**3.1 Gas defects**

Molten aluminium is particularly susceptible to adsorbing significant quantities of hydrogen gas from atmospheric moisture, which leads to a high concentration of dissolved hydrogen in the melt. This may be further exacerbated by alloying element like magnesium which may form oxidation reaction products that offer reduced resistance to hydrogen diffusion into the melt (Key to Metals, 2010). This causes blow holes and gas porosity which combine to reduce strength of the cast part. The micrograph in figure 5 shows a blow hole defect, it can appear at any region of the cast microstructure and is exacerbated by damp mould materials which give off steam during casting. Figure 6 shows gas porosity defects in an aluminium casting, these are much smaller than blow holes and tend to form in clusters around the region of the grain boundaries.

### **3.2 Melt oxidation**

Oxidation of the melt is another severe defect suffered by aluminium alloy castings. The elevated melt temperature promotes easy oxidation of the aluminium by ambient oxygen. The aluminium oxide thus formed is an undesirable non-metallic inclusion. Considerable efforts, through the use of in-mould filters, protected atmosphere, or alloying additions are often needed to reduce oxide formation and entrainment in the mould.

Fig. 4. The vacuum is maintained until the cavity is completely filled. Vacuum pressure is

Molten aluminium is particularly susceptible to adsorbing significant quantities of hydrogen gas from atmospheric moisture, which leads to a high concentration of dissolved hydrogen in the melt. This may be further exacerbated by alloying element like magnesium which may form oxidation reaction products that offer reduced resistance to hydrogen diffusion into the melt (Key to Metals, 2010). This causes blow holes and gas porosity which combine to reduce strength of the cast part. The micrograph in figure 5 shows a blow hole defect, it can appear at any region of the cast microstructure and is exacerbated by damp mould materials which give off steam during casting. Figure 6 shows gas porosity defects in an aluminium casting, these are much smaller than blow holes and tend to form in clusters

Oxidation of the melt is another severe defect suffered by aluminium alloy castings. The elevated melt temperature promotes easy oxidation of the aluminium by ambient oxygen. The aluminium oxide thus formed is an undesirable non-metallic inclusion. Considerable efforts, through the use of in-mould filters, protected atmosphere, or alloying additions are

often needed to reduce oxide formation and entrainment in the mould.

released causing un-solidified melt to flow back into the furnace

around the region of the grain boundaries.

**3.1 Gas defects** 

**3.2 Melt oxidation** 

Fig. 5. A Blow hole defect in an aluminum casting at 100× magnification

Fig. 6. Gas porosity in aluminium casting at 1000× magnification

Aluminium Countergravity Casting – Potentials and Challenges 9

turbulent cavity-fill (Jorstad, 2003). The process of air melting and pouring also inevitably introduces oxides, formed during melting, into the cast product. Significant inclusions segregation at grain boundaries are thus very common with gravity assisted sand casting.

Numerous advantages for metal casters are endemic to the countergravity casting technique. These may be broadly categorized into defect reduction and elimination and

For aluminium alloys, metal oxides formed and aggregated on the melt surface can be bypassed by taking clean melt from below the surface. The practice of de-slagging using a hand ladle or metal rod to scoop the slag layer off the melt surface unavoidably leaves pieces of slag in the melt which ultimately flows into the mould during casting. Countergravity casting also results in improved melt cleanliness, due to reduced turbulence

Shrinkage is virtually eliminated in the countergravity casting technique. This is because a constant supply of fresh melt is maintained in the mould during casting. Hence, as portions of the mould begin to solidify, the down-sprue is the last to start solidifying. The reservoir of molten melt in the crucible acts as a riser, ensuring a steady supply of melt into the mould during solidification. This effectively eliminates the need for risering. Figure 8 shows the cross-section of a countergravity cast rod. The absence of volumetric shrinkage defect is

In the countergravity technique, the gating system is considerably simplified as is depicted in figure 9. It consists merely of branches of flow channels emanating from the central sprue. This simplicity is possible because the interior of the mould is actually an extension of the vacuum system. The high pressure differential between the mould interior and the atmospheric pressure ensures that the molten metal will completely permeate every cavity in the mould. Complex in-gates, depending on gravity flow of melt are thus not needed.

Countergravity technique significantly decreases the amount of gates that must be re-melted (Flemings et al, 1997). This was actually one of the original goals of the countergravity technique at its inception. Fettling time and costs are reduced while high quality melt is

The countergravity technique has numerous potentials, derivable from its advanatges over the conventional metal casting techniques. As such it is gradually making in-roads into

**5. Potentials and applications of the countergravity casting technique** 

traditional investment casting applications and also in novel materials production.

**4. Advantages of the countergravity casting technique** 

during mould filling (Druschitz and Fitzgerald, 2000).

evident from the convex meniscus at the top of the rod section.

**4.2 Elimination of shrinkage defect** 

**4.3 Simplified gating system** 

**4.4 Economical** 

judiciously used.

This considerably simplifies the mould design.

casting economics.

**4.1 Cleaner melt** 
