Ti-6Al-4V合金鋳造品の繰り返し加熱冷却熱処理による組織の改良

抄録

The micro-structure of as-cast Ti-6Al-4V alloy is the acicular alpha or the Widmanstätten structure, and it is believed that this structure cannot be changed to other structures by the conventional heat treatment. As forging or rolling cannot be amployed after casting, it is desirable to obtain a pre-heat-treated structure that can be conventionally heat-treated.
The cyclic heating has been developed, by which the acicular alpha or Widmanstätten structure is decomposed to give a two-phase structure (alpha+beta) capable of being conventionally heat-treated.
Two to 50 cycles of heating from 500 to 850∼950°C at a heating rate of 5.8 to 225°C/min have been applied to the castings and the micro-structual changes and the diffusion of the alloying elements were examined by means of a microscope and an X-ray microanalyser.
The following conclusions are obtained.
(1) The cast microstructure is improved by the cyclic heating and became nearly equiaxed alpha grains similar as in the forgings.
(2) It is possible to heat-treat this microstructure by the conventional heating method, and the two-phase structure (alpha+beta) is obtained.
(3) The diffusion of Al and V atoms is observed to stabilize the alpha and beta phase.