Technical Field Invention relates to a method which ensures that grain refinement can be made more efficiently in aluminum cast alloys.

The invention particularly relates to a method which ensures grain refinement of aluminum alloys by adding sufficient amount of strontium (Sr) and boron (B) into the ladle containing aluminum alloy while the rotor is rotating after flux effect neutralized in degassing process. Prior Art

In present practice, Al-Ti-B master alloys are commonly used for grain refinement in Al-Si cast alloys. Nucleating mechanism in alloy is provided by TiB ₂ and AI ₃ Ti compounds as a result of using Al-Ti-B master alloys.

However, since Si element easily reacts with Ti element, TiB ₂ and AI ₃ Ti loose the efficiency of grain refinement after a while and joins in main matrix by forming a compound with Si. This limits the refinement of the grain size of Aluminum Alloys to a certain level.

Search for higher mechanical durability and more competitive designs require the necessity for having a finer grain structure at macrostructure level in Aluminum Alloys. This causes a finer grain size which is formed by AIB ₂ and SrB ₆ compounds. SrB ₆ has positive effect on the modification of eutectic phase as well. Mechanical and metallurgical characteristics of the material are improved through grain structure with finer grain size and well-modified eutectic phase. Higher number of grains and finer size of grain particularly cause the elongation values to be improved. The other positive effect is that the impact strength of alloy increases with the increase in toughness of the material. Improvements occur in mechanical properties.

Grain size in aluminum alloy wheels manufactured by low-pressure die casting method, hypoeutectic aluminum alloys, on present practices average grain size varies between 50Όμηι-1000μΓη. The grain size may vary depending on cooling speed in the range specified above and type of grain refining master alloys used. Grain size on commercial vehicles aluminum alloy wheels in the market are as mentioned above. Minimum grain size which can be formed by present commercial grain refiners, aluminum alloy wheels which are produced with low pressure die casting, metallurgical and mechanical properties have reached the limit in current design inputs and section thickness. Additionally, current grain size does not allow designs with higher strength to be made or finer and complex designs to be made. Present average grain size which is in the range of 500 pm - 1000 pm causes the porosities to be present on the product with larger and segregated form.

Also, fluidity in current grain size may be insufficient and cause feeding inadequacy in thin sections. In such cases, cooling speed of concerned section may only be reduced and thus, section is completely filled and fed. But finally, because of the lower solidification rate, grain size increase and mechanical properties decrease. Manufacturing method of aluminum alloy wheels by present low-pressure casting and grain refining activities carried out by Al-B. However eutectic modification does not occur due to the loss of free Sr. Considering the examinations conducted, boron (B) element added for grain refining combines necessary strontium (Sr) element for modification and forms SrB ₆ . Since Sr is essential for eutectic modification in Aluminum Alloys, absence of Sr results in unmodified eutectic.

In patent researches carried out pertaining to grain refinement methods applied in aluminum alloys, a patent no: EP1443122B1 has been found. The invention relates to a pressure casting alloy comprised of a suitable aluminum alloy for pressure casting of structure parts having high expansion character in case of casting, in addition to aluminum and unpreventable contamination, silicon with a weight percentage of 8.5 and 10.5, manganese with a weight percentage of 0.3 and 0.8, magnesium with maximum weight percentage of 0.06, iron with maximum weight percentage of 0.15, copper with maximum weight percentage of 0.03, zinc with maximum weight percentage of 0.10, titanium with maximum weight percentage of 0.15, molybdenum with a weight percentage of 0.05 and 0.5 and 30 and 300 ppm strontium for continuous enrichment or 5 and 30 ppm sodium and/or 1 and 30 ppm calcium.

According to another patent no: AT511397B1 , the invention relates to a method for grain refinement and structure modification for Al-Mg-Si alloys used in sand casting containing Ca, Na and Sr less than 0.001%, 5.0-10.0% Mg, 1.0-5.0% Si, 0.001-1.0% n, 0.01-0.2% Ti as main alloy or hard casting.

Inventions briefly summarized above are similar to the present art in terms of grain refining processes. Standard addition elements are added to aluminum master alloys. It contains the disadvantages in present art. In conclusion, improvements are made particularly in production of aluminum alloy wheels in grain refinement methods applied in production of aluminum alloy and new configurations are required for elimination of the abovementioned disadvantages and a solution to the present art. Explanation of the invention

Objective of the invention is to introduce an embodiment having different technical properties introducing a new development in this field considering the applications used in present art.

Another objective of the invention is to obtain an embodiment having finer grain size in aluminum alloys with Sr and B elements. Another objective of the invention is that grain sizes, obtained in production of aluminum alloy wheels by low pressure casting method, are in the range of 50μιη-5υΌμηι.

Another objective of the invention is that aluminum alloy is formed in homogeneous macrostructure.

Another objective of the invention is reduction of defects in casting such as shrinkage and gas porosity in alloy.

Another objective of the invention is increase in fluidity during casting process and capability of cast feeding.

Another objective of the invention is filling narrow sections easier during casting process.

Another objective of the invention is reduction of shrinkage-distortion problems depending on contraction during thermal process.

Another objective of the invention is to reduce the number of scrap ratio in production.

Another objective of the invention is to be able to reduce heat treatment process time by increase in surface area of grain boundaries due to grain refining; in other words, increase in the number of grains in per unit volume. Another objective of the invention is to obtain improvements in energy consumption and production period regarding to reduction in heat treatment process time.

Another objective of the invention is increase in mechanical properties of final product and accordingly, improvement and moderation in design as a result of finer grain structure of aluminum alloys. Another objective of the invention is to obtain commercially successful product with higher mechanical properties according to all these improvements.

Another objective of the invention is reducing the amount of additive elements for the aluminum alloy grain refinement up to 1/5-1/7 ratio. For the purpose achieving the mentioned objectives above; the invention relates to a method which ensures refining of grain structure of aluminum alloys. It is comprised of process steps for grain refining which sufficient amount of strontium (Sr) is added to the ladle containing aluminum alloy and boron (B) is added to said ladle after flux influence in degassing process is completed while the rotor is still rotating. Additionally, this method follows the steps in which total titanium content of aluminum alloy is controlled under 0.005% by weight, SrB ₆ is formed by binding of boron(B) and strontium (Sr) in aluminum alloy at the rate of 1 :1.35 by weight, free strontium (Sr) at the rate of 0.015% or more by weight remain in aluminum alloy for eutectic modification following SrB ₆ formation.

Boron (B) is added to the aluminum alloy as AIB compound. Strontium (Sr) is added to the aluminum alloy as AlSr compound.

Detailed Explanation of the Invention

Invention relates to a method which ensures refining of grain structure of aluminum casting alloys. It comprises the processes in which sufficient amount of strontium (Sr) is added to the ladle containing aluminum alloy and boron (B) is added to the ladle after flux effect in degassing process is finished while the rotor continues rotating for grain refining.

This method follows the steps in which total titanium content of aluminum alloy is controlled under 0.005% by weight, SrB ₆ is formed by binding of boron(B) and strontium (Sr) in aluminum alloy at the rate of 1 :1.35 by weight, free strontium (Sr) at the rate of 0.015% or more by weight remain in aluminum alloy for eutectic modification following SrB ₆ formation. Boron (B) is added to the aluminum alloy as AIB compound. Strontium (Sr) is added to the aluminum alloy as AlSr compound.

In SrB ₆ compound _, nearly 4ppm Sr binds 3 ppm Boron. This is equivalent to 1 :1.35 mass ratio. When Sr is added at a rate that is more than enough to saturate the existing B to Sr, the remaining free Sr yields the eutectic modification while the combined SrB ₆ provides grain refining.

Preferably AlSi7Mg is used as common aluminium alloy in wheel manufacturing. However, this process is likely to be successful in Al-Si, Al-Cu and Al-Mg alloys too. Prior to degassing, sufficient amount of AlSr is added to the ladle containing aluminum alloy and B is added during degassing period after the flux is activated completely. Since it is important to have an even distribution of B in the alloy it is important to keep the degassing rotor running for a required time after the B addition. The Boron amount in the cast piece can be examined via spectrometer to have a better understanding of SrB ₆ amount in the alloy but the obtained value comprises also AIB ₂ and TiB ₂ phases. Thus in order to achieve a successful grain refining and successor eutectic modification; minimum 35ppm B and 150 ppm Sr should be measured in any section of cast piece. Since the B affinity to Ti is higher than B affinity to Sr, any existing Ti element in the master alloy leads to form TiB _2. Although TiB ₂ is known to be one of the existing grain refining methods in aluminum alloys, the SrB6 formation is reduced significantly by the existence of Ti in the master alloy resulting loss in grain refining effectiveness. Therefore rather Ti free master alloys or master alloys containing Ti below 50ppm should be preferred to have an accomplished grain refining by SrB6.

This method can be used in aluminum alloys which includes Titanium element by an appropriate holding ladle and mixing system.

AlSr is added to the ladle beforehand and then, AIB is added after flux effect passes in degassing process while rotor is rotating. Afterwards, this ladle is transferred to casting bench.

Desired fine grained and modified structure can be maintained in a single charge. Totally, Xppm B + 1.35XppmSr + 100-200 ppm Sr (for modification) is added to the ladle.