Alloys or metallic compositions – Aluminum base – Titanium – zirconium – hafnium – vanadium – niobium – or tantalum...
Patent
1990-01-16
1992-04-14
Andrews, Melvyn J.
Alloys or metallic compositions
Aluminum base
Titanium, zirconium, hafnium, vanadium, niobium, or tantalum...
75680, 75683, C22C 102
Patent
active
051046164
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns both a method for manufacturing a master alloy to be added to aluminum melts in order to obtain a grain refining effect in cast products of aluminium and the resultant master alloy as such.
It is well known that when casting aluminium the molten metal must have certain sufficient crystal nuclei to obtain the desired grain size of the cast products. It is often necessary to increase the number of crystal nuclei through additions to the melt. This is usually achieved by adding to the melt a master alloy containing a very large number of nucleating particles, which disperse in the aluminium melt.
Titanium is the most common additive for grain refining of aluminium, and also a very efficient additive in this regard. At normal melting and casting temperatures, titanium concentrations above 0.2% form with aluminium the intermetallic phase Al.sub.3 Ti, although lower concentrations will also give a grain refining effect. In the production of a master alloy containing 1-15% Ti in aluminium, particles of Al.sub.3 Ti form together with some Ti in the solution, in accordance with generally accepted phase diagrams. It has also been discovered that an addition of boron to master alloys containing Ti will considerably improve the grain refining effect, especially when the Ti/B ratio is higher than 2.2. Boron forms particles of the type TiB.sub.2 which are assumed by some researchers to constitute crystal nucleation sites, while others claim that boron causes a decrease in the Al.sub.3 Ti dissolution rate in the aluminium melt, and thereby creates a more effective and durable grain refining action in which the intermetallic phase Al.sub.3 Ti is involved. This behaviour is considered to be further enhanced when the boride particles develop in intimate contact with the Al.sub.3 Ti-crystals (R. Kiusalaas "Relation between Phases present in Master Alloys of the Al-Ti-B type, Chemical Communications, University of Stockholm, 1986, No. 1).
It has been noted, however, that the addition of boron to aluminium causes certain disadvantages, due to the formation of hard boride particles; therefore, it is often desired to avoid boron in special-duty aluminiums, e.g. when producing material for beverage cans and for foil.
Therefore, efforts have been made to replace boron by other elements. Thus, from WO 86/05212 it can be seen that efforts have been made to produce a master alloy by introducing at least 0.1 and preferably around 1% of carbon as graphite powder into a titanium-rich aluminium melt to form a large number of TiC-particles, which by the inventors are considered to constitute active nucleation sites for aluminium. This basic idea was published by A. Cibula, J. Inst. Metals, 1949-50, 76, pp. 321-359, who, however, recognized the difficulties in introducing large amounts of carbon into aluminium, a problem which the inventors claim to have solved. The use of a master alloy containing .gtoreq.1% carbon in the form of TiC-particles does not, however, lessen the negative effect caused by the presence of hard particles in the final product.
Cibula made his observation in diluted melts (aluminium alloy melts, ready to cast) where the amount of transition elements, like titanium, was below the concentration at which an aluminide phase (in the actual case Al.sub.3 Ti) could form.
Some years later (1957 DE-B-10 27 407), the problem was solved by introducing carbon into aluminium melts via a gas stream containing hydrocarbons or a chlorinated hydro-carbon, on the basis of Cibula's observations.
The grain refining treatment was performed on diluted melts (ready to cast) at temperatures <800.degree. C. where the titanium concentration was below 0.2% Ti and hence TiAl.sub.3 -particles were not present. Carbon and/or boron was added in amounts such as to quantitatively transform all titanium in the melt to carbides and/or borides, in accordance with the object of the treatment. The use of N.sub.2 as a carrier gas was not considered to influence the intended reaction.
The present invention is based on the
REFERENCES:
patent: 4060411 (1977-11-01), Goto et al.
patent: 4812290 (1989-03-01), Sigworth
patent: 4842821 (1989-06-01), Banerji et al.
Backerud Lennart
Kiusalaas Rein
Klang Hans
Nagelvoort Edward H. K.
Noordegraaf Jan
Andrews Melvyn J.
Backerud Innovation AB
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