**4.1 Cleaner melt**

8 Recent Trends in Processing and Degradation of Aluminium Alloys

Shrinkage is the natural consequence of liquid to solid transformation of the melt during cooling and is common in most metals. Shrinkage is particularly severe in aluminium alloys. In aluminium alloys, the volumetric shrinkage ranges from 3.5% to 8% (Kaufman and Rooy,

This is often counteracted by strategic placement of risers. Figure 7 shows the typical

2004). This manifests as shrinkage cavities in larger portions of the casting.

Fig. 7. Typical appearance of volumetric shrinkage defect in an aluminium section

During pouring of the melt, there is considerable splashing and sloshing about of the melt. This entrains significant quantities of air and non-metallic inclusions in the mould. Such entrained material degrades casting quality. This problem is often mitigated by incorporation of complex gating systems designed using advanced Computational Fluid Dynamics (CFD) modules. Such casting simulation software is able to predict and avoid bubble streams in

Some of the problems outlined above have been resolved by advancements in pressure die casting, improved investment casting techniques and centrifugal casting. These techniques individually solve some, but often not all of the problems with gravity-assisted pour of an air-melt. For instance, in conventional die casting, melt is sprayed at high velocity into the die and cavity-atmosphere tends to be admixed and entrapped in castings during the

appearance of volumetric shrinkage defect in an aluminium section.

**3.3 Shrinkage** 

**3.4 Pouring defects** 

metals castings (Waterman, 2010).

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 during mould filling (Druschitz and Fitzgerald, 2000).

## **4.2 Elimination of shrinkage defect**

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