Effect of Si-addition as a grain refiner on microstructure and properties of Ti-6Al-4V Alloy

Two different Ti alloys were cast in a graphite mould using vacuum induction skull melting furnace. The first alloy was Ti-6Al-4V and the second was Ti-6Al-4V-0.5Si. Silicon as a grain refiner was added into Ti-6Al-4V alloy, and the effects of Si-addition on the microstructure and properties of the as-cast and swaged alloys were investigated. Hot swaging at 900 degrees C was performed on the cast samples and then two different thermal treatments were applied. The first treatment was done by heating the swaged samples at 1050 degrees C to produce fine lamella structure, while the second treatment was carried out at 1050 degrees C and then decreased the temperature to 800 degrees C for getting coarse lamella structure. An addition of 0.5% Si to Ti-6Al-4V alloy decreased the grain size of the as-cast sample from 627 to 337 mu m. There was an increase in ultimate tensile strength of about 25 MPa for the as-cast Ti-6Al-4V-0.5Si alloy compared to Ti-6Al-4V due to the refinement effect caused by Si addition. A maximum ultimate tensile strength of 1380 MPa and a minimum corrosion rate (1.35x10(-6) mm/a in Hank's solution and 5.78x10(-4) min/a in NaCl solution) were reported for the heat treated fine lamella structure of Ti-6Al-4V-0.5Si alloy. The wear rate was decreased to about 50% by adding 0.5% Si at low sliding speeds and to about 73% at high sliding speeds.