. Even so, the HPDC castings can never be relied on not to leak, and sometimes, not to fail unexpectedly. Their use for safety critical purposes should therefore only be accepted with very great caution. (In contrast, gravity sand and gravity die castings [permanent

mold castings] are typically favored for safety critical components).

Traditionally, small HPDC machines provide high productivity for small thin-walled products. The accuracy and surface finish are good, often eliminating machining, making the process favored by engineers. Recently, extremely large HPDC machines have been built to produce castings of several square meters area with walls only a millimeter or two in thickness, creating large pieces of automo-

There are some genuine reasons why vacuum is needed for the melting and casting of certain alloys and certain products. Sometimes, a limit on the oxidation of reactive metals or alloying elements is required. At other times the vacuum is needed to ensure the filling of extremely narrow and tapering sections as in turbine

Alternatively, vacuum casting is used, imagining that this will prevent the formation of defects during a top pour. This appears to be a widespread but dangerously incorrect assumption. The entrainment defects resulting in bifilm creation appear to be the same no matter what environment is used, whether this is air, inert gas or vacuum. The reason is that both the inert gas and the vacuum environments always contain sufficient oxygen and/or nitrogen to create oxide or nitride films on the surface of the pouring liquid, so that defects of identical size and geometry are formed if entrainment of the surface occurs – the only difference being the thickness of the resulting bifilms. Bifilms are generally so thin that they

*DOI: http://dx.doi.org/10.5772/intechopen.93739*

porosity in castings.

biles in one shot.

**6. Vacuum casting**

#### *Perspective Chapter: A Personal Overview of Casting Processes DOI: http://dx.doi.org/10.5772/intechopen.93739*

a second mold, and so on. Both sand and die systems can benefit from multiple impressions, giving multiple castings per filling.

For most HPDC machines, metal is spooned from an open holding furnace, and poured into a shot sleeve, from where it is rammed into a steel die by a piston. The steel die is sunk into a massive steel bolster, which is kept closed during the shock of the filling process by hydraulic rams developing hundreds or thousands of tons of force. This brutal description is not too far from reality, although the injection stroke and filling pattern is now often optimized by computer simulation to reduce air entrainment, which has resulted in significant improvements to the reduction in porosity in castings.

The turbulence during the injection process, in which the metal velocity usually exceeds 50 to 100 m/s, is so great that defects are necessarily created but are accepted as a feature of the process. Interestingly, the high density of bifilms is not necessarily the disadvantage that might be imagined; the long oxide flow tubes (the oxide tubes which surrounded the jets of metal entering the mold cavity) and other bifilms are aligned along the flow direction, giving a fibrous microstructure whose properties somewhat resemble the directional features of wood. The rapidity of the filling process, being completed within milliseconds, probably also suppresses the degradation of the casting by bifilms, whose constituent films have so little time to grow and are necessarily extremely thin. Their limited thickness may permit some bonding between the two films as a result of atomic rearrangements during their transformation from pure alumina to spinel as Mg in the alloy diffuses into the bifilm. The high pressure, keeping the two sides of the bifilm closely in contact is a further aid to bonding and, in any case, provides strength by the bifilm being enabled to resist shear force, because of jogs and wrinkles, if not direct tensile force. Even so, the HPDC castings can never be relied on not to leak, and sometimes, not to fail unexpectedly. Their use for safety critical purposes should therefore only be accepted with very great caution. (In contrast, gravity sand and gravity die castings [permanent mold castings] are typically favored for safety critical components).

Traditionally, small HPDC machines provide high productivity for small thin-walled products. The accuracy and surface finish are good, often eliminating machining, making the process favored by engineers. Recently, extremely large HPDC machines have been built to produce castings of several square meters area with walls only a millimeter or two in thickness, creating large pieces of automobiles in one shot.
