The Ti6Al4V Alloy Microstructure Modification Via Various Cooling Rates, its Influence on Hardness and Microhardness

Abstract

Titanium alloy Ti6Al4V falls into the group of a-b titanium alloys and its widely used for engineering application due to its unique mechanical properties in regular or corrosion environments. Mechanical properties of alloy are strictly depending on final microstructure. The microstructure can be varying by various heat-treatment procedures. Heating slightly over b-transus temperatures, 1050°C, with a dwell of 3 hours and cooling by various rates provide a wide possibility of microstructure modification. The cooling rates were represented by water quenching, air cooling, and furnace cooling. The microstructure has changed from lamellar a-phase in prior b-grains, through Widmanstatten microstructure to lamellar a'-martensite structure due to cooling rates. After applied heat-treatment, the Vickers hardness HV10/10 (STN EN ISO 6507) and Vickers microhardness HV0.2/10 (STN EN ISO 6507) were done. The hardness and microhardness test results were compared to the starting stage. The Vickers hardness increases in all states about 8% for furnace cooling, 18% for air cooling, and almost 40% for water quenching. The same situation was for Vickers microhardness which increases about almost 29% for furnace cooling, 16% for air cooling, and 25% for water quenching. The hardness measurement shows increasing mechanical properties after all cooling rates. However, heat-treatment also shows negatives by creating the acase layer and surface cracks, which is negative for the fatigue life of Ti6Al4V alloy.