Master Alloys for Aluminum Alloys: Part One

Sommario:

One of the most important goals of aluminum alloy production by casting is the refinement of grain size, because coarse grain size immediately reflects in lower property levels. In the case of fixed melting and casting conditions, this goal is usually achieved by different modification methods. These methods are conventionally based on the introduction of a small amount of special master alloys into a slightly overheated melt.

Grain refinement of aluminum provides a number of technical and economic advantages, including reduced ingot cracking, better ingot homogeneity, better mechanical deformation characteristics and improved mechanical properties. Grain refining elements, titanium and boron, were originally introduced into molten metal as refractory titanium alloy and as corrosive complex potassium metal fluoride salts. Use of these materials resulted in inconsistent performance and detrimental side effects, such as corrosion of furnace refractories, risk of inclusions and unpredictable grain refining response. These side effects and uncertainties were eliminated when master alloy companies together with aluminum producing companies developed a master alloy, which included aluminum, titanium and boron in precise quantities.

One of the most important goals of aluminum alloy production by casting is the refinement of grain size, because coarse grain size immediately reflects in lower property levels. In the case of fixed melting and casting conditions, this goal is usually achieved by different modification methods. These methods are conventionally based on the introduction of a small amount of special master alloys into a slightly overheated melt.

During crystallization, the nature and appearance of the master alloys play the main role in the grain size refinement:

  • element solubility in the aluminum solid solution, binary, or multicomponent compounds according to the stable and metastable phase stability diagrams,
  • surface properties (surface tension, adhesion between phases, adsorption, etc.),
  • intermetallide morphology,
  • their particle size distribution,
  • their distribution uniformity,
  • processing parameters (heating/cooling rates, temperature, and time).

The first issue remains the main one for the development and selection of master alloys for particular grain refinement or modifying effects. It is based on the stable phase diagrams, assessed theoretically and experimentally, as well as on the chemical thermodynamics of the alloy systems.

The basic idea of "master alloys" has its grounds in the fact that many alloying elements are very active (Sr, Ti, Mg) or are just unsuitable (Na, B) to be entered in a pure form into aluminum alloys. Also, the production of purified metals like strontium or sodium with the purpose of dissolving only a small amount of addition (0.01-0.2%) is obviously not a cost-effective method. Many active metals can be manufactured much more easily in their alloy form (Al-Ti, Al-Sr, Al-Mg) due to lower activity of the leading elements. The dissolution of the master alloy in the melt also proceeds under a different mechanism than that of pure metals.

The master alloys used most often for aluminum contain titanium and other transition metals. These master alloys have been extensively studied for many years and are available commercially from different manufacturers. It is necessary to note that the chemical composition of the master alloys is only one, but not the most important, parameter to characterize their efficiency as grain refiners.

Often, master alloys are compared on the basis of the intermetallide particle that already exist in the master alloy and act as seeding nuclei during crystallization of the melt, influencing, in this way, the grain growth. For example, for Al-Ti-B master alloys, the best modification effect was obtained with spheroid particles of TiAl3 of 300 μm and TiB2 < 3 μm.

In the case of Al-Si alloys, so-called primary silicon crystals as well as silicon-rich phases remain in the liquid even at high temperatures (800-1200°C). In this case, the morphology and the size distribution of silicon crystals will determine the final grain size of the alloy, and thus, modifiers should affect first the silicon rather than aluminum crystals.

Many master alloys (like Al-Sr) are used mainly for the purpose of modifying, and not primarily grain refinement. On the other hand, some additions, like sodium, do not eventually affect the crystallization process by forming some intermetallides -rather, they change surface tension, adsorption, and other similar properties locally, and in this way prevent or assist the crystallization of one or another phase type, its polytype, or compound.

There are also master alloys with combined effect, acting as both grain refiners and modifiers. For example, an Al-10% Sr-2% B alloy is used for this purpose.

Classification of Master Alloys

There is a variety of master alloys for aluminum and aluminum alloys, depending on the principal (leading) element. Most master alloys have a special color code and classification numbers provided by the Aluminum Association (AA) and the European Standardization Committee (CEN).

Master alloys are available in a variety of forms: waffle ingot, notched ingot, slab ingot, sheared ingot, button, splatter (flake), broken ingot, coiled rod, and cut rod, depending on the leading element, manufacturing procedure, and the purpose of the master alloy itself.

Besides "standard" or, better said, the most common master alloys, there are many experimentally developed master alloy types for a variety of applications. The majority of them seem to have no international standards or clear local regulations (e.g. factory developments for their own needs). For example, several master alloys with rare earth (RE) metals have been developed and tested in China for Al-Si and Al-Cu cast alloy grain refinement, etc.

In master alloy processing, the main issues are the liquidus surface of the system in question and the thermodynamics of the Al-M solutions. The "ideal" system should have:

  • optimal liquidus temperature (e.g. 50-100°C lower than that of the basic aluminum alloy),
  • low vapor pressure (in the case of magnesium and some non-metals),
  • reasonable activity of the alloying element (low activity helps to prepare the master alloy, higher activity helps in the reaction of the master alloy with the molten aluminum bath), and
  • suitable intermetallic compounds formed during either production or application of the master alloy.

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