Aluminium casting alloy

This invention relates to an aluminium casting alloy and more particularly to a hypoeutectic aluminium silicon alloy for use in shape casting.

Aluminium silicon alloys containing less than about 12% silicon are referred to as hypoeutectic alloys. In addition to alleviating the formation of defects such as hot tearing and porosity due to the shrinkage and gases, and also to minimising the presence of inclusions, two very significant ways in which the strength, ductility and performance of an aluminium casting alloy can be improved are through grain refinement of the primary aluminium phase and modification of the eutectic Al+Si structure. Upon cooling of molten hypoeutectic alloys, aluminium crystals form first through nucleation and growth, and later the second important event is the formation of the Al+Si eutectic mixture. It is understood that the (Al+Si) eutectic is an irregular and coupled eutectic, and it grows in the form of eutectic colonies, with silicon radiating from a single nucleating point and the tips of the silicon plates grow ahead of the aluminium, leading into the cooling liquid. It has been demonstrated that the (Al+Si) eutectic can nucleate on existing aluminium dendrites or substrate particles in the melt such as AlP, AlSiNa, Al 2 Si 2 Sr and other unidentified particles.

Grain refinement of primary aluminium is simply the process of adding nuclei and solutes with a strong constitutional undercooling effect to the melt prior to pouring such that upon the freezing process (i.e. solidification) the casting will expedite a refined microstructure with small equiaxed aluminium crystals. Grain refinement of primary aluminium crystals is accomplished generally by adding master alloys containing titanium and/or boron to the melt.

Eutectic modification on the other hand is the process of changing the morphology of the cast structure and in particular, that portion of the cast structure which freezes as a eutectic mixture of aluminium and silicon towards the end of solidification. Unmodified hypoeutectic aluminium silicon alloys are relatively non ductile or brittle and consist of primary aluminium dendrites with eutectic composed of coarse acicular or plate-like silicon phase in an aluminium matrix. The morphology of these silicon rich crystals in the eutectic mixture can be modified by small additions of elements such as sodium, strontium or antimony to the melt to alter the eutectic structure and to yield silicon rich crystals having fine, fibrous structure. However the addition of modifiers has been found to neutralise the potent nuclei for the eutectic colonies in the melts resulting in a significant increase of the undercooling in eutectic nucleation and depression of the eutectic growth temperature. This in turn increases the eutectic grain size and reduces nucleation frequency in forming modified aluminium silicon alloys. Furthermore, modification of the aluminium silicon alloys has also been reported to cause pore redistribution and an increase in casting porosity.

It is an object of the present invention to provide a hypoeutectic aluminium silicon alloy having an improved microstructure with good castability and improved porosity characteristics.

Accordingly, in one aspect the invention provides a hypoeutectic aluminium silicon alloy wherein the eutectic is modified by a master alloy consisting of an element selected from strontium, sodium, antimony, barium, calcium, yttrium, lithium, potassium, ytterbium and rare earth elements such as europium, mischmetal, such as lanthanum, cerium, praseodynium and neodynium and further refined by the addition of a master alloy containing nucleant particles for the eutectic colonies. It is preferable that the nucleant particles are selected from the group consisting of TiSi x , MnC x , AlP, AlB x , and CrB x which are added as particles or formed in situ in the melts. These nucleant particles promote a small eutectic grain size without altering fine fibrous silicon crystal structure.

In a preferred embodiment of the invention, the nucleant particles have a particle size of less than 100 μm and preferably less than 10 μm. The nucleant particles are preferably added to the melt by way of a master alloy containing the nucleant particles or formed in situ in the melts through preferred reactions, such as reactions between melt and master alloys.

In one aspect of the invention, there is provided a method of forming a hypoeutectic aluminium silicon alloy including the steps of:

forming an aluminium melt including adding greater than zero and less than about 12 wt % silicon, 20-3000 ppm, preferably 150-3000 ppm of a eutectic modifying element selected from the group consisting of strontium, sodium, antimony, barium, calcium, yttrium, lithium, potassium, ytterbium, europium and mischmetal, such as lanthanum, cerium, praseodynium and neodynium, more preferably 20-300 ppm when the eutectic modifying element is sodium, 50-300 ppm when the eutectic modifying element is strontium, 1000-3000 ppm when the eutectic modifying element is antimony; and either adding nucleant particles and/or causing nucleant particles to be formed in the melt, the nucleant particles being selected from the group of TiSi x , MnC x , AlP, AlB x and CrB x where x is an integer, 1 or 2.

The applicant has found that by the addition or in situ formation of these nucleant particles into a hypoeutectic aluminium silicon alloy modified by one of the elements referred to above, a modified aluminium silicon alloy having reduced porosity is produced. Furthermore, fine silicon rich crystals having a fibrous eutectic structure are produced.

In the case of CrB x , the addition rate of these particles to the melt was preferably greater than 2 wt %.

The applicant has found that the above mentioned colony refining additions are not affected by the presence of eutectic modification additions or vice versa if the addition conditions are controlled properly. Hence the TiSi x , MnC x , AlP, CrB x and AlB x present are able to act as nucleant particles for eutectic colonies.

In another aspect of the invention, there is provided an aluminium silicon alloy including:

greater than zero and less than about 12 wt % silicon 20-3000 ppm, preferably 150-3000 ppm of eutectic modifying element selected from the group consisting of strontium, sodium, antimony, barium, calcium, yttrium, lithium, potassium, ytterbium, europium and mischmetal such as lanthanum, cerium, praseodynium and neodynium preferably 20-3000 ppm when the eutectic modifying element is sodium; and balance aluminium; wherein eutectic grains are formed around nucleant particles being selected from the group consisting of TiSi x , MnC x , AlP, AlB x and CrB x where x is an integer of 1 or 2.

In a further aspect, there is provided use of a hypoeutectic alloy to produce an as cast material, the alloy consisting essentially of:

less than about 12 wt % silicon, 20-3000 ppm, preferably 150-3000 ppm of a eutectic modifying element selected from the group consisting of: strontium, sodium, antimony, barium, calcium, yttrium, lithium, potassium, ytterbium, europium and mischmetal, such as lanthanum, cerium, praseodynium and neodynium, more preferably 20-300 ppm when the eutectic modifying element is sodium, 50-300 ppm when the eutectic modifying element is strontium, 1000-3000 ppm when the eutectic modifying element is antimony; and balance aluminium; wherein eutectic grains are formed around nucleant particles being selected from the group consisting of TiSi x , MnC x , AlP, AlB x and CrB x where x is an integer of 1 or 2.

FIGS. 1( a )- 1 ( d ) show micrographs of quenched and fully solidified samples. FIG. 1( a ) is the base alloy, 1 ( b ) is the base alloy with the addition of 300 ppm Sr, 1 ( c ) is the base alloy modified with Sr and with 2% CrB x addition with 1 ( d ) the micrograph of a section of FIG. 1( c ). FIG. 1( f ) is the macrograph of base, modified with Sr and 4% CrB x addition and FIG. 1( e ) is the micrograph of a section of FIG. 1( f );

FIG. 2 illustrates the microstructures of master alloy additives of (a) CrB, (b) MnC and (c) TiSi;

FIG. 3 are macrographs of quenched samples and micrographs of fully solidified samples of different levels of phosphorus addition to Sr modified Al 10% Si alloys;

FIG. 4 are macrographs of Tatur castings cast from melts of unmodified and Sr modified with varying phosphorus addition levels;

FIG. 5 illustrates cooling curves of the Sr modified melts with varying P additions;

FIGS. 6( a )- 6 ( d ) are macrographs of samples quenched from different addition levels of B as Al-3% B to Sr modified alloy.

FIGS. 7( a )- 7 ( d ) are micrographs of the fully solidified samples of those shown in FIGS. 6( a )- 6 ( d ).

FIG. 8 is cooling curves measured of the samples shown in FIGS. 6( a )- 6 ( d ) and 7 ( a )- 7 ( d );

FIG. 9 is a schematic diagram illustrating the effect of addition of CrB x , P and AlB x on nucleation frequency and degree of modification.

It will be understood that the invention disclosed and defined in this specification extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. All of these different combinations constitute various alternative aspects of the invention.

An Al-10% Si-0.35% Mg alloy unless otherwise specified, was selected as a base alloy and it was prepared from commercial purity aluminium, silicon and magnesium in an induction furnace. After being held at about 750° C. for 10 minutes for homogenization, the base alloy melt was transferred to an electric resistance furnace, which was held at 730° C. After reaching thermal equilibrium, the melt was modified first by the addition of a refining element such as Sr, to neutralize the potent nuclei present in the melt. Weighted trial master alloy was then added to introduce or form new nuclei in situ in the melt. The melt was stirred twice after each addition. All additives were dried in an oven at 300° C. and then wrapped in aluminium foil before addition to ensure that they dissolved properly and evenly throughout the melt.

Thermal analysis and quenching trials were usually performed prior to and after eutectic modification as well as after addition of trial master alloys. Thermal analysis was performed first using a preheated graphite crucible and a centrally located, stainless steel-sheathed Type N thermocouple to help develop a strategy for the following quenching trials. The cooling rate for thermal analysis was about 1° C./s just prior to nucleation of the first solid. Two interrupted quenching tests, corresponding to the beginning and middle stages of eutectic solidification, were then carried out using a special stainless steel quenching cup sitting either in an insulation brick or in the air.

Samples for chemical analysis were also collected after each addition and prepared according to Australian standard (AS 2612) and analysed using a bench top spark optical emission spectrometer. For microstructural observation, the quenched samples were sectioned vertically along the thermocouple line while fully solidified TA samples were sectioned horizontally at the level of the thermocouple. Metallographic samples were mounted in resin and prepared using a standard procedure with a final polishing stage of 0.05 μm colloidal silica suspension. The macrographs were taken from etched samples using a high-resolution digital camera under indirect illumination conditions. The micrographs were taken in the median region of the section, 10 mm away from the bottom of the unetched samples.

Potential Nucleating Particles and Trial Master Alloys

It is understood that the (Al+Si) eutectic is an irregular and coupled eutectic, and it grows in the form of eutectic colonies, with silicon radiating from a single nucleating point and the tips of the silicon plates grow ahead of the aluminium, leading into the cooling liquid. It has been demonstrated that the (Al+Si) eutectic can nucleate on existing aluminium dendrites or substrate particles in the melt.

Since Si is generally believed to be the leading phase in (Al+Si) eutectic, the nucleation of the eutectic therefore comes to the nucleation of Si. Based on our knowledge of known nucleants and lattice mismatch, a list of potential nucleating particles for the eutectic colonies were compiled. Three particles including TiSi x , CrB x , MnC x were further selected from the list, and trial master alloys containing these three particles have been manufactured. During the implementation of this project, two more particles, AlB x , and AlP, have been further included in the test. Table 1 lists all the potential nucleating particles tested and the corresponding trial master alloys. While most nucleating particles are assumed to be present in the trial master alloys, some others have to form in-situ in the melts after addition of trial master alloys.