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The fy-Gem process involves the in-situ formation of heterogeneous boride nuclei in molten aluminum for the purpose of grain refinement. This three-year study has demonstrated grain refinement in various aluminum alloy types. While the wrought alloys will require further improvements to the process before it grain refines as well as existing grain...
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Introduction Relating Grain Size, Nucleant Particles and Solute Content Optimising Grain Refiner Additions Conclusions Acknowledgements
Grain refining of aluminum alloys using aluminum-base master alloy containing inoculant particles is practiced in the casthouses and ingot casting plants worldwide. The main reasons for grain refining are to reduce ingot cracking during casting and improve ingot fabricability during subsequent thermal mechanical processing. Industrial grain refinement practices are often established based on past experience and not necessarily optimized. Much progress has been made in recent years in understanding the mechanisms of grain refinement and the characteristics of the Al-Ti-B grain refiners. These advances now enable the establishment of grain refining practices for industrial applications based on a scientific basis. In this paper, an overview of current understanding of the mechanism of grain refinement will be presented. The basis of commercial grain refiners and the factors controlling the extent of grain refinement by the addition of a grain refiner under the normal casting conditions will be reviewed. The metal quality associated with TiB 2 particle agglomerates will be discussed. The trend of adapting commercial Al-Ti-C grain refiner for critical products applications and recent development of other grain refining techniques including gas-liquid in-situ reaction and acoustic cavitation will also be reviewed.
In the present research paper is suggested a new methodology to determine the growth restricting factor (Q) and grain size (GS) for various Al-alloys. The present method combines a thermodynamical component based on the liquidus behavior of each alloying element that is later incorporated into the well known growth restricting models for multi-component alloys. This approach that can be used to determine Q and/or GS based on the chemical composition and the slope of the liquidus temperature of any Al-alloy solidified in close to equilibrium conditions. This method can be modified further in order to assess the effect of cooling rate or thermomechanical processing on growth restricting factor and grain size. In the present paper is proposed a highly accurate (R2 = 0.99) and validated model for Al–Si alloys, but it can be modified for any other Al–X alloying system. The present method can be used for alloys with relatively high solute content and due to the use of the thermodynamics of liquidus this system considers the poisoning effects of single and multi-component alloying elements.
The effect of refiners containing Ti, B elements on the microstructure of purity aluminum has been studied in detail with optical microscopy, scanning electron microscopy (SEM) and energy dispersive analysis of X-ray (EDAX). It is shown that salts mixture containing 5Ti1B is the best refiner with the finest α-Al grain and more than 10h fading time. It is ascertained from the analysis of SEM–EDAX and thermodynamics analysis that the refining mechanism of salts mixture is mainly contributed to the heterogeneous nuclei of more fine TiAl3 particles dispersed in the melting, which come from the reaction between the salts and aluminum. While (Al,Ti)B2 has little or no refining effect, but it will reduce the size of TiAl3 since the TiAl3 nucleates and grows along the (Al,Ti)B2 particle. That is to say, B atom has also refining effect on the purity aluminum when it is added simultaneously with Ti atom.
Seven different aluminium alloys, 1050, 2014, 3003, 5083, 6060, 6061 and 6082, were chosen to develop a universal relationship between alloy content and grain refiner addition. It was found that the grain size was inversely related to the solute content as determined by the growth restriction factor. It was also found that the grain size was inversely proportional to the cube root of the number of nucleant particles present. This finding suggests that for all alloys, except those with poisoning elements, the grain size is related to the grain refiner content by the relationship d=a/3√%TiB2+b/Q, where d is the grain size, Q is the growth restriction factor and a and b are coefficients determined experimentally. This relationship can be used to minimise the cost of grain refiner additions, by decreasing the amount of grain refiner rod added in the launder and compensating for this by adding titanium to the alloying furnace. Using a grain refiner cost minimisation spreadsheet developed for this purpose it is found that assuming a current addition practice of 0.005% Ti as Al-5Ti-1B, savings of over $1/tonne can be made for some aluminium alloys, whilst achieving the same microstructure and staying within current alloy specifications. The alloys most likely to benefit from this procedure are those that contain a lower alloy content, such as 1000 series alloys and some 3000 and 6000 series alloys.
