In 2 to 3 papers it is mentioned that stirring has adverse effect on the quality of TiBAl. If stirring is there during reaction between halide salts of Ti and B and aluminum melt during TiBAl production the chances of agglomeration of borides are much higher.
The role of stirring is meager in aluminum composites where the reinforcement is synthesized within/inside molten aluminum by chemical reactions of one or more elements. Stirring serves the purpose of creating a vortex to suck the externally added particles in case of stir casting. If the same stirrer setup (same blade design and same stirring speed) is used for in-situ casting process, it will produce undesirable inclusions in the composite. The reaction between the halide salts (K2TiF6/K2ZrF6/KBF4) and molten aluminum will produce large amount of slag and dross which will be floating over the molten aluminum. Stirring action will pull the floating slag inside the aluminum melt and cause numerous inclusions in the casting which will reduce the strength drastically. A mild and momentary stirring can be applied intermittently during the reaction period to disperse the halide salts in the aluminum melt. A final stirring can be given to improve the dispersion after removing all slag just before pouring into a mold. Ultrasonic stirring and electromagnetic stirring can be used to improve dispersion and break nano clusters.
Yes, I agree the comments of Prof.Isaac .
In my point of view, i see the flux floating on the melt during the reaction period actually protects our melt against any bad effects from environment.In such case, if a melt is stirred mechanically then it will bring few issues such as addition of inclusions , contaminants , oxide formation etc like said by Prof .Isaac.
There is an another way usually we do ,that is Controlled stirring can be done with in protective environment using inert gas purging.
Dear Dr Isaac Dinaharan and Sasikumar Samuthiram , thank you very much for the reply with technical reason behind the negative effect of mechanical stirring during TiBAl production. I would like to know- what should be the optimum temperature and time for reaction and holding between halide salts and molten aluminum? Generally it is carried out between 760 to 800 deg cel and for 20 min. For how much time should the melt be hold after complete reaction so that all Ti and B reacts with Al to form TiAl3 (30-60 micron) particles and TiB2 (1-2 micron) without and agglomeration and entrapped oxides?
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