Method for grain refining of aluminium

Abstract: A method for grain refining aluminium and aluminium alloys in which a siliconboron alloy containing between 0.01 to 4.0% by weight of boron is added to molten aluminium or aluminium alloy in such an amount that the resulting melt of aluminium or aluminium alloy contains at least 50 ppm boron. The invention also relates to a grain refining alloy for aluminium and aluminium alloys in the form of a siliconboron alloy containing between 0.01 and 4.0% by weight of boron.

Description: The present invention relates to a method for the grain refining of aluminium and aluminium alloys. The grain structure of a metal or an alloy determines a number of important properties in the product. The grain refining of aluminium and aluminium based alloys is an example of how a structure consisting of small equiaxial grains has a number of advantages compared to a structure comprising larger grains. The most important of these are: Improved castability due to a more efficient flow of metal. Improved mechanical properties. Improved machinability. Improved surface quality. The grain size varies with the chemical composition of the alloy and with the casting method. The casting method determines a number of important factors, such as cooling rate, casting temperature, temperature gradient and the state of mixture in the melt both before and during solidification. It is not always possible to control or optimise these factors and it has therefore been found necessary to add grain refiners to the molten metal prior to casting. The addition of grain refiners "catalyses" the nucleation of aluminium crystals. Commercially available grain refiners contain in addition to aluminium, titanium and/or boron. By changing the composition of the grain refining alloys, it is possible to obtain significant differences in their ability to effect grain refining. The concept of grain refining can be divided into two phenomena; nucleation and the growth of crystals to a limited size. Grain refining alloys contain aluminium with titanium and/or boron in solid solution and particles such as TiAl 3 and/or TiB 2 /AlB 2 . It is generally accepted thai-grain refining is due to the heterogeneous nucleation of aluminium crystals on particles supplied through the grain refining alloy. It is, however, not known if the active particles are TiAl 3 or TiB 2 . The above described method for grain refining has, however, the disadvantages of an incubation time and the so-called fading effect. Incubation time means that the molten aluminium must be kept in a molten state for some time after the addition of the grain refiner in order to obtain the optimum effect, while the fading effect means that the grain refining effect decreases with the holding time. It is believed that the fading effect is caused by particles settling in the melt. A serious problem of grain refining of aluminium alloys which are to be used for rolling products is agglomeration of TiB 2 -particles, so-called clustering, which can cause holes in the foil. In addition, non-homogeneous grain structures have been observed, both in regard to grain size and crystal structure. It is an object of the present invention to provide a method for grain refining by which aluminium and aluminium alloys with a very small grain size can be obtained and in which the problem of fading is substantially reduced. Accordingly, the present invention provides a method for grain refining aluminium and aluminium alloys in which a solid siliconboron alloy containing from 0.01 to 4.0% by weight of boron is added to molten aluminium or aluminium alloy in such an amount that the resulting melt of aluminium or aluminium alloy contains at least 50 ppm boron. According to a preferred embodiment of the method, a solid siliconboron alloy containing between 0.02 and 1% of weight of boron is added to the molten aluminium or aluminium alloy. The siliconboron alloy is preferably added in such an amount that the resulting melt of aluminium or aluminium alloy contains at least 100 ppm boron. In has surprisingly been found that the method according to the present invention results in very small grains at a very low boron content. in aluminium and aluminium alloys while at the same time, the known fading effect is virtually non-existent. It is believe that the surprisingly good effect achieved by the present invention is due to the fact that the mechanism of the grain refining by the method of the present invention is different from the mechanism which is effective when using known grain refiners consisting of aluminium with titanium and/or boron. While the grain refining effect of these known grain refiners as mentioned above is believed to be caused by presence of particles of the type TiAl 3 and/or TiB 2 /AlB 2 in the grain refiners which are added to the aluminium melt and which particles causes nucleation in the melt, it has been found that by the preferred grain refiner and the method according to the present invention, the addition of siliconboron alloy causes solution of boron atoms in the aluminium melt. First by cooling the aluminium melt, AlB 2 particles are formed in situ in the melt. The AlB 2 particles have a lower density than TiB 2 and TiAl 3 particles and have therefore a lower tendency of settling in the aluminium melt. This can explain the fact that the well known fading effect, even after long holding times, does not occur in the method of the present invention. By the method of the present invention, aluminium alloys have been obtained with extremely small equiaxial grains. Thus for an AlSi-alloy containing 9. 6% by weight of Si, grain sizes of 200-300 µ m have been obtained at a boron content in the melt of 160 ppm. By grain refining the same alloy using a conventional aluminium based grain refining alloy containing 6% by weight of titanium, grain sizes of about 1800µm were obtained at a Ti-content of 0.10% by weight, and about 1300µm at a Ti-content of 0.2% by weight. As the preferred grain refining alloy used in the method according to the present invention contains silicon as the dominating component, the method of the present invention cannot be used for aluminium and aluminium alloys where the silicon content is very low. The grain refining alloy used can thus in practice not be used for aluminium and aluminium alloys which after grain refining contain less than 0.1% by weight of silicon. EXAMPLE 1 A number of 3kg high purity aluminium specimens were placed in salamander crucibles and melted in a resistance furnace. The furnace temperature was kept constant at 800°C. A siliconboron alloy containing about 1% by weight of boron in solid solution was added to four of the aluminium melts in such an amount that the final alloys contained about 9.6% by weight of Si and had boron content of 110 ppm, 160ppm, 550ppm and 680ppm respectively. For comparison purposes a melt of 3kg high purity aluminium was alloyed with high purity silicon to provide an alloy containing about 9.6% by weight of silicon. The high purity silicon used did not contain boron. The melts were cast at a constant cooling rate of 1K per second and the nucleation temperature and the growth temperature for the aluminium crystals were calculated from the cooling curves. The grain sizes for the cast specimens were measured according to the intercept method (D(TA)). In addition the grain size was measured according to Aluminium Associations: "Standard Test Procedure for Aluminium Grain Refiners" (D(AA)). According to this standard, the cooling rate is about 5K per second. The results are shown in Figure 1 and 2. Figure 1 shows the cooling curves for the melt containing 160ppm boron and for the melt that did not contain boron, and Figure 2 shows the nucleation temperature, Tn, the crystal growth temperature, Tg, and the grain size as a function of boron content in the aluminium alloys. From Figure 1 it can be seen that the start of the solidification process is very different in the case of the alloy which had been treated by the method of the present invention, compared to the Al-Si alloy without boron addition. Thus the Al-Si alloy without boron addition shows a supercooling before recalescence up to the crystal growth temperature. In contrast to this, the cooling curve for the alloy which had been grain refined according to the present invention flattens out at a substantially constant temperature level immediately after nucleation. From Figure 2 it can be seen that for the specimens containing boron, the nucleation temperature and crystal growth temperature seem to be independent of the boron concentration above a certain minimum value. Figure 2 further shows that the grain sizes obtained by the addition of the grain refiner according to the present invention are very small and in the range of 300µm. In can also be seen from Figure 2 that the grain size is independent of the boron content as long as the boron content is kept above a certain minimum value. Finally, Figure 2 shows that the cooling rate does not substantially affect the grain size for the aluminium alloys which have been grain refined according to the present invention. In order to investigate the fading effect, additional melts of the above mentioned compositions were cast 1 hour, 2 hours, 2.5 hours, 3.4 hours, 4 hours and 6.5 hours after the addition of the grain refiner. It was found that the nucleation and crystal growth temperature were not affected by the holding time. This shows that the fading effect is eliminated by use of the grain refiner according to the present invention. EXAMPLE 2 Two melts of 3kg high purity aluminium were produced in the same way as described in Example 1. A siliconboron alloy containing about 1% by weight of boron was added to the two melts in such an amount that the final alloys contained 1.1% by weight of silicon and 100 ppm boron. The melts were kept at 800°C for 0.5 and 1 hour respectively, whereafter the alloys were cast at a cooling rate of 1K per second. The cooling curves for the two alloys show that the supercooling before formation of aluminium crystals was about 0.5K which is substantially less than what is expected for such an alloy without boron content. This shows that the method and the grain refiner according to the present invention also is effective for aluminium having a relatively low silicon content. The grain size for the solidified specimens was measured according to the intercept method. The average grain size was measured to be about 900µm which is substantially less than what would be expected for an A1-1.1Si alloy which has not been grain refined. Microstructure investigation of the two specimens showed that a number of the aluminium crystals contained a primary AlB 2 particle at the centre.

Claims: A method for grain refining aluminium and aluminium alloys, characterised in that a solid siliconboron alloy containing from 0.01 to 4.0% by weight of boron is added to molten aluminium or aluminium alloy in such an amount that the resulting melt of aluminium or aluminium alloy contains at least 50 ppm boron. A method as claimed in Claim 1, characterised in that the solid siliconboron alloy added to the molten aluminium or aluminium alloy contains between 0.02 and 1% by weight of boron. A method as claimed in Claim 1 or Claim 2, characterised in that the solid siliconboron alloy is added in such an amount that the resulting melt of aluminium or aluminium alloy contains at least 100 ppm boron.

Inventors: ARNBERG LARS, HALVORSEN GUNNAR, TONDEL PER ARNE, ARNBERG, LARS, HALVORSEN, GUNNAR, TONDEL, PER ARNE
Applicants: ELKEM ALUMINIUM, ELKEM ALUMINIUM ANS
Application year: 1992
Main classification: C22C1
IPC codes: C22C21, C22C1, B22D21, B22D1