Mechanisms of porosity formation in aluminum.

This article describes the path porosity follows into aluminum castings and how you can control this costly problem.

Porosity in aluminum occurs in one of three ways: hydrogen emerging from the liquid solution, shrinkage during solidification or, as is usually the case, by a combination of these effects.

This article combines the experience of foundrymen with research of the theory and prediction of porosity formation based on math models and the application of the physical laws that govern solidification.

Although internal voids also can be caused by mold reactions, high-temperature oxidation, blow holes and entrapped gas, this two-part series focuses on hydrogen and shrinkage porosity formation.

Hydrogen Solubility 

Hydrogen is the only gas that is soluble in aluminum and its alloys. The solubility of hydrogen in aluminum varies directly with temperature and the square root of pressure. Its solubility is considerably greater in liquid than the solid state. Actual liquid and solid solubilities in pure aluminum just above and below the solidus are 0.69 and 0.04 c|m.sup.3~ per 100 grams, respectively. These values vary for most common casting alloys, but not dramatically.|1~

The relationship of hydrogen solubility and temperature is normally defined under equilibrium conditions. No more hydrogen than the equilibrium solubility value can be dissolved at any temperature. Controlling melting conditions and melt treatment can result in substantially reduced dissolved hydrogen levels.

During cooling and solidification, dissolved hydrogen exceeding low solid solubility may precipitate and form internal voids.

Hydrogen bubble formation is strongly resisted by surface tension forces, increased liquid cooling and solidification rates that affect diffusion, and an absence of nucleation sites for hydrogen precipitation such as entrained oxides.

Hydrogen's emergence obeys the laws of nucleation and growth, and is similar in these respects to the formation of other metallurgical phases during solidification.

Gas Formation

Hydrogen precipitation consists of: the diffusion of hydrogen atoms within the liquid pool; the formation of subcritical nuclei as a function of time and cooling; the random emergence of stable precipitates exceeding the critical size required for sustained growth; and continued growth of the precipitated phase as long as hydrogen atoms are free to diffuse to the melt/bubble interface. The result is a general distribution of voids occurring throughout the solidified structure.

Research has established a number of valuable rules and relationships that describe the tendency for hydrogen pore formation.|2~

* Regardless of composition or defined solidification condition, a critical or threshold dissolved hydrogen level must be exceeded for hydrogen porosity to occur.

* At any cooling rate, a corresponding hydrogen content exists in which a certain residual pore volume fraction results for each alloy.

* For any specific cooling rate, pore volume fraction and pore size declines with decreased hydrogen content above the threshold value.

* For a given alloy and hydrogen content, pore volume fraction and pore size decrease with the cooling rate.

The critical or threshold value of hydrogen concentration also depends on pressure. Higher solidification pressures increase threshold limits, while casting under vacuum reduces them.

For reasons analogous to the formation of shrinkage voids, it also can be argued that alloy characteristics in addition to those directly related to solubility and precipitation are relevant. Alloy- and solidification-dependent expressions contained in derivations of D'Arcy's law for aluminum solidification--such as the number and tortuosity of liquid paths that exist in a solidifying dendritic network--must influence pore size and distribution. The higher the product of these factors, the smaller and more finely distributed the porosity.

Mathematical treatment proposes that:

* a permanent mold part should display a higher threshold limit than an equivalent sand or investment cast part;

* large solidification range alloys should display lower thresholds and a greater tendency for hydrogen porosity formation;

* grain-refined castings display finer pore size and lower void volume fraction than parts that are not grain refined;

* modified aluminum-silicon alloy castings with an equivalent dissolved hydrogen content display lower hydrogen porosity levels;

* increased solidification rate decreases void size, and;

* parts cast under substantially reduced pressure display greater tendencies for hydrogen porosity formation.

This partial list of conclusions generally is agreed upon by foundrymen with the exception of modification influences. Modifiers such as strontium and sodium may be the source of dissolved hydrogen. These elements may increase hydrogen solubility and alter physical characteristics, which influence hydrogen pore formation such as surface tension.

Vacuum Solidification Tests

Foundries use various methods of molten metal vacuum testing samples to quickly and simply determine acceptability of the processed melt for any casting application. The basis for this test is the relationship between hydrogen solubility and pressure. Since hydrogen solubility is related directly to the square root of pressure, decreased pressure reduces hydrogen solubility, increasing the tendency for bubble formation in the sample.

The results of the reduced pressure test can then predict in relative terms the tendency for formation of hydrogen voids in the cast part at ambient pressure. The pressure/solubility relationship is relevant when negative pressures associated with shrinkage develop in the solidifying structure.

As in the case of metallic and intermetallic phases, hydrogen precipitation may occur as a result of either heterogeneous or homogeneous nucleation. The most powerful nucleates for hydrogen precipitation are oxides--especially those that trap gaseous phases through turbulence in gating, pouring, melt handling and treatment.

In the presence of such nuclei, hydrogen emerges without surface tension resistance at even low hydrogen levels. In the absence of nucleating phases such as oxides and gaseous species, surface tension forces are generally strong enough that precipitation may be resisted at even relatively high hydrogen levels.

Predicting Porosity

Vacuum solidification tests provide the opportunity of assessing whether bubble formation could be expected to occur by heterogeneous or homogeneous nucleation.|3~

If immediate bubble formation takes place when a vacuum is applied to the molten sample, it is assumed that the melt is contaminated by oxides and contains an indeterminate amount of hydrogen. When no evidence of gas evolution appears in the solidifying sample until the last stages of solidification, oxides probably aren't present and hydrogen is most likely present at a moderate to relatively high concentration.

If no evolution of gas occurs at an appropriate pressure for prediction of porosity formation, the melt is probably free of oxides and the hydrogen contained in liquid solution is below the threshold value for precipitation under solidification conditions. It may also indicate that the test was improperly performed.

Sources of Hydrogen

Despite degassing efforts by producers, hydrogen is generally present in alloyed remelt ingot in some amount. Hydrogen solution may result from the dissociation of moisture in the atmosphere, which allows atomic hydrogen diffusion into the melt. Moisture in any form--contamination on tools, flux tubes, metallurgical metals, grain refiners and master alloys that may be added to the heat--can affect hydrogen content up to the applicable solubility limit.

Turbulence, whether in melt treatment or in pouring, rapidly accelerates the rate at which hydrogen from atmospheric moisture is absorbed and, coincidentally, substantially degrades melt quality after effective melt treatment for hydrogen removal.

Whenever the protective oxide surface of the melt is disturbed, an increase in hydrogen content can be expected. Magnesium-containing alloys display an amorphous magnesium oxide that is more permeable to the diffusion of hydrogen from the atmosphere to the melt. It follows that periods of high humidity increase the problems foundrymen face in dealing with hydrogen contamination. Magnesium-containing alloys are more susceptible to hydrogen absorption than others.

It also follows that melt degradation through careless drawing, pouring and poor gating design can more than neutralize the most effective melt preparation measures.

Gas Removal

Degassing by using inert or active gases reduces hydrogen concentrations by diffusing it into bubbles of the fluxing gas corresponding to the partial pressure of hydrogen in the fluxing gas.

Spinning rotor techniques have been developed to offer more effective flux gas usage, improved reaction efficiencies and shorter reaction times to achieve relatively low hydrogen levels. The use of active fluxing gases and/or filtration removes oxides that permit acceptable quality castings to be produced from metal with higher hydrogen contents.

More rapid solidification results in a decrease in the amount of hydrogen which precipitates from liquid solution and in the size of the voids that form.

References

1. E.L. Rooy, E.F. Fischer, "Control of Aluminum Casting Quality for Vacuum Solidification Tests," AFS Transactions, vol 76, pp 237-240 (1968).

2. Q.T. Fang, P.N. Anyalebechi, "Effects of Solidification Conditions on Hydrogen Porosity Formation in Aluminum Alloy Castings," TMS Annual Meeting of Light Metals, p 477 (1988).

3. K.J. Brondyke and P.D. Hess, "Interpretation of Vacuum Gas Test Results for Aluminum Alloys," Transactions of AIME, vol 230, p 1542 (1964).

Rooy is a retired metallurgist from the Aluminum Company of America.