Microstructure and Mechanical Properties of BT20Titanium Alloy Casted in Sand Mold and Ceramic Shell Mold
|School||Harbin Institute of Technology|
|Keywords||BT20titanium alloy Zircon mould heat treatment microstructure mechanical properties|
In this paper, the BT20castings were produced in two kinds of mold as follows:Zircon mold and ceramic shell mold.They were melted and poured in vacuuminduction melting furnace. The smples of the two castings were treated underdifferent heat treatment schemes. Tensile tests of mechanical properties at roomtemperature were measured by an electrical universal material testing machine whilethe microstructure as-cast state and after heat treatment was analyzed by OM andSEM.Cut samples from different wall thickness of the two castings, then comparetheir as-cast state grain size and α lamellar width with each other, we found thatmold type and wall thickness would produce a great impact on the microstructure ofBT20Titanium alloy. The casting which was produced in Zircon sand mold hadthinner α lamellars than the one produced in ceramic shell mold because of itsbigger cooling rate, what’s more, the grain size in thin-walled department was finerthan thick-walled department due to their different thermal environments.Analysis of microstructure under air-cooling condition show that whenair-cooled from the α+β phase region, primary α phase decreased as temperaturerised, the minor α phase in β transformed matrix became wider at the same time andits width was no longer affected by the wall thickness.α lamellar of the samplesunder furnace cooling condition grew continuously and some of them exhibitedequiaxed morphology as temperature rose, moreover, the rose of temperature alsoreduced the number of α lamellars in α colony and decreased the feature of α colony.The different α lamellars width caused by different wall thickness becameprogressively less obvious with increasing temperature, α lamellars are the samewidth in different wall thickness after furnace cooling from β phase region, inaddition, the feature of α colony almost disappear.Tensile tests on samples of BT20alloy after heat treatment showed thatair-cooled from higher temperature of α+β phase region would result in poormechanical properties.It was because the microstructures were composed of a fewprimary α lamellars without colony feature and β transformed matrix around them,and this kind of microstructure caused the obvious reduction of elongation. However,air-cooled from the β phase region would own high strenth and considerableelongation. On the other hand, furnace cooling achived higher elongation but lowerin strength. Furnace cooling from α+β phase field had less influence on mechanicalproperties，however，furnace cooling from β phase field would reduce strength slightly but lessen elongation rapidly. Water quenched from β phase region wouldresult in significant increase of tensile strength which is30%higher than the furnacecooling one, but the elongation was limited.