Abstract: The local coarse and clear grain defects are found in the TA15 titanium alloy die forging formed in the dual phase region. The metallographic stitching technology and point by point statistical method are used to analyze the macrostructure and microstructure of different macro-grain regions. It is found that the microstructure of clear grain region and fuzzy grain region are typical duplex microstructure of the titanium alloy. Compared with the fuzzy grain region, the content of primary equiaxes α phase (α_P) in clear grain region is slightly lower, and the lamellar thickness of secondary α phase is slightly thicker, but there are a lot of coarse original β grains, whose average size is more than 3 times of that of fuzzy-grained region. According to papers, the coarse original β grains are the main factors leading to clear grain defects. The critical condition of clear grain on the surface of the forging is proposed. The same batch of original material is used to perform hot compression tests. The evolvement law variation of macrostructure and microstructure are obtained under different deformation processes. The prediction model of the original β grain size of TA15 titanium alloy is established, so the visualization prediction of macrostructure is realized. The results show that the dynamic transformation can lead to the coarsening of β grain and the clear grain of macrostructure. The clear grain is sensitive to temperature, deformation and strain rate. When other conditions are constant, the higher the deformation temperature, the larger the deformation amount and the smaller the strain rate, the higher the tendency of macrostructure to be clear grain.

Key words: titanium alloy, macrostructure, clear grain, prediction model, dynamic transformation