Aluminium fluidity

Castabilty

There are currently no methods, which could be pointed out as a means of determining the casting properties of an alloy in terms of measurable quantitative parameters [1]. Three specific characteristics of the alloy determine the performance of its casting properties:
• Fluidity
• Hot tearing
• Metal sticking on a die .

Fluidity

This is the ability of a material to flow into a given cavity and fill it. Foundry variables, such as mold coating, alloying additives, head pressure and especially the degree of superheating of the melt, affect the amount of fluidity. for instance, for castings in sand and permanent molds, an increase in the solidification interval of the alloy greatly affects the fluidity.

Hot tearing

It's a casting phenomenon, which often happens, for example, in sand casting. It depends on voltage, that occur during hardening. The behavior of the material during solidification also matters..

Metal sticking to a die

It happens, when liquid aluminum comes into contact with a steel die, for example, in injection molding. Due to the natural affinity of iron and aluminum, a reaction occurs on the surface, which results in the formation of intermetallic phases. Per Series “shots” a significant amount of aluminum sticks to these phases on the crystal surface. Thus, the head must be unplugged and cleaned, which is costly, if it happens too often.

Metal fluidity

Fluidity - this is a purely foundry term and it differs from the general scientific term "fluidity". In ordinary science, "fluidity" is a quantity, reverse viscosity and, so, it can be accurately measured numerically.

Foundry fluidity (also called "casting fluidity") is defined as the distance, on which the liquid metal penetrates into the cavity of the casting mold during metal casting, including, at aluminum casting. Therefore, fluidity is not absolute, relative magnitude.

When molten aluminum enters the mold cavity, then during this flow it loses its thermal energy. Finally, aluminum begins to solidify and stops flowing at some point in the cavity.

Fig. 1 – Thin-veins or intricate thin walled structures require a high degree of fluidity to avoid misruns (Source: EAA [3])

Influence of casting technology on metal fluidity

The main factors, affecting the fluidity of the metal, They are as follows.

• Initial metal temperature.
• Power of heat extraction from the melt by the material of the casting mold, including the effect of thermal insulation coatings on the mold.
• Kinetic energy of metal:
  1) Gravity casting (die casting) and sand casting rely on the flow of metal from top to bottom under its own weight.
  2) Low and high pressure casting use different, adjustable pressure for, to make the metal flow into the mold.

• Metal purity, that is, the presence of impurities in the alloy also has a great effect on the fluidity of the metal (figure 2).

Fig. 2 – Filtered (clean) versus unfiltered (dirty) metal fluidity comparison (Source: EAA [3])

The role of metal fluidity in casting

• Insufficient fluidity can lead to incomplete filling of the mold and unclear repetition of the product shape.
• Excessive fluidity can result in metal penetration through the sand mold or excessive metal splashing as it travels through casting channels..
• In practice, metal fluidity is controlled by alloy selection, melt temperature and mold temperature.
• Deterioration of metal fluidity may be an indication of contamination..
• One can expect a change in the fluidity of the metal with a change in the content of impurities, the introduction of additives for grain refinement or alloy modification.

Tests for fluidity

A spiral sample is the pouring of metal into a spiral sand mold (Fig.. 3). It is the oldest method for assessing the fluidity of a metal.. The metal is poured into a mold at a carefully controlled temperature and the length is measured, which metal went through.

The essence of a vacuum fluidity test is as follows. Ceramic or quartz tube connects to the vacuum system. This pipe is dipped into the tested melt at a given temperature. According to the metal lifting height (Fig. 4) or metal length (Fig. 5) judge its fluidity.

The multichannel sample is shown in Fig. 6.

Fig. 3 – The fluidity spiral, or sand spiral, is one of the oldest and simplest fluidity tests (Source: EAA [3])

Fig. 4 – Vacuum fluidity testing is more complex, but eliminates variables associated with moulded sand (Source: EAA [3])

Fig. 5 – Vacuum fluidity test [2]

Fig. 6 – Multichannel fluidity test in metal mold [2]

Influence of alloying elements on aluminum fluidity

The effect of the alloying elements (copper and silicon) content in aluminum on the fluidity is shown in the figure 7.

Fig. 7 – Foundry fluidity of Al with Si or Cu content (Source: EAA [3])

Influence of modifiers (strontium) on the fluidity of aluminum, grain grinders (titanium, titanium / boron) and impurities (iron) shown in the figure 8.

Fig. 8 – Effects on AA 320 alloy of changes in trace elements or master alloy additions (Source: EAA [3])

Effect of aluminum purity on its fluidity

The fluidity of aluminum is very sensitive to its purity level, as seen from the picture 9.

Fig. 9 – Rapid change in fluidity of pure Al with impurity content (Source: EAA [3])

Sources:

1. Aluminum Cast Alloys: Enabling Tools for Improved Performance / D. Apelian – North American Die Casting Association, 2009
2. Castability—Fluidity and Hot Tearing //ASM Handbook, Volume 15: Casting – 2008
3. European Aluminium Association, 2002

More about sand casting of aluminum:

Aluminum Alloy Casting: Properties, Processes And Applications – J. Gilbert Kaufman, Elwin L. Rooy