Metal founding – Process – With measuring – testing – inspecting – or condition determination
Patent
1998-08-04
2000-06-13
Lin, Kuang Y.
Metal founding
Process
With measuring, testing, inspecting, or condition determination
164 571, 1641541, 420552, B22D 2720, C22C 2104
Patent
active
060736771
DESCRIPTION:
BRIEF SUMMARY
A new method is disclosed to control the addition levels that will give optimum grain refinement in aluminium-based alloys. The method consists of first calculating the grain growth index for the composition of the alloy under consideration, and then determining how much additional grain size affecting agents, e.g. titanium and/or boron must be added to obtain desired results.
The procedure also makes it possible to e.g. determine the best titanium to boron ratio for grain refinement. Optionally, the method can be further improved by establishing the crystal coherency point. An algorithm or formula, is proposed to calculate the optimum refinement, and methods of grain refinement using this algorithm is also disclosed.
BACKGROUND
Primary grain size in material produced by a casting process depends on the nucleation frequency and on the growth rate of the first crystals formed during the solidification process. To control the grain size in order to obtain coarse grains, certain elements or compounds are avoided, while other such additives are made in order to obtain a fine grain size. However, when it concerns grain refinement, a quick and reliable method to measure and to control the properties as cast of a certain melt before casting has so far been missing. As a result the additives are often added in amounts that are much larger than what is necessary. Apart from the drawback of the unnecessary high costs of additives, these large additions often lead to problems with large agglomerated particles when recycling the material. Hence, there is a need for a method for obtaining castings comprising small nucleating particles which uses a minimum of grain-modifying additives.
As mentioned above, the addition of nucleating particles to stimulate the formation of crystals upon solidification is well-known. Examples of suitable nucleating particles are boride or carbide particles (aluminium), zirconium (magnesium) and TiC-particles (steel) etc. In many cases, it is also possible to control the growth parameter of crystals in solidifying metal melts.
As already mentioned, the present invention relates to optimising the grain refinement of aluminium alloys. It is based upon controlled additions of agents promoting grain refinement of aluminium, such as the elements Ti, Zr, B, N and C, mostly in the form of master alloys, which are added to the molten metal.
The master alloys are usually added in the form of small buttons or ingots, or when continuous additions are desirable (as in direct chill casting of billets or slabs) the addition is made by feeding a rod into the flowing melt stream. Various master alloy compositions and methods of manufacture and use have been proposed. (See, for example, patents U.S. Pat. Nos. 3,785,807, 3,933,476, 4,298,408, 4,612,073, 4,748,001, 4,812,290 and 5,055,256).
It should be stressed that all aluminium-titanium-boron (Al--Ti--B) master alloys, regardless of their composition, are a mixtures of two crystals interspersed in a matrix of solidified aluminium. These two phases are titanium diboride (Al,TiB.sub.2) and titanium aluminide (TiAl.sub.3). The whole range of boride particles from AlB.sub.2 -TiB.sub.2 may form during production of master alloys.
In alloys with excess Ti compared to what is needed to form TiB.sub.2 most boride particles have a composition close to TiB.sub.2. For the sake of simplicity this phase is considered in the following text.
Virtually all of the titanium and boron in master alloy grain refiners are contained in these crystals, because the solubility of boron and titanium in solid aluminium at room temperature is very small. This means that changing the master alloy composition only changes the relative proportion of these two crystals which are added to affect the grain refinement.
In spite of this simple fact, there has been an enormous amount of controversy, and disagreement about what Ti to B ratio the master alloy should contain for best grain refinement. This question was considered at some length in U.S. Pat. No. 4,612,073 and also in the paper
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Backerud Lennart
Johnsson Mats
Sigworth Geoffrey
Lin Kuang Y.
Opticast AB
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