Strontium and boron loss: a peril of aluminum degassing.

While degassing is an important tool for removing hydrogen from foundry aluminum, it also promotes a loss of beneficial alloys.

As all aluminum founders know, hydrogen in molten aluminum causes the formation of porosity as the casting solidifies. Therefore, one of the most important quality steps in aluminum casting is degassing the melt prior to pouring, to remove hydrogen and other gases from the metal.

The most prevalent method of eliminating gases is rotary impeller degassing [ILLUSTRATION FOR FIGURE 1 OMITTED]. This process is based on the principle that, due to the differential in hydrogen pressure, hydrogen will diffuse to the ascending bubbles of an inert gas (nitrogen or argon) injected into the bottom of the melt. That hydrogen then becomes trapped, and is carried out of the molten metal.

Of primary importance in this process is the gas bubble size. If gas is introduced via a lance - an alternative to rotary impeller degassing - large bubbles are produced, making the method inefficient. The best results are obtained with a rotary impeller degasser, which is submerged in the melt and spun as the gas is introduced through the bottom of the unit. In this way, it produces very fine bubbles, which are spread evenly throughout the bath.

For rotary impeller degassing, variables such as rotor speed, degassing time and flow rate are optimized to achieve maximum gas removal, while creating a minimum of metal turbulence within the bath. If this maximization is accomplished, the method is an effective way to remove hydrogen from molten aluminum, and is therefore widely used by aluminum foundries.

Testing for Drawbacks

Rotary impeller degassing of aluminum does, however, have its drawbacks.

In aluminum-silicon (Al-Si) hypoeutectic alloys, strontium is often added for modification of the Si structure during the alloying process. Titanium-boron is added immediately prior to degassing for grain refinement.

Unfortunately, rotary impeller degassing can speed the fade and dissipation of strontium and boron in molten aluminum. To study and measure this problem, testing was undertaken at an Australian foundry to gauge the effects of several variables on the loss of these important alloys prior to pouring.

Degassing of an Al-Si alloy (A356) [TABULAR DATA FOR TABLE 1 OMITTED] was done in a 990 lb (450-kg) crucible using two impeller designs - the "bubble chopper" type (which is like a propeller with short, squared blades) and the "hamburger" variety (a smooth-sided cylinder). For this testing, degassing was carried out with and without a covering flux over the metal to trap oxides and impurities. Metal temperatures at the beginning of each experiment were 1364-1400F (740-760C). For compositional analysis of the test metals, an optical emission spectrometer was used.

Also, to test the effect of holding time on strontium and boron loss, experiments were carried out on melts held in a 660-lb (300-kg) electric furnace. The compositional ranges for the Al-Si test alloy appear in Table 1.

Test Results

In the first experiment, no covering flux was used. It was found that, within eight min of degassing, about 12% of the strontium and 25-30% of the boron were lost from the melt [ILLUSTRATION FOR FIGURE 2 OMITTED]. As can be seen from these charts, the loss of strontium is approximately linear with time.

Figure 3 shows the loss of strontium against degassing time when cover flux (mainly containing potassium chloride, sodium chloride and flouride salts) was added. Compared with Fig. 2-A, in which no cover was used, it is apparent the loss of strontium increased considerably. The loss of boron, however, remained the same as in the melt without cover flux.

The effect of gas flow on strontium loss was next gauged, and it was found that as the flow of nitrogen increased, so did strontium loss. However, this increase slows down at higher flow rates. These results proved equally true when argon replaced nitrogen.

For every rotary impeller degasser, there is normally an optimal recommended rotor speed, usually varying between 350-600 rpm. These trials established that, within the optimal recommended speeds of the two degassers, there were only minor variations in alloy loss. Outside these ranges, there was only about a 5% difference between the highest and lowest speeds, provided the melt surface remained relatively undisturbed. Only at extreme speeds, when the shaft and rotor assembly go off balance and create turbulence inside the crucible, were strontium loss increases observed.

For boron, however, the same trials showed that the loss of boron increases as the rotor speed increases [ILLUSTRATION FOR FIGURE 3 OMITTED]. But when gas flow was adjusted to 10-22 liters per min, no significant increase in the rate of boron loss was noted.

Further tests to determine the effect of rotor design on degassing showed no significant differences in alloy loss rate.

Holding Time

It is well-known that elements such as strontium and boron will naturally fade from the melt during holding time.

In these trials, it was discovered that boron fades very rapidly: 33% in less than 10 min, and 75% in 30 min. This is due to a combination of heavier density boron compounds and a lack of dissolution in the metal. After three hr of holding, however, the metal was stirred up and 97% of the boron was recovered [ILLUSTRATION FOR FIGURE 4 OMITTED]. Researchers found that metal temperature had a minor impact on the fade rate of boron.

While the loss of strontium during holding time is mainly due to oxidation from exposure to air at the metal surface, it was found that the rate of fade increased as the bath temperature was raised. At temperatures below 1292F (700c), the rate of loss was minimal, but above that temperature, loss increased significantly.

Operational Implications

These experiments show that the loss of strontium and boron during degassing are considerable and, if not allowed for, might affect silicon modification and grain refinement. This will precipitate a deterioration of the mechanical properties of Al-Si castings.

One solution might be to add the strontium and titanium-boron alloys when degassing is finished, or a few minutes before the end of the process. The danger here is that such late additions may cause poor mixing, as well as the addition of hydrogen gas after de-gassing. Since the loss of strontium during degassing is primarily through surface oxidation, adding the alloy just before or during degassing might result in the floatation of large, needle-shaped undissolved alloy particles - partially contributing to some of the losses.

During degassing, strontium oxides are generated at the metal surface and remain there until removed with the dross. In these tests, when dross from a sample containing 120 parts per million (ppm) strontium was analyzed, it contained less than 20 ppm strontium as oxide. Later, after the same metal was degassed, the amount of strontium as oxide in the dross was 120 ppm.

The research found that when cover flux was added, the loss of strontium roughly doubled, while the rate of boron loss was not significantly affected. The strontium loss increase is mainly due to the reaction of chloride and flouride-type salts present in the flux. Support for this explanation is found in the fact that, when carbonate-type cover flux was used, the rate of strontium loss remained the same as when no cover flux was used.

Gas flow also showed a direct impact on the loss of strontium because the higher the gas flow, the more bubbles generated. As more bubbles break the protective oxide layer at the surface, the more aluminum is exposed to new oxidation. But, as long as there was no change in the turbulence of the bath, rotor speeds within the recommended limits of operation had no effect on the loss rate of strontium.

The loss of boron during degassing is a different process than that of strontium. Boron particles have no tendency for oxidation, and boron is probably lost due to the floatation of alloy particles to the bath surface, where they become trapped within the dross. This is a diminishing and irreversible process. A sample of dross taken after degassing contained 80 ppm boron, compared to an expected bath level of 20-25 ppm.

Unlike strontium, boron loss is directly affected by rotor speed, as higher speeds increase the amount lost. This is probably due to the higher metal circulation, which lifts heavier boron alloy particles to the surface. On the other hand, gas flow did not significantly affect the rate of boron loss, suggesting that the number of bubbles do not impact the number of boron particles that are lifted to the surface.