Grain Refinement and Modification of Al alloys

Research Scholars: A.K. Prasada Rao, G.S. Vinod Kumar, N. Subba Reddy, K. Venkateswarulu, S.A.Kori, K.V.S. Prasad and A. Arjuna Rao

 

Grain refinement has long been an active area of interest for our group. Our studies have been focused on a number of Al alloys, ranging from Al-Si to Al-Cu. The poisoning and fading phenomena during grain refinement have also been investigated.

Master alloys of Al-B, Al-Ti and Al-TiB have been prepared by reaction of molten Al with potassium salts and used for our studies. The effect of silicon addition on modification was also explored and the synergistic effects of modification and grain refinement on mechanical properties and microstructural characteristics were assessed.

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Our work has led to an understanding of poisoning of the fading phenomena observed during grain refinement. This proved to be very useful in designing B rich master alloys for effective grain refinement of Al-Si alloys with excess (>2%) silicon and other methods of counteracting the poisoning phenomena. In addition to the experimental work being carried out, we have also developed a model for estimating dissolution time of TiAl3particles in liquid aluminum.

 

The work carried out in our group with regards to grain refinement has tremendous commercial prospects. We have developed a highly efficient process for for the Preparation of Master Alloys for the Grain Refinement of Al and its alloys. The technology of manufacture of grain refiners has already been successfully transferred to Saru Aikoh Chemicals Pvt. Ltd. in 1999. Another commercial success story involves the development of Modifiner, A master alloy that can lead to simaltaneous grain refinement and modification of Al-Si alloys.

Using artificial neural network modeling the best combination of Ti and B in Al-Ti-B master alloy for achieving effective grain refinement for Al and Al-Si alloys has been identified.

 

The top figure shows the macrostructures of Aluminum after grain refinement of Al-5Ti-B master alloy

Figure in second shows the microstructure of Al-7Si alloy without grain refiner and modifier

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Figure at third shows the microstructure of Al-7Si alloy with grain refiner and modifier

 

Figure at bottom shows Neural network modeling of grain refiner for Al-7Si alloy

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Fast-acting and Long-lasting refiners for α-Al and modification of eutectic silicon by combined addition of Al-5Ti-2C (grain-refiner), Sr and Sb (modifiers)

It is well known that modification effect of Sr fades on longer melt holding after inoculation. It is also known that Sb results in good modification of eutectic silicon on longer melt holding. We have found that simultaneous addition of Al-5Ti-2C (grain-refiner), Sr and Sb to molten Al-7Si alloy results in fast acting(5min holding) and long lasting(120min holding) refinement ofa-Al and eutectic silicon. Microstructures shown in the inset in Fig. (a) and (b) represent the eutectic silicon morphology at higher magnification. It is evident from the figures that Sr is active (indicated from fibrous morphology of the eutectic silicon) at short holding time while Sb is dominant (indicated from the lamellar morphology of the eutectic silicon) at longer holding time, although both Sr and Sb are present in the melt for all holding times. It is also seen that the primary a-Al size is also refined due to the presence of Al-5Ti-2C grain refiner.

Without inoculation (500X)      After inoculation (5min)             After inoculation (120min)


A novel master alloy (inoculant) synthesized for simultaneous grain refinement and modification of hypo-eutectic Al-Si alloy (Si~7%)

A new Al-Ti-C-Sr inoculant master alloy developed resulted in simultaneous grain refinement and modification of A356 alloy. The figure below reveals the macrostructure of the A356 alloy before inoculation (designated as 0) and after inoculation (5, 30, 60, 120min holding times after melt inoculation). SEM photo micrographs shown below demonstrate the extent of modification of eutectic silicon in un-inoculated and inoculated A356 alloy. Here the grain refinement is obtained due to the presence of TiC particles in the inoculant alloy, which act as substrates for heterogeneous nucleation of -Al. While the Al4Sr plate-lets in the inoculent alloy dissolve in molten A356 alloy at 720oC, and provide Sr for modification of eutectic silicon. Finally, both the primaries as well as the eutectic phases are refined by inoculating the A356 alloy melt with the new master alloy alone. (Patenting of the process of preparation of this new inoculant master alloy is in progress).

 
Dr. B.S. Murty, FNAE, FASc, FNASc
Professor
Dept. of Metallurgical and Materials Engineering
Indian Institute of Technology Madras
Chennai 600 036, India

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